Ionic liquids are increasingly discussed as alternatives to conventional organic solvents for applications based on photoinduced electron transfer. For the rational design of such applications, reliable estimates of electron-transfer driving forces are essential. Based on the Born model of solvation, the moderate dielectric constants of ionic liquids (εr ≈ 8 – 15) suggest that they should resemble medium-polarity solvents such as dichloromethane or pyridine in photoinduced electron transfer and exhibit comparable solvation energies. Here, we test this assumption by experimentally comparing the solvation energies of three small organic solutes relevant to photochemistry in several imidazolium-based ionic liquids and in conventional dipolar solvents. Solvation energies were inferred from shifts of half-wave reduction potentials obtained from cyclic voltammetry. We find that, for the investigated solutes, ionic liquids provide solvation energies comparable to those of strongly polar solvents such as acetonitrile or dimethyl sulfoxide. While the organic solvents follow the qualitative trend predicted by the Born equation, ionic liquids deviate from it and yield much larger solvation energies compared to dipolar solvents of the same dielectric constant. This behavior is attributed to the intrinsically high ionic strength of ionic liquids, which enhances electrostatic screening and results in substantially larger solvation energies and consequently much larger driving forces for photoinduced electron transfer than would be expected based on their dielectric constants alone.
  
Herein, we reinvestigate the photophysics of ovalene, a prototypical nanographene for which conflicting spectroscopic results have been reported. Owing to its structural similarity and its identical D2h point-group symmetry, ovalene can essentially be viewed as a larger pyrene. We show that its optical transitions can be understood using the same model that is invoked to explain the excited states of pyrene. Absorption and (polarized)-emission measurements reveal that the S1 ← S0 (1B3u ← 1Ag) transition is forbidden, whereas the first prominent absorption band can be assigned to the allowed S2 ← S0 (1B2u ← 1Ag) transition, in contrast to recent reassignments. Temperature and time-dependent spectroscopic measurements show that the S1 and S2 states quickly establish a thermal pre-equilibrium, giving rise to thermally activated S2 → S0 emission at room-temperature. As a result, the fluorescence lifetime of ovalene decreases with increasing temperature while its fluorescence quantum yield increases. Contrary to the frequently cited small energy gap of ∼400 cm–1, our measurements reveal a significantly larger S2–S1 gap of approximately 1200 cm–1.
 
In electron donor-acceptor (D-A) molecules, the relative orientation of constituents has a dramatic influence over their performance. However, the D and A subunits are generally composed of planar aromatic backbones, and the effect of curvature is rarely explored. Here, we investigate how the twist of the aromatic core of a symmetric double-branched D-π-A molecule affects the nature and dynamics of its lower singlet excited state. We show that the twisting of the central donor not only affects the chiroptical properties, and increases the triplet yield, but also facilitates excited-state symmetry breaking (ESSB) and the trapping of the exciton on one D-π-A branch of the molecule. This enhancement is attributed to the decrease in the interbranch coupling upon distortion. Because of this, the loss of the coupling upon ESSB requires a smaller gain in solvation energy to be compensated for and, thus, exciton trapping occurs in a less polar solvent. Consequently, distortion can be viewed as an additional tuning knob for controlling the localisation of electronic excitation in large conjugated systems.
  
lectron donor–acceptor (D–A) polymers are emerging as promising candidates for the development of solid materials with tunable emission. Herein, we investigate the excited-state dynamics of polymers consisting of a central naphthalenediimide (NDI) acceptor with two polystyrene donor chains and copolymers with various secondary donors incorporated. We find strong differences in the dynamics when going from diluted polymer solutions to pure polymer films. In liquids, ultrafast intrachain electron transfer from a styrenic donor to the excited NDI, followed by sub-nanosecond charge recombination to the ground state is observed. Because of the tight packing in the film, ultrafast electron transfer occurs between donors and acceptors of different polymer chains. Emission is found to originate from the most strongly coupled D–A pairs, for which electron transfer is so fast that it leads to a lifetime broadening of the NDI absorption band. Because of this, these highly coupled pairs can be photoselected upon red-edge excitation. The charge-transfer state decays on the tens of ns timescale via radiative and non-radiative charge recombination to the ground state as well as via charge recombination to the triplet state of NDI. This latter pathway, which is detrimental to the fluorescence quantum yield, is almost suppressed with the strongest secondary donor. Finally, we show that excitation of the secondary donor instead of the NDI acceptor does not lead to the population of the charge-transfer state and thus does not contribute to the luminescence of the films.
 
  
A significant part of our understanding of the excited-state properties of polythiophenes comes from studies of smaller oligothiophenes, which have a better-defined structure. Among them, terthiophene (3T) was reported to have an excitation-wavelength dependent triplet quantum yield, ÎŚT. This was explained by the opening of a second intersystem crossing (ISC) pathway upon the high-energy excitation of distorted molecules. Here, we reinvestigate the excited-state dynamics of 3T in solvents of various viscosity and in a polymer film. Our results reveal that, although different subpopulations are excited upon high- and low-energy irradiation, the ISC dynamics, and hence ÎŚT, are the same. We show that the distorted molecules excited at short wavelength undergo rapid planarization independent of the viscosity of the environment before significant ISC takes place. The apparent increase of ÎŚT with increasing excitation wavelength reported earlier can be explained by the neglect of the early relaxation dynamics.
 
Although electron donor–acceptor complexes have been known for more than 70 years and are increasingly used in various applications, very little is still known about their structure in liquids. Here, we investigate the excited-state dynamics of a complex with two charge-transfer (CT) bands, which are usually discussed in terms of two distinct geometries, opening the possibility for photoselection. Apart from an initial ultrafast internal conversion to the lowest CT state upon high-energy band excitation, the ensuing dynamics do not depend on which CT transition has been excited, suggesting complexes of similar structures. The pure ground-state bleach dynamics, extracted using polarized transient absorption measurements, does not exhibit any hole-burning effect and is independent of the excitation wavelength, indicating an absence of photoselection. These results are rationalized using molecular dynamics simulations, which point to a broad distribution of structures with a significant oscillator strength for both transitions, contrary to the generally accepted picture.
  
Boramidine is a small water-soluble organic fluorophore that was recently introduced as a versatile building block of fluorescent probes. Herein, we show that boramidine is protonated in highly protic solvents. This behaviour explains the surprisingly large difference in the absorption spectrum reported previously when going from an organic to an aqueous environment. Transient absorption measurements reveal that the invariance of the fluorescence spectrum to the environment arises from an excited-state proton transfer to the solvent occurring a few ps after photoexcitation of the protonated boramidine. This photoacidity of boramidine is a further add-on to the polyvalence of this fluorophore.
  • Controlling the spatial distribution of electronic excitation in asymmetric D-A-D' and symmetric D'-A-D-A-D' electron donor-acceptor molecules
    E. Balanikas, T. Bianconi, P. Mancini, N.J. Tiwari, M. Sheokand, R. Misra, B. Carlotti and E Vauthey
    Chemical Science, 14 , 2025, p8443-8453
    DOI:10.1039/D5SC01257K | unige:184564 | Abstract | Article HTML | Article PDF | Supporting Info
  
Understanding how electronic energy is funnelled towards a specific location in a large conjugated molecule is of primary importance for the development of a site-specific photochemistry. To this end, we investigate here how electronic excitation redistributes spatially in a series of electron donor-acceptor (D-A) molecules containing two different donors, D and D', and organised in both linear D-A-D' and symmetric double-branch D'-A-D-A-D' geometries. Using transient IR absorption spectroscopy to probe the alkyne spacers, we show that for both types of systems in non-polar solvents, excitation remains delocalised over the whole molecule. In polar media, charge-transfer (CT) exciton in the linear D-A-D' systems localises rapidly at the end with the strongest donor. For the double-branch systems, excited-state symmetry breaking occurs and the CT exciton localises at the end of one of the two branches, even if the D' terminal donor is not the strongest one. This unexpected behaviour is explained by considering that the energy of a CT state depends not only on the electron donating and withdrawing properties of the donor and acceptor constituents, but also on the solvation energy. This study demonstrates the possibility to control the location of CT excitons in large conjugated systems by varying the nature of the donors and acceptors, the distance between them as well as the environment.
  
Boramidines are promising chromophores capable of circularly polarized luminescence (CPL). The synthesis, characterization, and photophysical analysis of novel BINOL- and H8-BINOL-tethered boramidines 1 and 2 are reported, leveraging the chiral perturbation strategy for CPL material design. These enantiopure compounds, prepared in a concise three-step synthesis, exhibit high fluorescence quantum yields (up to 95% in N2-saturated solutions) and luminescence dissymmetry factors (|glum| ~10-3). Transient absorption spectroscopy and quantum-chemical calculations provide insight into their singlet-triplet spin-orbit coupling and intersystem crossing mechanisms.
 
Whereas the photoinduced charge-transfer properties of electron donor–acceptor dyads are now well understood, those of symmetric conjugated architectures containing several identical donor–acceptor branches have started to be scrutinised much more recently. Here, we report on our investigation of the charge-transfer dynamics of a series of formally centrosymmetric triads consisting of a quadrupolar dihydropyrrolopyrrole core substituted with two identical diphenylethynyl lateral branches. Using a combination of time-resolved electronic and vibrational spectroscopies, we show that these molecules exhibit rich excited-state dynamics, which includes three different types of symmetry-breaking charge-transfer processes depending on the nature of the end substituents on the core and branches as well as on the solvent: (i) excited-state symmetry breaking within the core; (ii) charge transfer from the core to one of the two branches; (iii) charge transfer between the two branches. This investigation illustrates how the excited-state properties of symmetric conjugated molecules, including the nature and location of the exciton, can be controlled by fine tuning structural as well as environmental parameters.
  
Electronic excitation in quadrupolar conjugated molecules rapidly localizes on a single electron donor–acceptor (DA) branch when in polar environments. The loss of center of inversion upon this excited-state symmetry breaking (ES-SB) can be monitored by exploiting the relaxation of the exclusion rules for IR and Raman vibrational transitions. Here, we compare ES-SB in a right-angled (1) and a centrosymmetric (2) DAD dyes using time-resolved IR spectroscopy. We show that the localization of the excitation can also be identified with the bent molecule 1. We find that contrary to dye 2, subpopulations with localized and delocalized excitation coexist for 1 in weak to medium polar solvents. This difference originates from the torsional disorder present in the excited state of 1 but not of 2. Additionally, irreversible localization in a bent molecule is shown to require higher solvent polarity than in a centrosymmetric one.
 
The torsional disorder of conjugated dyes in the electronic ground state can lead to inhomogeneous broadening of the S1 ←S0 absorption band, allowing for the selective photoexcitation of molecules with different amounts of distortion. Here, we investigate how this affects electronic transitions to upper excited states. We show that torsion of a core-alkynylated push–pull dye can have opposite effects on the oscillator strength of its lowest-energy transitions. Consequently, photoselection of planar and twisted molecules can be achieved by exciting in distinct absorption bands. Whereas this has limited effect in liquids due to fast planarization of the excited molecules, it strongly affects the overall photophysics in a polymeric environment, where torsional motion is hindered, allowing for the photoselection of molecules with different fluorescence quantum yields and intersystem-crossing dynamics.
  
Fluorescent flippers have been introduced as small-molecule probes to image membrane tension in living systems. This study describes the design, synthesis, spectroscopic and imaging properties of flippers that are elongated by one and two alkynes inserted between the push and the pull dithienothiophene domains. The resulting mechanophores combine characteristics of flippers, reporting on physical compression in the ground state, and molecular rotors, reporting on torsional motion in the excited state, to take their photophysics to new level of sophistication. Intensity ratios in broadened excitation bands from differently twisted conformers of core-alkynylated flippers thus report on mechanical compression. Lifetime boosts from ultrafast excited-state planarization and lifetime drops from competitive intersystem crossing into triplet states report on viscosity. In standard lipid bilayer membranes, core-alkynylated flippers are too long for one leaflet and tilt or extend into disordered interleaflet space, which preserves rotor-like torsional disorder and thus weak, blue-shifted fluorescence. Flipper-like planarization occurs only in highly ordered membranes of matching leaflet thickness, where they light up and selectively report on these thick membranes with red-shifted, sharpened excitation maxima, high intensity and long lifetime.
 
The inclusion of boron atoms into chiral π-conjugated systems is an effective strategy to unlock unique chiroptical properties. Herein, the preparation and characterization of a configurationally stable axially-chiral boramidine are reported, showcasing absorption in the UV domain, deep-blue fluorescence ( up to 94%), and ca. |10-3| g_abs and g_lum values. Detailed photophysical studies and quantum-chemical calculations clearly elucidate the deactivation pathways of the emissive state to triplet excited states, involving increased spin–orbit coupling between the lowest singlet excited state and an upper triplet state.
  
Dye-sensitized photocatalytic systems (DSPs) have been extensively investigated for solar-driven hydrogen (H2) evolution. However, their application in carbon dioxide (CO2) reduction remains limited. Furthermore, current solar-driven CO2-to-CO DSPs typically employ rhenium complexes as catalysts. In this study, we have developed DSPs that incorporate noble metal-free components, specifically a zinc-porphyrin as photosensitizer (PS) and a cobalt-quaterpyridine as catalyst (CAT). Taking a significant stride forward, we have achieved an antenna effect for the first time in CO2-to-CO DSPs by introducing a Bodipy as an additional chromophore to enhance light harvesting efficiency. The energy transfer from Bodipy to zinc porphyrin resulted in remarkable stability (turn over number (TON)=759 vs. CAT), and high CO evolution activity (42?mmol?g?1?h?1 vs. CAT).
 
So far, symmetry-breaking charge separation (SB-CS) has been observed with a limited number of chromophores and is usually inhibited by the formation of an excimer. We show here that thanks to fine-tuning of the interchromophore coupling via structural control, SB-CS can be operative with pyrene, despite its high propensity to form an excimer. This is realized with a bichromophoric system consisting of two pyrenes attached to a crown ether macrocycle, which can bind cations of different sizes. By combining stationary and time-resolved spectroscopy together with molecular dynamics simulations, we demonstrate that the excited-state dynamics can be totally changed depending on the binding cation. Whereas strong coupling leads to rapid excimer formation, too weak coupling results in noninteracting chromophores. However, intermediate coupling, achieved upon binding of Mg2+, allows for SB-CS to be operative.
  
There is a growing interest in developing dye-sensitized photocatalytic systems (DSPs) to produce molecular hydrogen (H2) as alternative energy source. To improve the sustainability of this technology, we replaced the sacrificial electron donor (SED), typically an expensive and polluting chemical, with an alcohol oxidation catalyst. This study demonstrates the first dye-sensitized system using a diketopyrrolopyrrole dye covalently linked to 2,2,6,6-tetramethyl-1-piperidine-N-oxyl (TEMPO) based catalyst for simultaneous H2 evolution and alcohol-to-aldehyde transformation operating in water with visible irradiation.
 
A large number of multipolar dyes undergo excited-state symmetry breaking (ESSB) in polar media. During this process, electronic excitation, initially distributed evenly over the molecule, localizes, at least partially, on one donor–acceptor branch. To resolve its initial stage, ESSB is investigated with a donor–acceptor–donor dye in binary mixtures of nonpolar and polar solvents using time-resolved infrared absorption spectroscopy. The presence of a few polar molecules around the dye is sufficient to initiate ESSB. Although the extent of asymmetry in a mixture is close to that in a pure solvent of similar polarity, the dynamics are slower and involve translational diffusion. However, preferential solvation in the mixtures leads to a larger local polarity. Furthermore, inhomogeneous broadening of the S1 <- S0 absorption band of the dye is observed in the mixtures, allowing for a photoselection of solutes with different local environments and ESSB dynamics.
  
Electron transfer (ET) quenching in nonpolar media is not as well understood as in polar environments. Here, we investigate the effect of dipole–dipole interactions between the reactants using ultrafast broadband electronic spectroscopy combined with molecular dynamics simulations. We find that the quenching of the S1 state of two polar dyes, coumarin 152a and Nile red, by the polar N,N-dimethylaniline (DMA) in cyclohexane is faster by a factor up to 3 when exciting on the red edge rather than at the maximum of their S1 ← S0 absorption band. This originates from the inhomogeneous broadening of the band due to a distribution of the number of quencher molecules around the dyes. As a consequence, red-edge excitation photoselects dyes in a DMA-rich environment. Such broadening is not present in acetonitrile, and no excitation wavelength dependence of the ET dynamics is observed. The quenching of both dyes is markedly faster in nonpolar than polar solvents, independently of the excitation wavelength. According to molecular dynamics simulations, this is due to the preferential solvation of the dyes by DMA in cyclohexane. The opposite preferential solvation is predicted in acetonitrile. Consequently, close contact between the reactants in acetonitrile requires partial desolvation. By contrast, the recombination of the quenching product is slower in nonpolar than in polar solvents and exhibits much smaller dependence, if any, on the excitation wavelength.
 
Photoinduced symmetry-breaking charge separation (SB-CS) results in the generation of charge carriers through electron transfer between two identical molecules, after photoexcitation of one of them. It is usually studied in systems where the two reacting moieties are covalently linked. Examples of photoinduced bimolecular SB-CS with organic molecules yielding free ions remain scarce due to solubility or aggregation issues at the high concentrations needed to study this diffusion-assisted process. Here we investigate the excited-state dynamics of perylene (Pe) at high concentrations in solvents of varying polarity. Transient absorption spectroscopy on the subnanosecond to microsecond timescales reveal that self-quenching of Pe in the lowest singlet excited state leads to excimer formation in all solvents used. Additionally, bimolecular SB-CS, resulting in the generation of free ions, occurs concurrently to excimer formation in polar media, with a relative efficiency that increases with the polarity of the solvent. Moreover, we show that SB-CS is most efficient in room-temperature ionic liquids due to a charge-shielding effect leading to a larger escape of ions and due to the high viscosity that disfavours excimer formation.
  • Coupling to octahedral tilts in halide perovskite nanocrystals induces phonon-mediated attractive interactions between excitons
    N. Yazdani, M.I. Bodnarchuk, F. Bertolotti, N. Masciocchi, I. Fureraj, B. Guzelturk, B. Cotts, M. Zajac, G. Raino, M. Jansen, S.C. Boehme, M. Yarema, M.F. Lin, M. Kozina, A. Reid, X. Shen, S. Weathersby, X. Wang, E. Vauthey, A. Guagliardi, M.V. Kovalenko, V. Wood and A.M. Lindenberg
    Nature Physics, 20 , 2024, p47-53
    DOI:10.1038/s41567-023-02253-7 | unige:174617 | Abstract | Article HTML | Article PDF
Understanding the origin of electron-phonon coupling in lead halide perovskites is key to interpreting and leveraging their optical and electronic properties. Here we show that photoexcitation drives a reduction of the lead-halide-lead bond angles, a result of deformation potential coupling to low-energy optical phonons. We accomplish this by performing femtosecond-resolved, optical-pump-electron-diffraction-probe measurements to quantify the lattice reorganization occurring as a result of photoexcitation in nanocrystals of FAPbBr(3). Our results indicate a stronger coupling in FAPbBr(3) than CsPbBr(3). We attribute the enhanced coupling in FAPbBr(3) to its disordered crystal structure, which persists down to cryogenic temperatures. We find the reorganizations induced by each exciton in a multi-excitonic state constructively interfere, giving rise to a coupling strength that scales quadratically with the exciton number. This superlinear scaling induces phonon-mediated attractive interactions between excitations in lead halide perovskites.
  
A significant number of quadrupolar dyes behave as their dipolar analogues when photoexcited in polar environments. This is due to the occurrence of excited-state symmetry breaking (ES-SB), upon which the electronic excitation, initially distributed over the whole molecule, localises preferentially on one side. Here, we investigate the ES-SB properties of two A–D–A dyes, consisting of a pyrrolo-pyrrole donor (D) and either cyanophenyl or dicyanovinyl acceptors (A). For this, we use time-resolved vibrational spectroscopy, comparing IR absorption and femtosecond stimulated Raman spectroscopies. Although dicyanovinyl is a stronger electron-withdrawing group, ES-SB is not observed with the dicyanovinyl-based dye even in highly polar media, whereas it already takes place in weakly polar solvents with dyes containing cyanophenyl accepting groups. This difference is attributed to the large electronic coupling between the D–A branches in the former dye, whose loss upon symmetry breaking cannot be counterbalanced by a gain in solvation energy. Comparison with analogues of the cyanophenyl-based dye containing different spacers reveals that interbranch coupling does not so much depend on the distance between the D–A subunits than on the nature of the spacer. We show that transient Raman spectra probe different modes of these centrosymmetric molecules but are consistent with the transient IR data. However, lifetime broadening of the Raman bands, probably due to the resonance enhancement, may limit the application of this technique for monitoring ES-SB.
 
The nature of the lowest-energy electronic absorption band of crystal violet (CV) and particularly the origin of its high-energy shoulder have been debated since the middle of the past century. The most recent studies invoke a splitting of the S1 state upon symmetry breaking induced by interactions with the solvent and/or the counterion. Using a combination of stationary and time-resolved polarized spectroscopy together with quantum-chemical calculations, we show that torsional disorder in the ground-state results in an inhomogeneous broadening of the absorption band of CV. The center of the band is mostly due to symmetric molecules with a degenerate S1 state, whereas the edges originate from transitions to the S1 and S2 states of distorted symmetry-broken molecules. Transient-absorption measurements with different excitation wavelengths reveal that these two groups of molecules interconvert rapidly in liquid but not in a rigid environment.
  
Occurrence of chiral recognition in bimolecular photoinduced electron transfer (ET) is difficult to identify because of the predominant role of diffusion. To circumvent this problem, we apply a combination of ultrafast time-resolved fluorescence and transient electronic absorption to look for stereoselectivity in the initial, static stage of ET quenching, where diffusion is not relevant. The fluorophore and electron acceptor is a cationic hexahelicene, whereas the quencher has either stereocentered (tryptophan) or axial (binaphthol) chirality. We found that, in all cases, the quenching dynamics are the same, within the limit of error, for different diastereomeric pairs in polar and medium-polar solvents. The same absence of chiral effect is observed for the recombination of the radical pair, which results from the quenching. Molecular dynamics simulations suggest that the distribution of inter-reactant distance is independent of the chirality of the acceptor and the donor. Close contact resulting in large electronic coupling is predicted to be possible with all diastereomeric pairs. In this case, ET is an adiabatic process, whose dynamics do no longer depend on the coupling, but are rather controlled by high-frequency intramolecular modes.
  • Experiment and theory reveal similarities and differences between porphycenes substituted at the meso position with amino and nitro groups
    I. Mbakara, A. Gajewska, A. Listkowski, M. Kijak, K. Nawara, T. Kumpulainen, E. Vauthey and J. Waluk
    Physical Chemistry Chemical Physics, 24 , 2022, p29655-29666
    DOI:10.1039/D2CP04555A | unige:165882 | Abstract | Article HTML | Article PDF
Parent, unsubstituted porphycene and its two derivatives: 2,7,12,17-tetra-n-propylporphycene and 2,7,12,17-tetra-t-butylporphycene were substituted at the meso position with amino and nitro groups. These two families of porphycenes were characterized in detail with respect to their spectral, photophysical, and tautomeric properties. Two trans tautomers of similar energies coexist in the ground electronic state, but only one form dominates in the lowest excited singlet state. Absorption, magnetic circular dichroism (MCD), and emission anisotropy combined with quantum-chemical calculations led to the assignment of S1 and S2 transitions in both tautomers. Compared with the parent porphycene, the S1–S2 energy gap significantly increases; for one tautomeric form, the effect is twice as large as for the other. Both amino- and nitroporphycenes emit single fluorescence; previously reported dual emission of aminoporphycenes is attributed to a degradation product. Introduction of bulky t-butyl groups leads to a huge decrease in fluorescence intensity; this effect, arising from the interaction of the meso substituent with the adjacent t-butyl moiety, is particularly strong in the nitro derivative.
 
Conjugated molecules with phenylethynyl building blocks are usually characterised by torsional disorder at room temperature. They are much more rigid in the electronic excited state due to conjugation. As a consequence, the electronic absorption and emission spectra do not present a mirror-image relationship. Here, we investigate how torsional disorder affects the excited state dynamics of 9,10-bis(phenylethynyl)anthracene in solvents of different viscosities and in polymers, using both stationary and ultrafast electronic spectroscopies. Temperature-dependent measurements reveal inhomogeneous broadening of the absorption spectrum at room temperature. This is confirmed by ultrafast spectroscopic measurements at different excitation wavelengths. Red-edge irradiation excites planar molecules that return to the ground state without significant structural dynamics. In this case, however, re-equilibration of the torsional disorder in the ground state can be observed. Higher-energy irradiation excites torsionally disordered molecules, which then planarise, leading to important spectral dynamics. The latter is found to occur partially via viscosity-independent inertial motion, whereas it is purely diffusive in the ground state. This dissimilarity is explained in terms of the steepness of the potential along the torsional coordinate.
  
The excited-state properties of an amphiphilic porphyrin-fullerene dyad and of its porphyrin analogue adsorbed at the dodecane/water interface are investigated by using surface second-harmonic generation. Although the porphyrin is formally centrosymmetric, the second-harmonic spectra of both compounds are dominated by the intense Soret band of the porphyrin. Polarization-selective measurements and molecular dynamics simulations suggest an angle of about 45° between the donor-acceptor axis and the interfacial plane, with the porphyrin interacting mostly with the nonpolar phase. Time-resolved measurements reveal a marked concentration dependence of the dynamics of both compounds upon Q-band excitation, indicating the occurrence of intermolecular quenching processes. The significant differences in dynamics and spectra between the dyad and the porphyrin analogue are explained by a self-quenching of the excited dyad via an intermolecular electron transfer.
A series of nine soluble, symmetric chalcogenophenes bearing hexyl-substituted triphenylamines, indolocarbazoles, or phenylcarbazoles was designed and synthesized as potential two-photon absorption (2PA) initiators. A detailed photophysical analysis of these molecules revealed good 2PA properties of the series and, in particular, a strong influence of selenium on the 2PA cross sections, rendering these materials especially promising new 2PA photoinitiators. Structuring and threshold tests proved the efficiency and broad spectral versatility of two selenium-containing lead compounds as well as their applicability in an acrylate resin formulation. A comparison with commercial photoinitiators Irg369 and BAPO as well as sensitizer ITX showed that the newly designed selenium-based materials TPA-S and TPA-BBS outperform these traditional initiators by far both in terms of reactivity and dose. Moreover, by increasing the ultralow concentration of TPA-BBS, a further reduction of the polymerization threshold can be achieved, revealing the great potential of this series for application in two-photon polymerization (2PP) systems where only low laser power is available.
 
he emissive properties of symmetric molecules containing several donor-acceptor branches are often similar to those of the single-branched analogues. This is due to the at least partial localization of the excitation on one branch. Detailed understanding of this excited-state symmetry breaking (ES-SB) requires the ability to monitor this process in real time. Over the past few years, several spectroscopic approaches were shown to enable visualization of ES-SB and of its dynamics. They include the detection of new vibrational or electronic absorption bands associated with transitions that are forbidden in the symmetric excited state. Alternatively, ES-SB can be detected by observing transitions that become weaker or vanish upon localization of the excitation. Herein, we discuss these different approaches as well as their merits and weaknesses.
  
The current developments in photoredox chemistry are stimulating a renewed interest for bimolecular photoinduced electron transfer reactions. Their investigation, initiated in the 1960s using conventional photochemical tools, resulted in a relatively simple reaction scheme. More recent studies, using not only spectroscopic techniques with better time resolution and extended spectral/temporal windows but also molecular dynamics simulations, reveal a more complex picture. This Perspective focuses on the results of these latest studies with neutral organic reactants, highlighting the time dependence of the quenching rate, the effect of mutual orientation of the reactants on the electronic coupling, and their consequence on the nature of the reaction product. Remaining questions, such as the occurrence of distant electron transfer in nonviscous liquids are also addressed, and possible directions toward their answer are proposed.
 
The radical anion of 9,10-dicyanoanthracene (DCA) has been suggested to be a promising chromophore for photoredox chemistry, due to its nanosecond excited-state lifetime determined from indirect measurements. Here, we investigate the excited-state dynamics of the radical anion of three cyanoanthracenes, including DCA˙−, produced by photoinduced electron transfer in liquid using both pump–probe and pump–pump probe transient electronic absorption spectroscopy. All three excited radical ions are characterised by a 3–5 ps lifetime, due to efficient non-radiative deactivation to the ground state. The decay pathway most probably involves D1/D0 conical intersection(s), whose presence is favoured by the enhanced flexibility of the radical anions relative to their neutral counterparts. The origin of the discrepancy with the nanosecond lifetime of DCA˙−* reported previously is discussed. These very short lifetimes limit, but do not preclude, photochemical applications of the cyanoanthracene anions.
  
 
  
  • Dye-Sensitized Photoelectrosynthesis Cells for Benzyl Alcohol Oxidation Using a Zinc Porphyrin Sensitizer and TEMPO Catalyst
    E. Nikoloudakis, B.Pati Palas, G. Charalambidis, D.S. Budkina, S. Diring, A. Planchat, D. Jacquemin, E. Vauthey, A.G. Coutsolelos and F. Odobel
    ACS Catalysis, 11 (19) , 2021, p12075-12086
    DOI:10.1021/acscatal.1c02609 | unige:154975 | Abstract | Article HTML | Article PDF | Supporting Info
 
Excited-state symmetry breaking is investigated in a series of symmetric 9,10-dicyanoanthracenes linked to electron-donating groups on the 2 and 6 positions via different spacers, allowing for a tuning of the length of the donor-acceptor branches. The excited-state properties of these compounds are compared with their dipolar single-branch analogues. The changes in electronic structure upon their optical excitation are monitored by transient electronic spectroscopy in the visible and near-infrared regions as well as by transient vibrational spectroscopy in the mid-infrared. Our results reveal that, with the shortest branches, electronic excitation remains distributed almost symmetrically over the molecule even in polar environments. Upon increasing the donor–acceptor distance, excitation becomes unevenly distributed and, with the longest one, it fully localises on one branch in polar solvents. The influence of the branch length on the propensity of quadrupolar dyes to undergo excited-state symmetry breaking is rationalised in terms of the balance between interbranch coupling and solvation energy.
  
Excited-state symmetry breaking (ES-SB) is common to a large number of multibranched electron donor-acceptor (DA) molecules in polar environments. During this process, the electronic excitation, originally evenly distributed over the molecule, localizes, at least partially, on one branch. Due to the absence of an unambiguous spectroscopic signature in the UV-vis region, electronic transient absorption (TA) has not been the method of choice for real-time observation of this phenomenon. Herein, we demonstrate that the Laporte rule, which states that one-photon transitions conserving parity are forbidden in centrosymmetric molecules, provides such clear signature of ES-SB in electronic TA spectroscopy. Using a dicyanoanthracene-based D-A-D dye, we show that transitions from the S1 state of this molecule, which are initially Laporte forbidden, become allowed upon ES-SB. This leads to the rise of new TA bands, whose intensity provides a direct measure of the extent of asymmetry in the excited state.
  • Porous shape-persistent rylene imine cages with tunable optoelectronic properties and delayed fluorescence
    H.-H. Huang, K.S. Song, A. Prescimone, A. Aster, G. Cohen, R. Mannancherry, E. Vauthey, A. Coskun and T. Ĺ olomek
    Chemical Science, 12 , 2021, p5275-5285
    DOI:10.1039/D1SC00347J | unige:150993 | Abstract | Article HTML | Article PDF | Supporting Info
A simultaneous combination of porosity and tunable optoelectronic properties, common in covalent organic frameworks, is rare in shape-persistent organic cages. Yet, organic cages offer important molecular advantages such as solubility and modularity. Herein, we report the synthesis of a series of chiral imine organic cages with three built-in rylene units by means of dynamic imine chemistry and we investigate their textural and optoelectronic properties. Thereby we demonstrate that the synthesized rylene cages can be reversibly reduced at accessible potentials, absorb from UV up to green light, are porous, and preferentially adsorb CO2 over N2 and CH4 with a good selectivity. In addition, we discovered that the cage incorporating three perylene-3,4:9,10-bis(dicarboximide) units displays an efficient delayed fluorescence. Time-correlated single photon counting and transient absorption spectroscopy measurements suggest that the delayed fluorescence is likely a consequence of a reversible intracage charge-separation event. Rylene cages thus offer a promising platform that allows combining the porosity of processable materials and photochemical phenomena useful in diverse applications such as photocatalysis or energy storage.
  • Long-lived triplet charge-separated state in naphthalenediimide based donor-acceptor systems
    A. Aster, C. Rumble, A.-B. Bornhof, H.-H. Huang, N. Sakai, T. Ĺ olomek, S. Matile and E. Vauthey
    Chemical Science, 12 , 2021, p4908-4915
    DOI:10.1039/D1SC00285F | unige:150871 | Abstract | Article HTML | Article PDF | Supporting Info
  
1,4,5,8-Naphthalenediimides (NDIs) are widely used motifs to design multichromophoric architectures due to their ease of functionalisation, their high oxidative power and the stability of their radical anion. The NDI building block can be incorporated in supramolecular systems by either core or imide functionalization. We report on the charge-transfer dynamics of a series of electron donor–acceptor dyads consisting of a NDI chromophore with one or two donors linked at the axial, imide position. Photo-population of the core-centred π–π* state is followed by ultrafast electron transfer from the electron donor to the NDI. Due to a solvent dependent singlet–triplet equilibrium inherent to the NDI core, both singlet and triplet charge-separated states are populated. We demonstrate that long-lived charge separation in the triplet state can be achieved by controlling the mutual orientation of the donor–acceptor sub-units. By extending this study to a supramolecular NDI-based cage, we also show that the triplet charge-separation yield can be increased by tuning the environment.
  
This study addresses a practical aspect of hybrid dye-sensitized photoelectrochemical cells by exploring a simple method to prepare multicomponent systems. Building on a previously reported methodology based on a copper-free click chemistry dipolar cycloaddition of azide with activated alkyne, a naphthalene diimide (NDI) derivative substituted with two propiolic esters was clicked on a NiO photocathode already coated with a diketopyrrolopyrrole (DPP) dye bearing two azido groups. A detailed photophysical study by transient absorption spectroscopy demonstrates that optical excitation of DPP dye leads to an effective electron transfer chain from the NiO valence band to the NDI passing via the DPP dye, resulting in a long-lived charge-separated state (hole in NiO/NDI radical anion) of 170 μs. The p-type dye-sensitized solar cells were also fabricated with the above molecular components and confirm the occurrence of the electron transfer as the performances of the solar cells were improved in terms of Voc and Jsc compared to the DPP dye lacking the NDI unit. The above-clicked system was also compared to a covalently linked DPP–NDI dyad, whose performances are 30% superior to the clicked system probably due to longer mean distance between the NiO surface and the NDI with the dyad. This finding paves the way for the design of multicomponent hybrid dye-sensitized photoelectrochemical cells by chemistry on the electrode.
 
Singlet fission (SF), i.e., the splitting of a high-energy exciton into two lower-energy triplet excitons, has the potential to increase the efficiency for harvesting spectrally broad light. The path from the photopopulated singlet state to free triplets is complicated by competing processes that decrease the overall SF efficiency. A detailed understanding of the whole cascade and the nature of the photoexcited singlet state is still a major challenge. Here, we introduce a pentacene dimer with a flexible crown ether spacer enabling a control of the interchromophore coupling upon solvent-induced self-aggregation as well as cation binding. The systematic change of solvent polarity and viscosity and excitation wavelength, as well as the available conformational phase space, allows us to draw a coherent picture of the whole SF cascade from the femtosecond to microsecond time scales. High coupling leads to ultrafast SF (<2 ps), independent of the solvent polarity, and to highly coupled correlated triplet pairs. The absence of a polarity effect indicates that the solvent coordinate does not play a significant role and that SF is driven by intramolecular modes. Low coupling results in much slower SF (∼500 ps), which depends on viscosity, and leads to weakly coupled correlated triplet pairs. These two triplet pairs could be spectrally distinguished and their contribution to the overall SF efficiency, i.e., to the population of free triplets, could be determined. Our results reveal how the overall SF efficiency can be increased by conformational restrictions and control of the structural fluctuation dynamics.
  • Universal quenching of common fluorescent probes by water and alcohols
    J. Maillard, K. Klehs, C. Rumble, E. Vauthey, M. Heilemann and A. FĂźrstenberg
    Chemical Science, 12 , 2021, p1352-1362
    DOI:10.1039/D0SC05431C | unige:148739 | Abstract | Article HTML | Article PDF
Although biological imaging is mostly performed in aqueous media, it is hardly ever considered that water acts as a classic fluorescence quencher for organic fluorophores. By investigating the fluorescence properties of 42 common organic fluorophores recommended for biological labelling, we demonstrate that H2O reduces their fluorescence quantum yield and lifetime by up to threefold and uncover the underlying fluorescence quenching mechanism. We show that the quenching efficiency is significantly larger for red-emitting probes and follows an energy gap law. The fluorescence quenching finds its origin in high-energy vibrations of the solvent (OH groups), as methanol and other linear alcohols are also found to quench the emission, whereas it is restored in deuterated solvents. Our observations are consistent with a mechanism by which the electronic excitation of the fluorophore is resonantly transferred to overtones and combination transitions of high-frequency vibrational stretching modes of the solvent through space and not through hydrogen bonds. Insight into this solvent-assisted quenching mechanism opens the door to the rational design of brighter fluorescent probes by offering a justification for protecting organic fluorophores from the solvent via encapsulation.
  
The absorption band shape of chromophores in liquid solution at room temperature is usually dominated by pure electronic dephasing dynamics, which occurs on the sub-100 fs time scale. Herein, we report on a series of dyads consisting of a naphthalenediimide (NDI) electron acceptor with one or two phenyl-based donors for which photoinduced intramolecular electron transfer is fast enough to be competitive with pure electronic dephasing. As a consequence, the absorption band of the π-π* transition of these dyads is broader than that of the NDI alone to an extent that scales with the electron transfer rate. Additionally, this reaction is so fast that it leads to the impulsive excitation of a low-frequency vibrational mode of the charge-separated product. Quantum-chemical calculations suggest that this vibration involves the C-N donor-acceptor bond, which shortens considerably upon electron transfer.
 
Interfaces with room-temperature ionic liquids (ILs) play key roles in many applications of these solvents, but our understanding of their properties is still limited. We investigate how the addition of ILs in the aqueous subphase affects the adsorption of the cationic dye malachite green at the dodecane/water interface using stationary and time-resolved surface second harmonic generation. We find that the interfacial concentration of malachite green depends crucially on the nature of both anionic and cationic constituents. This concentration reports on the overall charge of the interface, which itself depends on the relative interfacial affinity of the ions. Our results reveal that the addition of ILs to the aqueous subphase has similar effects to the addition of conventional salts. However, the IL cations have a significantly higher propensity to adsorb than small inorganic cations. Furthermore, the IL constituents show a synergistic effect, as the interfacial concentration of each of them also depends on the interfacial affinity of the other.
  
Naphthalenediimides (NDIs) are privileged scaffolds par excellence, of use in functional systems from catalysts to ion channels, photosystems, sensors, ordered matter in all forms, tubes, knots, stacks, sheets, vesicles, and colored over the full visible range. Despite this extensively explored chemical space, there is still room to discover core-substituted NDIs with fundamentally new properties: NDIs with cyclic trisulfides (i.e., trisulfanes) in their core absüorb at 668?nm, emit at 801?nm, and contract into disulfides (i.e., dithietes) upon irradiation at <475?nm. Intramolecular 1,5-chalcogen bonds account for record redshifts with trisulfides, ring-tension mediated chalcogen-bond-mediated cleavage for blueshifts to 492?nm upon ring contraction. Cyclic oligochalcogenides (COCs) in the NDI core open faster than strained dithiolanes as in asparagusic acid and are much better retained on thiol exchange affinity columns. This makes COC-NDIs attractive not only within the existing multifunctionality, particularly artificial photosystems, but also for thiol-mediated cellular uptake.
 
Reliable estimation of the driving force for photoinduced electron transfer between neutral reactants is of utmost importance for most practical applications of these reactions. The driving force is usually calculated from the Weller equation, which contains a Coulomb term, C, whose magnitude in polar solvents is debated. We have performed umbrella sampling molecular dynamics simulations to determine C from the potentials of mean force between neutral and ionic donor/acceptor pairs of different sizes in solvents of varying polarity. According to the simulations, C in polar solvents is a factor of 2 more negative than typically calculated according to the Weller equation. Use of the π-stack contact distance in the Weller equation instead of the van der Waals radius recovers the correct value of C, but this is mostly fortuitous due to the compensating effects of overestimating the dielectric screening at contact and neglecting both charge dilution and desolvation.
  
 
  
  • Chemistry on the electrodes: post-functionalization and stability enhancement of anchored dyes on mesoporous metal oxide photoelectrochemical cells with copper-free Huisgen cycloaddition reaction
    Y. Bentounsi, K. Seintis, D. Ameline, S. Diring, D. Provost, E. Blart, Y. Pellegrin, D. Cossement, E. Vauthey and F Odobel
    J. Mater. Chem. A, 8 , 2020, p12633-12640
    DOI:10.1039/D0TA04982D | unige:138165 | Abstract | Article HTML | Article PDF
  • Mechanosensitive membrane probes: push-pull papillons
    H.V. Humeniuk, G. Licari, E. Vauthey, N. Sakai and S. Matile
    Supramol. Chem., 32 , 2020, p106-111
    DOI:10.1080/10610278.2019.1702193 | unige:131618 | Abstract | Article HTML | Article PDF
Design, synthesis and evaluation of push-pull N,N′-diphenyl-dihydrodibenzo[a,c]phenazines are reported. Consistent with theoretical predictions, donors and acceptors attached to the bent mechanophore are shown to shift absorption maxima to either red or blue, depending on their positioning in the chromophore. Redshifted excitation of push-pull fluorophores is reflected in redshifted emission of both bent and planar excited states. The intensity ratios of the dual emission in more and less polar solvents imply that excited-state (ES) planarization decelerates with increasing fluorophore macrodipole, presumably due to attraction between the wings of closed papillons. ES planarization of highly polarisable papillons is not observed in lipid bilayer membranes. All push-pull papillon amphiphiles excel with aggregation-induced emission (AIE) from bent ES as micelles in water and mechanosensitivity in viscous solvents. They are not solvatochromic and only weakly fluorescent (QY < 4%).
 
  
The nature of the electronic excited state of many symmetric multibranched donor–acceptor molecules varies from delocalized/multipolar to localized/dipolar depending on the environment. Solvent-driven localization breaks the symmetry and traps the exciton in one branch. Using a combination of ultrafast spectroscopies, we investigate how such excited-state symmetry breaking affects the photochemical reactivity of quadrupolar and octupolar A(-π-D)2,3 molecules with photoisomerizable A-π-D branches. Excited-state symmetry breaking is identified by monitoring several spectroscopic signatures of the multipolar delocalized exciton, including the S2←S1 electronic transition, whose energy reflects interbranch coupling. It occurs in all but nonpolar solvents. In polar media, it is rapidly followed by an alkyne-allene isomerization of the excited branch. In nonpolar solvents, slow and reversible isomerization corresponding to chemically-driven symmetry breaking, is observed. These findings reveal that the photoreactivity of large conjugated molecules can be tuned by controlling the localization of the excitation.
 
A significant number of quadrupolar dyes with a D-π-A-π-D or  A-π-D-π-A  structure, where D and A are electron donor and acceptor groups, were shown to undergo symmetry breaking (SB) upon optical excitation. During this process, electronic excitation, originally distributed evenly over the molecule, concentrates on one D−π–A branch, and the molecule becomes dipolar. This process can be monitored by time-resolved infrared spectroscopy and causes significant spectral dynamics. A theoretical model of excited-state SB developed earlier (Ivanov, A. I. J. Phys. Chem. C2018,122, 29165–29172) is extended to account for the temporal changes taking place in the IR spectrum upon SB. This model can reproduce the IR spectral dynamics observed in the -C≡C- stretching region with a D-π-A-π-D dye in two polar solvents using a single set of molecular parameters. This approach allows estimating the degree of asymmetry of the excited state in different solvents and its change during SB. Additionally, the relative contribution of different mechanisms responsible for the splitting of the symmetric and antisymmetric -C≡C-  stretching bands, which are both IR active upon SB, can be determined.
  
  • Halogen-Bond Assisted Photoinduced Electron Transfer
    B. Dereka, I. Fureraj, A. Rosspeintner and E. Vauthey
    Molecules, 24 (23) , 2019, p4361
    DOI:10.3390/molecules24234361 | unige:127199 | Abstract | Article HTML | Article PDF
The formation of a halogen-bond (XB) complex in the excited state was recently reported with a quadrupolar acceptor–donor–acceptor dye in two iodine-based liquids (J. Phys. Chem. Lett. 2017, 8, 3927–3932). The ultrafast decay of this excited complex to the ground state was ascribed to an electron transfer quenching by the XB donors. We examined the mechanism of this process by investigating the quenching dynamics of the dye in the S1 state using the same two iodo-compounds diluted in inert solvents. The results were compared with those obtained with a non-halogenated electron acceptor, fumaronitrile. Whereas quenching by fumaronitrile was found to be diffusion controlled, that by the two XB compounds is slower, despite a larger driving force for electron transfer. A Smoluchowski–Collins–Kimball analysis of the excited-state population decays reveals that both the intrinsic quenching rate constant and the quenching radius are significantly smaller with the XB compounds. These results point to much stronger orientational constraint for quenching with the XB compounds, indicating that electron transfer occurs upon formation of the halogen bond.
  • Tuning symmetry breaking charge separation in perylene bichromophores by conformational control
    A. Aster, G. Licari, F. Zinna, E. Brun, T. Kumpulainen, E. Tajkhorshid, J. Lacour and E. Vauthey
    Chemical Science, 10 , 2019, p10629-19639
    DOI:10.1039/C9SC03913A | unige:126803 | Abstract | Article HTML | Article PDF | Supporting Info
  
Understanding structure–property relationships in multichromophoric molecular architectures is a crucial step in establishing new design principles in organic electronics as well as to fully understand how nature exploits solar energy. Here, we study the excited state dynamics of three bichromophores consisting of two perylene chromophores linked to three different crown-ether backbones, using stationary and ultrafast electronic spectroscopy combined with molecular dynamics simulations. The conformational space available to the bichromophores depends on the structure and geometry of the crown-ether and can be significantly changed upon cation binding, strongly affecting the excited-state dynamics. We show that, depending on the conformational restrictions and the local environment, the nature of the excited state varies greatly, going from an excimer to a symmetry-broken charge separated state. These results can be rationalised in terms of a structure–property relationship that includes the effect of the local environment.
  
The excited-state dynamics of the radical anion of perylene (Pe) generated upon bimolecular photoinduced electron transfer (PET) with a donor was investigated using broadband pump–pump–probe spectroscopy. It was found to depend on the age of the anion, that is, on the time interval between the first pump pulse that triggers PET and the second one that excites the ensuing Pe anion (Pe•–). These differences, observed in acetonitrile but not in tetrahydrofuran, report on the evolution of the PET product from an ion pair to free ions. Two photoinduced charge recombination pathways of the ion pair to the neutral Pe*(S1) + donor state were identified: one occurring in a few picoseconds from Pe•–*(D1) and one taking place within 100–200 fs from Pe•–*(Dn>1). Both processes are sensitive to the interionic distance over different length scales and thus serve as molecular rulers.
  • Effect of symmetric and asymmetric substitution on the optoelectronic properties of 9,10-dicyanoanthracene
    F. GlĂścklhofer, A. Rosspeintner, P. Pasitsuparoad, S. Eder, J. FrĂśhlich, G. Angulo, E. Vauthey and F. Plasser
    Mol. Sys. Des. Eng., 4 , 2019, p251-261
    DOI:10.1039/C9ME00040B | unige:126823 | Abstract | Article HTML | Article PDF
 
The structural dynamics of an electron donor/acceptor complex (DAC) consisting of benzene and tetracyanoethylene (Bz/TCNE) solvated in CH2Cl2 have been investigated using ultrafast spectroscopy and mixed quantum/classical computer simulations. Population dynamics from visible and infrared transient absorption (TRIR) spectroscopy point to complex sub-10 ps dynamics followed by charge recombination on a 55 to 60 ps timescale. Structural dynamics involving large-scale reorganization of radical ion pairs are revealed using TRIR anisotropy measurements. A computational study combining quantum chemical calculations and classical molecular dynamics simulations was able to reproduce the experimental electronic absorption lineshape and TRIR anisotropy dynamics, allowing for a detailed investigation of the pair conformational dynamics. Contrary to the static single structure typically assumed in descriptions of DACs, we find that neither the ground nor excited state can be described using a single, well-defined species. Instead, the pair undergoes a rearrangement from disordered pi-stacks to edge-to-face T-shaped structures following excitation. Translational diffusion of the radical ion pairs following excitation was found to be heterogeneous and dependent on both pair separation and orientation coordinates. Given the sensitivity of charge-transfer reactions to the arrangement of donor/acceptor pairs, the structural heterogeneity and corresponding dynamics demonstrated herein must be taken into account in future modeling of charge recombination processes in DACs.
  
The influence of torsional disorder around the ethynyl pi-bridges of a linear D-pi-A-pi-D molecule on the nature of its S1 excited state was investigated using ultrafast time-resolved infrared spectroscopy. By tuning the pump wavelength throughout the S1<- S0 absorption band, subpopulations with different extents of asymmetry could be excited. In non-polar solvents, the equilibrated S1 state is symmetric and quadrupolar independently of the initial degree of distortion. Photoexcitation of distorted molecules is followed by planarization and symmetrization of the S1 state. Excited-state symmetry breaking is only observed in polar environments, where the equilibrated S1 state has a strong dipolar character. However, neither the extent nor the rate of symmetry breaking are enhanced in an initially distorted molecule. They are only determined by the polarity and the dynamic properties of the solvent.
  • Towards efficient initiators for two-photon induced polymerization: fine tuning of the donor/acceptor properties
    B. Holzer, M. Lunzer, A. Rosspeintner, G. Licari, M. Tromayer, S. Naumov, D. Lumpi, E. Horkel, C. Hametner, A. Ovsianikov, R. Liska, E. Vauthey and J. FrĂśhlich
    Mol. Sys. Des. Eng., 4 , 2019, p437-448
    DOI:10.1039/C8ME00101D | unige:126822 | Abstract | Article HTML | Article PDF | Supporting Info
In this work we present the design, synthesis and systematic investigation of the optical properties of symmetric triphenylamine (TPA)-substituted thiophenes. The use of electron-donating (-OMe, -tBu, -Me, -TMS), -neutral (-H) or -withdrawing (-F, -CN, -SO2Me) substituents gives rise to D-A-D based two-photon absorption (2PA) chromophores. The photophysical properties of these compounds, including one-photon absorption and 2PA using two-photon-excited fluorescence, were investigated in different organic solvents with varying polarity. The maximum 2PA cross sections prove to be strongly dependent on the nature of the TPA substituent and range between  ca. 173 GM (Goeppert-Mayer units) and 379 GM. Although most of the investigated substances also exhibit high fluorescence quantum yields, two-photon absorption screening tests of an acrylate monomer formulation revealed the efficiency of these materials as 2PA photoinitiators. These results are supported by quantum chemical calculations of the spin density distribution indicating that the mechanism of polymerization initiation using acrylate monomer is favored by strong localization of the unpaired electrons in the triplet state on the C2 carbon of the thiophene moiety.
 
The dynamics of the ion pairs produced upon fluorescence quenching of the electron donor 9,10-dimethylanthracene (DMeA) by phthalonitrile have been investigated in acetonitrile and tetrahydrofuran using transient absorption spectroscopy. Charge recombination to both the neutral ground state and the triplet excited state of DMeA is observed in both solvents. The relative efficiency of the triplet recombination pathway decreases substantially in the presence of an external magnetic field. These results were analyzed theoretically within the differential encounter theory, with the spin conversion of the geminate ion pairs described as a coherent process driven by the hyperfine interaction. The early temporal evolution of ion pair and triplet state populations with and without magnetic field could be well reproduced in acetonitrile, but not in tetrahydrofuran where fluorescence quenching involves the formation of an exciplex. A description of the spin conversion in terms of rates, i.e., incoherent spin transitions, leads to an overestimation of the magnetic field effect.
  
Intermolecular H‐bonding dynamics around a photoexcited quadrupolar dye is directly observed using transient 2D‐IR spectroscopy. Upon solvent‐induced symmetry breaking, the H‐bond accepting abilities of the two nitrile end‐groups change drastically, and in extremely protic (superprotic) solvents, a tight H‐bond complex forms at one end. The time evolution of the 2D C≡N lineshape in methanol points to rapid, 2–3 ps, spectral diffusion due to fluctuations of the H‐bonding network. Similar behavior is observed in a superprotic solvent shortly after photoexcitation of the dye. However, at later times, the completely inhomogeneous band does not exhibit spectral diffusion for at least 5 ps, pointing to a glass‐like environment around one side of the dye. About half of the excited dyes show this behavior attributed to the tight H‐bond complex, whereas the others are loosely bound. A weak cross peak indicates partial exchange between these excited state subpopulations.
  • Machine Learning for Analysis of Time-Resolved Luminescence Data
    Nikola Dordevic, Joseph S. Beckwith, Maksym Yarema, Olesya Yarema, Arnulf Rosspeintner, Nuri Yazdani, Juerg Leuthold, Eric Vauthey and Vanessa Wood
    ACS Photonics, 5 (12) , 2018, p4888-4895
    DOI:10.1021/acsphotonics.8b01047 | Abstract | Article HTML | Article PDF | Supporting Info
  
Time-resolved photoluminescence is one of the most standard techniques to understand and systematically optimize the performance of optical materials and optoelectronic devices. Here, we present a machine learning code to analyze time-resolved photoluminescence data and determine the decay rate distribution of an arbitrary emitter without any a priori assumptions. To demonstrate and validate our approach, we analyze computer-generated time-resolved photoluminescence data sets and show its benefits for studying the photoluminescence of novel semiconductor nanocrystals (quantum dots), where it quickly provides insight into the possible physical mechanisms of luminescence without the need for educated guessing and fitting.
  
  • Excited-state dynamics of a molecular dyad with two orthogonally-oriented fluorophores
    Romain Letrun, Bernhard Lang, Oleksandr Yushchenko, Roland Wilcken, Denis Svechkarev, Dmytro Kolodieznyi, Eberhard Riedle and Eric Vauthey
    Physical Chemistry Chemical Physics, 20 (48) , 2018, p30219-30230
    DOI:10.1039/C8CP05356A | Abstract | Article HTML | Article PDF | Supporting Info
The excited-state dynamics of a T-shaped bichromophoric molecule, consisting of two strong fluorophores, diphenyloxazole and diphenylpyrazoline, directly linked in an orthogonal geometry, was investigated. Despite the weak coupling ensured by this geometry and confirmed by the electronic absorption spectra, this dyad exhibits only weak fluorescence in both apolar and polar solvents, with fluorescence lifetimes ranging from 200 ps in CHX to 10 ps in ACN. Ultrafast spectroscopic measurements reveal that the fluorescence quenching in polar solvents is due to the population of a charge-separated state. In non-polar solvents, this process is energetically not feasible, and a quenching due to an efficient intersystem crossing (ISC) to the triplet manifold is proposed, based on quantum-chemical calculations. This process occurs via the spin–orbit charge-transfer (SOCT) ISC mechanism, which is enabled by the charge-transfer character acquired by the S1 state of the dyad upon structural relaxation and by the orthogonal arrangement of the molecular orbitals involved in the transition. The same mechanism is proposed to explain why the recombination of the charge-separated state is faster in medium than in highly polar solvents, as well as to account for the fast decay of the lowest triplet state to the ground state.
 
Bimolecular photoinduced electron transfer between perylene and two quenchers was investigated in an imidazolium room-temperature ionic liquid (RTIL) and in a dipolar solvent mixture of the same viscosity using transient absorption on the subpicosecond to submicrosecond time scales. Whereas charge separation dynamics were similar in both solvents, significant differences were observed in the temporal evolution of the ensuing radical ions: although small, the free-ion yield is significantly larger in the RTIL, and recombination of the ion pair to the triplet state of perylene is more efficient in the dipolar solvent. The temporal evolution of reactant, ion, and triplet state populations could be well reproduced using unified encounter theory. This analysis reveals that the observed differences can be explained by the strong screening of the Coulomb potential in the ion pair by the ionic solvent. In essence, RTILs favor free ions compared to highly dipolar solvents of the same viscosity.
  • Wavelength-optimized Two-Photon Polymerization Using Initiators Based on Multipolar Aminostyryl-1,3,5-triazines
    Maximilian Tromayer, Peter Gruber, Arnulf Rosspeintner, Aliasghar Ajami, Wolfgang Husinsky, Felix Plasser, Leticia GonzĂĄlez, Eric Vauthey, Aleksandr Ovsianikov and Robert Liska
    Scientific Reports, 8 (1) , 2018, p17273
    DOI:10.1038/s41598-018-35301-x | Abstract | Article HTML
Two-photon induced polymerization (2PP) based 3D printing is a powerful microfabrication tool. Specialized two-photon initiators (2PIs) are critical components of the employed photosensitive polymerizable formulations. This work investigates the cooperative enhancement of two-photon absorption cross sections (σ2PA) in a series of 1,3,5-triazine-derivatives bearing 1-3 aminostyryl-donor arms, creating dipolar, quadrupolar and octupolar push-pull systems. The multipolar 2PIs were successfully prepared and characterized, σ2PA were determined using z-scan at 800 nm as well as spectrally resolved two-photon excited fluorescence measurements, and the results were compared to high-level ab initio computations. Modern tunable femtosecond lasers allow 2PP-processing at optimum wavelengths tailored to the absorption behavior of the 2PI. 2PP structuring tests revealed that while performance at 800 nm is similar, at their respective σ2PA-maxima the octupolar triazine-derivative outperforms a well-established ketone-based quadrupolar reference 2PI, with significantly lower fabrication threshold at exceedingly high writing speeds up to 200 mm/s and a broader window for ideal processing parameters.
  • Cyclopeptidic photosensitizer prodrugs as proteolytically triggered drug delivery systems of pheophorbide A: part II  co-loading of pheophorbide A and black hole quencher
    Jordan Bouilloux, Oleksandr Yuschenko, Bogdan Dereka, Gianluca Boso, AndrĂŠj Babic, Hugo Zbinden, Eric Vauthey and Norbert Lange
    Photochemical & Photobiological Sciences, 17 (11) , 2018, p1739-1748
    DOI:10.1039/C8PP00318A | Abstract | Article HTML | Article PDF
Previously, we have shown that the use of a cyclopeptidic carrier could be of great interest for the design of fully characterized prodrugs for further use in photodynamic therapy. In order to further optimize the design, we decided to modify the highly quenched conjugate uPA-cPPP4/5 by co-loading a long-distance fluorescence quencher. For this purpose we tethered two black hole quenchers (BHQ3) together with two pheophorbide A moities onto the same PEGylated backbone and assessed the modified photophysical properties. In addition, to prove the reliability of our concept, we designed two analogues, uPA-cPPQ2+2/5 and CathB-cPPQ2+2/5, by using two different peptidic linkers as substrates for uPA and cathepsin B, respectively. These two conjugates proved to be much more water-soluble than their analogues bearing only Phas. These conjugates are not only highly quenched in their native state with regard to their fluorescence emission (up to 850 Âą 287 times less fluorescent for CathB-cPPQ2+2/5 as compared to the unquenched monosubstituted reference uPA-cPPP1/5), but also prevent singlet oxygen production (with a total quenching of the emission when the quenchers are co-loaded with photosensitizers) when the photosentistizers are excited. After proteolytic activation, these conjugates recover their photophysical properties in the same way as occurred for uPA-cPPP4/5, with up to a 120-fold increase in fluorescence emission for uPA-cPPQ2+2/5 after two hours of incubation with uPA.
  • Cyclopeptidic photosensitizer prodrugs as proteolytically triggered drug delivery systems of pheophorbide A: part I self-quenched prodrugs
    Jordan Bouilloux, Oleksandr Yuschenko, Bogdan Dereka, Gianluca Boso, Hugo Zbinden, Eric Vauthey, AndrĂŠj Babic and Norbert Lange
    Photochemical & Photobiological Sciences, 17 (11) , 2018, p1728-1738
    DOI:10.1039/C8PP00317C | Abstract | Article HTML | Article PDF
Herein, we report the synthesis of a new prodrug system consisting of regioselectively addressable functionalized templates bearing multiple pheophorbide A moieties for use in photodynamic therapy. These coupling reactions were achieved using copper-free “click” chemistry, namely a strain-promoted azide–alkyne cycloaddition. This new design was used to obtain well-defined quenched photosensitizer prodrugs with perfect knowledge of the number and position of loaded photosensitizers, providing structures bearing up to six photosentitizers and two PEG chains. These conjugates are ideally quenched in their native state regarding their fluorescence emission (up to 155 ± 28 times less fluorescent for an hexasubstituted conjugate than a monosubstituted non-quenched reference compound) or singlet oxygen production (decreased 8.7-fold in the best case) when excited. After 2 h of proteolytic activation, the fluorescence emission of a tetrasubstituted conjugate was increased 17-fold compared with the initial fluorescence emission.
  • Optical transient absorption experiments reveal the failure of formal kinetics in diffusion assisted electron transfer reactions
    Gonzalo Angulo, Arnulf Rosspeintner, Bernhard Lang and Eric Vauthey
    Physical Chemistry Chemical Physics, 20 (39) , 2018, p25531-25546
    DOI:10.1039/C8CP05153D | Abstract | Article HTML | Article PDF
The ultimate goal of chemical kinetics is to understand why a given reaction is fast or not. To this end it is necessary to count on robust and experimentally well tested theories. One of the difficulties, long recognized in the study of bimolecular reactions, is the role of the molecular displacement, i.e. diffusion. Nonetheless the field is still lacking a compelling amount of case studies contrasting physical models to experiments. By performing transient absorption experiments on the photo-induced electron transfer reaction between perylene and N,N-dimethylaniline in liquid solutions over many orders of magnitude in time, we try to understand the factors determining the kinetics and yields of the full photocycle. We present a method to overcome potential pitfalls in the extraction of the relevant quantities, the transient populations, from the experimental data due to the changes in band shapes and positions. The results are compared to simulations of two different theories: a reaction–diffusion approach based on the encounter theories, and a formal kinetic scheme. We conclude that while the former explains the observed trends in the kinetics with quencher concentration and viscosity exceptionally well, the latter fails. Moreover the analysis of the data with the assistance of encounter theory unveils effects that otherwise would pass unnoticed. This approach and its results exemplify the path to follow in other condensed media whenever diffusion is involved.
  • White-Fluorescent Dual-Emission Mechanosensitive Membrane Probes that Function by Bending Rather than Twisting
    Heorhii V. Humeniuk, Arnulf Rosspeintner, Giuseppe Licari, Vasyl Kilin, Luigi Bonacina, Eric Vauthey, Naomi Sakai and Stefan Matile
    Angewandte Chemie International Edition, 57 (33) , 2018, p10559-10563
    DOI:10.1002/anie.201804662 | Abstract | Article HTML | Article PDF
Bent N,N′‐diphenyl‐dihydrodibenzo[a,c]phenazine amphiphiles are introduced as mechanosensitive membrane probes that operate by an unprecedented mechanism, namely, unbending in the excited state as opposed to the previously reported untwisting in the ground and twisting in the excited state. Their dual emission from bent or “closed” and planarized or “open” excited states is shown to discriminate between micelles in water and monomers in solid‐ordered (So), liquid‐disordered (Ld) and bulk membranes. The dual‐emission spectra cover enough of the visible range to produce vesicles that emit white light with ratiometrically encoded information. Strategies to improve the bent mechanophores with expanded π systems and auxochromes are reported, and compatibility with imaging of membrane domains in giant unilamellar vesicles by two‐photon excitation fluorescence (TPEF) microscopy is demonstrated.
  
  • Highly Stable and Red-Emitting Nanovesicles Incorporating Lipophilic Diketopyrrolopyrroles for Cell Imaging
    Antonio Ardizzone, Davide Blasi, Danilo Vona, Arnulf Rosspeintner, Angela Punzi, Emiliano Altamura, Natascia Grimaldi, Santi Sala, Eric Vauthey, Gianluca M. Farinola, Imma Ratera, Nora Ventosa and Jaume Veciana
    Chemistry - A European Journal, 24 (44) , 2018, p11386-11392
    DOI:10.1002/chem.201801444 | Abstract | Article HTML | Article PDF
Diketopyrrolopyrroles (DPPs) have recently attracted much interest as very bright and photostable red‐emitting molecules. However, their tendency to form nonfluorescent aggregates in water through the aggregation‐caused quenching (ACQ) effect is a major issue that limits their application under the microscope. Herein, two DPP molecules have been incorporated into the membrane of highly stable and water‐soluble quatsomes (QS; nanovesicles composed of surfactants and sterols), which allow their nanostructuration in water and, at the same time, limits the ACQ effect. The obtained fluorescent organic nanoparticles showed superior structural homogeneity, along with long‐term colloidal and optical stability. A thorough one‐ (1P) and two‐photon (2P) fluorescence characterization revealed the promising photophysical features of these fluorescent nanovesicles, which showed a high 1P and 2P brightness. Finally, the fluorescent QSs were used for the in vitro bioimaging of Saos‐2 osteosarcoma cell lines; this demonstrates their potential as nanomaterials for bioimaging applications.
  • Ultrafast excited state decay of natural UV filters: from intermolecular hydrogen bonds to a conical intersection
    Peter S. Sherin, Yuri P. Tsentalovich, Eric Vauthey and Enrico Benassi
    Physical Chemistry Chemical Physics, 20 (22) , 2018, p15074-15085
    DOI:10.1039/C8CP02183J | Abstract | Article HTML | Article PDF
Kynurenines (KNs) are natural UV filters of the human eye lens, protecting the eye tissues from solar UV radiation. Key points of their effective protection are the intramolecular charge transfer (ICT) in the excited state and the fast dissipation of absorbed light energy into heat via the intermolecular H-bonds. Herein we report that the introduction of an unsaturated double bond in the amino acid side chain, operating as an ICT-enhancing electron donor group, drastically accelerates the internal conversion (IC) due to a conical intersection (CI) between the potential energy surfaces of the excited and ground states. Our photophysical study of a deaminated KN (carboxyketoalkene, CKA), an intrinsic product of KN thermal decomposition, demonstrates an unusually fast excited state decay in a broad range of solvents of different polarity and proticity. The detailed analysis of interactions in the excited state by different computational techniques revealed that the changes in molecular structure – the twist of the carbonyl group from the plane of the aromatic ring followed by the formation of two mutually orthogonal conjugated substructures – are responsible for the CI of the excited and ground state potential energy surfaces. Intermolecular H-bonds facilitate the transition to the CI, but do not play a crucial role in the fast decay of the excited state. An extremely fast and efficient IC in CKA opens the way for the design of new types of organic UV filters and their applications in material science, cosmetics and medicine.
  • Synergistic Anion-(pi)n-pi Catalysis on pi-Stacked Foldamers
    Anna-Bea Bornhof, Antonio BauzĂĄ, Alexander Aster, Marion Pupier, Antonio Frontera, Eric Vauthey, Naomi Sakai and Stefan Matile
    Journal of the American Chemical Society, 140 (14) , 2018, p4884-4892
    DOI:10.1021/jacs.8b00809 | Abstract | Article HTML | Article PDF
  
In this report, we demonstrate that synergistic effects between π–π stacking and anion−π interactions in π-stacked foldamers provide access to unprecedented catalytic activity. To elaborate on anion–(π)n–π catalysis, we have designed, synthesized and evaluated a series of novel covalent oligomers with up to four face-to-face stacked naphthalenediimides (NDIs). NMR analysis including DOSY confirms folding into π stacks, cyclic voltammetry, steady-state and transient absorption spectroscopy the electronic communication within the π stacks. Catalytic activity, assessed by chemoselective catalysis of the intrinsically disfavored but biologically relevant addition reaction of malonate half thioesters to enolate acceptors, increases linearly with the length of the stacks to reach values that are otherwise beyond reach. This linear increase violates the sublinear power laws of oligomer chemistry. The comparison of catalytic activity with ratiometric changes in absorption and decreasing energy of the LUMO thus results in superlinearity, that is synergistic amplification of anion−π catalysis by remote control over the entire stack. In computational models, increasing length of the π-stacked foldamers correlates sublinearly with changes in surface potentials, chloride binding energies, and the distances between chloride and π surface and within the π stack. Computational evidence is presented that the selective acceleration of disfavored but relevant enolate chemistry by anion−π catalysis indeed originates from the discrimination of planar and bent tautomers with delocalized and localized charges, respectively, on π-acidic surfaces. Computed binding energies of keto and enol intermediates of the addition reaction as well as their difference increase with increasing length of the π stack and thus reflect experimental trends correctly. These results demonstrate that anion–(π)n–π interactions exist and matter, ready for use as a unique new tool in catalysis and beyond.
  • Dipole moment and polarizability of tunable Intramolecular charge transfer states in heterocyclic π‑conjugated molecular dyads determined by computational and Stark spectroscopic study
    E.J. Rohwer, M. Akbarimoosavi, S.E. Meckel, X. Liu, Y. Geng, L.M. Lawson Daku, A. Hauser, A. Cannizzo, S. Decurtins, R.J. Stanley, S.-X. Liu and T. Feurer
    Journal of Physical Chemistry C, 122 , 2018, p9346-9355
    DOI:10.1021/acs.jpcc.8b02268 | Abstract | Article HTML | Article PDF | Supporting Info
The annulation of two redox-active molecules into a compact and planar structure paves the way toward a new class of electronically versatile materials whose physical properties can be tuned via a substitution of one of the constituting moieties. Specifically, we present tetrathiafulvalene–benzothiadiazole donor–acceptor molecules. The critical role played by the dielectric properties of these molecules is evident by the large spectral shifts of the ground-state absorption spectra in a range of solvents. Stark spectroscopy is performed to determine experimentally dipole and polarizability change over transitions in the visible range with particular attention to the transition from the highest-occupied molecular orbital (HOMO) to the lowest-unoccupied molecular orbital (LUMO). The experimental results are compared to the results of time-dependent density functional theory calculations, and we reciprocally validate results from calculation and experiment. This allows us to filter out effective models and reveal important insights. The calculations are initially performed in the gas phase and subsequently a polarizable continuum model is adopted to probe the influence of the solvent on the molecular dielectric properties. The results show a large charge displacement from the HOMO to the LUMO and confirm the intramolecular charge transfer nature of the lowest-energy transition. Substitution of the acceptor moiety with electron-withdrawing groups results in changes to the experimentally determined molecular properties consistent with the effects predicted by computational results. The dominant contribution to the electroabsorption signal is due to the change in dipole moment, which is measured to be roughly 20 D for all samples and forms a small angle with the transition dipole moment in a toluene solvent environment.
  
A planarizable push–pull molecular probe with mechanosensitive properties was investigated at several biomimetic interfaces, consisting of different phospholipid monolayers located between dodecane and an aqueous buffer solution, using the interface-specific surface-second-harmonic-generation (SSHG) technique. Whereas the SSHG spectra recorded at liquid-disordered interfaces were similar to the absorption spectra in bulk solutions, those measured at liquid-ordered phases exhibited a remarkable shift towards lower energies to an extent depending on the surface pressure of the phospholipid monolayer. On the basis of quantum-chemical calculations, this effect was accounted for by the planarization of the mechanosensitive probe. Polarization-resolved SSHG measurements revealed that the average orientation of the probe at the interface is an even more sensitive reporter of lateral pressure and order than the spectral shape. Additionally, time-resolved SSHG measurements pointed to slower dynamics upon intercalation inside the phospholipid monolayer, most likely due to the more constrained environment. This study demonstrates that the concept of mechanosensitive optical probes can be further exploited when combined with a surface-selective nonlinear optical technique.
  • Membrane bound COMT isoform is an interfacial enzyme: general mechanism and new drug design paradigm
    Aniket Magarkar, Petteri Parkkila, Tapani Viitala, Tatu Lajunen, Edouard Mobarak, Giuseppe Licari, Oana Cramariuc, Eric Vauthey, Tomasz RĂłg and Alex Bunker
    Chemical Communications, 54 (28) , 2018, p3440-3443
    DOI:10.1039/C8CC00221E | Abstract | Article HTML | Article PDF
The enzyme catechol-O-methyltransferase (COMT) has water soluble (S-COMT) and membrane associated (MB-COMT), bitopic, isoforms. Of these MB-COMT is a drug target in relation to the treatment of Parkinson's disease. Using a combination of computational and experimental protocols, we have determined the substrate selection mechanism specific to MB-COMT. We show: (1) substrates with preferred affinity for MB-COMT over S-COMT orient in the membrane in a fashion conducive to catalysis from the membrane surface and (2) binding of COMT to its cofactor ADOMET induces conformational change that drives the catalytic surface of the protein to the membrane surface, where the substrates and Mg2+ ions, required for catalysis, are found. Bioinformatics analysis reveals evidence of this mechanism in other proteins, including several existing drug targets. The development of new COMT inhibitors with preferential affinity for MB-COMT over S-COMT is now possible and insight of broader relevance, into the function of bitopic enzymes, is provided.
  • On the mechanism of rapid metal exchange between thiolate-protected gold and gold/silver clusters: a time-resolved in situ XAFS study
    Bei Zhang, Olga V. Safonova, Stephan Pollitt, Giovanni Salassa, Annelies Sels, Rania Kazan, Yuming Wang, GĂźnther Rupprechter, Noelia BarrabĂŠs and Thomas BĂźrgi
    Physical Chemistry Chemical Physics, 20 (7) , 2018, p5312-5318
    DOI:10.1039/C7CP08272J | Abstract | Article HTML | Article PDF | Supporting Info
  
The fast metal exchange reaction between Au38 and AgxAu38−x nanoclusters in solution at −20 °C has been studied by in situ X-ray absorption spectroscopy (time resolved quick XAFS) in transmission mode. A cell was designed for this purpose consisting of a cooling system, remote injection and mixing devices. The capability of the set-up is demonstrated for second and minute time scale measurements of the metal exchange reaction upon mixing Au38/toluene and AgxAu38−x/toluene solutions at both Ag K-edge and Au L3-edge. It has been proposed that the exchange of gold and silver atoms between the clusters occurs via the SR(-M-SR)n (n = 1, 2; M = Au, Ag) staple units in the surface of the reacting clusters during their collision. However, at no point during the reaction (before, during, after) evidence is found for cationic silver atoms within the staples. This means that either the exchange occurs directly between the cores of the involved clusters or the residence time of the silver atoms in the staples is very short in a mechanism involving the metal exchange within the staples.
  
The applicability of room-temperature ionic liquids (RTILs) as inert solvents is generally based on their electrochemical window. We herein show that this concept has its limitations if RTILs are exposed to an oxidizing environment in the presence of light. Acetonitrile solutions of RTILs with 1-methyl-3-ethylimidazolium as cation and five different anions, including thiocyanate (SCN–) and dicyanamide (DCA–), were investigated. Upon addition of organic electron acceptors to solutions of RTILs with SCN– or DCA–, charge-transfer (CT) absorption bands due to the formation of donor–acceptor complexes between the anion and the electron acceptor were observed. Time-resolved measurements from the femtosecond to the microsecond regimes were used to investigate the nature and the excited-state dynamics of these complexes upon excitation in the CT band. We show that even though the RTILs are seemingly inert according to their electrochemical properties, the dicyanamide and thiocyanate based RTILs can actively participate in photochemical reactions in oxidizing environments and therefore differ from the behavior expected for an inert solvent. This has not only important implications for the long-term stability of RTIL-based systems but can also lead to misinterpretation of photochemical studies in these solvents.
  • Influence of the hydrogen-bond interactions on the excited-state dynamics of a push-pull azobenzene dye: the case of Methyl Orange
    Christoph Nançoz, Giuseppe Licari, Joseph S. Beckwith, Magnus Soederberg, Bogdan Dereka, Arnulf Rosspeintner, Oleksandr Yushchenko, Romain Letrun, Sabine Richert, Bernhard Lang and Eric Vauthey
    Physical Chemistry Chemical Physics, 20 (10) , 2018, p7254-7264
    DOI:10.1039/C7CP08390D | Abstract | Article HTML | Article PDF | Supporting Info
  
The excited-state dynamics of the push–pull azobenzene Methyl Orange (MO) were investigated in several solvents and water/glycerol mixtures using a combination of ultrafast time-resolved fluorescence and transient absorption in both the UV-visible and the IR regions, as well as quantum chemical calculations. Optical excitation of MO in its trans form results in the population of the S2 ππ* state and is followed by internal conversion to the S1 nπ* state in ∼50 fs. The population of this state decays on the sub-picosecond timescale by both internal conversion to the trans ground state and isomerisation to the cis ground state. Finally, the cis form converts thermally to the trans form on a timescale ranging from less than 50 ms to several minutes. Significant differences depending on the hydrogen-bond donor strength of the solvents, quantified by the Kamlet Taft parameter α, were observed: compared to the other solvents, in highly protic solvents (α > 1), (i) the viscosity dependence of the S1 state lifetime is less pronounced, (ii) the S1 state lifetime is shorter by a factor of ≈1.5 for the same viscosity, (iii) the trans-to-cis photoisomerisation efficiency is smaller, and (iv) the thermal cis-to-trans isomerisation is faster by a factor of ≥103. These differences are explained in terms of hydrogen-bond interactions between the solvent and the azo nitrogen atoms of MO, which not only change the nature of the S1 state but also have an impact on the shape of ground- and excited-state potentials, and, thus, affect the deactivation pathways from the excited state.
The accurate description of transition metal complexes in liquid solutions is a challenging fundamental research problem, which must be tackled when it comes to understanding the role of the solvent in the photoinduced low-spin (LS) → high-spin (HS) transition in solvated Fe(II) complexes. We report an in-depth ab initio molecular dynamics (AIMD) study of the spin-state dependence of the structural and vibrational properties of the prototypical [Fe(bpy)3]2+ (bpy = 2,2'-bipyridine) LS complex in water. The description achieved for the LS and HS solution structures of aqueous [Fe(bpy)3]2+ significantly improves on and actually supersedes the one from our previous AIMD study [Lawson Daku and Hauser, J. Phys. Chem. Lett., 2010, 1, 1830], thanks to substantially longer simulation times and the use of the dispersion-corrected BLYP-D3 functional in place of the standard BLYP functional. The present results confirm the ≈0.19 Å lengthening of the Fe–N bonds and the increased thermal fluctuation of the molecular edifice stemming from the weakening of the Fe–N bonds upon the LS → HS change of states. Revisiting our previous finding on the solvation of  [Fe(bpy)3]2+, they indicate that the number of water molecules in its first hydration shell actually increases from ~15 in the LS state to ~17 in the HS state. The vibration modes and associated vibrational density of states (VDOS) of  [Fe(bpy)3]2+ have been determined from a generalized normal coordinate analysis. The VDOS of the Fe–N stretching and bending modes are located in the far-IR region. For LS [Fe(bpy)3]2+, the peak positions of the VDOS of the Fe–N stretching modes agree very well with the experimental Fe–N stretching frequencies. For HS  [Fe(bpy)3]2+, the spanned frequency range encompasses the Fe–N stretching frequency range reported for HS polypyridine Fe(II) complexes. The LS and HS IR spectra of the complex have also been calculated in the 0 ≤ ν ≤ 2500 cm-1 range from the dynamics of the Wannier function centers. The calculated LS IR spectrum matches available experimental data. The predicted HS–LS IR difference spectrum of aqueous [Fe(bpy)3]2+ shows mostly an increase in intensity upon the LS → HS change of states.
  • Correlating Boron-Hydrogen Stretching Frequencies with Boron-Hydrogen Bond Lengths in Closoboranes: An Approach Using DFT Calculations
    Hans Hagemann, Manish Sharma, Daniel Sethio and LatĂŠvi Max Lawson Daku
    Helvetica Chimica Acta, 101 (2) , 2018
    DOI:10.1002/hlca.201700239 | Abstract
  • Photocatalytic CO2 reduction by Cr-substituted Ba2(In2-xCrx )O5 ·(H2O)δ (0.04 ≤ x ≤ 0.60)
    Songhak Yoon, Michael Gaul, Sitansh Sharma, Kwanghyo Son, Hans Hagemann, Dirk Ziegenbalg, Udo Schwingenschlogl, Marc Widenmeyer and Anke Weidenkaff
    Solid State Sciences, in press , 2018
    DOI:10.1016/j.solidstatesciences.2018.02.005 | Abstract
Cr-substituted polycrystalline Ba2(In2-xCrx)O5·(H2O)δ powders (0.04 ≤ x ≤ 0.60) were synthesized by solid state reaction to investigate the relation of crystal structure, thermochemical, magnetic, and optical properties. The Cr-substitution results in an unit cell expansion and formation of the higher-symmetric tetragonal phase together with increased oxygen and hydrogen contents. Magnetic property measurements reveal that the diamagnetic pristine Ba2In2O5·(H2O)δ becomes magnetically ordered upon Cr-substitution. By UV–vis spectroscopy a gradual shift of the absorption-edge energy to lower values was observed. Numerical calculations showed that the observed bandgap narrowing was ascribed to the Cr induced states near the Fermi level. The correlation between the changes of crystal chemistry, magnetic, and optical properties of Cr-substituted Ba2(In2-xCrx)O5·(H2O)δ can be explained by the replacement of In by Cr. Consequently, an enhanced photocatalytic CO2 reduction activity was observed with increasing Cr substitution, compatible with the state-of-the-art high surface area TiO2 photocatalyst (P-25).
  • Excimer-Based On-Off Bis(pyreneamide) Macrocyclic Chemosensors
    Mahesh Vishe, TimothĂŠe Lathion, Simon Pascal, Oleksandr Yushchenko, Alexandre Homberg, Elodie Brun, Eric Vauthey, Claude Piguet and JĂŠrĂ´me Lacour
    Helvetica Chimica Acta, 101 , 2018
    DOI:10.1002/hlca.201700265 | Abstract | Article HTML
A series of bis(pyreneamide) macrocycles, synthesized in two steps from THF, THP, oxepane and 1,4-dioxane, are tested as chemosensors for a large range of mono-, di- and trivalent cations. In their native states, these macrocycles exhibit a strong excimer fluorescence that is quenched upon the addition of the metal ions (alkaline, alkaline earth, p-, d-, and f-block metals). UV-Vis spectrophotometric titrations, cyclic voltammetry, excimer fluorescence quenching and transient absorption spectroscopy experiments helped characterize the On-Off changes occurring upon binding and demonstrate that the highest stability constants are obtained with divalent cations Ca2+ and Ba2+ specifically.
  • Energy Transfer and Charge Separation Dynamics in Photoexcited Pyrene-Bodipy Molecular Dyads
    Mihalis Fakis, Joseph Beckwith, Kostas Seintis, Elisabeth Martinou, Christoph Nançoz, Nikolaos Karakostas, Ioannis D Petsalakis, George Pistolis and Eric Vauthey
    Physical Chemistry Chemical Physics, 20 , 2018, p837-849
    DOI:10.1039/C7CP06914F | Abstract | Article PDF | Supporting Info
The photophysical properties of two pyrene-bodipy molecular dyads, composed of a phenyl-pyrene (Py-Ph) linked to the meso position of a bodipy (BD) molecule with either H-atoms (BD1) or ethyl groups (BD2) at the 2, 6 positions, are investigated by stationary, nanosecond and femtosecond spectroscopy. The properties of these dyads (Py-Ph-BD1 and Py-Ph-BD2) are compared to those of their constituent chromophores in two solvents namely 1,2 dichloroethane (DCE) and acetonitrile (ACN). Stationary spectroscopy reveals a weak coupling among the subunits in both dyads. Excitation of the Py subunit eads to emission that is totally governed by the BD subunits in both dyads pointing to excitation energy transfer (EET) from the Py to BD chromophore. Femtosecond fluorescence and transient absorption spectroscopy reveal that EET takes place within 0.3-0.5 ps and is mostly independent of the solvent and the type of the BD subunit. The EET lifetime is in reasonable agreement with that predicted by FĂśrster theory. After EET has taken place, Py-Ph-BD1 in DCE and Py-Ph-BD2 in both solvents decay mainly radiatively to the ground state with 3.5 - 5.0 ns lifetimes which are similar to those of the individual BD chromophores. However, the excited state of Py-Ph-BD1 in ACN is quenched having a lifetime of 1 ns. This points to the opening of an additional non-radiative channel of the excited state of Py-Ph-BD1 in this solvent, most probably charge separation (CS). Target analysis of the TA spectra has shown that the CS follows an inverted kinetics and is substantially slower than the recombination of the charge-separated state. Occurrence of CS with Py-Ph-BD1 in ACN is also supported by energetic considerations. The above results indicate that only a small change in the structure of the BD units incorporated in the dyads, significantly affects the excited state dynamics leading either to a dyad with long lifetime and high fluorescence quantum yield or to a dyad with an intramolecular CS ability.
  
  • Photolabile Coumarins with Improved Efficiency through Azetidinyl Substitution
    Giovanni Bassolino, Christoph Nançoz, Zacharias Thiel, Estelle Marine Bois, Eric Vauthey and Pablo Rivera-Fuentes
    Chemical Science, 9 , 2018, p387-391
    DOI:10.1039/C7SC03627B | Abstract | Article PDF | Supporting Info
Azetidinyl substituents have been recently used to improve the fluorescence quantum yield of several classes of fluorophores. Herein, we demonstrate that other useful photochemical processes can be modulated using this strategy. In particular, we prepared and measured the quantum yield of photorelease of a series of 7-azetidinyl-4-methyl coumarin esters and compared it to their 7-diethylamino and julolidine-fused analogues. The efficiency of the photorelease reactions of the azetidinyl-substituted compounds was 2- to 5-fold higher than the corresponding diethylamino coumarins. We investigated the origin of this effect in model fluorophores and in the photoactivatable esters, and found that H-bonding with the solvent seems to be the prominent deactivation channel inhibited upon substitution with an azetidinyl ring. We anticipate that this substitution strategy could be used to modulate other photochemical processes with applications in chemical biology, catalysis and materials science.
  • Probing the Impact of Solvation on Photoexcited Spin Crossover Complexes with High-Precision X-ray Transient Absorption Spectroscopy
    Cunming Liu, Jianxin Zhang, LatĂŠvi M. Lawson Daku, David Gosztola, Sophie E. Canton and Xiaoyi Zhang
    Journal of the American Chemical Society, 139 (48) , 2017, p17518-17524
    DOI:10.1021/jacs.7b09297 | Abstract | Article HTML | Article PDF | Supporting Info
Investigating the photoinduced electronic and structural response of bistable molecular building blocks incorporating transition metals in solution phase constitutes a necessary stepping stone for steering their properties toward applications and performance optimizations. This work presents a detailed X-ray transient absorption (XTA) spectroscopy study of a prototypical spin crossover (SCO) complex [FeII(mbpy)3]2+ (where mbpy = 4,4′-dimethyl-2,2′-bipyridine) with an [FeIIN6] first coordination shell in water (H2O) and acetonitrile (CH3CN). The unprecedented data quality of the XTA spectra together with the direct fitting of the difference spectra in k space using a large number of scattering paths enables resolving the subtle difference in the photoexcited structures of an FeII complex in two solvents for the first time. Compared to the low spin (LS) 1A1 state, the average Fe–N bond elongations for the photoinduced high spin (HS) 5T2 state are found to be 0.181 ± 0.003 Å in H2O and 0.199 ± 0.003 Å in CH3CN. This difference in structural response is attributed to ligand–solvent interactions that are stronger in H2O than in CH3CN for the HS excited state. Our studies demonstrate that, although the metal center of [FeII(mbpy)3]2+ could have been expected to be rather shielded by the three bidentate ligands with quasi-octahedral coordination, the ligand field strength in the HS excited state is nevertheless indirectly affected by solvation effects that modifies the charge distribution within the Fe–N covalent bonds. More generally, this work highlights the importance of including solvation dynamics in order to develop a generalized understanding of the spin-state switching at the atomic level.
  • Characterization of di-4-ANEPPS with nano-black lipid membranes
    Maria Tsemperouli and Kaori Sugihara
    Nanoscale, in press , 2017
    DOI:10.1039/C7NR05863B | Abstract | Article PDF | Supporting Info
We report a platform based on lateral nano-BLMs, where electrical measurements and fluorescence microscopy setup are combined, for the calibration of di-4-ANEPPS, a common voltage sensitive dye (VSD). The advantage of the setup is 1) its flexibility in the choice of lipids and the applied voltages, 2) its high stability that enables high voltage (500 mV) application and long time measurements, and 3) its fluorescence microscopy readout, which can be directly correlated with other fluorescence microscopy experiments using VSDs (e.g. membrane potential measurements in living cells). Using the setup, we observed that the calibration curve of di-4-ANEPPS highly depends on the net electric charge of the lipids. The developed setup can be used to calibrate VSDs in different lipid environment for understanding their fundamental voltage-sensing mechanism in future.
  • A stable 3 V all-solid-state sodium-ion battery based on a closo -borate electrolyte
    L. DuchĂŞne, R.-S. KĂźhnel, E. Stilp, E.Cuervo Reyes, A. Remhof, H. Hagemann and C. Battaglia
    Energy & Environmental Science, 10 (12) , 2017, p2609-2615
    DOI:10.1039/C7EE02420G | Abstract
 
The influence of the length of the push–pull branches of quadrupolar molecules on their excited-state symmetry breaking was investigated using ultrafast time-resolved IR spectroscopy. For this, the excited-state dynamics of an A-π-D-π-A molecule was compared with those of an ADA analogue, where the same electron donor (D) and acceptor (A) subunits are directly linked without a phenylethynyl π-spacer. The spatial distribution of the excitation was visualized in real time by monitoring C≡C and C≡N vibrational modes localized in the spacer and acceptor units, respectively. In nonpolar solvents, the excited state is quadrupolar and the excitation is localized on the π-D-π center. In medium polarity solvents, the excitation spreads over the entire molecule but is no longer symmetric. Finally, in the most polar solvents, the excitation localizes on a single D-π-A branch, contrary to the ADA analogue where symmetry breaking is only partial.
  
Most quadrupolar molecules designed for large two-photon absorption cross section have been shown to undergo symmetry breaking upon excitation to the S1 state. This was originally deduced from their strong fluorescence solvatochromism and later visualized in real time using transient infrared spectroscopy. For molecules not containing clear IR marker modes, however, a specific real-time observation of the symmetry breaking process remains lacking. Here we show that this process can be resolved using broadband fluorescence upconversion spectroscopy by monitoring the instantaneous emission transition dipole moment. This approach is illustrated with measurements performed on two quadrupolar molecules, with only one of them undergoing excited-state symmetry breaking in polar solvents.
  • Direct Observation of a Photochemical Alkyne-Allene Reaction and of a Twisted and Rehybridized Intramolecular Charge-Transfer State in a Donor-Acceptor DyadOpen access paper
    Bogdan Dereka, Denis Svechkarev, Arnulf Rosspeintner, Maximilian Tromayer, Robert Liska, Aaron M. Mohs and Eric Vauthey
    Journal of the American Chemical Society, 139 , 2017, p16885-16893
    DOI:10.1021/jacs.7b09591 | Abstract | Article HTML | Article PDF | Supporting Info
  
The excited-state dynamics of an aniline–triazine electron donor–acceptor dyad with an alkyne spacer has been investigated using a combination of ultrafast broadband mid-IR and visible transient absorption and fluorescence spectroscopies. The transient IR data reveal the occurrence of an efficient alkyne to allene isomerization of the spacer with a time constant increasing from a few hundreds of femtoseconds to a few picoseconds with solvent viscosity. This process is faster than the vibrational cooling of the Franck–Condon excited state, indicative of nonequilibrium dynamics. The transient electronic absorption and fluorescence data evidence that this transformation is accompanied by a charge separation between the donor and the acceptor subunits. The allene character of the spacer implies an orthogonal orientation of the donor and acceptor moieties, similar to that proposed for twisted intramolecular charge-transfer states. Such states are often invoked in the excited-state dynamics of donor–acceptor dyads, but their involvement could never be unambiguously evidenced spectroscopically. The alkyne–allene isomerization involves not only a torsional motion but also a bending of the molecule due to the sp to sp2 rehybridization of one of the alkyne carbon atoms. This twisted and rehybridized intramolecular charge transfer (“TRICT”) state decays back to the planar and linear alkyne ground state on a time scale decreasing from a few hundred to ten picoseconds upon going from weakly to highly polar solvents. The different solvent dependencies reveal that the dynamics of the allene buildup are controlled by the structural changes, whereas the decay is limited by the charge recombination step.
  
Fluoride substitution in LiBH4 is studied by investigation of LiBH4-LiBF4 mixtures (9:1 and 3:1). Decomposition was followed by in-situ synchrotron radiation X-ray diffraction (in-situ SR-PXD), thermogravimetric analysis and differential scanning calorimetry with gas analysis (TGA/DSC-MS) and in-situ infrared spectroscopy (in-situ FTIR). Upon heating, fluoride substituted LiBH4 forms (LiBH4-xFx) and decomposition occurs, releasing diborane and solid decomposition products. The decomposition temperature is reduced more than fourfold relative to the individual constituents, with decomposition commencing at T / °C = 80 °C. The degree of fluoride substitution is quantified by sequential Rietveld refinement and shows a selective manner of substitution. In-situ FTIR experiments reveal formation of bands originating from LiBH4-xFx. Formation of LiF and observation of diborane release implies that the decomposing materials have a composition that facilitates formation of diborane and LiF, i.e. LiBH4-xFx (LiBH3F). An alternative approach for fluoride substitution was performed, by addition of Et3N∙3HF to LiBH4, yielding extremely unstable products. Spontaneous decomposition indicates fluoride substitution to have occurred. From our point of view, this is the most significant destabilization effect seen for borohydride materials so far.
The nucleation and growth properties of domains of molecules of the same state in open boundary three-dimensional (3D) spin-crossover systems of various shapes are discussed within the framework of the mechanoelastic model. The molecules are situated on face-centered-cubic lattices and are linked by springs through which they interact. Monte Carlo simulations imply that clusters nucleate from corners in the case of systems having well-developed faces and from kinks in the case of spherical samples, in accordance with available experimental data. In addition, a method to characterize the cooperativity in these systems is proposed, which by scanning the fluctuations in the 3D samples can be related directly to powder x-ray-diffraction experiments.
  • Regioselective and Enantiospecific Synthesis of Dioxepines by (Cyclopentadienyl)ruthenium-Catalyzed Condensations of Diazocarbonyls and Oxetanes
    LĂŠo Egger, Laure GuĂŠnĂŠe, Thomas BĂźrgi and JĂŠrĂ´me Lacour
    Advanced Synthesis & Catalysis, 359 (17) , 2017, p2918-2923
    Keywords: diazo compounds; insertion; metal carbenes; oxetanes; oxonium ylides; retention of configuration
    DOI:10.1002/adsc.201700638 | unige:96477 | Abstract | Article HTML | Article PDF
1,4-Dioxepines result from the decomposition of ι-diazo-β-keto esters in the presence of oxetanes using the catalytic combination of the (cyclopentadienyl)ruthenium complex [CpRu(CH3CN)3][BArF] and 1,10-phenanthroline. The regioselective [4+3] insertions follow an SN1-like mechanism and occur yet enantiospecifically (es 74%). The retention of configuration was ascertained by vibrational circular dichroism (VCD) and solid state analyses. Furans, products of [4+1] insertions, are only observed as traces in the above protocol. To promote their formation under CpRu catalysis, it is necessary to use a two-step process with γ-halogenated alcohols as substrates.
  • How good is the generalized Langevin equation to describe the dynamics of photo-induced electron transfer in fluid solution?
    Gonzalo Angulo, Jakub Jedrak, Anna Ochab-Marcinek, Pakorn Pasitsuparoad, Czeslaw Radzewicz, Pawel Wnuk and Arnulf Rosspeintner
    The Journal of Chemical Physics, 146 (24) , 2017, p244505
    Keywords: fluorescence; spectroscopy; chemical reactions; computer simulation; thermodynamic properties
    DOI:10.1063/1.4990044 | unige:98052 | Abstract | Article HTML | Article PDF | Supporting Info
The dynamics of unimolecular photo-triggered reactions can be strongly affected by the surrounding medium for which a large number of theoretical descriptions have been used in the past. An accurate description of these reactions requires knowing the potential energy surface and the friction felt by the reactants. Most of these theories start from the Langevin equation to derive the dynamics, but there are few examples comparing it with experiments. Here we explore the applicability of a Generalized Langevin Equation (GLE) with an arbitrary potential and a non-Markovian friction. To this end, we have performed broadband fluorescence measurements with sub-picosecond time resolution of a covalently linked organic electron donor-acceptor system in solvents of changing viscosity and dielectric permittivity. In order to establish the free energy surface (FES) of the reaction, we resort to stationary electronic spectroscopy. On the other hand, the dynamics of a non-reacting substance, Coumarin 153, provide the calibrating tool for the non-Markovian friction over the FES, which is assumed to be solute independent. A simpler and computationally faster approach uses the Generalized Smoluchowski Equation (GSE), which can be derived from the GLE for pure harmonic potentials. Both approaches reproduce the measurements in most of the solvents reasonably well. At long times, some differences arise from the errors inherited from the analysis of the stationary solvatochromism and at short times from the excess excitation energy. However, whenever the dynamics become slow, the GSE shows larger deviations than the GLE, the results of which always agree qualitatively with the measured dynamics, regardless of the solvent viscosity or dielectric properties. The method applied here can be used to predict the dynamics of any other reacting system, given the FES parameters and solvent dynamics are provided. Thus no fitting parameters enter the GLE simulations, within the applicability limits found for the model in this work.
 
Implementation, benchmarking, and representative applications of the new FDE-ADC(3) method for describing environmental effects on excited states as a combination of frozen density embedding (FDE) and the algebraic-diagrammatic construction scheme for the polarization propagator of third order (ADC(3)) are presented. Results of FDE-ADC(3) calculations are validated with respect to supersystem calculations on test systems with varying molecule–environment interaction strengths from dispersion up to multiple hydrogen bonds. The overall deviation compared to the supersystem calculations is as small as 0.029 eV for excitation energies, which is even smaller than the intrinsic error of ADC(3). The dependence of the accuracy on the choice of method and functional for the calculation of the environment and the nonelectrostatic part of the system–environment interaction is evaluated. In three representative examples, the FDE-ADC method is applied to investigate larger systems and to analyze excited state properties using visualization of embedded densities and orbitals.
  
The kinetic lability of hexadentate gallium-based tripods is sufficient to ensure thermodynamic self-assembly of luminescent heterodimetallic [GaLn(L3)3]6+ helicates on the hour time scale, where Ln is a trivalent 4f-block cation. The inertness is however large enough for preserving the triple-helical structure when [GaLn(L3)3]6+ is exposed to lanthanide exchange. The connection of a second gallium-based tripod further slows down the exchange processes to such an extent that spectroscopically active [CrErCr(L4)3]9+ can be diluted into closed-shell [GaYGa(L4)3]9+ matrices without metal scrambling. This feature is exploited for pushing molecular-based energy transfer upconversion (ETU) at room temperature.
 
A thorough understanding of the microscopic mechanism of excited-state proton transfer (ESPT) and the influence of the solvent environment on its dynamics are of great fundamental interest. We present here a detailed investigation of an ESPT to solvent (DMSO) using time-resolved broadband fluorescence and transient absorption spectroscopies. All excited-state species are resolved spectrally and kinetically using a global target analysis based on the two-step Eigen-Weller model. Reversibility of the initial short-range proton transfer producing excited contact ion pairs (CIP*) is observed unambiguously in fluorescence and must be explicitly considered to obtain the individual rate constants. Close inspection of the early dynamics suggests that the relative populations of the protonated form (ROH*) and CIP* are governed by solvent relaxation that influences the relative energies of the excited states. This constitutes a breakdown of the Eigen-Weller model, although the overall agreement between the data and the analysis using classical rate equations is excellent.
  
Cr-substituted and pristine Ba2In2O5·(H2O)x powders were synthesized by solid state reaction. The influence of Cr-substitution on the crystal structure, chemical composition, magnetic and optical properties were investigated. Powder X-ray diffraction (XRD), elemental analysis and TGA-MS reveal that with substitution of In for Cr, the unit cell volume and the unit cell parameter b increase together with the oxygen and hydrogen content. Magnetic property measurements indicate that Ba2In2O5·(H2O)x is diamagnetic in the temperature range of 2 K < T < 300 K becoming ferromagnetic upon Cr-substitution. In the UV–vis spectra of the Cr-substituted sample a distinctive shift of the absorption-edge energy from 430 to 690 nm was observed corresponding to a bandgap narrowing from 2.88 to 1.80 eV. The replacement of tetrahedral InO4 units by octahedral CrO6 units was found to be the main factor for the drastic change of the magnetic and optical properties.
 
Symmetry breaking of the excited state of a linear quadrupolar acceptor-donor-acceptor molecule was investigated using time-resolved infrared spectroscopy in 55 solvents allowing the influence of several solute–solvent interactions to be examined separately. No symmetry breaking was found in nonpolar solvents irrespective of their refractive index, indicating that differences in dispersion interactions between the two arms of the molecule do not suffice to induce an asymmetric distribution of the excitation. However, symmetry breaking was observed in nondipolar but quadrupolar solvents like benzene to an extent that can be as large as that found in medium dipolar solvents like THF. Whereas larger symmetry breaking occurs in the most dipolar solvents, the strongest are observed in protic solvents due to hydrogen bonding. Strong evidence of the formation of halogen bonds in the excited state is also presented, confirming the idea of symmetry-breaking-induced asymmetrical photochemistry.
  
Lithium amide–borohydrides Li[BH4]1–x[NH2]x possess liquid-like Li superionic conductivity at nearly ambient temperature. The fast Li+ diffusion facilitated by the localized motions of the anions is proposed to occur through a network of vacant tetrahedral sites, acting as conduction channels. To study the reorientational dynamics of the anions, we have performed quasielastic neutron scattering experiments on samples with different compositions (x = 2/3, 0.722, 0.737, 3/4) over a broad temperature and time range. To unambiguously disentangle the contributions of the two species, [BH4]− and [NH2]−, we took advantage of deuterium labeling and could clearly demonstrate that the quasielastic broadening is mainly determined by the [BH4]− reorientations. With the help of a newly developed model, supported by ab initio molecular dynamics calculations, we have identified three relaxation components, which account for generally anisotropic C3-rotations of the [BH4]− tetrahedra including jumps by a small angle from the equilibrium position.
  • Enantiospecific Elongation of Cationic Helicenes by Electrophilic Functionalization at Terminal Ends
    Romain Duwald, Simon Pascal, Johann Bosson, Stephane Grass, Celine Besnard, Thomas Buergi and Jerome Lacour
    Chemistry - A European Journal, 23 (55) , 2017, p13596-13601
    Keywords: carbenium ions; helicenes; regioselectivity; SEAr; SNAr; thermodynamic control
    DOI:10.1002/chem.201703441 | unige:97330 | Abstract | Article HTML | Article PDF | Supporting Info
  
A strategy for late-stage electrophilic functionalizations of cationic helicenes is exposed. Thanks to strongly acidic conditions that permit reversible electrophilic substitutions, regioselective acylations, sulfonylations or alkylations occur at the extremity(ies) of the helical cores. Extended [5] or [6]helicenes can then be generated from cationic [4]helicenes in successive one-pot elongation processes. Retention of configuration and excellent enantiospecificity (up to 99%) are observed for the helicene growth in the enantiopure series.
  
  • Triplet state CPL active helicene-dithiolene platinum bipyridine complexes
    Thomas Biet, Thomas Cauchy, Qinchao Sun, Jie Ding, Andreas Hauser, Patric Oulevey, Thomas BĂźrgi, Denis Jacquemin, Nicolas Vanthuyne, Jeanne Crassous and Narcis Avarvari
    Chemical Communications, 53 (66) , 2017, p9210-9213
    DOI:10.1039/C7CC05198K | unige:96312 | Abstract | Article HTML | Article PDF | Supporting Info
Chiral metal dithiolene complexes represent a family of chiral precursors, which can give rise to molecular materials with properties resulting from the interplay of chirality with conductivity, magnetism, and photophysics. We describe herein the first examples of chiral metal diimine dithiolene complexes, by the use of a platinum(II) centre coordinated by 2,2’-bipyridine and helicene-dithiolene ligands. Straightforward synthesis of racemic and enantiopure complexes allows the preparation of luminescent Pt(bipy) [4] and [6]helicene compounds for which the solid-state structure was determined as well. TD-DFT calculations support the assignment of the low energy bands observed in the UV-vis absorption spectra as mixed metal-ligand-to-ligand charge transfer transitions and confirm that the emission band results from the T1 excited state. Interestingly the enantiopure [6]helicene complexes show CPL activity at room temperature in acetonitrile solutions with anisotropy factors of 3×10-4.
  • CrIII as an alternative to RuII in metallo-supramolecular chemistry
    Davood Zare, Benjamin Doistau, Homayoun Nozary, CĂŠline Besnard, Laure GuĂŠnĂŠe, Yan Suffren, Anne-Laure PelĂŠ, Andreas Hauser and Claude Piguet
    Dalton Transactions, 46 , 2017, p8992-9009
    DOI:10.1039/c7dt01747b | unige:95682 | Article HTML | Article PDF | Supporting Info
  
Compared with divalent ruthenium coordination complexes, which are widely exploited as parts of multi-component photonic devices, optically active trivalent chromium complexes are under-represented in multi-metallic supramolecular architectures performing energy conversion because of the tricky preparation of stable heteroleptic CrIII building blocks. We herein propose a kind of remedy with the synthesis of a novel family of kinetically inert hetereloptic bis-terdentate mononuclear complexes, which can be incorporated into dinuclear rod-like diads as a proof-of-concept. The mechanism and magnitude of intermetallic Cr¡¡¡Cr communications have been unraveled by a combination of magnetic, photophysical and thermodynamic investigations. Alternated aromatic/alkyne connectors provided by Sonogashira coupling reactions emerge as the most efficient wires for long-distance communication between two chromium centres bridged by Janus-type back-to-back bis-terdentate receptors.
  
  • A Lithium Amide-Borohydride Solid-State Electrolyte with Lithium-Ion Conductivities Comparable to Liquid Electrolytes
    Yigang Yan, Ruben-Simon KĂźhnel, Arndt Remhof, LĂŠo DuchĂŞne, Eduardo Cuervo Reyes, Daniel Rentsch, Zbigniew Lodziana and Corsin Battaglia
    Advanced Energy Materials, 2017, p1700294
    Keywords: all-solid-state lithium-ion batteries, complex hydrides, solid-state electrolytes
    DOI:10.1002/aenm.201700294 | Abstract | Article HTML | Article PDF | Supporting Info
High ionic conductivity of up to 6.4 ×10−3S cm−1 near room temperature  (40 °C) in lithium amide-borohydrides is reported, comparable to values  of liquid organic electrolytes commonly employed in lithium-ion batteries.  Density functional theory is applied coupled with X-ray diffraction, calorimetry, and nuclear magnetic resonance experiments to shed light on the  conduction mechanism. A Li4Ti5O12 half-cell battery incorporating the lithium amide-borohydride electrolyte exhibits good rate performance up to 3.5 mA cm−2 (5 C) and stable cycling over 400 cycles at 1 C at 40 °C, indicating high bulk and interfacial stability. The results demonstrate the potential of lithium amide-borohydrides as solid-state electrolytes for high-power lithium-ion batteries.
 
The fluorescence quenching of organic dyes via H-bonding interactions is a well-known phenomenon. However, the mechanism of this Hydrogen-Bond Induced Nonradiative Deactivation (HBIND) is not understood. Insight into this process is obtained by probing in the infrared the O H stretching vibration of the solvent after electronic excitation of a dye with H-bond accepting cyano groups. The fluorescence lifetime of this dye was previously found to decrease from 1.5 ns to 110 ps when going from an aprotic solvent to the strongly protic hexafluoroisopropanol (HFP). Prompt strengthening of the H-bond with the dye was identified by the presence of a broad positive O-H band of HFP, located at lower frequency than the O-H band of the pure solvent. Further strengthening occurs within a few picoseconds before the excited H-bonded complex decays to the ground state in 110 ps. The latter process is accompanied by the dissipation of energy from the dye to the solvent and the rise of a characteristic hot solvent band in the transient spectrum. Polarization-resolved measurements evidence a collinear alignment of the nitrile and hydroxyl groups in the H-bonded complex, which persists during the whole excited-state lifetime. Measurements in other fluorinated alcohols and in chloroform/HFP mixtures reveal that the HBIND efficiency depends not only on the strength of the H-bond interactions between the dye and the solvent but also on the ability of the solvent to form an extended H-bond network. The HBIND process can be viewed as an enhanced internal conversion of an excited complex consisting of the dye molecule connected to a large H-bond network.
  
  • Search for Electron Delocalization from [Fe(CN)6]3- to the Dication of Viologen in (DNP)3[Fe(CN)6]2 · 10H2O
    Ahmed S. Abouelwafa, Andreas Hauser, Valeriu Mereacre, Yanhua Lan, Gary J. Long, Fernande Grandjean, Gernot Buth, Christopher E. Anson and Annie K. Powell
    Inorganic Chemistry, 56 (11) , 2017, p6477-6488
    DOI:10.1021/acs.inorgchem.7b00540 | unige:94916 | Abstract | Article HTML | Article PDF | Supporting Info
K3Fe(CN)6 reacts with the viologen 1,1′-bis(2,4-dinitrophenyl)-4,4′-bipyridinium dication, (DNP)2+, to form a supramolecular complex, (DNP)3[Fe(CN)6]2·10H2O (1). The crystal structure of 1 reveals that there are two [Fe(CN)6]3– anions within an organic framework of three (DNP)2+ cations with the shortest Fe(III)···Fe(III) distances of ca. 9.8 Å, distances that minimize extensive long-range magnetic exchange coupling interactions between the [Fe(CN)6]3– anions, and, thus, 1 is paramagnetic above ca. 17 K and exhibits weak ferromagnetic coupling between 17 and 3 K and antiferromagnetic coupling between 3 and 1.8 K. The long Fe(III)···Fe(III) distances permit slow spin–spin and slow spin–lattice paramagnetic relaxation, relative to the iron-57 Larmor precession frequency, as is evidenced by the Mössbauer spectra measured between 3 and 60 K; between 85 and 295 K, rapid paramagnetic relaxation is observed. Both the slow spin–spin and slow spin–lattice relaxation are mediated by the organic, π-conjugated viologen cations. The Fe–C distances, the Mössbauer isomer shifts, the temperature dependence of the magnetic susceptibility, and the 3 K magnetization results all indicate the presence of low-spin Fe(III) ions in the [Fe(CN)6]3– anions in 1. There is no unequivocal indication of the presence of any formal electron delocalization or transfer from the [Fe(CN)6]3– anion to the (DNP)2+ cations in the results obtained from X-ray crystallography, magnetic measurements, and Mössbauer spectra. Because of enhancement of the spin–orbit coupling by the heavy-atom or -ion effect, the Fe(III) ions in the [Fe(CN)6]3– anions interact with the (DNP)2+ cations, causing them to fluoresce with increasing intensity upon cooling from 90 to 25 K when excited at 300 nm. The resulting luminescence of the viologen (DNP)2+ cation induced by the [Fe(CN)6]3– anions indicates the presence of significant mixing of the molecular orbitals derived from the [Fe(CN)6]3– anions and the molecular orbitals associated with the (DNP)2+ cations to yield bonding supramolecular orbitals in 1, a mixing that is also observed between 50 and 3 K in the temperature dependence of the isomer shift of 1.
 
A CuAu38 bimetallic nanocluster was synthesized by adding a single copper atom to the Au38(2-PET)24 nanocluster. The absence of CuxAu38(2-PET)24 doped species was demonstrated by MALDI-TOF mass spectrometry. A separation of bimetallic clusters was attained for the first time where isomers of the E2 enantiomer of the Au38Cu1(2-PET)24 adduct were successfully isolated from their parent cluster using chiral HPLC. The CD of the isolated isomers revealed a change in their electronic structure upon copper addition. The luminescence of the Au38Cu1 adduct is significantly enhanced in comparison with the parent Au38 nanocluster. The stability of the newly formed adduct is strongly dependent on the coexistence of the Au38 nanoclusters.
Enhancement of infrared signals from polyelectrolyte adsorbed on gold nanoparticles (GNPs) was studied in situ by attenuated total reflection infrared spectroscopy. Nanoparticles and polyelectrolytes were deposited using layer-by-layer techniques, and the infrared signal was studied as a function of particle size, particle density, and distance from particle surface. It was observed that enhancement is more pronounced for larger nanoparticles and it decreases with increasing distance from the particle surface. Furthermore, at high GNP coverage, the signal from the first polyelectrolyte layer is particularly enhanced, and the signal increases slowly with time, in contrast to subsequent layers. We assign this to polyelectrolyte adsorption within narrow gaps between nanoparticles, where the electric field is enhanced. Furthermore, enhanced absorption was observed in the gap between the GNPs and the germanium internal reflection element, which was confirmed by polarized measurements. This enhancement is more pronounced for silver particles, and it represents a promising route for analysis of surfaces by infrared spectroscopy.
  
The ligand exchange reaction between heteroatom doped (Pd, Pt) Au25(2-PET)18 (2-PET = 2-phenylethylthiolate) clusters and enantiopure 1,1′-binaphthyl-2,2′-dithiol (BINAS) was monitored in situ using chiral high-performance liquid chromatography (HPLC). During the ligand exchange reactions, replacement of two protecting thiols (2-PET) with one new entering BINAS ligand on the cluster surface occurs. The rigid dithiol BINAS adsorbs in a specific mode that bridges the apex and one core site of two adjacent S(R)–Au–S(R)–Au–S(R) units. This is the most favorable binding mode and theoretically preserves the original structure. A kinetic investigation on these in situ ligand exchange reactions revealed a decrease in reactivity after multiple exchange. A comparison of relative rate constants demonstrates a similar exchange rate toward BINAS for both (Pd, Pt) systems. The possible structural deformation after incorporation of BINAS was investigated by X-ray absorption spectroscopy (XAS) at the S K-edge and Au L3-edge. First, a thorough assignment of all sulfur contributions to the XANES spectrum was performed, distinguishing for the first time long and short staple motifs. Following that, a structural comparison of doped systems using XANES and EXAFS confirmed the unaltered Au25 structure, except for some slight influence on the Au–S bonds. Additionally, an intact staple motif was confirmed after incorporation of rigid dithiol BINAS by both XANES and EXAFS. This finding agrees with a BINAS interstaple binding mode predicted by calculation, which does not perturb the cluster structure.
  • Computational study of the vibrational spectroscopy properties of boron-hydrogen compounds: Mg(B3H8)2 , CB9H10– and CB11H12–
    Daniel Sethio, LatĂŠvi Max Lawson Daku and Hans Hagemann
    International Journal of Hydrogen Energy, 42 (35) , 2017, p22496-22501
    Keywords: boron-hydrogen species; IR; Raman; vibrational frequencies; anharmonicity; density functional theory
    DOI:10.1016/j.ijhydene.2017.03.044 | unige:96315 | Abstract | Article PDF
  
We report the DFT study of the vibrational spectroscopy properties of Mg(B3H8)2, a potential intermediate in the decomposition of Mg(BH4)2, as well as those of CB11H12− and CB9H10−, whose salts can exhibit high ionic conductivities. Because the inclusion of anharmonicity is key to the accurate description of the vibrational properties of BH species [D. Sethio, L. M. Lawson Daku, H. Hagemann. Int. J. Hydrogen Energy, 41 (2016) 6814], the calculations were performed both in the harmonic and in the anharmonic approximation. The IR and Raman spectra of Cs(CB11H12) and Na2(B10H10) have also been measured. The calculated and experimental spectra are in good agreement. A comparative analysis of the vibrational spectroscopy properties is made for B3H8− and Mg(B3H8)2, B12H122− and CB11H12−, and for B10H102− and CB9H10−.
  
  • Artificial tubular connections between cells based on synthetic lipid nanotubesOpen access paper
    Alexander Kozintsev and Kaori Sugihara
    RSC Advances, 7 (33) , 2017, p20700-20708
    DOI:10.1039/C7RA02187A | unige:94060 | Abstract | Article HTML | Article PDF
Tunneling nanotubes (TNTs) have become a major topic of interest as a form of intercellular communication due to their recent discovery. However, research on this subject has often suffered from a lack of controllability in the generation of the nanotubular connections. In this work, we demonstrate a simplified approach to selectively create a direct nanotubular connection between eukaryotic cells by manually manipulating self-assembling lipid nanotubes (LNTs) from inverted hexagonal-phase lipid blocks. The technique requires minimal instrumentation for creating the LNT connection between cells compared to conventional approaches. Based on the diffusion of fluorescent lipids from LNTs into cell membranes (D = 0.032 ± 0.003 μm2 s−1), the probability of observing membrane fusion between LNTs and cell membranes was estimated as 30%. Among these cell–LNT junctions the resulting structure is open-ended roughly 75% of the time, as evidenced from observations of the diffusion of a water-soluble dye between two cells connected with this nanotubular structure.
 
We propose a simple method for predicting the spin state of homoleptic complexes of the Fe(II) d6 ion with chelating diimine ligands. The approach is based on the analysis of a single metric parameter within a free (noncoordinated) ligand: the interatomic separation between the N-donor metal-binding sites. An extensive analysis of existing complexes allows the determination of critical N¡¡¡N distances that dictate the regions of stability for the high-spin and low-spin complexes, as well as the intermediate range in which the magnetic bistability (spin crossover) can be observed. The prediction has been tested on several complexes that demonstrate the validity of our method.
  
  • A simple model of solvent-induced symmetry-breaking charge transfer in excited quadrupolar molecules
    Anatoly I. Ivanov, Bogdan Dereka and Eric Vauthey
    The Journal of Chemical Physics, 146 (16) , 2017, p164306
    Keywords: solvents; dielectric constant; excited states; charge transfer; band models
    DOI:10.1063/1.4982067 | unige:94061 | Abstract | Article HTML | Article PDF
A simple model has been developed to describe the symmetry-breaking of the electronic distribution of AL–D–AR type molecules in the excited state, where D is an electron donor and AL and AR are identical acceptors. The origin of this process is usually associated with the interaction between the molecule and the solvent polarization that stabilizes an asymmetric and dipolar state, with a larger charge transfer on one side than on the other. An additional symmetry-breaking mechanism involving the direct Coulomb interaction of the charges on the acceptors is proposed. At the same time, the electronic coupling between the two degenerate states, which correspond to the transferred charge being localised either on AL or AR, favours a quadrupolar excited state with equal amount of charge-transfer on both sides. Because of these counteracting effects, symmetry breaking is only feasible when the electronic coupling remains below a threshold value, which depends on the solvation energy and the Coulomb repulsion energy between the charges located on AL and AR. This model allows reproducing the solvent polarity dependence of the symmetry-breaking reported recently using time-resolved infrared spectroscopy.
  • Kinetic Resolution of 2-Substituted Indolines by N -Sulfonylation using an Atropisomeric 4-DMAP- N -oxide Organocatalyst
    James I. Murray, Nils J. FlodĂŠn, Adriano Bauer, Nico D. Fessner, Daniel L. Dunklemann, Opetoritse Bob-Egbe, Henry S. Rzepa, Thomas BĂźrgi, Jeffery Richardson and Alan C. Spivey
    Angewandte Chemie International Edition, 56 (21) , 2017, p5760-5764
    Keywords: asymmetric catalysis;indoline;kinetic resolution;organocatalysis;sulfonylation
    DOI:10.1002/anie.201700977 | unige:94104 | Abstract | Article HTML | Article PDF | Supporting Info
  
The first catalytic kinetic resolution by N-sulfonylation is described. 2-Substituted indolines are resolved (s=2.6–19) using an atropisomeric 4-dimethylaminopyridine-N-oxide (4-DMAP-N-oxide) organocatalyst. Use of 2-isopropyl-4-nitrophenylsulfonyl chloride is critical to the stereodiscrimination and enables facile deprotection of the sulfonamide products with thioglycolic acid. A qualitative model that accounts for the stereodiscrimination is proposed.
  
The adsorption of a DNA fluorescent probe belonging to the thiazole orange family at the dodecane/water and dodecane/phospholipid/water interfaces has been investigated using a combination of surface second harmonic generation (SSHG) and all-atomistic molecular dynamics (MD) simulations. Both approaches point to a high affinity of the cationic dye for the dodecane/water interface with a Gibbs free energy of adsorption on the order of −45 kJ/mol. Similar affinity was observed with a monolayer of negatively charged DPPG (1,2-dipalmitoyl-sn-glycero-3-phospho-rac-(1-glycerol)) lipids. On the other hand, no significant adsorption could be found with the zwitterionic DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) lipids. This was rationalized in terms of Coulombic interactions between the monolayer surface and the cationic dye. The similar affinity for the interface with and without DPPG, despite the favorable Coulombic attraction in the latter case, could be explained after investigating the interfacial orientation of the dye. In the absence of a monolayer, the dye adsorbs with its molecular plane almost flat at the interface, whereas in the presence of DPPG it has to intercalate into the monolayer and adopt a significantly different orientation to benefit from the electrostatic stabilization.
  • A highly stable sodium solid-state electrolyte based on a dodeca/deca-borate equimolar mixture
    LĂŠo DuchĂŞne, Ruben-Simon KĂźhnel, Daniel Rentsch, Arndt Remhof, Hans Hagemann and Corsin Battaglia
    ChemComm, 53 (30) , 2017, p4195-4198
    DOI:10.1039/C7CC00794A | unige:93532 | Abstract | Article HTML | Article PDF
  
Na2(B12H12)0.5(B10H10)0.5, a new solid-state sodium electrolyte is shown to offer high Na+ conductivity of 0.9 mS cm-1 at 20 °C, excellent thermal stability up to 300 °C, and a large electrochemical stability window of 3 V including stability towards sodium metal anodes, all essential prerequisites for a stable room-temperature 3 V all-solid-state sodium-ion battery.
  
Polar solvation dynamics of six 7-aminocoumarins and 4-aminophthalimide (4AP) are investi- gated using broadband FLuorescence UP–conversion Spectroscopy (FLUPS) combined with a global analysis based on time-dependent band-shape functions. The solvation dynamics of the coumarins in ethanol exhibit only minor differences but are, however, significantly different from that of 4AP. The band-shape parameters, width and asymmetry, exhibit much larger variation even among the coumarins and are correlated with the amount of excess excitation energy. Differences in the solvation dynamics of 4AP and a selected coumarin, C151, are also observed in dimethyl sulfoxide demonstrating the molecularity of solvation i.e. solvation depends on the solute and does not solely reflect the dynamic properties of the solvent. These differences are attributed to specific solute-solvent interactions due to hydrogen bonding. In a weakly interacting solvent, benzonitrile, the solvation dynamics of 4AP and C151 are nearly identical.
Alkaline or alkaline earth octahydrotriborate M(B3H8)x and dodecahydro-closo-dodecaborate MxB12H12 (M = Li, Na, Mg or Ca with x = 1 or 2) have recently attracted a lot of interest for hydrogen storage and solid electrolyte applications. Nevertheless, their syntheses are still a roadblock for large scale applications. In this paper we propose a novel approach for their syntheses starting from the cheapest borohydride NaBH4. The process involves first the solvothermal synthesis of tetrabutylammonium octahydrotriborate (C4H9)4NB3H8 (TBAB3H8) being the basis for the syntheses of the others boranes. Starting from TBAB3H8, we have synthesized pure and unsolvated NaB3H8 by salt metathesis reaction with sodium tetraphenylborate. Then, we have successfully obtained Na2B12H12 by solvothermal decomposition of NaB3H8. This approach has shown to be quantitative and reproducible, which could lead to the development of these boranes in real life applications.
 
Because of its crucial role in many areas of science and technology, photoinduced electron transfer is the most investigated photochemical reaction. Despite this, several important questions remain open. We present recent efforts to answer some of them, which concern both inter- and intramolecular processes. The decisive factor that allowed these issues to be successfully addressed was the use of time-resolved infrared (TRIR) spectroscopy.

Many different transient species, such as tight and loose ion pairs (TIPs and LIPs) and exciplexes, have been invoked to explain the dynamics of intermolecular photoinduced charge separation reactions (i.e., electron transfer between two neutral species) and the production of free ions. However, their structures are essentially unknown, and their exact roles in the reaction mechanism are unclear. Indeed, the commonly used transient electronic absorption spectroscopy does not give much structural insight and cannot clearly distinguish ion pairs from free ions, at least in the visible region. Unambiguous spectral signatures of TIPs, LIPs, and exciplexes could be observed in the IR using electron donor/acceptor (D/A) pairs with adequate vibrational marker modes. The ability to spectrally distinguish these intermediates allowed their dynamics to be disentangled and their roles to be determined. Structural information could be obtained using polarization-resolved TRIR spectroscopy. Our investigations reveal that moderately to highly exergonic reactions result in the formation of both TIPs and LIPs. TIPs are not only generated upon direct charge-transfer excitation of DA complexes, as usually assumed, but are also formed upon static quenching with reactant pairs at distances and orientations enabling charge separation without diffusion. On the other hand, dynamic quenching produces primarily LIPs. In the case of highly exergonic reactions, strong indirect evidence for the generation of ion pairs in an electronic excited state was found, accounting for the absence of an inverted region. Finally, weakly exergonic reactions produce predominantly exciplexes, which can evolve further into ion pairs or recombine to the neutral ground state.

The high sensitivity of specific vibrational modes to the local electronic density was exploited to visualize the photoinduced charge flow in symmetric A–(π–D)2- and D–(π–A)2-type molecules developed for their two-photon absorption properties. The electronic ground state and Franck–Condon S1 state of these molecules are purely quadrupolar, but the strong solvatochromism of their fluorescence points to a highly dipolar relaxed S1 state. This has been explained in terms of excited-state symmetry breaking induced by solvent and/or structural fluctuations. However, real-time observation of this process was missing. Direct visualization of symmetry-breaking charge transfer was achieved using TRIR spectroscopy by monitoring vibrations localized in the two arms of these molecules. A transition from a purely quadrupolar state to a symmetry-broken state on the timescale of solvent relaxation could be clearly observed in polar solvents, indicating that symmetry breaking occurs primarily via solvent fluctuations. In the case of the D–(π–A)2 molecule, this breaking results in different basicities at the two A ends and consequently in different affinities for H-bonds, which in turn leads to the formation of an asymmetric tight H-bonded complex in highly protic solvents.

  
  • Mechanosensitivity of polydiacetylene with a phosphocholine headgroup
    Roberto Diego Ortuso, Ugo Cataldi and Kaori Sugihara
    Soft Matter, 13 (8) , 2017, p1728-1736
    Keywords: polydiacetylene; mechanochromism; DC(8,9)PC; melittin; UV/VIS spectroscopy; fluorescence spectroscopy
    DOI:10.1039/C6SM02579J | unige:92341
We demonstrated colorimetric and fluorescence detection of peptide, melittin, with polydiacetylene (PDA) made of 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DC(8,9)PC). The PDA used in this work has a phosphocholine headgroup, which mimics peptide-cell membrane interactions better than the conventional PDA assays with carboxyl headgroup. The dose curve (colorimetric response vs melittin concentration) showed a half maximum response at the melittin concentration of 0.1 mg/ml, which is similar to the traditional PDA assays. It suggests that the replacement of the headgroup was achieved without sacrificing the sensitivity. From the dose curve, Hill coefficient was extracted as ÎąHill = 2.1. The value is in agreement with previous melittin studies with standard phospholipids, which reflects the benefit of having a biologically relevant headgroup. In addition, we found an unexpectedly slow spectral change when DC(8,9)PC-PDA was incubated with melittin. The origin of the time-dependent signal was studied by combining UV/VIS spectroscopy, fluorescence spectroscopy and dynamic light scattering.
 
A model for simulating the transient electronic absorption spectra of donor–acceptor dyads undergoing ultrafast intramolecular charge transfer in solution has been developed. It is based on the stochastic multichannel point-transition approach and includes the reorganization of high-frequency intramolecular modes (treated quantum mechanically) and of low frequency intramolecular and solvent modes (described classically). The relaxation of the slow modes is assumed to be exponential with time constants taken from experiments. The excited-state dynamics is obtained by simulating the population distribution of each quantum state after optical excitation and upon electronic and vibrational transitions. This model was used to simulate the transient electronic absorption spectra measured previously with a pyrylium phenolate in acetonitrile. A very good agreement between the simulated and measured spectra was obtained assuming a three-level model including the ground state, the optically excited state, and a dark state with large charge-transfer character and a substantially different geometry relative to that of the optically excited state. The merit of this approach to disentangle the contributions of both population changes and relaxation processes to the ultrafast spectral dynamics will be discussed.
  
  • Original oxo-centered bismuth oxo-arsenates; Critical effect of PO4 for AsO4 substitution
    Jacob Olchowka, Marie Colmont, Almaz Aliev, Thao Tran, P. Shiv Halasyamani, Hans Hagemann and Olivier MentrĂŠ
    CrystEngComm, 19 , 2017, p936-945
    DOI:10.1039/C6CE02466A | unige:92340 | Abstract | Article HTML | Article PDF | Supporting Info
This work deals with the synthesis and crystal structure study of new bismuth oxo-arsenates and their oxo-phosphates homologous: Bi6ZnO7(AsO4)2 vs Bi6ZnO7(PO4)2 and Bi3.667Cd3O4(AsO4)3 vs Bi3Cd4O4(PO4)3. Their crystal structures were solved using single crystal X-Ray Diffraction. These are two other examples of crystal structures built on ribbon-like polycations formed of the linkage of oxo-centered O(Bi,M)4 tetrahedra sharing edges and surrounded by isolated XO4 groups (X= As or P), where the O(Bi,M)4 units are derived from the fluorite topology structure. Dealing with Bi6ZnO7(PO4), its acentric space group was confirmed by preliminary second harmonic generation (SHG). The P/As substitution led to a centrosymmetric space group due to local reorientation of oxo-anions. This is strongly related to steric effects between AsO4 (d As-O= 1.6-1.7Å) and PO4 (d P-O= 1.4-1.5Å). Concerning Bi3.667Cd3O4(AsO4)3 and Bi3Cd4O4(PO4)3, they show a second example of the reorientation of the XO4 groups depending of the X chemical nature. Finally, we present an original topology of oxo-centered units obtained with Bi5KO5(AsO4). The photoluminescence properties of Bi5KO5(AsO4) and Bi6ZnO7(AsO4)2 were also investigated. The first one emits at room temperature in the reddish-orange range (single band peak at 615nm assigned to the Bi3+: 3P1→1S0 transition whereas the second exhibits a weak emission in the green range (peak at 530nm). Its intriguing temperature dependence is discussed in the paper.
  • Ultrafast Elementary Photochemical Processes of Organic Molecules in Liquid SolutionOpen access paper
    Tatu Kumpulainen, Bernhard Lang, Arnulf Rosspeintner and Eric Vauthey
    Chemical Reviews, 117 (16) , 2017, p10826-10939
    DOI:10.1021/acs.chemrev.6b00491 | unige:96318 | Abstract | Article HTML | Article PDF
  
Ultrafast photochemical reactions in liquids occur on similar or shorter time scales compared to the equilibration of the optically populated excited state. This equilibration involves the relaxation of intramolecular and/or solvent modes. As a consequence, the reaction dynamics are no longer exponential, cannot be quantified by rate constants, and may depend on the excitation wavelength contrary to slower photochemical processes occurring from equilibrated excited states. Such ultrafast photoinduced reactions do no longer obey the Kasha–Vavilov rule. Nonequilibrium effects are also observed in diffusion-controlled intermolecular processes directly after photoexcitation, and their proper description gives access to the intrinsic reaction dynamics that are normally hidden by diffusion. Here we discuss these topics in relation to ultrafast organic photochemical reactions in homogeneous liquids. Discussed reactions include intra- and intermolecular electron- and proton-transfer processes, as well as photochromic reactions occurring with and without bond breaking or bond formation, namely ring-opening reactions and cis–trans isomerizations, respectively.
  
  • A biocompatible macromolecular two-photon initiator based on hyaluronanOpen access paper
    Maximilian Tromayer, Peter Gruber, Marica Markovic, Arnulf Rosspeintner, Eric Vauthey, Heinz Redl, Aleksandr Ovsianikov and Robert Liska
    Polymer Chemistry, 8 (2) , 2017, p451-460
    DOI:10.1039/C6PY01787H | unige:91449 | Abstract | Article HTML | Article PDF | Supporting Info
The possibility of the direct encapsulation of living cells via two-photon induced photopolymerization enables the microfabrication of hydrogel scaffolds with high initial cell loadings and intimate matrix-cell contact. While highly efficient water-soluble two-photon initiators based on benzylidene ketone dyes have been developed, they exhibit considerable cyto- and phototoxicity. To address the problem of photoinitiator migration from the extracellular matrix into the cytosol, a two-photon initiator bound to a polymeric hyaluronan backbone (HAPI) was synthesized in this work. HAPI exhibited a distinct improvement of cytocompatibility compared to a reference two-photon initiator. Basic photophysical investigations were performed to characterize the absorption and fluorescence behavior of HAPI. Laser scanning microscopy was used to visualize and confirm the hindered transmembrane migration behavior of HAPI. The performance of HAPI was tested in two-photon polymerization at exceedingly high printing speeds of 100 mm s-1 producing gelatin-based complex 3D hydrogel scaffolds with a water content of 85%. The photodamage of the structuring process was low and viable MC3T3 cells embedded in the gel were monitored for several days after structuring.
  • Triggering the emission by the helical turn in thiadiazole-helicenes
    Thomas Biet, KĂŠvin Martin, Jihane Hankache, Nora Hellou, Andreas Hauser, Thomas BĂźrgi, Nicolas Vanthuyne, Tal Aharon, Marco Caricato, Jeanne Crassous and Narcis Avarvari
    Chemistry - A European Journal, 23 (2) , 2017, p437-446
    Keywords: chirality;circular dichroism;density functional calculations;helicenes;heterocycles
    DOI:10.1002/chem.201604471 | unige:91450 | Abstract | Article HTML | Article PDF | Supporting Info
Introduction of heterocycles in the helical skeleton of helicenes allows modulation of their redox, chiroptical and photophysical properties. Herein, we describe the straightforward preparation and structural characterization by single crystal X-ray diffraction of thiadiazole-[7]helicene, which has been resolved into (M) and (P) enantiomers by chiral HPLC, together with its S-shaped double [4]helicene isomer, as well as the smaller congeners thiadiazole-[5]helicene and benzothiadiazole-anthracene. A copper(II) complex with two thiadiazole-[5]helicene ligands has been structurally characterized and it shows the presence of both (M) and (P) isomers coordinated to the metal centre. The emission properties of the unprecedented heterohelicenes are highly dependent on the helical turn, as the [7]- and [5]helicene are poorly emissive, whereas their isomers, that is, the S-shaped double [4]helicene and thiadiazole-benzanthracene, are luminescent, with quantum efficiencies of 5.4% and 6.5%, respectively. DFT calculations suggest a quenching of the luminescence of enantiopure [7]helicenes through an intersystem crossing mechanism arising from the relaxed excited S1 state.
  • Ultrafast transient IR spectroscopy and DFT calculations of ruthenium(II) polypyridyl complexesOpen access paper
    Qinchao Sun, Bogdan Dereka, Eric Vauthey, Latevi Max Lawson Daku and Andreas Hauser
    Chemical Science, 8 (1) , 2017, p223-230
    DOI:10.1039/C6SC01220E | unige:90819 | Abstract | Article HTML | Article PDF | Supporting Info
Ultrafast time-resolved infrared spectroscopy of [Ru(bpy)3]2+ (bpy = 2,2’-bipyridine), [Ru(mbpy)3]2+ (mbpy = 6-methyl-2,2’-bipyridine), and [Ru(mphen)3]2+ (mphen = 2-methyl-1,10’-phenanthroline) in deuterated acetonitrile serves to elucidate the evolution of the system following pulsed excitation into the 1MLCT band at 400 nm. Whereas for [Ru(bpy)3]2+ no intermediate state can be evidenced for the relaxation of the corresponding 3MLCT state back to the ground state, for [Ru(mbpy)3]2+ and [Ru(mphen)3]2+ an intermediate state with a lifetime of about 400 ps is observed. The species associated IR difference spectra of this state are in good agreement with the calculated difference spectra of the lowest energy 3dd state using DFT. The calculated potential energy curves for all the complexes in the triplet manifold along the metal-ligand distance show that for [Ru(bpy)3]2+ the 3dd state is at higher energy than the 3MLCT state and that there is a substantial barrier between the two minima. For [Ru(mbpy)3]2+ and [Ru(mphen)3]2+, the 3dd state is at lower energy than the 3MLCT state.
Sum frequency mixing of fluorescence and ∼1300 nm gate pulses, in a thin β-barium borate crystal and non-collinear type II geometry, is quantified as part of a femtosecond fluorimeter [X.-X. Zhang et al., Rev. Sci. Instrum. 82, 063108 (2011)]. For a series of fixed phasematching angles, the upconversion efficiency is measured depending on fluorescence wavelength. Two useful orientations of the crystal are related by rotation around the surface normal. Orientation A has higher efficiency (factor ∼3) compared to B at the cost of some loss of spectral coverage for a given crystal angle. It should be used when subtle changes of an otherwise stationary emission band are to be monitored. With orientation B, the fluorescence range λF > 420-750 nm is covered with a single setting of the crystal and less gate scatter around time zero. The accuracy of determining an instantaneous emission band shape is demonstrated by comparing results from two laboratories.
We present experimental Raman optical activity (ROA) spectra of enantio-enriched single-walled carbon nanotubes (SWCNTs). Enantiomeric samples of (6,5) SWCNTs were prepared using nonlinear density gradient ultracentrifugation (DGU). Upon excitation at 2.33 eV, remarkably strong G-band signals are obtained due to strong resonance enhancement with the E22S transition of (6,5) SWCNTs. Enhancement allows measuring the vibrational optical activity (VOA) at unusually low concentrations. The obtained results are in good agreement with the single-excited-state theory (SES). To our knowledge, these are the first experimental VOA spectra of SWCNTs.
Multiple Ag atoms were doped inside Au38(SCH2CH2Ph)24 nanoclusters using the metal exchange method for the first time for the synthesis of AgxAu38–x(SCH2CH2Ph)24. MALDI-TOF mass spectrometry revealed the time dependence of the synthesis. Cluster species with different numbers of Ag atoms (different x values) migrate differently on a chromatography (HPLC) column, which allows one to isolate cluster samples with a narrowed distribution of exchanged metal atoms. The enantiomers of selected AgxAu38–x(SCH2CH2Ph)24 samples (average x = 6.5 and 7.9) have been separated by HPLC. Doping changes the electronic structure, as is evidenced by the significantly different CD spectra. UV–vis spectra of the doped sample also show diminished features. The temperature required for complete racemization follows Au38 > AgxAu38–x (x = 6.5) > AgxAu38–x (x = 7.9). To our surprise, the racemization of AgxAu38–x(SCH2CH2Ph)24 (x = 7.9) occurred even at 20 °C. Racemization involves a rearrangement of the staple motifs at the cluster surface. The results therefore show an increased flexibility of the cluster with increasing silver content. The weaker Ag–S bonds compared to Au–S are proposed to be at the origin of this observation. The experimentally determined activation energy for the racemization is ca. 21.5 kcal/mol (x = 6.5) and 19.5 kcal/mol (x = 7.9), compared to 29.5 kcal/mol for Au38(SCH2CH2Ph)24, suggesting no complete metal–S bond breaking in the process.
We describe for the first time the crystallization in thin films of a DNA copolymer composed of a low molecular weight chitosan backbone to which a sequence of nucleic acids is grafted (chitosan-g-ssDNA). As assessed by atomic force microscopy, optical microscopy and spectroscopy, crystallization occurs due to intermolecular hydrogen bonding in which the nucleic acid strands engage. The morphology of the crystals depends on the affinity for the surface of the polymer segments that compose the DNA copolymer hybrid. The nucleic acids adsorb on mica and silica on which side-branched structures are observed whereas chitosan interacts preferentially with gold, and dendritic crystals are assembled. Attenuated total reflectance infrared spectroscopy supports the high ordering of the structure and the establishment of strong intermolecular interactions by hydrogen bonding.
  • Control of the plasmonic resonance of a graphene coated plasmonic nanoparticle array combined with a nematic liquid crystalOpen access paper
    Luciano De Sio, Ugo Cataldi, Thomas BĂźrgi, Nelson Tabiryan and Timothy J. Bunning
    AIP Advances, 6 (7) , 2016, p75114
    DOI:10.1063/1.4959869 | unige:94066 | Abstract | Article HTML | Article PDF
We report on the fabrication and characterization of a switchable plasmonic device based on a conductive graphene oxide (cGO) coated plasmonic nanoparticle (NP) array, layered with nematic liquid crystal (NLC) as an active medium. A monolayer of NPs has been immobilized on a glass substrate through electrostatic interaction, and then grown in place using nanochemistry. This monolayer is then coated with a thin (less then 100nm) cGO film which acts simultaneously as both an electro-conductive and active medium. The combination of the conductive NP array with a separate top cover substrate having both cGO and a standard LC alignment layer is used for aligning a NLC film in a hybrid configuration. The system is analysed in terms of morphological and electro-optical properties. The spectral response of the sample characterized after each element is added (air, cGO, NLC) reveals a red-shift of the localized plasmonic resonance (LPR) frequency of approximately 62nm with respect to the NP array surrounded by air. The application of an external voltage (8Vpp) is suitable to modulate (blue shift) the LPR frequency by approximately 22nm.
 
The catalytic photo-oxidation of 3,4-dihydroxybenzoic acid on TiO2 has been studied by in situ ATR-FTIR in flowing water and in flowing wet air/nitrogen gas. In flowing water it was difficult to observe photodegradation intermediates despite photocatalytic action during UV illumination. In the flowing wet air/nitrogen system carboxylic acids and carbonates were observed. It was shown that water plays an important role in the formation of oxidation active species. Oxygen shows a prominent role for carboxylic acid degradation, but the photogenerated hole plays the important role for the 3,4-dihydroxybenzoic acid ring cleavage.
We have studied, both experimentally and theoretically, the Raman vibrational spectra of a series of n-alkanethiolate protected Au25(SCnH2n+1)18 clusters, with n = 2, 3, 4, 5, 6, 8, 10, 12, and 14. The C–H stretching region of the infrared spectra reveals that, while shorter chains are flexible, longer chains are more ordered with a propensity toward extended all-trans conformation. The different behavior of long and short chains is also reflected in the low-frequency Raman spectra of the clusters, which are broadened for the longer chains due to interchain interactions and formation of bundles. The experimental low-frequency modes in the Raman spectra, associated with Au–S stretching vibrations, change drastically and in an apparently unsystematic way as a function of chain length. For example, a band around 320 cm–1 associated with tangential Au–S stretching character shifts up in frequency, then down and then up again as the carbon chain is increased. DFT calculations reveal that this behavior is due to a nonlinear coupling of this mode to torsional and bending modes of the alkyl chain. The frequencies of these modes strongly depend on the chain length and, as a consequence, also their coupling with the Au–S stretching modes, which explains the erratic behavior of this band in the spectra. This behavior is well described by calculations on a mimic cluster model that considers only one staple motif. For the ethanethiolate-protected cluster, the entire cluster was included in the calculation of the Raman spectrum, and this allowed for the first time to compare directly experimental and calculated Raman spectra of the same cluster. Furthermore, our study shows that the entire ligand has to be considered for the calculation of the low frequency vibrations of the Au–S interface, as this spectral region is sensitive to coupling with low-frequency ligand modes.
  • Optical control of plasmonic heating effects using reversible photo-alignment of nematic liquid crystals
    Giovanna Palermo, Ugo Cataldi, Luciano De Sio, Thomas BĂźrgi, Nelson Tabiryan and Cesare Umeton
    Applied Physics Letters, 109 , 2016, p191906
    DOI:10.1063/1.4967377 | unige:94069 | Abstract | Article HTML | Article PDF
We demonstrate and characterize an optical control of the plasmonic heat delivered by a monolayer substrate of gold nanoparticles, obtained by modulating the effective refractive index of the neighboring dielectric medium. The effect, which exploits the dependence of the nematic liquid crystal (NLC) refractive index on the molecular director orientation, is realized by using a polarization dependent, light-induced molecular reorientation of a thin film of photo-alignment layer that the NLC is in contact with. For a suitable alignment, plasmonic pumping intensity values ranging from 0.25 W/cm2 to 6.30 W/cm2 can induce up to 17.4 °C temperature variations in time intervals of the order of seconds. The reversibility of the optically induced NLC molecular director orientation enables an active control of the plasmonic photo-induced heat.
  • Self-assembled Lipid Structures as Model Systems for Studying Electrical and Mechanical Properties of Cell Membranes
    Kaori Sugihara
    Chimia, 70 (11) , 2016, p805-809
    Keywords: electrochemistry; lipid nanotube; lipid self-assembly; mechanochromic polymer; pore-forming peptide
    DOI:10.2533/chimia.2016.805 | unige:90820 | Abstract
This mini review summarizes our recent works in the development of electrical and mechanical characterization tools for cell membranes. These research topics require the application of physical and chemical characterization tools to biological systems, thus are very interdisciplinary.
  
  • Room Temperature Magnetic Switchability Assisted by Hysteretic Valence Tautomerism in a Layered Two Dimensional Manganese-Radical Coordination Framework
    Anthony Lannes, Yan Suffren, Jean Bernard Tommasino, Rodica Chiriac, François Toche, Lhoussain Khrouz, Florian Molton, Carole Duboc, Isabelle Kieffer, Jean-Louis Hazemann, Christian Reber, Andreas Hauser and Dominique Luneau
    Journal of the American Chemical Society, 138 (50) , 2016, p16493-16501
    DOI:10.1021/jacs.6b10544 | unige:90821 | Abstract | Article HTML | Article PDF | Supporting Info
The manganese-nitronyl-nitroxide two dimensional coordination polymer {[Mn2(NITIm)3]ClO4}n (1) (NITImH = 2-(2-imidazolyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-3-oxide-1-oxyl) undergoes an unusual hysteretic thermoinduced valence tautomeric transition near room temperature, during which the manganese(II) ions are oxidized to manganese(III) and two of the three deprotonated radicals (NITIm-) are reduced to their diamagnetic aminoxyl form (denoted NITImRed2-). Upon cooling, the high-temperature species {[MnII2(NITIm)3]ClO4}n (1HT) turns into the low-temperature species {[MnIII2(NITImRed)2(NITIm)]ClO4}n (1LT) around 274 K, while on heating the process is reversed at about 287 K. This valence tautomeric phenomenon is supported by temperature-dependent magnetic susceptibility measurements, differential scanning calorimetry (DSC), crystal structure determination, UV-vis absorption, X-ray absorption (XAS) an emission (XES) and Electron Paramagnetic Resonance (EPR) spectroscopies in the solid-state.
  • Physicochemical and Electronic Properties of Cationic [6]Helicenes: from Chemical and Electrochemical Stabilities to Far-Red (Polarized) Luminescence
    Johann Bosson, Geraldine M. Labrador, Simon Pascal, François-Alexandre Miannay, Oleksandr Yushchenko, Haidong Li, Laurent Bouffier, Neso Sojic, Roberto C. Tovar, Gilles Muller, Denis Jacquemin, Adèle D. Laurent, Boris Le Guennic, Eric Vauthey and JĂŠrĂ´me Lacour
    Chemistry - A European Journal, 22 (51) , 2016, p18394-18403
    Keywords: circularly polarized luminescence;density functional calculations;electrochemistry;fluorescence;helicenes
    DOI:10.1002/chem.201603591 | unige:90268 | Abstract | Article HTML | Article PDF | Supporting Info
  
The physicochemical properties of cationic dioxa (1), azaoxa (2), and diaza (3) [6]helicenes demonstrate a much higher chemical stability of the diaza adduct 3 (pKR+=20.4, Ered1/2 =−0.72 V) compared to its azaoxa 2 (pKR+=15.2, Ered1/2=−0.45 V) and dioxa 1 (pKR+=8.8, Ered1/2=−0.12 V) analogues. The fluorescence of these cationic chromophores is established, and ranges from the orange to the far-red regions. From 1 to 3, a bathochromic shift of the lowest energy transitions (up to 614 nm in acetonitrile) and an enhancement of the fluorescence quantum yields and lifetimes (up to 31 % and 9.8 ns, respectively, at 658 nm) are observed. The triplet quantum yields and circularly polarized luminescence are also reported. Finally, fine tuning of the optical properties of the diaza [6]helicene core is achieved through selective and orthogonal post-functionalization reactions (12 examples, compounds 4–15). The electronic absorption is modulated from the orange to the far-red spectral range (560–731 nm), and fluorescence is observed from 591 to 755 nm with enhanced quantum efficiency up to 70 % (619 nm). The influence of the peripheral auxochrome substituents is rationalized by first-principles calculations.
  
  • Symmetry-Breaking Charge Transfer and Hydrogen Bonding: Toward Asymmetrical Photochemistry
    Bogdan Dereka, Arnulf Rosspeintner, Maciej Krzeszewski, Daniel T. Gryko and Eric Vauthey
    Angewandte Chemie International Edition, 55 (50) , 2016, p15624-15628
    Keywords: charge transfer;photochemistry;quadrupolar molecules;solvation;ultrafast vibrational spectroscopy
    DOI:10.1002/anie.201608567 | unige:90841 | Abstract | Article HTML | Article PDF | Supporting Info
Symmetry-breaking charge transfer upon photoexcitation of a linear A-π-D-π-A molecule (D and A being electron donating and accepting groups) could be visualized using ultrafast time-resolved infrared spectroscopy by monitoring the CN stretching modes on the A units. Whereas in apolar solvents, the S1 state remains symmetric and quadrupolar, symmetry breaking occurs within ca. 100 fs in polar solvents as shown by the presence of two CN bands, instead of one in apolar solvents, with a splitting that increases with polarity. In protic solvents, symmetry breaking is significantly amplified by H-bonding interactions, which are the strongest at the CN group with the highest basicity. In strongly protic solvents, the two CN bands transform in about 20 ps into new bands with a larger splitting, and the lifetime of the S1 state is substantially reduced. This is attributed to the formation of an excited asymmetric tight H-bond complex.
 
Terpyridine derivatives are of great interest due to their unique photophysical properties when used as antennas in metallic complexes. Several experimental and theoretical studies indicate strong charge-transfer character of the lowest electronic excited state, which could be exploited for predicting fluorescence quantum yields from the magnitude of the charge separation induced by electronic transitions. Focusing on substituted 4′-phenyl-2,2′:6′2′′-terpyridyl, we report on two measures of the charge separation obtained from high-level calculations in ground and excited states (length of the change of the dipole moment and the electron–hole distance). Our refined model confirms that the fluorescence quantum yield shows a global S-shape dependence on the magnitude of the charge separation, which can be quantified either by the change in dipole moments between the ground and excited states or by the associated charge–hole distances. This approach provides a remarkable tool for the molecular design of a fluorescent polyaromatic antenna.
  
Germanium – polyelectrolyte – gold nanoparticle composites were prepared and characterized using FTIR-ATR spectroscopy and scanning electron microscopy. The germanium (Ge) element served as internal reflection element and the buildup of the layered system was followed in situ. Positively charged polyelectrolyte poly (allylamine hydrochloride) (PAH) adsorbs spontaneously on negatively charged Ge. Citrate-stabilized gold nanoparticles can then be adsorbed onto the PAH layer. Upon illumination of the device with visible light a prominent absorption over the entire mid infrared region is observed which is due to intervalence band transitions in Ge. The strong infrared signals are evidence for holes in the valence band of the Ge semiconductor, which arise due to electron transfer to the gold nanoparticles (GNP). The electron transfer, as evidenced by the holes in Ge, is affected by the nature of the gap between the Ge and the GNP. Increasing the gap by adsorbing polyelectrolyte multilayers hinders the electron transfer. Also heating and vacuum have a pronounced effect. The device is proposed as a sensor, where the sensing event is transduced into an optical signal in the infrared, as demonstrated for a thiol molecule. The thiol has a large affinity for the gold and therefore affects the germanium – gold nanoparticle gap. This reduces the electron transfer and therefore the absorption in the infrared upon illumination with visible light. Removal of the thiol from the solution leads to a recovering of the signal.
  • Configurationally Stable Doubly Bridged Biphenyl Azocines through Copper-Catalyzed Double Carbene Insertions into the Corresponding Azepines
    Steven Harthong, Elodie Brun, StĂŠphane Grass, CĂŠline Besnard, Thomas BĂźrgi and JĂŠrĂ´me Lacour
    Synthesis, 48 (19) , 2016, p3254-3262
    Keywords: azepine; azocine; carbenes; configurational stability; [1,2]-Stevens rearrangement; ylides
    DOI:10.1055/s-0035-1562467 | unige:86678 | Abstract | Article HTML | Article PDF | Supporting Info
  
Doubly bridged biphenyl azocines can be prepared in a single step through copper-catalyzed reactions of a doubly bridged biphenyl azepine and diazodiester reagents. Double [1,2]-Stevens rearrangements occur at 100 °C to afford doubly tethered eight-membered rings (49 to 61%) as trans and cis regioisomers (1:1 ratio). These products present an axial chirality. ECD and VCD analyses of the separated enantiomers (CSP-HPLC) were used to assign the absolute configuration. High configurational stability is observed for both regioisomers as racemization does not occur after 1 week of heating at 208 °C in dodecane (ΔG ‡ > 41 kcal·mol–1). Interestingly, reactions performed at 40 °C retain a certain level of enantiospecificity (82–86%), avoiding, for the most part, thermal racemization of the starting material.
  • Symmetry Breaking in Chiral Ionic Liquids Evidenced by Vibrational Optical Activity
    Patric Oulevey, Sandra Luber, Birte Varnholt and Thomas BĂźrgi
    Angewandte Chemie International Edition, 55 (39) , 2016, p11787-11790
    Keywords: density functional theory;ionic liquids;molecular dynamics;optical activity;Raman spectroscopy
    DOI:10.1002/anie.201605792 | unige:87902 | Abstract | Article HTML | Article PDF
Ionic liquids (ILs) are receiving increasing interest for their use in synthetic laboratories and industry. Being composed of charged entities, they show a complex and widely unexplored dynamic behavior. Chiral ionic liquids (CILs) have a high potential as solvents for use in asymmetric synthesis. Chiroptical methods, owing to their sensitivity towards molecular conformation, offer unique possibilities to study the structure of these chiral ionic liquids. Raman optical activity proved particularly useful to study ionic liquids composed of amino acids and the achiral 1-ethyl-3-methylimidazolium counterion. We could substantiate, supported by selected theoretical methods, that the achiral counterion adopts an overall chiral conformation in the presence of chiral amino acid ions. These findings suggest that in the design of chiral ionic liquids for asymmetric synthesis, the structure of the achiral counter ion also has to be carefully considered.
  • Gold Nanowire Fabrication with Surface-Attached Lipid Nanotube Templates
    Kristina Jajcevic, Mohamed Chami and Kaori Sugihara
    Small, 12 (35) , 2016, p4830-4836
    Keywords: gold nanoparticles;lipid nanotubes;self-assembly;nanowires
    DOI:10.1002/smll.201600431 | unige:87903 | Abstract | Article HTML | Article PDF
A high-throughput approach to fabricate gold nanowires on surfaces with a lipid nanotube template is demonstrated. Streptavidin-coated gold nanoparticles are attached to the biotin-tagged lipid nanotubes. After the chemical fixation, the samples are dried and treated with oxygen plasma to remove the organic template and connect the particles. The created nanowires are characterized by cryo-transmission electron microscopy, atomic force microscopy, and electrical measurements.
  
A fast redistribution of metal atoms occurs upon mixing the AgxAu38−x and Au38 nanoclusters in solution, as observed by mass spectrometry. Physical separation of AgxAu38−x and Au38 species by a dialysis membrane prohibits the metal migration, which suggests that collisions between the reacting clusters are at the origin of the observation.
 
Here we reproduce the static and dynamical properties of spin-crossover complexes in the framework of the mechanoelastic model applied to triangular lattices. The switching processes between the high-spin and low-spin states are studied by combining the Monte Carlo method with the elastic lattice relaxation. The transition probabilities between the two states take into account intrinsic parameters, the values of which are approximated from experimental quantities (e.g., the energy gap, and the degeneracy ratio from the thermodynamic enthalpy and the entropy difference between the states), and the elastic force or elastic energy stored in the springs connecting the spin-changing centres. The value of the corresponding spring constant is estimated from the experimentally determined variation of the ligand-field strengths in the two spin states due to the cooperativity and the bulk modulus. Both simulated hysteresis loops and relaxation curves are in agreement with experimental data. Cooperativity related phenomena such as like-spin domain formation and the evolution of the interaction distribution with the HS fraction are also analysed.
  
Borohydrides have attained high interest in the past few years due to their high volumetric and gravimetric hydrogen content. Synthesis of di/trimetallic borohydride is a way to alter the thermodynamics of hydrogen release from borohydrides. Previously reported preparations of M(BH4)2 involved chloride containing species such as SrCl2. The presence of residual chloride (or other halide) ions in borohydrides may change their thermodynamic behavior and their decomposition pathway. Pure monometallic borohydrides are needed to study decomposition products without interference from halide impurities. They can also be used as precursors for synthesizing di/trimetallic borohydrides. In this paper we present a way to synthesize halide free alkaline earth metal (Sr, Ba) and europium borohydrides starting with the respective hydrides as precursors. Two novel high temperature polymorphs of Sr and Eu borohydrides and four polymorphs of Ba borohydride have been characterized by synchrotron X-ray powder diffraction, thermal analysis, and Raman and infrared spectroscopy and supported by periodic DFT calculations. The decomposition routes of these borohydrides have also been investigated. In the case of the decomposition of strontium and europium borohydrides, the metal borohydride hydride (M(BH4)H3, M = Sr, Eu) is observed and characterized. Periodic DFT calculations performed on room temperature Ba(BH4)2 revealed the presence of bidentate and tridentate borohydrides.
  • Twisted and tubular silica structures by anionic surfactant fibers encapsulation
    Mahshid Chekini, Laure GuĂŠnĂŠe, Valentina Marchionni, Manish Sharma and Thomas BĂźrgi
    Journal of Colloid and Interface Science, 477 , 2016, p166-175
    Keywords: chiral mesoporous silica; chiral anionic surfactant; L-arginine; encapsulation; self-assembly; organic fiber
    DOI:10.1016/j.jcis.2016.05.049 | unige:84470 | Abstract | Article HTML | Article PDF
  
Organic molecules imprinting can be used for introducing specific properties and functionalities such as chirality to mesoporous materials. Particularly organic self-assemblies can work as a scaffold for templating inorganic materials such as silica. During recent years chiral imprinting of anionic surfactant for fabrication of twisted rod-like silica structures assisted by co-structuring directing agent were thoroughly investigated. The organic self-assemblies of anionic surfactants can also be used for introducing other shapes in rod-like silica structures. Here we report the formation of amphiphilic N-miristoyl-L-alanine self-assemblies in aqueous solution upon stirring and at presence of L-arginine. These anionic surfactant self-assemblies form fibers that grow by increasing the stirring duration. The fibers were studied using transmission electron microscopy, infra-red spectroscopy and vibrational circular dichroism. Addition of silica precursor 1,2-bis(triethoxysilyl)ethylene and co-structuring directing agent N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride at different stages of fibers’ growth leads to formation of different silica structures. By controlling stirring duration, we obtained twisted tubular silica structures as a result of fibers encapsulation. We decorated these structures with gold nanoparticles by different methods and measured their optical activity.
  
  • Cooperative Enhancement versus Additivity of Two-Photon-Absorption Cross Sections in Linear and Branched Squaraine Superchromophores
    Harald Ceymann, Arnulf Rosspeintner, Maximilian Hubert Schreck, Carina MĂźtzel, Andreas Stoy, Eric Vauthey and Christoph Lambert
    Phys. Chem. Chem. Phys., 18 (24) , 2016, p16404-16413
    DOI:10.1039/C6CP02312F | unige:85610 | Abstract | Article HTML | Article PDF | Supporting Info
The linear and nonlinear optical properties of a series of oligomeric squaraine dyes were investigated by one-photon absorption spectroscopy (1PA) and two-photon absorption (2PA) induced fluorescence spectroscopy. The superchromophores are based on two indolenine squaraine dyes with transoid (SQA) and cisoid configuration (SQB). Using these monomers, linear dimers and trimers as well as star-shaped trimers and hexamers with benzene or triphenylamine cores were synthesised and investigated. The red-shifted and intensified 1PA spectra of all superchromophores could well be explained by exciton coupling theory. In the linear chromophore arrangements we also found superradiance of fluorescence but not in the branched systems. Furthermore, the 2PA showed enhanced cross sections for the linear oligomers but only additivity for the branched systems. This emphasizes that the enhancement of the 2PA cross section in the linear arrangements is probably caused by orbital interactions of higher excited configurations.
  • First time combination of frozen density embedding theory with the algebraic diagrammatic construction scheme for the polarization propagator of second order
    Stefan Prager, Alexander Zech, Francesco Aquilante, Andreas Dreuw and Tomasz A. Wesolowski
    The Journal of Chemical Physics, 144 (20) , 2016, p204103
    Keywords: excitation energies; hydrogen bonding; wave functions; Rydberg states; basis sets
    DOI:10.1063/1.4948741 | unige:84472 | Abstract | Article HTML | Article PDF
  
The combination of Frozen Density Embedding Theory (FDET) and the Algebraic Diagrammatic Construction (ADC) scheme for the polarization propagator for describing environmental effects on electronically excited states is presented. Two different ways of interfacing and expressing the so-called embedding operator are introduced. The resulting excited states are compared with supermolecular calculations of the total system at the ADC(2) level of theory. Molecular test systems were chosen to investigate molecule–environment interactions of varying strength from dispersion interaction up to multiple hydrogen bonds. The overall difference between the supermolecular and the FDE-ADC calculations in excitation energies is lower than 0.09 eV (max) and 0.032 eV in average, which is well below the intrinsic error of the ADC(2) method itself.
Exposure of PdAu24(2-PET)18 (2-PET: 2-phenylethylthiolate) to BINAS (1,1-binaphthyl-2,2-dithiol) leads to species of composition PdAu24(2-PET)18−2x(BINAS)x due to ligand exchange reactions. The BINAS adsorbs in a specific mode that bridges the apex and one core site of two adjacent S(R)–Au–S(R)–Au–S(R) units. Species with different compositions of the ligand shell can be separated by HPLC. Furthermore, site isomers can be separated. For the cluster with exactly one BINAS in its ligand shell only one isomer is expected due to the symmetry of the cluster, which is confirmed by High-Performance Liquid Chromatography (HPLC). Addition of a second BINAS to the ligand shell leads to several isomers. In total six distinguishable isomers are possible for PdAu24(2-PET)14(BINAS)2 including two pairs of enantiomers concerning the adsorption pattern. At least four distinctive isomers are separated by HPLC. Calculations indicate that one of the six possibilities is energetically disfavoured. Interestingly, diastereomers, which have an enantiomeric relationship concerning the adsorption pattern of chiral BINAS, have significantly different stabilities. The relative intensity of the observed peaks in the HPLC does not reflect the statistical weight of the different isomers. This shows, as supported by the calculations, that the first adsorbed BINAS molecule influences the adsorption of the second incoming BINAS ligand. In addition, experiments with the corresponding Pt doped gold cluster reveal qualitatively the same behaviour, however with slightly different relative abundances of the corresponding isomers. This finding points towards the influence of electronic effects on the isomer distribution. Even for clusters containing more than two BINAS ligands a limited number of isomers were found, which is in contrast to the corresponding situation for monothiols, where the number of possible isomers is much larger.
  
Considered at the beginning of the 21th century as being incompatible with the presence of closely bound high-energy oscillators, lanthanide-centered superexcitation, which is the raising of an already excited electron to an even higher level by excited-state energy absorption, is therefore a very active topic strictly limited to the statistical doping of low-phonon bulk solids and nanoparticles. We show here that molecular lanthanide-containing coordination complexes may be judiciously tuned to overcome these limitations and to induce near-infrared (NIR)-to-visible (VIS)-light upconversion via the successive absorption of two low-energy photons using linear-optical responses. Whereas single-ion-centered excited-state absorption mechanisms remain difficult to implement in lanthanide complexes, the skillful design of intramolecular intermetallic energy-transfer processes operating in multimetallic architectures is at the origin of the recent programming of erbium-centered molecular upconversion.
  • Looking for the Origin of Allosteric Cooperativity in Metallopolymers
    Lucille Babel, Thi Nhu Y. Hoang, Laure GuĂŠnĂŠe, CĂŠline Besnard, Tomasz A. Wesolowski, Marie Humbert-Droz and Claude Piguet
    Chemistry - A European Journal, 22 (24) , 2016, p8113-8123
    DOI:10.1002/chem.201600857 | unige:84475 | Abstract | Article HTML | Article PDF
  
The cooperativity index in a metallopolymer can be thought of as an estimation of the level of available water in nature. In a river, the water molecules can only feed immediate surroundings. Those in an aqueduct possess sufficient potential energy to extend this beneficial effect to wider areas. Twenty centuries ago, the Romans exploited this concept for extending their settlement in Western Europe. A reliable access to allosteric cooperativity indexes might push host–guest chemistry toward unexplored areas. More information can be found in the Full Paper by C. Piguet et al. (DOI: 10.1002/chem.201600857).

 

The basic concept of allosteric cooperativity used in biology, chemistry and physics states that any change in the intermolecular host–guest interactions operating in multisite receptors can be assigned to intersite interactions. Using lanthanide metals as guests and linear multi-tridentate linear oligomers of variable lengths and geometries as hosts, this work shows that the quantitative modeling of metal loadings requires the consideration of a novel phenomenon originating from solvation processes. It stepwise modulates the intrinsic affinity of each isolated site in multisite receptors, and this without resorting to allosteric cooperativity. An easy-to-handle additive model predicts a negative power law dependence of the intrinsic affinity on the length of the linear metallopolymer. Applied to lanthanidopolymers, the latter common analysis overestimates cooperativity factors by more than two orders of magnitude.

  
  • Functionalized Cationic [4]Helicenes with Unique Tuning of Absorption, Fluorescence and Chiroptical Properties up to the Far-Red Range Open access paper
    Irene Hernandez Delgado, Simon Pascal, Antoine Wallabregue, Romain Duwald, CĂŠline Besnard, Laure GuĂŠnĂŠe, Christoph Nançoz, Eric Vauthey, Roberto Tovar, Jamie Lunkley, Gilles Muller and Jerome Lacour
    Chemical Science, 7 (7) , 2016, p4685-4693
    DOI:10.1039/C6SC00614K | unige:84734 | Abstract | Article HTML | Article PDF | Supporting Info
Unprecendented regioselective post-functionalization of racemic and enantiopure cationic diaza [4]helicenes is afforded. The peripheral auxochrome substituents allow a general tuning of the electrochemical, photophysical and chiroptical properties of the helical dyes (26 examples). For instance, electronic absorption and circular dichroism are modulated from the orange to near-infrared spectral range (575-750 nm), fluorescence quantum efficiency is enhanced up to 0.55 (631 nm) and circularly polarized luminescence is recorded in the red (│glum│ ~ 10-3)
  • Reaction Pathways in Ca(BH4)2-NaNH2 and Mg(BH4)2-NaNH2 Hydrogen-rich Systems
    Fabrice Morelle, Lars Haahr Jepsen, Torben R. Jensen, Manish Sharma, Hans Hagemann and Yaroslav Filinchuk
    The Journal of Physical Chemistry C, 120 (16) , 2016, p8428-8435
    DOI:10.1021/acs.jpcc.6b00209 | unige:83818 | Abstract | Article PDF
  
Two reactive hydride composite systems, Ca(BH4)2–NaNH2 and Mg(BH4)2–NaNH2, were systematically studied by in situ synchrotron radiation powder diffraction, in situ Fourier transform infrared spectroscopy, thermogravimetric analysis and differential scanning calorimetry coupled with mass spectrometry. Metathesis reactions between the amides and borohydrides take place in both systems between 100°C and 150°C yielding amorphous materials with the proposed composition M(BH4)(NH2). Simultaneously, a fraction of NaNH2 decomposes to Na3N and ammonia via a complex pathway. The main gas released under 300°C is ammonia for both systems, while significant amounts of hydrogen are released only above 350°C.
  
  • A theoretical study of the spectroscopic properties of B2H6 and of a series of Bx species (x = 1-12, y = 3-14, z = 0-2): From BH3 to B12H122-
    Daniel Sethio, LatĂŠvi Max Lawson Daku and Hans Hagemann
    International Journal of Hydrogen Energy, 41 (16) , 2016, p6814-6824
    Keywords: boron-hydrogen species; 11B and 1H NMR chemical shifts; vibrational frequencies; anharmonicity; density functional theory
    DOI:10.1016/j.ijhydene.2016.02.121 | unige:83819 | Abstract | Article PDF
The characterization of boron-hydrogen compounds is an active research area which encompasses subjects as diverse as the chemistry and structures of closoboranes or the thermal decomposition mechanism of the borohydrides. Due to their high gravimetric hydrogen content, borohydrides are considered as potential hydrogen storage materials. Their thermal decompositions are multistep processes, for which the intermediate products are not easily identified. To help address this issue, we have extensively investigated the vibrational and NMR properties of 21 relevant Bm boron-hydrogen species (m = 1–12; n = 1–14; z = 0–2) within density functional theory. We could thus show that the B3LYP-D2 dispersion-corrected hybrid can be used in combination with the large cc-pVTZ basis set for the reliable prediction of the 11B and 1H NMR spectra of the boron-hydrogen species, and also for the reliable prediction of their IR and Raman spectra while taking into account the anharmonicity of their molecular vibrations.
 
Polydiacetylene (PDA) is a conductive polymer that has a mechanochromism. When the polymer is exposed to mechanical stresses, change in temperature (thermochromism), pH (ionochromism) etc., the structural perturbation can be seen by the change in its color. Although it presents interesting electrical and optical properties, the relationship between these signals has rarely been investigated. We studied the correlation between the electrical conductivity and the absorption spectra of PDA. Upon UV irradiation, PDA absorption spectra presented a blue shift, which coincided with the decrease in the electrical conductivity.
  
Most symmetric quadrupolar molecules designed for two-photon absorption behave as dipolar molecules in the S1 electronic excited state. This is usually explained by a breakup of the symmetry in the excited state. However, the origin of this process and its dynamics are still not fully understood. Here, excited-state symmetry breaking in a quadrupolar molecule with a D-π-A-π-D motif, where D and A are electron donating and accepting units, is observed in real time using ultrafast transient infrared absorption spectroscopy. The nature of the relaxed S1 state was found to strongly depend on the solvent polarity: (1) in nonpolar solvents, it is symmetric and quadrupolar; (2) in weakly polar media, the quadrupolar state observed directly after excitation transforms to a symmetry broken S1 state with one arm bearing more excitation than the other; and (3) in highly polar solvents, the excited state evolves further to a purely dipolar S1 state with the excitation localized entirely on one arm. The time scales associated with the transitions between these states coincide with those of solvation dynamics, indicating that symmetry breaking is governed by solvent fluctuations.
  • Spectroscopy and chemical bonding in transition metal complexes
    Andreas Hauser and Christian Reber
    in "Structure and Bonding" Springer, 172 , 2016, p291-312
    Keywords: transition metal complexes, spectroscopy , photophysics and photochemistry , high pressure , intersystem crossing , spin crossover , ultrafast methods
    DOI:10.1007/430_2015_195 | unige:89169 | Abstract | Article PDF
Optical spectroscopy of transition metal complexes plays an important role in establishing excited state electronic and nuclear structures and thus in the elucidation of the multitude of photophysical and photochemical relaxation processes. The most important advances in this area of research over the past decade are due to the development of new experimental techniques such as ultrafast spectroscopy as well as structure determination in conjunction with other methods such as high pressure and variable temperature techniques. In this contribution, several paradigmatic systems, namely of complexes if chromium(III), iron(II), ruthenium(II), nickel(II), platinum(II) and palladium(II), are discussed with regard to their excited electronic and nuclear structures and photophysical relaxation processes.
  • Frozen-density embedding theory with average solvent charge densities from explicit atomistic simulations
    Andrey Laktionov, Emilie Chemineau-Chalaye and Tomasz A. Wesolowski
    Phys. Chem. Chem. Phys., 18 (31) , 2016, p21069-21078
    DOI:10.1039/C6CP00497K | unige:86906 | Abstract | Article HTML | Article PDF
  
Besides molecular electron densities obtained within the Born–Oppenheimer approximation (ρB(r)) to represent the environment, the ensemble averaged density (〈ρB〉(r)) is also admissible in frozen-density embedding theory (FDET) [Wesolowski, Phys. Rev. A, 2008, 77, 12054]. This makes it possible to introduce an approximation in the evaluation of the solvent effect on quantum mechanical observables consisting of replacing the ensemble averaged observable by the observable evaluated at ensemble averaged ρB(r). This approximation is shown to affect negligibly the solvatochromic shift in the absorption of hydrated acetone. The proposed model provides a continuum type of representation of the solvent, which reflects nevertheless its local structure, and it is to be applied as a post-simulation analysis tool in atomistic level simulations
  
A molecular pentad comprised of a central multielectron donor and two flanking photosensitizer-acceptor moieties was prepared in order to explore the possibility of accumulating two positive charges at the central donor, using visible light as an energy input. Photoinduced charge accumulation in purely molecular systems without sacrificial reagents is challenging, because of the multitude of energy-wasting reaction pathways that are accessible after excitation with two photons. As expected, the main photoproduct in our pentad is a simple electron–hole pair, and it is tricky to identify the desired two-electron oxidation product on top of the stronger signal resulting from one-electron oxidation.
  • Characterization of Dimethylsulfoxide / Glycerol Mixtures: A Binary Solvent System for the Study of "Friction-Dependent" Chemical ReactivityOpen access paper
    Gonzalo Angulo, Marta Brucka, Mario Gerecke, GĂźnter Grampp, Damien Jeannerat, Jadwiga Milkiewicz, Yavor Mitrev, Czeslaw Radzewicz, Arnulf Rosspeintner, Eric Vauthey and Pawel Wnuk
    Physical Chemistry Chemical Physics, 18 (27) , 2016, p18460-18469
    DOI:10.1039/C6CP02997C | arXiv:1603.01521v1 | unige:83511 | Abstract | Article HTML | Article PDF | Supporting Info
  
The properties of binary mixtures of dimethylsulfoxide and glycerol, measured by several techniques, are reported. Special attention is given to those properties contributing or affecting chemical reactions. In this respect the investigated mixture behaves as a relatively simple solvent and it is especially well suited for studies on the influence of viscosity in chemical reactivity. This is due to the relative invariance of the dielectric properties of the mixture. However, special caution must be taken with specific solvation, as the hydrogen-bonding properties of the solvent changes with the molar fraction of glycerol.
  
SrAl2O4 doped with europium and dysprosium is a powerful and widely used afterglow material. Within this material strontium is found in two crystallographic different sites. Due to the similar ion radii and same charge, Eu2+-ions can occupy both sites, resulting in two different Eu2+-ions, one emitting in the blue and one in the green spectral range. The blue emission is thermally quenched at room temperature. In this paper we investigate the energy transfer between different Eu ions depending on the concentration and temperature using two different approaches: lifetime measurements and integrated intensity. We find an activation energy for the thermal quenching of the blue emission of 0.195 Âą 0.023 eV and a critical radius for the energy transfer of 3.0 Âą 0.5 nm. This results can help in designing better afterglow materials due to the fact that with energy transfer parts of the lost emission in the blue region at room temperature can be converted to the green site.
Late endosomes are a major trafficking hub in the cell at the crossroads between endocytosis, autophagy, and degradation in lysosomes. Herein is disclosed the first small molecule allowing their selective imaging and monitoring in the form of a diazaoxatriangulene fluorophore, 1a (hexadecyl side chain). The compound is prepared in three steps from a simple carbenium precursor. In nanospheres, this pH-sensitive (pKa = 7.3), photochemically stable dye fluoresces in the red part of visible light (601 and 578 nm, acid and basic forms, respectively) with a quantum yield between 14 and 16% and an excited-state lifetime of 7.7–7.8 ns. Importantly, the protonated form 1a·H+ provokes a specific staining of late endosome compartments (pH 5.0–5.5) after 5 h of incubation with HeLa cells. Not surprisingly, this late endosome marking depends on the intra-organelle pH, and changing the nature of the lipophilic chain provokes a loss of selectivity. Interestingly, fixation of the fluorophore is readily achieved with paraformaldehyde, giving the possibility to image both live and fixed cells.
 
The properties of a series of oxazole yellow dyes, including the dicationic YOPRO-1 and its homodimeric parent YOYO-1 and two monocationic dyes (YOSAC-1 and YOSAC-3), have been investigated at the dodecane/water interface using stationary and time-resolved surface second harmonic generation (SSHG) combined with quantum chemical calculations. Whereas YOYO-1 exists predominantly as a H-dimer in aqueous solution, the stationary SSHG spectra reveal that such dimers are not formed at the interface. No significant H-aggregation was observed with YOPRO-1, neither in solution nor at the interface. In the case of the monocationic YOSAC dyes, a distinct SSHG band due to H-aggregates was measured at the interface, whereas only weak aggregation was found in solution. These distinct aggregation behaviors can be explained by the different orientations of the dyes at the interface, as revealed from the analysis of polarization-resolved experiments, the doubly-charged dyes lying more flat on the interface than the singly charged ones. Although YOYO-1 and YOPRO-1 do not form H-dimer/aggregates at the interface, time-resolved SSHG measurements point to the occurrence of intra- and intermolecular interactions, respectively, which inhibit the ultrafast non-radiative decay of the excited dyes via large amplitude motion, and lead to a nanosecond excited-state lifetime. The distinct behavior evidenced here for YOYO-1 and YOSAC dyes points to their potential use as fluorescent or SHG interfacial probes.
  
  • Homogeneity properties of the embedding potential in frozen-density embedding theory
    Alexander Zech, Francesco Aquilante and Tomasz A. Wesolowski
    Molecular Physics, 114 (7-8) , 2016, p1199-1206
    Keywords: embedding, multi-level simulations, density functional theory, homogeneity, frozen-density embedding theory
    DOI:10.1080/00268976.2015.1125027 | unige:83820 | Abstract | Article HTML | Article PDF
In numerical simulations, based on frozen-density embedding theory, the independent variables describing the total system are the embedded wave function (ΨA) and the density  ρB(r)  representing the environment. Due to inhomogeneity of the non-electrostatic component of the total energy:  EnadxcT[ρA,ρB] ≠ ∫ ρA(r)  (δEnadxcT[ρA,ρB] / δρA(r))  dr   , the expectation value of the embedding potential is not equal to the corresponding component of the total energy. The differences  ΔnadxcT  = EnadxcT[ρA,ρB] –  ∫ ρA(r)  (δEnadxcT[ρA,ρB] / δρA(r))  dr are evaluated using local and semi-local approximations for the functional  EnadxcT[ρA, ρB] in two model systems representing embedded species weakly interacting with the environment. It is found that ΔnadxcT is typically one order of magnitude smaller than EnadxcT[ρA, ρB] and decreases with the overlap between ρA(r) and ρB(r) . The kinetic- and exchange-correlation contributions to ΔnadxcT cancel partially reducing its magnitude to mHartrees. Compared to local approximation for EnadxcT[ρA, ρB], the inhomogeneity is more pronounced in semi-local functionals.
 
Size-controlled nanocrystals (140 nm and 670 nm) and microcrystals (2.5 mm) of the three-dimensional oxalate network [Ru(bpy)3][NaCr(ox)3], ox = oxalate, bpy = 2,2’-bipyridine, were prepared by the reverse micelle technique. The photo-physical properties of the [Cr(ox)3]3- chromophores in the nanocrystals at low temperatures are significantly different from those of the same chromophore in 4 mm crystallites prepared by fast precipitation. For the latter, the absorption in the region of the R lines of the 4A2 → 2E transition is characterized by a sharp doublet. For the nanocrystals the inhomogeneous broadening of the two lines is considerably larger with tails on the low-energy side. Whereas the 4 mm crystallites at low temperatures just show equally sharp emission from the R1 line, the emission intensity from the nanocrystallites is shifted into the low-energy tail. Time resolved fluorescence line narrowing spectra and luminescence decay curves demonstrate that this is due to efficient directional energy migration from the center of the nanocrystals towards the surface
The persistent phosphorescence and thermoluminescence of SrAl2O4:Eu2+:Dy3+ is reported for a variety of different excitation wavelengths and excitation temperatures, to provide new insights in the mechanism of the trapping and detrapping. These measurements reveal that the trapping is strongly dependent on the wavelength and temperature. First, with increasing loading temperature, the thermoluminescence peak shifts to lower temperatures which corresponds to a change of trap population. Secondly, the integrated thermoluminescent intensity increases with increasing loading temperature. All wavelength and temperature dependent experiments indicate that the loading of the traps is a thermally activated processes. Utilizing different wavelengths for loading, this effect can be enhanced or reduced. Furthermore excitation with UV-B-light reveals a tendency for detrapping the phosphor, reducing the resulting thermoluminescent intensity and changing the population of the traps.
The transfer of chirality from one set of molecules to another is fundamental for applications in chiral technology and has likely played a crucial role for establishing homochirality on earth. Here we show that an intrinsically chiral gold cluster can transfer its handedness to an achiral molecule adsorbed on its surface. Solutions of chiral Au38(2-PET)24 (2-PET=2-phenylethylthiolate) cluster enantiomers show strong vibrational circular dichroism (VCD) signals in vibrations of the achiral adsorbate. Density functional theory (DFT) calculations reveal that 2-PET molecules adopt a chiral conformation. Chirality transfer from the cluster to the achiral adsorbate is responsible for the preference of one of the two mirror images. Intermolecular interactions between the adsorbed molecules on the crowded cluster surface seem to play a dominant role for the phenomena. Such chirality transfer from metals to adsorbates likely plays an important role in heterogeneous enantioselective catalysis.
Thiolate-protected gold clusters are promising candidates for imaging applications due to their interesting, size-dependent properties. Their high stability and the ability to functionalize the clusters with biocompatible ligands render the clusters interesting for various imaging techniques such as fluorescence microscopy or second-harmonic generation microscopy. The latter nonlinear optical effect has not yet been observed on this type of ultrasmall nanoparticle. We hereby present second- and third-harmonic generation and multiphoton fluorescence of two thiolate-protected gold clusters: Au25(SCH 2CH2Ph)18 and Au38(SCH2CH2Ph)24. At a fundamental wavelength of 800 nm, the Au38(SCH2CH2Ph)24 cluster is active. In contrast, Au25(SCH2CH2Ph)18 does not yield significant SHG signal. We ascribe this to the center of inversion in the Au25 cluster. Measurements on chiral Au25(capt)18 (capt: captopril) gave an SHG response, supporting this interpretation. We also observed third-harmonic generation at a fundamental wavelength of 1200 nm. At 800 and 1100 nm, the clusters decompose after short illumination time but are stable at illumination at 1200 nm. This may be exploited in combined deep tissue imaging and photothermal heating for theranostics applications
  • Amorphous Nanoparticle Assemblies by Bottom-Up Principles
    Alastair Cunningham, Mahshid Chekini and Thomas Burgi
    in "Active Plasmonic Nanomaterials" Luciano de Sio, Pan Stanford, 2015, p101-103
    DOI:10.1201/b18647-4 | Abstract | Article PDF
Self-assembly of plasmonic nanoprticles is a fast, robust, and cheap route to obtain large-scale materials with promising optical properties. Although the materials obtained in this way are usually amorphous, the approach has the advantage that three-dimensional assemblies of small particles can be otained with control of the average distance at the nanoscale. In this chapter, we will focus on one particular strategy to assemble plasmonic particles that relies on the interplay between charged particles, surfaces, and polyelectrolytes. Several geometries will be discussed with an emphasis on the optical properties that are dictated by the coupling between plasmons in different geometries.
  • Plasmonic coupling between nanostructures: from periodic and rigid to random and flexible systems
    Ugo Cataldi, Roberto Caputo, Yuriy Kurylyak, Gerard Klein, Mahshid Chekini, Cesare Umeton and Thomas Burgi
    in "Active Plasmonic Nanomaterials" Luciano De Sio, Pan Stanford, 2015, p363-365
    DOI:10.1201/b18647-13 | Abstract | Article HTML | Article PDF
In this chapter, the mechanism of plasmonic coupling of the near-fields that takes place when nanostructure are very close to each other is described. After a theoretical introduction illustrating the main physical principles governing plasmonic coupling, several experimental systems are considered. Interestingly, experimnetal results show that both periodic and rigid, random, and flexible systems of gold nanoparticles exhibit a universal scaling behaviour and verify the plamonic ruler equation.
We have carried out an experiment on a flexible polymeric substrate, coated with a monolayer of gold nanoparticles, which demonstrates how the combined effect of nanoparticle growth and stretching influences the average normalized gap between particles, thus modifying the extinction spectra of the sample. The study paves the way for the realization of a plasmonic strain sensor based on the plasmonic coupling of gold nanoparticles deposited onto elastomeric films: application of a mechanical stretching induces a change of colour of the device and a fine control of the applied strain allows a continuous tuning of the colour.
Elongated plasmonic nanoparticles show superior optical properties when compared to spherical ones. Facile, versatile and cost-effective bottom-up approaches for fabrication of anisotropic nanoparticles in solution have been developed. However, fabrication of 2-D plasmonic templates from elongated nanoparticles with spatial arrangement at the surface is still a challenge. We used controlled seed-mediated growth in the presence of porous and functionalized surface of flexible polydimethylsiloxane (PDMS) templates to provide directional growth and formation of elongated gold nanoparticles (AuNPs). Atomic force microscopy (AFM) and spectroscopy revealed embedding of the particles within the functionalized porous surface of PDMS. Nanoparticles shapes were observed with transmission electron microscope (TEM), UV–Vis spectroscopy, and X-ray powder diffraction (XRPD) measurements, which revealed an overall orientation of particles at the surface. Anisotropic and oriented particles on a flexible substrate are of interest for sensing applications.
  
  • Fluorescence enhancement in large-scale self-assembled gold nanoparticle double arrays
    M. Chekini, R. Filter, J. Bierwagen, A. Cunningham, C. Rockstuhl and T. BĂźrgi
    Journal of Applied Physics, 118 (23) , 2015, p233107
    Keywords: nanoparticles; fluorescence; wave attenuation; gold; fluorescence spetcra
    DOI:10.1063/1.4938025 | unige:79113 | Abstract | Article HTML | Article PDF
Localized surface plasmon resonances excited in metallic nanoparticles confine and enhance electromagnetic fields at the nanoscale. This is particularly pronounced in dimers made from two closely spaced nanoparticles. When quantum emitters, such as dyes, are placed in the gap of those dimers, their absorption and emission characteristics can be modified. Both processes have to be considered when aiming to enhance the fluorescence from the quantum emitters. This is particularly challenging for dimers, since the electromagnetic properties and the enhanced fluorescence sensitively depend on the distance between the nanoparticles. Here, we use a layer-by-layer method to precisely control the distances in such systems. We consider a dye layer deposited on top of an array of goldnanoparticles or integrated into a central position of a double array of goldnanoparticles. We study the effect of the spatial arrangement and the average distance on the plasmon-enhanced fluorescence. We found a maximum of a 99-fold increase in the fluorescence intensity of the dye layer sandwiched between two goldnanoparticle arrays. The interaction of the dye layer with the plasmonic system also causes a spectral shift in the emission wavelengths and a shortening of the fluorescence life times. Our work paves the way for large-scale, high throughput, and low-cost self-assembled functionalized plasmonic systems that can be used as efficient light sources.
 
Attached to electron-rich aromatic systems, sulfides are very weak acceptors; however, attached to electron-poor aromatics, they turn into quite strong donors. Here, we show that this underappreciated dual nature of sulfides deserves full consideration for the design of functional systems. Tested with newly designed and synthesized planarizable push−pull mechanophores, sulfide acceptors in the twisted ground state are shown to prevent oxidative degradation and promote blue-shifting deplanarization. Turned on in the planar excited state, sulfide donors promote red-shifting polarization. Impressive Stokes shifts are the result. Demonstrating the usefulness of time-resolved broadband emission spectra to address significant questions, direct experimental evidence for the ultrafast (3.5 ps), polarity-independent and viscosity-dependent planarization from the twisted Franck−Condon S1 state to the relaxed S1 state could be secured.
  
  • Structure-Property Studies of P-Triarylamine-Substituted Dithieno[3,2-b:2',3'-d]phospholes
    Hannes Puntscher, Paul Kautny, Berthold Stoeger, Antoine Tissot, Christian Hametner, Hans Hagemann, Johannes FrĂśhlich, Thomas Baumgartner and Daniel Lumpi
    RSC Advances, 5 (114) , 2015, p93797-93807
    DOI:10.1039/C5RA13651B | unige:77112 | Abstract | Article HTML | Article PDF
The synthesis of 10 novel P-substituted dithienophosphole oxide compounds applying phenylcarbazole and indolocarbazole donors is presented. Based on photo-physical and theoretical investigations, the study reveals that the pyramidal geometry of the phosphorus allows for the synthesis of charge transfer materials by introducing strong exocyclic donor groups but suppresses intramolecular charge transfer below a certain donor strength threshold, which is an appealing structural feature for the design of donor–acceptor materials. The triplet energies of the phenylcarbazole based compounds are in the range of 2.49–2.65 eV, sufficiently high for potential applications as host materials in PhOLEDs. By contrast, the introduction of indolocarbazole, the weakest employed donor, yields materials exhibiting a significantly higher triplet energy of up to 2.87 eV and a remarkably low singlet–triplet splitting (0.18 eV). In addition an interesting example of an intramolecular electronic through-space interaction has been observed for the ortho-linked phenylcarbazole derivative.
Other than lowest-energy stationary embedded wave functions obtained in Frozen-Density Embedding Theory (FDET) [T. A. Wesolowski, Phys. Rev. A 77, 012504 (2008)] can be associated with electronic excited states but they can be mutually non-orthogonal. Although this does not violate any physical principles — embedded wave functions are only auxiliary objects used to obtain stationary densities — working with orthogonal functions has many practical advantages. In the present work, we show numerically that excitation energies obtained using conventional FDET calculations (allowing for non-orthogonality) can be obtained using embedded wave functions which are strictly orthogonal. The used method preserves the mathematical structure of FDET and self-consistency between energy, embedded wave function, and the embedding potential (they are connected through the Euler-Lagrange equations). The orthogonality is built-in through the linearization in the embedded density of the relevant components of the total energy functional. Moreover, we show formally that the differences between the expectation values of the embedded Hamiltonian are equal to the excitation energies, which is the exact result within linearized FDET. Linearized FDET is shown to be a robust approximation for a large class of reference densities.
  • ChemInform Abstract: A Mild and Efficient CH2-Extrusion Reaction for the Enantiospecific Synthesis of Highly Configurationally Stable Troeger Bases.
    Sandip A. Pujari, Celine Besnard, Thomas Buergi and Jerome Lacour
    ChemInform, 46 (43) , 2015
    Keywords: bridged compounds (heterocyclic compounds);ring contraction
    DOI:10.1002/chin.201543111 | unige:76521 | Abstract | Article HTML | Article PDF
Oxidation of ethano-Troeger bases (I) and (III) with DDQ in wet nitromethane results in a highly enantiospecific methylene extrusion to afford ring contracted products (II) and (IV) which are significantly more configurationally stable than regular Troeger bases.
  • Characterization of Salophen Co(III) Acetate Ionophore for Nitrite Recognition
    GastĂłn A. Crespo, Majid Ghahraman Afshar, Noelia BarrabĂŠs, Marcin Pawlak and Eric Bakker
    Electrochimica Acta, 179 , 2015, p16-23
    Keywords: ionophore-based membranes; nitrite; salophen; electrically charged ionophore
    DOI:10.1016/j.electacta.2015.03.180 | unige:75939 | Abstract | Article PDF
The ionophore (salph)Co(III)OAc is used here for the first time as a recognition molecule for nitrite in lipophilic membranes. The nature of the recognition process is evaluated by UV-Vis, ATR-IR and potentiometry, while the purity of the compound is confirmed by LC-MS and cyclic voltammetry. Experimental evidence suggests a replacement of the acetate ligand by nitrite. The formation constant between (salph)Co(III)OAc and NO2−, β∼108, is estimated potentiometrically with the sandwich membrane method. The analytical parameters, LOD∼3 μM, linear range of four orders of magnitude, stability over four days, and the selectivity over chloride (log KNO2,ClPOT∼-4.1) are similar to previous ionophores for the same ion. Undiluted urine samples were spiked with incremental concentrations of nitrite to characterize the matrix effect. Beyond the observed analytical performance, we aim here to offer a rational study to understand an unconventional charged ionophore recognition process.
  • Pd2Au36(SR)24 cluster: structure studies
    Bei Zhang, Sameh Kaziz, Houhua Li, Dawid Wodka, Sami Malola, Olga V. Safonova, Maarten Nachtegaal, Clement Mazet, Igor Dolamic, Jordi Llorca, Elina Kalenius, Latevi Max Lawson Daku, Hannu Häkkinen, Thomas Buergi and Noelia Barrabes
    Nanoscale, 7 (40) , 2015, p17012-17019
    DOI:10.1039/C5NR04324G | unige:75923 | Abstract | Article HTML | Article PDF | Supporting Info
The location of the Pd atoms in Pd2Au36(SC2H4Ph)24, is studied both experimentally and theoretically. X-ray photoelectron spectroscopy (XPS) indicates oxidized Pd atoms. Palladium K-edge extended X-ray absorption fine-structure (EXAFS) data clearly show Pd-S bonds, which is supported by far infrared spectroscopy. By comparing theoretical EXAFS spectra in R space and circular dichroism spectra of the staple, surface and core doped structures with experimental spectra.
 
Among the different potential hydrogen storage materials, borohydrides have been largely investigated because of their high gravimetric and volumetric hydrogen content. In the analysis of borohydrides, vibrational spectroscopy plays an important role since it gives information on the local structure of the BH4– ion inside the solid. Here the GF method, developed by Wilson, is used in order to determine the local symmetry of BH4– in solid borohydrides starting from their vibrational spectra. Two different cases of deformations of BH4– are considered. In the first case, the effects of small angular variations on the vibrational spectra of borohydrides will be taken into account; starting from the splitting of the bands corresponding to the deformation modes, the angular deformations will be estimated. In the second one, the BH4– under chemical pressure (in different cubic alkali halides) is considered; in this case, the symmetry of the BH4– remains Td, while the bond lengths change according to the pressure experienced. Different practical examples will be illustrated.
  
  • Properties of the gold—sulphur interface: from self-assembled monolayers to clusters
    Thomas BĂźrgi
    Nanoscale, 7 (38) , 2015, p15553-15567
    DOI:10.1039/C5NR03497C | unige:76251 | Abstract | Article HTML | Article PDF
The gold–sulphur interface of self-assembled monolayers (SAMs) was extensively studied some time ago. More recently tremendous progress has been made in the preparation and characterization of thiolate-protected gold clusters. In this feature article we address different properties of the two systems such as their structure, the mobility of the thiolates on the surface and other dynamical aspects, the chirality of the structures and characteristics related to it and their vibrational properties. SAMs and clusters are in the focus of different communities that typically use different experimental approaches to study the respective systems. However, it seems that the nature of the Au–S interfaces in the two cases is quite similar. Recent single crystal X-ray structures of thiolate-protected gold clusters reveal staple motifs characterized by gold ad-atoms sandwiched between two sulphur atoms. This finding contradicts older work on SAMs. However, newer studies on SAMs also reveal ad-atoms. Whether this finding can be generalized remains to be shown. In any case, more and more studies highlight the dynamic nature of the Au–S interface, both on flat surfaces and in clusters. At temperatures slightly above ambient thiolates migrate on the gold surface and on clusters. Evidence for desorption of thiolates at room temperature, at least under certain conditions, has been demonstrated for both systems. The adsorbed thiolate can lead to chirality at different lengths scales, which has been shown both on surfaces and for clusters. Chirality emerges from the organization of the thiolates as well as locally at the molecular level. Chirality can also be transferred from a chiral surface to an adsorbate, as evidenced by vibrational spectroscopy.
  • Modulation of Active Sites in Supported Au38(SC2H4Ph)24 Cluster Catalysts: Effect of Atmosphere and Support Material
    Bei Zhang, Sameh Kaziz, Houhua Li, Miguel Gonzalez Hevia, Dawid Wodka, ClĂŠment Mazet, Thomas BĂźrgi and NoĂŤlia BarrabĂŠs Rabanal
    The Journal of Physical Chemistry C, 119 (20) , 2015, p11193-11199
    DOI:10.1021/jp512022v | unige:72892 | Abstract | Article HTML | Article PDF
We investigate the distinctly different interaction of thiolate-protected cluster Au38(SC2H4Ph)24with two diverse support materials Al2O3 and CeO2. The catalytic surfaces have been heated in different atmospheres, and the removal of the thiolate ligands has been studied. Thermogravimetry (TG), temperature-programmed process coupled with mass spectrometer (TPRDO-MS), and X-ray absorption spectroscopy (XAFS) studies were performed to understand the desorption of thiol ligands depending on conditions and support material. Depending on the atmosphere and the support material the fate of the thiol ligands is different upon heating, leading to metallic Au in the case of Al2O3 and to cationic Au with CeO2. The thiolate removal seems to be a two-step procedure. The catalytic activity of these Au38-supported clusters was studied for the aerobic oxidation of cyclohexane. Conversion was higher for the gold clusters supported on CeO2. Surprisingly, a significant amount of cyclohexanethiol was found, revealing the active participation of the thiolate ligand in catalytic reactions. The observation also indicates that breaking and formation of C–S bonds can be catalyzed by the gold clusters.
 
can be formed during the thermal decomposition of metal borohydrides (M(BH4)x). Halogen ions such as fluoride or chloride can contribute to destabilize the ions. Hydride and fluoride mixed species like will be probable products after hydrogen release from mixed boro-hydride-fluoride or borohydride-borofluoride systems (, ). Various number of isomers are possible for (n = 2–11). DFT calculations were performed on isolated ions of all the possible isomers for (n = 0–3, 9–12), using B3LYP functionals and 6-31G(d,p) basis set. Relative stability, vibrational and NMR spectroscopy of these isomers are discussed and compared with available experimental data.
  
The dynamics of a moderately exergonic photoinduced charge separation has been investigated by ultrafast time-resolved infrared absorption with the dimethylanthracene/phthalonitrile donor/acceptor pair in solvents covering a broad range of polarity. A distinct spectral signature of an exciplex could be identified in the −C≡N stretching region. On the basis of quantum chemistry calculations, the 4–5 times larger width of this band compared to those of the ions and of the locally excited donor bands is explained by a dynamic distribution of exciplex geometry with different mutual orientations and distances of the constituents and, thus, with varying charge-transfer character. Although spectrally similar, two types of exciplexes could be distinguished by their dynamics: short-lived, “tight”, exciplexes generated upon static quenching and longer-lived, “loose”, exciplexes formed upon dynamic quenching in parallel with ion pairs. Tight exciplexes were observed in all solvents, except in the least polar diethyl ether where quenching is slower than diffusion. The product distribution of the dynamic quenching depends strongly on the solvent polarity: whereas no significant loose exciplex population could be detected in acetonitrile, both exciplex and ion pair are generated in less polar solvents, with the relative population of exciplex increasing with decreasing solvent polarity. These results are compared with those reported previously with donor/acceptor pairs in different driving force regimes to obtain a comprehensive picture of the role of the exciplexes in bimolecular photoinduced charge separation.
 
We report that anion−π and cation−π interactions can occur on the same aromatic surface. Interactions of this type are referred to as ion pair−π interactions. Their existence, nature, and significance are elaborated in the context of spectral tuning, ion binding in solution, and activation of cell-penetrating peptides. The origin of spectral tuning by ion pair−π interactions is unraveled with energy-minimized excited-state structures: The solvent- and pH-independent red shift of absorption and emission of push–pull fluorophores originates from antiparallel ion pair−π attraction to their polarized excited state. In contrast, the complementary parallel ion pair−π repulsion is spectroscopically irrelevant, in part because of charge neutralization by intriguing proton and electron transfers on excited push–pull surfaces. With time-resolved fluorescence measurements, very important differences between antiparallel and parallel ion pair−π interactions are identified and quantitatively dissected from interference by aggregation and ion pair dissociation. Contributions from hydrogen bonding, proton transfer, π–π interactions, chromophore twisting, ion pairing, and self-assembly are systematically addressed and eliminated by concise structural modifications. Ion-exchange studies in solution, activation of cell-penetrating peptides in vesicles, and computational analysis all imply that the situation in the ground state is complementary to spectral tuning in the excited state; i.e., parallel rather than antiparallel ion pair−π interactions are preferred, despite repulsion from the push–pull dipole. The overall quite complete picture of ion pair−π interactions provided by these remarkably coherent yet complex results is expected to attract attention throughout the multiple disciplines of chemistry involved.
A bimetallic dodecaborate LiNaB12H12 has been successfully synthesized for the first time, through a sintering process of LiBH4, NaBH4 and B10H14. LiNaB12H12 has a cubic Pa-3 space group symmetry at room temperature, and transforms into a high temperature phase with Fm-3m symmetry at 488 K, which is lower than that of Li2B12H12 and Na2B12H12. The ionic conductivity at 550 K reaches 0.79 S/cm, which is approximately 8 times higher than that of Na2B12H12 and 11 times higher than that of Li2B12H12. The Li/Na compositional and thus an induced positional disorder in LiNaB12H12 are suggested to be responsible for the reduced phase transition temperature and the improved super ionic conductivity compared to its monometallic counterparts.
  • The influence of boric acid on improved persistent luminescence and thermal oxidation resistance of SrAl2O4:Eu2+
    Songhak Yoon, Jakob Bierwagen, Matthias Trottmann, Bernhard Walfort, Nando Gartmann, Anke Weidenkaff, Hans Hagemann and Simone Pokrant
    Journal of Luminescence, 167 , 2015, p126-131
    Keywords: persistent luminescence; SrAl2O4; boric acid; thermal oxidation resistance; oxygen vacancies
    DOI:10.1016/j.jlumin.2015.06.021 | unige:74192 | Abstract | Article HTML | Article PDF | Supporting Info
Persistent luminescence of SrAl2O4:Eu2+ has attracted considerable attention due to their high initial brightness, long-lasting time and excellent thermal stability. Here the influence of boric acid on the persistent luminescence and thermal oxidation resistance of SrAl2O4:Eu2+ was investigated in detail. Crystal structural analysis and scanning electron microscopy revealed that with the addition of boron, the unit cell volume decreased and the morphology of the particles became more irregular with sharp edges. Thermogravimetric analysis showed better thermal oxidation resistance accompanied by a change in oxygen vacancy concentration when boron acid is used. Photoluminescence spectra and afterglow decay curves confirm an improved afterglow performance for boron-added SrAl2O4:Eu2+. Thermoluminesence allowed monitoring the changes in the trap states due to the presence of B. Our results imply that the substantial improvement of afterglow performance and the thermal stability in SrAl2O4:Eu2+ can be attributed to the incorporation of boron into the aluminate network.
  
The fluorescence quantum yield of a red naphthalenediimide dye (rNDI) with amino and Br core substituents has been found to decrease by a factor of almost 2 by going from S1 ← S0 to S2 ← S0 excitation. Time-resolved spectroscopic measurements reveal that this deviation from the Kasha−Vavilov’s rule is due to an ultrafast, < 200 fs, intersystem- crossing (ISC) from the S2 state to the triplet manifold, due to the ππ* → nπ* character of the transition and to the presence of the heavy Br atom. In non-core substituted naphthalenediimide (pNDI), ISC is slower, ∼2 ps, and was found to be reversible on a time scale shorter than that of vibrational cooling. The fluorescence and triplet quantum yields of rNDI, thus, can be substantially changed by a simple variation of the excitation wavelength. 
 
The femto- to microsecond excited-state dynamics of an electron donor–acceptor–donor triad, consisting of two red core-substituted naphthalenediimides (rNDI) and one colorless naphthalenediimide (pNDI), in solution has been compared to that of a supramolecular surface architecture, constituted of coaxial stacks of rNDI and pNDI and prepared by self-organizing surface initiated polymerization (SOSIP). In the triad, charge separation between an excited rNDI and pNDI takes place in highly polar solvents only and for a subensemble of molecules, around 30%, with a folded conformation. Other processes, such as singlet and triplet excitation energy transfer from pNDI to rNDI and intersystem crossing, are also operative. Additionally, bimolecular symmetry-breaking charge separation upon triplet–triplet annihilation is observed on the microsecond time scale in polar solvent. In the surface architecture, excitation of an rNDI is followed by an ultrafast symmetry breaking-charge separation resulting in a charge-transfer exciton, which either recombines or dissociates into a charge-separated state with the electron and the hole in different stacks. The same charge-separated state can also be populated upon excitation of pNDI, either via a charge-transfer pNDI exciton or after excitation energy transfer to rNDI. Charge recombination in the SOSIP film takes place on a wide range of time scales, ranging from a few picoseconds to several hundreds of microseconds.
  
The structural phase transitions occurring in a series of perovskite-type complex hydrides based on the tetrahydroborate anion BH4- are investigated by means of in situ synchrotron x-ray powder diffraction, vibrational spectroscopy, thermal methods and ab initio calculations in the solid state. Structural dynamics of the BH4 anion are followed with quasi-elastic neutron scattering. We show that unexpected temperature-induced lattice instabilities in perovskite-type ACa(BH4)3 (A = K, Rb, Cs) have their origin in close hydridic di-hydrogen contacts. The rich lattice dynamics lead to coupling between internal B-H vibrations and phonons, resulting in distortions in the high-temperature polymorph that are identical in symmetry to well-known instabilities in oxide perovskites, generally condensing at lower temperatures. It is found that anion-substitution BH4-  ↔ X- (X = Halide) can relax distortions in ACa(BH4)3 by eliminating coulomb repulsive H- • • • H- effects. The interesting nature of phase transition in ACa(BH4)3 enters an unexplored field of weak interactions in ceramic-like host lattices and is the principal motivation for this study. Close di-hydrogen contacts suggest new concepts to tailor crystal symmetries in complex hydride perovskites in the future.
  • Frozen-Density Embedding Strategy for Multilevel Simulations of Electronic StructureOpen access paper
    Tomasz A. Wesolowski, Sapana Shedge and Xiuwen Zhou
    Chemical Reviews, 115 (12) , 2015, p5891-5928
    DOI:10.1021/cr500502v | unige:74195 | Abstract | Article HTML | Article PDF
1. Introduction  2. Frozen-Density Embedding Theory  3. Extensions and Formalisms Related to FDET  4. Approximations in FDET for Multilevel Simulations  5. Numerical Simulations Using Approximated FDET Embedding Potentials  6. Concluding Remarks
  • Access to Enantioenriched 2,3- and 2,5-Dihydrofurans with a Fully Substituted C2 Stereocenter by Pd-Catalyzed Asymmetric Intermolecular Heck ReactionOpen access paper
    Gustavo Manuel Borrajo-Calleja, Vincent Bizet, Thomas Buergi and Clement Mazet
    Chemical Science, 6 (8) , 2015, p4807-4811
    DOI:10.1039/C5SC01460C | unige:74172 | Abstract | Article HTML | Article PDF | Supporting Info
A palladium catalyzed intermolecular asymmetric Heck reaction with cyclic trisubstituted olefins is reported. The use of two different chiral ligands provides access to valuable 2,3- and 2,5-dihydrofurans with a fully substituted C2 stereocenter with high levels of regio- and enantiocontrol.
  • Remote stereoselective deconjugation of α,β-unsaturated esters by simple amidation reactions Open access paper
    Mahesh Vishe, Radim Hrdina, Amalia Isabel Poblador-Bahamonde, CĂŠline Besnard, Laure GuĂŠnĂŠe, Thomas Buergi and Jerome Lacour
    Chemical Science, 6 (8) , 2015, p4923-4928
    DOI:10.1039/C5SC01118C | unige:74171 | Abstract | Article HTML | Article PDF | Supporting Info
The thermodynamically disfavored isomerization of ι,β-unsaturated esters to deconjugated β,γ-unsaturated analogues occurs readily when coupled to an amidation. Within the framework of macrocyclic derivatives, it is shown that 15, 16, and 18 membered macrocycles react with tBuOK and anilines to generate in one-pot the β,γ-unsaturated amides (yields up to 88%). Importantly, single (chiral) diastereomers are isolated (d.r. > 49:1, 1H NMR) irrespective of the size and nature of the rings showing an effective transmission of remote stereochemistry during the isomerization process. CSP-chromatographic resolution and absolute configuration determination by VCD are achieved.
 
Two donor bridge–acceptor molecules with terminal triarylamine and Ru(bpy)32+ (bpy = 2,2′-bipyridine) redox partners were synthesized and investigated by cyclic voltammetry, optical absorption, luminescence, and transient absorption spectroscopy. The two dyads differ only by the central bridging unit, which was tetramethoxybenzene (tmb) in one case and unsubstituted phenylene (ph) in the other case. Photoirradiation of the Ru(bpy)32+ complex of the two dyads triggers intramolecular electron transfer from the triarylamine to the 3MLCT-excited metal complex, and this process occurs with time constants of 1.5 and 6.8 ns for the tmb- and ph-bridged dyads, respectively. Thermal electron transfer in the reverse direction then leads to disappearance of the photoproduct with a time constant of 10 ns in both dyads. The faster rate of photoinduced charge transfer in the tmb-bridged dyad can be understood in the framework of a hole-tunneling model in which the electron-rich tmb bridge imposes a more shallow barrier than the less electron-rich ph spacer. Until now tmb-based molecular wires have received very little attention, and alkoxy substituents have been mostly used for improving the solubility of oligo-p-phenylene vinylene (OPV) and oligo-p-phenylene ethynylene (OPE) wires. Our study illustrates how four alkoxy-substituents on a phenylene backbone can have a significant influence on the charge-transfer properties of a molecular wire, and this is relevant in the greater context of a future molecular electronics technology.
  • A Mild and Efficient CH2-Extrusion Reaction for the Enantiospecific Synthesis of Highly Configurationally Stable Tröger Bases
    Sandip A. Pujari, CĂŠline Besnard, Thomas BĂźrgi and JĂŠrĂ´me Lacour
    Angewandte Chemie International Edition, 54 (26) , 2015, p7520-7523
    Keywords: configuration determination;enantioselectivity;heterocycles;ring contraction;stereochemistry
    DOI:10.1002/anie.201500435 | unige:73259 | Abstract | Article HTML | Article PDF | Supporting Info
A novel CH2-extrusion reaction leading to the transformation of ethano-Tröger bases into disubstituted methano derivatives is reported (yields up to 93 %). Under mild and metal-free oxidative conditions, a loss of CH2 and a ring contraction are provoked. Despite two bond cleavages at stereogenic nitrogen and carbon centers and a temporary rupture of the bicyclic structure, a very high enantiospecificity (es≥98 %) is observed for this unusual reaction.
  
  • Excited state interactions between the chiral Au38L24 cluster and covalently attached porphyrin
    Birte Varnholt, Romain Letrun, Jesse J. Bergkamp, Yongchun Fu, Oleksandr Yushchenko, Silvio Decurtins, Eric Vauthey, Shi-Xia Liu and Thomas Buergi
    Phys. Chem. Chem. Phys., 17 , 2015, p14788-14795
    DOI:10.1039/C5CP01638J | unige:73046 | Abstract | Article PDF | Supporting Info
A protected S-acetylthio porphyrin was synthesized and attached to the Au38(2-phenylethanethiolate)24 cluster in a ligand exchange reaction. Chiral high performance liquid chromatography of the functionalized cluster yielded enantiomeric pairs of clusters probably differing in the binding site of the porphyrin. As proven by circular dichroism, the chirality was maintained. Exciton coupling between the cluster and the chromophore is observed. Zinc can be incorporated into the porphyrin attached to the cluster, as evidence by absorption and fluorescence spectroscopy, however, the reaction is slow.Quenching of the chromophores fluorescence is observed, which can be explained by energy transfer from the porphyrin to the cluster. Transient absorption spectra on the Au38(2-phenylethanethiolate)24 and the functionalized cluster probe the bleach of the gold cluster due to ground state absorption and characteristic excited state absorption signals. Zinc incorporation does not have a pronounced effect on the photophysical behaviour. Decay times are typical for the molecular behaviour of small monolayer protected gold clusters.
  • Electron-deficient fullerenes in triple-channel photosystems
    Javier LĂłpez-Andarias, Altan Bolag, Christoph Nançoz, Eric Vauthey, Carmen Atienza, Naomi Sakai, Nazario MartĂ­n and Stefan Matile
    ChemComm, 51 (35) , 2015, p7543-7545
    DOI:10.1039/C5CC01551K | unige:55656 | Abstract | Article HTML | Article PDF | Supporting Info
  
Fullerenes of increasing electron deficiency are designed, synthesized and evaluated in multicomponent surface architectures to ultimately build gradients in LUMO levels with nine components over 350 meV down to −4.22 eV.
  
  • Structural, Photophysical and Magnetic Properties of Transition Metal Complexes Based on the Dipicolylamino-chloro-1,2,4,5-tetrazine Ligand
    Iuliia Nazarenko, Flavia Pop, Qinchao Sun, Andreas Hauser, Francesc Lloret, Miguel Julve, Abdelkrim El-Ghayoury and Narcis Avarvari
    Dalton Transactions, 44 (19) , 2015, p8855-8866
    DOI:10.1039/c5dt00550g | unige:72655 | Abstract | Article HTML | Article PDF | Supporting Info
The ligand 3-chloro-6-dipicolylamino-1,2,4,5-tetrazine (Cl-TTZ-dipica) 1, prepared by the direct reaction between 3,6-dichloro-1,2,4,5-tetrazine and di(2-picolyl)-amine, afforded a series of four neutral transition metal complexes formulated as [Cl-TTZ-dipica-MCl2]2, with M = Zn(II) 2a, Cd(II) 2b, Mn(II) 2c and Co(II) 2d, when reacted with the corresponding metal chlorides. The dinuclear structure of the isostructural complexes was disclosed by single crystal X-ray analysis, clearly indicating the formation of [MII-(m-Cl)2MII] motifs and the involvement of the amino nitrogen atom in semi-coordination with the metal centers, thus leading to distorted octahedral coordination geometries. Moreover, the chlorine atoms, either coordinated to the metal or as substituent on the tetrazine ring, engage respectively in specific anion-p intramolecular and intermolecular interactions with the electron poor tetrazine units in the solid state, thus controlling the supramolecular architecture. Modulation of the emission properties is observed in the case of the Zn(II) and Cd(II) complexes when compared to the free ligand. A striking difference is observed in the magnetic properties of the Mn(II) and Co(II) complexes. An antiferromagnetic coupling takes place in the dimanganese(II) compound (J = -1.25 cm-1) while the Co(II) centers are ferromagnetically coupled in the corresponding complex (J = +0.55 cm-1), the spin Hamiltonian being defined as H = -JSA.SB.
  • Analysis of first order reversal curves in the thermal hysteresis of spin-crossover nanoparticles within the mechanoelastic model
    Laurentiu Stoleriu, Alexandru Stancu, Pradip Chakraborty, Andreas Hauser and Cristian Enachescu
    Journal of Applied Physics, 117B , 2015, p307
    DOI:10.1063/1.4914953 | unige:55062 | Article HTML | Article PDF
The recently obtained spin-crossover nanoparticles are possible candidates for applications in the recording media industry as materials for data storage, or as pressure and temperature sensors. For these applications, the intermolecular interactions and interactions between spin-crossover nanoparticles are extremely important, as they may be essential factors in triggering the transition between the two stable phases: the high-spin and low-spin ones. In order to find correlations between the distributions in size and interactions and the transition temperatures distribution, we apply the FORC (First Order Reversal Curves) method, using simulations based on a mechanoelastic model applied to 2D triangular lattices composed of molecules linked by springs and embedded in a surfactant. We consider two Gaussian distributions: one is the size of the nanoparticles and another is the elastic interactions between edge spin-crossover molecules and the surfactant molecules. In order to disentangle the kinetic and non-kinetic parts of the FORC distributions, we compare the results obtained for different temperature sweeping rates. We also show that the presence of few larger particles in a distribution centered around much smaller particles dramatically increases the hysteresis width.
  • Discrete polynuclear manganese(II) complexes with thiacalixarene ligands: synthesis, structures and photophysical properties
    Yan Suffren, Niall O'Toole, Andreas Hauser, Erwann Jeanneau, Arnaud Brioude and Cedric Desroches
    Dalton Transactions, 44 (17) , 2015, p7991-8000
    DOI:10.1039/C5DT00827A | unige:72656 | Abstract | Article HTML | Article PDF | Supporting Info
  
The synthesis, crystal structure and photophysical properties of the new compound [Mn4(ThiaSO2)2F][K(18-crown-6)], ThiaSO2 = p-tertbutylsulphonylcalix[4]arene, are presented and compared to the ones of [Mn4(ThiaSO2)2F]K. The strong orange luminescence is attributed to the Mn2+ centred 4T1 → 6A1 transition. Its temperature and pressure dependence and quenching by molecular dioxygen are reported. The latter is attributed to energy transfer from the 4T1 state exciting dioxygen to its 1Σ+g state. In the solid state, the quenching is much more efficient in [Mn4(ThiaSO2)2F][K(18-crown-6)] than in [Mn4(ThiaSO2)2F]K. This is attributed to the open pore structure of the former allowing fast diffusion of dioxygen into the crystal lattice.
  • Magnetic properties of the tetragonal RCuGa3 (R=Pr, ND and Gd) single crystals
    R. Nagalakshmi, Ruta Kulkarni, S.K. Dhar, A. Thamizhavel, V. Krishnakumar, Marian Reiffers, Ivan Curlik, Hans Hagemann, Dominique Lovy and S. Nallamuthu
    Journal of Magnetism and Magnetic Materials, 386 , 2015, p37-43
    Keywords: intermetallic; spin glasses; anisotropy; entropy
    DOI:10.1016/j.jmmm.2015.03.017 | unige:72657 | Abstract | Article HTML | Article PDF
Single crystals of tetragonal RCuGa3 (R=La, Pr, Nd and Gd), with BaNiSn3 type structure (space group I4 mm), have been grown by high temperature solution growth technique using Ga as flux. Their magnetic properties were determined by heat capacity and the measurement of magnetization and electrical resistivity along [100] and [001] directions. Except LaCuGa3, the compounds order magnetically. PrCuGa3 undergoes a ferromagnetic transition with Curie temperature of 4.6 K. NdCuGa3 shows a bulk magnetic transition at 3.3 K. The data on GdCuGa3 indicate combined characteristics of spin glass and antiferromagnetic behavior at low temperatures. From the Schottky heat capacity data the crystal electric field level energy spectra have been determined. Further we have performed temperature dependent measurements of electron spin resonance (ESR) on GdCuGa3 between 11 K and room temperature. The ESR data indicate an enhancement of magnetic fluctuations associated with spin reorientation and both homogeneous and inhomogeneous thermal broadening of the linewidth.
LNTs are unique 3D structures made only of safe and abundant biomaterials by self-assembly. The current bottleneck for developing applications using LNTs is the lack of an easy technique to pattern them on substrates. We report a method to free-draw single lipid nanotube (LNT) patterns in any shape on surfaces with 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) that takes an inverted hexagonal (HII) phase. We used pre-self-assembled LNTs or HII lipid blocks as a lipid reservoir from which new LNTs were pulled by applying a point load with a micromanipulator. The extreme simplicity of our technique originates from the fundamental nature of DOPE lipids that prefer a HII phase, while all the conventional approaches use PC lipids that form a lamellar phase. By adjusting the surface properties with polyelectrolyte multilayers, the created single LNT objects are able to remain adhered to the surface for over a week. Importantly, it could be shown that two vesicles loaded with caged fluorescent molecules were able to fuse well with a LNT, enabling diffusive transport of uncaged fluorescent molecules from one vesicle to the other.
  
In this report, “fluorescent flippers” are introduced to create planarizable push–pull probes with the mechanosensitivity and fluorescence lifetime needed for practical use in biology. Twisted push–pull scaffolds with large and bright dithienothiophenes and their S,S-dioxides as the first “fluorescent flippers” are shown to report on the lateral organization of lipid bilayers with quantum yields above 80% and lifetimes above 4 ns. Their planarization in liquid-ordered (Lo) and solid-ordered (So) membranes results in red shifts in excitation of up to +80 nm that can be transcribed into red shifts in emission of up to +140 nm by Förster resonance energy transfer (FRET). These unique properties are compatible with multidomain imaging in giant unilamellar vesicles (GUVs) and cells by confocal laser scanning or fluorescence lifetime imaging microscopy. Controls indicate that strong push–pull macrodipoles are important, operational probes do not relocate in response to lateral membrane reorganization, and two flippers are indeed needed to “really swim,” i.e., achieve high mechanosensitivity.
  • Directional Energy Migration in Nanoparticles of Crystalline Metal Complexes
    Elia Previtera, Antoine Tissot, Robert W. Johns and Andreas Hauser
    Advanced Materials, 27 (11) , 2015, p1832-1836
    Keywords: Cr(III);energy migration;luminescence;nanocrystals;oxalate networks
    DOI:10.1002/adma.201405179 | unige:48204 | Abstract | Article HTML | Article PDF | Supporting Info
  
Size-controlled micro- and nanocrystals of a [Ru(bpy)3][NaCr(ox)3] coordination network are prepared using reverse micelles. Compared with the bulk reference, the 2E emission of the Cr(III) ions indicates an efficient directional energy migration toward the surface of the nanocrystals.
  
  • Mapping the Ultrafast Changes of Continuous Shape Measures in Photoexcited Spin Crossover Complexes without Long-Range Order
    S.E. Canton, X. Zhang, L.M. Lawson Daku, Y. Liu, J. Zhang and S. Alvarez
    The Journal of Physical Chemistry C, 119 (6) , 2015, p3322-3330
    DOI:10.1021/jp5117189 | unige:46831 | Abstract | Article HTML | Article PDF
Establishing a tractable yet complete reaction coordinate for the spin-state interconversion in d4–d7 transition metal complexes is an integral aspect of controlling the dynamics that govern their functionality. For spin crossover phenomena, the limitations of a single-mode approximation that solely accounts for an isotropic increase in the metal–ligand bond length have long been recognized for all but the simple octahedral monodentate FeII compounds. However, identifying the coupled deformations that also impact on the unimolecular rate constants remains experimentally and theoretically challenging, especially for samples that do not display long-range order or when crystallization profoundly alters the dynamics. Owing to the rapid progress in ultrafast X-ray absorption spectroscopy (XAS), it is now possible to obtain transient structural information in any physical phase with unprecedented details. Using picosecond XAS and DFT modeling, the structure adopted by the photoinduced high-spin state of solvated [Fe(terpy)2]2+ (terpy: 2,2′:6′,2″-terpyridine) has been recently established. Based on these results, the methodology of the continuous shape measure is applied to classify and quantify the short-lived distortion of the first coordination shell. The reaction coordinate of the spin-state interconversion is clearly identified as a double axial bending. This finding sets a benchmark for gauging the influence of first-sphere and second-sphere interactions in the family of FeII complexes that incorporate terpy derivatives. Some implications for the optimization of related photoactive FeII complexes are also outlined.
Characterizing structural distortions in the metastable spin states of d4–d7 transition metal ion complexes is crucial to understand the nature of their bistability and eventually control their switching dynamics. In particular, the impact of the Jahn–Teller effect needs to be assessed for any electronic configuration that could be effectively degenerate, as in e.g. the high-spin (HS) manifold of highly symmetric homoleptic FeII complexes. However, capturing its manifestations remains challenging since crystallization generally alters the molecular conformations and their interconversion. With the rapid progress of ultrafast X-ray absorption spectroscopy, it is now possible to collect data with unprecedented signal-to-noise ratio, opening up for detailed structural characterization of transient species in the homogeneous solution phase. By combining the analysis of picosecond X-ray absorption spectra with DFT simulations, the structure of the photoinduced HS state is elucidated for solvated [Fe(terpy)2]2+ (terpy = 2,2′:6′,2″-terpyridine). This species can be viewed as the average 5B structure in D2 symmetry that originates from a dynamic Jahn–Teller effect in the HS manifold. These results evidence the active role played by this particular instance of vibronic coupling in the formation of the HS state for this benchmark molecule. Ultimately, correlating the interplay between intramolecular and intermolecular degrees of freedom to conformational strain and distortions in real time should contribute to the development of advanced functionalities in transition metal ion complexes.
  • Rigid Urea and Self-Healing Thiourea Ethanolamine Monolayers
    Cristina Stefaniu, Pierre-LĂŠonard Zaffalon, Alessio Carmine, Quentin Verolet, Samuel Fernandez, Tomasz A. Wesolowski, Gerald Brezesinski and Andreas Zumbuehl
    Langmuir, 31 (4) , 2015, p1296-1302
    DOI:10.1021/la5039987 | unige:55058 | Abstract | Article HTML | Article PDF | Supporting Info
  
A series of long-tail alkyl ethanolamine analogs containing amide-, urea-, and thiourea moieties was synthesized and the behavior of the corresponding monolayers was assessed on the Langmuir–Pockels trough combined with grazing incidence X-ray diffraction experiments and complemented by computer simulations. All compounds form stable monolayers at the soft air/water interface. The phase behavior is dominated by strong intermolecular headgroup hydrogen bond networks. While the amide analog forms well-defined monolayer structures, the stronger hydrogen bonds in the urea analogs lead to the formation of small three-dimensional crystallites already during spreading due to concentration fluctuations. The hydrogen bonds in the thiourea case form a two-dimensional network, which ruptures temporarily during compression and is recovered in a self-healing process, while in the urea clusters the hydrogen bonds form a more planar framework with gliding planes keeping the structure intact during compression. Because the thiourea analogs are able to self-heal after rupture, such compounds could have interesting properties as tight, ordered, and self-healing monolayers.
  
The absorption spectra of Sm2+ doped in MFX (M=Sr, Ba; X=Cl, Br) crystals were studied within the range of 20,000–35,000 cm−1 as a function of temperature and host. The absorption bands observed were described with a simple model developed by Wood and Kaiser using group theory. The temperature and host dependence on the 7F0→5D3 Fano resonance lines were investigated. BaFCl:Sm2+ system showed a “normal” 7F0→5D3 transition at 4 K in spite of similar crystal structure and absorption profile with other MFX hosts. New Fano resonances were observed in the absorption spectra at higher energies (23,000–25,000 cm−1) for all MFX:Sm2+ systems at 4 K which persist up to room temperature. Preliminary energy level calculation showed that these resonance lines involve the interaction between higher excited 5LJ states of 4 f6configuration and 4 f55d1 configuration.
 
Bis(thiomethyl)- and bis(thiohexyl)-tetrathiafulvalene-bromo-benzothiadiazoles, containing electron donor tetrathiafulvalene (TTF) and electron acceptor benzothiadiazole (BTD) units, have been prepared by Stille coupling reactions between the TTF-SnMe3 precursors and BTD-Br2. In another series of experiments, TTF-acetylene-BTD compounds have been synthesized by Sonogashira coupling between either TTF-acetylenes and BTD-Br2 in low yields, or TTF-iodine and BTD-acetylene in moderate yields. In the compound TTF-C≡C-BTD the TTF and BTD units are coplanar in the solid state, as shown by the single crystal X-ray structure, and there is segregation in the packing between the donor and acceptor units. All the derivatives have good electron donor properties, as determined by cyclic voltammetry measurements, and they can also be reversibly reduced thanks to the presence of the BTD moiety. UV-visible spectroscopy and photophysical investigations show the presence of an intramolecular charge transfer (ICT) band and an emission band originating from the charge transfer. Both the absorption and the emission are modulated by the substitution scheme and the insertion of the acetylenic bridge.
  
  • Structural Investigation of the High Spin→Low Spin Relaxation Dynamics of the Porous Coordination Network [Fe(pz)Pt(CN)4] · 2.6 H2O
    Teresa Delgado, Antoine Tissot, CĂŠline Besnard, Laure GuĂŠnĂŠe, Philip Pattison and Andreas Hauser
    Chemistry - A European Journal, 21 (9) , 2015, p3664-3670
    Keywords: cooperative effects;LIESST;organic electronics;relaxation dynamics;spin crossover;X-ray diffraction studies
    DOI:10.1002/chem.201405405 | unige:48201 | Abstract | Article HTML | Article PDF | Supporting Info
The Hoffman-type coordination compound [Fe(pz)Pt(CN)4]⋅2.6 H2O (pz=pyrazine) shows a cooperative thermal spin transition at around 270 K. Synchrotron powder X-Ray diffraction studies reveal that a quantitative photoinduced conversion from the low-spin (LS) state into the high-spin (HS) state, based on the light-induced excited spin-state trapping effect, can be achieved at 10 K in a microcrystalline powder. Time-resolved measurements evidence that the HS→LS relaxation proceeds by a two-step mechanism: a random HS→LS conversion at the beginning of the relaxation is followed by a nucleation and growth process, which proceeds until a quantitative HS→LS transformation has been reached.
  • Controlling singlet–triplet splitting in carbazole–oxadiazole based bipolar phosphorescent host materials
    Paul Kautny, Zhongbin Wu, Berthold StĂśger, Antoine Tissot, Ernst Horkel, Jiangshan Chen, Dongge Ma, Hans Hagemann, Johannes FrĂśhlich and Daniel Lumpi
    Organic electronics, 17 , 2015, p216-228
    Keywords: PHOLED; high triplet energy; low efficiency roll-off; sterically induced torsion; structure-property relationship
    DOI:10.1016/j.orgel.2014.11.027 | unige:44949 | Article HTML | Article PDF | Supporting Info
  
A rational molecular design strategy for carbazole–oxadiazole based bipolar host materials was developed to improve the device efficiency of blue phosphorescent organic light-emitting diodes (PHOLED). Steric effects of strategically placed methyl groups led to an increase of triplet energies (o-2MPCzPOXD: 2.66 eV and o-3MPCzPOXD: 2.73 eV versus the initial host material o-PczPOXD: 2.62 eV) while less pronouncedly affecting singlet energies and, therefore, retaining low driving voltages, high power efficiencies and remarkably low efficiency roll-offs in PHOLEDs. The maximum quantum efficiencies (EQE) for blue devices (FIrpic) were significantly raised for o-2MPCzPOXD (13.6%) and o-3MPCzPOXD (11.5%) versus o-PCzPOXD (9.0%) although yielding comparable values for green devices (Ir(ppy)3; 12.9% and 15.4% versus 13.2%). Supported by theoretical calculations a structure–property relationship was established from photo-physical properties, PHOLED performance measurements and structural characterization from single crystal data.
  
Europium doped crystalline Ba7F12Cl2 phosphors have been prepared at temperatures between 650 and 900 °C using alkali chloride fluxes, yielding both disordered (with the incorporation of small amounts of Na) and ordered crystal modifications. The white emission spectrum excited in the near UV consists roughly of two broad emission bands at ca 450 and 590 nm, as well as weak sharp Eu2+ 4f-4f emission bands around 360 nm. The incorporation of Eu2+ is further studied using EPR spectroscopy on single crystals, and reveals a significant zero field splitting. The emission spectrum can be significantly tuned by varying the excitation wavelength between 300 and 390 nm. Fine tuning may also be achieved by chemical substitutions to form Ba7-xMyF12Cl2-zBrz (M = Na, Ca,Eu). Quantitative measurements of the light produced using commercial near UV LEDs show that the color temperature ranges between 4000 and 9700 K with CIE chromaticity coordinates close to the ideal values of x=y=0.333. The best color rendering index (CRI) found was 0.83, and the highest light to light conversion yield was 171 lumen/W. These results show that the title compound is a very promising candidate for white light generation using near UV LED excitation.
Borohydrides are actively considered as potential hydrogen storage materials. In this context fundamental understanding of breaking and forming B-H bond is essential. Isotope exchange reactions allow isolating some parts of this reaction without introducing major structural or chemical changes. Experiments were performed on Ca(BH4)2and Ca(BD4)2 as a function of temperature and pressure. A complete exchange can be realized in about 9h at 200 °C using a deuterium pressure of 20 bar. The activation energy, estimated using first order kinetics, for the forward reaction (Ca(BH4)2 → Ca(BD4)2) was found to be 82.1 ± 2.7 kJ/mol (P = 35 bar) and the one for the backward reaction (Ca(BD4)2 → Ca(BH4)2) was found to be 98.5 ± 8.3 kJ/mol (P = 35 bar). Pressure dependent study shows that the reaction rate increases with increasing pressure up to 35 bar. This behavior is consistent with first adsorption step prior to diffusion into the solid and isotope exchange according to the scheme described below.
 
The excited-state dynamics of two molecular dyads, consisting of zinc (1) and free-base (2) porphyrin connected via a peptide linker to a core-substituted naphthalenediimide (NDI) have been investigated using optical spectroscopy. These dyads exhibit rich photophysics because of the large number of electronic excited states below 3 eV. In the case of 1 in apolar solvents, excitation energy transfer from the vibrationally hot singlet excited porphyrin to the NDI takes place with a 500 fs time constant. Electronic energy ends up in the NDI-localized triplet state, which decays to the ground state on a microsecond timescale. In polar solvents, ground-state recovery is faster by 5 orders of magnitude because of the occurrence of charge separation followed by recombination. On the other hand, excitation energy transfer in 2 takes place in the opposite direction, namely from the NDI to the porphyrin, which then undergoes intersystem crossing to the triplet state, followed by triplet energy transfer back to the NDI. Therefore, four distinct local electronic excited states are consecutively populated after excitation of the NDI unit of 2, with the energy shuttling between the two ends of the dyad.
  
  • Substituent and Solvent Effects on the Excited State Deactivation Channels in Anils and Boranils
    Jacek Dobkowski, Pawel Wnuk, Joanna Buczynska, Maria Pszona, Grazyna Orzanowska, Denis Frath, Gilles Ulrich, Julien Massue, Sandra Mosquera-VĂĄzquez, Eric Vauthey, Czeslaw Radzewicz, Raymond Ziessel and Jacek Waluk
    Chemistry - A European Journal, 21 (3) , 2015, p1312-1327
    Keywords: anils;boranils;photochemistry;photophysics;ultrafast spectroscopy
    DOI:10.1002/chem.201404669 | unige:44951 | Abstract | Article HTML | Article PDF | Supporting Info
Differently substituted anils (Schiff bases) and their boranil counterparts lacking the proton-transfer functionality have been studied using stationary and femtosecond time-resolved absorption, fluorescence, and IR techniques, combined with quantum mechanical modelling. Dual fluorescence observed in anils was attributed to excited state intramolecular proton transfer. The rate of this process varies upon changing solvent polarity. In the nitro-substituted anil, proton translocation is accompanied by intramolecular electron transfer coupled with twisting of the nitrophenyl group. The same type of structure is responsible for the emission of the corresponding boranil. A general model was proposed to explain different photophysical responses to different substitution patterns in anils and boranils. It is based on the analysis of changes in the lengths of CN and CC bonds linking the phenyl moieties. The model allows predicting the contributions of different channels that involve torsional dynamics to excited state depopulation.
  • Smaller than a nanoparticle with the design of discrete polynuclear molecular complexes displaying near-infrared to visible upconversion
    Davood Zare, Yan Suffren, Laure GuĂŠnĂŠe, Svetlana V. Eliseeva, Homayoun Nozary, Lilit Aboshyan-Sorgho, StĂŠphane Petoud, Andreas Hauser and Claude Piguet
    Dalton Transactions, 44 (6) , 2015, p2529-2540
    DOI:10.1039/C4DT02336F | unige:46187 | Abstract | Article HTML | Article PDF | Supporting Info
  
This work shows that the operation of near-infrared to visible light-upconversion in a discrete molecule is not limited to non-linear optical processes, but may result from superexcitation processes using linear optics. The design of nine-coordinate metallic sites made up of neutral N-heterocyclic donor atoms in kinetically inert dinuclear [GaEr(L1)3]6+ and trinuclear [GaErGa(L2)3]9+ helicates leads to [ErN9] chromophores displaying unprecedented dual visible nanosecond Er(4S3/2→4I15/2) and near-infrared microsecond Er(4I13/2→4I15/2) emissive components. Attempts to induce one ion excited-state absorption (ESA) upconversion upon near-infrared excitation of these complexes failed because of the too-faint Er-centred absorption cross sections. The replacement of the trivalent gallium cation with a photophysically-tailored pseudo-octahedral [CrN6] chromophore working as a sensitizer for trivalent erbium in [CrEr(L1)3]6+ improves the near-infrared excitation efficiency, leading to the observation of a weak energy transfer upconversion (ETU). The connection of a second sensitizer in [CrErCr(L2)3]9+ generates a novel mechanism for upconversion, in which the superexcitation process is based on the CrIII-sensitizers. Two successive Cr→Er energy transfer processes (concerted-ETU) compete with a standard Er-centred ETU, and a gain in upconverted luminescence by a factor larger than statistical values is predicted and observed.
  
The electronic absorption spectrum of 3-hydroxyflavone (3HF) in various solvents exhibits a long-wavelength (LW) band, whose origin has been debated. The excited-state dynamics of neutral and basic solutions of 3HF in alcohols upon excitation in this LW band has been investigated using a combination of fluorescence up-conversion and transient electronic and vibrational absorption spectroscopies. The ensemble of results reveals that, in neutral solutions, LW excitation results in the population of two excited species with similar fluorescence spectra but very different lifetimes, namely 40–100 ps and 2–3 ns, depending on the solvent. In basic solutions, the relative concentrations of these species change considerably in favor of that with the short-lived excited state. On the basis of the spectroscopic data and quantum chemistry calculations, the short lifetime is attributed to the excited state of 3HF anion, whereas the long one is tentatively assigned to an excited hydrogen-bonded complex with the solvent. Excited-state intermolecular proton transfer from the solvent to the anion yielding the tautomeric form of 3HF is not operative, as the excited anion decays to the ground state via an efficient nonradiative transition.
  • On the Role of Ligand-Field States for the Photophysical Properties of Ruthenium(II) Polypyridyl Complexes
    Qinchao Sun, Sandra Mosquera-Vazquez, Yan Suffren, Jihane Hankache, N. Amstutz, LatĂŠvi Max Lawson Daku, Eric Vauthey and Andreas Hauser
    Coordination Chemistry Reviews, 282-283 , 2015, p87-99
    Keywords: Ruthenium(II) polypyridyl complexes; ligand-field states; luminescence quenching; photophysical properties; high-pressure
    DOI:10.1016/j.ccr.2014.07.004 | unige:42120 | Abstract | Article HTML | Article PDF
The role of ligand-field states for the photophysical properties of d6 systems has been discussed in a large number of publications over the past decades. Since the seminal paper by Houten and Watts, for instance, the quenching of the 3MLCT luminescence in ruthenium(II) polypyridyl complexes is attributed to the presence of the first excited ligand-field state, namely a component of the 3T1(t2g5eg1) state, at similar energies. If this state lies above the 3MLCT state, the luminescence is quenched via thermal population at elevated temperatures only. If it lies well below, then the luminescence is quenched down to cryogenic temperatures. In this contribution we present transient absorption spectra on non-luminescent ruthenium polypyridyl complexes such as [Ru(m-bpy)3]2+, m-bpy = 6-methyl-2,2’-bipyridine, in acetonitrile at room temperature, which reveal an ultra-rapid depopulation of the 3MLCT state but a much slower ground state recovery. We propose that in this and related complexes the methyl groups force longer metal-ligand bond lengths, thus resulting in a lowering of the ligand-field strength such that the 3dd state drops to below the 3MLCT state, and that furthermore the population of this state from the 3MLCT state occurs faster than its decay to the ground state. In addition we demonstrate that in this complex the luminescence can be switched on by external pressure, which we attribute to a destabilisation of the ligand-field state by the pressure due to its larger molecular volume compared to the ground state as well as the 3MLCT state.
Over recent years, research on thiolate-protected gold clusters Aum(SR)n has gained significant interest. Milestones were the successful determination of a series of crystal structures (Au102(SR)44, Au25(SR)18, Au38(SR)24, Au36(SR)24, and Au28(SR)20). For Au102(SR)44, Au38(SR)24, and Au28(SR)20, intrinsic chirality was found. Strong Cotton effects (circular dichroism, CD) of gold clusters protected by chiral ligands have been reported a long time ago, indicating the transfer of chiral information from the ligand into the cluster core. Our lab has done extensive studies on chiral thiolate-protected gold clusters, including those protected with chiral ligands. We demonstrated that vibrational circular dichroism can serve as a useful tool for the determination of conformation of the ligand on the surface of the cluster. The first reports on crystal structures of Au102(SR)44 and Au38(SR)24 revealed the intrinsic chirality of these clusters. Their chirality mainly arises from the arrangement of the ligands on the surface of the cluster cores. As achiral ligands are used to stabilize the clusters, racemic mixtures are obtained. However, the separation of the enantiomers by HPLC was demonstrated which enabled the measurement of their CD spectra. Thermally induced inversion allows determination of the activation parameters for their racemization. The inversion demonstrates that the gold–thiolate interface is anything but fixed; in contrast, it is rather flexible. This result is of fundamental interest and needs to be considered in future applications. A second line of our research is the selective introduction of chiral, bidentate ligands into the ligand layer of intrinsically chiral gold clusters. The ligand exchange reaction is highly diastereoselective. The bidentate ligand connects two of the protecting units on the cluster surface and thus effectively stabilizes the cluster against thermally induced inversion. A minor (but significant) influence of chiral ligands to the CD spectra of the clusters is observed. The studied system represents the first example of an intrinsically chiral gold cluster with a defined number of exchanged ligands, full control over their regio- and stereochemistry. The methodology allows for the selective preparation of mixed-ligand cluster compounds and a thorough investigation of the influence of single ligands on the cluster’s properties. Overall, the method enables even more detailed tailoring of properties. Still, central questions remain unanswered: (1) Is intrinsic chirality a ubiquitous feature of thiolate-protected gold clusters? (2) How does chirality transfer work? (3) What are the applications for chiral thiolate-protected gold clusters? In this Account, we summarize the main findings on chirality in thiolate-protected gold cluster of the past half decade. Emphasis is put on intrinsically chiral clusters and their structures, optical activity, and reactivity.
The Au102(p-MBA)44 cluster (p-MBA: para-mercaptobenzoic acid) is observed as a chiral compound comprised of achiral components in its single-crystal structure. So far the enantiomers observed in the crystal structure are not isolated, nor is the circular dichroism spectrum known. A chiral phase transfer method is presented which allows partial resolution of the enantiomers by the use of a chiral ammonium bromide, (−)-1R,2S-N-dodecyl-N-methylephedrinium bromide ((−)-DMEBr). At sufficiently low concentration of (−)-DMEBr, the phase transfer from water to chloroform is incomplete. Both the aqueous and organic phases show optical activity of near mirror image relationship. Differences in the spectra are ascribed to the formation of diastereomeric salts. At high concentrations of (−)-DMEBr, full phase transfer is observed. The organic phase, however, still displays optical activity. We assume that one of the diastereomers has very strong optical activity, which overrules the cancelation of the spectra with opposite sign. Comparison with computations further corroborates the experimental data and allows a provisional assignment of handedness of each fraction.
  • Stacked and Tunable Large-Scale Plasmonic Nanoparticle Arrays for Surface-Enhanced Raman Spectroscopy
    Stefan MĂźhlig, Dana Cialla, Alastair Cunningham, Anne März, Karina Weber, Thomas BĂźrgi, Falk Lederer and Carsten Rockstuhl
    The Journal of Physical Chemistry C, 118 (19) , 2014, p10230-10237
    DOI:10.1021/jp409688p | unige:94111 | Abstract | Article HTML | Article PDF
Surface-enhanced Raman spectroscopy takes advantage of plasmonic substrates that sustain resonances at tunable frequencies with a reproducibly extraordinary field enhancement. Low-cost and large-scale fabrication of these substrates is further required. Here, we present stacked large-scale arrays of strongly coupled gold nanoparticles as promising candidates for such substrates. These arrays are fabricated by bottom-up techniques that fulfill the aforementioned requirements. The distance between adjacent arrays in the stack is controlled with high precision using a discrete number of monolayers of molecules that enable the spectral position of the plasmonic resonances to be tuned. Although the nanoparticles are randomly arranged in each array, the spatial proximity of the stacked arrays enables a strong coupling among nanoparticles to be achieved in adjacent arrays. The huge field enhancements due to these strongly coupled gold nanoparticles are shown to enhance the Raman signal. We show that effectively the optical response from these stacked arrays and the Raman signals can be understood in a simplifying picture where only an individual nanoparticle dimer is considered. The possibility to tune the plasmonic resonances of the substrate across the visible spectrum makes our material a plasmonic substrate of choice for many applications where light–matter interactions need to be intensified.
  • Growing gold nanoparticles on a flexible substrate to enable simple mechanical control of their plasmonic coupling
    Ugo Cataldi, Roberto Caputo, Yuriy Kurylyak, GĂŠrard Klein, Mahshid Chekini, Cesare Umeton and Thomas BĂźrgi
    Journal of Materials Chemistry C, 2 (37) , 2014, p7927
    DOI:10.1039/C4TC01607F | unige:94112 | Abstract | Article HTML | Article PDF
A simple method is presented to control and trigger the coupling between plasmonic particles using both a growing process of gold nanoparticles (GNPs) and a mechanical strain applied to the elastomeric template where these GNPs are anchored. The large scale samples are prepared by first depositing and then further growing gold nanoparticles on a flexible PDMS tape. Upon stretching the tape the particles move further apart in the direction of the stretching and closer together in the direction perpendicular to it. The synergy between the controlled growth of GNPs and the mechanical strain, leads to a drastic shift of the plasmon band and a color change of the sample. Furthermore, the stretching by only a few percent of the amorphous and initially isotropic sample results in a strong polarization-dependent plasmon shift. At smaller gap sizes between neighboring particles, induced by stretching the PDMS tape, the plasmon shift strongly deviates from the behaviour expected considering the plasmon ruler equation. This shows that multipolar coupling effects significantly contribute to the observed shift. Overall, these results indicate that a macroscopic mechanical strain allows one to control the coupling and therefore the electromagnetic field at the nanoscale.
  • Steroid Receptor RNA Activator (SRA) Modification by the Human Pseudouridine Synthase 1 (hPus1p): RNA Binding, Activity, and Atomic ModelOpen access paper
    Tiphaine Huet, François-Alexandre Miannay, Jeffrey R. Patton, StĂŠphane Thore and Paul C. Driscoll
    PLoS ONE, 9 (4) , 2014
    DOI:10.1371/journal.pone.0094610 | unige:36716 | Abstract | Article HTML | Article PDF
The most abundant of the modified nucleosides, and once considered as the “fifth” nucleotide in RNA, is pseudouridine, which results from the action of pseudouridine synthases. Recently, the mammalian pseudouridine synthase 1 (hPus1p) has been reported to modulate class I and class II nuclear receptor responses through its ability to modify the Steroid receptor RNA Activator (SRA). These findings highlight a new level of regulation in nuclear receptor (NR)-mediated transcriptional responses. We have characterised the RNA association and activity of the human Pus1p enzyme with its unusual SRA substrate. We validate that the minimal RNA fragment within SRA, named H7, is necessary for both the association and modification by hPus1p. Furthermore, we have determined the crystal structure of the catalytic domain of hPus1p at 2.0 Å resolution, alone and in a complex with several molecules present during crystallisation. This model shows an extended C-terminal helix specifically found in the eukaryotic protein, which may prevent the enzyme from forming a homodimer, both in the crystal lattice and in solution. Our biochemical and structural data help to understand the hPus1p active site architecture, and detail its particular requirements with regard to one of its nuclear substrates, the non-coding RNA SRA.
  • Convergent Synthesis, Resolution, and Chiroptical Properties of Dimethoxychromenoacridinium Ions
    JĂŠrĂ´me Gouin, Thomas BĂźrgi, Laure GuĂŠnĂŠe and JĂŠrĂ´me Lacour
    Organic Letters, 16 (14) , 2014, p3800-3803
    DOI:10.1021/ol501692r | unige:38927 | Abstract | Article HTML | Article PDF | Supporting Info
Cationic azaoxa[4]helicenes can be prepared in a single step from a common xanthenium precursor by addition of nucleophilic amines under monitored conditions (160 °C, 2 min, MW). The (−)-(M) and (+)-(P) enantiomers can be separated by chiral stationary-phase chromatography. Determination of the absolute configuration and racemization barrier (ΔG⧧ (433 K) 33.3 ± 1.3 kcal·mol–1) was achieved by VCD and ECD spectroscopy, respectively.
  • Non-uniform Continuum Model for Solvated Species Based on Frozen-Density Embedding Theory: The Study Case of Solvatochromism of Coumarin 153
    Sapana V. Shedge, Xiuwen Zhou and Tomasz A. Wesolowski
    Chimia, 68 (9) , 2014, p609-614
    Keywords: continuum models; density functional theory; multi-level simulations; orbital-free embedding theory; solvatochromism
    DOI:10.2533/chimia.2014.609 | unige:75040 | Abstract
Recent application of the Frozen-Density Embedding Theory based continuum model of the solvent, which is used for calculating solvatochromic shifts in the UV/Vis range, are reviewed. In this model, the solvent is represented as a non-uniform continuum taking into account both the statistical nature of the solvent and specific solute–solvent interactions. It offers, therefore, a computationally attractive alternative to methods in which the solvent is described at atomistic level. The evaluation of the solvatochromic shift involves only two calculations of excitation energy instead of at least hundreds needed to account for inhomogeneous broadening. The present review provides a detailed graphical analysis of the key quantities of this model: the average charge density of the solvent (<ρB>) and the corresponding Frozen-Density Embedding Theory derived embedding potential for coumarin 153.
  • Crystal-clear - The '2014 Most Superlative Crystal Growth Contest' for School Classes
    Didier Perret, Hans Hagemann, Radovan Cerny, Christoph Renner, Enrico Giannini, Laure GuĂŠnĂŠe, CĂŠline Besnard, David GĂŠrard and Lionel Windels
    Chimia, 68 (12) , 2014, p893-895
    Keywords: crystal growth contest; crystallography and society; education; incentive science; interrnational year of crystallography 2014; school science; potassium dihydrogen phosphate; outreach initiative
    DOI:10.2533/chimia.2014.893 | unige:46708 | Article PDF
  
To celebrate the International Year of Crystallography among the general public, a consortium of chemists, physicists and crystallographers of the University of Geneva organised in Spring 2014 an incentive crystal growth contest for Geneva scholars aged 4 to 19. Starting from a kit containing a salt and user instructions, classes had to prepare a crystal that met specific criteria according to their category of age. The composition of the salt – potassium dihydrogen phosphate (KDP) – was only disclosed to the participants during the final Awards Ceremony. This contest positively exceeded our expectations with almost 100 participating classes (ca. 1800 participants) and 54 specimens received over all categories.
  
  • Structure and properties of complex hydride perovskite materials
    Pascal Schouwink, Morten B. Ley, Antoine Tissot, Hans Hagemann, Torben R. Jensen, LubomĂ­r Smrcok and Radovan CernĂ˝
    Nature Communications, 5 , 2014, p5706
    DOI:10.1038/ncomms6706 | unige:43536 | Abstract | Article HTML
Perovskite materials host an incredible variety of functionalities. Although the lightest element, hydrogen, is rarely encountered in oxide perovskite lattices, it was recently observed as the hydride anion H−, substituting for the oxide anion in ​BaTiO3. Here we present a series of 30 new complex hydride perovskite-type materials, based on the non-spherical ​tetrahydroborate anion ​BH4− and new synthesis protocols involving rare-earth elements. Photophysical, electronic and ​hydrogen storage properties are discussed, along with counterintuitive trends in structural behaviour. The electronic structure is investigated theoretically with density functional theory solid-state calculations. BH4-specific anion dynamics are introduced to perovskites, mediating mechanisms that freeze lattice instabilities and generate supercells of up to 16 × the unit cell volume in AB(BH4)3. In this view, homopolar hydridic di-hydrogen contacts arise as a potential tool with which to tailor crystal symmetries, thus merging concepts of molecular chemistry with ceramic-like host lattices. Furthermore, anion mixing ​BH4−←X− (X−=Cl−, Br−, I−) provides a link to the known ABX3 halides.
  • Excited State and Injection Dynamics of Triphenylamine Sensitizers Containing a Benzothiazole Electron-Accepting Group on TiO2 and Al2O3 Thin Films
    Mihalis Fakis, Peter HrobĂĄrik, Oleksandr Yushchenko, Ivica SigmundovĂĄ, Marius Koch, Arnulf Rosspeintner, Elias Stathatos and Eric Vauthey
    The Journal of Physical Chemistry C, 118 (49) , 2014, p28509-28519
    DOI:10.1021/jp509971q | unige:43537 | Abstract | Article HTML | Article PDF
  
The excited state and electron injection dynamics of three new organic sensitizers, comprising a triphenylamine moiety connected by an ethenylene (C−C double-bond) or ethynylene (C−C triple-bond) π-spacer to an electron-withdrawing benzothiazole bearing a cyanoacrylic acid anchoring group, have been studied using a combination of steady-state and femtosecond-resolved spectroscopies. The measurements were carried out for the three dyes in predominantly neutral and completely deprotonated forms in liquid solutions and bound on nanocrystalline TiO2 and Al2O3 thin films. In addition, quantum-chemical calculations were performed to predict absorption spectra of the sensitizers and their corresponding cation radicals. Time-resolved fluorescence (TRF) measurements on TiO2 indicate that electron injection takes place on a <0.2 ps time scale. Transient electronic absorption (TA) measurements provide evidence for the formation of radical cations not only in dye-sensitized TiO2 films but also in Al2O3 ones. The cation lifetime in Al2O3 is significantly shorter compared to TiO2, indicating a faster recombination of injected electrons with the dye cations. In addition, the ground-state bleach band in dye-sensitized TiO2 films experiences a gradual red-shift, which is indicative of a transient Stark effect. Finally, femtosecond transient absorption measurements in the IR region point to an ultrafast generation of injected electrons for all dyes. A faster recombination of the injected electrons with the dye cations is observed for the sensitizer decorated with auxiliary electron-donating methoxy groups on the triphenylamine moiety.
  • Single Crystal EPR Studies of Radicals Produced by Radiolysis of Organophosphorus Compounds
    Michel Geoffroy
    in "Applications of EPR in Radiation Research" Anders Lund - Masaru Shiotani Eds., Springer, 2014, p33-66
    DOI:10.1007/978-3-319-09216-4_2 | unige:42196
The main radical species produced by radiolysis of organophosphorus compounds are described in this chapter. Their identification is generally based on an analysis of the g and hyperfine tensors obtained from EPR experiments performed on irradiated single crystals. Special emphasis is placed on the properties of the 31P hyperfine tensor, which is often decisive in determining the structure of these radicals. Radiogenic species mentioned in the beginning of this review correspond to simple phosphorus-centered radicals (PR2, PR3−, PR4, PR3+, and R2PO). Then, more delocalized systems are reported (allylic structures, captodatively stabilized radicals, symmetrical radical ions containing a P–P bond). The effects of radiolysis on compounds containing low-coordinate phosphorus atoms (e.g. phosphaalkenes) are also described as well as the formation of radical pairs in irradiated phosphated sugars. The last part of the chapter deals with metallated radicals formed by radiolysis of metallic complexes M(CO)5P(H)Ph2 (with M = Mo, Cr, W). In some cases, phosphorus-centered radicals are compared with their arsenic analogues. For several systems the focus lies on dynamical effects; this is the case, for example, for the triptycenephosphinyl radical, which undergoes internal rotation around a P–C bond. Molecular rearrangements after radiolysis of some organophosphorus compounds (e.g. diphosphenes) are also reported.
  
  • Complementary Surface Second Harmonic Generation and Molecular Dynamics Investigation of the Orientation of Organic Dyes at a Liquid/Liquid Interface
    Denis Svechkarev, Dmitry Kolodezny, Sandra Mosquera-VĂĄzquez and Eric Vauthey
    Langmuir, 30 (46) , 2014, p13869-13876
    DOI:10.1021/la503121g | unige:43539 | Abstract | Article HTML | Article PDF
The second-order nonlinear response of two dyes adsorbed at the dodecane/water interface was investigated by surface second harmonic generation (SSHG). These dyes consist of the same chromophoric unit, 2-pyridinyl-5-phenyloxazole, with an alkyl chain located at the two opposite ends. The analysis of the polarization dependence of the SSHG intensity as usually performed points to similar tilt angles of the two dyes with respect to the interface but does not give information on the absolute direction. Molecular dynamics (MD) simulations reveal that both dyes lie almost flat at the interface but have opposite orientations. A refined SSHG data analysis with the width of the orientational distribution yields tilt angles that are in very satisfactory agreement with the MD simulations.
 
Because of their key role in many areas of science and technology, bimolecular photoinduced electron transfer reactions have been intensively studied over the past five decades. Despite this, several important questions, such as the absence of the Marcus inverted region or the structure of the primary reaction product, have only recently been solved while others still remain unanswered. Ultrafast spectroscopy has proven to be extremely powerful to monitor the entire electron transfer process and to access, with the help of state-of-the-art theoretical models of diffusion-assisted reactions, crucial information like e.g. the intrinsic charge separation dynamics beyond the diffusion limit. Additionally, extension of these experimental techniques to other spectral regions than the UV-visible, such as the infrared, has given a totally new insight into the nature, the structure and the dynamics of the key reaction intermediates, like exciplexes and ions pairs. In this perspective, we highlight these recent progresses and discuss several aspects that still need to be addressed before a thorough understanding of these processes can be attained.
  • Copper-Catalyzed Propargylic Substitution of Dichloro Substrates: Enantioselective Synthesis of Trisubstituted Allenes and Formation of Propargylic Quaternary Stereogenic Centers
    Hailing Li, David Grassi, Laure GuĂŠnĂŠe, Thomas BĂźrgi and Alexandre Alexakis
    Chemistry - A European Journal, 20 (50) , 2014, p16694-16706
    Keywords: allenes;asymmetric catalysis;copper;Grignard reagents;P ligands
    DOI:10.1002/chem.201404668 | unige:43540 | Abstract | Article HTML | Article PDF
An easy and versatile Cu-catalyzed propargylic substitution process is presented. Using easily prepared prochiral dichloro substrates, readily available Grignard reagents together with catalytic amount of copper salt and chiral ligand, we accessed a range of synthetically interesting trisubstituted chloroallenes. Substrate scope and nucleophile scope are broad, providing generally high enantioselectivity for the desired 1,3-substitution products. The enantioenriched chloroallenes could be further transformed into the corresponding trisubstituted allenes or terminal alkynes bearing all-carbon quaternary stereogenic centers, through the copper-catalyzed enantiospecific 1,1/1,3-substitutions. The two successive copper-catalyzed reactions could be eventually combined into a one-pot procedure and different desired allenes or alkynes were obtained respectively with high enantiomeric excesses.
  
  • Enantioselective Catalytic Fluorinative Aza-semipinacol Rearrangement
    Fedor Romanov-Michailidis, Marion Pupier, CĂŠline Besnard, Thomas BĂźrgi and Alexandre Alexakis
    Organic Letters, 16 (19) , 2014, p4988-4991
    DOI:10.1021/ol5022355 | unige:41312 | Abstract | Article HTML | Article PDF
An efficient and highly stereoselective fluorinative aza-semipinacol rearrangement is described. The catalytic reaction requires use of Selectfluor in combination with the chiral, enantiopure phosphate anion derived from acid L3. Under optimized conditions, cyclopropylamines A were transformed into β-fluoro cyclobutylimines B in good yields and high levels of diastereo- and enantiocontrol. Furthermore, the optically active cyclobutylimines were reduced diastereoselectively with L-Selectride in the corresponding fluorinated amines C, compounds of significant interest in the pharmacological industry.
  • CO2-promoted hydrolysis of KBH4 for efficient hydrogen co-generation
    I. Dovgaliuk, H. Hagemann, T. Leyssens, M. Devillers and Y. Filinchuk
    International Journal of Hydrogen Energy, 39 (34) , 2014, p19603-19608
    Keywords: potassium borohydride; tetraborate; hydrolysis; crystal structure; hydrogen storage
    DOI:10.1016/j.ijhydene.2014.09.068 | unige:41528 | Abstract | Article PDF
Hydrolysis of metal borohydrides in the presence of CO2 has not been studied so far, although carbon dioxide contained in air is known to accelerate hydrogen generation. KBH4 hydrolysis promoted by CO2 gas put through an aqueous solution was studied by time-resolved ATR-FTIR spectroscopy, showing a transformation of BH4− into B4O5(OH)42−, and a drastically accelerated hydrogen production which can be completed within minutes. This process can be used to produce hydrogen on-board from exhaust gases (CO2 and H2O). We found a new intermediate, K9[B4O5(OH)4]3(CO3)(BH4)·7H2O, forming upon hydrolysis on air via a slow adsorption of the atmospheric CO2. The same intermediate can be crystallized from partly hydrolyzed solutions of KBH4 + CO2, but not from the fully reacted sample saturated with CO2. This phase was studied by single-crystal and powder X-ray diffraction, DSC, TGA, Raman, IR and elemental analysis, all data are fully consistent with the presence of the three different anions and of the crystallized water molecules. Its crystal structure is hexagonal, space group P-62c, with lattice parameters a = 11.2551(4), c = 17.1508(8) Å. Formation of the intermediate produces 16 mol of H2 per mole of adsorbed CO2 and thus is very efficient both gravimetrically and volumetrically. It allows also for an elimination of carbon dioxide from exhaust gases.
  • Synthesis and Crystal Structures of a Stable, a Metastable and a High Temperature Modification of Pb2NaIO6
    Frank Kubel, Mariana Pantazi, Nicole Wandl and Hans Hagemann
    Zeitschrift fĂźr anorganische und allgemeine Chemie, 640 (15) , 2014, p3184-3189
    Keywords: lead periodates; irreversible phase transition; metastable compounds; anisotropic diffraction peak broadening
    DOI:10.1002/zaac.201400208 | unige:43538 | Abstract | Article HTML | Article PDF
In analogy to the synthesis of polycrystalline M2NaIO6 (M = Ca, Sr, Ba) by precipitation in water at 90 °C, the title compound was first prepared as a metastable compound. The stable modification of Pb2NaIO6 was obtained by a heat treatment to 400 °C followed by cooling to room temperature. The crystal structure was refined from powder diffraction data [space group P21/c (14), a = 5.9040(2), b = 5.7526(2), c = 10.1104(3) Å, β = 125.341(1)°]. On heating, at ca. 125 °C, a phase transition to a cubic high temperature modification was observed. The crystal structure was refined from XRD data measured at 200 °C [space group Fm3m (225), a = 8.2678(1) Å]. Depending on the precipitation temperature between 90 °C and 0 °C, several metastable modifications were obtained, which can be distinguished by significantly different lattice parameters. The XRD pattern of a powder precipitated at room temperature is pseudocubic. The crystal structure was refined at room temperature in P21/c with a = 5.8201(4), b = 5.8473(4), c = 10.0798(5) Å, β = 125.074(3)°. This modification behaves almost as a cubic lattice on heating as found from XRD and DSC measurements.
 
Pd2Au36(SC2H4Ph)24 clusters have been prepared, isolated and separated in their enantiomers. Compared to the parent Au38(SC2H4Ph)24 cluster the doping leads to a significant change of the circular dichrosim spectrum, however, the anisotropy factors are of similar magnitude in both cases. Isolation of the enantiomers allowed us to study the racemi-zation of the chiral cluster, which reflects the flexibility of the ligand shell composed of staple motifs. The doping leads to a substantial lowering of the racemization temperature. The change in activation parameters due to the doping may be solely due to modification of the electronic structure.
  
The compound Ba5I2O12 was synthesized by heating a precipitate of dissolved Ba(OH)2·8H2O and H5IO6. Rb2O was added to increase the crystallite size. The crystal structure was determined from conventional laboratory X-ray diffraction data by using a real-space structure solution approach followed by a Rietveld refinement. No constraints on positions were used. The structure analysis gave an orthorhombic symmetry with a = 19.7474(2) Å, b = 5.9006(1) Å and c = 10.5773(1) Å. The final RBragg value in space group Pnma (62) was 1.0 %. The structure can be described by layers of a metal and iodine arrangement forming almost pentagonal holes. Raman measurements were correlated with the two IO6 octahedra. Two further barium periodate patterns were observed and indexed.
  • Where does the Raman optical activity of [Rh(en)3]3+ come from? Insight from a combined experimental and theoretical approach
    Marie Humbert-Droz, Patric Oulevey, LatĂŠvi Max Lawson Daku, Sandra Luber, Hans Hagemann and Thomas BĂźrgi
    Phys. Chem. Chem. Phys., 16 (42) , 2014, p23260-23273
    DOI:10.1039/C4CP02145B | unige:40863 | Abstract | Article HTML | Article PDF
  
Backscattered Raman optical activity (ROA) spectra are measured for Δ- and Λ-tris-(ethylenediamine)rhodium(III) chloride in aqueous solution. In addition, the spectra of the four possible conformers in the Λ configuration are investigated by ab initio calculations. The Λ(δδδ) conformer is in best agreement with experimental spectra and examined in more details. The two most stable conformers according to the calculations are not compatible with the experimental ROA spectrum. Insights into the origin of observed band intensities are obtained by means of group coupling matrices. The influence of the first solvation shell is explored via an ab initio molecular dynamics simulation. Taking explicit solvent molecules into account further improves the agreement between calculation and experiment. Analysis of selected normal modes using group coupling matrices shows that solvent molecules lead to normal mode rotation and thus contribute to the ROA intensity, whereas the contribution of the Rh can be neglected.
  • Modular Synthesis of pH-Sensitive Fluorescent Diaza[4]helicenes
    Antoine Wallabregue, Petr Sherin, Joyram Guin, CĂŠline Besnard, Eric Vauthey and JĂŠrĂ´me Lacour
    European Journal of Organic Chemistry, (29) , 2014, p6431-6438
    Keywords: fluorescence; pH sensing; hHelicenes; chiral resolution; reaction mechanisms
    DOI:10.1002/ejoc.201402863 | unige:40860 | Abstract | Article HTML | Article PDF
Configurationally stable diaza[4]helicenes have been prepared in two steps by using a particularly facile N–N bond-cleavage reaction. The synthetic procedure uses hydrazine (NH2NH2) for the introduction of a single nitrogen atom. The strategy is general, modular and highly tolerant to functional groups. A mechanistic rationale is proposed for the spontaneous N–N bond-cleavage reaction. The resulting helical quinacridines are dyes that present absorption and emission properties that can be modulated as a function of pH; the pink quinacridine and green protonated forms (pKa ≈ 9.0) display distinct optical features in the near-IR region. Single enantiomers were obtained by chiral stationary phase HPLC resolution. The absolute configurations were assigned by comparison of the ECD spectra of the conjugated acids with those of known dialkylquinacridinium derivatives. A rather high racemization barrier was measured by means of variable-temperature ECD experiments (ΔG‡ = 30.7 ± 4.0 kcal mol–1 at 140 °C).
  • Nonuniform Continuum Model for Solvatochromism Based on Frozen-Density Embedding Theory
    Sapana Vitthal Shedge and Tomasz A. Wesolowski
    ChemPhysChem, 15 (15) , 2014, p3291-3300
    Keywords: density functional calculations;molecular dynamics;molecular modeling;solvatochromism;UV/Vis spectroscopy
    DOI:10.1002/cphc.201402351 | unige:41527 | Abstract | Article PDF
Frozen-density embedding theory (FDET) provides the formal framework for multilevel numerical simulations, such that a selected subsystem is described at the quantum mechanical level, whereas its environment is described by means of the electron density (frozen density; ρB( r →) ) The frozen density ρB( r →) is usually obtained from some lower-level quantum mechanical methods applied to the environment, but FDET is not limited to such choices for ρB( r →). The present work concerns the application of FDET, in which ρB( r →) is the statistically averaged electron density of the solvent <ρB( r →)> . The specific solute–solvent interactions are represented in a statistical manner in <ρB( r →)>. A full self-consistent treatment of solvated chromophore, thus involves a single geometry of the chromophore in a given state and the corresponding <ρB( r →)>. We show that the coupling between the two descriptors might be made in an approximate manner that is applicable for both absorption and emission. The proposed protocol leads to accurate (error in the range of 0.05 eV) descriptions of the solvatochromic shifts in both absorption and emission.
  
  • Synthesis and Self-Assembly of a DNA Molecular Brush
    Dawid Kedracki, Mahshid Chekini, Plinio Maroni, Helmut Schlaad and Corinne Nardin
    Biomacromolecules, 15 (9) , 2014, p3375-3382
    DOI:10.1021/bm5008713 | unige:39951 | Abstract | Article HTML | Article PDF
We report herein on the polymer-crystallization-assisted thiol-ene photosynthesis of an amphiphilic comb/graft DNA copolymer, or molecular brush, composed of a hydrophobic poly(2-oxazoline) backbone and hydrophilic short single-stranded nucleic acid grafts. Coupling efficiencies are above 60% and thus higher as compared with the straight solid-phase-supported synthesis of amphiphilic DNA block copolymers. The DNA molecular brushes self-assemble into sub-micron-sized spherical structures in water as evidenced by light scattering as well as atomic force and electron microscopy imaging. The nucleotide sequences remain functional, as assessed by UV and fluorescence spectroscopy subsequent to isoindol synthesis at the surface of the structures. The determination of a vesicular morphology is supported by encapsulation and subsequent spectroscopy monitoring of the release of a water-soluble dye and spectroscopic quantification of the hybridization efficiency (30% in average) of the functional nucleic acid strands engaged in structure formation: about one-half of the nucleotide sequences are available for hybridization, whereas the other half are hindered within the self-assembled structure. Because speciation between complementary and non complementary sequences in the medium could be ascertained by confocal laser scanning microscopy, the stable self-assembled molecular brushes demonstrate the potential for sensing applications.
  • Anion-π and Cation-π Interactions on the Same Surface
    Kaori Fujisawa, CĂŠsar Beuchat, Marie Humbert-Droz, Adam Wilson, Tomasz A. Wesolowski, Jiri Mareda, Naomi Sakai and Stefan Matile
    Angewandte Chemie International Edition, 53 (42) , 2014, p11266-11269
    Keywords: computational chemistry|peptides|π interactions|push-pull chromophores|spectral tuning
    DOI:10.1002/anie.201407161 | unige:40859 | Abstract | Article HTML | Article PDF
Herein, we address the question whether anion–π and cation–π interactions can take place simultaneously on the same aromatic surface. Covalently positioned carboxylate–guanidinium pairs on the surface of 4-amino-1,8-naphthalimides are used as an example to explore push–pull chromophores as privileged platforms for such “ion pair–π” interactions. In antiparallel orientation with respect to the push–pull dipole, a bathochromic effect is observed. A red shift of 41 nm found in the least polar solvent is in good agreement with the 70 nm expected from theoretical calculations of ground and excited states. Decreasing shifts with solvent polarity, protonation, aggregation, and parallel carboxylate–guanidinium pairs imply that the intramolecular Stark effect from antiparallel ion pair–π interactions exceeds solvatochromic effects by far. Theoretical studies indicate that carboxylate–guanidinium pairs can also interact with the surfaces of π-acidic naphthalenediimides and π-basic pyrenes.
  • Direct coupling of carbenium ions with indoles and anilines for the synthesis of cationic π-conjugated dyes
    RĂŠmi Vanel, François-Alexandre Miannay, Eric Vauthey and Jerome Lacour
    ChemComm, 50 (81) , 2014, p12169-12172
    DOI:10.1039/C4CC05193A | unige:40250 | Abstract | Article HTML | Article PDF
A C-C bond forming reaction occurs spontaneously between tris-(2,6-dimethoxyphenyl)carbenium ion and indoles / anilines. The carbocation acts both as electrophile and oxidant. Effective cationic π-conjugated dyes are formed resulting in a strong hyper- and bathochromism
  • Excited-State Dynamics of an Environment-Sensitive Push–Pull Diketopyrrolopyrrole: Major Differences between the Bulk Solution Phase and the Dodecane/Water Interface
    Sabine Richert, Sandra Mosquera Vazquez, Marek Grzybowski, Daniel T. Gryko, Alexander Kyrychenko and Eric Vauthey
    Journal of Physical Chemistry B, 118 (33) , 2014, p9952-9963
    DOI:10.1021/jp506062j | unige:39940 | Abstract | Article HTML | Article PDF
  
The excited-state dynamics of a diketopyrrolopyrrole (DPP) derivative with push–pull substituents has been investigated in a variety of solvents and at the dodecane/water and dodecane/heavy-water interfaces using a combination of ultrafast spectroscopic techniques, including transient electronic absorption and time-resolved surface second-harmonic generation. Whereas the photophysics of a nonpolar DPP analogue is mostly independent of the solvent, the fluorescence decay of the push–pull DPP accelerates strongly by going from aprotic to protic solvents. As this effect increases with the polarity and the hydrogen-bond-donating ability of the solvent, it is attributed to the occurrence of H-bond-assisted nonradiative deactivation induced by the charge-transfer character of the excited state that favors the coupling of the molecule to the H-bond network of the solvent. At the dodecane/water interface, the excited-state lifetime is longer by a factor of ca. 20 than that estimated in pure water and increases further by a factor of about 3 when going to the dodecane/heavy-water interface. This isotope effect, that is more than twice as strong as that measured in bulk solutions, and molecular dynamic simulations indicate that the slowing down of the dynamics at the interface cannot be solely ascribed to a reduced accessibility of the DPP molecule to the aqueous phase. The slower excited-state decay is rather assigned to the conjunction of several effects, such as the strengthening of the H-bond network formed by the interfacial water molecules and the lower local polarity of the interfacial region.
  
By using a polyelectrolyte layer gold nanoparticles have been assembled onto a Ge internal reflection element. Upon illumination with visible and near infrared light a strong infrared absorption has been observed, which can be traced to intervalence band transitions in Ge. This reveals the existence of holes in the Ge near its valence band edge. The switching between bright and dark states is faster than 160 Îźs and the device acts as an infrared modulator. The effect develops with a peculiar kinetics, which may indicate the development of an interfacial layer between germanium and gold that allows efficient electron transfer upon illumination.
  • Light-induced spin-state switching in the mixed crystal series of the 2D coordination network {[Zn1-xFex(bbtr)3](BF4)2}: optical spectroscopy and cooperative effects
    Pradip Chakraborty, Cristian Enachescu, Arnaud Humair, Leo Egger, Teresa Delgado, Antoine Tissot, Laure GuĂŠnĂŠe, CĂŠline Besnard, Robert Bronisz and Andreas Hauser
    Dalton Transactions, 43 (47) , 2014, p17786-17796
    DOI:10.1039/C4DT01728E | unige:42340 | Abstract | Article HTML | Article PDF
  
Depending on the iron(II) concentration, the mixed crystals of {[Zn1-xFex(bbtr)3](BF4)2}∞, bbtr = 1,4-di(1,2,3-triazol-1-yl)butane, 0.01 ≤ x ≤ 1, show macroscopic light-induced bistability between the high-spin and the low-spin state. In the highly diluted system with x = 0.01 and up to x = 0.31, the photoinduced low-spin state always relaxes back to the high-spin state independent of the initial light-induced low-spin fraction. In the highly concentrated mixed crystals with x = 0.67, 0.87 and 1, the strong cooperative effects coupled to a crystallographic phase transition result in light-induced bistability with decreasing critical light-induced low-spin fraction and increasing hysteresis width for increasing iron(II) concentrations. The lower limit for the light-induced bistability is estimated at x ≈ 0.5.
  
  • Atomic shell structure from the Single-Exponential Decay Detector
    Piotr De Silva, Jacek Korchowiec and Tomasz A. Wesolowski
    The Journal of Chemical Physics, 140 (16) , 2014, p164301
    Keywords: scalar field theory; electron gas; wave functions; chemical bonds; electronic structure
    DOI:10.1063/1.4871501 | unige:39941 | Abstract | Article HTML | Article PDF
The density of atomic systems is analysed via the Single-Exponential Decay Detector (SEDD). SEDD is a scalar field designed to explore mathematical, rather than physical, properties of electron density. Nevertheless, it has been shown that SEDD can serve as a descriptor of bonding patterns in molecules as well as an indicator of atomic shells [P. de Silva, J. Korchowiec, and T. A. Wesolowski, ChemPhysChem13, 3462 (2012)]. In this work, a more detailed analysis of atomic shells is done for atoms in the Li–Xe series. Shell populations based on SEDD agree with the Aufbau principle even better than those obtained from the Electron Localization Function, which is a popular indicator of electron localization. A link between SEDD and the local wave vector is given, which provides a physical interpretation of SEDD.
Frozen-Density-Embedding Theory (FDET) is a formalism to obtain the upper bound of the ground-state energy of the total system and the corresponding embedded wavefunction by means of Euler-Lagrange equations [T. A. Wesolowski, Phys. Rev. A77(1), 012504 (2008)]. FDET provides the expression for the embedding potential as a functional of the electron density of the embedded species, electron density of the environment, and the field generated by other charges in the environment. Under certain conditions, FDET leads to the exact ground-state energy and density of the whole system. Following Perdew-Levy theorem on stationary states of the ground-state energy functional, the other-than-ground-state stationary states of the FDET energy functional correspond to excited states. In the present work, we analyze such use of other-than-ground-state embedded wavefunctions obtained in practical calculations, i.e., when the FDET embedding potential is approximated. Three computational approaches based on FDET, that assure self-consistent excitation energy and embedded wavefunction dealing with the issue of orthogonality of embedded wavefunctions for different states in a different manner, are proposed and discussed.
  • [CpRu]-Catalyzed Carbene Insertions into Epoxides: 1,4-Dioxene Synthesis through SN1-Like Chemistry with Retention of Configuration
    Thierry Achard, Cecilia Tortoreto, Amalia I. Poblador-Bahamonde, Laure GuĂŠnĂŠe, Thomas BĂźrgi and JĂŠrĂ´me Lacour
    Angewandte Chemie International Edition, 53 (24) , 2014, p6140-6144
    DOI:10.1002/anie.201402994 | unige:37229 | Abstract | Article HTML | Article PDF
  
Rather than lead to the usual deoxygenation pathway, metal carbenes derived from ι-diazo-β-ketoesters undergo three-atom insertions into epoxides using a combination of 1,10-phenanthroline and [CpRu(CH3CN)3][BArF] as the catalyst. Original 1,4-dioxene motifs are obtained as single regio- and stereoisomers. A perfect syn stereochemistry (retention, e.r. up to 97:3) is observed for the ring opening, which behaves as an SN1-like transformation.
  • Improved persistent luminescence of CaTiO3:Pr by fluorine substitution and thermochemical treatment
    Songhak Yoon, Eugenio H. Otal, Alexandra E. Maegli, Lassi Karvonen, Santhosh K. Matam, Stefan G. Ebbinghaus, Bernhard Walfort, Hans Hagemann, Simone Pokrant and Anke Weidenkaff
    Journal of Alloys and Compounds, 613 , 2014, p338-343
    Keywords: CaTiO3:Pr; fluorination; persistent luminescence; afterglow; X-ray diffraction
    DOI:10.1016/j.jallcom.2014.06.041 | unige:38546 | Abstract | Article HTML | Article PDF
Fluorine-substituted CaTiO3:Pr phosphors were prepared by a solid-state reaction. Rietveld refinements of powder X-ray diffraction patterns revealed that increasing fluorine-substitution leads to the gradual shrinkage of the unit-cell. Enhanced afterglow intensities were observed with fluorine-substitution. Furthermore, the effect of annealing atmosphere was investigated by thermochemical treatment in different atmospheres (Ar, air and NH3). UV-Vis diffuse reflectance spectra and photoluminescence excitation spectra revealed that Pr4+ in the pristine CaTi(O,F)3:Pr phosphor was partially reduced to Pr3+ under NH3 flow leading to an intensity improvement of ca. 450% compared to CaTiO3:Pr. The substantial improvement of afterglow intensity by fluorine substitution and annealing in NH3 is considered to be connected with the generation of oxygen vacancies and the partial reduction of Pr4+ to Pr3+.
  
  • Structural Information on the Au-S Interface of Thiolate-Protected Gold Clusters: A Raman Spectroscopy Study
    Birte Varnholt, Patric Oulevey, Sandra Luber, Chanaka Kumara, Amala Dass and Thomas BĂźrgi
    The Journal of Physical Chemistry C, 118 (18) , 2014, p9604-9611
    DOI:10.1021/jp502453q | unige:37937 | Abstract | Article HTML | Article PDF
The Raman spectra of a series of monolayer-protected gold clusters were investigated with special emphasis on the Au–S modes below 400 cm–1. These clusters contain monomeric (SR-Au-SR) and dimeric (SR-Au-SR-Au-SR) gold–thiolate staples in their surface. In particular, the Raman spectra of [Au25(2-PET)18]0/–, Au38(2-PET)24, Au40(2-PET)24, and Au144(2-PET)60 (2-PET = 2-phenylethylthiol) were measured in order to study the influence of the cluster size and therefore the composition with respect to the monomeric and dimeric staples. Additionally, spectra of Au25(2-PET)18–2x(S-/rac-BINAS)x (BINAS = 1,1′-binaphthyl-2,2′-dithiol), Au25(CamS)18 (CamS = 1R,4S-camphorthiol), and AunBINASm were measured to identify the influence of the thiolate ligand on the Au–S vibrations. The vibrational spectrum of Au38(SCH3)24 was calculated which allows the assignment of bands to vibrational modes of the different staple motifs. The spectra are sensitive to the size of the cluster and the nature of the ligand. Au–S–C bending around 200 cm–1 shifts to slightly higher wavenumbers for the dimeric as compared to the monomeric staples. Radial Au–S modes (250–325 cm–1) seem to be sensitive toward the staple composition and the bulkiness of the ligand, having higher intensities for long staples and shifting to higher wavenumbers for sterically more demanding ligands. The introduction of only one BINAS dithiol has a dramatic influence on the Au–S vibrations because the molecule bridges two staples which changes their vibrational properties completely.
 
The dynamics of photoinduced electron transfer between polar acceptors and donors has been investigated in apolar solvents using femtosecond-resolved fluorescence spectroscopy. It was found to be ultrafast and to continuously accelerate by varying the excitation wavelength from the maximum to the red edge of the absorption band of the acceptor, the overall difference being as large as a factor 4–5. This violation of the Kasha–Vavilov rule is explained by a correlation between the composition of the acceptor environment and its transition energy, that is, the more donors around an acceptor, the longer its absorption wavelength, and the faster the quenching. Because of preferential solvation, this dependence is already observed at low quencher concentrations. This effect, which requires quenching to be faster than the fluctuations of the environment composition, should be quite general for photoinduced charge transfer processes in low-polarity, viscous, or rigid media, such as those used in organic optoelectronic devices.
  
The excited-state dynamics of rhodamine 6G (R6G) has been investigated in aqueous solution using ultrafast transient absorption spectroscopy and at the dodecane/water interface using the femtosecond time-resolved surface second harmonic generation (SSHG) technique. As the R6G concentration exceeds ca. 1 mM in bulk water, both R6G monomers and aggregates are excited to a different extent when using pump pulses at 500 and 530 nm. The excited-state lifetime of the monomers is shortened compared to dilute solutions because of the occurrence of excitation energy transfer to the aggregates, which themselves decay nonradiatively to the ground state with a ca. 70 ps time constant. At the dodecane/water interface, both monomers and aggregates contribute to the SSHG signal to an extent that depends on the bulk concentration, the pump and probe wavelengths, and the polarization of probe and signal beams. The excited-state lifetime of the monomers at the interface is of the order of a few picoseconds even at bulk concentrations where it is as large as several nanoseconds. This is explained by the relatively high interfacial affinity of R6G that leads to a large interfacial concentration, favoring aggregation and thus rapid excitation energy transfer from monomers to aggregates.
 
Inorganic compounds with BH4- ions are the subject of many recent investigations in the context of potential hydrogen storage materials. In this work, Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectra of a series of reference and research compounds (including deuterated samples) are collected and made available to the research community.
  • Anisotropic magnetic, transport and thermodynamic properties of novel tetragonal Ce2RhGa12 compound
    S. Nallamuthu, T.P. Rashid, V. Krishnakumar, CĂŠline Besnard, Hans Hagemann, Marian Reiffers and R. Nagalakshmi
    Journal of Alloys and Compounds, 604 , 2014, p379-383
    Keywords: intermetallic; anisotropy; heavy fermions; heat capacity; magnetization
    DOI:10.1016/j.jallcom.2014.03.067 | unige:37941 | Article HTML
We report on a comprehensive study of the magnetization, resistivity and heat capacity on the single crystals of Ce2RhGa12 synthesized using Ga flux. Single crystal X-ray diffraction data confirm the tetragonal Pb/nbm structure of Ce2RhGa12, which is isostructural to Ce2PdGa12. Ce2RhGa12 orders antiferromagnetically at TN = 3.5 K and exhibits anisotropic magnetic behavior, inferred from the magnetization and resistivity data, taken along the two principal crystallographic directions of the crystal, viz., along [100] and [001]. The anisotropic magnetic response of Ce2RhGa12 establishes [001] as the easy axis of magnetization, and a weak meta-magnetic transition is also observed in the magnetic isotherm at 2K along the same axis. A sharp peak in specific heat signals the bulk antiferromagnetic transition at TN = 3.5 K, which shifts to lower temperatures in low applied fields. The electrical resistivity along the two directions shows metallic behavior from 300K down to 1.8K and establishes Ce2RhGa12 as a normal, trivalent cerium compound.
  
  • The role of ligand-field states in the ultrafast photophysical cycle of the prototypical iron(II) spin-crossover compound [Fe(ptz)6](BF4)2Very Important Paper
    Andrea Marino, Pradip Chakraborty, Marina Servol, Maciej Lorenc, Eric Collet and Andreas Hauser
    Angewandte Chemie International Edition, 53 (15) , 2014, p3863-3867
    Keywords: intersystem crossing;LIESST;ligand-field states;spin crossover;ultrafast spectroscopy
    DOI:10.1002/anie.201310884 | unige:38562 | Abstract | Article HTML | Article PDF
Light-induced excited spin state trapping (LIESST) in iron(II) spin-crossover compounds, i.e., the light-induced population of the high-spin (S=2) state below the thermal transition temperature, was discovered thirty years ago. For irradiation into metal-ligand charge transfer (MLCT) bands of the low-spin (S=0) species the acknowledged sequence takes the system from the initially excited 1MLCT to the high-spin state via the 3MLCT state within ~150 fs, thereby bypassing low-lying ligand-field (LF) states. Nevertheless, these play role, as borne out by the observation of LIESST and reverse-LIESST on irradiation directly into the LF bands for systems with only high-energy MLCT states. Herein we elucidate the ultrafast reverse-LIESST pathway by identifying the lowest energy S=1 LF state as intermediate state with a lifetime of 39 ps for the light-induced high-spin to low-spin conversion on irradiation into the spin-allowed LF transition of the high-spin species in the NIR.
 
The dynamics of bimolecular photoinduced electron-transfer reactions has been investigated with three donor/acceptor (D/A) pairs in tetrahydrofuran (THF) and acetonitrile (ACN) using a combination of ultrafast spectroscopic techniques, including time-resolved infrared absorption. For the D/A pairs with the highest driving force of electron transfer, all transient spectroscopic features can be unambiguously assigned to the excited reactant and the ionic products. For the pair with the lowest driving force, three additional transient infrared bands, more intense in THF than in ACN, with a time dependence that differs from those of the other bands are observed. From their frequency and solvent dependence, these bands can be assigned to an exciplex. Moreover, polarization-resolved measurements point to a relatively well-defined mutual orientation of the constituents and to a slower reorientational time compared to those of the individual reactants. Thanks to the minimal overlap of the infrared signature of all transient species in THF, a detailed reaction scheme including the relevant kinetic and thermodynamic parameters could be deduced for this pair. This analysis reveals that the formation and recombination of the ion pair occur almost exclusively via the exciplex.
  
  • Probing the Anisotropic Distortion of Photoexcited Spin Crossover Complexes with Picosecond X‑ray Absorption Spectroscopy
    Sophie E. Canton, Xiaoyi Zhang, LatĂŠvi M. Lawson Daku, Amanda L. Smeigh, Jianxin Zhang, Yizhu Liu, Carl-Johan Wallentin, Klaus Attenkofer, Guy Jennings, Charles A. Kurtz, David Gosztola, Kenneth Wärnmark, Andreas Hauser and Villy SundstrĂśm
    Journal of Physical Chemistry C, 118 (8) , 2014, p4536-4545
    DOI:10.1021/jp5003963 | unige:37983 | Abstract | Article HTML | Article PDF
For numerous spin crossover complexes, the anisotropic distortion of the first coordination shell around the transition metal center governs the dynamics of the high-spin/lowspin interconversion. However, this structural parameter remains elusive for samples that cannot be investigated with crystallography. The present work demonstrates how picosecond X-ray absorption spectroscopy is able to capture this specifi c deformation in the photoinduced high-spin state of solvated [Fe(terpy)2 ]2+ , a complex which belongs to the prominent family of spin crossover building blocks with nonequivalent metal− ligand bonds. The correlated changes in Fe−NAxial , Fe− NDistal , and bite angle NDistal− Fe− NAxial  extracted from the measurements are in very good agreement with those predicted by DFT calculations in D2d  symmetry. The outlined methodology is generally applicable to the characterization of ultrafast nuclear rearrangements around metal centers in photoactive molecular complexes and nanomaterials, including those that do not display long-range order.
 
The excited-state dynamics of two multichromophoric arrays composed of a naphthalene diimide centre and four zinc or free-base porphyrins substituted on the naphthalene core via aniline bridges has been investigated using a combination of stationary and ultrafast spectroscopies. These pentads act as efficient antennae as they absorb over the whole visible region, with a band around 700 nm, associated with a transition to the S1 state delocalised over the whole arrays, and bands at higher energy due to transitions centred on the porphyrins. In non-polar solvents, population of these porphyrin states is followed by sub-picosecond internal conversion to the S1 state. The existence of a charge-separated state located above the S1 state could enhance this process. The decay of the S1 state is dominated by non-radiative deactivation on the 100 ps timescale, most probably favoured by the small S1-S0 energy gap and the very high density of vibrational states of these very large chromophores. In polar solvents, the charge-separated state lies just below the S1 state. It can be populated within a few picoseconds by a thermally-activated hole transfer from the S1 state as well as via sub-picosecond non-equilibrium electron transfer from vibrationally hot porphyrin excited states. Because of the small energy gap between the charge-separated state and the ground state, charge recombination is almost barrierless and occurs within a few picoseconds. Despite their very different driving forces, charge separation and recombination occur on similar timescales. This is explained by the electronic coupling that differs considerably for both processes.
  
Retinal is the light-absorbing biochromophore responsible for the activation of vision pigments and light-driven ion-pumps. Nature has evolved molecular tuning mechanisms that significantly shift the optical properties of the retinal pigments to enable their absorption of visible light. Using large-scale quantum chemical calculations at the density functional theory level combined with the frozen density embedding theory approach, we show here how the protein environment of vision pigments tune the absorption of retinal to the 2.3-2.6 eV (480-530 nm) region by electrostatically dominated interactions between the chromophore and the surrounding protein residues. The calculations accurately reproduce the experimental absorption maxima of rhodopsin (2.49 eV/498 nm), and the red, green, and blue color pigments (2.3-2.9 eV/430-530 nm). We further identify key interactions responsible for the red- and blue-shifting effects by mutating the rhodopsin structure in silico, and find that deprotonation of the retinyl is likely to be responsible for the blue shifted absorption in the blue cone vision pigment.
  • Oxadiazole based bipolar host materials employing planarized triarylamine donors for RGB PHOLEDs with low efficiency roll-off
    Paul Kautny, Daniel Lumpi, Yanping Wang, Antoine Tissot, Johannes Bintinger, Ernst Horkel, Berthold Stoeger, Christian Hametner, Hans Hagemann, Dongge Ma and Johannes FrĂśhlich
    Journal of Materials Chemistry C, 2 (11) , 2014, p2069-2081
    DOI:10.1039/c3tc32338b | unige:34399 | Abstract | Article HTML | Article PDF
  
A series of 6 novel triarylamine-containing oxadiazole compounds (o-PCzPOXD, o-ICzPOXD, o-TPATOXD, o-PCzTOXD, o-ICzTOXD, o-CzTOXD) have been designed, synthesized and characterized concerning applications as host materials in PHOLED devices. To further improve the ortho-linkage concept, the impact of incorporating planarized electron-donating triarylamine (TAA) structures on intramolecular charge transfer was examined. The effect was evaluated for two series of electron-accepting oxadiazole scaffolds, realizing ortho-linkage on the benzene (POXD) and the thiophene (TOXD) core. Thermal analysis shows increased glass-transition temperatures for planarized structures indicating an improved morphological stability. A higher degree of planarization also results in significantly increased singlet and triplet energy values, revealing the impact on the intramolecular charge transfer. Employing the developed materials, red (o-TPATOXD: CEmax: 28.8 cd A-1, EQEmax: 16.9%), green (o-PCzPOXD: CEmax: 62.9 cd A-1, EQEmax: 17.1%) and blue (o-PCzPOXD: CEmax: 29.8 cd A-1, EQEmax: 13.4%) devices were achieved showing remarkably low efficiency roll-off for planarized donors. Hence, this is the first report of efficient blue devices for this specific class of host materials. It is proposed that the results correlate with an increasing ortho-linkage effect and decreasing donor strength of the TAA moiety by planarization and, thus, tackling one of the major challenges in PHOLED research: improving both triplet energy and compound stability.
  
To access the intrinsic, diffusion free, rate constant of bimolecular photoinduced electron transfer reactions, fluorescence quenching experiments have been performed with 14 donor/acceptor pairs, covering a driving-force range going from 0.6 to 2.4 eV, using steady-state and femtosecond time-resolved emission, and applying a diffusion-reaction model that accounts for the static and transient stages of the quenching for the analysis. The intrinsic electron transfer rate constants are up to 2 orders of magnitude larger than the diffusion rate constant in acetonitrile. Above ~1.5 eV, a slight decrease of the rate constant is observed, pointing to a much weaker Marcus inverted region than those reported for other types of electron transfer reactions, such as charge recombination. Despite this, the driving force dependence can be rationalized in terms of Marcus theory.
  • Vapor pressure measurements of Mg(BH4)2 using Knudsen torsion effusion thermo graphic method
    L.-N.N. Nforbi, A. Talekar, K.H. Lau, R. Chellapa, W.-M. Chien, D. Chandra, H. Hagemann, Y. Filinchuk, J.-C. Zhao and Andre Levchenko
    International Journal of Hydrogen Energy, 39 (5) , 2014, p2175-2186
    Keywords: Mg(BH4)2; hydrogen desorption under dynamic vacuum; torsion effusion vapor pressure measurements; vaporization thermodynamics
    DOI:10.1016/j.ijhydene.2013.11.071 | unige:33261 | Abstract | Article PDF
  
The vapor pressure and molecular weight of effusing vapors of α, β, and amorphous Mg(BH4)2 were determined by Torsion-effusion gravimetric method, under dynamic vacuum. A Cahn balance in the system yielded the rate of the weight loss. Molecular weights measured revealed if the effusion was congruent or there was disproportionation. The vaporization behavior of crystalline Mg(BH4)2, was measured up to 533 K at pressures of ∼10−5 torr. It was found that Mg(BH4)2 disproportionates to form predominantly H2 gas (∼95%) with a small amount of Mg(BH4)2 (∼5%) in the gas phase. The combined average molecular weight measured is 4.16 g/mol. The equations for vapor pressures for crystalline Mg(BH4)2 are given by: log PTotal(bar) = 9.2303 − 7286.2/T, log PMg(BH4)2 (bar) = 8.2515 - 7286.2 / T , and log PH2 (bar) = 9.1821 - 7286.2 / T. The partial pressures of the gaseous species were determined as PMg2(4BH)(g)/PT=0.105 and PH2(g)/PT=0.895. Enthalpies of vaporization for the effusing gases were calculated to be ΔH = +558.0 kJ/mol H2 and ΔH = +135 kJ/mol Mg(BH4)2. The standard Gibbs free energy changes, ΔG°(kJ/mol), for the complete decomposition reaction (Mg(BH4)2(s) → Mg(s) + 2B(s) + 4H2(g)), sublimation reaction (Mg(BH4)2(s) → Mg(BH4)2(g)) and the disproportionation reaction for Mg(BH4)2 are reported in this paper. The decomposition pathway of amorphous Mg(BH4)2 was also carried out between 388.2 K and 712.8 K showing multistep decomposition of a-Mg(BH4)2 Different reaction products were obtained depending on the method used in the vaporization experiment. The behavior of the amorphous Mg(BH4)2(s) is very different from those for the two crystalline phases (α and β). The vapor pressure behavior and thermodynamics of vaporization of different phases of Mg(BH4)2 are presented.
  • How to choose the frozen density in Frozen-Density Embedding Theory-based numerical simulations of local excitations?
    Marie Humbert-Droz, Xiuwen Zhou, Sapana V. Shedge and Tomasz A. Wesolowski
    Theoretical Chemistry Accounts, 133 (1) , 2014, p1405
    Keywords: frozen-density; embedding theory; linear-response; time-dependent density functional
    DOI:10.1007/s00214-013-1405-1 | unige:33263 | Abstract | Article HTML | Article PDF
According to Frozen-Density Embedding Theory, any observable evaluated for the embedded species is a functional of the frozen density (ρB —the density associated with the environment). The environment-induced shifts in the energies of local excitations in organic chromophores embedded in hydrogen-bonded environments are analyzed. The excitation energies obtained for ρB , which is derived from ground-state calculations for the whole environment applying medium quality basis sets (STO–DZP) or larger, vary in a narrow range (about 0.02 eV which is at least one order of magnitude less than the magnitude of the shift). At the same time, the ground-state dipole moment of the environment varies significantly. The lack of correlation between the calculated shift and the dipole moment of the environment reflects the fact that, in Frozen-Density Embedding Theory, the partitioning of the total density is not unique. As a consequence, such concepts as “environment polarization” are not well defined within Frozen-Density Embedding Theory. Other strategies to generate ρB (superposition of densities of atoms/molecules in the environment) are shown to be less robust for simulating excitation energy shifts for chromophores in environments comprising hydrogen-bonded molecules.
  • Large π-Conjugated Chromophores Derived from Tetrathiafulvalene
    Hongpeng Jia, Jie Ding, Andreas Hauser, Silvio Decurtins and Shi-Xia Liu
    Asian Journal of Organic Chemistry, 3 (2) , 2014, p198-202
    Keywords: dipyrido[3,2-a:2',3'-c]phenazines;photophysics;redox chemistry;ruthenium;tetrathiafulvalenes
    DOI:10.1002/ajoc.201300144 | unige:34423 | Abstract | Article HTML | Article PDF
A large pi-conjugated chromophore composed of two dipyrido[3,2-a:2’,3’-c]phenazine (dppz) units directly fused to the central tetrathiafulvalene (TTF) core, has been prepared as a bridging ligand, and its strong binding ability to Ru2+ forming a new dinuclear complex is presented. The electronic absorption and luminescence and the electrochemical behaviour of the free ligand as well as the Ru2+ complex have been investigated in detail. The free ligand shows a very strong band in the UV region consistent with ligand centred π-π* transitions and an intense broad band in the visible region corresponding to an intramolecular charge transfer (ILCT) transition. Upon coordination, a metal-to-ligand charge transfer (MLCT) appears at 22520 cm-1 while the ILCT band is bathochromically shifted by 1620 cm-1. These electrochemically amphoteric chromophores have also been characterized by spectroelectrochemical methods. The oxidized radical species of the free ligand show a strong tendency to undergo aggregation, in which long-distance attractive interactions overcome the electrostatic repulsion. Moreover, these two new chromophores reveal an ILCT fluorescence with large solvent-dependent Stokes shifts and quantum efficiencies of 0.052 for the free ligand and 0.016 for its dinuclear Ru2+ complex in CH2Cl2.
  • Discriminability of tryptophan containing dipeptides using quantum control
    S. Afonina, O. Nenadl, A. Rondi, L. Bonacina, J. Extermann, D. Kiselev, I. Dolamic, T. Burgi and J.P. Wolf
    Applied Physics B, 111 (4) , 2013, p541-549
    DOI:10.1007/s00340-013-5370-0 | unige:37054 | Abstract | Article HTML | Article PDF
We show that the coherent manipulation of molecular wavepackets in the excited states of trp-containing dipeptides allows efficient discrimination among them. Optimal dynamic discrimination fails, however, for some dipeptide couples. When considering the limited spectral resources at play (3 nm bandwidth at 266 nm), we discuss the concept of discriminability, which appears uncorrelated to both static spectra and relaxation lifetimes.
  • Model-free Investigation of Ultrafast Bimolecular Chemical Reactions: Bimolecular Photo Induced Electron TransferOpen access paper
    Bernhard Lang, Arnulf Rosspeintner and Eric Vauthey
    EPJ Web of Conferences, 41 , 2013, p5041
    DOI:10.1051/epjconf/20134105041 | unige:94113 | Abstract | Article PDF
Using photoinduced bimolecular electron transfer reactions as example we demonstrate how diffusion controlled bimolecular chemical reactions can be studied in a model-free manner by quantitatively combining different ultrafast spectroscopical tools.
Attenuated total reflection infrared (ATR-IR) spectroscopy is used to study the adsorption of gold and silver nanoparticles and the layer-by-layer (LBL) growth of polyelectrolyte multilayers on a Ge ATR crystal. The Ge ATR crystal is first functionalized using positively charged polyelectrolyte poly(allylamine hydrochloride) (PAH). Then citrate-stabilized gold or silver nanoparticles are adsorbed onto the modified Ge ATR crystal. When gold or silver nanoparticles are adsorbed, a drastic increase of the water signal is observed which is attributed to an enhanced absorption of IR radiation near the nanoparticles. This enhancement was much larger for the silver nanoparticles (SNP). On top of the nanoparticles multilayers of oppositely charged polyelectrolytes PAH and poly(sodium 4-styrenesulfonate) (PSS) were deposited, which allowed to study the enhancement of the IR signals as a function of the distance from the nanoparticles. Furthermore, adsorption of a thiol, N-acetyl-l-cysteine, on the nanoparticles confirmed the enhancement. In the case of SNP an absorbance signal of about 15% was observed, which is a factor of about 40 times larger compared to typical signals measure without nanoparticles.
  • Modeling Transition Metal Complexes in the Frameworkof the Spin-Crossover Phenomenon: a DFT Perspective
    Latevi Max Lawson Daku
    Current Inorganic Chemistry, 3 (3) , 2013, p242-259
    Keywords: density functional theory, spin crossover, transition metal complexes
    DOI:10.2174/1877944103666140110231029 | unige:73072
Using the study of the low-spin complex [Fe(bpy)3]2+ in the gas phase and in condensed phases as a guideline, we examine different aspects of the application of DFT to the study of transition metal complexes in the framework of spin crossover or related phenomena.
  • Crystal structure solution of an elusive polymorph of Dibenzylsquaramide
    Anna Portell, Xavier AlcobĂŠ, LatĂŠvi M. Lawson Daku, Radovan Cerny and Rafel Prohens
    Powder Diffraction, 28 (S2) , 2013, p470-480
    Keywords: dibenzylsquaramide; crystal structure; X-ray powder diffraction
    DOI:10.1017/S0885715613000821 | unige:35159 | Abstract | Article PDF
The crystal structure of the third polymorph of dibenzylsquaramide (Portell, A. et al., 2009), (fig. 1) has been determined from laboratory X-ray powder diffraction data by means of direct space methods using the computing program FOX. (Favre-Nicolin and Černý, 2002) The structure resolution has not been straightforward due to several difficulties on the indexing process and in the space group assignment. The asymmetric unit contains two different conformers, which has implied an additional difficulty during the Rietveld (Rietveld, 1969) refinement. All these issues together with particular structural features of disquaramides are discussed.
  
  • Near-Infrared to Visible Light-Upconversion in Molecules: From Dream to Reality
    Yan Suffren, Davood Zare, Svetlana V. Eliseeva, Laure GuĂŠnĂŠe, Homayoun Nozary, TimothĂŠe Lathion, Lilit Aboshyan-Sorgho, StĂŠphane Petoud, Andreas Hauser and Claude Piguet
    Journal of Physical Chemistry C, 117 (51) , 2013, p26957-26963
    DOI:10.1021/jp4107519 | unige:34037 | Abstract | Article HTML | Article PDF
Light-upconversion via stepwise energy transfer from a sensitizer to an activator exploits linear optics for converting low-energy infrared or near-infrared incident photons to higher energy emission occurring in the part of the electromagnetic spectrum ranging from visible to ultraviolet. Stepwise excitation is restricted to activators possessing intermediate long-lived excited states such as those found for trivalent lanthanide cations dispersed in solid-state matrices. When the activator is embedded in a molecular complex, efficient non-radiative relaxation processes usually reduce excited state lifetimes to such an extent that upconversion becomes too inefficient to be detected under practical excitation intensities. Theoretical considerations suggest that the combination of millisecond timescale sensitizers with a central lanthanide activator located in supramolecular complexes circumvents this bottleneck by creating a novel pathway reminiscent of the energy transfer upconversion mechanism observed in doped solids. Application of this novel concept to chromium/erbium pairs in discrete triple-stranded helicates demonstrates that strong-field trivalent chromium chromophores irradiated with near-infrared photons produce upconverted green erbium-centered emission both in the solid state and in solution.
  • A Pt(II) complex with both a phenanthroline and a tetrathiafulvalene-extended dithiolate ligand: Synthesis, crystal structure, electro-chemical and spectroscopic properties
    Chunyang Jia, Jie Ding, Shi-Xia Liu, GaĂŤl Labat, Antonia Neels, Andreas Hauser and Silvio Decurtins
    Polyhedron, 55 , 2013, p87-91
    Keywords: tetrathiafulvalene; intramolecular charge-transfer; spectroelectrochemistry
    DOI:10.1016/j.poly.2013.02.064 | unige:32100 | Article HTML | Article PDF
  
The reaction of 4,5-bis(2'-cyano-ethylsulfanyl)-4',5'-dipropylthiotetrathiafulvalene with [Pt(phen)Cl2] (phen = 1,10-phenanthroline) with CsOH as base in CH3OH–THF affords the target complex 1 in 44% yield. This complex crystallizes in the monoclinic space group P21/c, M = 790.01, a = 12.1732(12), b = 15.851(2), c = 14.5371(16) Å, b = 107.693(12)˚, V = 2672.4(5) Å3 and Z = 4. It undergoes two reversible single-electron oxidation and two irreversible reduction processes. An intense electronic absorption band at 15200 cm-1 (658 nm) in CH2Cl2 is assigned to the intramolecular mixed metal/ligand-to-ligand charge transfer (LLCT) from a tetrathiafulvalene-extended dithiolate-based HOMO to a phenanthroline-based LUMO. This band shifts hypsochromically with increasing solvent polarity. Systematic changes in the optical spectra upon oxidation allow precise tuning of the oxidation states of 1 and reversible control over its optical properties. Irradiation of 1 at 15625 cm-1 (640 nm) in glassy solution below 150 K results in emission from the 3LLCT excited state.
  
  • Ultrafast Excited-State Dynamics of Donor-Acceptor Biaryls: Comparison between Pyridinium and Pyrylium Phenolates
    Romain Letrun, Marius Koch, Marina L. Dekhtyar, Vladimir V. Kurdyukov, Alexei I. Tolmachev, Wolfgang Rettig and Eric Vauthey
    The Journal of Physical Chemistry A, 117 (49) , 2013, p13112-13126
    DOI:10.1021/jp409646g | unige:32101 | Abstract | Article HTML | Article PDF
The excited-state dynamics of two donor–acceptor biaryls that differ by the strength of the acceptor, a pyridinium or a pyrylium moiety, have been investigated using a combination of steady-state solvatochromic absorption, ultrafast fluorescence, as well as visible and infrared transient absorption spectroscopies. The negative solvatochromic behavior of pyridinium phenolate indicates that the permanent electric dipole moment experiences a decrease upon S1 ← S0 excitation, implying that the ground state possesses more zwitterionic character than the excited state. In contrast, pyrylium phenolate exhibits a weakly positive solvatochromic behavior corresponding to a small increase in the dipole moment upon excitation, implying more zwitterionic character in the excited than the ground state. Both compounds are therefore situated at different sides of the cyanine-limit structure, which has equally polar ground and excited states. Despite these differences, both molecules exhibit qualitatively similar excited-state properties. They are characterized by a very short fluorescence lifetime, increasing from about 1 to 20 ps, when varying solvent viscosity from 0.4 to 11 cP. There are, however, characteristic differences between the two compounds: The excited-state lifetimes of the pyrylium dye are shorter and also depend somewhat on polarity. The ensemble of spectroscopic data can be explained with a model where the emitting Franck–Condon excited state relaxes upon twisting around the single bond between the aryl units to a point where the excited- and ground-state surfaces are very close or intersect. After internal conversion to the ground state, the distorted molecule relaxes back to its equilibrium planar configuration, again largely dependent upon solvent viscosity. However, in this case, the kinetics for the pyrylium dye are slower than for the pyridinium dye and the polar solvent-induced acceleration is significantly stronger than in the excited state. This difference of kinetic behavior between the two compounds is a direct consequence of the change of the electronic structure from anormal to an overcritical merocyanine evidenced by steady-state spectroscopy.
  • Excited-State Dynamics of Charged Dyes at Alkane/Water Interfaces in the Presence of Salts and Ionic Surfactants
    Marina Fedoseeva, Piotr Fita and Eric Vauthey
    Langmuir, 29 (48) , 2013, p14865-14872
    DOI:10.1021/la402191p | unige:32123 | Abstract | Article HTML | Article PDF
  
The excited-state dynamics of the cationic dye malachite green (MG) and of the dianionic dye eosin B at the dodecane/water interface has been investigated using femtosecond time-resolved surface second harmonic generation (TR-SSHG). By using different probe wavelengths, the contributions of monomeric and aggregated MG to the signal could be spectroscopically distinguished. The effect of the addition of a small amount of surfactants was found to strongly depend on the relative charges of surfactant and dye. For surfactant/dye pairs with opposite charges, the TR-SSHG signal is dominated by the contribution from aggregates, whereas for pairs with the same charges, the signal intensity becomes vanishingly small. These effects are explained in terms of electrostatic interactions between surfactants and dyes that favor either attraction of the dye toward the interface or its repulsion toward the bulk. As a very similar behavior is observed with MG upon addition of NaSCN, we conclude that, in this case, this effect reflects the affinity of SCNÂŻ for the interface. On the other hand, the guanidinium cation was found to have a different effect than that of a positively charged surfactant on the SSHG signal of MG, indicating this cation does not accumulate in the interfacial region.
  • Acylgermanes: Photoinitiators and Sources for Ge-Centered Radicals. Insights into their Reactivity
    Dmytro Neshchadin, Arnulf Rosspeintner, Markus Griesser, Bernhard Lang, Sandra Mosquera-Vazquez, Eric Vauthey, Vitaly Gorelik, Robert Liska, Christian Hametner, Beate Ganster, Robert Saf, Norbert Moszner and Georg Gescheidt
    Journal of the American Chemical Society, 135 (46) , 2013, p17314-17321
    DOI:10.1021/ja404433u | unige:31278 | Abstract | Article HTML | Article PDF
Acylgermanes have been shown to act as efficient photoinitiators. In this investigation we show how dibenzoyldiethylgermane 1 reacts upon photoexcitation. Our real-time investigation utilizes femto- and nanosecond transient absorption, time-resolved EPR (50 ns), photo-chemically induced dynamic nuclear polarization, DFT calculations, and GC-MS analysis. The benzoyldiethylgermyl radical G• is formed via the triplet state of parent 1. On the nanosecond time scale this radical can recombine or undergo hydrogen-transfer reactions. Radical G• reacts with butyl acrylate at a rate of 1.2 ± 0.1 × 108 and 3.2 ± 0.2 × 108 M–1 s–1, in toluene and acetonitrile, respectively. This is ˜1 order of magnitude faster than related phosphorus-based radicals. The initial germyl and benzoyl radicals undergo follow-up reactions leading to oligomers comprising Ge–O bonds. LC-NMR analysis of photocured mixtures containing 1 and the sterically hindered acrylate 3,3-dimethyl-2-methylenebutanoate reveals that the products formed in the course of a polymerization are consistent with the intermediates established at short time scales.
  
The far infrared spectra of a series of well-defined gold clusters covered by 2-phenylethanetiolate were studied. The spectra of the cluster are different but the differences are subtle. The Au-S stretching vibrations give rise to bands around 300 cm-1 and below. The relative intensity of these bands changes but they shift only slightly for different clusters. A low-frequency band was identified that is sensitive to the conformation (trans / gauche) of the 2-phenylethanetiolate ligand.
 
We describe the experimental investigation of time-resolved magnetic field effects in exciplex-forming organic donor–acceptor systems. In these systems, the photoexcited acceptor state is predominantly deactivated by bimolecular electron transfer reactions (yielding radical ion pairs) or by direct exciplex formation. The delayed fluorescence emitted by the exciplex is magnetosensitive if the reaction pathway involves loose radical ion pair states. This magnetic field effect results from the coherent interconversion between the electronic singlet and triplet radical ion pair states as described by the radical pair mechanism. By monitoring the changes in the exciplex luminescence intensity when applying external magnetic fields, details of the reaction mechanism can be elucidated. In this work we present results obtained with the fluorophore-quencher pair 9,10-dimethylanthracene/N,N-dimethylaniline (DMA) in solvents of systematically varied permittivity. A simple theoretical model is introduced that allows discriminating the initial state of quenching, viz., the loose ion pair and the exciplex, based on the time-resolved magnetic field effect. The approach is validated by applying it to the isotopologous fluorophore-quencher pairs pyrene/DMA and pyrene-d10/DMA. We detect that both the exciplex and the radical ion pair are formed during the initial quenching stage. Upon increasing the solvent polarity, the relative importance of the distant electron transfer quenching increases. However, even in comparably polar media, the exciplex pathway remains remarkably significant. We discuss our results in relation to recent findings on the involvement of exciplexes in photoinduced electron transfer reactions.
  
  • Ligand exchange Reaction on Au38(SR)24, Separation of Au38(SR)23(SR')1 Regioisomers and Migration of Thiolates
    Lule Beqa, Damien Dechamps, Stephane Perrio, Annie-Claude Gaumont, Stefan Knoppe and Thomas BĂźrgi
    The Journal of Physical Chemistry C, 117 (41) , 2013, p21619-21625
    DOI:10.1021/jp408455x | unige:30763 | Abstract | Article HTML | Article PDF
The ligand exchange reaction between Au38(2-PET)24 (2-PET: 2-phenylethanethiolate) clusters and enantiopure planar chiral [2.2]paracyclophane-4-thiol 1 (PCP-4-SH) was studied using High Performance Liquid Chromatography (HPLC) and mass spectrometry. It is shown that even at the initial stage of the reaction at least three out of the four symmetry-unique sites are exchanged leading to different regioisomers of composition Au38(2-PET)23(PCP-4-S)1. Using HPLC it was possible to isolate one specific regioisomer. The latter is stable at room temperature and at slightly elevated temperatures. However, at 80° C the adsorbed thiolate (PCP-4-S) moves between different symmetry-unique sites. These observations have implications for the preparation of mixed ligand shell clusters with specific ligand patterns.
  • Molecular Dynamics Simulations of Liquid Phase Interfaces: Understanding the Structure of the Glycerol/Water–Dodecane System
    Frank R. Beierlein, Andreas M. Krause, Christof M. Jäger, Piotr Fita, Eric Vauthey and Timothy Clark
    Langmuir, 29 (38) , 2013, p11898-11907
    DOI:10.1021/la4021355 | unige:30152 | Abstract | Article HTML | Article PDF
Modern spectroscopic techniques such as time-resolved second-harmonic-generation spectroscopy allow molecules to be examined selectively directly at phase interfaces. Two-phase systems formed by glycerol/water and alkane layers have previously been studied by time-resolved second-harmonic-generation spectroscopic measurements. In this molecular dynamics study, a triphenylmethane dye was inserted at the glycerol/water–alkane interface and was used as a probe for local properties such as viscosity. We now show how extensive simulations over a wide range of concentrations can be used to obtain a detailed view of the molecular structure at the glycerol/water–alkane interface. Glycerol is accumulated in a double layer adjacent to the alkane interface, which results in increased viscosity of the glycerol/water phase in the direct vicinity of the interface. We also show that conformational ensembles created by classical molecular-dynamics simulations can serve as input for QM/MM calculations, yielding further information such as transition dipoles, which can be compared with spectroscopic measurements.
  • Structures and Chiroptical Properties of the BINAS-monosubstituted Au38(SCH3)24 cluster
    Bertha Molina, Ariadna SĂĄnchez-Castillo, Stefan Knoppe, Ignacio L Garzon, Thomas BĂźrgi and Alfredo Tlahuice-Flores
    Nanoscale, 5 (22) , 2013, p10956-10962
    DOI:10.1039/c3nr03403h | unige:31281 | Abstract | Article HTML | Article PDF
  
The structure and optical properties of a set of R-1,1´-binaphthyl-2,2´-dithiol (R-BINAS) monosubstituted A-Au38(SCH3)24 clusters are studied by means of time dependent density functional theory (TD-DFT). While it was proposed earlier that BINAS selectively binds to monomer motifs (SR-Au-SR) covering the Au23 core, our calculations suggest a binding mode that bridges two dimer (SR-Au-SR-Au-RS) motifs. The more stable isomers show a negligible distortion induced by BINAS adsorption on the Au38(SCH3)24 cluster which is reflected by similar optical and Circular Dichroism (CD) spectra to those found for the parent cluster. The results furthermore show that BINAS adsorption does not enhance the CD signals of the Au38(SCH3)24 cluster.
  • Electronic Structure and Optical Properties of the Thiolate-Protected Au28(SMe)20 Cluster
    Stefan Knoppe, * Sami Malola, Lauri Lehtovaara, Thomas BĂźrgi and Hannu Häkkinen
    Journal of Physical Chemistry A, 117 (40) , 2013, p10526-10533
    DOI:10.1021/jp407494v | unige:30762 | Abstract | Article HTML | Article PDF
The recently reported crystal structure of the Au28(TBBT)20 cluster (TBBT: para-tert-butylbenzenethiolate) is analyzed with (Time-Dependent-) Density Functional Theory (TD-DFT). Bader charge analysis reveals a novel trimeric Au3(SR)4 binding motif. The cluster can be formulated as Au14(Au2(SR)3)4(Au3(SR)4)2. The electronic structure of the Au146+ core and the ligand-protected cluster were analyzed and their stability can be explained by formation of distorted eight-electron superatoms. Optical absorption and Circular Dichroism (CD) spectra were calculated and compared to the experiment. Assignment of handedness of the intrinsically chiral cluster is possible.
  
  • Experimental Evidence of Ultrafast Quenching of the 3MLCT Luminescence in Ruthenium(II) Tris-bipyridyl Complexes via a 3dd State
    Qinchao Sun, Sandra Mosquera-Vazquez, Latevi Max Lawson Daku, Laure GuĂŠnĂŠe, Harold A. Goodwin, Eric Vauthey and Andreas Hauser
    Journal of the American Chemical Society, 135 (37) , 2013, p13660-13663
    DOI:10.1021/ja407225t | unige:29641 | Abstract | Article HTML | Article PDF
Ultrafast transient absorption spectroscopy serves to identify the 3dd state as intermediate quencher state of the 3MLCT luminescence in the non-luminescent ruthenium complexes [Ru(m-bpy)3]2+ (m-bpy = 6-methyl-2,2′-bipyridine) and [Ru(tm-bpy)3]2+ (tm-bpy = 4,4′,6,6′-tetramethyl-2′,2′-bipyridine). For [Ru(m-bpy)3]2+, the population of the 3dd state from the 3MLCT state occurs within 1.6 ps, while the return to the ground state takes 450 ps. For [Ru(tm-bpy)3]2+, the corresponding values are 0.16 and 7.5 ps, respectively. According to DFT calculations, methyl groups added in the 6 and 6′ positions of bipyridine stabilize the 3dd state by ∼4000 cm–1 each, compared to [Ru(bpy)3]2+.
  • Hydrogen-fluorine exchange in NaBH4-NaBF4
    Line Rude, Uffe Filso, Vincenza D'Anna, Alexandra Spyratou Stratmann, Bo Richter, Satoshi Hino, Olena Zavorotynska, Marcello Baricco, Magnus H. Sørby, Bjorn C. Hauback, Hans Hagemann, Flemming Besenbacher, Jørgen Skibsted and Torben R. Jensen
    Physical Chemistry Chemical Physics, 15 , 2013, p18185-18194
    DOI:10.1039/c3cp52815d | unige:30153 | Abstract | Article PDF
  
Hydrogen-fluorine exchange in the NaBH4–NaBF4 system is investigated with a range of experimental methods combined with DFT calculations and a possible mechanism for the reactions is proposed. Fluorine substitution is observed by in-situ synchrotron radiation powder X-ray diffraction (SR-PXD) as a new Rock salt type compound with idealized composition NaBF2H2 in the temperature range T = 200 to 215 °C. Combined use of solid-state 19F MAS NMR, FT-IR and DFT calculations supports the formation of a BF2H2− complex ion, reproducing the observation of a 19F chemical shift at 144.2 ppm, which is different from that of NaBF4 at 159.2 ppm, along with the new absorption bands observed in the IR spectra. After further heating, the fluorine substituted compound becomes X-ray amorphous and decomposes to NaF at ~310 ºC. This work shows that fluorine-substituted borohydrides tend to decompose to more stable compounds, e.g. NaF, BF3 or amorphous products such as closo-boranes, e.g. Na2B12H12. The NaBH4-NaBF4 composite decomposes at lower temperatures (300 °C) compared to NaBH4 (476 °C), as observed by thermogravimetric analysis. NaBH4-NaBF4 (1:0.5) preserves 30 % of the hydrogen storage capacity after three hydrogen release and uptake cycles compared to 8 % for NaBH4 measured by the Sievert’s method under identical conditions, but more than 50 % using prolonged hydrogen absorption time. The reversible hydrogen storage capacity tends to decrease possibly due to the formation of NaF and Na2B12H12. On the other hand, the additive sodium fluoride appears to facilitate hydrogen uptake, prevent foaming, phase segregation and loss of material from the sample container for samples of NaBH4-NaF.
  
A major problem in the extraction of the reaction probability in bimolecular processes is the disentanglement from the influence of molecular diffusion. One of the strategies to overcome it makes use of reactive solvents in which the reactants do not need to diffuse to encounter each other. However, most of our quantitative understanding of chemical reactions in solution between free partners is based on the assumption that they can be approximated by spheres because rotation averages their mutual orientations. This condition may not be fulfilled when the reaction takes place on time scales faster than that of molecular reorientation. In this work, the fluorescence quenching of two very similar polyaromatic hydrocarbons with different electric dipole moments is measured. The concentration of a liquid electron-donating quencher is varied from very dilute solutions to pure quencher solutions. In both cases, the thermodynamics of the reactions are very similar and, according to the Marcus expression, the kinetics are expected to proceed at similar rates. However, one of them is 10 times faster in the pure quencher solution. This difference starts at relatively low quencher concentrations. An explanation based on the fluorophore–solvent dipole–dipole interaction and the consequent orientational solvent structure is provided. The orientational correlation between fluorophore and quencher is calculated by means of computer simulations. Important differences depending on the fluorophore dipole moment are found. The kinetics can be explained quantitatively with a reaction–diffusion model that incorporates the effects of the presence of the dipole moment and the rotational diffusion, only in the highest quencher concentration case, but not in dilute solutions, most likely due to fundamental limitations of the kinetic theory.
The ligand exchange reaction between monodisperse Au25(2-PET)18 (2-PET: 2-phenylethylthiolate) clusters and 1,1′-binaphthyl-2,2′-dithiol (BINAS) was long thought to induce decomposition of the cluster (Si et al., J. Phys. Chem. C, 2009). We repeated the experiment and analyzed the reaction products using MALDI-TOF mass spectrometry. The spectra clearly indicate successful ligand exchange, bidentate binding of the BINAS ligand and intact Au25 clusters. The reaction products are identified as Au25(2-PET)18−2x(BINAS)x (x = 1–4) for a 24 h reaction with a 50-fold molar excess of BINAS. Two likely binding motifs are discussed. Analysis of atomic distances in both the cluster and the free ligand indicates interstaple binding connecting the central sulfur atom of the protecting (SRAu)2SR with the outer sulfur atom of a second unit. The results presented have implications on the binding position of BINAS in Au38(SR)24−2x(BINAS)x clusters.
  • On the flexibility of the gold-thiolate interface: Racemization of the Au40(SR)24 cluster
    Birte Varnholt, Igor Dolamic, Stefan Knoppe and Thomas BĂźrgi
    Nanoscale, 5 , 2013, p9568-9571
    DOI:10.1039/c3nr03389a | unige:30154 | Abstract | Article PDF
  
The two enantiomers of the Au40(2-PET)24 cluster were collected using HPLC and analyzed by MALDI-TOF mass spectrometry, UV-vis- and CD-spectroscopy. The flexibility of the cluster surface allows racemization of the intrinsically chiral cluster at elevated temperatures (80 – 130 °C) which was monitored following the optical activity. The determined activation energy (25 kcal/mol) lies in the range of previously reported values for Au38 nanoclusters whereas the activation entropy deviates significantly from the one in Au38. The latter may indicate that the racemization can take place via different mechanisms.
A new multiconfigurational quantum chemical method, SplitGAS, is presented. The configuration interaction expansion, generated from a generalized active space, GAS, wave function is split in two parts, a principal part containing the most relevant configurations and an extended part containing less relevant, but not negligible, configurations. The partition is based on an orbital criterion. The SplitGAS method has been employed to study the HF, N2, and Cr2 molecules. The results on these systems, especially on the challenging, multiconfigurational Cr2 molecule, are satisfactory. While SplitGAS is comparable with the GASSCF method in terms of memory requirements, it performs better than the complete active space method followed by second-order perturbation theory, CASPT2, in terms of equilibrium bond length, dissociation energy, and vibrational properties.
  
  • Broadband ultraviolet-visible transient absorption spectroscopy in the nanosecond to microsecond time domain with sub-nanosecond time resolution
    Bernhard Lang, Sandra Mosquera-Vazquez, Dominique Lovy, Peter Sherin, Vesna Markovic and Eric Vauthey
    Review of Scientific Instruments, 84 (7) , 2013, p73107
    Keywords: delays, high-speed optical techniques, measurement by laser beam, neodymium, optical sensors, photodetectors, photoexcitation, photolysis, probes, Q-switching, solid lasers, time-domain analysis, timing jitter, ultraviolet spectroscopy, visible spectroscopy
    DOI:10.1063/1.4812705 | unige:28965 | Abstract | Article HTML | Article PDF
A combination of sub-nanosecond photoexcitation and femtosecond supercontinuum probing is used to extend femtosecond transient absorption spectroscopy into the nanosecond to microsecond time domain. Employing a passively Q-switched frequency tripled Nd:YAG laser and determining the jitter of the time delay between excitation and probe pulses with a high resolution time delay counter on a single-shot basis leads to a time resolution of 350 ps in picosecond excitation mode. The time overlap of almost an order of magnitude between fs and sub-ns excitation mode permits to extend ultrafast transient absorption (TA) experiments seamlessly into time ranges traditionally covered by laser flash photolysis. The broadband detection scheme eases the identification of intermediate reaction products which may remain undetected in single-wavelength detection flash photolysis arrangements. Single-shot referencing of the supercontinuum probe with two identical spectrometer/CCD arrangements yields an excellent signal-to-noise ratio for the so far investigated chromophores in short to moderate accumulation times.
  • Complexes with Redox-Active Ligands: Synthesis, Structure, and Electrochemical and Photophysical Behavior of the Ru(II) Complex with TTF-Annulated Phenanthroline
    Lawrence K. Keniley, Nathalie Dupont, Lipika Ray, Jie Ding, Kirill Kovnir, Jordan M. Hoyt, Andreas Hauser and Michael Shatruk
    Inorganic Chemistry, 52 (14) , 2013, p8040-8052
    DOI:10.1021/ic4006949 | unige:28963 | Abstract | Article HTML | Article PDF
  
Ru(II) complexes with chelating ligands, 4′,5′-ethylenedithiotetrathiafulvenyl[4,5-f][1,10]phenanthroline (L1), 1,3-dithiole-2-thiono[4,5-f][1,10]phenanthroline (L2), and 1,3-dithiole-2-ono[4,5-f][1,10]phenanthroline (L3), have been prepared and their structural, electrochemical, and photophysical properties investigated. Density functional theory (DFT) calculations indicate that the highest occupied molecular orbital of [Ru(bpy)2(L1)](PF6)2 (1) is located on the tetrathiafulvalene (TTF) subunit and appears ≈0.6 eV above the three Ru-centered d orbitals. In agreement with this finding, 1 exhibits three reversible oxidations: the two at lower potentials take place on the TTF subunit, and the one at higher potential is due to the Ru3+/Ru2+ redox couple. Complexes [Ru(bpy)2(L2)](PF6)2 (2) and [Ru(bpy)2(L3)](PF6)2 (3) exhibit only the Ru3+/Ru2+-related oxidation. The optical absorption spectra of all complexes reveal a characteristic metal-to-ligand charge transfer (MLCT) band centered around 450 nm. In addition, in the spectrum of 1 the MLCT band is augmented by a low-energy tail that extends beyond 500 nm and is attributed to the intraligand charge transfer (ILCT) transition of L1, according to time-dependent DFT calculations. The substantial decrease in the luminescence quantum yield of 1 compared to those of 2 and 3 is attributed to the reductive quenching of the emissive state via electron transfer from the TTF subunit to the Ru3+ center, thus allowing nonradiative relaxation to the ground state through the lower-lying ILCT state. In the presence of O2, complex 1 undergoes a photoinduced oxidative cleavage of the central C═C bond of the TTF fragment, resulting in complete transformation to 3. This photodegradation process was studied with 13C NMR and optical absorption spectroscopy.
  
  • Light Induced Bistability in the 2D Coordination Network {[Fe(bbtr)3](BF4)2}∞: Wavelength-Selective Addressing of Molecular Spin States
    Pradip Chakraborty, SĂŠbastien Pillet, El-Eulmi Bendeif, Cristian Enachescu, Robert Bronisz and Andreas Hauser
    Chemistry - A European Journal, 19 (34) , 2013, p11418-11428
    Keywords: cooperative effects;iron;photoinduced bistability;photoswitching;spin crossover
    DOI:10.1002/chem.201301257 | unige:29524 | Abstract | Article PDF
Whereas the neat polymeric Fe(II) compound {[Fe(bbtr)3](ClO4)2}∞ (bbtr=1,4-di(1,2,3-triazol-1-yl)butane) shows an abrupt spin transition centered at 107 K facilitated by a crystallographic symmetry breaking, in the covalently linked 2D coordination network of {[Fe(bbtr)3](BF4)2}∞, Fe(II) stays in the high-spin state down to 10 K. However, strong cooperative effects of elastic origin result in reversible, persistent and wavelength-selective photoswitching between the low-spin and high-spin manifolds. This compound thus shows true light-induced bistability below 100 K. The persistent bidirectional optical switching behavior is discussed as a function of temperature, irradiation time and intensity. Crystallographic studies reveal a photo-induced symmetry breaking and serve to establish the correlation between structure and cooperative effects. The static and kinetic behavior is explicated within the framework of the mean-field approximation.
  • Stabilization of Thiolate-Protected Gold Clusters against Thermal Inversion: Diastereomeric Au38(SCH2CH2Ph)24-2x(R-BINAS)x
    Stefan Knoppe, Sophie Michalet and Thomas BĂźrgi
    Journal of Physical Chemistry C, 117 (29) , 2013, p15354-15361
    DOI:10.1021/jp4040908 | unige:29224 | Abstract | Article PDF
Intrinsically chiral thiolate-protected gold clusters were recently separated into their enantiomers and their circular dichroism (CD) spectra were measured. Introduction of the chiral R-1,1’-binaphthyl-2,2’-dithiol (BINAS) into the ligand layer of rac-Au38(2-PET)24 clusters (2-PET: 2-phenylethylthiolate, SCH2CH2Ph) was shown to be diastereoselective. In this contribution, we isolated and characterized the diastereomeric reaction products of the first exchange step, A-Au38(2-PET)22(R-BINAS)1 and C-Au38(2-PET)22(R-BINAS)1 (A/C, anti-clockwise/clockwise) and the second exchange product, A-Au38(2-PET)20(R-BINAS)2. The absorption spectra show minor, but significant influence of the BINAS ligand. Overall, the spectra are less defined compared to Au38(2-PET)24, which is ascribed to symmetry breaking. The CD spectra are similar to those of the parent Au38(2-PET)24 enantiomers, readily allowing the assignment of handedness of the ligand layer. Nevertheless, some characteristic differences are found between the diastereomers. The anisotropy factors are slightly lower after ligand exchange. The second exchange step seems to confirm the trend. Inversion experiments were performed and compared to the racemization of Au38(2-PET)24. It was found that the introduction of the BINAS ligand effectively stabilizes the cluster against inversion, which involves a rearrangement of the thiolates on the cluster surface. It therefore seems that introduction of the di-thiol reduces the flexibility of the gold-sulfur interface.
  • Determination of the molecular structure of the short-lived light-induced high-spin state in the spin-crossover compound [Fe(6-mepy)3tren](PF6)2
    Pradip Chakraborty, Antoine Tissot, Lisa Peterhans, Laure GuĂŠnĂŠe, CĂŠline Besnard, Philip Pattison and Andreas Hauser
    Physical Review B, 87 (21) , 2013, p214306
    DOI:10.1103/PhysRevB.87.214306 | unige:28964 | Abstract | Article PDF
In the spin-crossover compound [Fe(6-mepy)3tren](PF6)2, (6-mepy)3tren = tris{4-[(6-methyl)-2-pyridyl]-3-aza-butenyl}amine, the high-spin state can be populated as metastable state below the thermal transition temperature via irradiation into the metal to ligand charge transfer absorption band of the low-spin species. At 10 K, the lifetime of this metastable state is only 1 s. Despite this, it is possible to determine an accurate excited state structure by following the evolution of relevant structural parameters by synchrotron X-ray diffraction under continuous irradiation with increasing intensity. The difference in metal-ligand bond length between the high-spin and the low-spin state is found to be 0.192 Å obtained from an analysis of the experimental data using the mean-field approximation to model cooperative effects.
  • Real-Time Observation of the Formation of Excited Radical Ions in Bimolecular Photoinduced Charge Separation: Absence of the Marcus Inverted Region Explained
    Marius Koch, Arnulf Rosspeintner, Katrin Adamczyk, Bernhard Lang, Jens Dreyer, Erik T.J. Nibbering and Eric Vauthey
    Journal of the American Chemical Society, 135 (26) , 2013, p9843-9848
    DOI:10.1021/ja403481v | unige:28966 | Abstract | Article HTML | Article PDF
  
Unambiguous evidence for the formation of excited ions upon ultrafast bimolecular photoinduced charge separation is found using a combination of femtosecond time-resolved fluorescence up-conversion, infrared and visible transient absorption spectroscopy. The reaction pathways are tracked by monitoring the vibrational energy redistribution in the product after charge separation and subsequent charge recombination. For moderately exergonic reactions, both donor and acceptor are found to be vibrationally hot, pointing to an even redistribution of the energy dissipated upon charge separation and recombination in both reaction partners. For highly exergonic reactions, the donor is very hot, whereas the acceptor is mostly cold. The asymmetric energy redistribution is due to the formation of the donor cation in an electronic excited state upon charge separation, confirming one of the hypotheses for the absence of the Marcus inverted region in photoinduced bimolecular charge separation processes
  • A modified cyclen azaxanthone ligand as a new fluorescent probe for Zn2+
    Hela Nouri, Cyril Cadiou, Latevi Max Lawson Daku, Andreas Hauser, Sylviane Chevreux, Isabelle DĂŠchamps-Olivier, Fabien Lachaud, Riadh Ternane, Malika Trabelsi-Ayadi, Françoise Chuburu and Gilles Lemercier
    Dalton Transactions, 42 , 2013, p12157-12164
    DOI:10.1039/c3dt51216a | unige:29225 | Abstract | Article PDF
A new cyclen derivative L, bearing a methyl-chromeno-pyridinylidene hydrazone moiety, was synthesized and studied in MeOH, as potential fluorescent “OFF-on-ON” sensors for Zn(II). Photocphysical properties of this ligand being PET regulated, L was only weakly emissive in the absence of metal ions (OFF). L fluorescence was increased modestly upon addition of one equivalent of Zn(II), and further increased upon addition of a second equivalent. Therefore, Zn:L behaved as a highly sensitive ON sensor for zinc. This efficiency was correlated to Zn(II) coordination via the hydrazone moiety of the fluorophore, producing an efficient CHelation-Enhanced Fluorescence (CHEF) effect. A complementary theoretical study carried out with DFT calculations further elucidated of the optical properties.
  • Photoinduced Relaxation Dynamics in Iron(II) Spin-Crossover Nanoparticles: The Significance of Crystallinity
    Pradip Chakraborty, Marie-Laure Boillot, Antoine Tissot and Andreas Hauser
    Angewandte Chemie International Edition, 52 (28) , 2013, p7139-7142
    Keywords: crystallinity; nanoparticles; optical spectroscopy; spin crossover; spin relaxation
    DOI:10.1002/anie.201301562 | unige:29226 | Abstract | Article HTML | Article PDF
A switch in time: A fast precipitation technique was used to prepare 75 nm FeII spin-crossover nanocrystals. Their photoswitching dynamics, based on the light-induced excited spin-state trapping effect, has been investigated by means of optical spectroscopy. A significant variation of the switching proprieties is observed compared to similar but amorphous nanoparticles.
  
  • Stimuli Responsive Hybrid Magnets: Tuning the Photoinduced Spin-Crossover in Fe(III) Complexes Inserted into Layered Magnets
    Miguel Clemente-LeĂłn, Eugenio Coronado, Maurici LĂłpez-JordĂ , JoĂŁo C. Waerenborgh, CĂŠdric Desplanches, Hongfeng Wang, Jean-François LĂŠtard, Andreas Hauser and Antoine Tissot
    Journal of the American Chemical Society, 135 (23) , 2013, p8655-8667
    DOI:10.1021/ja402674x | unige:28703 | Abstract | Article HTML | Article PDF
The insertion of a [Fe(sal2-trien)]+ complex cation into a 2D oxalate network in the presence of different solvents results in a family of hybrid magnets with coexistence of magnetic ordering and photoinduced spin-crossover (LIESST effect) in compounds [FeIII(sal2-trien)][MnIICrIII(ox)3]¡CHCl3 (1¡CHCl3), [FeIII(sal2-trien)][MnIICrIII(ox)3]¡CHBr3 (1¡CHBr3), and [FeIII(sal2-trien)][MnIICrIII(ox)3]¡CH2Br2 (1¡CH2Br2). The three compounds crystallize in a 2D honeycomb anionic layer formed by MnII and CrIII ions linked through oxalate ligands and a layer of [Fe(sal2-trien)]+ complexes and solvent molecules (CHCl3, CHBr3, or CH2Br2) intercalated between the 2D oxalate network. The magnetic properties and MÜssbauer spectroscopy indicate that they undergo long-range ferromagnetic ordering at 5.6 K and a spin crossover of the intercalated [Fe(sal2-trien)]+ complexes at different temperatures T1/2. The three compounds present a LIESST effect with a relaxation temperature TLIESST inversely proportional to T1/2. The isostructural paramagnetic compound, [FeIII(sal2-trien)][ZnIICrIII(ox)3]¡CH2Cl2 (2¡CH2Cl2) was also prepared. This compound presents a partial spin crossover of the inserted FeIII complex as well as a LIESST effect. Finally, spectroscopic characterization of the FeIII doped compound [Ga0.99Fe0.01(sal2trien)][MnIICrIII(ox)3]¡CH2Cl2 (3¡CH2Cl2) shows a gradual and complete thermal spin crossover and a LIESST effect on the isolated FeIII complexes. This result confirms that cooperativity is not a necessary condition to observe the LIESST effect in an FeIII compound.
  • Extracting Information about Chemical Bonding from Molecular Electron Densities via Single Exponential Decay Detector (SEDD)
    Piotr De Silva, Jacek Korchowiec, Nirmal J.S. Ram and Tomasz A. Wesolowski
    Chimia, 67 (4) , 2013, p253-256
    Keywords: aromaticity; bonding; conceptual DFT; electron density; electron localization
    DOI:10.2533/chimia.2013.253 | unige:28399 | Abstract | Article PDF
  
The recently introduced molecular descriptor (Single Exponential Decay Detector - SEDD) [P. de Silva, J. Korchowiec, T. A. Wesolowski, ChemPhysChem 2012, 13, 3462] is used to visualize bonding patterns in molecules. In each point of space SEDD is simply related to the electron density:

SEDD(r) = ln[1/ρ2(∇(∇ρ/ρ)2)2}.

Either experimental or computed densities ρ(r) can be used to evaluate SEDD. Here, maps of SEDD are obtained from theoretical densities and reveal such features as core electrons, chemical bonds, lone pairs and delocalization in aromatic systems. It is shown that SEDD provides fingerprints of aromaticity, which can be separated into geometric and electronic effects.

  • Non-additive kinetic energy and potential in analytically solvable systems and their approximated counterparts
    Tomasz A. Wesolowski and Andreas Savin
    in "Recent Progress in Orbital-free Density Functional Theory (Recent Advances in Computational Chemistry Vol. 6)" T.A. Wesolowski and Y.A. Wang Eds., World Scientific, 6 , 2013, p275-295
    DOI:10.1142/9789814436731_0009 | unige:28400 | Abstract | Article PDF
The one-electron equation for orbitals embedded in frozen electron density (Eqs. 20-21 in [Wesolowski and Warshel, J. Phys. Chem, 97 (1993) 8050]) in its exact and approximated version is solved for an analytically solvable model system. The system is used to discuss the role of the embedding potential in preventing the collapse of a variationally obtained electron density onto the nucleus in the case when the frozen density is chosen to be that of the innermost shell. The approximated potential obtained from the second-order gradient expansion for the kinetic energy prevents such a collapse almost perfectly but this results from partial compensation of flaws of its components. It is also shown that that the quality of a semi-local approximation to the kinetic-energy functional, a quantity needed in orbital-free methods, is not related to the quality of the non-additive kinetic energy potential - a key component of the effective embedding potential in one-electron equations for embedded orbitals.
  • Semilocal approximations for the kinetic energy
    Fabien Tran and Tomasz A. Wesolowski
    in "Recent Progress in Orbital-free Density Functional Theory (Recent Advances in Computational Chemistry Vol. 6)" T.A. Wesolowski and Y.A. Wang Eds., World Scientific, 6 , 2013, p429-442
    DOI:10.1142/9789814436731_0016 | unige:28401 | Abstract | Article PDF
Approximations to the non-interacting kinetic energy Ts[ρ], which take the form of semilocal analytic expressions are collected. They are grouped according to the quantities on which they explicitly depend. Additionally, the approximations for quantities related to Ts[ρ] (kinetic potential and non-additive kinetic energy), for which the analytic expressions for the “parent” approximation for the functional Ts[ρ] are unknown, are also given.
  
Four-dimensional (4D) electron microscopy (EM) uniquely combines the high spatial resolution to pinpoint individual nano-objects, with the high temporal resolution necessary to address the dynamics of their laser-induced transformation. Here, using 4D-EM, we demonstrate the in situ irreversible transformation of individual nanoparticles of the molecular framework Fe(pyrazine)Pt(CN)4. The newly formed material exhibits an unusually large negative thermal expansion (i.e. contraction), which is revealed by time-resolved imaging and diffraction. Negative thermal expansion is a unique property exhibited by only few materials. Here we show that the increased flexibility of the metal–cyanide framework after the removal of the bridging pyrazine ligands is responsible for the negative thermal expansion behavior of the new material. This in situ visualization of single nanostructures during reactions should be extendable to other classes of reactive systems.
  • Chiroptical Properties of Intrinsically Chiral Thiolate-protected Gold Clusters
    Stefan Knoppe and Thomas BĂźrgi
    Chimia, 67 (4) , 2013, p236-239
    DOI:10.2533/chimia.2013.236 | unige:27716 | Abstract | Article PDF
Chiral thiolate-protected gold clusters of atomic precision have gained increasing interest in recent years due to their potential use in catalysis, sensing or bioapplications. While the protection of gold clusters with chiral ligands is a rather trivial task, it was found that the clusters can bear intrinsically chiral features, most obvious in the arrangement of the protecting ligands on the surface of the cluster. Recent efforts showed the separation of the enantiomers of such intrinsically chiral gold clusters. This technique can be used for the prediction of chirality in structurally unknown clusters. Activation barriers for the racemization of Au38(SR)24 were determined. As this involves a huge rearrangement of the ligands, the flexibility of the gold-thiolate interface is demonstrated. Furthermore, the ligand exchange reactions between intrinsically chiral clusters and bidentate chiral thiols were studied. A limited, regioselective exchange was found. Most importantly, the reaction is diastereoselective and allows tailoring of gold clusters that are protected with a defined layer of ligands.
  • Single-nanoparticle phase transitions visualized by four-dimensional electron microscopy
    Renske M. Van der Veen, Oh-Hoon Kwon, Antoine Tissot, Andreas Hauser and Ahmed H. Zewail
    Nature Chemistry, 5 (5) , 2013, p395-402
    DOI:10.1038/NCHEM.1622 | unige:27715 | Abstract | Article HTML | Article PDF
The advancement of techniques that can probe the behaviour of individual nanoscopic objects is of paramount importance in various disciplines, including photonics and electronics. As it provides images with a spatiotemporal resolution, four-dimensional electron microscopy, in principle, should enable the visualization of single-nanoparticle structural dynamics in real and reciprocal space. Here, we demonstrate the selectivity and sensitivity of the technique by visualizing the spin crossover dynamics of single, isolated metal–organic framework nanocrystals. By introducing a small aperture in the microscope, it was possible to follow the phase transition and the associated structural dynamics within a single particle. Its behaviour was observed to be distinct from that imaged by averaging over ensembles of heterogeneous nanoparticles. The approach reported here has potential applications in other nanosystems and those that undergo (bio)chemical transformations.
 
The crystal structures of the M2NaIO6 series (M = Ca, Sr, Ba), prepared at 650 °C by ceramic methods, were determined from conventional laboratory X-ray powder diffraction data. Synthesis and crystal growth were made by oxidizing I– with O2(air) to I7+ followed by crystal growth in the presence of NaF as mineralizator, or by the reaction of the alkali-metal periodate with the alkaline-earth metal hydroxide. All three compounds are insoluble and stable in water. The barium compound crystallizes in the cubic space group Fm3m (no. 225) with lattice parameters of a = 8.3384(1) Å, whereas the strontium and calcium compounds crystallize in the monoclinic space group P21/c (no. 14) with a = 5.7600(1) Å, b = 5.7759(1) Å, c = 9.9742(1) Å, β = 125.362(1)° and a = 5.5376(1) Å, b = 5.7911(1) Å, c = 9.6055(1) Å, β = 124.300(1)°, respectively. The crystal structure consists of either symmetric (for Ba) or distorted (for Sr and Ca) perovskite superstructures. Ba2NaIO6 contains the first perfectly octahedral [IO6]5– unit reported. The compounds of the ortho-periodates are stable up to 800 °C. Spectroscopic measurements as well as DFT calculations show a reasonable agreement between calculated and observed IR- and Raman-active vibrations.
  
  • Tetrathiafulvalene-1,3,5-triazines as (Multi)Donor-Acceptor Systems with Tunable Charge Transfer: Structural, Photophysical, and Theoretical Investigations
    Flavia Pop, François RiobĂŠ, Sabine Seifert, Thomas Cauchy, Jie Ding, Nathalie Dupont, Andreas Hauser, Marius Koch and Narcis Avarvari
    Inorganic Chemistry, 52 (9) , 2013, p5023-5034
    DOI:10.1021/ic3027336 | unige:27865 | Abstract | Article HTML | Article PDF
Palladium-catalyzed cross-coupling reactions between chlorinated 1,3,5-triazines (TZ) and tetrathiafulvalene (TTF) trimethyltin derivatives afford mono- and C3 symmetric tris(TTF)-triazines as donor–acceptor compounds in which the intramolecular charge transfer (ICT) is modulated by the substitution scheme on TTF and TZ and by chemical or electrochemical oxidation. The TTF-TZ-Cl2 and (SMe)2TTF-TZ-Cl2 derivatives show fully planar structures in the solid state as a consequence of the conjugation between the two units. Electrochemical and photophysical investigations, supported by theoretical calculations, clearly demonstrate that the lowest excited state can be ascribed to the intramolecular charge transfer (ICT) π(TTF)→π*(TZ) transition. The tris(TTF) compound [(SMe)2TTF]3-TZ shows fluorescence when excited in the ICT band, and the emission is quenched upon oxidation. The radical cations TTF+• are easily observed in all of the cases through chemical and electrochemical oxidation by steady-state absorption experiments. In the case of [(SMe)2TTF]3-TZ, a low energy band at 5000 cm–1, corresponding to a coupling between TTF+• and TTF units, is observed. A crystalline radical cation salt with the TTF-TZ-Cl2 donor and PF6– anion, prepared by electrocrystallization, is described.
Ultrafast photochemical processes can occur in parallel with the relaxation of the optically populated excited state toward equilibrium. The latter involves both intra- and intermolecular modes, namely vibrational and solvent coordinates, and takes place on timescales ranging from a few tens of femtoseconds to up to hundreds of picoseconds, depending on the system. As a consequence, the reaction dynamics can substantially differ from those usually measured with slower photoinduced processes occurring from equil-ibrated excited states. For example, the decay of the excited-state population may become strongly nonexponential and depend on the excitation wavelength, contrary to the Kasha and Vavilov rules. In this article, we first give a brief account of our current understanding of vibrational and solvent relaxation processes. We then present an overview of important classes of ultrafast photochemical reactions, namely electron and proton transfer as well as isomerization, and illustrate with several examples how nonequilibrium effects can affect their dynamics.
  • Chiral Selectivity in the Binding of [4]Helicene Derivatives to Double-Stranded DNA
    Oksana Kel, Alexandre FĂźrstenberg, Nathalie Mehanna, Cyril Nicolas, BenoĂŽt Laleu, Martin Hammarson, Bo Albinsson, JerĂ´me Lacour and Eric Vauthey
    Chemistry - A European Journal, 19 (22) , 2013, p7173-7180
    Keywords: chirality;circular dichroism;DNA recognition;fluorescent probes;helical structures;time-resolved spectroscopy
    DOI:10.1002/chem.201203915 | unige:27926 | Abstract | Article HTML | Article PDF
  
The interaction of a series of chiral cationic [4]helicene derivatives, which differ by their substituents, with double-stranded DNA has been investigated by using a combination of spectroscopic techniques, including time-resolved fluorescence, fluorescence anisotropy, and linear dichroism. Addition of DNA to helicene solutions results to a hypochromic shift of the visible absorption bands, an increase of fluorescence quantum yield and lifetime, a slowing down of fluorescence anisotropy decay, and a linear dichroism in flow-oriented DNA, which unambiguously points to the binding of these dyes to DNA. Both helicene monomers and dimeric aggregates, which form at higher concentration, bind to DNA, the former most probably upon intercalation and the latter upon groove binding. The binding constant depends substantially on the dye substituents and is, in all cases, larger with the M than the P enantiomer, by factors ranging from 1.2 to 2.3, depending on the dye.
The time resolution of photon detection systems is important for a wide range of applications in physics and chemistry. It impacts the quality of time-resolved spectroscopy of ultrafast processes and has a direct influence on the best achievable time resolution of time-of-flight detectors in high-energy and medical physics. For the characterization of photon detectors, it is important to measure their exact timing properties in dependence of the photon flux and the operational parameters of the photodetector and its accompanying electronics. We report on the timing of silicon photomultipliers (SiPM) as a function of their bias voltage, electronics threshold settings and the number of impinging photons. We used ultrashort laser pulses at 400 nm wavelength with pulse duration below 200 fs. We focus our studies on different types of SiPMs (Hamamatsu MPPC S10931-025P, S10931-050P and S10931-100P) with different SPAD sizes (25Οm, 50Οm and 100Οm) coupled to the ultrafast discriminator amplifier NINO. For the SiPMs, an optimum in the time resolution regarding bias and threshold settings can be reached. For the 50Οm type, we achieve a single photon time resolution of 80 ps sigma, and for saturating photon fluxes better than 10 ps sigma.
The photophysics and photochemistry of kynurenic acid (KNA) and kynurenine yellow (KNY) in neutral aqueous solutions were investigated using time-resolved optical spectroscopy. Both molecules have similar quinoline-like structures, the only difference being the absence of conjugation in the nitrogen containing cycle in KNY. The main channel of S1 excited state decay in the case of partially-unconjugated KNY is the solvent assisted S1 → S0 radiationless transition via intermolecular hydrogen bonds (ΦIC = 0.96), whereas, in the case of fully-conjugated KNA, it is intersystem crossing to the triplet state (ΦT = 0.82). The major intermediate products of the singlet excited KNY deactivation are the triplet state (ΦT = 0.022) and, most probably, the enol form (Φenol = 0.012), which decay with the formation of 2,3-dihydro-4-hydroxyquinoline and 4-hydroxyquinoline, respectively. The results obtained show that KNA and KNY, which are products of the decomposition of the UV filter kynurenine, are significantly more photoactive and less photostable than the parent molecule.
  
  • Effect of Temperature and Pressure on Emission Lifetime of Sm2+ Ion Doped in MFX (M=Sr, Ba; X=Br, I) Crystals
    Prodipta Pal, Tiphaine PenhouĂŤt, Vincenza D'Anna and Hans Hagemann
    Journal of Luminescence, 142 , 2013, p66-74
    Keywords: Sm2+; high pressure lifetime; temperature lifetime; SrFBr; BaFBr; BaFI
    DOI:10.1016/j.jlumin.2013.03.011 | unige:27714 | Abstract | Article PDF
The emission lifetime of Sm2+ ions doped in MFX (M=Ba, Sr; X=Br, I) crystals was investigated as a function of pressure and temperature. The decay of the 5DJ(J=0,1,2) levels showed single exponential relaxation. The analysis of these experiments yielded the position of the lowest 4f55d1 state as well as non-radiative rate constants. These values were compared with those for Sm2+ doped in other matlockite host crystals. The single exponential decrease of the 5D0,1 lifetime as a function of pressure was described considering the increased radiative decay rates of these 5D0,1 levels through electronic mixing between the 4f55d1 and 5DJstates.
  • Study of Surfactant Alcohols with various Chemical Motives at the Hydrophilic/Hydrophobic Interface
    Pierre-Leonard Zaffalon, Vincenza D'Anna, Hans Hagemann and Andreas Zumbuehl
    RSC Advances, 3 (20) , 2013, p7237-7244
    DOI:10.1039/c3ra40704g | unige:27864 | Abstract | Article PDF
The melting behavior, the solubility, and the influence of hydrogen bonds were analyzed for a series of single- and double-tailed surfactant alcohols. Various effects such as the presence of free amides or the intermolecular spacing were found to be important factors for increasing or decreasing the melting temperature of a surfactant. Furthermore, we present a model for the packing of diamido-lipids and study the temperature-dependence of the IR signals.
 
A comparison of the vibrational spectra of many inorganic borohydrides allows us to distinguish compounds with isolated BH4- ions and compounds containing complex ions such as Sc(BH4)4-. The characteristic spectral features of both types of compounds are identified, showing that the B–H bonding is quite different in both cases. A detailed analysis of the vibrations of the isolated BH4- ions provides new information about their local structure. Angular deformations of individual borohydride ion are analyzed quantitatively. It appears that the compounds containing isolated BH4- ions belong to those with the most electropositive cations and the highest decomposition temperature, while the complex borohydrides show significantly lower decomposition temperatures and possible diborane formation.
  • Synthesis and Redox and Photophysical Properties of Benzodifuran–Spiropyran Ensembles
    Hui Li, Jie Ding, Songjie Chen, Christoph Beyer, Shi-Xia Liu, Hans-Achim Wagenknecht, Andreas Hauser and Silvio Decurtins
    Chemistry - A European Journal, 19 (20) , 2013, p6459-6466
    Keywords: asymmetric synthesis;click chemistry;electron transfer;photochromism;redox chemistry;spiro compounds
    DOI:10.1002/chem.201204043 | unige:27859 | Abstract | Article PDF
Two benzodifuran (BDF)-coupled spiropyran (SP) systems and their BDF reference compounds were obtained in good yields through Huisgen–Meldal–Sharpless “click” chemistry and then subjected to investigation of their electrochemical and photophysical properties. In both SP and merocyanine (MC) forms of the coupled molecules, the BDF-based emission is quenched to around 1 % of the quantum yield of emission from the BDF reference compounds. Based on electrochemical data, this quenching is attributed to oxidative electron-transfer quenching. Irradiation at 366 nm results in ring opening to the MC forms of the BDF-coupled SP compounds and the SP reference compound with a quantum efficiency of about 50 %. The rate constants for the thermal ring closing are approximately 3.4×10−3 s−1. However, in the photostationary states the MC fractions of the coupled molecules are substantially lower than that of the reference SP compound, attributed to the observed acceleration of the ring-closing reaction upon irradiation. As irradiation at 366 nm invariably also excites higher-energy transitions of the BDF units in the coupled compounds, the ring-opening reaction is accelerated relative to the SP reference, which results in lower MC fractions in the photostationary state. Reversible photochromism of these BDF-coupled SP compounds renders them promising in the field of molecular switches.
  
  • Tetrathiafulvalene-s-tetrazine: versatile platform for donor-acceptor systems and multifunctional ligands
    Flavia Pop, Jie Ding, LatĂŠvi Max Lawson Daku, Andreas Hauser and Narcis Avarvari
    RSC Advances, 3 , 2013, p3218-3221
    DOI:10.1039/c3ra21702g | unige:27369 | Abstract | Article HTML | Article PDF
The structurally characterized tetrathiafulvalene-1,2,4,5-tetrazine donor–acceptor system shows redox tuneable intramolecular charge transfer, solvatochromic and electrochromic behaviour. Attachment of a dipicolyl-amine chelating unit affords a multifunctional ligand, which allows the preparation of the ZnCl2 complex in which an anion-π interaction is seen.
  • Electron Transfer between Hydrogen-Bonded Pyridylphenols and a Photoexcited Rhenium(I) Complex
    William Herzog, Catherine Bronner, Susanne LĂśffler, Bice He, Daniel Kratzert, Dietmar Stalke, Andreas Hauser and Oliver S. Wenger
    ChemPhysChem, 14 (6) , 2013, p1168-1176
    Keywords: electron transfer;luminescence;photochemistry;proton transfer;transient absorption
    DOI:10.1002/cphc.201201069 | unige:27484 | Abstract | Article PDF
Two pyridylphenols with intramolecular hydrogen bonds between the phenol and pyridine units have been synthesized, characterized crystallographically, and investigated by cyclic voltammetry and UV/Vis spectroscopy. Reductive quenching of the triplet metal-to-ligand charge-transfer excited state of the [Re(CO)3(phen)(py)]+ complex (phen=1,10-phenanthroline, py=pyridine) by the two pyridylphenols and two reference phenol molecules is investigated by steady-state and time-resolved luminescence spectroscopy, as well as by transient absorption spectroscopy. Stern–Volmer analysis of the luminescence quenching data provides rate constants for the bimolecular excited-state quenching reactions. H/D kinetic isotope effects for the pyridylphenols are on the order of 2.0, and the bimolecular quenching reactions are up to 100 times faster with the pyridylphenols than with the reference phenols. This observation is attributed to the markedly less positive oxidation potentials of the pyridylphenols with respect to the reference phenols (≈0.5 V), which in turn is caused by proton coupling of the phenol oxidation process. Transient absorption spectroscopy provides unambiguous evidence for the photogeneration of phenoxyl radicals, that is, the overall photoreaction is clearly a proton-coupled electron-transfer process.
  • Towards accurate estimates of the spin-state energetics of spin-crossover complexes within density functional theory: a comparative case study of cobalt(ii) complexes
    Alfredo Vargas, Itana Krivokapic, Andreas Hauser and LatĂŠvi Max Lawson Daku
    Physical Chemistry Chemical Physics, 15 (11) , 2013, p3752-3763
    DOI:10.1039/c3cp44336a | unige:26498 | Abstract | Article HTML | Article PDF
  
We report a detailed DFT study of the energetic and structural properties of the spin-crossover Co(II) complex [Co(tpy)2]2+ (tpy = 2,2′:6′,2′′-terpyridine) in the low-spin (LS) and the high-spin (HS) states, using several generalized gradient approximation and hybrid functionals. In either spin-state, the results obtained with the functionals are consistent with one another and in good agreement with available experimental data. Although the different functionals correctly predict the LS state as the electronic ground state of [Co(tpy)2]2+, they give estimates of the HS–LS zero-point energy difference ΔE0HL (tpy)  which strongly depend on the functional used. This dependency on the functional was also reported for the DFT estimates of the zero-point energy difference ΔE0HL (bpy)  in the HS complex [Co(bpy)3]2+ (bpy = 2,2′-bipyridine) [A. Vargas, A. Hauser and L. M. Lawson Daku, J. Chem. Theory Comput., 2009, 5, 97]. The comparison of the ΔE0HL (tpy)  and ΔE0HL (bpy)  estimates showed that all functionals correctly predict an increase of the zero-point energy difference upon the bpy → tpy ligand substitution, which furthermore weakly depends on the functionals, amounting to (ΔE0HL)bpy->tpy  ≈ +2670 cm-1 . From these results and basic thermodynamic considerations, we establish that, despite their limitations, current DFT methods can be applied to the accurate determination of the spin-state energetics of complexes of a transition metal ion, or of these complexes in different environments, provided that the spin-state energetics is accurately known in one case. Thus, making use of the availability of a highly accurate ab initio estimate of the HS–LS energy difference in the complex [Co(NCH)6]2+ [L. M. Lawson Daku, F. Aquilante, T. W. Robinson and A. Hauser, J. Chem. Theory Comput., 2012, 8, 4216], we obtain for [Co(tpy)2]2+ and [Co(bpy)3]2+best estimates of ΔE0HL (bpy) ≈ -2800 cm-1  and ΔE0HL (tpy) ≈ 0 cm-1 , in good agreement with the known magnetic behaviour of the two complexes.
  • A Straightforward Synthesis and Structure-Activity Relationship of Highly Efficient Initiators for Two-Photon Polymerization
    Zhiquan Li, Niklas Pucher, Klaus Cicha, Jan Torgersen, Samuel C. Ligon, Aliasghar Ajami, Wolfgang Husinsky, Arnulf Rosspeintner, Eric Vauthey, Sergej Naumov, Tom Scherzer, JĂźrgen Stampfl and Robert Liska
    Macromolecules, 46 (2) , 2013, p352-361
    DOI:10.1021/ma301770a | unige:26500 | Abstract | Article HTML | Article PDF | Â
The development of practical two-photon absorption photoinitiators (TPA PIs) has been slow due to their complicated syntheses often reliant on expensive catalysts. These shortcomings have been a critical obstruction for further advances in the promising field of two-photon-induced photopolymerization (TPIP) technology. This paper describes a series of linear and cyclic benzylidene ketone-based two-photon initiators containing double bonds and dialkylamino groups synthesized in one step via classical aldol condensation reactions. Systematic investigations of structure–activity relationships were conducted via quantum-chemical calculations and experimental tests. These results showed that the size of the central ring significantly affected the excited state energetics and emission quantum yields as well as the two-photon initiation efficiency. In the TPIP tests the 4-methylcyclohexanone-based initiator displayed much broader ideal processing windows than its counterparts with a central five-membered ring and previously described highly active TPA PIs. Surprisingly, a writing speed as high as 80 mm/s was obtained for the microfabrication of complex 3D structures employing acrylate-based formulations. These highly active TPA PIs also exhibit excellent thermal stability and remain inert to one-photon excitation. Straightforward synthesis combined with high TPA initiation efficiency makes these novel initiators promising candidates for commercialization.
  
The excited-state dynamics of two energy donor–bridge–acceptor (D–B–A) systems consisting of a zinc tetraphenylporphyrin (ZnP) and a free base tetraphenylporphyrin (FbP) bridged by oligo-p-phenyleneethynylene units with different substituents has been investigated using ultrafast spectroscopy. These systems differ by the location of the lowest singlet excited state of the bridge, just above or below the S2 porphyrin states. In the first case, Soret band excitation of the porphyrins is followed by internal conversion to the local S1 state of both molecules and by a S1 energy transfer from the ZnP to the FbP end on the 10 ns time scale, as expected for a center-to-center distance of about 4.7 nm. On the other hand, if the bridge is excited, the energy is efficiently transferred within 1 ps to both porphyrin ends. Selective bridge excitation is not possible with the second system, because of the overlap of the absorption bands. However, the time-resolved spectroscopic data suggest a reversible conversion between the D*(S2)–B–A and D–B*(S1)–A states as well as a transition from the D–B*(S1)–A to the D–B–A* states on the picosecond time scale. This implies that the local S2energy of the ZnP end can be transported stepwise to the FbP end, i.e., over about 4.7 nm, within 1 ps with an efficiency of more than 0.2.
  • Improved photoluminescence and afterglow of CaTiO3:Pr3+ by ammonia treatment
    Songhak Yoon, Eugenio H. Otal, Alexandra E. Maegli, Lassi Karvonen, Santhosh K. Matam, Stefan Riegg, Stefan G. Ebbinghaus, Juan C. Fallas, Hans Hagemann, Bernhard Walfort, Simone Pokrant and Anke Weidenkaff
    Optical Materials Express, 3 (2) , 2013, p248-259
    Keywords: fluorescent materials; rare-earth doped materials
    DOI:10.1364/OME.3.000248 | unige:26402 | Abstract | Article HTML | Article PDF
  
The phosphor CaTiO3:Pr3+ was synthesized via a solid-state reaction in combination with a subsequent annealing under flowing NH3. Comparatively large off-center displacements of Ti in the TiO6 octahedra were confirmed for as-synthesized CaTiO3:Pr3 by XANES. Raman spectroscopy showed that the local crystal structure becomes highly symmetric when the powders are ammonolyzed at 400 °C. Rietveld refinement of powder X-ray diffraction data revealed that the samples ammonolyzed at 400 °C have the smallest lattice strain and at the same time the largest average Ti-O-Ti angles were obtained. The samples ammonolyzed at 400 °C also showed the smallest mass loss during the thermal re-oxidation in thermogravimetric analysis (TGA). Enhanced photolumincescence brightness and an improved decay curve as well as the highest reflectance were obtained for the samples ammonolyzed at 400 °C. The improved photoluminescence and afterglow by NH3 treatment are explained as a result of the reduced concentration of oxygen excesses with simultaneous relaxation of the lattice strain.
  
  • Tetrathiafulvalene-Benzothiadiazoles as Redox-Tunable Donor-Acceptor Systems: Synthesis and Photophysical Study
    Flavia Pop, Anneliese Amacher, Narcis Avarvari, Jie Ding, Latevi Max Lawson Daku, Andreas Hauser, Marius Koch, JĂźrg Hauser, Shi-Xia Liu and Silvio Decurtins
    Chemistry - A European Journal, 19 (7) , 2013, p2504-2514
    Keywords: charge transfer;donor-acceptor systems;fluorescence;photophysics;redox chemistry
    DOI:10.1002/chem.201202742 | unige:26401 | Abstract | Article PDF
Electrochemical and photophysical analysis of new donor–acceptor systems 2 and 3, in which a benzothiadiazole (BTD) unit is covalently linked to a tetrathiafulvalene (TTF) core, have verified that the lowest excited state can be ascribed to an intramolecular-charge-transfer (ICT) π(TTF)→π*(benzothiadiazole) transition. Owing to better overlap of the HOMO and LUMO in the fused scaffold of compound 3, the intensity of the 1ICT band is substantially higher compared to that in compound 2. The corresponding CT fluorescence is also observed in both cases. The radical cation TTF+. is easily observed through chemical and electrochemical oxidation by performing steady-state absorption experiments. Interestingly, compound 2 is photo-oxidized under aerobic conditions.
  • Modular Synthesis, Orthogonal Post-Functionalization, Absorption, and Chiroptical Properties of Cationic [6]Helicenes
    Franck Torricelli, Johann Bosson, CĂŠline Besnard, Mahshid Chekini, Thomas BĂźrgi and JĂŠrĂ´me Lacour
    Angewandte Chemie International Edition, 52 (6) , 2013, p1796-1800
    Keywords: arenes;helical structures;synthetic methods;UV/Vis spectroscopy;vicarious nucleophilic substitution
    DOI:10.1002/anie.201208926 | unige:26214 | Abstract | Article HTML | Article PDF
  
Novel cationic diaza-, azaoxo-, and dioxo[6]helicenes are readily prepared and functionalized selectively by orthogonal aromatic electrophilic and vicarious nucleophilic substitutions (see scheme). Reductions, cross-coupling, or condensation reactions introduce additional diversity and allow tuning of the absorption properties up to the near-infrared region. The diaza salts can be resolved into single enantiomers.
  
  • Analysis of the Experimental Data for Pure and Diluted [FexZn1–x(bbtr)3](ClO4)2 Spin-Crossover Solids in the Framework of a Mechanoelastic Model
    Pradip Chakraborty, Cristian Enachescu and Andreas Hauser
    European Journal of Inorganic Chemistry, 2013 (5-6) , 2013, p770-780
    Keywords: spin crossover;mechanoelastic model;doping;iron;zinc
    DOI:10.1002/ejic.201201193 | unige:26499 | Abstract | Article PDF
The mechanoelastic model is applied to reproduce the experimental relaxation and thermal transition curves as determined for crystals of pure and diluted {[FexZn1–x(bbtr)3](ClO4)2}∞ [bbtr = 1,4-di(1,2,3-triazol-1-yl)butane] spin-crossover systems. In the mechanoelastic model, the spin-crossover complexes are situated in a hexagonal planar lattice, which is similar to the 2D coordination polymer with (3,6) network topology of [Fe(bbtr)3](ClO4)2. These complexes are linked by springs, which simulate the elastic interactions between them. Owing to the change in volume of the complexes during the spin transition, an elastic force accompanies the switch of every complex. This force propagates through the entire lattice and causes a shift of all molecules in the system and thus results in a new nuclear configuration. First, the ability of the model to reproduce various shapes of thermal transition and relaxation curves in pure compounds is analyzed; these range from gradual to very steep and include hysteresis behavior for the former and from single exponential to sigmoidal or with several steps for the latter. A structural phase transition can also be accounted for by changing the shape of the sample at a fixed temperature from a regular to an elongated hexagon. Furthermore, the effect of adding Zn as a dopant in a mixed crystal series is discussed. The role of dopants on the cluster evolution is also analyzed directly and by using the correlation factor.
  • A Donor–Acceptor Tetrathiafulvalene Ligand Complexed to Iron(II): Synthesis, Electrochemistry, and Spectroscopy of [Fe(phen)2(TTF-dppz)](PF6)2
    Nathalie Dupont, Ying-Fen Ran, Shi-Xia Liu, Jakob Grilj, Eric Vauthey, Silvio Decurtins and Andreas Hauser
    Inorganic Chemistry, 52 (1) , 2013, p306-312
    DOI:10.1021/ic3019277 | unige:25113 | Abstract | Article HTML | Article PDF
  
The synthesis and photophysical properties of the complex [Fe(phen)2(TTF-dppz)]2+ (TTF-dppz = 4′,5′-bis-(propylthio)tetrathiafulvenyl[i]dipyrido[3,2-a:2′,3′-c]phenazine, phen = 1,10-phenanthroline) are described. In this complex, excitation into the metal–ligand charge transfer bands results in the population of a high-spin state of iron(II), with a decay lifetime of approximately 1.5 ns, in dichloromethane, at room temperature. An intraligand charge transfer state can also be obtained and has a lifetime of 38 ps. A mechanism for the different states reached is proposed based on transient absorption spectroscopy.
  
The excited-state dynamics of two triads consisting of a naphthalenediimide (cNDI) substituted at the core by two zinc (ZnP) or free-base tetraphenylporphyrins (FbP) was investigated by ultrafast fluorescence and transient absorption spectroscopy. The electronic absorption spectra of the triads are almost the composites of those of the constituents, pointing to a weak electronic coupling and to a localization of the excitation energy on one of the porphyrins. In cyclohexane, the excited-state dynamics of the triads are essentially the same as those of the individual porphyrins, with the exception of the Soret emission of the ZnP triad, whose lifetime exhibits a more than 10 fold shortening compared to ZnP. A similarly ultrafast fluorescence decay was measured in tetrahydrofuran and benzonitrile. In these two solvents, charge separation from the excited porphyrin to the cNDI was found to take place with ~1 ps and ~25 ps time constants in the ZnP and FbP triads, respectively. The build up of the charge-separated state population in the ZnP triad is independent on the excitation wavelength, indicating that charge separation takes place from the lowest singlet excited state. Charge recombination occurs with a time constant around 8 ps in both triads, i.e. is slower than charge separation in the ZnP triad but faster in the FbP triad. These differences are rationalized in terms of the driving forces for charge separation and recombination.
  • A Comparison of Sensitized Ln(III) Emission using Pyridine- and Pyrazine-2,6-Dicarboxylates - Part II
    Evan G Moore, Jakob Grilj, Eric Vauthey and Paola Ceroni
    Dalton Transactions, 42 (6) , 2013, p2075-2083
    DOI:10.1039/c2dt32229c | unige:26400 | Abstract | Article HTML | Article PDF
The synthesis, X-ray structures and photophysical properties of several new Ln(III) complexes with pyrazine-2,6-dicarboxylic acid (H2PYZ) that demonstrate excellent stability and solubility in non-aqueous solution are reported, and compared to structurally analogous complexes with pyridine-2,6-dicarboxylic acid (H2DPA). The Eu(III) and Yb(III) complexes demonstrate efficient metal centered luminescence in the visible and Near Infra-Red (NIR) regions respectively. Low temperature (77 K) phosphorescence measurements using the corresponding Gd(III) complex allowed the photophysical properties of the sensitizer to be rationalized, together with corresponding TD-DFT studies for a model complex. Lastly, we have evaluated the sensitization efficiencies for these complexes, and have undertaken femtosecond transient absorption (TA) measurements in order to evaluate the relative importance of the intersystem crossing and energy transfer processes involved with sensitized Ln(III) emission via the antennae effect.
  • First-Principles Simulation of Absorption Bands of Fluorenone in Zeolite L
    Xiuwen Zhou, Tomasz A. Wesolowski, Gloria Tabacchi, Ettore Fois, Gion Calzaferri and AndrĂŠ Devaux
    Physical Chemistry Chemical Physics, 15 (1) , 2013, p159-167
    DOI:10.1039/c2cp42750h | unige:24518 | Abstract | Article PDF
  
The absorption spectrum of fluorenone in zeolite L is calculated from first-principles simulations. The broadening of each band is obtained from the explicit treatment of the interactions between the chromophore and its environment in the statistical ensemble. The comparison between the simulated and measured spectra reveals the main factors affecting the spectrum of the chromophore in hydrated zeolite L. Whereas each distinguishable band is found to originate from a single electronic transition, the bandwidth is determined by the statistical nature of the environment of the fluorenone molecule. The K+...O=C motif is retained in all conformations. Although the interactions between K+ and the fluorenone carbonyl group result in an average lengthening of the C=O bond and in a redshift of the lowest energy absorption band compared to gas phase or non-polar solvents, the magnitude of this shift is noticeably smaller than the total shift. An important factor affecting the shape of the band is fluorenone’s orientation, which is strongly affected by the presence of water. The effect of direct interactions between fluorenone and water is, however, negligible.
  
  • Effect of Pressure on the Free Ion and Crystal Field Parameters of Sm2+ in BaFBr and SrFBr Hosts
    Prodipta Pal, Tiphaine PenhouĂŤt, Vincenza D'Anna and Hans Hagemann
    Journal of Luminescence, 134 , 2013, p678-685
    Keywords: Sm2+; high pressure luminescence; crystal field; BaFBr; SrFBr
    DOI:10.1016/j.jlumin.2012.07.010 | unige:24061 | Abstract | Article HTML | Article PDF
The emission spectra of Sm2+ doped in BaFBr and SrFBr hosts were measured at 10 K from ambient pressure to 8 GPa. The crystal field energy levels determined from the emission spectra were used to extract the free ion parameters (Fk and Μ ) and crystal field parameters (Bqk). The variation of Fk and Μ as a function of pressure was studied systematically and was discussed in relation to the central field and symmetry restricted covalency models. The change of the spin orbit coupling parameter (Μ) with pressure for SrFBr:Sm2+ showed very different behavior than in other matlockite hosts. Moreover the variation of Bqk under pressure was studied. The pressure dependence of the Bqk was described quantitatively using the Superposition Model (SM) with the help of structural parameters as a function of pressure, obtained from periodic DFT calculations. The validity of the SM was tested for Sm2+ in BaFBr and SrFBr. It is shown that this model does not apply to SrFBr, in contrast to other matlockite host materials.
  • Chromium(III)-trisoxalate, a versatile building block for luminescent materials
    Mia Milos and Andreas Hauser
    Journal of Luminescence, 133 , 2013, p15-20
    Keywords: chromium(III)-tris-oxalate; energy migration; fluorescence line narrowing; spectral diffusion; persistent spectral hole burning
    DOI:10.1016/j.jlumin.2011.12.053 | unige:24060 | Article HTML | Article PDF
  
Chromium(III)-trisoxalate,[Cr(ox)3]3- (ox = C2O42-), incorporated into polymeric networks of composition [NaCr(ox)3][MII(bpy)3] and [NaCr(ox)3][MIII(bpy)3]ClO4 (bpy= 2,2'-bipyridine, MII = Zn, Fe, Ru; MIII = Rh, Cr), results in interesting features ranging from phonon-assisted and resonant energy migration within the R1 line the 2E state to persistent spectral side-hole burning via the latter, and manifestations of specific nearest-neighbour π–π interactions between bipyridine and oxalate.
  
The ligand exchange reaction between racemic Au38(2-PET)24 (2-PET: 2-phenylethylthiolate) clusters and enantiopure 1,1’-binapththyl-2,2’-dithiol (BINAS) was monitored in situ using a chiral HPLC approach. In the first exchange step, a clear preference of R-BINAS towards the left-handed enantiomer of Au38(2-PET)24 is observed (about four times faster than reaction with the right-handed enantiomer). The second exchange step is drastically slowed compared to the first step. BINAS substitution deactivates the cluster for further exchange, which is attributed to (stereo)electronic effects. The results constitute the first example of a ligand exchange reaction in a thiolate-protected gold cluster with directed enrichment of a defined species in the product mixture. This may open new possibilities for the design of nanomaterials with tailored properties.
  • In situ ATR-IR spectroscopy study of adsorbed protein: Visible light denaturation of bovine serum albumin on TiO2
    Ahmed Bouhekka and Thomas BĂźrgi
    Applied Surface Science, 261 , 2012, p369-374
    Keywords: in situ spectroscopy; TiO2; visible light; BSA adsorption; denaturation; protein structure
    DOI:10.1016/j.apsusc.2012.08.017 | unige:24517 | Abstract | Article PDF
In this work in situ Fourier transform infrared-attenuated total reflection (FTIR-ATR) spectroscopy in a flow-through cell was used to study the effect of visible light irradiation on bovine serum albumin (BSA) adsorbed on porous TiO2 films. The experiments were performed in water at concentrations of 10−6 mol/l at room temperature. The curve fitting method of the second derivative spectra allowed us to explore details of the secondary structure of pure BSA in water and conformation changes upon adsorption as well as during and after illumination by visible light. The results clearly show that visible light influences the conformation of adsorbed BSA. The appearance of a shift of the amide I band, in the original spectra, from 1653 cm−1 to 1648 cm−1, is interpreted by the creation of random coil in the secondary structure of adsorbed BSA. The second derivative analysis of infrared spectra permits direct quantitative analysis of the secondary structural components of BSA, which show that the percentage of α-helix decreases during visible light illumination whereas the percentage of random coil increases.
  • Au40(SR)24 Cluster as a Chiral Dimer of 8-Electron Superatoms: Structure and Optical Properties
    Sami A. Malola, Lauri Lehtovaara, Stefan Knoppe, Kuo-Juei Hu, Richard E. Palmer, Thomas BĂźrgi and Hannu J. Häkkinen
    Journal of the American Chemical Society, 134 (48) , 2012, p19560-19563
    DOI:10.1021/ja309619n | unige:24516 | Abstract | Article HTML | Article PDF
We predict and analyze density-functional theory (DFT) -based structures for the recently isolated Au40(SR)24 cluster. Combining structural information extracted from ligand-exchange reactions, circular dichroism and transmission electron microscopy leads us to propose two families of low-energy structures that have a chiral Au-S framework on the surface. These families have a common geometrical motif where a non-chiral Au26 bi-icosahedral cluster core is protected by 6 RS-Au-SR and 4 RS-Au-SR-Au-SR oligomeric units, analogously to the “Divide and Protect” motif of known clusters Au25(SR)18-/0, Au38(SR)24 and Au102(SR)44. The strongly prolate shape of the proposed Au26 core is supported by transmission electron microscopy. Density-of-state-analysis shows that the electronic structure of Au40(SR)24 can be interpreted in terms of a dimer of two 8-electron superatoms, where the 8 shell electrons are localized at the two icosahedral halves of the metal core. The calculated optical and chiroptical characteristics of the optimal chiral structure are in a fair agreement with the reported data for Au40(SR)24.
  • Heteroleptic FeII Complexes of 2,2′-Biimidazole and Its Alkylated Derivatives: Spin-Crossover and Photomagnetic Behavior
    Hoa V. Phan, Pradip Chakraborty, Meimei Chen, Yitzi M. Calm, Kirill Kovnir, Lawrence K. Keniley, Jordan M. Hoyt, Elisabeth S. Knowles, CĂŠline Besnard, Mark W. Meisel, Andreas Hauser, Catalina Achim and Michael Shatruk
    Chemistry - A European Journal, 18 (49) , 2012, p15805-15815
    Keywords: iron complexes;LIESST effect;N ligands;photomagnetism;spin crossover
    DOI:10.1002/chem.201202045 | unige:24449 | Abstract | Article PDF
  
Three iron(II) complexes, [Fe(TPMA)(BIM)](ClO4)2⋅0.5H2O (1), [Fe(TPMA)(XBIM)](ClO4)2 (2), and [Fe(TPMA)(XBBIM)](ClO4)2 ⋅0.75CH3OH (3), were prepared by reactions of FeII perchlorate and the corresponding ligands (TPMA=tris(2-pyridylmethyl)amine, BIM=2,2′-biimidazole, XBIM=1,1′-(α,α′-o-xylyl)-2,2′-biimidazole, XBBIM=1,1′-(α,α′-o-xylyl)-2,2′-bibenzimidazole). The compounds were investigated by a combination of X-ray crystallography, magnetic and photomagnetic measurements, and Mössbauer and optical absorption spectroscopy. Complex 1 exhibits a gradual spin crossover (SCO) with T1/2=190 K, whereas 2 exhibits an abrupt SCO with approximately 7 K thermal hysteresis (T1/2=196 K on cooling and 203 K on heating). Complex 3 is in the high-spin state in the 2–300 K range. The difference in the magnetic behavior was traced to differences between the inter- and intramolecular interactions in 1 and 2. The crystal packing of 2features a hierarchy of intermolecular interactions that result in increased cooperativity and abruptness of the spin transition. In 3, steric repulsion between H atoms of one of the pyridyl substituents of TPMA and one of the benzene rings of XBBIM results in a strong distortion of the FeII coordination environment, which stabilizes the high-spin state of the complex. Both 1 and 2 exhibit a photoinduced low-spin to high-spin transition (LIESST effect) at 5 K. The difference in the character of intermolecular interactions of 1 and 2 also manifests in the kinetics of the decay of the photoinduced high-spin state. For 1, the decay rate constant follows the single-exponential law, whereas for 2 it is a stretched exponential, reflecting the hierarchical nature of intermolecular contacts. The structural parameters of the photoinduced high-spin state at 50 K are similar to those determined for the high-spin state at 295 K. This study shows that N-alkylation of BIM has a negligible effect on the ligand field strength. Therefore, the combination of TPMA and BIM offers a promising ligand platform for the design of functionalized SCO complexes.
  
  • Double active control of the plasmonic resonance of a gold nanoparticle array
    Luciano De Sio, Alastair Cunningham, Vanessa Verrina, Caterina Maria Tone, Roberto Caputo, Thomas BĂźrgi and Cesare Umeton
    Nanoscale, 4 (24) , 2012, p7619-7623
    DOI:10.1039/c2nr31426f | unige:24450 | Abstract | Article HTML | Article PDF
A two-fold active control of the plasmonic resonance of randomly distributed gold nanoparticles (GNPs) has been achieved. GNPs have been immobilized on an Indium Tin Oxide (ITO) coated glass substrate and then covered with a liquid crystalline compound. The system has been investigated by means of atomic force and scanning electron microscopy, revealing the presence of isolated and well distributed GNPs. The application of an external electric field to the sample has a two-fold consequence: the re-orientation of the hybrid-aligned liquid crystal layer and the formation of a carrier accumulation layer in the proximity of the ITO substrate. The refractive indices of both liquid crystal and accumulation layers are influenced by the applied field in a competitive way and produce a “dancing behavior” of the GNP’s plasmonic resonance spectral position.
  • Accurate Spin-State Energetics of Transition Metal Complexes: I. CCSD(T), CASPT2 and DFT Study of [M(NCH)6]2+ (M = Fe, Co)
    Latevi Max Lawson Daku, Francesco Aquilante, Timothy William Robinson and Andreas Hauser
    Journal of Chemical Theory and Computation, 8 (11) , 2012, p4216-4231
    DOI:10.1021/ct300592w | unige:24068 | Abstract | Article PDF
Highly accurate estimates of the high-spin/low-spin energy difference ΔEHLel in the high-spin complexes [Fe(NCH)6]2+ and [Co(NCH)6]2+ have been obtained from the results of CCSD(T) calculations extrapolated to the complete basis set limit. These estimates are shown to be strongly influenced by scalar relativistic effects. They have been used to assess the performances of the CASPT2 method and of 30 density functionals of the GGA, meta-GGA, global hybrid, RSH and double-hybrid types. For the CASPT2 method, the results of the assessment support the proposal [Kepenekian, M.; Robert, V.; Le Guennic, B. J. Chem. Phys.2009, 131, 114702] that the ionization potential–electron affinity (IPEA) shift defining the zeroth-order Hamiltonian be raised from its standard value of 0.25 au to 0.50–0.70 au for the determination of ΔEHLel in Fe(II) complexes with a [FeN6] core. At the DFT level, some of the assessed functionals proved to perform within chemical accuracy (±350 cm-1) for the spin-state energetics of [Fe(NCH)6]2+, others for that of [Co(NCH)6]2+, but none of them simultaneously for both complexes. As demonstrated through a reparametrization of the CAM-PBE0 range-separated hybrid, which led to a functional that performs within chemical accuracy for the spin-state energetics of both complexes, performing density functionals of broad applicability may be devised by including in their training sets highly accurate data like those reported here for [Fe(NCH)6]2+ and [Co(NCH)6]2+.
The strong coupling between planar arrays of gold and silver nanoparticles mediated by a near-field interaction is investigated both theoretically and experimentally to provide an in-depth study of symmetry breaking in complex nanoparticle structures. The asymmetric composition allows to probe for bright and dark eigenmodes, in accordance with plasmon hybridization theory. The strong coupling could only be observed by separating the layers by a nanometric distance with monolayers of suitably chosen polymers. The bottom-up assembly of the nanoparticles as well as the stratified structures themselves gives rise to an extremely flexible system that, moreover, allows the facile variation of a number of important material parameters as well as the preparation of samples on large scales. This flexibility was used to modify the coupling distance between arrays, showing that both the positions and relative intensities of the resonances observed can be tuned with a high degree of precision. Our work renders research in the field of “plasmonic molecules” mature to the extent that it could be incorporated into functional optical devices.
  • Tautomerization in 2,7,12,17-Tetraphenylporphycene and 9-Amino-2,7,12,17-tetraphenylporphycene: Influence of Asymmetry on the Direction of the Transition Moment
    Piotr Fita, Maria Pszona, Grazyna Orzanowska, David SĂĄnchez-GarcĂ­a, Santi Nonell, Eric Vauthey and Jacek Waluk
    Chemistry - A European Journal, 18 (41) , 2012, p13160-13167
    Keywords: hydrogen transfer;porphycenes;tautomerism;transition moments
    DOI:10.1002/chem.201201432 | unige:23089 | Abstract | Article PDF
Femtosecond transient absorption anisotropy studies have been performed for two porphycenes of different symmetry. In 2,7,12,17-tetraphenylporphycene, the chemical identity of two trans forms implies a change in the S0–S1 transition-moment direction upon tautomerization. Exploiting this phenomenon, the rates of double hydrogen transfer in both the S0 and S1 states (1.4×1012 s−1 and 2.7×1011 s−1, respectively) have been determined by performing time-resolved anisotropy studies. In the asymmetric 9-amino-2,7,12,17-tetraphenylporphycene, tautomerization occurs with a similar rate in the ground state. In the S1 state, the reaction is hindered in its vibrationally relaxed form, but the excitation spectra suggest that it may occur for an unrelaxed molecule. Unlike all porphycenes that have been studied so far, 9-amino-2,7,12,17-tetraphenylporphycene does not reveal significant changes in anisotropy owing to intramolecular double hydrogen transfer; rather, the transition-moment directions are similar in the two tautomeric forms. Analysis of the molecular orbitals allows for an explanation of the “locking” of the transition moments: it is due to a large splitting of the two HOMO orbitals, which retain the order of their energies in the two tautomers.
 
This study addresses the free energy dependence of charge recombination following photoinduced bimolecular electron transfer in three different solvents of either inert (acetonitrile and benzyl acetate) or reactive (N,N-dimethylaniline) character. Femtosecond time-resolved fluorescence and transient absorption have been used to determine the time scales for charge recombination. In pure N,N-dimethylaniline, charge recombination is found to be substantially slower than charge separation in a range of driving forces covering 1.5 eV. In all three solvents, the so-called Marcus inverted region is clearly observed for charge recombination. Additionally, the charge recombination step is found to be influenced by the solvent relaxation dynamics. A diffusion-reaction equation approach using an electron transfer model accounting for solvent relaxation is used to rationalize the experimental results.
  
  • A Spectroscopic and Computational Study of a Photoinduced Cross-Dehydrogenative Coupling Reaction of a Stable Semiquinone Radical
    Jakob Grilj, Tanya Kumanova Todorova, Chenyi Yi, Shi-Xia Liu, Eric Vauthey and Silvio Decurtins
    Chemistry - A European Journal, 18 (43) , 2012, p13605-13608
    Keywords: density functional calculations;photochemistry;radicals;transient absorption;transition states
    DOI:10.1002/chem.201201463 | unige:23743 | Abstract | Article PDF
Norrish-type-II reaction on a semiquinone radical: Stable semiquinone radicals serve as novel molecular platforms on which a Norrish-type-II photoreaction can be initiated. A detailed reaction scheme involving a 1,5-hydrogen transfer followed by a cyclization step that finally leads to a new C—C bond formation could be verified. Transient absorption spectroscopy and DFT calculations trace convincingly the intermediates and transition states along the reaction path (see scheme).
In methods based on frozen-density embedding theory or subsystem formulation of density functional theory, the non-additive kinetic potential (vtnad(r)) needs to be approximated. Since vtnad(r) is defined as a bifunctional, the common strategies rely on approximating vtnad[ρA,ρB](r). In this work, the exact potentials (not bifunctionals) are constructed for chemically relevant pairs of electron densities (ρA and ρB) representing: dissociating molecules, two parts of a molecule linked by a covalent bond, or valence and core electrons. The method used is applicable only for particular case, where ρA is a one-electron or spin-compensated two-electron density, for which the analytic relation between the density and potential exists. The sum ρA + ρB is, however, not limited to such restrictions. Kohn-Sham molecular densities are used for this purpose. The constructed potentials are analyzed to identify the properties which must be taken into account when constructing approximations to the corresponding bifunctional. It is comprehensively shown that the full von Weizsäcker component is indispensable in order to approximate adequately the non-additive kinetic potential for such pairs of densities.
  • Dye Bonding to TiO2: In Situ Attenuated Total Reflection Infrared Spectroscopy Study, Simulations, and Correlation with Dye-Sensitized Solar Cell Characteristics
    Barbara VĂślker, Florian WĂślzl, Thomas BĂźrgi and Dominic Lingenfelser
    Langmuir, 28 (31) , 2012, p11354-11363
    DOI:10.1021/la302197z | unige:22919 | Abstract | Article HTML | Article PDF
  
Processing dye-sensitized solar cells gains more and more importance as interest in industrial applications grows daily. For large-scale processing and optimizing manufacturing in terms of environmental acceptability as well as time and material saving, a detailed knowledge of certain process steps is crucial. In this paper we concentrate on the sensitizing step of production, i.e., the anchoring of the dye molecules onto the TiO2 semiconductor. A vacuum-tight attentuated total reflection infrared (ATR-IR) flow-through cell was developed, thus allowing measurements using a vacuum spectrometer to monitor infiltration of dye molecules into the porous TiO2 film in situ at high sensitivity. In particular, the influence of the anchor and backbone of perylene dye molecules as well as the influence of solvents on the adsorption process was investigated. The experiments clearly show that an anhydride group reacts much slower than an acid group. A significantly lower amount of anhydride dye can be adsorbed on the films. Ex situ transmission experiments furthermore indicate that the availability of OH groups on the TiO2 surface may limit dye adsorption. Also the backbone and base frame of the dye can influence the adsorption time drastically. Electrical cell characteristics correlate with the amount of adsorbed dye molecules determined by in situ ATR-IR measurements. The latter is also sensitive toward the diffusion of the dye through the porous layer. To gain a deeper understanding of the interplay between diffusion and adsorption, simulations were performed that allowed us to extract diffusion and adsorption constants. Again it was demonstrated that the anchoring group has a strong effect on the adsorption rate. The influence of the solvent was also studied, and it was found that both adsorption and desorption are affected by the solvent. Protic polar solvents are able to remove bound dye molecules, which is a possible pathway of cell degradation. Most importantly, the analysis shows the potential of this approach for the evaluation of molecules or additives concerning their characteristics important for cell processing.
  
  • Thermal and Light-Induced Spin Switching Dynamics in the 2D Coordination Network of {[Zn1-xFex(bbtr)3](ClO4)2}∞: The Role of Cooperative Effects
    Pradip Chakraborty, Cristian Enachescu, Christophe Walder, Robert Bronisz and Andreas Hauser
    Inorganic Chemistry, 51 (18) , 2012, p9714-9722
    DOI:10.1021/ic301006c | unige:22920 | Abstract | Article HTML | Article PDF
The thermal spin transition, the photoexcitation, and the subsequent spin relaxation in the mixed crystal series of the covalently linked two-dimensional network {[Zn1-xFex(bbtr)3](ClO4)2}∞ (x = 0.02–1, bbtr =1,4-di(1,2,3-triazol-1-yl)-butane) are discussed. In the neat compound, the thermal spin transition with a hysteresis of 13 K is accompanied by a crystallographic phase transition (Kusz, J.; Bronisz, R.; Zubko, M.; Bednarek, H. Chem. Eur. J.2011, 17, 6807). In contrast, the diluted crystals with x ≤ 0.1 stay essentially in the high-spin state down to low temperatures and show typical first order relaxation kinetics upon photoexcitation, and the structural phase transition is well separated from the spin transition. With increasing Fe(II) concentration, steeper thermal transitions and sigmoidal relaxation curves indicate increasingly important cooperative effects. Already at x = 0.38, the spin relaxation is governed by cooperative interactions between Fe(II) centers, and the crystallographic phase transition begins to influence the spin transition. The kinetic behavior of the thermal spin transition is reproduced within the framework of a dynamic mean-field model.
  • Excited-State Dynamics of Organic Dyes at Liquid/Liquid Interfaces
    M. Fedoseeva, S. Richert and E. Vauthey
    Langmuir, 28 , 2012, p11291-11301
    DOI:10.1021/la301505e | unige:22047 | Abstract | Article HTML | Article PDF
  
Liquid/liquid interfaces play a crucial role in numerous areas of science. However, direct spectroscopic access to this thin (∟1 nm) region is not possible with conventional optical methods. After a brief review of the most used techniques to perform interfacial optical spectroscopy, we will focus on time-resolved surface second harmonic generation, which allows the measurement of the excited-state dynamics of probe molecules at interfaces. By comparing these dynamics with those measured in bulk solutions, precious information on the properties of the interfacial region can be obtained. To illustrate this, several studies performed in our group will be presented.
  
  • Revealing the Bonding Pattern from the Molecular Electron Density Using Single Exponential Decay Detector: An Orbital-Free Alternative to the Electron Localization Function
    Piotr De Silva, Jacek Korchowiec and Tomasz A. Wesolowski
    ChemPhysChem, 13 (15) , 2012, p3462-3465
    Keywords: bonding;computational chemistry;electron density;electron localization function;single exponential decay detector
    DOI:10.1002/cphc.201200500 | unige:23744 | Abstract | Article PDF
We introduce a new tool (single exponential decay detector: SEDD) to extract information about bonding and localization in atoms, molecules, or molecular assemblies. The practical evaluation of SEDD does not require any explicit information about the orbitals. The only quantity needed is the electron density (calculated or experimental) and its derivatives up to the second order.
  • A Synthetic and Mechanistic Investigation of the Chromium Tricarbonyl-Mediated Masamune–Bergman Cyclization. Direct Observation of a Ground-State Triplet p-Benzyne Biradical
    Kai E.O. Ylijoki, SĂŠverine Lavy, Angelika Fretzen, E.Peter KĂźndig, ThĂŠo Berclaz, GĂŠrald Bernardinelli and CĂŠline Besnard
    Organometallics, 31 (15) , 2012, p5396-5404
    DOI:10.1021/om300427j | unige:22151 | Abstract | Article HTML | Article PDF
A new room-temperature chromium tricarbonyl-mediated cycloaromatization of enediynes is reported. The reaction occurs with both cyclic and acyclic enediynes in the presence of [Cr(CO)3(η6-naphthalene)] and both a coordinating solvent and a hydrogen atom source, providing chromium–arene complexes in reasonable yield and good diastereocontrol. The mechanism of the reaction has been probed through DFT computational and spectroscopic methods. These studies suggest that direct C1–C6 bond formation from an η6-enediyne complex is the lowest-energy path, forming a metal-bound p-benzyne biradical. NMR spectroscopy suggests that enediyne alkene coordination occurs in preference to alkyne coordination, forming a THF-stabilized olefin intermediate; subsequent alkyne coordination leads to cyclization. While biradical quenching occurs rapidly and primarily via the singlet biradical, the triplet state biradical is detectable by EPR spectroscopy, suggesting intersystem crossing to a triplet ground state.
  • Optimizing Millisecond Time Scale Near-Infrared Emission in Polynuclear Chrome(III)-Lanthanide(III) Complexes
    Lilit Aboshyan-Sorgho, Homayoun Nozary, Annina Aebischer, Jean-Claude G. BĂźnzli, , Kevin R. Kittilstved, Andreas Hauser, Svetlana V. Eliseeva, StĂŠphane Petoud and Claude Piguet
    Journal of the American Chemical Society, 134 (30) , 2012, p12675-12684
    DOI:10.1021/ja304009b | unige:22645 | Abstract | Article HTML | Article PDF
  
This work illustrates a simple approach for optimizing long-lived near-infrared lanthanide-centered luminescence using trivalent chromium chromophores as sensitizers. Reactions of the segmental ligand L2 with stoichiometric amounts of M(CF3SO3)2 (M = Cr, Zn) and Ln(CF3SO3)3 (Ln = Nd, Er, Yb) under aerobic conditions quantitatively yield the D3-symmetrical trinuclear [MLnM(L2)3](CF3SO3)n complexes (M = Zn, n = 7; M = Cr, n = 9), in which the central lanthanide activator is sandwiched between the two transition metal cations. Visible or NIR irradiation of the peripheral Cr(III) chromophores in [CrLnCr(L2)3]9+ induces rate-limiting intramolecular intermetallic Cr→Ln energy transfer processes (Ln = Nd, Er, Yb), which eventually produces lanthanide-centered near-infrared (NIR) or IR emission with apparent lifetimes within the millisecond range. As compared to the parent dinuclear complexes [CrLn(L1)3]6+, the connection of a second strong-field [CrN6] sensitizer in [CrLnCr(L2)3]9+ significantly enhances the emission intensity without perturbing the kinetic regime. This work opens novel exciting photophysical perspectives via the buildup of non-negligible population densities for the long-lived doubly excited state [Cr*LnCr*(L2)3]9+ under reasonable pumping powers.
Thiolate-protected gold nanoparticles and clusters combine size-dependent physical properties with the ability to introduce (bio)chemical functionality within their ligand shell. The engineering of the latter with molecular precision is an important prerequisite for future applications. A key question in this respect concerns the flexibility of the gold – sulfur interface. Here we report the first study on racemization of an intrinsically chiral gold nanocluster, Au38(SCH2CH2Ph)24, which goes along with a drastic rearrangement of its surface involving place exchange of several thiolates. This racemization takes place at modest temperatures (40 – 80 °C) without significant decomposition. The experimentally determined activation energy for the inversion reaction is ca 28 kcal/mol, which is surprisingly low considering the large rearrangement. The activation parameters furthermore indicate that the process occurs without complete Au-S bond breaking.
  
The effect of viscosity on the bimolecular electron transfer quenching of a series of coumarins by N,N-dimethylaniline was investigated using steady-state and time-resolved fluorescence spectroscopy. The data reveal that the static and transient stages of the quenching become dominant as viscosity increases. When extracting the quenching rate constants using a simple Stern–Volmer analysis, a decrease of the rate constant with increasing driving force is observed above ~2 cP. However, this apparent Marcus inverted region, already reported several times with the same system in micelles and room temperature ionic liquids, totally vanishes when analyzing the data with a model accounting for the static and transient stages of the quenching. It appears that the apparent Marcus inverted region arises from the neglect of these quenching regimes together with the use of fluorophores with different excited-state lifetimes.
 
The excited-state dynamics of a series of Wurster’s salts (p-phenylenediamine radical cations) with different subtituents on the nitrogen atoms was investigated under a variety of experimental conditions using a combination of ultrafast spectroscopic techniques. At room temperature, the lifetime of the lowest excited state of all radical cations is on the order of 200 fs, independently of the solvent, that is, water, nitriles, alcohols, and room-temperature ionic liquid. On the other hand, all cations, except that with the bulky nitrogen substituents, become fluorescent below 120 K. The observed dynamics can be accounted for by the presence of a conical intersection between the D1 and D0 states. For the cations with a small nitrogen substituent, this conical intersection could be accessed through a twist of one amino group, as already suggested for Wurster’s Blue. However, this coordinate cannot be invoked for the cation with bulky nitrogen subtituents, and more probably, pyramidalization of the nitrogen center and/or deformation of the phenyl ring play an important role. Consequently, the excited-state dynamics of these structurally very similar Wurster’s salts involves different decay mechanisms.
  • Unravelling the Hydration Structure of ThX4 (X = Br, Cl) Water Solutions by Molecular Dynamics Simulations and X-ray Absorption Spectroscopy
    Riccardo Spezia, Cesar Beuchat, Rodolphe Vuilleumier, Paola D'Angelo and Laura Gagliardi
    The Journal of Physical Chemistry B, 116 (22) , 2012, p6465-6475
    DOI:10.1021/jp210350b | unige:21867 | Abstract | Article HTML | Article PDF
The hydration of Th(IV) in ThCl4 and ThBr4 water solutions at different salt concentrations was studied in order to understand the structure of Th(IV) in liquid water and the effect of Br– and Cl– anions on its hydration structure. Several theoretical methods were employed: density functional theory and classical molecular dynamics based on both semiempirical polarizable potentials and ab initio derived polarizable potentials. The results of the computations were combined with extended X-ray absorption fine structure (EXAFS) experimental data. The results of this study show that in pure water the Th–O distance of 2.45 Å corresponds to a first shell coordination number between 9 and 10. In the salt solutions, while Br– does not affect directly the hydration of Th(IV) also at relatively high concentrations, Cl–, on the other hand, is more structured around Th(IV), in agreement with recent high-energy X-ray scattering experiments. Counterions, even at relatively high concentrations (0.8 m), do not enter in the first solvation shell of Th(IV), but they induce an increase of water molecules in the first and second hydration shells of Th(IV).
  
Although liquid/liquid and air/liquid interfaces are omnipresent, very little is known up to now about the dynamics of processes occurring at such interfaces. As a detailed understanding of these processes could be of invaluable technological, environmental, and medical importance, considerable effort has been invested over the last two decades in developing new interface-selective techniques that allow for gaining further insight into the dynamics of these processes. Whereas several major results have been achieved that helped to contribute to a deeper understanding, there are still many aspects concerning the properties of liquid interfaces that are not yet fully understood. In this Perspective, the work that has been carried out so far on photoinduced interfacial dynamics will be reviewed and the current challenges in this still emerging field of research discussed.
  • Pure-state noninteracting v-representability of electron densities from Kohn-Sham calculations with finite basis sets
    Piotr De Silva and Tomasz Wesolowski
    Physical Review A, 85 (3) , 2012, p32518
    DOI:10.1103/PhysRevA.85.032518 | unige:21865 | Abstract | Article PDF
  
Within the linear combination of atomic orbitals (LCAO) approximation, one can distinguish two different Kohn-Sham potentials. One is the potential available numerically in calculations, and the other is the exact potential corresponding to the LCAO density. The latter is usually not available, but can be obtained from the total density by a numerical inversion procedure or, as is done here, analytically using only one LCAO Kohn-Sham orbital. In the complete basis-set limit, the lowest-lying Kohn-Sham orbital suffices to perform the analytical inversion, and the two potentials differ by no more than a constant. The relation between these two potentials is investigated here for diatomic molecules and several atomic basis sets of increasing size and quality. The differences between the two potentials are usually qualitative (wrong behavior at nuclear cusps and far from the molecule even if Slater-type orbitals are used) and δ-like features at nodal planes of the lowest-lying LCAO Kohn-Sham orbital. Such nodes occur frequently in LCAO calculations and are not physical. Whereas the behavior of the potential can be systematically improved locally by the increase of the basis sets, the occurrence of nodes is not correlated with the size of the basis set. The presence of nodes in the lowest-lying LCAO orbital can be used to monitor whether the effective potential in LCAO Kohn-Sham equations can be interpreted as the potential needed for pure-state noninteracting v-representability of the LCAO density. Squares of such node-containing lowest-lying LCAO Kohn-Sham orbitals are nontrivial examples of two-electron densities which are not pure-state noninteracting v-representable.
  
Chirality unveiled: Thiolate-protected Au40(SR)24 clusters were enantioenriched using an HPLC approach. CD spectra show strong mirror-image responses, indicating the intrinsic chirality of a cluster of unknown structure protected with achiral ligands.
  • Rapid screening of phytosterols in orange juice by solid-phase microextraction on polyacrylate fibre derivatisation and gas chromatographic-mass spectrometric
    SĂŠbastien Balme and Fazil O. GĂźlaçar
    Food Chemistry, 132 (1) , 2012, p613-618
    DOI:10.1016/j.foodchem.2011.10.097 | unige:21624 | Abstract | Article HTML | Article PDF
The potential of solid-phase microextraction on polyacrylate coated fibre, with sequential or simultaneous trimethylsilyl derivatisation followed by gas chromatographic–mass spectrometric analysis, was evaluated for a rapid determination of the distribution of the phytosterols in aqueous food matrixes. Influences of different parameters (bis(trimethylsilyl)trifluoro-acetamide and sterol exposure time, sterol concentration and experimental protocol) on the recovery of sterols were investigated to determine optimum conditions which were tested for sterol extraction and analysis from orange juice. Best selectivity, sterol recovery and derivatisation yields were obtained by extraction and simultaneous derivatisation through immersion of the SPME-PA fibre in the orange juice (10 min, 65 °C) after headspace absorption of BSTFA (30 min, 65 °C) on the fibre. Nevertheless the method developed cannot be used for quantitative analysis. But the possibility to effect rapid screen of phytosterol containing in complex media have been shown.
 
Silver nanoclusters protected by 2-phenylethanethiol (1), 4-fluorothiophenol (2), and l-glutathione (3) ligands were successfully synthesized. The optical properties of the prepared silver nanoclusters were studied. The absorption signal of Ag@SCH2CH2Ph in toluene can be found at 469 nm, and Ag@SPhF in THF shows two absorption bands at 395 and 462 nm. Ag@SG in water absorbs at 478 nm. Mie theory in combination with the Drude model clearly indicates the peaks in the spectra originate from plasmonic transitions. In addition, the damping constant as well as the dielectric constant of the surrounding medium was determined. In addition, the CD spectra of silver nanoclusters protected by the three ligands (1–3) were also studied. As expected, only the clusters of type 3 gave rise to chiroptical activity across the visible and near-ultraviolet regions. The location and strength of the optical activity suggest an electronic structure of the metal that is highly sensitive to the chiral environment imposed by the glutathione ligand. The morphology and size of the prepared nanoclusters were analyzed by using transmission electron microscopy (TEM). TEM analysis showed that the particles of all three types of silver clusters were small than 5 nm, with an average size of around 2 nm. The analysis of the FTIR spectra elucidated the structural properties of the ligands binding to the nanoclusters. By comparing the IR absorption spectra of pure ligands with those of the protected silver nanoclusters, the disappearance of the S–H vibrational band (2535–2564 cm–1) in the protected silver nanoclusters confirmed the anchoring of ligands to the cluster surface through the sulfur atom. By elemental analysis and thermogravimetric analysis, the Ag/S ratio and, hence, the number of ligands surrounding a Ag atom could be determined.
  
  • Bimetallic Borohydrides in the System M(BH4)2–KBH4 (M = Mg, Mn): On the Structural Diversity
    Pascal Schouwink, Vincenza D'Anna, Morten Brix Ley, LatĂŠvi Max Lawson Daku, Bo Richter, Torben R. Jensen, Hans Hagemann and Radovan CernĂ˝
    The Journal of Physical Chemistry C, 116 (20) , 2012, p10829-10840
    DOI:10.1021/jp212318s | unige:21580 | Abstract | Article HTML | Article PDF
Four novel bimetallic borohydrides have been discovered, K2M(BH4)4 (M = Mg or Mn), K3Mg(BH4)5, and KMn(BH4)3, and are carefully investigated structurally as well as regarding their decomposition reaction mechanism by means of in situ synchrotron radiation powder X-ray diffraction (SR-PXD), vibrational spectroscopies (Raman and IR), thermal analysis (TGA and DTA), and ab initio density functional theory (DFT) calculations. Mechano-chemical synthesis (ball-milling) using the reactants KBH4, α-Mg(BH4)2, and α-Mn(BH4)2 ensures chlorine-free reaction products. A detailed structural analysis reveals significant similarities as well as surprising differences among the two isomorphs K2M(BH4)4, most importantly concerning the extent to which the complex anion [M(BH4)4]2– is isolated in the structure. Anisotropic thermal expansion and an increase in symmetry at high temperatures in K3Mg(BH4)5 is ascribed to the motion of BH4 groups inducing hydrogen repulsive effects, and the dynamics of K3Mg(BH4)5 are investigated. Decomposition in the manganese system proceeds via the formation of KMn(BH4)3, the first perovkite type borohydride reported to date.
  • Strong non-linear effects in the chiroptical properties of the ligand-exchanged Au38 and Au40 clusters
    Stefan Knoppe, Amala Dass and Thomas BĂźrgi
    Nanoscale, 4 (14) , 2012, p4211-4216
    DOI:10.1039/C2NR30498H | unige:21864 | Abstract | Article HTML | Article PDF
Ligand exchange reactions on size-selected Au38(2-PET)24 and Au40(2-PET)24 clusters (2-PET: 2-phenylethylthiol) with mono- and bidentate chiral thiols was performed. The reactions were monitored with MALDI mass spectrometry and the arising chiroptical properties were compared to the number of incorporated chiral ligands. Only a small fraction of chiral ligands is needed to induce significant optical activity to the clusters. The use of monodentate camphor-10-thiol (CamSH) leads to comparably fast exchange on both clusters. The arising optical activity is weak. In contrast, the use of bidentate 1,1’-binaphthyl-2,2’-dithiol (BINAS) is slow, but the optical activity measured is strong. Moreover, a non-linear behaviour between optical activity and number of chiral ligands is found in the BINAS case for both Au38 and Au40, which may indicate different exchange rates of enantiopure BINAS with the enantiomers of inherently chiral clusters. This is ascribed to effects arising from the bidentate nature of BINAS. This is the first study where chiroptical effects are directly correlated with the composition of the ligand shell.
  • First enantioseparation and circular dichroism spectra of Au38 clusters protected with achiral ligands
    Igor Dolamic, Stefan Knoppe, Amala Dass and Thomas BĂźrgi
    Nature Communications, 3 , 2012, p798
    DOI:10.1038/ncomms1802 | unige:20260 | Abstract
  
Bestowing chirality to metals is central in fields such as heterogeneous catalysis and modern optics. Although the bulk phase of metals is symmetric, their surfaces can become chiral through adsorption of molecules. Interestingly, even achiral molecules can lead to locally chiral, though globally racemic, surfaces. A similar situation can be obtained for metal particles or clusters. Here we report the first separation of the enantiomers of a gold cluster protected by achiral thiolates, Au38(SCH2CH2Ph)24, achieved by chiral high-performance liquid chromatography. The chirality of the nanocluster arises from the chiral arrangement of the thiolates on its surface, forming 'staple motifs'. The enantiomers show mirror-image circular dichroism responses and large anisotropy factors of up to 4×10−3. Comparison with reported circular dichroism spectra of other Au38 clusters reveals that the influence of the ligand on the chiroptical properties is minor.
  • Asymmetric C(sp3)-H/C(Ar) coupling reactions. Highly enantio-enriched indolines via regiodivergent reaction of a racemic mixture
    Dmitry Katayev, Masafumi Nakanishi, Thomas BĂźrgi and E.Peter KĂźndig
    Chemical Science, 3 (5) , 2012, p1422-1425
    DOI:10.1039/c2sc20111a | unige:19404 | Abstract | Article HTML | Article PDF
N-Aryl, N-branched alkyl carbamates react with in situ generated chiral Pd-NHC catalysts by coupling a Pd-Ar moiety with an aliphatic C–H bond at high temperature to give enantioenriched 2-substituted and 2,3-disubstituted indolines. Prochiral precursors give single products with very high asymmetric induction. Chiral racemic precursors react in a regiodivergent reaction of a racemic mixture to yield enantioenriched indolines resulting from either methyl C–H activation or asymmetric methylene C–H activation. In favorable cases this can result in a complete separation of an enatiomeric mixture into two different highly enantioenriched indolines.
  
Colloidal Mn2+-doped semiconductor nanocrystals such as Mn2+:ZnSe have attracted broad attention for potential applications in phosphor and imaging technologies. Here, we report saturation of the sensitized Mn2+ photoluminescence intensity at very low continuous-wave (CW) and quasi-CW photoexcitation powers under conditions that are relevant to many of the proposed applications. Time-resolved photoluminescence measurements and kinetic modeling indicate that this saturation arises from an Auger-type nonradiative cross relaxation between an excited Mn2+ ion and an exciton within the same nanocrystal. A lower limit of k = 2 × 1010 s–1 is established for the fundamental rate constant of the Mn2+(4T1)-exciton cross relaxation.
  • Photoinduced Symmetry-Breaking Charge Separation
    Eric Vauthey
    ChemPhysChem, 13 , 2012, p2001-2011
    Keywords: excitation energy hopping;multichromophoric systems;photoinduced electron transfer;photochemistry;ultrafast spectroscopy
    DOI:10.1002/cphc.201200106 | unige:21591 | Abstract | Article PDF
  
Molecular systems where several apparently equivalent charge separation pathways exist upon photoexcitation are presented. They encompass MQn (n≥2) architectures, where M is a chromophore and Q an electron transfer quencher (either donor or acceptor), and M–M systems where M acts as both electron donor and acceptor. In all cases, charge separation involves symmetry breaking. The conditions for such process to be operative as well as the origin of the symmetry breaking are discussed.
  
  • A mixed-cation mixed-anion borohydride NaY(BH4)2Cl2
    Dorthe B. RavnsbĂŚk, Morten B. Ley, Young-Su Lee, Hans Hagemann, Vincenza D'Anna, Young Whan Cho, Yaroslav Filinchuk and Torben R. Jensen
    International Journal of Hydrogen Energy, 37 (10) , 2012, p8428-8438
    Keywords: hydrides; X-ray diffraction; solid state structure; solid phase synthesis; transition metal
    DOI:10.1016/j.ijhydene.2012.02.130 | unige:21592 | Article HTML | Article PDF
Hydrogen production from waste feedstocks using supercritical water gasification (SCWG) is a promising approach towards cleaner fuel production and a solution for hard to treat wastes. In this study, the catalytic co-gasification of starch and catechol as models of carbohydrates and phenol compounds was investigated in a batch reactor at 28 MPa, 400–500 °C, from 10 to 30 min. The effects of reaction conditions, and the addition of calcium oxide (CaO) as a carbon dioxide (CO2) sorbent and TiO2 as catalyst on the gas yields and product distribution were investigated. Employing TiO2 as a catalyst alone had no significant effect on the H2 yield but when combined with CaO increased the hydrogen yield by 35% and promoted higher total organic carbon (TOC) reduction efficiencies. The process liquid effluent was characterized using GC–MS, with the results showing that the major non-polar components were phenol, substituted phenols, and cresols. An overall reaction scheme is provided.
  • Ligand dependence of the synthetic approach and chiroptical properties of a magic cluster protected with a bicyclic thiolate
    Stefan Knoppe, Nuwan Kothalawala, Vijay Jupally, Amala Dass and Thomas BĂźrgi
    ChemComm, 48 , 2012, p4630-4632
    DOI:10.1039/C2CC00056C | unige:20258 | Abstract | Article HTML | Article PDF
Chiral gold clusters stabilised by enantiopure thiolates were prepared, size-selected and characterised by Circular Dichroism and mass spectrometry. The product distribution is found to be ligand dependent. Au25 clusters protected with camphorthiol show clear resemblance of their chiroptical properties with their glutathionate analogue.
 
The photophysical properties of a series of helicene cations in various solvents have been investigated using stationary and time-resolved spectroscopy. These compounds fluoresce in the near infrared region with a quantum yield ranging between 2 and 20% and a lifetime between 1 and 12 ns, depending of the solvent. No clear solvent dependence could be recognized except for a decrease of fluorescence quantum yield and lifetime with increasing hydrogen-bond donating ability of the solvent. In water, the helicene cations undergo aggregation. This effect manifests itself by the presence of a slow fluorescence decay component, whose amplitude increases with dye concentration, and by a much slower decay of the polarization anisotropy in water compared to an organic solvent of similar viscosity. However, aggregation has essentially no effect on the stationary fluorescence spectrum, whereas relatively small changes can be seen in the absorption spectrum. Analysis of the dependence of aggregation on the dye concentration reveals that the aggregates are mostly dimers and that the aggregation constant is substantially larger for hetero- than homochiral dimers.
  
  • The influence of defects formed by Ca excess and thermal post-treatments on the persistent luminescence of CaTiO3:Pr
    Eugenio H. Otal, Alexandra E. Maegli, Nina Vogel-Schäuble, Bernhard Walfort, Hans Hagemann, Songhak Yoon, Albert Zeller and Anke Weidenkaff
    Optical Materials Express, 2 (4) , 2012, p405-412
    DOI:10.1364/OME.2.000405 | unige:18791 | Abstract | Article HTML | Article PDF
Red emitting CaTiO3:Pr phosphors with a nominal composition of Ca0.998+xPr0.002TiO3+δ (0.02≤x≤0.04) were prepared by solid state reactions with different thermal post treatments and characterized by X-ray diffraction, transmission electron microscopy and photoluminescence. The Ca excess exhibited complete solubility up to 4% in the samples treated at 1400 °C but segregation in the form of Ruddlesden-Popper phases (Ca3Ti2O7 - Ca4Ti3O10) was observed in samples prepared at 1500 °C. The increase in temperature for stoichiometric samples showed a monotonic increase of decay time due to the reduction of non-radiative recombination defects. It was found that the Ca excess favored the formation of oxygen vacancies which are known to act as trap. In the samples treated at 1400 °C, 3% of Ca excess showed to be the best concentration to increase the decay time of persistent luminescence. For the samples treated at 1500 °C, the segregation of Ruddlesden-Popper phases left a constant amount of Ca soluble in all the CaTiO3 samples. This constant concentration of Ca caused the same density of defects and, consequently, the same decay time in all samples.
  • Lanthanide-mediated triangular cationic assemblies: structural and physico-chemical properties
    Soumaila Zebret, Nathalie Dupont, CĂŠline Besnard, GĂŠrald Bernardinelli and Josef Hamacek
    Dalton Transactions, 41 (16) , 2012, p4817-4824
    DOI:10.1039/c2dt12227h | unige:20257 | Abstract | Article HTML | Article PDF
This contribution investigates LnIII complexes formed with a small ditopic ligand, L1, and their structural, thermodynamic and photophysical properties. The spectrophotometric and NMR titrations evidence the triangular assemblies [Ln3(L1-H)3]6+ at stoichiometric conditions and their properties are discussed in relation to L2-containing analogues. In addition, the dinuclear species, [Ln2(L1-H)]5+, is observed with an excess of metal.
  • Optical sensitization and upconversion in discrete polynuclear chromium–lanthanide complexes
    Lilit Aboshyan-Sorgho, Martine Cantuel, Stephane Petoud, Andreas Hauser and Claude Piguet
    Coordination Chemistry Reviews, 256 (15-16) , 2012, p1644-1663
    DOI:10.1016/j.ccr.2011.12.013 | unige:21642 | Abstract | Article PDF
Due to its extreme kinetic inertness, trivalent chromium, Cr(III), has been rarely combined with labile trivalent lanthanides, Ln(III), to give discrete self-assembled (supra)molecular polynuclear complexes. However, the plethora of accessible metal-centered excited states possessing variable lifetimes and emissive properties, combined with the design of efficient intramolecular Cr(III) ↔ Ln(III) energy transfer processes open attractive perspectives for programming directional light-conversion within these heterometallic molecules. Efforts made to address this exciting challenge for both light-sensitization and light-upconversion are discussed in this article.
  • Crystal chemistry in the barium fluoride chloride system
    Hans Hagemann, Vincenza D'Anna, Latevi Max Lawson Daku and Frank Kubel
    Crystal Growth & Design, 12 (3) , 2012, p1124-1131
    DOI:10.1021/cg201588s | unige:18790 | Abstract | Article PDF
  
The crystal chemistry of the barium fluoride chloride system is studied both experimentally and theoretically. Different synthetic approaches yield nanocrystalline materials as well as large single crystals. The crystalline phases identified so far are BaFCl, Ba12F19Cl5 and Ba7F12Cl2 (in two modifications) and compared with analogous compounds. It is demonstrated that the compound Ba2F3Cl reported by Fessenden and Lewin 50 years ago corresponds to Ba7F12Cl2. The phase diagram of the BaCl2 – BaF2 system is reinvestigated for fluoride mole fractions between 0.5 and 1. The peritectic formation of Ba12F19Cl5 is observed. Periodic DFT calculations are performed for all structures in this system, including a hypothetical structure for Ba2F3Cl, based on the experimental structure of Ba2H3Cl. The energy of formation of the different barium fluoride chloride compounds from BaCl2 and BaF2 (normalized for one barium atom per formula unit), as calculated by DFT at 0K, is within only about ± 15 kJ/mol. Comparison with recent experimental results on calcium and strontium hydride chloride (bromide) compounds, suggest the possibility of a mutual exclusion between the M2X3Y and M7X12Y2 (M = Ca, Sr, Ba, Pb, X = H, F, Y = Cl,Br) structures. The single crystal structure of PbFBr is also reported.
  
In the covalently linked 2D coordination network {[Fe(bbtr)3](BF4)2}∞, bbtr = 1,4-di(1,2,3-triazol-1-yl)butane, the iron(II) centers stay in the high-spin (HS) state down to 10 K. They can, however, be quantitatively converted to the low-spin (LS) state by irradiating into the near-IR spin allowed 5dd band and back again by irradiating into the visible 1dd band. The compound shows true light-induced bistability below 100 K, thus, having the potential for persistent bidirectional optical switching at elevated temperatures.
 
The fluorescence quenching of 3-cyanoperylene upon electron transfer from N,N-dimethylaniline in three room-temperature ionic liquids (RTILs) and in binary solvent mixtures of identical viscosity has been investigated using steady-state and time-resolved fluorescence spectroscopy. This study was stimulated by previous reports of bimolecular electron transfer reactions faster by one or several orders of magnitude in RTILs than in conventional polar solvents. These conclusions were usually based on a comparison with data obtained in low-viscous organic solvents and extrapolated to higher viscosities and not by performing experiments at similar viscosities as those of the RTILs, which we show to be essential. Our results reveal that (i) the diffusive motion of solutes in both types of solvents is comparable, (ii) the intrinsic electron transfer step is controlled by the solvent dynamics in both cases, being slower in the RTILs than in the conventional organic solvent of similar viscosity, and (iii) the previously reported reaction rates much larger than the diffusion limit at low quencher concentration in RTILs originate from a neglect of the static and transient stages of the quenching, which are dominant in solvents as viscous as RTILs.
  
  • NMR Study of Reorientational Motion in Alkaline-Earth Borohydrides: β and γ Phases of Mg(BH4)2 and α and β Phases of Ca(BH4)2
    Alexei V. Soloninin, Olga A. Babanova, Alexander V. Skripov, Hans Hagemann, Bo Richter, Torben R. Jensen and Yaroslav Filinchuk
    Journal of Physical Chemistry C, 116 (7) , 2012, p4913-4920
    DOI:10.1021/jp210509g | unige:18736 | Abstract | Article HTML | Article PDF
To study the reorientational motion of BH4 groups in β and γ phases of Mg(BH4)2 and in α and β phases of Ca(BH4)2, we have performed nuclear magnetic resonance (NMR) measurements of the 1H and 11B spin–lattice relaxation rates in these compounds over wide ranges of temperature and resonance frequency. It is found that at low temperatures the reorientational motion in β phases of Mg(BH4)2 and Ca(BH4)2 is considerably faster than in other studied phases of these alkaline-earth borohydrides. The behavior of the measured spin–lattice relaxation rates in both β phases can be satisfactorily described in terms of a Gaussian distribution of activation energies Ea with the average Ea values of 138 meV for β-Mg(BH4)2and 116 meV for β-Ca(BH4)2. The α phase of Ca(BH4)2 is characterized by the activation energy of 286 ± 7 meV. For the novel porous γ phase of Mg(BH4)2, the main reorientational process responsible for the observed spin–lattice relaxation rate maximum can be described by the activation energy of 276 ± 5 meV. The barriers for reorientational motion in different phases of alkaline-earth borohydrides are discussed on the basis of changes in the local environment of BH4 groups.
  • Computational Insights into Uranium Complexes Supported by Redox-Active α-Diimine Ligands
    Giovanni Li Manni, Justin R. Walensky, Steven J. Kraft, William P. Forrest, Lisa M. PĂŠrez, Michael B. Hall, Laura Gagliardi and Suzanne C. Bart
    Inorganic Chemistry, 51 (4) , 2012, p2058-2064
    DOI:10.1021/ic202522w | unige:18735 | Abstract | Article HTML | Article PDF
The electronic structures of two uranium compounds supported by redox-active α-diimine ligands, (MesDABMe)2U(THF) (1) and Cp2U(MesDABMe) (2) (MesDABMe = [ArN═C(Me)C(Me)═NAr]; Ar = 2,4,6-trimethylphenyl (Mes)), have been investigated using both density functional theory and multiconfigurational self-consistent field methods. Results from these studies have established that both uranium centers are tetravalent, that the ligands are reduced by two electrons, and that the ground states of these molecules are triplets. Energetically low-lying singlet states are accessible, and some transitions to these states are visible in the electronic absorption spectrum.
  • Photophysical Properties of {[Au(CN)2]−}2 Dimers Trapped in a Supramolecular Electron-Acceptor Organic Framework
    Ahmed S. Abouelwafa, Christopher E. Anson, Andreas Hauser, Howard H. Patterson, François Baril-Robert, Xiaobo Li and Annie K. Powell
    Inorganic Chemistry, 51 (3) , 2012, p1294-1301
    DOI:10.1021/ic201109u | unige:18374 | Abstract | Article HTML | Article PDF
  
Dicyanoaurate reacts with the organic acceptor molecule, 1,1′-bis-(2,4-dinitrophenyl)-4,4′-bipyridinium, DNP, to form a supramolecular complex with the general formula {[Au(CN)2]2DNP}·4H2O. The complex was characterized by X-ray crystallography, and its photophysical properties were investigated in the solid-state. Although the initial (DNP)Cl2 compound does not show photoluminescence behavior and the dicyanoaurate shows photoluminescence only in the UV range, the resulting supramolecular complex displays two simultaneous, essentially independent, photoluminescence bands in the visible range originating from individual contributions of the DNP unit and the dicyanoaurate dimers. This unusual simultaneous photoluminescence behavior displayed by both the dicyanoaurate donor units and the redox-active 4,4′-bipyridinium acceptor have lifetimes of 0.5 μs and several hundred μs, respectively.
  
The photophysical properties of the free neutral radical galvinoxyl were studied by a combination of femtosecond time-resolved spectroscopy and quantum chemical calculations. The electronic absorption spectrum is dominated by an intense band at 430 nm that is ascribed to the D9,10←D0 transitions. Upon photoexcitation at 400 nm, the population of the D9,10 states decays within less than 200 fs to the electronic ground state. This ultrafast internal conversion does not involve intramolecular modes with large amplitude motion as the measured dynamics does not show any significant dependence on the environment, but is most probably facilitated by a high density of electronic states of different character. Depending on the solvent, a weak transient band due to the galvinoxylate anion is also observed. This closed-shell species, which is fluorescent although its deactivation is also dominated by non-radiative decay, is generated upon biphotonic ionization of the solvent and electron capture. The ultrashort excited-state lifetime of the galvinoxyl radical precludes photoinduced disproportionation previously claimed to be at the origin of the formation of both anion and cation.
  • Assessing Metal-Metal Multiple Bonds in Cr—Cr, Mo—Mo, and W—W Compounds and a Hypothetical U—U Compound: A Quantum Chemical Study Comparing DFT and Multireference Methods
    Giovanni Li Manni, Allison L. Dzubak, Abbas Mulla, David W. Brogden, John F. Berry and Laura Gagliardi
    Chemistry - A European Journal, 18 (6) , 2012, p1737-1749
    Keywords: bond theory;density functional calculations;electronic states;metal|metal interactions;quantum chemistry
    DOI:10.1002/chem.201103096 | unige:18373 | Abstract | Article PDF
To gain insights into the trends in metal–metal multiple bonding among the Group 6 elements, density functional theory has been employed in combination with multiconfigurational methods (CASSCF and CASPT2) to investigate a selection of bimetallic, multiply bonded compounds. For the compound [Ar-MM-Ar] (Ar=2,6-(C6H5)2-C6H3, M=Cr, Mo, W) the effect of the Ar ligand on the M2 core has been compared with the analogous [Ph-MM-Ph] (Ph=phenyl, M=Cr, Mo, W) compounds. A set of [M2(dpa)4] (dpa=2,2′-dipyridylamide, M=Cr, Mo, W, U) compounds has also been investigated. All of the compounds studied here show important multiconfigurational behavior. For the Mo2 and W2 compounds, the σ2π4δ2 configuration dominates the ground-state wavefunction, contributing at least 75 %. The Cr2 compounds show a more nuanced electronic structure, with many configurations contributing to the ground state. For the Cr, Mo, and W compounds the electronic absorption spectra have been studied, combining density functional theory and multireference methods to make absorption feature assignments. In all cases, the main features observed in the visible spectra may be assigned as charge-transfer bands. For all compounds investigated the Mayer bond order (MBO) and the effective bond order (EBO) were calculated by density functional theory and CASSCF methods, respectively. The MBO and EBO values share a similar trend toward higher values at shorter normalized metal–metal bond lengths.
  • Shifts in Excitation Energies Induced by Hydrogen Bonding: A Comparison of the Embedding and Supermolecular Time-Dependent Density Functional Theory Calculations with the Equation-of-Motion Coupled-Cluster Results
    Georgios Fradelos, Jesse J. Lutz, Tomasz A. Wesolowski, Piotr Piecuch and Marta Wloch
    in "Progress in Theoretical Chemistry and Physics" Advances in the Theory of Quantum Systems in Chemistry and Physics, ed. P. Hoggan, E. Brändas, J. Maruani, P. Piecuch and G. Delgado-Barrio, 22 , 2012, p219-248
    DOI:10.1007/978-94-007-2076-3_13 | unige:17800 | Abstract | Article PDF
Shifts in the π → π∗ excitation energy of the cis-7-hydroxyquinoline chromophore induced by hydrogen bonding with small molecules, obtained with the frozen-density embedding theory (FDET), are compared with the results of the high-level equation-of-motion coupled-cluster (EOMCC) calculations with singles, doubles, and noniterative triples, which provide the reference ab initio data, the supermolecular time-dependent density functional theory (TDDFT) calculations, and the available experimental data. It is demonstrated that the spectral shifts resulting from the FDET calculations employing nonrelaxed environment densities and their EOMCC counterparts are in excellent agreement with one another, whereas the analogous shifts obtained with the supermolecular TDDFT approach do not agree with the EOMCC reference data. Among the discussed issues are the effects of higher-order correlations on the excitation energies and complexation-induced excitation energy shifts resulting from the EOMCC calculations, and the choice of the approximants that represent the nonadditive kinetic energy contributions to the embedding potential of FDET.
  • Attenuated total reflection infrared (ATR-IR) spectroscopy, modulation excitation spectroscopy (MES), and vibrational circular dichroism (VCD)
    Thomas BĂźrgi
    in "Biointerface Characterization by Advanced IR Spectroscopy" Edited by:C.-M. Pradier And Y.J. Chabal, Ch. 5 , 2011, p115-144
    DOI:10.1016/B978-0-444-53558-0.00005-9 | unige:98056 | Abstract
This chapter describes the potential and limitations of attenuated total reflection infrared (ATR-IR) spectroscopy, modulation excitation spectroscopy (MES), and vibrational circular dichroism (VCD) for the investigation of bio-interfaces. The three techniques are completely independent of each other although ATR-IR and MES can be combined. ATR-IR is based on the total internal reflection of a light beam at an interface formed between two media. The resulting field at the interface is used for spectroscopy. The evanescent nature of this field allows one to probe the layer close to the interface formed between the internal reflection element (IRE) and the sample. Therefore, the field probes only a small part of the bulk fluid phase, such that molecules at the interface can be studied even in the presence of strongly absorbing solvents such as water. This makes the technique particularly interesting for applications in biology, where water is the solvent. The electric field can furthermore be polarized, which can be used for orientation measurements. MES is based on the stimulation of the sample under investigation by a periodic change of an external parameter such as temperature, pressure, pH, or concentration. The periodic system response to this stimulation (excitation) can be followed using a spectroscopic technique like ATR-IR by recording time-resolved spectra. The latter are then transformed into phase-resolved spectra using a phase-sensitive detection (PSD) scheme. MES leads to a drastic suppression of noise, which is very welcome when studying molecules at interfaces by spectroscopic techniques. Furthermore, MES helps to disentangle complex overlapping spectra arising from several species.
  • Optical properties of a fabricated self-assembled bottom-up bulk metamaterialOpen access paper
    S. MĂźhlig, C. Rockstuhl, V. Yannopapas, T. BĂźrgi, N. Shalkevich and F. Lederer
    Optics Express, 19 (10) , 2011, p9607-9616
    DOI:10.1364/OE.19.009607 | unige:98050 | Abstract | Article HTML | Article PDF
We investigate the optical properties of a true three-dimensional metamaterial that was fabricated using a self-assembly bottom-up technology. The metamaterial consists of closely packed spherical clusters being formed by a large number of non-touching gold nanoparticles. After presenting experimental results, we apply a generalized Mie theory to analyze its spectral response revealing that it is dominated by a magnetic dipole contribution. By using an effective medium theory we show that the fabricated metamaterial exhibits a dispersive effective permeability, i.e. artificial magnetism. Although this metamaterial is not yet left-handed it might serve as a starting point for achieving bulk metamaterials by using bottom-up approaches.
  • Supramolecular construction of vesicles based on core-substituted naphthalene diimide appended with triethyleneglycol motifs
    Sheshanath V. Bhosale, Chintan H. Jani, Cecilia H. Lalander, Steven J. Langford, Igor Nerush, Joseph G. Shapter, Diego Villamaina and Eric Vauthey
    Chemical Communications, 47 (29) , 2011, p8226
    DOI:10.1039/c1cc11318f | unige:94115 | Abstract | Article HTML | Article PDF | Supporting Info
The self-assembly of core-substituted naphthalene diimides bearing triethylene glycol motifs leads to the formation of stable vesicles in DMSO and CHCl3/MeOH (6 : 4, v/v) solvents. The vesicles were evaluated by means of UV/vis and fluorescence spectroscopy, transmission electron microscopy, atomic force microscopy and dynamic light scattering.
  • Crystal growth and structure determination of the novel tetragonal compound Ce2RhGa12
    R. Nagalakshmi, R. Kulkarni, S.K. Dhar, A. Thamizhavel, V. Krishnakumar, C. Besnard, H. Hagemann and M. Reiffers
    Chemistry of metals and alloys, 4 (3/4) , 2011, p229-233
    Keywords: Crystal structure, intermetallic compound
    unige:23118 | Abstract | Article PDF
Single crystals of Ce2RhGa12 have been synthesized using Ga flux and their structure was determined by single-crystal X-ray diffraction. Ce2RhGa12 crystallizes in the tetragonal space group P4/nbm (No. 125), and is isostructural to Ce2PdGa12, with Z = 2 and lattice parameters a = 6.0405 Å and c = 15.706 Å. Data were collected at the Swiss Norwegian Beam Line at the European Synchrotron Facility, Grenoble, France. Laue diffraction was carried out to confirm the quality of the single crystal and showed well-defined spots and tetragonal symmetry.
  • Naphthalene- and perylenediimides with hydroquinones, catechols, boronic esters and imines in the core
    Andrea Fin, Irina Petkova, David Alonso Doval, Naomi Sakai, Eric Vauthey and Stefan Matile
    Organic & Biomolecular Chemistry, 9 (24) , 2011, p8246-8252
    DOI:10.1039/c1ob05702b | unige:17558 | Abstract | Article HTML | Article PDF
The green-fluorescent protein of the jellyfish operates with the most powerful phenolate donors in the push–pull fluorophore. To nevertheless achieve red fluorescence with the same architecture, sea anemone and corals apply oxidative imination, a process that accounts for the chemistry of vision as well. The objective of this study was to apply these lessons from nature to one of the most compact family of panchromatic fluorophores, i.e. core-substituted naphthalenediimides (cNDIs). We report straightforward synthetic access to hydroxylated cNDI and cPDI cores by palladium-catalyzed cleavage of allyloxy substituents. With hydroxylated cNDIs but not cPDIs in water-containing media, excited-state intramolecular proton transfer yields a second bathochromic emission. Deprotonation of hydroquinone, catechol and boronic ester cores provides access to an impressive panchromism up to the NIR frontier at 640 nm. With cNDIs, oxidative imination gives red shifts up to 638 nm, whereas the expanded cPDIs already absorb at 754 nm upon deprotonation of hydroquinone cores. The practical usefulness of hydroquinone cNDIs is exemplified by ratiometric sensing of the purity of DMF with the “naked eye” at a sensitivity far beyond the “naked nose”. We conclude that the panchromatic hypersensitivity toward the environment of the new cNDIs is ideal for pattern generation in differential sensing arrays.
  • Modified ene-yne compounds: a novel functional material with nonlinear optical properties
    Daniel Lumpi, Berthold StĂśger, Christian Hametner, Frank Kubel, Georg Reider, Hans Hagemann, Alfred Karpfen and Johannes FrĂśhlich
    CrystEngComm, 13 (24) , 2011, p7194-7197
    DOI:10.1039/C1CE06093G | unige:17799 | Article HTML | Article PDF
  
The title compound, an achiral flexible molecule containing a 1,2,3-triazole structure as the acceptor subunit, crystallizes as a single enantiomorph in the space group P212121. The material exhibits nonlinear optical properties and is capable of second harmonic generation. Thus, the developed molecular scaffold represents an interesting novel type of NLO chromophore.
  
  • Role of the aromatic bridge on radical ions formation during reduction of diphosphaalkenes
    Manuel Lejeune, Philippe Grosshans, ThĂŠo Berclaz, Helena Sidorenkova, CĂŠline Besnard, Phil Pattison and Michel Geoffroy
    New Journal of Chemistry, 35 (11) , 2011, p2510-2520
    DOI:10.1039/c1nj20314b | unige:17483 | Abstract | Article HTML | Article PDF
Two molecules containing two phenylphosphaalkene moieties linked by an anthracene (1) or by a naphthalene (2) ring have been synthesized and their crystal structures have been determined. While electrochemical measurements show that these two systems are easily reduced, EPR spectra indicate that, at room temperature, the electronic structures of the two reduction compounds 1˙− and 2˙− are quite different. In 1˙−, in good accordance with DFT predictions, the unpaired electron is delocalized on the full molecule while in 2˙− it is confined on a single phosphaalkene moiety. This difference is attributed to the short distance between the two phenylphosphaalkene groups in 2˙− which hinders their reorientation after addition of an electron. The role of this motion is consistent with the fact that two additional paramagnetic species are detected at 145 K: the dianion 22− characterized by a rather small exchange coupling constant and the radical monoanion 2*˙− resulting from the formation of a one electron P–P bond. These two species are probably reaction intermediates which can lead to the formation of biphosphane.
  • Thermal hysteresis in spin-crossover compounds studied within the mechanoelastic model and its potential application to nanoparticles
    C. Enachescu, P. Chakraborty, L. Stoleriu, A. Stancu and A. Hauser
    Physical Review B, 84 (13) , 2011, p134102
    DOI:10.1103/PhysRevB.84.134102 | unige:17482 | Abstract | Article PDF
  
The recently developed mechanoelastic model is applied to characterize the thermal transition in spin-crossover complexes, with special attention given to the case of spin-crossover nanoparticles. In a two-dimensional system, hexagonal-shaped samples with open boundary conditions are composed of individual molecules that are linked by springs and can switch between two states, namely, the high-spin (HS) and the low-spin (LS) states. The switching of an individual molecule during the spin transition is decided by way of a Monte Carlo standard procedure, using transition probabilities depending on the temperature, the energy gap between the two states, the enthalpy difference, the degeneracy ratio, and the local pressure determined by the elongation or compression of its closest springs. The influence of external parameters, such as temperature sweeping rate and pressure, or intrinsic features of the system, such as the value of its spring constant, on the width of the thermal hysteresis, its shape, and its position are discussed. The particular case of spin-crossover nanoparticles is treated by considering them embedded into a polymer environment, which essentially affects the molecules situated at the edges and faces by decreasing their transition probabilities from HS to LS. Finally, the pressure hysteresis, obtained by varying the external pressure at constant temperature is discussed.
  
  • Targeting π-Conjugated Multiple Donor–Acceptor Motifs Exemplified by Tetrathiafulvalene-Linked Quinoxalines and Tetrabenz[bc,ef,hi,uv]ovalenes: Synthesis, Spectroscopic, Electrochemical, and Theoretical Characterization
    Hong-Peng Jia, Jie Ding, Ying-Fen Ran, Shi-Xia Liu, Carmen Blum, Irina Petkova, Andreas Hauser and Silvio Decurtins
    Chemistry - An Asian Journal, 6 (12) , 2011, p3312-3321
    Keywords: charge transfer;donor-acceptor systems;nanographene;redox chemistry;tetrathiafulvalene
    DOI:10.1002/asia.201100515 | unige:18010 | Abstract | Article PDF
An efficient synthetic approach to a symmetrically functionalized tetrathiafulvalene (TTF) derivative with two diamine moieties, 2-[5,6-diamino-4,7-bis(4-pentylphenoxy)-1,3-benzodithiol-2-ylidene]-4,7-bis(4-pentylphenoxy)-1,3-benzodithiole-5,6-diamine (2), is reported. The subsequent Schiff-base reactions of 2 afford large π-conjugated multiple D–A arrays, for example the triad 2-[4,9-bis(4-pentylphenoxy)-1,3-dithiolo[4,5-g]quinoxalin-2-ylidene]-4,9-bis(4-pentylphenoxy)-1,3-dithiolo[4,5-g]quinoxaline (8) and the corresponding tetrabenz[bc,ef,hi,uv]ovalene-fused pentad 1, in good yields and high purity. The novel redox-active nanographene 1 is so far the largest known TTF-functionalized polycyclic aromatic hydrocarbon with a well-resolved 1H NMR spectrum. The electrochemically highly amphoteric pentad 1 and triad 8 exhibit various electronically excited charge-transfer states in different oxidation states leading to intense optical intramolecular charge transfer (ICT) absorbances over a wide spectral range. The chemical and electrochemical oxidations of 1 result in an unprecedented TTF•+ radical cation dimerization, leading to the formation of [1•+]2 at room temperature in solution due to the stabilizing effect arising from strong π–π interactions. Moreover, ICT fluorescence is observed with large solvent-dependent Stokes shifts and quantum efficiencies of 0.05 for 1 and 0.035 for 8 in CH2Cl2.
  • Study of the relaxation in diluted spin crossover molecular magnets in the framework of the mechano-elastic model
    Cristian Enachescu, Laurentiu Stoleriu, Alexandru Stancu and Andreas Hauser
    Journal of Applied Physics, 109 (7) , 2011, p711
    Keywords: magnetic impurities, metastable states, molecular magnetism, Monte Carlo methods, spin dynamics
    DOI:10.1063/1.3556702 | unige:17481 | Abstract | Article HTML | Article PDF
We model here the behavior of spin transition compounds, considering molecules arranged in a 2D hexagonal lattice and interacting via springs. The role of impurities in the clustering and nucleation phenomena is analyzed, as well as the manner in which the impurities affect the relaxation curves. The switching of the individual molecules is checked using a Monte Carlo procedure. When a molecule changes its state, it also modifies its volume, and the new equilibrium positions of all the molecules are calculated. As in previously reported experiments, it is found here that bigger impurities slow down the relaxation from the metastable high-spin state to the low-spin state, while smaller impurities act in an opposite way. It is shown that if the concentration of the impurities is higher than a certain threshold, then they act as a barrier, trammeling the fast evolution of domains developing from the edges.
 
PURPOSE. To compare the photochemical properties of UV filter molecules present in the human lens (kynurenine, KN; 3-hy- droxykynurenine, 3OHKN; 3-hydroxykynurenine O-gluco- side, 3OHKG; 4-(2-aminophenyl)-4-oxobutanoic acid, AHA; and glutathionyl-kynurenine, GSH-KN) with the use of the following parameters: excited singlet lifetime, fluorescence quantum yield, triplet quantum yield, photodecomposition quantum yield.

METHODS. The excited singlet lifetimes were measured with the use of fluorescence upconversion (time resolution, 210 fs) and pump-probe transient absorption (time resolution, 200 fs) methods. The fluorescence quantum yields were determined relative to an aqueous solution of quinine bisulfate. The triplet quantum yields were measured with the use of nanosecond laser flash photolysis. The photodecomposition quantum yields were determined by steady state photolysis followed by the high-performance liquid chromatography analysis.

RESULTS. The secondary UV filters—AHA and GSH-KN are better photosensitizers than the primary ones -KN, 3OHKN and 3OHKG: the singlet state lifetimes of the secondary UV filters are longer, and the quantum yields of fluorescence and triplet state formation are higher.

CONCLUSIONS. With aging, the ratio primary/secondary UV filters in the human lens decreases from approximately 10:1 to 2:1. The obtained results demonstrate that the quality of the secondary UV filters is inferior compared to the primary ones, which may result in a higher susceptibility of old lenses to UV light. That might be an important factor for the development of the age-related cataract.

  • Self-Organizing Surface-Initiated Polymerization: Facile Access to Complex Functional Systems
    Naomi Sakai, Marco Lista, Oksana Kel, Shinichiro Sakurai, Daniel Emery, Jiri Mareda, Eric Vauthey and Stefan Matile
    Journal of the American Chemical Society, 133 (39) , 2011, p15224-15227
    DOI:10.1021/ja203792n | unige:17068 | Abstract | Article HTML | Article PDF
Facile access to complex systems is crucial to generate the functional materials of the future. Herein, we report self-organizing surface-initiated polymerization (SOSIP) as a user-friendly method to create ordered as well as oriented functional systems on transparent oxide surfaces. In SOSIP, self-organization of monomers and ring-opening disulfide exchange polymerization are combined to ensure the controlled growth of the polymer from the surface. This approach provides rapid access to thick films with smooth, reactivatable surfaces and long-range order with few defects and high precision, including panchromatic photosystems with oriented four-component redox gradients. The activity of SOSIP architectures is clearly better than that of disordered controls.
  
  • Porous and Dense Magnesium Borohydride Frameworks: Synthesis, Stability, and Reversible Absorption of Guest SpeciesVery Important Paper
    Yaroslav Filinchuk, Bo Richter, Torben R. Jensen, Vladimir Dmitriev, Dmitry Chernyshov and Hans Hagemann
    Angewandte Chemie International Edition, 50 (47) , 2011, p11162-11166
    Keywords: framework materials;host-guest systems;hydrides;hydrogen storage;polymorphism
    DOI:10.1002/anie.201100675 | unige:17480 | Abstract | Article PDF
Highly occupied: A highly porous form of Mg(BH4)2 (see picture; Mg green, BH4 blue, unit cells shown in red) reversibly absorbs H2, N2, and CH2Cl2. At high pressures, this material transforms into an interpenetrated framework that has 79 % higher density than the other polymorphs. Mg(BH4)2 can act as a coordination polymer that has many similarities to metal–organic frameworks.
  • A novel ruthenium(ii) complex for two-photon absorption-based optical power limiting in the near-IR range
    MickaĂŤl Four, Didier Riehl, Olivier Mongin, Mireille Blanchard-Desce, LatĂŠvi Max Lawson Daku, Juliette Moreau, JĂŠrĂ´me Chauvin, Jacques A. Delaire and Gilles Lemercier
    Physical Chemistry Chemical Physics, 13 (38) , 2011, p17304-17312
    DOI:10.1039/c1cp21661a | unige:17238 | Abstract | Article HTML | Article PDF
  
In this article, the synthesis of a novel high-conjugated ligand and its corresponding Ru(II) complex PTFTF:Ru is reported, along with the linear and nonlinear optical characterizations. Two-photon absorption based optical power limiting properties (OPL), especially in the near infrared, are described and compared to those of the analogous complexes previously published. Combined with a preliminary theoretical approach, this allows us to highlight several key parameters for OPL optimization in such molecular systems and more particularly the spectral overlap between TPA and excited-state absorption.
  
  • Optimizing Sensitization Processes in Dinuclear Luminescent Lanthanide Oligomers. Selection of Rigid Aromatic Spacers.
    Jean-François Lemonnier, Laure GuĂŠnĂŠe, CĂŠsar Beuchat, Tomasz Adam Wesolowski, Prasun Mukherjee, David Hennessey Waldeck, Kristy Alexa Gogick, Stephane Petoud and Claude Piguet
    Journal of the American Chemical Society, 133 (40) , 2011, p16219-16234
    DOI:10.1021/ja206806t | unige:17237 | Abstract | Article HTML | Article PDF
This work illustrates a simple approach for optimizing the lanthanide luminescence in molecular dinuclear lanthanide complexes and identifies a particular multidentate europium complex as the best candidate for further incorporation into polymeric materials. The central phenyl ring in the bis-tridentate model ligands L3–L5, which are substituted with neutral (X = H, L3), electron-withdrawing (X = F, L4), or electron-donating (X = OCH3, L5) groups, separates the 2,6-bis(benzimidazol-2-yl)pyridine binding units of linear oligomeric multi-tridentate ligand strands that are designed for the complexation of luminescent trivalent lanthanides, Ln(III). Reactions of L3–L5 with [Ln(hfac)3(diglyme)] (hfac– is the hexafluoroacetylacetonate anion) produce saturated single-stranded dumbbell-shaped complexes [Ln2(Lk)(hfac)6] (k = 3–5), in which the lanthanide ions of the two nine-coordinate neutral [N3Ln(hfac)3] units are separated by 12–14 Å. The thermodynamic affinities of [Ln(hfac)3] for the tridentate binding sites in L3–L5 are average (6.6 ≤ log(β2,1Y,Lk) ≤ 8.4) but still result in 15–30% dissociation at millimolar concentrations in acetonitrile. In addition to the empirical solubility trend found in organic solvents (L4 > L3 >> L5), which suggests that the 1,4-difluorophenyl spacer in L4 is preferable, we have developed a novel tool for deciphering the photophysical sensitization processes operating in [Eu2(Lk)(hfac)6]. A simple interpretation of the complete set of rate constants characterizing the energy migration mechanisms provides straightforward objective criteria for the selection of [Eu2(L4)(hfac)6] as the most promising building block.
  • Thermodynamics, Structure and Properties of Polynuclear Lanthanide Complexes with a Tripodal Ligand: Insight into their Self-Assembly
    Josef Hamacek, CĂŠline Besnard, Tiphaine Penhouet and
    Chemistry - A European Journal, 17 (24) , 2011, p6753-6764
    Keywords: helical structures;lanthanides;polynuclear complexes;thermodynamics;tripodal ligands
    DOI:10.1002/chem.201100173 | unige:17236 | Abstract | Article HTML | Article PDF
Self-assembly processes between a tripodal ligand and LnIII cations have been investigated by means of supramolecular analytical methods. At an equimolar ratio of components, tetranuclear tetrahedral complexes are readily formed in acetonitrile. The structural analysis of the crystallographic data shows a helical wrapping of binding strands around metallic cations. The properties of this series of highly charged 3D compounds were examined by using NMR spectroscopy and optical methods in solution and in the solid state. In the presence of excess metal, a new trinuclear complex was identified. The X-ray crystal structure elucidated the coordination of metallic cations with two ligands of different conformations. By varying the metal/ligand ratio, a global speciation of this supramolecular system has been evidenced with different spectroscopic methods. In addition, these rather complicated equilibria were successfully characterised with the thermodynamic stability constants. A rational analysis of the self-assembly processes was attempted by using the thermodynamic free energy model and the impact of the ligand structure on the effective concentration is discussed.
  • Lanthanide-Mediated Supramolecular Cages and Host–Guest Interactions
    Badr El Aroussi, Laure GuĂŠnĂŠe, Prodipta Pal and Josef Hamacek
    Inorganic Chemistry, 50 (17) , 2011, p8588-8597
    DOI:10.1021/ic201156q | unige:17235 | Abstract | Article HTML | Article PDF
  
The structure and thermodynamic properties of lanthanide complexes with a new tripodal ligand L2 have been elucidated using different physicochemical methods. At stoichiometric ratios, the tetrahedral three-dimensional complexes with lanthanide cations are formed in acetonitrile with good stabilities. Despite minor structural changes comparing to previously investigated tripodal ligands, the resulting assembly exhibits different features revealed with the crystal structure of [Eu4L24](OH)(ClO4)11 (orthorhombic, Pbcn). Interestingly, the highly charged edifice contains an inner cage encapsulating a perchlorate anion. Such lanthanide mediated cage-like assemblies are rare, and may be of interest for different sensing applications. Indeed, the anionic guest can be exchanged with different anions. The related host–guest equilibria were investigated with NMR techniques. Various aspects of these reactions are qualitatively discussed.
  • Self-consistency in frozen-density embedding theory based calculations
    Francesco Aquilante and Tomasz A. Wesolowski
    Journal of Chemical Physics, 135 (8) , 2011, p84120
    Keywords: density functional theory, HF calculations, hydrogen bonds, SCF calculations, variational techniques
    DOI:10.1063/1.3624888 | unige:16946 | Abstract | Article HTML | Article PDF
The bi-functional for the non-electrostatic part of the exact embedding potential of frozen-density embedding theory (FDET) depends on whether the embedded part is described by means of a real interacting many-electron system or the reference system of non-interacting electrons (see [Wesolowski, Phys. Rev. A. 77, 11444 (2008)]). The difference δΔFMD[ρA] / δρA(r), where ΔFMD[ρA] is the functional bound from below by the correlation functional Ec[ρA] and from above by zero. Taking into account ΔFMD[ρA] in both the embedding potential and in energy is indispensable for assuring that all calculated quantities are self-consistent and that FDET leads to the exact energy and density in the limit of exact functionals. Since not much is known about good approximations for ΔFMD[ρA], we examine numerically the adequacy of neglecting ΔFMD[ρA] entirely. To this end, we analyze the significance of δΔFMD[ρA] / δρA(r) in the case where the magnitude of ΔFMD[ρA] is the largest, i.e., for Hartree-Fock wavefunction. In hydrogen bonded model systems, neglecting δΔFMD[ρA] / δρ(r) in the embedding potential marginally affects the total energy (less than 5% change in the interaction energy) but results in qualitative changes in the calculated hydrogen-bonding induced shifts of the orbital energies. Based on this estimation, we conclude that neglecting δΔFMD[ρA] / δρA(r) may represent a good approximation for multi-reference variational methods using adequate choice for the active space. Doing the same for single-reference perturbative methods is not recommended. Not only it leads to violation of self-consistency but might result in large effect on orbital energies. It is shown also that the errors in total energy due to neglecting δΔFMD[ρA] / δρA(r) do not cancel but rather add up to the errors due to approximation for the bi-functional of the non-additive kinetic potential.
  
  • Importance of the Intermolecular Pauli Repulsion in Embedding Calculations for Molecular Properties: The Case of Excitation Energies for a Chromophore in Hydrogen-Bonded Environments
    Georgios Fradelos and Tomasz A. Wesolowski
    Journal of Physical Chemistry A, 115 (35) , 2011, p10018-10026
    DOI:10.1021/jp203192g | unige:16945 | Abstract | Article HTML | Article PDF
In embedding methods such as those labeled commonly as QM/MM, the embedding operator is frequently approximated by the electrostatic potential generated by nuclei and electrons in the environment. Such approximation is especially useful in studies of the potential energy surface of embedded species. The effect on energy of neglecting the non-Coulombic component of the embedding operator is corrected a posteriori. The present work investigates applicability of such approximation in evaluation of electronic excitation energy, the accuracy of which depends directly on that of the embedding potential. For several model systems involving cis-7-hydroxiquinoline hydrogen-bonded to small molecules, we demonstrate that such truncation of the embedding operator leads to numerically unstable results upon increasing the size of the atomic basis sets. Approximating the non-Coulombic component of the embedding potential using the expression derived in Frozen-Density Embedding Theory ([Wesolowski and Warshel, J. Phys. Chem.1993, 97, 8050] and subsequent works) by means of even a simple bifunctional dependent on the electron density of the chromophore and its hydrogen-bonded environment, restores the numerical stability of the excitation energies that reach a physically meaningful limit for large basis sets.
  • The generalized active space concept in multiconfigurational self-consistent field methods
    Dongxia Ma, Giovanni Li Manni and Laura Gagliardi
    Journal of Chemical Physics, 135 (4) , 2011, p44128
    Keywords: configuration interactions, gadolinium, manganese compounds, organic compounds, SCF calculations, wave functions
    DOI:10.1063/1.3611401 | unige:16781 | Abstract | Article HTML | Article PDF
  
A multiconfigurational self-consistent field method based on the concept of generalized active space (GAS) is presented. GAS wave functions are obtained by defining an arbitrary number of active spaces with arbitrary occupation constraints. By a suitable choice of the GAS spaces, numerous ineffective configurations present in a large complete active space (CAS) can be removed, while keeping the important ones in the CI space. As a consequence, the GAS self-consistent field approach retains the accuracy of the CAS self-consistent field (CASSCF) ansatz and, at the same time, can deal with larger active spaces, which would be unaffordable at the CASSCF level. Test calculations on the Gd atom, Gd2 molecule, and oxoMn(salen) complex are presented. They show that GAS wave functions achieve the same accuracy as CAS wave functions on systems that would be prohibitive at the CAS level.
  
  • Ultrafast spectroscopic investigation of a fullerene poly(3-hexylthiophene) dyad
    N. Banerji, J. Seifter, M. Wang, E. Vauthey, F. Wudl and A.J. Heeger
    Physical Review B, 84 , 2011, p75206-75206
    DOI:10.1103/PhysRevB.84.075206 | unige:16782 | Abstract | Article PDF
We present the femtosecond spectroscopic investigation of a covalently linked dyad, PCB-P3HT, formed by a segment of the conjugated polymer P3HT (regioregular poly(3-hexylthiophene)) that is end capped with the fullerene derivative PCB ([6,6]-phenyl-C61-butyric acid ester), adapted from PCBM. The fluorescence of the P3HT segment in tetrahydrofuran (THF) solution is reduced by 64% in the dyad compared to a control compound without attached fullerene (P3HT-OH). Fluorescence upconversion measurements reveal that the partial fluorescence quenching of PCB-P3HT in THF is multiphasic and occurs on an average time scale of 100 ps, in parallel to excited-state relaxation processes. Judging from ultrafast transient absorption experiments, the origin of the quenching is excitation energy transfer from the P3HT donor to the PCB acceptor. Due to the much higher solubility of P3HT compared to PCB in THF, the PCB-P3HT dyad molecules self-assemble into micelles. When pure C60 is added to the solution, it is incorporated into the fullerene-rich center of the micelles. This dramatically increases the solubility of C60 but does not lead to significant additional quenching of the P3HT fluorescence by the C60 contained in the micelles. In PCB-P3HT thin films drop-cast from THF, the micelle structure is conserved. In contrast to solution, quantitative and ultrafast (<150 fs) charge separation occurs in the solid-state films and leads to the formation of long-lived mobile charge carriers with characteristic transient absorption signatures similar to those that have been observed in P3HT:PCBM bulk heterojunction blends. While π -stacking interactions between neighboring P3HT chains are weak in the micelles, they are strong in thin films drop-cast from ortho-dichlorobenzene. Here, PCB-P3HT self-assembles into a network of long fibers, clearly seen in atomic force microscopy images. Ultrafast charge separation occurs also for the fibrous morphology, but the transient absorption experiments show fast loss of part of the charge carriers due to intensity-induced recombination and annihilation processes and monomolecular interfacial trap-mediated or geminate recombination. The yield of the long-lived charge carriers in the highly organized fibers is however comparable to that obtained with annealed P3HT:PCBM blends. PCB-P3HT can therefore be considered as an active material in organic photovoltaic devices.
 
Even flow: Photoinduced symmetry-breaking charge separation takes place in a few picoseconds in a 1,3-bis(perylene)propane dyad in polar solvents. Polarized transient absorption measurements show that the direction of the charge flow is random and entirely governed by the fluctuations of the solvent orientation around the dyad.
  
  • Synthesis and structure-activity relationship of several aromatic ketone-based two-photon initiators
    Zhiquan Li, Marton Siklos, Niklas Pucher, Klaus Cicha, Aliasghar Ajami, Wolfgang Husinsky, Arnulf Rosspeintner, Eric Vauthey, Georg Gescheidt, JĂźrgen Stampfli and Robert Liska
    Journal of Polymer Science Part A: Polymer Chemistry, 49 (17) , 2011, p3688-3699
    DOI:10.1002/pola.24806 | unige:16780 | Abstract | Article HTML | Article PDF
Several novel aromatic ketone-based two-photon initiators containing triple bonds and dialkylamino groups were synthesized and the structure-activity relationships were evaluated. Branched alkyl chains were used at the terminal donor groups to improve the solubility in the multifunctional monomers. Because of the long conjugation length and good coplanarity, the evaluated initiators showed large two-photon cross section values, while their fluorescence lifetimes and quantum yields strongly depend on the solvent polarity. All novel initiators exhibited high activity in terms of two-photon-induced microfabrication. This is especially true for fluorenone-based derivatives, which displayed much broader processing windows than well-known highly active initiators from the literature and commercially available initiators. While the new photoinitiators gave high reactivity in two-photon-induced photopolymerization at concentration as low as 0.1% wt, these compounds are surprisingly stable under one photon condition and nearly no photo initiation activity was found in classical photo DSC experiment.
  • Ba2.2Ca0.8Mg4F14, a new “solid solution stabilized” matrix for an intense blue phosphor
    F. Kubel, M. Pantazi and H. Hagemann
    Crystal Research and Technology, 46 (9) , 2011, p899-905
    Keywords: solid solution stabilized compound, fluoride, blue phosphor
    DOI:10.1002/crat.201000624 | unige:16784 | Article PDF
  
Barium calcium magnesium fluoride (Ba2(BaxCa1-x)Mg4F14, x=0.19-0.26) has been synthesized at 850 °C from precursors prepared by the solution precipitation method. Single crystals with composition of Ba2.200(2)Ca0.800(2)Mg4F14were obtained after prolonged heating. Lattice parameters from single crystal data are a = 12.4203(8) and c = 7.4365(5) Å [tetragonal, space group P42/mnm (No. 136)]. They increase with increasing barium concentration within a given stability window. The structure is built of a network of MgF6 octahedra forming a pyrochlore related channel system and isolated fluorine ions. Within the channels, heavy alkaline earth ions are located. The wide channel is filled with off-center positioned barium ions. The channel with a narrow cross section hosts both ions, Ca2+and Ba2+. The structure is isotypic with Pb3Nb4O12F2 but has a different coordination around Ba/Ca and Pb, respectively. Doped with ∼1% Eu(II), the compound shows intense blue luminescence under UV activation.
  • Comment on "Accurate frozen-density embedding potentials as a first step towards a subsystem description of covalent bonds" [J. Chem. Phys. 132, 164101 (2010)]
    Tomasz A. Wesolowski
    Journal of Chemical Physics, 135 (2) , 2011, p27101
    DOI:10.1063/1.3609108 | unige:16763 | Abstract | Article HTML | Article PDF
Several assertions which are incorrect or might be misleadingly interpreted as well as omissions of issues concerning the non-additive kinetic energy potential made by Fux et al. are analyzed. They concern issues of great importance for any computational method based on the orbital-free embedding theory: evaluation of the total energy, approximating the non-additive kinetic potential, exact properties of non-additive kinetic energy potential. In a nutshell, the authors do not distinguish between two different quantities: the functional, i.e., the correspondence assigning the non-additive kinetic potential to a pair of electron densities and the function (the potential itself).
  • An induced fit mechanism regulates p53 DNA binding kinetics to confer sequence specificity
    Tom J. Petty, Soheila Emamzadah, Lorenzo Costantino, Irina Petkova, Eric Vauthey and Thanos D. Halazonetis
    The EMBO Journal, 30 , 2011, p2167-2176
    DOI:10.1038/emboj.2011.127 | unige:16761 | Abstract | Article HTML | Article PDF
The p53 tumour suppressor gene, the most frequently mutated gene in human cancer, encodes a transcription factor that contains sequence-specific DNA binding and homo-tetramerization domains. Interestingly, the affinities of p53 for specific and non-specific DNA sites differ by only one order of magnitude, making it hard to understand how this protein recognizes its specific DNA targets in vivo. We describe here the structure of a p53 polypeptide containing both the DNA binding and oligomerization domains in complex with DNA. The structure reveals that sequence- specific DNA binding proceeds via an induced fit mechan- ism that involves a conformational switch in loop L1 of the p53 DNA binding domain. Analysis of loop L1 mutants demonstrated that the conformational switch allows DNA binding off-rates to be regulated independently of affinities. These results may explain the universal prevalence of conformational switching in sequence-specific DNA binding proteins and suggest that proteins like p53 rely more on differences in binding off-rates, than on differences in affinities, to recognize their specific DNA sites.
  • Photoinduced electron transfer reactions: from the elucidation of old problems towards the exploration of interfaces
    Marina Fedoseeva, Jakob Grilj, Oksana Kel, Marius Koch, Romain Letrun, Vesna Markovic, Irina Petkova, Sabine Richert, Arnulf Rosspeintner, Peter Sherin, Diego Villamaina, Bernhard Lang and Eric Vauthey
    Chimia, 65 , 2011, p350-352
    DOI:10.2533/chimia.2011.350 | unige:16760 | Abstract | Article PDF
The activities of our research group in the field of photoinduced electron transfer reactions are discussed and illustrated by several examples
  
  • Self-Assembled Plasmonic Core-Shell Clusters with an Isotropic Magnetic Dipole Response in the Visible Range
    Stefan MĂźhlig, Alastair Cunningham, Sebastian Scheeler, Claudia Pacholski, Thomas BĂźrgi, Carsten Rockstuhl and Falk Lederer
    ACS Nano, 5 (8) , 2011, p6586-6592
    DOI:10.1021/nn201969h | unige:16819 | Abstract | Article HTML | Article PDF
We theoretically analyze, fabricate, and characterize a three-dimensional plasmonic nanostructure that exhibits a strong and isotropic magnetic response in the visible spectral domain. Using two different bottom-up approaches that rely on self-organization and colloidal nanochemistry, we fabricate clusters consisting of dielectric core spheres, which are smaller than the wavelength of the incident radiation and are decorated by a large number of metallic nanospheres. Hence, despite having a complicated inner geometry, such a core–shell particle is sufficiently small to be perceived as an individual object in the far field. The optical properties of such complex plasmonic core–shell particles are discussed for two different core diameters.
  • MgxMn(1−x)(BH4)2 (x = 0–0.8), a cation solid solution in a bimetallic borohydride
    Radovan Cerny, Nicolas Penin, Vincenza D'Anna, Hans Hagemann, Etienne Durand and Jakub Ruzicka
    Acta Materialia, 59 (13) , 2011, p5171-5180
    Keywords: hydrides; hydrogen storage; synchrotron radiation
    DOI:10.1016/j.actamat.2011.04.052 | unige:16759 | Abstract | Article HTML | Article PDF
A solid solution of magnesium and manganese borohydrides was studied by in situ synchrotron radiation X-ray powder diffraction and infrared spectroscopy. A combination of thermogravimetry, mass and infrared spectroscopy, and atomic emission spectroscopy were applied to clarify the thermal gas desorption of pure Mn(BH4)2 and a solid solution of composition Mg0.5Mn0.5(BH4)2. MgxMn(1−x)(BH4)2 (x = 0–0.8) conserves the trigonal structure of Mn(BH4)2 at room temperature. Manganese is dissolved in the hexagonal structure of α-Mg(BH4)2, with the upper solubility limit not exceeding 10 mol.% at room temperature. There exists a two-phase region of trigonal and hexagonal borohydrides within the compositional rangex = 0.8–0.9 at room temperature. Infrared spectra show splitting of various vibrational modes, indicating the presence of two cations in the trigonal MgxMn(1−x)(BH4)2 solid solutions, as well as the appearance of a second phase, hexagonal α-Mg(BH4)2, at higher magnesium contents. All vibrational frequencies are shifted to higher values with increasing magnesium content. The decomposition temperature of the trigonal MgxMn(1−x)(BH4)2 (x = 0–0.8) does not vary significantly as a function of the magnesium content (433–453 K). The desorbed gas contains mostly hydrogen and 3–7.5 mol.% diborane B2H6, as determined from analyses of the Mn(BH4)2 and Mg0.5Mn0.5(BH4)2 samples. An eutectic relation between α-Mg(BH4)2 and LiBH4 is observed. The solid solution MgxMn(1−x)(BH4)2 is a promising material for hydrogen storage as it decomposes at a similar temperature to Mn(BH4)2, i.e. at a much lower temperature than pure Mg(BH4)2 without significantly losing hydrogen weight capacity thanks to substitution of Mn by Mg up to 80 mol.%. The questions of diborane release and reversibility remain to be addressed.
  • «La chimie, ça reste une science expérimentale!» Entretien avec le professeur Jacques Weber
    Jacques Weber
    Chimia, 65 (5) , 2011, p362-365
    DOI:10.2533/chimia.2011.362
  • Novel sodium aluminium borohydride containing the complex anion [Al(BH4,Cl)4]−
    Inge Lindemann, Roger Domènech Ferrer, Lothar Dunsch, Radovan Cerny, Hans Hagemann, Vincenza D'Anna, Yaroslav Filinchuk, Ludwig Schultz and Oliver Gutfleisch
    Faraday Discussions, 151 , 2011, p231-242
    DOI:10.1039/C0FD00024H | unige:16758 | Abstract | Article HTML | Article PDF
  
The synthesis of a novel alkali-metal aluminium borohydride NaAl(BH4)xCl4−x from NaBH4 and AlCl3 using a solid state metathesis reaction is described. Structure determination was carried out using synchrotron powder diffraction data and vibrational spectroscopy. An orthorhombic structure (space group Pmn21) is formed which contains Na+ cations and complex [Al(BH4,Cl)4]−anions. Due to the high chlorine content (1 ≤ x ≤ 1.43) the hydrogen density of the borohydride is only between 2.3 and 3.5 wt.% H2 in contrast to the expected 14.6 wt.% for chlorine free NaAl(BH4)4. The decomposition of NaAl(BH4)xCl4−x is observed in the target range for desorption at about 90 °C by differential scanning calorimetry (DSC), in situ Raman spectroscopy and synchrotron powder X-ray diffraction. Thermogravimetric analysis (TG) shows extensive mass loss indicating the loss of H2 and B2H6 at about 90 °C followed by extensive weight loss in the form of chloride evaporation.
  • YMn2Hx and RMn2−yFeyH6 (R = Y, Er) studied by Raman, infrared and inelastic neutron scattering spectroscopies
    V. Paul-Boncour, S.F. Parker, H. Hagemann, S.M. Filipek, R. Wierzbicki and M. Latroche
    Faraday Discussions, 151 , 2011, p307-314
    DOI:10.1039/C0FD00019A | unige:16757 | Abstract | Article HTML | Article PDF
YMn2 forms either interstitial YMn2Hx hydrides for x ≤ 4.5 or a complex YMn2H6 hydride when submitted to high hydrogen pressure. These compounds have been studied by inelastic neutron scattering (INS) in order to clarify the different modes of H vibration. The INS spectra of YMn2Hx hydrides are strongly dependent on the H content. YMn2H6 and YMn2D6 show broad bands, also observed by Raman and IR spectroscopy, assigned to H–Mn–H (or D) and Mn–H bending and stretching modes. Both ErMn2D6 and ErMn1.8Fe0.2D6 show, in addition to the H vibration mode, an intense band at 215 cm−1 which has been attributed to a magnetic excitation of Er3+ in view of its momentum transfer dependence.
  
  • Size Exclusion Chromatography for Semi-Preparative Scale Separation of Au38(SR)24 and Au40(SR)24 and larger clusters
    Stefan Knoppe, Julien Boudon, Igor Dolamic, Amala Dass and Thomas BĂźrgi
    Analytical Chemistry, 83 (13) , 2011, p5056-5061
    DOI:10.1021/ac200789v | unige:16756 | Abstract | Article HTML | Article PDF
Size Exclusion Chromatography (SEC) on a semi-preparative scale (10 mg and more) was used to size-select ultrasmall gold nanoclusters (< 2 nm) from polydisperse mixtures. In particular, the ubiquitous byproducts of the etching process towards Au38(SR)24 (SR: thiolate) clusters were separated and gained in high monodispersity (based on mass spectrometry). The isolated fractions were characterized by UV/Vis spectroscopy, MALDI mass spectrometry and electron microscopy. Most notably, the separation of Au38(SR)24 and Au40(SR)24 clusters is demonstrated.
Nanofluids have been proposed to improve the performance of microchannel heat sinks. In this paper, we present a systematic characterization of aqueous silica nanoparticle suspensions with concentrations up to 31  vol %. We determined the particle morphology by transmission electron microscope imaging and its dispersion status by dynamic light scattering measurements. The thermophysical properties of the fluids, namely, their specific heat, density, thermal conductivity, and dynamic viscosity were experimentally measured. We fabricated microchannel heat sinks with three different channel widths and characterized their thermal performance as a function of volumetric flow rate for silica nanofluids at concentrations by volume of 0%, 5%, 16%, and 31%. The Nusselt number was extracted from the experimental results and compared with the theoretical predictions considering the change of fluids bulk properties. We demonstrated a deviation of less than 10% between the experiments and the predictions. Hence, standard correlations can be used to estimate the convective heat transfer of nanofluids. In addition, we applied a one-dimensional model of the heat sink, validated by the experiments. We predicted the potential of nanofluids to increase the performance of microchannel heat sinks. To this end, we varied the individual thermophysical properties of the coolant and studied their impact on the heat sink performance. We demonstrated that the relative thermal conductivity enhancement must be larger than the relative viscosity increase in order to gain a sizeable performance benefit. Furthermore, we showed that it would be preferable to increase the volumetric heat capacity of the fluid instead of increasing its thermal conductivity.
  • Multi-scale modelling of solvatochromic shifts from frozen-density embedding theory with non-uniform continuum model of the solvent: the coumarin 153 case
    Xiuwen Zhou, Jakub W. Kaminski and Tomasz A. Wesolowski
    Physical Chemistry Chemical Physics, 13 (22) , 2011, p10565-10576
    DOI:10.1039/C0CP02874F | unige:15943 | Abstract | Article HTML | Article PDF
  
For nine solvents of various polarity (from cyclohexane to water), the solvatochromic shifts of the lowest absorption band of coumarin 153 are evaluated using a computational method based on frozen-density embedding theory [Wesolowski and Warshel, J. Chem Phys., 1993, 97, 9050, and subsequent articles]. In the calculations, the average electron density of the solvent 〈ρB(r→)〉 is used as the frozen density. 〈ρB(r→)〉 is evaluated using the statistical-mechanical approach introduced in Kaminski et al., J. Phys. Chem. A, 2010, 114, 6082. The small deviations between experimental and calculated solvatochromic shifts (the average deviation equals to about 0.02 eV), confirm the adequacy of the key approximations applied: (a) in the evaluation of the average effect of the solvent on the excitation energy, using the average density of the solvent instead of averaging the shifts over statistical ensemble and (b) using the approximant for the bi-functional of the non-electrostatic component of the orbital-free embedding potential, are adequate for chromophores which interact with the environment by non-covalent bonds. The qualitative analyses of the origin of the solvatochromic shifts are made using the graphical representation of the orbital-free embedding potential.
  • Embedding vs supermolecular strategies in evaluating the hydrogen-bonding-induced shifts of excitation energies
    Georgios Fradelos, Jesse J. Lutz, Tomasz Adam Wesolowski, Piotr Piecuch and Marta Wloch
    Journal of Chemical Theory and Computation, 7 (6) , 2011, p1647-1666
    DOI:10.1021/ct200101x | unige:16755 | Abstract | Article PDF
Shifts in the excitation energy of the organic chromophore, cis-7-hydroxyquinoline (cis-7HQ), corresponding to the π→π* transition in cis-7HQ and induced by the complexation with a variety of small hydrogen-bonded molecules, obtained with the frozen-density embedding theory (FDET), are compared with the results of the supermolecular equation-of-motion coupled-cluster (EOMCC) calculations with singles, doubles, and non-iterative triples, which provide the reference theoretical data, the supermolecular time-dependent density functional theory (TDDFT) calculations, and experiment. Unlike in the supermolecular EOMCC and TDDFT cases, where each complexation-induced spectral shift is evaluated by performing two separate calculations, one for the complex and another one for the isolated chromophore, the FDET shifts are evaluated as the differences of the excitation energies determined for the same many-electron system, representing the chromophore fragment with two different effective potentials. By considering eight complexes of cis-7HQ with up to three small hydrogen-bonded molecules, it is shown that the spectral shifts resulting from the FDET calculations employing non-relaxed environment densities and their EOMCC reference counterparts are in excellent agreement with one another, whereas the analogous shifts obtained with the supermolecular TDDFT method do not agree with the EOMCC reference data. The average absolute deviation between the complexation-induced shifts, which can be as large, in absolute value, as about 2000 cm-1, obtained using the non-relaxed FDET and supermolecular EOMCC approaches that represent two entirely different computational strategies, is only about 100 cm-1, i.e., on the same order as the accuracy of the EOMCC calculations. This should be contrasted with the supermolecular TDDFT calculations, which produce the excitation energy shifts that differ from those resulting from the reference EOMCC calculations by about 700 cm-1 on average. Among the discussed issues are the choice of the electronic density defining the environment with which the chromophore interacts, which is one of the key components of FDET considerations, the basis set dependence of the FDET, supermolecular TDDFT, and EOMCC results, the usefulness of the monomer vs supermolecular basis expansions in FDET considerations, and the role of approximations that are used to define the exchange-correlation potentials in FDET and supermolecular TDDFT calculations.
  
The femtosecond-resolved evolution of the emission spectrum of the important conjugated polymer poly(3-hexylthiophene) (P3HT) is presented. Detailed fluorescence up-conversion spectroscopy was performed on P3HT solid-state films and on P3HT in chlorobenzene solution. Two excitation wavelengths and several emission wavelengths, covering the entire fluorescence spectrum, were used. The data were complemented by polarization-sensitive measurements. Our global analysis allowed a reconstruction of the time-resolved emission spectra with 200 fs temporal resolution, so that spectral changes due to the early relaxation processes following π–π* interband absorption in the pristine polymer could be comprehensively characterized. Absorption occurs in isolated polymer chains in solution and in the solid state (including interchain interactions) for the film. In both cases, we find evidence of delocalization of the electrons and holes formed in the energy bands directly after photoexcitation with excess energy. This is followed by ultrafast (~100 fs) self-localization of the primary photoexcitation and by relatively slow exciton formation (~1 ps). Further relaxation occurs with time constants ranging from hundreds of femtoseconds to tens of picoseconds, due to exciton hopping to sites with lower energy and to a slow conformational planarization of the polymer backbone. Depolarization, a spectral red shift, and important changes in the vibronic structure are observed as a consequence of this relaxation. Finally, relaxed intrachain and interchain singlet excitons are formed in solution and film, respectively, on a 100–200 ps time scale. They decay with a ~500 ps time constant, by intersystem crossing in solution and by nonradiative recombination in the film. Our results are consistent with and strongly support the conclusions we obtained from a similar time-resolved fluorescence study of the polymer PCDTBT (J. Am. Chem. Soc.2010, 132, 17459): ultrafast charge separation in polymer:fullerene blends seems to occur before localization of the primary excitation to form a bound exciton.
 
The fluorescence lifetime of the radical cation of N,N,N′,N′-tetramethyl-p-phenylenediamine (Wurster's blue) decreases from 260 ps at 82 K to 200 fs at room temperature. Calculations indicate a small barrier between the excited-state minimum (D1 min) and a conical intersection (CI) of the excited and ground state potentials. The intersection is reached within 200 fs upon torsion of one of the C—N bonds.
  • Fluorescent monolayer protected gold nanoparticles – Preparation and structure elucidation
    P. Angelova, N. Kuchukova, G.M. Dobrikov, I. Timtcheva, K. Kostova, I. Petkova and E. Vauthey
    Journal of Molecular Structure, 993 (1-3) , 2011, p185-192
    DOI:10.1016/j.molstruc.2010.12.019 | unige:15785 | Abstract | Article HTML
A novel N-substituted 4-methoxy-1,8-naphthalimide (NAFTA 8) especially designed for fluorescent labeling of gold nanoparticles has been synthesized. NAFTA 8 bears a long methylene chain at the imide N atom and has a terminal SH group, which enables its chemical binding to gold nanostructures. The longest wavelength absorption maximum of NAFTA 8 in chloroform is at 370 nm, the fluorescent maximum is at 430 nm and the fluorescent quantum yield is 0.95. The newly synthesized fluorophore is applied for functionalization of gold nanoparticles with diameter 1.5 ¹ 0.5 nm prepared through chemical reduction. The obtained Monolayer Protected Clusters are characterized by elemental analysis, TEM, XPS, FT-IR, absorption and fluorescence spectroscopy. The performed investigations provide evidence for the formation of chemical bond between the thiol ligand and the gold surface. They also show that the obtained metal/dielectric 3D structures are highly fluorescent.
Using bottom-up and self-assembly processes, large scale layered arrays of strongly coupled gold nanoparticles with controllable dimensions were fabricated. By carefully adjusting the distance between adjacent gold nanoparticle arrays, it is possible to control the coupling of the localized surface plasmon polariton resonance as sustained by individual gold nanoparticles. A greater interaction is observed at smaller separations, leading to a well pronounced shift in the spectral position of resonances that can be adjusted with high precision. Simulations showed good agreement with experimental observations in an in-depth investigation of such structures, suggesting minimal separations of only one nanometer are achieved.
  
  • Hydrogen-Bond-Assisted Excited-State Deactivation at Liquid/Water Interfaces
    Piotr Fita, Marina Fedoseeva and Eric Vauthey
    Langmuir, 27 , 2011, p4645-4652
    DOI:10.1021/la104801h | unige:15783 | Abstract | Article HTML | Article PDF
The excited-state dynamics of eosin B (EB) at dodecane/water and decanol/water interfaces has been investigated with polarization-dependent and time-resolved surface second harmonic generation. The results of the polarization-dependent measurements vary substantially with (1) the EB concentration, (2) the age of the sample, and (3) the nature of the organic phase. All of these effects are ascribed to the formation of EB aggregates at the interface. Aggregation also manifests itself in the time-resolved measurements as a substantial shortening of the excited-state lifetime of EB. However, independently of the dye concentration used, the excited-state lifetime of EB at both dodecane/water and decanol/water interfaces is much longer than in bulk water, where the excited-state population undergoes hydrogen-bond-assisted non-radiative deactivation in a few picoseconds. These results indicate that hydrogen bonding between EB and water molecules at liquid/water interfaces is either much less efficient than in bulk water or does not enhance non-radiative deactivation. This strong increase of the excited-state lifetime of EB at liquid/water interfaces opens promising avenues of applying this molecule as a fluorescent interfacial probe.
  • Near-Infrared to Visible Light Upconversion in a Trinuclear d-f-d ComplexVery Important Paper
    Lilit Aboshyan-Sorgho, CĂŠline Besnard, Phil Pattison, Kevin R. Kittilstved, Annina Aebischer, Jean-Claude G. BĂźnzli, Andreas Hauser and Claude Piguet
    Angewandte Chemie International Edition, 50 , 2011, p4108-4112
    Keywords: helical structures;lanthanides;photochemistry;self-assembly;upconversion
    DOI:10.1002/anie.201100095 | unige:15714 | Abstract | Article PDF
  
The connection of two CrIII sensitizers around a central ErIII acceptor in a self-assembled cation provides high local metal concentrations that favor efficient nonlinear energy transfer upconversion luminescence (see picture). Upon selective low-energy near-infrared irradiation of CrIII-centered transitions, 1 displays an unprecedented molecular two-photon upconverted green ErIII-centered emission.
  • The kinetics features of a thermal spin transition characterized by optical microscopy on the example of [Fe(bbtr)3](ClO4)2 single crystals: Size effect and mechanical instability
    Christian Chong, Ahmed Slimani, François Varret, Kamel Boukheddaden, Eric Collet, Jean-Claude Ameline, Robert Bronisz and Andreas Hauser
    Chemical Physics Letters, 504 (1-3) , 2011, p29-33
    DOI:10.1016/j.cplett.2011.01.041 | unige:16831 | Abstract | Article HTML | Article PDF | Â | Â | Â | Â | Â
We investigated by optical microscopy the thermal spin transition in single crystals of [Fe(bbtr)3](ClO4)2 (bbtr = 1,4-di(1,2,3-triazol-1-yl) butane). The growth of the low-spin phase was observed for different crystal orientations and sizes. The process always started from a corner of the crystal but its further development depended on the size, shape and thermal history of the crystal. In crystals of smaller size, under isothermal conditions, the low-spin phase developed in a continuous way, through the propagation of a rather well defined transformation front, with a higher propagation velocity inside the planes perpendicular to the c axis. In larger crystals the spontaneous occurrence of inhomogeneous stresses led to a stepwise propagation process.
  
The photocatalytic degradation of l-asparagine and l-glutamic acid over Au/TiO2 and TiO2 catalysts was investigated in situ by attenuated total reflection infrared (ATR-IR) in combination with modulation excitation spectroscopy. Oxalate was detected on the catalyst surface, which has not been reported before for degradation of amino acids by studies focusing on intermediates in solution. The ATR-IR spectra provide valuable information on the fate of the nitrogen. Ammonium was detected, in agreement with previous studies. Most importantly, strong signals of cyanide were observed, and this assignment has been corroborated by 15N labeling experiments. Cyanide was not reported before, to the best of our knowledge, for the photocatalytic degradation of amino acids. Cyanide was formed in the presence and the absence of gold particles on the TiO2 surface. The cyanide leads to leaching of gold via Au(CN)2− species that were detected in solution by mass spectrometry.
  • Shining Light at Working Interfaces and Chiral Nanoparticles
    Thomas BĂźrgi
    Chimia, 65 (3) , 2011, p157-167
    Keywords: attenuated total reflection;chiral surfaces;nanoparticles;photocatalysis;spectroscopy
    DOI:10.2533/chimia.2011.157 | unige:15782 | Abstract | Article PDF
In this article we present an overview of our recent research in the fields of in situ spectroscopy, nanomaterials and chirality. Our research focuses around the spectroscopic investigation of chemical reactions taking place at solid-liquid interfaces. This research goesh and in hand with the development of experimental techniques that enable us to study interface phenomena in situ. Using such techniques we try to shed light on photocatalytic reactions like the decomposition of organic pollutants in water or the reduction of carbon dioxide. We are moreover interested in chiral surfaces and their ability to discriminate betweenen antiomers. Again this relies on special techniques that highlight the enantiodiscriminating surface-adsorbate interactions. We further more seek to transfer chirality from adsorbates to metal nanoparticles. The latter are probed by chiroptical techniques, particularly also vibrational circular dichroism (VCD). Finally, we aim at preparing metamaterials with tailored optical properties by organizing plasmonic particles in two and three dimensions.
 
The photophysics of two dyes from the xanthene family, eosin B (EB), and eosin Y (EY) has been investigated in various solvents by femtosecond transient absorption spectrosco- py, first, to clarify the huge disparity of the EB fluorescence lifetimes reported in literature, and, second, to understand the mechanism responsible for the ultrafast excited-state deactivation of EB in water. The excited-state lifetime of EB was found to be much shorter in water and in other protic solvents, due to the occurrence of hydrogen-bond assisted nonradiative deactivation. This mechanism is associated with the hydrogen bonds between the solvent molecules and the nitro groups of EB, which become stronger upon optical excitation due to the charge-transfer character of the excited-state. This process is not operative with EY, where the nitro groups are replaced by bromine atoms. Therefore, the excited-state lifetime of EB in solution is directly related to the strength of the solvent as a hydrogen-bond donor, offering the possibility to build a corresponding scale based on the fluorescence quantum yield or lifetime of EB. This scale of hydrogen-bonding strength could be especially useful for studies of liquid interfaces by time-resolved surface second harmonic generation.
  
  • Effect of the Addition of a Fused Donor−Acceptor Ligand on a Ru(II) Complex: Synthesis, Characterization, and Photoinduced Electron Transfer Reactions of [Ru(TTF-dppz)2(Aqphen)]2+
    Nathalie Dupont, Ying-Fen Ran, Hong-Peng Jia, Jakob Grilj, Jie Ding, Shi-Xia Liu, Silvio Decurtins and Andreas Hauser
    Inorganic Chemistry, 50 (8) , 2011, p3295-3303
    DOI:10.1021/ic101951n | unige:15004 | Abstract | 2(Aqphen)]2+" target="_blank">Article HTML | Article PDF
The synthesis and the photophysical properties of the complex [Ru(TTF-dppz)2(Aqphen)]2+(TTF = tetrathiafulvalene, dppz = dipyrido-[3,2-a:2′,3′-c]phenazine, Aqphen = anthraquinone fused to phenanthroline via a pyrazine bridge) are described. In this molecular triad excitation into the metal−ligand charge transfer bands results in the creation of a long-lived charge separated state with TTF acting as electron donor and anthraquinone as terminal acceptor. The lifetime of the charge-separated state is 400 ns in dichloromethane at room temperature. A mechanism for the charge separation involving an intermediate charge-separated state is proposed based on transient absorption spectroscopy.
  • Ground and Excited State Double Hydrogen Transfer in Symmetric and Asymmetric Potentials: Comparison of 2,7,12,17-Tetra-n-propylporphycene with 9-Acetoxy-2,7,12,17-tetra-n-propylporphycene
    Piotr Fita, Piotr Garbacz, Michal Nejbauer, Czeslaw Radzewicz and Jacek Waluk
    Chemistry - A European Journal, 17 (13) , 2011, p3672-3678
    Keywords: density functional theory;hydrogen transfer;porphycenes;tautomerism;time-resolved anisotropy
    DOI:10.1002/chem.201002931 | unige:14939 | Abstract | Article PDF
Analysis of time-resolved anisotropy of transient absorption enabled determination of room temperature ground and excited state rate constants for intramolecular double hydrogen transfer in two similar porphycenes, one of them with symmetric and the other, with asymmetric character of a double minimum potential for hydrogen motion. The perturbation preserves a quasi-symmetric minimum in S0, but the rate decreases approximately two times. In S1, the perturbed potential becomes strongly asymmetric, and the downhill hydrogen transfer occurs with a rate higher than that observed for a symmetrical compound.
  • Nonradiative Deactivation of Excited Hemicyanines Studied with Submolecular Spatial Resolution by Time-Resolved Surface Second Harmonic Generation at Liquid−Liquid Interfaces
    Gaelle Martin-Gassin, Diego Villamaina and Eric Vauthey
    Journal of the American Chemical Society, 133 , 2011, p2358-2361
    DOI:10.1021/ja109813j | unige:14979 | Abstract | Article HTML | Article PDF
  
The excited-state dynamics of aminostilbazolium dyes is known to be dominated by nonradiative deactivation through large-amplitude motion. In order to identify the coordinate(s) responsible for this process, the excited-state lifetimes of two dialkylaminostyryl-methylpyridinium iodides have been measured at liquid−liquid interfaces using time-resolved surface second harmonic generation. We found that the decay time of the excited-states of both compounds was increasing with the viscosity of the apolar phase, consisting of n-alkanes of varying length, but was unaffected by that of the polar phase, made of water/glycerol mixtures. This indicates that the nonradiative deactivation is associated with the twist of the dialkylaniline group, which is located in the apolar part of the molecule.
  
  • Low-Spin→High-Spin Relaxation Dynamics in the Highly Diluted Spin-Crossover System [FexZn1−x(bbtr)3](ClO4)2
    Itana Krivokapic, Pradip Chakraborty, Cristian Enachescu, Robert Bronisz and Andreas Hauser
    Inorganic Chemistry, 50 (5) , 2011, p1856-1861
    DOI:10.1021/ic102269y | unige:14980 | Abstract | Article HTML | Article PDF
Whereas the neat polymeric iron(II) compound [Fe(bbtr)3](ClO4)2, bbtr = 1,4-di(1,2,3-triazol-1-yl)butane, shows a quantitative spin transition triggered by a crystallographic phase transition centered at 107 K with a 13 K wide hysteresis, the iron(II) complexes in the diluted mixed crystals [FexZn1−x(bbtr)3](ClO4)2, x = 0.02 and 0.1, stay predominantly in the 5T2 high-spin state down to cryogenic temperatures. However, the 1A1 low-spin state can be populated as metastable state via irradiation into the spin-allowed 5T2→5E ligand-field transition of the high-spin species in the near-infrared. The quantum efficiency of the light-induced conversion is approximately 10% at low temperatures and decreases rapidly above 160 K. The lifetime of the light-induced low-spin state decreases from 15 days at 40 K to 30 ns at 220 K, that is, by 14 orders of magnitude. In the high-temperature regime the activation energy for the low-spin→high-spin relaxation is 1840(20) cm−1.
The structural and vibrational properties of the isostructural compounds Ca2FeH6 and Sr2RuH6 are determined by periodic DFT calculations and compared with their previously published experimental crystal structures as well as new experimental vibrational data. The analysis of the vibrational data is extended to the whole series of alkaline-earth iron and ruthenium hydrides A2TH6 (A = Mg,Ca,Sr; T = Fe, Ru) in order to identify correlations between selected frequencies and the T-H bond length. The bulk moduli of Ca2FeH6 and Sr2RuH6 have also been determined within DFT. Their calculated values prove to compare well with the experimental values reported for Mg2FeH6 and several other compounds of this structure.
  • Experimental evidence of librational vibrations determining the stability of calcium borohydride
    A. Borgschulte, R. Gremaud, A. ZĂźttel, P. Martelli, A. Remhof, A.J. Ramirez-Cuesta, K. Refson, E.G. Bardaji, W. Lohstroh, M. Fichtner, H. Hagemann and M. Ernst
    Physical Review B, 83 (2) , 2011, p24102
    DOI:10.1103/PhysRevB.83.024102 | unige:14989 | Abstract | Article PDF
  
The high energy of hydrogen vibrations in solids is the origin of their strong impact on thermodynamic properties. The peculiar structure of complex hydrides amplifies this impact. We shed light on the vibrational properties of three allotropes of Ca(BH4)2 using density-functional theory calculations, infrared spectroscopy, and inelastic neutron scattering. We show that the vibrational properties of Ca(BH4)2 depend on the specific phase and are hitherto the origin of their differences in stability.
  • Strong correlation treated via effective hamiltonians and perturbation theory
    Giovanni Li Manni, Francesco Aquilante and Laura Gagliardi
    Journal of Chemical Physics, 134 (3) , 2011, p34114
    Keywords: configuration interactions, perturbation theory, SCF calculations, wave functions
    DOI:10.1063/1.3532927 | unige:14855 | Abstract | Article HTML | Article PDF
We propose a new approach to determine a suitable zeroth-order wavefunction for multiconfigurational perturbation theory. The same ansatz as in complete active space (CAS) wavefunction optimization is used but it is split in two parts, a principal space (A) and a much larger extended space (B). LÜwdin's partitioning technique is employed to map the initial eigenvalue problem to a dimensionality equal to that of (A) only. Combined with a simplified expression for the (B) portion of the wavefunction, we are able to drastically reduce the storage and computational demands of the wavefunction optimization. This scheme is used to produce reference wavefunctions and energies for subsequent second-order perturbation theory (PT2) corrections. Releasing the constraint of computing the exact CAS energy and wavefunction prior to the PT2 treatment introduces a nonstandard paradigm for multiconfigurational methods. Based on the results of test calculations, we argue that principal parts with only few percents of the total number of CAS configurations could provide final multiconfigurational PT2 energies of the same accuracy as in the standard paradigm. In the future, algorithmic improvements for this scheme will bring into reach active spaces much beyond the present limit of CAS-based methods, therefore allowing for accurate studies of systems featuring strong correlation.
  
  • New fundamental experimental studies on α-Mg(BH4)2 and other borohydrides
    Hans Hagemann, Vincenza D'Anna, Jean-Philippe Rapin, Radovan CernĂ˝, Yaroslav Filinchuk, Ki Chul Kim, David S. Sholl and Stewart F. Parker
    Journal of Alloys and Compounds, 509 (2) , 2011
    Keywords: hydrogen storage materials; vibrational spectroscopy; synchrotron X-ray diffraction
    DOI:10.1016/j.jallcom.2010.10.068 | unige:16970 | Article HTML | Article PDF
Several new studies of Mg(BH4)2 are reported. A 1:1 LiBH4:Mg(BH4)2 mixture was studied by in situ synchrotron X-ray diffraction and reveals an eutectic behavior with the eutectic composition more rich in Mg(BH4)2, and the eutectic temperature lower than 456 K. No dual cation compound was observed in this experiment.

New vibrational spectra including INS data have been obtained and are compared with theoretical DFT calculations and recent NMR studies, showing good agreement.

  • Polarized Raman and Hyperpolarizability studies of Hydroxyethylammonium (L) tartrate monohydrate for quadratic nonlinear optics
    R. Nagalakshmi, V. Krishnakumar, Hans Hagemann and S. Muthunatesan
    Journal of Molecular Structure, 988 , 2011, p17-23
    Keywords: solution growth; X ray diffraction; vibrational spectroscopy; polarized Raman, hyperpolarizability
    DOI:10.1016/j.molstruc.2010.11.056 | unige:14817 | Abstract | Article PDF
Single crystals of Hydroxyethylammonium L – tartrate monohydrate [HEALT] have been grown by slow evaporation technique using water as a solvent. The structural and vibrational properties of the crystals were studied. Besides these characterizations ab initio quantum chemical calculations have been performed at HF/6-31G (d) level to derive first order hyperpolarizability. It is shown that the first order hyperpolarizability is found to be 14.2 times more than that of urea. The characteristic vibrational frequencies obtained from polarized Raman spectra in different scattering configurations have been assigned based on the complete factor group analysis. Vibrational analysis of IR and Raman reveals that the charge transfer interaction must be responsible for nonlinear optical (NLO) properties of the present system. The UV absorption measurements have also been carried out to confirm the utility of the material for optical applications.
The importance of the nonelectrostatic component of the embedding potential is investigated by comparing the complexation induced shifts of the iso-g obtained in embedding calculations to its supermolecular counterparts. The analyses are made in view of such multilevel simulations, for which supermolecular strategy is either impractical or impossible, such as the planned simulations for the whole enzyme ferredoxin oxidoreductase. For the biliverdin radical surrounded by a few amino acids, it is shown that the embedding potential comprising only Coulomb terms fails to reproduce even qualitatively the shifts evaluated from supermolecular calculations. The nonelectrostatic component of the exact embedding potential is a bifunctional of two electron densities [Wesolowski and Warshel, J. Phys. Chem. 1993, 97, 8050; Wesolowski, Phys. Rev. A 2008, 77, 012504]. Therefore we analyze in detail both the quality of the approximant for the bifunctional and the importance of the choice of the electron densities at which it is evaluated in practical calculations.
  • Multiconfigurational Second-Order Perturbation Theory Restricted Active Space (RASPT2) Method for Electronic Excited States: A Benchmark Study
    Vicenta Sauri, Luis Serrano-AndrĂŠs, Abdul Rehaman Moughal Shahi, Laura Gagliardi, Steven Vancoillie and Kristine Pierloot
    Journal of Chemical Theory and Computation, 7 (1) , 2011, p153-168
    DOI:10.1021/ct100478d | unige:14808 | Abstract | Article HTML | Article PDF
The recently developed second-order perturbation theory restricted active space (RASPT2) method has been benchmarked versus the well-established complete active space (CASPT2) approach. Vertical excitation energies for valence and Rydberg excited states of different groups of organic (polyenes, acenes, heterocycles, azabenzenes, nucleobases, and free base porphin) and inorganic (nickel atom and copper tetrachloride dianion) molecules have been computed at the RASPT2 and multistate (MS) RASPT2 levels using different reference spaces and compared with CASPT2, CCSD, and experimental data in order to set the accuracy of the approach, which extends the applicability of multiconfigurational perturbation theory to much larger and complex systems than previously. Relevant aspects in multiconfigurational excited state quantum chemistry such as the valence−Rydberg mixing problem in organic molecules or the double d-shell effect for first-row transition metals have also been addressed.
 
The photophysics and excited-state dynamics of two dyads consisting of either a free-base or a zinc-tetraphenylporphyrin linked through a rigid bridge to a core-substituted naphthalenediimide (NDI) have been investigated by femtosecond-resolved spectroscopy. The absorption and fluorescence spectra differ substantially from those of the individual units, pointing to a substantial coupling and to a delocalisation of the excitation over the whole molecule, as confirmed by quantum chemistry calculations. A strong dependence of their excited-state dynamics on the solvent polarity has been observed. In toluene, the fluorescence quantum yield of the dyads is of the order of a few percent and the main decay channel of the emitting state is proposed as intersystem-crossing to the triplet state. However, in a medium polarity solvent like dichloromethane, the emitting state undergoes charge separation from the porphyrin to the NDI unit within 1–3 ps, and the ensuing charge-separated state recombines in about 10–20 ps. This solvent dependence can be explained by the weak driving force for charge separation in polar solvents and the large electronic coupling between the porphyrin and NDI moieties, making charge separation a solvent-controlled adiabatic process.
  • Identification of New Aromatic Compounds in the New Zealand Manuka Honey by Gas Chromatography-Mass Spectrometry
    Sawsan Daher and Fazil O. GĂźlaçar
    E-Journal of Chemistry, 7 (S1) , 2010, p7-14
    Keywords: Manuka honey; aromatic compounds; solid phase microextraction; GC-CMS
    DOI:10.1155/2010/472769 | unige:21623 | Abstract | Article PDF
Analysis of aromatic compounds in the New Zealand manuka honey was carried out by solid phase microextraction followed by gas chromatography-mass spectrometry. A total of 38 compounds were detected. Seven of them such as; 1,4-bis(x-methoxyphenyl)-but-2-en-1-one, 1,5-bis(x-methoxyphenyl)-pent-3-en-1-one, 1,4-bis(x-methoxyphenyl)-1-pentanone, 1,6-bis(x-methoxyphenyl)-3-heptene, 1,6-bis(x-methoxyphenyl)-hex-2(3 or 4)-en-1-one and 2(3, 4 or 5)-hydroxy-1,6-bis(x-methoxyphenyl)-1-hexanone, had never before been identified as natural products. Their structures were deduced from the mass spectral data. Seven other compounds; 2,3-dimethoxynaphthalene, 4-(x-methoxyphenyl)-1-phenyl-1-butanone, desoxyanisoin, 2,6-dimethoxybenzoic acid benzyl ester, 4,4'-dimethoxystilbene, 3,3,4,5,5,8-hexamethyl-2,3,5,6-tetrahydro-s-indacene-1,7-dione and 1,5-bis(4-methoxyphenyl)-pentane-1,5-dione, were found in honey for the first time. Methyl syringate, ortho-methoxyacetophenone and 3-phenyllactic acid were the most abundant components.
  • Kinetic Control in the Chiral Recognition of Three-Bladed Propellers
    ClĂŠment Bonnot, Emmanuel Aubert, Natalie Banerji, JĂŠrĂ´me Lacour, Enrique Espinosa and Jean-Claude Chambron
    Chemistry - A European Journal, 16 (19) , 2010, p5706-5711
    Keywords: cryptands;helical structures;ion pairs;kinetics;solvent effects
    DOI:10.1002/chem.200903058 | unige:6579 | Abstract | Article PDF
The ion pair of the stereolabile C3-symmetric, i+o proton complex [1⋅H]+ of diaza-macropentacycle 1 and the configurationally stable Δ-TRISPHAT ([Δ-3]−) anion exists in the form of two diastereomers, namely, [Δ-(1⋅H)][Δ-3] and [Λ-(1⋅H)][Δ-3], the ratio of which, in terms of diastereomeric excess (de) decreases in the order [D8]THF (28 %)>CD2Cl2 (22 %)>CDCl3 (20 %)>[D8]toluene (16 %)>C6D6 (7 %)>[D6]acetone (0 %) at thermodynamic equilibrium. Except in the case of [D6]acetone, the latter is reached after a period of time that increases from 1 h ([D8]THF) to 24 h (CDCl3). Moreover, the initial value of the de of [1⋅H][Δ-3] in CDCl3, before the thermodynamic equilibrium is reached, depends on the solvent in which the sample has been previously equilibrated (sample “history”). This property has been used to show that the crystals of [1⋅H][Δ-3] formed by slow evaporation of CH2Cl2/CH3OH mixtures had 100 % de, which indicates that [1⋅H][Δ-3] has enjoyed a crystallization-induced asymmetric transformation. Structural studies in solution (NMR spectroscopy) and in the gas phase by calculations at the semiempirical PM6 level of theory suggest that the optically active anion is docked on the i+ (endo) external side of the proton complex such that one of the aromatic rings of [Δ-3]− is inserted into a groove of [1⋅H]+, a second aromatic ring being placed astride the outside i+ pocket. Solvent polarity controls the thermodynamics of inversion of the [1⋅H]+ propeller. However, both polarity and basicity control its kinetics. Therefore, the rate-limiting steps correspond to the ion-pair separation/recombination and [1⋅H]+/1deprotonation/protonation processes, rather than the inversion of [1⋅H]+, the latter being likely to take place in the deprotonated form (1).
  • Orbital-Free Embedding Effective Potential in Analytically Solvable Cases
    Andreas Savin and Tomasz A. Wesolowski
    in "Progress in Theoretical Chemistry and Physics" Advances in the Theory of Atomic and Molecular Systems, P. Piecuch, J. Maruani, G. Delgado-Barrio and S. Wilson, Springer, 19 , 2010, p311-326
    Keywords: embedding potential; density functional theory; kinetic energy functional; orbital-free embedding
    DOI:10.1007/978-90-481-2596-8 | unige:14990
The effective embedding potential introduced by Wesolowski and Warshel [J. Phys. Chem., 97 (1993) 8050] depends on two electron densities: that of the environment (n B ) and that of the investigated embedded subsystem (n A ). In this work, we analyze this potential for pairs n A  and n B , for which it can be obtained analytically. The obtained potentials are used to illustrate the challenges in taking into account the Pauli exclusion principle.
  
  • Unidirectional Photoisomerization of Styrylpyridine for Switching the Magnetic Behavior of an Iron(II) Complex: A MLCT Pathway in Crystalline Solids
    Antoine Tissot, Marie-Laure Boillot, SĂŠbastien Pillet, Epiphane Codjovi, Kamel Boukheddaden and LatĂŠvi Max Lawson Daku
    Journal of Physical Chemistry C, 114 (49) , 2010, p21715-21722
    DOI:10.1021/jp106583f | unige:14853 | Abstract | Article HTML | Article PDF
The photoreactivity of two iron(II)−styrylpyridine frameworks Fe(stpy)4(NCSe)2 (stpy = 4-styrylpyridine) has been investigated for the very first time in a crystalline solid. A quantitative cis-to-trans isomerization of stilbenoids is shown to occur in the confined environment of the inorganic solid. The photochromic reaction was driven by a visible excitation into the metal-to-ligand charge transfer absorption of the high-spin all-cis complex. The solid-state transformation is accompanied by a unit-cell volume increase and an amorphization. Interestingly, the photoproduct formed by irradiating the high-spin all-cis reactant undergoes a spin conversion when the temperature is decreased. This observation is related to the “ligand-driven light-induced spin change” effect in a constrained environment.
  • Unsymmetrical Tripodal Ligand for Lanthanide Complexation: Structural, Thermodynamic, and Photophysical Studies
    Badr El Aroussi, Nathalie Dupont, GĂŠrald Bernardinelli and Josef Hamacek
    Inorganic Chemistry, 49 (2) , 2010, p606-615
    DOI:10.1021/ic901757u | unige:5248 | Abstract | Article HTML | Article PDF
  
Two tridentate and one bidentate binding strands have been anchored on a carbon atom to provide a new unsymmetrical tripodal ligand L for Ln(III) coordination. The ligand itself adopts a single conformation in solution stabilized by intramolecular hydrogen bonds evidenced in the solid state. The reaction of L with trivalent lanthanides provides different coordination complexes depending on the metal/ligand ratio. The speciation studies with selected lanthanides were performed in solution by means of NMR, ESMS, and spectrophotometric titrations. Differences in coordination properties along the lanthanide series were evidenced and may be associated with the changes in the ionic size. However, thermodynamic stability constants for the species of the same stoichiometry do not significantly vary. In addition, the structure of the dinuclear complex [Eu2L2]6+ has been elucidated in the solid state, where the complex crystallizes predominantly as an M-isomer. The crystal structure shows the coordination of two different ligands to each europium cation through tridentate strands, and the europium nine-coordinate sphere is completed with three solvent molecules. Finally, the results of photophysical investigations of [Eu2L2]6+ are in close agreement with the structural parameters determined by crystallography.
  
The nature and time evolution of the primary excitations in the pristine conjugated polymer, PCDTBT, are investigated by femtosecond-resolved fluorescence up-conversion spectroscopy. The extensive study includes data from PCDTBT thin film and from PCDTBT in chlorobenzene solution, compares the fluorescence dynamics for several excitation and emission wavelengths, and is complemented by polarization-sensitive measurements. The results are consistent with the photogeneration of mobile electrons and holes by interband π-π* transitions, which then self-localize within about 100 fs and evolve to a bound singlet exciton state in less than 1 ps. The excitons subsequently undergo successive migrations to lower energy localized states, which exist as a result of disorder. In parallel, there is also slow conformational relaxation of the polymer backbone. While the initial self-localization occurs faster than the time resolution of our experiment, the exciton formation, exciton migration, and conformational changes lead to a progressive relaxation of the inhomogeneously broadened emission spectrum with time constants ranging from about 500 fs to tens of picoseconds. The time scales found here for the relaxation processes in pristine PCDTBT are compared to the time scale (<0.2 ps) previously reported for photoinduced charge transfer in phase- separated PCDTBT:fullerene blends (Phys. Rev. B 2010, 81, 125210). We point out that exciton formation and migration in PCDTBT occur at times much longer than the ultrafast photoinduced electron transfer time in PCDTBT:fullerene blends. This disparity in time scales is not consistent with the commonly proposed idea that photoinduced charge separation occurs after diffusion of the polymer exciton to a fullerene interface. We therefore discuss alternative mechanisms that are consistent with ultrafast charge separation before localization of the primary excitation to form a bound exciton.
  • Photoinduced Processes in Fluorene-Bridged Rhenium–Phenothiazine Dyads – Comparison of Electron Transfer Across Fluorene, Phenylene, and Xylene Bridges
    Mathieu E. Walther, Jakob Grilj, David Hanss, Eric Vauthey and Oliver S. Wenger
    European Journal of Inorganic Chemistry, 2010 (30) , 2010, p4843-4850
    DOI:10.1002/ejic.201000645 | unige:14738 | Abstract | Article HTML | Article PDF
The photoinduced processes occurring after pulsed laser excitation of a series of donor–bridge–acceptor molecules comprising a phenothiazine electron donor, variable-length fluorene bridges, and a rhenium(I) electron acceptor were investigated. A dyad with a single fluorene bridge unit exhibits electron transfer from phenothiazine to the rhenium(I) complex upon photoexcitation, whereas in dyads with fluorene oligomers bridge-localized triplet excited states are formed rather than electron transfer products. In the monofluorene-bridged system with a donor–acceptor distance of ca. 15 Å, electron transfer occurs with a time constant of 1.9 ns. The equidistant electron transfer between the same donor and acceptor is considerably slower across a biphenyl bridge (3.9 ns) or a bi-p-xylene spacer (20 ns). This finding is interpreted in terms of different tunneling barrier heights associated with the charge transfer across the three different types of molecular bridges.
  • Structure and Characterization of KSc(BH4)4
    Radovan Cerny, Dorthe B. RavnsbĂŚk, Godwin Severa, Yaroslav Filinchuk, Vincenza D'Anna, Hans Hagemann, DĂśrthe Haase, Jørgen Skibsted, Craig M. Jensen and Torben R. Jensen
    Journal of Physical Chemistry C, 114 (45) , 2010, p19540-19549
    Keywords: borohydrides; hydrogen storage; metathesis; X-ray diffraction
    DOI:10.1021/jp106280v | unige:14680 | Abstract | Article HTML | Article PDF
A new potassium scandium borohydride, KSc(BH4)4, is presented and characterized by a combination of in situ synchrotron radiation powder X-ray diffraction, thermal analysis, and vibrational and NMR spectroscopy. The title compound, KSc(BH4)4, forms at ambient conditions in ball milled mixtures of potassium borohydride and ScCl3 together with a new ternary chloride K3ScCl6, which is also structurally characterized. This indicates that the formation of KSc(BH4)4 differs from a simple metathesis reaction, and the highest scandium borohydride yield (~31 mol %) can be obtained with a reactant ratio KBH4:ScCl3 of 2:1. KSc(BH4)4 crystallizes in the orthorhombic crystal system, a = 11.856(5), b = 7.800(3), c = 10.126(6) Å, V = 936.4(8) Å3 at RT, with the space group symmetry Pnma. KSc(BH4)4 has a BaSO4 type structure where the BH4 tetrahedra take the oxygen positions. Regarding the packing of cations, K+, and complex anions, [Sc(BH4)4]−, the structure of KSc(BH4)4 can be seen as a distorted variant of orthorhombic neptunium, Np, metal. Thermal expansion of KSc(BH4)4 in the temperature range RT to 405 K is anisotropic, and the lattice parameter b shows strong nonlinearity upon approaching the melting temperature. The vibrational and NMR spectra are consistent with the structural model, and previous investigations of the related compounds ASc(BH4)4 with A = Li, Na. KSc(BH4)4 is stable from RT up to ~405 K, where the compound melts and then releases hydrogen in two rapid steps approximately at 460−500 K and 510−590 K. The hydrogen release involves the formation of KBH4, which reacts with K3ScCl6 and forms a solid solution, K(BH4)1−xClx. The ternary potassium scandium chloride K3ScCl6 observed in all samples has a monoclinic structure at room temperature, P21/a, a = 12.729(3), b = 7.367(2), c = 12.825(3) Å, β = 109.22(2)°, V = 1135.6(4) Å3, which is isostructural to K3MoCl6. The monoclinic polymorph transforms to cubic at 635 K, a = 10.694 Å (based on diffraction data measured at 769 K), which is isostructural to the high temperature phase of K3YCl6.
 
The thiolate-for-thiolate ligand exchange reaction between the stable Au38(2-PET)24 and Au40(2-PET)24 (2-PET: 2-phenylethanethiol) clusters and enantiopure BINAS (BINAS: 1,1′-binaphthyl-2,2′-dithiol) was investigated by circular dichroism (CD) spectroscopy in the UV/vis and MALDI mass spectrometry (MS). The ligand exchange reaction is incomplete, although a strong optical activity is induced to the resulting clusters. The clusters are found to be relatively stable, in contrast to similar reactions on [Au25(2-PET)18]− clusters. Maximum anisotropy factors of 6.6 × 10−4 are found after 150 h of reaction time. During the reaction, a varying ratio between Au38 and Au40 clusters is found, which significantly differs from the starting material. As compared to Au38, Au40 is more favorable to incorporate BINAS into its ligand shell. After 150 h of reaction time, an average of 1.5 and 4.5 BINAS ligands is found for Au38 and Au40 clusters, respectively. This corresponds to exchange of 3 and 9 monodentate 2-PET ligands. To show that the limited exchange with BINAS is due to the bidentate nature of the ligand, exchange with thiophenol was performed. The monodentate thiophenol exchange was found to be faster, and more ligands were exchanged when compared to BINAS.
  • First emission studies of Tc2X82- systems (X = Cl, Br)
    Breeze N. Briggs, David R. McMillin, Tanya K. Todorova, Laura Gagliardi, Frederic Poineau, Kenneth R. Czerwinski and Alfred P. Sattelberger
    Dalton Transactions, 39 , 2010, p11322-11324
    DOI:10.1039/C0DT00751J | unige:14694 | Abstract | Article HTML | Article PDF
The emission spectra of the solids [n-Bu4N]2Tc2X8 (X = Cl, Br) have been investigated at room temperature and 77 K. In each case, the emission originates in the 1δ–δ* excited state, as with the rhenium homologues, but has a shorter lifetime.
  • Hydration of Lanthanide Chloride Salts: A Quantum Chemical and Classical Molecular Dynamics Simulation Study
    Cesar Beuchat, Daniel Hagberg, Riccardo Spezia and Laura Gagliardi
    Journal of Physical Chemistry B, 114 (47) , 2010, p15590-15597
    DOI:10.1021/jp105590h | unige:14689 | Abstract | Article HTML | Article PDF
We present the results of a quantum chemical and classical molecular dynamics simulation study of some solutions containing chloride salts of La3+, Gd3+, and Er3+ at various concentrations (from 0.05 to 5 M), with the purpose of understanding their structure and dynamics and analyzing how the coordination varies along the lanthanide series. In the La−Cl case, nine water molecules surround the central La3+ cation in the first solvation shell, and chloride is present only in the second shell for all solutions but the most concentrated one (5 M). In the Gd3+ case, the coordination number is ~8.6 for the two lowest concentrations (0.05 and 0.1 M), and then it decreases rapidly. In the Er3+ case, the coordination number is 7.4 for the two lowest concentrations (0.05 and 0.1 M), and then it decreases. The counterion Cl− is not present in the first solvation shell in the La3+ case for most of the solutions, but it becomes progressively closer to the central cation in the Gd3+ and Er3+ cases, even at low concentrations.
  
A combination of in situ synchrotron powder diffraction, energy minimization (DFT), and Raman and infrared spectroscopy confirmed porous interpenetrated 3D-framework structures of recently discovered alkali-metal−zinc borohydrides, AZn2(BH4)5 (A = Li, Na). In the less zinc rich NaZn(BH4)3 the 3D-framework structural model has been confirmed but with a slightly modified description giving an isolated triangular anion, [Zn(BH4)3]−, rather than a 1D anionic chain, {[Zn(BH4)3]n}n−. Another polymorph of NaZn(BH4)3, isostructural to a new compound, LiZn(BH4)3, is proposed by energy minimization. Both compounds, the new NaZn(BH4)3 polymorph and LiZn(BH4)3, are, however, not observed experimentally at ambient pressure and in the temperature range of 100−400 K. The alkali-metal−zinc borohydride NaZn(BH4)3 containing the triangular anion [Zn(BH4)3]− is an equivalent of recently characterized alkali-metal−scandium borohydrides NaSc(BH4)4 and LiSc(BH4)4 based on the tetrahedral [Sc(BH4)4]− complex anion.
  • Comment on: “Nucleation and Growth of BaFxCl2-x Nanorods”
    Frank Kubel and Hans-Rudolf Hagemann
    Chemistry - A European Journal, 16 (42) , 2010, p12526-12527
    Keywords: barium halides; crystal growth; crystal structure; nanocrystals
    DOI:10.1002/chem.200900300 | unige:14777 | Abstract | Article PDF
  
The crystal structure of recently reported Ba,F,Cl nanorods is shown to correspond to the structure of Ba7F12Cl2 (see picture), which can be prepared by several growth techniques.
  • Significant variation of the singlet-quintet intersystem crossing rate constant in an iron(II) high-spin complex as a function of temperature
    Itana Krivokapic, Pradip Chakraborty, Robert Bronisz, Cristian Enachescu and Andreas Hauser
    Angewandte Chemie, 49 (45) , 2010, p8509-8512
    DOI:10.1002/anie.201004500 | unige:14713
In the dilute mixed-crystal system [Zn1−xFex(bbtr)3](ClO4)2, x=2 % (bbtr=1,4-di(1,2,3-triazol-1-yl)butane), the iron(II) centers are predominantly in the high-spin state. The low-spin state can be populated as a metastable state by irradiation with near-IR light; the rate constant of the low-spin→high-spin relaxation spans 14 orders of magnitude between 40 and 220 K
  
  • C 3 symmetric tris(phosphonate)-1,3,5-triazine ligand: homopolymetallic complexes and its radical anion
    Catalin Maxim, Adil Matni, Michel Geoffroy, Marius Andruh, Nigel G.R. Hearns, Rodolphe ClĂŠrac and Narcis Avarvari
    New Journal of Chemistry, 34 (10) , 2010, p2319-2327
    DOI:10.1039/C0NJ00204F | unige:14779 | Abstract | Article HTML | Article PDF
The ligand 2,4,6-tris(dimethoxyphosphonate)-1,3,5-triazine L has been synthesized and its single crystal X-ray structure determined. The occurrence of P=O···π intermolecular interactions, suggested by the short P=O··· triazine distances of 3.16–3.35 Å, is observed. The electrochemical reduction of the ligand shows its electron acceptor character by the formation of a stable radical anion. The hyperfine structure observed in the EPR spectra, combined with a theoretical DFT study, evidences the full delocalization of the unpaired electron mainly on the triazine core, with some participation of the phosphonate groups. Theoretical calculations are in agreement with the experimental values of the hyperfine coupling constants of 11.81 G for Aiso–31P and 1.85 G for Aiso–14N. Homopolymetallic complexes, formulated as {L[Cu(hfac)2]3} (1), 1∞{L2[Co(hfac)2]3} (2) and 1∞{L2[Mn(hfac)2]3} (3) (hfac = hexafluoroacetylacetonate), have been synthesized and structurally characterized.
  • The cis-[RuII(bpy)2(H2O)2]2+ Water-Oxidation Catalyst Revisited
    Xavier Sala, Mehmed Z. Ertem, Laura Vigara, Tanya K. Todorova, Weizhong Chen, Reginaldo C. Rocha, Francesco Aquilante, Christopher J. Cramer, Laura Gagliardi and Antoni Llobet
    Angewandte Chemie International Edition, 49 (42) , 2010, p7745-7747
    Keywords: density functional calculations; energy conversion; reaction mechanisms; ruthenium; water oxidation
    DOI:10.1002/anie.201002398 | unige:14721 | Abstract | Article PDF
The only operating mechanism in the oxidation of water to dioxygen catalyzed by the mononuclear cis-[RuII(bpy)2(H2O)2]2+ complex when treated with excess CeIV was unambiguously established. Theoretical calculations together with 18O-labeling experiments (see plot) revealed that it is the nucleophilic attack of water on a Ru=O group.
  • Competition Between Photo-Excitation and Relaxation in Spin Crossover Complexes in The Frame of a Mechano-Elastic Model
    C. Enachescu, L. Stoleriu, A. Stancu and A. Hauser
    Physical Review B, 82 , 2010, p104114
    DOI:10.1103/PhysRevB.82.104114 | unige:14682 | Abstract | Article PDF
In this paper we use a recently proposed elastic model in order to study the competition between linear photoexcitation and cooperative relaxation in spin-crossover molecular magnets. The difference in molecular size between the two possible spin states, that is, the high-spin and the low-spin states, respectively, induces distortions of the crystal lattice. These determine the elastic interactions between molecules, treated here as connecting springs that are either compressed or extended from their equilibrium length, thus modulating the local probability for the high-spin→low-spin relaxation. The crossover of individual molecules within the lattice is checked by a standard Monte Carlo procedure. Using very simple assumptions and a minimum number of parameters, photoexcitation curves and hysteresis loops under continuous irradiation below the thermal transition temperature can thus be simulated. The formation of clusters is analyzed and the presence of inhomogeneities in the system is investigated.
  • Cluster evolution in spin crossover systems observed in the frame of a mechano-elastic model
    C. Enachescu, M. Nishino, S. Miyashita, L. Stoleriu, A. Stancu and A. Hauser
    Europhysics Letters, 91 (2) , 2010, p27003
    Keywords: Spin crossover, general studies of phase transitions, domain effects, magnetization curves, hysteresis
    DOI:10.1209/0295-5075/91/27003 | unige:14746
  
In this paper we study the cluster formation and evolution in spin crossover systems during the thermal transition in the frame of a mechano-elastic model applied to open boundary hexagonal lattices. The switching processes between the high-spin (HS) and low-spin (LS) state are studied by a method combining a Monte Carlo standard procedure on the spin state and the lattice relaxation. In the present study, we adopt the transition probabilities of the spin state taking into account the energy gap between the two states, the degeneracy ratio and the local pressure determined by the elongations of the closest springs. It is found that clusters of molecules in the same state tend to grow starting from corners, as in available experimental data. Some qualitative differences between the processes of cluster formation for the two hysteresis branches, i.e., HS to LS and LS to HS are pointed out. Moreover, we have studied the dependence of cluster formation on the strength of the elastic interactions, and also on the system size. The size dependence of the ratio between the system size and the maximum cluster length is very weak, which indicates the appearance of macroscopic domains.
  • Effect of External Pressure on the Excitation Energy Transfer from [Cr(ox)3]3- to [Cr(bpy)3]3+ in [Rh1-xCrx(bpy)3][NaM1-yCry(ox)3]ClO4
    Mia Milos, Prodipta Pal and Andreas Hauser
    ChemPhysChem, 11 (14) , 2010, p3161-3166
    Keywords: [Cr(bpy)3]3+; [Cr(ox)3]3−; 3D oxalate networks; excitation energy transfer; high pressures
    DOI:10.1002/cphc.201000324 | unige:14715 | Abstract | Article HTML | Article PDF
Resonant excitation energy transfer from [Cr(ox)3]3- to [Cr(bpy)3]3+ in the doped 3D oxalate networks [Rh1-xCrx(bpy)3][NaMIII1-yCry(ox)3]ClO4 (ox=C2O4-, bpy=2,2’-bipyridine, M=Al,Rh) is due to two types of interaction, namely super exchange coupling and electric dipole–dipole interaction. The energy transfer probability for both mechanisms is proportional to the spectral overlap of the 2E→4A2 emission of the [Cr(ox)3]3- donor and the 4A2→2T1 absorption of the [Cr(bpy)3]3+ acceptor.The spin-flip transitions of (pseudo-)octahedral Cr3+ are known to shift to lower energy with increasing pressure. Because the shift rates of the two transitions in question differ, the spectral overlap between the donor emission and the acceptor absorption is a function of applied pressure. For [Rh1-xCrx(bpy)3][Na-M1-yCry(ox)3]ClO4 the spectral overlap is thus substantially reduced on increasing pressure from 0 to 2.5 GPa. As a result, the energy transfer probability decreases with increasing pressure as evidenced by a decrease in the relative emission intensity from the [Cr(bpy)3]3+ acceptor.
  
  • Dynamic Perspective on the Function of Thermoresponsive Nanopores from in Situ AFM and ATR-IR Investigations
    Ana Maria Popa, Silvia Angeloni, Thomas BĂźrgi, Jeffrey A. Hubbell, Harry Heinzelmann and RaphaĂŤl Pugin
    Langmuir, 26 (19) , 2010, p15356-15365
    DOI:10.1021/la102611k | unige:14783 | Abstract | Article HTML | Article PDF
This article describes the morphological and chemical characterization of stimuli-responsive functionalized silicon surfaces provided in parallel by atomic force spectroscopy (AFM) and Fourier transform infrared spectroscopy (FT-IR) enhanced by the single-beam sample reference attenuated total reflection method (SBSR-ATR). The stimuli-responsive behavior of the surfaces was obtained by grafting-to in melt carboxyl-terminated poly-N-isopropylacryl amides (PNIPAAM) with different degree of polymerization (DP) on epoxide-functionalized silicon substrates. The unprecedented real time and in situ physicochemical insight into the temperature-triggered response of the densely packed superficial brushes allowed for the selection of a PNIPAAM with a specific DP as a suitable polymer for the fabrication of silicon membranes exhibiting switchable nanopores. The fabrication process combines the manufacture of nanoporous silicon surfaces and their subsequent chemical functionalization by the grafting-to in melt of the selected polymer. Then, relevant information was obtained in what concerns the chemical modifications behind the topographical changes that drive the functioning of PNIPAAM-based hybrid nanovalves as well as the timescale on which the opening and closing of the nanopores occur.
 
Vibrational circular dichroism (VCD) spectra of small size-selected gold nanoparticles covered by both enantiomers of 1,1′-binaphthyl-2,2′-dithiol (BINAS) were measured. VCD spectra of particles covered by the opposite enantiomers of BINAS show a mirror image relationship. The VCD spectrum of adsorbed BINAS is different from the one of free BINAS and its disulfide form, but it resembles more the dithiol form. Detailed analysis reveals that the angle between the two binaphthyl rings of BINAS is close to 90° for the adsorbed BINAS, similar to what is found for the free molecule. VCD spectra are quite insensitive to the particles size, in contrast to the electronic CD spectra, which change drastically as the particle size increases. This indicates that the vibrational characteristic is a local property. A model of BINAS adsorbed on a Au10 cluster was used to calculate VCD spectra. As for free BINAS and the disulfide the calculated spectrum of the adsorbed BINAS is in very good agreement with the measured one. This shows the potential of VCD spectroscopy to gain insight into the conformation of chiral molecules adsorbed on small metal particles.
  • Pigments based on silica-coated gold nanorods: Synthesis, colouring strength, functionalisation, extrusion, thermal stability and colour evolution
    Cyrille Gautier, Alastair Cunningham, Lynda Si-Ahmed, Gilles Robert and Thomas BĂźrgi
    Gold Bulletin, 43 (2) , 2010, p94-104
    Keywords: gold, silica, nanorods, thermal stability, porosity, coloring strength
    unige:14683 | Article PDF
The intense plasmon absorption bands of gold nanorods (GNRs) with peak extinction coefficients up to 6.4 x 109 M-1 cm-1 as well as their expected high stability make GNRs promising candidates for the colouration of bulk materials. The comparison of the integrated absorption in the visible region of GNRs with those of commercial organic pigments shows that the colouring strength of GNRs is 4 to 8 times higher. In order to improve their stability, GNRs were encapsulated in a silica shell of around 15 nm thickness using an optimized StĂśber method. The silica surface was modified with octadecylsilane to enable their dispersion in non-polar media. Different plastics were successfully coloured with a tiny quantity of bare and functionalised GNRs@SiO2. These rods were homogeneously dispersed using extrusion. The shape of the rods was effectively stabilised by the silica shell at high temperature during the extrusion process. Surprisingly, a slight modification of the rods colour was observed due to a decrease of the refractive index in the mesoporous silica shell. However, this effect is greatly limited after the functionalisation.
  
The thermal conductivity of concentrated colloids in fluid, glass, and gel states was analyzed. SiO2 colloids at 10−31 vol % and Al2O3 colloids at 4.8 vol % in the fluid, the gel, and the glassy states were studied by dynamic light scattering, rheology, and transmission electron microscopy. Thermal conductivity of the three states was measured as a function of volume fraction. For the fluid and gel states the thermal conductivity increases almost linearly with concentration, reaching roughly 18% enhancement for silica at a volume fraction of 31 vol %. In contrast, in the glass state thermal conductivity strongly decreases with increasing volume fraction.
  • Photophysics and Photochemistry of the UV Filter Kynurenine Covalently Attached to Amino Acids and to a Model Protein
    P.S. Sherin, J. Grilj, L.V. Kopylova, V.V. Yanshole, Y.P. Tsentalovich and E. Vauthey
    Journal of Physical Chemistry B, 114 (36) , 2010, p11909-11919
    DOI:10.1021/jp104485k | unige:14788 | Abstract | Article HTML | Article PDF
  
The photophysics and photochemistry of kynurenine (KN) covalently bound to the amino acids lysine, cysteine, and histidine, the antioxidant glutathione, and the protein lysozyme have been studied by optical spectroscopy with femto- and nanosecond time resolution. The fluorescence quantum yield of the adducts of KN to amino acids is approximately 2 times higher than that of the free KN in solution; KN attached to protein exhibits a 7-fold increase in the fluorescence quantum yield. The S1 state dynamics of KN-modified lysozyme reveals a multiphasic decay with a broad dispersion of time constants from 1 ps to 2 ns. An increase of the triplet yield of KN bound to lysozyme is also observed; the triplet state undergoes fast intramolecular decay. The obtained results reveal an increase of the photochemical activity of KN after its covalent attachment to amino acids and proteins, which may contribute to the development of oxidative stress in the human lensessthe main causative factor for the cataract onset.
  • DFT and CASPT2 Analysis of Polymetallic Uranium Nitride and Oxide Complexes: How Theory Can Help When X-Ray Analysis Is Inadequate
    Tanya K. Todorova, Laura Gagliardi, Justin R. Walensky, Kevin A. Miller and William J. Evans
    Journal of the American Chemical Society, 132 (35) , 2010, p12397-12403
    DOI:10.1021/ja103588w | unige:14729 | Abstract | Article HTML | Article PDF
Recent studies of organouranium chemistry have provided unusual pairs of similar polymetallic molecules containing (N)3− and (O)2− ligands, namely [(C5Me5)U(μ-I)2]3(μ3-N), 1, and [(C5Me5)U(μ-I)2]3(μ3-O), 2, and chair and boat conformations of [(C5Me5)2U(μ-N)U(μ-N3)(C5Me5)2]4, 3. These compounds were analyzed by density functional theory and multiconfigurational quantum chemical studies to differentiate nitride versus oxide in molecules for which the crystallographic data were not definitive and to provide insight into the electronic structure and unique chemical bonding of these polymetallic compounds. Calculations were also performed on [(C5Me5)2UN3(μ-N3)]3, 4, and [(C6F5)3BNU(N[Me]Ph)3], 5, for comparison with 1 and 3. On the basis of these results, the complex, [(C5Me5)U(μ3-E)]8, 6, for which only low-quality X-ray crystallographic data are available, was analyzed to predict if E is nitride or oxide.
  
  • Thermal Desorption, Vibrational Spectroscopic, and DFT Computational Studies of the Complex Manganese Borohydrides Mn(BH4)2 and [Mn(BH4)4]2−
    Godwin Severa, Hans Hagemann, Mose Longhini, Jakub W. Kaminski, Tomasz A. Wesolowski and Craig M. Jensen
    Journal of Physical Chemistry C, 114 (36) , 2010, p15516-15521
    DOI:10.1021/jp101675q | unige:14754 | Abstract | Article HTML | Article PDF
The mechanochemical reaction of LiBH4 with MnCl2 produces the neutral complex Mn(BH4)2. Thermal desorption studies show that the mechanochemical reaction of NaBH4 with MnCl2produces a different species, apparently Na2Mn(BH4)4, that undergoes dehydrogenation of a much lower weight percent H at a ~20 °C higher temperature than the neutral Mn(BH4)2. Vibrational spectroscopy also reveals that a complex manganese borohydride(s) in addition to Mn(BH4)2 are formed from the mechanochemical reactions. Analysis of the vibrational spectra in conjunction with DFT calculations on a model Mn(BH4)42− complex suggest bidentate binding of the [BH4]− ligands to the Mn center in the anionic complex. The calculated highest frequencies of the B−H stretching modes (corresponding to the “free” B−H bonds) agree well with the experimental frequencies and support the presence of this structural feature.
  • Bis(tetrathiafulvalenes) with aromatic bridges: electron delocalization in the oxidized species through EPR and theoretical studies
    François RiobĂŠ, Narcis Avarvari, Philippe Grosshans, Helena Sidorenkova, ThĂŠo Berclaz and Michel Geoffroy
    Physical Chemistry Chemical Physics, 12 , 2010, p9650-9660
    DOI:10.1039/C001014F | unige:14784 | Abstract | Article HTML | Article PDF
  
A series of bis(TTF) donors containing aromatic linkers between the two TTF units has been synthesized in order to investigate on the electronic structure of the oxidized species from an experimental and theoretical point of view. A mono(TTF)-pyridine compound has been also prepared and characterized by single-crystal X-ray diffraction analysis. Oxidation of a solution of 2,6-bis(TTF)-pyridine (TTF-Pyr-TTF) or of 1,3-bis(TTF)-benzene (TTF-Bz-TTF) in CH2Cl2 with less than 0.1 equivalent of [Cp2Fe][PF6] gives rise to a seven-line EPR spectrum consistent with the hyperfine structure calculated by DFT for the corresponding radical monocation. Increasing the proportion of oxidant leads to a four-line hyperfine structure, similar to the quartet pattern observed after oxidation of mono(TTF)-pyridine (Pyr-TTF) or mono(TTF)-benzene (Bz-TTF). In good accordance with the very weak value of J calculated by DFT for the dicationic biradicals these four-line spectra are attributed to [2,6-bis(TTF)-pyridine]2+ and [1,3-bis(TTF)-benzene]2+. Similar experimental results are obtained for 1,4-bis(TTF)-benzene. In this case, however, electrochemical oxidation leads to the monoradical at low potential and to the diradical at higher potential, while only the diradical could be observed by electrochemical oxidation of 2,6-bis(TTF)-pyridine or of 1,3-bis(TTF)-benzene. 
  
  • Pressure and Temperature Influence on the Desorption Pathway of the LiBH4−MgH2 Composite System
    Ulrike BĂśsenberg, Dorthe B. RavnsbĂŚk, Hans Hagemann, Vincenza D'Anna, Christian Bonatto Minella, Claudio Pistidda, Wouter Van Beek, Torben R. Jensen, Rdiger Bormann and Martin Dornheim
    Journal of Physical Chemistry C, 114 (35) , 2010, p15212-15217
    DOI:10.1021/jp104814u | unige:14742 | Abstract | Article HTML | Article PDF
The decomposition pathway in LiBH4−MgH2 reactive hydride composites was investigated systematically as a function of pressure and temperature. Individual decomposition of MgH2 and LiBH4 is observed at higher temperatures and low pressures (T ≥ 450 °C and p(H2) ≤ 3 bar), whereas simultaneous desorption of H2 from LiBH4 and formation of MgB2 was observed at 400 °C and a hydrogen backpressure of p(H2) = 5 bar. The simultaneous desorption of H2 from LiBH4 and MgH2 without intermediate formation of metallic Mg could not be observed. In situ X-ray diffraction (XRD) and infrared (IR) spectroscopy reveal the present crystalline and amorphous phases.
  • Zipper assembly of SHJ photosystems: focus on red naphthalenediimides, optoelectronic finetuning and topological matching
    R. Bhosale, R.S.K. Kishore, V. Ravikumar, O. Kel, E. Vauthey, N. Sakai and S. Matile
    Chemical Science, 1 , 2010, p357-368
    DOI:10.1039/C0SC00177E | unige:14679 | Abstract | Article HTML | Article PDF
The objective of this study was to synthesize multichromophoric donor-acceptor systems with non-halogenated red (RO) naphthalenediimides (NDIs) attached along p-oligophenyl (POP) and oligophenylethynyl (OPE) scaffolds, and to evaluate their usefulness for zipper assembly of artificial photosystems. Compared to halogenated red NDIs (RCl, RBr), the HOMO of RO is 0.2 eV higher and the HOMO/LUMO gap 0.1 eV smaller, the latter introducing a shade of pink. Consistent with higher HOMO levels, RO zippers generate less photocurrent than RBr zippers in their respective action spectra. RO zippers are less sensitive to topological mismatch than RBr zippers and thus more robust and broadly applicable. Transient absorption measurements reveal efficient electron transfer from excited OPE donors to RO acceptors and less efficient hole injection from excited RO donors into OPE acceptors. Both processes demonstrate compatibility with OMARG-SHJ photosystems (supramolecular n/p-heterojunctions with oriented multicolored antiparallel redox gradients). Decreasing hole transfer with decreasing HOMO energy differences further demonstrates that SHJ-type hole injection disappears gradually (rather than abruptly). Losses in photonic energy during this process can thus be minimized by optoelectronic finetuning, but eventual gains in open circuit voltages risk coming with complementary losses in short circuit current.
  • Salt Effect on the Formation of Dye Aggregates at Liquid/Liquid Interfaces Studied by Time-Resolved Surface Second Harmonic Generation
    M. Fedoseeva, P. Fita, A. Punzi and E. Vauthey
    Journal of Physical Chemistry C, 114 , 2010, p13774-13781
    DOI:10.1021/jp104334g | unige:14748 | Abstract | Article HTML | Article PDF
  
The present study reports on the effect caused by sodium salts added to a solution of malachite green in a liquid/liquid interfacial system probed by the time-resolved surface second harmonic generation (TRSSHG) technique. This effect is known as “salting-out effect” and is shown to reveal two main issues: salts added to the bulk, first, cause a reduction of the dye solubility and, second, stimulate the adsorption of malachite green cations at the interface, changing the equilibrium constant between the dye molecules adsorbed at the interface and those being dissolved in the bulk. The increased adsorption at the interface is observed in the TRSSHG experiment as a relative increase of the aggregates’ contribution to the measured time profile. However, depending on the nature and properties of salt anions, the mechanisms responsible for enhancing the population of interfacial aggregates can differ. This study explains such mechanisms for NaCl and NaSCN: addition of NaCl leads to an increase of the malachite green adsorption at the interface followed by the formation of aggregates, whereas the addition of NaSCN leads rather to the formation of aggregates already in the bulk with their further migration toward the interface. A simple quantitative description of the salting-out effect based on a modified Frumkin-Fowler-Guggenheim model also has been proposed. It has been shown to give a good agreement with the experiment with NaCl, i.e., when the formation of dye aggregates in the bulk solution can be neglected.
The properties of xanthurenic acid (XAN) in ground and photoexcited states have been studied using steady-state and time-resolved optical methods as well as quantum chemistry calculations. In neutral aqueous solution and in alcohols, XAN is present in a single tautomeric form (keto form), whereas in aprotic solvents and probably in basic aqueous solutions, more than one tautomeric form is present. UV irradiation of aqueous and alcoholic solutions of XAN results in a very rapid solvent-assisted tautomerization to the enol form, the later undergoes solvent-assisted transformation back to the keto form. The photolysis of XAN in aprotic solvents gives rise to the formation of numerous intermediate forms of XAN in both triplet and ground states. Under intense laser irradiation, XAN undergoes biphotonic ionization, the precursor for ionization being the excited singlet state.
  
  • Ionic layered BaFCl and Ba1−xSrxFCl compounds: Physical- and chemical-pressure effects
    Vincenza D'Anna, LatĂŠvi Max Lawson Daku, Hans Hagemann and Frank Kubel
    Physical Review B, 82 (2) , 2010, p24108
    DOI:10.1103/PhysRevB.82.024108 | unige:14745 | Abstract | Article PDF
The effect on crystal structure and vibrational frequencies of physical pressure in BaFCl and chemical pressure in Ba1−xSrxFCl solid solutions is studied using periodic density-functional theory (DFT) calculations performed within the local-density approximation (LDA) and the generalized gradient approximation (GGA). These results are compared with previously published experimental data for BaFCl in conjunction with new experimental data for Ba1−xSrxFCl and show overall a good agreement with experiment. The GGA method outperforms the LDA method for the description of BaFCl under pressure. However, the two DFT methods perform equally well for the description of the solid solutions, which have been studied within the virtual-crystal approximation. They also give consistent values of the energy of formation of Ba1−xSrxFCl, which can be correlated with the experimentally observed melting points. The comparison of the calculated mode Grüneisen parameters shows that, for the investigated systems, the effect of the chemical pressure and that of the physical pressure are not identical.
  • Modeling Solvatochromic Shifts Using the Orbital-Free Embedding Potential at Statistically Mechanically Averaged Solvent Density
    Jakub W. Kaminski, Sergey Gusarov, Tomasz A. Wesolowski and Andriy Kovalenko
    Journal of Physical Chemistry A, 114 (20) , 2010, p6082-6096
    DOI:10.1021/jp100158h | unige:14717 | Abstract | Article HTML | Article PDF
The correspondence between the exact embedding potential and the pair of the electron densities—that of the embedded molecule and that of its environment [Wesolowski and Warshel, J. Phys. Chem. 1993, 97, 8050]—is used to generate the average embedding potential and to subsequently calculate the solvatochromic shifts in a number of organic chromophores in solvents of various polarities. The averaged embedding potential is evaluated at a fictitious electron density of the solvent, which is obtained by means of “dressing up” with electrons the classical site distributions derived from the statistical-mechanical, 3D molecular theory of solvation (aka 3D-RISM method) [Kovalenko In Molecular Theory of Solvation; Hirata, Ed.; Understanding Chemical Reactivity; 2003, Vol 24], self-consistently coupled with the electronic structure of the solute. The proposed approach to modeling solvatochromic shifts can be situated between the implicit and explicit type of models for the solvent. Numerical examples are given for the lowest-lying n → π* and π → π* excitations.
 
A series of pyridinium phenoxides that differ by the dihedral angle between the pyridinium and the phenoxide rings because of substituents with increasing steric encumbrance has been investigated by ultrafast spectroscopy. Like the related betaine-30, these molecules are characterised by a zwitterionic electronic ground state and a weakly polar S1 state. Their fluorescence lifetime was found to lie between 200 to 750 fs, decreasing with increasing dihedral angle, and increasing with solvent viscosity. This was assigned to a non-radiative deactivation of the emissive state coupled to a large amplitude motion involving the dihedral angle. The transient absorption spectra suggested that emission occurs from the Franck–Condon S1 state, which decays to a dark excited state, that itself most probably corresponds to the relaxed S1 state. Finally, this relaxed state decays to the vibrationally hot ground state through an intramolecular charge separation process with a time constant ranging between 0.4 and 3 ps, increasing with the dihedral angle and with the solvent relaxation time. These variations were discussed in terms of the Jortner–Bixon model of electron transfer, where the charge separation dynamics depends on both electronic coupling and solvent relaxation. The results suggested that charge separation slows down with increasing dihedral angle.
  
This feature article reviews research of core-substituted naphthalenediimides (cNDIs) in a comprehensive yet easily readable manner. Their synthesis, electrochemistry and spectroscopy are covered first with emphasis on the ability of cNDIs with electron donating substituents to absorb and fluoresce in all colors without global structural changes and cNDIs with electron withdrawing substituents to reach unprecedented extents of π-acidity. The section on supramolecular chemistry covers face-to-face π-stacks and peripheral hydrogen bonds, that on molecular recognition moves from pH and fluoride sensors to the binding to telomeric DNA in vivo and intercalation into π-stacks and sticky tweezers. cNDIs can recognize and transport anions by functional anion–π interactions. The section on electron transport describes cNDIs as air-stable n-semiconductors with high charge mobility and use as OFETs. Photoinduced electron transport by rainbow cNDIs has been used for the creation of artificial photosystems in solution, in bilayer membranes and on solid substrates. Examples include multicolor light harvesting architectures, organic solar cells, photosystems that can open up into ion channels, and supramolecular n/p-heterojunctions with antiparallel redox gradients. The review is highly interdisciplinary but should appeal most to organic, biosupramolecular and physical chemists.
The objective of this study was to evaluate the possibility of photoinduced stack/rod electron transfer in surface “zipper” architectures composed of stacks of blue (B) naphthalenediimides (NDIs) along strings of oligophenylethynyl (OPE) rods. The synthesis and characterization of anionic and cationic multichromophoric OPE-B systems are reported. Absorption spectra suggest that in OPE-B systems, planarity and thus absorption and conductivity of the OPE can possibly be modulated by intramolecular stacking of the surrounding NDIs, although interfering contributions from aggregation remain to be differentiated. Among surface architectures constructed with OPE-B and POP-B systems by zipper and layer-by-layer (LBL) assembly, photocurrents generated by OPE-B zippers exhibit the best critical thickness and fill factors. These findings confirm the existence and functional relevance of topologically matching zipper architectures. In OPE-B zippers, OPEs generate much more photocurrent than the blue NDIs. Ultrafast electron transfer from OPEs to NDIs accounts for these photocurrents, providing wavelength-controlled access to rod–stack charge separation, and thus to formal supramolecular n/p-heterojunctions (SHJs). NDI excitation is not followed by the complementary hole transfer to the OPE rod. Scaffolds with higher HOMOs will be needed to integrate blue NDIs into SHJ photosystems.
  • Structural, Spectroscopic, and Multiconfigurational Quantum Chemical Investigations of the Electron-Rich Metal−Metal Triple-Bonded Tc2X4(PMe3)4 (X = Cl, Br) Complexes
    Frederic Poineau, Paul M. Forster, Tanya K. Todorova, Laura Gagliardi, Alfred P. Sattelberger and Kenneth R. Czerwinski
    Inorganic Chemistry, 49 (14) , 2010, p6646-6654
    DOI:10.1021/ic100641j | unige:14719 | Abstract | Article HTML | Article PDF
The compounds Tc2Cl4(PMe3)4 and Tc2Br4(PMe3)4 were formed from the reaction between (n-Bu4N)2Tc2X8 (X = Cl, Br) and trimethylphosphine. The Tc(II) dinuclear species were characterized by single-crystal XRD, UV−visible spectroscopy, and cyclic voltammetry techniques, and the results are compared to those obtained from density functional theory and multiconfigurational (CASSCF/CASPT2) quantum chemical studies. The compound Tc2Cl4(PMe3)4 crystallizes in the monoclinic space group C2/c [a = 17.9995(9) Å, b = 9.1821(5) Å, c = 17.0090(9) Å, β = 115.4530(10)°] and is isostructural to M2Cl4(PMe3)4 (M = Re, Mo, W) and to Tc2Br4(PMe3)4. The metal−metal distance (2.1318(2) Å) is similar to the one found in Tc2Br4(PMe3)4 (2.1316(5) Å). The calculated molecular structures of the ground states are in excellent agreement with the structures determined experimentally. Calculations of effective bond orders for Tc2X82− and Tc2X4(PMe3)4 (X = Cl, Br) indicate stronger π bonds in the Tc24+ core than in Tc26+ core. The electronic spectra were recorded in benzene and show a series of low intensity bands in the range 10000−26000 cm−1. Assignment of the bands as well as computing their excitation energies and intensities were performed at both TD-DFT and CASSCF/CASPT2 levels of theory. Calculations predict that the lowest energy band corresponds to the δ* → σ* transition, the difference between calculated and experimental values being 228 cm−1 for X = Cl and 866 cm−1 for X = Br. The next bands are attributed to δ* → π*, δ → σ*, and δ → π* transitions. The cyclic voltammograms exhibit two reversible waves and indicate that Tc2Br4(PMe3)4 exhibits more positive oxidation potentials than Tc2Cl4(PMe3)4. This phenomenon is discussed and ascribed to stronger metal (d) to halide (d) back bonding in the bromo complex. Further analysis indicates that Tc(II) dinuclear species containing π-acidic phosphines are more difficult to oxidize, and a correlation between oxidation potential and phosphine acidity was established.
  • Al3Li4(BH4)13: A Complex Double-Cation Borohydride with a New Structure
    Inge Lindemann, Roger Domènech Ferrer, Lothar Dunsch, Yaroslav Filinchuk, Radovan CernĂ˝, Hans Hagemann, Vincenza D'Anna, LatĂŠvi Max Lawson Daku, Ludwig Schultz and Oliver Gutfleisch
    Chemistry - A European Journal, 16 , 2010, p8707-8712
    Keywords: borohydrides; density functional calculations; hydrogen storage; metathesis; X-ray diffraction
    DOI:10.1002/chem.201000831 | unige:14778 | Abstract | Article PDF
  
The new double-cation Al-Li-borohydride is an attractive candidate material for hydrogen storage due to a very low hydrogen desorption temperature (~70 °C) combined with a high hydrogen density (17.2 wt %). It was synthesised by high-energy ball milling of AlCl3 and LiBH4. The structure of the compound was determined from image-plate synchrotron powder diffraction supported by DFT calculations. The material shows a unique 3D framework structure within the borohydrides (space group=P-43n, a=11.3640(3) Å). The unexpected composition Al3Li4(BH4)13 can be rationalized on the basis of a complex cation [(BH4)Li4]3+ and a complex anion [Al(BH4)4]-. The refinements from synchrotron powder diffraction of different samples revealed the presence of limited amounts of chloride ions replacing the borohydride on one site. In situ Raman spectroscopy, differential scanning calorimetry (DSC), thermogravimetry (TG) and thermal desorption measurements were used to study the decomposition pathway of the compound. Al-Li-borohydride decomposes at ~70 °C, forming LiBH4. The high mass loss of about 20 % during the decomposition indicates the release of not only hydrogen but also diborane.
  • Nuclear Magnetic Resonance Study of Reorientational Motion in α-Mg(BH4)2
    Alexander V. Skripov, Alexei V. Soloninin, Olga A. Babanova, Hans Hagemann and Yaroslav Filinchuk
    Journal of Physical Chemistry C, 114 (28) , 2010, p12370-12374
    DOI:10.1021/jp1037206 | unige:14686 | Abstract | Article HTML | Article PDF
To study the reorientational motion of BH4 groups in the low-temperature (α) phase of Mg(BH4)2, we have performed nuclear magnetic resonance (NMR) measurements of the 1H and 11B spin−lattice relaxation rates in this compound over wide ranges of temperature (82−443 K) and resonance frequency (14−90 MHz for 1H and 14−28 MHz for 11B). It is found that the thermally activated reorientational motion in α-Mg(BH4)2 is characterized by a coexistence of at least three jump processes with strongly differing activation energies. Taking into account the anisotropy of the local environment of BH4 groups in α-Mg(BH4)2, these jump processes can be attributed to different types of reorientation. The nearly linear coordination of BH4 groups by two Mg atoms suggests that the fastest jump process corresponds to the rotation around the 2-fold axis connecting B and two Mg atoms, whereas the slowest process is associated with the rotation around two other 2-fold axes perpendicular to the Mg−B−Mg line.
  
  • Raman Spectroscopy Measurements of the Pressure−Temperature Behavior of LiAlH4
    Juan C. Fallas, Wen-Ming Chien, Dhanesh Chandra, Vamsi K. Kamisetty, Erik D. Emmons, Aaron M. Covington, Raja Chellappa, Stephen A. Gramsch, Russell J. Hemley and Hans Hagemann
    Journal of Physical Chemistry C, 114 (27) , 2010, p11991-11997
    DOI:10.1021/jp1015017 | unige:14747 | Abstract | Article HTML | Article PDF
The pressure/temperature phase diagram of LiAlH4 has been constructed by using Raman spectroscopy data. In situ high pressure−temperature experiments were carried out using resistively heated diamond anvil cells up to 150 °C and 7 GPa. Room temperature phase transitions of monoclinic α-LiAlH4 → δ-LiAlH4 were observed at ~3.2 GPa. As the temperature is increased to ~100 °C, both the α and δ phases transform to β-LiAlH4 and remain stable up to 5.5 GPa. At temperatures greater than 300 °C, a new γ-LiAlH4 phase forms. Data of Konovalov (1995) has been used to define the phase boundary between β- and γ-LiAlH4 phases. We present a pressure−temperature phase diagram of LiAlH4 based using diamond anvil cells coupled with Raman spectroscopy.
 
The mechanism of the photoinduced low-spin → high-spin spin crossover is actively being investigated in Fe(II) complexes in solution using ultrafast spectroscopies. These studies accurately inform on the reaction coordinate of the Fe(II) chromophore upon photoexcitation. However, they leave open questions regarding the role of the solvent. Here, we report the description from a fully ab initio molecular dynamics study of the structure of [Fe(bpy)3]2+ in water and of the organization of its solvation shell in the low-spin and the high-spin states. In particular, the low-spin → high-spin change of states is shown to be accompanied (i) by a 0.191 Å lengthening of the Fe−N bond, in agreement with experiment, and (ii) by an increased thermal fluctuation of the molecular edifice, which both result from the weakening of the Fe−N bond. Furthermore, our results suggest that about two water molecules are expelled from the first solvation shell of [Fe(bpy)3]2+, which consists of water molecules intercalated between the bpy ligands.
  
  • Cation Size and Anion Anisotropy in Structural Chemistry of Metal Borohydrides. The Peculiar Pressure Evolution of RbBH4
    Yaroslav Filinchuk, Alexandr V. Talyzin, Hans Hagemann, Vladimir Dmitriev, Dmitry Chernyshov and Bertil Sundqvist
    Inorganic Chemistry, 49 (11) , 2010, p5285-5292
    DOI:10.1021/ic100359v | unige:14769 | Abstract | Article HTML | Article PDF
The pressure evolution of RbBH4 has been characterized by synchrotron powder X-ray diffraction and Raman spectroscopy up to 23 GPa. Diffraction experiments at ambient temperature reveal three phase transitions, at 3.0, 10.4, and 18 GPa (at 2.6, 7.8, and ~20 GPa from Raman data), at which the space group symmetry changes in the order Fm-3m(Z=4) → P4/nmm(2) → C222(2) → I-42m(4). Crystal structures and equations of state are reported for all four phases. The three high-pressure structure types are new in the crystal chemistry of borohydrides. RbBH4 polymorphs reveal high coordination numbers (CNs) for cation and anion sites, increasing with pressure from 6 to 8, via an intermediate 4 + 4 coordination. Different arrangements of the tetrahedral BH4 group in the Rb environment define the crystal symmetries of the RbBH4 polymorphs. The structural evolution in the MBH4 series is determined by the cation’s size, as it differs drastically for M = Li (CNs = 4, 6), Na (CN = 6), and Rb. The only structure common to the whole MBH4 family is the cubic one. Its bulk modulus linearly decreases as the ionic radius of M increases, indicating that the compressibility of the material is mainly determined by the repulsive BH4···BH4 interactions.
Deuterium−hydrogen exchange in solid α-Mg(BH4)2 is demonstrated. Compared to the previously reported exchange reactions in the alkali borohydrides, the temperature at which isotope exchange starts to take place is significantly lower (132 °C vs 200 °C for LiBH4). The activation energy for the deuterium−hydrogen exchange reaction is estimated to be 51 ± 15 kJ/mol. Almost complete isotope exchange was observed by treating solid Mg(BH4)2 for 72 h at 172 °C with 42 bar of D2. Preliminary experiments indicate that under these conditions Ca(BH4)2 also undergoes isotope exchange.
 
Inorganic borohydrides are actively studied in view of potential hydrogen storage applications. These compounds can be obtained by a variety of reactions ranging from high temperature reactions of the elements to exchange reactions in solution or in solid state. Different approaches will be discussed and compared.
  • On the Analysis of the Cr-Cr Multiple Bond in Several Classes of Dichromium Compounds
    G. La Macchia, G. Li Manni, T.K. Todorova, M. Brynda, F. Aquilante, B.O. Roos and L. Gagliardi
    Inorganic Chemistry, 49 (11) , 2010, p5216-5222
    DOI:10.1021/ic100345b | unige:14714 | Abstract | Article HTML | Article PDF
Since the discovery of a formal quintuple bond in Ar′CrCrAr′ (CrCr = 1.835 Å) by Power and co-workers in 2005, many efforts have been dedicated to isolating dichromium species featuring quintuple-bond character. In the present study we investigate the electronic configuration of several, recently synthesized dichromium species with ligands using nitrogen to coordinate the metal centers. The bimetallic bond distances of Power’s compound and Cr2-diazadiene (1) (CrCr = 1.803 Å) are compared to those found for Cr2(μ-η2-ArNC(R)NAr)2 (2) (CrCr = 1.746 Å; R = H, Ar = 2,6-Et2C6H3), Cr2(μ-η2-ArXylNC(H)NArXyl)3 (3) (CrCr = 1.740reduced/1.817neutral Å; ArXyl= 2,6-C6H3-(CH3)2), Cr2(μ-η2-TippPyNMes)2 (4) (CrCr = 1.749 Å; TippPyNMes = 6-(2,4,6-triisopropylphenyl)pyridin-2-yl (2,4,6-trimethylphenyl)amide), and Cr2(μ-η2-DippNC(NMe2)N-Dipp)2 (5) (CrCr = 1.729 Å, Dipp = 2,6-i-Pr2C6H3). We show that the correlation between the CrCr bond length and the effective bond order (EBO) is strongly affected by the nature of the ligand, as well as by the steric hindrance due to the ligand structure (e.g., the nature of the coordinating nitrogen). A linear correlation between the EBO and CrCr bond distance is established within the same group of ligands. As a result, the CrCr species based on the amidinate, aminopyridinate, and guanidinate ligands have bond patterns similar to the Ar′CrCrAr′ compound. Unlike these latter species, the dichromium diazadiene complex is characterized by a different bonding pattern involving Cr−Nπ interactions, resulting in a lower bond order associated with the short metal−metal bond distance. In this case the short CrCr distance is most probably the result of the constraints imposed by the diazadiene ligand, implying a Cr2N4 core with a closer CrCr interaction.
  
  • Ab initio static and molecular dynamics study of the absorption spectra of the 4-styrylpyridine photoswitch in its cis and trans forms
    LatĂŠvi Max Lawson Daku, Jorge Linares and Marie-Laure Boillot
    Physical Chemistry Chemical Physics, 12 , 2010, p6107-6123
    DOI:10.1039/b920850j | unige:13187 | Abstract
We report a thorough investigation of the absorption spectra of the cis and trans isomers of the 4-styrylpyridine photoswitch based on TDDFT calculations. The spectra of both isomers were analysed first from the results of excitation calculations performed on their optimised geometries. The main absorption band of the cis isomer is thus predicted to be due to the S0 → S1 and S0 → S2 transitions, while the main absorption band of the trans isomer is predicted to originate exclusively from the S0 → S1 transition. The convolution of the calculated oscillator strengths with Gaussians helped mimic the broadening of the electronic transitions. However, it proved necessary to use Gaussians with a large full width at half maximum of 5000 cm-1; and, compared to experiment, the calculated main absorption bands of the two isomers are significantly red-shifted and far too symmetric. Consequently, as required for the detailed analysis of the finite-temperature absorption spectrum of a molecule as flexible as 4-styrylpyridine, the influence of the thermal fluctuations has been taken into account by calculating the spectra as time averages over Car–Parrinello molecular dynamics trajectories. For both isomers, this led to a noticeable improvement in the relative positions of the calculated and experimental main absorption bands, and the asymmetry of the calculated bands brings them in better agreement with the experimental ones. Furthermore, these last results show that, actually, the S0 → S1 and S0 → S2 transitions both contribute significantly to the finite-temperature main absorption bands of the two isomers. Finally, in order to also take the vibrational broadening into account, the Franck–Condon factors of the relevant vibrations were calculated within the displaced harmonic oscillator approximation. By thus taking both the thermal and the vibrational broadening into account for the calculation of the absorption bands, the agreement between experiment and theory could be further improved.
  • Computational study of the cooperative effects of nitrogen and silicon atoms on the singlet-triplet energy spacing in 1,3-diradicals and the reactivity of their singlet states
    Takeshi Nakamura, Laura Gagliardi and Manabu Abe
    Journal of Physical Organic Chemistry, 23 (4) , 2010, p300-307
    DOI:10.1002/poc.1643 | unige:6524 | Abstract | Article HTML | Article PDF
Quantum chemical calculations were performed to investigate the cooperative effect of the nitrogen and silicon atoms on the singlet-triplet energy spacing and the reactivity of the singlet state in 1,2-diazacyclopentane-3,5-diyls and 1,2-diaza-4-silacyclopentane-3,5-diyls. The largest singlet-triplet energy gap (ΔEcST = -36.1 kcal/mol) found so far in localized 1,3-diradicals was in the C2v symmetry of 4,4-difluoro-1,2-diaza-4-silacyclopentane-3,5-diyl at the UB3LYP/6-31G(d) level of theory. The cooperative effect was also found in the energy differences of singlet diradicals with the corresponding ring-closing compounds, bicyclo[2.1.0]pentane derivatives. The singlet state of the 1,2-diaza-4-silacyclopentane-3,5-diyls was calculated to be energetically more stable than the ring-closing compound. The notable finding on the stability of the singlet diradicals may be attributed to the resonance structures that specifically stabilize the singlet state of diradicals. The computational studies predict that the singlet 1,2-diaza-4-silacyclopentane-3,5-diyl is a persistent molecule under conditions without intermolecular-trapping reagents.
  • Ionization Energies for the Actinide Mono- and Dioxides Series, from Th to Cm: Theory versus Experiment
    Ivan Infante, Attila Kovacs, Giovanni La Macchia, Abdul Rehaman Moughal Shahi, John K. Gibson and Laura Gagliardi
    Journal of Physical Chemistry A, 114 (19) , 2010, p6007-6015
    DOI:10.1021/jp1016328 | unige:14774 | Abstract | Article HTML | Article PDF
  
The results of a computational study with multiconfigurational quantum chemical methods on actinide monoxides (AnO) and dioxides (AnO2) for An = Th, Pa, U, Np, Pu, Am, and Cm, are presented. First and second ionization energies were determined and compared with experimental values, when available. The trend along the series is analyzed in terms of the electronic configurations of the various species. The agreement with experiment is excellent in most cases. Of particular interest is the first ionization of PuO2. We applied cutting-edge theoretical methods to refine the ionization energy, but our computed data fall in the range of ~6 eV and not in the ~7 eV region as the experiment dictates. Such a system requires further computational and experimental attention.
Playing with a full deck: Single-crystal X-ray and neutron diffraction data show that the Th center in the title complex 1 (see structure; Th orange, B beige, N purple, C black, H blue) forms bonds with 15 H atoms, thus making 1 the first crystallographically characterized example of a complex with a Werner coordination number of fifteen. DFT calculations suggest that 1 adopts the fully symmetric 16-coordinate structure in the gas phase.
In the 3D network [Rh(bpy)3][NaCr(ox)3]ClO4 (ox = oxalate, bpy = 2,2'-bipyridine) phonon-assisted as well as resonant energy migration within the R1 line of the 4A2 → 2E transition of Cr3+ has been identified. The latter is dominant below 4.2 K, and in a fluorescence line narrowing spectrum, it manifests itself in a multiline pattern across the inhomogeneous line width with spacings corresponding to the zero-field splitting of the 4A2 ground state (Milos, M.; Kairouani, S.; Rabaste, S.; Hauser, A. Coord. Chem. Rev. 2008, 252, 2540). H. Riesen demonstrated efficient spectral hole burning within the R1 line of Cr3+ doped at low concentrations into partially deuterated NaMg[Al(ox)3]·9H2O (Riesen, H. Coord. Chem. Rev. 2006 250, 1737). Here we show that at higher Cr3+ concentrations in the same host, both phenomena can be observed simultaneously, the resonant energy migration thus creating an additional series of persistent side holes.
  
The photophysics of aminoperylene (APe) in various solvents, including a room-temperature ionic liquid, has been investigated by steady-state and femtosecond transient absorption spectroscopies. The ultrafast excited-state dynamics originates from the solvation of the polar S1 state and not from a transition from a locally-excited to a charge-transfer state, as found with perylene-dimethylaniline. Addition of acid yields the protonated form APeH+, which exhibits similar photophysical properties than perylene, due to the suppression of the charge-transfer character of the S0–S1 transition. However, excited-state proton transfer, resulting to the formation of APe in the S1 state, is observed in methanol.
  • Structural Study of Mixed Crystals of [Zn1-xRux(bpy)3][NaCr(ox)3] Probed by High-Resolution Absorption Spectroscopy and High-Pressure Experiments
    Mia Milos, Tiphaine Penhouet, Prodipta Pal and Andreas Hauser
    Inorganic Chemistry, 49 (7) , 2010, p3402-3408
    DOI:10.1021/ic902514w | unige:6468 | Abstract | Article HTML | Article PDF
  
In the mixed crystal series of the cubic three-dimensional networks of composition [Zn1−xRux(bpy)3][NaCr(ox)3] (0 ≤ x ≤1, ox = C2O42−, bpy = 2,2′-bipyridine), high-resolution absorption spectroscopy in the region of the 4A2→2E transition (R-lines) reveals the creation of five specific spectroscopic sites for the [Cr(ox)3]3− complex. The concentration of these spectroscopic sites follows a binomial distribution of [Zn(bpy)3]2+ and [Ru(bpy)3]2+ among the four nearest neighbors of a given [Cr(ox)3]3− complex within the network. The tris-bipyridine complexes occupying those positions have an optimal π−π interaction with the oxalate ligands of the tris-oxalate chromophore. The energy of each spectroscopic [Cr(ox)3]3− site depends on the total concentration of [Ru(bpy)3]2+ in the mixed crystal and on its specific distribution among the four nearest neighbors. Single crystal X-ray diffraction indicates a reduction of the unit cell volume when [Zn(bpy)3]2+ (a = 15.6365(18) Å) is substituted by [Ru(bpy)3]2+ (a = 15.5098(6) Å). This alone would lead to a red-shift of the R lines in analogy to the red-shift of 25.2 cm−1/GPa due to the decrease of the metal ligand Cr−O bond length as observed in high-pressure luminescence experiments. However, specific π−π interactions with the nearest neighbors have the opposite effect and shift the transition in discrete jumps to higher energies with increasing [Ru(bpy)3]2+ mole fraction.
  
The electronic structure and the photophysical properties of the vanadium(III)ion in pseudo-octahedral oxygen coordination is reviewed. V3+ has received much interest from spectroscopists in recent years due to the advancement of state-of-the-art experimental techniques such as inelastic neutron scattering and high-field electron paramagnetic resonance spectroscopy that directly interrogate its large ground state zero-field splittings (ZFSs) and to rational parameterization of the ligand fieldp arameters using the angular overlap model. However, for V3+ these ZFSs can be large enough to also be probed directly by high-resolution electronic absorption spectroscopy of intra-configurational (t22g → t22g) spin-forbidden transitions in the near-IR and visible regions. The luminescent properties of V3+ with hexa-oxo and tris-bidentate di-oxo-coordination are quite disappointing compared to its neighbor in the periodic table, Cr3+, in similar environments. The efficient non-radiative pathways in these compounds are reviewed and compared to recent work on V3+ doped into NaMgAl(ox)3⋅9H2O. The poor luminescence quantum efficiencies of V3+ oxo complexes is explained by strong coupling of multi-phonon processes with a dynamic Jahn-Teller distortion originating from the 3E trigonal component of the 3T1g ground state.
  • On the Coherent Description of Diffusion-Influenced Fluorescence Quenching Experiments II: Early Events
    Gonzalo Angulo, Daniel R. Kattnig, Arnulf Rosspeintner, GĂźnter Grampp and Eric Vauthey
    Chemistry - A European Journal, 16 , 2010, p2291-2299
    DOI:10.1002/chem.200901693 | unige:6467 | Abstract | Article HTML | Article PDF
In a previous article we showed how to perform and analyze steady-state and nanosecond time-resolved experiments on fluorescence quenching by electron transfer in a coherent manner. Now, by making use of a superior time resolution, we explore the first stages of this kind of reaction. The novel information gained enables us to merge the results on the viscosity and the driving-force dependencies of the reaction rate. A unique set of parameters for a single reaction channel suffices to describe all the results in the frame of differential encounter theory for diffusion-influenced, bimolecular, remote electron-transfer reactions. The inclusion of the solvent structure is crucial for the understanding of the reaction kinetics. To the authors best knowledge, this is the first time that such a comprehensive set of data has been successfully and jointly explained in the field, with physically sound parameters for electron-transfer reactions.
  • Ultrafast Decay of the Excited Singlet States of Thioxanthone by Internal Conversion and Intersystem Crossing
    Gonzalo Angulo, Jakob Grilj, Eric Vauthey, Luis Serrano-AndrĂŠs, Òscar Rubio-Pons and Patrice Jacques
    ChemPhysChem, 11 , 2010, p480-488
    DOI:10.1002/cphc.200900654 | unige:5125 | Abstract | Article HTML | Article PDF
The experimental ultrafast photophysics of thioxanthone in several aprotic organic solvents at room temperature is presented, measured using femtosecond transient absorption together with high-level ab initio CASPT2 calculations of the singlet- and triplet-state manifolds in the gas phase, including computed state minima and conical intersections, transition energies, oscillator strengths, and spin-orbit coupling terms. The initially populated singlet ππ* state is shown to decay through internal conversion and intersystem crossing processes via intermediate nπ* singlet and triplet states, respectively. Two easily accessible conical intersections explain the favorable internal conversion rates and low fluorescence quantum yields in nonpolar media. The presence of a singlet-triplet crossing near the singlet ππ* minimum and the large spin-orbit coupling terms also rationalize the high intersystem crossing rates. A phenomenological kinetic scheme is proposed that accounts for the decrease in internal conversion and intersystem crossing (i.e. the very large experimental crescendo of the fluorescence quantum yield) with the increase of solvent polarity.
  • Effect of additives on the synthesis and reversibility of Ca(BH4)2
    C. Rongeat, V. D'Anna, H. Hagemann, A. Borgschulte, A. ZĂźttel, L. Schultz and O. Gutfleisch
    Journal of Alloys and Compounds, 493 (1-2) , 2010, p281-287
    Keywords: Hydrogen storage; calcium borohydride; reactive ball milling; Raman and infrared spectroscopy
    DOI:10.1016/j.jallcom.2009.12.080 | unige:6412 | Abstract | Article PDF
  
Metal borohydrides are potential materials for solid state hydrogen due to their high gravimetric and volumetric hydrogen densities. Among them, Ca(BH4)2 is particularly interesting because of the predicted suitable thermodynamic properties. In this work, we investigate a new synthesis route using high pressure reactive ball milling. Starting from CaH2 and CaB6 with a TiCl3 or TiF3 as additive, a reaction yield of 19% is obtained after 24 h milling at room temperature and 140 bar H2. The presence of Ca(BH4)2 is confirmed by the presence of the stretching mode of the [BH4]- group in the infrared spectra of the as-milled samples. Using in-situ XRD, we observe the recrystallisation of a poorly crystallised Ca(BH4)2 phase present after milling. The reversible decomposition/formation of Ca(BH4)2 is obtained with higher yield (57%) using higher temperature and TiF3 as additive but not with TiCl3 despite its similar electronic structure. The differences observed using different additives and the influence of the anion are discussed.
  • NaSc(BH4)4: A Novel Scandium-Based Borohydride
    Radovan Cerny, Godwin Severa, Dorthe B. RavnsbĂŚk, Yaroslav Filinchuk, Vincenza D'Anna, Hans Hagemann, DĂśrthe Haase, Craig M. Jensen and Torben R. Jensen
    Journal of Physical Chemistry C, 114 , 2010, p1357-1364
    Keywords: Hydrogen storage material, crystal structure, vibrational spectra
    DOI:10.1021/jp908397w | unige:4849 | Abstract | Article HTML | Article PDF
A new alkaline transition-metal borohydride, NaSc(BH4)4, is presented. The compound has been studied using a combination of in situ synchrotron radiation powder X-ray diffraction, thermal analysis, and vibrational and NMR spectroscopy. NaSc(BH4)4 forms at ambient conditions in ball-milled mixtures of sodium borohydride and ScCl3. A new ternary chloride Na3ScCl6 (P21/n, a = 6.7375(3) Å, b = 7.1567(3) Å, c = 9.9316(5) Å, β = 90.491(3)°, V = 478.87(4) Å3), isostructural to Na3TiCl6, was identified as an additional phase in all samples. This indicates that the formation of NaSc(BH4)4 differs from a simple metathesis reaction, and the highest scandium borohydride yield (22 wt %) was obtained with a reactant ratio of ScCl3/NaBH4 of 1:2. NaSc(BH4)4 crystallizes in the orthorhombic crystal system with the space group symmetry Cmcm (a = 8.170(2) Å, b = 11.875(3) Å, c = 9.018(2) Å, V = 874.9(3) Å3). The structure of NaSc(BH4)4 consists of isolated homoleptic scandium tetraborohydride anions, [Sc(BH4)4]–, located inside slightly distorted trigonal Na6 prisms (each second prism is empty, triangular angles of 55.5 and 69.1°). The experimental results show that each Sc3+ is tetrahedrally surrounded by four BH4 tetrahedra with a 12-fold coordination of H to Sc, while Na+ is surrounded by six BH4 tetrahedra in a quite regular octahedral coordination with a (6 + 12)-fold coordination of H to Na. The packing of Na+ cations and [Sc(BH4)4]– anions in NaSc(BH4)4 is a deformation variant of the hexagonal NiAs structure type. NaSc(BH4)4 is stable from RT up to ∼410 K, where the compound melts and then releases hydrogen in two rapidly occurring steps between 440 and 490 K and 495 and 540 K. Thermal expansion of NaSc(BH4)4 between RT and 408 K is anisotropic, and lattice parameter b shows strong anomaly close to the melting temperature.
  
  • Tuning the Excited-State Dynamics of GFP-Inspired Imidazolone Derivatives
    Irina Petkova, Georgi Dobrikov, Natalie Banerji, Guillaume Duvanel, Robert Perez, Vladimir Dimitrov, Peter Nikolov and Eric Vauthey
    Journal of Physical Chemistry A, 114 (1) , 2010, p10-20
    DOI:10.1021/jp903900b | unige:4850 | Abstract | Article HTML | Article PDF
The excited-state dynamics of five derivatives of the GFP-chromophore, which differ by the position and nature of their substituents, has been investigated in solvents of various viscosity and polarity and in rigid media using femtosecond-resolved spectroscopy. In polar solvents of low viscosity, like acetonitrile or methanol, the fluorescence decays of all compounds are multiexponential, with average lifetimes of the order of a few picoseconds, whereas in rigid matrices (polymer films and low temperature glasses), they are single exponential with lifetimes of the order of a few nanoseconds and fluorescence quantum yields close to unity. Global analysis of the fluorescence decays recorded at several wavelengths and of the transient absorption spectra reveals the presence of several excited-state populations with slightly different fluorescence and absorption spectra and with distinct lifetimes. These populations are attributed to the existence of multiple ground-state conformers. From the analysis of the dependence of the excited-state dynamics on the solvent and on the nature of the substituents, it follows that the nonradiative deactivation of all these excited chromophores involves an intramolecular coordinate with large amplitude motion. However, depending on the solvent and substituent, additional channels, namely, inter- and intramolecular hydrogen bond assisted nonradiative deactivation, are operative. This allows tuning of the excited-state lifetime of the chromophore. Finally, an ultrafast photoinduced intramolecular charge transfer is observed in polar solvents with one derivative bearing a dimethylaminophenyl substituent.
  • An Electrochemical and Photophysical Study of a Covalently Linked Inorganic–Organic Dyad
    Axel Kahnt, Leo-Philipp Heiniger, Shi-Xia Liu, Xiaoyan Tu, Zhiping Zheng, Andreas Hauser, Silvio Decurtins and Dirk M. Guldi
    ChemPhysChem, 11 , 2010, p651-658
    Keywords: cluster compounds, cyclic voltammetry, energy transfer, fullerenes, radiation chemistry
    DOI:10.1002/cphc.200900728 | unige:6465 | Abstract | Article PDF
  
A molecular donor-acceptor dyad comprising a hexarhenium cluster core, [Re6(Îź3-Se)8]2+, and a fullerene moiety which are covalently linked through a pyridine ligand was synthesized and fully characterized. The electrochemical and photophysical properties are reported. The detailed study includes cyclic voltammetry, steady-state absorption and fluorescence spectroscopy, radiation chemistry and transient absorption spectroscopy. A light-induced electron transfer between the inorganic cluster moiety and the fullerene can be excluded. However, a light-induced energy transfer from the rhenium cluster to the fullerene is proposed.
  • Time-resolved x-ray absorption spectroscopy: Watching atoms dance
    Chris J Milne, Van-Thai Pham, Wojciech Gawelda, Renske M Van der Veen, Amal El Nahhas, Steven L Johnson, Paul Beaud, Gerhard Ingold, Frederico Lima, Dimali A Vithanage, Maurizio Benfatto, Daniel Grolimund, Camelia Borca, Maik Kaiser, Andreas Hauser, Rafael Abela, Christian Bressler and Majed Chergui
    Journal of Physics: Conference Series, 190 , 2009, p12052
    DOI:10.1088/1742-6596/190/1/012052 | Abstract | Article PDF
The introduction of pump-probe techniques to the field of x-ray absorption spectroscopy (XAS) has allowed the monitoring of both structural and electronic dynamics of disordered systems in the condensed phase with unprecedented accuracy, both in time and in space. We present results on the electronically excited high-spin state structure of an Fe(II) molecular species, [FeII(bpy)3]2+, in aqueous solution, resolving the Fe-N bond distance elongation as 0.2 Å. In addition an analysis technique using the reduced χ2 goodness of fit between FEFF EXAFS simulations and the experimental transient absorption signal in energy space has been successfully tested as a function of excited state population and chemical shift, demonstrating its applicability in situations where the fractional excited state population cannot be determined through other measurements. Finally by using a novel ultrafast hard x-ray 'slicing' source the question of how the molecule relaxes after optical excitation has been successfully resolved using femtosecond XANES.
  • Luminescence of Sm2+ Doped in BaFBr
    Prodipta Pal and Hans Hagemann
    Terrae Rarae, 2009, p36-37
    Keywords: rare-earth elements; samarium; luminescence; high pressure; crystal field
    DOI:10.3286/tr.200906
The luminescence of Sm2+-doped BaFBr has been measured as a function of temperature and pressure. The 7FJ crystal field levels have been identified and the corresponding crystal field parameters evaluated. Temperature dependent lifetime measurements allow to locate the energy of the lowest 4f55d1 level. Pressure dependent measurements up to 8 GPa show linear red shifts of the 5D2,1,0 levels. These shifts are about 3 times stronger than the well known ruby pressure shifts and highlight thus the potential use as pressure sensors below 8 GPa.
  • Correction: The First Crystallographic and Spectroscopic Characterization of a 3d -Metal Borohydride: Mn(BH4)2
    Radovan Cerny, Nicolas Penin, Hans Hagemann and Yaroslav Filinchuk
    Journal of Physical Chemistry C, 113 (32) , 2009, p14582-14582
    DOI:10.1021/jp9062687 | Abstract | Article HTML | Article PDF
Pages 9003−9007. The author improved the information in the CIF file in Supporting Information. The manuscript was published on the Web on April 9, 2009 (ASAP) and in print (Volume 113, Issue 20). The correct version was published on the Web on July 14, 2009.
  • On the Thermal Conductivity of Gold Nanoparticle Colloids
    Natallia Shalkevich, Werner Escher, Thomas BĂźrgi, Bruno Michel, Lynda Si-Ahmed and Dimos Poulikakos
    Langmuir, 26 (2) , 2009, p663-670
    DOI:10.1021/la9022757 | unige:14723 | Abstract | Article HTML | Article PDF
Nanofluids (colloidal suspensions of nanoparticles) have been reported to display significantly enhanced thermal conductivities relative to those of conventional heat transfer fluids, also at low concentrations well below 1% per volume (Putnam, S. A., et at. J. Appl. Phys. 2006, 99, 084308; Liu, M.-S. L., et al. Int. J. Heat Mass Transfer. 2006, 49; Patel, H. E., et al. Appl. Phys. Lett. 2003, 83, 2931−2933). The purpose of this paper is to evaluate the effect of the particle size, concentration, stabilization method and particle clustering on the thermal conductivity of gold nanofluids. We synthesized spherical gold nanoparticles of different size (from 2 to 45 nm) and prepared stable gold colloids in the range of volume fraction of 0.00025−1%. The colloids were inspected by UV−visible spectroscopy, transmission electron microscope (TEM) and dynamic light scattering (DLS). The thermal conductivity has been measured by the transient hot-wire method (THW) and the steady state parallel plate method (GAP method). Despite a significant search in parameter space no significant anomalous enhancement of thermal conductivity was observed. The highest enhancement in thermal conductivity is 1.4% for 40 nm sized gold particles stabilized by EGMUDE (triethyleneglycolmono-11-mercaptoundecylether) and suspended in water with a particle-concentration of 0.11 vol%.
  
  • Reversible formation of gold nanoparticle–surfactant composite assemblies for the preparation of concentrated colloidal solutions
    Natallia Shalkevich, Andrey Shalkevich, Lynda Si-Ahmed and Thomas BĂźrgi
    Physical Chemistry Chemical Physics, 11 (43) , 2009, p10175-10179
    DOI:10.1039/B912571J | unige:14787 | Article HTML | Article PDF
We have developed a simple method for the preparation of nearly mono-dispersed stable gold colloids with a fairly high concentration using a two step procedure. First we synthesize citrate capped gold nanoparticles and then exchange the citrate ions with triethyleneglycolmono-11-mercaptoundecylether (EGMUDE). This leads to the immediate precipitation and formation of composite assemblies. The gold nanoparticles were successfully self-redispersed after a few days. The prepared gold colloid can be easily concentrated up to 20 times by separation of the flocculated part. UV-visible spectra, transmission electron microscopy (TEM), and dynamic light scattering (DLS) were used to characterize the products thus formed.
  • Enantioselective alkylidenecyclopropanation of norbornenes with terminal alkynes catalyzed by palladium–phosphinous acid complexes
    David Gatineau, Delphine Moraleda, Jean-Valère Naubron, Thomas BĂźrgi, Laurent Giordano and GĂŠrard Buono
    Tetrahedron: Asymmetry, 20 (16) , 2009, p1912-1917
    DOI:10.1016/j.tetasy.2009.07.017 | unige:14704 | Article HTML | Article PDF
Pd(II)-coordinated phosphinous acids catalyzed the formal enantioselective [2+1] cycloaddition of norbornene derivatives with terminal alkynes. The absolute configuration of (+)-3aa was assigned using VCD.
  • Solid-supported amphiphilic triblock copolymer membranes grafted from gold surface
    Ekaterina Rakhmatullina, Alexandre Mantion, Thomas BĂźrgi, Violeta Malinova and Wolfgang Meier
    Journal of Polymer Science Part A: Polymer Chemistry, 47 (1) , 2009, p1-13
    Keywords: amphiphiles; amphiphilic brushes; atom transfer radical polymerization (ATRP); biomimetic; biomimetic membrane; block copolymers; membranes; solvent-responsive behavior; surface-initiated ATRP
    DOI:10.1002/pola.23116 | unige:14753 | Abstract | Article HTML | Article PDF
  
Surface-initiated ATRP was applied for the step-by-step growth of the biomimetic amphiphilic block copolymer membrane on a gold support. Different thicknesses of membranes were achieved through the variation of the polymerization conditions. The OH-groups of the hydrophilic polymer blocks can be further functionalized to tune the membrane properties. Synthesis, characterization, and solvent-responsive properties of the amphiphilic triblock copolymer membrane are presented.
  • Chiral Gold Nanoparticles
    Cyrille Gautier and Thomas BĂźrgi
    ChemPhysChem, 10 (3) , 2009, p483-492
    Keywords: chirality; circular dichroism; gold; nanostructures; organic|inorganic hybrid composites
    DOI:10.1002/cphc.200800709 | unige:14706 | Abstract | Article HTML | Article PDF
Nanoparticle chirality has attracted much attention recently, and the application of chiral nanoparticles to chiral technologies (see figure) is also of interest. This Minireview deals with advances in the preparation and characterization of chiral gold nanoparticles. Origins of the chiroptical properties and potential applications are discussed. Monolayer-protected gold nanoparticles have many appealing physical and chemical properties such as quantum size effects, surface plasmon resonance, and catalytic activity. These hybrid organic–inorganic nanomaterials have promising potential applications as building blocks for nanotechnology, as catalysts, and as sensors. Recently, the chirality of these materials has attracted attention, and application to chiral technologies is an interesting perspective. This minireview deals with the preparation of chiral gold nanoparticles and their chiroptical properties. On the basis of the latter, together with predictions from quantum chemical calculations, we discuss different models that were put forward in the past to rationalize the observed optical activity in metal-based electronic transitions. We furthermore critically discuss these models in view of recent results on the structure determination of some gold clusters as well as ligand-exchange experiments examined by circular dichroism spectroscopy. It is also demonstrated that vibrational circular dichroism can be used to determine the structure of a chiral adsorbate and the way it interacts with the metal. Finally, possible applications of these new chiral materials are discussed.
  
Ligand exchange on [Au25(SCH2CH2Ph)18−] [TOA+] is studied with two chiral ligands R/S-BINAS and NILC/NIDC in THF with induction of metal-based optical activity. Under the applied condition the ligand exchange is only partial, showing that also within a mixed ligand shell significant optical activity can be induced. The ligand exchange resulted in the change of particle size as observed by UV−vis spectroscopy.
  • Luminescence of Sm2+ Doped in BaFBr
    Prodipta Pal and Hans Hagemann
    in "Proceedings of the 7th International Conference on f Elements,Terrae Rarae" Gerd Meyer, 06 , 2009, p1-2
    Keywords: rare-earth elements | samarium | luminescence | high pressure | crystal Field
    DOI:10.3286/tr.200906 | unige:6525

The luminescence of Sm2+-doped BaFBr has been measured as a function of temperature and pressure. The 7FJ crystal field levels have been identified and the corresponding crystal field parameters evaluated. Temperature dependent lifetime measurements allow to locate the energy of the lowest 4f55d1 level.

Pressure dependent measurements up to 8 GPa show linear red shifts of the 5D2,1,0 levels. These shifts are about 3 times stronger than the well known ruby pressure shifts and highlight thus the potential use as pressure sensors below 8 GPa.

  • The Chemical Society of Geneva, a Vital Link Between the Academy and the City
    Jean-Pierre Barras and Hans Hagemann
    Chimia, 63 (12) , 2009, p843-845
    DOI:10.2533/chimia.2009.843 | unige:4774 | Abstract | Article PDF
  
The historical background and the current activities of the Chemical Society of Geneva are described. Founded 88 years ago from the merger of three student associations, the Society today connects some 190 chemists and biochemists from all professional horizons. The Society's main goal is to promote molecular sciences studies and applications in Geneva. In addition to regularly organizing scientific lectures and visits, it sponsors major scientific seminars and grants annual awards to secondary school pupils and bachelor students.
  • Physical Chemistry at the University of Geneva
    Hans Hagemann, Tomasz A. Wesolowski, ThĂŠo Berclaz, Laura Gagliardi, Michel Geoffroy, Andreas Hauser, Hans Bill, Armand Buchs, Fazil GĂźlaçar, Edwin Anthony C. Lucken, Jacques Weber and Eric Vauthey
    Chimia, 63 (12) , 2009, p798-806
    DOI:10.2533/chimia.2009.807 | unige:4773 | Abstract | Article PDF
A brief historical overview of physical chemistry at the University of Geneva as well as a description of the present research activities at the department of physical chemistry are presented.
  
  • Synthesis and Characterization of NaBD3H, A Potential Structural Probe for Hydrogen Storage Materials
    H. Hagemann, V. D'Anna, P. Carbonnière, E. Gil Bardaji and M. Fichtner
    Journal of Physical Chemistry A, 113 , 2009, p13932-13936
    Keywords: Borohydrides, Synthesis, IR spectra, DFT calculations
    DOI:10.1021/jp904991j | unige:6181 | Abstract | Article HTML | Article PDF
Specifically labeled NaBD3H has been synthesized and characterized using X-ray diffraction, NMR, and vibrational spectroscopy. The isotopic purity of the compound, as estimated from NMR spectra, was found to be about 85% with the compound NaBD2H2 as the second product. IR spectra confirm the relatively strong intensity of the single B–H stretching mode predicted from DFT calculations. Anharmonic DFT calculations show that for the BD3H- ion Fermi resonances with the single B–H stretching mode are very limited, making this mode a promising structural probe for complex borohydrides which can be prepared by metathetical reactions.
  • Local Viscosity of Binary Water+Glycerol Mixtures at Liquid/Liquid Interfaces Probed by Time-Resolved Surface Second Harmonic Generation
    Piotr Fita, Angela Punzi and Eric Vauthey
    Journal of Physical Chemistry C, 113 (48) , 2009, p20705-20712
    DOI:10.1021/jp906676x | unige:4801 | Abstract | Article HTML | Article PDF
  
The excited-state relaxation of malachite green and brilliant green in solvents of various viscosity has been investigated at liquid/liquid interfaces and in bulk solutions by surface second harmonic generation and transient absorption spectroscopy. Mixtures of water and glycerol in various proportions have been used as solvents of variable viscosity. Transient absorption measurements in bulk revealed that both dyes are suitable as a probe of local viscosity for water+glycerol mixtures and that two of three processes following the optical excitation exhibit the same power dependence on solvent viscosity. This observation leads to assignment of the processes to a twist and twist-back of the aromatic rings attached to the central carbon atom of the dye. Therefore, identification of the intermediate state observed in the radiationless deactivation pathway with the twisted form of the dye has been supported. The time profiles of the second harmonic signal recorded at water+glycerol/dodecane interfaces have been found to be monoexponential at low dye concentrations (below 10-5 M) and biexponential at higher concentrations, and therefore the origin of the slower component has been attributed to the relaxation of dye aggregates adsorbed at the interface. The decay times measured at interfaces increased with increasing amount of glycerol in the mixture, but the rise was slower than in bulk solution. Therefore, the viscosity at the interfacial region, higher than that of the bulk solution, is mainly determined by structural modification of the solvent resulting from interactions between the two liquids that constitute the interface and addition of glycerol affects viscosity, only to a lesser extent. We have also shown that if the viscosity of the upper layer is much higher (at least 1 order of magnitude) than that of water or short alkanes, a slow-down of the relaxation is observed. This contradicts earlier findings and means that large amplitude motion of all three rings is involved in the deactivation of the excited molecule, but the rotation of the phenyl ring, which is smaller than the alkyl-substituted aniline groups, becomes a bottleneck for the relaxation in very viscous environments.
  
  • Infrared Spectra of Small Insertion and Methylidene Complexes in Reactions of Laser-Ablated Palladium Atoms with Halomethanes
    Han-Gook Cho, Lester Andrews, Bess Vlaisavljevich and Laura Gagliardi
    Organometallics, 28 (24) , 2009, p6871-6879
    DOI:10.1021/om900750t | unige:4800 | Abstract | Article HTML | Article PDF
Palladium carbene complexes, CX2=PdX2, are prepared along with the insertion products, CX3–PdX, in reactions of laser-ablated Pd atoms with tetrahalomethanes and identified from matrix infrared spectra and density functional frequency calculations. The carbon–metal bonds of the CCl2=PdCl2 and CClF=PdCl2 complexes are essentially double bonds with effective bond orders of 1.9, near those for the Pt and Ni analogues, as calculated by CASPT2 methods. On the other hand, only insertion complexes are generated from mono-, di-, and trihalomethane precursors. While the carbenes have staggered allene-type structures, many insertion complexes containing C–Cl bonds reveal distinct bridged structures, which indicate effective coordination of Cl to the metal center.
  • Cooperative Effect of Hydrogen-Bonded Chains in the Environment of a π → π* Chromophore
    Georgios Fradelos, Jakub W. Kaminski, Tomasz A. Wesolowski and Samuel Leutwyler
    Journal of Physical Chemistry A, 113 (36) , 2009, p9766-9771
    DOI:10.1021/jp906483z | unige:6178 | Abstract | Article HTML | Article PDF
  
Laser resonant two-photon ionization UV spectra provide clear evidence that the effect of increasing the length of the hydrogen-bonded chain consisting of molecules such as NH3, H2O, or CH3OH on the π → π* excitations of cis-7-hydroxyquinoline (cis-7HQ) is strongly cooperative [Thut; et al. J. Phys. Chem. A 2008, 112, 5566.] A theoretical analysis of the experimental data is provided to identify the origin of this cooperativity for four chains. The computational method to determine the changes of the electronic structure of a molecule due to interactions with its environment uses the nonempirical expression for the embedding potential [Wesolowski; WarshelJ. Phys. Chem. 1993, 97, 8050.] It is concluded that the electronic coupling between the molecules at the ends of the chain, which are hydrogen-bonded to cis-7HQ, plays a crucial role in this cooperativity.
  • Second-order Perturbation Theory with Complete and Restricted Active Space Reference Functions Applied to Oligomeric Unsaturated Hydrocarbon
    Abdul Rehaman Moughal Shahi, Christopher J. Cramer and Laura Gagliardi
    Physical Chemistry Chemical Physics, 2009 (11) , 2009, p10964-10972
    DOI:10.1039/B912607D | unige:6177 | Abstract | Article HTML | Article PDF
Multiconfigurational second-order perturbation theory based on either a complete active space reference wave function (CASSCF/CASPT2) or a restricted active space reference wave function (RASSCF/RASPT2) has been applied to compute one-electron ionization potentials and vertical electronic energy differences of oligomers of length n formed from ethylene (n = 1-10), acetylene (n = 1-5), and phenylene (n = 1-3) subunits. The RASSCF/RASPT2 approach offers an accuracy similar to CASSCF/CASPT2 at significantly reduced computational expense (both methods show good agreement with experimental data where available). It is shown that RASPT2 extends the range of CASPT2-like approaches by permitting the use of larger active spaces.
  • The effect of electron-withdrawing groups in the fragmentation of the radical anions of benzyl phenyl ethers
    Xavier Asensio, Angels GonzĂĄlez-Lafont, Jordi Marquet, JosĂŠ M. Lluch and Michel Geoffroy
    Journal of Molecular Structure: THEOCHEM, 913 (1-3) , 2009, p928-935
    DOI:10.1016/j.theochem.2009.07.047 | unige:6176 | Article PDF
The objective of this paper is to compare the influence of two electron-withdrawing groups (–CN and –NO2) in the fragmentation of 2-methoxy-4-nitrophenyl benzyl ether and 4-cyanophenyl benzyl ether once these species are reduced. The stability of the corresponding radical anions depends essentially on those substituents. In our design we have chosen two species that are able to attach the electron but which have each a different capacity to retain it. An experimental study is carried out by EPR spectroscopy to analyze the different electronic nature of the generated radical anions. In addition, electronic structure calculations at the DFT level of theory have been performed to study those radical anions and their fragmentation pathways. The thermodynamics and kinetics of the two fragmentations are discussed and compared in detail.
  
  • Topologically Matching Supramolecular n/p-Heterojunction Architectures
    Rajesh Bhosale, Alejandro Perez-Velasco, Velayutham Ravikumar, Ravuri S.K. Kishore, Oksana Kel, Alberto Gomez-Casado, Pascal Jonkheijm, Jurriaan Huskens, Plinio Maroni, Michal Borkovec, Tomohisa Sawada, Eric Vauthey, Naomi Sakai and Stefan Matile
    Angewandte Chemie International Edition, 48 (35) , 2009, p6461-6464
    DOI:10.1002/anie.200902551 | unige:4796 | Abstract | Article HTML | Article PDF
Matching matters when building supramolecular n/p-heterojunction photosystems on solid supports that excel with efficient photocurrent generation, important critical thickness, smooth surfaces, and flawless responsiveness to functional probes for the existence of operational intra- and interlayer recognition motifs.
  • Ordered and Oriented Supramolecular n/p-Heterojunction Surface Architectures: Completion of the Primary Color Collection
    Ravuri S.K. Kishore, Oksana Kel, Natalie Banerji, Daniel Emery, Guillaume Bollot, Jiri Mareda, Alberto Gomez-Casado, Pascal Jonkheijm, Jurriaan Huskens, Plinio Maroni, Michal Borkovec, Eric Vauthey, Naomi Sakai and Stefan Matile
    Journal of the American Chemical Society, 131 (31) , 2009, p11106-11116
    DOI:10.1021/ja9030648 | unige:6173 | Abstract | Article HTML | Article PDF
In this study, we describe synthesis, characterization, and zipper assembly of yellow p-oligophenyl naphthalenediimide (POP-NDI) donor−acceptor hybrids. Moreover, we disclose, for the first time, results from the functional comparison of zipper and layer-by-layer (LBL) assembly as well as quartz crystal microbalance (QCM), atomic force microscopy (AFM), and molecular modeling data on zipper assembly. Compared to the previously reported blue and red NDIs, yellow NDIs are more π-acidic, easier to reduce, and harder to oxidize. The optoelectronic matching achieved in yellow POP-NDIs is reflected in quantitative and long-lived photoinduced charge separation, comparable to their red and much better than their blue counterparts. The direct comparison of zipper and LBL assemblies reveals that yellow zippers generate more photocurrent than blue zippers as well as LBL photosystems. Continuing linear growth found in QCM measurements demonstrates that photocurrent saturation at the critical assembly thickness occurs because more charges start to recombine before reaching the electrodes and not because of discontinued assembly. The found characteristics, such as significant critical thickness, strong photocurrents, large fill factors, and, according to AFM images, smooth surfaces, are important for optoelectronic performance and support the existence of highly ordered architectures.
  • The Ru-Hbpp Water Oxidation Catalyst
    Fernando Bozoglian, Sophie Romain, Mehmed Ertem, Tanya Todorova, Joaquim Mola, Cristina Sens, Montse Rodriguez, Isabel Romero, Jordi Benet-Buchholz, Xavier Fontrodona,  Christopher Cramer, Laura Gagliardi and Antoni Llobet
    Journal of the American Chemical Society, 141 (42) , 2009, p15176-15187
    DOI:10.1021/ja9036127 | unige:6171 | Abstract | Article HTML | Article PDF
A thorough characterization of the Ru−Hbpp (in,in-{[RuII(trpy)(H2O)]2(μ-bpp)}3+ (trpy is 2,2′:6′,2′′-terpyridine, bpp is bis(2-pyridyl)-3,5-pyrazolate)) water oxidation catalyst has been carried out employing structural (single crystal X-ray), spectroscopic (UV−vis and NMR), kinetic, and electrochemical (cyclic voltammetry) analyses. The latter reveals the existence of five different oxidation states generated by sequential oxidation of an initial II,II state to an ultimate, formal IV,IV oxidation state. Each of these oxidation states has been characterized by UV−vis spectroscopy, and their relative stabilities are reported. The electron transfer kinetics for individual one-electron oxidation steps have been measured by means of stopped flow techniques at temperatures ranging from 10 to 40 °C and associated second-order rate constants and activation parameters (ΔH‡ and ΔS‡) have been determined. Room-temperature rate constants for substitution of aqua ligands by MeCN as a function of oxidation state have been determined using UV−vis spectroscopy. Complete kinetic analysis has been carried out for the addition of 4 equiv of oxidant (CeIV) to the initial Ru−Hbpp catalyst in its II,II oxidation state. Subsequent to reaching the formal oxidation state IV,IV, an intermediate species is formed prior to oxygen evolution. Intermediate formation and oxygen evolution are both much slower than the preceding ET processes, and both are first order with regard to the catalyst; rate constants and activation parameters are reported for these steps. Theoretical modeling at density functional and multireference second-order perturbation theory levels provides a microscopic mechanism for key steps in intermediate formation and oxygen evolution that are consistent with experimental kinetic data and also oxygen labeling experiments, monitored via mass spectrometry (MS), that unambiguously establish that oxygen−oxygen bond formation proceeds intramolecularly. Finally, the Ru−Hbpp complex has also been studied under catalytic conditions as a function of time by means of manometric measurements and MS, and potential deactivation pathways are discussed.
  • Infrared Spectra of Small Insertion and Methylidene Complexes in Reactions of Laser-Ablated Nickel Atoms with Halomethanes
    Han-Gook Cho, Lester Andrews, Bess Vlaisavljevich and Laura Gagliardi
    Organometallics, 28 (19) , 2009, p5623-5632
    DOI:10.1021/om900498m | unige:6464 | Abstract | Article HTML | Article PDF
  
Nickel carbene complexes, CX2 = NiX2, are prepared along with the insertion products, CX3 – NiX, in reactions of laser-ablated Ni atoms with tetrahalomethanes. These reaction products are identified from matrix infrared spectra and density functional frequency calculations. In agreement with the previously studied Pt cases, the carbon – nickel bonds of the Ni carbene complexes are essentially double bonds with CASPT2-computed effective bond orders of 1.8 – 1.9. On the other hand, only insertion complexes are generated from dihalomethane and trihalomethane precursors. The nickel carbenes have staggered structures, and several insertion complexes containing C – Cl bonds reveal distinct bridged structures similar to those observed in the corresponding Fe products, which indicate effective coordination of Cl to the metal center. The unique F-bridged CH2F – NiCl structure is also observed.
  • What Active Space Adequately Describes Oxygen Activation by a Late Transition Metal? CASPT2 and RASPT2 Applied to Intermediates from the Reaction of O2 with a Cu(I)-α-Ketocarboxylate
    Stefan M. Huber, Abdul Rehaman Moughal Shahi, Francesco Aquilante, Christopher J. Cramer and Laura Gagliardi
    Journal of Chemical Theory and Computation, 5 (11) , 2009, p2967-2976
    DOI:10.1021/ct900282m | unige:6169 | Abstract | Article HTML | Article PDF
Multiconfigurational second-order perturbation theory calculations based on a complete active space reference wave function (CASPT2), employing active spaces of increasing size, are well converged at the level of 12 electrons in 12 orbitals for the singlet−triplet state−energy splittings of three supported copper−dioxygen and two supported copper−oxo complexes. Corresponding calculations using the restricted active space approach (RASPT2) offer similar accuracy with a significantly reduced computational overhead provided an inner (2,2) complete active space is included in the overall RAS space in order to account for strong biradical character in most of the compounds. The effects of the different active space choices and the outer RAS space excitations are examined, and conclusions are drawn with respect to the general applicability of the RASPT2 protocol.
  • Amidinato– and Guanidinato–Cobalt(I) Complexes : Characterization of Exceptionally Short Co–Co Interactions
    Cameron Jones, Christian Schulten, Richard P. Rose, Andreas Stasch,  Simon Aldridge, William D. Woodul, Keith S. Murray, Boujemaa Moubaraki, Marcin Brynda, Giovanni La Macchia and Laura Gagliardi
    Angewandte Chemie International Edition, 48 (40) , 2009, p7406-7410
    Keywords: cobalt, magnetic properties, metal-metal interactions, N ligands
    DOI:10.1002/anie.200900780 | unige:6168 | Abstract | Article HTML | Article PDF
CoCo loco! Ligand-bridged dimers (see picture) with the shortest known Co-Co interactions are the first amidinato and guanidinato cobalt(I) complexes. The nature of the interactions has been probed by magnetic and theoretical investigations, and has been shown to be multiconfigurational. Preliminary reactivity studies of the complexes have also been carried out.
  
  • Ground-State Electronic Structure of Vanadium(III) Trisoxalate in Hydrated Compounds
    Kevin R. Kittilstved, Lilit Aboshyan Sorgho, Nahid Amstutz, Philip L.W. Tregenna-Piggott and Andreas Hauser
    Inorganic Chemistry, 48 (16) , 2009, p7750-7764
    Keywords: high-resolution spectroscopy, zero-field splittings, anisotropic ligand fields, hydrogen bonding
    DOI:10.1021/ic900613p | unige:3542 | Abstract | Article HTML | Article PDF
The ground-state electronic structures of K3V(ox)3·3H2O, Na3V(ox)3·5H2O, and NaMgAl1–xVx(ox)3·9H2O (0 < x <= 1, ox = C2O42–) have been studied by Fourier–transform electronic absorption and inelastic neutron scattering spectroscopies. High-resolution absorption spectra of the 3Γ(t2g2) → 1Γ(t2g2) spin-forbidden electronic origins and inelastic neutron scattering measurements of the pseudo-octahedral [V(ox)3]3– complex anion below 30 K exhibit both axial and rhombic components to the zero-field-splittings (ZFSs). Analysis of the ground-state ZFS using the conventional S = 1 spin Hamiltonian reveals that the axial ZFS component changes sign from positive values for K3V(ox)3·3H2O (D ≈ +5.3 cm–1) and Na3V(ox)3·5H2O (D ≈ +7.2 cm–1) to negative values for NaMgAl1–xVx(ox)3·9H2O (D ≈ –9.8 cm–1 for x = 0.013, and D ≈ –12.7 cm–1 for x = 1) with an additional rhombic component, |E|, that varies between 0.8 and 2 cm–1. On the basis of existing crystallographic data, this phenomenon can be identified as due to variations in the axial and rhombic ligand fields resulting from outer-sphere H-bonding between crystalline water molecules and the oxalate ligands. Spectroscopic evidence of a crystallographic phase change is also observed for K3V(ox)3·3Y2O (Y = H or D) with three distinct lattice sites below 30 K, each with a unique ground-state electronic structure.
  • Excited-State Dynamics of Hybrid Multichromophoric Systems: Toward an Excitation Wavelength Control of the Charge Separation Pathways
    Natalie Banerji, Guillaume Duvanel, Alejandro Perez-Velasco, Santanu Maity, Naomi Sakai, Stefan Matile and Eric Vauthey
    Journal of Physical Chemistry A, 113 (29) , 2009, p8202-8212
    DOI:10.1021/jp903572r | unige:3180 | Abstract | Article HTML | Article PDF
  
The photophysical properties of two hybrid multichromophoric systems consisting of an oligophenylethynyl (OPE) scaffold decorated by 10 red or blue naphthalene diimides (NDIs) have been investigated using femtosecond spectroscopy. Ultrafast charge separation was observed with both red and blue systems. However, the nature of the charge-separated state and its lifetime were found to differ substantially. For the red system, electron transfer occurs from the OPE scaffold to an NDI unit, independently of whether the OPE or an NDI is initially excited. However, charge separation upon OPE excitation is about 10 times faster, and takes place with a 100 fs time constant. The average lifetime of the ensuing charge-separated state amounts to about 650 ps. Charge separation in the blue system depends on which of the OPE scaffold or an NDI is excited. In the first case, an electron is transferred from the OPE to an NDI and the hole subsequently shifts to another NDI unit, whereas in the second case symmetry-breaking charge separation between two NDI units occurs. Although the charges are located on two NDIs in both cases, different recombination dynamics are observed. This is explained by the location of the ionic NDI moieties that depends on the charge separation pathway, hence on the excitation wavelength. The very different dynamics observed with red and blue systems can be accounted for by the oxidation potentials of the respective NDIs that are higher and lower than that of the OPE scaffold. Because of this, the relative energies of the two charge-separated states (hole on the OPE or an NDI) are inverted.
  • Effect of Salt on the Excited-State Dynamics of Malachite Green in Bulk Aqueous Solutions and at Air/Water Interfaces: a Femtosecond Transient Absorption and Surface Second Harmonic Generation Study
    Angela Punzi, GaĂŤlle Martin-Gassin, Jakob Grilj and Eric Vauthey
    Journal of Physical Chemistry C, 113 (27) , 2009, p11822-11829
    DOI:10.1021/jp9018662 | unige:3179 | Abstract | Article HTML | Article PDF
The ultrafast excited-state dynamics of Malachite Green (MG) in bulk aqueous solutions and at air/water interfaces, in particular the effect of the presence of various sodium salts in the aqueous phase, has been investigated by transient absorption and surface second harmonic generation. In bulk solutions, a slowing down of the ground-state recovery that can be unambiguously ascribed to the formation of aggregates of various sizes is observed at high (>0.3 M) salt concentrations only, with the exception of NaSCN where an effect is already found at 0.05 M. At the interface, small amounts of salt result in two effects: 1) an increase of the stationary surface second harmonic signal and 2) a slowing down of the ground-state recovery of MG. These phenomena are explained by the formation of aggregates due to an increase of the interfacial MG concentration upon addition of salt. The dependencies of both effects on salt concentration are correlated and vary with the anion as SCN− > Br− > SO4− > Cl−. This order is almost the opposite as that in the Hofmeister series for the salting-out strength.
  
  • Site-Dependent Excited-State Dynamics of a Fluorescent Probe Bound to Avidin and Streptavidin
    Alexandre FĂźrstenberg, Oksana Kel, Julieta Gradinaru, Thomas R. Ward, Daniel Emery, Guillaume Bollot, Jiri Mareda and Eric Vauthey
    ChemPhysChem, 10 (9-10) , 2009, p1517-1532
    Keywords: electron transfer • femtochemistry • fluorescent probes • molecular dynamics • time-resolved spectroscopy
    DOI:10.1002/cphc.200900132 | unige:3554 | Abstract | Article HTML | Article PDF
The excited-state dynamics of biotin–spacer–Lucifer-Yellow (LY)constructs bound to avidin (Avi) and streptavidin (Sav) was investigatedusing femtosecond spectroscopy. Two different locations in the proteins,identified by molecular dynamics simulations of Sav, namely the entrance of the binding pocket andthe protein surface, were probed by varying the length of thespacer. A reduction of the excited-state lifetime, stronger inSav than in Avi, was observed with the long spacer construct.Transient absorption measurements show that this effect originatesfrom an electron transfer quenching of LY, most probablyby a nearby tryptophan residue. The local environment of theLY chromophore could be probed by measuring the time-dependent polarisation anisotropy and Stokes shift of the fluorescence. Substantial differences in both dynamics were observed.The fluorescence anisotropy decays analysed by using thewobbling-in-a-cone model reveal a much more constrained environment of the chromophore with the short spacer. Moreover, the dynamic Stokes shift is multiphasic in all cases, with a~ 1 ps component that can be ascribed to diffusive motion ofbulk-like water molecules, and with slower components withtime constants varying not only with the spacer, but with theprotein as well. These slow components, which depend strongly on the local environment of the probe, are ascribed to themotion of the hydration layer coupled to the conformationaldynamics of the protein.
  • Matrix Infrared Spectroscopic and Computational Investigation of Late Lanthanide Metal Hydride Species MHx(H2)y (M = Tb−Lu, x = 1−4, y = 0−3)
    Xuefeng Wang, Lester Andrews, Ivan Infante and Laura Gagliardi
    Journal of Physical Chemistry A, 113 (45) , 2009, p12566-12572
    DOI:10.1021/jp9043754 | unige:6167 | Abstract | Article HTML | Article PDF
  
Laser-ablated late lanthanide metal atoms were condensed with pure hydrogen at 4 K, and new infraredabsorptions are assigned to binary metal hydrides on the basis of deuterium substitution and density functionaltheory frequency calculations. The dominant absorptions in the 1330-1400 cm-1 region are identified asLnH3 complexes with very weak ligand bands near 3900 cm-1. With ytterbium, YbH and YbH2 were themajor initial products, but YbH3 increased at their expense upon sample irradiation. Evidence is also presentedfor the LuH and ErH molecules and the tetrahydride anions in solid hydrogen.
  
  • A Compactly Fused π-Conjugated Tetrathiafulvalene-Perylenediimide Donor-Acceptor Dyad
    Michael Jaggi, Carmen Blum, Nathalie Dupont, Jakob Grilj, Shi-Xia Liu, JĂźrg Hauser, Andreas Hauser and Silvio Decurtins
    Organic Letters, 11 (14) , 2009, p3096-3099
    DOI:10.1021/ol901186n | unige:3553 | Abstract | Article HTML | Article PDF
The synthesis and structural characterization of a tetrathiafulvalene-fused perylenediimide molecular dyad is presented. Its largely extendedπ-conjugation provides intense optical absorption bands over a wide spectral range. The planar functional molecule exhibits a short-livednonluminescent excited state attributed to intramolecular charge separation.
  • Model for Elastic Relaxation Phenomena in Finite 2D Hexagonal Molecular Lattices
    Cristian Enachescu, Laurentiu Stoleriu, Alexandru Stancu and Andreas Hauser
    Physical Review Letters, 102 (25) , 2009, p257204
    DOI:10.1103/PhysRevLett.102.257204 | unige:3178 | Abstract | Article PDF
The relaxation in a spin transition compound is modeled on the basis of molecules interacting by theway of connecting springs and situated in a bidimensional open boundary hexagonal lattice. The switch ofindividual molecules is randomly checked using a standard Monte Carlo procedure. The switchingprobability depends on the energy gap between the two states in the absence of interactions and on theelongations of the nearest springs. The main characteristics of the experimental relaxation curves arereproduced and clustering and nucleation phenomena are detected.
  • Density-Functional Theory Study of the Stereochemistry of Chloroiron(III) and Chloromanganese(III) Complexes of a Bridled Chiroporphyrin
    LatĂŠvi Max Lawson Daku, Anna Castaings and Jean-Claude Marchon
    Inorganic Chemistry, 48 (12) , 2009, p5164-5176
    DOI:10.1021/ic900031c | unige:3552 | Abstract | Article HTML | Article PDF
  
Transition metal complexes of chiroporphyrins, in which two adjacent meso substituents are linked by a strap of eightmethylene groups, [M(BCP8)], can exist as either an ιιιι or ιβιβ atropisomer depending on the nature of thecoordinated metal cation. This remarkable conformational versatility was investigated by density-functional theorycalculations for the d5 chloroiron(III) complex in the low-spin and high-spin states and for the d4 high-spinchloromanganese(III) complex. The lowest-lying electronic state of all of the conformers of the chloroiron(III) bridledchiroporphyrin is found to be the high-spin state. For the chloroiron(III) complex in the low-spin or the high-spin stateand for the high-spin chloromanganese(III) complex, the most stable form is predicted to be the ιιιι conformer inwhich the chloride axial ligand is located within the cavity provided by the bridles. The predicted stereochemistries arecompared with those similarly obtained (i) for the chloroiron(III) and chloromanganese(III) complexes of thetetramethylchiroporphyrin, which is devoid of straps, and (ii) for the d10 zinc(II) and low-spin d8 nickel(II) BCP8complexes, on the basis of the effects tied to the occupancy of the stereochemically active dx2-y2-type antibondingorbital level, to the restraints imposed by the straps, and to the presence of the axial chloride ligand.
  
  • Rigid Bis(tetrathiafulvalenes) Doubly Bridged by Phosphino Groups and Derivatives: Synthesis and Intramolecular Mixed Valence State
    Ion Danila, FrĂŠdĂŠric Biaso, Helena Sidorenkova, Michel Geoffroy, Marc FourmiguĂŠ, Eric Levillain and Narcis Avarvari
    Organometallics, 28 (13) , 2009, p3691-3699
    DOI:10.1021/om900107y | unige:3551 | Abstract | Article HTML | Article PDF
The synthesis and structural characterization of the Ν5-bis(phosphine sulfide) and the bimetalliccomplexes bis[phosphino-M(CO)5] (M = Mo, W) of the 3,4-dimethyltetrathiafulvalene (ortho-DMTTF)-based rigid dimer (PPh)2(o-DMTTF)2, containing a central 1,4-dihydro-1,4-diphosphi-nine ring, are described. Single-crystal X-ray analyses have been performed for the trans isomers(PhPX)2(o-DMTTF)2 (X = S, Mo(CO)5, and W(CO)5) and for the cis isomer [PhPW(CO)5]2-(o-DMTTF)2. Planar or slightly folded boat-type conformations are observed for the central six-membered ring, together with different packings characterized by short intermolecular S ¡ ¡ ¡ Scontacts. The optical signature of the oxidized species in the case of the free ligand (PPh)2-(o-DMTTF)2 has been evidenced by UV-vis spectroelectrochemistry measurements. SolutionEPR measurements on the radical cation species of (PPh)2(o-DMTTF)2 definitely assess the fulldelocalization of the unpaired electron over both electroactive TTF units, with an associatedcoupling of 0.48 G with 12 equivalent protons. The EPR signal of the dication proves the radicalnature of this species, in favor of a triplet ground state. The radical cation of the cis-[PhPW(CO)5]2-(o-DMTTF)2 isomer was also investigated by EPR, for which the observed hyperfine structuredemonstrates the extended delocalization of the electron, together with a larger coupling constantwith the phosphorus nuclei. DFT calculations for the radical cation of (PPh)2(o-DMTTF)2 afford aboat-type conformation for the central ring and a SOMO consistent with a full delocalization of theelectron over both TTF units. Moreover, the calculations indicate that in the case of the dication of(PPh)2(o-DMTTF)2 the triplet state is more stable by 11.7 kcal mol-1 than the singlet state.
  • Lattice anharmonicity and structural evolution of LiBH4: an insight from Raman and X-Ray diffraction experiments
    H. Hagemann, Y. Filinchuk, D. Chernyshov and W. Van Beek
    Phase Transitions, 82 (4) , 2009, p344-355
    Keywords: Raman spectroscopy; hydrogen storage materials; X-ray diffraction; in situ experiments
    DOI:10.1080/01411590802707688 | unige:3550 | Abstract | Article HTML | Article PDF
  
New in situ Raman and synchrotron X-ray diffraction data (between 300 and400 K) in conjunction with separate temperature-dependent Raman data(between 7 and 400 K) are presented. The low-frequency Raman spectra showgood agreement with theoretical values obtained previously using periodic DFTcalculations. The temperature-dependent spectra reveal the presence of significantanharmonicity of librational modes neither predicted theoretically nor notedin previous experiments. The splitting of the internal deformation mode ν2 (of Esymmetry in the free ion) decreases continuously with increasing temperature,but drops abruptly at the first-order orthorhombic to hexagonal phase transitionobserved at 381 K. The temperature dependence of the linewidth of the internaldeformation mode ν2 reveals coupling to reorientational motions of theborohydride ion in the orthorhombic phase. The thermal evolution of bothcrystal structure and vibration frequencies agree with the phase diagramsuggested by the Landau theory.
  • Understanding, Controlling and Programming Cooperativity in Self-assembled Polynuclear Complexes in Solution
    Thomas Riis-Johannessen, Natalia Dalla Favera, Tanya K. Todorova, Stefan M. Huber, Laura Gagliardi and Claude Piguet
    Chemistry - A European Journal, 15 (46) , 2009, p12702-12718
    Keywords: cooperativity ; polynuclear complexes ; self-assembly ; solvation ; thermodynamics
    DOI:10.1002/chem.200900904 | unige:4122 | Abstract | Article HTML | Article PDF
Deviations from statistical binding, that is cooperativity, in self-assembled polynuclear complexes partly result from intermetallic interactions ΔEM,M, whose magnitudes in solution depend on a balance between electrostatic repulsion and solvation energies. These two factors have been reconciled in a simple point-charge model, which suggests severe and counter-intuitive deviations from predictions based solely on the Coulomb law when considering the variation of ΔEM,M with metallic charge and intermetallic separation in linear polynuclear helicates. To demonstrate this intriguing behaviour, the ten microscopic interactions that define the thermodynamic formation constants of some twenty-nine homometallic and heterometallic polynuclear triple-stranded helicates obtained from the coordination of the segmental ligands L1-L11 with Zn2+ (a spherical d-block cation) and Lu3+ (a spherical 4f-block cation), have been extracted by using the site binding model. As predicted, but in contrast with the simplistic coulombic approach, the apparent intramolecular intermetallic interactions in solution are found to be i) more repulsive at long distance (ΔELu,Lu1-4 > ΔELu,Lu1-2), ii) of larger magnitude when Zn2+ replaces Lu3+ (ΔEZn,Lu1-2 > ΔELu,Lu1-2) and iii) attractive between two triply charged cations held at some specific distance (ΔELu,Lu1-3 < 0). The consequences of these trends are discussed for the design of polynuclear complexes in solution.
  • Systematic truncation of the virtual space in multiconfigurational perturbation theory
    Francesco Aquilante, Tanya Kumanova Todorova, Laura Gagliardi, Thomas Bondo Pedersen and BjĂśrn Olof Roos
    Journal of Chemical Physics, 131 (3) , 2009, p34113
    Keywords: coupled cluster calculations, eigenvalues and eigenfunctions, matrix decomposition, perturbation theory, physics computing
    DOI:10.1063/1.3157463 | unige:3177 | Abstract | Article HTML | Article PDF
A method is suggested which allows truncation of the virtual space in Cholesky decomposition-based multiconfigurational perturbation theory (CD-CASPT2) calculations with systematic improvability of the results. The method is based on a modified version of the frozen natural orbital (FNO) approach used in coupled cluster theory. The idea is to exploit the near-linear dependence among the eigenvectors of the virtual-virtual block of the second-order Møller–Plesset density matrix. It is shown that FNO-CASPT2 recovers more than 95% of the full CD-CASPT2 correlation energy while requiring only a fraction of the total virtual space, especially when large atomic orbital basis sets are in use. Tests on various properties commonly investigated with CASPT2 demonstrate the reliability of the approach and the associated reduction in computational cost and storage demand of the calculations.
  • A model for spectral diffusion induced by resonant energy migration applied to the 4A2→2E transition of Cr3+ in [Cr(ox)3]3- (ox = C2O42-)
    Mia Milos and Andreas Hauser
    Journal of Luminescence, 129 (12) , 2009, p1901-1904
    DOI:10.1016/j.jlumin.2009.04.093 | unige:6165 | Abstract | Article PDF
In the 3D oxalate networks [NaCr(ox)3][Rh(bpy)3]ClO4 and [NaCr(ox)3][Ru(bpy)3] (ox=oxalate, bpy=2,2′-bipyridine) three different types of energy migration within the 4A2→2E transition can be identified. One is a resonant process between spectral members spaced by the ground-state zero-field splitting (ZFS). This leads to the sequential appearance of additional sharp lines spaced by the ground-state ZFS in the fluorescence line narrowing spectrum across the inhomogeneous line. The second one is a quasi-resonant process between spectral neighbours and manifests itself by rapid spectral diffusion. The third one is the well-known phonon-assisted process setting in at higher temperature.
  
  • Electronic structure of V3+ in NaMgAl(ox)3·9H2O probed by Fourier transform spectroscopy
    Kevin R. Kittilstved and Andreas Hauser
    Journal of Luminescence, 129 (12) , 2009, p1493-1496
    Keywords: Vanadium trisoxalate; V3+ electronic structure; Fourier transform spectroscopy; Near-IR luminescence
    DOI:10.1016/j.jlumin.2009.02.021 | unige:6164 | Abstract | Article HTML | Article PDF
High-resolution Fourier transform absorption and luminescence spectroscopy reveal axial and rhombic zero-field splittings of the spin-forbidden electronic origins of V3+ in NaMgAl(ox)3·9H2O (ox=oxalate) single crystals below 25 K. The temperature dependence of the integrated absorption of the split features display behavior consistent with a Boltzmann distribution within the zero-field split 3Â2 ground state of V3+. Weak luminescence is observed in the near-IR from the lowest energy spin-forbidden transition with a luminescence lifetime of less than 0.5 μs at 11 K and an estimated quantum efficiency of the order of 10-5
 
A radical species characterized by a large g-anisotropy and a clearly resolved hyperfine structure with 95/97Mo and 31P nuclei is formed, at 77 K, by radiolysis of a single crystal of Mo(CO)5PPh3. The corresponding EPR signals disappear irreversibly with increasing temperature and the angular dependence of the various coupling constants imply a spin delocalization of not, vert, ∟60% and not, vert, ∟4% on the molybdenum and the phosphorus atoms, respectively and are, a priori, consistent with the trapping of a one-electron deficient centre. The ability of DFT to predict the EPR tensors for a 17-electron Mo(I) species is verified by calculating the g-tensor and the various 14N and 13C coupling tensors previously reported by Hayes for [Mo(CN)5NO]3-. Calculations at the B3LYP/ZORA/SOMF level of theory show that, in contrast to Mo(CO)5PH3, one-electron oxidation of Mo(CO)5PPh3 causes an appreciable change in the geometry of the complex. The g-tensor and the 95/97Mo and 31P isotropic and anisotropic coupling constants calculated for [Mo(CO)5PPh3]+¡ confirm the trapping of this species in the irradiated crystal of Mo(CO)5PPh3; they also show that the conformational modifications induced by the electron release are probably hindered by the nearby complexes.
  
The first crystal structure of a 3d-metal borohydride is presented.Solvent-free homoleptic manganese borohydride Mn(BH4)2 forms at ambient conditions in ball-milled mixtures of alkali metal borohydrides and MnCl2. It crystallizes in the trigonal crystal system with the space group symmetry P3112 and is stable from 90 to 450 K, where the compound melts. Thermal expansion of Mn(BH4)2 between 90 and 400 K is highly anisotropic and strongly nonuniform. The structure of Mn(BH4)2 shows interesting similarity to α-Mg(BH4)2: the two structures are made of similar layers L with the composition M4(BH4)10 per cell. The layers are stacked along the c-axis, and rotated by 120° by the 31 axis in Mn(BH4)2 and by 60° by the 61 axis in α-Mg(BH4)2. Three identical layers are stacked along one unit cell vector c in Mn(BH4)2, while six layers are stacked in α-Mg(BH4)2. In Mn(BH4)2 the layers L are connected directly, and share atoms. In α-Mg(BH4)2 the layers L are intercalated by a thin layer L', which contains one Mg atom per layer per cell. The layer L is chiral, and both borohydrides crystallize in chiral space groups. Similar to α-Mg(BH4)2, the structure of Mn(BH4)2 is not densely packed and contains isolated voids with the estimated volume of 21 Å3 each, which occupy in total 6% of the space. The resemblance between Mn(BH4)2 and α-Mg(BH4)2 is also reflected in their Raman and infrared spectra.
The effective embedding potential introduced by Wesolowski and Warshel [J. Phys. Chem., 97 (1993) 8050] depends on two electron densities: that of the environment (n B ) and that of the investigated embedded subsystem (n A ). In this work, we analyze this potential for pairs n A and n B , for which it can be obtained analytically. The obtained potentials are used to illustrate the challenges in taking into account the Pauli exclusion principle.
Minimization of the Hohenberg-Kohn total energy functional EHK [ρ] in the presence of the constraint ρ - ρB  ≥ 0, where ρB is some arbitrarily chosen electron density comprising integer number of electrons is considered. To access better numerical accuracy of approximations to EHK [ρ] in practice, the search for optimal ρ - ρB is performed using auxiliary quantities such as orbitals of a reference system of non-interacting electrons [Wesolowski and Warshel, J Phys Chem 1993, 97, 8050] or a wavefunction-like object corresponding to interacting electrons [Wesolowski, Phys Rev A 2008, 77, 012504]. In both cases, the condition ρ - ρB  ≥ 0 leads to a local potential (orbital-free effective embedding potential) of the same general form if expressed by means of universal density functionals. In this work, it is shown that the same local potential is obtained if the search for optimal ρ - ρB is performed among one-particle reduced density matrices.
Conventionally, solving one-electron equations for embedded orbitals[Eqs. (20) and (21) in Wesolowski and Warshel, J Phys Chem, 1993, 97, 8050] proceedsby a self-consistent procedure in which the whole effective potential, including itsembedding component, is updated in each iteration. We propose an alternative scheme(splitSCF), which uses the linearized embedding potential in the inner iterative loop andthe outer-loop is used to account for its deviations from linearity. The convergence ofthe proposed scheme is investigated for a set of weakly bound intermolecularcomplexes representing typical interactions with the environment. The outer loop isshown to converge very fast. No more than 3-4 iterations are needed. Errors due toskipping the outer loop completely and using the electron density obtained in theabsence of the environment in the linearized embedding potential are investigated indetail. It is shown that this computationally attractive simplification, used already innumerical simulations by others, is adequate not only for van der Waals and hydrogen-bondedcomplexes but even if the complex comprises charged components, i.e., wherestrong electronic polarization takes place. In charge-transfer type of complexes, largerchanges of electron of density upon complex formation occur and the abovesimplification is not recommended. Figure (a) The splitSCF scheme: In the inner loop(i-index), the embedding potential vemb[ρA,ρB]is evaluated for A taken from the previous iteration in theouter loop (j-index) and remains constant, whereas thevKS[ρA] component is recalculated as A changes. (b) The conventional SCF scheme: Both vKS[ρA] andvemb[ρA,ρB] are recalculated as A changes.
  • The photophysics of salicylic acid derivatives in aqueous solution
    Ivan P. Pozdnyakov, Anatolio Pigliucci, Nikolai Tkachenko, Victor F. Plyusnin, Eric Vauthey and Helge Lemmetyinen
    Journal of Physical Organic Chemistry, 22 (5) , 2009, p449-454
    DOI:10.1002/poc.1489 | unige:3174 | Abstract | Article PDF
The photophysics of a number salicylic acid derivatives (SADs) in aqueous solutions was investigated in a wide range of pH by time-correlated single photon counting (λex = 350 nm, τresp = 300 ps) and fluorescence up-conversion (λex = 266 nm, τresp = 300 fs) techniques. The acid-base equilibrium constants in the ground (pKa) and the excited states (pKa*), the fluorescence quantum yields as well as the lifetimes of anionic, neutral, and cationic forms of SADs were determined. Evidence of ultrafast excited-state intramolecular proton transfer (ESIPT) leading to the formation of the proton-transferred excited state of SADs was obtained from the fluorescence up-conversion measurement. The nature of the ESIPT process is discussed.
  • Photoinduced Ring-Opening of a Photochromic Dihydroindolizine Derivative Monitored with Femtosecond Visible and Infrared Spectroscopy
    Omar F. Mohammed, Saleh A. Ahmed, Eric Vauthey and Erik T.J. Nibbering
    Journal of Physical Chemistry A, 113 (17) , 2009, p5061-5065
    DOI:10.1021/jp811248u | unige:3547 | Abstract | Article HTML | Article PDF
  
We present results of a femtosecond spectroscopy study of the ring-opening dynamics of the photochromiccompound trimethyl-1′H-spiro[fluorene-9,1′-pyrrolo[1,2-b]pyridazines]-2′,3′,6′-tricarboxylate (also known asdihydroindolizine and abbreviated as DHI) in solvents of different polarities. We follow the ring-openingdynamics of photoexcited DHI by probing the transient response in the visible region between 450 and 700nm, as well as in the fingerprint region between 1100 and 1800 cm-1. We conclude that photoexcited DHIconverts into the ring-opened betaine isomer while remaining in the electronic excited state. Subsequentelectronic excited-state decay on a time scale of 40-80 ps results in regeneration of ground-state DHI (0.75-0.9quantum yield) or betaine photoproduct, the exact value for DHI quantum yield recoveries and rates beingsolvent dependent. Figure Steady state of DHI in ACN-d3, DCM-d2, and TCE (A).Transient spectra of DHI at different pulse delays after 400 nm laserexcitation in ACN-d3 (B), in DCM-d2 (C), and in TCE (D).
  • Crystal Structure of Octabromoditechnetate(III) and a Multiconfigurational Quantum Chemical Study of the δ-> δ* Transition in Quadruply Bonded [M2X8]2- Dimers (M = Tc, Re; X = Cl, Br)
    Frederic Poineau, Laura Gagliardi, Paul M. Forster, Alfred P. Sattelberger and Kenneth R. Czerwinski
    Dalton Transactions, 2009 , 2009, p5954-5959
    DOI:10.1039/b902106j | unige:3173 | Abstract | Article HTML | Article PDF
The technetium(III) compound (n-Bu4N)2[Tc2Br8] was prepared by metathesis of (n-Bu4N)2[Tc2Cl8] with concentrated aqueous HBr in acetone and recrystallized from acetone–diethyl ether solution (2 : 1 v/v). The acetone solvate obtained, (n-Bu4N)2[Tc2Br8]·4[(CH3)2CO] ( 1), crystallizes in the monoclinic space group P21/n with a = 13.8959(8) Å, b = 15.2597(9) Å, c = 15.5741(9) Å, β = 109.107(1)°, R1 = 0.028, and Z = 4. The Tc–Tc distance (2.1625(9) Å) and the average Tc–Br distances (2.4734(7) Å) are in excellent agreement with those previously determined by EXAFS spectroscopy. These and other experimental data on quadruply metal–metal bonded group 7 [M2X8]2- dimers (M = Tc, Re; X = Cl, Br) are compared to the results of a set of multi-configurational quantum chemical studies. The calculated molecular structures of the ground states are in very good agreement with the structures determined experimentally. The theory overestimates the * transition energies by some 1000 cm-1, but mimics the trends in δ – δ* energies across the series.
  
The excited-state dynamics of kynurenine (KN) has been examined in various solvents by femtosecond-resolved optical spectroscopy. The lifetime of the S1 state of KN amounts to 30 ps in aqueous solutions, increases by more than 1 order of magnitude in alcohols, and exceeds 1 ns in aprotic solvents such as DMSO and DMF, internal conversion (IC) being shown to be the main deactivation channel. The IC rate constant is pH independent but increases with temperature with an activation energy of about 7 kJ/mol in all solvents studied. The dependence on the solvent proticity together with the observation of a substantial isotope effect indicates that hydrogen bonds are involved in the rapid nonradiative deactivation of KN in water. These results give new insight into the efficiency of KN as a UV filter and its role in cataractogenesis.
Terms of donor-acceptor complex states participating in transitions with the excitation of the CT1 and CT2 bands (vertical arrows). The dotted arrow is the radiationless transition. Dotted lines are vibrational sublevels of the ground electronic state. Dashed arrows show the directions of system relaxation before and after nonthermal transitions.
  • Atomic Cholesky decompositions: a route to unbiased auxiliary basis sets for density fitting approximation with tunable accuracy and efficiency
    Francesco Aquilante, Thomas Pedersen, Laura Gagliardi and Roland Lindh
    Journal of Chemical Physics, 130 (15) , 2009, p154107
    DOI:10.1063/1.3116784 | unige:3545 | Abstract | Article HTML | Article PDF
  
Cholesky decomposition of the atomic two-electron integral matrix has recently been proposed as a procedure for automated generation of auxiliary basis sets for the density fitting approximation [F. Aquilante et al., J. Chem. Phys. 127, 114107 (2007)]. In order to increase computational performance while maintaining accuracy, we propose here to reduce the number of primitive Gaussian functions of the contracted auxiliary basis functions by means of a second Cholesky decomposition. Test calculations show that this procedure is most beneficial in conjunction with highly contracted atomic orbital basis sets such as atomic natural orbitals, and that the error resulting from the second decomposition is negligible. We also demonstrate theoretically as well as computationally that the locality of the fitting coefficients can be controlled by means of the decomposition threshold even with the long-ranged Coulomb metric. Cholesky decomposition-based auxiliary basis sets are thus ideally suited for local density fitting approximations.
  
  • Imidazole-Annulated Tetrathiafulvalenes exhibting pH-Tuneable Intramolecular Charge Transfer and Redox Properties
    Jincai Wu, Nathalie Dupont, Shi-Xia Liu, Antonia Neels, Andreas Hauser and Silvio Decurtins
    Chemistry - An Asian Journal, 4 (3) , 2009, p392-399
    Keywords: charge transfer, donor-acceptor systems
    DOI:10.1002/asia.200800322 | unige:3544 | Abstract | Article HTML | Article PDF
In order to study the electronic interactions in donor-acceptor ensembles as a function of pH, an efficient synthetic route to three imidazole-annulated tetrathiafulvalene (TTF) derivatives 1-3 is reported. Their electronic absorption spectra, in view of photoinduced intramolecular charge transfer, and their electrochemical behavior were investigated, and pKa values for the two protonation processes on the acceptor unit were determined in organic solvents by photometric titration. The influence of the TTF moiety on these values is discussed.
  • Self-Assembly of a Trinuclear Luminescent Europium Complex
    Soumaila Zebret, Nathalie Dupont, GĂŠrald Bernardinelli and Josef Hamacek
    Chemistry - A European Journal, 15 (14) , 2009, p3355-3358
    Keywords: europium • luminescence • self-assembly • supramolecular chemistry • trinuclear complexes
    DOI:10.1002/chem.200802676 | unige:3543 | Abstract | Article HTML | Article PDF
  
Triangular luminescent box: Self-assembly of a new multidentate receptor with europium cations results in the formation of trinuclear discrete complexes. X-ray crystallography shows that nine-coordinate cations are linked by ligands to provide a triangular complex in the solid state and in solution. Despite the coordinated solvent molecules, this topologically unusual complex exhibits remarkable luminescent properties.
  
  • Pronounced Electrochemical Amphotericity of a Fused Donor–Acceptor Compound: A Planar Merge of TTF with a TCNQ-Type Bithienoquinoxaline
    Xavier GuĂŠgano, Alexander L. Kanibolotsky, Carmen Blum, Stijn F.L. Mertens, Shi-Xia Liu, Antonia Neels, Hans Hagemann, Peter J. Skabara, Samuel Leutwyler, Thomas Wandlowski, Andreas Hauser and Silvio Decurtins
    Chemistry - A European Journal, 15 (1) , 2009, p63-66
    DOI:10.1002/chem.200802011 | unige:3555 | Abstract | Article HTML | Article PDF
Electronic absorption spectrum of 1 in DMF solution at room temperature, together with the calculated oscillator strengths.
  • Revisited conformational analysis of perhydro-3a,6a,9a-triazaphenalene based on Raman analysis
    Christian Chapuis, Hans Hagemann, Wolfgang Fieber, Robert Brauchli and Jean-Yves De Saint Laumer
    Journal of Physical Organic Chemistry, 22 (4) , 2009, p282-288
    Keywords: triazaphenalene; conformation; Raman; anomeric; stereoelectronic
    DOI:10.1002/poc.1465 | unige:15001 | Abstract | Article PDF
We have demonstrated experimentally by Raman analysis that 1a exists as a mixture of ttt/cct conformers at 22°C in liquid or CCl4 solution, thus contrasting with the initial IR, NMR and X-ray analyses, which were strongly in favour of (ttt)-1a. The global stereoelectronic stabilization is ca. 4.0-4.2 kcal/mol for both parallel N lone pairs (lp) in (cct)-1a, based on a ΔH of 0.75 kcal/mol (±10%), as measured by Raman spectroscopy from 22 to 90°C, as well as the roughly estimated MM2 MeNHEt and MeNHCH2NH2 gauche interactions. DFT calculations using B3LYP/6-31G* yield a standard ΔH value of 1.04 kcal/mol, in good agreement with the experiment, and predict a ttt/cct ratio of ca. 77:23 at 25°C. Broadening of the 13C-NMR signals was observed in either CCl4 or CS2 or even CDCl3 solutions between -20 and -40°C.
  • Insight into Mg(BH4)2 with Synchrotron X-ray Diffraction: Structure Revision, Crystal Chemistry, and Anomalous Thermal Expansion
    Yaroslav Filinchuk, Radovan Cerny and Hans Hagemann
    Chemistry of Materials, 21 (5) , 2009, p925-933
    DOI:10.1021/cm803019e | unige:3531 | Abstract | Article HTML | Article PDF
  
Geometry of [Mg(BH4)4] units in R-Mg(BH4)2, coordination of Mg2 atom is given as an example. Two nearly planar BH2-Mg-H2B fragments are situated at nearly 90° dihedral angle. The shortest H ¡ ¡ ¡ H distances are highlighted in green.
  
  • Protease responsive nanoprobes with tethered fluorogenic peptidyl 3-arylcoumarin substrates
    Katharina Welser, Jakob Grilj, Eric Vauthey, Jonathan W. Aylott and Weng C. Chan
    ChemComm, 2009, p671-673
    DOI:10.1039/b816637d | unige:3556 | Abstract | Article HTML | Article PDF
Protease responsive nanosensors were obtained by the attachment of unique green fluorescent bifunctional 3-arylcoumarin-derived fluorogenic substrates to poly(acrylamide-co-N-(3-aminopropyl)methacrylamide) nanoparticles, in which proteolysis results in substantial signal amplification.
 
The kinetics of the dual fluorescence of several derivatives of dimethylaminobenzonitrile (DMABN) has been compared using fs-fluorescence upconversion experiments. Variation of the size and twist angle of the donor (dialkylamino group) suggest a large amplitude solvent-viscosity controlled diffusional twisting motion towards larger twist angles as the rate limiting step. Large rate differences were observed for an ester group as acceptor. Temperature dependent studies indicate that these differences are not connected with different activation barriers but with changes in the Arrhenius preexponential factor. It is argued that conical intersections along the reaction path can bring about these entropy changes.
  • Generating CuII-Oxyl/CuIII-Oxo Species from CuI- α-Ketocarboxylate Complexes and O2: In silico studies on ligand effects and C-H-activation reactivity
    Stefan M. Huber, Mehmed Z. Ertem, Francesco Aquilante, Laura Gagliardi, William B. Tolman and Christopher J. Cramer
    Chemistry - A European Journal, 15 (19) , 2009, p4886-4895
    DOI:10.1002/chem.200802338 | unige:3742 | Abstract | Article HTML | Article PDF
Theoretically speaking: The mechanistic details associated with the generation and reaction of [CuO]+ species from CuI-Îą-ketocarboxylate complexes, especially with respect to modifications of the ligand supporting the copper center, were investigated (see scheme). Theoretical models were used to characterize the electronic structures of different [CuO]+ species and their reactivity in CH activation and O-atom transfer reactions.
  
This Letter discusses the nature of the chemical bond between two chromium atoms in different di-chromium complexes with the metal atoms in different oxidation states. Starting with the Cr diatom, with its formally sextuple bond and oxidation number zero, we proceed to analyse the bonding in some Cr(I)-Cr(I) XCrCrX complexes with X varying from F, to Phenyl, and Aryl. The bond distance in these complexes varies over a large range: 1.65-1.83 Å and we suggest explanations for these variations. A number of dichromium complexes with bond distances around or shorter than 1.80 Å have recently been synthesized and we study one of these complexes, Cr2(diazadiene)2 and show how the Cr-Cr bond order is related to the oxidation number and the ligand bonding, factors that are all involved in the determination of the short Cr-Cr bond length: 1.80 Å. The discussion is based on the use of multicon?gurational wave func- tions, which give a qualitatively correct description of the electronic structure in these multiply bonded systems.
  • Binding motifs for lanthanide-hydrides: a combined experimental and theoretical study of the MHx(H2)y species (M= La-Gd; x=2-4; y=0-6)
    Ivan Infante, Laura Gagliardi, Xuefeng Wang and Lester Andrews
    Journal of Physical Chemistry A, 113 (11) , 2009, p2446-2455
    DOI:10.1021/jp8099658 | unige:3741 | Abstract | Article HTML | Article PDF
The results of a combined spectroscopic and computational study of lanthanide hydrides with the general formula MHx(H2)y, where M = La, Ce, Pr, Nd, Sm, Eu, and Gd, x = 1−4, and y = 0−6 are reported. To understand the nature of the dihydrogen complexes formed with lanthanide metal hydride molecules, we have first identified the binary MHx species formed in the ablation/deposition process and then analyzed the dihydrogen supercomplexes, MHx(H2)y. Our investigation shows that the trihydrides bind dihydrogen more weakly than the dihydrides and that the interaction between the central lanthanide and the H2 molecules occurs via a 6s electron transfer from the lanthanide to the H2 molecules. Evidence is also presented for the SmH and EuH diatomic molecules and the tetrahydride anions in solid hydrogen.
Multiconfigurational quantum chemical methods (CASSCF/CASPT2) have been used to study the chemiionization reactions Ce + O → CeO+ + e- and Ce + O2 → CeO2+ + e-. Selected spectroscopic constants for CeOn and CeOn+ (n = 1, 2), as well as reaction enthalpies of the chemiionization reactions of interest, have been computed and compared with experimental values. In contrast to the lanthanum case, for both Ce + O2(X3Σg-) and Ce + O2( a1Δg), the Ce + O2 → CeO2+ + e- reaction is shown to be exothermic, and thus, contributes to the experimental chemielectron spectra. The apparent discrepancy between the computed reaction enthalpies and the high kinetic energy offset values measured in the chemielectron spectra is rationalized by arguing that chemielectrons are produced mainly via two sequential reactions (Ce + O2 → CeO + O, followed by Ce + O → CeO+ + e-) as in the case of lanthanum. For Ce + O2 (a1Δg), a chemielectron band with higher kinetic energy than that recorded for Ce + O2( X3Σg-) is obtained. This is attributed to production of O( 1D) from the reaction Ce + O2( a1Δg) → CeO + O( 1D), followed by chemiionization via the reaction Ce + O( 1D) → CeO+ + e-. Accurate potential energy curves for the ground and a number of excited states of CeO and CeO+ have been computed, and a mechanism for the chemiionization reactions investigated experimentally was proposed.
  • Mono- and Bis(tetrathiafulvalene)-1,3,5-Triazines as Covalently Linked Donor-Acceptor Systems: Structural, Spectroscopic, and Theoretical Investigations
    François RiobĂŠ, Philippe Grosshans, Helena Sidorenkova, Michel Geoffroy and Narcis Avarvari
    Chemistry - A European Journal, 15 (2) , 2009, p380-387
    Keywords: charge transfer | donor-acceptor systems | EPR spectroscopy | nitrogen heterocycles | structure elucidation | tetrathiafulvalenes
    DOI:10.1002/chem.200801851 | unige:3558 | Abstract | Article HTML | Article PDF
  
Reaction of 2,4,6-trichloro-1,3,5-triazine with lithiated tetrathiafulvalene (TTF) in stoichiometric conditions, followed by treatment with sodium methanolate, provides mono- and bis(TTF)-triazines as new covalently linked (multi)donor-acceptor systems. Single-crystal X-ray analyses reveal planar structures for both compounds, with formation of peculiar segregated donor and acceptor stacks for the mono(TTF)-triazine compound, while mixed TTF-triazine stacks establish in the case of the bis(TTF) derivative. Cyclic voltammetry measurements show reversible oxidation of the TTF units, at rather low potential, with no splitting of the oxidation waves in the case of the dimeric TTF, whereas irreversible reduction of the triazine core is observed. Intramolecular charge transfer is experimentally evidenced through solution electronic absorption spectroscopy. Time-dependent DFT calculations allow the assignment of the charge transfer band to singlet transitions from the HOMO of the donor(s) to the LUMO of the acceptor. Solution EPR measurements correlated with theoretical calculations were performed in order to characterize the oxidized species. In both cases the spectra show very stable radical species and contain a triplet of doublet pattern, in agreement with the coupling of the unpaired electron with the three TTF protons. The dication of the bis(TTF)-triazine is paramagnetic, but no spin-spin exchange interaction could be detected.
  
  • Influence of Guest−Host Interactions on the Structural, Energetic, and Mössbauer Spectroscopy Properties of Iron(II)tris(2,2′-bipyridine) in the Low-Spin and High-Spin States: A Density-Functional Theory Study of the Zeolite-Y Embedded Complex
    Alfredo Vargas, Andreas Hauser and LatĂŠvi Max Lawson Daku
    Journal of Chemical Theory and Computation, 5 (1) , 2009, p97-115
    DOI:10.1021/ct800284g | unige:3559 | Abstract | Article HTML | Article PDF
Density functional theory is applied within a supramolecular approach to the study of the guest−host interactions in [Fe(bpy)3]2+@Y and their influence on the structural, energetic, and 57Fe Mössbauer spectroscopy properties of the encapsulated [Fe(bpy)3]2+ complex in the low- and high-spin states. The structures of the isolated complex and the supramolecular model used for [Fe(bpy)3]2+@Y were optimized in both spin-states using different generalized gradient approximation (PBE, HCTH, OLYP) and hybrid (B3LYP*, O3LYP) functionals. The results obtained are consistent with one another and show that, in either spin-state, the structure of [Fe(bpy)3]2+ shrinks and distorts upon encapsulation. Still, the structural changes experienced by the complex in a given spin-state remain limited, especially in that they do not lead to a substantial variation of the 57Fe quadrupole splitting, whose calculated values are in very good agreement with avalaible experimental data. The decomposition of the guest−host interaction energy into its electrostatic, Pauli and orbital contributions shows that the bonding between the complex and the supercage is more electrostatic than covalent. The ability of modern functionals to accurately describe the interactions explains the remarkable consistency of the results obtained with the various functionals. In particular, although the functionals perform very differently for the determination of the high-spin/low-spin energy difference ΔEHLel in [Fe(bpy)3]2+ and [Fe(bpy)3]2+@Y, they consistently predict that the encapsulation entails a destabilization of the high-spin state with regard to the low-spin state of Δ(ΔEHLel) = 2500 cm−1. Using for [Fe(bpy)3]2+ the CASPT2 value of ΔEHLel = 3700 cm−1 [Pierloot, K.; Vancoillie, S. J. Chem. Phys.2006, 125, 124303; Pierloot, K.; Vancoillie, S. J. Chem. Phys.2008, 128, 034104], we obtain for the high-spin/low-spin energy difference in [Fe(bpy)3]2+@Y, a best ab initio estimate of ΔEHLel = 6200 cm−1.
  • Spin-transition in nearly cubic site in [FeII(L)3][PF6]2
    Vibha Mishra, Rabindranath Mukherjee, Jorge Linares, Epiphane Codjovi, François Varret and LatĂŠvi Max Lawson Daku
    Hyperfine Interactions, 188 (1-3) , 2009, p71-78
    Keywords: spin-transition ; 57Fe MĂśssbauer spectroscopy ; iron(II) complex ; ligand-field ; density functional theory
    DOI:10.1007/s10751-008-9891-6 | unige:3560 | Abstract | Article PDF
  
The spin-transition (1A1↔5T2) behaviour of a new mononuclear iron(II) compound [FeII(L)3][PF6]2[L = 2-[3-(2′-pyridyl)pyrazole-1-ylmethyl]pyridine] has been investigated by 57Fe Mössbauer spectroscopy. Analysis of the Mössbauer spectra revealed low value of the quadrupole splitting of the high-spin state which reflects iron(II) to be in nearly cubic lattice site. Mössbauer spectra under light show the light-induced excited spin state trapping effect and the observed quadrupole splitting of the metastable high-spin state is found little sensitive to the high-spin fraction value. DFT calculations are in progress to document the almost cubic nature of the ligand-field acting on the iron atom.
  • Artificial tongues and leaves
    Natalie Banerji, Rajesh Bhosale, Guillaume Bollot, Sara M. Butterfield, Alexandre FĂźrstenberg, Virginie Gorteau, Shinya Hagihara, Andreas Hennig, Santanu Maity, Jiri Mareda, Stefan Matile, Federico Mora, Alejandro Perez-Velasco, Velayutham Ravikumar, Ravuri S.K. Kishore, Naomi Sakai, Duy-Hien Tran and Eric Vauthey
    Pure and Applied Chemistry, 80 (8) , 2008, p1873-1882
    Keywords: lipid bilayer; nanoarchitecture; photosynthesis; photovoltaics; scaffolds; sensors
    DOI:10.1351/pac200880081873 | unige:8008 | Abstract | Article PDF
The objective with synthetic multifunctional nanoarchitecture is to create large suprastructures with interesting functions. For this purpose, lipid bilayer membranes or conducting surfaces have been used as platforms and rigid-rod molecules as shape-persistent scaffolds. Examples for functions obtained by this approach include pores that can act as multicomponent sensors in complex matrices or rigid-rod π-stack architecture for artificial photosynthesis and photovoltaics.
  • Chiral 1,1'-binaphthyl-2,2'-dithiol-stabilized gold clusters: Size separation and optical activity in the UV-vis
    Cyrille Gautier, Rossana Taras, Serafino Gladiali and Thomas BĂźrgi
    Chirality, 20 (3-4) , 2008, p486-493
    Keywords: circular dichroism; optical activity; 1,1′-binaphthyl-2,2′-dithiol; nanoparticles; gold; size separation
    DOI:10.1002/chir.20488 | unige:14705 | Abstract | Article PDF
Gold particles covered with 1,1′-binaphthyl-2,2′-dithiol (BINAS) were prepared. Using size exclusion chromatography, it was possible for the first time to separate the sample into fractions with different sizes and colors. Transmission electron microscopy shows that the particles are very small, in the order of 1 nm or slightly above. The absorption spectra of the separated samples show rich structure. The particles show size-dependent optical activity in metal-based electronic transitions. The shape of both the absorption and circular dichroism spectra of one of the smallest fractions exhibits similarities with the spectra reported for Au11 covered by 2,2′-bis(diphenylphosphino)-1,1′-biphenyl although the spectra are shifted to shorter wavelengths in the case of the dithiol. The anisotropy factors, Δϵ/ϵ of these particles are as large as 4 × 10−3, which is larger than the values reported for gold particles stabilized by phosphines and water-soluble thiols. This indicates that BINAS is particularly well-suited to impart chirality on to gold particles.
  • Adsorption of Thiol-Protected Gold Nanoparticles on TiO2 and Their Behavior under UV Light Irradiation
    Igor Dolamic, Cyrille Gautier, Julien Boudon, Natallia Shalkevich and Thomas BĂźrgi
    Journal of Physical Chemistry C, 112 (15) , 2008, p5816-5824
    DOI:10.1021/jp711442m | unige:14681 | Abstract | Article HTML | Article PDF
A combination of in situ attenuated total reflection infrared (ATR-IR) spectroscopy, UV−vis spectroscopy and transmission electron microscopy was used to study the adsorption of thiol-protected gold nanoparticles on TiO2 films and the behavior of the resulting composite films upon UV irradiation. The gold nanoparticles were covered by charged thiols N-acetyl-l-cysteine and l-glutathione and had a mean core diameter of about 1 nm. The TiO2 film was prepared by deposition of a slurry of TiO2 nanoparticles with a particles size of 21 nm. The combination of the two spectroscopic techniques showed that the adsorption of the gold nanoparticles onto the TiO2 films is significantly limited by intrafilm diffusion. Upon illumination the IR spectra revealed the removal of the adsorbed thiolates and the appearance of sulfates. These species were also observed when N-acetyl-l-cysteine adsorbed on TiO2 was illuminated, i.e., in the absence of gold. In the latter case oxalate was observed in large quantity on the TiO2 surface, in contrast to the illumination of the N-acetyl-l-cysteine-protected gold particles. This indicates a different pathway for the decomposition of the adsorbed thiol when adsorbed on the gold or directly on the TiO2 surface. In situ UV−vis spectroscopy also shows the formation of larger particles upon illumination, which is confirmed by transmission electron microscopy.
  
Using simple organic synthetic transformations, a novel diazaoxatricornan derivative, the 12c-methyl-12-phenyl-8-propyl-12,12c-dihydro-8H-4-oxa-8,12-diazadibenzo[cd,mn]pyrene (6a), was prepared. This novel chiral cup-shaped molecule was isolated in racemic form and in excellent yield after the addition of methyl lithium to the BF4 salt of a novel unsymmetrical diazaoxatriangulenium cation. Compound 6a was found to be stable under classical laboratory conditions—something not obvious considering the extreme stability of the carbenium ion precursor, the electron-rich nature of the core, and the strain induced by the pyramidalization of the central carbon. The enantiomers were readily separated by chiral stationary phase chromatography, and the absolute configuration of (−)-(S)-6a was determined by a comparison of the experimental and theoretical vibrational circular dichroism (VCD) spectra. This isolation of (−)-(S)-6a and (+)-(R)-6a constitutes thus the first report of a nonracemic closed-capped chiral bowl molecule for which the chirality is due to the intrinsic dissymmetry of the central core of the structure only.
 
The thiolate-for-thiolate ligand exchange was performed on well-defined gold nanoparticles under an inert atmosphere without any modification of the core size. This reaction is faster than the well-known core etching. Surprisingly, if a chiral thiol is exchanged for its opposite enantiomer, the optical activity in the metal-based electronic transitions is reversed although the form of the CD spectra remains largely unchanged. The extent of inversion corresponds to the overall ee of the chiral ligand in the system. This shows that the chiral arrangement of metal atoms in the metal particle (surface) can not withstand the driving force imposed by the ligand of opposite absolute configuration. If the incoming thiol has a different structure, the electronic transitions in the metal core are slightly modified whereas the absorption onset remains unchanged. These results emphasize the influence of the thiols on the structure of the gold nanoparticles and give insight on the ligand exchange pathways.
  • Sensitivity enhancement and dynamic behavior analysis by modulation excitation spectroscopy: Principle and application in heterogeneous catalysis
    Atsushi Urakawa, Thomas BĂźrgi and Alfons Baiker
    Chemical Engineering Science, 63 (20) , 2008, p4902-4909
    DOI:10.1016/j.ces.2007.06.009 | unige:14732 | Article PDF
Modulation excitation spectroscopy (MES) allows sensitive and selective detection and monitoring of the dynamic behavior of species directly involved in a reaction. The method, combined with proper in situ spectroscopy, is powerful for elucidating complex systems and noisy data as often encountered in heterogeneous catalytic reactions at solid–liquid and solid–gas interfaces under working conditions. The theoretical principle and actual data processing of MES are explained in detail. Periodic perturbation of the system by an external parameter, such as concentration and temperature, is utilized as stimulation in MES. The influence of stimulation shape upon response analysis is explained. Furthermore, an illustrative example of MES, enantioselective hydrogenation at a solid-liquid interface, is presented.
  • Liquid-Crystalline Thiol- and Disulfide-Based Dendrimers for the Functionalization of Gold Nanoparticles
    StĂŠphane Frein, Julien Boudon, Mireille Vonlanthen, Toralf Scharf, JoaquĂ­n BarberĂĄ, Georg SĂźss-Fink, Thomas BĂźrgi and Robert Deschenaux
    Helvetica Chimica Acta, 91 (12) , 2008, p2321-2337
    Keywords: liquid-crystals; dendrimers; gold nanoparticles; nanoparticles
    DOI:10.1002/hlca.200890253 | unige:14703 | Abstract | Article PDF
Liquid-crystalline dendrons carrying either a thiol or disulfide function which display nematic, smectic A, columnar, or chiral nematic phases have been synthesized. Their mesomorphic properties are in agreement with the nature of the mesogenic units and structure of the dendrons. The first-generation poly(aryl ester) dendron containing two cyanobiphenyl mesogenic units was used to functionalize gold nanoparticles. For full coverage, a smectic-like supramolecular organization on the nanometer scale is observed, when the gold nanoparticles are spread onto carbon-coated copper grids. This result indicates that the dendritic ligands reported here act as self-organization promoters.
  • What governs nitrogen configuration in substituted aminophosphines?
    Matthew D. Wodrich, Alfredo Vargas, , Gabriel Merino and ClĂŠmence Corminboeuf
    Journal of Physical Organic Chemistry, 22 (2) , 2008, p101-109
    Keywords: stereoelectronic effects • electron delocalization • hyperconjugation • density functional theory
    DOI:10.1002/poc.1431 | unige:3181 | Abstract | Article PDF
The trigonal planar geometry of the nitrogen atom in commonly used phosphoramidite ligands is not in line with the traditional valence shell electron pair repulsion (VSEPR) model. In this work, the effects governing nitrogen configuration in several substituted aminophosphines, A2PNB2 (A or B = H, F, Cl, Br, Me, OMe, BINOP), are examined using modern computational analytic tools. The electron delocalization descriptions provided by both electron localization function (ELF) and block localized wavefunction analysis support the proposed relationships between conformation and negative hyperconjugative interactions. In the parent H2PNH2, the pyramidal nitrogen configuration results from nitrogen lone pair electron donation into the σ* P — H orbital. While enhanced effects are seen for F2PNMe2, placing highly electronegative fluorine substituents on nitrogen (i.e., Me2PNF2) eliminates delocalization of the nitrogen lone pair. Understanding and quantifying these effects can lead to greater flexibility in designing new catalysts.
  
  • 3MLCT excited states in Ru(II) complexes: Reactivity and related two-photon absorption applications in the near-infrared spectral range
    Gilles Lemercier, Adeline Bonne, MickaĂŤl Four and LatĂŠvi M. Lawson Daku
    Comptes Rendus Chimie, 11 (6-7) , 2008, p709-715
    Keywords: Ruthenium(II) complexes; Polypyridyl ligands; Two-photon absorption; 3MLCT; Dioxygen sensors; Photodynamic therapy
    DOI:10.1016/j.crci.2007.11.012 | unige:3561 | Article PDF
The synthesis and characterization of new 1,10-phenanthroline-based chromophores LT1, LT2 and LD1 featuring fluorene unit(s) are reported. Their absorption and emission as well as their two-photon absorption properties in the 450–650 nm spectral range are discussed in comparison with the parent 1,10-phenanthroline and already described ligands L1 and L2.
  • Low-lying phonons in NaBH4 as seen with inelastic scattering of synchrotron radiation
    D. Chernyshov, A. Bosak, V. Dmitriev, Y. Filinchuk and H. Hagemann
    Physical Review B, 78 , 2008, p172104
    Keywords: Hydrides, Phonons, Inelastic X-ray scattering, elasticity
    DOI:10.1103/PhysRevB.78.172104 | unige:3562 | Abstract | Article PDF | Article PS (gzipped)
  
(a) IXS spectrum recorded at (310) (solid diamonds) compared to the resolution function (solid line) . (b) Phonon dispersion in high-symmetry directions; experimental points empty symbols connected by a guide for the eyes (solid lines) are compared with the ab initio calculations (dashed lines) for the F43m structure. The estimated experimental errors are less than the symbol size.
  • Direct Femtosecond Observation of Tight and Loose Ion Pairs upon Photoinduced Bimolecular Electron Transfer
    Omar F. Mohammed, Katrin Adamczyk, Natalie Banerji, Jens Dreyer, Bernhard Lang, Erik T.J. Nibbering and Eric Vauthey
    Angewandte Chemie International Edition, 47 (47) , 2008, p9044-9048
    DOI:10.1002/anie.200803164 | unige:3182 | Abstract | Article HTML | Article PDF
Polarization-sensitive ultrafast infrared measurements on photoinduced electron transfer in donor-acceptor pairs in polar acetonitrile show distinct contributions from loose and tight ion pairs. Highly anisotropic signals from tight ion pairs reveal the importance of mutual orientation of the reactants (see picture) and thus the need to refine theoretical models based on spherical species that solely involve reaction distances.
  
  • Bond Length and Bond Order in One of the Shortest Cr−Cr Bonds
    Giovanni La Macchia, Francesco Aquilante, Valera Veryazov, Bjorn O. Roos and Laura Gagliardi
    Inorganic Chemistry, 47 (24) , 2008, p11455-11457
    DOI:10.1021/ic801537w | unige:3563 | Abstract | Article HTML | Article PDF
Multiconfigurational quantum chemical calculations on the R-diimines dichromium compound confirm that the Cr−Cr bond, 1.80 Å, is among the shortest CrI−CrI bonds. However, the bond between the two Cr atoms is only a quadruple bond rather than a quintuple bond. The reason why the bond is so short has to be attributed to the strain in the NCCN ligand moieties.
  • Ruthenium(II) Coordination Chemistry of a Fused Donor-Acceptor Ligand: Synthesis, Characterization and Photoinduced Electron Transfer Reactions of [{Ru(bpy)2}n(TTF-ppb)](PF6)2n (n = 1, 2)
    Christine Goze, Nathalie Dupont, Elvira Beitler, Claudia Leiggener, Hongpeng Jia, Philippe Monbaron, Shi-Xia Liu, Antonia Neels, Andreas Hauser and Silvio Decurtins
    Inorganic Chemistry, 47 (23) , 2008, p11010-11017
    DOI:10.1021/ic801252t | unige:3564 | Abstract | Article HTML | Article PDF
  
A π-extended, redox-active bridging ligand 4′,5′-bis(propylthio)tetrathiafulvenyl[i]dipyrido[2,3-a:3′,2′-c]phenazine (L) was prepared via direct Schiff-base condensation of the corresponding diamine−tetrathiafulvalene (TTF) precursor with 4,7-phenanthroline-5,6-dione. Reactions of L with [Ru(bpy)2Cl2] afforded its stable mono- and dinuclear ruthenium(II) complexes 1 and 2. They have been fully characterized, and their photophysical and electrochemical properties are reported together with those of [Ru(bpy)2(ppb)]2+ and [Ru(bpy)2(μ-ppb)Ru(bpy)2]4+ (ppb = dipyrido[2,3-a:3′,2′-c]phenazine) for comparison. In all cases, the first excited state corresponds to an intramolecular TTF → ppb charge-transfer state. Both ruthenium(II) complexes show two strong and well-separated metal-to-ligand charge-transfer (MLCT) absorption bands, whereas the 3MLCT luminescence is strongly quenched via electron transfer from the TTF subunit. Clearly, the transient absorption spectra illustrate the role of the TTF fragment as an electron donor, which induces a triplet intraligand charge-transfer state (3ILCT) with lifetimes of approximately 200 and 50 ns for mono- and dinuclear ruthenium(II) complexes, respectively.
  
  • Intramolecular Charge-Transfer Dynamics in Covalently Linked Perylene−Dimethylaniline and Cyanoperylene−Dimethylaniline
    Natalie Banerji, Gonzalo Angulo, Igor Barabanov and Eric Vauthey
    Journal of Physical Chemistry A, 112 (40) , 2008, p9665-9674
    DOI:10.1021/jp803621z | unige:3565 | Abstract | Article HTML | Article PDF
The excited-state dynamics of covalently linked electron donor−acceptor systems consisting of N,N-dimethylaniline (DMA) as electron donor and either perylene (Pe) or cyanoperylene (CNPe) as acceptor has been investigated in a large variety of solvents, including a room-temperature ionic liquid, by using femtosecond time-resolved fluorescence and absorption spectroscopy. The negligibly small solvent dependence of the absorption spectrum of both compounds and the strong solvatochromism of the fluorescence are interpreted by a model where optical excitation results in the population of a locally excited state (LES) and emission takes place from a charge-separated state (CSS). This interpretation is supported by the fluorescence up-conversion and the transient absorption measurements that reveal substantial spectral dynamics in polar solvents only, occurring on time scales going from a few hundreds of femtoseconds in acetonitrile to several tens of picoseconds in the ionic liquid. The early transient absorption spectra are similar to those found in nonpolar solvents and are ascribed to the LES absorption. The late spectra due to CSS absorption show bands that are red-shifted relative to those of the radical anion of the acceptor moiety by an amount that depends on solvent polarity, pointing to partial charge separation. Global analysis of the time-resolved data indicates that the charge separation dynamics in PeDMA is essentially solvent controlled, whereas that in CNPeDMA is faster than diffusive solvation, this difference being accounted for by a larger driving force for charge separation in the latter. On the other hand, the CSS lifetime of PeDMA is of the order of a few nanoseconds independently of the solvent, whereas that of CNPeDMA decreases with increasing solvent polarity from a few nanoseconds to a few hundreds of picoseconds. Comparison of these results with previously published data on the fluorescence quenching of Pe and CNPe in pure DMA shows that the charge separation and the ensuing charge recombination occur on similar time scales independently of whether these processes are intra- or intermolecular.
 
The ground and excited states of neutral and cationic PuO and PuO2 have been studied with multiconfigurational quantum chemical methods followed by second order perturbation theory, the CASSCF/CASPT2 method. Scalar relativistic effects and spin–orbit coupling have been included in the treatment. As literature values for the ionization energy of PuO2 are in the wide range of ~6.6 eV to ~10.1 eV, a central goal of the computations was to resolve these discrepancies; the theoretical results indicate that the ionization energy is near the lower end of this range. The calculated ionization energies for PuO, PuO+ and PuO2+ are in good agreement with the experimental values.
A strategy to construct approximants to the kinetic-energy-functional dependent component (v[ρA,ρB](→r)) of the effective potential in one-electron equations for orbitals embedded in a frozen-density environment [Eqs. (20) and (21) in Wesolowski and Warshel, J. Phys. Chem. 97, (1993) 8050 ] is proposed. In order to improve the local behavior of the orbital-free effective embedding potential near nuclei in the environment, the exact behavior of vt[ρA,ρB](→r) at ρA→0 and ∫ρBd→r = 2 is taken into account. As a result, the properties depending on the quality of this potential are invariably improved compared to the ones obtained using conventional approximants which violated the considered exact condition. The approximants obtained following the proposed strategy and especially the simplest one constructed in this work are nondecomposable, i.e., cannot be used to obtain the analytic expression for the functional of the total kinetic energy.
  
  • LiSc(BH4)4: A Novel Salt of Li+ and Discrete Sc(BH4)4− Complex Anions
    Hans Hagemann, Moise Longhini, Jakub W. Kaminski, Tomasz A. Wesolowski, Radovan Cerny, Nicolas Penin, Magnus H. Sørby, Bjørn C. Hauback, Godwin Severa and Craig M. Jensen
    Journal of Physical Chemistry A, 112 (33) , 2008, p7551-7555
    DOI:10.1021/jp803201q | unige:3567 | Abstract | Article HTML | Article PDF
LiSc(BH4)4 has been prepared by ball milling of LiBH4 and ScCl3. Vibrational spectroscopy indicates the presence of discrete Sc(BH4)4− ions. DFT calculations of this isolated complex ion confirm that it is a stable complex, and the calculated vibrational spectra agree well with the experimental ones. The four BH4− groups are oriented with a tilted plane of three hydrogen atoms directed to the central Sc ion, resulting in a global 8 + 4 coordination. The crystal structure obtained by high-resolution synchrotron powder diffraction reveals a tetragonal unit cell with a = 6.076 Å and c = 12.034 Å (space group P-42c). The local structure of the Sc(BH4)4− complex is refined as a distorted form of the theoretical structure. The Li ions are found to be disordered along the z axis.
 
IR and Raman data were obtained from α-, β-, and mixed (β,γ)-Ca(BH 4) 2 samples and from the deuterated β,γ phase mixture. The results obtained with α phase indicate that the DFT calculated values for the B−H stretching modes and the lattice vibrations are fairly close to the experimental values. The spectral behavior at temperatures around the transition to the β phase shows a continuous transition and suggests the presence of disorder caused by reorientational motions of the [BH 4] − ion in the β phase. The data indicate that there are more deformation bands observed for the mixed (β,γ) samples than for the α phase which indicates structural variations between the β and the γ phases.
  
  • Ultrafast Photoinduced Charge Separation in Naphthalene Diimide Based Multichromophoric Systems in Liquid Solutions and in a Lipid Membrane
    Natalie Banerji, Alexandre FĂźrstenberg, Sheshanath Bhosale, Adam L. Sisson, Naomi Sakai, Stefan Matile and Eric Vauthey
    Journal of Physical Chemistry B, 112 (30) , 2008, p8912-8922
    DOI:10.1021/jp801276p | unige:3569 | Abstract | Article HTML | Article PDF
The photophysical properties of multichromophoric systems consisting of eight red or blue naphthalene diimides (NDIs) covalently attached to a p-octiphenyl scaffold, as well as a blue bichromophoric system with a biphenyl scaffold, have been investigated in detail using femtosecond time-resolved spectroscopy. The blue octachromophoric systems have been recently shown to self-assemble as supramolecular tetramers in lipid bilayer membranes and to enable generation of a transmembrane proton gradient upon photoexcitation (Bhosale, S.; Sisson, A. L.; Talukdar, P.; FĂźrstenberg, A.; Banerji, N.; Vauthey, E.; Bollot, G.; Mareda, J.; RĂśger, C.; WĂźrthner, F.; Sakai, N.; Matile, S. Science2006, 313, 84). A strong reduction of the fluorescence quantum yield was observed when going from the single NDI units to the multichromophoric systems in methanol, the effect being even stronger in a vesicular lipid membrane. Fluorescence up-conversion measurements reveal ultrafast self-quenching in the multichromophoric systems, whereas the formation of the NDI radical anion, evidenced by transient absorption measurements, points to the occurrence of photoinduced charge separation. The location of the positive charge could not be established unambiguously from the transient absorption measurements, but energetic considerations indicate that charge separation should occur between two NDI units in the blue systems, whereas both an NDI unit and the p-octiphenyl scaffold could act as electron donor in the red system. The lifetime of the charge-separated state was found to increase from 22 to 45 ps by going from the bi- to the octachromophoric blue systems in methanol, while a 400 ps decay component was observed in the lipid membrane. This lifetime lengthening is explained in terms of charge migration that is most efficient when the octachromophoric systems are assembled as supramolecular tetramers in the lipid membrane. Furthermore, the average charge-separated state lifetime of the red system in methanol is even larger and amounts to 750 ps. This effect cannot be simply explained in terms of Marcus inverted regime as the driving force for charge recombination in the red system is only slightly larger than in the blue one. A better spatial separation of the charges in the red system stemming from the localization of the hole on the p-octiphenyl scaffold could additionally contribute to the slowing down of charge recombination.
 
NaBH4·2H2O and NaBH4 were studied by single-crystal X-ray diffraction and vibrational spectroscopy. In NaBH4·2H2O, the BH4- anion has a nearly ideal tetrahedral geometry and is bridged with two Na+ ions through the tetrahedral edges. The structure does not contain classical hydrogen bonds, but reveals strong dihydrogen bonds of 1.77-1.95 Å. Crystal structures and vibrational spectra of NaBr·2H2O and NaBH4·2H2O reveal many similarities. The unit cell volume of NaBH4·2H2O increases linearly with temperature between 200 and 313 K. At 313-315 K, the hydrate decomposes into NaBH4 and H2O, which react to release hydrogen.
  
  • Simultaneous Generation of Different Types of Ion Pairs upon Charge-Transfer Excitation of a Donor−Acceptor Complex Revealed by Ultrafast Transient Absorption Spectroscopy
    Omar F. Mohammed and Eric Vauthey
    Journal of Physical Chemistry A, 112 (26) , 2008, p5804-5809
    DOI:10.1021/jp801004b | unige:3571 | Abstract | Article HTML | Article PDF
The excited-state dynamics of the methylperylene/tetracyanoethylene (MPe/TCNE) donor−acceptor complex has been investigated in various solvents using femtosecond transient absorption spectroscopy. The transient spectra reveal the formation of two types of ion pairs: The first (IP1), constituting the major fraction of the total ion-pair population, is characterized by a broad and red-shifted absorption spectrum compared to that of the free MPe cation and by a subpicosecond lifetime, whereas the second (IP2) has a spectrum closer to that of MPe cation and a lifetime of a few picoseconds. A substantial polarization anisotropy was observed with IP1 but not with IP2, indicating a relatively well-defined structure for the former. The reaction scheme that best accounts for the observed dynamics and its solvent dependence involves the simultaneous excitation of complexes that differ by their electronic coupling. The more coupled complexes have a high absorption coefficient and thus yield IP1, which undergoes ultrafast charge recombination, whereas the less coupled complexes have a lower probability to be excited and lead to the longer-lived IP2.
  • Towards inert and pre-organized d-block-containing receptors for trivalent lanthanides: The synthesis and characterization of triple-helical monometallic OsII and bimetallic OsII-LnIII complexes
    Thomas Riis-Johannessen, Nathalie Dupont, Gabriel Canard, GĂŠrald Bernardinelli, Andreas Hauser and Claude Piguet
    Dalton Transactions, 28 , 2008, p3661-3677
    DOI:10.1039/b718885d | unige:3572 | Abstract | Article HTML | Article PDF
The mononuclear OsII complex [Os( L1)3](PF6)2 ( L1 = 5-methyl(1-methylbenzimidazol-2-yl)pyridine) is an obvious candidate for the design of an inert d-block-based tripodal receptor capable of binding and photosensitizing trivalent lanthanides (LnIII). It has thus been prepared and its two enantiomeric meridional (Δ-mer and Λ-mer) and facial (rac-fac) isomers have been separated by ion-exchange chromatography. The optical isomers have been characterized by CD spectroscopy and assignments of absolute configuration confirmed by an X-ray crystallographic study of Λ-mer-[Os( L1)3](PF6)2·1.5MeCN (monoclinic, P21, Z = 4). Comparison of the latter structure with that of racemic fac-[Os( L1)3](PF6)2 (monoclinic, C2/c, Z = 8) and [Os(bipy)3](PF6)2 (where bipy = 2,2' -bipyridine) shows minimal structural variations, but differences are observed in the photophysical and electrochemical properties of the respective compounds. Luminescence emissions from OsII complexes of L1 are typically lower in energy, with shorter lifetimes and lower quantum yields than their bipy analogues, whilst metal-centred oxidation processes are more facile due to the enhanced π-donor ability of L1. The key relationships between these parameters are discussed. Finally, though challenged by (i) the low reactivity of many osmium precursors and (ii) the irreversible formation of competing side products, the synthesis and purification of the heterobimetallic triple-stranded helicate HHH-[OsLu( L2)3](CF3SO3)5 has been realised, in which L2 is a segmental ligand containing the same bidentate unit as that found in L1 further connected to a tridentate binding site adapted for complexing LnIII. Its solid-state structure has been established by X-ray crystallography (triclinic, P1-, Z = 2).
The La + O and La + O2 chemiionization reactions have been investigated with quantum chemical methods. For La + O2(X3Σg) and La + O2(a1Δg), the chemiionization reaction La + O2 → LaO2+ + e− has been shown to be endothermic and does not contribute to the experimental chemielectron spectra. For the La + O2(X3Σg) reaction conditions, chemielectrons are produced by La + O2 → LaO + O, followed by La + O → LaO+ + e−. This is supported by the same chemielectron band, arising from La + O → LaO+ + e−, being observed from both the La + O(3P) and La + O2(X3Σg) reaction conditions. For La + O2(a1Δg), a chemielectron band with higher electron kinetic energy than that obtained from La + O2(X3Σg) is observed. This is attributed to production of O(1D) from the reaction La + O2(a1Δg) → LaO + O(1D), followed by chemiionization via the reaction La + O(1D) → LaO+ + e−. Potential energy curves are computed for a number of states of LaO, LaO* and LaO+ to establish mechanisms for the observed La + O → LaO+ + e− chemiionization reactions.
 
We report a comprehensive THz, infrared and optical study of Nb-doped SrTiO3 as well as dc conductivity and Hall effect measurements. Our THz spectra at 7 K show the presence of an unusually narrow (<2  meV) Drude peak. For all carrier concentrations the Drude spectral weight shows a factor of three mass enhancement relative to the effective mass in the local density approximation, whereas the spectral weight contained in the incoherent midinfrared response indicates that the mass enhancement is at least a factor two. We find no evidence of a particularly large electron-phonon coupling that would result in small polaron formation.
  
  • Helicate Extension as a Route to Molecular Wires
    David Schultz, FrĂŠdĂŠric Biaso, Abdul Rehaman Moughal Shahi, Michel Geoffroy, Kari Rissanen, Laura Gagliardi, Christopher J. Cramer and Jonathan R. Nitschke
    Chemistry - A European Journal, 14 (24) , 2008, p7180-7185
    DOI:10.1002/chem.200800503 | unige:3184 | Abstract | Article HTML | Article PDF
We describe the preparation of a helicate containing four closely spaced, linearly arrayed copper(I) ions. This product may be prepared either directly by mixing copper(I) with a set of precursor amine and aldehyde subcomponents, or indirectly through the dimerization of a dicopper(I) helicate upon addition of 1,2-phenylenediamine. A notable feature of this helicate is that its length is not limited by the lengths of its precursor subcomponents: each of the two ligands wrapped around the four copper(I) centers contains one diamine, two dialdehyde, and two monoamine residues. This work thus paves the way for the preparation of longer oligo- and polymeric structures. DFT calculations and electrochemical measurements indicate a high degree of electronic delocalization among the metal ions forming the cores of the structures described herein, which may therefore be described as "molecular wires".
  • [CpNi(diselenolene)] Neutral Radical Complexes: Electron Paramagnetic Resonance and Density Functional Theory Investigations
    Philippe Grosshans, Prashant Adkine, Helena Sidorenkova, Mitsushiro Nomura, Marc FourmiguĂŠ and Michel Geoffroy
    Journal of Physical Chemistry A, 112 (17) , 2008, p4067-4073
    DOI:10.1021/jp800584v | unige:3574 | Abstract | Article HTML | Article PDF
  
77Se-enriched CpNi(bds) (bds = 1,2-benzenediselenolate), has been synthesized and its g tensor and 77Se hyperfine tensors have been obtained from its frozen solution electron paramagnetic resonance (EPR) spectrum. These parameters are consistent with those calculated by density functional theory (DFT); it is shown that 10% of the spin is localized on each selenium and that the direction associated to the maximum 77Se couplings is aligned along the gmin direction, perpendicular to the Ni(bds) plane. EPR measurements and DFT calculations are also carried out on the 77Se enriched complex CpNi(dsit) as well on the two dithiolene analogues CpNi(bdt) and CpNi(dmit). The optimized structures of the isolated CpNi(bds) and CpNi(bdt) complexes have been used to generate the idealized dimers (bds)NiCp¡¡¡CpNi(bds) and (bdt)NiCp¡¡¡CpNi(bdt) characterized by Cp¡¡¡Cp overlap. The exchange parameters J calculated at the DFT level for these systems are in reasonable accord with the experimental values. The influence of the geometry of the dimer on its magnetic properties is assessed by calculating the variation of J as a function of the relative orientation of the two Ni(diselenolene) or Ni(dithiolene) planes.
A large set of electronic states of scandium dimer has been calculated using high-level theoretical methods such as quantum diffusion Monte Carlo (DMC), complete active space perturbation theory as implemented in GAMESS-US, coupled-cluster singles, doubles, and triples, and density functional theory (DFT). The 3Σu and 5Σu states are calculated to be close in energy in all cases, but whereas DFT predicts the 5Σu state to be the ground state by 0.08 eV, DMC and CASPT2 calculations predict the 3Σu to be more stable by 0.17 and 0.16 eV, respectively. The experimental data available are in agreement with the calculated frequencies and dissociation energies of both states, and therefore we conclude that the correct ground state of scandium dimer is the 3Σu state, which breaks with the assumption of a 5Σu ground state for scandium dimer, believed throughout the past decades.
  
  • Energy migration within the 2E state of Cr3+
    Mia Milos, SĂŠlim Kairouani, SĂŠbastien Rabaste and Andreas Hauser
    Coordination Chemistry Reviews, 252 (23-24) , 2008, p2540-2551
    Keywords: resonant and phonon-assisted excitation energy migration; [Cr(ox)3]3-; fluorescence line narrowing; spectral diffusion
    DOI:10.1016/j.ccr.2008.04.006 | unige:3575 | Article PDF
Excitation energy migration is an important phenomenon at high concentration of luminescent chromophores. In crystalline solids it results in a quenching of the intrinsic luminescence of the chromophore as the excitation energy migrates to impurity centres or other forms of trap sites. As concluded from the extensively studied systems where Cr3+ is doped as the active chromophore into inert host lattices, energy migration in crystalline solids is usually a phonon-assisted process, in which the simultaneous creation or annihilation of phonons helps to bridge the energy miss-match in the energy levels of two neighbouring chromophores within a inhomogeneously broadened absorption band. However, in the three-dimensional network systems [Ru(bpy)3][NaCr(ox)3] and [Rh(bpy)3][NaCr(ox)3]ClO4, it proved possible to unambiguously identify three different mechanisms for energy migration within the R1 line of the 4A2 → 2E transition of Cr3+. In addition to the common temperature dependant phonon-assisted process, a resonant process between the zero-field split components of the 4A2 ground state leading to a multi-line pattern in a fluorescence line narrowing spectrum and a quasi-resonant process within the same component leading to fast spectral diffusion can be identified at very low temperature. The parameters governing these processes are discussed and the behaviour of the model systems is compared to more conventional doped oxides and related systems.
  • The interaction between the spin transition and a crystallographic phase transition in the spin-crossover compound [Fe(bbtr)3](ClO4)2: Nucleation, formation of domains and fluctuations
    Itana Krivokapic, Cristian Enachescu, Robert Bronisz and Andreas Hauser
    Inorganica Chimica Acta, 361 (12-13) , 2008, p3616-3622
    DOI:10.1016/j.ica.2008.03.064 | unige:3576 | Abstract | Article PDF
  
The thermal and the light-induced spin transition in [Fe(bbtr)3](ClO4)2 (bbtr = 1,4-di(1,2,3-triazol-1-yl)) as well as the high-spin → low-spin relaxation following the light-induced population of the high-spin state below the thermal transition temperature are discussed in relation to the accompanying crystallographic phase transition. The experimental data have exclusively been obtained using optical single crystal absorption spectroscopy.
  • Spin transition and relaxation dynamics coupled to a crystallographic phase transition in a polymeric iron(II) spin-crossover system
    Itana Krivokapic, Cristian Enachescu, Robert Bronisz and Andreas Hauser
    Chemical Physics Letters, 455 (4-6) , 2008, p192-196
    DOI:10.1016/j.cplett.2008.02.088 | unige:74 | Article HTML | Article PDF
The spin-crossover compound [Fe(bbtr)3](ClO4)2 (bbtr = 1,4-di(1,2,3-triazol-1-yl)butane) forms a polymeric hexagonal sheet structure. It shows an abrupt thermal spin transition with 13 K wide hysteresis around 105 K, as evidenced by single crystal optical spectroscopy. The transition temperature for the thermal high-spin→low-spin transition on cooling as well as the relaxation kinetics just below Tc↓ depend upon the history of the sample. This is typical for a nucleation and growth mechanism and domain formation. In contrast, the high-spin→low-spin relaxation following the light-induced population of the high-spin state at low temperatures is governed by the intersystem crossing process.
  
A series of mixed crystals with general formula Ba7-xNayF12Cl2-zBrz in the ordered modification (space group P-6) has been studied by single crystal x-ray diffraction. Depending on synthesis conditions, the disorder in the channels (i.e. occupation of 0 0 z sites) can be changed. The disorder is found to be correlated with the refined Na content, and its effect on Ba-Cl(Br) bond length is discussed.
  • Zipper Assembly of Vectorial Rigid-Rod π-Stack Architectures with Red and Blue Naphthalenediimides: Toward Supramolecular Cascade n/p-Heterojunctions
    Adam L. Sisson, Naomi Sakai, Natalie Banerji, Alexandre FĂźrstenberg, Eric Vauthey and Stefan Matile
    Angewandte Chemie International Edition, 47 (20) , 2008, p3727-3729
    DOI:10.1002/anie.200800203 | unige:72 | Abstract | Article HTML | Article PDF
  
Supramolecular 3D organization on gold with interdigitating intra- and interlayer recognition motifs (see picure, black p-oligophenyl rods; red, blue naphthalenediimide (NDI) stacks) is designed to access supramolecular cascade n/p-heterojunctions or the adaptable directionality needed to control fill factors in current-voltage curves.
  
The photophysics and excited-state dynamics of nitroperylene (NPe) in solvents of various polarities and viscosities, including a room-temperature ionic liquid, have been investigated by femtosecond-resolved transient absorption spectroscopy. The excited-state absorption spectrum was found to depend substantially on solvent polarity. In the most polar solvents, it is very similar to that of the NPe radical cation generated upon bimolecular quenching by an electron acceptor, denoting a substantial charge-transfer character of the S1 state. Contrary to smaller nitroaromatic compounds, NPe in the S1 state does not undergo ultrafast intersystem crossing (ISC) but decays mainly by internal conversion (IC). In nonprotic solvents, IC involves low-frequency modes with large amplitude motion associated with the nitro group and depends on both the solvent viscosity and polarity. It takes place on a 100 ps time scale in acetonitrile, while in cyclohexane, it is slow enough for ISC to become competitive. Moreover, both the fluorescence quantum yield and the excited-state dynamics were found to differ, depending on which side of the S0−S1 absorption band excitation was performed. This dependence is explained by the inhomogeneous nature of the absorption spectrum arising from a distribution of twist angles of the nitro group relative to the aromatic plane. On the other hand, such excitation wavelength effects were not observed in protic solvents, where the excited-state lifetime was found to be substantially shorter than that in nonprotic solvents. This behavior is rationalized in terms of a H-bonding interaction, which limits the torsional disorder of NPe and favors ultrafast nonradiative deactivation of the excited state. Transient absorption measurements performed for comparative purpose with nitropyrene in acetonitrile confirm the occurrence of ultrafast ISC in smaller nitroaromatic compounds.
  • The Restricted Active Space Followed by Second Order Perturbation Theory Method: Theory and Application to the Study of CuO2 and Cu2O2 Systems
    Per Ă…ke Malmqvist, Kristine Pierloot, Abdul Rehaman Moughal Shahi, Christopher J. Cramer and Laura Gagliardi
    Journal of Chemical Physics, 128 (20) , 2008, p204109
    DOI:10.1063/1.2920188 | unige:66 | Abstract | Article HTML | Article PDF | Article PS (gzipped)
A multireference second-order perturbation theory using a restricted active space self-consistent field wave function as reference (RASPT2/RASSCF) is described. This model is particularly effective for cases where a chemical system requires a balanced orbital active space that is too large to be addressed by the complete active space self-consistent field model with or without second-order perturbation theory (CASPT2 or CASSCF, respectively). Rather than permitting all possible electronic configurations of the electrons in the active space to appear in the reference wave function, certain orbitals are sequestered into two subspaces that permit a maximum number of occupations or holes, respectively, in any given configuration, thereby reducing the total number of possible configurations. Subsequent second-order perturbation theory captures additional dynamical correlation effects. Applications of the theory to the electronic structure of complexes involved in the activation of molecular oxygen by mono- and binuclear copper complexes are presented. In the mononuclear case, RASPT2 and CASPT2 provide very similar results. In the binuclear cases, however, only RASPT2 proves quantitatively useful, owing to the very large size of the necessary active space.
 
Some dimetal fullerenes M2@C60 (M = Cr, Mo, W) have been studied with computational quantum chemistry methods. The transition metal diatomic molecules Cr2, Mo2, W2 form exohedral complexes with C60, while U2 forms a highly symmetric endohedral compound and it is placed in the center of the C60 cavity. This highly symmetric structure is an artifact due to the small size of the C60 cavity, which constrains U2 at the center. If a larger cavity is used, like C70 or C84, U2 preferentially binds the internal walls of the cavity and the U−U bond no longer exists.
  
  • Large Differences in Secondary Metal - Arene Interactions in the Transition Metal Dimers ArMMAr (Ar = Terphenyl; M = Cr, Fe or Co): Implications for Cr-Cr Quintuple Bonding
    Giovanni La Macchia, Laura Gagliardi, Philip P. Power and Marcin Brynda
    Journal of the American Chemical Society, 130 (15) , 2008, p5104-5114
    DOI:10.1021/ja0771890 | unige:68 | Abstract | Article HTML | Article PDF
Quantum mechanical calculations, using both CASPT2 and DFT methods, for the model systems (MeMMMe, PhMMPh, (MeMMMe)(C6H6)2, Ar§MMAr§, Ar#MMAr#; M = Cr, Fe, Co; Ar§ = C6H4-2(C6H5), Ar# = C6H3-2,6(C6H3-2,6-Me2)2) are described. These studies were undertaken to provide a multireference description of the metal−metal bond in the simple dimers MeMMMe and PhMMPh (M = Cr, Fe, Co) and to determine the extent of secondary metal−arene interaction involving the flanking aryl rings of the terphenyl ligands in quintuply bonded Ar′CrCrAr′ (Ar′ = C6H3-2,6(C6H3-2,6-Pri2)2). We show that in the Cr−Cr species the Cr−arene interaction is a feeble one that causes only a small weakening of the quintuple bond. In sharp contrast, in the analogous Fe and Co species strong η6-arene interactions that preclude significant metal−metal bonding are predicted.
  • Stereoelectronic Effects on Molecular Geometries and State-Energy Splittings of Ligated Monocopper Dioxygen Complexes
    Christopher J. Cramer, Jeffrey R. Gour, Armagan Kinal, Marta Wloch, Piotr Piecuch, Abdul Rehaman Moughal Shahi and Laura Gagliardi
    Journal of Physical Chemistry A, 112 (16) , 2008, p3754-3767
    DOI:10.1021/jp800627e | unige:22 | Abstract | Article HTML | Article PDF
The relative energies of side-on versus end-on binding of molecular oxygen to a supported Cu(I) species, and the singlet versus triplet nature of the ground electronic state, are sensitive to the nature of the supporting ligands and, in particular, depend upon their geometric arrangement relative to the O2 binding site. Highly correlated ab initio and density functional theory electronic structure calculations demonstrate that optimal overlap (and oxidative charge transfer) occurs for the side-on geometry, and this is promoted by ligands that raise the energy, thereby enhancing resonance, of the filled Cu dxz orbital that hybridizes with the in-plane π* orbital of O2. Conversely, ligands that raise the energy of the filled Cu dz2 orbital foster a preference for end-on binding as this is the only mode that permits good overlap with the in-plane O2 π*. Because the overlap of Cu dz2 with O2 π* is reduced as compared to the overlap of Cu dxz with the same O2 orbital, the resonance is also reduced, leading to generally more stable triplet states relative to singlets in the end-on geometry as compared to the side-on geometry, where singlet ground states become more easily accessible once ligands are stronger donors. Biradical Cu(II)-O2 superoxide character in the electronic structure of the supported complexes leads to significant challenges for accurate quantum chemical calculations that are best addressed by exploiting the spin-purified M06L local density functional, single-reference completely renormalized coupled-cluster theory, or multireference second-order perturbation theory, all of which provide predictions that are qualitatively and quantitatively consistent with one another.
  • A Dichromium(II) Bis(η8-pentalene) Double-Sandwich Complex with a Spin Equilibrium: Synthetic, Structural, Magnetic and Theoretical Studies
    Gabor Balazs, F.Geoffrey N. Cloke, Laura Gagliardi, Jennifer C. Green, Andrew Harrison, Peter B. Hitchcock, Abdul Rehaman Moughal Shahi and Owen T. Summerscales
    Organometallics, 27 (9) , 2008, p2013-2020
    DOI:10.1021/om701153t | unige:65 | Abstract | Article HTML | Article PDF
  
The new bis(pentalene) complex Cr2(η5:η5-C8H41,4-SiiPr3)2 has been synthesized and characterized; it is found to exhibit paramagnetism at room temperature, and solid-state magnetic studies show that the dimer is best modeled as containing a pair of antiferromagnetically interacting S = ½ centers with the separation between the singlet ground state and triplet excited state being 2.23 kJ mol−1. Structural data show a Cr−Cr distance of 2.2514(15) Å, consistent with a strong metal−metal interaction. The bonding has been further investigated by density functional, hybrid, and CASPT2 methods. The metal−metal interaction is best described by a double bond with each metal having an 18-electron count. Theory predicts the singlet and triplet states to lie close in energy but puts the triplet state at a slightly lower energy than the singlet. The energy difference predicted by CASPT2 is closest to the experimental value.
  
  • Synthesis of a stable radical anion via the one electron reduction of a 1,1-bis-phosphinosulfide alkene derivative
    Thibault Cantat, FrĂŠdĂŠric Biaso, AurĂŠlien Momin, Louis Ricard, Michel Geoffroy, Nicolas MĂŠzailles and Pascal Le Floch
    ChemComm, (7) , 2008, p874-876
    DOI:10.1039/b715380e | unige:42 | Abstract | Article HTML | Article PDF
A new type of stable radical ligand featuring a 1,1-bis-phosphinosulfide alkene backbone has been prepared and characterized on the basis of X-ray diffraction, EPR and DFT studies.
Laser-ablated Th atoms react with molecular hydrogen to give thorium hydrides and their dihydrogen complexes during condensation in excess neon and hydrogen for characterization by matrix infrared spectroscopy. The ThH2, ThH4, and ThH4(H2)x (x = 1−4) product molecules have been identified through isotopic substitution (HD, D2) and comparison to frequencies calculated by density functional theory and the coupled-cluster, singles, doubles (CCSD) method and those observed previously in solid argon. Theoretical calculations show that the Th−H bond in ThH4 is the most polarized of group 4 and uranium metal tetrahydrides, and as a result, a strong attractive “dihydrogen” interaction was found between the oppositely charged hydride and H2 ligands ThH4(H2)x. This bridge-bonded dihydrogen complex structure is different from that recently computed for tungsten and uranium hydride super dihydrogen complexes but is similar to that recently called the “dihydrogen bond” (Crabtree, R. H. Science 1998, 282, 2000). Natural electron configurations show small charge flow from the Th center to the dihydrogen ligands.
 
The codeposition of laser-ablated tungsten atoms with neat hydrogen at 4 K forms a single major product with a broad 2500 cm-1 and sharp 1860, 1830, 1782, 1008, 551, and 437 cm-1 absorptions, which are assigned to the WH4(H2)4 complex on the basis of isotopic shifts and agreement with isotopic frequencies calculated by density functional theory. This D2d structured complex was computed earlier to form exothermically from W atoms and hydrogen molecules. Annealing the matrix allows hydrogen to evaporate and the complex to aggregate and ultimately to decompose. Comparison of the H−H stretching mode at 2500 cm-1 and the W−H2 stretching mode at 1782 cm-1 with 2690 and 1570 cm-1 values for the Kubas complex W(CO)3(PR3)2(H2) suggests that the present physically stable WH4(H2)4 complex has more strongly bound dihydrogen ligands. Our CASPT2 calculations suggest a 15 kcal/mol average binding energy per dihydrogen molecule in the WH4(H2)4 complex.
Gradient-dependent approximations to the functional of the kinetic energy of non-interacting electrons (Ts[ρ]), which reflect various properties of the exact functional, are considered. For specially constructed pairs of electron densities, for which the analytic expression for the differences of Ts[ρ] is known, it is shown that the accuracy of the quantities derivable from a given approximation to Ts[ρ]: energy differences and their functional derivatives, does not reflect that of Ts[ρ] itself. The comparisons between the exact values of the kinetic energy in such cases are proposed as an independent condition/criterion for appraisal of approximations to Ts[ρ].
  • Synthesis of new ethynylbipyridine-linked mono- and bis-tetrathiafulvalenes: electrochemical, spectroscopic, and Ru(II)-binding studies
    Christine Goze, Shi-Xia Liu, Claudia Leiggener, Lionel Sanguinet, Eric Levillain, Andreas Hauser and Silvio Decurtins
    Tetrahedron, 64 (7) , 2008, p1345-1350
    Keywords: Tetrathiafulvalene; Ruthenium(II) complex; Photophysical properties; Cyclic voltammetry; Donoreacceptor systems
    DOI:10.1016/j.tet.2007.11.056 | unige:32 | Abstract | Article HTML | Article PDF
Two new ethynylbipyridine-linked mono- and bis-tetrathiafulvalene (TTF) derivatives, together with a Ru(II) complex, were synthesized using Sonogashira coupling reactions and characterized by UV/vis spectroscopy and cyclic voltammetry. They display a clear electrochemically amphoteric behavior consisting of two reversible single-electron oxidation waves (typical for TTF derivatives) and one reversible single-electron reduction wave (bpy) and act as donor–acceptor (D–A) systems. Furthermore, for the Ru(II) complex, a quite intense fluorescence originating from the 3MLCT state is observed.
  
  • Sm2+ as a probe of crystal field in fluorides and fluorohalides: Effect of pressure and temperature
    Tiphaine Penhouet and Hans Hagemann
    Journal of Alloys and Compounds, 451 (1-2) , 2008, p74-76
    Keywords: Alkaline earth fluorides; Samarium; Luminescence spectroscopy; f–f transitions; Crystal structure
    DOI:10.1016/j.jallcom.2007.04.121 | unige:31 | Abstract | Article HTML | Article PDF
The luminescence of Sm2+ substituting for Sr2+ or Ba2+ has been studied in SrFBr, BaFBr, BaFI and SrAlF5.

The pressure induced shifts of the intra-configurational 5D0,1 → 7F0,1,2 observed in the MFX crystals are about three times larger than those observed in ruby, confirming thus some potential of these systems as pressure sensors.

The comparison of excitation spectra in MFX shows that the position of the lowest 4f55d1 band shifts strongly to the red passing from SrFBr to BaFI. In BaFI, one observes simultaneously intra-configurational and inter-configurational emission.

The non-degenerate 5D0 → 7F0 emission of Sm2+ in SrAlF5 confirms the presence of four crystallographic sites for Sr. Site selective spectra show clear differences for the different sites. Spectra as a function of pressure reveal different pressure shifts for the different sites.

  • Solvent and Spectral Effects in the Ultrafast Charge Recombination Dynamics of Excited Donor-Acceptor Complexes
    Serguei V. Feskov, Vladimir N. Ionkin, Anatoly I. Ivanov, Hans Hagemann and Eric Vauthey
    Journal of Physical Chemistry A, 112 (4) , 2008, p594-601
    DOI:10.1021/jp709587x | unige:30 | Abstract | Article HTML | Article PDF
The charge recombination dynamics of excited donor−acceptor complexes consisting of hexamethylbenzene (HMB), pentamethylbenzene (PMB), and isodurene (IDU) as electron donors and tetracyanoethylene (TCNE) as electron acceptor in various polar solvents has been investigated within the framework of the stochastic approach. The model accounts for the reorganization of intramolecular high-frequency vibrational modes as well as for the solvent reorganization. All electron-transfer energetic parameters have been determined from the resonance Raman data and from the analysis of the stationary charge transfer absorption band, while the electronic coupling has been obtained from the fit to the charge recombination dynamics in one solvent. It appears that nearly 100% of the initially excited donor−acceptor complexes recombine in a nonthermal (hot) stage when the nonequilibrium wave packet passes through a number of term crossings corresponding to transitions toward vibrational excited states of the electronic ground state. Once all parameters of the model have been obtained, the influence of the dynamic solvent properties (solvent effect) and of the carrier frequency of the excitation pulse (spectral effect) on the charge recombination dynamics have been explored. The main conclusions are (i) the model provides a globally satisfactory description for the IDU/TCNE complex although it noticeably overestimates the spectral effect, (ii) the solvent effect is quantitatively well described for the PMB/TCNE and HMB/TCNE complexes but the model fails to reproduce their spectral effects, and (iii) the positive spectral effect observed with the HMB/TCNE complex cannot be described within the framework of two-level models and the charge redistribution in the excited complexes should most probably be taken into account.
Variational methods to treat a many-electron system embedded in the environment, which is represented by means of only its electron density, are considered. It is shown that the embedding operator is a local potential in the case where the electron-electron repulsion is treated exactly and the trial embedded wave function takes the multideterminantal form with a fixed number of determinants. The local embedding potential is constructed by imposing that it leads to the same electron density as the one which minimizes the Hohenberg-Kohn functional. For the limiting cases of single-determinant and configuration interaction forms of the embedded wave function, the expressions for the local embedding potential using commonly known density functionals are given. The relation between the derived local embedding potential and the effective embedding potential in the case of the embedded Kohn-Sham system [T. A. Wesołowski and A. Warshel, J. Phys. Chem. 97, 8050 (1993)] is discussed in detail.
 
SrMgF4 was prepared by precipitation in aqueous solution. Alkaline earth metal acetates and ammonium fluoride were used as precursors. After drying and annealing the samples at different temperatures and times, single phase SrMgF4 was obtained. By varying the annealing conditions, the mean crystallite size could be adjusted. Furthermore, the thermally treated samples displayed UV-excited intensive broad band luminescence in the visible region. The emissions colour and intensity can be adjusted by the tempering conditions. X-Ray diffraction, TEM-microscopy, fluorescence and IR-spectroscopy were used for analysis.
Americium and curium oxides AmOn and CmOn (n = 1, 2) were studied using state-of-the-art multiconfigurational, relativistic, quantum chemical methods. Spectroscopic properties for the ground state and several excited states of the four target compounds were determined. The computed dissociation energy of AmO (4.6 eV) agrees fairly well with estimates derived from experimental studies (5.73 Âą 0.37 eV) while the computed dissociation energy of CmO (7.1 eV) agrees well with the experimental value (7.5 eV). The computed ionization energy of AmO (6.3 eV) is in good agreement with the current experimental value (5.9 Âą 0.2 eV).
  • Theoretical prediction of linear free energy relationships using proton nucleomers
    Giovanni La Macchia, Laura Gagliardi, Geoffrey S. Carlson, Ashley N. Jay, Erik Davis and Christopher J. Cramer
    Journal of Physical Organic Chemistry, 21 (2) , 2008, p136-145
    DOI:10.1002/poc.1297 | unige:27 | Abstract | Article PDF
Values of σ and σ+, for use in linear free energy relationships, are determined for para hydrogen atoms having nuclear charges other than 1 (nucleomers). Hammett ρ values for a variety of free energies of activation, reaction, and other extrathermodynamic properties (e.g., vibrational frequencies) are computed therefrom and compared to those computed using typical para functional groups. The nucleomer correlations show excellent qualitative agreement with standard correlations but the quantitative agreement is less good, typically underestimating the standard ρ-value by 10-60%.
  
Biological homochirality on earth and its tremendous consequences for pharmaceutical science and technology has led to an ever increasing interest in the selective production, the resolution and the detection of enantiomers of a chiral compound. Chiral surfaces and interfaces that can distinguish between enantiomers play a key role in this respect as enantioselective catalysts as well as for separation purposes. Despite the impressive progress in these areas in the last decade, molecular-level understanding of the interactions that are at the origin of enantiodiscrimination are lagging behind due to the lack of powerful experimental techniques to spot these interactions selectively with high sensitivity. In this article, techniques based on infrared spectroscopy are highlighted that are able to selectively target the chiral properties of interfaces. In particular, these methods are the combination of Attenuated Total Reflection InfraRed (ATR-IR) with Modulation Excitation Spectroscopy (MES) to probe enantiodiscriminating interactions at chiral solid–liquid interfaces and Vibrational Circular Dichroism (VCD), which is used to probe the structure of chirally-modified metal nanoparticles. The former technique aims at suppressing signals arising from non-selective interactions, which may completely hide the signals of interest due to enantiodiscriminating interactions. Recently, this method was successfully applied to investigate enantiodiscrimination at self-assembled monolayers of chiral thiols on gold surfaces. The nanometer size analogues of the latter—gold nanoparticles protected by a monolayer of a chiral thiol—are amenable to VCD spectroscopy. It is shown that this technique yields detailed structural information on the adsorption mode and the conformation of the adsorbed thiol. This may also turn out to be useful to clarify how chirality can be bestowed onto the metal core itself and the nature of the chirality of the latter, which is manifested in the metal-based circular dichroism activity of these nanoparticles.
  • Probing chiral recognition in liquid chromatography by absolute configuration modulation ATR-IR spectroscopy
    R. Wirz, D. Ferri, T. BĂźrgi and A. Baiker
    Spectroscopy Europe, 19 (1) , 2007, p8-16
    unige:14678
  • Photocatalysis of dicarboxylic acids over TiO2: An in situ ATR-IR study
    Igor Dolamic and Thomas BĂźrgi
    Journal of Catalysis, 248 (2) , 2007, p268-276
    Keywords: in situ spectroscopy; photocatalysis; attenuated total reflection; TiO2, malonic acid
    DOI:10.1016/j.jcat.2007.03.020 | unige:14767 | Article HTML | Article PDF
  
Attenuated total reflection infrared (ATR-IR) spectroscopy in a flow-through cell was used to study the photocatalytic mineralization of malonic acid and succinic acid over P25 TiO2 in situ. The experiments were performed in water at concentrations of 1.5×10−4 mol/L and pH 3.5 at room temperature. Changes on the catalyst surface were observed within a few minutes. The first step in the mineralization of malonic acid is a photo-Kolbe reaction of adsorbed malonate. Part of the resulting C2 species is converted into oxalate and finally into carbon dioxide, and part desorbs from the surface. The branching ratio for the two pathways is 50:50. The mineralization reaction was also observed in the absence of dissolved oxygen, but at a slower rate. In the presence of dissolved 18O2, labeled oxygen was incorporated into the adsorbed oxalate. A dominant pathway in the mineralization of succinic acid involves the transformation to oxalate via malonate. Thus, it is proposed that a favored pathway for dicarboxylic acid mineralization is a photo-Kolbe reaction, followed by oxidation of the carbon-centered radical to a carboxylate, which corresponds to the overall formal shortening of the alkyl chain by one CH2 unit.
  • Synthesis and application of gold nanoparticles
    Qiao-Ling Li and Thomas BĂźrgi
    Xiandai Huagong (Modern Chemical Industry), 27 Suppl , 2007, p378-381
    unige:14664
  
We introduce zipper assembly as a simple and general concept to create complex functional architectures on conducting surfaces. Rigid-rod π-stack architecture composed of p-oligophenyl rods and blue naphthalenediimide (NDI) stacks is selected as an example. First, short p-quaterphenyl initiators with four anionic NDIs are deposited on gold. Then, long p-octiphenyl propagators with eight cationic NDIs are added. The lower half of the propagator π-stacks with the initiator, whereas the upper half of the molecule remains free. These cationic sticky-ends zip up with anionic propagators to produce anionic sticky-ends, and so on. Zipper assembly on gold nanoparticles is demonstrated by the appearance of the absorption of face-to-face NDI π-stacks and the shift of the surface plasmon resonance band with increasing layer thickness. Complete inhibition by zipper capping demonstrates that zipper assembly affords complex architectures that are more ordered than those obtained by conventional layer-by-layer (LBL) approaches. Zipper assembly on gold electrodes produces increasing photocurrents with increasing number of zipped layers. The photocurrents obtained by this method are much higher than those obtained by conventional LBL controls; zipper termination by capping cleanly stops any increase in photocurrent.
  • ESR/DFT study of bis-iminophosphorane cation radicals
    Adil Matni, Leila Boubekeur, Philippe Grosshans, Nicolas MĂŠzailles, GĂŠrald Bernardinelli, Pascal Le Floch and Michel Geoffroy
    Magnetic Resonance in Chemistry, 45 (12) , 2007, p1011-1017
    Keywords: ESR ; DFT ; 1H ; 14N ; 31P ; hyperfine structure ; spin delocalization ; bis-iminophosphoranes ; cation radical
    DOI:10.1002/mrc.2079 | unige:3578 | Abstract | Article PDF
  
Bis-iminophosphoranes containing various types of linkers between two R3P=N moieties were electrochemically oxidized at controlled potential in situ in the electron spin resonance (ESR) cavity. For linkers constituted of phenylenes, conjugated phenylenes or merely a dicyanoethylenic bond, this oxidation led to well-resolved ESR spectra which were characterized by their g values and by their 1H, 14N and 31P isotropic hyperfine constants. These coupling constants agree with those calculated by DFT for the corresponding cation radicals. Experimental and theoretical results clearly indicate that in these species the unpaired electron is mostly delocalized on the bridge and on the nitrogen atoms while the spin density on the phosphorus atoms is particularly small. Cyclic voltammetry and ESR spectra show that the nature of the bridge between the two iminophosphoranes considerably influences the oxidation potential of the compound as well as the stability of the radical cation. Information about the conformation of the precursor containing two Ph3P=N moieties separated by a —C(CN)=C(CN)—group was obtained from its crystal structure.
  • Molecular Characterization and Subcellular Localization of Macrophage Infectivity Potentiator, a Chlamydia trachomatis Lipoprotein
    Laurence Neff, Sawsan Daher, Patrick Muzzin, Ursula Spenato, Fazil GĂźlaçar, Cem Gabay and Sylvette Bas
    Journal of Bacteriology, 189 (13) , 2007, p4739-4748
    DOI:10.1128/JB.01889-06 | unige:3186 | Abstract | Article HTML | Article PDF
Macrophage infectivity potentiator (MIP) was originally reported to be a chlamydial lipoprotein from experiments showing incorporation of radiolabeled palmitic acid into native and recombinant MIP; inhibition of posttranslational processing of recombinant MIP by globomycin, known to inhibit signal peptidase II; and solubility of native MIP in Triton X-114. However, the detailed structural characterization of the lipid moiety on MIP has never been fully elucidated. In this study, bioinformatics and mass spectrometry analysis, as well as radiolabeling and immunochemical experiments, were conducted to further characterize MIP structure and subcellular localization. In silico analysis showed that the amino acid sequence of MIP is conserved across chlamydial species. A potential signal sequence with a contained lipobox was identified, and a recombinant C20A variant was prepared by replacing the probable lipobox cysteine with an alanine. Both incorporation of U-14C-esterified glycerol and [U-14C]palmitic acid and posttranslational processing that was inhibitable by globomycin were observed for recombinant wild-type MIP but not for the recombinant C20A MIP variant. The fatty acid contents of native and recombinant MIP were analyzed by gas chromatography-mass spectrometry, and the presence of amide-linked fatty acids in recombinant MIP was investigated by alkaline methanolysis. These results demonstrated a lipid modification in MIP similar to that of other prokaryotic lipoproteins. In addition, MIP was detected in an outer membrane preparation of Chlamydia trachomatis elementary bodies and was shown to be present at the surfaces of elementary bodies by surface biotinylation and surface immunoprecipitation experiments.
  
  • Optical and structural properties of a Eu(II)-doped silico-aluminate with channel structure and partial site occupation
    Andreas Rief, Franck Kubel and Hans Hagemann
    Zeitschrift fĂźr Naturforschung, 62b (12) , 2007, p1535-1542
    Keywords: disorder, structure refinement, silico-aluminate,luminescence
    unige:3579 | Abstract | Article PDF
A new barium silico-aluminate phase with the stoichiometry Ba13.35(1) Al30.7 Si5.3 O70 has been found and characterized. The compound crystallizes in the space group P63 /m (No. 176) with a = 15.1683(17) Å, c = 8.8708(6) Å, V = 1767.5(4) Å3 , Z = 1, Rw = 0.026, 32 refined parameters. A 3-dimensional matrix of Al/SiO4 tetrahedra with Ba(II) ions located in channels along the c axis builds up the structure. One of these channels is partially filled with Ba(II) ions (CN 6+3) in Wyckoff position 2a, leaving ∼ 1/3 of the positions empty. The second and third type of Ba(II) ions occupy channels orientated along the c axis with CN 4+2+2 and 4+3+1, respectively. The structure shows a rare clustered arrangement of six tetrahedra filled exclusively by Al(III) and therefore is an exception to Loewenstein’s rule. The other tetrahedral positions show an Al to Si ratio of ∼ 4 : 1. The Al/Si–O bond lengths in the tetrahedral Al/Si positions drawn vs. site occupation show linear behavior similar to the prediction by Vegard’s rule for solid solutions. After doping with Eu(II) the compound shows bright orange-yellow luminescence with an unusual large shift of the Eu(II) emission band.
  • Dual Luminescence and Long-lived Charge Separated states in Donor-Acceptor Assemblies based on Tetrathiafulvalene Fused Ruthenium(II)-Polypyridine Complexes
    Claudia Leiggener, Nathalie Dupont, Shi-Xia Liu, Christine Goze, Silvio Decurtins, Elvira Breitler and Andreas Hauser
    Chimia, 61 (10) , 2007, p621-625
    DOI:10.2533/chimia.2007.621 | unige:3580 | Abstract | Article PDF
  
The creation of long-lived charge-separated states in donor-acceptor assemblies has been the goal of many studies aimed at mimicking the primary processes in photosynthesis. Here we present such assemblies based on tetrathiafulvalene (TTF) as electron donor and a dipyridophenazine (dppz) unit as electron acceptor in the form of a fused ligand (TTF-dppz) coordinated to ruthenium(II) via the dipyrido coordination site and with 2,2′-bipyridine (bpy) as auxiliary ligand, namely [Ru(bpy)3−x(TTF-dppz)x]2+ (x = 1−3). For x = 2, irradiation into the metal to dppz charge transfer transition results in electron transfer from TTF to ruthenium, thus creating a charge-separated state best described by [(TTF+-dppz)Ru(dppz−-TTF)(bpy)]2+ with a lifetime of 2.5 μs in dichloromethane.
  
  • Ultrafast Excited-State Dynamics in Biological Environments
    Alexandre FĂźrstenberg and Eric Vauthey
    Chimia, 61 (10) , 2007, p617-620
    DOI:10.2533/chimia.2007.617 | unige:3581 | Abstract | Article PDF
We discuss and illustrate by several examples how the ultrafast excited-state dynamics of a chromophore can be altered when changing its environment from a homogenous solution to a biological molecule such as proteins or nucleic acids.
  • Copper(I)-α-Ketocarboxylate Complexes: Characterization and O2 Reactions That Yield Copper-Oxygen Intermediates Capable of Hydroxylating Arene
    Sungjun Hong, Stefan M. Huber, Laura Gagliardi, Christopher J. Cramer and William B. Tolman
    Journal of the American Chemical Society, 129 (46) , 2007, p14190-14192
    DOI:10.1021/ja0760426 | unige:3583 | Abstract | Article HTML | Article PDF
  
A series of copper(I)−α-ketocarboxylate complexes have been prepared and shown to exhibit variable coordination modes of the α-ketocarboxylate ligand. Reaction with O2 induces decarboxylation of this ligand, and the derived copper−oxygen intermediate(s) has been intercepted, resulting in hydroxylation of an arene substituent on the supporting N-donor ligand. Theoretical calculations have provided intriguing mechanistic notions for the process, notably implicating hydroxylation pathways that involve novel [CuI−OOC(O)R] and [CuII−O-• ↔ CuIII = O2-]+ species.
Molecular dynamics simulations of Cm(III) in water were performed at two different temperatures, namely, T = 300 K and T = 473 K. Fully ab initio intermolecular potentials were employed. At the lower temperature, T = 300 K, nine water molecules coordinate preferentially the Cm(III) ion in the first coordination sphere, while at the higher temperature, T = 473 K, the preferential coordination number is eight instead of nine. The number of water molecules in the second coordination sphere is not uniquely defined, but the most probable number is 16.
  
The excited-state dynamics of the DNA intercalator YO-PRO-1 and of three derivatives has been investigated in water and in DNA using ultrafast fluorescence spectroscopy. In the free form, the singly charged dyes exist both as monomers and as H-dimers, while the doubly charged dyes exist predominantly as monomers. Both forms are very weakly fluorescent: the monomers because of ultrafast nonradiative deactivation, with a time constant on the order of 3−4 ps, associated with large amplitude motion around the methine bridge, and the H-dimers because of excitonic interaction. Upon intercalation into DNA, large amplitude motion is inhibited, H-dimers are disrupted, and the molecules become highly fluorescent. The early fluorescence dynamics of these dyes in DNA exhibits substantial differences compared with that measured with their homodimeric YOYO analogues, which are ascribed to dissimilarities in their local environment. Finally, the decay of the fluorescence polarization anisotropy reveals ultrafast hopping of the excitation energy between the intercalated dyes. In one case, a marked change of the depolarization dynamics upon increasing the dye concentration is observed and explained in terms of a different binding mode.
 
The topology of the ground-state potential energy surface of M(CN)6 with orbitally degenerate 2T2g (M = TiIII (t2g1), FeIII and MnII (both low-spin t2g5)) and 3T1g ground states (M = VIII (t2g2), MnIII and CrII (both low-spin t2g4)) has been studied with linear and quadratic Jahn−Teller coupling models in the five-dimensional space of the εg and τ2g octahedral vibrations (Tg⊗(εg+τ2g) Jahn−Teller coupling problem (Tg = 2T2g, 3T1g)). A procedure is proposed to give access to all vibronic coupling parameters from geometry optimization with density functional theory (DFT) and the energies of a restricted number of Slater determinants, derived from electron replacements within the t2g1,5 or t2g2,4 ground-state electronic configurations. The results show that coupling to the τ2g bending mode is dominant and leads to a stabilization of D3d structures (absolute minima on the ground-state potential energy surface) for all complexes considered, except for [Ti(CN)6]3-, where the minimum is of D4h symmetry. The Jahn−Teller stabilization energies for the D3d minima are found to increase in the order of increasing CN−M π back-donation (TiIII < VIII < MnIII < FeIII < MnII < CrII). With the angular overlap model and bonding parameters derived from angular distortions, which correspond to the stable D3d minima, the effect of configuration interaction and spin−orbit coupling on the ground-state potential energy surface is explored. This approach is used to correlate Jahn−Teller distortion parameters with structures from X-ray diffraction data. Jahn−Teller coupling to trigonal modes is also used to reinterpret the anisotropy of magnetic susceptibilities and g tensors of [Fe(CN)6]3-, and the 3T1g ground-state splitting of [Mn(CN)6]3-, deduced from near-IR spectra. The implications of the pseudo Jahn−Teller coupling due to t2g−eg orbital mixing via the trigonal modes (τ2g) and the effect of the dynamic Jahn−Teller coupling on the magnetic susceptibilities and g tensors of [Fe(CN)6]3- are also addressed.
  
  • Optical investigation of spin-crossover in cobalt(II) bis-terpy complexes
    Cristian Enachescu, Itana Krivokapic, Mohamed Zerara, Jose Antonio Real, Nahid Amstutz and Andreas Hauser
    Inorganica Chimica Acta, 360 (13) , 2007, p3945-3950
    DOI:10.1016/j.ica.2007.06.022 | unige:3585 | Abstract | Article HTML | Article PDF
The spin transition of the [Co(terpy)2]2+ complex (terpy = 2,2′:6′,2″-terpyridine) is analysed based on experimental data from optical spectroscopy and magnetic susceptibility measurements. The single crystal absorption spectrum of [Co(terpy)2](ClO4)2 shows an asymmetric absorption band at 14 400 cm−1 with an intensity typical for a spin-allowed d–d transition and a temperature behaviour typical for a thermal spin transition. The single crystal absorption spectra of suggest that in this compound, the complex is essentially in the high-spin state at all temperatures. However, the increase in intensity observed in the region of the low-spin MLCT transition with increasing temperature implies an unusual partial thermal population of the low-spin state of up to about 10% at room temperature. Finally, high-spin → low-spin relaxation curves following pulsed laser excitation for [Co(terpy)2](ClO4)2 dispersed in KBr discs, and as a comparison for the closely related [Co(4-terpyridone)2](ClO4)2 spin-crossover compound are given.
  • Photoinduced Energy Transfer Processes within Dyads of Metallophthalocyanines Compactly Fused to a Ruthenium(II) Polypyridine Chromophore
    Marco Haas, Shi-Xia Liu, Axel Kahnt, Claudia Leiggener, Dirk M. Guldi, Andreas Hauser and Silvio Decurtins
    Journal of Organic Chemistry, 72 (20) , 2007, p7533-7543
    DOI:10.1021/jo0710477 | unige:3188 | Abstract | Article HTML | Article PDF
An unsymmetric, peripherally octasubstituted phthalocyanine (Pc) 1, which contains a combination of dipyrido[3,2-f:2‘,3‘-h] quinoxaline and 3,5-di-tert-butylphenoxy substituents, has been obtained via a statistical condensation reaction of two corresponding phthalonitriles. Synthetic procedures for the selective metalation of the macrocyclic cavity and the periphery of 1 were developed, leading to the preparation of the key precursor metallophthalocyanines 3−5 in good yields. Two different strategies were applied to the synthesis of compact dyads MPc−Ru(II) 6−8 (M = Mg(II), Co(II), Zn(II)). Intramolecular electronic interactions in these dyads were studied by absorption, emission, and transient absorption spectroscopy. Upon photoexcitation, these dyads exhibit efficient intramolecular energy transfer from the Ru(II) chromophore to the MPc moiety.
  • Transition Metal-and Actinide-Containing Systems Studied with Multiconfigurational Quantum Chemical Methods
    Laura Gagliardi
    in "Reviews in Computational Chemistry" Kenny B. Lipkowitz and Thomas R. Cundari, John Wiley & Sons, Inc., Hoboken, NJ, 25 , 2007, p249-284
    unige:4035
 
The vibrational spectra of UBz and ThBz have been measured in solid argon. Complementary quantum chemical calculations have allowed the assignments of the vibrational spectra. According to the calculations, AcBz are stable molecules, as well as other species like BzAcBz and BzAc2Bz. Experimentally, there is no evidence for the sandwich compounds BzAcBz and BzAc2Bz due to the limitations in the reagent concentrations.
Capacitively coupled contactless conductivity detection (C4D) is a new technique providing high sensitivity in capillary electrophoresis (CE) especially for small ions that can otherwise only be determined with indirect methods. In this work, direct determination and validation of valproic acid (VPA) in biological fluids was achieved using CE with C4D. VPA is of pharmacological interest because of its use in epilepsy and bipolar disorder. The running electrolyte solution used consisted of 10 mM 2-(N-morpholino)ethane sulfonic acid (MES)/dl-histidine (His) and 50 μM hexadecyltrimethylammonium bromide (HTAB) at pH 6.0. Caproic acid (CA) was selected as internal standard (IS). Analyses of VPA in serum, plasma and urine samples were performed in less than 3 min. The interference of the sample matrix was reduced by deproteinization of the sample with acetonitrile (ACN). The effect of the solvent type and ratio on interference was investigated. The limits of detection (LOD) and quantitation (LOQ) of VPA in plasma samples were determined as 24 and 80 ng/ml, respectively. The method is linear between the 2 and 150 μg/ml, covering well the therapeutic range of VPA (50–100 μg/ml).
  
  • Acid catalysed backbone rearrangement of cholesta-2,4,6-triene: On the origin of ring A and ring B aromatic steroids in recent sediments
    Patrick SchĂźpfer, Yvan Finck, FrĂŠdĂŠric Houot and Fazil O. GĂźlaçar
    Organic Geochemistry, 38 (4) , 2007, p671-681
    DOI:10.1016/j.orggeochem.2006.11.004 | unige:3587 | Abstract | Article HTML | Article PDF
Rearrangement of cholesta-2,4,6-triene in the presence of p-toluenesulfonic acid in acetic acid at 70 °C leads to 4-methyl-19-nor-cholesta-1,3,5(10)-triene and 1(10 → 6)-abeo-14β-cholesta-5,7,9(10)-triene in less than 2 h. Postulated mechanisms of formation of these products are supported by molecular mechanics calculations of the relative stabilities of reaction intermediates. The results suggest that Δ5,7-sterols, the most common natural precursors of triunsaturated steroidal hydrocarbons in contemporary sediments, constitute another major source for monoaromatic A and B steroids in addition to Δ5-sterols.
Interaction energies for a representative sample of 39 intermolecular complexes are calculated using two computational approaches based on the subsystem formulation of density functional theory introduced by Cortona (Phys. Rev. B 44:8454, 1991), adopted for studies of intermolecular complexes (Wesolowski and Weber in Chem. Phys. Lett. 248:71, 1996). The energy components (exchange-correlation and non-additive kinetic) expressed as explicit density functionals are approximated by means of gradient-free- (local density approximation) of gradient-dependent- (generalized gradient approximation) approximations. The sample of the considered intermolecular complexes was used previously by Zhao and Truhlar to compare the interaction energies derived using various methods based on the Kohn-Sham equations with high-level quantum chemistry results considered as the reference. It stretches from rare gas dimers up to strong hydrogen bonds. Our results indicate that the subsystem-based methods provide an interesting alternative to that based on the Kohn-Sham equations. Local density approximation, which is the simplest approximation for the relevant density functionals and which does not rely on any empirical data, leads to a computational approach comparing favorably with more than twenty methods based on the Kohn-Sham equations including the ones, which use extensively empirical parameterizations. For various types of non-bonding interactions, the strengths and weaknesses of gradient-free and gradient-dependent approximations to exchange-correlation and non-additive kinetic energy density functionals are discussed in detail.
The influence of pressure on the structural and vibrational properties of a2RuH6has been investigated using periodic density functional theory calculations performed at the local density approximation (LDA) and generalized gradient approximation (GGA) levels. At ambient pressure, the calculated structure and vibrational frequencies are in satisfactory agreement with experimental data. The calculated em>P-Vcurves could be fitted with the Vinet equation of state, yielding em>B0=67.6and em>B0=58.5  GPaat the LDA and GGA levels, respectively, and em>B0′=4.0at both theoretical levels. The unit cell parameter is found to decrease faster with increasing pressure than the Ru–H bond length. The calculated pressure dependence of the vibrational frequencies agrees well with experiment for em>ν5(T2g)but not for em>ν9(A1g)
 
Unexpected structural complexity: Well-crystallized Mg(BH4)2 powder is obtained, allowing the structure to be determined from synchrotron X-ray and neutron diffraction data. Mg(BH4)2 is a novel and remarkably complex three-dimensional framework in which each Mg2+ ion (blue) is tetrahedrally coordinated by four [BH4]- tetrahedra (B red, H gray; see picture).
  • Fused Donor–Acceptor Ligands in RuII Chemistry: Synthesis, Electrochemistry and Spectroscopy of [Ru(bpy)3-n(TTF-dppz)n](PF6)2
    Christine Goze, Claudia Leiggener, Shi-Xia Liu, Lionel Sanguinet, Eric Levillain, Andreas Hauser and Silvio Decurtins
    ChemPhysChem, 8 , 2007, p1504-1512
    DOI:10.1002/cphc.200700066 | unige:3192 | Abstract | Article HTML | Article PDF
Three ruthenium(II) polypyridine complexes of general formula [Ru(bpy)3-n(TTF-dppz)n](PF6)2 (n=1-3, bpy=2,2'-bipyridine), with one, two or three redox-active TTF-dppz (4',5'-bis(propylthio)tetrathiafulvenyl[i]dipyrido[3,2-a:2',3'-c]phenazine) ligands, were synthesised and fully characterised. Their electrochemical and photophysical properties are reported together with those of the reference compounds [Ru(bpy)3](PF6)2, [Ru(dppz)3](PF6)2 and [Ru(bpy)2(dppz)](PF6)2 and the free TTF-dppz ligand. All three complexes show intraligand charge-transfer (ILCT) fluorescence of the TTF-dppz ligand. Remarkably, the complex with n=1 exhibits luminescence from the Ru2+dppz metal-to-ligand charge-transfer (3MLCT) state, whereas for the other two complexes, a radiationless pathway via electron transfer from a second TTF-dppz ligand quenches the 3MLCT luminescence. The TTF fragments as electron donors thus induce a ligand-to-ligand charge-separated (LLCS) state of the form TTF-dppz--Ru2+→ -dppz-TTF+. The lifetime of this LLCS state is approximately 2.3 μs, which is four orders of magnitude longer than that of 0.4 ns for the ILCT state, because recombination of charges on two different ligands is substantially slower.
The excited-state dynamics of oligomeric phenyleneethynylenes (OPEs) of various length and substitution has been investigated by femtosecond time-resolved spectroscopy. The fluorescence lifetime of the OPEs decreases with the number of phenyleneethynylene units up to about 9. This effect is due to an increase of the oscillator strength for the S1–S0 transition. Dynamic features occurring within a few tens of picoseconds and ascribed to structural relaxation directly after population of the S1 state can be observed in non-viscous solvents. The effect of torsional disorder on the fluorescence intensity is shown to depend strongly on the nature of the substituent on the phenyl groups. All these effects are qualitatively discussed with a simple exciton model.
  
Design, synthesis and evaluation of advanced rigid-rod π-stack photosystems with asymmetric scaffolds are reported. The influence of push–pull rods on self-organization, photoinduced charge separation and photosynthetic activity is investigated and turns out to be surprisingly small overall.
  • Structure-Fluorescence Contrast Relationship in Cyanine DNA Intercalators: Toward Rational Dye Design
    Alexandre FĂźrstenberg, Todor G. Deligeorgiev, Nikolai I. Gadjev, Aleksey A. Vasilev and Eric Vauthey
    Chemistry - A European Journal, 13 (30) , 2007, p8600-8609
    DOI:10.1002/chem.200700665 | unige:3590 | Abstract | Article HTML | Article PDF
  
The fluorescence enhancement mechanisms of a series of DNA stains of the oxazole yellow (YO) family have been investigated in detail using steady-state and ultrafast time-resolved fluorescence spectroscopy. The strong increase in the fluorescence quantum yield of these dyes upon DNA binding is shown to originate from the inhibition of two distinct processes: 1) isomerisation through large-amplitude motion that non-radiatively deactivates the excited state within a few picoseconds and 2) formation of weakly emitting H-dimers. As the H-dimers are not totally non-fluorescent, their formation is less efficient than isomerisation as a fluorescent contrast mechanism. The propensity of the dyes to form H-dimers and thus to reduce their fluorescence contrast upon DNA binding is shown to depend on several of their structural parameters, such as their monomeric (YO) or homodimeric (YOYO) nature, their substitution and their electric charge. Moreover, these parameters also have a substantial influence on the affinity of the dyes for DNA and on the ensuing sensitivity for DNA detection. The results give new insight into the development and optimisation of fluorescent DNA probes with the highest contrast.
  
  • Investigation of the Influence of Solute-Solvent Interactions on the Vibrational Energy Relaxation Dynamics of Large Molecules in Liquids
    Anatolio Pigliucci, Guillaume Duvanel, Latevi Max Lawson Daku and Eric Vauthey
    Journal of Physical Chemistry A, 111 (28) , 2007, p6135-6145
    DOI:10.1021/jp069010y | unige:3591 | Abstract | Article HTML | Article PDF
The influence of solute−solvent interactions on the vibrational energy relaxation dynamics of perylene and substituted perylenes in the first singlet excited-state upon excitation with moderate (<0.4 eV) excess energy has been investigated by monitoring the early narrowing of their fluorescence spectrum. This narrowing was found to occur on timescales ranging from a few hundreds of femtoseconds to a few picoseconds. Other processes, such as a partial decay of the fluorescence anisotropy and the damping of a low-frequency oscillation due to the propagation of a vibrational wavepacket, were found to take place on a very similar time scale. No significant relationship between the strength of nonspecific solute−solvent interactions and the vibrational energy relaxation dynamics of the solutes could be evidenced. On the other hand, in alcohols the spectral narrowing is faster with a solute having H-bonding sites, indicating that this specific interaction tends to favor vibrational energy relaxation. No relationship between the dynamics of spectral narrowing and macroscopic solvent properties, such as the thermal diffusivity, could be found. On the other hand, a correlation between this narrowing dynamics and the number of low-frequency modes of the solvent molecules was evidenced. All these observations cannot be discussed with a model where vibrational energy relaxation occurs via two consecutive and dynamically well-separated steps, namely ultrafast intramolecular vibrational redistribution followed by slower vibrational cooling. On the contrary, the results indicate that both intra- and intermolecular vibrational energy redistribution processes are closely entangled and occur, at least partially, on similar timescales.
  • Intramolecular Mixed-Valence State Through Silicon or Germanium Double Bridges in Rigid Bis(Tetrathiafulvalenes)
    FrĂŠdĂŠric Biaso, Michel Geoffroy, Enric Canadell, Pascale Auban-Senzier, Eric Levillain, Marc FourmiguĂŠ and Narcis Avarvari
    Chemistry - A European Journal, 13 (19) , 2007, p5394-5400
    Keywords: conducting materials; EPR spectroscopy; main group elements; mixed-valent compounds; tetrathiafulvalenes
    DOI:10.1002/chem.200700237 | unige:3592 | Abstract | Article HTML | Article PDF
  
The synthesis and characterization of two ortho-dimethyltetrathiafulvalene (o-DMTTF)-based rigid dimers containing dimethylsilicon (Me2Si) or dimethylgermanium (Me2Ge) linkers are described. Single-crystal X-ray analysis reveals planar geometry for the central 1,4-disilicon or 1,4-digermanium six-membered rings. DFT calculations provide optimized conformations in agreement with the experimental ones, and also emphasize the role of the heteroatomic linkers in the conjugation between the two redox active units. Cyclic voltammetry measurements show sequential oxidation into radical cation, and then dication species. Solution EPR measurements on the radical-cation species indicate full delocalization of the unpaired electron over both electroactive TTF units, with an associated coupling of 0.42 G with twelve equivalent protons. DFT calculations afford fully planar geometry for the radical-cation species and confirm the experimental isotropic coupling constant. Single-crystal X-ray analyses of two charge-transfer compounds obtained upon chemical oxidation, formulated as [(Me2Si)2(o-DMTTF)2]-1/2[TCNQ]¡1/2[TCNQF4] and [(Me2Ge)2(o-DMTTF)2]¡[TCNQ], demonstrate the occurrence of genuine mixed-valence radical-cation species, as well as a three-dimensional network of short S¡¡¡S intermolecular contacts. Temperature-dependent conductivity measurements demonstrate semiconducting behavior for both charge-transfer compounds, with an increase of the absolute value of the conductivity upon applying external pressure. Band structure calculations reveal peculiar pseudo-two-dimensional electronic structures, also confirming electronic interactions through SiMe2 and GeMe2 bridges.
  • Hohenberg-Kohn-Sham Density Functional Theory: The formal basis for a family of succesful and still evolving computational methods for modelling interactions in complex chemical systems
    T.A. Wesolowski
    in "Molecular Materials with Specific Interactions: Modeling and Design" A.W. Sokalski, Ed., Springer Verlag, 2007, p153-202
    DOI:10.1007/1-4020-5372-X | unige:4034
The emergence of a family of computational methods, known under the label ‘density functional theory∈dex theory! density functional ’ or ‘DFT’, revolutionalized the field of computer modelling of complex molecular systems. Many computational schemes belonging to the DFT family are currently in use. Some of them are designed to be universal (nonempirical) whereas other to treat specific systems and/or properties (empirical). This review starts with the introduction of the formal elements underlying all these methods: Hohenberg-Kohn theorems∈dex theorem! Hohenberg-Kohn , reference system∈dex reference system of noninteracting electrons∈dex reference system! noninteracting electrons , exchange-correlation energy∈dex energy functional! exchange-correlation functional∈dex functional , and the Kohn-Sham equations∈dex equation! Kohn-Sham . The main roads to approximate the exchange-correlation-energy functional based on: local density approximation∈dex approximation! local density (LDA), generalized gradient approximation∈dex approximation! generalized gradient (GGA), meta-GGA∈dex energy functional! exchange-correlation! meta-GGA , and adiabatic connection∈dex adiabatic connection formula (hybrid functionals∈dex energy functional! exchange-correlation! hybrid ), are outlined. The performance of these approximations in describing molecular properties of relevance to intermolecular interaction∈dex interactions! intermolecular s and their interactions with environment in condensed phase (ionization potential∈dex potential! ionization s, electron∈dex electron affinities∈dex electron! affinity , electric moments∈dex electric moment , polarizabilities∈dex polarizability ) is reviewed. Developments concerning new methods situated within the general Hohenberg-Kohn-Sham framework or closely related to it are overviewed in the last section
  
The subsystem formulation of density functional theory is used to obtain equilibrium geometries and interaction energies for a representative set of noncovalently bound intermolecular complexes. The results are compared with literature benchmark data. The range of applicability of two considered approximations to the exchange-correlation- and nonadditive kinetic energy components of the total energy is determined. Local density approximation, which does not involve any empirical parameters, leads to excellent intermolecular equilibrium distances for hydrogen-bonded complexes (maximal error 0.13 Å for NH3−NH3). It is a method of choice for a wide class of weak intermolecular complexes including also dipole-bound and the ones formed by rare gas atoms or saturated hydrocarbons. The range of applicability of the chosen generalized gradient approximation, which was shown in our previous works to lead to good interaction energies in such complexes, where π-electrons are involved in the interaction, remains limited to this group because it improves neither binding energies nor equilibrium geometries in the wide class of complexes for which local density approximation is adequate. An efficient energy minimization procedure, in which optimization of the geometry and the electron density of each subsystem is made simultaneously, is proposed and tested.
  • Excited-State Dynamics of Donor-Acceptor Bridged Systems Containing a Boron-Dipyrromethene Chromophore: Interplay between Charge Separation and Reorientational Motion
    Guillaume Duvanel, Natalie Banerji and Eric Vauthey
    Journal of Physical Chemistry A, 111 (25) , 2007, p5361-5369
    DOI:10.1021/jp071560o | unige:3594 | Abstract | Article HTML | Article PDF
  
The excited-state dynamics of a series of electron donor−acceptor bridged systems (DABS) consisting of a boron−dipyrromethene chromophore covalently linked to a dinitro-substituted triptycene has been investigated using femtosecond time-resolved spectroscopy. The chromophores differ by the number of bromine atom substituents. The fluorescence lifetime of the DABS without any bromine atom is strongly reduced when going from toluene to polar solvents, this shortening being already present in chloroform. This effect is about 10 times weaker with a single bromine atom and negligible with two bromine atoms on the chromophore. The excited-state lifetime shortening is ascribed to a charge transfer from the excited chromophore to a nitrobenzene moiety, the driving force of this process depending on the number of bromine substituents. The occurrence of this process is further confirmed by the investigation of the excited-state dynamics of the chromophore alone in pure nitrobenzene. Surprisingly, no correlation between the charge separation time constant and the dielectric properties of the solvents could be observed. However, a good correlation between the charge separation time constant and the diffusional reorientation time of the chromophore alone, measured by fluorescence anisotropy, was found. Quantum chemistry calculations suggest that quasi-free rotation about the single bond linking the chromophore to the triptycene moiety permits a sufficient coupling of the donor and the acceptor to ensure efficient charge separation. The charge separation dynamics in these molecules is thus controlled by the reorientational motion of the donor relative to the acceptor.
  
[M(CO)4PPh3]•− (M = Mo, W) were trapped at 77 K in X-irradiated single crystals of M(CO)5PPh3 and studied by EPR. Structures of [M(CO)4PPh3]•− (M = Cr, Mo, W) were optimized by DFT; predicted g and 31P-hyperfine tensors agree with experiments for M = Mo, W. The anions adopt a slightly distorted pyramidal structure with PPh3 in basal position and the spin mostly delocalized in a metal-dz2 orbital and carbon-pz orbitals of carbonyls. The EPR tensors are slightly modified by annealing, they suggest that new constraints in the matrix distort the structure of [M(CO)4PPh3]•− (M = Cr, Mo, W).
  • Self-sorting Subcomponent Rearrangement During Crystallization
    Marie Hutin, Christopher J. Cramer, Laura Gagliardi, Abdul Rehaman Moughal Shahi, GĂŠrald Bernardinelli, Radovan Cerny and Jonathan R. Nitschke
    Journal of the American Chemical Society, 129 (28) , 2007, p8774-8780
    DOI:10.1021/ja070320j | unige:3596 | Abstract | Article HTML | Article PDF
  
The incorporation of enantiopure 1-amino-2,3-propanediol as a subcomponent into a dicopper double helicate resulted in perfect chiral induction of the helicate's twist. DFT calculations allowed the determination of the helicity of the complex in solution. The same helical induction, in which S amines induced a Λ helical twist, was observed in the solid state by X-ray crystallography. Electronic structure calculations also revealed that the unusual deep green color of this class of complexes was due to a metal-to-ligand charge transfer excitation, in which the excited state possesses a valence delocalized Cu23+ core. The use of a racemic amine subcomponent resulted in the formation of a dynamic library of six diastereomeric pairs of enantiomers. Surprisingly, this library converted into a single pair of enantiomers during crystallization. We were able to observe this process reverse upon redissolution, as initial ligand exchange was followed by covalent imine metathesis.
  • Capturing Transient Electronic and Molecular Structures in Liquids by Picosecond X-Ray Absorption Spectroscopy
    W. Gawelda, V.T. Pham, A. El Nahhas, M. Kaiser, Y. Zaushitsyn, S.L. Johnson, D. Grolimund, R. Abela, A. Hauser, C. Bressler and M. Chergui
    American Institute of Physics Conference Proceedings, 882 , 2007, p31-36
    DOI:10.1063/1.2644425 | unige:4058 | Abstract | Article PDF
We describe an advanced setup for time-resolved x-ray absorption fine structure (XAFS) Spectroscopy with picosecond temporal resolution. It combines an intense femtosecond laser source synchronized to the x-ray pulses delivered into the microXAS beamline of the Swiss Light Source (SLS). The setup is applied to measure the short-lived high-spin geometric structure of photoexcited aqueous Fe(bpy)3 at room temperature.
  
  • An Experimental and Computational Study on Intramolecular Charge Transfer: A Tetrathiafulvalene Fused Dipyridophenazine Molecule
    Chunyang Jia, Shi-Xia Liu, Christian Tanner, Claudia Leiggener, Antonia Neels, Lionel Sanguinet, Eric Levillain, Samuel Leutwyler, Andreas Hauser and Silvio Decurtins
    Chemistry - A European Journal, 13 (13) , 2007, p3804-3812
    Keywords: charge transfer; donor-acceptor systems; nitrogen heterocycles; photophysical properties; tetrathiafulvalene
    DOI:10.1002/chem.200601561 | unige:3597 | Abstract | Article HTML | Article PDF
To study the electronic interactions in donor-acceptor (D-A) ensembles, D and A fragments are coupled in a single molecule. Specifically, a tetrathiafulvalene (TTF)-fused dipyrido[3,2-a:2',3'-c]phenazine (dppz) compound having inherent redox centers has been synthesized and structurally characterized. Its electronic absorption, fluorescence emission, photoinduced intramolecular charge transfer, and electrochemical behavior have been investigated. The observed electronic properties are explained on the basis of density functional theory.
 
The fluorine superhyperfine (shf) tensor measured in aFCl:La2+has been found to be practically isotropic, a result which is certainly anomalous when compared to that for em>d9centers with one unpaired electron in a em>x2−y2orbital. This puzzling fact has been explored by means of density functional calculations. Obtained results confirm that in the em>C4vequilibrium geometry the unpaired electron lies in a em>b1(∼x2−y2)orbital which overlaps with the sorbitals of four −ligands. For explaining the origin of the near isotropy, which is well reproduced by the present calculations, the simple em>D4hand i>C4vaF42− F42− and gF42−centers have also been investigated. Although the obtained results stress the high dependence of the isotropic shf constant i>Ason the metal-ligand distance i>R a near isotropy of the shf tensor is only reached for aF42−(but not for F42− under i>C4vsymmetry which corresponds to the actual symmetry of the a2+center in the BaFCl lattice. The origin of this peculiar situation is shown to come from the mixing between dand forbitals of a2+allowed in i>C4vsymmetry thus stressing the role played by forbitals in bonding properties. Writing i>As=CR−nsit is shown that for the i>D4haF42−and F42−complexes the exponent i>nsis around 20, while it is only equal to 4 for gF42− This huge difference is shown to stem from the quite distinct slope of the radial i>dwave function at the equilibrium distance for the two i>d1centers and the i>d9gF42−unit. Finally, the present calculations strongly support that the intense band peaked at 7  890  cm−1recorded in the optical absorption spectrum of aFCl:La2+is indeed a d→4ftransition.
  • A Combined Experimental and Theoretical Study of Uranium Polyhydrides with New Evidence for the Large Complex UH4(H2)6
    Juraj Raab, Roland H. Lindh, Xuefeng Wang, Lester Andrews and Laura Gagliardi
    Journal of Physical Chemistry A, 111 (28) , 2007, p6383-6387
    DOI:10.1021/jp0713007 | unige:3194 | Abstract | Article HTML | Article PDF
Several monouranium and diuranium polyhydride molecules were investigated using quantum chemical methods. The infrared spectra of uranium and hydrogen reaction products in condensed neon and pure hydrogen were measured and compared with previous argon matrix frequencies. The calculated molecular structures and vibrational frequencies were used to identify the species present in the matrix. Major new absorptions were observed and compared with the previous argon matrix study. Spectroscopic evidence was obtained for the novel complex, UH4(H2)6, which has potential interest as a metal hydride with a large number of hydrogen atoms bound to uranium. Our calculations show that the series of complexes UH4(H2)1,2,4,6 are stable.
  
  • Tuning the Polarization Along Linear Polyaromatic Strands for Rationally Inducing Mesomorphism in Lanthanide Nitrate Complexes
    Emmanuel Terazzi, Laure GuĂŠnĂŠe, , GĂŠrald Bernardinelli, Bertrand Donnio, Daniel Guillon and Claude Piguet
    Chemistry - A European Journal, 13 (6) , 2007, p1674-1691
    Keywords: electrostatic potentials; intermolecular interactions; lanthanides; liquid crystals; metallomesogens
    DOI:10.1002/chem.200601389 | unige:3599 | Abstract | Article HTML | Article PDF
The opposite orientation of the ester spacers in the rodlike ligands L 4C12 (benzimidazole-OOC-phenyl) and L 5C12 (benzimidazole-COO-phenyl) drastically changes the electronic structure of the aromatic systems, without affecting their meridional tricoordination to trivalent lanthanides, LnIII, and their thermotropic liquid crystalline (i.e., mesomorphic) behaviors. However, the rich mesomorphism exhibited by the complexes [Ln(L 4C12)(NO3)3] (Ln=La-Lu) vanishes in [Ln(L 5C12)(NO3)3], despite superimposable molecular structures and comparable photophysical properties. Density functional theory (DFT) and time-dependant DFT calculations performed in the gas phase show that the inversion of the ester spacers has considerable effects on the electronic structure and polarization of the aromatic groups along the strands, which control residual intermolecular interactions responsible for the formation of thermotropic liquid-crystalline phases. As a rule of thumb, an alternation of electron-poor and electron-rich aromatic rings favors intermolecular interactions between the rigid cores and consequently mesomorphism, a situation encountered for L 4C12, L 5C12, [Ln(L 4C12)(NO3)3], but not for [Ln(L 5C12)(NO3)3]. The intercalation of an additional electron-rich diphenol ring on going from [Ln(L 5C12)(NO3)3] to [Ln(L 6C12)(NO3)3] restores mesomorphism despite an unfavorable orientation of the ester spacers, in agreement with our simple predictive model.
  • Ab initio static and molecular dynamics study of 4-styrylpyridine: Structure, energy and reactivity of the cis and trans isomers in the ground state
    LatĂŠvi Max Lawson Daku, Jorge Linares and Marie-Laure Boillot
    ChemPhysChem, 8 (9) , 2007, p1402-1416
    Keywords: ab initio calculations; density functional calculations; isomerization; molecular dynamics; transition states
    DOI:10.1002/cphc.200700117 | unige:3195 | Abstract | Article HTML | Article PDF
We report an in-depth theoretical study of 4-styrylpyridine in its singlet S0 ground state. The geometries and the relative stabilities of the trans and cis isomers were investigated within density functional theory (DFT) as well as within Hartree-Fock (HF), second-order Møller-Plesset (MP2), and coupled cluster (CC) theories. The DFT calculations were performed using the B3LYP and PBE functionals, with basis sets of different qualities, and gave results that are very consistent with each other. The molecular structure is thus predicted to be planar at the energy minimum, which is associated with the trans conformation, and to become markedly twisted at the minimum of higher energy, which is associated with the cis conformation. The results of the calculations performed with the post-HF methods approach those obtained with the DFT methods, provided that the level of treatment of the electronic correlation is high enough and that sufficiently flexible basis sets are used. Calculations carried out within DFT also allowed the determination of the geometry and the energy of the molecule at the biradicaloid transition state associated with the thermal cis ↔trans isomerization and at the transition states associated with the enantiomerization of the cis isomer and with the rotations of the pyridinyl and phenyl groups in the trans and cis isomers. Car-Parrinello molecular dynamics simulations were also performed at 50, 150, and 300 K using the PBE functional. The studies allowed us to evidence the highly flexible nature of the molecule in both conformations. In particular, the trans isomer was found to exist mainly in a nonplanar form at finite temperatures, while the rotation of the pyridinyl ring in the cis isomer was incidentally observed to take place within ≈1 ps during the simulation carried out at 150 K on this isomer.
 
The ground state properties and absorption spectra of N-benzylideneaniline (NBA) have been studied at the density functional (DFT) and at the time-dependent density functional (TD-DFT) level of the theory. The equilibrium geometries of the E and Z isomers in the ground state and their vibrational frequencies have been computed. Furthermore, the excitation energies of the lowest excited singlet and triplet states and the potential energy curves along the torsion and the inversion isomerization coordinates were evaluated. The results are discussed in light of the available experimental data.
We argue with Kryachko's criticism [Int J Quantum Chem 2005, 103, 818] of the original proof of the second Hohenberg-Kohn theorem. The Kato cusp condition can be used to refute a "to-be-refuted" statement as an alternative to the original proof by Hohenberg and Kohn applicable for Coulombic systems. Since alternative ways to prove falseness of the "to-be-refuted" statement in a reduction ad absurdum proof do not exclude each other, Kryachko's criticism is not justified.
  • Quantum Chemical Characterization of Low-energy States of Calicene in the Gas Phase and Solution
    Giovanni Ghigo, Abdul Rehaman Moughal Shahi, Laura Gagliardi, Lee M. Solstad and Christopher J. Cramer
    Journal of Organic Chemistry, 72 (8) , 2007, p2823-2831
    DOI:10.1021/jo062420y | unige:3197 | Abstract | Article HTML | Article PDF
The ground and excited electronic state properties of calicene (triapentafulvalene or 5-(cycloprop-2-en-1-ylidene)cyclopenta-1,3-diene) have been studied with a variety of density functional models (mPWPW91, PBE, TPSS, TPSh, B3LYP) and post-Hartree−Fock models based on single (MP2 and CCSD(T)) and multideterminantal (CASPT2) reference wave functions. All methods agree well on the properties of ground-state calicene, which is described as a conjugated double bond system with substantial zwitterionic character deriving from a charge-separated mesomer in which the three- and five-membered rings are both aromatic. Although the two rings are joined by a formal double bond, contributions from the aromatic mesomer reduce its bond order substantially. A rotational barrier of 40−41 kcal mol-1 is predicted in the gas phase and solvation effects reduce the barrier to 37 and 33 kcal mol-1 in benzene and water, respectively, because of increased zwitterionic character in the twisted transition-state structure. Multi-state CASPT2 (MS-CASPT2) is used to characterize the first few excited singlet and triplet states and indicates that the most important transition occurs at 4.93 eV (251 nm). A cis−trans photoisomerization about the inter-ring double bond is found to be inefficient.
  • Structural Determination of a Short-lived Iron(II) Complex by Picosecond X-ray Absorption Spectroscopy
    Wojciech Gawelda, Van-Thai Pham, Maurizio Benfatto, Yuri Zaushitsyn, Maik Kaiser, Daniel Grolimund, Steven L. Johnson, Rafael Abela, Andreas Hauser, Christian Bressler and Majed Chergui
    Physical Review Letters, 98 , 2007, p57401
    DOI:10.1103/PhysRevLett.98.057401 | unige:3198 | Abstract | Article PDF | Article PS (gzipped)
Structural changes of the iron(II)-tris-bipyridine ([FeII(bpy)3]2+) complex induced by ultrashort pulse excitation and population of its short-lived (≤0.6  ns) quintet high spin state have been detected by picosecond x-ray absorption spectroscopy. The structural relaxation from the high spin to the low spin state was followed over the entire lifetime of the excited state. A combined analysis of the x-ray-absorption near-edge structure and extended x-ray-absorption fine structure spectroscopy features delivers an Fe-N bond elongation of 0.2 Å in the quintet state compared to the singlet ground state.
  
  • Light-induced phase separation in the [Fe(ptz)6](BF4)2 spin-crossover single crystal
    F. Varret, K. Boukheddaden, C. Chong, A. Goujon, B. Gillon, J. Jeftic and A. Hauser
    Europhysics Letters, 77 (3) , 2007, p30007
    DOI:10.1209/0295-5075/77/30007 | unige:3601 | Abstract | Article HTML | Article PDF
We present novel insight on like-spin domains (LSD) in cooperative spin transition solids by following the photo-transformation and the subsequent relaxation of a [Fe(ptz)6](BF4)2 single crystal in the vicinity of the light-induced instability. Self-organization under light is observed, accompanied by Barkhausen-like noise and jumps which reveal the presence of elastic interactions between LSDs. The light-induced phase separation process is discussed in terms of a dynamic potential providing spinodal instability in the corresponding temperature range. This useful concept is applicable to all types of switchable molecular solids.
Maxing out at six: The maximum bond order that can be achieved between two equal atoms in the periodic system is six. The picture shows the potential energy curves for the diatoms Cr2, Mo2, and W2, where the latter two are sextuply bonded molecules (d=internuclear distance in atomic units).
 
Recent advances in computational actinide chemistry are reported in this tutorial review. Muticonfigurational quantum chemical methods have been employed to study the gas phase spectroscopy of small actinide molecules. Examples of actinide compounds studied in solution are also presented. Finally the multiple bond in the diuranium molecule and other diactinide compounds is described.
The endohedral fullerene CH4@C84 has been studied using density functional theory (DFT) and second-order Møller-Plesset perturbation theory (MP2). In addition to the structure with a C— bond of CH4 in a tetrahedral pocket conformation, we find an alternative minimum, very close in energy (6.3-9.5 kJ/mol higher according to the level of theory), with the methane inverted, which we call the antipocket conformation. Computed IR spectra are reported for CH4@C84 and also for the reference system CH4@C60. The calculated vibrational levels, in a harmonic approximation, reveal close-lying translational, librational, and shell-vibrational modes. The results are also presented for the isoelectronic species NH4+@C60.
Single crystal dixanthinium tetrachlorozincate has been grown from dilute chloridric acid. Polarized Raman spectrum of the single crystalline sample, FT-Raman and FT-IR spectra of the polycrystalline samples have been examined and the bands assigned to the appropriate modes predicted by a factor group analysis for the space group Pmn21. The crystal structure has been confirmed by powder XRD measurements.
  
Mixed single PbFBr1−xIx crystals have been prepared. X-ray powder diffraction structure determinations show that all samples crystallize with the matlockite structure. However, the single crystal structure of PbFBr0.5I0.5 involves not only fractional occupation of one site corresponding to the stoichiometry, but also split positions of the Pb2+ ion. Raman spectra reveal the presence of new additional bands with respect to PbFBr and PbFI. DFT calculations of lattice vibrations for PbFI show good agreement with experimental spectra. The calculated phonon dispersion curve suggests that for the mixed crystals the centre of inversion is conserved locally. These combined results suggest the presence of domains with ordered F–Pb–Br–Br–Pb–F and F–Pb–I–I–Pb–F layers in the mixed crystals. Calculations on PbFBr0.5I0.5 show that this suggested structure is more stable than the structure consisting of the F–Pb–Br–I–Pb–F arrangement.
  • Spin-crossover in cobalt(II) imine complexes
    Itana Krivokapic, Mohamed Zerara, LatĂŠvi Max Lawson Daku, Alfredo Vargas, Cristian Enachescu, Christina Ambrus, Philip Tregenna-Piggott, Nahid Amstutz, Elmars Krausz and Andreas Hauser
    Coordination Chemistry Reviews, 251 , 2007, p364-378
    DOI:10.1016/j.ccr.2006.05.006 | unige:3202 | Abstract | Article HTML | Article PDF
Whereas there are hundreds of known iron(II) spin-crossover compounds, only a handful of cobalt(II) spin-crossover compounds have been discovered to date, and hardly an in depth study on any of them exists. This review begins with an introduction into the theoretical aspects to be considered when discussing spin-crossover compounds in general and cobalt(II) systems in particular. It is followed by case studies on [Co(bpy)3]2+ and [Co(terpy)2]2+ (bpy = 2,2′-bipyridine, terpy = 2,2′:6′,2″-terpyridine) presenting and discussing results from magnetic susceptibility measurements, X-ray crystallography, optical spectroscopy, and EPR spectroscopy.
  • Enantiodiscrimination between an N-acetyl-L-cysteine SAM and proline: an in situ spectroscopic and computational study
    M. Bieri and T. BĂźrgi
    ChemPhysChem, 7 (2) , 2006, p514-523
    Keywords: chirality; density functional calculations; enantioselectivity; IR spectroscopy; self-assembly
    DOI:10.1002/cphc.200500474 | unige:14692 | Abstract | Article PDF
A combination of attenuated total reflection infrared (ATR-IR) and modulation excitation spectroscopy (MES) is used to study the enantiodiscriminating interactions between proline and a chiral, self-assembled monolayer (SAM) of N-acetyl-L-cysteine on gold. The N-acetyl-L-cysteine SAM consists of a mixture of protonated and deprotonated molecules. Whereas both species are influenced by adsorbed proline, only the deprotonated molecules are involved in enantiodiscrimination. Density functional theory (DFT) calculations reveal that electrostatics dominates the interaction between the two molecules. By modulating the absolute configuration of proline over the chiral SAM, and a subsequent phase-sensitive detection of the periodically varying signals in the ATR-IR spectra, the small spectral differences between the diastereomeric complexes are spotted. The resulting difference spectrum is in qualitative agreement with the spectrum predicted by the DFT calculations.
  • Ferrocene-containing optically active liquid-crystalline side-chain polysiloxanes with planar chirality
    Julie Brettar, Thomas BĂźrgi, Bertrand Donnio, Daniel Guillon, Rolf Klappert, Toralf Scharf and Robert Deschenaux
    Advanced Functional Materials, 16 (2) , 2006, p260-267
    Keywords: Chiral polymers; Liquid crystals; Optically active materials; Polysiloxanes
    DOI:10.1002/adfm.200500223 | unige:14766 | Abstract | Article PDF
  
Optically active liquid-crystalline side-chain polysiloxanes have been prepared by grafting planar chiral ferrocene-based vinyl monomers onto commercially available polyhydrosiloxane. Two ferrocene monomers have been synthesized: a linear-type monomer, which displays a monotropic chiral smectic C (SC*) phase and enantiotropic smectic A (SA) and chiral N (N*) phases, and a laterally branched monomer, which shows an enantiotropic N* phase. X-ray diffraction analysis indicates a monomolecular organization of the monomeric units within the smectic layers. The polymers retain the liquid-crystalline phases of their corresponding monomers. The UV-vis and circular dichroism (CD) spectra are in agreement with the structure of the monomers and polymers. The molar absorption coefficient (ϵ) and molar circular dichroic absorption coefficient (Δϵ) values of the polymers are proportional to the number of monomeric units grafted onto them. The absolute configuration of the ferrocene carboxylic acid intermediate, used to synthesize the monomers, has been determined on the basis of CD spectra. The helical twisting power (HTP) of the nematogenic monomer and polymer have been determined in E7, and indicate that such materials could be used as chiral dopants. Finally, this study demonstrates that the nature of chiral phases can be controlled by structural engineering of the organic groups only, with ferrocene acting as the source of chirality.
  • Simultaneous in situ monitoring of surface and gas species and surface properties by modulation excitation polarization-modulation infrared reflection-absorption spectroscopy: CO oxidation over Pt film
    Atsushi Urakawa, Thomas BĂźrgi, Hans-Peter Schläpfer and Alfons Baiker
    Journal of Chemical Physics, 124 (5) , 2006, p54717-11
    DOI:10.1063/1.2159484 | unige:14673 | Abstract | Article HTML | Article PDF
A method for in situ monitoring of surface and gas species utilizing separately the difference and sum reflectivity of two polarizations, normal and parallel to the surface, measured by polarization-modulation infrared reflection-absorption spectroscopy is presented. Surface and gas-phase spectra were separately but simultaneously obtained from the reflectivities. The technique is combined with modulation excitation spectroscopy to further enhance the sensitivity, and a small-volume cell was designed for this purpose. CO oxidation over a 40 nm Pt film on aluminum was investigated under moderate pressure (atmospheric pressure, 5% CO, and 5%–40% O2) at 373–433 K. The surface species involved in the oxidation process and the gas-phase species, both reactant (CO) and product (CO2), could be simultaneously monitored and analyzed quantitatively. In addition, the reflectivity change of the sample during the reaction was assigned to a near-surface bulk property change, that is, surface reconstruction to the oxide phase. Under an O2-rich atmosphere, two reactive phases, denoted as low- and high-activity phases, were identified. A large amount of atop CO was observed during the low-activity phase, while the adsorbed CO completely disappeared during the high-activity phase. The presence of an infrared-inactive CO2 precursor formed by the reaction between surface oxide and gaseous CO during the high-activity phase was inferred. The desorption of the CO2 precursor is facilitated under a CO-rich atmosphere, most likely, by surface reconstruction to metallic Pt and a competitive adsorption of CO on the surface.
  
The adsorption of L-glutathione (γ-Glu-Cys-Gly) from ethanol on gold surfaces was studied in situ by both attenuated total reflection infrared (ATR-IR) spectroscopy and using a quartz crystal microbalance (QCM). The molecule is firmly anchored to the gold surface through the thiol group. Different IR signals of adsorbed L-glutathione, notably the amide I and ν(–COOH), show significantly different behavior with time, which reveals that their increase is not related to adsorption (mass uptake) alone. This indicates that structural transformations take place during the formation of the self-assembled monolayer (SAM). In particular, the intensity of the acid signal increases quickly only within the first couple of minutes. The complexity of the self-assembling process is confirmed by QCM measurements, which show fast mass uptake within about 100 s followed by a considerably slower regime. The structural change superimposed on the mass uptake is, based on the in situ time-resolved ATR-IR measurements, assigned to the interaction of the acid group of the Gly moiety with the surface. The latter group is protonated in ethanol but deprotonates upon interaction with the gold surface. The protonation–deprotonation equilibrium is sensitive to external stimuli, such as the presence of dissolved L-glutathione molecules. The interaction of the acid group with the surface and concomitant deprotonation proceeds via two distinguishable steps, the first being a reorientation of the molecule, followed by the deprotonation.
  • Kinetic analysis using square-wave stimulation in modulation excitation spectroscopy: Mixing property of a flow-through PM-IRRAS cell
    A. Urakawa, T. BĂźrgi and A. Baiker
    Chemical Physics, 324 (2-3) , 2006, p653-658
    DOI:10.1016/j.chemphys.2005.12.009 | unige:14731 | Article PDF
Square-wave stimulation used in modulation excitation spectroscopy [D. Baurecht, U.P. Fringeli, Rev. Sci. Instrum. 72 (2001) 3782] can have significant advantages over a simple sinusoidal-wave due to the high odd-frequency terms contained in square-wave, particularly when a system response is close to linear. Phase-sensitive detection (PSD) affords separating the signals of the different frequency terms with a high signal-to-noise ratio by averaging a number of modulation cycles. A modulation excitation experiment applying square-wave stimulation provides the same information as several experiments applying sinusoidal-wave stimulations at the same frequency as the square-wave stimulation and at higher frequencies. The amplitude and the phase lag of a response obtained by PSD at fundamental and higher frequencies using square-wave stimulation are related to the ones obtained by sinusoidal-wave stimulation using transfer function of a general system. Mixing property of a PM-IRRAS (polarization–modulation infrared reflection–absorption spectroscopy) flow-through cell was studied by a simple mixing tank model using square-wave concentration stimulation. The advantages of square-wave stimulation are shown by the characterization of the mixing property.
  • Modulation Excitation PM-IRRAS: A New Possibility for Simultaneous Monitoring of Surface and Gas Species and Surface Properties
    Atsushi Urakawa, Thomas BĂźrgi and Alfons Baiker
    Chimia, 60 (4) , 2006, p231-233
    Keywords: co oxidation; fas-solid interface; in situ spectroscopy; modulation excitation spectroscopy; pm-irras
    DOI:10.2533/000942906777674949 | unige:14687 | Abstract
Polarization-modulation infrared reflection-absorption spectroscopy (Pm-irras) is a sensitive tool for the analysis of species residing at gas-solid and gas-liquid interfaces. the polarization-modulation allows excellent back-ground compensation and the analysis of surface/interface species under moderate pressure (e.g. atmospheric pressure of ir-absorbing gases) is possible. we demonstrate a new possibility to extract simultaneously information of gas and solid phases in addition to surface species from the Pm-irras experiments, using co oxidation over Pt film as an example. modulation excitation spectroscopy (mes) has been combined with this technique to enhance the sensitivity and to analyze the kinetic behavior of species. the surface species involved in the oxidation process, the state of Pt, and the gas phase species (co and co2) could be simultaneously monitored in situ and analyzed quantitatively. the technique can serve as a valuable tool for investigations of various dynamic phenomena occurring at gas-solid interfaces.
  • Chromatographic Resolution, Solution and Crystal Phase Conformations, and Absolute Configuration of tert-Butyl(dimethylamino)phenylphosphine-Borane Complex
    Jean-Valère Naubron, Laurent Giordano, FrĂŠdĂŠric Fotiadu, Thomas BĂźrgi, Nicolas Vanthuyne, Christian Roussel and GĂŠrard Buono
    Journal of Organic Chemistry, 71 (15) , 2006, p5586-5593
    DOI:10.1021/jo0605647 | unige:14781 | Abstract | Article HTML | Article PDF
  
The enantiomers of tert-butyl(dimethylamino)phenylphosphine−borane complex 2 have been separated by HPLC using cellulose tris-p-methylbenzoate as chiral stationary phase. The borane protection could be removed without racemization and the P-configuration of the free aminophosphine 1 has shown to be stable in solution. Infrared (IR) and vibrational circular dichroism (VCD) spectra have been measured in CD2Cl2 solution for both enantiomers. B3LYP/6-31+G(d) DFT calculations allowed a prediction that complex (S)-2 exists as three conformers in equilibrium and computed population-weighted IR and VCD spectra. Predicted and experimental IR and VCD spectra compared very well and indicate that enantiomer (+)-2 has the S absolute configuration. This assignment has been confirmed by an X-ray diffraction study on a single crystal of (+)-2. The crystal structure of enantiomerically pure 2 appears to be very close to the most stable computed conformer which proved to be predominant in solution.
The photoassisted mineralization, i.e., conversion to CO2 and water, of malonic acid over P25 TiO2 was investigated by in situ attenuated total reflection infrared (ATR-IR) spectroscopy in a small volume flow-through cell. Reassignment of the vibrational bands of adsorbed malonic acid, assisted by deuterium labeling, reveals two dissimilar carboxylate groups within the molecule. This indicates adsorption via both carboxylate groups, one in a bridging or bidentate and the other in monodentate coordination. During irradiation the coverage of malonic acid strongly decreases, and oxalate is observed on the surface in at least two different adsorption modes. The major oxalate species observed during irradiation is characterized by monodentate coordination of both carboxylate groups. In the dark, however, part of these species adopts another adsorption mode, possibly interacting only with one carboxylate group. During band gap illumination a large fraction of the surface is not covered by acid. Oxalate is a major intermediate in the mineralization of malonic acid. However, the observed transient kinetics of adsorbed malonic and oxalic acid indicates additional pathways not involving oxalate. The rate constant for oxalate decomposition is slightly larger than the one for oxalate formation from malonic acid. As the oxalate is desorbing slowly from the surface its concentration in the liquid phase is small, despite the fact that it is a major intermediate in the mineralization of malonic acid.
  
We have prepared gold nanoparticles covered with N-isobutyryl-l-cysteine and N-isobutyryl-d-cysteine, respectively. These particles with a mean particle size smaller than 2 nm are highly soluble in water and are amenable to chiroptical techniques such as vibrational circular dichroism (VCD) and circular dichroism (CD) spectroscopy. Density functional theory shows that the VCD spectra are sensitive toward the conformation of the adsorbed thiol. Based on the comparison between the experimental VCD spectrum and the calculated VCD spectra for different conformers, a preferential conformation of the thiol adsorbed on the gold particles can be proposed. In this conformation the carboxylate group interacts with the gold particle in addition to the sulfur. The particles could furthermore be separated according to their charge and size into well-defined compounds. The optical absorption spectra revealed a well-quantized electronic structure and a systematic red-shift of the absorption onset with increasing gold core size, which was manifested in a color change with particle size. Some compounds showed basically identical absorption spectra as analogous gold particles protected with l-glutathione. This shows that these particles have identical core sizes (10−12, 15 and 18 gold atoms, respectively) and indicates that the number and arrangement of the adsorbed thiol are the same, independent of the two thiols, which have largely different sizes. Some separated compounds show strong optical activity with opposite sign when covered with the d- and l-enantiomer, respectively, of N-isobutyryl-cysteine. The origin of the optical activity in the metal-based transitions is discussed. The observations are consistent with a mechanism based on a chiral footprint on the metal core imparted by the adsorbed thiol.
  • D-Penicillamine Adsorption on Gold: An in Situ ATR-IR Spectroscopic and QCM Study
    Marco Bieri and Thomas BĂźrgi
    Langmuir, 22 (20) , 2006, p8379-8386
    DOI:10.1021/la061454y | unige:14691 | Abstract | Article HTML | Article PDF
The adsorption of penicillamine from ethanol on gold was studied in situ by attenuated total reflection infrared (ATR-IR) and quartz crystal microbalance (QCM) experiments. Both ATR-IR and QCM reveal a fast mass uptake. In ethanol, the molecule adopts a zwitterionic form. Upon adsorption, part of the molecules deprotonate at the amine group, which is a relatively slow process that goes along with a strong shift of the νas(COO-) mode. Both ATR-IR and QCM confirm a physisorbed layer. ATR-IR furthermore shows that the latter consists of zwitterionic molecules only, whereas both zwitterionic and anionic species are found in the chemisorbed layer. The infrared spectra of the physisorbed and chemisorbed layers are rather different, and the molecules within both layers seem to be oriented with respect to the surface. The ATR-IR spectra furthermore indicate that all three functional groups of penicillamine (i.e., thiol, carboxylate, and amine) interact with the surface, and density functional theory calculations support this finding. QCM also shows that the molecule uses considerably more space on the surface than molecules of similar size, which supports a three-point interaction. The latter leads to a strong anchoring of the molecule to the metal, which may explain the exceptional capability of penicillamine to bind metals.
  • Probing Chiral Nanoparticles and Surfaces by Infrared Spectroscopy
    Cyrille Gautier, Marco Bieri, Igor Dolamic, Silvia Angeloni, Julien Boudon and Thomas BĂźrgi
    Chimia, 60 (11) , 2006, p777-782
    Keywords: chiral surfaces; enantiodiscrimination; nanoparticles; sams; vibrational circular dichroism
    DOI:10.2533/chimia.2006.777 | unige:14749
  
Chiral metal surfaces and nanoparticles have the potential to be used for the selective production, the resolution and the detection of enantiomers of a chiral compound, which renders them highly attractive in view of the tremendous consequences of homochirality on earth. Their capability to distinguish between enantiomers of a chemical compound relies on their structure and the ability to form intermolecular interactions. However, molecular-level understanding of the interactions that are at the origin of enantiodiscrimination is lagging behind due to the lack of powerful experimental techniques that are able to spot these interactions selectively with high sensitivity. In this article two techniques based on infrared spectroscopy are presented that are able to selectively target the chiral properties of nanoparticles and interfaces. These are the combination of attenuated total reflection infrared (ATR-IR) with modulation excitation spectroscopy (MES) to probe enantiodiscriminating interactions at chiral solid-liquid interfaces and vibrational circular dichroism (VCD), which is used to probe the structure of chirally modified metal nanoparticles.
  • Attenuated Total Reflection Infrared Spectroscopy of Solid Catalysts Functioning in the Presence of Liquid-Phase Reactants
    Thomas BĂźrgi and Alfons Baiker
    in "Advances in Catalysis" Ed. by H. KnĂźpffer, Academic Press, 50 , 2006, p227-283
    DOI:10.1016/S0360-0564%2806%2950005-7 | unige:14641 | Article HTML | Article PDF
Attenuated total reflection (ATR) infrared (IR) spectroscopy is a powerful tool for investigation of solid catalysts, allowing the detection of liquid-phase products (for on-line reaction monitoring) and the investigation of species adsorbed on the catalyst, during reaction and in the presence of strongly absorbing solvents. Flat model catalysts such as metal films as well as powder catalysts can be investigated. In favorable situations, even changes of the catalyst structure can be followed. In this review, some fundamental concepts of ATR spectroscopy are summarized, and practical aspects, such as cell design and sample preparation, are discussed. The potential and limitations of the method are illustrated with examples. Furthermore, powerful techniques aimed at enhancing signal-to-noise ratios and long-term stability are described, which make use of phase-sensitive detection of periodically varying signals and accurate reference measurements. Until now, only a rather limited number of investigations have been reported that use the ATR technique to study heterogeneous catalytic reactions at solid–liquid interfaces, but the method holds good promise because it is comparatively inexpensive and versatile and can provide a large amount of information.
  
Density functional theory (DFT) has progressively emerged in the last 40 years as a leading methodology for the modelling and simulation of chemical systems. In this paper, some historical landmarks in the development of this method are outlined, emphasizing on its main characteristic being an electron density-based theory. This is in contrast with wavefunction-based methodologies which were exclusively employed previously. Interestingly, DFT has been first applied to solids, with a rather late recognition by chemists and molecular scientists. After this historical survey, several applications of DFT to the structure and properties of zeolites are reviewed as a tribute to Dr Annick Goursot.
  • Neutral, Radical [CpNi(dithiolene)] Complexes with Flexible, Nonplanar Seven-Membered Rings: [CpNi{S2C2S2(CH2)2X}] (X = CH2, CF2, C=CH2, S, C=O)
    Mitsushiro Nomura, Michel Geoffroy, Prashant Adkine and Marc FourmiguĂŠ
    European Journal of Inorganic Chemistry, 2006 (24) , 2006, p5012-5021
    Keywords: nickel ; S ligands ; radicals ; metallacycles ; magnetic properties
    DOI:10.1002/ejic.200600649 | unige:3634 | Abstract | Article HTML | Article PDF
  
Neutral, radical [CpNi(dithiolene)] complexes fused with seven-membered rings, formulated as [CpNi{S2C2S2(CH2)2X}] (X = CH2, CF2, C=CH2, S), have been synthesized in 30-60 % yields from the reactions of nickelocene with the corresponding neutral, square-planar, (dithiolene)nickel complexes [Ni{S2C2S2(CH2)2X}2]. [CpNi{S2C2S2(CH2)2X}] (X = C=O) was prepared from nickelocene and [1,3]dithiolo[4,5-b][1,4]dithiepine-2,6-dione under thermal or photochemical conditions. All complexes exhibit reversible oxidation and reduction waves to the cation and anion form, respectively. The terminal groups (X) in the seven-membered ring shift their redox potentials to anodic potentials in the following order: CF2 > C=O > S > C=CH2 > CH2. The singlet EPR responses of [CpNi{S2C2S2(CH2)2(X)}] appear at g ≈ 2.0514-2.0529 in dichloromethane solution at room temperature. An NIR absorption is observed at λmax ≈ 798-848 nm (ε ≈ 1700-2400 sup>-1 cm-1) in dichloromethane solution. The X-ray structures of the five complexes show two-legged piano-stool geometries around the central nickel atom (formally NiIII) and strong distortions from planarity of the seven-membered C2S2(CH2)2X rings. In the solid state, those radical (S = 1/2) species adopt either one-dimensional alternating chain-like motifs (X = CH2, C=CH2, S) or dimeric entities characterized by a singlet-triplet magnetic behavior (X = CF2, C=O).
  • Chiroptical and Computational Studies of a Bridled Chiroporphyrin and of Its Nickel(II), Copper(II), and Zinc(II) Complexes
    GĂŠraldine Maheut, Anna Castaings, Jacques PĂŠcaut, LatĂŠvi Max Lawson Daku, Gennaro Pescitelli, Lorenzo Di Bari and Jean-Claude Marchon
    Journal of the American Chemical Society, 128 (19) , 2006, p6347-6356
    DOI:10.1021/ja054926o | unige:3289 | Abstract | Article HTML | Article PDF
Circular dichroism (CD) spectra and density functional theory (DFT) calculations are reported for a series of conformationally bistable chiroporphyrins with 8-methylene bridles MBCP-8, which can display either an ιιιι or an ιβιβ orientation of their meso substituents. From DFT geometry optimizations, the most stable form of ZnBCP-8 is found to be the ιιιι conformer. By passing to NiBCP-8, there is a strong stabilization of the ιβιβ conformation with respect to the ιιιι conformation, consistent with the X-ray structures of ιιιι-ZnBCP-8 and ιβιβ-NiBCP-8. A correlation between the sign of the CD signal in the Soret region and the conformation of the BCP-8 compounds is reported: the ιιιι conformers H2BCP-8 and ZnBCP-8 show a positive CD signal, whereas the ιβιβ conformers NiBCP-8 and CuBCP-8 exhibit a negative signal. The possible contributions to the rotational strengths of ιβιβ-NiBCP-8 and ιιιι-ZnBCP-8, calculated on the basis of their crystal structures, have been analyzed. The CD signals are found to result from a combination of both the inherent chirality of the porphyrin and of extrinsic contributions due to the chiral bridles. These results may have a broad significance for understanding the chiroptical properties of chiral porphyrins and hemoproteins and for monitoring stimuli-responsive, conformationally bistable chiroporphyrin compounds.
  
  • Photoswitching of the Dielectric Constant of the Spin-Crossover Complex [Fe(L)(CN)2]H2O
    SĂŠbastien Bonhommeau, Thomas Guillon, LatĂŠvi Max Lawson Daku, Philippe Demont, JosĂŠ Sanchez Costa, Jean-François LĂŠtard, GĂĄbor MolnĂĄr and Azzedine Bousseksou
    Angewandte Chemie International Edition, 45 (10) , 2006, p1625-1629
    DOI:10.1002/anie.200503252 | unige:3635 | Abstract | Article HTML | Article PDF
Photoswitching of the dielectric constant has been observed for the first time in the spin-crossover complex [Fe(L)(CN)2]¡H2O (L=2,13-dimethyl-6,9-dioxa-3,12,18-triazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene, see picture). The electrical detection of a photoinduced change in spin state could allow the use of such complexes in optical information-storage devices.
Frequency shifts of the Ag  I  4d105s  2S1∕2(F=0,MF=0) to 4d95s2  2D5∕2(F′=2,MF′=0) electric-quadrupole transition at 330.6  nm due to external fields are calculated using multiconfigurational self-consistent field methods. As this forbidden transition is free from first order Doppler and Zeeman effects, it is under investigation for the realization of an atomic optical clock. The calculated perturbations are the light shift, the blackbody frequency shift, and the quadratic Zeeman shift. Results show that a total uncertainty of 10−18 could be reach without confining the atoms in a Lamb-Dicke regime in an optical lattice.
The bifunctional of the nonadditive kinetic energy in the reference system of noninteracting electrons ( Tnads [ρA, ρB] = Ts[ρA + ρB] − Ts[ρA] − Ts[ρB]) is the key quantity in orbital-free embedding calculations because they hinge on approximations to Tnads [ρA,ρB]. Since Tnads [ρA,ρB] is not linear in ρA, the associated potential (functional derivative) Tnads [ρ,ρB]/δρ|ρ=ρA(r→) changes if ρA varies. In this work, for two approximations to Tnads [ρA,ρB], which are nonlinear in ρA (gradient-free and gradient-dependent), their linearized versions are constructed, and the resulting changes (linearization errors) in various properties of embedded systems (orbital energies, dipole moments, interaction energies, and electron densities) are analyzed. The considered model embedded systems represent typical nonbonding interactions: van der Waals contacts, hydrogen bonds, complexes involving charged species, and intermolecular complexes of the charge-transfer character. For van der Waals and hydrogen bonded complexes, the linearization of Tnads [ρA,ρB] affects negligibly the calculated properties. Even for complexes, for which large complexation induced changes of the electron density can be expected, such as the water molecule in the field of a cation, the linearization errors are about 2 orders of magnitude smaller than the interaction induced shifts of the corresponding properties. Linearization of Tnads [ρA,ρB] is shown to be inadequate for the complexes of a strong charge-transfer character. Compared to gradient-free approximation to Tnads [ρA,ρB], introduction of gradients increases the linearization error.
About 3 Âľm thick tungsten trioxide film electrodes consisting of partly sintered, 40-80 nm in diameter, particles deposited on conducting glass substrates exhibit high photon-to-current conversion efficiencies for the photooxidation of water, exceeding 70% at 400 nm. This is facilitated by a ca. 40% film porosity resulting in high contact area with the electrolyte. It is shown that the activity of the WO3 electrodes towards photooxidation of water is enhanced by addition of even small amounts of halide (Cl-, Br-) ions to the acidic electrolyte. Photoelectrolysis experiments performed either in acidic electrolytes containing chloride or bromide anions or in a 0.5 M NaCl solution, under simulated 1.5 AM solar illumination, demonstrated long term stability of the photocurrents. Oxygen remains the main product of the photoanodic reaction even in a 0.5 M NaCl solution, a composition close to the sea water, with chlorine accounting for ca. 20% of current efficiency.
  • Photoinduced bimolecular electron transfer investigated by femtosecond time-resolved infrared spectroscopy
    Omar F. Mohammed, Natalie Banerji, Bernhard Lang, Erik T J Nibbering and Eric Vauthey
    Journal of Physical Chemistry A, 110 (51) , 2006, p13676-13680
    DOI:10.1021/jp066079x | unige:3292 | Abstract | Article HTML | Article PDF
Ultrafast infrared transient absorption spectroscopy is used to study the photoinduced bimolecular electron transfer reaction between perylene in the first singlet excited state and 1,4-dicyanobenzene in acetonitrile and dichloromethane. Following vibrational marker modes on both donor and acceptor sides in real time provides direct insight into the structural dynamics during the reaction. A band narrowing on a time scale of a few tens of picoseconds observed on the antisymmetric CN stretching vibration of the dicyanobenzene radical anion indicates that a substantial part of the excess energy is channeled into vibrational modes of the product, despite the fact that the reaction is weakly exergonic. An additional narrowing of the same band on a time scale of several hundreds of picoseconds observed in acetonitrile only is interpreted as a signature of the dissociation of the geminate ion pairs into free ions.
  • Ultrafast Dynamics of Fluorescent DNA Intercalators
    9. A. FĂźrstenberg, M.D. Julliard, T.G. Deligeorgiev, N.I. Gadjev, A.A. Vassilev and E. Vauthey
    in "Femtochemistry VII: Fundamental Ultrafast Processes in Chemistry, Physics, and Biology" A. W. Castleman Jr., M. L. Kimble Eds., Elsevier, 2006, p391-395
    unige:4037
 
Multiconfigurational quantum chemical methods (CASSCF/CASPT2) have been used to study the chemical bond in the actinide diatoms Ac2, Th2, Pa2, and U2. Scalar relativistic effects and spin−orbit coupling have been included in the calculations. In the Ac2 and Th2 diatoms the atomic 6d, 7s, and 7p orbitals are the significant contributors to the bond, while for the two heavier diatoms, the 5f orbitals become increasingly important. Ac2 is characterized by a double bond with a 3∑g-(0g+) ground state, a bond distance of 3.64. Å, and a bond energy of 1.19 eV. Th2 has quadruple bond character with a 3Dg(1g) ground state. The bond distance is 2.76 Å and the bond energy (D0) 3.28 eV. Pa2 is characterized by a quintuple bond with a 3∑g-(0g+) ground state. The bond distance is 2.37 Å and the bond energy 4.00 eV. The uranium diatom has also a quintuple bond with a 7Og (8g) ground state, a bond distance of 2.43 Å, and a bond energy of 1.15 eV. It is concluded that the strongest bound actinide diatom is Pa2, characterized by a well-developed quintuple bond.
  • On the nature of the metal-metal multiple bond
    L. Gagliardi and B.O. Roos
    in "Lecture Series on Computer and Computational Sciences" Brill Academic Publisher, 6 , 2006, p6-22
  • Effect of cetyltrimethylammonium bromide on the migration of polyaromatic hydrocarbons in capillary electrokinetic chromatography
    G. Kavran Belin, F.B. Erim and F.O. GĂźlaçar
    Talanta, 69 (3) , 2006, p596-600
    DOI:10.1016/j.talanta.2005.10.025 | unige:3293 | Abstract | Article HTML | Article PDF
The separation of different ring numbered polyaromatic hydrocarbons (PAHs) was accomplished by using cetyltrimethylammonium bromide (CTAB) in capillary electrokinetic chromatography. In order to increase the solubilities and selectivities of PAHs, acetonitrile (ACN) was used as an organic modifier. Under the optimised conditions, 11 aromatic compounds were separated within 14.5 min in a running electrolyte containing 10 mM phosphate, 30 mM CTAB, and 40% ACN at pH 6.0. The effects of CTAB and ACN concentrations, voltage and pH on the resolution were investigated. Reproducibilities of migration times range between 0.55 and 1.27 R.S.D.% and peak areas between 1.02 and 7.23 R.S.D.%. Limit of detections (LODs) range between 0.09 and 2.24 μg ml−1. This new and fast separation method of PAHs was applied to cooked oil sample.
  
One-electron reduction of a diphosphafulvenium dication gives the first stable diphosphafulvenium monoradical cation (see scheme). An X-ray crystal structure analysis, EPR measurements, and DFT calculations clearly show that reduction takes place at the exocyclic double bond and that the excess of electron density is stabilized by the two electron-withdrawing phosphonium groups (see SOMO; P orange, C dark gray, H light gray).
  • Electronic Structure, Spectroscopic Properties, and Reactivity of Molybdenum and Tungsten Nitrido and Imido Complexes with Diphosphine Coligands: Influence of the trans Ligand
    Klaus Mersmann, Andreas Hauser, Nicolai Lehnert and Felix Tuczek
    Inorganic Chemistry, 45 , 2006, p5044-5056
    DOI:10.1021/ic060141n | unige:3294 | Abstract | Article HTML | Article PDF
A series of molybdenum and tungsten nitrido, [M(N)(X)(diphos)2], and imido complexes, [M(NH)(X)(diphos)2)]Y, (M = Mo, W) with diphosphine coligands (diphos = dppe/depe), various trans ligands (X = N3-, Cl-, NCCH3) and different counterions (Y- = Cl-, BPh4-) is investigated. These compounds are studied by infrared and Raman spectroscopies; they are also studied with isotope-substitution and optical-absorption, as well as emission, spectroscopies. In the nitrido complexes with trans-azido and -chloro coligands, the metal−N stretch is found at about 980 cm-1; upon protonation, it is lowered to about 920 cm-1. The 1A1 → 1E (n → π*) electronic transition is observed for [Mo(N)(N3)(depe)2] at 398 nm and shows a progression in the metal−N stretch of 810 cm-1. The corresponding 3E → 1A (π* → n) emission band is observed at 542 nm, exhibiting a progression in the metal−N stretch of 980 cm-1. In the imido system [Mo(NH)(N3)(depe)2]BPh4, the n → π* transition is shifted to lower energy (518 nm) and markedly decreases in intensity. In the trans-nitrile complex [Mo(N)(NCCH3)(dppe)2]BPh4, the metal−N(nitrido) stretching frequency increases to 1016 cm-1. The n → π* transition now is found at 450 nm, shifting to 525 nm upon protonation. Most importantly, the reduction of this nitrido trans-nitrile complex is drastically facilitated compared to its counterparts with anionic trans-ligands (Epred = −1.5 V vs Fc+/Fc). On the other hand, the basicity of the nitrido group is decreased (pKa{[Mo(NH)(NCCH3)(dppe)2](BPh4)2} = 5). The implications of these findings with respect to the Chatt cycle are discussed.
  • Density-functional theory investigation of the geometric, energetic and optical properties of the cobalt(II)tris(2,2'-bipyridine) complex in the high-spin and in the Jahn-Teller active low-spin state
    Alfredo Vargas, Mohamed Zerara, Elmars Krausz, Andreas Hauser and LatĂŠvi Max Lawson Daku
    Journal of Chemical Theory and Computation, 2 , 2006, p1342-1359
    DOI:10.1021/ct6001384 | unige:3638 | Abstract | Article HTML | Article PDF
  
State-of-the-art generalized gradient approximation (GGA) (PBE, OPBE, RPBE, OLYP, and HCTH), meta-GGA (VSXC and TPSS), and hybrid (B3LYP, B3LYP*, O3LYP, and PBE0) functionals are compared for the determination of the structure and the energetics of the D3 [Co(bpy)3]2+ complex in the 4A2 and 4E trigonal components of the high-spin 4T1g( t52g e2g ) state and in the low-spin 2E state of octahedral 2Eg( t62g e1g) parentage. Their comparison extends also to the investigation of the Jahn−Teller instability of the 2E state through the characterization of the extrema of C2 symmetry of this spin state's potential energy surface. The results obtained for [Co(bpy)3]2+ in either spin manifold are very consistent among the functionals used and are in good agreement with available experimental data. The functionals, however, perform very differently with respect to the spin-state energetics because the calculated values of the high-spin/low-spin energy difference ΔEelHL vary between −3212 and 3919 cm-1. Semilocal functionals tend to give too large ΔEelHL values and thus fail to correctly predict the high-spin state as the ground state of the isolated complex, while hybrid functionals tend to overestimate the stability of the high-spin state with respect to the low-spin state. Reliable results are, however, obtained with the OLYP, HCTH, B3LYP*, and O3LYP functionals which perform best for the description of the isolated complex. The optical properties of [Co(bpy)3]2+ in the two spin states are also analyzed on the basis of electronic excitation calculations performed within time-dependent density functional response theory. The calculated absorption and circular dichroism spectra agree well with experimental results.
  • Preparation and characterization of 3-(4,5-ethylenedithio-1,3-dithiol-2-ylidene)naphthopyranone: a luminescent redox-active donor–acceptor compound
    Stefan Dolder, Shi-Xia Liu, Xavier GuĂŠgano, Mihail Atanasov, Claude A. Daul, Claudia Leiggener, Andreas Hauser, Antonia Neels and Silvio Decurtins
    Tetrahedron, 62 (48) , 2006, p11106-11111
    Keywords: Naphthopyranone; TTF; Donor–acceptor compound; Cyclic voltammetry; TDDFT calculations; Luminescence
    DOI:10.1016/j.tet.2006.09.032 | unige:3295 | Abstract | Article HTML | Article PDF
A new 1,3-dithiol-2-ylidene substituted naphthopyranone 2 has been synthesized and characterized. UV–vis spectroscopic and cyclic voltammetry results, interpreted on the basis of density functional theory, show that 2 displays an intramolecular charge-transfer transition and acts like a donor–acceptor (D–A) system. Furthermore, a weak fluorescence originating from the excited charge-transfer state is observed.
  • Synthesis and properties of the bifunctional luminophors 3-[4-(4'-N,N-dimethylaminophenyl)butyl]perylene and 3-(4'-N,N-dimethylaminophenyl)perylene
    Igor I. Barabanov, Valerii V. Korolev, Nina P. Gritsan and Eric Vauthey
    Mendeleev Communications, 16 (4) , 2006, p226-228
    DOI:10.1070/MC2006v016n04ABEH002299 | unige:3305 | Abstract | Article PDF
Luminophors of the perylene series containing the N,N-dimethylaniline residue in their molecules have been synthesised for the first time; spectral and luminescent properties of these compounds have been studied
  
  • [CpNi(dithiolene)] (and Diselenolene) Neutral Radical Complexes
    Mitsushiro Nomura, Thomas Cauchy, Michel Geoffroy, Prashant Adkine and Marc FourmiguĂŠ
    Inorganic Chemistry, 45 (20) , 2006, p8194-8204
    DOI:10.1021/ic0608546 | unige:3639 | Abstract | Article HTML | Article PDF
Various preparations of the neutral radical [CpNi(dddt)] complex (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) were investigated with CpNi sources, [Cp2Ni], [Cp2Ni](BF4), [CpNi(CO)]2, and [CpNi(cod)](BF4), and dithiolene transfer sources, O=C(dddt), the naked dithiolate (dddt2-), the monoanion of square-planar Ni dithiolene complex (NBu4)[Ni(dddt)2], and the neutral complex [Ni(dddt)2]. The reaction of [CpNi(cod)](BF4) with (NBu4)[Ni(dddt)2] gave the highest yield for the preparation of [CpNi(dddt)] (86%). [CpNi(ddds)] (ddds = 5,6-dihydro-1,4-dithiin-2,3-diselenolate), [CpNi(dsdt)] (dsdt = 5,6-dihydro-1,4-diselenin-2,3-dithiolate), [CpNi(bdt)] (bdt = 1,2-benzenedithiolate), and [CpNi(bds)] (bds = 1,2-benzenediselenolate) were synthesized by the reactions of [Cp2Ni] with the corresponding neutral Ni dithiolene complexes [Ni(ddds)2]2, [Ni(dsdt)2], [Ni(bdt)2], and [Ni(bds)2], respectively. The five, formally NiIII, radical complexes oxidize and reduce reversibly. They exhibit, in the neutral state, a strong absorption in the NIR region, from 1000 nm in the dddt/ddds/dsdt series to 720 nm in the bdt/bds series with ξ values between 2500 and 5000 M-1 cm-1. The molecular and solid state structures of the five complexes were determined by X-ray structure analyses. [CpNi(dddt)] and [CpNi(ddds)] are isostructural, while [CpNi(dsdt)] exhibits a closely related structure. Similarly, [CpNi(bdt)] and [CpNi(bds)] are also isostructural. Correlations between structural data and magnetic measurements show the presence of alternated spin chains in [CpNi(dddt)], [CpNi(ddds)], and [CpNi(dsdt)], while a remarkably strong antiferromagnetic interaction in [CpNi(bdt)] and [CpNi(bds)] is attributed to a Cp¡¡¡Cp face-to-face σ overlap, an original feature in organometallic radical complexes.
  • Quantum Chemical Characterization of the Bonding of N-Heterocyclic Carbenes to Cp2MI Compounds [M = Ce(III), U(III)]
    Laura Gagliardi and Christopher J. Cramer
    Inorganic Chemistry, 45 (23) , 2006, p9442-9447
    DOI:10.1021/ic061306v | unige:3296 | Abstract | Article HTML | Article PDF
The binding of N-heterocyclic carbenes to Ce(III) and U(III) compounds is characterized by quantum chemical methods. Density functional methods are in qualitative agreement with experiment that binding to U(III) is more favorable than to Ce(III); after correcting for basis-set superposition error, quantitative agreement with experiment is achieved with a multireference second-order perturbation theory approach accounting for relativistic effects. The small computed (and observed) preference derives from a combination of several small effects, including differences in electronic binding energies, rovibrational partition functions, and solvation free energies. Prospects for ligand modification to improve the differentiation between lanthanides and actinides are discussed on the basis of computational predictions.
  • Theoretical Characterization of End-On and Side-On Peroxide Coordination in Ligated Cu2O2 Models
    Christopher J. Cramer, Armagan Kinal, Marta Wloch, Piotr Piecuch and Laura Gagliardi
    Journal of Physical Chemistry A, 110 (40) , 2006, p11557-11568
    DOI:10.1021/jp064232h | unige:3640 | Abstract | Article HTML | Article PDF
  
The relative energetics of Ο-Ρ1:Ρ1 (trans end-on) and Ο-Ρ2:Ρ2 (side-on) peroxo isomers of Cu2O2 fragments supported by 0, 2, 4, and 6 ammonia ligands have been computed with various density functional, coupled-cluster, and multiconfigurational protocols. There is substantial disagreement between the different levels for most cases, although completely renormalized coupled-cluster methods appear to offer the most reliable predictions. The significant biradical character of the end-on peroxo isomer proves problematic for the density functionals, while the demands on active space size and the need to account for interactions between different states in second-order perturbation theory prove challenging for the multireference treatments. In the latter case, it proved impossible to achieve any convincing convergence.
  
  • On the crystallochemical origin of the disordered form of Ba7(EuII)F12Cl2 and the structural changes induced at high temperature
    F. Kubel and H. Hagemann
    Crystal Research and Technology, 41 (10) , 2006, p1005-1012
    Keywords: disordered solids ; structural changes ; luminescence
    DOI:10.1002/crat.200610712 | unige:3641 | Abstract | Article PDF
The crystal structure of the disordered modification of Ba7F12Cl2 has been carefully re-examined on several new crystals prepared under different conditions of synthesis. All single crystal structure refinements reveal a residual electron density of ~3 e-/Å3 in the 0,0,0 position which is explained by the introduction of a small amount of sodium ions in the crystal. The title compound transforms from a disordered to an ordered modification at ~800 °C. New structural data show a change in space group from P63/m to P6. During this process, a slight chemical change and the formation of nano-channels in the crystals is observed by electron microscopy. These changes were further studied by electron microprobe analysis, by spectroscopic methods and thermal analysis.
Nice to see U2! The [PhUUPh] molecule (see picture; U pink, C gray, H white) has been studied by multiconfigurational quantum chemical methods. It was found that a quintuple bond is formed between the two uranium atoms with a U—U bond length of 2.29 Å. The phenyl ligand was used to mimic a bulky terphenyl ligand, which could be a promising candidate for the stabilization of multiply bonded uranium compounds.
  • EPR and DFT Studies of the Structure of Phosphinyl Radicals Complexed by a Pentacarbonyl Transition Metal
    Bassirou Ndiaye, Shrinivasa Bhat, Abdelaziz Jouaiti, ThĂŠo Berclaz, GĂŠrald Bernardinelli and Michel Geoffroy
    Journal of Physical Chemistry A, 110 (31) , 2006, p9736-9742
    DOI:10.1021/jp061960w | unige:3298 | Abstract | Article HTML | Article PDF
Paramagnetic complexes M(CO)5P(C6H5)2, with M = Cr, Mo, W, have been trapped in irradiated crystals of M(CO)5P(C6H5)3 (M = Cr, Mo, W) and M(CO)5PH(C6H5)2 (M = Cr, W) and studied by EPR. The radiolytic scission of a P−C or a P−H bond, responsible for the formation of M(CO)5P(C6H5)2, is consistent with both the number of EPR sites and the crystal structures. The g and 31P hyperfine tensors measured for M(CO)5P(C6H5)2 present some of the characteristics expected for the diphenylphosphinyl radical. However, compared to Ph2P•, the 31P isotropic coupling is larger, the dipolar coupling is smaller, and for Mo and W compounds, the g-anisotropy is more pronounced. These properties are well predicted by DFT calculations. In the optimized structures of M(CO)5P(C6H5)2 (M = Cr, Mo, W), the unpaired electron is mainly confined in a phosphorus p-orbital, which conjugates with the metal dxz orbital. The trapped species can be described as a transition metal-coordinated phosphinyl radical.
 
Quantum chemistry can today be employed to invent new molecules and explore unknown molecular bonding. An overview of novel species containing metals bound to polynitrogen clusters is presented. The prediction of metal polyhydrides is discussed. Finally, some species containing gold that behaves as a halogen are described, together with recent advances in actinide chemistry and exploration of the nature of the actinide–actinide chemical bonding.
Vibrational spectra of BH4- and its isotopic analogues in a crystalline environment of alkali metals cations (K+, Rb+, Cs+) have been investigated beyond the harmonic approximation using a variational approach supported by computations of B3LYP type anharmonic force fields. From the comparison of the observed and simulated IR spectra, the influence of the anharmonic couplings on the band position and on the relative intensity of the allowed vibrational transitions is discussed. Here, the effect of the crystalline environment induces a blue shift of about 50 and 100 cm-1 respectively for the bending and stretching modes of BH4-. Furthermore, anharmonic effects, which are exclusively well reproduced by a variational approach, are needed to yield reliable positions and relative amplitudes of IR allowed combination and overtone transitions. This leads to theoretical results fitting their experimental counterpart between 6 and 30 cm-1 in the investigated series.
  
  • Early Excited State Dynamics of 6-Styryl-Substituted Pyrylium Salts Exhibiting Dual Fluorescence
    Anatolio Pigliucci, Peter Nikolov, Abdul Rehaman, Laura Gagliardi, Christopher J. Cramer and Eric Vauthey
    Journal of Physical Chemistry A, 110 (33) , 2006, p9988-9994
    DOI:10.1021/jp063214x | unige:3643 | Abstract | Article HTML | Article PDF
A series of 6-styryl-2,4-diphenylpyrylium salts exhibiting dual fluorescence has been investigated by fluorescence up-conversion in conjunction with quantum chemical calculations. The short-wavelength emission is due to an excited state localized on the pyrylium fragment and the long-wavelength emission arises from a charge-transfer state delocalized over the whole molecule. The two fluorescing states do not exhibit a precursor−successor relationship. The rise time of the short-wavelength fluorescence is smaller than 200 fs, and that of the long-wavelength emission depends on the electron-donating property of the styryl group substituent. The rise is almost prompt with the weaker donors but is slower, wavelength and viscosity dependent with the strongest electron-donating group. A model involving a S2/S1 conical intersection is proposed to account for these observations.
Femtosecond and nanosecond laser flash photolysis was used to determine the photophysical and photochemical processes in aqueous solutions of Fe(III) ion and 5-sulfosalicylic acid (SSA) containing the FeSSA complex and the free ligand. Excitation of the FeSSA complex in the charge transfer band (λmax = 505 nm) is followed by an ultrafast relaxation to the ground electronic state with two characteristic times of 0.26 and 1.8 ps. The shorter time constant is ascribed to internal conversion to the vibrationally hot electronic ground state of FeSSA and the 1.8 ps time constant is assigned to the vibrational cooling of the ground state. The UV irradiation of the solution (308 nm) leads to the excitation of both the free ligand and the FeSSA complex. The latter relaxes rapidly and the free ligand undergoes intersystem crossing to the triplet state. This system undergoes an irreversible photochemical reaction originating from an electron transfer (k = (9 ± 2) × 108 M−1 s−1) from the free ligand in the triplet state to the FeSSA complex. This electron transfer is accompanied by an energy transfer between these species (k = (1.3 ± 0.2) × 109 M−1 s−1).
  • Effect of the f-Orbital Delocalization on the Ligand-Field Splitting Energies in Lanthanide-Containing Elpasolites
    Mohamed Zbiri, Claude A. Daul and Tomasz A. Wesolowski
    Journal of Chemical Theory and Computation, 2 , 2006, p1106-1111
    Keywords: ligand-field theory, lanthanide, optical centre, embedding, density functional theory, elpasolite
    DOI:10.1021/ct060035a | unige:3644 | Abstract | Article HTML | Article PDF
  
The ligand-field induced splitting energies of f-levels in lanthanide-containing elpasolites are derived using the first-principles universal orbital-free embedding formalism [Wesolowski and Warshel, J. Phys. Chem. 1993, 97, 8050]. In our previous work concerning chloroelpasolite lattice (Cs2NaLnCl6), embedded orbitals and their energies were obtained using an additional assumption concerning the localization of embedded orbitals on preselected atoms leading to rather good ligand-field parameters. In this work, the validity of the localization assumption is examined by lifting it. In variational calculations, each component of the total electron density (this of the cation and that of the ligands) spreads over the whole system. It is found that the corresponding electron densities remain localized around the cation and the ligands, respectively. The calculated splitting energies of f-orbitals in chloroelpasolites are not affected noticeably in the whole lanthanide series. The same computational procedure is used also for other elpasolite lattices (Cs2NaLnX6, where X=F, Br, and I)—materials which have not been fabricated or for which the ligand-field splitting parameters are not available.
  • Photoproduction of Proton Gradients with pi-Stacked Fluorophore Scaffolds in Lipid Bilayers.
    Sheshanath Bhosale, Adam L. Sisson, Pinaki Talukdar, Alexandre FĂźrstenberg, Natalie Banerji, Eric Vauthey, Guillaume Bollot, Jiri Mareda, Cornelia RĂśger, Frank WĂźrthner, Naomi Sakai and Stefan Matile
    Science, 313 (5783) , 2006, p84-86
    DOI:10.1126/science.1126524 | unige:3301 | Abstract | Article HTML | Article PDF
Rigid p-octiphenyl rods were used to create helical tetrameric π-stacks of blue, red-fluorescent naphthalene diimides that can span lipid bilayer membranes. In lipid vesicles containing quinone as electron acceptors and surrounded by ethylenediaminetetraacetic acid as hole acceptors, transmembrane proton gradients arose through quinone reduction upon excitation with visible light. Quantitative ultrafast and relatively long-lived charge separation was confirmed as the origin of photosynthetic activity by femtosecond fluorescence and transient absorption spectroscopy. Supramolecular self-organization was essential in that photoactivity was lost upon rod shortening (from p-octiphenyl to biphenyl) and chromophore expansion (from naphthalene diimide to perylene diimide). Ligand intercalation transformed the photoactive scaffolds into ion channels.
  • One-electron Equations for Embedded Electron Density: Challenge for Theory and Practical Payoffs in Multi-Level Modeling of Complex Polyatomic Systems
    T.A. Wesolowski
    in " Computational Chemistry: Reviews of Current Trends" World Scientific, 10 , 2006, p1-82
    Keywords: multi-level, computer simulations, density functional theory, embedding, condensed matter
    unige:4036 | Article PDF
The idea of describing a many-electron system using only its electron density, i.e. without constructing its wavefunction, was initiated in the works of Thomas and Fermi. Hohenberg-Kohn theorems of modern density functional theory transformed this idea into an exact theory. The Kohn-Sham formalism, widely used in computer simulations of polyatomic systems today, is based on these theorems but is not orbital-free. It reintroduces orbitals to minimize errors in approximating the total energy. The present review concerns an alternative formalism based also on Hohenberg-Kohn theorems, in which orthogonal orbitals are used not for the whole system but only for subsystems [Cortona, Phys. Rev. B, 44 (1991) 8454]. These orbitals are derived from Kohn-Sham-like one-electron equations, called here Kohn-Sham Equations with Constrained Electron Density (KSCED), in which all terms representing the interactions between the subsystems are expressed as universal functionals of electron density. This formulation provides the formal basis for the orbital-free embedding in first-principles based multi-level simulations of complex systems, in which the orbital-level is retained for a selected subsystem, whereas its environment is described at the orbital-free level [Wesolowski and Warshel, J. Phys. Chem., 97 (1993) 8050]. The formal aspects, development and testing of relevant approximate density functionals, and the possible use of the orbital-free embedding in multi-level modelling are covered in detail in this review. Examples of applications, especially those concerning the electronic structure of embedded systems in the condensed phase are provided.
  
Multiconfigurational quantum chemical methods show that a quintuple bond is present between the two CrI units in the model complex [PhCrCrPh]. The Cr—Cr (1.75 Å) and Cr—Ph (2.02 Å) bonds are shorter than those in the recently reported compound [Ar'CrCrAr'] (Ar'=2,6-(2,6-iPr2C6H3)2C6H3; 1.83 and 2.15 Å, respectively). This difference is attributed to the additional Cr—Ar' interactions.
Nanosecond laser flash photolysis, absorption and fluorescent spectroscopy were used to study the influence of pH on the photophysical and photochemical processes of 5-sulfosalicylic acid (SSA) in aqueous solutions. Information on the excited singlet state intramolecular proton transfer (ESIPT) of the SSA ions could be deduced from the dependence of the quantum yield and the spectral maximum of SSA fluorescence on the pH of the medium. The main photochemical active form of SSA at pH < 10 is the dianion (HSSA2−). Excitation of this species gives rise to the HSSA2− triplet state, to the SSA•2− radical anion and to the hydrated electron. In a neutral medium, the main decay channels of these intermediates are T–T annihilation, recombination and capture by the HSSA2− dianion, respectively. A decrease of pH leads to an increase of the second-order rate constants of disappearance of both HSSA2− triplet state and SSA•2− radical anion due to their protonation.
 
Experimental (IR and Raman) and theoretical (Kohn-Sham calculations) methods are used in a combined analysis aimed at refining the available structural data concerning the molecular guests in channels formed by stacked dibenzo-18-crown-6 (DB18C6) crown ether. The calculations are performed for a simplified model comprising isolated DB18C6 unit and its complexes with either H2O or H3O+ guests, which are the simplest model ingredients of a one-dimensional diluted acid chain, to get structural and energetic data concerning the formation of the complex and to assign the characteristic spectroscopic bands. The oxygen centers in the previously reported crystallographic structure are assigned to either H2O or protonated species.
  • Ultrafast Excited State Dynamics of the Perylene Radical Cation Generated upon Bimolecular Photoinduced Electron Transfer Reaction
    StĂŠphane Pagès, Bernhard Lang and Eric Vauthey
    Journal of Physical Chemistry A, 110 (24) , 2006, p7547-7553
    DOI:10.1021/jp0615252 | unige:3303 | Abstract | Article HTML | Article PDF
The ultrafast ground state recovery (GSR) dynamics of the radical cation of perylene, Pe•+, generated upon bimolecular photoinduced electron transfer in acetonitrile, has been investigated using pump−pump−probe spectroscopy. With 1,4-dicyanobenzene as electron acceptor, the free ion yield is substantial and the GSR dynamics of Pe•+ was found to depend on the time delay between the first and second pump pulses, Δt12, i.e., on the “age” of the ion. At short Δt12, the GSR dynamics is biphasic, and at Δt12 larger than about500 ps, it becomes exponential with a time constant around 3 ps. With trans-1,2-dicyanoethylene as acceptor, the free ion yield is essentially zero and the GSR dynamics of Pe•+ remains biphasic independently of Δt12. The change of dynamics observed with 1,4-dicyanobenzene is ascribed to the transition from paired to free solvated ion, because in the pair, the excited ion has an additional decay channel to the ground state, i.e., charge recombination followed by charge separation. The rate constants deduced from the analysis of these GSR dynamics are all fully consistent with this hypothesis.
  
  • Ultrafast Excited-State Dynamics of DNA Fluorescent Intercalators: New Insight into the Fluorescence Enhancement Mechanism
    Alexandre FĂźrstenberg, Marc D. Julliard, Todor G. Deligeorgiev, Nikolai I. Gadjev, Aleksey A. Vasilev and Eric Vauthey
    Journal of the American Chemical Society, 128 (23) , 2006, p7661-7669
    DOI:10.1021/ja0609001 | unige:3648 | Abstract | Article HTML | Article PDF
The excited-state dynamics of the DNA bisintercalator YOYO-1 and of two derivatives has been investigated using ultrafast fluorescence up-conversion and time-correlated single photon counting. The free dyes in water exist in two forms: nonaggregated dyes and intramolecular H-type aggregates, the latter form being only very weakly fluorescent because of excitonic interaction. The excited-state dynamics of the nonaggregated dyes is dominated by a nonradiative decay with a time constant of the order of 5 ps associated with large amplitude motion around the monomethine bridge of the cyanine chromophores. The strong fluorescence enhancement observed upon binding of the dyes to DNA is due to both the inhibition of this nonradiative deactivation of the nonaggregated dyes and the dissociation of the aggregates and thus to the disruption of the excitonic interaction. However, the interaction between the two chromophoric moieties in DNA is sufficient to enable ultrafast hopping of the excitation energy as revealed by the decay of the fluorescence anisotropy. Finally, these dyes act as solvation probes since a dynamic fluorescence Stokes shift was observed both in bulk water and in DNA. Very similar time scales were found in bulk water and in DNA.
  • Photoexcitation and Relaxation Dynamics of Catecholato–Iron(III) Spin-Crossover Complexes
    Cristian Enachescu, Andreas Hauser, Jean-Jacques Girerd and Marie-Laure Boillot
    ChemPhysChem, 7 , 2006, p1127-1135
    Keywords: charge transfer, iron, laser spectroscopy, photophysics, spin crossover
    DOI:10.1002/cphc.200500671 | unige:3649 | Abstract | Article HTML | Article PDF
  
The photophysical properties of the ferric catecholate spin-crossover compounds [(TPA)Fe(R-Cat)]X (TPA=tris(2-pyridylmethyl)amine; X=PF6-, BPh4-; R-Cat=catecholate dianion substituted by R=NO2, Cl, or H) are investigated in the solid state. The catecholate-to-iron(III) charge-transfer bands are sensitive both to the spin state of the metal ion and the charge-transfer interactions associated with the different catecholate substituents. Vibronic progressions are identified in the near-infrared (NIR) absorption of the low-spin species. Evidence for a low-temperature photoexcitation process is provided. The relaxation dynamics between 10 and 100 K indicate a pure tunneling process below ≈40 K, and a thermally activated region at higher temperatures. The relaxation rate constants in the tunneling regime at low temperature, kHL(T→0), vary in the range from 0.58 to 8.84 s-1. These values are in qualitative agreement with the inverse energy-gap law and with structural parameters. A comparison with ferrous spin-crossover complexes shows that the high-spin to low-spin relaxation is generally faster for ferric complexes, owing to the smaller bond length changes for the latter. However, in the present case the corresponding rate constants are smaller than expected based on the single configurational coordinate model. This is attributed to the combined influence of the electronic configuration and the molecular geometry.
  • Low-temperature lifetimes of metastable high-spin states in spin-crossover and in low-spin iron(II) compounds: The rule and exceptions to the rule
    Andreas Hauser, Cristian Enachescu, LatĂŠvi Max Lawson Daku, Alfredo Vargas and Nahid Amstutz
    Coordination Chemistry Reviews, 250 (13-14) , 2006, p1642-1652
    DOI:10.1016/j.ccr.2005.12.006 | unige:3304 | Abstract | Article HTML | Article PDF
The high-spin → low-spin relaxation in spin-crossover compounds can be described as non-adiabatic multi-phonon process in the strong coupling limit, in which the low-temperature tunnelling rate increases exponentially with the zero-point energy difference between the two states. Based on the hypothesis that the experimental bond length difference between the high-spin and the low-spin state of ~0.2 Å is also valid for low-spin iron(II) complexes, extrapolation of the single configurational coordinate model allows an estimate of the zero-point energy difference for low-spin complexes from kinetic data. DFT calculations on low-spin [Fe(bpy)3]2+ support the structural assumption. However, for low-spin [Fe(terpy)2]2+ the relaxation rate constant shows an anomalous behaviour in so far as it is more in line with spin-crossover systems. This is attributed to very anisotropic bond length changes associated with the spin state change, and the subsequent breakdown of the single mode model.
  • A redox-active tri star molecule: merging of TTF and HAT chemistry
    Chunyang Jia, Shi-Xia Liu, Christian Tanner, Claudia Leiggener, Lionel Sanguinet, Eric Levillain, Samuel Leutwyler, Andreas Hauser and Silvio Decurtins
    ChemComm, (17) , 2006, p1878-1880
    DOI:10.1039/b601173j | unige:3307 | Abstract | Article HTML | Article PDF
A planar π-conjugated heteroaromatic molecule 1 has been synthesized and fully characterized; it combines two characteristics, a charge-transfer transition originating from its inherent donor–acceptor nature in its neutral state and an intervalence charge-transfer transition in its 12+ mixed-valence state.
  
The dynamic Stokes shift of coumarin 153 has been measured in two room-temperature ionic liquids, 1-(3-cyanopropyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-propyl-3-methylimidazolium tetrafluoroborate, using the fluorescence up-conversion technique with a 230 fs instrumental response function. A component of about 10−15% of the total solvation shift is found to take place on an ultrafast time scale < 10 ps. The amplitude of this component is substantially less than assumed previously by other authors. The origin of the difference in findings could be partly due to chromophore-internal conformational changes on the ultrafast time scale, superimposed to solvation-relaxation, or due to conformational changes of the chromophore ground state in polar and apolar environments. First three-pulse photon-echo peak-shift experiments on indocyanine green in room-temperature ionic liquids and in ethanol indicate a difference in the inertial component of the early solvent relaxation of <100 fs.
Computer simulation methods using orbital level of description only for a selected part of the larger systems are prone to the artificial charge leak to the parts which are described without orbitals. The absence of orbitals in one of the subsystems makes it impossible to impose explicitly the orthogonality condition. Using the subsystem formulation of density functional theory, it is shown that the absence of explicit condition of orthogonality between orbitals belonging to different subsystems, does not cause any breakdown of this type of description for the chosen intermolecular complexes (F−H2O and Li+H2O), for which a significant charge-leak problem could be a priori expected.
  • The Characterisation of Molecular Alkali-Metal Azides
    J.Steven Ogden, John M. Dyke, William Levason, Francesco Ferrante and Laura Gagliardi
    Chemistry - A European Journal, 12 (13) , 2006, p3580-3586
    Keywords: alkali metals; azides; IR spectroscopy; matrix isolation; theoretical calculations
    DOI:10.1002/chem.200501101 | unige:3306 | Abstract | Article HTML | Article PDF
Matrix isolation infrared (IR) studies have been carried out on the vaporisation of the alkali-metal azides MN3 (M = Na, K, Rb and Cs). The results show that under high vacuum conditions, molecular KN3, RbN3 and CsN3 are present as stable high-temperature vapour species, together with variable amounts of nitrogen gas and the corresponding metal atoms. The characterisation of these molecular azides is supported by ab initio molecular orbital calculations and density functional theory (DFT) calculations, and for CsN3 in particular, by the detection of the isotopomers Cs(14N15N14N) and Cs(15N14N14N). The IR spectra are assigned to a "side-on" (C2v) structure by comparison with the spectral features predicted both by vibrational analysis and calculation. The most intense IR features for KN3, RbN3 and CsN3 isolated in nitrogen matrices lie at 2005, 2004.4 and 2002.2 cm-1, respectively, and correspond to the N3 asymmetric stretch. The N3 bending mode in CsN3 is identified at 629 cm-1. An additional feature routinely observed in these experiments occurred at approximately 2323 cm-1 and is assigned to molecular N2, perturbed by the close proximity of an alkali-metal atom. The position of this band appeared to show very little cation dependence, but its intensity correlated with the extent of sample thermal decomposition.
 
Several controversial questions in the field of bimolecular photoinduced electron transfer reactions in polar solvents are first briefly reviewed. Results obtained in our group using ultrafast spectroscopy and giving a new insight into these problems will then be described. They concern the driving force dependence of the charge separation distance, the formation of the reaction product in an electronic excited state, the absence of normal region for weakly exergonic charge recombination processes and the excitation wavelength dependence of the CR dynamics of donor–acceptor complexes.
  • Mechanism of Nitrate Reduction by Desulfovio desulfuricans Nitrate Reductase - A Theoretical Investigation
    Monica Leopoldini, Nino Russo, Marirosa Toscano, Marcin Dulak and Tomasz Adam Wesolowski
    Chemistry - A European Journal, 12 (9) , 2006, p2532-2541
    Keywords: density functional calculations; enzyme models; nitrate reductase; ONIOM; orbital-free embedding
    DOI:10.1002/chem.200500790 | unige:3284 | Abstract | Article HTML | Article PDF
The oxidative half-reaction of oxygen atom transfer from nitrate to an MoIV complex has been investigated at various levels of theory. Two models have been used to simulate the enzyme active site. In the second, more advanced model, additional amino acid residues capable of significantly affecting the catalytic efficiency of the enzyme were included. B3LYP/6-31+G*, ONIOM, and orbital-free embedding approaches have been used to construct the potential energy profile and to qualitatively compare the results of a QM/MM study with those obtained by a full quantum mechanical strategy. The study has confirmed the utility of the orbital-free embedding method in the description of enzymatic processes.
  
  • Theoretical Models on the Cu2O2 Torture Track: Mechanistic Implications for Oxytyrosinase and Small-Molecule Analogues
    Christopher J. Cramer, Marta Wloch, Piotr Piecuch, Cristina Puzzarini and Laura Gagliardi
    Journal of Physical Chemistry A, 11 (5) , 2006, p1991-2004
    DOI:10.1021/jp056791e | unige:3631 | Abstract | Article HTML | Article PDF
Accurately describing the relative energetics of alternative bis(μ-oxo) and μ-η2:η2 peroxo isomers of Cu2O2 cores supported by 0, 2, 4, and 6 ammonia ligands is remarkably challenging for a wide variety of theoretical models, primarily owing to the difficulty of maintaining a balanced description of rapidly changing dynamical and nondynamical electron correlation effects and a varying degree of biradical character along the isomerization coordinate. The completely renormalized coupled-cluster level of theory including triple excitations and extremely efficient pure density functional levels of theory quantitatively agree with one another and also agree qualitatively with experimental results for Cu2O2 cores supported by analogous but larger ligands. Standard coupled-cluster methods, such as CCSD(T), are in most cases considerably less accurate and exhibit poor convergence in predicted relative energies. Hybrid density functionals significantly underestimate the stability of the bis(μ-oxo) form, with the magnitude of the error being directly proportional to the percentage Hartree−Fock exchange in the functional. Single-root CASPT2 multireference second-order perturbation theory, by contrast, significantly overestimates the stability of bis(μ-oxo) isomers. Implications of these results for modeling the mechanism of C−H bond activation by supported Cu2O2 cores, like that found in the active site of oxytyrosinase, are discussed.
Results from quantum chemical calculations that predict the existence of a series of diuranium molecules are reported. Two diuranium chlorides, U2Cl6 and U2Cl82-, and three different carboxylates, U2(OCHO)4, U2(OCHO)6, and U2(OCHO)4Cl2 have been studied. All species have been found to be stable with a multiply bonded U2 unit.
  • EPR and DFT studies of the one-electron reduction product of phospholium cations
    Prashant Adkine, Thibault Cantat, Eliane Deschamps, Louis Ricard, Nicolas MĂŠzailles, Pascal Le Floch and Michel Geoffroy
    Physical Chemistry Chemical Physics, 8 , 2006, p862-868
    DOI:10.1039/b513736p | unige:3632 | Abstract | Article HTML | Article PDF
  
Cyclic voltammetry and EPR spectroscopy show that cationic phospholium groups are good electron acceptors whose reduction leads to a neutral radical where the unpaired electron is mainly delocalized on the carbon atoms of the five-membered ring. DFT calculations together with the crystal structure of phospholiums indicate that the electron addition causes a drastic diminution of the exocyclic CPC angle. The SOMO of reduced phospholium is compared to the SOMO of the phosphole radical anion.
  
  • Optical and vibrational properties of (MnF6)4- complexes in cubic fluoroperovskites: insight through embedding calculations using Kohn-Sham equations with constrained electron density
    J.M. Garcia-Lastra, T.A. Wesolowski, M.T. Barriuso, J.A. Aramburu and M. Moreno
    Journal of Physics: Condensed Matter, 18 (5) , 2006, p1519-1534
    DOI:10.1088/0953-8984/18/5/004 | unige:3633 | Abstract | Article PDF
The local structure and optical and vibrational properties associated with Mn2+-doped cubic AMF3 (A = K, Rb; M = Mg, Zn, Cd) fluoroperovskites are studied by means of embedding calculations using Kohn–Sham equations with constrained electron density. It is shown that while an electronic parameter like 10Dq essentially depends on the Mn2+–F− distance, the local vibration frequencies ωi (i = a1g, eg modes) are dominated by the interaction between F− ligands and nearest M2+ ions lying along bonding directions. The high ωa values observed for KMgF3:Mn2+ and KZnF3:Mn2+, the huge variations of ωe and ωa frequencies when the host lattice is changed, as well as the increase of Huang–Rhys factors and the Stokes shift following the host lattice parameter, are shown to be related to this elastic coupling of the MnF64− complex to the rest of the host lattice. The present results support the conclusion that the Stokes shift is determined by the interaction of the excited 4T1g state with a1g and eg local modes while the coupling with the t2g shear mode is not relevant. The variations of local vibrational frequencies and the Stokes shift induced by a hydrostatic pressure on a given system are shown to be rather different to those produced by the chemical pressure associated with distinct host lattices.
  • Inter- and Intramolecular Interactions in Some Supramolecular Photochemical Systems
    Sandra Delahaye, Claudia Loosli, Shi-Xia Liu, Silvio Decurtins, Gael Labat, Antonia Neels, Andreas Loosli, Thomas R. Ward and Andreas Hauser
    Advanced Functional Materials, 16 (2) , 2006, p286-295
    Keywords: organometallic complexes ; photophysical effects ; phthalocynanines ; ruthenium complexes ; supramolecular materials
    DOI:10.1002/adfm.200500329 | unige:3286 | Abstract | Article PDF
In supramolecular systems, the interaction between different units modulates their photophysical properties. a) For platinum(II) complexes with ligands that have extended π systems, π-stacking and direct metal-metal interactions result in the formation of excimers with characteristically red-shifted luminescence. Time-resolved emission spectra show clear evidence of dual luminescence. b) In phthalocyanines to which electron-donating tetrathiafulvalene (TTF) groups have been fused, the luminescence is strongly quenched by intramolecular electron transfer. The luminescence can be switched on by oxidation of the TTF groups. c) The luminescence of ruthenium tris-bipyridyl derivatives is strongly influenced by the environment. Linked to biotin, the luminescence quantum yield of such a complex is enhanced by 30 % upon binding to avidin. Furthermore, the binding to avidin induces a circular-dichroism signal from the π-π* transition of the initially racemic ruthenium tris-bipyridyl derivative.
The formalism based on the total energy bifunctional (E[ρI,ρII]) is used to derive interaction energies for several hydrogen-bonded complexes (water dimer, HCN–HF, H2CO–H2O, and MeOH–H2O). Benchmark ab initio data taken from the literature were used as a reference in the assessment of the performance of gradient-free [local density approximation (LDA)] and gradient-dependent [generalized gradient approximation (GGA)] approximations to the exchange-correlation and nonadditive kinetic-energy components of E[ρI,ρII]. On average, LDA performs better than GGA. The average absolute error of calculated LDA interaction energies amounts to 1.0 kJ/mol. For H2CO–H2O and H2O–H2O complexes, the potential-energy curves corresponding to the stretching of the intermolecular distance are also calculated. The positions of the minima are in a good agreement (less than 0.2 Å) with the reference ab initio data. Both variational and nonvariational calculations are performed to assess the energetic effects associated with complexation-induced deformations of molecular electron densities.
We report the synthesis, crystal structure and electrochemical behaviour of a complex in which the Ph group of the phosphaalkene PhC(H)=PMes* (Mes*: 2,4,6-tri-tert-butylphenyl) is coordinated to a chromium tricarbonyl group. The EPR spectra resulting from electrochemical and chemical reductions are described and the experimental g and hyperfine tensors (31P)T, as determined from the EPR data, are compared with those predicted by DFT calculations for the radical anion (Cr(CO)3, PhC(H)=PMes)·−. The structural changes caused by the addition of an electron to the neutral complex are described, together with an estimation of the contribution of Cr(CO)3 to the stabilization of the radical anion.
  • Interaction of Water, Alkyl Hydroperoxide, and Allylic Alcohol with a Single-Site Homogeneous Ti-Si Epoxidation Catalyst: A Spectroscopic and Computational Study
    Atsushi Urakawa, Thomas BĂźrgi, Peter Skrabal, Felix Bangerter and Alfons Baiker
    Journal of Physical Chemistry B, 109 (6) , 2005, p2212-2221
    DOI:10.1021/jp048999q | unige:14730 | Abstract | Article HTML | Article PDF
Tetrakis(trimethylsiloxy)titanium (TTMST, Ti(OSiMe3)4) possesses an isolated Ti center and is a highly active homogeneous catalyst in epoxidation of various olefins. The structure of TTMST resembles that of the active sites in some heterogeneous Ti−Si epoxidation catalysts, especially silylated titania−silica mixed oxides. Water cleaves the Ti−O−Si bond and deactivates the catalyst. An alkyl hydroperoxide, TBHP (tert-butyl hydroperoxide), does not cleave the Ti−O−Si bond, but interacts via weak hydrogen-bonding as supported by NMR, DOSY, IR, and computational studies. ATR−IR spectroscopy combined with computational investigations shows that more than one, that is, up to four, TBHP can undergo hydrogen-bonding with TTMST, leading to the activation of the O−O bond of TBHP. The greater the number of TBHP molecules that form hydrogen bonds to TTMST, the more electrophilic the O−O bond becomes, and the more active the complex is for epoxidation. An allylic alcohol, 2-cyclohexen-1-ol, does not interact strongly with TTMST, but the interaction is prominent when it interacts with the TTMST−TBHP complex. On the basis of the experimental and theoretical findings, a hydrogen-bond-assisted epoxidation mechanism of TTMST is suggested.
In situ attenuated total reflection (ATR) infrared and UV–vis spectroscopy are combined to yield simultaneous time-resolved information on dissolved reaction products, adsorbed species, and the catalyst during the oxidation of ethanol and 2-propanol on a 5% Pd/Al2O3 catalyst. The oxidation is initiated by change from hydrogen- to oxygen-saturated solvent flow. 2-Propanol oxidation is observed only in the transient period, whereas ethanol oxidation is also observed in the steady state. This may be ascribed to overoxidation of the catalyst in the former case. In a mixture of the two alcohols the same thing is observed. Competitive adsorption in the steady state may explain this behavior. For ethanol oxidation ethyl acetate is also observed during the transient period. The UV–vis spectra reveal a fast reversible change of the catalyst with switching between hydrogen and oxygen and a slow irreversible change during ethanol oxidation. The latter is ascribed to the change in Pd particle structure, which hardly affects, however, catalyst activity on the time scale of about 1 h.
  • L-Glutathione Chemisorption on Gold and Acid/Base Induced Structural Changes: A PM-IRRAS and Time-Resolved in Situ ATR-IR Spectroscopic Study
    Marco Bieri and Thomas BĂźrgi
    Langmuir, 21 (4) , 2005, p1354-1363
    DOI:10.1021/la047735s | unige:14761 | Abstract | Article HTML | Article PDF
  
Adsorption of the tripeptide l-glutathione (γ-glu-cys-gly) on gold surfaces was investigated by polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and attenuated total reflection (ATR) infrared spectroscopy. PM-IRRAS was used to study ex situ the adsorbate layer prepared from aqueous solutions at different pH, whereas ATR-IR was applied to study in situ adsorption from ethanol in the presence and absence of acid and base. ATR-IR was furthermore combined with modulation spectroscopy in order to investigate the reversible changes within the adsorbate layer induced by acid and base stimuli, respectively. The molecule is firmly anchored on the gold surface via the thiol group of the cys part. However, the ATR-IR spectra in ethanol indicate a further interaction with the gold surface via the carboxylic acid group of the gly part of the molecule, which deprotonates upon adsorption. Hydrochloric acid readily protonates the two acid groups of the adsorbed molecule. During subsequent ethanol flow the acid groups deprotonate again, a process which proceeds in two distinct steps: a fast step associated with the deprotonation of the acid in the glu part of the molecule and a considerably slower step associated with deprotonation of the acid in the gly moiety. The latter process is assisted by the interaction of the corresponding acid group with the surface. The spectra furthermore indicate a rearrangement of the hydrogen bonding network within the adsorbate layer upon deprotonation. Depending on the protonation state during adsorption of l-glutathione, the response toward identical protonation−deprotonation stimuli is significantly different. This is explained by the ionic state-dependent shape of the molecule, as supported by density functional theory calculations. The different shapes of the individual molecules during layer formation thus influence the structure of the adsorbate layer.
The scope of the asymmetric hydrogenation of functionalized ketones over cinchona-modified platinum was extended to achiral α-hydroxyketones. Cinchonidine showed by far the best catalytic performance affording an enantiomeric excess between 57 and 82% depending on the substrate. O-methoxy-cinchonidine showed poor enantioselection. O-phenoxy-cinchonidine favoured the opposite enantiomer compared to cinchonidine. Solvents with empirical solvent parameters ET N – ranging from 0.10 to 0.65 were tested. Tert-butylmethylether proved to be the most suitable. The highest ratio of substrate/cinchonidine where no loss in e.e. was observed was at around 540, independent of the structure of the α-hydroxyketone. The oxygen in α-position to the ketone seems to play an important role in the enantioselection as well as a phenyl ring or a rigid cis-conformation. The dependence of the enantiomeric excess on the modifier structure and the inversion of the sense of enantiodifferentiation is interpreted in terms of repulsive interactions, which become more evident as the steric demand of the functional group (OH, O-Me, O-Ph) of the modifier increases. The findings indicate that a hydrogen bond in the modifier reactant complex involving the hydroxyl functionality of cinchonidine is not crucial in order to achieve high enantioselectivity.
  • Enantioselective hydrogenation of aromatic ketones over cinchona-modified rhodium: a new opportunity?
    Otmar J. Sonderegger, Gabriel M.-W. Ho, Thomas BĂźrgi and Alfons Baiker
    Journal of Catalysis, 230 (2) , 2005, p499-506
    DOI:10.1016/j.jcat.2005.01.008 | unige:14725 | Article HTML | Article PDF
Alumina-supported rhodium modified with cinchonidine has been investigated with regard to its applicability in the enantioselective hydrogenation of various aromatic ketones possessing an Îą-hydroxy or Îą-methoxy group. The study revealed that depending on the substrate, rhodium can outperform the catalytic behavior of platinum. With one of the substrates, 2-hydroxy-1-(4-methoxy-phenyl)-ethanone (4), an enantiomeric excess (ee) of 80% at 89% conversion was reached, which is the highest ee reported so far for chirally modified rhodium. However, completely different conditions are required to achieve optimal catalytic performance with rhodium, compared with platinum. Rhodium requires a much higher modifier concentration, and high hydrogen pressure is favorable. The higher modifier concentration required is traced to the much higher activity of rhodium for the hydrogenation of the quinoline ring, which is assumed to be the anchoring moiety of the cinchona modifiers on the platinum group metals. Changing the modifier from cinchonidine to O-phenoxy-cinchonidine resulted in a switch of the major enantiomer of the product, as exemplified for 2-hydroxyacetophenone (1), which showed a switch from 73% ee in favor of the (R)-product to 68% ee for the (S)-product when the modifier was changed from cinchonidine to O-phenoxy-cinchonidine.
The interaction of proline with self-assembled monolayers (SAMs) of l-glutathione (γ-glu-cys-gly) on gold was investigated by a combination of attenuated total reflection (ATR-IR) infrared and modulation excitation spectroscopy (MES). The latter technique makes use of phase-sensitive detection of periodically varying signals and allows discrimination between species with different kinetics such as dissolved proline and adsorbed molecules. By applying a convection−diffusion model coupled to adsorption and desorption, it was possible to extract relative adsorption and desorption rates from the experimental data for the two enantiomers of proline, fully accounting for mass transport within the flow-through cell. The results show that, in particular, the desorption kinetics is different for the two enantiomers. Therefore, the l-glutathione SAM can discriminate between enantiomers, d-proline being stronger bound. The IR spectra reveal that upon interaction with proline the adsorbed l-glutathione is protonated at the gly part of the molecule, which, in the absence of proline, is bound to the gold surface as carboxylate. The observed protonation of adsorbed l-glutathione upon interaction with proline goes along with a structural change of the former, which seems to play an important role for enantiodiscrimination.
  
  • Conformational rigidity: a necessary prerequisite of chiral modifiers used in heterogeneous enantioselective catalysis?
    Elisabeth Orglmeister, Thomas BĂźrgi, Tamas Mallat and Alfons Baiker
    Journal of Catalysis, 232 (1) , 2005, p137-142
    Keywords: Acid effect; Asymmetric heterogeneous catalysis; Pt/Al2O3; Ketopantolactone; Hydrogenation; Ab initio calculation; NOE; Modifier conformation
    DOI:10.1016/j.jcat.2005.03.005 | unige:14782 | Article HTML | Article PDF
In the hydrogenation of ketopantolactone, the (R,R) and (R,S) diastereomers of a new chiral modifier, pantoyl-naphthylethylamine, afforded 74 and 40% ee, respectively, to (R)-pantolactone. On the basis of NOE studies and theoretical calculations, the different properties of the diastereomers and in particular the effect of acid on the modifier structure are deduced from differences in conformational rigidity and steric constraint. In case of the (R,R)-diastereomer, a loose, extended structure in apolar solvent changes to a compact conformation via an additional intramolecular hydrogen bond, resulting in a more defined “chiral pocket” available for the reactant on the Pt surface.
  • Competition at Chiral Metal Surfaces: Fundamental Aspects of the Inversion of Enantioselectivity in Hydrogenations on Platinum
    Norberto Bonalumi, Angelo Vargas, Davide Ferri, Thomas BĂźrgi, Tamas Mallat and Alfons Baiker
    Journal of the American Chemical Society, 127 (23) , 2005, p8467-8477
    DOI:10.1021/ja050424z | unige:14765 | Abstract | Article HTML | Article PDF
  
O-Phenylcinchonidine (PhOCD) is known to efficiently induce inversion of enantioselectivity with respect to cinchonidine (CD) in the enantioselective hydrogenation of various activated ketones on Pt/Al2O3. To understand the origin of the switch of enantioselective properties of the catalyst, the adsorption of PhOCD has been studied by in situ ATR-IR spectroscopy, in the presence of organic solvent and dissolved hydrogen, i.e., under conditions used for catalytic hydrogenation. The adsorption structures and energies of the anchoring group of CD and PhOCD were calculated on a Pt 38 cluster, using relativistically corrected density functional theory (DFT). Both approaches indicate that both modifiers are adsorbed via the quinoline ring and that the spatial arrangement of the quinuclidine skeleton is critical for the chiral recognition. New molecular level information on the conformation of CD relative to PhOCD adsorbed on a surface is extracted from the ATR spectra and supported by DFT calculations. The result is a clearer picture of the role played by the phenyl group in defining the chiral space created by the modifiers on Pt. Moreover, when CD was added to a pre-equilibrated adsorbed layer of PhOCD, a chiral adsorbed layer was formed with CD as the dominant modifier, indicating that CD adsorbs more strongly than PhOCD. Conversely, when PhOCD was added to preadsorbed CD, no significant substitution occurred. The process leading to nonlinear effects in heterogeneous asymmetric catalysis has been characterized by in situ spectroscopy, and new insight into a heterogeneous catalytic R−S switch system is provided.
  
The adsorption of N-acetyl-l-cysteine from ethanol solution on gold has been studied by in situ attenuated total reflection infrared (ATR-IR) spectroscopy, polarization modulation infrared reflection absorption spectroscopy, and a quartz crystal microbalance. After an initial fast adsorption, in situ ATR-IR revealed two considerably slower processes, besides further adsorption. The appearance of carboxylate bands and the partial disappearance of the carboxylic acid bands demonstrated that part of the molecules on the surface underwent deprotonation. In addition, the C=O stretching vibration of the carboxylic acid group shifted to lower and the amide II band to higher wavenumbers, indicating hydrogen-bonding interactions within the adsorbate layer. Based on the initial ATR-IR spectrum, which did not reveal deprotonation, the orientation of the molecule within the adsorbate layer was determined. For this, density functional theory was used to calculate the transition dipole moment vectors of the vibrational modes of N-acetyl-l-cysteine. The projections of the latter onto the z-axis of the fixed surface coordinate system were used to determine relative band intensities for different orientations of the molecule. The analysis revealed that the amide group is tilted with respect to and points away from the surface, whereas the carboxylic acid is in proximity to the surface, which is also supported by a shift of the C−O−H bending mode. This position of the acid group favors its deprotonation assisted by the gold surface and easily enables intermolecular interactions. Periodic acid stimuli revealed reversible protonation/deprotonation of part of the adsorbed molecules. However, only non-hydrogen-bonded carboxylic acid groups showed a response toward the acid stimuli.
  • Synthesis and Characterization of Tetrahedral Ru3O Clusters with Intrinsic Framework Chirality: A Chiral Probe of the Intact Cluster Catalysis Concept
    Ludovic Vieille-Petit, Georg SĂźss-Fink, Bruno Therrien, Thomas R. Ward, Helen Stœckli-Evans, GaĂŤl Labat, Lydia Karmazin-Brelot, Antonia Neels, Thomas BĂźrgi, Richard G. Finke and Collin M. Hagen
    Organometallics, 24 (25) , 2005, p6104-6119
    DOI:10.1021/om050643t | unige:14737 | Abstract | Article HTML | Article PDF
To bring evidence for or against the hypothesis of catalytic hydrogenation by intact trinuclear arene ruthenium clusters containing an oxo cap, cationic Ru3O clusters with three different arene ligands (intrinsically chiral tetrahedra) have been synthesized as racemic mixtures. By introduction of a chiral auxiliary substituent at one of the three different arene ligands, the separation of the two diastereomers was possible. The chiral Ru3O framework was evidenced by X-ray crystallography, by circular dichroism in the UV and IR regions, and by chiral shift reagents in the NMR spectra. The catalytic hydrogenation of the prochiral substrate methyl 2-acetamidoacrylate using a chiral Ru3O cluster showed no asymmetric induction, suggesting that the catalytically active species is not the intact Ru3O cluster.
  • Vibrational circular dichroism of N-acetyl-L-cysteine protected gold nanoparticles
    Cyrille Gautier and Thomas BĂźrgi
    ChemComm, (43) , 2005, p5393-5395
    DOI:10.1039/b509346e | Abstract
Vibrational circular dichroism is used to determine the conformation of a thiol adsorbed on gold nanoparticles.
  • Applied density functional theory and the deMon codes 1964-2004
    D.R. Salahub, A. Goursot, J. Weber, A.M. KĂśster and A. Vela
    in "Theory and Applications of Computational Chemistry : The First 40 Years. A Volume of Technical and Historical Perspectives" C.E. Dykstra, G. Franking, K.S. Kim and G.E. Scuseria, eds., Elsevier, Amsterdam, 2005, p1079
    unige:4033
  • Reactivity of Copper(I) Complexes with Tripodal Ligands towards O2: Structures of a Precursor [L3CuI(NCCH3)](BF4), L3 = Tris(3-isopropyl-4,5-trimethylenepyrazolyl)methane and of its Oxidation Product [L3CuII(-OH)2CuIIL3](BF4)2 with Strong Antiferromagnetic Spin-Spin Coupling
    Wolfgang Kaim, Christoph Titze, Thilo Schurr, Monika Sieger, LatĂŠvi Max Lawson Daku, Jeanne Jordanov, DarĂ­o Rojas, Ana M. GarcĂ­a and Jorge Manzur
    Zeitschrift fĂźr anorganische und allgemeine Chemie, 631 (13-14) , 2005, p2568-2574
    DOI:10.1002/zaac.200500053 | unige:3316 | Abstract | Article PDF
The molecular structure of the highly oxygen-sensitive complex [L3CuI(NCCH3)](BF4) (1) reveals approximately symmetrical coordination by the fac-tridentate (tripodal) ligand L3 = tris(3-isopropyl-4,5-trimethylenepyrazolyl)methane and a rather short CuI-N(acetonitrile) distance of 1.865(5) Å. In CH2Cl2 at -78 °C the colourless compound reacts with O2 to yield a labile purple intermediate (λmax 517 nm) - presumably a peroxodicopper(II) complex - which decomposes at -30 °C. No such intermediate was observed on reaction of the CuI complex of bis(2-pyridylmethyl)benzylamine with O2 at -80 °C. However, an EPR spectrum with g|| = 2.17 and g_|_ = 2.03 without 63,65Cu hyperfine splitting was observed at low temperatures. Exposure of the precursor 1 to air under ambient conditions yields dinuclear [L3Cu||(μ-OH)2CuIIL3](BF4)2 (2) which exhibits an EPR detectable dissociation into monomers in CH2Cl2 solution. The structure of the hexakis(dichloromethane) solvate of 2 with Cu-Cu and Cu-O distances of 3.055 and 1.94Å, respectively, is typical for dihydroxo-bridged dicopper compounds with square-pyramidal Cu|| configuration (τ = 0.03), adopting an anti arrangement. In agreement with the relatively wide Cu-O-Cu angles of 103.5° an analysis of the temperature dependence of the magnetic susceptibility revealed a rather strong (J = -633 cm-1) antiparallel spin-spin coupling. The effect is ascribed to the steric bulk of the ligand L3.
  • Vibrational and electronic circular dichroism of Δ-TRISPHAT [tris(tetrachlorobenzenediolato)phosphate(V)] anion
    Delphine Bas, Thomas BĂźrgi, JerĂ´me Lacour, JerĂ´me Vachon and Jacques Weber
    Chirality, 17 (S1) , 2005, p143-148
    DOI:10.1002/chir.20131 | unige:3628 | Abstract | Article PDF
  
Herein is reported an experimental and theoretical study of the circular dichroism properties of TRISPHAT (1) anion. ECD analysis of the [tetramethylammonium][Δ-1] salt confirms the absolute configuration assignment obtained through X-ray crystallographic analysis of the parent cinchonidium salt. The structure, infrared, and vibrational circular dichroism (VCD) spectra derived from density functional theory (DFT) calculations are compared with experimental data.
  
  • Enantioseparation of baclofen with highly sulfated β-cyclodextrin by capillary electrophoresis with laser-induced fluorescence detection
    Gamze Kavran Belin, Serge Rudaz and Jean-Luc Veuthey
    Journal of Separation Science, 28 (16) , 2005, p2187-2192
    DOI:10.1002/jssc.200500100 | unige:3629 | Abstract | Article PDF
The enantioseparation of baclofen (4-amino-3-p-chlorophenylbutyric acid) was achieved by CE-LIF with highly sulfated β-CD (HS-β-CD) as chiral selector. Naphthalene-2,3-dicarboxaldehyde was used for the derivatization of nonfluorescent baclofen. HS-β-CD (2%) containing 50 mM borate buffer at pH 9.5 was chosen as the optimal running electrolyte and applied to the analysis of baclofen enantiomers in human plasma. The linearity of calibration curves (R 2 ≥ 0.998) for R-(-) and S-(+)-baclofen was in the 0.1-2.0 μM concentration range. After a simple ACN-protein precipitation, the LOD of baclofen in plasma sample was found as low as 50 nM.
  • Optical properties of thin films of [Ru(bpy)3][NaCr(ox)3] polymeric networks
    Sebastien Rabaste, Nahid Amstutz, Andreas Hauser and Anne Pillonnet
    Applied Physics Letters, 87 (25) , 2005, p251904-251907
    DOI:10.1063/1.2143409 | unige:3261 | Abstract | Article HTML | Article PDF
The optical properties of a thin film of the [Ru(bpy)3][NaCr(ox)3] network structure obtained by pulsed laser deposition are described. The luminescence shows the characteristic doublet of R lines at 14 400 cm−1 of the spin-forbidden ligand field transition 2E(t2g3)→4A2(t2g3) of the [Cr(ox)3]3− chromophore. The resonant energy migration within the R1 line shows that the three-dimensional crystallographic structure is preserved during the coating process. The observation of the R lines of [Cr(bpy)3]3+ at 13 710 cm−1 indicates that a small fraction of Cr3+ ions migrate from the oxalate network to the tris-bipyridine cation site in the cavities of the network.
  • Electron Paramagnetic Resonance: Nonlinear Least-Squares Fitting of the Hamiltonian Parameters from Powder Spectra with the Levenberg-Marquardt Algorithm
    E.J. SouliĂŠ and T. Berclaz
    Applied Magnetic Resonance, 29 (2) , 2005, p401-416
    DOI:10.1007/BF03167026 | unige:15798 | Abstract | Article PDF
In many instances, the deduction of spectroscopic parameters from electron paramagnetic resonance spectra depends on spectrum simulation and parameter optimization. We have developed two software packages based on the approximate formulae of Iwasaki for the calculation of line positions and on the Levenberg-Marquardt algorithm for nonlinear least-squares optimization. Our software applies to systems having an anisotropicg-tensor and an arbitrary number of hyperfine interactions with nuclei. They are written in the FORTRAN 77 programming language. At present, neither the nuclear quadrupolar interaction nor the nuclear Zeeman interaction terms are handled. The programs CRISAJU and EPRPOWDERFIT apply to the cases of single crystals and powders, respectively. For use in the latter, thanks to the software ODYSSEE which implements automatic differentiation of algorithms, an ancillary subroutine, which contributes to the performance of the optimization, was created automatically.
 
The recombination dynamics of ion pairs generated upon electron transfer quenching of perylene in the first singlet excited state by tetracyanoethylene in acetonitrile is quantitatively described by the extended unified theory of photoionization/recombination. The extension incorporates the hot recombination of the ion pair passing through the level-crossing point during its diffusive motion along the reaction coordinate down to the equilibrium state. The ultrafast hot recombination vastly reduces the yield of equilibrated ion pairs subjected to subsequent thermal charge recombination and separation into free ions. The relatively successful fit of the theory to the experimentally measured kinetics of ion accumulation/recombination and free ion yield represents a firm justification of hot recombination of about 90% of primary generated ion pairs.
UNIL088 is a water-soluble prodrug of cyclosporine A (CsA) developed for topical eye delivery. Such a prodrug has to fulfil two paradoxical requirements as it must be rapidly hydrolysed under physiological conditions but also retain a long shelf-life in aqueous media. This study has been conducted to explore the stability of UNIL088 formulated as an eyedrop solution. The stability study of the prodrug was performed over a pH range of 5–7 at 20 °C and at various ionic strengths. The molecule was more stable at pH 5 than at pH 7 with conversion rate constant of 3.2 × 10−3 and 26.0 × 10−3 days−1, respectively. The effect of temperature was studied at four different temperatures and activation energy was determined. Conversion of UNIL088 followed a pseudo-first-order kinetic with an activation energy of 79.4 kJ mol−1. Due to its low solubility, CsA generated precipitated in the solution. The average size of CsA precipitates, determined by photon spectroscopy, was 0.22 and 1.08 μm at 7 and 14 days, respectively. The hydrolysis mechanism was partially elucidated by identification of the intermediate pSer-Sar-CsA.
  
  • How useful are vibrational frequencies of isotopomeric O2 fragments for assessing local symmetry? Some simple systems and the vexing case of a galactose oxidase model
    Christopher R. Kinsinger, Benjamin F. Gherman, Laura Gagliardi and Christopher J. Cramer
    Journal of Biological Inorganic Chemistry, 10 (7) , 2005, p778-789
    Keywords: copper superoxide ; copper peroxide ; Raman spectroscopy ; galactose oxidase ; density functional theory
    DOI:10.1007/s00775-005-0026-0 | unige:3651 | Abstract | Article HTML | Article PDF
The tendency for mixed-isotope O2 fragments to exhibit different stretching frequencies in asymmetric environments is examined with various levels of electronic structure theory for simple peroxides and peroxyl radicals, as well as for a variety of monocopper–O2 complexes. The study of the monocopper species is motivated by their relevance to the active site of galactose oxidase. Extensive theoretical work with an experimental model characterized by Jazdzewski et al. (J. Biol. Inorg. Chem. 8:381–393, 2003) suggests that the failure to observe a splitting between 16O18O and 18O16O isotopomers cannot be taken as evidence against end-on O2 coordination. Conformational analysis on an energetic basis, however, is complicated by biradical character inherent in all of the copper–O2 singlet structures.
Quantum chemical calculations show that metal−hydride molecules are more compact when they are placed inside a fullerene cage than when they are isolated molecules. The metal−hydrogen bond distance in ZrH4 becomes 0.15 Å shorter when it is placed inside a C60 cage. Metal−polyhydride molecules with a large number of H atoms such as ScH15 and ZrH16, which are not bound as isolated molecules, are predicted to be bound inside a fullerene cage. It is also shown that two TiH16 clusters are bound inside a bicapped (9,0) carbon nanotube. Possible ways to make metal−hydrides inside C60 and nanotubes are suggested.
  • Multiconfigurational Theoretical Study of the Octamethyldimetalates of Cr(II), Mo(II), W(II), and Re(III): Revisiting the Correlation between the M-M Bond Length and the δ→δ* Transition Energy
    Francesco Ferrante, Laura Gagliardi, Bruce E. Bursten and Alfred P. Sattelberger
    Inorganic Chemistry, 44 (34) , 2005, p8476-8480
    DOI:10.1021/ic050406i | unige:3309 | Abstract | Article HTML | Article PDF
Four compounds containing metal−metal quadruple bonds, the [M2(CH3)8]n- ions (M = Cr, Mo, W, Re and n = 4, 4, 4, 2, respectively), have been studied theoretically using multiconfigurational quantum-chemical methods. The molecular structure of the ground state of these compounds has been determined and the energy of the δ → δ* transition has been calculated and compared with previous experimental measurements. The high negative charges on the Cr, Mo, and W complexes lead to difficulties in the successful modeling of the ground-state structures, a problem that has been addressed by the explicit inclusion of four Li+ ions in these calculations. The ground-state geometries of the complexes and the δ → δ* transition have been modeled with either excellent agreement with experiment (Re) or satisfactory agreement (Cr, Mo, and W).
  • Intra- vs Intermolecular Photoinduced Electron Transfer Reactions of a Macrocyclic Donor-Acceptor Dyad
    Evan G. Moore, Paul V. Bernhardt, Alexandre FĂźrstenberg, Mark J. Riley and Eric Vauthey
    Journal of Physical Chemistry A, 109 (51) , 2005, p11715-11723
    DOI:10.1021/jp0547414 | unige:3263 | Abstract | Article HTML | Article PDF
The synthesis, structural characterization, and photophysical behavior of a 14-membered tetraazamacrocycle with pendant 4-dimethylaminobenzyl (DMAB) and 9-anthracenylmethyl groups is reported (L3, 6-((9-anthracenylmethyl)amino)-trans-6,13-dimethyl-13-((4-dimethylaminobenzyl)amino)-1,4,8,11-tetraazacyclotetradecane). In its free base form, this compound displays rapid intramolecular photoinduced electron transfer (PET) quenching of the anthracene emission, with both the secondary amines and the DMAB group capable of acting as electron donors. When complexed with Zn(II), the characteristic fluorescence of the anthracene chromophore is restored as the former of these pathways is deactivated by coordination. Importantly, it is shown that the DMAB group, which remains uncoordinated and PET active, acts only very weakly to quench emission, by comparison to the behavior of a model Zn complex lacking the pendant DMAB group, [ZnL2]2+ (Chart 1). By contrast, Stern−Volmer analysis of intermolecular quenching of [ZnL2]2+ by N,N-dimethylaniline (DMA) has shown that this reaction is diffusion limited. Hence, the pivotal role of the bridge in influencing intramolecular PET is highlighted.
 
Mixed matlokite hosts of composition BaFBrxI1−x(0≤x≤1) (pure and doped with Sm2+, Eu2+) were studied with x-ray crystallography, luminescence, Raman, and electron paramagnetic resonance (EPR) spectroscopy. Results are presented for BaFBr0.5I0.5 which demonstrate that a ferrielectric domain structure is formed due to the fact that the heavy halogen ions form separate sublattices with randomly distributed domain walls. The space group of a domain is P4  mm (No. 99). The EPR data from Eu2+ allowed to determine the volume fraction of domains.
  • Metal Analyses in Environmental and Pharmaceutical Samples by Capillary Electrophoresis with Methyl 3-Amino-3-(pyridin-3-yl)propanoate Dihydrochloride as a New Ion-Pairing Reagent
    Gamze Kavran Belin and Fazil Osman GĂźlaçar
    Helvetica Chimica Acta, 88 (8) , 2005, p2322-2332
    DOI:10.1002/hlca.200590167 | unige:3264 | Abstract | Article PDF
Separation and determination of some common metal ions was achieved with methyl 3-amino-3-(pyridin-3-yl)propanoate dihydrochloride (MAPP) as an ion-pairing reagent and pyridine as a detectable counter-ion for indirect UV detection at 254 nm. The effects of the complexing reagent and chromophore concentrations, applied voltage, and organic solvent content on the separation were investigated. The optimized separation was carried out in a running electrolyte containing 16 mM MAPP and 20 mM pyridine at pH 4.0 and was successfully applied to the qualitative and quantitative analysis of Li+, Na+, Mg2+, Ca2+, Ba2+, Ni2+, and Zn2+ in pharmaceutical vitamin preparations and various water samples.
  • Orbital-free embedding applied to the calculation of induced dipole moments in CO2.. X( X=He, Ne, Ar, Kr, Xe, Hg) van der Waals complexes
    Christoph R. Jacob, Tomasz A. Wesolowski and Lucas Visscher
    Journal of Chemical Physics, 123 (17) , 2005, p174104
    DOI:10.1063/1.2107567 | unige:3265 | Abstract | Article PDF | Article PS (gzipped)
The orbital-free frozen-density embedding scheme within density-functional theory [ T. A. Wesolowski and A. Warshel, J. Phys. Chem. 97, 8050 (1993) ] is applied to the calculation of induced dipole moments of the van der Waals complexes CO2⋯X (X = He, Ne, Ar, Kr, Xe, Hg). The accuracy of the embedding scheme is investigated by comparing to the results of supermolecule Kohn-Sham density-functional theory calculations. The influence of the basis set and the consequences of using orbital-dependent approximations to the exchange-correlation potential in embedding calculations are examined. It is found that in supermolecular Kohn-Sham density-functional calculations, different common approximations to the exchange-correlation potential are not able to describe the induced dipole moments correctly and the reasons for this failure are analyzed. It is shown that the orbital-free embedding scheme is a useful tool for applying different approximations to the exchange-correlation potential in different subsystems and that a physically guided choice of approximations for the different subsystems improves the calculated dipole moments significantly.
  
  • Quantum chemical calculations show that the uranium molecule U2 has a quintuple bond
    Laura Gagliardi and BjĂśrn O. Roos
    Nature, 433 (7028) , 2005, p848-851
    DOI:10.1038/nature03249 | unige:3652 | Article HTML | Article PDF
Covalent bonding is commonly described by Lewis's theory, with an electron pair shared between two atoms constituting one full bond. Beginning with the valence bond description for the hydrogen molecule, quantum chemists have further explored the fundamental nature of the chemical bond for atoms throughout the periodic table, confirming that most molecules are indeed held together by one electron pair for each bond. But more complex binding may occur when large numbers of atomic orbitals can participate in bond formation. Such behaviour is common with transition metals. When involving heavy actinide elements, metal–metal bonds might prove particularly complicated. To date, evidence for actinide–actinide bonds is restricted to the matrix-isolation of uranium hydrides, including H2U–UH2, and the gas-phase detection and preliminary theoretical study of the uranium molecule, U2. Here we report quantum chemical calculations on U2, showing that, although the strength of the U2 bond is comparable to that of other multiple bonds between transition metals, the bonding pattern is unique. We find that the molecule contains three electron-pair bonds and four one-electron bonds (that is, 10 bonding electrons, corresponding to a quintuple bond), and two ferromagnetically coupled electrons localized on one U atom each—so all known covalent bonding types are contributing.
The U+O chemi-ionization reaction has been investigated by quantum chemical methods. Potential-energy curves have been calculated for several electronic states of UO and UO+. Comparison with the available spectroscopic and thermodynamic values for these species is reported and a mechanism for the chemi-ionization reaction U+O→UO++e− is proposed. The U+O and Sm+O chemi-ionization reactions are the first two metal-plus-oxidant chemi-ionization reactions to be studied theoretically in this way.
Quantum chemical calculations, based on multiconfigurational wave functions and including relativistic effects, show that the U22+ system has a large number of low-lying electronic states with S of 0 to 2 and Λ ranging from zero to ten. These states share a very small bond length of about 2.30 Å, compared to 2.43 Å in neutral U2. The Coulomb explosion to 2 U+ lowers the energy by only 1.6 eV and is separated by a broad barrier.
  • The electronic spectra of 2-(2'-hydroxybenzoyl)pyrrole and 2-(2'-methoxybenzoyl)pyrrole: a theoretical study
    Giovanni Ghigo, Maurizio Ciofalo, Laura Gagliardi, Gianfranco La Manna and Christopher J. Cramer
    Journal of Physical Organic Chemistry, 18 (11) , 2005, p1099-1106
    DOI:10.1002/poc.972 | unige:3653 | Abstract | Article PDF
  
The gas-phase electronic spectra of 2-(2'-hydroxybenzoyl)pyrrole and 2-(2'-methoxybenzoyl)pyrrole have been determined using multiconfigurational perturbation theory (CASPT2). Solvatochromic spectral shifts for these molecules have been measured in cyclohexane and methanol and the electrostatic components of these shifts have been estimated using the vertical electrostatic model (VEM 4.2) developed for the configuration interaction with single excitations model implemented with the intermediate neglect of differential overlap Hamiltonian (CIS/INDO/S2). Comparison between theory and experiment and an interpretation of the main spectral differences between the two substituted pyrroles and their solvation are presented.
The electronic spectrum of the UO2 molecule has been determined using multiconfigurational wave functions together with the inclusion spin−orbit coupling. The molecule has been found to have a (5fφ)(7s), 3Φ2u, ground state. The lowest state of gerade symmetry, 3H4g, corresponding to the electronic configuration (5f)2 was found 3330 cm-1 above the ground state. The computed energy levels and oscillator strengths were used for the assignment of the experimental spectrum in the energy range 17 000−19 000 and 27 000−32 000 cm-1.
The coordination environment of uranyl in water has been studied using a combined quantum mechanical and molecular dynamics approach. Multiconfigurational wave function calculations have been performed to generate pair potentials between uranyl and water. The quantum chemically determined energies have been used to fit parameters in a polarizable force field with an added charge transfer term. Molecular dynamics simulations have been performed for the uranyl ion and up to 400 water molecules. The results show a uranyl ion with five water molecules coordinated in the equatorial plane. The U−O(H2O) distance is 2.40 Å, which is close to the experimental estimates. A second coordination shell starts at about 4.7 Å from the uranium atom. No hydrogen bonding is found between the uranyl oxygens and water. Exchange of waters between the first and second solvation shell is found to occur through a path intermediate between association and interchange. This is the first fully ab initio determination of the solvation of the uranyl ion in water.
  
In this study, we investigate the performance of the frozen-density embedding scheme within density-functional theory [ J. Phys. Chem. 97, 8050 (1993) ] to model the solvent effects on the electron-spin-resonance hyperfine coupling constants (hfcc’s) of the H2NO molecule. The hfcc’s for this molecule depend critically on the out-of-plane bending angle of the NO bond from the molecular plane. Therefore, solvent effects can have an influence on both the electronic structure for a given configuration of solute and solvent molecules and on the probability for different solute (plus solvent) structures compared to the gas phase. For an accurate modeling of dynamic effects in solution, we employ the Car-Parrinello molecular-dynamics (CPMD) approach. A first-principles-based Monte Carlo scheme is used for the gas-phase simulation, in order to avoid problems in the thermal equilibration for this small molecule. Calculations of small H2NO-water clusters show that microsolvation effects of water molecules due to hydrogen bonding can be reproduced by frozen-density embedding calculations. Even simple sum-of-molecular-densities approaches for the frozen density lead to good results. This allows us to include also bulk solvent effects by performing frozen-density calculations with many explicit water molecules for snapshots from the CPMD simulation. The electronic effect of the solvent at a given structure is reproduced by the frozen-density embedding. Dynamic structural effects in solution are found to be similar to the gas phase. But the small differences in the average structures still induce significant changes in the computed shifts due to the strong dependence of the hyperfine coupling constants on the out-of-plane bending angle.
  • Effect of the Excitation Wavelength on the Ultrafast Charge Recombination Dynamics of Donor-Acceptor Complexes in Polar Solvents
    Olivier Nicolet, Natalie Banerji, StĂŠphane Pagès and Eric Vauthey
    Journal of Physical Chemistry A, 109 (37) , 2005, p8236-8245
    DOI:10.1021/jp0532216 | unige:3268 | Abstract | Article HTML | Article PDF
The effect of the excitation wavelength on the charge recombination (CR) dynamics of several donor−acceptor complexes (DACs) composed of benzene derivatives as donors and of tetracyanoethylene or pyromellitic dianhydride as acceptors has been investigated in polar solvents using ultrafast time-resolved spectroscopy. Three different wavelength effects have been observed. (1) With complexes exhibiting two well-separated charge-transfer bands, the CR dynamics was found to be slower by a factor of about 1.5 upon excitation in the high-energy band. This effect was measured in both fast and slow relaxing solvents and was discussed in terms of different DAC geometries. (2) When the CR is faster than diffusive solvation, a slowing down of the CR with increasing excitation wavelength accompanied by an increase of the nonexponential character of the dynamics was measured. This effect appears only when exciting on the red edge of the charge-transfer absorption band. (3) When the driving force for CR is small, both nonequilibrium (hot) and thermally activated CR pathways can be operative. The results obtained with such a complex indicate that the relative contribution of these two paths depends on the excitation wavelength.
  • One-Electron Equations for Embedded Electron Density and Their Applications to Study Electronic Structure of Atoms and Molecules in Condensed Phase
    Marcin Dulak, Ruslan Kevorkyants, Fabien Tran and Tomasz A Wesolowski
    Chimia, 59 (7-8) , 2005, p488-492
    DOI:10.2533/000942905777676146 | unige:3269 | Abstract | Article PDF
Recent applications of one-electron equations for embedded electron density introduced originally for multi-level modeling of solvated molecules (T.A. Wesolowski, A. Warshel, J. Phys. Chem. 1993, 97, 8050) are reviewed. The considered applications concern properties directly related to the electronic structure of molecules (or an atom) in condensed phase such as: i) localized electronic excitations in a chromophore involved in a hydrogen-bonded intermolecular complex; ii) UV/Vis spectra of acetone in water; and iii) energy levels of f-orbitals for lanthanide cations in a crystalline environment. For each case studied, the embedding potential is represented graphically and its qualitative features are discussed.
The absorption spectra of aminocoumarin C151 in water and n-hexane solution are investigated by an explicit quantum chemical solvent model. We improved the efficiency of the frozen-density embedding scheme, as used in a former study on solvatochromism (J. Chem. Phys. 2005, 122, 094115) to describe very large solvent shells. The computer time used in this new implementation scales approximately linearly (with a low prefactor) with the number of solvent molecules. We test the ability of the frozen-density embedding to describe specific solvent effects due to hydrogen bonding for a small example system, as well as the convergence of the excitation energy with the number of solvent molecules considered in the solvation shell. Calculations with up to 500 water molecules (1500 atoms) in the solvent system are carried out. The absorption spectra are studied for C151 in aqueous or n-hexane solution for direct comparison with experimental data. To obtain snapshots of the dye molecule in solution, for which subsequent excitation energies are calculated, we use a classical molecular dynamics (MD) simulation with a force field adapted to first-principles calculations. In the calculation of solvatochromic shifts between solvents of different polarity, the vertical excitation energy obtained at the equilibrium structure of the isolated chromophore is sometimes taken as a guess for the excitation energy in a nonpolar solvent. Our results show that this is, in general, not an appropriate assumption. This is mainly due to the fact that the solute dynamics is neglected. The experimental shift between n-hexane and water as solvents is qualitatively reproduced, even by the simplest embedding approximation, and the results can be improved by a partial polarization of the frozen density. It is shown that the shift is mainly due to the electronic effect of the water molecules, and the structural effects are similar in n-hexane and water. By including water molecules, which might be directly involved in the excitation, in the embedded region, an agreement with experimental values within 0.05 eV is achieved.
  • DFT study of mixed-valent Mn(II/III) hexacyanide cluster
    Claude Daul, CĂŠdrick Rauzy, Silvio Decurtins, Patrick Franz and Andreas Hauser
    International Journal of Quantum Chemistry, 101 (6) , 2005, p753-760
    Keywords: cluster; cyanide bridging ligand; DFT; manganese complexes; Prussian blue analogue
    DOI:10.1002/qua.20333 | unige:3620 | Abstract | Article HTML | Article PDF
  
The cubic Prussian blue analogue Mn3[Mn(CN)6]2 · 15 H2O, which has the advantage of being transparent and magnetic (TN = 35 K) at the same time, has been investigated by density functional theory (DFT) calculations. The three-dimensional structure is built of MnII ions linked to MnIII ions by μ-bridging cyanides, to form a crystal structure, which is related to the NaCl type. In a first step, the relative stabilities of the mononuclear complexes [Mn(CN)6]z- (z = 2 to 4) have been studied as a function of the oxidation state, spin configuration, and the linkage isomerism of the cyanide ligand. The results we have obtained by this investigation are in good agreement with our chemical expertise. In addition, the calculations have been extended to the dinuclear [Mn2(CN)11]z- (z = 5 and 6) clusters. Furthermore, we used DFT to model the magnetic properties as well as the 3T1 → 1T2 transition, which has been observed by single-crystal near-IR spectra of Mn3[Mn(CN)6]2 · 15 H2O.
  • Thermal- and Photoinduced Spin-State Switching in an Unprecedented Three-Dimensional Bimetallic Coordination Polymer
    Virginie Niel, Amber L. Thompson, AndrĂŠs E. Goeta, Cristian Enachescu, Andreas Hauser, Ana Galet, M.Carmen MuĂąoz and JosĂŠ A. Real
    Chemistry - A European Journal, 11 (7) , 2005, p2047-2060
    Keywords: argentophilic interactions; coordination modes; polymers; spin crossover
    DOI:10.1002/chem.200400930 | unige:3271 | Abstract | Article HTML | Article PDF
The compound {Fe(pmd)[Ag(CN)2][Ag2(CN)3]} (pmd=pyrimidine) was synthesized and characterized. Magnetic, calorimetric and single crystal visible spectroscopic studies demonstrate the occurrence of a two-step high-spin (HS) ↔ low-spin (LS) transition. The critical temperatures are Tc1=185 and Tc2=148 K. Each step involves ~50 % of the iron centers, with the low-temperature step showing a hysteresis of 2.5 K. The enthalpy and entropy variations associated with the two steps are ΔH1=3.6±0.4 kJ mol-1 and ΔS1=19.5±3 J K-1 mol-1; ΔH2=4.8±0.4 kJ mol-1 and ΔS2=33.5±3 J K-1 mol-1. Photomagnetic and visible spectroscopy experiments show that below 50 K, where the LS state is the thermodynamically stable state, the compound can be switched quantitatively to the HS state using green-red light (550-650 nm). HS-to-LS relaxation experiments in the dark at temperatures between 15 and 55 K show that the relaxation takes place via a two-step cooperative process, which was analyzed in the context of the mean field theory. The crystal structure has been studied at 290, 220, 170, 90 and 30 K together with 30 K after irradiation. The compound adopts monoclinic symmetry (P21/c, Z=16) at all temperatures. There are five [FeN6] pseudo-octahedral sites linked by pmd, [Ag(CN)2]- and [Ag2(CN)3]- bridging ligands to form an unprecedented three-dimensional (6,6) topology. The structural analysis allows for an understanding of the microscopic mechanism of the two-step behavior of the thermally induced spin transition as well as the corresponding relaxation of the photoexcited compound based on the individual changes of the five sites. Synergy between metallophilic interactions and the spin transition is also shown by the variation of the Ag…Ag distances. Correlations between the variation of the unit-cell volume and the change of Ag…Ag interactions within each step with the asymmetric change of the anomalous heat capacity have also been inferred.
  • Comparison of density functionals for energy and structural differences between the high- [5T2g:(t2g)4(eg)2] and low- [1A1g:(t2g)6(eg)0] spin states of iron(II) coordination compounds. II. More functionals and the hexaminoferrous cation, [Fe(NH3)6]2+
    Antony Fouqueau, Mark E. Casida, LatĂŠvi Max Lawson Daku, Andreas Hauser and Frank Neese
    Journal of Chemical Physics, 122 (4) , 2005, p44110
    DOI:10.1063/1.1839854 | unige:3272 | Abstract | Article HTML | Article PDF
The ability of different density functionals to describe the structural and energy differences between the high- [5T2g:(t2g)4(eg)2] and low- [1A1g:(t2g)6(eg)0] spin states of small octahedral ferrous compounds is studied. This work is an extension of our previous study of the hexaquoferrous cation, [Fe(H2O)6]2+, [J. Chem. Phys. 120, 9473 (2004)] to include a second compound—namely, the hexaminoferrous cation, [Fe(NH3)6]2+—and several additional functionals. In particular, the present study includes the highly parametrized generalized gradient approximations (GGAs) known as HCTH and the meta-GGA VSXC [which together we refer to as highly parametrized density functionals (HPDFs)], now readily available in the GAUSSIAN03 program, as well as the hybrid functional PBE0. Since there are very few experimental results for these molecules with which to compare, comparison is made with best estimates obtained from second-order perturbation theory-corrected complete active space self-consistent field (CASPT2) calculations, with spectroscopy oriented configuration interaction (SORCI) calculations, and with ligand field theory (LFT) estimations. While CASPT2 and SORCI are among the most reliable ab initio methods available for this type of problem, LFT embodies many decades of empirical experience. These three methods are found to give coherent results and provide best estimates of the adiabatic low-spin–high-spin energy difference, ΔELHadia, of 12 000–13 000 cm−1 for [Fe(H2O)6]2+ and 9 000–11 000 cm−1 for [Fe(NH3)6]2+. All functionals beyond the purely local approximation produce reasonably good geometries, so long as adequate basis sets are used. In contrast, the energy splitting, ΔELHadia, is much more sensitive to the choice of functional. The local density approximation severely over stabilizes the low-spin state with respect to the high-spin state. This “density functional theory (DFT) spin pairing-energy problem” persists, but is reduced, for traditional GGAs. In contrast the hybrid functional B3LYP underestimates ΔELHadia by a few thousands of wave numbers. The RPBE GGA of Hammer, Hansen, and Nørskov gives good results for ΔELHadia as do the HPDFs, especially the VSXC functional. Surprisingly the HCTH functionals actually over correct the DFT spin pairing-energy problem, destabilizing the low-spin state relative to the high-spin state. Best agreement is found for the hybrid functional PBE0.
  • Tuning the Decay Time of Lanthanide-Based Near Infrared Luminescence from Micro- to Milliseconds through d->f Energy Transfer in Discrete Heterobimetallic Complexes
    StĂŠphane Torelli, Daniel Imbert, Martine Cantuel, GĂŠrald Bernardinelli, Sandra Delahaye, Andreas Hauser, Jean-Claude G. BĂźnzli and Claude Piguet
    Chemistry - A European Journal, 11 (11) , 2005, p3228-3242
    Keywords: energy transfer; helicates; heterobimetallic complexes; lanthanides; near infrared luminescence
    DOI:10.1002/chem.200401158 | unige:3273 | Abstract | Article HTML | Article PDF
Inert and optically active pseudo-octahedral CrIIIN6 and RuIIN6 chromophores have been incorporated by self-assembly into heterobimetallic triple-stranded helicates HHH-[CrLnL3]6+ and HHH-[RuLnL3]5+. The crystal structures of [CrLnL3](CF3SO3)6 (Ln=Nd, Eu, Yb, Lu) and [RuLnL3](CF3SO3)5 (Ln=Eu, Lu) demonstrate that the helical structure can accommodate metal ions of different sizes, without sizeable change in the intermetallic M…Ln distances. These systems are ideally suited for unravelling the molecular factors affecting the intermetallic nd→4f communication. Visible irradiation of the CrIIIN6 and RuIIN6 chromophores in HHH-[MLnL3]5/6+ (Ln=Nd, Yb, Er; M=Cr, Ru) eventually produces lanthanide-based near infrared (NIR) emission, after directional energy migration within the complexes. Depending on the kinetic regime associated with each specific d-f pair, the NIR luminescence decay times can be tuned from micro- to milliseconds. The origin of this effect, together with its rational control for programming optical functions in discrete heterobimetallic entities, are discussed.
  
  • Assessment of Density Functionals for the High-Spin/Low-Spin Energy Difference in the Low-Spin Iron(II) Tris(2,2'-bipyridine) Complex
    LatĂŠvi Max Lawson Daku, Alfredo Vargas, Andreas Hauser, Antony Fouqueau and Mark Earl Casida
    ChemPhysChem, 6 (7) , 2005, p1393-1410
    Keywords: density functional calculations; iron(II) complexes; time-resolved spectroscopy; high-spin->low-spin relaxation; spin crossover
    DOI:10.1002/cphc.200400584 | unige:3621 | Abstract | Article HTML | Article PDF
In the iron(II) low-spin complex [Fe(bpy)3]2+, the zero-point energy difference between the 5T2g(t42ge2g) high-spin and the 1A1g(t62g) low-spin states, ΔE0HL, is estimated to lie in the range of 2500-5000 cm-1. This estimate is based on the low-temperature dynamics of the high-spin→low-spin relaxation following the light-induced population of the high-spin state and on the assumption that the bond-length difference between the two states ΔrHL is equal to the average value of ≈0.2 Å, as found experimentally for the spin-crossover system. Calculations based on density functional theory (DFT) validate the structural assumption insofar as the low-spin-state optimised geometries are found to be in very good agreement with the experimental X-ray structure of the complex and the predicted high-spin geometries are all very close to one another for a whole series of common GGA (PB86, PW91, PBE, RPBE) and hybrid (B3LYP, B3LYP*, PBE1PBE) functionals. This confirmation of the structural assumption underlying the estimation of ΔE0HL from experimental relaxation rate constants permits us to use this value to assess the ability of the density functionals for the calculation of the energy difference between the HS and LS states. Since the different functionals give values from -1000 to 12000 cm-1, the comparison of the calculated values with the experimental estimate thus provides a stringent criterion for the performance of a given functional. Based on this comparison the RPBE and B3LYP* functionals give the best agreement with experiment.
  • Synthesis and Electrochemical and Photophysical Studies of Tetrathiafulvalene-Annulated Phthalocyanines
    Claudia Loosli, Chunyang Jia, Shi-Xia Liu, Marco Haas, Marylène Dias, Eric Levillain, Antonia Neels, Gael Labat, Andreas Hauser and Silvio Decurtins
    Journal of Organic Chemistry, 70 (13) , 2005, p4988-4992
    DOI:10.1021/jo0501801 | unige:3274 | Abstract | Article HTML | Article PDF
The synthesis of tetrakis(tetrathiafulvalene)-annulated metal-free and metallophthalocyanines 5−8 via the tetramerization of the phthalonitrile derivative 4 is reported. All of them have been fully characterized by electronic absorption spectroscopy, thin-layer cyclic voltammetry, mass spectrometry, and elemental analysis. Their solution electrochemical data show two reversible four-electron oxidation waves, indicating that these fused systems are strong π-electron donors, which give rise to tetra- or octaradical cation species. For the metal-free phthalocyanine 5, additionally a reversible one-electron wave was found in the negative direction arising from the reduction of the macrocycle. Moreover, the tetrathiafulvalene unit acts as an efficient reductive electron-transfer quencher for the phthalocyanine emission, but upon its oxidation, an intense luminescence is switched on.
 
Electrochemical oxidation of (C6H5)3P=N(C6H5) and (C6H5)3P=N(C6H5) leads to EPR spectra which reveal the dimeric structure of the resulting radical cation. In contrast to this behaviour, oxidation of bis-iminophosphorane leads to [(C6H5)3P=N(C6H4)N=P(C6H5)3]•+: In these species the unpaired electron is delocalized on the N(C6H4)N moiety and the persistency of the radical depends upon the relative position of the two P=N bonds.
  • Time-Resolved Spectroscopy of the Metal-to-Metal Charge Transfer Excited State in Dinuclear Cyano-Bridged Mixed-Valence Complexes
    Brendan P. Macpherson, Paul V. Bernhardt, Andreas Hauser, StĂŠphane Pagès and Eric Vauthey
    Inorganic Chemistry, 44 (15) , 2005, p5530-5536
    DOI:10.1021/ic0506512 | unige:3275 | Abstract | Article HTML | Article PDF
Visible pump−probe spectroscopy has been used to identify and characterize short-lived metal-to-metal charge transfer (MMCT) excited states in a group of cyano-bridged mixed-valence complexes of the formula [LCoIIINCMII(CN)5]-, where L is a pentadentate macrocyclic pentaamine (L14) or triamine-dithiaether (L14S) and M is Fe or Ru. Nanosecond pump−probe spectroscopy on frozen solutions of [L14CoIIINCFeII(CN)5]- and [L14SCoIIINCFeII(CN)5]- at 11 K enabled the construction of difference transient absorption spectra that featured a rise in absorbance in the region of 350−400 nm consistent with the generation of the ferricyanide chromophore of the photoexcited complex. The MMCT excited state of the Ru analogue [L14CoIIINCRuII(CN)5]- was too short-lived to allow its detection. Femtosecond pump−probe spectroscopy on aqueous solutions of [L14CoIIINCFeII(CN)5]- and [L14SCoIIINCFeII(CN)5]- at room temperature enabled the lifetimes of their CoII−FeIII MMCT excited states to be determined as 0.8 and 1.3 ps, respectively.
  • Tuning the Photophysical Behavior of Luminescent Cyclam Derivatives by Cation Binding and Excited State Redox Potential
    Evan G. Moore, Paul V. Bernhardt, Alexandre FĂźrstenberg, Mark J. Riley, Trevor A. Smith and Eric Vauthey
    Journal of Physical Chemistry A, 109 (17) , 2005, p3788-3796
    DOI:10.1021/jp044221t | unige:3276 | Abstract | Article HTML | Article PDF
The emission from two photoactive 14-membered macrocyclic ligands, 6-((naphthalen-1-ylmethyl)-amino)-trans-6,13-dimethyl-13-amino-1,4,8,11-tetraaza-cyclotetradecane (L1) and 6-((anthracen-9-ylmethyl)-amino)-trans-6,13-dimethyl-13-amino-1,4,8,11-tetraaza-cyclotetradecane (L2) is strongly quenched by a photoinduced electron transfer (PET) mechanism involving amine lone pairs as electron donors. Time-correlated single photon counting (TCSPC), multiplex transient grating (TG), and fluorescence upconversion (FU) measurements were performed to characterize this quenching mechanism. Upon complexation with the redox inactive metal ion, Zn(II), the emission of the ligands is dramatically altered, with a significant increase in the fluorescence quantum yields due to coordination-induced deactivation of the macrocyclic amine lone pair electron donors. For [ZnL2]2+, the substituted exocyclic amine nitrogen, which is not coordinated to the metal ion, does not quench the fluorescence due to an inductive effect of the proximal divalent metal ion that raises the ionization potential. However, for [ZnL1]2+, the naphthalene chromophore is a sufficiently strong excited-state oxidant for PET quenching to occur.
  
  • Ground States, Excited States, and Metal-Ligand Bonding in Rare Earth Hexachloro Complexes: A DFT-Based Ligand Field Study
    M. Atanasov, C. Daul, H.U. GĂźdel, T.A. Wesolowski and M. Zbiri
    Inorganic Chemistry, 44 (8) , 2005, p2954-2963
    DOI:10.1021/ic040105t | unige:3623 | Abstract | Article HTML | Article PDF
Metal (4f)−ligand (Cl 3p) bonding in LnCl63- (Ln = Ce to Yb) complexes has been studied on the basis of 4f→4f and Cl,3p→4f charge-transfer spectra and on the analysis of these spectra within the valence bond configuration interaction model to show that mixing of Cl 3p into the Ln 4f ligand field orbitals does not exceed 1%. Contrary to this, Kohn−Sham formalism of density functional theory using currently available approximations to the exchange-correlation functional tends to strongly overestimate 4f−3p covalency, yielding, for YbCl63-, a much larger mixing of Cl 3p→4f charge transfer into the f13 ionic ground-state wave function. Thus, ligand field density functional theory, which was recently developed and applied with success to complexes of 3d metals in our group, yields anomalously large ligand field splittings for Ln, the discrepancy with experiment increasing from left to the right of the Ln 4f series. It is shown that eliminating artificial ligand-to-metal charge transfer in Kohn−Sham calculations by a procedure described in this work leads to energies of 4f−4f transitions in good agreement with experiment. We recall an earlier concept of Ballhausen and Dahl which describes ligand field in terms of a pseudopotential and give a thorough analysis of the contributions to the ligand field from electrostatics (crystal field) and exchange (Pauli) repulsion. The close relation of the present results with those obtained using the first-principles based and electron density dependent effective embedding potential is pointed out along with implications for applications to other systems.
  • Using the Diphosphanyl Radical as a Potential Spin Label: Effect of Motion on the EPR Spectrum of an R1(R2)P--PR1 Radical
    Laurent Cataldo, Cosmina Dutan, Sushil K. Misra, Sandra Loss, HansjĂśrg GrĂźtzmacher and Michel Geoffroy
    Chemistry - A European Journal, 11 (11) , 2005, p3463-3468
    Keywords: EPR spectroscopy; internal rotation; phosphorus; radicals; spin labeling
    DOI:10.1002/chem.200401276 | unige:3277 | Abstract | Article PDF
The EPR spectrum of the novel radical Mes*(CH3)P—PMes* (Mes*=2,4,6-(tBu)3C6H2) was measured in the temperature range 100-300 K, and was found to be drastically temperature dependent as a result of the large anisotropy of the 31P hyperfine tensors. Below 180 K, a spectrum of the liquid solution is accurately simulated by calculating the spectral modifications due to slow tumbling of the radical. To achieve this simulation, an algorithm was developed by extending the well-known nitroxide slow-motion simulation technique for the coupling of one electron spin to two nuclear spins. An additional dynamic process responsible for the observed line broadening was found to occur between 180 K and room temperature; this broadening is consistent with an exchange between two conformations. The differences between the isotropic 31P couplings associated with the two conformers are shown to be probably due to an internal rotation about the P—P bond.
  • Approximating the kinetic energy functional Ts[ρ]: lessons from four-electron systems.
    Tomasz A. Wesolowski
    Molecular Physics, 103 (6-8) , 2005, p1165-1167
    Keywords: kinetic energy functional, four electron systems
    DOI:10.1080/00268970512331339341 | unige:3278 | Abstract | Article HTML | Article PDF
It is shown that for pairs of electron densities (ρα and ρα‾) obtained from mixing orbital densities in a spin-compensated four-electron system, the kinetic energy functional of the non-interacting reference system (Ts[ρ]) satisfies the general inequality Ts[ρα  +ρα‾] ≥ Ts[ρα] + Ts[ρα‾]. This condition is discussed in the context of the gradient expansion approximation to Ts[ρ] and its possible use in variational orbital-free calculations. In particular, it is shown that the second-order term of the analytic form given by von Weizsäcker violates this inequality for the considered pairs.
  • Transient Grating Investigations at Liquid-Liquid Interfaces
    Angela Punzi, Pierre Brodard and Eric Vauthey
    Chimia, 59 (3) , 2005, p116-118
    DOI:10.2533/000942905777676777 | unige:3624 | Abstract | Article PDF
  
A new four-wave-mixing technique with evanescent optical fields generated by total internal reflection at a liquid-liquid interface is described. Several applications of this method to measure thermoacoustic and dynamic properties near liquid-liquid interfaces are presented.
An optical investigation of the properties of europium (II) ions introduced in Ba2Mg3F10 single crystals is presented. The spectra and time-dependence of both the inter-configurational (f–d) and the intra-configurational (f–f) transitions are described. The emission spectrum consists of two emitting Eu2+ centres, each showing one broad f–d band and one 6P7/2 quartet. The thermal equilibrium between the f–f and f–d emitting states is investigated and found to take place on a microsecond timescale for one of the two Eu2+ centres. The spectroscopic results combined with numerical modelling of the influence of the host crystal on the 6P7/2 energy scheme allow the assignment of each f–f and f–d emission to its corresponding Eu2+ centre.
  
Transient grating experiments performed with evanescent fields resulting from total internal reflection at an interface between a polar absorbing solution and an apolar transparent solvent are described. The time evolution of the diffracted intensity was monitored from picosecond to millisecond time scales. The diffracted signal originates essentially from two density phase gratings:  one in the absorbing phase induced by thermal expansion and one in the transparent solvent due to electrostriction. A few nanoseconds after excitation, the latter grating is replaced by a thermal grating due to thermal diffusion from the absorbing phase. The speed of sound and the acoustic attenuation measured near the interface are found to be essentially the same as in the bulk solutions. However, after addition of a surfactant in the polar phase, the speed of sound near the interface differs substantially from that in the bulk with the same surfactant concentration. This effect is interpreted in terms of adsorption at the liquid/liquid interface. Other phenomena, which are not observed in bulk experiments, such as acoustic echoes and a fast oscillation of the signal intensity, are also described.
  • The merits of the frozen-density embedding scheme to model solvatochromic shifts.
    J. Neugebauer, M.J. Louwerse, E.J. Baerends and T.A. Wesolowski
    Journal of Chemical Physics, 122 (9) , 2005, p94115
    Keywords: density functional theory; excited states; molecular electronic states; solvent effects; charge exchange; electron correlations; organic compounds; molecular dynamics method
    DOI:10.1063/1.1858411 | unige:3279 | Abstract | Article HTML | Article PDF
We investigate the usefulness of a frozen-density embedding scheme within density-functional theory [ J. Phys. Chem. 97, 8050 (1993) ] for the calculation of solvatochromic shifts. The frozen-density calculations, particularly of excitation energies have two clear advantages over the standard supermolecule calculations: (i) calculations for much larger systems are feasible, since the time-consuming time-dependent density functional theory (TDDFT) part is carried out in a limited molecular orbital space, while the effect of the surroundings is still included at a quantum mechanical level. This allows a large number of solvent molecules to be included and thus affords both specific and nonspecific solvent effects to be modeled. (ii) Only excitations of the system of interest, i.e., the selected embedded system, are calculated. This allows an easy analysis and interpretation of the results. In TDDFT calculations, it avoids unphysical results introduced by spurious mixings with the artificially too low charge-transfer excitations which are an artifact of the adiabatic local-density approximation or generalized gradient approximation exchange-correlation kernels currently used. The performance of the frozen-density embedding method is tested for the well-studied solvatochromic properties of the n→π* excitation of acetone. Further enhancement of the efficiency is studied by constructing approximate solvent densities, e.g., from a superposition of densities of individual solvent molecules. This is demonstrated for systems with up to 802 atoms. To obtain a realistic modeling of the absorption spectra of solvated molecules, including the effect of the solvent motions, we combine the embedding scheme with classical molecular dynamics (MD) and Car-Parrinello MD simulations to obtain snapshots of the solute and its solvent environment, for which then excitation energies are calculated. The frozen-density embedding yields estimated solvent shifts in the range of 0.20–0.26 eV, in good agreement with experimental values of between 0.19 and 0.21 eV.
 
The photophysics of the dye Lucifer Yellow ethylenediamine (LYen) has been investigated in various polar solvents. The main deactivation pathways of its first singlet excited state are the fluorescence and the intersystem crossing. In water, non-radiative decay by intermolecular proton transfer becomes a significant deactivation channel. The early fluorescence dynamics, which was investigated in liquids and in reverse micelles, was found to depend substantially on the environment. An important static quenching of LYen by tryptophan and indole occurring in the subpicosecond timescale was observed. The use of the fluorescence dynamics of LYen as a local probe is illustrated by preliminary results obtained with a biotinylated Lucifer Yellow derivative complexed with avidin.
  • Effects of milling, doping and cycling of NaAlH4 studied by vibrational spectroscopy and X-ray diffraction
    S. Gomes, G. Renaudin, H. Hagemann, K. Yvon, M.P. Sulic and C.M. Jensen
    Journal of Alloys and Compounds, 390 (1-2) , 2005, p305-313
    Keywords: Hydrogen storage materials; Infrared and Raman spectroscopy; X-ray diffraction; Alanates
    DOI:10.1016/j.jallcom.2004.08.036 | unige:3280 | Abstract | Article PDF
The effects of milling and doping NaAlH4 with TiCl3, TiF3 and Ti(OBun)4, and of cycling doped NaAlH4 have been investigated by infrared (IR) and Raman spectroscopy and X-ray powder diffraction. Milling and doping produce similar effects. Both decrease the crystal domain size (~900 Å for milled and ~700 Å for doped, as compared to ~1600 Å for unmilled and undoped NaAlH4) and increase anisotropic strain (by a factor >2.5, mainly along c). They also influence structure parameters such as the axial ratio c/a, cell volume and atomic displacement amplitudes. They show IR line shifts by ~15 cm−1 to higher frequencies for the Al–H asymmetric stretching mode ν3, and by ~20 cm−1 to lower frequencies for one part of the H–Al–H asymmetric bending mode ν4, thus suggesting structural changes in the local environment of the [AlH4]− units. The broad ν3 bands become sharpened which suggests a more homogeneous local environment of the [AlH4]− units, and there appears a new vibration at 710 cm−1. The Raman data show no such effects. Cycling leads to an increase in domain size (1200–1600 Å), IR line shifts similar to doped samples (except for TiF3: downward shift by ~10 cm−1) and a general broadening of the ν3 mode that depend on the nature of the dopants. These observations support the idea that some Ti diffusion and substitution into the alanate lattice does occur, in particular during cycling, and that this provides the mechanism through which Ti-doping enhances kinetics during re-crystallisation.
  
  • Synthesis and X-ray Crystal Structure of a Cationic Homoleptic (SPS)2Rh(III) Complex and EPR Study of Its Reduction Process
    Marjolaine Doux, Nicolas MĂŠzailles, Louis Ricard, Pascal Le Floch, Prashant Adkine, ThĂŠo Berclaz and Michel Geoffroy
    Inorganic Chemistry, 44 (4) , 2005, p1147-1152
    DOI:10.1021/ic049046+ | unige:3627 | Abstract | Article HTML | Article PDF
Oxidation of the square planar Rh(I) complex [Rh(SPSMe)(PPh3)] (SPSMe = 1-methyl-1-P-2,6-bis(diphenylphosphinosulfide)-3,5-(bisphenyl)-phosphinine) (1) based on mixed SPS-pincer ligand with hexachloroethane yielded the Rh(III) dichloride complex [Rh(SPSMe)(PPh3)Cl2] (2), which was structurally characterized. The homoleptic Rh(III) complex [Rh(SPSMe)2][Cl] (4) was obtained via the stoichiometric reaction of SPSMe anion (3) with [Rh(tht)3Cl3] (tht = tetrahydrothiophene). Complex 4, which was characterized by X-ray diffraction, was also studied by cyclic voltammetry. Complex 4 can be reversibly reduced at E = −1.16 V (vs SCE) to give the neutral 19-electron Rh(II) complex [Rh(SPSMe)2] (5). Accordingly, complex 5 could be synthesized via chemical reduction of 4 with zinc dust. EPR spectra of complex 5 were obtained after electrochemical or chemical reduction of 4 in THF or CH2Cl2. Hyperfine interaction with two equivalent 31P nuclei was observed in liquid solution, while an additional coupling with a spin 1/2 nucleus, probably 103Rh, was detected in frozen solution. The 31P couplings are consistent with DFT calculations that predict a drastic increase in the axial P−S bond lengths when reducing (SPSMe)2Rh(III). In the reduced complex, the unpaired electron is mainly localized in a rhodium dz2 orbital, consistent with the g-anisotropy measured at 100 K.
  • Transient Holographic Grating Techniques in Chemical Dynamics
    Eric Vauthey
    in "Encyclopedia of modern optics" R. D. Guenther, D. G. Steel and L. Bayvel Eds; Oxford: Elsevier, 1 , 2005, p73-82
    unige:3931
Geometry and interaction energy in complexes of the Ph-L type (L = Ar, N2, CO, H2O, NH3, CH4, CH3OH, CH3F) involving neutral or cationic phenol were determined using the density functional theory formalism based on the minimization of the total energy bifunctional and gradient-dependent approximations for its exchange-correlation and nonadditive kinetic-energy parts. For the neutral complexes the calculated interaction energies range from 1 kcal/mol for the Ph-Ar complex to about 10 kcal/mol for Ph-NH3. The interactions are stronger if the cationic phenol is involved (up to 25 kcal/mol). It is found, except for neutral Ph-Ar, that the hydrogen-bonded structure is more stable than the π-bound one. Calculated interaction energies (De) correlate well with the experimental dissociation energies (D0).
  • The basis set effect on the results of the minimization of the total energy bifunctional E[ρA,ρB]
    M. Dulak and T.A. Wesolowski
    International Journal of Quantum Chemistry, 101 (5) , 2005, p543-549
    Keywords: basis sets; total energy bifunctional; orbital-free embedding; weak intermolecular complexes
    DOI:10.1002/qua.20309 | unige:3315 | Abstract | Article HTML | Article PDF
The basis set effect on the results of the minimization of the total energy bifunctional E[ρA, ρB] approximated at the local density approximation level is analyzed for several weak intermolecular complexes. The considered complexes formed by hydrocarbons at the equilibrium geometry were previously studied by means of the same formalism using large decontracted basis sets consisting of Gaussian-type atomic orbitals limited to s-, p-, and d-functions. In this work, we use our two new computer implementations of the formalism to analyze the basis set effects accompanying changing the basis sets from Gaussian-type orbitals to Slater-type orbitals and including f-functions. We show that the interaction energies, their components, and the energies of the highest occupied molecular orbital converge within a range of 0.07 kcal/mol, 0.08 kcal/mol, and 0.06 eV, respectively.
  • Tetrathiafulvalene-phosphine-based iron and ruthenium carbonyl complexes: Electrochemical and EPR studies
    Cyril Gouverd, FrĂŠdĂŠric Biaso, Laurent Cataldo, ThĂŠo Berclaz, Michel Geoffroy, Eric Levillain, Narcis Avarvari, Marc FourmiguĂŠ, François X. Sauvage and Corinne Wartelle
    Physical Chemistry Chemical Physics, 7 , 2005, p85-93
    DOI:10.1039/b409958c | unige:3282 | Abstract | Article HTML | Article PDF
The radical cation of the redox active ligand 3,4-dimethyl-3',4'-bis-(diphenylphosphino)-tetrathiafulvalene ( P2) has been chemically and electrochemically generated and studied by EPR spectroscopy. Consistent with DFT calculations, the observed hyperfine structure (septet due to the two methyl groups) indicates a strong delocalization of the unpaired electron on the central S2C=CS2 part of the tetrathiafulvalene (TTF) moiety and zero spin densities on the phosphine groups. In contrast with the ruthenium(0) carbonyl complexes of P2 whose one-electron oxidation directly leads to decomplexation and produces P2•+, one-electron oxidation of [Fe( P2)(CO)3] gives rise to the metal-centered oxidation species [Fe(I)( P2)(CO)3], characterized by a coupling with two 31P nuclei and a rather large g-anisotropy. The stability of this complex is however modest and, after some minutes, the species resulting from the scission of a P–Fe bond is detected. Moreover, in presence of free ligand, [Fe(I)( P2)(CO)3] reacts to give the complex [Fe(I)( P2)2(CO)] containing two TTF fragments. The two-electron oxidation of [Fe( P2)(CO)3] leads to decomplexation and to the P2•+ spectrum. Besides EPR spectroscopy, cyclic voltammetry as well as FTIR spectroelectrochemistry are used in order to explain the behaviour of [Fe( P2)(CO)3] upon oxidation. This behaviour notably differs from that of the Ru(0) counterpart. This difference is tentatively rationalized on the basis of structural arguments.
  • Inversion of enantioselectivity in the platinum-catalyzed hydrogenation of substituted acetophenones
    Reto Hess, Angelo Vargas, Tamas Mallat, Thomas BĂźrgi and Alfons Baiker
    Journal of Catalysis, 222 (1) , 2004, p117-128
    Keywords: Asymmetric hydrogenation; Ring-substituted acetophenones; Ethers of cinchonidine; Inversion of enantioselectivity; 3,5-Di(trifluoromethyl)acetophenone
    DOI:10.1016/j.jcat.2003.10.021 | unige:14708 | Article HTML | Article PDF
The enantioselective hydrogenation of ring-substituted acetophenones that possess no functional group in the α-position to the keto group represents the latest extension of the application range of the Pt–cinchona system. The influence of the type of solvent, pressure, temperature, and modifier/substrate/Pt molar ratios was investigated in the hydrogenation of 3,5-di(trifluoromethyl)acetophenone. Modification of a 5 wt% Pt/Al2O3 catalyst by cinchonidine (CD) afforded the corresponding (S)-1-phenylethanol (69.5% ee). Working in strongly polar solvents, addition of trifluoroacetic acid in a weakly polar solvent, and replacing CD by its ether derivatives resulted in the inversion of enantioselectivity. Addition of CD or any of its derivatives always led to a lower reaction rate, contrary to the generally observed rate acceleration in the hydrogenation of α-functionalized activated ketones over the same catalyst system. Another fundamental difference to the hydrogenation of α-functionalized activated ketones is that both the quinuclidine N and the OH functions of CD influence the stereochemical outcome of the reaction, as clarified by using O- and N-substituted derivatives of CD. Ab initio calculations confirmed these remarkable mechanistic differences. Inversion of enantioselectivity in the presence of strongly polar and acidic solvents is attributed to special interactions with the OH function of CD, and to the formation of a CD–acid ion pair, respectively. A possible explanation for the moderate ee's in the hydrogenation of ring-substituted acetophenones is that a reaction pathway without involvement of the OH function of CD is also feasible. This competing pathway is even faster and provides low ee to the opposite enantiomer.
  • The absolute configuration of heptahelicene: a VCD spectroscopy study
    Thomas BĂźrgi, Atsushi Urakawa, Bahar Behzadi, Karl-Heinz Ernst and Alfons Baiker
    New Journal of Chemistry, 28 (3) , 2004, p332-334
    DOI:10.1039/b312877f | unige:14877 | Abstract | Article HTML | Article PDF
The comparison between experimental and calculated VCD spectra allowed the unequivocal assignment of the absolute configuration of heptahelicene C30H18 as P(+).
  • Adsorption of activated ketones on platinum and their reactivity to hydrogenation: a DFT study
    Angelo Vargas, Thomas BĂźrgi and Alfons Baiker
    Journal of Catalysis, 222 (2) , 2004, p439-449
    DOI:10.1016/j.jcat.2003.12.006 | unige:14735 | Article HTML | Article PDF
The adsorption of several ketones interesting for the enantioselective hydrogenation on cinchona-modified platinum has been modeled using relativistically corrected density functional theory. Two metal clusters, containing 19 and 31 Pt atoms, respectively, have been used to model a Pt(111) surface. The two adsorption modes Ρ1 and Ρ2 have been described, and their importance for the mechanism of hydrogenation has been pointed out. The effect of an ester group in ι position and of ι-fluorination of a ketone on its adsorption has been studied, and an explanation for the reactivity enhancement due to the ketone substitution has been proposed.
  • Epoxidation of cyclic allylic alcohols on titania-silica aerogels studied by attenuated total reflection infrared and modulation spectroscopy
    Andreas Gisler, Thomas BĂźrgi and Alfons Baiker
    Journal of Catalysis, 222 (2) , 2004, p461-469
    DOI:10.1016/j.jcat.2003.12.009 | unige:14772 | Article HTML | Article PDF
  
Epoxidation of cyclohex-2-en-1-ol and cyclooct-2-en-1-ol on titania–silica aerogel catalysts using t-butylhydroperoxide (TBHP) as oxidant was studied by in situ attenuated total reflection (ATR) Fourier transform infrared spectroscopy. Probing of the catalytic liquid–solid interface revealed different adsorption behaviors for the two allylic alcohols on the aerogel. Cyclohexenol was found to adsorb stronger and less reversible on the catalyst surface and Ti sites than cyclooctenol. The spectroscopic measurements under working conditions support the previously proposed hydroxy-assisted mechanism for the formation of cyclohexenol oxide and the silanol-assisted mechanism for cyclooctenol epoxidation. The evidence of the former is traced to the occurrence of a framework vibration upon adsorption of cyclohexenol, whereas the latter is supported by large negative bands of the silanol groups at 3700 and 980 cm−1 in the case of cyclooctenol epoxidation.
  • Identification of catalyst surface species during asymmetric platinum-catalyzed hydrogenation in a "supercritical" solvent
    Michael S. Schneider, Atsushi Urakawa, Jan-Dierk Grunwaldt, Thomas BĂźrgi and Alfons Baiker
    ChemComm, 6 , 2004, p744-745
    DOI:10.1039/b315591a | unige:14722 | Abstract | Article HTML | Article PDF
The enantioselective hydrogenation of several isatine derivatives over cinchonidine modified Pt/Al2O3 was investigated. A maximum enantiomeric excess (e.e.) of 45% was found for (R)-5,7-dimethylisatin. The enantiomeric excess was limited by racemization catalyzed by the basic cinchonidine in solution, leading to low enantiomeric excess at high cinchonidine concentration. The modifier in solution also catalyzed the formation of the corresponding isatide. High cinchonidine concentration favored isatide formation, whereas low cinchonidine concentration and high hydrogen pressure favored alcohol formation. The isatide, formed from the isatin reactant and the alcohol, underwent disproportionation. Though both hydrogenation and isatide formation are fast reactions, isatide formation was considerably faster at least at the beginning of the reaction. Substitution of the isatin reactant had relatively little effect on enantiomeric excess but affected considerably the rate of racemization.
The behavior of ethyl pyruvate during adsorption on vapor deposited alumina-supported platinum films and on a commercial 5 wt % Pt/Al2O3 catalyst has been studied in the absence and presence of coadsorbed cinchonidine, which is usually applied as a chiral modifier in the platinum-catalyzed enantioselective hydrogenation of α-ketoesters. The in situ ATR−IR measurements, performed at room temperature using hydrogen-saturated CH2Cl2 as solvent, revealed that upon adsorption on the platinum some of the ethyl pyruvate decomposes leading to strongly adsorbed CO and other fragmentation products. The CO originating from decomposition of ethyl pyruvate reached approximately 14% of the amount of adsorbable CO on the free platinum surface and is proposed to be adsorbed preferentially on energetically favored sites such as edges and corners. The presence of cinchonidine (10-4 M) lead to a drastic decrease of the decomposition rate of ethyl pyruvate by a factor of about 60 under the conditions used. Competitive adsorption experiments of CO and cinchonidine in the presence of hydrogen indicated that cinchonidine can displace the adsorbed CO, confirming the strong anchoring of cinchonidine on the platinum surface, which is a prerequisite for its action as a chiral modifier. The findings of the adsorption studies provide a plausible explanation for the earlier made observation that the sequence of admission of α-ketoester, chiral modifier, and hydrogen affects the catalytic performance of platinum-catalyzed enantioselective hydrogenation. The decomposition is likely to occur also with other α-ketoesters and may have a bearing on the initial transient period, typically observed during hydrogenation of such compounds on cinchona-modified platinum catalysts.
  • Adsorption of cinchonidine on platinum: a DFT insight in the mechanism of enantioselective hydrogenation of activated ketones
    Angelo Vargas, Thomas BĂźrgi and Alfons Baiker
    Journal of Catalysis, 226 (1) , 2004, p69-82
    Keywords: Cinchonidine; Adsorption; DFT; Platinum; Enantioselective hydrogenation; DFT; Activated ketones; Mechanism
    DOI:10.1016/j.jcat.2004.05.011 | unige:14736 | Article HTML | Article PDF
The adsorption of cinchonidine on platinum has been calculated with relativistically corrected density-functional theory, by first studying the interaction of the 1(S)-(4-quinolinyl)ethanol with a platinum cluster of 31 metal atoms, and by successive addition and separate optimization of the quinuclidine moiety. The conformations of the alkaloid on the surface were analyzed and their possible interactions with a surface chemisorbed methylpyruvate and acetophenone are discussed. A chiral space that is able to selectively accommodate surface enantiomers and to promote their rapid hydrogenation in a ligand-accelerated fashion has been determined. The role of the O-alkylation of the alkaloid in the modulation of enantioselectivity has been rationalized within the new interaction model.
  • Enantioselective hydrogenation of furancarboxylic acids: a spectroscopic and theoretical study
    Mihaela Maris, Thomas BĂźrgi, Tamas Mallat and Alfons Baiker
    Journal of Catalysis, 226 (2) , 2004, p393-400
    DOI:10.1016/j.jcat.2004.05.036 | unige:14718 | Article HTML | Article PDF
FTIR and NMR spectroscopy and ab initio calculations were applied to understand the nature of enantioselection in the hydrogenation of the heteroaromatic ring in furan- and benzofurancarboxylic acids over cinchonidine-modified Pd. Most probably, cinchonidine adsorbs on Pd, via its quinoline moiety, approximately parallel to the surface, and the protonated quinuclidine N atom and the OH function of the alkaloid form a cyclic complex with the deprotonated acid dimer (2:1 acid:cinchonidine). The acid dimer adsorbs via the electron-rich furan ring and the carboxylate groups close to parallel to the Pd surface; the furan O atom points toward the OH function of cinchonidine. In this position, hydrogen uptake from the Pd surface results in the (S)-enantiomer as the major product. Another cyclic complex (1:1) involving cinchonidine and only one acid molecule is also feasible in solution, but this rigid structure is thermodynamically less favored, and it may be difficult to fulfill the geometric constraints imposed by adsorption on the metal surface.
  
  • Absolute Configuration Modulation Attenuated Total Reflection IR Spectroscopy: An in Situ Method for Probing Chiral Recognition in Liquid Chromatography
    Ronny Wirz, Thomas BĂźrgi, Wolfgang Lindner and Alfons Baiker
    Analytical Chemistry, 76 (18) , 2004, p5319-5330
    DOI:10.1021/ac049428x | unige:14790 | Abstract | Article HTML | Article PDF
A method to selectively probe the different adsorption of enantiomers at chiral solid−liquid interfaces is applied, which combines attenuated total reflection infrared spectroscopy and modulation spectroscopy. The spectral changes on the surface are followed while the absolute configuration of the adsorbate is changed periodically. Demodulated spectra are calculated by performing a subsequent digital phase-sensitive data analysis. The method is sensitive solely to the difference of the interaction of the two enantiomers with the chiral surface, and the small spectral changes are amplified by the phase-sensitive data analysis. Its potential is demonstrated by investigating an already well-studied system in liquid chromatography, namely, the enantiomer separation of N-3,5-dinitrobenzoyl-(R,S)-leucine (DNB-(R,S)-Leu) using tert-butylcarbamoyl quinine (tBuCQN) as the chiral selector immobilized on the surface of porous silica particles. The performed experiments and density functional theory calculations confirm an interaction model that was proposed earlier based on solution NMR and XRD in the solid state. It emerges that the ionic interaction is the strongest one, but the main reason for the potential for enantioseparation of the chiral stationary phase (CSP) is the distinct formation of a hydrogen bond of the (S)-enantiomer with the chiral selector. This H-bond is established between the amide N−H of DNB-(S)-Leu with the carbamate C=O of the CSP. The (R)-enantiomer instead shows no specific hydrogen bonds. Only the unspecific ionic bonding between the protonated quinine part of the tBuCQN and the carboxylate of the DNB-(R)-Leu (holds also for DNB-(S)-Leu) is observed.
In situ attenuated total reflection infrared spectroscopy in a flow-through cell combined with online UV−vis spectroscopy was used to investigate the oxidation of 2-propanol over Pd/Al2O3 catalyst. The state of the catalyst was driven fast between reduced and oxidized by admitting alternately dissolved hydrogen and oxygen, and the response of the catalytic solid−liquid interface was followed in time. Besides the oxidation product acetone and the water that forms, when hydrogen and oxygen are simultaneously adsorbed on the catalyst surface, an additional species was observed with a characteristic band at ~1065 cm-1. On the basis of the transient character of the adsorbate and density functional theory calculations, we assign this species to adsorbed 2-propoxide. Its observation indicates that the second dehydrogenation step is rate limiting in an oxidative dehydrogenation mechanism. The results furthermore show that adsorbed hydrogen and oxygen limit the dissociative adsorption of 2-propanol and that 2-propoxide can be hydrogenated back to the reactant in the presence of adsorbed hydrogen.
Modification of a metal surface by a strongly adsorbed chiral organic molecule has proven to be an interesting strategy for heterogeneous chiral catalysis. Platinum chirally modified by cinchona alkaloids, successfully applied for the enantioselective hydrogenation of α-ketoesters, is probably the most prominent catalyst based on this concept. Despite considerable research efforts toward understanding of this complex catalytic system, the proposed mechanistic models are still debated. Here we discuss how enantiodifferentiation can be induced on a catalytically active surface and validate the models proposed for the platinum−cinchona system in the light of the existing molecular knowledge.
Cumulated triple bonds: A new class of molecules, the isoelectronic series of NUIr, (depicted) has been postulated and theoretically studied. The bond between the actinide and the 5d-metal atom is very short and is shown to correspond to triple bonding. The N — U bond is also a triple bond.
 
The fragmentation behaviour of the ion MeP(O)OMe+ has been investigated using quantum mechanical calculations at the B3LYP and MP2 levels to support experiments made with an Ion Trap Mass Spectrometer. Two mechanisms for the loss of CH2O are found, one involving a 1,3-H migration to phosphorus and the other a 1,2-methyl migration to give P(OMe)2+ followed by a 1,3-H migration. In each case an ion-dipole complex is formed that rapidly dissociates to yield CH2O. The relative importance of each route has been previously determined experimentally via isotopic labelling experiments, and the theoretical results are found to be consistent with these experimental results. The mechanisms suggested in the earlier work involving a 1,4 H migration to O are shown to be energetically unfavourable.
  • A theoretical study of the lowest electronic states of azobenzene: the role of torsion coordinate in the cis-trans photoisomerization
    Laura Gagliardi, Giorgio Orlandi, Fernando Bernardi, Alessandro Cembran and Marco Garavelli
    Theoretical Chemistry Accounts, 111 (2-6) , 2004, p363-372
    DOI:10.1007/s00214-003-0528-1 | unige:3319 | Article PDF
In the present paper we report the results of a multiconfigurational computational study on potential-energy curves of azobenzene along the NN twisting to clarify the role of this coordinate in the decay of the S2(ππ*) and S1(nπ*) states. We have found that there is a singlet state, S3 at the trans geometry, on the basis of the doubly excited configuration n2π*2, that has a deep minimum at about 90° of twisting, where it is the lowest excited singlet state. The existence of this state provides an explanation for the short lifetime of S2(ππ*) and for the wavelength-dependence of azobenzene photochemistry. We have characterized the S1(nπ*) state by calculating its vibrational frequencies, which are found to correspond to the recently observed transient Raman spectrum. We have also computed the potential-energy curve for the triplet T1(nπ*) at the density functional theory B3LYP level, which indicates that in this state the isomerization occurs along the twisting coordinate.
In this paper, we identify the most efficient decay and isomerization route of the S1, T1, and S0 states of azobenzene. By use of quantum chemical methods, we have searched for the transition states (TS) on the S1 potential energy surface and for the S0/S1 conical intersections (CIs) that are closer to the minimum energy path on the S1. We found only one TS, at 60° of CNNC torsion from the E isomer, which requires an activation energy of only 2 kcal/mol. The lowest energy CIs, lying also 2 kcal/mol above the S1 minimum, were found on the torsion pathway for CNNC angles in the range 95−90°. The lowest CI along the inversion path was found ca. 25 kcal/mol higher than the S1 minimum and was character1 state decay involves mainly the torsion route and that the inversion mechanism may play a role only if the molecule is excited with an excess energy of at least 25 kcal/mol with respect to the S1 minimum of the E isomer. We have calculated the spin−orbit couplings between S0 and T1 at several geometries along the CNNC torsion coordinate. These spin−orbit couplings were about 20−30 cm-1 for all the geometries considered. Since the potential energy curves of S0 and T1 cross in the region of twisted CNNC angle, these couplings are large enough to ensure that the T1 lifetime is very short (~10 ps) and that thermal isomerization can proceed via the nonadiabatic torsion route involving the S0−T1−S0 crossing with preexponential factor and activation energy in agreement with the values obtained from kinetic measures.
The Sm+O chemiionization reaction has been investigated theoretically using a method that allows for correlation and relativistic effects. Potential energy curves have been calculated for several electronic states of SmO and SmO+. Comparison with available spectroscopic and thermodynamic values for these species is reported and a mechanism for the chemiionization reaction Sm+O is proposed. The importance of spin–orbit coupling in the excited states of SmO, in allowing this chemiionization reaction to take place, has been revealed by these calculations. This paper shows the metal-plus-oxidant chemiionization reaction.
  
Quantum chemical calculations suggest that a series of molecules with the general formula MAu6 are stable, where M is a a group 6 atom, Cr, Mo, W, respectively. These species have a structure analogous to the corresponding MH6 compounds, while they differ from the MX6, where X is a halogen. The further reaction MAu6 + 3Au2→MAu12 is strongly exothermic.
A new method is presented, which makes it possible to partition molecular properties like multipole moments and polarizabilities, into atomic and interatomic contributions. The method requires a subdivision of the atomic basis set into occupied and virtual basis functions for each atom in the molecular system. The localization procedure is organized into a series of orthogonalizations of the original basis set, which will have as a final result a localized orthonormal basis set. The new localization procedure is demonstrated to be stable with various basis sets, and to provide physically meaningful localized properties. Transferability of the methyl properties for the alkane series and of the carbon and hydrogen properties for the benzene, naphtalene, and anthracene series is demonstrated.
Quantum chemical calculations predict the existence of new molecular species with general formula MH12, where M is a group 6 atom. The previous MHn species had n values up to 9. The new systems with n = 12 would be a new record for metal hydrides.
  • Contribution to the analysis of the predissociated rovibronic structure of the symmetric isotopomers 16O3 and 18O3 of ozone near 10,400 cm-1: 3A2(A20) <- X1A1(O00) and 3B2 <- X1A1
    G. Wannous, A.J. Bouvier, Z. El Helou, X.F.D. Chillier, S. Churassy, R. Bacis, A. Campargue, G. Weirauch and R.H. Judge
    Spectrochimica Acta, Part A: Molecular and Biomolecular Spectroscopy, 60 (4) , 2004, p889-898
    Keywords: Ozone; 16O3; 18O3; Electronic spectroscopy; Wulf transition; Intra cavity laser absorption spectroscopy (ICLAS)
    DOI:10.1016/S1386-1425(03)00316-0 | unige:3324 | Abstract | Article HTML | Article PDF
The absorption spectrum of ozone was recorded at low temperatures (down to −135 °C) by high resolution Fourier transform spectrometry and intra cavity laser absorption spectroscopy (ICLAS) near 10,400 cm−1. A preliminary analysis of the rotational structure of the absorption spectra of 16O3 and 18O3 shows that this spectral region corresponds to a superposition of two different electronic transitions, one with a very broad rotational structure, showing for the first time the asymmetric stretching frequency mode ν3 of the electronic state 3A2, the other formed by a completely diffuse band, probably the 201 band of a new transition due to the triplet electronic state 3B2. Predissociation effects induce large broadening of the rotational lines for the transition centered at 10,473 cm−1 identified as the 302 band of the 3A2 ← X1A1 electronic transition. The rotational structure cannot be analyzed directly but instead the band contour method was used to confirm the symmetry of the transition and to estimate the spectroscopic constants for the 16O isotopomer. The origin of the band is at 10,473±3 cm−1 and the value of the 16O3(3A2) antisymmetric stretching frequency mode is equal to 460±2 cm−1. We believe that the diffuse band is due to the 3B2 state and is located at about 10,363±3 cm−1 for 16O3 and 10,354±3 cm−1 for 18O3. The isotopic rules confirm the different results obtained for 18O3 and 16O3.
  • Photophysical properties of three-dimensional transition metal tris-oxalate network structures
    Andreas Hauser, Marianne E. Von Arx, Vaughan S. Langford, SĂŠlim Kairouani, Ueli Oetliker and Anne Pillonnet
    in "Topics in Current Chemistry, Transition Metal and Rare Earth Compounds. Excited States, Transitions, and Interactions, Vol III" (ed. H. Yersin), Springer, Berlin, 241 , 2004
    Keywords: oxalate networks ; [Cr(ox)3]3– ; [Cr(bpy)3]3+ ; 2E state ; resonant energy transfer ; phonon-assisted energy transfer ; Förster transfer ; Exchange interaction
    DOI:10.1007/b96860 | unige:3941
Excitation energy transfer processes play an important role in many areas of physics, chemistry and biology. The three-dimensional oxalate networks of composition [MIII(bpy)3][MIMIII(ox)3]ClO4 (bpy=2,2-bipyridine, ox=oxalate, MI=alkali ion) allow for a variety of combinations of different transition metal ions. The combination with chromium(III) on both the tris-bipyridine as well as the tris-oxalate site constitutes a model system in which it is possible to differentiate unambiguously between energy transfer from [Cr(ox)3]3– to [Cr(bpy)3]3+ due to dipole-dipole interaction on the one hand and exchange interaction on the other hand. Furthermore it is possible to just as unambiguously differentiate between the common temperature dependent phonon-assisted energy migration within the 2E state of [Cr(ox)3]3–, and a unique resonant process.
  • Crystalline, Mixed-Valence Manganese Analogue of Prussian Blue: Magnetic, Spectroscopic, X-ray and Neutron Diffraction Studies
    Patrick Franz, Christina Ambrus, Andreas Hauser, Dmitry Chernyshov, Marc Hostettler, JĂźrg Hauser, Lukas Keller, Karl Krämer, Helen Stoeckli-Evans, Philip Pattison, Hans-Beat BĂźrgi and Silvio Decurtins
    Journal of the American Chemical Society, 126 (50) , 2004, p16472-16477
    DOI:10.1021/ja0465451 | unige:3247 | Abstract | Article HTML | Article PDF
  
The compound of stoichiometry Mn(II)3[Mn(III)(CN)6]2·zH2O (z = 12−16) (1) forms air-stable, transparent red crystals. Low-temperature single crystal optical spectroscopy and single crystal X-ray diffraction provide compelling evidence for N-bonded high-spin manganese(II), and C-bonded low-spin manganese(III) ions arranged in a disordered, face-centered cubic lattice analogous to that of Prussian Blue. X-ray and neutron diffraction show structured diffuse scattering indicative of partially correlated (rather than random) substitutions of [Mn(III)(CN)6] ions by (H2O)6 clusters. Magnetic susceptibility measurements and elastic neutron scattering experiments indicate a ferrimagnetic structure below the critical temperature Tc = 35.5 K.
  • Ultrafast spectroscopy on bimolecular photoinduced electron transfer reactions
    Ana Morandeira, Alexandre FĂźrstenberg, StĂŠphane Pagès, Bernhard Lang and Eric Vauthey
    The Spectrum, 17 (4) , 2004, p14-19
    unige:3248 | Article PDF
  • Thermodynamic and Kinetic Stability of Synthetic Multifunctional Rigid-Rod β- Barrel Pores: Evidence for Supramolecular Catalysis
    Svetlana Litvinchuk, Guillaume Bollot, Jiri Mareda, Abhigyan Som, Dawn Ronan, Muhammad Raza Shah, Philippe Perrottet, Naomi Sakai and Stefan Matile
    Journal of the American Chemical Society, 126 (32) , 2004, p10067-10075
    DOI:10.1021/ja0481878 | unige:3612 | Abstract | Article HTML | Article PDF
The lessons learned from p-octiphenyl β-barrel pores are applied to the rational design of synthetic multifunctional pore 1 that is unstable but inert, two characteristics proposed to be ideal for practical applications. Nonlinear dependence on monomer concentration provided direct evidence that pore 1 is tetrameric (n = 4.0), unstable, and “invisible,” i.e., incompatible with structural studies by conventional methods. The long lifetime of high-conductance single pores in planar bilayers demonstrated that rigid-rod β-barrel 1 is inert and large (d ≈ 12 Å). Multifunctionality of rigid-rod β-barrel 1 was confirmed by adaptable blockage of pore host 1 with representative guests in planar (8-hydroxy-1,3,6-pyrenetrisulfonate, KD = 190 μM, n = 4.9) and spherical bilayers (poly-l-glutamate, KD ≤ 105 nM, n = 1.0; adenosine triphosphate, KD = 240 μM, n = 2.0) and saturation kinetics for the esterolysis of a representative substrate (8-acetoxy-1,3,6-pyrenetrisulfonate, KM = 0.6 μM). The thermodynamic instability of rigid-rod β-barrel 1 provided unprecedented access to experimental evidence for supramolecular catalysis (n = 3.7). Comparison of the obtained kcat = 0.03 min-1 with the kcat ≈ 0.18 min-1 for stable analogues gave a global KD ≈ 39 μM3 for supramolecular catalyst 1 with a monomer/barrel ratio ≈ 20 under experimental conditions. The demonstrated “invisibility” of supramolecular multifunctionality identified molecular modeling as an attractive method to secure otherwise elusive insights into structure. The first molecular mechanics modeling (MacroModel, MMFF94) of multifunctional rigid-rod β-barrel pore hosts 1 with internal 1,3,6-pyrenetrisulfonate guests is reported.
  • Mechanism of Exciplex Decay: The Quantum Yields and the Rate Constants of Radical Ion Formation from Exciplexes with Partial Charge Transfer
    D.N. Dogadkin, E.V. Dolotova, I.V. Soboleva, M.G. Kuzmin, V.F. Plyusnin, I.P. Pozdnyakov, V.P. Grivin, E. Vauthey, P. Brodard and O. Nicolet
    High Energy Chemistry, 38 (6) , 2004, p392-400
    DOI:10.1023/B:HIEC.0000048237.12132.26 | unige:3249 | Abstract | Article PDF
The dynamics of exciplex and radical ion formation was studied in donor–acceptor systems with G * et > –0.1 eV. It was shown that the quenching of excited singlet states of aromatic molecules by electron donors in polar solvents led to the formation of radical ions via exciplex dissociation resulting to complete charge separation. Intersystem crossing and internal conversion into the ground state (back electron transfer) compete with this process. The quantum yields and the rate constants of the radical ion formation were measured.
  • Combination of gas chromatography-mass spectrometry and mass spectral deconvolution for structural elucidation of an unusual C29-steroid detected in a complex sedimentary matrix
    Yvan Finck, Nebahat Aydin, Caroline Pellaton, Georges Gorin and Fazil O. GĂźlaçar
    Journal of Chromatography A, 1049 (1-2) , 2004, p227-231
    Keywords: deconvolution; AMDIS; 4 14-Dimethycholestane; Monterey
    DOI:10.1016/j.chroma.2004.08.004 | unige:3613 | Abstract | Article HTML | Article PDF
A complex sedimentary sample from the Monterey Formation (CA, USA) has been submitted to GC–MS analysis followed by mass spectral deconvolution using Automated Mass Spectral Deconvolution and Identification System (AMDIS). Adjusting the parameters of the software allowed for the extraction of the spectrum of an unusual steroidal hydrocarbon coeluting with the major compound of the chromatogram. Following a careful interpretation of the “extracted” mass spectrum, the structure of the unknown has been postulated to be the 4,14-dimethylcholestane (DMC). Possible origins of this rare steroid are briefly discussed. Thus, application of AMDIS appears to be particularly suitable for the GC–MS analysis of natural complex mixtures characterized by a high number of analytes present in low amounts.
  • Capillary electrokinetic separation of polycyclic aromatic hydrocarbons using cetylpyridinium bromide
    G. Kavran Belin, F.B. Erim and F.O. GĂźlaçar
    Polycyclic Aromatic Compounds, 24 (4-5) , 2004, p343-352
    Keywords: capillary electrophoresis; cationic surfactant; cetylpyridinium bromide; polycyclic aromatic hydrocarbons
    DOI:10.1080/10406630490468504 | unige:3250 | Abstract | Article HTML | Article PDF
Polycyclic aromatic hydrocarbons (PAHs) including isomeric pairs were separated in capillary electrokinetic chromatography using a cationic surfactant cetylpyridinium bromide (CPBr) as additive. With addition of 2 mM CPBr into the running electrolyte, dynamic coating occurs in the capillary and EOF is reversed. Changes of electroosmotic and electrophoretic mobilities with increasing CPBr concentration were investigated. Under optimum separation conditions, running electrolyte contains 50% MeCN, 20 mM acetate, and 40 mM CPBr at pH = 4.0. Using high concentration of organic solvent, aggregation of surfactants into micelles is prevented. Significant retentions indicate solvophobic, n- and π-electron interactions between CPBr monomers and PAHs.
  • Calculated volume and energy profiles for water exchange on t2g6 rhodium(III) and iridium(III) hexaaquaions : conclusive evidence for an Ia mechanism
    D. De Vito, J. Weber and A.E. Merbach
    Inorganic Chemistry, 43 (3) , 2004, p858-864
    DOI:10.1021/ic035096n | unige:3325 | Abstract | Article HTML | Article PDF
An Ia mechanism was assigned for water exchange on the hexaaquaions Rh(OH2)63+ and Ir(OH2)63+ on the basis of negative ΔV‡ experimental values (−4.2 and −5.7 cm3 mol-1, respectively). The use of ΔV‡ as a mechanistic criterion was open to debate primarily because ΔV‡ could be affected by extension or compression of the nonparticipating ligand bond lengths on going to the transition state of an exchange process. In this paper, volume and energy profiles for two distinct water exchange mechanisms (D and Ia) have been computed using quantum chemical calculations which include hydration effects. The activation energy for Ir(OH2)63+ is 32.2 kJ mol-1 in favor of the Ia mechanism (127.9 kJ mol-1), as opposed to a D pathway; the value for the Ia mechanism being close to ΔH‡ and ΔG‡ experimental values (130.5 kJ mol-1 and 129.9 kJ mol-1 at 298 K, respectively). Volumes of activation, computed using Connolly surfaces and for the Ia pathway (ΔV‡calc = −3.9 and −3.5 cm3 mol-1, respectively, for Rh3+ and Ir3+), are in agreement with the experimental values. Further, it is demonstrated for both mechanisms that the contribution to the volume of activation due to the changes in bond lengths between Ir(III) and the spectator water molecules is negligible: −1.8 for the D, and −0.9 cm3 mol-1 for Ia mechanism. This finding clarifies the debate about the interpretation of ΔV‡ and unequivocally confirms the occurrence of an Ia mechanism with retention of configuration and a small a character for both Rh(III) and Ir(III) hexaaquaions.
  • Tuning facial-meridional isomerisation in monometallic nine-coordinate lanthanide complexes with unsymmetrical tridentate ligands
    T. Le Borgne, P. Altmann, N. AndrĂŠ, J.-C.G. BĂźnzli, G. Bernardinelli, , J. Weber and C. Piguet
    Dalton Transactions, (5) , 2004, p723-733
    DOI:10.1039/b316035a | unige:3251 | Abstract | Article HTML | Article PDF
The unsymmetrical tridentate benzimidazole–pyridine–carboxamide units in ligands L1–L4 react with trivalent lanthanides, LnIII, to give the nine-co-ordinate triple-helical complexes [Ln(Li)3]3+ (i=1–4) existing as mixtures of C3-symmetrical facial and C1-symmetrical meridional isomers. Although the β13 formation constants are 3–4 orders of magnitude smaller for these complexes than those found for the D3-symmetrical analogues [Ln(Li)3]3+ (i=5–6) with symmetrical ligands, their formation at the millimolar scale is quantitative and the emission quantum yield of [Eu(L2)3]3+ is significantly larger. The fac-[Ln(Li)3]3+↔mer-[Ln(Li)3]3+ (i =1–4) isomerisation process in acetonitrile is slow enough for Ln=LuIII to be quantified by 1H NMR below room temperature. The separation of enthalpic and entropic contributions shows that the distribution of the facial and meridional isomers can be tuned by the judicious peripheral substitution of the ligands affecting the interstrand interactions. Molecular mechanics (MM) calculations suggest that one supplementary interstrand -stacking interaction stabilises the meridional isomers, while the facial isomers benefit from more favourable electrostatic contributions. As a result of the mixture of facial and meridional isomers in solution, we were unable to obtain single crystals of 13 complexes, but the X-ray crystal structures of their nine-co-ordinate precursors [Eu(L1)2(CF3SO3)2(H2O)](CF3SO3)(C3H5N)2(H2O) ( 6, C45H54EuF9N10O13S3, monoclinic, P21/c, Z=4) and [Eu(L4)2(CF3SO3)2(H2O)](CF3SO3)(C4H4O)1.5 ( 7, C51H66EuF9N8O15.5S3, triclinic, P, Z=2) provide crucial structural information on the binding mode of the unsymmetrical tridentate ligands.
  • Unusual regio-and enantioselective [1,2]-Stevens rearrangement of a spirobi[dibenzazepinium] cation
    L. Vial, M.-H. Gonçalves, , J. Weber, G. Bernardinelli and J. Lacour
    Synlett, (9) , 2004, p1565-1568
    Keywords: ammonium, rearrangements, regioselectivity, stereoselectivity, ylides
    DOI:10.1055/s-2004-829069 | unige:3256 | Abstract | Article HTML | Article PDF
Highly symmetric spirobi[dibenzazepinium] cation 3 reacts with P4-t-Bu to form exclusively a ring-expanded tertiary amine; this unusual reactivity can be traced back to the geometry of the ylide.
  
The electron transfer quenching dynamics of excited perylene and cyanoperylene in various donating solvents has been investigated by using ultrafast fluorescence up-conversion and multiplex transient grating. The strongly nonexponential fluorescence decays have been analyzed by using the orientational model described in the first article of this series (J. Phys. Chem. A 2003, 107, 5375). It appears that the solvent dependence of the quenching dynamics is strongly connected to the number of surrounding donor molecules enabling ultrafast electron transfer. This number depends mainly on the driving force for electron transfer, on steric interactions, and on the occurrence of dipole−dipole interactions with the acceptor. The quenching product is an exciplex with a strong charge-transfer character. The complicated wavelength dependence of the fluorescence dynamics in the exciplex region, as well as the spectral dynamics observed in the transient grating data, is attributed to dipolar solvation, which leads to an increase of the charge-transfer character of the exciplex. The strong donor dependence of the exciplex lifetime is very similar to that reported earlier for the charge recombination time of geminate ion pairs in acetonitrile, and can be rationalized in terms of different intramolecular reorganization energies and electronic coupling constants.
  • Effect of Conformational Changes on a One-Electron Reduction Process: Evidence of a One-Electron PP Bond Formation in a Bis(phosphinine)
    Sylvie Choua, Cosmina Dutan, Laurent Cataldo, ThĂŠo Berclaz, Michel Geoffroy, Nicolas MĂŠzailles, Audrey Moores, Louis Ricard and Pascal Le Floch
    Chemistry - A European Journal, 10 (16) , 2004, p4080-4090
    Keywords: aromaticity; density functional calculations; electron transfer; EPR spectroscopy; fluxionality; P ligands
    DOI:10.1002/chem.200400073 | unige:3252 | Abstract | Article HTML | Article PDF
EPR spectra show that one-electron reduction of bis(3-phenyl-6,6-(trimethylsilyl)phosphinine-2-yl)dimethylsilane (1) on an alkali mirror leads to a radical anion that is localized on a single phosphinine ring, whereas the radical anion formed from the same reaction in the presence of cryptand or from an electron transfer with sodium naphthalenide is delocalized on the two phosphinine rings. Density functional theory (DFT) calculations show that in the last species the unpaired electron is mainly confined in a loose P — P bond (3.479 Å), which results from the overlap of two phosphorus p orbitals. In contrast, as attested by X-ray spectroscopy, the P — P distance in neutral 1 is large (5.8 Å). As shown by crystal structure analysis, addition of a second electron leads to the formation of a classical P — P single bond (P — P 2.389 Å). Spectral modifications induced by the presence of cryptand or by a change in the reaction temperature are consistent with the formation of a tight ion pair that stabilizes the radical structure localized on a single phosphinine ring. It is suggested that the structure of this pair hinders internal rotation around the C — Si bonds and prevents 1 from adopting a conformation that shortens the intramolecular P — P distance. The ability of the phosphinine radical anion to reversibly form weak P — P bonds with neutral phosphinines in the absence of steric hindrance is confirmed by EPR spectra obtained for 2,6-bis(trimethylsilyl)-3-phenylphosphinine (2). Moreover, as shown by NMR spectroscopy, in this system, which contains only one phosphinine ring, further reduction leads to an intermolecular reaction with the formation of a classical P — P bond.
The packing preferences of dimers formed by nitrogen-containing planar polycyclic aromatic hydrocarbons ((C30H15N)2 and (C36H15N)2) were studied by means of theoretical calculations. Potential energy curves corresponding to various relative motions of the monomers (vertical displacement, rotating, slipping, and combinations of them) were derived. It was found that the monomers in such π-stacked dimers are rather strongly held together (the interaction energy is about −9 kcal/mol) in an off-centered arrangement. It emerges as a general picture that the aligned structures are less stable than the ones where the nitrogen atoms, as the centers of the considered monomers, are not on top of each other but offset by 1.8−2.7 Å. Displacing the centers further results in a rapid reduction of the interaction energy. Within these relatively large relative motions (up to about 3 Å) of the monomers, however, no significant loss of stability of the dimers is noted. In the case of C30H15N, changing the orientation of the enantiotopic faces in the dimer formation leads to two nonequivalent minimum energy structures of similar energies but notably different geometries. The most stable structure of both dimers studied resembles that of two adjacent layers of graphite. We conclude, therefore, that the studied molecules could be considered as good building block candidates for the fabrication of columnar organic conductors.
  • Application of the density functional theory derived orbital-free embedding potential to calculate the splitting energies of lanthanide cations inchloroelpasolite crystals
    Mohamed Zbiri, Mihail Atanasov, Claude Daul, Juan Maria Garcia-Lastra and Tomasz A. Wesolowski
    Chemical Physics Letters, 397 (4-6) , 2004, p441-446
    DOI:10.1016/j.cplett.2004.09.010 | unige:3614 | Abstract | Article HTML | Article PDF
  
Ligand field splitting energies of lanthanides Ln3+ (Ln = from Ce to Yb) in octahedral environment are calculated using the Hohenberg–Kohn theorems based orbital-free embedding formalism. The lanthanide cation is described at orbital level whereas its environment is represented by means of an additional term in the Kohn–Sham-like one-electron equations expressed as an explicit functional of two electron densities: that of the cation and that of the ligands. The calculated splitting energies, which are in good agreement with the ones derived from experiment, are attributed to two main factors: (i) polarization of the electron density of the ligands, and; (ii) ion–ligand Pauli repulsion.
The theoretically calculated dimerization-induced shifts of the lowest excitation energies in two model systems, adenine−thymine and guanine−cytosine base pairs, are analyzed. The applied formalism is based on first principles and allows one to study the influence of the microscopic environment of a given molecule on its ground- [Wesolowski, T. A.; Warshel, A. J. Phys. Chem. 1993, 97, 8050] and excited-state [Casida, M. E.; Wesolowski, T. A. Int. J. Quantum Chem. 2004, 96, 577] properties. The assessment of the relative importance of such effects as (a) Coulomb interactions, (b) orbital interactions, (c) electronic polarization of the environment, and (d) electron density overlap effects is straightforward in this formalism. In the applied formalism, electron density overlap effects can be further decomposed into the exchange−correlation component which provides a small attractive contribution and the repulsive kinetic energy-dependent component. It is shown that the shifts can be attributed to the electrostatic interactions and the repulsive overlap-dependent term in the embedding potential. The electronic polarization of the environment plays a significant role (up to 30% of the total shift) only in transitions involving the orbitals localized on hydrogen bond donor groups. For all analyzed shifts, the contribution of the intermolecular orbital interactions is negligible. The analysis of this work provides strong evidence supporting the use of the widely applied embedding-molecule strategy in computational studies of chromophores in a condensed phase even in such cases where only one end of the hydrogen bond is included in the quantum mechanical part.
A model of nonequilibrium charge recombination from an excited adiabatic state of a donor-acceptor complex induced by the nonadiabatic interaction operator is considered. The decay of the excited state population prepared by a short laser pulse is shown to be highly nonexponential. The influence of the excitation pulse carrier frequency on the ultrafast charge recombination dynamics of excited donor-acceptor complexes is explored. The charge recombination rate constant is found to decrease with increasing excitation frequency. The variation of the excitation pulse carrier frequency within the charge transfer absorption band of the complex can alter the effective charge recombination rate by up to a factor 2. The magnitude of this spectral effect decreases strongly with increasing electronic coupling.
  • Ligand-field theoretical considerations
    Andreas Hauser
    in "Topics in Current Chemistry, Spincrossover in Transition Metal Compounds, Vol I" (eds P. GĂźtlich, H. A. Goodwin), Springer, Berlin, 233 , 2004, p49-58
    Keywords: spin crossover ; ligand field theory ; optical properties ; vibronic structure ; configurational coordinate
    DOI:10.1007/b40394-9 | unige:3942
The phenomenon of the thermal spin transition, as observed for octahedral transition metal complexes having a d 4 to d 7 electronic configuration, can be fully rationalised on the basis of ligand field theory. In order to arrive at a self-consistent description of the vibronic structure of spin crossover compounds, it is essential to take into account the fact that the population of anti-bonding orbitals in the high-spin state results in a substantially larger metal-ligand bond length than for the low-spin state. Whereas the electron-electron repulsion is not affected to any great extent by such a bond length difference, the ligand field strength for iron(II) spin crossover compounds can be estimated to be almost twice as large in the low-spin state as compared to the one for the high-spin state. In fact, the dependence of the ligand field strength on the metal-ligand distance may be considered the quantum mechanical driving force for the spin crossover phenomenon.
  • Light-induced spin-crossover and the high-spin -> low-spin relaxation
    Andreas Hauser
    in "Topics in Current Chemistry, Spincrossover in Transition Metal Compounds, Vol II" (eds P. GĂźtlich, H. A. Goodwin), Springer, Berlin, 234 , 2004, p155-198
    Keywords: thermal and light-induced spin crossover ; high-spin->ow-spin relaxation ; Intersystem crossing ; external pressure ; chemical pressure ; cooperative effects ; Iron(II) complexes
    DOI:10.1007/b95416 | unige:3943
The discovery of a light-induced spin transition at cryogenic temperatures in a series of iron(II) spin-crossover compounds in 1984 has had an enormous impact on spin-crossover research. Apart from being an interesting photophysical phenomenon in its own right, it provided the means of studying the dynamics of the intersystem crossing process between the high-spin and the low-spin state in a series of compounds and over a large temperature range. It could thus be firmly established that intersystem crossing in spin-crossover compounds is a tunnelling process, with a limiting low-temperature lifetime below 50 K and a thermally activated region above 100 K. This review begins with an elucidation of the mechanism of the light-induced spin transition, followed by an in depth discussion of the chemical and physical factors, including cooperative effects, governing the lifetimes of the light-induced metastable states.
  
  • Comparison of density functionals for energy and structural differences between the high- [5T2g: (t2g)4(eg)2] and low- [1A1g: (t2g)6(eg)0] spin states of the hexaquoferrous cation [Fe(H2O)6]2+
    Antony Fouqueau, SĂŠbastien Mer, Mark E. Casida, Latevi Max Lawson Daku, Andreas Hauser, Tsonka Mineva and Frank Neese
    Journal of Chemical Physics, 120 (20) , 2004, p9473-9486
    DOI:10.1063/1.1710046 | unige:3615 | Abstract | Article HTML | Article PDF | Article PS (gzipped)
A comparison of density functionals is made for the calculation of energy and geometry differences for the high- [5T2g: (t2g)4(eg)2] and low- [1A1g: (t2g)6(eg)0] spin states of the hexaquoferrous cation [Fe(H2O)6]2+. Since very little experimental results are available (except for crystal structures involving the cation in its high-spin state), the primary comparison is with our own complete active-space self-consistent field (CASSCF), second-order perturbation theory-corrected complete active-space self-consistent field (CASPT2), and spectroscopy-oriented configuration interaction (SORCI) calculations. We find that generalized gradient approximations (GGAs) and the B3LYP hybrid functional provide geometries in good agreement with experiment and with our CASSCF calculations provided sufficiently extended basis sets are used (i.e., polarization functions on the iron and polarization and diffuse functions on the water molecules). In contrast, CASPT2 calculations of the low-spin–high-spin energy difference ΔELH = ELS−EHS appear to be significantly overestimated due to basis set limitations in the sense that the energy difference of the atomic asymptotes (5D→1I excitation of Fe2+) are overestimated by about 3000 cm−1. An empirical shift of the molecular ΔELH based upon atomic calculations provides a best estimate of 12 000–13 000 cm−1. Our unshifted SORCI result is 13 300 cm−1, consistent with previous comparisons between SORCI and experimental excitation energies which suggest that no such empirical shift is needed in conjunction with this method. In contrast, after estimation of incomplete basis set effects, GGAs with one exception underestimate this value by 3000–4000 cm−1 while the B3LYP functional underestimates it by only about 1000 cm−1. The exception is the GGA functional RPBE which appears to perform as well as or better than the B3LYP functional for the properties studied here. In order to obtain a best estimate of the molecular ΔELH within the context of density functional theory (DFT) calculations we have also performed atomic excitation energy calculations using the multiplet sum method. These atomic DFT calculations suggest that no empirical correction is needed for the DFT calculations.
  • Cobalt(II)-tris-2,2'-bipyridine as a Spin-Crossover Complex: Evidence for Cooperative Effects in Three-Dimensional Oxalate Networks
    Mohamed Zerara and Andreas Hauser
    ChemPhysChem, 5 (3) , 2004, p395-399
    Keywords: cooperative effects; host-guest systems; magnetic properties; spin crossover; X-ray diffraction
    DOI:10.1002/cphc.200301044 | unige:3257 | Abstract | Article HTML | Article PDF
In the three-dimensional oxalate network structures of composition [CoxMII1-x(bpy)3][MICr(ox)3], the spin state of the [Cox(bpy)3]2+ complex can be tuned by means of chemical pressure. With MI=Na it is a classic high-spin complex. Substitution of Na by Li stabilises the complex and it becomes a spin-crossover complex. Dilution with MII=Fe reinforces this effect, and MII=Zn reverses it.
  • Ruthenium(II) as a Novel "Labile" Partner in Thermodynamic Self-Assembly of Heterobimetallic d-f Triple-Stranded Helicates
    StĂŠphane Torelli, Sandra Delahaye, Andreas Hauser, GĂŠrald Bernardinelli and Claude Piguet
    Chemistry - A European Journal, 10 (14) , 2004, p3503-3516
    Keywords: helical structures; isomerization; lanthanides; ruthenium; self-assembly
    DOI:10.1002/chem.200400092 | unige:3258 | Abstract | Article HTML | Article PDF
Unsymmetrical substituted bidentate benzimidazol-2-ylpyridine ligands L2 and L3 react with [Ru(dmso)4Cl2] in ethanol to give statistical 1:3 mixtures of fac-[Ru(Li)3]2+ and mer-[Ru(Li)3]2+ (i=2, 3; ΔGΘisomerisation=-2.7 kJ mol-1). In more polar solvents (acetonitrile, methanol), the free energy of the facial ↔ meridional isomerisation process favours mer-[Ru(Li)3]2+, which is the only isomer observed in solution at the equilibrium (ΔGΘisomerisation≤-11.4 kJ mol-1). Since the latter process takes several days for [Ru(L2)3]2+, fac-[Ru(L2)3]2+ and mer-[Ru(L2)3]2+ have been separated by chromatography, but the 28-fold increase in velocity observed for [Ru(L3)3]2+ provides only mer-[Ru(L3)3](ClO4)2 after chromatography (RuC60H51N9O8Cl2, monoclinic, P21/n, Z=4). The facial isomer can be stabilised when an appended tridentate binding unit, connected at the 5-position of the benzimidazol-2-ylpyridine unit in ligand L1, interacts with nine-coordinate lanthanides(III). The free energy of the facial↔meridional isomerisation is reversed (ΔGΘisomerisation≥11.4 kJ mol-1), and the Ru — N bonds are labile enough to allow the quantitative thermodynamic self-assembly of HHH-[RuLu(L1)3]5+ within hours ([RuLu(L1)3](CF3SO3)4.5Cl0.5(CH3OH)2.5: RuLuC106H109Cl0.5N21O19S4.5F13.5, triclinic, P, Z=2). Electrochemical and photophysical studies show that the benzimidazol-2-ylpyridine units in L1-L3 display similar π-acceptor properties to, but stronger π-donor properties than, those found in 2,2'-bipyridine. This shifts the intraligand π→π* and the MLCT transitions toward lower energies in the pseudo-octahedral [Ru(Li)3]2+ (i=2, 3) chromophores. The concomitant short lifetime of the 3MLCT excited state points to efficient, thermally activated quenching via low-energy Ru-centred d-d states, a limitation which is partially overcome by mechanical coupling in HHH-[RuLu(L1)3]5+.
  • Effect of the excitation pulse carrier frequency on the ultrafast charge recombination dynamics of donor-acceptor complexes: Stochastic simulations and experiments
    Roman G. Fedunov, Serguei V. Feskov, Anatoly I. Ivanov, Olivier Nicolet, StĂŠphane Pagès and Eric Vauthey
    Journal of Chemical Physics, 121 (8) , 2004, p3643-3656
    DOI:10.1063/1.1772362 | unige:3616 | Abstract | Article HTML | Article PDF | Article PS (gzipped)
  
The influence of the excitation pulse carrier frequency on the ultrafast charge recombination dynamics of excited donor-acceptor complexes has been explored both theoretically and experimentally. The theoretical description involves the explicit treatment of both the optical formation of the nuclear wave packet on the excited free energy surface and its ensuing dynamics. The wave packet motion and the electronic transition are described within the framework of the stochastic point-transition approach. It is shown that the variation of the pulse carrier frequency within the absorption band can significantly change the effective charge recombination dynamics. The mechanism of this phenomenon is analyzed and a semiquantitative interpretation is suggested. The role of the vibrational coherence in the recombination dynamics is discussed. An experimental investigation of the ultrafast charge recombination dynamics of two donor-acceptor complexes in valeronitrile also is presented. The decays of the excited state population were found to be highly nonexponential, the degree of non-exponentiality depending on the excitation frequency. For one complex, the charge recombination dynamics was found to slow down upon increasing the excitation frequency, while the opposite behavior was observed with the other complex. These experimental observations follow qualitatively the predictions of the simulations.
The fluorescence dynamics of perylene in the presence of tetracyanoethylene in acetonitrile was studied experimentally and theoretically, taking into consideration that the quenching is carried out by remote electron transfer in the Marcus inverted region. The initial stage was understood as a convolution of the pumping pulse with the system response accounting for the fastest (kinetic) electron transfer accompanied by vibrational relaxation. The subsequent development of the process was analyzed with differential encounter theory using different models of transfer rates distinguished by their mean square values. The single channel transfer having a bell-shaped rate with a maximum shifted far from the contact produces the ground state ion pair. It was recognized as inappropriate for fitting the quenching kinetics at moderate and long times equally well. A good fit was reached when an additional near contact quenching is switched on, to account for the parallel electron transfer to the electronically excited state of the same pair. The concentration dependence of the fluorescence quantum yield is well fitted using the same rates of distant transfer as for quenching kinetics while the contact approximation applied to the same data was shown to be inadequate.
An investigation of the ultrafast excited state dynamics of triporphyrin and hexaporphyrin arrays consisting of covalently linked Zn tetraphenylporphine (ZnTPP) and free base tetraphenylporphine (FbTPP) units is reported. The interchromophoric distance in the hexamers is of the order of 13 Å, while it varies from 26 to 70 Å in the trimers. These arrays exhibit several features that differ substantially from those of the monomeric units:  a broadening of the Soret band, a shortening of the S2 lifetime of the ZnTPP chromophores, and additional ultrafast decay components of the S1 fluorescence. In the hexaporphyrin arrays, most of these features are attributed to the presence of excitonic states that result from the strong coupling between the Bx,y transition dipoles. The time constants for S1 energy transfer between ZnTPP chromophores as well as between ZnTPP and FbTPP moieties, deduced from anisotropic and isotropic time-resolved fluorescence measurements, were found to be of the order of a few tens of picoseconds. Moreover, back energy transfer from the FbTPP to ZnTPP units is also observed. At high to moderate excitation intensity, S1−S1 annihilation becomes an important decay mechanism of the excited state population of the hexaporphyrins. In the triporphyrins, the differences relative to the monomer are ascribed to the interaction with the phenylacetylene linkers, which lifts the degeneracy of the S2 states. S2 and S1 energy transfer were found to take place in the triporphyrin with the shortest linker only. In the other triporphyrins, an efficient energy transfer from the linker to the porphyrin units was observed.
  • Synthesis and Structure of the new Fluoride Bromide Ba6.668(2)Ca0.332(2)F12Br2 and Solid Solutions with Composition Ba7-xCaxF12(ClyBr1-y)2 with x=~0.5, 0<1
    B. FrĂźhmann, F. Kubel, H. Hagemann and H. Bill
    Zeitschrift fĂźr anorganische und allgemeine Chemie, 630 (9) , 2004, p1484-1488
    Keywords: inorganic compounds, crystal structure, solid solutions
    DOI:10.1002/zaac.200400108 | unige:3714 | Abstract | Article PDF
Crystals of the chemical composition Ba7F12Cl2 were modified by adding a small amount of Ca2+ to allow the synthesis of the corresponding bromine compound Ba[Ca]7F12Br2. These samples were prepared in a NaBr flux and characterized by single crystal x-ray diffraction. The new structure crystallizes in a disordered arrangement in the hexagonal space group P63/m (176). The calcium ion has a coordination number of 6.
Solid solutions on the heavy halide position can be synthesised in a NaCl/NaBr flux to obtain the compounds Ba7-xCaxF12(ClyBr1-y)2 with x = ~0.5 and 0 < y < 1.
Regardless the amount of calcium used in the preparation process, the Ca stoichiometry in the compound is always between 0.3 and 0.5. The lattice parameters differ depending on the Ca- and Br-content between 1053.81(5) ≤ a = b ≤ 1058.93(3) pm and 421.21 ≤ c ≤ 426.78(3) pm.
  
  • Comparative Infra-Red, Raman, and Natural Bond Orbital Analyses of King's Sultam
    H. Hagemann, M. Dulak, C. Chapuis, T.A. Wesolowski and J. Jurczak
    Helvetica Chimica Acta, 87 (7) , 2004, p1748-1766
    DOI:10.1002/hlca.200490158 | unige:3520 | Abstract | Article PDF
By means of 1H-NOESY- and Raman-spectroscopic analyses, we experimentally demonstrated the presence of the equatorial N — Me conformer of King's sultam 4b in solution, resulting from a rapid equilibrium. As a consequence, the value of the N lone-pair anomeric stabilization should be revised to 1.5-1.6 kcal/mol. Independently from the N tilting, natural bond orbital (NBO)-comparative analyses suggest that the S d* orbitals do not appear as primordial and stereospecific acceptors for the N lone pair. Second, the five-membered-ring sultams do not seem to be particularly well-stabilized by the S — C σ* orbital in the N-substituted pseudo-axial conformation, as opposed to an idealized anti-periplanar situation for the six-membered-ring analogues. In this latter case, the other anti-periplanar C — C σ* and C(1') — H/C(2') σ*orbitals are as important, if not more, when compared to the S — C σ* participation. In the pseudo-equatorial conformation, γ-sultams particularly benefit from the N lone-pair hyperconjugation with the anti-periplanar S — O1 σ* and C(2) — H/C or C(1') — H/C σ* orbitals. This is also the case for δ-sultams when the steric requirement of the N-substituent exceeds 1.6 kcal/mol. When both axial and equatorial conformations are sterically too exacting, the N-atom is prone to sp2 hybridization or/and conformational changes (i.e., 12c). In that case also, the mode of stereoelectronic stabilization differs from γ- to δ-sultams.
  • Structural and spectroscopic studies on the alkali borohydrides MBH4 (M=Na,K,Rb,Cs)
    G. Renaudin, S. Gomes, H. Hagemann, L. Keller and K. Yvon
    Journal of Alloys and Compounds, 375 (1-2) , 2004, p98-106
    Keywords: hydrogen storage materials, crystal structure, neutron diffraction, infrared and Raman spectroscopy
    DOI:10.1016/j.jallcom.2003.11.018 | unige:3713 | Abstract | Article HTML | Article PDF
Alkali borohydrides MBH4 and their deuterides have been investigated by X-ray and neutron powder diffraction (M=K,Rb,Cs) and by infrared and Raman spectroscopy (M=Na,K,Rb,Cs). At room temperature the compounds crystallize with a cubic high temperature (HT) structure having Fm3m  symmetry in which the [BH4]− complexes are disordered. At low temperature (LT) the potassium compound transforms into a tetragonal low temperature structure having P42/n mc symmetry in which the [BH4]− complexes are ordered such as in the isotypic sodium congener. The B---H distances within the complex as measured on the deuteride at 1.5 K are 1.205(3) Å. Indications for a partial ordering in the rubidium and cesium compounds exist but are not sufficient for a full structural characterization. Infrared and Raman spectra at room temperature are fully assigned for both hydrides and deuterides, including the overtones and combination bands, the Fermi resonance type interactions and the 10B to 11B splitting due to the presence of natural boron in the samples.
  • Quantum Chemistry "Without Orbitals" - An Old Idea and Recent Developments
    T.A. Wesolowski
    Chimia, 58 (5) , 2004, p311-315
    Keywords: density functional theory; kinetic energy functional; orbital-free embedding; orbitals
    DOI:10.2533/000942904777677885 | unige:3712 | Abstract | Article PDF
The role of one-electron functions known as orbitals in various theoretical methods used to describe molecules and complex materials at a quantum mechanical level is outlined in a historical perspective. A hierarchy consisting of three types of general formalism, ordered according to the importance of orbital-dependent expressions in the total energy, is presented. Two such formalisms, less known to the general chemistry community, are discussed in detail together with their recent applications in modelling complex systems: a) the orbital-free formulation of density functional theory, which does not use orbitals at all and which can be seen as the modern realization of the original ideas of Thomas and Fermi, and b) the density partitioning based formalism, in which the orbitals are used only for smaller parts of a larger system (subsystems). The emphasis is placed on the second type of formalism, a topic of strong interest of our Geneva group.
The uranyl salophene complex and its co-complexes with several anions (H2PO4-, HSO4-, NO2-, OH-, Cl-, F-) in the gas phase are investigated theoretically. Equilibrium geometries of relevant species and complexation-induced structural changes are discussed. The 13C NMR chemical shifts calculated at the gas-phase optimized geometry agree very well with experimental liquid-phase results. The optimized geometry agrees also very well with available crystallographic data. This indicates that the gas-phase structures derived from theoretical calculations can be considered representative also for the condensed phase. For all anions, except H2PO4-, the calculated gas-phase binding energies correlate well with experimental Gibbs free energies of complexation. The possible role of the solvent in the case of H2PO4- complexation is discussed.
 
Temperature-dependent emission spectra of Sm2+-doped SrMgF4 have been obtained in the temperature range from 50 to 300 K. At 50 K, six bands are observed for the very strong 5D0→7F0 transition, in agreement with the reported sixfold crystal superstructure. The overall splitting of more than 70 cm−1 highlights the important structural differences of the six Sr sites. Upon heating progressively to room temperature, the spectra change progressively with a more pronounced change between 270 and 300 K. These observations suggest the possibility of a complex structural behavior for SrMgF4 which will require new experiments.
  • Femtosecond times-resolved studies on bimolecular electron transfer processes
    S. Pagès, B. Lang and E. Vauthey
    in "Femtochemistry and Femtobiology" M. Martin and J. T. Hynes Eds., Elsevier, 2004, p319
    unige:4027
  • Influence of solute-solvent interactions on the quenching dynamics of perylene derivatives in an electron donating solvent
    A. Morandeira, A. FĂźrstenberg and E. Vauthey
    in "Femtochemistry and Femtobiology" M. Martin and J. T. Hynes Eds., Elsevier, 2004, p327
  • Effect of excitation wavelength on the charge recombination dynamics of excited donor acceptor complexes
    O. Nicolet, A.I. Ivanov and E. Vauthey
    in "Femtochemistry and Femtobiology" M. Martin and J. T. Hynes Eds., Elsevier, 2004, p331
    unige:4028
  
  • [CpRu((R)-Binop-F)(H2O)][SbF6], a New Fluxional Chiral Lewis Acid Catalyst: Synthesis, Dynamic NMR, Asymmetric Catalysis, and Theoretical Studies
    V. Alezra, G. Bernardinelli, C. Corminboeuf, U. Frey, E.P. KĂźndig, A.E. Merbach, C.M. Saudan, F. Viton and J Weber
    Journal of the American Chemical Society, 126 (15) , 2004, p4843-4853
    DOI:10.1021/ja0374123 | unige:3518 | Abstract | Article HTML | Article PDF
The C2-symmetric electron-poor ligand (R)-BINOP-F (4) was prepared by reaction of (R)-BINOL with bis(pentafluorophenyl)-phosphorus bromide in the presence of triethylamine. The iodo complex [CpRu((R)-BINOP-F)(I)] ((R)-6) was obtained by substitution of two carbonyl ligands by (R)-4 in the in situ-prepared [CpRu(CO)2H] complex followed by reaction with iodoform. Complex 6 was reacted with [Ag(SbF6)] in acetone to yield [CpRu((R)-BINOP-F)(acetone)][SbF6] ((R)-7). X-ray structures were obtained for both (R)-6 and (R)-7. The chiral one-point binding Lewis acid [CpRu((R)-BINOP-F)][SbF6] derived from either (R)-7 or the corresponding aquo complex (R)-8 activates methacrolein and catalyzes the Diels−Alder reaction with cyclopentadiene to give the [4 + 2] cycloadduct with an exo/endo ratio of 99:1 and an ee of 92% of the exo product. Addition occurs predominantly to the methacrolein Cα-Re face. In solution, water in (R)-8 exchanges readily. Moreover, a second exchange process renders the diastereotopic BINOP-F phosphorus atoms equivalent. These processes were studied by the application of variable-temperature 1H, 31P, and 17O NMR spectroscopy, variable-pressure 31P and17O NMR spectroscopy, and, using a simpler model complex, density functional theory (DFT) calculations. The results point to a dissociative mechanism of the aquo ligand and a pendular motion of the BINOP-F ligand. NMR experiments show an energy barrier of 50.7 kJ mol-1 (12.2 kcal mol-1) for the inversion of the pseudo-chirality at the ruthenium center.
  • Ultrafast Spectroscopic Investigation of the Charge Recombination Dynamics of Ion Pairs Formed upon Highly Exergonic Bimolecular Electron-Transfer Quenching: Looking for the Normal Region
    StĂŠphane Pagès, Bernhard Lang and Eric Vauthey
    Journal of Physical Chemistry A, 108 (4) , 2004, p549-555
    DOI:10.1021/jp036796g | unige:3517 | Abstract | Article HTML | Article PDF
  
The charge recombination dynamics of the ion pairs formed upon electron-transfer quenching of perylene by tetracyanoethylene in acetonitrile has been investigated using ultrafast fluorescence upconversion, transient absorption, and transient grating techniques. For this donor/acceptor pair, charge separation is highly exergonic (ΔGCS= −2.2 eV), but charge recombination is weakly exergonic (ΔGCR = −0.6 eV). It was found that for more than 90% of the ion pair population, charge recombination is ultrafast and occurs in less than 10 ps. This decay component could not be observed in a previous investigation with a lower time resolution. The results indicate that the primary quenching product is a contact ion pair and not a solvent-separated ion pair as generally assumed for highly exergonic electron-transfer quenching processes. A possible explanation for this apparent divergence is that the contact ion pair is initially formed in an electronic excited state. Only a very minor fraction of the ion pair population undergoes the slow charge recombination predicted by Marcus theory for weakly exergonic charge-transfer processes (normal region).
The components of nucleus-independent chemical shift (NICS) tensors for Dnhn-annulenes are discussed as indexes of the aromatic character of electronic π systems. The component corresponding to the principal axis perpendicular to the ring plane, NICSzz, is found to be a good measure for the characterisation of the π system of the ring. Isotropic NICS values at ring centres contain large influences from the σ system and from all three principal components of the NICS tensor. At large distances away from the ring center, NICSzz, which is dominated by contributions from the π system, characterizes NICS well.
  
  • Antiaromaticity in Bare Deltahedral Silicon Clusters Satsifying Wade's and Hirsch's Rules: An Apparent Correlation of Antiaromaticity with High Symmetry
    R.B. King, C. Corminboeuf, T. Heine and P.v.R. Schleyer
    Journal of the American Chemical Society, 126 (2) , 2004, p430-431
    Keywords: aromaticity, inorganic cluster
    DOI:10.1021/ja036259+ | unige:3516 | Abstract | Article HTML | Article PDF
Entirely unlike the aromatic closo BnHn2- borane dianions, isoelectronic Si62- and Si122- are antiaromatic. Their Oh and Ih symmetries are responsible, as the other deltahedral silicon dianion clusters do not exhibit this behavior. These high symmetries prevent mixing among the degenerate lone pair and skeletal orbitals, leading to paratropic behavior.
  • Synthesis and crystal structures of Ba6Mg11F34 and the solid solutions Ba6Mg11-xM(II)xF34 (M(II)=Mn,Fe)and luminescence of Ba6Mg11F34:Eu2+
    W. Kerbe, M. Weil, F. Kubel and H. Hagemann
    Materials Research Bulletin, 39 (3) , 2004, p343-355
    Keywords: fluorides, X-ray diffraction, crystal structure, luminescence
    DOI:10.1016/j.materresbull.2003.12.006 | unige:3707 | Abstract | Article HTML | Article PDF
Ba6Mg11F34, a new compound of the pseudobinary BaF2–MgF2 system, has been synthesized by solid state techniques from stoichiometric amounts of BaF2 and MgF2 and its crystal structure determined by single crystal X-ray diffraction (space group P1 , a=7.5084(6), b=9.9192(8), c=10.0354(8) Å, α=81.563(2), β=72.402(2), γ=71.198(1)°, 3899 structure factors, 233 parameters, R(F2>2σ(F2))=0.018, wR(F2 all) = 0.046). It is isotypic with the copper(II) analogue, Ba6Cu11F34. The main features of the structure are a network of [MgF6] octahedra and three different [BaFx] polyhedra with x=12, 11+1 and 13. Ba6Mg11−xFexF34 and Ba6Mg11−xMnxF34 solid solutions were prepared and their composition determined by single crystal structure analyses. The luminescence properties of Ba6Mg11F34 doped with Eu2+ were studied using fluorescence spectroscopy. The observed luminescence was pale blue with a maximum at 465 nm.
 
BaFCl single crystals doped with Sm3+ ions were studied by using the EPR method. Several types of paramagnetic Sm3+ centres were found. The parameters of the corresponding spin Hamiltonians were determined. Structural models and ground states of the observed centres are proposed.
  • Generalization of the Kohn-Sham Equations with Constrained Electron Density (KSCED) Formalism and its Time-Dependent Response Theory Formulation.
    M.E. Casida and T.A. Wesolowski
    International Journal of Quantum Chemistry, 96 (6) , 2004, p577-588
    Keywords: DFT; TDDFT; KSCED; subsystem quantum mechanics; embedding
    DOI:10.1002/qua.10744 | unige:3706 | Abstract | Article HTML | Article PDF
The Kohn-Sham equations with constrained electron density (KSCED) embedding formalism of Wesolowski and coworkers was originally developed and is good for the case of two weakly interacting molecular regions with weakly overlapping densities, such as might be expected in describing solvation. A generalization is given here for the case of three molecular regions with strongly overlapping densities with the idea that this generalized theory can offer a better description of embedding in the context of situations that might be encountered in, for example, chemisorption on surfaces or active sites in enzymes. This three-partition generalization includes the original two-partition formalism as a special case. Time-dependent response theory equations are then developed for the two- and three-partition theories for application to the problem of the calculation of polarizabilities and other response properties, including excitation spectra, of embedded molecules or molecular structures.
  
Raman spectra of the alkali borohydride series MBH4 (M=Li, Na, K, Rb, Cs) have been measured as a function of temperature in the range 300–540 K. For the cubic modification of M=Na, K, Rb and Cs, the analysis of the Raman line widths suggests that the energy barrier of reorientation of the [BH4]− anions decreases as a function of cation size in the sequence Na: 12.1(5), K: 9.2(4), Rb: 8.8(3) and Cs: 8.2(4) kJ/mol. For the hexagonal high temperature modification of LiBH4, the data suggest two energy barriers of reorientation at ~5 and ~ 60 kJ/mol, respectively.
 
The asymmetric hydrogenation of cyclohexane-1,2-dione over cinchonidine-modified platinum was investigated. Despite the fact that the first hydrogenation step is close to nonenantioselective, a high enantiomeric excess is obtained for the (R)-α-hydroxyketone due to kinetic resolution. In the second hydrogenation step one out of the four reactions of the network is substantially accelerated with respect to the others and with respect to the reaction in the absence of modifier, leading to an enantiomeric excess of (1R,2R)-trans-cyclohexane-1,2-diol of over 80%. Comparison with recently reported asymmetric hydrogenation of α-hydroxyethers indicates striking similarities, which hint at similar reactant–modifier interaction in both cases. The importance of cis versus trans conformation of the reactant for the reactant–modifier interaction emerges from a comparison of suggested reaction intermediates for cyclohexane-1,2-dione and butane-2,3-dione hydrogenation, respectively.
  • Probing enantiospecific interactions at chiral solid-liquid interfaces by absolute configuration modulation infrared spectroscopy
    R. Wirz, T. BĂźrgi and A. Baiker
    Langmuir, 19 (3) , 2003, p785-792
    DOI:10.1021/la026568y | unige:14740 | Abstract | Article HTML | Article PDF
A method to selectively probe the different adsorption of enantiomers at chiral solid−liquid interfaces is presented, which combines attenuated total reflection infrared spectroscopy and modulation spectroscopy. The weak spectral changes upon adsorption of enantiomers at a chiral interface are followed in time, while periodically changing the absolute configuration of the admitted chiral molecule. A subsequent digital phase-sensitive data analysis reveals spectral differences arising due to the different diastereomeric interactions of the two enantiomers with the chiral interface. The main advantage of the method compared to conventional difference spectroscopy is the enhanced signal-to-noise ratio. The method is selective for differences in diastereomeric interactions of the enantiomers. Its potential is demonstrated by studying the adsorption of ethyl lactate on a chiral stationary phase, which is amylose tris[(S)-α-methylbenzylcarbamate] coated onto silica gel. d-Ethyl lactate interacts stronger with the chiral stationary phase. In particular the spectral shifts reveal a stronger N−H···O=C hydrogen bonding interaction between amide group of the chiral stationary phase and the ester group of the ethyl lactate. The spectra also indicate that one of the three (S)-α-methylbenzylcarbamate side chains of the amylose derivative is predominantly involved in the interaction with the ethyl lactate. Furthermore, the experimental observations indicate that more than one interaction mode is populated at room temperature and that interaction with the ethyl lactate may induce a conformational change of the amide group of the chiral stationary phase.
The epoxidation of cyclohexene over titania–silica aerogel catalysts using t-butylhydroperoxide (TBHP) was investigated by in situ attenuated total reflection (ATR) infrared spectroscopy. In order to distinguish between relevant and spectator species and to increase sensitivity, ATR was combined with modulation spectroscopy. In the latter technique the catalyst system is periodically perturbed by a forced concentration change. The interaction of cyclohexene with the aerogels is found to be weak. In contrast, TBHP adsorbs strongly on the catalysts on two different sites. Modulation experiments reveal that TBHP adsorbed on Si-OH groups is a spectator, whereas the one adsorbed on the Ti-sites is involved in the catalytic cycle. The latter species is stronger adsorbed and the associated signals increase with the Ti content of the catalyst. Adsorption of the TBHP on the Ti sites results in a strong shift of the C-O stretching vibration and changes in the Ti-O-Si modes of the catalyst. The study furthermore reveals vastly different pore diffusion for cyclohexene and TBHP due to their different interaction with the polar catalyst surface. In the modulation experiments the reaction product appears retarded with respect to the admittance of the reactants, which indicates that pore diffusion and kinetics of adsorption and desorption are important factors for the catalysis. Methylation of the aerogel has a beneficial effect on the catalysis, which can be traced to the different pore size distribution and polarity with respect to the unmodified catalyst. When the flow-through ATR cell is slowly heated, a change in the framework vibrations of the catalyst occurs simultaneously with the onset of reaction, indicating reaction induced structural changes.
Prominent nonlinear effects in enantioselectivity were observed with a transient technique when ethyl pyruvate was hydrogenated over Pt/Al2O3 in the presence of two cinchona alkaloids, which alone afford the opposite enantiomers of ethyl lactate in excess. The changes in reaction rate and ee, detected after injection of the second alkaloid, varied strongly with type and amount of the alkaloid, and with the order of their addition to the reaction mixture. For example, under ambient conditions in acetic acid cinchonidine (CD) afforded 90% ee to (R)-ethyl lactate and addition of equimolar amount of quinidine (QD) reduced the ee to (R)-ethyl lactate only to 88%, though QD alone provided 94% ee to (S)-lactate in a slightly faster reaction. The stronger adsorption of CD on Pt in the presence of hydrogen and acetic acid was proved by UV–vis spectroscopy. The different adsorption strengths result in an enrichment of CD on the Pt surface and also in a crucial difference in the dominant adsorption geometries. CD is assumed to adsorb preferentially via the quinoline rings laying approximately parallel to the Pt surface. In this position it can interact with ethyl pyruvate during hydrogen uptake and control the enantioselectivity. The weaker adsorbing QD adopts mainly a position with the quinoline plane being tilted relative to the Pt surface and these species are not involved in the enantioselective reaction. Competing hydrogenation of the alkaloid, and steric and electronic interactions among the adsorbed species, can also influence the alkaloid efficiency and the product distribution. Hydrogenation of the quinoline rings at low alkaloid concentration resulted in unprecedented swings in the enantiomeric excess.
  • Palladium-catalyzed enantioselective hydrogenation of 2-pyrones: evidence for competing reaction mechanisms
    W.-R. Huck, T. BĂźrgi, T. Mallat and A. Baiker
    Journal of Catalysis, 219 (1) , 2003, p41-51
    Keywords: Asymmetric; Hydrogenation; Cinchonidine; Cinchonine; Palladium; 2-Pyrone
    DOI:10.1016/S0021-9517%2803%2900166-0 | unige:14712 | Article HTML | Article PDF
The enantioselective hydrogenation of 4-hydroxy-6-methyl-2-pyrone (1a), 3,6-dimethyl-4-hydroxy-2-pyrone (2a), 4-methoxy-6-methyl-2-pyrone (3a), and 4,6-dimethyl-2-pyrone (4a) was studied over a 5 wt% Pd/TiO2 catalyst. Various cinchona alkaloids and their O- and N-methyl derivatives were applied as chiral modifiers. The catalytic experiments combined with FTIR, NMR, and NOESY-NMR spectroscopic analysis and ab initio calculations revealed an interesting feature of the reactions: the ee is determined by competing reactant–modifier interactions. These interactions may involve the OH function and the quinuclidine N of the alkaloid modifier. When the reactant possesses an acidic OH group (1a and 2a), the reaction via the energetically most stable bidentate complex controls the enantioselectivity. Protic or basic solvents diminish the ee in these reactions by stabilizing a single-bonded (acid–base type) interaction. Different mechanisms are proposed for the hydrogenation of the nonacidic pyrones 3a and 4a. These models can well interpret the catalytic results but require further confirmation. Besides, the studies provided the first experimental evidence for an intrinsic rate acceleration coupled with the enantiodifferentiating process over chirally modified Pd.
  • In Situ Attenuated Total Reflection Infrared Spectroscopy: A Sensitive Tool for the Investigation of Reduction-Oxidation Processes on Heterogeneous Pd Metal Catalysts
    Thomas BĂźrgi, Ronny Wirz and Alfons Baiker
    Journal of Physical Chemistry B, 107 (28) , 2003, p6774-6781
    DOI:10.1021/jp027672r | unige:14649 | Abstract | Article HTML | Article PDF
Alternative exposure of Pd thin films and Pd/TiO2 catalysts to dissolved hydrogen and oxygen leads to significant changes in the reflectivity of infrared radiation as observed in attenuated total reflection spectroscopy. The reflectivity decreases and the absorbance increases upon changing from oxygen- to hydrogen-saturated solvent. Reflectivity calculations based on the Drude model for the Pd thin film show that a slight change in the concentration of the free electrons of the metal could be at the origin of the observed effect. Alternatively, the reversible formation of a surface oxide layer can lead to a similar observation. The reflectivity changes can be used to follow the changes of the metal catalyst, similar to potential measurements, however without the need to work in conducting media. They can be correlated with the observation of adsorbed species and the formation of reaction products. The potential of the method for in situ studies of catalytic solid−liquid interfaces is demonstrated for the oxidation of 2-propanol and ethanol. Upon changing from reducing to oxidizing conditions, the observation of reaction products is slightly offset with respect to the observed reflectivity change in both cases, whereas the frequency of the CO vibration shifts at the same time as the reflectivity increases.
  • High pressure view-cell for simultaneous in situ infrared spectroscopy and phase behavior monitoring of multiphase chemical reactions
    M.S. Schneider, J.-D. Grunwaldt, T. BĂźrgi and A. Baiker
    Review of Scientific Instruments, 74 (9) , 2003, p4121-4128
    DOI:10.1063/1.1597961 | unige:14667 | Abstract | Article PDF
A new type of high pressure spectroscopy view-cell for investigation of multiphase reactions is presented. It allows visual observation of the reaction mixture at conditions up to 200 °C and 200 bar. Measurements of the reactor cell’s upper part by transmission spectroscopy with variable path length and of the cell’s bottom part by attenuated total reflection (ATR) spectroscopy can be performed quasi-simultaneously. By coating the internal reflection element with a catalyst film, in situ investigations of heterogeneous catalysts can be performed. The potential of this new experimental setup is demonstrated using examples of heterogeneous and homogeneous catalytic reactions. For the heterogeneously catalyzed hydrogenation of ethyl pyruvate over Pt/Al2O3 in “supercritical” ethane the reaction progress could be monitored by spectroscopic investigation of the fluid phase. Quantitative evaluation of the spectra combined with digital imaging of the reaction mixture allowed simultaneous determination of phase behavior and reaction kinetics. ATR-IR spectra of the catalyst film could be measured at the same time. In the homogeneously catalyzed formylation of morpholine with “supercritical” carbon dioxide and hydrogen, not only number and nature, but also the composition of the different phases could be determined. The catalyst was found to be confined to the liquid phase. Although the aim of these preliminary studies was to test the functionality of the new cell, already significant new insight on the investigated catalytic reactions could be gained.
The interaction of 2-methoxypropene, a vinyl ether, with heterogeneous acid catalysts containing sulfonic acid groups covalently bound to SiO2 (Deloxan ASP, Degussa) and sulfuric acid adsorbed on TiO2-modified amorphous SiO2 (Degussa), respectively, was investigated by in situ attenuated total reflection infrared spectroscopy. Rapid hydrolysis is observed, which does not, however, require the acid sites. The resulting acetone is adsorbed predominantly on SiOH groups. Promoted by the acid sites a further transformation is observed on the catalysts. Based on the time behavior of the ATR signals of acetone and the product the further reaction likely involves the condensation of 2-methoxypropene and acetone. During the buildup of the reaction product hydronium ions disappear from the catalyst surface. Upon desorption of the reaction product the hydronium ions become prominent again on the catalyst containing adsorbed sulfuric acid. This behavior is less pronounced on the catalyst, which contains sulfonic acid groups. The two investigated catalysts contain vastly different relative concentrations of Brønsted and Lewis acid sites, which can explain the difference in the relative concentration of intermediate and product at the interface in the observed consecutive reaction.
  
The combination of ATR−IR and modulation spectroscopy allowed for the study of the interaction of ketopantolactone with Pt/Al2O3 films chirally modified by cinchonidine under hydrogenation conditions. The spectra reveal a significant influence of ketopantolactone on the adsorption of the modifier and indicate a N−H−O hydrogen bond between modifier and reactant. The latter was corroborated by a comparative study with N-methyl cinchonidine chloride modified Pt/Al2O3.
  • ATR-IR Flow-Through Cell for Concentration Modulation Excitation Spectroscopy: Diffusion Experiments and Simulations
    Atsushi Urakawa, Ronny Wirz, Thomas BĂźrgi and Alfons Baiker
    Journal of Physical Chemistry B, 107 (47) , 2003, p13061-13068
    DOI:10.1021/jp0359889 | unige:14674 | Abstract | Article HTML | Article PDF
A new ATR-IR cell was designed, and its performance was characterized by modulation excitation spectroscopy (MES). The new cell allows concentration modulation at relatively high frequency without unnecessary phase delay in the response. The response delay due to convection and diffusion was studied at different flow rates and modulation frequencies by experiments and simulations. The diffusion behavior of a small relatively fast-diffusing molecule, acetonitrile, was compared with that of a large slow-diffusing molecule, hemoglobin, in water. Experimentally, significant differences in their diffusion behavior were observed. The flow and diffusion behavior of the probe molecules was described using two different models, the diffusion layer model and the convection−diffusion model, and the theoretical results were compared with the experiments. The diffusion layer model allows estimating an effective diffusion layer thickness near the surface of the internal reflection element. However, the simulated response is significantly different from the experimental one. On the other hand, the convection−diffusion model describes the flow and diffusion behavior of the solute molecules with high accuracy. This work forms the basis for the investigation of chemical and physical kinetics such as surface reaction and diffusion by MES. It also suggests criteria for appropriate experimental conditions in ATR-IR MES experiments.
  • In situ spectroscopy of catalytic solid-liquid interfaces and chiral surfaces
    Thomas BĂźrgi
    Chimia, 57 (10) , 2003, p623-627
    DOI:10.2533/000942903777678885 | unige:14743 | Abstract
  
This contribution gives an overview of our recent effort to probe catalytic solid-liquid interfaces in situ and to investigate recognition processes at chiral surfaces. Attenuated total reflection infrared spectroscopy in a dedicated low volume flow-through cell is used to investigate the working catalytic interface. The latter technique is combined with modulation spectroscopy, which relies on the perturbation of the system under investigation by a periodically varying external parameter. A digital phase-sensitive detection results in high quality spectra. The method furthermore yields kinetic information and helps disentangle complex spectra. The described tool is therefore ideally suited for the investigation of complex systems. Applications in the fields of heterogeneous catalysis and recognition at chiral solid-liquid interfaces are presented. Our aim is a better molecular level understanding of these processes and, based on this knowledge, a rational design of better catalyst materials.
  • Heavy element quantum chemistry - the multiconfigurational approach
    B.O. Roos, P.-A. Malmqvist and L. Gagliardi
    in "Fundamental World of Quantum Chemistry" E. J. Brandas and E. S. Kryachko, Eds.Kluwer, Dordrecht, 2 (Ch. 16) , 2003
    unige:4031
  • On the reaction of a uranium atom with a nitrogen molecule: a theoretical attempt
    Laura Gagliardi, Gianfranco La Manna and BjĂśrn O. Roos
    Faraday Discussions, 124 , 2003, p63-68
    DOI:10.1039/b211224h | unige:3724 | Abstract | Article HTML | Article PDF
An attempt has been made to study the reaction between a uranium atom and a nitrogen molecule theoretically using multiconfigurational wave functions. The C2v part of the reaction surface has been computed for several electronic states of various spin multiplicities. The system proceeds from a neutral uranium atom in its (5f)3(6d)(7s)2, 5L ground state to the linear molecule NUN, which has a 1Σ+g ground state and uranium in a formal U(VI) oxidation state. The effect of spin–orbit coupling has been estimated at crucial points along the reaction. These preliminary results shows that the system proceeds from a quintet state for U + N2, via a triplet transition state to the final closed shell molecule. An eventual energy barrier for the insertion reaction is caused by the spin–orbit coupling energy.
Results are presented from a theoretical study of the lower electronic states of the CUO molecule. Multiconfigurational wave functions have been used with dynamic correlation added using second order perturbation theory. Extended basis sets have been used, which for uranium were contracted including scalar relativistic effects. Spin–orbit interaction has been included using the state-interaction approach. The results predict that the ground state of linear CUO is Φ2 with the closed shell Σ+0 state 0.5 eV higher in energy. This is in agreement with matrix isolation spectroscopy, which predicts Φ2 as the ground state when the matrix contains noble gas atoms heavier than Ne. In an Ne matrix, the experiments indicate, however, that CUO is in the Σ+0 state. The change of ground state due to the change of the matrix surrounding CUO cannot be explained by the results obtained in this work and remains a mystery.
  
One of the prototype compounds for metal−metal multiple bonding, the Re2Cl82- ion, has been studied theoretically using multiconfigurational quantum chemical methods. The molecular structure of the ground state has been determined. It is shown that the effective bond order of the Re−Re bond is close to three, due to the weakness of, in particular, the δ bond. The electronic spectrum has been calculated with the inclusion of spin−orbit coupling. Observed spectral features have been reproduced with good accuracy, and a number of new assignments are suggested.
  • Predicted Group 4 Tetra-azides M(N3)4 (M = Ti-Hf, Th): The First Examples of Linear M-NNN Coordination
    Laura Gagliardi and Pekka PyykkĂś
    Inorganic Chemistry, 42 (9) , 2003, p3074-3078
    DOI:10.1021/ic034122e | unige:3722 | Abstract | Article HTML | Article PDF
Quantum chemical calculations suggest that group 4 tetra-azides M(N3)4, where M = Ti, Zr, Hf, and Th, are stable species. They present a unique structural feature; namely, the M−N−N−N fragments are linear. These species are energetically more stable than the corresponding isomers with general formula η5-N5 −M−η7-N7, and the Th species, Th(N3)4, is the most stable of all. Possible mixed nitride azides NMN3 were also investigated.
Quantum chemical calculations suggest that a series of molecules with the general formula MAu4 are stable, where M = U, Th, and a group-4 atom. They correspond to Au in the formal valence state −1 and indicate that gold can act as a ligand similar to the halogen series. Of the MAu4 species studied, UAu4, the first predicted mixed gold uranium compound, has a short M−Au bond distance, 2.71 Å, which would locate Au between Br and I from the bond length point of view in the U-tetrahalide series. Energetically, the U−Au bond is weaker than the corresponding U−Br and U−I bonds.
 
Quantum chemical calculations suggest that inverse sandwich compounds with the general formula MN7 M', where M is an alkali metal (K,Rb,Cs), N7 is a ten-π-electron ring, and M' is an alkaline-earth metal (Ca,Sr,Ba), are local C7v minima. Among these systems, the CsN7Ba molecule is the stablest of all and presents a barrier of 35 kcal/mol to dissociation towards CsNBa and three N2 molecules. Substantial 5d character is found in the bonding. Possible ways of making these high-energy compounds are discussed.
  • Non-equilibrium charge recombination dynamics of excited donor-acceptor complexes
    Olivier Nicolet and Eric Vauthey
    in "Ultrafast Phenomena XIII" Springer Series in Chemical Physics, 2003, p432-434
    unige:4032
  • Luminescence from Vacuum-Ultraviolet irradiated Cosmic Ice Analogs and Residues
    Murthy S. Gudipati, Jason P. Dworkin, Xavier F.D. Chillier and Louis J. Allamandola
    Astrophysical Journal, 583 (1) , 2003, p514-523
    Keywords: astrochemistry;comets: general;dust, extinction;ISM: molecules;methods: laboratory;molecular data
    DOI:10.1086/345349 | unige:3719 | Abstract | Article HTML | Article PDF
Here we report a study of the optical luminescent properties for a variety of vacuum-ultraviolet (VUV)-irradiated cosmic ice analogs and the complex organic residues produced. Detailed results are presented for the irradiated, mixed molecular ice: H2O : CH3OH : NH3 : CO (100 : 50 : 1 : 1), a realistic representation for an interstellar/precometary ice that reproduces all the salient infrared spectral features associated with interstellar ices. The irradiated ices and the room-temperature residues resulting from this energetic processing have remarkable photoluminescent properties in the visible (520-570 nm). The luminescence dependence on temperature, thermal cycling, and VUV exposure is described. It is suggested that this type of luminescent behavior might be applicable to solar system and interstellar observations and processes for various astronomical objects with an ice heritage. Some examples include grain temperature determination and vaporization rates, nebula radiation balance, albedo values, color analysis, and biomarker identification.
  • Investigation of the Reduced High-Potential Iron-Sulfur Protein from Chromatium vinosum and Relevant Model Compounds: A Unified Picture of the Electronic Structure of [Fe4S4]2+ Systems through Magnetic and Optical Studies
    LatĂŠvi M. Lawson Daku, Jacques PĂŠcaut, Alix Lenormand-Foucaut, BĂŠatrice Vieux-Melchior, Peter Iveson and Jeanne Jordanov
    Inorganic Chemistry, 42 (21) , 2003, p6824-6850
    DOI:10.1021/ic034494n | unige:3717 | Abstract | Article HTML | Article PDF
Magnetization measurements and variable temperature optical spectroscopy have been used to investigate, within the 4−300 K temperature range, the electronic structure of the reduced high-potential iron protein (HiPIP) from Chromatium vinosum and the model compounds (Cat)2[Fe4S4(SR)4], where RS- = 2,4,6-triisopropylphenylthiolate (1), 2,6-diphenylphenylthiolate (2), diphenylmethylthiolate (3), 2,4,6-triisopropylbenzylthiolate (4, 4‘), 2,4,6-triphenylbenzylthiolate (5, 5‘), 2,4,6-tri-tert-butylbenzylthiolate (6), and Cat+ = +NEt4 (1, 2, 3, 4‘, 5‘, 6), +PPh4 (4, 5). The newly synthesized 22-, 32-, 52-, and 62- complexes are, as 12- and 42-, excellent models of the reduced HiPIPs:  they exhibit the [Fe4S4]3+/2+ redox couple, because of the presence of bulky ligands which stabilize the [Fe4S4]3+ oxidized core. Moreover, the presence of SCH2 groups in 42-, 52-, and 62-, as in the [Fe4S4] protein cores, makes them good biomimetic models of the HiPIPs. The X-ray structure of 2 is reported:  it crystallizes in the orthorhombic space group Pcca with no imposed symmetry and a D2d-distorted geometry of the [Fe4S4]2+ core. Fit of the magnetization data of the reduced HiPIP and of the 1, 2, 3, 4, 5, and 6 compounds within the exchange and double exchange theoretical framework leads to exchange coupling parameters J = 261−397 cm-1. A firm determination of the double exchange parameters B or, equivalently, the transfer integrals β = 5B could not be achieved that way. The obtained |B| values remain however high, attesting thus to the strength of the spin-dependent electronic delocalization which is responsible for lowest lying electronic states being characterized by delocalized mixed-valence pairs of maximum spin 9/2. Electronic properties of these systems are then accounted for by the population of a diamagnetic ground level and excited paramagnetic triplet and quintet levels, which are respectively J and 3J above the ground level. Optical studies of 1, 2, 4‘, 5‘, and 6 but also of (NEt4)2[Fe4S4(SCH2C6H5)4] and the isomorph (NEt4)2[Fe4S4(S-t-Bu)4] and (NEt4)2[Fe4Se4(S-t-Bu)4] compounds reveal two absorption bands in the near infrared region, at 705−760 nm and 1270−1430 nm, which appear to be characteristic of valence-delocalized and ferromagnetically coupled [Fe2X2]+ (X = S, Se) units. The |B| and |β| values can be directly determined from the location at 10|B| of the low-energy band, and are respectively of 699−787 and 3497−3937 cm-1. Both absorption bands are also present in the 77 K spectrum of the reduced HiPIP, at 700 and 1040 nm (Cerdonio, M.; Wang, R.-H.; Rawlings, J.; Gray, H. B. J. Am. Chem. Soc. 1974, 96, 6534−6535). The blue shift of the low-energy band is attributed to the inequivalent environments of the Fe sites in the protein, rather than to an increase of |β| when going from the models to the HiPIP. The small differences observed in known geometries of [Fe4S4]2+ clusters, especially in the Fe−Fe distances, cannot probably lead to drastic changes in the direct Fe−Fe interactions (parameter β) responsible for the delocalization phenomenon. These differences are however magnetostructurally significant as shown by the 261−397 cm-1 range spanned by J. The cluster's geometry, hence the efficiency of the Fe−μ3-S−Fe superexchange pathways, is proposed to be controlled by the more or less tight fit of the cluster within the cavity provided by its environment.
  • Synthetic multifunctional pores: deletion and inversion of anion/cation selectivity using pM and pH
    Naomi Sakai, Nathalie Sorde, Gopal Das, Philippe Perrottet, David Gerard and Stefan Matile
    Organic and Biomolecular Chemistry, 1 (7) , 2003, p1226-1231
    DOI:10.1039/b210604c | unige:3716 | Abstract | Article HTML | Article PDF
We report the characterization of multifunctional rigid-rod β-barrel ion channels with either internal aspartates or arginine–histidine dyads by planar bilayer conductance experiments. Barrels with internal aspartates form cation selective, large, unstable and ohmic barrel-stave (rather than toroidal) pores; addition of magnesium cations nearly deletes cation selectivity and increases single-channel stability. Barrels with internal arginine–histidine dyads form cation selective (PK+/PCl–= 2.1), small and ohmic ion channels with superb stability (single-channel lifetime > 20 seconds). Addition of "protons" results in inversion of anion/cation selectivity (PCl–/PK+= 3.8); addition of an anionic guest (HPTS) results in the blockage of anion selective but not cation selective channels. These results suggest that specific, internal counterion immobilization, here magnesium (but not sodium or potassium) cations by internal aspartates and inorganic phosphates by internal arginines (but not histidines), provides access to synthetic multifunctional pores with attractive properties.
  • In Vitro Monitoring of Poly(ortho ester) Degradation by Electron Paramagnetic Resonance Imaging
    Sergio Capancioni, Khadija Schwach-Abdellaoui, Werner Kloeti, Werner Herrmann, Holger Brosig, Hans-Hubert Borchert, Jorge Heller and Robert Gurny
    Macromolecules, 36 (16) , 2003, p6135-6141
    DOI:10.1021/ma034365q | unige:3715 | Abstract | Article HTML | Article PDF
Electron paramagnetic resonance (EPR) imaging was applied to investigate further the in vitro degradation process of poly(ortho esters) containing 30 mol % lactic acid units in the polymer backbone (POE70LA30) and developed for controlled drug delivery. The objective of this study was the direct and continuous determination of pH values inside the degrading POE70LA30. pH-sensitive nitroxide spin probes 4-amino-2,2,5,5-tetramethyl-3-imidazoline-1-yloxy, 2,2,3,4,5,5-hexamethylimidazolidine-1-yloxy, and 2,2,4,5,5-pentamethyl-3-imidazoline-1-yloxy were calibrated in buffer solutions in order to cover a pH range between 1.0 and 8.0. Nitroxide spin probes were incorporated in POE70LA30, and polymer samples were incubated in 0.1 M phosphate buffer (pH 7.4) at 37 °C. At selected times, polymer samples were removed for the determination of pH values inside the eroding POE70LA30 by EPR at a frequency of 9.4 GHz. EPR imaging showed that the in vitro degradation of POE70LA30 followed a two-phase process:  in the first week of incubation, diffusion of water, and in consequence polymer degradation, were limited to the surface of the hydrophobic POE70LA30 where pH values between 6.0 and 7.4 were measured. After 1 week of incubation, water diffused into the core of the sample, allowing the determination of pH values inside the eroding POE70LA30 until complete erosion. Results indicated the formation of a pH gradient, with the most acidic environment inside the eroding sample where the lowest pH value of 3.8 was measured and higher pH at the surface. It was also possible to observe a polymer erosion front moving down within the polymer matrix in the course of time. The pH value of 3.8 measured inside the degrading POE70LA30 remained constant until polymer samples disintegrated at day 23, where no EPR signal was detectable. In conclusion, EPR imaging allows the noninvasive spatially resolved observation of pH changes within POE70LA30, and results confirmed that the in vitro erosion mechanism of POE70LA30 was neither bulk erosion nor pure surface erosion.
The fluorescence decays of several exciplexes with partial charge transfer have been investigated in solvents of various polarity. The measured lifetimes are found to be in reasonable agreement with the activation enthalpy and entropy of exciplex decay obtained earlier from the temperature dependence of the exciplex emission quantum yields. For exciplexes with 9-cyanophenanthrene substantial contribution of the higher local excited state into the exciplex electronic structure is found and borrowed intensity effect enhances the exciplex emission rate constants.
Free ion formation in acetonitrile is examined through transient photoconductivity for a set of ketones excited at different wavelengths. According to the photophysical parameters of the ketones and the incident photon energy, two mechanisms can be operative: triplet–triplet annihilation (bimolecular process) and/or photoionization (monomolecular biphotonic process). By using a tunable laser, excited state mediated photoionization was studied. From the threshold energy (Ethr) required for this process to occur, ionization potentials in solution (IS) were deduced and compared to the corresponding values in gas phase (IG). A simple energetic model enables the determination of the oxidation potential (Eox) of the ketones that are compared to the corresponding values obtained through electrochemical measurements.
  • Photodegradation of Sulfonylurea Molecules: Analytical and Theoretical DFT Studies
    C. Corminboeuf, F. Carnal, J. Weber, J.-M. Chovelon and H. Chermette
    Journal of Physical Chemistry A, 107 (47) , 2003, p10032-10038
    Keywords: TD-DFT, photodegradation
    DOI:10.1021/jp035776b | unige:3703 | Abstract | Article HTML | Article PDF
The use and the number of sulfonylurea herbicides have increased since the early 1980s. A good understanding of their degradation is of ecological importance, since environmental pollutants can be issued from them. It is claimed that microbial degradation and chemical hydrolysis present the main degradation pathways but photodegradation cannot be neglected. Time-dependent density functional theory has been used to help in the elucidation of the photochemical behavior of sulfonylureas.
  • Analysis of Aromatic Delocalization: Individual Molecular Orbital Contributions to Nucleus-Independent Chemical Shifts
    T. Heine, P.v.R. Schleyer, C. Corminboeuf, G. Seifert, R. Reviakine and J. Weber
    Journal of Physical Chemistry A, 107 (33) , 2003, p6470-6475
    Keywords: chemical shifts, aromaticity, delocalization, annulenes, Londo-HĂźckel
    DOI:10.1021/jp035163z | unige:3702 | Abstract | Article HTML | Article PDF
Individual molecular orbital (MO) contributions to the magnetic shielding of atoms as well as to the nucleus-independent chemical shifts (NICS) of aromatic compounds can be computed by the widely used gauge-including atomic orbital (GIAO) method. Detailed analyses of magnetic shielding MO-NICS contributions provide interpretive insights that complement and extend those given by the localized MO (“dissected NICS”, LMO-NICS) method. Applications to (4n + 2) π-electron systems, ranging from [n] annulenes to Dnh S3, S5, and N6H62+ rings as well as to D2h cyclobutadiene, show the extent to which their diatropic character results from the σ framework and from the π orbitals. The diatropicity of both these contributions decreases with the number of nodes of the wave function around the ring. The highest-energy orbitals can become paratropic. This is generally the case with the σ orbitals, but is found only for “electron-rich” π systems such as sulfur rings. MO-NICS contributions, which can be interpreted using London−Hückel theory, correlate with inverse ring size.
  
As shown by detailed nucleus-independent chemical shift (NICS) analyses of the contributions of each molecular orbital, the very recently reported gas-phase all-metal Al4Li3- anion and its relatives (Kuznetsov, A.E.; Birch, K.A.; Boldyrev, A.I.; Li, X.; Zhai, A.I.; Wang, L.S. Science 2003, 300, 622) are aromatic rather than antiaromatic. The paratropic (antiaromatic) four-π-electron contribution is overcome by the predominating diatropic effects of σ aromaticity. However, true antiaromatic all-metal clusters, such as Sn62- (Schiemenz, B.; Huttner, G. Angew. Chem., Int. Ed. Engl. 1993, 32, 297), do exist.
 
A tetragonal La2+ center (symmetry C4v) was identified in single crystals of BaFCl and SrFCl doped with lanthanum with the aid of electron paramagnetic resonance (EPR)/electron-nuclear double resonance (ENDOR). This center forms a donor-acceptor couple with initially present F(F-) centers. Switching takes place by illumination of appropriate wavelength. The kinetics of the process was monitored by EPR as La2+ and the unswitched F center are paramagnetic. The results of our experimental investigation of this kinetics are presented. A foregoing spectroscopic characterization of the La2+ center allowed one to identify a d-d (the B1-E) transition, a charge-transfer band (for BaFCl at 10940cm-1 and at 17890cm-1, respectively) and to obtain a value of 710cm-1 for the spin-orbit coupling constant in the ground state. In order to narrow the choice of possible acceptor-donor partners a detailed EPR/optical search was further done to identify a number of lattice defects and oxygen centers—in addition to a La-oxygen molecular structure.
  • Fine tuning the electronic properties of [M(bpy)3]2+ complexes by chemical pressure (M = Fe2+, Ru2+, Co2+, bpy = 2,2'-bipyridine)
    Andreas Hauser, Nahid Amstutz, Sandra Delahaye, Sabine Schenker, Aasma Sadki, Regula Sieber and Mohamed Zerara
    in "Structure and Bonding" (ed. Th. SchĂśnherr), Springer, Berlin, 106 , 2003, p81
    unige:4029
  • Excited Spin State Trapping (LIESST, NIESST)
    Andreas Hauser and Philipp GĂźtlich
    in "Comprehensive Coordination Chemistry II" (eds. T. J. Meyer, A. B. P. Lever), Elsevier, Amsterdam, 2 , 2003, p427
    unige:4030
  
  • Bond-Stretch Isomerism: Trapped Isomeric Structures of Hexacoordinate Copper(II) Bispidine Chromophores along a Jahn-Teller Active Vibrational Coordinate
    Peter Comba, Andreas Hauser, Marion Kerscher and Hans Pritzkow
    Angewandte Chemie International Edition, 42 (37) , 2003, p4536-4540
    Keywords: coordination chemistry; copper; Jahn-Teller effects; N ligands
    DOI:10.1002/anie.200351900 | unige:3511 | Abstract | Article HTML | Article PDF
Copper(II) complexes of the pentadentate bispidine ligands exist in two isomeric forms (see structure) with bond-length differences up to 0.5 Å. The stabilization of either isomer may be achieved by a variation of the substituent at N7.
The 132,173-cyclopheophorbide a enol (CPP) is shown to convert mainly to a ~1:1 mixture of (132R/S) chlorophyllones a (Chlone), when chromatographed over silica gel or alumina supports. 151-hydroxychlorophyllonelactone a and some other chlorophyll a related compounds are also tentatively identified as minor transformation products of CPP. This raises the possibility that the chlorophyllones reported in recent sediments may be analytical artifacts from CPP. However, data for the surface sediments from Lake Motte as well as literature data for other contemporary sediments show that, (i) they are not artifacts, (ii) considering that CPP is the intermediate compound in the formation of chlorophyllones from chlorophyll a, the hydroxylation of CPP in the sedimentary environment involves an enzymatic process leading preferentially to 132S chlorophyllone a.
In this paper we discuss on the quantum efficiency in spin crossover compounds. Spin crossover solids are text-book examples of photo switchable materials that present a thermal spin transition from the diamagnetic low-spin state, thermodynamically stable at low temperatures, to the paramagnetic high-spin state becoming the thermodynamically stable state at elevated temperature. By irradiating them with an appropriate wavelength, they can pass from the stable low spin state to the metastable high spin state at temperatures below the thermal transition temperature. For the compound [Fe(pic)3]Cl2¡EtOH, the question regarding the quantum efficiency of the photo-conversion process that is the number of molecules converted by one single photon and its possible dependency on irradiation intensity gave rise to a controversy. The experimental results presented in this paper demonstrate that the quantum efficiency of the photo-conversion at 11 K is on the order of unity, with no noticeable dependency of the quantum efficiency on light intensity. It does, however, depend to a small extent on the fraction of complexes already converted to the high-spin state.
  • External pressure and light influence on internal pressure in a spin-crossover solid [Zn : Fe(ptz)6](BF4)2
    Jelena Jeftic, Claude Ecolivet and Andreas Hauser
    High Pressure Research, 23 (3) , 2003, p359-363
    DOI:10.1080/0895795031000139127 | unige:3689 | Abstract | Article PDF
The relation between the internal pressure during spin-crossover is compared to the chemical pressure induced by dilution with zinc. Further, the light of a specific LIESST (Light Induced Excited Spin State Trapping) wavelength is used to induce partial stabilisation of high-spin state and thus shift temperature of the spin-crossover towards lower values. The de-coupling of the spin-crossover and structural phase transition is discussed.
  • Rates of Electronic Energy Transfer in Conformationally Flexible Bichromophoric Macrocyclic Complexes: A Combined Experimental and Molecular Modeling Study
    E.G. Moore, P.V. Bernhardt, A. Pigliucci, M.J. Riley and E. Vauthey
    Journal of Physical Chemistry A, 107 (41) , 2003, p8396-8403
    DOI:10.1021/jp035781f | unige:3509 | Abstract | Article HTML | Article PDF
  
Electronic energy transfer (EET) rate constants between a naphthalene donor and anthracene acceptor in [ZnL4a](ClO4)2 and [ZnL4b](ClO4)2 were determined by time-resolved fluorescence where L4a and L4b are the trans and cis isomers of 6-((anthracen-9-yl-methyl)amino)-6,13-dimethyl-13-((naphthalen-1-yl-methyl)amino)-1,4,8,11-tetraazacyclotetradecane, respectively. These isomers differ in the relative disposition of the appended chromophores with respect to the macrocyclic plane. The trans isomer has an energy transfer rate constant (kEET) of 8.7 × 108 s-1, whereas that of the cis isomer is significantly faster (2.3 × 109 s-1). Molecular modeling was used to determine the likely distribution of conformations in CH3CN solution for these complexes in an attempt to identify any distance or orientation dependency that may account for the differing rate constants observed. The calculated conformational distributions together with analysis by 1H NMR for the [ZnL4a]2+ trans complex in the common trans-III N-based isomer gave a calculated Förster rate constant close to that observed experimentally. For the [ZnL4b]2+ cis complex, the experimentally determined rate constant may be attributed to a combination of trans-III and trans-I N-based isomeric forms of the complex in solution.
  
  • Kinetic Stabilization of Primary Hydrides of Main Group Elements. The Synthesis of an Air-Stable, Crystalline Arsine and Silane
    M. Brynda, G. Bernardinelli, C. Dutan and M. Geoffroy
    Inorganic Chemistry, 42 (21) , 2003, p6586-6588
    DOI:10.1021/ic034367r | unige:3508 | Abstract | Article HTML | Article PDF
Two new, “user-friendly” derivatives of triptycene containing AsH2 and SiH3 fragments were synthesized. Both solids are crystalline, air-stable compounds characterized by elevated melting points and resistance toward moisture. The highly reactive As−H and Si−H bonds are protected by the presence of the surrounding phenylene hydrogen atoms, which ensure a remarkable kinetic stabilization of these primary hydrides. After X-ray irradiation of a single crystal of triptycenesilane, a persistent silyl radical was trapped and characterized.
  • Sterically Encumbered Diphosphaalkenes and a Bis(diphosphene) as Potential Multiredox-Active Molecular Switches: EPR and DFT Investigations
    Cosmina Dutan, Shashin Shah, Rhett C. Smith, Sylvie Choua, ThĂŠo Berclaz, Michel Geoffroy and John D. Protasiewicz
    Inorganic Chemistry, 42 (20) , 2003, p6241-6251
    DOI:10.1021/ic030079j | unige:3507 | Abstract | Article HTML | Article PDF
  
The reduction products of two diphosphaalkenes (1 and 2) and a bis(diphosphene) (3) containing sterically encumbered ligands and corresponding to the general formulas Ar−X==Y−Ar‘−Y==X−Ar, have been investigated by EPR spectroscopy. Due to steric constraints in these molecules, at least one of the dihedral angles between the CXYC plane and either the Ar plane or the Ar‘ plane is largely nonzero and, hence, discourages conformations that are optimal for maximal conjugation of P==X (or P==Y) and aromatic π systems. Comparison of the experimental hyperfine couplings with those calculated by DFT on model systems containing no cumbersome substituents bound to the aromatic rings shows that addition of an electron to the nonplanar neutral systems causes the X==Y−Ar‘−Y==X moiety to become planar. In contrast to 1 and 2, 3 can be reduced to relatively stable dianion. Surprisingly the two-electron reduction product of 3 is paramagnetic. Interpretation of its EPR spectra, in the light of DFT calculations on model dianions, shows that in [3]2- the plane of the Ar‘ ring is perpendicular to the CXYC planes. Due to interplay between steric and electronic preferences, the Ar−X==Y−Ar‘−Y==X−Ar array for 3 is therefore dependent upon its redox state and acts as a “molecular switch”.
The kinetic energy functional Ts[ρ] in a reference system of non-interacting electrons is a key quantity in density functional theory. Approximating it as an explicit functional of the electron density ρ is the object of continuous interest since the earliest days of quantum mechanics (Thomas–Fermi electron gas theory). A simple proof of the exact inequality Ts[ρA + ρB] − Ts[ρA] − Ts[ρB] ≥ 0 valid for a special class of spin-compensated pairs of electron densities ρA and ρB (vAB-representable pairs) is provided. The derived relation is discussed to rationalize some of the results of the past attempts to approximate Ts[ρ]. It is also discussed as a tool for deriving approximations to the functional Ts[ρ] and/or the bi-functional Tnads[ρA, ρB] = Ts[ρA + ρB] − Ts[ρA] − Ts[ρB].
The electronic structure of La2CuO4 has been investigated using first-principles cluster calculations and the chemical shieldings at the copper nucleus have been determined with several state-of-the-art quantum chemical methods. We have also calculated the copper shieldings for CuCl, which is often used as a reference substance for copper nuclear magnetic resonance shifts measurements, and found an appreciable paramagnetic contribution in agreement with precise measurements. The calculated chemical shift at the copper nucleus in La2CuO4 for an applied field parallel to the CuO2 planes is generally smaller than, but still in reasonable agreement with, the values derived from experiment with the assumption that the spin susceptibility vanishes at zero temperature. For the field perpendicular to the planes, the quantum chemical result is substantially smaller than the experimental data but in accord with a perturbation theoretical estimate. Inconsistencies in previous representations and interpretations of the copper magnetic shift data are pointed out and corrected.
  
  • The solution structure of homotrimetallic lanthanide helicates investigated with novel model-free multi-centre paramagnetic NMR methods
    N. Ouali, J.-P. Rivera, , J. Weber and C. Piguet
    Dalton Transactions, (7) , 2003, p1251-1263
    DOI:10.1039/b212352e | unige:3505 | Abstract | Article HTML | Article PDF
The combination of one contact and three pseudo-contact contributions to the NMR hyperfine paramagnetic shift of each proton in the triple-stranded helicates [Ln3(L1)3]9+(Ln = Ce–Yb except Pm, Gd) produce intractable 1H NMR spectra whose assignment is limited by the large electronic contribution to the nuclear relaxation processes. The detailed analysis of the NMR spectra for the diamagnetic complexes [Ln3(L1)3]9+(Ln = La, Lu, Y) shows that the triple-helical structure found in the solid state is maintained in solution. Extension of the classical one-nucleus crystal-field dependent model-free method for paramagnetic D3-symmetrical homotrimetallic lanthanide complexes possessing two different metallic sites (i.e. two second-rank crystal-field parameters: B20central and B20terminal) allows (i) the complete interpretation of the paramagnetic signals for Ln = Ce–Yb and (ii) the detection of a concomitant abrupt change of the contact terms Fi and of the pseudo-contact terms Si=B20centralG1i+B20terminal(G2i+G3i) occurring near the middle of the lanthanide series. The derivation and application of a novel three-nuclei crystal-field independent method eventually demonstrates that the helicates [Ln3(L1)3]9+ adopt a single D3-symmetrical structure along the complete lanthanide series in solution, which ascribes the discontinuity observed for Si to a concomitant decrease of the two crystal-field parameters. Comparison with structural models is limited by the extreme sensitivity of the structural factors Cikl and Dikl to minor geometrical variations affecting the wrapping of the ligand strands, but calculations of the geometrical factors Gmi(m= 1–3) for [Ln3(L1)3]9+ in solution confirm the formation of a regular triple-helical structure. Generalization of this novel three-nuclei method for addressing the solution structure of rhombic lanthanide complexes is discussed.
  • Clusters of alkali and alkaline earth metals : a new synthetic approach, single crystal structures, theoretical calculations and potential applications
    K.M. Fromm, E.D. Gueneau, G. Bernardinelli, H. Goesmann, J. Weber, M.J. Mayor-Lopez, P. Boulet and H. Chermette
    Journal of the American Chemical Society, 125 (12) , 2003, p3593-3604
    DOI:10.1021/ja0205737 | unige:3684 | Abstract | Article HTML | Article PDF
A new synthetic approach, reacting alkaline earth metal iodides with butyllithium, lithium hydroxide, and/or lithium butoxide under salt elimination, is presented, giving access to some interesting clusters of calcium, strontium, and barium, partially in combination with lithium. The so far largest calcium cluster Li[{Ca7(Ο3-OH)8I6(thf)12}2(Ο2-I)]¡3THF, 4, and the new strontium compound [Sr3I3(OH)2(thf)9]I, 5, are shown to feature common building blocks of OH-capped M3 triangles. On the basis of mainly electrostatic interactions, these clusters are not volatile. By introducing LiOtBu, the two clusters [IM(OtBu)4{Li(thf)}4(OH)] (6, M = Sr; 7, M = Ba) are prepared, 7 exhibiting volatility as an important physical property, which makes it a potential precursor for chemical vapor deposition. The structural relationship between 4, 5, 6, and 7 and their respective starting materials is shown, and possible reaction mechanisms are proposed. Exhibiting surprising and new structural motifs, the bonding modes of these clusters are investigated by the electron localization function as well as by ab initio calculations.
  • Density functional theory study of homologous organometallic molecules of the [RhXL2]2 (X=Cl, Br, or I); L=CO, PH3, or PF3) type.
    P. Seuret, J. Weber and T.A. Wesolowski
    Molecular Physics, 101 (16) , 2003, p2537-2543
    DOI:10.1080/0026897031000112497 | unige:3502 | Abstract | Article PDF
  
Density functional theory generalized gradient approximation calculations, which were tested in our previous detailed study of [RhCl(PF3)2]2 (Seuret et al., 2003, Phys. Chem. chem. Phys., 5, 268-274), were applied for a series of homologous organometallic compounds of the [RhXL2]2 (X = Cl, Br, or I; L = CO, PH3, or PF3) type. Various properties of the studied compounds were obtained. Optimized geometries of [RhCl(PH3)2]2 and [RhCl(CO)2]2 are in very good agreement with available experimental data. Geometries of other compounds as well as other properties (thermochemistry of selected fragmentation channels, barriers to structural changes, frontier orbitals) which are not available experimentally were predicted. All the considered compounds are not planar. Enforcing planarity of the central [RhX]2 moiety requires only a small energetic cost ranging from 2.2 to 3.9 kcal mol-1. The analysis of frontier orbitals indicates that the metals provide the most favourable site for the electrophilic attack in all considered compounds. The analysis of the shape of the lowest unoccupied molecular orbitals indicates that the halogens and ligands provide the most favourable site for the nucleophilic attack for [RhCl(CO)2]2 or [RhCl(PF3)2]. For [RhBr(PF3)2]2, [RhI(PF3)2]2 and [RhCl(PH3)2]2, the nucleophilic attack on the halogen is less probable. Except for [RhCl(CO)2]2, the least energetically expensive decomposition channel involves initial separation of ligands. For [RhCl(CO)2]2, its decomposition into the RhCl(CO)2 fragments was found to be the least energetically expensive fragmentation reaction which is probably one of the reasons for the known catalytic activity of this compound.
  • Connecting Terminal Carboxylate Groups in Nine-Coordinate Lanthanide Podates: Consequences on the Thermodynamic, Structural, Electronic, and Photophysical Properties
    Jean-Michel Senegas, GĂŠrald Bernardinelli, Daniel Imbert, Jean-Claude G. BĂźnzli, , Jacques Weber and Claude Piguet
    Inorganic Chemistry, 42 (15) , 2003, p4680-4695
    DOI:10.1021/ic034231t | unige:3236 | Abstract | Article HTML | Article PDF
The hydrolysis of terminal tbutyl-ester groups provides the novel nonadentate podand tris{2-[N-methylcarbamoyl-(6-carboxypyridine-2)-ethyl]amine} (L13) which exists as a mixture of slowly interconverting conformers in solution. At pH = 8.0 in water, its deprotonated form [L13 − 3H]3- reacts with Ln(ClO4)3 to give the poorly soluble and stable podates [Ln(L13 − 3H)] (log(β110) = 6.7−7.0, Ln = La−Lu). The isolated complexes [Ln(L13 − 3H)](H2O)7 (Ln = Eu, 8; Tb, 9; Lu, 10) are isostructural, and their crystal structures show Ln(III) to be nine-coordinate in a pseudotricapped trigonal prismatic site defined by the donor atoms of the three helically wrapped tridentate binding units of L13. The Ln−O(carboxamide) bonds are only marginally longer than the Ln−O(carboxylate) bonds in [Ln(L13 − 3H)], thus producing a regular triple helix around Ln(III) which reverses its screw direction within the covalent Me−TREN tripod. High-resolution emission spectroscopy demonstrates that (i) the replacement of terminal carboxamides with carboxylates induces only minor electronic changes for the metallic site, (ii) the solid-state structure is maintained in water, and (iii) the metal in the podate is efficiently protected from interactions with solvent molecules. The absolute quantum yields obtained for [Eu(L13 − 3H)] ( ΦtotEu = 1.8 × 10-3) and [Tb(L13 − 3H)] ( ΦtotEu = 8.9 × 10-3) in water remain modest and strongly contrast with that obtained for the lanthanide luminescence step (ΦEu = 0.28). Detailed photophysical studies assign this discrepancy to the small energy gap between the ligand-centered singlet (1ππ*) and triplet (3ππ*) states which limits the efficiency of the intersystem crossing process. Theoretical TDDFT calculations suggest that the connection of a carboxylate group to the central pyridine ring prevents the sizable stabilization of the triplet state required for an efficient sensitization process. The thermodynamic and electronic origins of the advantages (stability, lanthanide quantum yield) and drawbacks (solubility, sensitization) brought by the “carboxylate effect” in lanthanide complexes are evaluated for programming predetermined properties in functional devices.
The charge recombination dynamics of geminate ion pairs formed by electron transfer quenching of cyanoanthracene derivatives by bromo- and iodo-anisole in acetonitrile was investigated using various ultrafast spectroscopic techniques. Without a heavy atom, the only charge recombination pathway is that leading to the neutral ground state. With heavy atom substituted anisoles, charge recombination to the local triplet state of the excited precursor is observed. Time constants for triplet charge recombination ranging from 400 ps to less than 10 ps, depending on the heavy atom and on the energy gap between the ion pair and the triplet state, have been measured. This heavy atom effect was observed with ion pairs formed upon electron-transfer quenching with driving force going from −0.15 to −0.6 eV, suggesting that these intermediates are in fact exciplexes. A new scheme for producing free ions with a high yield using this effect and a secondary electron donor is also demonstrated.
  • A Highly Configurationally Stable [4]Heterohelicenium Cation
    Christelle Herse, Delphine Bas, Frederik C. Krebs, Thomas BĂźrgi, Jacques Weber, Tomasz Wesolowski, Bo W. Laursen and JĂŠrĂ´me Lacour
    Angewandte Chemie International Edition, 42 (27) , 2003, p3162-3166
    Keywords: asymmetric synthesis, carbenium ions, chiral resolution, circular dichroism, helicenes
    DOI:10.1002/anie.200351443 | unige:3238 | Abstract | Article HTML | Article PDF
A [4](hetero)helicenium cation was resolved using the hexacoordinated phosphorus-containing binphat anion (see picture: N, blue; O, red; C, gray). Its absolute configuration was determined by vibrational circular dichroism spectroscopy. The barrier of interconversion of its enantiomers is higher than that of [6]helicene.
  
  • Fluorescence Quenching in Electron-Donating Solvents. 1. Influence of the Solute-Solvent Interactions on the Dynamics
    Ana Morandeira, Alexandre FĂźrstenberg, Jean-Claude Gumy and Eric Vauthey
    Journal of Physical Chemistry A, 107 (28) , 2003, p5375-5383
    DOI:10.1021/jp0343133 | unige:3610 | Abstract | Article HTML | Article PDF
The electron transfer (ET) quenching dynamics of excited perylene (Pe), cyanoperylene (PeCN), methanolperylene (PeOH), and methylperylene (PeMe) in N,N-dimethylaniline (DMA) has been investigated using ultrafast fluorescence up-conversion. Measurements of the rotational dynamics of PeCN and PeMe in nonpolar and polar inert solvents using optically heterodyned polarization spectroscopy are also presented. The fluorescence decay in DMA is strongly nonexponential and about 10 times faster with PeCN than with the other electron acceptors. The quenching dynamics has been analyzed with a model distinguishing three types of donor molecules surrounding the acceptor:  those with optimal orientation for ET and those requiring orientational or translational diffusion prior to ET. According to this model, which can account for the whole fluorescence decay, the faster quenching dynamics of PeCN is not due to a larger ET rate constant, but to a larger number of donor molecules, typically three to four, with an optimal orientation. This is explained by the effect of dipole−dipole interaction between PeCN and the donor molecules, which favors mutual orientations with a large electronic coupling. With the other acceptors, this interaction is either not present or does not lead to ET active geometries. The occurrence of this interaction is substantiated by the rotational dynamics measurements.
  • Phosphaalkenes with Inverse Electron Density: Electrochemistry, Electron Paramagnetic Resonance Spectra, and Density Functional Theory Calculations of Aminophosphaalkene Derivatives
    Patrick Rosa, Cyril Gouverd, GĂŠrald Bernardinelli, ThĂŠo Berclaz and Michel Geoffroy
    Journal of Physical Chemistry A, 107 (24) , 2003, p4883-4892
    DOI:10.1021/jp030023a | unige:3239 | Abstract | Article HTML | Article PDF
Cyclic voltammetry of Mes*P==C(NMe2)2 (1) and Mes*P==C(CH3)NMe2 (2) shows that, in solution in DME, these compounds are reversibly oxidized at 395 and 553 mV, respectively. Electrochemical oxidation or reaction of 1 (or 2) with [Cp2Fe]PF6 leads to the formation of the corresponding radical cation, which was characterized by its electron paramagnetic resonance (EPR) spectra. Experimental 31P and 13C isotropic and anisotropic coupling constants agree with density functional theory (DFT) calculations showing that the unpaired electron is strongly localized on the phosphorus atom, in accord with the description Mes*P•−(C(NMe2)2)+. Electrochemical reduction of 1 is essentially irreversible and leads to a radical species largely delocalized on the C(NMe2)2 moiety; this neutral radical results from the protonation of the phosphorus atom and corresponds to Mes*(H)P−•C(NMe2)2. No paramagnetic species is obtained by reduction of 2. The presence of the amino groups, responsible for the inverted electron distribution at the P−C double bond (P-−C+), confers on 1 and 2 redox properties that are in very sharp contrast with those observed for phosphaalkenes with a normal π electron distribution (P+−C-):  no detection of the radical anion but easy formation of a rather persistent radical cation. For 1, this radical cation could even be isolated as a powder, 1•+PF6-. As shown by DFT calculations, this behavior is consistent with the decrease of the double bond character of the phosphorus−carbon bond caused by the presence of the amino groups.
  • Density Functional Study of a Helical Organic Cation
    D. Bas, , J. Weber and T.A. Wesolowski
    Chimia, 57 (4) , 2003, p173-174
    Keywords: basis set, computational chemistry, density functional theory, exchange-correlation functional, helical cation
    DOI:10.2533/000942903777679460 | unige:3240 | Abstract | Article PDF
We report on the first stage of our theoretical study of the quino[2,3,4-kl]acridinium,1,13-dimethoxy-5,9-dipropyl-cation. This molecule, involved in the synthesis of novel triazaangulenium dyes of high chemical stability, is a chiral [4]-helicenium. The structure and the IR spectrum of the quino[2,3,4-kl]acridinium,1,13-dimethoxy-5,9-dimethyl-cation derived from theoretical calculations which use various density functional theory methods, are compared with the geometry derived from X-ray diffraction measurements and the experimental IR spectrum. Our study shows that the chosen variant of DFT methods (Becke88 for exchange, P86 for correlation, 3-21G** basis set) reproduces the experimental geometry within 0.004 Å and the IR frequencies within 15 cm−1.
  • Vibrational Relaxation Dynamics of Polyatomic Molecules in Solution
    A. Pigliucci and E. Vauthey
    Chimia, 57 (4) , 2003, p200-203
    DOI:10.2533/000942903777679406 | unige:3241 | Abstract | Article PDF
Ultrafast time-resolved fluorescence measurements have been carried out to investigate vibrational relaxation dynamics of perylene derivatives in solution. The early results obtained with cyanoperylene in acetonitrile are presented and discussed.
The excitation of a charge transfer band by a laser pulse of finite duration and the ensuing charge recombination are calculated in the framework of the perturbation theory. The influence of the spectral characteristics of the laser pulse on the charge recombination dynamics is investigated for models including several nuclear modes that differ greatly in their timescales. It is shown that, in the area of applicability of the perturbation theory, the variation of the pulse carrier frequency inside the absorption band can significantly change the effective charge recombination rate constant.
  • Electron transfer between two sylil-substituted phenylene rings: EPR/ENDOR spectra, DFT calculations, and crystal structure of the one-electron reduction compound of a di(m-silylphenylenedisiloxane)
    C. Dutan, S. Choua, T. Berclaz, M. Geoffroy, N. MĂŠzailles, A. Moores, L. Ricard and P. Le Floch
    Journal of the American Chemical Society, 125 (15) , 2003, p4487-4494
    DOI:10.1021/ja0209060 | unige:3243 | Abstract | Article HTML | Article PDF
Reduction of a solution of octamethylcyclo-di(m-silylphenylenedisiloxane) 4 in THF on a potassium mirror leads to EPR/ENDOR spectra characterized by a large coupling (~20 MHz) with two protons, similar to the spectra obtained after reduction of the m-disilylbenzene derivative 5, consistent with a localization of the extra electron on a single ring of 4. The spectra recorded after reduction of 4 at low temperature in the presence of an equimolar amount of 18-crown-6 exhibit couplings of ~10 MHz with four protons and indicate that embedding the counterion in crown-ether provokes the delocalization of the unpaired electron on the two phenyl rings of 4. The measured hyperfine interactions agree with those calculated by DFT for the optimized structure of 4•-. Direct information on the structure of this anion is obtained from the X-ray diffraction of crystals grown at -18 °C in reduced solutions containing 4, potassium, and crown ether in a THF/hexane mixture. Both DFT and crystal structures clearly indicate the geometry changes caused by the addition of an electron to 4: the interphenyl distance drastically decreases, leading to a partial overlap of the two rings. The structure of 4•- is a model for an electron transfer (ET) transition state between the two aromatic rings. The principal reason for the adoption of this structure lies in the bonding interaction between the LUMO (π* orbitals) of these two fragments; moreover, the constraints of the macrocycle probably contribute to the stabilization of this structure.
  • Intramolecular motion in the triptycenegermanyl radical: single crystal EPR study at variable temperature and DFT calculations
    M. Brynda, C. Dutan, T. Berclaz, M. Geoffroy and G. Bernardinelli
    Journal of Physics and Chemistry of Solids, 64 (6) , 2003, p939-946
    Keywords: organic compounds; EPR; molecular dynamics; quantum chemical method
    DOI:10.1016/S0022-3697(02)00452-3 | unige:3611 | Abstract | Article HTML | Article PDF
  
X-irradiation of single crystals of Tp–GeH3 (Tp: triptycene) led to the trapping of the radical Tp–√GeH2. The angular variations of the resulting EPR spectra were recorded at 300 and 77 K. The drastic temperature dependence of the spectra was caused by both a strong anisotropy of the g-tensor and a rotation of the √GeH2 moiety around the C–Ge bond. The determination of the EPR tensors as well as the analysis of this motion required to take the presence of disorder in the crystal into account. In accordance with DFT calculations, Tp–√GeH2 is shown to be pyramidal and to adopt, in its lowest energy structure, a staggered conformation. Rotation around the C–GeH2 bond is blocked at 90 K and is almost free above 110 K. The experimental barrier, obtained after simulation of the EPR spectra as a function of the rotational correlation time, is equal to 1.3 kcal mol−1, which is slightly inferior to the barrier calculated by DFT (3.6 kcal mol−1). Calculations performed on Tp–CH3, Tp–GeH3 and Tp–√GeH2 show that the rotation barrier ΔErot around the C–Ge bond drastically decreases by passing from the germane precursor to the germanyl radical and that ΔErot increases by passing from the germane to its carbon analogous. Structural parameters involved in these barrier differences are examined.
  • The Change of Aromaticity along a Diels-Alder Reaction Path
    ClĂŠmence Corminboeuf, Thomas Heine and Jacques Weber
    Organic Letters, 5 (7) , 2003, p1127-1130
    Keywords: quantification of aromaticity in a Diels-Alder reaction path using NMR calculations
    DOI:10.1021/ol034203e | unige:3326 | Abstract | Article HTML | Article PDF
Quinodimethanes are highly reactive toward dienophiles since Diels−Alder cycloaddition results in an aromatic product. Density functional-based 13C, 1H NMR, NICS, and MO-NICS calculations indicate that the increase of aromatic character of the developing benzenoid ring along the reaction path is especially pronounced after the transition state is reached, even though the number of π orbitals decreases. The forming aliphatic ring exhibits large ring current effects during the reaction.
  • Evaluation of aromaticity: A new dissected NICS model based on canonical orbitals
    ClĂŠmence Corminboeuf, Thomas Heine and Jacques Weber
    Physical Chemistry Chemical Physics, 5 (2) , 2003, p246-251
    Keywords: quantification of aromaticity based on theoretical NMR calculations
    DOI:10.1039/b209674a | unige:3327 | Abstract | Article HTML | Article PDF
Several methods to address aromaticity in terms of nucleus-independent chemical shifts (NICS) are compared. These include NICS at the ring centre NICS(0), NICS 1 Å above the ring plane NICS(1), aromatic ring current shielding (ARCS), and dissected NICS, i.e. NICS calculated from selected π orbitals NICSπ, again in the ring plane and 1 Å above. The methods are tested on the basis of density-functional theory (DFT) and the individual gauge for local orbitals (IGLO) technique. Applications include simple organic rings (C4H4, C4H42+, C6H6, C5H5–, C7H7+) and transition metal carbonyl complexed molecules Fe(CO)3C4H4 and Cr(CO)3C6H6.
An approximate one-electron functional for the classical Coulomb energy J[ρ] is presented. The analytical form of the terms appearing in the functional is justified by the scaling relations of the exact form of the classical Coulomb energy, and, the coefficients in front of each term are determined by a least-squares fit of the exact values for rare-gas atoms. It is shown that the approximation, tested on a set of neutral atoms (2≤Z≤54), can predicts energies with accuracy and leads to a potential vJ(r)=δJ[ρ]/δρ(r) which is in qualitative agreement with the exact one.
A transient grating setup with evanescent wave probing has been developed to investigate ultrafast processes at liquid–liquid interfaces. In order to evaluate the selectivity of this method to the interface, the speed of sound in the low refractive index medium has been measured as a function of the penetration depth of the probe pulse. Our preliminary results indicate an increase of the speed of sound in methanol with decreasing the probe depth from 100 to 70 nm. However, no correlation was found in acetonitrile in the same range. Modifications of the experiment for improving the selectivity to the interface are proposed.
  
  • EPR and optical spectroscopy of Yb3+ ions in CaF2 and SrF2
    M.L. Falin, K.I. Gerasimov, V.A. Latypov, A.M. Leushin, H. Bill and D. Lovy
    Journal of Luminescence, 102-103 , 2003, p239-242
    Keywords: crystal fields; optical spectroscopy; Yb3+; CaF2; SrF2
    DOI:10.1016/S0022-2313(02)00505-7 | unige:3657 | Abstract | Article HTML | Article PDF
The Yb3+ paramagnetic center of the trigonal symmetry (“oxygen” paramagnetic center T2) in CaF2 and SrF2 single crystals is studied by EPR and optical spectroscopy. The Stark level energies of the Yb3+ multiplets are established from absorption, luminescence and excitation luminescence spectra and the crystal field parameters are calculated.
Experimental and theoretical techniques have been applied to study the decomposition of the [RhCl(PF3)2]2 molecule which is known as a precursor in electron beam induced deposition (EBID) of Rh. Mass spectrometry (MS) has been carried out to study the electron ionisation and fragmentation of isolated molecules. Auger electron spectroscopy has been used to characterize the EBID deposit. The MS data indicate the presence of free phosphorus and rhodium ions. This is in agreement with the analysis of the composition of the EBID deposit containing: 60% Rh, 12–25% P, 2–13% Cl, no F, 3–20% O and N. Theoretical calculations (density functional theory) has been used to characterize the precursor molecule and to derive the enthalpies of several simple decomposition reactions. The calculated geometries are in a good agreement with the available X-ray crystallographic data. The [RhCl(PF3)2]2 appears not to be rigid: the PF3 groups can rotate with a relatively low barrier (0.6 kcal mol–1) whereas the barrier for the butterfly-like motion of (RhCl)2 moiety is only 3.5 kcal mol–1. According to the theoretical results, the lowest energy pathway of the decomposition corresponds to a consecutive loss of PF3 ligands, resulting in a (RhCl)2 moiety (without phosphorus). The same conclusion is also valid for the ionised precursor. Experimental data combined with the theoretical results concerning the energetics of the considered various simple decomposition processes indicate that the electron induced dissociation of the precursor cannot be seen as a simple one-step decomposition process.
We analyze the performance of gradient-free local density approximation (LDA) and gradient-dependent generalized gradient approximation (GGA) functionals in a density functional theory variational calculations based on the total energy bifunctional (E[ρ1,ρ2]). These approximations are applied to the exchange-correlation energy and to the nonadditive component of the kinetic energy of the complex. Benchmark ab initio interaction energies taken from the literature for 25 intermolecular complexes for which the interaction energies fall into the 0.1–3.0 kcal/mol range are used as reference. At the GGA level, the interaction energies derived from E[ρ1,ρ2] are more accurate than the Kohn–Sham ones. LDA leads to very good interaction energies for such complexes where the ρ1,ρ2 overlap is very small (Ne-Ne, Ar-Ar, for instance) but it is not satisfactory for such cases where the overlap is larger. Introduction of gradient-dependent terms into the approximate part of E[ρ1,ρ2] improves significantly the overall accuracy of the interaction energies. Gradient-dependent functionals applied in E[ρ1,ρ2] lead to the average error and the average absolute error of the interaction energies amounting to 0.08 kcal/mol and 0.29 kcal/mol, respectively.
  • Enantioselective hydrogenation on cinchona-modified metal catalysts: Mechanistic implication of acid additive
    W.-R. Huck, T. BĂźrgi, T. Mallat and A. Baiker
    Journal of Catalysis, 205 (1) , 2002, p213-216
    DOI:10.1006/jcat.2001.3436 | unige:14710 | Article HTML | Article PDF
The enantioselective hydrogenation of 4-hydroxy-6-methyl-2-pyrone in the presence of acetic acid and trifluoroacetic acid has been studied on cinchonidine-modified Pd/TiO2. Catalytic experiments and theoretical calculations indicate the formation of a cinchonidine–trifluoroacetic acid cyclic ion pair. We propose that this is the actual modifier, which interacts with 4-hydroxy-6-methyl-2-pyrone in the enantiodifferentiating step. The new mechanistic model is assumed to be valid also for other reactions over cinchona-modified Pt or Pd, in the presence of trifluoroacetic acid.
 
Cyclic cinchonidine ∶ acid complexes (1 ∶ 1 and 1 ∶ 2) of the chiral modifier cinchonidine (CD) and an alkenoic acid, tiglic acid, in dichloromethane solvent have been observed by FTIR spectroscopy. Both the OH and the quinuclidine N atom of CD are involved in the hydrogen bond with the acid molecule(s). Such dual-site modifier–reactant interactions play an important role in the enantioselective hydrogenation of alkenoic acids over CD-modified Pd catalysts. The stability of these 1 ∶ 1 and 1 ∶ 2 complexes has been probed by addition of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), a stronger base than CD. DBU builds ion pairs with the acid (with 1 ∶ 1 and 1 ∶ 2 stoichiometry) and a hydrogen bond with the OH of CD. However, despite the large difference in basicity between CD and DBU, 1 ∶ 2 CD ∶ acid complexes can still be detected when more than 0.5 equivalent DBU was added with respect to the acid, at which ratio the enantiomeric excess (ee) drops dramatically. Hence, the molecular structure of CD favours formation of cyclic complexes via a dual-site interaction, which is not possible for DBU ∶ acid complexes, and stabilises 1 ∶ 2 CD ∶ acid species, which are proposed to be responsible for enantiodifferentiation.
  • Environmental catalysis on iron oxide-silica aerogels: Selective oxidation of NH3 and reduction of NO by NH3
    P. Fabrizioli, T. BĂźrgi and A. Baiker
    Journal of Catalysis, 206 (1) , 2002, p143-154
    DOI:10.1006/jcat.2001.3475 | unige:14696 | Article HTML | Article PDF
The catalytic properties of mesoporous iron oxide–silica aerogels prepared by a sol–gel process combined with ensuing supercritical extraction with CO2 was investigated in the selective oxidation (SCO) of ammonia and the selective reduction (SCR) of NO by ammonia. The main parameters changed in the aerogel preparation were the type of base used as gelation agent, the iron content, and the calcination temperature. The aerogels differed significantly in acidity and iron dispersion. Diffuse reflectance infrared Fourier transform spectroscopy studies of ammonia adsorption at different temperatures revealed that ammonia was bound to Brønsted- and Lewis-type sites, the latter being dominant at 300°C. A fraction of low coordinated Fe2+ sites were probed by NO adsorption measurements. Lewis-type sites were found to be associated with low-coordinated iron sites. Catalytic tests were performed in a continuous fixed-bed reactor in the temperature 210–550°C range and at ambient pressure. The catalytic activity of the aerogels in SCO correlated with the abundance of more strongly bound ammonia adsorbed on Lewis sites (low coordinated iron). High selectivity to nitrogen (97%) could be reached up to 500°C, whereas at higher temperature the formation of N2O and NO became significant. The apparent activation energy of N2 formation ranged from 69 to 94 kJ/mol, whereby catalysts with higher selectivity and activity showed lower activation energy. In SCR, selectivity to nitrogen was for all aerogels >98% at T<460°C, and activation energies varied from 38 to 53 kJ/mol. The catalytic activity for SCR did not correlate with the population density of Lewis sites. We propose that SCO predominantly occurs on Lewis sites consisting of highly dispersed iron atoms of low coordination, whereas in SCR these sites do not play an important role.
  • Reactions of gas phase H atoms with ethylene, acetylene and ethane adsorbed on Ni(111)
    T. BĂźrgi, T.R. Trautman, M. Gostein, D.L. Lahr, K.L. Haug and S.T. Ceyer
    Surface Science, 501 (1-2) , 2002, p49-73
    Keywords: Electron energy loss spectroscopy (EELS); Chemisorption; Physical adsorption; Surface chemical reaction; Nickel; Alkanes; Alkenes; Alkynes
    DOI:10.1016/S0039-6028%2801%2901755-1 | unige:14879 | Article HTML | Article PDF
The products of the reaction of the most energetic form of hydrogen, gas phase H atoms, with ethylene, acetylene and ethane adsorbed on a Ni(1 1 1) surface at 60 K are probed. Adsorbed ethylidyne (CCH3) is identified by high resolution electron energy loss spectroscopy to be the major product (30% yield) in all three cases. Adsorbed acetylene is a minor product (3% yield) and arises as a consequence of a dynamic equilibrium between CCH3 and C2H2 in the presence of gas phase H atoms. The observation of the same product for the reaction of H atoms with all three hydrocarbons implies that CCH3 is the most stable C2 species in the presence of coadsorbed hydrogen. The rates of CCH3 production are measured as a function of the time of exposure of H atoms to each hydrocarbon. A simple kinetic model treating each reaction as a pseudo-first order reaction in the hydrocarbon coverage is fit to these data. A mechanism for the formation of CCH3 via a CHCH2 intermediate common to all three reactants is proposed to describe this model. The observed instability of the CH2CH3 species relative to C2H4 plays a role in the formulation of this mechanism as does the observed stability of CHCH2 species in the presence of coadsorbed hydrogen. The CH2CH3 and the CHCH2 species are produced by the translational activation of ethane and the dissociative ionization of ethane and ethylene, respectively. In addition, the binding energy and the vibrational spectrum of ethane adsorbed on Ni(1 1 1) are determined and exceptionally high resolution vibrational spectra of adsorbed ethylene and acetylene are presented.
  
Iron oxide aerogels were synthesized from tetramethoxysilicon(IV) (TMOS) or tetraethoxysilicon(IV) (TEOS) and iron nitrate using an acid-catalyzed solution–sol–gel method combined with subsequent extraction of the alcoholic solvent with supercritical CO2. The main parameters varied in the sol–gel synthesis were: the type of N-base used as the gelation agent (N,N-diethylaniline, trihexylamine, ammonium carbonate, ammonia), the concentration of the iron precursor, and the water content. The silicon precursor was prehydrolyzed to improve its reactivity. After calcination at 600 °C, the structural and chemical properties of the aerogels containing 0–20wt% nominal Fe2O3 were characterized by means of nitrogen adsorption, X-ray diffraction (XRD), transmission and scanning electron microscopy, temperature programmed reduction, X-ray photoelectron spectroscopy (XPS), UV-Vis, DRIFT and EPR spectroscopy. XRD and electron microscopy indicated that all aerogels were amorphous, irrespective of the sol–gel conditions used. The aerogels were predominantly mesoporous, with pore size maxima ranging between 20–50 nm, but also exhibited some microporosity. For the 10 wt% iron oxide samples, the specific pore volumes ranged from 0.7 to 2 cm3 g−1 and BET-surface areas from 150 to 636 m2 g−1, depending on conditions. With increasing iron content, the BET surface area decreased from 740 to 340 m2 g−1, accompanied by increasing microporosity. XPS revealed significant silicon enrichment on the surface. Spectroscopic investigations (UV-Vis, EPR) uncovered different iron-containing species, ranging from tetrahedrally coordinated iron atoms incorporated in the silica matrix to iron oxide nanoclusters. Formation of isolated iron atoms was favored with low iron content samples. The N-base used to force gelation had a significant effect on the morphology and population density of Fe(OH)Si in the aerogels.
The influence of acetic acid (AcOH) and trifluoroacetic acid (TFA) on the hydrogenation of ethyl-4,4,4-trifluoroacetoacetate has been investigated by using Pt/Al2O3 modified by cinchonidine and O-methylcinchonidine. We have shown that the sometimes dramatic changes in enantioselectivity and rate cannot simply be interpreted by protonation of the alkaloid modifier. We propose a new three-step reaction pathway, involving interaction of the carboxylic acid with the reactant and the chiral modifier. The mechanism is supported by IR spectroscopic identification of cyclic TFA–modifier ion pairs. This new approach can rationalise the poorly understood role of acids in the enantioselective hydrogenation of activated ketones over cinchona-modified platinum metals.
  • Manganese oxide-silica aerogels: Synthesis and structural and catalytic properties in the selective oxidation of NH3
    P. Fabrizioli, T. BĂźrgi and A. Baiker
    Journal of Catalysis, 207 (1) , 2002, p88-100
    DOI:10.1006/jcat.2002.3532 | unige:14697 | Abstract | Article PDF
Manganese oxide–silica mixed oxide aerogels with different morphological and chemical properties were prepared using the sol–gel method and ensuing extraction of the solvent with supercritical CO2. Two types of manganese precursor, varying hydrolysis conditions of the silica and manganese precursors, influence of base addition for gelation, and calcination temperatures were investigated. Base addition had a strong effect on textural properties, producing high-surface-area, mesoporous aerogels, whereas these properties were only marginally affected by kind of manganese precursor used. The presence of different manganese oxide species was evidenced by X-ray diffraction, Raman and diffuse reflectance infrared Fourier transform spectroscopy, and temperature-programmed reduction. Mn4+, Mn3+, and Mn2+ oxide species were found after calcination at 600°C in air. Sol–gel processing with manganese(II) nitrate resulted in highly dispersed mixed oxides. Basic gelation of these sols strongly influenced the state of the manganese, leading to crystallites of hausmannite and to amorphous Mn5O8 in the calcined samples. Aerogels derived from the less reactive Mn(III) (acac)3 did not contain any manganese oxide crystallites when prepared under the same basic conditions. The catalytic performance of the aerogels in the selective oxidation of ammonia strongly depended on the state of the manganese. Samples containing crystalline Mn3O4 were more active than amorphous aerogels with dispersed manganese oxide species and afforded high selectivity to N2O. The presence of amorphous Mn5O8 further increased the activity and the selectivity to nitrous oxide, reaching 74% at 360°C. Nitrogen formation was found to be related to the amount of strongly Lewis-bound ammonia. The amorphous aerogels showing more Lewis-bound ammonia produced mainly nitrogen below 480°C, affording a selectivity of 78% at 360°C.
  • Origin of rate acceleration in enantioselective hydrogenation of a-functionalised ketones over cinchona alkaloid modified platinum
    Angelo Vargas, Thomas BĂźrgi and Alfons Baiker
    New Journal of Chemistry, 26 , 2002, p807-810
    DOI:10.1039/b202182j | unige:14675 | Abstract | Article HTML | Article PDF
  
The origin of the rate acceleration in enantioselective hydrogenation of α-functionalised ketones over cinchona alkaloid modified platinum has been studied using a combined experimental and theoretical approach, and the rate acceleration is traced to a lowering of the energy of the carbonyl π orbitals in the diastereomeric complex formed between reactant and modifier.
  • Relation between electronic structure of a-substituted ketones and their reactivity in racemic and enantioselective platinum-catalyzed hydrogenation
    Angelo Vargas, Thomas BĂźrgi, Matthias Von Arx, Reto Hess and Alfons Baiker
    Journal of Catalysis, 209 (2) , 2002, p489-500
    DOI:10.1006/jcat.2002.3653 | unige:14734 | Article HTML | Article PDF
The relation between the electronic structure of Îą-substituted ketones and their reactivity in the racemic and enantioselective platinum-catalyzed hydrogenation has been investigated using a combined theoretical and experimental approach. A correlation between the keto carbonyl orbital energy and the hydrogenation rate has been found, which rationalizes the effect of the substituent on the rate of hydrogenation. The uncovered relationship between the keto carbonyl orbital energy and the hydrogenation rate provides a rational explanation for the often observed rate acceleration that occurs when cinchona-modified platinum is used as a enantioselective hydrogenation catalyst. The previously suggested model for enantiodiscrimination based on the different stability of the diastereomeric complexes formed between the reactant and the cinchona modifier is discussed in the light of the new kinetic findings.
Vibrational circular dichroism (VCD) spectra of the chiral modifiers cinchonidine, an alkaloid, and (R)-2-(pyrrolidin-1-yl)-1-(1-naphthyl)ethanol (PNE) were measured and simulated. For cinchonidine independent information from NMR investigations on the distribution of conformers was used to simulate VCD spectra from calculated spectra of the individual conformers. Agreement with experiment is reasonably good. For the structurally similar synthetic modifier PNE VCD spectra show that an open conformer predominates in solution. The difference between the most stable conformers of cinchonidine and PNE in solution is the intramolecular hydrogen bond found in the latter, which forms due to the enhanced flexibility of the pyrrolidinyl moiety in PNE as compared to the quinuclidine moiety in cinchonidine. The similar enantiodifferentiating power of cinchonidine and PNE as chiral modifiers in the heterogeneous enantioselective hydrogenation of ethyl pyruvate indicates that the rigidity of this part of the molecule is not a prerequisite for enantioselection. It is furthermore shown that binding of a non-chiral carboxylic acid to the alkaloid induces VCD in vibrations associated with the acid. Observation of this induced VCD allows probing of the chiral binding site.
  • In situ ATR-IR study of the adsorption of cinchonidine on Pd/Al2O3. Differences and similarities with adsorption on Pt/Al2O3
    D. Ferri, T. BĂźrgi and A. Baiker
    Journal of Catalysis, 210 (1) , 2002, p160-170
    DOI:10.1006/jcat.2002.3660 | unige:14700 | Article HTML | Article PDF
Pd/Al2O3 model catalysts have been prepared by physical vapour deposition and characterised by means of XPS, STM, and in situ ATR–IR spectroscopy. Morphological changes in the Pd film induced by dissolved hydrogen leads to enhanced infrared absorption and could be followed with both STM measurements and IR spectroscopy. Adsorption of CO, pyridine, quinoline, 2-methylquinoline, and the chiral auxiliary cinchonidine has been studied in situ at 283 K in CH2Cl2 solvent. Two different species have been observed for cinchonidine on Pd. One is oriented with the quinoline moiety nearly parallel to the Pd surface, likely through the π-system, whereas in the second the σ-bonding through the N lone pair prevails and induces a tilting of the ring with respect to Pd. No indication of the presence of α-quinolyl species has been found, in contrast to adsorption on Pt/Al2O3 catalysts. Compared to adsorption on Pt, cinchonidine is more weakly bound on Pd under hydrogenation conditions. Also, the relative stability of the π- and N lone pair-bonded species is different for the two metals, with the π-bonded species being relatively more stable on Pt. Similarities and differences found in the adsorption of the chiral modifier on the two metals are discussed and traced mainly to the different d-orbital diffuseness of Pd and Pt.
  • On the role of oxygen in the liquid phase aerobic oxidation of alcohols on palladium
    Csilla Keresszegi, Thomas BĂźrgi, Tamas Mallat and Alfons Baiker
    Journal of Catalysis, 211 , 2002, p244
    DOI:10.1006/jcat.2002.3723 | unige:14661 | Article HTML | Article PDF
The mechanism of alcohol oxidation was investigated using the conversion of cinnamyl alcohol (1) over Pd-based catalysts as a sensitive test reaction. Studies in a slurry reactor revealed that dehydrogenation and oxidative dehydrogenation of 1 follow the same reaction pathways independent of the presence or absence of oxygen and reaction conditions. Hydrogenation and hydrogenolysis side reactions indicated the presence of hydrogen on the metal surface during reactions. Catalyst deactivation in Ar is attributed to decarbonylation reactions and site blocking by CO. Introduction of molecular oxygen induced a dramatic enhancement of alcohol conversion rate by a factor of up to 285 due to oxidative removal of CO. Strong adsorption of CO on Pd/Al2O3 and its rapid removal by oxygen were corroborated by in situ ATR-IR spectroscopy. All these observations conform to a model according to which oxidation of 1 follows the classical dehydrogenation mechanism, and the key role of oxygen is the continuous oxidative removal of CO and other degradation products from the active sites. This oxidative cleaning of the metal surface allows a high rate of alcohol dehydrogenation even when the oxidation of the co-product hydrogen is slow and incomplete. It is likely that the observed effects are not limited to the oxidation of 1 on Pd, and regeneration of the active sites by oxygen generally plays an important role during aerobic oxidation of alcohols on platinum metals.
  • In situ infrared spectroscopy of catalytic solid-liquid interfaces using phase-sensitive detection: Enantioselective hydrogenation of a pyrone over Pd/TiO2
    Thomas BĂźrgi and Alfons Baiker
    Journal of Physical Chemistry B, 106 (41) , 2002, p10649-10658
    DOI:10.1021/jp0255987 | unige:14640 | Abstract | Article HTML | Article PDF
The potential of modulation excitation spectroscopy and phase-sensitive detection in combination with attenuated total reflection (ATR) for in situ infrared spectroscopy of catalytic solid−liquid interfaces is demonstrated. The method is based on the periodic variation of an external parameter such as reactant concentration. The periodically varying signals are subsequently demodulated using a phase-sensitive detection scheme. In this way, the small periodically varying signals are separated from the large static ones, yielding high quality difference spectra. Species, which have different response to the excitation, i.e., species with different kinetics, can easily be separated in the spectra. The method is applied to the enantioselective hydrogenation of 4-methoxy-6-methyl-2-pyrone over a 5 wt % Pd/TiO2 powder catalyst modified by cinchonidine. Upon modulation of the reactant concentration, the ATR spectra exhibit varying signals from dissolved reactant, product as well as from adsorbed species. Part of the signals are associated with carboxylates adsorbed on the TiO2. The kinetics of these species are distinctly different from the one of the primary hydrogenation product. The carboxylates are formed from alcoholysis of the lactone, which is obtained by a second hydrogenation step. The enantiomeric excess was also measured phase sensitive. Its time dependence indicates a negative influence of the carboxylates on enantioselection.
The adsorption of carboxylic acids (formic, acetic, and pyruvic acid) from corresponding solutions in CH2Cl2 solvent on Al2O3 and TiO2 thin films has been studied by attenuated total reflection infrared spectroscopy. The metal-oxide films were vapor-deposited on a Ge internal reflection element, which was mounted into a specially designed flow cell. The system allowed in situ monitoring of the processes occurring at the solid-liquid interface. The metal-oxide films were characterized by X-ray photoelectron spectroscopy, ellipsometry, and atomic force microscopy. Formic acid and acetic acid adsorbed predominantly as bridging species on alumina surfaces. Adsorbed free acids were not observed under a flow of neat solvent. Based on the position of the νAS(COO) and of the keto-group stretching vibration of the pyruvate ion, pyruvic acid is proposed to coordinate to the Al2O3 surface in a monodentate fashion, whereas, on TiO2, a bidentate species is preferred. Comparison of the adsorption behavior on the vapor-deposited alumina film and on an ι-Al2O3 layer deposited from a water suspension of the corresponding metal-oxide powder indicated that pyruvic acid adsorbs in a similar mode, irrespective of the metal-oxide deposition technique.
The results of a study on the ground-state of monocarbonate, bicarbonate, and tricarbonate complexes of neptunyl using multiconfigurational second-order perturbation theory (CASSCF/CASPT2) are presented. The equilibrium geometries of the complexes corresponding to neptunium in the formal oxidation state (V) have been fully optimized at the CASPT2 level of theory in the presence of an aqueous environment modeled by a reaction field Hamiltonian with a spherical cavity. Some water molecules have been explicitly included in the calculation. This study is consistent with the hypothesis that the monocarbonate complex has a pentacoordinated structure with three water molecules in the first coordination shell and that the bicarbonate complex has a hexacoordinated structure, with two water molecules in the first coordination shell. The typical bond distances are in good agreement with experimental results. The tricarbonate complex was studied with explicit counterions, which resulted in somewhat longer Np−carbonate bond distances than experiment indicates.
  
Ab initio calculations at the B3LYP and MP2 levels suggest that a series of compounds with the general formula N5MN7 (M = Ti, Zr, Hf, Th) are locally stable. These compounds are thermodynamically at least as stable as the recently suggested ScN7 molecule. N5ThN7 seems the most stable of all. It lies 21.5 kcal/mol below a transition state, corresponding to the opening of one N−N bond in the N7 ring, and only 132 kcal/mol above Th + 6 N2, or 22 kcal/(mol N2).
  • A theoretical study of the 21Ag -> 11Ag two-photon transition and its vibronic band in trans-stilbene
    Jonna StĂĽlring, Laura Gagliardi, Per-Åke Malmqvist and Roland Lindh
    Molecular Physics, 100 (11) , 2002, p1791-1796
    DOI:10.1080/00268970110112327 | unige:3727 | Abstract | Article PDF
The two-photon spectrum of the 21Ag ← 11Ag transition in trans-stilbene has been calculated at the complete active space self-consistent field (CASSCF) level of theory. Energies were obtained at the complete active space second-order perturbation (CASPT2) level of theory, while the geometries of both the initial and final states were optimized at the CASSCF level. The energy and the geometry optimizations were performed using an active space of 14 electrons in 14 active π orbitals. The vibrational frequencies of both states and the two-photon transition (TPT) cross-section were calculated with a smaller active space where the two lowest π orbitals were kept inactive. A newly implemented algorithm, in the quantum chemical package Molcas was used to determine the two-photon transition intensity. This method requires only the linear response of the CASSCF wavefunction. Furthermore, the vibronic structure of this TPT was studied. The Franck-Condon factors were obtained by calculating the overlap between the vibrational states involved, which were determined from the force fields of both the initial and final states, at the CASSCF level of theory. The results are in agreement with experiment.
Quantum chemical calculations of vibrational frequencies of Stilbene were followed by the computation of the potential energy surface for the two rotors related to the single bonds. By the flexible model approach applied to the computed surface we have confirmed previous assignment of mode 37 and determined frequency of the elusive mode 48. The same analysis was performed not only for the ground, but also for the excited electronic state. The shape of the potential energy surface in S0 is in agreement with that of styrene and the barrier height obtained from the fitting in S1 is increased with respect to S0, as expected.
 
The existence of a series of triatomic molecules with the general formula MNM‘, where M is an alkaline metal (K, Rb, Cs), and M‘ is an alkaline earth metal (Ca, Sr, Ba), has been predicted by quantum chemical methods. Among these, the CsNBa molecule shows a feature not found before, the presence of a multiple bond between barium and nitrogen. As a consequence of this novel bonding situation, the molecule is linear. The same holds for all Ba triatomics, MNBa, independent of the nature of the alkali M atom, and for all Sr compounds, MNSr. The presence of a multiple bond makes CsNBa, and other related Ba and Sr molecules, particularly stable and appealing experimentally. The systems with the alkaline earth metal M‘ = Ca, on the other hand, turned out to be bent. Calculations have also been performed on the negative ions BaN- and CaN-, which form a well-defined entity in the MNM‘ systems (M‘ = Ba, Ca). The results show that the two ions have a different electronic structure in the ground state, which is one reason for the different properties of the MNM‘ systems and explains why the molecules containing the BaN- moiety are linear, while those containing CaN- are bent.
The results of a theoretical study of the ground state, 11Ag, and of the lowest 1Bu states oftrans-stilbene are presented. The vertical and adiabatic excitation energies of the lowest 1Bustates have been computed using multiconfigurational SCF theory, followed by second-order perturbation theory. It is shown that the two lowest excited states are separated by a small energy gap in the Franck−Condon region. They are the 11Bu, characterized by the HOMO→LUMO single excitation substantially localized on the ethylenic moiety, and the 21Bu, formed by a combination of one electron excitations localized mainly on the benzene rings. The most intense transition is found to be the lowest in energy when the interaction between different states is included at the level of second-order perturbation theory. The vibronic structure of emission and absorption spectra of the two lowest 1Bu states have been determined within the Franck−Condon approximation. The spectrum calculated for the 11Bustate agrees with the experimental spectrum, while the low intensity band computed for the 21Bu state has no experimental counterpart. It is concluded that this band is buried in the strong 11Bu absorption and therefore not observed.
  • Ruthenium(II) complex of bis(2,2'-bipyridine)(6,7-dicyano-dipyrido[3,2-a:2',3'-c]phenazine): synthesis, structure, electrochemical and luminescence studies
    Julia Rusanova, Silvio Decurtins, Eduard Rusanov, Helen Stoeckli-Evans, Sandra Delahaye and Andreas Hauser
    Dalton Transactions, (23) , 2002, p4318-4320
    DOI:10.1039/b210440g | unige:3700 | Abstract | Article HTML | Article PDF
A novel ruthenium complex with a 6,7-dicyanosubstituted dppz ligand has been synthesised: its crystal structure and physico-chemical studies are reported.
Quantum-chemical calculations, at the self-consistent-charge density-functional-based non-orthogonal tight binding (SCC-DFTB) level, are used to provide the input for unimolecular reaction rate theory calculations to predict the temperatures at which rapid, i.e., microsecond timescale, equilibration between mono-cyclic and bi-cyclic carbon clusters can occur. The computational results are discussed in the form of a set of trends for their variation with the size of the cluster, the length of the carbon–carbon bond broken or formed, the vibrational frequencies, the energy differences and the rate constants. The temperatures used experimentally to prepare fullerenes and nanotubes are compatible with the rapid equilibration of rings and bi-cyclic rings, a factor that explains the lack of defects in these higher forms of carbon clusters and the general trend towards the formation of the most stable fullerene for a given nuclearity.
  • First Principles Study of the Structure and Chemistry of Mg-Based Hydrotalcite-Like Anionic Clays
    Andrea Trave, Annabella Selloni, Annick Goursot, Didier Tichit and Jacques Weber
    Journal of Physical Chemistry B, 106 (47) , 2002, p12291-12296
    DOI:10.1021/jp026339k | unige:3696 | Abstract | Article HTML | Article PDF
We use variable-cell first principles molecular dynamics as an optimization tool to investigate the structural and electronic properties of Mg-based anhydrous hydrotalcite-like compounds. The formation energy as a function of the ratio R between di- and trivalent cations shows a minimum at R ~ 3, in good agreement with experimental stability ranges for these materials. At the same value R ~ 3, a maximum is found in the calculated interlayer distance, suggesting a correlation between energetic stability and structure. The energies and character of the electronic states of hydrotalcites containing different interlayer anions and trivalent cations have been compared. The nature of the anions is found to have a major influence on the electronic properties. In particular, OH- anions, rather than, e.g., Cl-, lead to a significantly smaller HOMO−LUMO gap, with a LUMO spatially more localized in the interlayer region. These features are related to the observed differences in the catalytic properties of hydrotalcites containing OH- vs Cl- anions.
  • Non-equilibrium charge recombination dynamics of excited donor-acceptor complexes
    Olivier Nicolet and Eric Vauthey
    Trends in Optics and Photonics, 72 , 2002, p78-79
    unige:3695
The absorption and photostimulated spectra of single crystals of the new substance Ba12F19Cl5 doped with Europium ions were studied. Creation of color-center-type absorption bands was observed under C band UV irradiation of the doped crystals. These samples show photostimulated luminescence when subsequently excited with the 20,492 cm−1 line of an Ar ion laser. Our experiments support the assignment that the PSL signal is from the Eu2+ ions. This system may be of interest as an UV storage phosphor.
  • Chemical pressure
    Andreas Hauser, Nahid Amstutz, Sandra Delahaye, Asmaâ Sadki, Sabine Schenker, Regula Sieber and Mohamed Zerara
    Chimia, 56 (12) , 2002, p685-689
    Keywords: chemical pressure; [Co(bpy)3]2+; [Fe(bpy)3]2+; guest host interaction; intersystem crossing; luminescence; radiationless deactivation; [Ru(bpy)3]2+; spin-crossover
    DOI:10.2533/000942902777679858 | unige:3691 | Abstract | Article PDF
The physical and photophysical properties of three classic transition metal complexes, namely [Fe(bpy)3]2+, [Ru(bpy)3]2+, and [Co(bpy)3]2+, can be tuned by doping them into a variety of inert crystalline host lattices. The underlying guest-host interactions are discussed in terms of a chemical pressure.
  • Rational determination of transfer free energies of small drugs across the water-oil interface
    Delphine Bas, Delphine Dorizon-Duval, StĂŠphanie Moreau, Pierre Bruneau and Christophe Chipot
    Journal of Medicinal Chemistry, 45 (1) , 2002, p151-159
    DOI:10.1021/jm010289a | unige:3688 | Abstract | Article HTML | Article PDF
The application of statistical simulations to the estimation of transfer free energies of pharmacologically relevant organic molecules is reported. Large-scale molecular dynamics simulations have been carried out on a series of four solutes, viz. antipyrine, caffeine, ganciclovir, and α-d-glucose, at the water−dodecane interface as a model of a biological water−membrane interfacial system. Agreement with experimentally determined partition coefficients is remarkable, demonstrating that free energy calculations, when executed with appropriate protocols, have reached the maturity to predict thermodynamic quantities of interest to the pharmaceutical world. The computational effort that warrants accurate, converged free energies remains, however, in large measure, incompatible with the high-throughput exploration of large sets of pharmacologically active drugs sought by industrial settings. Compared to the cost-effective, fast estimation of simple partition coefficients, the present free energy calculations, nevertheless, offer a far more detailed information about the underlying energetics of the system when the solute is translocated across the water−dodecane interface, which can be valuable in the context of de novo drug design.
  
The adsorption of methanol on V2O5 and its mild oxidation to formaldehyde has been studied applying density functional theory. The model used throughout is a cluster model saturated by hydrogen atoms. It is shown that the adsorption of methanol is energetically favored if the cluster is partially reduced (i.e., protonated because of the dissociative adsorption of water). Methanol behaves as a soft base and adsorbs as a methoxonium cation. The proposed mechanism is based on two steps, the first being the dissociation of methanol to form a methoxy group on the surface. This dissociation occurs between the oxygen and the carbon atoms of methanol. Finally, for the second step, which corresponds to the desorption of formaldehyde, the calculations show that filling of the vanadyl oxygen vacancy created by formaldehyde desorption is crucial to cope with an energetically feasible reaction pathway.
  • Resonant and Phonon-Assisted Excitation Energy Transfer within the R1 Lines of [Cr(Ox)3]3- in a System with Two Crystallographically Non-equivalent Lattice Sites
    Marianne E. Von Arx and Andreas Hauser
    Journal of Physical Chemistry A, 106 (31) , 2002, p7106-7112
    DOI:10.1021/jp020174y | unige:3683 | Abstract | Article HTML | Article PDF
The [Ru(bpy)3][LiCr(ox)3] system (bpy = 2,2‘-bipyridine, ox = oxalate) has two crystallographically non-equivalent [Cr(ox)3]3- sites. In steady-state resonant and nonresonant fluorescence line narrowing (FLN) experiments on the R1 lines of the two non-equivalent [Cr(ox)3]3- chromophores, multiline spectra are observed at 1.6 K. Such multiline spectra are clear evidence for resonant energy transfer processes within the inhomogeneously broadened R1 lines. In addition, time-resolved experiments show that also site-to-site energy transfer occurs, which turns out to be resonant, too, however with a non-negligible phonon-assisted contribution even at 1.5 K.
  • Resonant Energy Transfer in the Mixed Crystal Series [Rh(bpy)3][NaAlxCr1-x(ox)3]ClO4 (bpy = 2,2'-bipyridine, ox = Oxalate, x = 0.05-1)
    Marianne E. Von Arx, Vaughan S. Langford, Ueli Oetliker and Andreas Hauser
    Journal of Physical Chemistry A, 106 (31) , 2002, p7099-7105
    DOI:10.1021/jp0201736 | unige:3682 | Abstract | Article HTML | Article PDF
Efficient resonant energy transfer occurs within the R1 line of the 4A2 → 2E transition of the [Cr(ox)3]3- chromophore in mixed crystal [Rh(bpy)3][NaAl1-xCrx(ox)3]ClO4 (x = 0.05−0.9, ox = oxalate, bpy = 2,2‘-bipyridine). This manifests itself in the form of multiline patterns in resonant fluorescence line narrowing (FLN) experiments at 1.5 K. The conditions for such a resonant process to occur are that the inhomogeneous line width of the R1 line is larger than the zero-field splitting of the ground state, which, in turn, is larger than the homogeneous line width of the transition. The number of lines and their relative intensities depend critically upon the [Cr(ox)3]3- concentration and the excitation wavelength within the inhomogeneous distribution. The basic model for resonant energy transfer as presented by von Arx et al. (Phys. Rev B 1996, 54, 15800) is extended to include the effects of diluting the chromophores in an inert host lattice and of nonresonant R2 excitation. In addition, Monte Carlo simulations serve to explain the temporal evolution of the multiline pattern following pulsed excitation.
 
[Fe(pic)3]Cl2¡EtOH (pic = 2-picolylamine) is a spin-crossover compound that can be converted from the low-spin state to the high-spin state at temperatures below the thermal transition temperature by way of light irradiation in the visible part of the electromagnetic spectrum. For this compound, the question regarding the quantum efficiency of this photoconversion process and its possible dependence on irradiation intensity gave rise to some controversy. The experimental results presented in this paper demonstrate that the quantum efficiency of the photoconversion at 11 K is on the order of unity, with no noticeable dependence on irradiation intensity. It does, however, depend to some extent on the fraction of complexes already converted to the high-spin state.
  • Analysis of paramagnetic NMR spectra of triple-helical lanthanide complexes with 2,6-dipicolinic acid revisited : a new assignment of structural changes and crystal-field effects 25 years later
    Nadjet Ouali, Bernard Bocquet, StĂŠphane Rigault, , Jacques Weber and Claude Piguet
    Inorganic Chemistry, 41 (6) , 2002, p1436-1445
    DOI:10.1021/ic010801i | unige:3223 | Abstract | Article HTML | Article PDF
Variable-temperature 1H and 13C NMR measurements of the D3-symmetrical triple-helical complexes [Ln(L1-2H)3]3- (L1 = pyridine-2,6-dicarboxylic acid; Ln = La−Lu) show evidence of dynamic intermolecular ligand-exchange processes whose activation energies depend on the size of the metal ion. At 298 K, the use of diastereotopic probes in [Ln(L3-2H)3]3- (L3 = 4-ethyl-pyridine-2,6-dicarboxylic acid) shows that fast intramolecular P ↔ M interconversion between the helical enantiomers occurs on the NMR time scale. Detailed analyses of the paramagnetic NMR hyperfine shifts according to crystal-field independent techniques demonstrate the existence of two different helical structures, one for large lanthanides (Ln = La−Eu) and one for small lanthanides (Ln = Tb−Lu), in complete contrast with the isostructurality proposed 25 years ago. A careful reconsideration of the original crystal-field-dependent analysis shows that an abrupt variation of the axial crystal-field parameter A202> parallels the structural change leading to some accidental compensation effects that prevent the detection of structural variations according to the classical one-nucleus method. Crystal structures in the solid state and density functional theory calculations in the gas phase provide structural models that rationalize the paramagnetic NMR data. A regular triple-helical structure is found for small lanthanides (Ln = Tb−Lu) in which the terdentate chelating ligands are rigidly tricoordinated to the metals. A flexible and distorted structure is evidenced for Ln = La−Eu in which the central pyridine rings interact poorly with the metal ion. The origin of the simultaneous variation of structural parameters and crystal-field and hyperfine constants near the middle of the lanthanide series is discussed together with the use of crystal-field-independent techniques for the interpretation of paramagnetic NMR spectra in axial lanthanide complexes.
Geometries and 29Si NMR chemical shifts are calculated for silanes SinH2n+2, n=1,…,5, methylsilanes SiHnMe4−n, methoxysilanes SiHn(OMe)4−n, and methylmethoxysilanes SiMen(OMe)4−n, n=0,…,4. Geometries and 29Si NMR chemical shifts are in satisfying agreement with experiment within LCGTO-DFT at the DZVP/LDA level for geometries and IGLO-III/GGA (GGA=PW91,PBE) level for shielding constants, which is an improvement to B88PW86, P86PW86 and B3LYP results. If an auxiliary basis is applied to express the Coulomb potential, g-functions have to be included to reproduce SiOSi angles and 29Si NMR chemical shifts correctly.
Calculations of 13C nuclear shieldings for low-energy isomers of C36H2x (x=2,3) suggest that it should be possible to use experimental13C shifts, when these become available, to distinguish the isomeric form of the underlying fullerene cage and, in the case of isomers based on the six-fold symmetrical cylindrical fullerene cage 36:15, the degree of polar hydrogenation.
  • Ultrafast Photochemistry
    A. Morandeira, A. FĂźrstenberg, O. Nicolet, S. Pagès, B. Lang and E. Vauthey
    Chimia, 56 (12) , 2002, p690-694
    Keywords: photoinduced electron transfer; solvation dynamics; ultrafast processes; upper excited states; vibrational relaxation
    DOI:10.2533/000942902777679849 | unige:3225 | Abstract | Article PDF
Several aspects of ultrafast photochemistry in the condensed phase are discussed and illustrated by three examples from our laboratory.
  • Development of Novel Computational Strategies to Match the Challenges of Supramolecular Chemistry, Biochemistry, and Materials Science
    T.A. Wesolowski
    Chimia, 56 (12) , 2002, p707-711
    Keywords: density functional theory, embedding, non-covalent interactions, physisorption, solvatation
    DOI:10.2533/000942902777679876 | unige:3226 | Abstract
Recent formal developments and applications of the 'freeze-and-conquer' strategy proposed by Wesolowski and Warshel in 1993 to study large systems at quantum mechanical level are reviewed. This universal approach based on density functional theory allows one to link, via the orbital-free embedding potential, two parts of a larger system described at different levels of accuracy leading thus to significant savings in computational costs. As a result, applicability of conventional methods of quantum chemistry can be extended to even larger systems. It is shown that the 'freeze-and-thaw' approach applying the first-principles based approximation to the orbital-free embedding potential recently developed in our group provides a powerful and universal technique to study such embedded molecules (or molecular complexes), which are not linked with their microscopic environment by covalent bonds.
  • Formation and structure of Rh(0) complexes of phosphinine-containing macrocycles: EPR and DFT investigations
    Laurent Cataldo, Sylvie Choua, ThĂŠo Berclaz, Michel Geoffroy, Nicolas MĂŠzailles, Narcis Avarvari, François Mathey and Pascal Le Floch
    Journal of Physical Chemistry A, 106 (12) , 2002, p3017-3022
    DOI:10.1021/jp014339z | unige:3227 | Abstract | Article HTML | Article PDF
Electrochemical and chemical reductions of Rh(I) complexes of LP4 (a macrocycle containing four phosphinine rings) and of LP2S2 (a macrocycle containing two phosphinine rings and two thiophene rings) lead, in liquid solution, to EPR spectra exhibiting large hyperfine couplings with 31P nuclei. An additional coupling (27 MHz) with 103Rh is detected, in the liquid state, for the spectrum obtained with [LP2S2Rh(0)]; moreover, resolved 31P hyperfine structure is observed in the frozen solution spectrum of this latter complex. DFT calculations performed on Rh(I) complexes of model macrocycles L‘P4 and L‘P2S2 indicate that, in these systems, the metal coordination is planar and that one-electron reduction induces a small tetrahedral distortion. The calculated couplings, especially the dipolar tensors predicted for [L‘P2S2Rh(0)], are consistent with the experimental results. Although the unpaired electron is mostly delocalized on the ligands, the replacement of two phosphinines by two thiophenes tends to increase the rhodium spin density (ρRh =0.35 for [L‘P2S2Rh(0)]). It is shown that coordination to Rh as well as one-electron reduction of the resulting complex provoke appreciable changes in the geometry of the macrocycle.
  • Electron Paramagnetic Resonance, Optimization and Automatic Differentiation
    Edgar J. SouliĂŠ, Christèle Faure, ThĂŠo Berclaz and Michel Geoffroy
    in "Automatic Differentiation of Algorithms" Springer, New-York, 2002, p99-106
    unige:3401
  • Aromatic bent-core liquid crystals : an opportunity for introducing terdentate binding units into mesophases
    H. Nozary, J.-P. Rivera, P. Tissot, C. Piguet, , J. Weber, G. Bernardinelli, J.-C.G. BĂźnzli, R. Deschenaux, B. Donnio and D. Guillon
    Chemistry of Materials, 14 (3) , 2002, p1075-1090
    DOI:10.1021/cm011162c | unige:3222 | Abstract | Article HTML | Article PDF
Lipophilic linear semirigid side arms containing two or three successive phenyl rings separated by carboxylate spacers have been connected to the 5 or 6 positions of bent aromatic terdentate 2,6-bis(benzimidazol-2-yl)pyridine binding units to give extended V-shaped (L11) and I-shaped receptors (L12, L12b, and L13). The carboxylate spacers limit the flexibility of the side arms and provide crossed arrangements of the successive aromatic rings in the crystal structure of L12b (C63H61N5O10; triclinic, P↑, Z = 2) in agreement with semiempirical calculations performed on optimized gas-phase geometries. Moreover, the carboxylate spacers in L11−L13 prevent efficient electronic delocalization between the connected aromatic rings and act as weak π acceptors producing a slight increase of the energy of the 1ππ* and3ππ* levels centered on the terdentate binding unit. Intermolecular π-stacking interactions observed in the crystal of L12b are invoked to rationalize (i) the peculiar excimer emission ofL11 in the solid state and (ii) the rich and varied calamitic (I-shaped L12, L12b, and L13) and columnar (V-shaped L11) mesomorphism observed at high temperature. The ColR mesophase detected for L11 demonstrates that V-shaped bent terdentate binding units are compatible with liquid-crystalline behavior. Complexation of L11 with lanthanide(III) produces I-shaped complexes [Ln(L11)(NO3)3] (Ln = La, Eu, Gd, Tb, and Lu) possessing a large axial anisometry as found in the crystal structure of [Lu(L11)(CF3CO2)3(H2O)] (LuC81H87N5O17F9; triclinic, P↑,Z = 2), which exists in the solid state as H-bonded dimers. No mesomorphism is detected for the complexes as a result of the large perpendicular expansion brought by the metallic coordination site, but the high energy of the ligand-centered 3ππ* prevents Eu(5D0) → L11back transfer in the Eu(III) complex, which thus exhibits sizable red luminescence at room temperature, a crucial point for the design of luminescent materials.
  
The dynamics of charge recombination (CR) of ion pairs formed upon electron-transfer quenching of Zn tetraphenylporphine (ZnTPP) in the S2 state has been investigated by fluorescence upconversion. These ion pairs have two possible CR pathways:  (A) a highly exergonic CR to the neutral ground state and (B) a moderately exergonic CR leading to the formation of ZnTPP in the S1 state. Upon addition of quencher, the S2 fluorescence decreases considerably, while the S1 fluorescence is unaffected, indicating unambiguously that CR occurs via path B. A large fraction of the S2 fluorescence quenching occurs in less than 100 fs. CR to the S1 state of ZnTPP takes place with time constants around 400 fs.
The dynamics of charge recombination (CR) of excited donor−acceptor complexes composed of methoxy-substituted benzenes and pyromellitic dianhydride were investigated in four different solvents using both the multiplex transient grating and the transient absorption techniques. At constant driving force, the CR dynamics are substantially faster than those with methyl-substituted benzenes as donors. In acetonitrile (ACN), the CR time constant decreases from 3.5 ps with anisole down to 240 fs with tetramethoxybenzene. In valeronitrile, the CR is always slower than in ACN but is, in most cases, faster than diffusional solvation. The free energy, the solvent, and the temperature dependence of the CR dynamics can be qualitatively well reproduced using the hybrid model of Barbara and co-workers after incorporation of the contribution of inertial motion to solvation. The ability of this model to account for the absence of normal region at small driving force is also examined.
  • Dynamics of S2 Fluorescence Quenching and of Charge Recombination in the Resulting Ion Pair
    L. Engeli, A. Morandeira, P.-A. Muller and E. Vauthey
    in "Femtochemistry and Femtobiology" A. Douhal and J. Santamaria Eds., World Scientific Publishing: Singapore, 2002, p314
    unige:3779
  • Ultrafast Excited State Dynamics in Multiporphyrin Arrays
    A. Morandeira, E. Vauthey, A. Mongin and A. Gossauer
    in "Femtochemistry and Femtobiology" A. Douhal and J. Santamaria Eds., World Scientific Publishing: Singapore, 2002, p789
    unige:3780
Polycrystalline LiBH4 has been studied by Raman spectroscopy in the temperature interval 295–412 K and the frequency range 2700–130 cm−1. The Raman active modes are consistent with the presence of a (BH4)− ion having a distorted tetrahedral configuration. As the temperature is increased the sudden disappearance of mode splitting points to the onset of a structural phase transition that leads to a higher local symmetry of the (BH4)− tetrahedron. The transition occurs at ~384 K, is of first-order and has a hysteresis of about 8 K. A strong and discontinuous broadening of bands remaining after the transition suggests the onset of large vibrational amplitudes of the (BH4)− tetrahedra about their trigonal axis.
 
The EPR spectrum obtained at room temperature after electrochemical or chemical reduction of a solution of Ar–P=C=C=P–Ar in THF exhibits hyperfine interaction (165 MHz) with two equivalent 31P nuclei. Additional couplings with two equivalent 13C are observed with Ar–P=13C=13C=P–Ar. The 31P anisotropic coupling constants could be obtained from spectra recorded at low temperature. They indicate that the unpaired electron is mainly localized (78%) on the two phosphorus atoms. Quantum chemical calculations (DFT and ab initioSCI) were performed on the various isomers of the two radical anions: [H–P=C=C=P–H]•– and [H–P=CH–CH=P–H]•–. Although the optimized geometries of these two species are clearly different, neither of them leads to13C/31P hyperfine tensors in conflict with the experimental results. The absence of any 1H splitting on the EPR spectrum together with the quasi-reversibility of the reduction wave make the identification of [Ar–P=C=C=P–Ar]•– more probable. 
  • Dynamic phenomena in barrelenephosphinyl radicals: a complementary approach by density matrix analysis of EPR spectra and DFT calculations
    M. Brynda, C. Dutan, T. Berclaz and M. Geoffroy
    Current Topics in Biophysics, 26 (1) , 2002, p35-42
    unige:3230
The paper shows the possibilities of the complementary use of the density matrix formalism for the simulation of the anisotropic EPR spectra and the DFT potential energy surface calculations to obtain a detailed picture of the motions of radical molecules. The combined approach is illustrated by a comparative EPR study of three phosphorus derivatives of barrelene. Three compounds were chosen as the model molecules for the observation of different temperature dependent dynamics of radical fragment. Each molecule based on the same barrelene skeleton has a different set of substituents which by influencing the local chemical environment are likely to modify the internal dynamics. The temperature dependent EPR spectra are simulated by means of the density matrix formalism and the geometry of radicals are calculated with DFT. The motion is described in terms of rotational barriers, DFT calculated energy profiles and hypothetical intramolecular distortions. These two approaches lead to a similar microscopic picture of the intramolecular radical motion.
The repartition of molecular hydrogen in space, and its depletion on solid particles in particular, is an important question of modern astrophysics. In this paper, we report a theoretical study of the physisorption of molecular hydrogen, H2, on a major component of the interstellar dust known as polycyclic aromatic hydrocarbons (PAHs). Two different density functional theory approaches were used:  (i) the conventional Kohn−Sham theory and (ii) the subsystem-based approach (Kohn−Sham equations with constrained electron density, KSCED) developed in our group. The approximate exchange-correlation energy functional applied in all calculations and the nonadditive kinetic-energy functional needed in KSCED have a generalized gradient approximation form and were chosen on the basis of our previous studies. The results of both approaches show similar trends:  weak dependence of the calculated interaction energies on the size of the PAH and negligible effect of the complexation of two PAH molecules on the adsorption of molecular hydrogen. The KSCED interaction energy calculated for the largest considered PAH (ovalene), amounting to 1.27 kcal/mol, is in excellent agreement with experimental estimates ranging from 1.1 to 1.2 kcal/mol, whereas the one derived from supermolecular Kohn−Sham calculations is underestimated by more than 50%. This result is in line with our previous studies, which showed that the generalized gradient approximation applied within the KSCED framework leads to interaction energies of weakly bound complexes that are superior to the corresponding results of supermolecular Kohn−Sham calculations.
  • Link between the Kinetic- and Exchange-Energy Functionals in the Generalized Gradient Approximation
    F. Tran and T.A. Wesolowski
    International Journal of Quantum Chemistry, 89 (5) , 2002, p441-446
    Keywords: density functional theory; exchange-energy functional; kinetic-energy functional; orbital-free theories
    DOI:10.1002/qua.10306 | unige:3232 | Abstract | Article HTML | Article PDF
An approximate kinetic-energy functional of the generalized gradient approximation form was derived following the "conjointness conjecture" of Lee, Lee, and Parr. The functional shares the analytical form of its gradient dependency with the exchange-energy functionals of Becke and Perdew, Burke, and Ernzerhof. The two free parameters of this functional were determined using the exact values of the kinetic energy of He and Xe atoms. A set of 12 closed-shell atoms was used to test the accuracy of the proposed functional and more than 30 others taken from the literature. It is shown that the conjointness conjecture leads to a very good class of kinetic-energy functionals. Moreover, the functional developed in this work is shown to be one of the most accurate despite its simple analytical form.
  • Spin-densities in charge-transfer complexes derived from DFT calculations using an orbital-free embedding scheme for interacting subsystems.
    T.A. Wesolowski and J. Weber
    in "Recent Advances in Density Functional Methods, Vol. III, Proceedings of the DFT99 Conference, Rome, Italy, September 6-10th, 1999" V. Barone, A. Bencini, and P.Fantucci, Eds., World Scientific, III , 2002, p371-386
    unige:4042
The approximate nonempirical kinetic-energy functional proposed by Tal and Bader is analyzed for polyatomic systems. The performance of this functional and the functionals derived from the gradient expansion approximation truncated to zeroth, second, and fourth order is investigated for a testing set of 68 neutral and charged molecules. It is shown that the Tal–Bader functional, despite the simplicity of the idea behind its construction, leads to significantly better total kinetic energies than the gradient expansion approximation functionals. The local behavior of the kinetic-energy density derived from the Tal–Bader functional is also discussed.
A Comment on the Letter by Thorsten Klßner et al., Phys. Rev. Lett. 86, 5954 (2001). The authors of the Letter offer a Reply.
  • Intermolecular interaction energies from the total energy bi-functional. A case study of carbazole complexes.
    T.A. Wesolowski, and J. Weber
    Journal of Chemical Physics, 116 (15) , 2002, p6411-6421
    DOI:10.1063/1.1462613 | unige:3235 | Abstract | Article HTML | Article PDF
An approach in which the total energy of interacting subsystems is expressed as a bifunctional depending explicitly on two functions: electron densities of the two molecules forming a complex (ρ1 and ρ2) was used to determine the equilibrium geometry and the binding energy of several weak intermolecular complexes involving carbazole and such atoms or molecules as Ne, Ar, CH4, CO, and N2. For these complexes, the experimental dissociation energies fall within the range from 0.48 to 2.06 kcal/mol. Since the effect of the intermolecular vibrations on the dissociation energy is rather small, the experimental measurements provide an excellent reference set. The obtained interaction energies are in a good agreement with experiment and are superior to the ones derived from conventional Kohn–Sham calculations. A detailed analysis of relative contribution of the terms which are expressed using approximate functionals (i.e., exchange-correlationExc[ρ1+ρ2] and nonadditive kinetic energy Tsnad[ρ1,ρ2] = Ts[ρ1+ρ2]−Ts[ρ1]−Ts[ρ2]) is made. The nonvariational version of the applied formalism is also discussed.
  
Characterization of ternary and quaternary metal hydrides by Raman spectroscopy appears to be rather scarce due primarily to the decomposition of the metal hydrides by the energy of the laser excitation source. We report the results of some recent room temperature Raman measurements collected with a 2–10 mW 488 nm laser source for M2RuH6 where M=Ca, Sr and Eu. The assignments from this study are combined with existing vibrational data for other metal hydrides.
  • ATR spectroscopy of thin films
    U.P. Fringeli, D. Baurecht, M. Siam, G. Reiter, M. Schwarzott, T. BĂźrgi and P. BrĂźesch
    in "Handbook of thin film materials, Volume 2: Characterization and spectroscopy of thin films" H. S. Nalwa, Academic Press, London, 2001
    Article HTML
  • Model of reactant-modifier interaction in enantioselective hydrogenation of ethyl pyruvate on platinum-cinchona catalyst: Extension to synthetic chiral modifiers
    Angelo Vargas, Thomas BĂźrgi and Alfons Baiker
    Journal of Catalysis, 197 (2) , 2001, p378-384
    DOI:10.1006/jcat.2000.3109 | unige:14733 | Abstract | Article PDF
The previously proposed model for reactant–modifier interaction in the enantioselective hydrogenation of activated carbonyl compounds over platinum chirally modified by cinchona alkaloids has been extended to platinum modified by synthetic pyrrolidinyl–naphthyl–ethanol modifiers. As in the case of cinchonidine, the most used modifier, the model predicts enantiomeric excess in nearly quantitative agreement with experiment. Excellent agreement is achieved despite the fact that structural assumptions had to be made and the platinum surface was not explicitly taken into account. The one-to-one interaction between modifier and reactant was calculated at the ab initio level. A comparison of the results for different modifiers leads to the conclusion that steric repulsion caused by the anchoring group plays an important role in the enantiodifferentiating interaction. The favoured formation of the (R)-product is traced to the fact that the pro-(S) complex leading to the (S)-product upon hydrogenation is more destabilised due to repulsive interaction than the pro-(R) complex. The model calculations are a useful tool for designing effective modifiers and for gaining insight into the mechanism of enantiodifferentiation.
  • TPS, XPS and QEXAFS investigation of the sulfidation behavior of tungsten on fluorine-promoted alumina
    M. Sun, T. BĂźrgi, R. Cattaneo and R. Prins
    Journal of Catalysis, 197 (1) , 2001, p172-181
    DOI:10.1006/jcat.2000.3079 | unige:14727 | Abstract | Article PDF
The sulfidation behavior of alumina-supported W catalysts was investigated by means of temperature-programmed sulfidation, quick extended X-ray adsorption fine structure measurements, and X-ray photoelectron spectroscopy of two series of tungsten catalysts, one made from ammonium metatungstate and the other from ammonium tetrathiotungstate. The effect of the fluorination of the alumina support on the sulfidation behavior of W on these two series of catalysts was also studied. The sulfidation of catalysts prepared with ammonium metatungstate passes through intermediates of W oxysulfides; the sulfided catalysts are mixtures of W oxysulfides and WS2. After sulfidation at 400°C and atmospheric pressure for 4 h, the degree of sulfidation is only about 50%. Fluorination slightly increases the degree of sulfidation. When ammonium tetrathiotungstate is used as the precursor, fully sulfided catalysts can be obtained. Fluorination accelerates the transformation of WS3 sulfide to WS2.
 
Platinum particles supported on graphite have been investigated by scanning tunneling microscopy (STM). For one monolayer thick Pt particles the individual Pt atoms form a characteristic intensity pattern due to a mismatch between the Pt and graphite lattice. Based on density functional theory calculations and model structures of Pt on graphite it is argued that the observed STM imaging contrast has its origin in the tip induced elastic deformation of the graphite underneath the Pt particle. The Pt–graphite potential is much stiffer than the graphite–graphite potential. The calculations furthermore indicate that Pt adsorption is favored over top rather than hole sites and that the barrier for diffusion is very low.
  • Non-destructive sol-gel immobilization of metal(salen) catalysts in silica aerogels and xerogels
    Eamonn F. Murphy, Leo Schmid, Thomas BĂźrgi, Marek Maciejewski, Alfons Baiker, Detlef GĂźnther and Michael Schneider
    Chemistry of Materials, 13 (4) , 2001, p1296-1304
    DOI:10.1021/cm001187w | unige:14716 | Abstract | Article HTML | Article PDF
Nondestructive immobilization of cobalt and copper Schiff base complexes in silica aero- and xerogels was achieved via the sol−gel method using a precursor N,N‘-ethylenebis(salicylidenaminato) (salen) ligand modified with pendant silyl ethoxy groups. Aerogels were obtained by semicontinuous extraction of the wet gels with supercritical CO2 and xerogels by conventional drying. Cobalt and copper(salen) containing silica gels were characterized by FTIR, UV−vis, and XPS spectroscopy, laser ablation-ICP-MS, and EPR studies. Aero- and xerogel incorporated salen compounds exhibited similar spectroscopic properties to cobalt/copper(salen) precursors and known metal(salen) compounds. BET measurements confirmed the importance of supercritical CO2 drying in maintaining the mesoporous structure of the aerogel. Laser ablation-ICP-MS and EPR studies of the aerogels showed that a uniform distribution of the isolated metal(salen) complex was achieved via molecular mixing using the sol−gel method. Stability of these materials was demonstrated by thermogravimetric analyses in air and leaching studies conducted under typical liquid-phase oxidation conditions. XPS analyses showed surface relative atomic concentrations in the modified gels to be similar before and following leaching studies.
  • Asymmetric hydrogenation of 4-hydroxy-6-methyl-2-pyrone: Role of acid-base interactions in the mechanism of enantiodifferentiation
    W.-R. Huck, T. BĂźrgi, T. Mallat and A. Baiker
    Journal of Catalysis, 200 (1) , 2001, p171-180
    DOI:10.1006/jcat.2001.3205 | unige:14709 | Abstract | Article PDF
Enantioselective hydrogenation of the pseudo-aromatic 4-hydroxy-6-methyl-2-pyrone to the corresponding 5,6-dihydropyrone has been studied over cinchonidine-modified Pd/Al2O3 and Pd/TiO2 catalysts. A mechanistic model for enantiodifferentiation is proposed, involving two H-bond interactions (N–H···O and O–H···O) between the deprotonated reactant and the protonated chiral modifier. The model can rationalize (i) the sense of enantiodifferentiation, i.e., the formation of (S)-product in the presence of cinchonidine as modifier; (ii) the complete loss of enantioselectivity when the acidic OH group of the reactant is deprotonated by a base stronger than the quinuclidine N of the alkaloid; and (iii) the poor enantiomeric excesses obtained in good H-bond donor or acceptor solvents. NMR and FTIR investigations, and ab initio calculations, of reactant–modifier interactions support the suggested model. Several factors, such as catalyst prereduction conditions, trace amounts of water, presence of strong bases and acids, and competing hydrogenation of acetonitrile to ethylamines, were found to affect the efficiency of this catalytic system.
  • Pt and Pt/Al2O3 thin films for investigation of catalytic solid-liquid interfaces by ATR-IR spectroscopy: CO adsorption, H2-induced reconstruction and surface enhanced absorption
    D. Ferri, T. BĂźrgi and A. Baiker
    Journal of Physical Chemistry B, 105 (16) , 2001, p3187-3195
    DOI:10.1021/jp002268i | unige:14654 | Abstract | Article HTML | Article PDF
Model platinum catalysts have been designed to study the platinum−solvent interface in situ using attenuated total reflection (ATR) infrared spectroscopy. Pt and Pt/Al2O3 thin films were evaporated on a Ge internal reflection element (IRE) and characterized by XRD, XPS, AFM, STM, and IR spectroscopy. Changes within the adsorbate layer of the Pt catalyst during cleaning with O2 and H2 were followed. After cleaning, the catalyst surface was probed by CO adsorption from CH2Cl2. For the Pt/Al2O3 film the spectrum of adsorbed CO showed a band at 2000 cm-1, which is typical for Pt/Al2O3 catalysts. The stretching vibration of linearly bonded CO exhibited a coverage-dependent frequency shift due to vibrational coupling, thus showing the existence of large clean domains on the reactive catalyst surface even in the presence of an organic solvent. CO adsorption from CH2Cl2 was slow before the cleaning process. However, subsequent admission of H2 resulted in an instantaneous and drastic increase of the CO absorption signal. The origin of this effect is a structural change of the Pt particles induced by dissolved hydrogen, which was directly monitored by ATR spectroscopy using CO as probe molecule. STM investigations showed sintering of the Pt particles upon hydrogen treatment in CH2Cl2 at room temperature, which leads to a surface-enhanced infrared absorption (SEIRA).
  • TPS, XPS, QEXAFS, and XANES investigation of the sulfidation NiW/Al2O3-F catalysts
    Mingyong Sun, Thomas BĂźrgi, Riccardo Cattaneo, Dick Van Langeveld and Roel Prins
    Journal of Catalysis, 201 (2) , 2001, p258-269
    DOI:10.1006/jcat.2001.3257 | unige:14728 | Abstract | Article PDF
The sulfidation behavior of alumina-supported Ni–W catalysts was investigated by means of temperature-programmed sulfidation (TPS), X-ray photoelectron spectroscopy (XPS), quick extended X-ray absorption fine structure (QEXAFS), and X-ray absorption near-edge structure spectroscopy (XANES). Either ammonium tetrathiotungstate or ammonium metatungstate was used as the precursor of tungsten, and nickel nitrate was the source of nickel. The effect of fluorination of the alumina support on the sulfidation behavior of tungsten and nickel on these two series of catalysts was studied as well. The sulfidation of the catalysts prepared from ammonium metatungstate passes through W(VI) oxysulfide intermediates. Fluorination of the alumina support aids the sulfidation of tungsten and nickel at low temperature and promotes the transformation of the W(VI) oxysulfide intermediates to WS2. After sulfidation at 400°C and atmospheric pressure for 4 h, about 50% of tungsten and 60% of nickel in the catalysts prepared from ammonium metatungstate were sulfided. EXAFS showed that ammonium tetrathiotungstate supported on alumina decomposes to oxidic tungsten during the second impregnation with nickel nitrate. Nevertherless, sulfidation of the catalysts prepared from ammonium tetrathiotungstate is much easier. It also passes through W(VI) oxysulfide intermediates, and fluorination aids the formation WS2. In the sulfided catalysts prepared from ammonium tetrathiotungstate and nickel nitrate, 100% of tungsten and nickel is in the sulfided state, but a small amount of tungsten is in a {WS3} state, with fully sulfided W(VI), rather than in the WS2 state. The fluorine-containing catalyst contains a larger fraction of WS2 than the fluorine-free catalyst.
Attenuated total reflection (ATR) spectra of adsorbates and solvent on thin metal films were investigated with emphasis on the band-shape of absorption bands. Distorted band-shapes are found even far from the critical angle. Strong absorption bands are more distorted. The band-shape strongly depends on the optical constants of the metal film and its thickness. The distortion increases with increasing thickness and increasing refractive index of the thin metal film. For a 10 nm thick Pt film the measured band-shapes for liquid water and ethanol are in good agreement with theoretical predictions using the bulk optical constants for Pt. For CO adsorbed on a 1 nm Pt film a distorted band-shape is observed whereas calculations assuming bulk optical constants predict band-shape distortion only for considerably thicker Pt films. The effective optical constants for very thin metal films deviate considerably from the bulk values, due to the island structure of the film and non-adiabatic effects can lead to distorted band-shapes. Structural changes within a Pt film, induced by hydrogen treatment, leads to a change in the band-shape for adsorbed CO. The results show that band-shape analysis is a valuable tool for in situ ATR IR spectroscopy of metal films.
  • Chiral modification of platinum catalysts by cinchonidine adsorption studied by in situ ATR-IR spectroscopy
    Davide Ferri, Thomas BĂźrgi and Alfons Baiker
    ChemComm, 2001, p1172-1173
    DOI:10.1039/b102884g | Abstract | Article HTML | Article PDF
Adsorption of cinchonidine on a platinum model catalyst studied by in situ ATR-IR spectroscopy revealed that the adsorption mode depends on surface coverage and is affected by concomitant adsorption and fragmentation of solvent molecules.
Base-catalyzed H/D-exchange for ι- and β-isophorone (1 and 2, resp.) was monitored by NMR spectroscopy to identify the number and nature of reactive sites. Results show that ι-isophorone (1) undergoes H/D exchange at up to four different sites depending on reaction conditions. β-Isophorone (2), on the other hand, exhibits activity at two sites, predominantly at the ι-position, under comparable conditions. Quantum-chemical calculations indicate that the thermodynamically more-stable anions formed upon proton abstraction from isophorone are not favored kinetically in all cases. Thermodynamically unfavorable H/D-exchange at the ι-position in 1, which is observed experimentally, is explained via intermediate formation of γ-isophorone (3) with subsequent conjugation to the ι-isomer. Differences observed in the reactivities of the two isomers and differences in reactivity of 1 under various conditions in reactions involving proton abstraction as an initial step may be partly explained on the basis of these results.
The reactions of hydrogen atoms adsorbed on a Ni(111) surface (surface-bound H) and hydrogen atoms just below the surface (bulk H) with coadsorbed acetylene are probed under ultrahigh vacuum conditions. Bulk H is observed to react with acetylene upon emerging onto the surface at 180 K. Gas-phase hydrogenation products, ethylene and ethane, are produced as well as an adsorbed species, ethylidyne. Ethylidyne is identified by high-resolution electron energy loss spectroscopy. Surface-bound H reacts with adsorbed acetylene above 250 K to produce a single product, adsorbed ethylidyne. No gas-phase hydrogenation products, such as ethylene or ethane, are observed. The reaction of surface-bound H is extremely slow, with a rate constant determined from measurements of the initial reaction rate to be in the range of 10-5−10-3 (ML s)-1 for a temperature range of 250−280 K. The activation energy for the rate-determining step, which is shown to be the addition of the first surface-bound H to acetylene to form an adsorbed vinyl species, increases from 9 to 17 kcal/mol as the total coverage decreases from 0.92 to 0.74 ML. The reaction rate cannot be described by a simple first-order dependence on the coverage of either reactant, indicating the presence of strong interactions between reactants. Measurements of the equilibrium constant reveal strong interactions between the reactant surface H and the product ethylidyne, possibly resulting in island formation. Mechanisms for the formation of ethylidyne by the reactions of both surface-bound and bulk H are proposed, as well as mechanisms for the formation of ethylene and ethane by bulk H. The different product distributions resulting from the reaction of acetylene with the two forms of hydrogen are discussed in terms of the large energy difference between bulk and surface-bound H.
An in situ attenuated total reflection study of the chiral solid−liquid interface created by cinchonidine adsorption on a Pt/Al2O3 model catalyst is presented. Experiments were performed in the presence of dissolved hydrogen, that is under conditions used for the heterogeneous enantioselective hydrogenation of α-functionalized ketones. Cinchonidine adsorbs via the quinoline moiety. The adsorption mode is coverage dependent and several species coexist on the surface. At low concentration (10-6M) a predominantly flat adsorption mode prevails. At increasing coverage two different tilted species, α-H abstracted and N lone pair bonded cinchonidine, are observed. The latter is only weakly bound and in a fast dynamic equilibrium with dissolved cinchonidine. At high concentration (10-4−10-3 M) all three species coexist on the Pt surface. A slow transition from an adsorbate layer with a high fraction of α-H abstracted cinchonidine to one with a high fraction of N lone pair bonded cinchonidine is observed with the cinchonidine concentration being the driving force for the process. The reverse transition in the absence of dissolved cinchonidine is fast. Cinchonidine competes with solvent decomposition products for adsorption sites on the Pt, which may contribute to the observed solvent dependence of the heterogeneous enantioselective hydrogenation of ketones by cinchonidine-modified Pt.
  • Nature of active sites in sol-gel TiO2-SiO2 epoxidation catalysts
    C. Beck, T. Mallat, T. BĂźrgi and A. Baiker
    Journal of Catalysis, 204 (2) , 2001, p428-439
    DOI:10.1006/jcat.2001.3407 | unige:14688 | Abstract | Article PDF
A series of titania–silica aerogels with 0–100 wt% TiO2 content were synthesized and characterized by N2 physisorption, DRIFT, UV-Vis, XPS, and 29Si CP/MAS NMR analysis. It is shown that kinetic analysis of the epoxidation of 2-cyclohexene-1-ol (1) with TBHP is an informative test reaction providing insight in the nature of active sites. The surface area, pore volume, hydrophobicity, and relative abundance of Ti–O–Si linkages in the aerogels decreased with increasing Ti/Si ratio. Parallel to these changes, the initial rate of epoxide formation per Ti site (TOF) and the epoxide selectivity decreased but the productivity of the catalysts went through a maximum at 10 wt% TiO2. We propose that due to kinetic effects in the sol–gel synthesis the whole range of active Ti sites may be present in the mixed oxides, spanning from tetrahedral Ti isolated by four SiO groups to octahedral Ti surrounded by six TiO groups in titania nanodomains. Ether formation from 1 was catalyzed by Brønsted sites present only on high titania-containing aerogels. Oligomerization was a major side reaction on all catalysts including Ti-free silica. Si-free titania was the most active in allylic oxidation of 1 to cyclohexenone. Silylation, or amine (Me2BuN) addition to the reaction mixture, eliminated ether formation and suppressed oligomerization.
  
  • Magnetostructural correlations and spin model of (VO)2P2O7
    LatĂŠvi Max Lawson Daku, Serguei Borshch, Vincent Robert and Bernard Bigot
    Physical Review B, 63 (17) , 2001, p174439
    DOI:10.1103/PhysRevB.63.174439 | unige:3660 | Abstract | Article PDF
We report a theoretical density functional analysis of the exchange interactions in (VO)2P2O7 using molecular fragments. The calculations confirm that the magnetic structure must be decribed on the basis of linear dimer chains. The strongest exchange interaction is found through O-P-O bridges. The magnitude of the exchange parameters is governed not only by V-V distance but also by the whole structure along the superexchange pathway. The two chains present in the structure of (VO)2P2O7 are magnetically inequivalent. For the monoclinic phase of (VO)2P2O7, important variations in the calculated parameters for dimers with identical bridges are observed within one chain. The magnetic structure of this chain should be described not by two but by three or even four coupling constants.
This article presents a numerical quadrature intended primarily for evaluating integrals in quantum chemistry programs based on molecular orbital theory, in particular density functional methods. Typically, many integrals must be computed. They are divided up into different classes, on the basis of the required accuracy and spatial extent. Ideally, each batch should be integrated using the minimal set of integration points that at the same time guarantees the required precision. Currently used quadrature schemes are far from optimal in this sense, and we are now developing new algorithms. They are designed to be flexible, such that given the range of functions to be integrated, and the required precision, the integration is performed as economically as possible with error bounds within specification. A standard approach is to partition space into a set of regions, where each region is integrated using a spherically polar grid. This article presents a radial quadrature which allows error control, uniform error distribution and uniform error reduction with increased number of radial grid points. A relative error less than 10−14 for all s-type Gaussian integrands with an exponent range of 14 orders of magnitude is achieved with about 200 grid points. Higher angular lquantum numbers, lower precision or narrower exponent ranges require fewer points. The quadrature also allows controlled pruning of the angular grid in the vicinity of the nuclei.
 
The results of a study on the ground states of tricarbonato complexes of dioxouranate using multiconfigurational second-order perturbation theory (CASSCF/CASPT2) are presented. The equilibrium geometries of the complexes corresponding to uranium in the formal oxidation states VI and V, [UO2(CO3)3]4- and [UO2(CO3)3],5- have been fully optimized in D3h symmetry at second-order perturbation theory (MBPT2) level of theory in the presence of an aqueous environment modeled by a reaction field Hamiltonian with a spherical cavity. The uranyl fragment has also been optimized at CASSCF/CASPT2, to obtain an estimate of the MBPT2 error. Finally, the effect of distorting the D3h symmetry to C3 has been investigated. This study shows that only minor geometrical rearrangements occur in the one-electron reduction of [UO2(CO3)3]4- to [UO2(CO3)3],5- confirming the reversibility of this reduction.
The results of a theoretical study on the formation of the nitrogen cluster N10 from the ionic species N5+ and N5− are presented. The possibility to form N8 from N5+ and N3−has also been studied but no stable form was found. Structural and vibrational data are given for the different clusters. It is suggested that the anion N5− might be stable enough to be synthesized. The calculations have been carried out using multiconfigurational self-consistent-field wave functions and second-order perturbation theory.
  • Spin-orbit coupling within a two-component density functional theory approach: theory, implementation and first applications
    Laura Gagliardi, Bernd Schimmelpfennig, Laurent Maron, Ulf Wahlgren and Andrew Willetts
    Chemical Physics Letters, 344 (1-2) , 2001, p207-212
    DOI:10.1016/S0009-2614(01)00761-8 | unige:3730 | Abstract | Article HTML | Article PDF
An implementation of spin–orbit coupling within a two-component generalization of the density functional code MAGIC is described. The spin–orbit operator is represented in the effective one-electron mean-field approximation and included into the Fock matrix within an iterative self-consistent scheme. First tests have been carried out for the spin–orbit splitting of several atoms. The spin–orbit effect on the bond distance and harmonic frequency of some diatomics has also been determined. This scheme allows to include spin–orbit in a simple way and can be efficiently used to treat large systems.
  
Three all-nitrogen chemical species in bulk compounds are experimentally known from the three last centuries: N2, N3-, and N5+. The last one was predicted in 1991. Furthermore there is evidence for tetrahedral N4 in matrixes. Could further "nitrogens" exist? In recent years, the hypothetical existence of poly-nitrogen clusters has been the object of several theoretical investigations (refs 5−16 and references therein). Besides their theoretical interest, these structures have drawn attention because of their possible use as high energy-density materials (HEDM), that is, the large ratio between the energy released in a fragmentation reaction and the specific weight.
The structure and vibrational frequencies of the UO2 molecule have been determined using multiconfigurational wave functions (CASSCF/CASPT2), together with a newly developed method to treat spin−orbit coupling. The molecule has been found to have a (5fφ)(7s), 3Φu, Ω = 2 ground state with a U−O bond distance of 1.77 Å. The computed antisymmetric stretching σu frequency is 923 cm-1 with a 16/18 isotope ratio of 1.0525 which compares with the experimental values of 915 cm-1 and 1.0526, respectively. Calculations of the first adiabatic ionization energy gave the value 6.17 eV, which is 0.7 eV larger than the currently accepted experimental result. Reasons for this difference are suggested.
  • Magnetic Susceptibility Tensor and Heme Contact Shifts Determinations in the Rhodobacter capsulatus Ferricytochrome c': NMR and Magnetic Susceptibility Studies
    Pascale Tsan, Michael Caffrey, LatĂŠvi Max Lawson Daku, Michael Cusanovich, Dominique Marion and Pierre Gans
    Journal of the American Chemical Society, 123 (10) , 2001, p2243-2250
    DOI:10.1021/ja0011663 | unige:3718 | Abstract | Article HTML | Article PDF
The 1H and 15N resonances of the carbon monoxide complex of ferrocytochrome c‘ ofRhodobacter capsulatus, a ferrous diamagnetic heme protein, have been extensively assigned by TOCSY−HSQC, NOESY−HSQC, and HSQC−NOESY−HSQC 3D heteronuclear experiments performed on a 7 mM sample labeled with 15N. Based on short-range and medium-range NOEs and HN exchange rates, the secondary structure consists of four helices: helix 1 (3−29), helix 2 (33−48), helix 3 (78−101), and helix 4 (103−125). The 15N, 1HN, and 1Hαchemical shifts of the CO complex form are compared to those of the previously assigned oxidized (or ferric) state. From the chemical shift differences between these redox states, the orientation and the anisotropy of the paramagnetic susceptibility tensor have been determined using the crystallographic coordinates of the ferric state. The χ-tensor is axial, and the orientation of the z-axis is approximately perpendicular to the heme plane. The paramagnetic chemical shifts of the protons of the heme ligand have been determined and decomposed into the Fermi shift and dipolar shift contributions. Magnetic susceptibility studies in frozen solutions have been performed. Fits of the susceptibility data using the model of Maltempo (Maltempo, M. M. J. Chem. Phys. 1974, 61, 2540−2547) are consistent with a rather low contribution of the S = 3/2 spin state over the range of temperatures and confirm the value of the axial anisotropy. Values in the range 10.4−12.5 cm-1 have been inferred for the axial zero-field splitting parameter (D). Analysis of the contact shift and the susceptibility data suggests that cytochrome c‘ of Rb. capsulatus exhibits a predominant high-spin character of the iron in the oxidized state at room temperature.
The first-order Raman spectrum of K2S was measured over the temperature range from 10 to 742 K. The temperature dependence of the linewidth can be explained using results from the anharmonic lattice dynamics approach. Both the cubic and the quartic anharmonic interactions are of importance for this system. At 293 K, the Raman line is at ωT2g=128 cm−1 with a full width at half maximum ΓT2g=2.9 cm−1.
Calculated binding energies and spectroscopic properties of C70 dimers are presented. The two most stable isomers of the set of conceivable [2 + 2] cycloaddition products are isoenergetic, and both are compatible with NMR, infrared, and Raman data on the product recently synthesized by Lebedkin et al.
  • Quasi-static nature of the light induced thermal hysteresis in [Fe(ptz)6](BF4)2 spin-transition solid
    J. Jeftic, M. Matsarski, A. Hauser, A. Goujon, E. Codjovi, J. Linares and F. Varret
    Polyhedron, 20 (11-14) , 2001, p1599-1606
    Keywords: light induced thermal hysteresis; [Fe(ptz)6](BF4)2; spin-transition solid
    DOI:10.1016/S0277-5387(01)00660-X | unige:3510 | Abstract | Article HTML | Article PDF
  
The quasi-static nature of a light induced thermal hysteresis was studied on the spin-transition compound [Fe(ptz)6](BF4)2, by means of optical spectroscopy and magnetic measurements in the temperature interval between 10 and 80 K. Various experimental procedures are discussed in relation to the competition between the two processes considered, namely the photoexitation and the high-spin→low-spin relaxation. A detailed discussion of the experimental parameters, which should be considered in order to avoid erroneous interpretations of LITH, is given.
The cobalt(II) complexes prepared with a series of enantiopure ligands (1-3) containing the bis(oxazolinyl)pyridine unit have been studied. The ligands form high spin octahedral complexes as shown by the X-ray crystal structure of the homochiral complex [Co(R,R-1)2](ClO4)2(CH3CN)3. The diastereoselectivity of complex formation has been studied: equimolar mixtures of RR and SS ligands show mixtures of homochiral and heterochiral complexes for 2 and 3, but the phenyl-substituted ligand 1 shows exclusive formation of the heterochiral species. This selectivity is correlated with structural and electronic properties of the complexes.
  
  • Phase Transitions in Mesostructured Silica/Surfactant Composites: Surfactant Packing and the Role of Charge Density Matching
    Sarah H. Tolbert, Christopher C. Landry, Galen D. Stucky, Bradley F. Chmelka, Poul Norby, Jonathan C. Hanson and Alain Monnier
    Chemistry of Materials, 13 (7) , 2001, p2247-2256
    DOI:10.1021/cm0003727 | unige:3216 | Abstract | Article HTML | Article PDF
Time-resolved X-ray diffraction is utilized to follow phase transitions in nanostructured silica/surfactant composites in real time under hydrothermal conditions. The data allow us both to obtain kinetic parameters and to observe intermediate phases. In all cases, changes in the packing of the organic component of these composites drives the transformation, indicating that surfactant packing is a dominant factor in determining the overall structure of these materials. For materials heated in pure water, however, high activation energies for transformation were measured, suggesting that large kinetic barriers can stabilize structures against surfactant-driven rearrangements. Matching between the interfacial charge density of the inorganic silica framework and the charge density of the surfactant headgroups is also found to affect the kinetics of transformation. Lamellar-to-hexagonal transitions, which complement condensation-induced changes in charge density, are observed to be continuous, while hexagonal-to-lamellar transitions, which proceed contrary to these charge density changes, are discontinuous. For materials heated in their high-pH synthesis solutions, more complex phase behaviors are observed. Hexagonal (p6mm) structures transform either to a bicontinuous cubic phase (Ia3d) or to a lamellar structure. Lamellar phases are observed at either long or short polymerization times, while cubic phases dominate at intermediate polymerization times. The production of these different phases can be understood by considering the interplay between organic packing, charge density matching, and changing activation energies. At short times, high charge on the inorganic framework favors transformation to the low-curvature lamellar structures. At very long times, silica condensation both reduces this charge density and cross-links the framework. This cross-linking raises kinetic barriers for transformation and again favors the topologically simpler hexagonal-to-lamellar transition. Transformations to the cubic phase are only observed at intermediate times, when these effects are balanced.
  • Phase Transformations in Mesostructured Silica/Surfactant Composites. Mechanisms for Change and Applications to Materials Synthesis
    C.C. Landry, S.H. Tolbert, K.W. Gallis, A. Monnier, G.D. Stucky, P. Norby and J.C Hanson
    Chemistry of Materials, 13 (5) , 2001, p1600-1608
    DOI:10.1021/cm000373z | unige:3217 | Abstract | Article HTML | Article PDF
In this study, phase transformation of the hexagonal mesostructure MCM-41 to the cubic mesostructure MCM-48 is examined by in situ X-ray diffraction (XRD) of the transforming mesostructure and by XRD of products from bulk transformation experiments in Parr autoclaves. Transformations were studied under conditions of high pH and temperatures between 100 and 190 °C. Heating events took place after the hexagonal mesophase had assembled, but before it had fully polymerized. On the basis of these and previous results on transformations in silica−surfactant−water and surfactant−water systems, a model is proposed to explain the expected hexagonal → cubic transformation as well as the brief existence of a lamellar phase during the transformation. Additional experiments to establish synthetic parameters for the transformation included varying the silicon alkoxide source, replacing the supernatant prior to heating, and adding fluoride or aluminum to the reaction mixture. The results, taken together, illustrate the strong cooperativity between the organic and inorganic regions in controlling the assembly of the mesostructure and provide a better understanding of the effects that control phase transformations in these systems.
  • EPR and optical spectroscopy of SrF2 doped with Yb3+
    M.L. Falin, K.I. Gerasimov, A.M. Leushin, H. Bill, D. Lovy and T. Sanadze
    Journal of Alloys and Compounds, 323-324 , 2001, p692-695
    Keywords: insulators; crystal growth; point defects; crystal and ligand fields; EPR and optical spectroscopy
    DOI:10.1016/S0925-8388(01)01040-4 | unige:3331 | Abstract | Article HTML | Article PDF
Results of EPR and optical spectroscopic investigation of the trigonal paramagnetic Yb3+ ion in SrF2 (‘oxygen’ paramagnetic center — T2) are presented. The energy level scheme of the center is determined from its optical spectra and the parameters of the crystal field potential are calculated.
  • Absorption and emission spectroscopy of matrix-isolated benzo[g,h,i]perylene. An experimental and theoretical study for astrochemical applications
    X.F.D. Chillier, P. Boulet, H. Chermette, F. Salama and J. Weber
    Journal of Chemical Physics, 115 (4) , 2001, p1769-1776
    DOI:10.1063/1.1376632 | unige:3332 | Abstract | Article HTML | Article PDF
The absorption and emission spectra of benzo[g,h,i]perylene, a six ring polycyclic aromatic hydrocarbon molecule (C22H12), embedded in a rare gas matrix are reported. Time dependent emission shows that this molecule exhibits sharp phosphorescence in the red. Supporting theoretical calculations using the recently developed time-dependent density-functional response theory formalism (TD–DFRT) allow a tentative assignment for the observed transitions. The astrochemical significance of the results is briefly discussed.
  • DFT investigation of metal complexes containing a nitrosyl ligand. 1. ground state and metastable states
    P. Boulet, M. Buchs, H. Chermette, C. Daul, F. Gilardoni, F. Rogemond, C.W. Schläpfer and J. Weber
    Journal of Physical Chemistry A, 105 (39) , 2001, p8991-8998
    DOI:10.1021/jp010988z | unige:3658 | Abstract | Article HTML | Article PDF
  
Nitrosyl metal complexes, such as the sodium nitroprusside, have attracted chemists' interest for more than 30 years. The existence of long-lived metastable states easily populated by irradiation are the principal reason for this interest. Those long-lived states are interesting either for technical applications or for fundamental research. In this work, we present a comparative density functional theory (DFT) study of the ground state of two different nitrosyl compounds:  sodium nitroprusside and cyclopentadienylnitrosylnickel(II).
  • DFT investigation of metal complexes containing a nitrosyl ligand. 2. excited states
    P. Boulet, M. Buchs, H. Chermette, C. Daul, F. Gilardoni, F. Rogemond, C.W. Schläpfer and J. Weber
    Journal of Physical Chemistry A, 105 (39) , 2001, p8999-9003
    DOI:10.1021/jp010989r | unige:3333 | Abstract | Article HTML | Article PDF
The photochemical reactions of the nitroprusside and the CpNiNO complexes are explained on the basis of ΔSCF and time-dependent density functional theory (TD-DFT) calculations. Both similarities and differences in the photochemical processes are highlighted.
  • Theoretical prediction of IR spectra of guest molecules in zeolites : the stretching frequency of CO adsorbed at various cationic sites in ZSM-5
    T.A. Wesolowski, A. Goursot and J. Weber
    in "Studies in Surface Science and Catalysis" Proceedings of the 13th Int. Zeolite Conference, July 8-13, 2001, Montpellier, France, Elsevier (Amsterdam), 2001, p135
    unige:3784
  • Development of a tight-binding treatment for zeolites
    M. Elstner, A. Goursot, Z. Hajnal, T. Heine and J. Weber
    in "Studies in Surface Science and Catalysis" Proceedings of the 13th Int. Zeolite Conference, July 8-13, 2001, Montpellier, France, Elsevier (Amsterdam), 2001, p135
    unige:3785
The photochemistry of the CpNiNO complex has been investigated using density functional theory. The whole potential energy curve along the NiNO angle coordinate is presented for the first time with both ground and metastable states, and transition states connecting the minima. The excited states of the GS, MSI, and MSII species have been calculated using time-dependent density functional theory. Furthermore, the structure of the excited states pertaining to the photochemistry of CpNiNO has been optimized. From these results it is shown that the backward GS ← MSII ← MSI reaction is more efficient than the forward GS → MSII → MSI scheme.
The density-functional approach based on the partition into subsystems was applied to study the benzene dimer. For several structures, the calculated interaction energy and intermolecular distance were compared with the previous theoretical results. A good agreement with high level ab initio correlated methods was found. For instance, the interaction energies obtained in this work and the CCSD(T) method agree within 0.1 - 0.6 kcal/mol depending on the structure of the dimer. The structure with the largest interaction energy is T-shaped, in agreement with CCSD(T) results. The T-shaped structure of benzene dimer was suggested by several experimental measurements. The calculated interaction energy of 2.09 kcal/mol agrees also well with experimental estimates based on the dissociation energy which ranges from 1.6¹0.2 to 2.4¹0.4 kcal/mol and the estimated zero-point vibration energy of 0.3 - 0.5 kcal/mol.
  • Properties of CO adsorbed in ZSM5 Zeolite. Density Functional Theory Study Using the Embedding Scheme Based on Electron Density Partitioning.
    T.A. Wesolowski, A. Goursot and J. Weber
    Journal of Chemical Physics, 115 (10) , 2001, p4791-4797
    DOI:10.1063/1.1389275 | unige:3219 | Abstract | Article HTML | Article PDF
The CO molecule is frequently used as a probe in studies of zeolites where it adsorbs on metal cations. Compared with the free CO molecule, the stretching frequency of CO adsorbed in a zeolite is blue-shifted. The magnitude of the shift depends on the cation. The theoretical studies by Ferrari et al. [J. Chem. Phys., 105, 4129 (1996)] show that the isolated cation does not provide a good model of the zeolite because the calculated shifts are significantly overestimated. In this work, the effects of the interactions between theMe+CO (Me=Li, Na, or K) complex and the zeolite framework on the properties of CO adsorbed on the cation site are investigated. The properties of the investigated complexes are studied using the embedded molecule approach applying the orbital-free effective embedding potential derived within the subsystem formulation of density functional theory. In order to identify the major microsopic effects affecting the properties of the bound probe molecule, a hierarchy of cluster models is used to represent the zeolite framework. For the largest cluster model applied, the calculated frequency shifts agree within few cm−1 with experimental data. 
  
  • Absorption spectra of several metal complexes revisited by the time-dependent density-functional theory-response theory formalism
    P. Boulet, H. Chermette, C. Daul, F. Gilardoni, F. Rogemond, J. Weber and G. Zuber
    Journal of Physical Chemistry A, 105 (5) , 2001, p885-894
    DOI:10.1021/jp003041q | unige:3659 | Abstract | Article HTML | Article PDF
Vertical excitations calculated for the CrO42- , MnO42-  , RuO4, CrF6, FeCp2, RuCp2 and CpNiNO species are compared to experimental spectra. The results obtained from the time-dependent density-functional theory−response theory (TD-DFRT) method are compared to both previously reported ΔSCF calculations and experiment. The results show that, in general, excited states of metal oxide and metallocene compounds are well described by TD-DFRT. However, serious difficulties are met with the CrF6 system.
The predictive power of DFT, HF, and MP2 29Si NMR chemical shift calculations for silane molecules, including fluoro- and methylsilanes (SinH2n+2 (n = 1, ..., 5), SinF2n+2 (n = 1, ..., 3), and SiHmX4-m (X = F, CH3)) is compared. A systematic accumulation of error proportional to the number of hydrogen neighbors to silicon sites is observed for DFT for all applied exchange-correlation functionals, whereas MP2 is not affected by this problem. A proposed empirical correction scheme for DFT provides excellent agreement with experiment with any exchange-correlation functional employed in this study.
  • Biphasic Behaviour in the High-Spin -> Low-Spin Relaxation of [Fe(btpa)](PF6)2 in solution (btpa = N,N,N',N'-Tetrakis(2-pyridylmethyl)-6,6'-bis(aminomethyl)-2,2'-bipyridine)
    S. Schenker, P.C. Stein, J.A. Wolny, C. Brady, J.J. McGarvey, H. Toftlund and A. Hauser
    Inorganic Chemistry, 40 (1) , 2001, p134-139
    DOI:10.1021/ic000656t | unige:3336 | Abstract | Article HTML | Article PDF
The light-induced high-spin → low-spin relaxation for the Fe(II) spin-crossover compounds [Fe(btpa)](PF6)2 and [Fe(b(bdpa))](PF6)2 in solution, where btpa is the potentially octadentate ligand N,N,N‘,N‘-tetrakis(2-pyridylmethyl)-6,6‘-bis(aminomethyl)-2,2‘-bipyridine and b(bdpa) is the analogous hexadentate ligand N,N‘-bis(benzyl)-N,N‘-bis(2-pyridylmethyl)-6,6‘-bis(aminomethyl)-2,2‘-bipyridine, respectively, has been studied by temperature-dependent laser flash photolysis. [Fe(b(bdpa))](PF6)2 shows single-exponential 5T2 → 1A1 relaxation kinetics, whereas [Fe(btpa)](PF6)2 exhibits solvent-independent biphasic relaxation kinetics. The fast process of [Fe(btpa)](PF6)2 with a rate constant, kHL, of 2.5 × 107 s-1 at 295 K and an activation energy, Ea, of 1294(26) cm-1 in methanol can be assigned to the 5T2 → 1A1relaxation as well. The slow process with a kHL(295 K) of 3.7 × 105 s-1 and a Ea of 2297(32) cm-1 in methanol - which is the slowest light-induced relaxation process observed so far for an Fe(II) spin-crossover complex in solution - is assigned to a coupling of the 5T2 → 1A1relaxation process to a geometrical rearrangement within the pendent pyridyl arms.
The rate constants of back-electron-transfer (BET) reaction within geminate ion pairs generated upon static ET quenching of cyano-substituted anthracenes by aromatic amines and methoxy-substituted benzenes (MSB) at high concentration in acetonitrile have been measured directly using ultrafast multiplex transient grating spectroscopy. The free energy of BET, ΔGBET, was varied between −3.0 and −0.6 eV, a range corresponding, in principle, to the inverted, barrierless, and normal regimes. When plotted vs ΔGBET, the measured rate constants, kBET, exhibit a large scattering. Good fits of the semiclassical expression for nonadiabatic ET are obtained if the rate constants are sorted according to the electron donor. The resulting electronic coupling matrix elements V are larger and the solvent reorganization energies smaller than those reported for BET within solvent-separated ion pairs, suggesting that BET takes place between ions in contact. However, in the low exergonicity region, the observed BET rate constants are slower than those reported for contact ion pairs formed by charge-transfer excitation. The dynamics of BET within radical pairs generated upon ET quenching of the N-methylacridinium cation has also been investigated, and the role of the electrostatic interaction within geminate ion pairs is discussed.
An investigation of the charge recombination (CR) dynamics of geminate ion pairs formed upon electron transfer quenching of azulene, benz[a]azulene, and xanthione in the second singlet excited state by several electron donors, using ultrafast time resolved spectroscopy and photoconductivity is reported. The ion pairs have two possible CR pathways:  (i) a highly exergonic CR to the neutral ground state or (ii) a moderately exergonic CR leading to the formation of the neutral acceptor in the first singlet excited state. This investigation shows strong evidence of the predominance of the second pathway. CR in ion pairs formed with the azulenes is faster by a factor of more than 50 than in ion pairs having a similar energy but with the first CR pathway only. The electron transfer quenching of xanthione in the second singlet excited state by several weak donors does not lead to a significant reduction of the triplet yield of this molecule. The relevance of these results to explain the absence of the inverted region in highly exergonic bimolecular charge separation reactions is discussed.
The excited-state dynamics of the radical cations of perylene (PE•+), tetracene (TE•+), and thianthrene (TH•+), as well as the radical anions of anthraquinone (AQ•-) and tetracenequinone (TQ•-), formed by γ irradiation in low-temperature matrices (PE•+, TH•+, AQ•-, and TQ•-) or by oxidation in sulfuric acid (PE•+, TE•+, and TH•+) have been investigated using ultrafast pump−probe spectroscopy. The longest ground-state recovery time measured was 100 ps. The excited-state lifetime of PE•+ is substantially longer in low-temperature matrices than in H2SO4, where the effects of perdeuteration and of temperature on the ground-state recovery dynamics indicate that internal conversion is not the major decay channel of PE•+*. The data suggest that both PE•+* and TE•+* decay mainly through an intermolecular quenching process, most probably a reversible charge transfer reaction. Contrarily to AQ•-*, TQ•-* exhibits an emission in the visible which, according to theoretical calculations, occurs from an upper excited state.
  • Formation of a phosphorus-phosphorus bond by successive one-electron reductions of a two-phosphinines-containing macrocycle: Crystal structures, EPR and DFT investigations
    L. Cataldo, S. Choua, T. Berclaz, M. Geoffroy, N. MĂŠzailles, L. Ricard, F. Mathey and P. Le Floch
    Journal of the American Chemical Society, 123 (27) , 2001, p6654-6661
    DOI:10.1021/ja010331r | unige:3220 | Abstract | Article HTML | Article PDF
Chemical and electrochemical reductions of the macrocycle 1 lead to the formation of a radical monoanion anion [1]•- whose structure has been studied by EPR in liquid and frozen solutions. In accord with experimental 31P hyperfine tensors, DFT calculations indicate that, in this species, the unpaired electron is mainly localized in a bonding σ P−P orbital. Clearly, a one-electron bond (2.763 Å) was formed between two phosphorus atoms which, in the neutral molecule, were 3.256 Å apart (crystal structure). A subsequent reduction of this radical anion gives rise to the dianion [1]2- which could be crystallized by using, in the presence of cryptand, Na naphthalenide as a reductant agent. As shown by the crystal structure, in [1]2-, the two phosphinine moieties adopt a phosphacyclohexadienyl structure and are linked by a P−P bond whose length (2.305(2) Å) is only slightly longer than a usual P−P bond. When the phosphinine moieties are not incorporated in a macrocycle, no formation of any one-electron P−P bond is observed: thus, one-electron reduction of 3 with Na naphthalenide leads to the EPR spectrum of the ion pair [3]•- Na+; however, at high concentration, these ion pairs dimerize, and, as shown by the crystal structure of [(3)2]2-[{Na(THF)2}2]2+ a P−P bond is formed (2.286(2) Å) between two phosphinine rings which adopt a boat-type conformation, the whole edifice being stabilized by two carbon−sodium−phosphorus bridges.
  • EPR and theoretical studies of the reduction product of the fulvenephosphaallene system
    M. Chentit, H. Sidorenkova, S. Choua, M. Geoffroy, Y. Ellinger and G. Bernardinelli
    Journal of Organometallic Chemistry, 634 (2) , 2001, p136-144
    Keywords: phosphaallenes; EPR; DFT; radical anion; phosphaallylic radical
    DOI:10.1016/S0022-328X(01)01098-1 | unige:3606 | Abstract | Article PDF
  
Fluoren-9-ylidenemethylene-(2,4,6-tri-tert-butyl-phenyl)phosphane (2), a new type of phosphaallene with the terminal carbone incorporated in a cyclopentadienyl ring, has been synthesized and its crystal structure has been determined. The 31P and 13C (central carbon) hyperfine tensors of the reduction compound of this phosphaallene have been measured on the EPR spectra recorded after electrochemical reduction of a solution of 2 in THF. Structures of the model molecules HP=C=Cp (where Cp is a cyclopentadienyl ring), [HP=C=Cp]√− and [HP---CH=Cp]√ have been optimized by DFT and the hyperfine couplings of the paramagnetic species have been calculated by DFT and SCI methods. The comparison between the experimental and the theoretical results shows that, in solution, the radical anion [2]√− is readily protonated and that the EPR spectra are due to the phosphaallylic radical.
Stable for at least one week below -30°C: crystals of 1, the first highly persistent diphosphanyl radical, have been isolated and characterized. This phosphorus-centered radical exhibits hyperfine coupling whose anisotropy is considerably larger than that for well-established nitrogen radicals (hydrazyls, nitroxides). This feature is of potential interest for studies of fast molecular movements. Mes*=2,4,6-tBu3C6H2.
Ternary V2O5–WO3/TiO2 catalysts were prepared by sequential or simultaneous grafting steps of vanadia and tungsta onto titania and were compared with similarly prepared binary catalysts. Different grafting sequences including alternating grafting of vanadia and tungsta were compared.

During the grafting process the metal oxide precursor reacted with hydroxyl groups of the other grafted metal oxide. The interaction of vanadia with tungsta species resulted in a different reduction behavior of both metal oxides compared to the binary catalysts as indicated by temperature-programmed reduction with hydrogen. The strength of interaction of the grafted species depended on preparation sequence and metal loading.

At low coverage (‹monolayer) catalyst properties were found to depend strongly on loading, but relatively little on the grafting mode, as indicated by vibrational spectroscopy. Laser Raman experiments at different laser power revealed reversible effects due to temperature induced structural changes of surface vanadia species. For all catalysts, even at a loading of more than one and a half monolayers, no evidence of crystalline vanadia or tungsta could be found. After calcination of WO3/TiO2 catalyst at 1023 K instead of 573 K and subsequent grafting with vanadia, new species with hydroxyl groups showing a vibrational frequency below 3600 cm−1 were formed. The increase of the calcination temperature had no significant influence on the reduction of vanadia by hydrogen.

The conformational behaviour of several α-ketoesters was investigated using solution FTIR in combination with ab initio calculations. The α-ketoesters show marked differences in the O=C–C=O torsional potential energy surface depending on the substituent at the α-keto group. In general the torsional potential is characterised by broad minima corresponding to s-cis and s-trans conformations and low interconversion barriers. The s-trans conformation is more stable but the fraction of s-cis is considerable at room temperature and increases with solvent polarity due to the higher dipole moment of the latter. Hydrogen bonding with alcoholic solvents also leads to a stabilisation of the s-cis conformer. The interaction of ethyl pyruvate with R3N+–H is much stronger when ethyl pyruvate adopts an s-cis conformation due to strong ion–dipole interaction. This type of interaction between ethyl pyruvate and protonated cinchonidine is considered to be crucial for the enantio-differentiation in the heterogeneous enantioselective hydrogenation of ethyl pyruvate over cinchonidine modified platinum in acidic media.
  • Enhanced enantioselectivity in ethyl pyruvate hydrogenation due to competing enantioselective aldol reaction catalyzed by cinchonidine
    D. Ferri, T. BĂźrgi, K. Borszeky, T. Mallat and A. Baiker
    Journal of Catalysis, 193 (1) , 2000, p139-144
    DOI:10.1006/jcat.2000.2870 | unige:14698 | Abstract | Article PDF
IR and NMR experiments revealed that the enantioselective hydrogenation of ethyl pyruvate in nonacidic solvents is complicated by the simultaneously occurring self-condensation (aldol reaction) of the reactant. Both enantioselective reactions are catalyzed by the chiral base cinchona alkaloid, but the hydrogenation is faster by several orders of magnitude than the aldol reaction. Catalytic experiments proved that the aldol products are not spectator species. The enol form of the major aldol product protonates the quinuclidine N of cinchonidine and enhances the enantiomeric excess of the hydrogenation reaction. The significance of this observation with respect to kinetic and mechanistic studies is discussed.
  • Adsorption mode of ethyl pyruvate on platinum: An in situ XANES study
    T. BĂźrgi, F. Atamny, A. Knop-Gericke, M. Hävecker, T. Schedel-Niedrig, R. SchlĂśgl and A. Baiker
    Catalysis Letters, 66 (3) , 2000, p109-112
    Keywords: ethyl pyruvate; in situ XANES; enantioselective hydrogenation; platinum
    DOI:10.1023/A:1019047622959 | unige:14638 | Abstract | Article HTML | Article PDF
The adsorption of ethyl pyruvate on Pt(111) has been studied by in situ XANES measurements in the presence and absence of hydrogen. Depending on the hydrogen and ethyl pyruvate pressure, the C and O K‐edge spectra exhibit distinctly different angular dependence. Without hydrogen ethyl pyruvate is oriented preferentially perpendicular to the surface, indicating bonding via the O lone pairs. In the presence of hydrogen the mean orientation is more tilted towards the surface. Likely, ethyl pyruvate also interacts with Pt via its π system under these conditions. The observed angle‐dependent shift of the energy of the π* and σ* resonances indicates the coexistence of differently adsorbed ethyl pyruvate species. The experimental findings demonstrate the importance of the in situ approach for unraveling the adsorption mode of ethyl pyruvate in the enantioselective hydrogenation over cinchona‐alkaloid‐modified Pt.
The adsorption of ethyl pyruvate on Pt(111) at low temperature was investigated by XP and UP spectroscopy. The assignment of the photoelectron spectra was assisted by calculation of correlated ionization potentials. Comparison of the XP and UP spectra of the condensed and chemisorbed layer indicates a strong ethyl pyruvate adsorption bond in the latter. Upon chemisorption, the HOMO of ethyl pyruvate, which is a lone-pair orbital delocalized over both C=O groups, is stabilized by about 0.7 eV with respect to the other orbitals, which is characteristic for a lone-pair bonding mechanism. The same bonding mechanism was found for coverages far below saturation. The XP spectra further indicate that the ketone C=O is more strongly involved in the chemisorption bond than the carboxyl C=O of ethyl pyruvate. The packing density of the saturated chemisorbed ethyl pyruvate layer, as determined by XPS, is high. This points toward an upright or tilted orientation of ethyl pyruvate in this layer, in line with the observed bonding mechanism.
  • Model for enantioselective hydrogenation of α-ketoesters over chirally modified Pt revisited: Influence of a-ketoester conformation
    T. BĂźrgi and A. Baiker
    Journal of Catalysis, 194 (2) , 2000, p445-451
    DOI:10.1006/jcat.2000.2958 | unige:14809 | Abstract | Article PDF
Interaction complexes between cinchonidine modifier and methyl pyruvate reactant proposed for the enantioselective hydrogenation over platinum catalysts have been calculated using ab initio methods. For s-trans-methyl pyruvate it was found that the complex yielding (R)-methyl lactate upon hydrogenation was more stable than the corresponding pro-(S) complex. The calculated energy difference of 1.8 kcal/mol corresponds to an enantiomeric excess of 92%, in good agreement with experiment. For the analogous complexes of s-cis-methyl pyruvate the energy difference is only 0.2 kcal/mol in favour of pro-(R), corresponding to 17% enantiomeric excess. Due to the larger dipole moment of the s-cis conformer of methyl pyruvate its hydrogen-bonded complexes with cinchonidine are considerably more stable than the corresponding s-trans complexes. However, the predicted low enantiomeric excess for the s-cis conformer is in contrast with experiment. Possible reasons for this behaviour are discussed.
  
We present a theoretical analysis of the temperature dependence of the vanadyl pyrophosphate VO2P2O7 31P NMR spectra. Four distinct phosphorus sites responsible for four signals are identified in the crystal structure. The magnetic states of the crystal are described by two alternative models: the spin ladder and the dimer chain. Within both models, finite clusters with and without periodic conditions are considered. The fit of the experimental NMR data allows us to define combinations of hyperfine coupling parameters which are found to be similar in both spin models.
 

  
The total rate constant k1 has been determined at P = 1 Torr nominal pressure (He) and at T = 298 K for the vinyl-methyl cross-radical reaction: (1) CH3 + C2H3 → Products. The measurements were performed in a discharge flow system coupled with collision-free sampling to a mass spectrometer operated at low electron energies. Vinyl and methyl radicals were generated by the reactions of F with C2H4 and CH4, respectively. The kinetic studies were performed by monitoring the decay of C2H3 with methyl in excess, 6 < [CH3]0/ [C2H3]0 < 21. The overall rate coefficient was determined to be k1(298 K) = (1.02 ± 0.53) × 10-10cm3 molecule-1 s-1 with the quoted uncertainty representing total errors. Numerical modeling was required to correct for secondary vinyl consumption by reactions such as C2H3 + H and C2H3 + C2H3. The present result for k1 at T = 298 K is compared to two previous studies at high pressure (100-300 Torr He) and to a very recent study at low pressure (0.9-3.7 Torr He). Comparison is also made with the rate constant for the similar reaction CH3 + C2H5 and with a value for k1 estimated by the geometric mean rule employing values for k(CH3 + CH3) and k(C2H3 + C2H3). Qualitative product studies at T = 298 K and 200 K indicated formation of C3H6, C2H2, and C3H5 as products of the combination-stabilization, disproportionation, and combination-decomposition channels, respectively, of the CH3 + C2H3 reaction. We also observed the secondary C4H8 product of the subsequent reaction of C3H5 with excess CH3; this observation provides convincing evidence for the combination-decomposition channel yielding C3H5 + H. RRKM calculations with helium as the deactivator support the present and very recent experimental observations that allylic C-H bond rupture is an important path in the combination reaction. The pressure and temperature dependencies of the branching fractions are also predicted.
  • Pressure, temperature and light influence on spin transition solids
    Jelena Jeftic, Andreas Hauser, François Varret and Claude Ecolivet
    High Pressure Research, 18 (1-6) , 2000, p195-201
    DOI:10.1080/08957950008200968 | unige:3692 | Abstract | Article PDF
Present paper is an overview of our efforts during the past few years to understand complicated corelations of physical phenomena related to pressure in Fe(I1) solid state spin transition systems. Some principal results concerning p, T, λ-experiments are extracted. In the context of correlation of the crystallographic phase transition with simultaneous HS → LS relaxation and LS → HS photopopulation, we show the latest results: Brillouin and magnetic measurements on the crystal [Fe(pt6](BF6)2.
  • Luminescence and Energy Transfer of [Ru(bpy)3]2+, [Cr(ox)3]3-, and [Os(bpy)3]2+ in Three-Dimensional Oxalato-Networks
    M.E. Von Arx, E. Burattini, A. Hauser, L. Van Pieterson, R. Pellaux and S Decurtins
    Journal of Physical Chemistry A, 104 (5) , 2000, p883-893
    DOI:10.1021/jp993368v | unige:3504 | Abstract | Article HTML | Article PDF
  
Luminescence and energy transfer in [Zn1-xRux(bpy)3][NaAl1-yCry(ox)3] (x ≈ 0.01, y = 0.006 − 0.22; bpy = 2,2‘-bipyridine, ox = C2O42-) and [Zn1-x-yRuxOsy(bpy)3][NaAl(ox)3] (x ≈ 0.01, y = 0.012) are presented and discussed. Surprisingly, the luminescence of the isolated luminophores [Ru(bpy)3]2+ and [Os(bpy)3]2+ in [Zn(bpy)3][NaAl(ox)3] is hardly quenched at room temperature. Steady-state luminescence spectra and decay curves show that energy transfer occurs between [Ru(bpy)3]2+ and [Cr(ox)3]3- and between [Ru(bpy)3]2+ and [Os(bpy)3]2+ in [Zn1-xRux(bpy)3][NaAl1-yCry(ox)3] and [Zn1-x-yRuxOsy(bpy)3] [NaAl(ox)3], respectively. For a quantitative investigation of the energy transfer, a shell type model is developed, using a Monte Carlo procedure and the structural parameters of the systems. A good description of the experimental data is obtained assuming electric dipole−electric dipole interaction between donors and acceptors, with a critical distance Rc for [Ru(bpy)3]2+ to [Cr(ox)3]3- energy transfer of 15 Å and for [Ru(bpy)3]2+ to [Os(bpy)3]2+ energy transfer of 33 Å. These values are in good agreement with those derived using the Förster−Dexter theory.
The crystal chemistry of the Sm3+ to Sm2+ reduction in tetraborate lattices was investigated. In crystalline SrB4O7 in air it is mainly Sm2+ that is incorporated from a melt or glass containing predominantly Sm3+. For the process in air, a reduction and pick-up mechanism is assumed to take place at the crystal/nutrient interface. Stabilization of Sm2+ in SrB4O7 at high temperature and in an oxidizing atmosphere seems to be a particular property of the system, because no Sm2+ inclusion could be observed along the series MB4O7 (M = Ca, Ba, Cd, Pb), if similar reaction conditions were applied. So far, there is only one other oxide lattice (BaB8O13) known where at high temperatures significant amounts of Sm2+ are obtained for reactions in the air.Single crystals of SrB4O7 : Sm2+ were grown by the Czochralski method (keff for Sm is 0.5). Optical hole burning experiments for the transition 5D1–7F0 were performed at 80 K. A hole with a width of 0.21 cm–1 and a depth of 5.25% was formed for the first time for Sm2+ in a borate crystal excited by the beam of a single frequency dye laser. A rather small inhomogeneous linewidth of 0.28 cm–1 allowed the burning of a single hole only.
  • Extended Rodlike Polyaromatic Receptors with Bent Tridentate Units Complexed to Lanthanide Metal Ions
    Homayoun Nozary, Claude Piguet, Jean-Pierre Rivera, Paul Tissot, GĂŠrald Bernardinelli, Nathalie Vulliermet, Jacques Weber and Jean-Claude G. BĂźnzli
    Inorganic Chemistry, 39 (23) , 2000, p5286-5298
    DOI:10.1021/ic000338u | unige:3212 | Abstract | Article HTML | Article PDF
  • Optical microscopy of single ions and morphological inhomogeneities in Sm-doped CaF2 thin films
    R. Rodrigues-Herzog, F. Trotta, H. Bill, J.-M. Segura, B. Hecht and H.-J. GĂźntherĂśdt
    Physical Review B, 62 (16) , 2000, p11163-11169
    DOI:10.1103/PhysRevB.62.11163 | unige:3340 | Abstract | Article PDF | Article PS (gzipped)
We have investigated the luminescence of CaF2 thin films doped with very low concentrations of Sm2+ ions using scanning confocal optical microscopy at low temperatures. The film morphology was studied independently by atomic force microscopy. The Sm2+ ions are homogeneously distributed in the films and show photobleaching. Unexpectedly, on the film surface strongly luminescent small topographic features are observed that are found to contain Sm3+ by spectral analysis. The formation of Sm3+ is probably due to the presence of oxygen during film growth. In the lowest doped films on-off blinking behavior of isolated luminescent spots provides strong evidence for the first observation of single ions in a crystal.
Crystals of ordered Ba6EuF12Cl2 were found to form during high temperature flux growth. The structure was refined in the hexagonal space group P 6 to RF(RFW) = 0.024(0.024) for 326 reflections and 46 parameters. Lattice parameters are a = b = 1059.27(8) pm and c = 416.36(2) pm; Z = 1. The structure is isotypic to Ba7F12Cl2. No solid solution of Ba/Eu was observed, the Eu2+ ions are located in the channels formed by 3 + 6 fluorine ions, occupying only one of the three metal sites of the Ba7F12Cl2 structure.
  • Synthesis and structure of the disordered modification of Pb7F12Cl2
    F. Kubel, H. Hagemann and H. Bill
    Journal of Solid State Chemistry, 149 , 2000, p56-59
    Keywords: crystal structure, inorganic synthesis, lead halides
    DOI:10.1006/jssc.1999.8494 | unige:3341 | Abstract | Article PDF
We recently discovered a new compound with composition Ba7F12Cl2. It was possible to show that the variation of the synthesis conditions makes it possible to obtain a disordered and an ordered modification with different lattice parameters and space groups (P63/m [176] and P6  [174]). For Pb7F12Cl2 an ordered modification is reported in the literature. In this paper we present the synthesis and structural characterization from X-ray diffraction data of the disordered modification of Pb7F12Cl2. Single crystals were grown from a flux and the structure was refined in the hexagonal space group P63/m to R(Rw)=0.043(0.038) for 284 reflections and 26 parameters. Lattice parameters are a=b=1021.90(8) pm and c=361.93(6) pm with Z=1. Propeller-type arrangements with chlorine as axis and fluorines as blades are observed. The ordered modification of Pb7F12Cl2 was prepared by a new hydrothermal synthesis. Differences between both modifications are found in the lattice constants and atomic occupation parameters for the atom type Pb2 and the connected fluorine ions.
  
  • Magnesium-Adenosine Diphosphate binding sites in wild-type creatine kinase and in mutants:role of aromatic residues probed by Raman and Infrared spectroscopies
    Hans Hagemann, Olivier Marcillat, RenĂŠ Buchet and Christian Vial
    Biochemistry, 39 (31) , 2000, p9251-9256
    Keywords: Raman spectroscopy, IR spectroscopy, enzyme, creatine kinase
    DOI:10.1021/bi000009d | unige:3605 | Abstract | Article HTML | Article PDF
Two distinct methods were used to investigate the role of Trp residues during Mg-ADP binding to cytosolic creatine kinase (CK) from rabbit muscle:  (1) Raman spectroscopy, which is very sensitive to the environment of aromatic side-chain residues, and (2) reaction-induced infrared difference spectroscopy (RIDS) and photolabile substrate (ADP[Et(PhNO2)]), combined with site-directed mutagenesis on the four Trp residues of CK. Our Raman results indicated that the environment of Trp and of Tyr were not affected during Mg-ADP binding to CK. Analysis of RIDS of wild-type CK, inactive W227Y, and active W210,217,272Y mutants suggested that Trp227 was not involved in the stacking interactions. Results are consistent with Trp227 being essential to prevent water molecules from entering in the active site [as suggested by Gross, M., Furter-Graves, E. M., Wallimann, T., Eppenberger, H. M., and Furter, R. (1994) Protein Sci. 3, 1058−1068] and that another Trp could in addition help to steer the nucleotide in the binding site, although it is not essential for the activity of CK. Raman and infrared spectra indicated that Mg-ADP binding does not involve large secondary structure changes. Only 3−4 residues absorbing in the amide I region are directly implicated in the Mg-ADP binding (corresponding to secondary structure changes less than 1%), suggesting that movement of protein domains due to Mg-nucleotide binding do not promote large secondary structure changes.
  • Comment on "Anisotropic intermolecuar interactions in van der Waals and hydrogen-bonded complexes: What can we get from density-functional calculations?" [J. Chem. Phys. 111, 7727 (1999)]
    T.A. Wesolowski
    Journal of Chemical Physics, 113 (4) , 2000, p1666-1667
    DOI:10.1063/1.481955 | unige:3342 | Abstract | Article HTML | Article PDF
In this comment, it is pointed out that the generalized gradient approximation (GGA) functionals considered by Milet et al. [ J. Chem. Phys. 111, 7727 (1999)] share the same exchange part (B88) which violates significantly the Lieb–Oxford bound. Violation of this exact condition was shown to result in significant errors of the exchange energy in the case of weakly overlapping electron densities [Wesołowski et al., J. Phys. Chem. A 101, 7818 (1997); Zhang et al., J. Chem. Phys. 107, 7921 (1997)]. Numerical examples are given to illustrate that such exchange functionals which better satisfy the Lieb–Oxford bound lead to better interaction energies also for the complexes studied by Milet et al.
The use of hybrid ab initio QM/MM methods in studies of metalloenzymes and related systems presents a major challenge to computational chemists. Methods that include the metal ion in the quantum mechanical region should also include the ligands of the metal in this region. Such a treatment, however, should be very demanding if one is interested in performing the configurational averaging needed for proper calculations of activation free energies. In the present work we examine the ability of the frozen DFT (FDFT) and the constrained DFT (CDFT) approaches to be used in ab initio studies of metal-catalyzed reactions, while allowing for an effective QM (rather than a QM/MM) treatment of the reacting complex. These approaches allow one to treat the entire enzyme by ab initio DFT methods, while confining the SCF calculations to a relatively small subsystem and keeping the electron density of the rest of the system frozen (or constrained). It is found that the FDFT and CDFT models can reproduce the trend obtained by a full DFT calculation of a proton transfer between two water molecules in a (Im)3Zn2+(H2O)2 system. This and related test cases indicate that our approximated models should be capable of providing a reliable representation of the energetics of metalloenzymes. The reasons for the special efficiency of the FDFT approach are clarified, and the strategies that can be used in FDFT studies of metalloenzymes are outlined.
  • Coupled-cluster calculations on ferrocene and its protonated derivatives : the final word on the mechanism of protonation of ferrocene ?
    M.J. Mayor-Lopez, H.P. LĂźthi, H. Koch, and J. Weber
    Journal of Chemical Physics, 113 (18) , 2000, p8009-8014
    DOI:10.1063/1.1316031 | unige:3344 | Abstract | Article HTML | Article PDF | Article PS (gzipped)
The mechanism of the protonation of ferrocene, the simplest model for the electrophilic attack on a metallocene, has been studied extensively. However, neither experiment nor computation have reached agreement on the details of the mechanism. The different model calculations applied [Hartree–Fock, Möller–Plesset, and density functional theory (HF, MP2, and DFT) with different functionals] come to contradicting conclusions. As a complement to our previous work, we report the results obtained for neutral and protonated ferrocene using the coupled-cluster method [CCSD(T)] with polarized double- and triple-zeta basis sets. These calculations show that the metal-protonated and the agostic forms represent minima on the potential energy surface, whereas the ring-protonated form is higher in energy with no minimum structure identified. With regard to the reaction, these results indicate an exo reaction path. The CCSD(T) results are in good agreement with the predictions made by the DFT calculations, whereas the results obtained from the Hartree–Fock and MP2 computations appear to be incorrect.
 
  • Progresses towards the advanced computational chemistry of increasingly complex systems
    J. Weber
    Chimia, 54 (1-2) , 2000, p51-51
    Keywords: computational chemistry; COST; density functional theory
    unige:3345 | Abstract | Article PDF
Incorporation of [Co(bpy)3]2+ into the cavities of the three-dimensional oxalate network structure in [Co(bpy)3][LiCr(ox)3] produces chemical pressure that destabilises the normal high-spin ground state 4T1 to such an extent that the [Co(bpy)3]2+ complex becomes a spin-crossover complex. It shows a temperature-dependent equilibrium between the 2E low-spin and the 4T1 high-spin states.
  
The dynamics of charge recombination within geminate ion pairs formed by electron transfer (ET) quenching of excited aromatic hydrocarbons by aliphatic and aromatic amines was investigated using picosecond transient grating spectroscopy. With aliphatic donors, the rate constant of back ET, kBET, shows a substantial decrease with increasing steric encumbrance around the N atom. No correlation between kBET and the exergonicity of the process was observed. This effect is ascribed to a decrease of the electronic coupling matrix element, V, which is affected by both the distance between the N atom of the donor and the aromatic plane of the acceptor and by the delocalization of the hole upon increasing the bulkiness of the alkyl substituents. With aromatic amines, kBETis substantially slower than with the unhindered amines. This is also explained in terms of a smaller value of V because of charge delocalization.
  • Deuterium Isotope Effect on the Charge Recombination Dynamics of Contact Ion Pairs Formed by Electron-Transfer Quenching in Acetonitrile
    P.-A. Muller, C. Hoegemann, X. Allonas, P. Jacques and E. Vauthey
    Chemical Physics Letters, 326 (3-4) , 2000, p321-327
    DOI:10.1016/S0009-2614(00)00791-0 | unige:3346 | Abstract | Article HTML | Article PDF
The deuterium isotope effect on the fluorescence lifetime of contact ions pairs (CIP) composed of 9,10-dicyanoanthracene and weak aromatic electron donors in acetonitrile has been investigated. For all pairs studied, an increase of the emission lifetime with increasing deuterium substitution on both the electron acceptor and the electron donor was observed. The free ion yield shows the same variation upon deuteration. It is concluded that the efficiency of free ions formation is essentially determined by the competition, within the CIP, between charge recombination and dissociation into free ions and that loose ion pairs do not play a significant role.
  • Investigation of the Ultrafast Dynamics of Charge Recombination of Ion Pairs Using Multiplex Transient Grating Spectroscopy
    E. Vauthey
    in "Ultrafast Phenomena XII" (T. Elsaesser, S. Mukamel, M. M. Murnane and N.F. Scherer Eds), Springer: Berlin, 2000, p485
    unige:3786
  • Applications of Transient Grating Techniques for Investigating Ultrafast Processes
    E. Vauthey
    EPA Newsletter, 70 , 2000, p30-57
    unige:3347
  • One-electron reduction product of biphosphinine derivative and of its Ni(0) complex: crystal structure, EPR/ENDOR and DFT investigations on (tmbp).- and [Ni(tmbp)2].-
    S. Choua, H. Sidorenkova, T. Berclaz, M. Geoffroy, P. Rosa, N. MĂŠzailles, L. Ricard, F. Mathey and P. Le Floch
    Journal of the American Chemical Society, 122 (49) , 2000, p12227-12234
    DOI:10.1021/ja002125+ | unige:3604 | Abstract | Article HTML | Article PDF
The radical anion (tmbp)•-, where tmbp = 4,4‘,5,5‘-tetramethyl-2,2‘-biphosphinine, was generated by reduction of tmbp on a potassium mirror. EPR/ENDOR spectra and DFT calculations show that, in contrast to the neutral species, this anion is planar and that the unpaired electron is mainly delocalized on the PCCP fragment with a large participation of the phosphorus pπ orbitals. This planar structure was confirmed by the first crystal structure of an anionic biphosphinine:  [tmbp][Li(2.2.1)]. Reduction of [Ni(tmbp)2] led to the 19-electron complex whose g and 31P hyperfine tensors were obtained from EPR in liquid and frozen solutions. These results, together with DFT calculations on [Ni(bp)2] and [Ni(bp)2]•-, indicate that, by accepting an extra electron, the neutral nickel complex distorts toward a more planar geometry and that the dihedral angle between the two phosphinine rings of each ligand slightly increases. In the reduced Ni complex, the unpaired electron is mainly delocalized on the ligands, in a molecular orbital which retains the characteristics of the SOMO found for the reduced isolated ligand. A charge decomposition analysis (CDA) shows that, in [Ni(bp)2], metal−ligand back-donation strongly contributes to the metal−ligand bonding.
The g, 31P and 1H hyperfine tensors of the dibenzobarrelene phosphinyl radical, trapped in an X-irradiated single crystal of dibenzobarrelene phosphine, were estimated at 45 and 300 K. They indicate that among the three locations of the phosphinyl hydrogen expected from DFT calculations, only two are occupied at 40 K and that the third one remains practically vacant, even at 300 K. The temperature dependence of the EPR spectrum was simulated by assuming jumps between two P–H bond orientations (energy barrier ~= 0.5 kcal mol−1) which correspond to the conformation of the PH2 moiety in the only rotamer present in the dibenzobarrelene phosphine crystal.
Electron paramagnetic resonance, electron-nuclear double resonance, and optical spectroscopy of the tetragonal Yb3+ center in KMgF3 are reported here. The results of these experiments allow us to conclude that a previously given structural model as well as the interpretation of the optical spectrum of this center are incorrect. A model is presented and experimentally and theoretically justified. In particular, the values of the hyperfine and transferred hyperfine interaction parameters were determined as well as an experiment-based energy-level scheme. Its parametrization is performed by including simultaneously the crystal field and the spin-orbit interaction within the 7F term. Furthermore, a theoretical analysis of the transferred hyperfine interaction (THFl) parameters is presented. It is further shown from optics and from microscopic calculations of the THFI parameters that g∥ and g⊥have opposite signs and that the rule of correspondence between the cubic g factor and g̃=1/3(gx+gy+gz) does not depend on the relative magnitude of the cubic and low-symmetry crystal field acting on the rare-earth ion.
 
The optical and paramagnetic properties of X-irradiated silver doped SrF2 crystals were investigated. The freshly irradiated crystals show a complex absorption spectrum between approximately 200 and 650 nm. Subsequently, systematic heat treatments were applied and absorption, photoluminescence and its polarization dependence, thermo- and radio-luminescence experiments have been undertaken. The resulting experimental data were mutually correlated with the aid of the factor analysis technique and six different origins of the observed spectra were identified. Models of the underlying silver-defect structures are discussed and crystal preparation is further presented.
  • Injection-molding versus extrusion as manufacturing technique for the preparation of biodegradable implants
    A Rothen-Weinhold, K Besseghir, E Vuaridel, E Sublet, N Oudry, F Kubel and R Gurny
    European Journal of Pharmaceutics and Biopharmaceutics, 48 (2) , 1999, p113-121
    Keywords: somatostatin analogue; polylactic acid; implant; injection-molding; extrusion; controlled release
    DOI:10.1016/S0939-6411(99)00034-X | Abstract | Article HTML | Article PDF
Polylactic acid (PLA) is a biocompatible and biodegradable material with wide utility for many applications, including the design of controlled-release systems for pharmaceutical agents. The factors determining the degradation kinetics of these systems include the composition and the molecular mass of the polymer, the morphology and the structure of the device, and the influence of thermal processes. The processing of the polymer determines the structure and design of the device, and influences to a high degree its morphology, namely its microporous structure, polymeric chain orientation and crystallinity. In this work, we aimed to compare the influence of two different implant manufacturing techniques, extrusion and injection-molding, on the in vitro degradation of the polymeric matrix. Both kinds of implants were loaded with a somatostatin analogue. Decrease in molecular weight, and polydispersity evolution during an accelerated in vitro degradation test were studied by size exclusion chromatography. Morphological changes in the polymeric matrix during degradation were followed after defined time intervals by means of scanning electron microscopy. Crystallinity studies were performed by differential scanning calorimetry and by X-ray analysis. Peptide stability in the polymeric matrix after both manufacturing methods was evaluated. Peptide release profiles, obtained in vitro during a week dissolution test, from both implant samples, were studied. It was shown that both molecular weight and polydispersity decreased after extrusion or injection-molding. This decrease was more pronounced with the latter technique. Crystallinity studies demonstrated that the crystalline network was not destroyed after both manufacturing methods. Peptide release profiles obtained in vitro were in good accordance with scanning electron microscopy. It was found that both manufacturing techniques had to be considered, although the extruded implants degraded more rapidly in vitro than the injection-molded ones.
  • Cobalt-catalyzed amination of 1,3-propanediol. Effects of catalyst promotion and use of supercritical ammonia as solvent and reactant
    A. Fischer, M. Maciejewski, T. BĂźrgi, T. Mallat and A. Baiker
    Journal of Catalysis, 183 (2) , 1999, p373-383
    DOI:10.1006/jcat.1999.2408 | unige:14702 | Article PDF
The catalytic synthesis of 1,3-diaminopropane from 1,3-propanediol and ammonia was studied in a continuous fixed-bed reactor in the pressure range 50 to 150 bar. The unsupported Co-based catalysts applied were characterized by N2physisorption, XRD, XPS, TPR, and ammonia adsorption using pulse thermal analysis and DRIFT spectroscopy. The latter investigations revealed that the best catalyst, 95 wt% Co–5 wt% Fe, contained only very weak acidic sites, unable to chemisorb ammonia. The absence of strong acidic and basic sites was crucial to suppress the various acid/base-catalyzed side reactions (retro-aldol reaction, hydrogenolysis, alkylation, disproportionation, dimerization, oligomerization). Other important requirements for improved diaminopropane formation were the use of excess ammonia (molar ratio NH3/diol>20) and the presence of the metastable β-Co phase. A small amount of Fe additive could efficiently hinder the transformation of this phase into the thermodynamically stable α-Co phase and thus prevent catalyst deactivation up to 10 days on stream. Application of supercritical ammonia almost doubled the selectivity to amino alcohol and diamine. The selectivity enhancement in the near-critical region is attributed to elimination of the interphase mass transport limitations and to the resulting higher surface ammonia concentration.
  • Enantioselective hydrogenation over chirally modified platinum. New insight into the adsorption mode of the modifier
    Thomas BĂźrgi, Zhaohui Zhou, Niklaus KĂźnzle, Tamas Mallat and Alfons Baiker
    Journal of Catalysis, 183 (2) , 1999, p405-408
    DOI:10.1006/jcat.1999.2411 | unige:14812 | Abstract | Article PDF
A new modifier, 2-phenyl-9-deoxy-10,11-dihydrocinchonidine, has been synthesized for the enantioselective hydrogenation of ketopantolactone and Îą-ketoesters over chirally modified Pt/alumina. The results indicate flat adsorption of cinchonidine with the quinoline ring oriented parallel to the surface and, furthermore, give some insight into the conformation of the modifier within the transition state complex. Comparison of the structures and catalytic behaviors of 9-deoxycinchonidine and the new modifier allows to exclude the previously proposed perpendicular or tilted adsorption of the quinoline ring via the N atom.
Cinchona alkaloids play a major role as chiral auxiliaries in asymmetric catalysis. Acetic acid is known to be an excellent solvent in the enantioselective hydrogenation over chirally modified platinum metals. The crucial interaction between the chiral auxiliary and the solvent has been investigated using the cinchonidine–acetic acid pair. Solutions containing cinchonidine and acetic acid were studied by means of NMR and IR spectroscopy as well as by ab initio Hartree–Fock calculations. In the presence of the acid cinchonidine is protonated at the quinuclidine N and adopts an open conformation where the quinuclidine N points away from the quinoline moiety. In the most stable 1∶1 and 2∶1 acetic acid–cinchonidine complexes both the N–H+ and O–H groups of cinchonidine are involved in hydrogen bonding. The most stable 1∶1 complex is found to be cyclic. The relative arrangement of the N–H+ and O–H groups of protonated cinchonidine is ideally suited to bind an acetate anion, and the interaction hardly affects the cinchonidine conformation. Several 2∶1 acid–base complexes coexist in solution. The IR spectra give evidence for the existence of a 2∶1 cyclic complex. Calculated structures, relative energies and vibrational frequencies are in good agreement with the experiment. The findings rationalise the importance of the O–H group of cinchonidine for the enantiodifferentiation in the enantioselective hydrogenation of α,β-unsaturated carboxylic acids over cinchonidine-modified Pd.
  • Enantioselective hydrogenation of a,b-unsaturated carboxylic acid over cinchonidine modified palladium: Nature of modifier - reactant interaction
    K. Borszeky, T. BĂźrgi, Z. Zhaohui, T. Mallat and A. Baiker
    Journal of Catalysis, 187 (1) , 1999, p160-166
    DOI:10.1006/jcat.1999.2584 | unige:14693 | Abstract | Article PDF
The mechanism of enantiodifferentiation in the hydrogenation of alkenoic acids over cinchona-modified Pd has been investigated using the tiglic acid → 2-methyl-butanoic acid transformation as test reaction. Application of simple derivatives of cinchonidine, modified at the (C-9)–OH and/or the quinuclidine nitrogen, proved that both functional groups are involved in the enantiodiscriminating step. Addition of a strong base (1,8-diazabicyclo[5.4.0]undec-7-ene, DBU) to tiglic acid prior to hydrogenation revealed that one cinchonidine molecule interacts with a dimer of tiglic acid on the metal surface. Ab initio calculations corroborate the existence of an energetically favored acid dimer–cinchonidine intermediate stabilized by hydrogen bonding, involving both the OH and the quinuclidine nitrogen of cinchonidine.
  • Unusual Contact Shifts and Magnetic Tensor Orientation in Rhodobacter capsulatus Ferrocytochrome c': NMR, Magnetic Susceptibility, and EPR Studies
    Pascale Tsan, Michael Caffrey, LatĂŠvi Max Lawson Daku, Michael Cusanovich, Dominique Marion and Pierre Gans
    Journal of the American Chemical Society, 121 (9) , 1999, p1795-1805
    DOI:10.1021/ja9820745 | unige:2936 | Abstract | Article HTML | Article PDF
In contrast to high-spin ferrous paramagnetic heme proteins, the chemical shifts of the heme protons are very unusual in the ferrocytochromes c‘. Magnetic susceptibility studies ofRhodobacter capsulatus ferrocytochrome c‘ in frozen solutions have been performed and indicate an S = 2 spin state and a large negative axial (D) zero-field splitting parameter (−18.3 cm-1) as well as a significant rhombic (E) value (−4.9 cm-1). The 1H and 15N resonances have been extensively assigned by TOCSY−HSQC, NOESY−HSQC, and HSQC−NOESY−HSQC 3-D heteronuclear experiments performed on a 8 mM sample labeled with 15N. Based on short-range and medium-range NOEs and HN exchange rates, the secondary structure consists of four helices: helix-1 (3−30), helix-2 (34−49), helix-3 (78−97), and helix-4 (103−117). The 15N, HN, and Hα chemical shifts of the reduced (or ferro) state are compared to those previously assigned for the diamagnetic carbon monoxide complex form. From the chemical shift differences between these redox states, the orientation and the anisotropy of the paramagnetic susceptibility tensor have been determined using the crystallographic coordinates of the ferric state. Values of −23 and −3 cm-1 have been inferred for D and E, and the z-axis of the tensor is tilted approximately 30° from the normal to the heme. The paramagnetic chemical shifts of the heme protons have been determined and split up into Fermi shift and the dipolar shift contributions. The pattern of the contact shifts is very unusual, exhibiting a 2-fold symmetry, and is discussed in terms of molecular orbital interactions between the porphyrin macrocycle and the imidazole ring.
  • Crystal structures of Eu(II) substituted barium magnesium fluorides: Ba0.78(3)Eu0.22(3)MgF4 and Ba5.20(6)Eu0.80(6)Mg7F26
    F. Kubel, H. Hagemann and H. Bill
    Zeitschrift fĂźr Kristallographie, 214 , 1999, p139-142
    Keywords: Experimental study | XRD | Crystal structure | Barium fluorides | Magnesium fluorides | Europium fluorides | Ion substitution | Chemical composition | Inorganic compounds | Rare earth compounds
    unige:3710
Barium in BaMgF[4] and in Ba[6]Mg[7]F[26] can be partially replaced by Sr or Eu. The single crystal structural analysis of the title compounds (a = 409.06 pm, b = 1452.7 pm, c = 579.02 pm, space group Cmc2[1] for Ba[0.78(3)]Eu[0.22(3)]MgF[4] [Z = 2, R[w] = 0.023 for 998 reflections] and a = 583.47 pm, b = 1209.09 pm and c = 1506.56 pm, space group Immm for Ba[5.20(6)]Eu[0.80(6)]Mg[7]F[26]) MgF[4] [Z = 2, R[w] = 0.019 for 2694 reflections] confirms the substitution of barium by europium. A split refinement of the substituted barium site in both crystals reveals a trend of europium to get closer to the nearest fluoride neighbors by simultaneously reducing the coordination sphere.
  • Low temperature crystal growth and structure of ordered Ba7F12Cl2
    F. Kubel, H. Bill and H. Hagemann
    Zeitschrift fĂźr Naturforschung, 54b , 1999, p515-518
    Keywords: Bariumfluorochloride, Hexagonal Crystal Structure, Crystal Growth, Low Temperature Gel Method
    unige:3709 | Abstract
Crystals of composition Ba7F12Cl2 were obtained by a reaction at room temperature between Ba2+/Cl-/F- in a gel of agar-agar/water. The hexagonal crystals have space group P6, a=1064.69(8), c=417.89(5)pm, V=410.24(8) 106 pm3 and Z=1. The anions form a propeller type network located in tunnels parallel to the chex axis; the chloride ions are located at the center on the propeller axes. The Ba2+ ions are coordinated by a (distorted) tricapped trigonal environment of fluoride and chloride anions. Disorder is present for one particular Ba2+ site. The average structure is isotypic with the structure of Pb7F12Cl2.
  • Correlations of the distribution of spin states in spin crossover compounds
    H. Spiering, T. Kohlhaas, H. Romstedt, A. Hauser, C. Bruns-Yilmaz, J. Kusz and P. GĂźtlich
    Coordination Chemistry Reviews, 190-192 , 1999, p629-647
    Keywords: phase transition; spin crossover; correlation
    DOI:10.1016/S0010-8545(99)00109-5 | unige:3693 | Abstract | Article HTML | Article PDF
Short range correlations of the distribution of high spin (HS) and low spin (LS) states show up in thermal spin transition curves, decay curves of the light induced metastable HS state (LIESST state), and in structural features during the spin transitions. Correlations are due to short range interactions between the spin crossover molecules. Short range interactions may compete with omnipresent long range interactions and give rise to interesting spin transition phenomena. In this paper, the effect of correlations on the thermal spin transition in the mixed crystal system [FexZn1−x(pic)3]Cl2·EtOH (pic=picolylamine) is discussed. In particular the step in the thermal transition curve is a direct consequence of such correlations. In addition, the decay of the metastable HS state of the pure iron compound at ca. 20 K can be significantly changed by preparing metastable HS states with a random distribution over the lattice sites. Both experiments could be well reproduced by Monte Carlo simulations. In the orthorhombic modification of the compound Fe[5NO2-sal-N(1,4,7,10)]([2,2′-(2,5,8,11-tetraazadodeca-1,11-diene-1,12-diyl)4-nitrophenolato] (2-)-N2, N2′,N2′′,N2′′′,O1, O1′]Fe(II)) a commensurable superstructure has been found. This compound represents the first example of a stable infinite range correlation of the spin states over the lattice sites.
  • Patterns during photoexcitation and high-spin -> low-spin relaxation in [Fe(Ptz)6](BF4)2 spin transition crystal
    Jelena Jeftic, François Varret, Andreas Hauser, Olivier Roubeau, Marie Matsarski and Jean-Pierre Rivera
    Molecular Crystals and Liquid Crystals, 335 , 1999, p511-520
    Keywords: spin transition, photography, laser photoexcitation, instability
    DOI:10.1080/10587259908028892 | unige:17026 | Abstract | Article PDF
The propagation of the high-spin (HS) → low-spin (LS) relaxation at 53 K in a single crystal of the iron (II) spin-crossover compound [Fe(ptz)6](BF4)2 was followed by photography, after inducing the local photoexcitation to the metastable HS state at 20 K using the single wavelength (457 nm Ar± ion laser) irradiation. The photoinduced formation of the HS—LS patterns with a characteristic diameter of some 0.1 mm was observed to occur inhomogeneously at a macroscopic scale already during photoexcitation. The contrast between the HS (transparent) and the LS (purple) regions was amplified during relaxation. The effect is described in terms of a transient instability, for which a microscopic model in the mean-field approximation is proposed. The mechanism for the development of patterns at the macroscopic scale is discussed.
Crystals of ordered and disordered Ba7F12Cl2 were prepared by flux growth and solid state reactions. These new structures were characterized by single crystal and powder X-ray diffraction. The disordered variant which shows disorder on one of the cation sites was obtained from a BaF2 + BaCl2 + NaCl/NaF flux. It has hexagonal space group P63/m (176) with one formula unit per unit cell. The lattice constants are a = b = 1059.55(5) pm and c = 420.10(4) pm (at 21 °C). The structure was refined to R(Rw) = 0.026(0.030) for 346 independent reflections and 26 parameters.
Slow cooling of a mixture of BaF2 and LiCl yields the ordered variant. This one crystallizes in the hexagonal space group P6 (174) with one formula per unit cell. Lattice constants at 21 °C are a = b = 1063.46(2) pm and c = 417.52(1) pm. The structure was refined to R(Rw) = 0.017 (0.017) for 638 independent reflections and 45 parameters.
The structural arrangement and the interatomic distances of the two variants are mutually similar. The barium atoms have coordination number nine. Propeller-type arrangements with a chloride ion on the axis and the fluoride ions as blades are observed. These latter ones are interconnected into 'channels' of tricapped fluoride prisms. Occupation disorder of the barium sites in the channels of the disordered variant makes the main difference between the two. An unexpectedly high X-ray density obtained for both variants of Ba7F12Cl2 can be correlated to the density of other barium fluorohalides having a coordination number of nine for the barium ion.
  • EPR/ENDOR study of an X-irradiated single crystal of 1-triphenylphosphoranylidene-2-propanone : the role of hydrogen bonds in the trapping of radiogenic radical
    T. Berclaz, G. Bernardinelli, M. Geoffroy, G. Rao and Z. Tancic
    Radiation physics and chemistry, 56 (5-6) , 1999, p539-545
    DOI:10.1016/S0969-806X(99)00295-9 | unige:2736 | Abstract | Article HTML | Article PDF
As shown from the crystal structure, the oxygen atom of Ph3P=CH---C(O)CH3 forms both intra and intermolecular hydrogen bonds. X-irradiation of this compounds produces a room-temperature-stable radical which was studied by single crystal EPR/ENDOR spectroscopy. Comparison of the experimental hyperfine couplings with those obtained from ab initio calculations shows that the radical cation Ph3P+---CH=C(OH)CH2 is formed under radiolysis. The principal directions of the hyperfine tensors indicate that, in this process, some of the hydrogen bonds are broken and that the radical undergoes a drastic reorientation around the Ph3P---C bond.
  • Air-stable crystalline primary phosphines and germanes : synthesis and crystal structures of dibenzobarellenephosphine and tribenzobarellenegermane
    Marcin Brynda, Michel Geoffroy and GĂŠrald Bernardinelli
    ChemComm, (11) , 1999, p961-962
    DOI:10.1039/a901083a | unige:2735 | Abstract | Article PDF
  • Crystal structure of dilithiumsulfide, Li2S
    Franck Kubel, Bernard Bertheville and Hans Bill
    Zeitschrift fĂźr Kristallographie - New Crystal Structures, 214 (3) , 1999, p302
    Keywords: experimental study | XRD | crystal structure | binary compounds | lithium sulfides | inorganic compounds
    unige:2738
Li2S, cubic, Fiiβm (No. 225), a = 5.7158(1) A, V= 186.7 A3, Z = 4, REi(F) = 0.009, Rv.(F) = 0.009, T= 293 K.
  • Binding of Organic Cations to a Cyclophane Host As Studied with Molecular Dynamics Simulations and Free Energy Calculations
    M.A.L. Eriksson, and P.A. Kollman
    Journal of Physical Chemistry B, 103 (21) , 1999, p4474-4480
    DOI:10.1021/jp983513p | unige:2937 | Abstract | Article HTML | Article PDF
We have studied the binding of two organic cations, an iminium (IM) and a guanidinium (GU), to a cyclophane host P4--4Na+, using molecular dynamics simulations and free energy calculations. A proper treatment of the long-range electrostatic forces is essential for the stability of these highly charged complexes, and a simple cutoff at 12 Å results in an artifactual dissociation of the IM−P4--4Na+ complex. Since the host is highly aromatic and the guests cationic, cation−π interactions play an important role in the complex stability. In free energy calculations, using a simple additive force field, we calculate that the relative free energy of association of IM and GU binding to the host is 2.3 kcal/mol favoring IM, which is of the correct sign but 1.4 kcal/mol too small in magnitude. Differences in van der Waals interaction energies are mainly responsible for the different binding strengths, and the host adopts different shapes when accommodating IM compared to GU. To approximately estimate the contribution to the complex stability from the polarization energy, we calculated the in vacuo interaction energies in the two complexes, using a nonadditive force field, previously shown to accurately describe alkali cation−aromatic interaction energies in vacuo. Adding the contribution from the polarization energy upon forming the two complexes in this calculation to the estimate from the free energy calculation, we obtain an improved relative binding free energy (−4.0 kcal/mol), which is in close agreement with the experimental value of −3.7 kcal/mol.
Persistent spectral hole burning was performed on the 7F0–5D1 transition of Sm2+ in thin films of SrFCl. Depending on the substrate and the growth conditions, a total hole depth between 47% and 70% was reached. The holes were Lorentzians of width 4(±0.3) cm−1. The time evolution of the hole depth was studied. It is described by two exponentials: a short time decay (t1 = 0.37 days) and a long time decay (t2 = 20.4 days) with a 20% infinite time limit. One- and two-photon burning mechanisms act.
  • Superexchange and Dipole-Dipole Energy Transfer from the [Cr(ox)3]3- of 3D Oxalate Networks to Encapsulated [Cr(bpy)3]3+
    Vaughan S. Langford, Marianne E. Von Arx and Andreas Hauser
    Journal of Physical Chemistry A, 103 (36) , 1999, p7161-7169
    DOI:10.1021/jp9911301 | unige:2739 | Abstract | Article HTML | Article PDF
Electronic energy transfer from [Cr(ox)3]3- (ox = oxalate) in three-dimensional (3D) anionic oxalate networks to encapsulated [Cr(bpy)3]3+ (bpy = 2,2‘-bipyridine) cations at 1.5 K was investigated by time-resolved luminescence spectroscopy. Two series of mixed crystals of nominal compositions [NaAl1-xCrx(ox)3][Rh0.99Cr0.01(bpy)3]ClO4 (x = 0, 0.01, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, and 1) and [NaAl0.99Cr0.01(ox)3][Rh1-yCry(bpy)3]ClO4 (y = 0, 0.01, 0.02, 0.03, 0.04, and 0.05) were utilized. Energy transfer from [Cr(ox)3]3- to [Cr(bpy)3]3+ occurs by two mechanisms. Rapid, short-range transfer (ket > 106 s-1) is attributed to superexchange coupling between the Cr3+ ions via π overlap of the oxalate and bipyridine ligands. In addition, at low [Cr(ox)3]3- concentrations (nominally x = 0.01) a very much slower process with a maximum ket ≈ 200 s-1 is identified in the time-resolved spectra and attributed to a dipole−dipole mechanism. Furthermore, the resonant [Cr(ox)3]3- to [Cr(ox)3]3- energy migration previously reported by von Arx et al. (Phys. Rev. (1996), B54, 15800) assists [Cr(ox)3]3- to [Cr(bpy)3]3+ transfer as the [Cr(ox)3]3- concentration increases.
  
  • Cooperative phenomena and light-induced bistability in iron(II) spin-crossover compounds
    Andreas Hauser, Jelena Jeftic, Harald Romstedt, Roland Hinek and Hartmut Spiering
    Coordination Chemistry Reviews, 190-192 , 1999, p471-491
    Keywords: iron(II) coordination compounds; spin-crossover; cooperative effects; high-spin<->low-spin relaxation; bistability
    DOI:10.1016/S0010-8545(99)00111-3 | unige:2740 | Abstract | Article HTML | Article PDF
In iron(II) spin-crossover compounds, the transition from the 1A1 low-spin state at low temperatures to the 5T2 high-spin state at elevated temperatures is accompanied by a large increase in metal-ligand bond lengths. The resulting elastic interactions may be pictured as an internal pressure which is proportional to the concentration of the low-spin species. Because pressure stabilises the low-spin state relative to the high-spin state this results in a positive feedback. Thermal transition curves in neat iron(II) spin-crossover compounds are thus invariable much steeper than in diluted mixed crystals, and the high-spin→low-spin relaxation following the light-induced population of the high-spin state at low temperatures is self-accelerating. Strong interactions give rise to a thermal hysteresis, and light-induced bistabilities may be observed for compounds with initially a high-spin ground state and the potential for a light-induced population of the low-spin state. For such compounds, the increasing internal pressure may stabilise the low-spin state sufficiently so that it becomes the molecular ground state above some critical light-induced low-spin fraction. Secondary effects of the elastic interactions include crystallographic phase transitions, inhomogeneous distributions of sites, and anomalies such as steps in the transition curve.
The formalism of the Kohn–Sham equations with constrained electron density is extended to the spin-polarized case. The isotropic hyperfine coupling constants (Aiso(Mg)) of Mg+ embedded in a Ne or Ar matrix represented using a cluster are calculated and compared to that of free Mg+. For the largest basis set used, the calculated values (222.9 and 210.4 gauss for Ar and Ne, respectively) agree with experimental measurements (222.4 and 211.6). The shifts of Aiso(Mg) relative to the values for free Mg+ are rather basis-set-independent.
The ground- and excited-state properties of both [Ru(bz)2]2+ and crystalline bis(η6-benzene)ruthenium(II) p-toluenesulfonate are investigated using the density functional theory. A symmetry-based technique is employed to calculate the energies of the multiplet structure splitting of the singly excited triplet states. For the crystalline system, a Buckingham potential is introduced to describe the intermolecular interactions between the [Ru(bz)2]2+ system and its first shell of neighbor molecules. The overall agreement between experimental and calculated ground- and excited-state properties is good, as far as the absolute transition energies, the Stokes shift, and the geometry of the excited states are concerned. The calculated d-d excitation energies of the isolated cluster are typically 1000-2000 cm-1 too low. An energy lowering is obtained in a1g → e1g(3E1g) excited state when the geometry of [Ru(bz)2]2+ is bent along the e1u Renner-Teller active coordinate. It vanishes as the crystal packing is taken into account.
  • Theoretical investigation of the adsorption of methanol on the (110) surface of γ-alumina
    David A. De Vito, François Gilardoni, Lioubov Kiwi-Minsker, , StĂŠphane Porchet, Albert Renken and Jacques Weber
    Journal of Molecular Structure: THEOCHEM, 469 (1-3) , 1999, p7-14
    Keywords: density functional theory; cluster calculations; ab initio calculations; chemisorption; IR frequency shifts
    DOI:10.1016/S0166-1280(98)00511-9 | unige:2742 | Abstract | Article HTML | Article PDF
The adsorption of methanol on the (110) surface of Îł-alumina was investigated using both ab initio and density functional theory quantum chemical methods. A [Al3O9H10]+ cluster model comprising one tetrahedral and two octahedral aluminum cations were used to describe the surface and the mechanism of adsorption of methanol. This has allowed us to rationalize the stable structures of adsorbate and the mode of bonding. The IR frequency shifts between the gas phase and the adsorbed species were also calculated and they exhibit good agreement with experiment.
The structure and stretching frequency of the CO molecule physisorbed on the MgO(100) surface were investigated using the recently developed formalism of Kohn-Sham equations with constrained electron density (KSCED). The KSCED method makes it possible to divide a large system into two subsystems and to study one of them using Kohn-Sham-like equations in which the effective potential takes into account the interactions between subsystems. Compared to the standard Kohn-Sham formalism, the KSCED method involves an additional functional due to the non-additivity of the kinetic energy. The surface was represented using a cluster ((MgO5)8− or Mg9O9) embedded in an array of electric point-charges. The KSCED calculations led to a blue-shift of the stretching frequency of the C-down adsorbed CO molecule amounting to 47–21 cm−1 depending on the distance from the surface. At the C–Mg distance of 2.42 Å, which corresponds to a typical minimum of the potential energy curve derived from supermolecule Kohn-Sham calculations applying gradient-corrected functionals, the KSCED frequency shift amounts to 35 cm−1 in excellent agreement with the most recent experiments. The CO stretching frequency of the O-down adsorbed CO molecule is red-shifted. The effects of cluster size and choice of the functionals on the KSCED frequencies, geometries and energies were analyzed. For C–Mg distances varying between 2.3 and 3.0 Å, changing the cluster size affects the frequencies by less than 4 cm−1 and the CO bond length by less than 0.0003 Å. At C–Mg distances larger than 2.4 Å, the change of the cluster size negligibly affects the KSCED interaction energies. The KSCED formalism makes it possible to study directly the effects associated with relaxation of the surface's electron density upon adsorbing CO. It is shown that these effects might contribute up to 30% of the KSCED interaction energy, but that they do not result in significant changes of either the geometries or frequencies.
  • Spin Transition in Fe(II) Compounds.
    P. GĂźtlich, A. Hauser and H. Spiering
    in "Inorganic Electronic Structure and Spectroscopy Vol 2" (E.I. Solomon, A.B.P. Lever eds.) Wiley, New York, 1999, p575
    unige:3787 | Abstract
A study of the dynamics of electronic energy transfer (EET) in arrays containing three, four, and six tetraphenylporphine units connected with phenylethynyl spacers is reported. For arrays containing the same chromophores, the EET rate constant was determined from the reorientational dynamics of the transition dipole using the crossed grating technique. EET time constants ranging from 150 ps up to 33 ns were measured, depending on the distance between the chromophores and on the metal ion complexed in the porphyrins. For the trimeric planar arrays, the interchromophoric distance varies by a factor of 2, while the ratio of the through space to through bond distances is constant. By comparing the measured EET rate constants with those calculated using Förster theory, the contributions of the Coulombic, through space, mechanism and of the exchange, through bond, mechanism could be estimated. For the arrays with the shortest spacer (through space distance of 23 Å), EET occurs through both exchange and Coulombic interactions with a ratio of about 3:1. This ratio increases up to about 10 as the distance is increased to 34.5 Å. At 46.5 Å, the ratio decreases and it appears that the Coulombic interaction becomes the dominant mechanism at longer distances. In the tetrahedral compound, the presence of a central saturated carbon strongly alters the electronic conducting properties of the spacer and makes the exchange mechanism inoperative.
The deactivation of the S1(π,π*) excited state of nickel tetraphenylporphine has been investigated using various transient grating techniques. By measuring the density changes of the sample occurring during this process, the excited state, that is responsible for the ground state recovery time of 250 ps, was determined to lie 1.18±0.13 eV above the ground state. This value suggests that this state is the1(d,d) state.
The solvation dynamics of an organic dye, IR140, in methanol, ethanol, and in a series of six alkanenitriles has been investigated using the transient grating technique. In all solvents, the dynamics exhibit ultrafast, almost solvent-independent, components ascribed to inertial solvation, and a slower viscosity-dependent component, due to diffusive solvation. The relative amplitudes of these components depend on both the solvent and on the wavelength at which the experiment is performed. The contribution of inertial motion increases with decreasing size of the solvent molecules and with decreasing wavelength. It appears that diffusive motion is associated with a loose solvent shell, while inertial motion dominates when the solvation layer is dense.
A "CO-like matrix", showing coordination analogous to that of carbonyl groups, is provided by silacalix[4]phosphinine macrocycles. Reaction with AuI leads to the first gold(I) complexes of macrocycles, which can be reduced with sodium or potassium to the paramagnetic gold(0) complexes (an example is shown), as evidenced by cyclic voltammetry and EPR spectroscopy.
Single crystals of the new host Ba12F19Cl5 doped with Eu2+ were grown and studied by electron paramagnetic resonance (EPR) and luminescence emission spectroscopy. Three different Eu2+ sites were observed. Two of them had orthorhombic point symmetry while the last one was monoclinic. Physico-chemical and symmetry arguments allowed us to establish correspondence between the different Eu2+centres and the host cation lattice sites. All three centres presented in their ground state important crystal field splitting. The 80 K luminescence emission spectrum consisted of one broad unsymmetrical f-d band peaking at 22 700 cm-1. No 4f-4f transitions of the Eu2+ ion were observed between room temperature and 80 K.
 
We have studied the solid-liquid equilibrium of the system Sr1–yBayFCl1–xBrx using DTA and X-ray diffraction techniques. The entire composition range in this system yields solid solutions which crystallize in the PbFCl (Matlockite) structure type. The melting points of the entire composition range have been parametrized (within 5°C rms error) using a biquadratic fit of the available data obtained by experiment and from the literature.
The interaction of ethylene adsorbed on Ni(111) with gas-phase H atoms has been investigated. The major adsorbed reaction product is identified by high-resolution electron energy loss spectroscopy to be ethylidyne (C−CH3). This study is the first direct spectroscopic observation of a C−CH3 species adsorbed on Ni in an ultrahigh-vacuum environment. Spectra of four isotopomers, C−CH3, 13C−13CH3, C−CD3, and 13C−13CD3, are reported, and a complete and consistent vibrational assignment of their fundamental modes is presented. Based on this assignment, a force field is derived from the measured vibrational frequencies using a normal-modes analysis and is found to be in good agreement with that deduced from IR spectra of an ethylidyne species in an organometallic complex. Inspection of the eigenvectors of the normal-mode displacements reveals that substantial mixing of harmonic bond motions is the origin of the unusual upshift in frequency of the C−C stretching mode upon deuteration. A quantitative determination of the relative dynamic bond dipole moments demonstrates that the changes in intensity and dipole activity of the C−C stretching and symmetric CH3 deformation modes upon deuteration, phenomena common to all C−CD3 spectra, also arise from extensive mixing of bond motions. A detailed analysis of the spectra strongly suggests a C3v or C3 local environment for ethylidyne and a 3-fold hollow adsorption site.
We report that both surface-bound H atoms and bulk H atoms, upon moving out from the bulk of a Ni single crystal to its surface of a (111) orientation, are reactive with adsorbed C2H2, but the two kinds of H atoms have unique product distributions. Both bulk H and surface-bound H react with C2H2 to produce adsorbed ethylidyne, CCH3, while only bulk H hydrogenates C2H2 to gas-phase ethylene and ethane, the products of interest in acetylene hydrogenation catalysis for the purification of ethylene streams. Their distinct reactivities arise from both their different directions of approach to the π orbitals of the unsaturated hydrocarbon and their substantially different energetics. These observations demonstrate that H embedded in the metal catalyst is a reactant in alkyne hydrogenation and is not solely a source of surface-bound H which then reacts with acetylene, as proposed from correlations between the hydrogenation activity of Raney Ni and Pd catalysts and the amount of H absorbed in these catalysts. The reactivities of these two kinds of H atoms are clearly distinguished in this experiment because of the capability to synthesize either bulk H or surface-bound H cleanly in an ultrahigh vacuum environment.
The conformation of cinchonidine in solution has been investigated by NMR techniques as well as theoretically. Three conformers of cinchonidine are shown to be substantially populated at room temperature, Closed(1), Closed(2), and Open(3), with the latter being the most stable in apolar solvents. The stability of the closed conformers relative to that of Open(3), however, increases with solvent polarity. In polar solvents the three conformers have similar energy. The relationship between relative energies and the dielectric constant of the solvent is not linear but resembles the form of an Onsager function. Ab initio and density functional reaction field calculations using cavity shapes determined by an isodensity surface are in good agreement with experiment for solvents which do not show strong specific interaction with cinchonidine. The role of the conformational behavior of cinchonidine is illustrated using the example of the platinum-catalyzed enantioselective hydrogenation of ketopantolactone in different solvents.
  • Optical spectroscopy of the Ag+ ion in NaF: Experimental results and analysis of manifestations of the Jahn-Teller effect
    H. Bill, G.J. Hollingsworth, D.S. McClure, B. Moine and Ch. Pedrini
    Journal of Chemical Physics, 109 (17) , 1998, p7328-7337
    Keywords: sodium compounds | silver | photoluminescence | two-photon spectra | impurity absorption spectra | ultraviolet spectra | Jahn-Teller effect
    DOI:10.1063/1.477362 | unige:2948 | Abstract | Article PDF | Article PS (gzipped)
The 4d10→4d95s1 transitions of the Ag+ ion in single crystals of NaF were investigated by one- and two-photon spectroscopy and luminescence experiments. The one-photon absorption spectrum extends from 238 to below 180 nm. At low temperature, several bands show resolved fine structure, but compared to NaF:Cu+, there is much less, and the lines are broader. The Jahn–Teller effect is much larger in Ag+ than in Cu+ and has been identified in three of the excited states. Trapping in Jahn–Teller minima is shown to change the emission kinetics radically compared to Cu+. The off-center force from the d9p states has much less effect on Ag+ than on Cu+. All of the evidence shows that Ag+ is more strongly coupled to the lattice than is Cu+.
  • Thin film materials with Sm(III) far room temperature hole burning: design and photomechanism studies
    Raivo Jaaniso, Tea Avarmaa, Markko Paas, Marc Schnieper, FrĂŠdĂŠric Trotta and Hans Bill
    Molecular Crystals and Liquid Crystals, 314 (17) , 1998, p155-160
    Keywords: spectral hole burning | disordered crystals | thin films | samarium | molecular beam deposition | pulsed laser deposition
    DOI:10.1080/10587259808042472 | unige:2949 | Abstract | Article PDF
Multicomponent thin films with spectral hole burning capacity at room temperature were synthesized by using molecular beam and pulsed laser deposition techniques All materials were activated by Sm2+ in low-symmetry alkaline earth sites, the synthesis involved the control of ionic diffiision rate during multilayer growth and special reduction of Samarium. Enhancement of hole burning rate by 1-2 orders is obtained in nanocrystalline films as compared to bulk and microcrystalline materials New hypothetic mechanism involving the creation of Sm-defect (photochromic) centers is proposed for reversible photoburning.
  • A theoretical study of the N8 cubane to N8 pentalene isomerization reaction
    Laura Gagliardi, Stefano Evangelisti, Per-Olof Widmark and BjĂśrn O. Roos
    Theoretical Chemistry Accounts, 97 (1-4) , 1998, p136-142
    Keywords: CASPT2 | N8 isomers | transition states | perturbation theory | high energy density material | multiconfigurational SCF theory
    DOI:10.1007/s002140050246 | unige:2950 | Abstract | Article PDF
The isomerization reaction of cubic N8 to the planar bicyclic structure analogous to pentalene has been investigated using multiconfigurational self-consistent field and second-order perturbation theory (CASPT2). Comparative calculations using density functional theory have also been performed. Five local minima on the energy surface have been found, and the transition states between each two consecutive minima have been determined. The results show that all steps in the isomerization process, except one, can proceed via a set of transition states with moderately high energy barriers (10–20kcal/mol).
The HF/3s2pld and MP2/3s2pld structures, energies and vibrational frequencies were calculated for ten N8 isomers, corresponding to ten analogous CH structures. Comparative calculations using density functional theory (DFT), with a cc-pVTZ basis set, were also performed. All ten structures were found to be local minima on the energy hypersurface at the Hartree-Fock (HF) level, whereas at the second-order MĂśller-Plesset (MP2) level nine structures were stable. At the DFT level, eight local minima were found. The total energies were recomputed using 4s3p2dlf basis sets at the HF and MP2 levels of theory.
  • A two-centre implementation of the Douglas-Kroll transformation in relativistic calculations
    Laura Gagliardi, Nicholas C. Handy, Andrew G. Ioannou, Chris-Kriton Skylaris, Steven Spencer, Andrew Willetts and Adrian M. Simper
    Chemical Physics Letters, 283 (3-4) , 1998, p187-193
    DOI:10.1016/S0009-2614(97)01346-8 | unige:2952 | Abstract | Article PDF
An implementation of the Douglas–Kroll (DK) transformation is described within a new relativistic quantum chemistry code, MAGIC, which performs calculations on systems containing heavy atoms. This method reduces the computational cost in terms of memory requirements that are associated with completeness identities in the DK implementation by factorizing the one-electron matrices into smaller ones that depend only on two atoms at a time. Examples are presented.
  • A Relativistic Density Functional Study on the Uranium Hexafluoride and Plutonium Hexafluoride Monomer and Dimer Species
    Laura Gagliardi, Andrew Willetts, Chris-Kriton Skylaris, Nicholas C. Handy, Steven Spencer, Andrew G. Ioannou and Adrian M. Simper
    Journal of the American Chemical Society, 120 (45) , 1998, p11727-11731
    DOI:10.1021/ja9811492 | unige:2953 | Abstract | Article HTML | Article PDF
A study on the UF6 monomer and dimer was carried out within the density functional method. The U−F distance in the UF6 monomer was optimized at different levels of theory, pointwise, assuming octahedral geometry, (1) by using an all-electron basis for both U and F in a nonrelativistic calculation; (2) by using a relativistic effective core potential (RECP) on U and nonrelativistic effective core potential (ECP) on the fluorines; and (3) by using RECP on the U atom and an all-electron basis on the F atoms. Atomization energies of 23.11, 33.92, and 35.66 eV were obtained at the three levels, respectively. Relativistic effects lead to about a 50% increase in the atomization energy. For the UF6 dimer, the potential energy curve, as a function of the intermolecular U−U distance, was computed at level 2, and the rotational barrier between the two monomers was determined. Similar calculations were performed on the corresponding PuF6 species. Comparisons are made with experiment and other theoretical studies, where available.
  • An efficient method for calculating effective core potential integrals which involve projection operators
    Chris-Kriton Skylaris, Laura Gagliardi, Nicholas C. Handy, Andrew G. Ioannou, Steven Spencer, Andrew Willetts and Adrian M. Simper
    Chemical Physics Letters, 296 (5-6) , 1998, p445-451
    DOI:10.1016/S0009-2614(98)01077-X | unige:2954 | Abstract | Article HTML | Article PDF
An efficient approach for evaluating effective core potential integrals which involve projection operators has been implemented in the MAGIC quantum chemistry program. The methodology is presented and its performance is examined through illustrative calculations on transition metal and actinide compounds.
  • Modulation of iron reduction potential by deprotonation at a remote site
    Riccardo F. Carina, Ludovica Verzegnassi, GĂŠrald Bernardinelli and Alan F. Williams
    ChemComm, 296 (24) , 1998, p2681-2682
    DOI:10.1039/a807321j | unige:2756 | Abstract | Article PDF
Remote site deprotonation of a coordinated imidazole ligand switches the reduction potential of coordinated iron over a narrow pH range from +0.920 to –0.460 V.
We present a local density functional study of Cd4S and Cd4S4 clusters inside sodalite cages of different compositions (Al:Si ratios). The composition of the framework determines the cluster → cage charge transfer and strongly affects the atomic structure of the inclusion. The energy gap and the character of the highest occupied (HOMO) and lowest unoccupied (LUMO) electronic states depend on the size and stoichiometry of the included clusters, as well as on the overall stoichiometry of the composite. The calculated gap for Cd4S inclusions in aluminosilicate and aluminate sodalite (at half and full packing respectively) is 2.5 eV (i.e., about twice the calculated gap of bulk CdS), while for Cd4S4 in aluminosilicate and pure silica sodalite (at half packing) it is 1.7-1.9 eV (i.e., about 1.5 times the gap of bulk CdS). Our results indicate that simple confinement arguments are usually insufficient to predict the behavior of semiconductor−zeolite composites.
  • Density Functional Study of Protonated, Acetylated, and Mercurated Derivatives of Ferrocene: Mechanism of the Electrophilic Substitution Reaction
    MarĂ­a J. Mayor-Lopez, Jacques Weber, Berit Mannfors and Allan F. Cunningham
    Organometallics, 17 (23) , 1998, p4983-4991
    DOI:10.1021/om980218i | unige:2758 | Abstract | Article HTML | Article PDF
The mechanism of the electrophilic substitution reaction of ferrocene has been investigated using density functional theory. In particular, reactions with two hard electrophiles (protonation and acetylation) and one soft electrophile (mercuration) have been studied at the LDA and B-PW91 levels of theory using a triple-Îś STO basis set. A general description of the reactions has been obtained, leading to results in agreement with experiment. Acetylation is found to occur via exo attack, whereas mercuration follows an endo mechanism. In the case of protonation, evidence for a rapid equilibrium between metal-protonated and agostic ring-protonated ferrocene is obtained, and no clear conclusion concerning the exo or endo mechanism can be deduced. The calculated proton affinities corresponding to both metal-protonated and agostic ring-protonated structures are in excellent agreement with experiment.
  • Structure and reactivity of [Mo3-µ3S-(µS2)3]4+ complexes : quantum chemical calculations, X-ray structural characterization and Raman spectroscopic measurements
    Maria J. Mayor-Lopez, Jacques Weber, Kaspar Hegetschweiler, Marc D. Meienberger, Felix Joho, Stefano Leoni, Reinhard Nesper, Guido J. Reiss, Walter Frank, Boris A. Kolesov, Vladimir P. Fedin and Vladimir E. Fedorov
    Inorganic Chemistry, 37 (11) , 1998, p2633-2644
    DOI:10.1021/ic971214t | unige:2956 | Abstract | Article HTML | Article PDF
A series of compounds containing the [Mo3-μ3S-(μS2)3-(dtc)3]+ complex (dtc = diethyldithiocarbamate) with the anions I- (1), I- and Br- (2), S2- (3), ClO4- (4), NO3- (5), and SO42- (6) was prepared and characterized by elemental analysis, NMR, IR, and Raman spectroscopy, and FAB mass spectrometry. The previously reported crystal structure of 1 was reinvestigated. The X-ray analysis revealed the incorporation of CH2Cl2 in the crystal having the composition [Mo3S7(dtc)3]I·0.5CH2Cl2 (1a), which was in contradiction to the previous protocol. The corresponding ClO4- compound (4a) is isotypic. Crystal data: C15.5H31Cl2Mo3N3O4S13, orthorhombic space group Aba2, a = 25.816(5) Å, b = 17.761(4) Å, c = 16.250(3) Å, Z = 8. For 1a, 4a, 6, and the previously analyzed 2 and 3 the crystal structures revealed characteristic interactions between the anions X and the three axial (out-of-plane) sulfur atoms Sax of the disulfido bridges. The Raman data showed a significant decrease of the Seq−Sax stretch resonance frequency in the order 4, 5, 6 > 1 > 3. This decrease is paralleled with a slight increase of the Seq−Sax bond length and with a significant shortening of the X···Sax distances when compared to the sum of the corresponding van der Waals radii. A comprehensive quantum chemical study, using both density functional theory and semiempirical calculations, revealed that for hard counterions such as NO3- and ClO4- the Sax···X interactions can be understood in terms of an almost entirely electrostatic interaction, whereas for soft nucleophiles such as I- and S2- significant covalency is observed. In addition, the general reaction of [Mo3S7]4+ complexes with a nucleophile was modeled. With regard to the side-on bonding of the μ-S2 groups to Mo, the calculations indicated a significantly higher bond energy for the axial (out-of-plane) sulfur atoms, explaining the much higher lability of the sulfur atoms in the equatorial (in-plane) position. Analogous differences for the ligating atoms of the peripheral ligands, having a cis and trans position with respect to μ3-S, are less pronounced.
  • Presentation of the Section for Chemical Research (SCR) of the New Swiss Chemical Society (NSCS)
    Jacques Weber
    Chimia, 52 (4) , 1998, p154-155
    unige:2759 | Abstract | Article PDF
The Section for Chemical Research (SCR) of the NSCS intends to provide a forum for chemists active in research, so as to promote exchange of ideas and collaboration. The SCR organizes or supports symposia, seminars, and scientific meetings, with a special emphasis on the Fall Meeting of the NSCS, which is de facto the largest annual forum of Swiss chemistry. Particular attention is paid by the SCR to the promotion of research results obtained by young chemists so as to help them starting their career. At present, the SCR membership stands at 465 members.
  • Numerical evalutation of the internal orbitally resolved chemical hardness tensor : second order chemical reactivity through thermal density functional theory
    Martin G. Grigorov, Jacques Weber, Nathalie Vulliermet, Henry Chermette and Jean M.J. Tronchet
    Journal of chemical physics, 108 (21) , 1998, p8790-8798
    Keywords: water | organometallic compounds | chemical reactions | density functional theory | chemical potential | tensors
    DOI:10.1063/1.476325 | unige:2760 | Abstract | Article PDF | Article PS (gzipped)
In a previous work we have presented a numerical procedure for the calculation of the internal chemical hardness tensor at the molecular orbital resolution level from standard density functional calculations. In this article we describe an improvement of our method using the thermal extensions of density functional theory. Furthermore, new concepts are introduced in the orbitally resolved theory of chemical reactivity. Traditional molecular orbital theories of chemical reactivity are based only on considerations concerning the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) of molecules, supposed to describe the behavior towards electrophiles, respectively, nucleophiles. By applying our methodology to two test molecular systems, namely water and ferrocene, we show how chemical reactivity can be differentiated against hard and soft electrophiles (acids) and hard and soft nucleophiles (bases). As a by-product of the numerical algorithms being used, a self-consistent method for calculating the molecular chemical potential is also described.
Experiments in which the Raman linewidth was measured as a function of temperature (7-1183 K) and pressure (0-400 bar) were performed on the (111) and (100) planes of single crystals of the cubic anti-fluorite Li2S. The temperature dependence of the lattice constant was determined by x-ray diffraction (11-295 K). From these results and published Brillouin scattering data for this host, the volume thermal expansion coefficient as a function of temperature was obtained as well as the isothermal compressibility and the isothermal Raman mode Grüneisen parameter. Using the thermodynamic approach within the quasi-harmonic approximation, we show that below 400 K the volume effects describe well the temperature dependence of the Raman linewidth whereas above this temperature there are direct anharmonic effects appearing. Above approximately 850 K new Raman lines appear that are A⊥ and E polarized.
  • Bent tridentate receptors in calamitic mesophases with predetermined photophysical properties : new luminescent lanthanide-containing materials
    Homayoun Nozary, Claude Piguet, Paul Tissot, GĂŠrald Bernardinelli, Jean-Claude G. BĂźnzli, Robert Deschenaux and Daniel Guillon
    Journal of the American Chemical Society, 120 (47) , 1998, p12274-12288
    DOI:10.1021/ja982545n | unige:2752 | Abstract | Article HTML | Article PDF
A new synthetic strategy has been developed to introduce bent and rigid tridentate 2,6-bis(benzimidazol-2‘-yl)pyridine cores into rodlike ligands L11-17. The crystal structure of the nonmesogenic ligand L13 (C39H37N5O4, triclinic, P, Z = 2) shows the expected trans−trans conformation of the tridentate binding unit, which provides a linear arrangement of the semirigid aromatic sidearms. The crystal structure of the related mesogenic ligand L16 (C61H81N5O4, triclinic, P, Z = 2) demonstrates the fully extended conformation adopted by the lipophilic side chains, leading to a slightly helically twisted I-shaped molecule. A rich and varied mesomorphism results which can be combined with the simultaneous tuning of electronic and photophysical properties via a judicious choice of the spacers between the rigid central core and the semirigid lipophilic sidearms. Ligands L13,14 react with Ln(NO3)3·xH2O to give quantitatively and selectively the neutral 1:1 complexes [Ln(Li)(NO3)3] (Ln = La to Lu), which are stable in the solid state at room temperature but partially dissociate in acetonitrile to give the cationic species [Ln(Li)(NO3)2]+. The crystal structure of [Lu(L13)(NO3)3]·3CH3CN (30, LuC45H46N11O13, monoclinic, C2/c, Z = 8) reveals a U-shaped arrangement of the ligand strand arising from the cis−cis conformation of the coordinated tridentate binding unit. This drastic geometric change strongly affects the thermal behavior and the photophysical and electronic properties of the lipophilic complexes [Ln(L14)(NO3)3]. Particular attention has been focused on structure−properties relationships, which can be modulated by the size of the lanthanide metal ions.
  • Applications of Density Functional Theory to Biological Systems
    T.A. Wesolowski and J. Weber
    in "Molecular Orbital Calculations Applied to Biochemical Systems" (A.-M. Sapse ed.), Oxford University Press, 1998, p85-132
  • Teaching Computational Chemistry
    J. Weber, and H.P. LĂźthi
    in "Encyclopedia of Computational Chemistry" (P.v.R. Schleyer, N.L. Allinger, T. Clark, J. Gasteiger, P.A. Kollman, H.F. Schaefer and P.R. Schreiner, eds.), Wiley, Chichester, 1998, p2960
  • Investigation of the structural and acidic properties of bulk offretite using first principles molecular dynamics
    J. Weber, L. Campana, A. Selloni, A. Pasquarello, I. Papai and A. Goursot
    in "Thermodynamic Modeling and Materials Data Engineering" (J.P. Caliste, A. Truyol and J.H. Westbrook, eds.), Springer, Berlin, 1998, p129-134
Theoretical studies on structure and stretching frequency of the CO molecule physisorbed on the MgO(100) or ZnO(1010) surfaces are reported. The properties of the adsorbed molecule were investigated by means of the recently developed formalism of Kohn-Sham equations with constrained electron density (KSCED). The KSCED method makes it possible to divide a large system into two subsystems and to study one of them using Kohn-Sham-like equations with an effective potential which takes into account the interactions between subsystems. This method (KSCED) was shown to be adequate to study the properties of the CO molecule adsorbed on the MgO(100) surface as reported in a previous paper (Wesolowski et. al.: J. Mol. Struct., THEOCHEM, in press). The effect of the interactions with the surface on the CO stretching frequency and geometry was analyzed for vertically bound (C-down) CO at the Zn-site of the ZnO(1010) surface. The ZnO(1010) surface was represented using several cluster models: Zn2+, (ZnO3)4-, or Zn9O9 embedded in a matrix of point charges. The KSCED frequency shift of the CO stretching vibration is blue-shifted and in good agreement with experiment.
The migrative insertion of CO into the Ni–CH=CH2 bond has been investigated by both static and dynamic density functional methods. The stationary points of the potential surface for the migrative insertion of CO into the Ni–CH=CH2 bond have been characterized using Cl(CO)2Ni–CH=CH2 as a model compound. Such a reaction has been found exothermic by 16 kJ mol−1, with an energy barrier of 9 kJ mol−1. Dynamic simulations have also been performed on Cl(CO)2Ni–CH=CH2 and show that the migrative insertion begins from the cis isomer and occurs via a simultaneous detachment of the vinyl group from the metal and formation of the vinyl–carbonyl bond.
  • A comparison of ground- and excited-state properties of gas phase and crystalline ruthenocene using density functional theory
    F. Gilardoni, J. Weber, A. Hauser and C. Daul
    Journal of Chemical Physics, 109 (4) , 1998, p1425-1434
    DOI:10.1063/1.476693 | unige:2943 | Abstract | Article PDF
The ground- and excited-state properties of both gas phase and crystalline ruthenocene, Ru(cp)2, are investigated using density functional theory. A symmetry-based technique is employed to calculate the energies of the multiplet splittings of the singly excited triplet states. For the crystalline system, a Buckingham potential is introduced to describe the intermolecular interactions between a given Ru(cp)2 molecule and its first shell of neighbors. The overall agreement between experimental and calculated ground- and excited-state properties is very good as far as absolute transition energies, the Stokes shift and the geometry of the excited states are concerned. An additional energy lowering in the 3B2 component of the 5a1′→4e1″ excited state is obtained when the pseudolinear geometry of Ru(cp)2 is relaxed along the low-frequency bending vibration.
The condensed Fukui functions fk of maleimide (1H-pyrrole-2,5-dione) have been calculated using a numerical integration scheme implemented in the deMon program package. The condensed functions show that soft nucleophiles interact with the Îą carbon atoms, whereas hard nucleophiles interact with the carbonyl carbon atoms, in accordance with the experimental evidence. The present method yields extremely few dispersed values of fk, whatever the basis sets, the numerical grids, and the exchange-correlation functionals used. Finally, the validity of the method has been successfully tested on a set of organic and organometallic molecules.
  • Density functional theory with an approximate kinetic energy functional applied to study structure and stability of weak van der Waals complexes
    T.A. Wesolowski, Y. Ellinger and J. Weber
    Journal of Chemical Physics, 108 (15) , 1998, p6078-6083
    DOI:10.1063/1.476018 | unige:2757 | Abstract | Article PDF
n view of further application to the study of molecular and atomic sticking on dust particles, we investigated the capability of the “freeze-and-thaw” cycle of the Kohn–Sham equations with constrained electron density (KSCED) to describe potential energy surfaces of weak van der Waals complexes. We report the results obtained for C6H6⋯X(X=O2, N2, and CO) as test cases. In the KSCED formalism, the exchange-correlation functional is defined as in the Kohn–Sham approach whereas the kinetic energy of the molecular complex is expressed differently, using both the analytic expressions for the kinetic energy of individual fragments and the explicit functional of electron density to approximate nonadditive contributions. As the analytical form of the kinetic energy functional is not known, the approach relies on approximations. Therefore, the applied implementation of KSCED requires the use of an approximate kinetic energy functional in addition to the approximate exchange-correlation functional in calculations following the Kohn–Sham formalism. Several approximate kinetic energy functionals derived using a general form by Lee, Lee, and Parr [Lee et al., Phys. Rev. A. 44, 768 (1991)] were considered. The functionals of this type are related to the approximate exchange energy functionals and it is possible to derive a kinetic energy functional from an exchange energy functional without the use of any additional parameters. The KSCED interaction energies obtained using the PW91 [Perdew and Wang, in Electronic Structure of Solids ’91, edited by P. Ziesche and H. Eschrig (Academie Verlag, Berlin, 1991), p. 11] exchange-correlation functional and the kinetic energy functional derived from the PW91 exchange functional agree very well with the available experimental results. Other considered functionals lead to worse results. Compared to the supermolecule Kohn–Sham interaction energies, the ones derived from the KSCED calculations depend less on the choice of the approximate functionals used. The presented KSCED results together with the previous Kohn–Sham ones [Wesołowski et al., J. Phys. Chem. A 101, 7818 (1997)] support the use of the PW91 functional for studies of weakly bound systems of our interest.
  • Synthesis and reactivity of tethered h1:h6-(phosphinoarene)ruthenium dichlorides
    B. Therrien, T.R. Ward, M. Pilkington, C. Hoffmann, F. Gilardoni and J. Weber
    Organometallics, 17 (3) , 1998, p330-337
    DOI:10.1021/om970735l | unige:2761 | Abstract | Article HTML | Article PDF
The coordination properties of ortho- and meta-substituted [(2-diphenylphosphanylethyl)phenyl]methanol 4a and 4b toward ruthenium(II) have been investigated. To ensure coordination of both the arene and the tethered phosphine, the labile ruthenium arene dimer [RuCl2(EtO2CC6H5)]2 (7) was synthesized and structurally characterized. Both the ortho and meta isomers [Ru(4a)Cl2] (9a) and [Ru(4b)Cl2] (9b) were characterized by X-ray crystallography. The lack of reactivity of the benzylic alcohol functionality in complexes 9a and 9b toward various P and C electrophiles is rationalized with extended HĂźckel calculations.
Temperature-dependent laser flash photolysis experiments on the low-spin iron(II) systems [M1−xFex(bpy)3](PF6)2 (M=Cd, Mn and Zn,x≈0.01, bpy=2,2′-bipyridine) under external pressure are presented. Below 50 K the high-spin→low-spin relaxation is an almost temperature-independent tunnelling process. Above that temperature it tends towards a thermally activated behaviour. A change of the host from cadmium to zinc results in an increase of the low-temperature tunnelling rate constant by two orders of magnitude. An external pressure of 1 kbar accelerates the low-temperature tunnelling process by a factor of 2. [Mn1−xFex(bpy)3](PF6)2 and [Zn1−xFex(bpy)3](PF6)2show a phase transition at ≈1.1 kbar, which increases the tunnelling rate by a factor of about 6.
  • High-spin -> low-spin relaxation in [Zn1-xFex(6-mepy)3-y(py)ytren](PF6)2
    S. Schenker, A. Hauser, W. Wang and I.Y. Chan
    Journal of Chemical Physics, 109 (22) , 1998, p9870-9878
    DOI:10.1063/1.477681 | unige:2763 | Abstract | Article PDF
The thermal spin transition in the diluted mixed crystal [Zn1−xFex(6-mepy)3tren](PF6)2 (x = 0.00025, (6-mepy)3tren = tris{4-[(6-methyl)-2-pyridyl]-3-aza-3-butenyl}amine) is studied at 1 bar and 1 kbar by temperature-dependent absorption spectroscopy. From thermodynamic analysis of the high-spin (HS) fractions, values for ΔHHL0 and ΔSHL0 of 1551(50) cm−1 and 7.5(5) cm−1/K and a molecular volume of reaction, ΔVHL0, of 22(2) Å3result. Reconsideration of the cooperative effects in the neat [Fe(6-mepy)3tren](PF6)2from Adler et al. [Hyperfine Interact. 47, 343 (1989)] result in a lattice shift, Δ, of 208(15) cm−1 and an interaction constant, Γ, of 109(15) cm−1. Temperature-dependent laser flash photolysis experiments in the spin-crossover system [Zn1−xFex(6-mepy)3tren](PF6)2 and the LS system [Zn1−xFex(py)3tren](PF6)2 in the pressure range between 1 bar and 1 kbar are presented. Above ≈100 K the HS→LS (low-spin) relaxations behave classically, whereas they become almost temperature independent below 50 K. At ambient pressure, the low-temperature tunneling rate constant in[Zn1−xFex(py)3tren](PF6)2 is more than three orders of magnitude larger than the one in[Zn1−xFex(6-mepy)3tren](PF6)2. External pressure of 27 kbar accelerates the low-temperature tunneling process by almost nine orders of magnitude. The kinetic results are discussed within the theory of nonadiabatic multiphonon relaxation.
  • High-spin → low-spin relaxation in the two-step spin-crossover compound [Fe(pic)3]Cl2·EtOH (pic = 2-picolylamine)
    H. Romstedt, A. Hauser and H. Spiering
    Journal of Physics and Chemistry of Solids, 59 (2) , 1998, p265-275
    Keywords: Fe(II) compunds, high spin-low spin relaxation, two-step spin transition
    DOI:10.1016/S0022-3697(97)00142-X | unige:2764 | Abstract | Article PDF
The spin-crossover compound [Fe(pic)3]Cl2EtOH (pic = 2-picolylamine) shows an unusual two-step spin transition. This is thought to be caused by specific nearest-neighbour interactions and short-range correlations and requires a theoretical treatment of the elastic interactions between the spin-changing molecules beyond the mean-field approximation. Such short-range correlations also influence the high-spin → low-spin relaxation following the light-induced population of the high-spin state at cryogenic temperatures, leading to characteristic deviations from the predictions of a mean-field treatment. These deviations are directly observable by comparison of the full and unperturbed relaxation curves with curves for which the short-range correlations were destroyed using an appropriate irradiation technique. Monte Carlo simulations including both nearest-neighbour and long-range interactions give a description of the observed relaxation curves which is consistent with the thermal spin equilibrium.
The enthalpy of formation of the free ions generated by the electron transfer between benzophenone in the lower triplet state and 1,4-diazabicyclo[2,2,2]octane in acetonitrile has been measured using transient thermal phase grating. This enthalpy is smaller by more than 0.14±0.08 eV than the enthalpy of formation of the geminate ion pair obtained from a previous ps thermal phase grating investigation. Therefore, the dissociation of a geminate ion pair into free ions in endothermic. However, separation is exergonic if translational entropy in taken into account. A small and positive value of C, the correction term in the Rehm—Weller equation, is obtained if this term is considered as a free energy. The principles of the transient grating technique as a tool for investigating photoinduced processes in solution are briefly reviewed.
  • Direct Investigation of the Dynamics of Charge Recombination Following the Fluorescence Quenching of 9,10-Dicyanoanthracene by Various Electron Donors in Acetonitrile
    E. Vauthey, C. Hoegemann and X. Allonas
    Journal of Physical Chemistry A, 102 (38) , 1998, p7362-7369
    DOI:10.1021/jp981367h | unige:2945 | Abstract | Article HTML | Article PDF
The dynamics of the intermediate generated upon diffusional electron transfer (ET) quenching of 9,10-dicyanoanthracene by electron donors of varying oxidation potential in acetonitrile has been investigated using several transient grating techniques. With most of the donor/acceptor pairs studied, the transient grating spectrum cannot be differentiated from those of the free ions. Exciplex fluorescence, with the same lifetime as that of the ion pair, is observed with all donors. To extract from the measured kinetics the rate constant of exciplex dissociation, kEXdia , and of back ET, kEXBET , within these exciplexes, three different schemes have been considered. The best agreement is obtained by assuming that charge recombination predominantly takes place within the exciplex. The obtained kEXBET values are substantially different from the BET rate constants deduced indirectly from the free-ion yields and with a donor-independent rate constant of separation. For each class of donors, kEXBET exhibits a logarithmic free energy dependence with a slope of about −2 eV-1. Moreover, kEXdia is not constant but increases continuously with diminishing donor's oxidation potential.
  • Investigation of the Competition between Electron and Energy Transfer in the Quenching of Aromatic Ketones in the Triplet State using Picosecond Transient Grating Spectroscopy
    C. Hoegemann and E. Vauthey
    Journal of Physical Chemistry A, 102 (49) , 1998, p10051-10059
    DOI:10.1021/jp981810o | unige:2946 | Abstract | Article HTML | Article PDF
The competition between electron transfer (ET) and triplet energy transfer (TT) in the quenching of benzophenone, xanthone, and anthraquinone in the triplet state by molecules with both a sufficiently small oxidation potential and low triplet state was investigated in the picosecond to microsecond time scales. In the longer time scale, the product distribution depends strongly on the relative exergonicity of ET and TT processes, the yield of the lower energy product being at least four times larger than that of the other product. Picosecond transient grating measurements reveal that if TT is more exergonic than ET, the TT product is predominantly formed by two sequential ET reactions, i.e., by spin-allowed back ET within the triplet geminate ion pair formed upon ET quenching. However, if ET is more exergonic than TT, no conversion from the TT product to the ET product could be detected. In this case, the product distribution in the microsecond time scale seems to reflect the competition between the two processes. When both processes are exergonic, ET appeared to be always faster than TT. This is in agreement with the severe orbital overlap requirement for TT via the Dexter exchange mechanism.
  • Picosecond Time Resolved Dispersion Spectroscopy Using the Interference between Population and Thermal Phase Gratings
    C. Hoegemann and E. Vauthey
    Israel Journal of Chemistry, 3 , 1998, p181
    unige:2947
  • Synthesis and conformational analysis of a novel type of spin labelled bicyclonucleoside based on a tetrahydrofurano(2,3-c)pyrrolidine sugar skeleton
    J.M.J. Tronchet, L. Brenas, F. Barbalat-Rey, M. ZsĂŠly and M. Geoffroy
    Nucleosides, Nucleotides and Nucleic Acids, 17 (6) , 1998, p1019-1031
    DOI:10.1080/07328319808004218 | unige:2765 | Abstract | Article PDF
Bicyclonucleosides bearing a 5-deoxy-5-N-hydroxyamino-3,N5-(1,1-ethano)-β-o-furanosyl sugar moiety (15-18) have been prepared by glycosidation of the corresponding bicyclosugars obtained via an intramolecular reverse Cope elimination. The configuration of the asymmetric carbon of the 1,1-ethano bridge is the most important factor directing the conformation of the N-hydroxypyrrolidine ring and its invertomers ratio as shown by variable temperature H NMR experiments.
  • Hindered rotation around a C-.PH bond A single-crystal EPR study of the diphenyldibenzobarrelenephosphinyl radical
    M. Brynda, T. Berclaz, M. Geoffroy, G. Ramakrishnan and G. Bernardinelli
    Journal of Physical Chemistry A, 102 (43) , 1998, p8245-8250
    DOI:10.1021/jp9816519 | unige:2766 | Abstract | Article HTML | Article PDF
A new phosphine, the diphenyldibenzobarrelenephosphine 2, was designed to study the barrier to rotation of the P−H group around the C−•P bond. After homolytic scission of a P−H bond by radiolysis, the EPR spectrum of the resulting phosphinyl radical, trapped in a single crystal of 2, was studied at 77 K and at room temperature. The directions of the 31P hyperfine eigenvectors were compared with the bond orientations of the undamaged compound as determined from its crystal structure. The temperature dependence of the EPR spectrum was analyzed by using the density matrix formalism; this showed that interaction between the phosphinyl hydrogen and the phenyl ring bound to the ethylenic bond is determinant for explaining the potential energy profile. DFT investigations are consistent with these experimental results.
Cyclic voltammetry shows that monophosphaallene ArPCC(C6H5)2 (where Ar = C6H2tBu3-2,4,6), 1a, undergoes irreversible reduction at 2266 mV in THF. The EPR spectra of the reduction products are obtained in liquid and frozen solutions after specific 13C enrichment of the allenic carbon atoms. The resulting hyperfine tensors are compared with those obtained from ab initio MP2, MCSCF, CI, and DFT calculations for the radical anion (HPCCH2)-• and for the monophosphaallylic radical (HP•−CHCH2) ↔ (HPCH−•CH2). The most elaborate treatments of the hyperfine structure (CI and DFT) indicate that the species observed by EPR is the monophosphaallylic radical.
  • Spin labelled dinucleosides
    J.M.J. Tronchet, E. Grand, M. ZsĂŠly, R. Giovannini and M. Geoffroy
    Carbohydrate Letters, 3 (3) , 1998, p161-168
    unige:2768
  • Blocked sugar analogues bearing two or three N(sp3)-= bonds
    J.M.J. Tronchet, M. Koufaki, F. Barbalat-Rey and M. Geoffroy
    Carbohydrate Letters, 3 (4) , 1998, p255-262
    unige:2769
  • Electron Paramagnetic Resonance studies of radicals and ions with a low-coordinated phosphorus atom
    M. Geoffroy
    Recent research developments in physical chemistry, 2 (4) , 1998, p311-321
    unige:2770
Liquid phase EPR spectra of a diphosphaallenic radical anion have been Recorded after electrochemical reduction of a solution of ArPCPAr in THF at 293 K (Ar = 2,4,6-But3C6H2). The hyperfine coupling interactions of two 31P and one 13C nuclei (in the case of Ar13CPAr) are discussed in the light of AM1 calculations carried out on (ArPCPAr)–, of ab initio calculations performed on the model radical anion (HPCPH)– at the MP2 and MCSCF levels of theory and of DFT calculations on (HPCPH)–. The structure of the radical anion is compared with that of the neutral molecule.
  • Electron nuclear double resonance of the cubic Dy3+ center in KZnF3 single crystal
    M.L. Falin, V.A. Latypov, H. Bill and D. Lovy
    Applied Magnetic Resonance, 14 (4) , 1998, p427-434
    DOI:10.1007/BF03161852 | unige:2957 | Abstract | Article PDF
ENDOR measurements on the 19F- nuclei in the first four shells of KZnF3 containing Dy3+ ions in the cubic site are reported. The values and signs of the hyperfine and transferred hyperfine interaction parameters are determined. The local deformation of the crystal lattice in the vicinity of the impurity ion is estimated. The theoretical analysis of the THFI parameters for the first coordination shell of the F- ions has been carried out. For the Dy3+ ion the influence of spin polarization of the closed 5s and 5p shells is considered for the first time. Spin polarization is shown to play a significant role in the mechanism of rare-earth ion-ligand coupling. 
  
A recent investigation of the (BaF2–MgF2) phase diagram produced several new compounds which are suitable hosts for Rare Earth impurities. We present results on single crystals of Ba2Mg3F10 doped with Eu2+. The local structure and optical properties of this system were investigated by luminescence emission and by EPR. We observed two different Eu2+ sites. Both show Cs point symmetry and an important ground state splitting. Correlating our EPR and optical results with the new Ba2Mg3F10 structure data allowed the assignment of each of them to a specific barium lattice site. The luminescence emission of both the 4f7–4f65d and the 4f7–4f7 transitions is observed. The relative importance of the two emissions is strongly temperature dependent. The emission intensities of the intra f-shell 6P7/2→8S7/2transitions increase strongly on going from 295 K to 77 K. Thus, the lowest levels of the 4f65d configuration are approximately degenerate with the 6P7/2 manifold.
 
Eu2+ was introduced into pure and oxygen codoped BaMgF4 single crystals. A detailed EPR study of this ion (S=7/2) was realized on both types of systems. The result is that only one spectrum was observed involving a strong crystal field. The associated site symmetry of the impurity is Cs. It occupies very closely a Barium lattice site as was established by correlating the EPR results with those of a refined X-ray structure analysis on a Ba0.8Eu0.2MgF4 single crystal realized in our laboratory. The oxygen codoped crystals exhibited this same Eu2+EPR spectrum (the only one). Optical emission and excitation experiments were performed between 13 000 and 53 000 cm−1. The results due to the Eu2+ impurity are given and discussed qualitatively within the 4f7 ↔ 4f65d1 scheme.
Accurate dissociation energies were determined for gas-phase complexes between 1-naphthol and benzene, d6-benzene and cyclohexane, using the stimulated emission pumping resonant two-photon ionization spectroscopy technique in supersonic jets. The dissociation energies obtained for the electronic ground state are surprisingly large being D0 = 5.07¹0.07 kcal/mol for 1-naphthol ¡ benzene, 5.08¹0.06 kcal/mol for 1-naphthol ¡ d6-benzene, and 6.92¹0.03 kcal/mol for 1-naphthol ¡ cyclohexane, respectively. The dissociation energies scale well with the parallel molecular polarizabilities.
  • Effect of single point mutations of the human tachykinin NK1 receptor on antagonist affinity
    Kenneth Lundstrom, Anthony B. Hawcock, Alfredo Vargas, Peter Ward, Pam Thomas and Alan Naylor
    European Journal of Pharmacology, 337 (1) , 1997, p73-81
    Keywords: Semliki Forest virus system; Tachykinin NK1 receptor; Mutagenesis, site-directed; Binding assays; Functional coupling
    DOI:10.1016/S0014-2999(97)01226-0 | unige:2964 | Abstract | Article HTML | Article PDF
Molecular modelling and site-directed mutagenesis were used to identify eleven amino acid residues which may be involved in antagonist binding of the human tachykinin NK1 receptor. Recombinant receptors were expressed in mammalian cells using the Semliki Forest virus system. Wild type and mutant receptors showed similar expression levels in BHK and CHO cells, verified by metabolic labelling. Binding affinities were determined for a variety of tachykinin NK1 receptor antagonists in SFV-infected CHO cells. The binding affinity for GR203040, CP 99,994 and CP 96,345 was significantly reduced by mutant Q165A. The mutant F268A significantly reduced the affinity for GR203040 and CP 99,994 and the mutant H197A had reduced affinity for CP 96,345. All antagonists seemed to bind in a similar region of the receptor, but do not all rely on the same binding site interactions. Functional coupling to G-proteins was assayed by intracellular Ca2+ release in SFV-infected CHO cells. The wild type receptor and all mutants except A162L and F268A responded to substance P stimulation.
  • Paramagnetism of Tetranuclear Complexes between TCNX Ligands (TCNE, TCNQ, TCNB) and Four Pentaammineruthenium or Dicarbonyl(pentamethylcyclopentadienyl)manganese Fragments
    Eberhard WaldhĂśr, Wolfgang Kaim, LatĂŠvi Max Lawson Daku and Jeanne Jordanov
    Inorganic Chemistry, 32 (15) , 1997, p3248-3252
    DOI:10.1021/ic960998s | unige:2965 | Abstract | Article HTML | Article PDF
The tetranuclear complexes {(μ4-TCNX)[Ru(NH3)5]4}(A)8 and (μ4-TCNX)[Mn(CO)2(C5Me5)]4 [A = PF6 or CF3SO3; TCNX = TCNE (tetracyanoethene), TCNQ (7,7,8,8-tetracyano-p-quinodimethane), or TCNB (1,2,4,5-tetracyanobenzene)] were studied by variable-temperature (2−300 K) SQUID susceptometry. Mono- and dinuclear species [(PhCN)Ru(NH3)5](PF6)2 (PhCN = benzonitrile) and {(μ-L)[Ru(NH3)5]2}(PF6)4 (L = 1,4-dicyanobenzene (terephthalodinitrile) or pyrazine) were also investigated for comparison and were found to be essentially diamagnetic. Despite the even electron count, both the ruthenium and manganese tetranuclear complexes are paramagnetic, albeit with different spin−spin exchange coupling patterns. The manganese systems are characterized by exchange-coupled S = 1 states at the individual metal centers, whereas the magnetic behavior of the tetranuclear ruthenium compounds results from an exchange-coupling interaction between two S = 1/2 sites, identified as RuIII/RuII mixed-valence pairs.
  • A full configuration interaction study of the low-lying states of the BH molecule
    Laura Gagliardi, Gian Luigi Bendazzoli and Stefano Evangelisti
    Molecular Physics, 91 (5) , 1997, p861-872
    DOI:10.1080/002689797170969 | unige:2961 | Abstract | Article PDF
A full configuration interaction study on the BH molecule is presented. The potential energy curves of 20 different electronic states have been calculated correlating the four valence electrons. On the two most important states, i.e. the X1Sigma+ and A1Pi states, a complete study has been performed. This includes the effect of core electron correlation, estimated via truncated configuration interaction techniques. The dissociation energy of the molecule in the two states and the height of the predissociative barrier in the A1Pi state have been determined with basis sets of increasing quality.
We present a method for the direct generation of the lists of strings, suited for integral-driven full-CI (FCI) algorithms. This method generates the string lists each time they are used, and hence sensibly reduces the memory requirements, compared to our previous method that precalculates the lists. It was also extended to permit a truncation of the string space, according to the level of excitation.
  • Crystal structures, magnetic structures and photophysics in supra-molecular transition-metal oxalate compounds
    Silvio Decurtins, Helmut W. Schmalle, RenĂŠ Pellaux, Peter Fischer and Andreas Hauser
    Molecular Crystals and Liquid Crystals, 305 , 1997, p227-237
    DOI:10.1080/10587259708045060 | unige:2963 | Abstract | Article PDF
Polymeric two- and three-dimensional, homo- and heterometallic oxalatebridged coordination compounds offer exciting opportunities, mainly in the fields of molecular magnetism and photophysics. Given that a large variety of magnetic phenomena have been reported so far from these molecular magnets, very limited experience is gained from elastic neutron scattering experiments. Therefore, with two examples, we will address the topic of the elucidation of magnetic structures by means of the neutron scattering technique. In addition, due to the possibility of the variation of different metal ions in varying oxidation states, interesting photophysical processes can be observed within the extended three-dimensional host/guest systems.
A new compound in the binary phase diagram of BaF2 and MgF2 was synthesized. After Czrochalsky growth of BaMgF4, needle shaped crystals were found on the surface of the cooled residual mass. The compound Ba6Mg7F26 crystallizes in the orthorhombic space group Immm with lattice parameters of a = 585.35(4), b = 1214.95(7), and c = 1511.09(9) pm.
The global structure can be described as build up of perovskite like blocks and rutile related parts. Magnesium ions have an octahedral fluoride environment with minimum (maximum) bond distances of 192.1(2) (227.4(3)) pm.
Two different barium environments with coordination number (C.N.) of 12 + 1 (Ba1) and 12 (Ba2) were found. The different fluoride ions can have C.N. of 6, 4 or 3 with the nearest metal atoms.
Preliminary Raman spectra show significant differences between BaMgF4 and Ba6Mg7F26.
  • Calculations of chemical processes in solution by density functional and other quantum mechanical techniques
    R.P. Muller, T.A. Wesolowski and A. Warshel
    in "Density functional methods: Applications in chemistry and materials science" M. Springborg, ed. John Wiley and Sons, Ltd., 1997, p189-206
    unige:3932
  • Transition from surface vibrations to liquidlike dynamics at an incompletely melted semiconductor surface
    N. Takeuchi, A. Selloni and E. Tosatti
    Physical review B, 55 (23) , 1997, p15405-15407
    DOI:10.1103/PhysRevB.55.15405 | unige:2776 | Abstract | Article PDF
We present results of a first-principles molecular-dynamics simulation of the vibrational spectrum of a model semiconductor surface where, close to the bulk melting temperature TM, the top bilayer becomes diffusive. The specific example chosen is Ge(111). In place of the ordinary surface vibrations, we obtain a spectrum more akin to that of a confined liquid. In particular, we predict a dramatic decrease in the intensity of high-frequency (bond-stretching) surface mode intensity, associated with a local melting-induced breaking of hard covalent bonds in the top bilayer.
  • Response to "Comment on Density functional theory study of some structural and energetic properties of small lithium clusters", [J. chem. phys. 107, 1032(1997)]
    Georges Gardet, François Rogemond and Henry Chermette
    Journal of chemical physics, 107 (3) , 1997, p1034
    DOI:10.1063/1.474473 | unige:2777 | Abstract | Article PDF | Article PS (gzipped)
It is shown that whereas the spherical and spheroidal jellium models are inadequate to describe lithium clusters, only the ellipsoidal jellium model is adequate. The corresponding result, obtained by Yannouleas and Landman, was unpublished at the time of submission of our paper.
IR spectra of anthracene and pyrene derivatives, serving as models for isolated, linear and isolated, compact PAHs, respectively, have been calculated using ab-initio quantum mechanical methods. The separate and combined effects of ionization and multiple dehydrogenation have been studied. This study confirms and refines the trends of our preliminary paper on the smallest possible PAH, naphthalene. If small PAHs are responsible for any UIR bands, they should be ionized and partially dehydrogenated, with a few triple bonds at the periphery of the carbon skeleton. In the appendix are given the complete IR spectra of all the isomers of the derivatives of anthracene and pyrene calculated for the purpose of this study. Tables I are for anthracene and Tables II for pyrene. Positions of the the missing hydrogens in the dehydrogenated species are referred as in Figures 1 and 2 of the original publication.
In orthorhombic Ba6Mg7F26, the Ba2+ ion can be partially replaced by Sr2+ to form a mixed, disordered compound. This new compound has a refined composition of Ba5.24 (4)Sr0.76 (4)Mg7F26. The volume decreases after the partial substitution from 1074.64 (10) to 1064.31 (11) Å3. The structure has two barium sites, Ba1 on Wyckoff site 8(l) with Cs symmetry and a coordination number of 12 + 1, and Ba2 on site 4(j) with C2v symmetry and a coordination number of 12; only the Ba2+ on site 4(j) is partially replaced by Sr2+. The distorted octahedral fluoride environment around magnesium shows a tendency to become more irregular in the Sr-substituted compound.
The crystal structures of cubic and tetragonal NiCr2O4 and of tetragonal CuCr2O4 have been refined and their cell parameters have been measured, using single-crystal and powder X-ray diffraction. It has been observed that the differences in the Jahn–Teller distortions in both compounds are reflected in their M2+ –O tetrahedral environments and in their cell parameters. A calculation of the atomic shifts during the tetragonal–cubic phase transition showed that the average shifts for the oxygen atoms are 0.10(1) and 0.04(1) Å for the nickel and chromium atoms, respectively. Crystals of both compounds jump when they go through the phase transition. This behaviour is especially spectacular for NiCr2O4 since the phase transition takes place at 320 K, this temperature being reached when the crystals are illuminated. A comparison with organic jumping crystals is presented, and characteristics of the chromite crystals are discussed.
  • Interstellar silicon-nitrogen chemistry. 4. Which reaction paths to HSiN and HNSi ? : an extensive ab initio investigation with crucial consequences for molecular astrophysics
    O. Parisel, M. Hanus and Y. Ellinger
    Journal of physical chemistry A, 101 (3) , 1997, p299-309
    DOI:10.1021/jp9618441 | unige:2774 | Abstract | Article HTML | Article PDF
In order to provide a possible explanation for the lack of detection of both HSiN and HNSi in the interstellar medium, an ab initio study of the Si+ + NH3 reaction is presented: it includes accurate energetic considerations and sketches dynamics discussions as well. It is unambiguously concluded that the X1A1 ground state of the SiNH2+ cation is the only exit channel of this reaction assuming interstellar conditions. The rotational and vibrational constants of this species are reported to stimulate its experimental and astrophysical searches. Upon dissociative recombination, it is likely that SiNH2+ can evolve toward HNSi: unfortunately, the dramatic weakness of the dipole moment of the latter species (0.05 D) makes it an unlikely candidate for today's radiotelescopes. At variance with HNSi, the high dipole moment value of HSiN (4.5 D) would make it a much more attractive candidate for astrophysical searches, but under interstellar conditions, we show that it can derive neither from the unimolecular HNSi ↔ HSiN equilibration nor from the Si+ + NH3, N + SiH3+ or N+ + SiH3 reactions as sometimes incorrectly stated in the astrophysical models that deduce interstellar silicon chemistry from that of carbon. Throughout this study, the very hazardous character of conclusions deduced from isoelectronic considerations should be considered as the leading feature: the finishing stroke to such isoelectronic analogies is given by our study of the H+ + HNSi ↔ HSiN + H+ reactions which leads to the conclusion that HSiN might be unlikely to survive interstellar hydrogenation processes.
A helium gas pressure cell for pressures up to 1 kbar (0.1 GPa) has been developed in conjunction with a closed-cycle He refrigerator allowing variable temperatures between 15 and 300 K. Both cell and refrigerator are equipped with optical windows suitable for photophysical measurements, such as temperature- and pressure-dependent absorption spectroscopy or laser flash photolysis. Examples of measurements on iron(II) spin-crossover systems are given. In these compounds, comparatively small external pressures induce significant changes in the thermodynamic equilibrium as well as in the relaxation dynamics.
  • H2 adsorption/desorption at Si(111)-(7 x 7) : a density functional study
    A. Vittadini and A. Selloni
    Surface Science, 383 (2-3) , 1997
    Keywords: chemisorption | density functional calculations | hydrogen | low index single crystal surfaces | models of surface chemical reactions | silicon | solid-gas interfaces | thermal desorption
    DOI:10.1016/S0039-6028(97)00251-3 | unige:2780 | Abstract | Article PDF
The interaction of H2 with the Si(111)-(7 × 7) surface is investigated by means of density functional slab calculations on a (4 × 2) reconstructed model surface. A viable mechanism for β1 desorption is identified, which involves thermally activated SiH2 units at adatom sites. This mechanism leads to adsorption and desorption barriers of 1.0 and 2.4 eV, respectively, in agreement with experiment. An explanation for the two components observed in the β1 peak of temperature-programmed desorption spectra is proposed. The lattice deformation energy at the transition state for desorption is large (not, vert, similar0.6 eV). If we assume that this remains in the surface after H2 desorption, the low translational energy of desorbing molecules which has been observed experimentally can in part be explained.
  • First-principles molecular dynamics investigations of the stability of zeolite offretite under various Si4+/(Al3+,H+) substitutions
    L. Campana, A. Selloni, J. Weber and A. Goursot
    Il Nuovo Cimento D, 19 (11) , 1997, p1649-1655
    unige:2781
A local density functional study of Si41O( Al31, H1)-substituted offretites is presented. Proton siting and dynamical properties are investigated within the First-Principles Molecular Dynamics method, using a periodically repeated unit cell. Results for monoaluminated offretites, e.g., with one Al per unit cell, show that the proton is located inside the channel of the zeolite, where it is accessible to incoming molecules for reaction. Calculated vibrational spectra of the framework, extracted from a dynamical simulation, reproduce experimental data well. The determined OH stretching frequencies show a rather weak dependence on the H1 position. A comparison of these frequencies with those of offretites containing three Al per unit cell does not indicate a significative chan
  • Di-[mu]-bromo-bis{[(2,4,6-tris-tert-butyl-phenyl)phosphanediylmethyl-P]phenyl-C2}-dipalladium
    Mostafa Chentit, Michel Geoffroy and GĂŠrald Bernardinelli
    Acta Crystallographica C, 53 , 1997, p866-868
    DOI:10.1107/S0108270197003235 | unige:2782 | Article PDF
The crystal structure of the title compound, [Pd2Br2(C25H34P)2], a new binuclear phospha-alkene compound containing a trivalent P atom shows a centrosymmetric dimeric arrangement. The Pd2Br2 core is planar and adopts an irregular diamond shape. The coordination of the Pd atom is square planar. No stacking interactions were observed in the molecular packing.
  • A density functional especially designed for hydrogen-only systems
    H. Chermette, H. Razafinjanahary and L. Carrion
    Journal of chemical physics, 107 (24) , 1997, p10643-10651
    Keywords: hydrogen neutral atoms | hydrogen neutral molecules | dissociation energies | ab initio calculations | density functional theory | atom-molecule reactions | positive ions
    DOI:10.1063/1.474180 | unige:2783 | Abstract | Article PDF | Article PS (gzipped)
An hybrid functional which includes a larger amount of pure exchange has been specially designed for the description of hydrogen-only systems. Both the H abstraction by H2 and the H+2n+1 clusters have been investigated. Comparison with experimental values shows that the proposed functional gives dissociation energies and vibrational frequencies better than previous ab initio calculations. The results compare favorably with those obtained by a coupled clusters method [CCSD(T)], also performed in this work for sake of reference data.
  
  • Conformational studies of 2-butanol using temperature-dependent Raman measurements and MM3 calculations
    H. Hagemann, J. Mareda, C. Chiancone and H. Bill
    Journal of Molecular Structure, 410-411 , 1997, p357-360
    Keywords: Raman spectroscopy; conformation; conformers; molecular mechanics
    DOI:10.1016/S0022-2860(96)09584-1 | unige:3534 | Abstract | Article PDF
Temperature-dependent Raman measurements between 190 and 358 K yield conformational enthalpy differences between 550 and 690 cal mol−1 for racemic liquid 2-butanol. The conformational properties of 2-butanol were also studied using MM2 and MM3 calculations. The conformer 1, where C1 and C4 are in anti position, was found to be the most stable comformer with the MM3 calculations. Conformer 2, which has C1 and O5 in anti arrangement, has ca. 500 cal mol−1 higher energy than 1. Comparisons of the calculated MM3 vibrational frequencies with the Raman spectra suggest that the most stable conformer in the liquid phase also adopts a Cl,C4 anti conformation.
The present investigation is concerned with the possible effects of material-related properties (molecular mass, glass transition and melting temperatures, crystallinity, tacticity) and particle-related properties (shape, size, specific surface area) on the compression characteristics of the chosen model polymer powder: poly(vinyl chloride) (PVC). Four grades were selected known in literature for providing compacts of varied mechanical strength. The compression characteristics were determined using an instrumented single-punch tableting machine. The differences in tableting characteristics could not be ascribed to any of the material-related properties, but a direct relationship was observed between the compact strength and the specific surface area of the particles, as measured by nitrogen adsorption. The compact hardness was thus only dependent on the inter- and the intraparticulate contact area, which in turn is dictated by the very peculiar morphology of the grains of the PVC powders, whether prepared by emulsion or suspension polymerization.
  • Magnetism and photophysics in supramolecular transition-metal compounds
    S. Decurtins, H.W. Schmalle, R. Pellaux, A. Hauser, M.E. Von Arx and P. Fischer
    Synthetic metals, 85 (1-3) , 1997, p1689-1694
    Keywords: self-organization in macromolecules; neutron diffraction; magnetic phase transition
    DOI:10.1016/S0379-6779(97)80399-2 | unige:2786 | Abstract | Article PDF
  
Based on a synthetic strategy, extended anionic, homo and bimetallic oxalato-bridged transition-metal compounds with two (2D) and three-dimensional (3D) connectivities can be synthesized and crystallized. Thereby, the choice of the templating counterions will determine the crystal chemistry. Since the oxalato bridge is a mediator for both antiferro and ferromagnetic interactions between similar and dissimilar metal ions, long-range magnetic ordering will occur. Examples of the determination of magnetic structures in 2D and 3D compounds by means of elastic neutron scattering methods will be discussed. In addition, due to the possibility of the variation of different metal ions in varying oxidation states, interesting photophysical processes can be observed within the extended three-dimensional host/guest systems.
  • Mechanism for SiCl2 formation and desorption and the growth of pits in the etching of Si(100) with chlorine
    G.A. De Wijs, A. De Vita and A. Selloni
    Physical Review Letters, 78 (25) , 1997, p4877-4880
    DOI:10.1103/PhysRevLett.78.4877 | unige:2787 | Abstract | Article PDF
A mechanism for SiCl2 formation and desorption in the etching of Si(100)-(2×1) at low chlorine coverages is analyzed using first-principles calculations. We find that the two monochlorinated Si atoms of a surface dimer can rearrange into a metastable SiCl2(a) adsorbed species plus a Cl-free Si atom. Desorption of SiCl2 occurs via a two-step mechanism, in which the adsorbed species is preliminarily stabilized by the diffusion away of the free Si atom. The energy barrier to form SiCl2(a) is lower on a dimer next to a dimer vacancy than in an undamaged region of the surface, consistent with recent STM observation of preferential linear growth of etch pits along dimer rows.
The growth of thin films made from Samarium-doped alkaline earth fluoro halides (AEFH) of composition SrxCa1−xFCl:Sm2+ (0 ⩽ x ⩽ 1) is presented and the possibilities are studied to increase significantly the inhomogeneous width of the Sm2+ optical zero phonon transitions. The best films were obtained when grown with a molecular beam deposition (MBD) method involving two separate molecular beams: one for the alkaline earth fluoride, the other one for the alkaline earth halide (Cl or Br). The results demonstrate that the double beam MBD technique employed is able to produce pure and mixed Matlockite films with targeted composition. The results of mainly optical studies of the samarium f–f transitions and of other complementary techniques are used to assess the composition and homogeneity of the films. With the aid of a model the composition dependence of the positions of specific optical f–f emission lines is established. Their inhomogeneous linewidth is compared with that of corresponding emission lines obtained from bulk samples of the same chemical composition. The linewidths of the films are only slightly larger (∼1.5–2 times). Thus, the film morphology cannot be exploited to increase substantially the inhomogeneous broadening of the luminescence lines. A novel approach to increase this broadening was devised, theoretically modeled and successfully tested by using multilayered sandwich-type thin films in conjunction with interdiffusion. Films with cation disorder of composition SrxCa1−xFCl (x = 0.5 /0/ 0.5/ 0/..) were grown. The 5D1→7F0 Sm2+ emission linewidth is thereby increased to 70 cm−1 full width half maximum. A width of 100cm−1 may be obtained within the composition range x = 0, x = 1. This represents an enhancement by a factor of 3–5 in comparison with the largest values obtained in appropriate mixed bulk AEFH of constant composition. A factor >50 is gained in comparison with pure bulk AEFH hosts. The room temperature (RT) homogeneous linewidths, on the other hand, are similar to those found in bulk mixed crystals of constant composition. The intrafilm host cation diffusion during film growth of the sandwich structures was further studied. A diffusion constant of 2⋅10−19m2s−1 for the Sr and Ca ions was deduced from this observation. These films are among the most promising materials for optical mass data storage through RT hole-burning.
  • Synthesis, crystal structures and spectroscopic investigations on samarium-doped mixed Ba1-δSrδMgF4 crystals
    F. Kubel, H. Hagemann and H. Bill
    Materials Research Bulletin, 32 (3) , 1997, p263-269
    Keywords: fluorides; optical materials; X-ray diffraction; crystal structure; luminescence
    DOI:10.1016/S0025-5408(96)00190-0 | unige:2958 | Abstract | Article PDF
Single crystal X-ray diffraction analysis was performed on crystals with composition Ba1 − δSrδMgF4 (δ ≤ 0.55). The complete structure was analyzed for single domain crystals with nominal (refined) values of δ = 0.25 (0.27(2)) and 0.5 (0.55(2)). Interatomic distances vary in a characteristic manner, when smaller strontium ions replace the barium ions. Optical studies of Sm(II) doped samples show significant inhomogeneous line broadening and confirm the disorder On the Ba Site.
  • Pressure effects on the HS -> LS relaxation in [Zn1-xFex(6-mepy)3tren](PF6)2
    Wei Wang, I.Y. Chan, Sabine Schenker and Andreas Hauser
    Journal of Chemical Physics, 106 (9) , 1997, p3817-3820
    DOI:10.1063/1.473436 | unige:2789 | Abstract | Article PDF
Laser flash photolysis experiments were performed on the mixed crystal [Zn1−xFex(6-mepy)3tren](PF6)2 (x=0.00025) at 10 K in the pressure range between 1 bar and 20 kbar. An external pressure of 20 kbar accelerates the low-temperature tunneling process by almost eight orders of magnitude.
Applicability of the approximate kinetic energy functionals to study hydrogen-bonded systems by means of the formalism of Kohn–Sham equations with constrained electron density (KSCED) [Cortona, Phys. Rev. B 44, 8454 (1991); Wesołowski and Warshel, J. Phys. Chem. 97, 8050 (1993); Wesołowski and Weber, Chem. Phys. Lett. 248, 71 (1996)] is analyzed. In the KSCED formalism, the ground-state energy of a molecular complex is obtained using a “divide-and-conquer” strategy, which is applied to the Kohn–Sham-like equations to obtain the electron density of a fragment embedded in a larger system. The approximate kinetic energy functional enters into the KSCED formalism in two ways. First, the effective potential in which the electrons of each fragment move contains a component which is expressed by means of a functional derivative of an approximate kinetic energy functional (functional derivative of the non-additive kinetic energy). Second, the KSCED energy functional contains a component (non-additive kinetic energy) which is expressed using the approximate kinetic energy functional. In this work, the KSCED energies and densities of (H2O)2, (HF)2, (HCl)2, and HFNCH are compared to the ones obtained using the standard supermolecule Kohn–Sham approach. The following factors determining the agreement between the KSCED and supermolecule Kohn–Sham results are analyzed: the analytical form of the gradient-dependent terms in the approximate kinetic energy functional and the number of atom-centered orbitals used to expand electron density of fragments. The best agreement between the supermolecule Kohn–Sham and the KSCED results is obtained with the kinetic energy functional derived following the route of Lee, Lee, and Parr [Lee et al., Phys. Rev. A 44, 768 (1991)] from the exchange functional of Perdew and Wang [Perdew and Wang, in Electronic Structure of Solids ’91, edited by P. E. Ziesche and H. Eschrig (Academie Verlag, Berlin, 1991), p. 11]. The difference between the KSCED and the supermolecule Kohn–Sham energies of studied complexes amounts to less than 0.35 kcal/mol at the equilibrium geometry.
The edge-bridged tetrahedral geometry of five-coordinate d0 complexes [MD2L3] with strong π-donors D is analyzed with extended Hückel methodology as well as density functional theory. It is shown that this geometry, also encountered in bent metallocene systems [MCp2L3], can be considered as a distortion of a regular trigonal bipyramid arising from a second-order Jahn−Teller distortion of e‘ symmetry (in D3h) and corresponds to a deformation along a reversed-Berry pathway. This model was tested with a structure-correlation analysis of all experimentally determined [MD2L3] structures, thus allowing a mapping of the reversed-Berry pathway. The catalytic potential of these complexes and their isolobal relationship to [MCp2L3] are emphasized.
  • Trans- and cis- water reactivities in d6 octahedral ruthenium(II) pentaaqua complexes : experimental and density functional theory studies
    Nicolas Aebischer, Elena Sidorenkova, Mauro Ravera, GĂĄbor Laurenczy, Domenico Osella, Jacques Weber and AndrĂŠ E. Merbach
    Inorganic Chemistry, 36 (26) , 1997, p6009-6020
    DOI:10.1021/ic970783y | unige:2792 | Abstract | Article HTML | Article PDF
The hexaaqua complex of ruthenium(II) represents an ideal starting material for the synthesis of isostructural compounds with a [Ru(H2O-ax)(H2O-eq)4L]2+ general formula. We have studied a series of complexes, where L = H2O, MeCN, Me2SO, H2CCH2, CO, and F2CCH2. We have evaluated the effect of L on the cyclic voltammetric response, on the rate and mechanism of exchange reaction of the water molecules, and on the structures calculated with the density functional theory (DFT). As expected, the formal redox potential, E°‘(+2/+3), increases with the π-accepting capabilities of the ligands. For L = N2, the oxidation to Ru(III) is followed by a fast substitution of dinitrogen by a solvent molecule, revealing the poor stability of the Ru(III)−N2 bond. The water exchange reactions have been followed by 17O NMR spectroscopy. The variable-pressure and variable-temperature kinetic studies made on selected examples are all in accordance with a dissociative activation mode for exchange. The positive activation volumes obtained for the axial and equatorial water exchange reactions on [Ru(H2O)5(H2CCH2)]2+ (ΔVax and ΔVeq = +6.5 ± 0.5 and +6.1 ± 0.2 cm3 mol-1) are the strongest evidence of this conclusion. The increasing cis-effect series was established according to the lability of the equatorial water molecules and is as follows: F2CCH2 CO < Me2SO < N2 < H2CCH2 < MeCN < H2O. The increase of the lability is accompanied by a decrease of the E°‘ values, but no change was found in the calculated Ru−H2Oeq bond lengths. The increasing trans-effect series, established from the lability of the axial water molecule, is the following: N2 MeCN < H2O < CO < Me2SO < H2CCH2 < F2CCH2. A variation of the Ru−H2Oax bond lengths is observed in the calculated structures. However, the best correlation is found between the lability and the calculated Ru−H2Oax bond energies. It appears, also, that a decrease of the electronic density along the Ru−Oax bond and the increase of the lability can be related to an increase of the π-accepting capability of the ligand. For L = N2, the calculations have shown that the Ru(II)−N2 bond is weak. Consequently, the water exchange reaction proceeds through a different mechanism, where first the N2 ligand is substituted by one water molecule to produce the hexaaqua complex of Ru(II). The water exchange takes place on this compound before re-formation of the [Ru(H2O)5N2]2+ complex.
As recently reported by Klopper and Lüthi, there is a discrepancy between experiment and high-level quantum chemical calculations as to the value of the heterolytic metal–ligand bond disruption enthalpy of ferrocene. Indeed their ab initio calculations lead to a best estimate of 655 kcal/mol, whereas the experimental value is 635 kcal/mol. We report here results obtained using density functional theory. In addition to ferrocene, other metallocenes such as vanadocene, manganocene, nickelocene and ruthenocene, have also been investigated. Gradient-based corrections are crucial for a quantitative description of bond dissociation, our best estimate for ferrocene being 663 kcal/mol.
We have used density functional theory, both within the local density (LDA) and generalized gradient (GGA) approximations, to study the structure, energetics, and vibrational properties of zeolite offretite in the presence of different monovalent cations (H+, Na+, K+, and Cu+). We find that the spatial locations of the most favorable cation-binding sites are similar for the different cations, being related to the minima of the electrostatic potential. However, the relative stability of the sites does depend on the nature of the counterion, as well as on the Al/Si ratio and on the mutual interactions between cations. At low Al/Si ratios, the preferred site for H+ is in the channel, where it is accessible for reaction with incoming molecules. For both Na+ and Cu+, the most stable site is within the 6-fold ring of the gmelinite cage, but for Na+, two other sites are present within a few tenths of a kilocalorie/mole from the lowest site (small site selectivity). For K+, two sites, one inside the cancrinite cage and the other near the 8-fold ring of the gmelinite cage, are very close in energy, consistent with the X-ray experiments on natural hydrated and dehydrated offretites. Dynamical simulations have been carried out for H− and Na−offretite. The vibrational spectrum of the framework agrees well with the available experiment. OH stretching frequencies calculated for a number of different H+ locations show that more "open" positions, e.g., in the channel, have higher frequencies, in agreement with experiment.
  • Comparative study of benzene··· X (X = O2, N2, CO) complexes using density functional theory : the importance of an accurate exchange-correlation energy density at high reduced density gradients
    T.A. Wesolowski, O. Parisel, Y. Ellinger and J. Weber
    Journal of Physical Chemistry A, 101 (42) , 1997, p7818-7825
    DOI:10.1021/jp970586k | unige:2795 | Abstract | Article HTML | Article PDF
Although density functional theory (DFT) is more and more commonly used as a very efficient tool for the study of molecules and bulk materials, its applications to weakly bonded systems remain rather sparse in the literature, except studies that consider hydrogen bonding. It is, however, of essential interest to be able to correctly describe weaker van der Waals complexes. This prompted us to investigate more precisely the reliability of several widely-used functionals. The equilibrium geometries and the binding energies of C6H6···X (X = O2, N2, or CO) complexes are determined within the standard Kohn−Sham approach of DFT using different exchange−correlation functionals and at the MP2 level of theory for comparison. It is comprehensively concluded that extreme care must be taken in the choice of the functional since only those that behave properly at large and intermediate values of the reduced density gradient s give relevant results. The PW91 exchange functional, the enhancement factor of which does not diverge at increasing s, appears as the most reliable for the studied systems. It is furthermore demonstrated that the quality of the DFT results is determined by the exchange energy component of the total energy functional.
Quantum chemical calculations have been performed using density functional theory to model the mechanism of selective catalytic reduction of NO by NH3 on vanadium oxide. The reaction is initiated by NH3 adsorption on a Brønsted site modeled as a dimer cluster model representative of vanadium oxide, containing a terminal VO adjacent to a V−OH group. The calculations indicate that the adsorbed NH3 behaves as NH4+, which is supported by calculated IR spectra. Subsequently NO reacts with this activated NH3 to yield NH2NO and finally the reaction products N2 and H2O. The present results give support to a dual-site Eley−Rideal-type mechanism involving a Brønsted site and agree with isotopic labeling studies.
  • Enantioselective hydrogenation of ketopantolactone
    M. SchĂźrch, O. Schwalm, T. Mallat, J. Weber and A. Baiker
    Journal of Catalysis, 169 (1) , 1997, p275-286
    DOI:10.1006/jcat.1997.1674 | unige:2797 | Abstract | Article PDF
The enantioselective hydrogenation of ketopantolactone toR-(−)-pantolactone was investigated on 5 wt% Pt/Al2O3chirally modified with cinchonidine. The influence of catalyst pretreatment conditions, hydrogen pressure, temperature, solvent polarity, and catalyst, reactant, and modifier concentrations was studied in a slurry reactor. An enantiomeric excess (ee) of 79% at full conversion was achieved in toluene after optimization of pressure, temperature, and amount of modifier. Good ee could be obtained only after rigorous removal of traces of oxygen and water during catalyst pretreatment and from the hydrogenation reaction mixture. Molecular modeling studies (performed using molecular mechanics, semiempirical, andab initiomethods) provided a feasible structure for the diastereomeric transition complex formed between cinchonidine and ketopantolactone and an explanation for the observed enantiodifferentiation in apolar medium. The calculations indicate that formation of the complex affordingR-(−)-pantolactone is energetically favored with cinchonidine, whereas the near enantiomer cinchonine favorsS-pantolactone, in agreement with experimental observations. Interestingly, in apolar solvents, where the alkaloid modifier is not protonated, the modeling suggests similar structures for the diastereomeric transition complexes for the hydrogenation of ketopantolactone and methyl pyruvate.
  • A QSAR study confirming the heterogeneity of the HEPT derivatives series regarding their interaction with HIV reverse transcriptase
    J.M.J. Tronchet, M. Grigorov, N. Dolatshahi, F. Moriaud and J. Weber
    European Journal of Medicinal Chemistry, 32 (4) , 1997, p279-299
    Keywords: HEPT; HIV; TSAR; CATALYST; reverse transcriptase; neural network
    DOI:10.1016/S0223-5234(97)89081-2 | unige:2798 | Abstract | Article PDF
QSAR concerning the anti-HIV and cytotoxic activities of a series of HEPT analogues has been established using a Hansch-type approach (TSAR™), a neural network approach (TSAR) and a pharmacophore search method (CATALYST™). The techniques employed allowed reliable activity predictions and confirmed the heterogeneity of this series of compounds, which was previously established in biochemical experiments.
Quantum chemical calculations using the density functional theory were performed to model the mechanism of selective catalytic reduction of NO by NH3 on a supported vanadium oxide monolayer. In the first step, the adsorption of NH3 on a bimetallic cluster representative of vanadium oxide, containing a terminal VO adjacent to a VOH group, was investigated. The calculations indicate that NH3 may be strongly adsorbed on VOH (BrĂśnsted acid site) as NH+4(ads); subsequently, NO reacts with this activated NH3 to yield the reaction products N2 and H2O. The present results give support to a dual-site Eley-Rideal-type mechanism involving a BrĂśnsted site.
The purpose of the present work was to develop a method allowing one to extract the information needed for the construction of the internal chemical hardness tensor at the molecular orbital level from standard density functional calculations. This method is based on the Janak theorem and on the extension of the Slater transition-state concept. A detailed discussion of the current ideas about the validity of the Janak theorem is presented as well as of the established relations of this subject with the ensemble V-representability problem. The internal chemical hardness tensor has been obtained for water molecule as an example system. Its structure is consistent with the criteria for the internal molecular stability.
The antimalarial activity of a series of synthetic 1,2,4-trioxanes is correlated with molecular structure by using a pharmacophore search method (CATALYST). The technique is shown to have predictive accuracy and confirms that docking between an active trioxane and the receptor, heme, is the crucial step for drug action.
We analyze differences between ground-state electron densities of a model molecular complex obtained by solving the Kohn-Sham equations with constrained electron density (KSCED) [Wesolowski and Warshel, J. Phys. Chem. 97, 8050 (1993)] and as a solution of the standard Kohn-Sham equations applied to the whole complex. The differences between KSCED and KS electron densities which result from the inaccuracy of the kinetic energy functional applied in the KSCED equations are discussed. The model molecular complex consists of collinear H2 and HCN molecules separated by a short distance. Several kinetic energy functional approximations are applied within the KSCED framework and the differences between resulting electron densities are studied. The KSCED electron densities depend directly on the functional derivative of the kinetic energy functional applied in the KSCED equations. Among the studied functionals, the one proposed by Zhao et al. [Phys. Rev. A 47, 918 (1993)] and the gradient-dependent functional proposed by Perdew and Wang [Phys. Rev. B 33, 8800 (1986)] led to the smallest differences between KS and KSCED electron densities, thus having the most accurate functional derivative. Both functionals allow one to extend the range of applicability of the KSCED equations to system geometries where an overlap of electron densities of the partners of the complex is significant.
  • Cooperativity in the Iron(II) Spin-Crossover Compound [Fe(ptz)6](PF6)2 under the Influence of External Pressure (ptz = 1-n-Propyltetrazole)
    J. Jeftic, R. Hinek, S.C. Capelli and A. Hauser
    Inorganic Chemistry, 36 (14) , 1997, p3080-3087
    DOI:10.1021/ic961404o | unige:2803 | Abstract | Article HTML | Article PDF
The iron(II) spin-crossover compound [Fe(ptz)6](PF6)2 (ptz = 1-propyltetrazole) crystallizes in the triclinic space group P†, with a = 10.6439(4) Å, b = 10.8685(4) Å, c = 11.7014(4) Å, α = 75.644(1)°, β = 71.671(1)°, γ = 60.815(1)°, and Z = 1. In [Fe(ptz)6](PF6)2, the thermal spin transition is extremely steep because of cooperative effects of elastic origin. The transition temperature at ambient pressure is 74(1) K. An external pressure of 1 kbar shifts the transition temperature to 102(1) K, corresponding to a stabilization of the low-spin state, which is smaller in volume. The volume difference between the high-spin and the low-spin state, ΔV°HL, is 24(2) Å3/molecule. The interaction constant Γ, as a measure of cooperativity, is within experimental error independent of external pressure and has a value of 101(5) cm-1. In contrast to the case of the related compound [Fe(ptz)6](BF4)2 (Decurtins et al. Inorg. Chem. 1985, 24, 2174), there is no hysteresis due to a first-order crystallographic phase transition, nor is there a hysteresis induced by external pressure as in the mixed crystal [Zn1-xFex(ptz)6](BF4)2, x = 0.1 (Jeftić et al. J. Phys. Chem. Solids 1996, 57, 1743). However, in [Fe(ptz)6](PF6)2, the interaction constant Γ is found to be very close to the critical value above which a hysteresis solely due to the cooperative effects is expected. In addition, high-spin → low-spin relaxation measurements were performed under external pressures of up to 1 kbar in the temperature interval between 50 and 60 K. An external pressure of 1 kbar accelerates the high-spin → low-spin relaxation by 1 order of magnitude.
  • Pressure Study of the Thermal Spin Transition and the High-Spin -> Low-Spin Relaxation in the R3 and P1 Crystallographic Phases of [Zn1-xFex(ptz)6](BF4)2 Single Crystals (x = 0.1, 0.32, and 1; ptz = 1-n-propyltetrazole)
    J. Jeftic and A. Hauser
    Journal of Physical Chemistry B, 101 (49) , 1997, p10262-10270
    DOI:10.1021/jp972083k | unige:2804 | Abstract | Article HTML | Article PDF
In the iron(II) spin-crossover compound [Fe(ptz)6](BF4)2, the thermal spin transition is accompanied by a crystallographic phase transition showing a hysteresis with Tc↓ = 128 K and Tc↑ = 135 K at ambient pressure [Franke, P. L.; Haasnot, J. G.; Zuur, A. P. Inorg. Chim. Acta 1982, 59, 5]. The hysteresis is due to an interplay between the spin-transition and the R3 → P† crystallographic phase transition with a large low-spin fraction stabilizing the P† phase at low temperatures. In the mixed crystal [Zn1-xFex(ptz)6](BF4)2, x = 0.1, with the iron complexes imbedded into the isomorphous zinc lattice, the crystallographic phase transition can be induced by an external pressure [Jeftić, J.; Romstedt, H.; Hauser, A. J. Phys. Chem. Solids 1996, 57, 1743]. Thus the P† phase is additionally stabilized by external pressure. The interaction constant Γ, which describes cooperative effects between the spin-changing complexes, differs for the two crystallographic phases. Values for Γ(P†) of 144(8) cm-1 and the volume difference ΔV0HL of 29(4) Å3 are determined from a simultaneous fit to a series of transition curves for different pressures and iron content x in the P† phase. These values are compared to the corresponding values for the R3 phase, viz. Γ(R3) of 170(9) cm-1 and ΔV0HL(R3) of 26(3) Å3. Surprisingly Γ(R3) is larger than Γ(P†) despite the fact that ΔV0HL(R3) is smaller than ΔV0HL(P1). The high-spin → low-spin relaxation at temperatures above ~80 K is thermally activated, while below ~40 K temperature independent tunnelling takes place. An external pressure of 1 kbar accelerates the high-spin → low-spin relaxation exponentially by 1 order of magnitude in the tunnelling region in both crystallographic phases and regardless of x. In the concentrated material the high-spin → low-spin relaxation is self-accelerating due a buildup of an internal pressure [Hauser, A. Chem. Phys. Lett. 1992, 192, 65]. Both cooperative effects and external pressure result in a shift of the maximum of the 1A1 → 1T1 absorption band.
  • Investigation of the Photoinduced Electron Transfer Rection between 9,10-Dicyanoanthracene and 1-Methylnaphthalene in Acetonitrile using Picosecond Transient Grating Spectroscopy
    E. Vauthey
    Journal of Physical Chemistry A, 101 (9) , 1997, p1635-1639
    DOI:10.1021/jp961853+ | unige:2959 | Abstract | Article HTML | Article PDF
A spectroscopic and kinetic study of the photoinduced electron transfer (ET) reaction between 9,10-dicyanoanthracene (DCA) and 1-methylnaphthalene (MNA) in acetonitrile using the transient grating technique is reported. Apart from the bands assigned to 1DCA* and DCA•-, the transient spectrum exhibits a band located at 580 nm and ascribed to MNA2•+. This species is generated upon reaction of a second donor molecule with a 1:1 complex to form a 1:2 complex (DCA•-·MNA2•+). Using chloranil as electron acceptor, the rate constant of this reaction has been measured to be 6.8 × 109 M-1 s-1. From the donor concentration dependence of the kinetics of DCA•- diffraction intensity and of the free ion yield, the rates of back ET to the ground state within the 1:1 and the 1:2 complexes have been determined to be equal to 1.8 × 108 and 16.9 × 108 s-1, respectively, while their rate constants of separation into free ions are 1.6 × 108 and 1.1 × 108 s-1, respectively. These values have been obtained assuming a reaction scheme in which both forward and backward ET essentially take place at contact distance. In the case of the 1:1 complex, however, charge recombination within a loose ion pair cannot be ruled out.
The luminescence of the scorpion's outer shell has been shown to be due to fluorescence of very short lifetime (nanoseconds). The emission and excitation spectra have been determined, and the potential biological significance of this photoluminescence is discussed.
The reliability of the transient grating technique to determine the energetics of processes following second order kinetics, such as free ion recombination, is investigated theoretically. As the second order process takes place, the grating of the reactant population becomes anharmonic, complicating the determination of the population dynamics from the time profile of the diffracted intensity. However, the thermal phase grating is anharmonic from the beginning of the reaction, but is harmonic when the process is completed. As a consequence, the energetics of the reaction can be deduced accurately from the maximum amplitude of the diffracted intensity without having to take the anharmonicity of the grating into account, as long as thermal diffusion is slow. To achieve this, the crossing angle of the pump pulses on the sample has to be smaller than 1°. Otherwise, an arduous fitting procedure is required to extract the energetic parameters from the experimental data.
A study of the dynamics of ground-state recovery of the perylene radical cation (Pe•+), of perylene radical anion (Pe•-), and of anthraquinone radical anion (AQ•-) is reported. In boric acid glass, the excited-state lifetime of Pe•+ is 35 ± 3 ps, while in concentrated sulfuric acid, it is smaller than 15 ps, the time resolution of the experimental setup. The excited-state lifetime of Pe•+, Pe•-, and AQ•- generated by photoinduced intermolecular electron-transfer reaction in MeCN is shorter than 15 ps. In the case of Pe•-, the uncomplete ground-state recovery is ascribed to the occurrence of electron photoejection. The free ion yield in the intermolecular electron-transfer reaction between 9,10-dicyanoanthracene (DCA) and two electron acceptors was measured in a two-pulse experiment, where the second pulse excited the ensuing DCA•-. This excitation has no influence on the magnitude of the free ion yield, indicating a short excited-state lifetime of DCA•-* relative to the time scale of back electron transfer and ionic dissociation. A red emission, ascribed to the fluorescence of protonated Pe, was detected in boric acid glass and sulfuric acid. No fluorescence that could be clearly ascribed to Pe•+* could be observed.
  • Spin labelled 2',3'-seconucleoside derivatives
    J.M.J. Tronchet, M. ZsĂŠly, D. Cabrini, F. Barbalat-Rey, N. Dolatshahi and M. Geoffroy
    Carbohydrate Letters, 2 , 1997, p389-394
    unige:2805
Cyclic voltammetry of phosphafulvene and dibenzophosphafulvene shows that in DMF these compounds are reduced at -1.200 and -1.349 V, respectively. The EPR spectra of the corresponding radical anions, formed by electrochemical reduction or by reaction on a potassium mirror, are Recorded between 110 K and room temperature. The g and 31P hyperfine tensors are measured and compared to those previously obtained for a phosphaalkene radical anion. Abinitio investigations on model phosphaalkene and phosphafulvene radical anions show that, in accord with the experimental results, the electronic structure of these two species are quite different: whereas the unpaired electron is delocalized on the whole PC(H)R moiety in the phosphaalkenic anion, it is markedly localized on the phosphorus atom in the phosphafulvene anion. Calculated spin densities and charge distributions for phosphafulvene and azafulvene anions are compared.
  • Phosphaalkene derivatives of furane and thiophene : synthesis, crystal structure, electrochemistry and EPR study of their radical anions
    A. Jouaiti, A. Al Badri, M. Geoffroy and G. Bernardinelli
    Journal of Organometallic Chemistry, 529 (1-2) , 1997, p143-149
    Keywords: phosphaalkene; electron spin resonance; electrochemistry; crystal structure; radical anion
    DOI:10.1016/S0022-328X(96)06526-6 | unige:2807 | Abstract | Article PDF
Two phosphaalkenes containing either a furane or a thiophene ring bound to the carbon atom of the −Pdouble bond; length as m-dashC < bond have been synthesized. The crystal structure of the furane derivative has been determined and the electrochemistry of both compounds has been investigated. THF solutions of these compounds react at 255 K with a potassium mirror to yield the corresponding radical anions which have been studied by EPR in both the liquid and solid states. The resulting hyperfine constants are compared with the values predicted by ab initio calculations on radical anions formed from model phosphaalkenes.
  • The structure of diphosphaallenic radical cations as evidenced by EPR experiments and ab initio calculations
    M. Chentit, H. Sidorenkova, A. Jouaiti, G. Terron, M. Geoffroy and Y. Ellinger
    Journal of the Chemical Society. Perkin transactions 2, 1997 (5) , 1997, p921-926
    unige:2808 | Abstract | Article PDF
The isotropic hyperfine coupling constants of the diphosphaalkene radical cation have been measured by EPR spectroscopy after electrochemical oxidation of ArP]] C]] PAr (and ArP]] 13C]] PAr) in tetrahydrofuran (THF). The two 31P constants as well as the 13C coupling are close to 90 MHz. Taking HPCPH as a model compound, the structure has been assessed, by extensive ab initio calculations including correlation effects at the MP2 and MCSCF levels of theory. It is found that oxidation of the allenic ]P]] C]] P] structure leads to the formation of two rotamers with HPPH dihedral angles of 458 and 1358. These two structures are compatible with the Jahn–Teller distortion of allene. The calculated hyperfine constants support the EPR results.
  • Radiation damage in Pt(II) complexes : EPR study of an x-irradiated single crystal of Pt(1,3-dimethyl.imidazoline-2-thione)4Cl2.4H2O
    Michel Wermeille, Michel Geoffroy, Sushil Misra, Philippe Arrizabalaga and Gerald Bernardinelli
    Radiation Physics and Chemistry, 49 (3) , 1997, p347-356
    DOI:10.1016/S0969-806X(96)00146-6 | unige:2809 | Abstract | Article PDF
Several radiation defects have been detected by EPR in a single-crystal of Pt(dmimt)4Cl2.4H2O (dmit = 1,3-dimethyl-imidazoline-2-thione). In order to identify these rediogenic species, the structure of the crystal has been resolved and the angular dependence of the EPR signals has been analysed. The resulting g tensors and 195Pt hyperfine tensors have been determined and the orientations of their principal axes have been compared to those of the bond directions of the precursor. It is shown that both Pt(I) and Pt(III) complexes are trapped, whereas Pd(dmimt)42+ present as an impurity, leads only to the Pd(I) species. The temperature dependende of the EPR spectra gives information about the relative stability of the paramagnetic species and shows that the formation of some species, especially the Pt(III) complexes, requires drastic modifications of the parent Pt(II) cation.
  
  • Ionic conductivity of Na2S single crystals between 295 and 1350K Experimental setup and first results
    B. Bertheville, D. Lovy, H. Bill and F. Kubel
    Journal of Physics and Chemistry of Solids, 58 (10) , 1997, p1569-1577
    Keywords: inorganic compounds | crystal growth | X-ray diffraction | dielectric properties | electrical conductivity
    DOI:10.1016/S0022-3697(97)00101-7 | unige:3535 | Abstract | Article PDF
We study the ionic conductivity versus temperature and frequency of large Na2S single crystals by using a calibrated impedance apparatus. The experimental setup used for the ionic conductivity measurements up to 1350 K and its calibration are described. The apparatus allows to measure complex impedances between 0.1 Ω and 10 GΩ. The high temperature conductivity data were analyzed in terms of the conventional Frenkel defect model. We assume that cation vacancies and cation interstitials are the dominant intrinsic defects. The energy of motion was found to be 0.61 ± 0.05 eV for a cation vacancy. The energy of formation of a Frenkel defect pair is 2.51 ± 0.05 eV. Results are given that show clear evidence of a superionic behaviour close to the melting point, similar to the one found in alkaline earth fluorides and several halides. Furthermore, X-ray diffraction experiments on a high optical quality single crystal were performed. The cell parameter and the population parameter of Na+ were accurately determined (6.5373 Å and 0.988, respectively).
  • Defect association, Jahn-Teller or off-center effect in CaF2:Copper, BaF2:Silver ?
    ValĂŠrie Lefevre, Alain Monnier, Marc Schnieper, Dominique Lovy and Hans Bill
    Zeitschrift fĂźr Physikalische Chemie, 200 , 1997, p265-273
    unige:2810 | Abstract
The results of a detailed optical and paramagnetic-resonance study performed on copper in CaF2 and silver in BaF2 are presented. Two different Ag+ centers were identified in BaF2. One is associated with an interstitial F- ion whereas the other one has a cubic surrounding. The Cu2+ ion in CaF2 was shown to reorient at 4.2 K between 6 equivalent minima of D2h symetry. This fact is interpreted with the aid of a T2gx(T2g+Eg) type Jahn-Teller effect. The nonlinear mixed coupling terms are shown to play an important role. The Cu+ impurity in CaF2 is presumably off-center in the F- sublattice without associated defect or impurity.
  • Role of π-Bonding for Trigonal Level Splittings in Chromium(III) Complexes. 4. Doublet States and Zeeman Level Splittings in [Cr(bpy)3]3+
    Thomas SchĂśnherr, Michail Atanasov and Andreas Hauser
    Inorganic Chemistry, 35 (7) , 1996, p2077-2084
    DOI:10.1021/ic950150o | unige:6514 | Abstract | Article HTML | Article PDF
A detailed analysis of Zeeman splittings of highly resolved spin-forbidden transitions in [Cr(bpy)3](PF6)3 is presented. Assignments of vibronic bands are made based on low-temperature absorption, emission, and infrared spectra. The pattern of doublet states, obtained for H = 0 and H = 5 T, is consistent with angular overlap model (AOM) calculations, which allow one to consider σ- and π-interactions between the metal-d and relevant ligand orbitals and the particular angular geometry of the chromophore simultaneously. The observed level splittings are found to result from the combined effect of trigonal distortion and contributions of the symmetry adapted dπ-orbitals involved due to coupling with corresponding counterparts from the bidentate ligand (phase coupling). The larger splitting of the lowest excited state 2Eg(Oh) in the analogous ClO4- salt is due to the more distorted geometry of the [CrN6] moiety. Related properties of the bipyridine ligand, which turn out to show donor behavior in the present compounds, and the acetylacetonate ligand are discussed, and AOM parameters for the metal−ligand π-interaction are correlated with results of MO calculations.
  • A Complete Active-Space Self-Consistent-Field Study on Cubic N8
    S. Evangelisti and L. Gagliardi
    Il Nuovo Cimento D, 18 (12) , 1996, p1395-1405
    DOI:10.1007/BF02453781 | unige:2969 | Abstract | Article PDF
Complete Active-Space Self-Consistent-Field (CAS-SCF) calculations for cubic N8 are presented. We studied the N8↔4N2 reaction inD 4h symmetry and found its energy release and activation barrier with three different atomic basis sets. The energy release for this reaction is predicted to be around 526 kcal/mol, while the energy barrier to dissociation is estimated about 159 kcal/mol. These results are in substantial agreement with previousab initio estimates.
  • Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001
    David S. McKay, Everett K. Gibson Jr., Kathie L. Thomas-Keprta, Hojatollah Vali, Christopher S. Romanek, Simon J. Clemett, Xavier F.D. Chillier, Claude R. Maechling and Richard N. Zare
    Science, 273 (5277) , 1996, p924-930
    DOI:10.1126/science.273.5277.924 | unige:2970 | Abstract | Article HTML
Fresh fracture surfaces of the martian meteorite ALH84001 contain abundant polycyclic aromatic hydrocarbons (PAHs). These fresh fracture surfaces also display carbonate globules. Contamination studies suggest that the PAHs are indigenous to the meteorite. High-resolution scanning and transmission electron microscopy study of surface textures and internal structures of selected carbonate globules show that the globules contain fine-grained, secondary phases of single-domain magnetite and iron sulfides. The carbonate globules are similar in texture and size to some terrestrial bacterially induced carbonate precipitates. Although inorganic formation is possible, formation of the globules by biogenic processes could explain many of the observed features, including the PAHs. The PAHs, the carbonate globules, and their associated secondary mineral phases and textures could thus be fossil remains of a past martian biota.
  • ENDOR spectra of an X-irradiated single crystal of the R3P=CH-C(O)CH3 Wittig reagent
    T. Berclaz, M. Geoffroy, G. Rao and Z. Tancic
    Bulletin of Magnetic Resonance, 17 (1-4) , 1996, p296-297
    unige:2836
A new member belonging to the binary phase diagram of BaF2 and BaCl2 was synthesized. The single domain crystals of Ba12F19Cl5 can be prepared from a nonstoichiometric flux with molar ratio of 1 : 1 between BaFCl and BaF2. The compound crystallizes at room temperature in the non-centrosymmetric hexagonal space group P62m with a = b = 1408.48(14) and c = 427.33(5) pm. Three different barium environements with coordination number of nine are found. The barium fluorine distances vary between 250.59(6) - a short distance compared to other Ba — F distances - and 302.7(1) pm and barium chlorine distances between 331.55(3) and 336.19(15) pm. This compound is further characterized using Raman spectroscopy.
 
In the structure Ba12F19Cl5 [hexagonal space group P62m] the two chlorides on the sites Cl(1) and Cl(2) can partially be replaced by bromide ions. Single crystals of the type Ba12F19Clδ Br5-δ  with a chloride to bromide ratio up to 2 : 3 could be obtained by cooling a flux of 75 mol% BaF2 and 25 mol% BaX2 with X = Cl, Br. The crystal quality decreases with increasing bromide concentration. Structural parameters of five selected single crystals with different chloride/bromide ratio were studied by single crystal X-ray diffraction methods. The refined total Cl-/Br- population ratio in the crystals is close to the one of the flux. The lattice parameters and interatomic distances change in various ways, when the smaller chloride ion is replaced by the bigger bromide ion. The refinements show a statistical disorder on the halide sites with preferential bromide substitution on site Cl(1).
The compound Sr4OCl6 was synthesized and its crystal structure was determined by using a ferroelectrically single domain crystal. The structure is similar to those of other M4OCl6 compounds where M=Ca,Yb,Eu,Sm and Ba. Interatomic distances are compared with these compounds. The Raman spectra of Sr4OCl6 and Ba4OCl6 are compared.
Resonant fluorescence line narrowing of the R1 line of the [Cr(ox)3]3− chromophore in [Rh(bpy)3][NaCr(ox)3]ClO4 at 1.6 K neither gives rise to the usual three-line pattern nor to spectral diffusion. Instead multi-line spectra with spacings equal to the zero-field splitting of the ground state are observed. This phenomenon is attributed to efficient non-radiative resonant energy transfer within the R1 line.
  • La théorie de la fonctionnelle de la densité : un outil pour l'étude théorique de complexes de coordination
    Henry Chermette
    L'actualitĂŠ chimique, 261 (7) , 1996, p10-15
    Keywords: theoretical study | density functional method | historical aspects | coordination bond | application | electronic structure | iron
    unige:2818
An overview of the possibilities brought by the density functional theory to the coordination chemistry is given. The new trends in the development of the theory are outlined.
  • Synthesis and characterisation of a heterodinuclear ruthenium(II)-palladium(II) complex with two different cyclometallating sites
    Abdelaziz Jouaiti, Michel Geoffroy and Jean-Paul Collin
    Inorganica Chimica Acta, 245 (1) , 1996, p69-73
    Keywords: ruthenium complexes | palladium complexes | cyclometallated ligand complexes | heterodinuclear complexes
    DOI:10.1016/0020-1693(95)04799-9 | unige:2819 | Abstract | Article PDF
A bis-cyclometallating ligand bearing two different terdentate coordination sites (N---C---N: dipyridyI-benzene; P---C---P: diphosphaalkenebenzene moieties) has been synthesised. Selective reactions of appropriate metal complex precursors afforded a heterodinuclear ruthenium(II)-palladium(II) complex characterised by 1H, 13P NMR spectroscopy and FAB-MS techniques. We have compared its electrochemical and spectroscopic properties (absorption and emission) with the individual ruthenium(II) and palladium(II) subunits.
  • Second-order perturbation theory using correlated orbitals. II. A coupled MCSCF perturbation strategy for electronic spectra and its applications to ethylene, formaldehyde and vinylidene
    O. Parisel and Y. Ellinger
    Chemical Physics, 205 , 1996, p323-349
    DOI:10.1016/0301-0104(95)00430-0 | unige:2820 | Abstract | Article PDF
In this second paper, the philosophy of coupling multiconfigurational variational wave functions to perturbation treatments (MC/P methodology) is extended to the calculation of electronic spectra. The corresponding methodology is presented with emphasis on its flexibility and an overview of other available approaches is given. The contracted MC/P scheme is then applied to ethylene H2C=CH2, formaldehyde H2C=O vinylidene H2C=C. It is shown that combining well-designed averaged zeroth-order MCSCF wave functions to a barycentric Møller-Plesset (BMP) partition of the electronic Hamiltonian provides accurate spectra, contrary to Epstein-Nesbet partitions. The MC/BMP transition energies compare with experimental data within a few hundreds of cm−1. These results have been obtained using a polarized double-zeta quality basis set augmented by a set of semi-diffuse functions (6–31 + G*) and by an extra set of diffuse orbitals to account for Rydberg states. Since non-dynamic correlations effects that are important for a proper description of the manifold of the excited states of interest are included in the MCSCF zeroth-order space will all remaining correlation effects (non-dynamic and dynamic) are treated at the perturbation level, the present study lets anticipate applications of the MC/P methodology to medium size systems without much computational trouble.
The characteristics of natural organic matter (NOM), which affect its interactions with and therefore the transport of colloids and particles, depend on the evolution of its individual components. Information on the temporal variations of the components of the organic matter of a well-studied eutrophic lake (Bret, VD, Switzerland) was extracted using pyrolysis/gas chromatography/mass spectrometry. The influence of aquagenic organic matter was found to be at a maximum in summer. The soil-derived organic matter, on the other hand, was found in larger proportions in winter and spring. The appearance of hydroxypropanone in the pyrolysis fragments in the summer months indicates that this portion of the organic matter is aquagenic and very fresh. The dominance of furaldehyde in pyrolysates during the rest of the year indicates the presence of polysac charides may be of either aquagenic or pedogenic origin. The absence of lignin fragments found in the NOM in this lake suggests that lignin-containing components of terrestrial organic matter are not leached out from the soil in significant quantities. A correlation was found between 3-day cumulative rainfall and the proportion of terrestrial components in the identified organic matter in the spring. This correlation disap peared in the summer, probably because of a higher vegetal cover and the masking effect of the high aquagenic productivity. These factors are also likely to be of importance in more complicated lacustrine systems.
  • On the HCN/HNC abundance ratio : a theoretical study of the H + CNH <-> HCN + H exchange reaction
    D. Talbi, Y. Ellinger and E. Herbst
    Astronomy and Astrophysics, 314 , 1996, p688-692
    unige:2822 | Abstract | Article PDF
The hydrogen exchange reaction H+CNH->HCN+H may be a key step in gas-phase interstellar nitrogen chemistry. It is one of the reactions supposed to cause increasing HNC depletion with increasing temperature in dense interstellar clouds. In this paper we report the results of extensive ab-initio calculations on the H+CNH<->H+HCN system that partially confirm this hypothesis. It is shown that both forward and reverse reactions possess activation barriers. However, the activation energy of the H+CNH channel (4.2+/-1.0kcal/mol.) is four times smaller than for the endothermic HCN+H path. Calculations on the rate of the forward reaction show that tunneling under the entrance channel barrier allows a small rate coefficient at the temperatures under 100K and that the rate coefficient increases steadily with increasing temperature for T>100K.
  • Nucleosides and acyclonucleosides bearing a N-hydroxyureido group
    Jean M.J. Tronchet, Martina ZsĂŠly, Mohammed Iznaden, Françoise Barbalat-Rey, Michel Geoffroy and GĂŠrald Bernardinelli
    Carbohydrate letters, 2 , 1996, p101-108
    unige:2823
Hydroxyurea (HU), a ribonucleotide reductase inhibitor has been used in the treatment of some malignant and viral diseases and seems now to be promising, in association with 2,3dideoxynucleosides, for the management of AIDS. In an attempt to increase the specificity of action of this radical scavenger, or at least, to study the topological aspects of its reactivity, we introduced the N-hydroxyureido group into nucleosides by using Mitsunobu reaction or by reacting a nucleoside nitrogen nucleophile with a carbonyl electrophile. From the currently available antiviral testing results, concerning the nucleoside analogues it appears that the most noticable activity exert against Varicella Zoster virus (VZV). One acyclonucleoside derivative was found to be very active against the virus HIV-1, its therapeutic index is better than 100.000. We prepared peptid-like dinucleotide analogues33,36 also in which the internucleosidic bridge consists of a spacer of approximately the same length as in the natural compounds. These compounds could be tested as inhibitors of nucleotide-protein interactions, we supposed that they are able to disrupt zinc finger parts of nucleocapsid. Antiviral activity of these dinucleotides were tested in vitro against HIV-1, HIV-2, HSV-1, HSV-2, CMV, VZV and EBV but in no case EC50 values inferior to 10 ÂľM was found.
  • An NQR study of inclusion compounds formed between small chlorine-containing molecules and bis(N-alkylimidazolidine-2-thione)Cu(I) halide complexes
    Ya-Nan Zhao, Sundara Ramaprabhu and Edwin A.C. Lucken
    Zeitschrift fĂźr Naturforschung A, 51 , 1996, p677-685
    unige:2824
  • NQR and Mössbauer spectroscopy
    Edwin A.C. Lucken, Fernande Grandjean and Gary J. Long
    in "Physical methods in supramolecular chemistry" Pergamon, New York, 1996, p225-276
    unige:3948
The pseudo rotation of PF5 has been investigated using both static and dynamic density functional theory (DFT) methods. The lowest energy path is the Berry pseudorotation, corresponding to the concerted exchange of two apical and two equatorial ligands. The potential energy surface has been derived and the transition state localised. In ab initio molecular dynamics the Berry pseudorotation has been observed and occurs with a typical period of 0.6 ps at 750 K. Analysis of the trajectories and comparison of the spectral density with the vibrational frequencies is presented.
  • Methane concentration profiles in a lake with a permanently anoxic hypolimnion (Lake Lugano, Switzerland-Italy)
    Rongming Liu, Annette Hofmann, Fazil O. GĂźlaçar, Pierre-Yves Favarger and Janusz Dominik
    Chemical Geology, 133 (1-4) , 1996, p201-209
    Keywords: lake Lugano methane | water column | anoxic water | fluxes | biogenic methane
    DOI:10.1016/S0009-2541(96)00090-3 | unige:2812 | Abstract | Article PDF
Methane concentrations were measured in the water column of the northern basin of Lake Lugano in 1993 and 1994. Methane profiles show three distinct zones: (1) low concentrations generally below 0.1 mmol m−3 from the surface to 80–90-m depth; (2) a. sharp rise in concentration up to 50 mmol m−3 from 80–90- to not, vert, similar 150-m depth; and (3) from not, vert, similar 150-m depth to the bottom with a lower concentration gradient and with the highest methane concentrations in near-bottom water was not, vert, similar 80 mmol m−3. These profiles result from a combination of several factors such as fluxes from sediments, spatially variable vertical mixing, and aerobic and anaerobic bacterial oxidation in the water column. δ13C values indicate a biogenic origin for the methane. The methane inventory in the anoxic hypolimnion is not, vert, similar 2000 metric tonnes, yet its transfer to the surface water is almost completely cancelled by oxidation at the permanent redox interface situated at not, vert, similar 90-m water depth. Increased methane concentrations in near-surface water are not related to the deep-water methane reservoir but probably result from bacterial production of methane in the photic zone. Surface water is over saturated with methane in respect to the atmosphere. Anthropogenic eutrophication in temperate zone lakes has affected the carbon budget and methane production and storage. The potential impact of these changes on global emissions to the atmosphere should be evaluated.
For the HNC/HCN interconversion we show that the push-pull hydrogen exchange reaction H+CNHright harpoon over leftHCNHright harpoon over leftHCN+H is favoured over internal isomerization; the formation of H2CN or CNH2 followed by rearrangement to HCNH and subsequent elimination are more energy demanding processes. Both push-pull forward and reverse reactions present activation barriers. However, the activation energy on the H+CNH entrance channel (4.2Âą1.0 kcal/mol) is four times smaller than on the HCN+H path. As a consequence, it can be anticipated that there will be a range of temperatures where the H+CNH reaction will be efficient while the reverse HCN+H process is still inhibited. This process, much less endothermic than internal isomerization, should become an important path for HNC/HCN conversion with increasing temperature in star forming regions.
The recent detection of SiN in the outer envelope of the IRC+10216 carbon star has renewed the interest for the gas phase interstellar silicon chemistry. In this contribution, we present a theoretical study of the H2SiN+ molecular ion, the silicon hydrogenated counterpart of the previously studied SiNH + 2. On many points, the differences relative to the SiNH + 2 isomer have been found to be dramatic. As an example, the dipole moment is computed to be 3.8 D while being only 0.5 D in SiNH + 2. The radio, infrared and electronic signatures have been evaluated at a quantitative level. The rotational constants and vibrational frequencies have been determined using Möller–Plesset MPn (n=2,3,4), coupled cluster (CCSDT) and complete active space self-consistent field (CASSCF) methods for H2SiN+ and some of its isotopomers. These quantities have been corrected using a scaling procedure derived from previous studies on the HNSi, HSiN, HSiNH2, H2SiNH, and SiNH + 2 species in order to provide quantitative results. The failure of single-reference perturbation theories to predict a relevant infrared spectrum is discussed. Intense bands around 550, 950, and 2300 cm–1 are predicted. The electronic spectrum has been obtained using a coupled multiconfiguration SCF–perturbation treatment (MC/P): It is characterized by a large number of excited states, none of them having a strong transition moment. The lowest excited state is predicted to lie 0.54 eV above the ground state, but the first allowed transition having a nonnegligible oscillator strength has to be searched at 6.44 eV
  • Interstellar silicon-nitrogen chemistry. 1, The microwave and the infrared signatures of the HSiN, HNSi, HSiNH?2, HNSiH2 and HSiNH+ species
    O. Parisel, M. Hanus and Y. Ellinger
    Chemical Physics, 212 (2-3) , 1996, p331-351
    DOI:10.1016/S0301-0104(96)00216-9 | unige:2814 | Abstract | Article PDF
The experimental and the theoretical interests for the silicon chemistry have been renewed by the recent detection of SiN in space. In this contribution a theoretical study of the HSiN, HNSi, HSiNH2 and HNSiH2 molecular systems is presented that aims to help in the interpretation of available experimental results as well as in the attribution of new interstellar lines. The main goal of this report remains, however, the calibration of ab initio calculations on still-unknown silicon-nitrogen systems: the infrared and the microwave signatures of the HSiNH+ cation are reported as a direct application. The signatures of the five molecules under investagation have been computed at increasing levels of post-Hartree-Fock theories, using up to a 6–311 + + G** atomic orbital expansion. Accurate geometries and Be rotational constants have been determined at the Möller-Plesset MPn(n = 2, 3, 4), CASSCF and CCSD(T) theoretical plateaus for HNSi. The comparison with experimental data allows then to derive the scaling factors needed to obtain accurate rotational constants for related species: they are applied as such on the crude constants determined for HSiN, HSiNH2, HNSiH2, and finally HSiNH2 in its floppy linear singlet ground state and in its lowest cis-bent a3A′ state as well. Dipole moments are reported in order to assess the feasability for these species to be detected owing to their rotational signatures either in the laboratory or in space using millimetric radioastronomy techniques. Infrared (IR) signatures are computed at the same levels of theory and compared to the recent matrix isolation experiments devoted to HSiN, HNSi, HSiNH2 and HNSiH2. The calculations unambiguosly confirm that all these species have been effectively produced and observed. They also lead to the determination of accurate IR scaling factors that are significantly larger than the usual ones. Such an approach allows then to quantitatively predict the IR spectra of the still-unknown HSiNH+ entity. The study of the IR spectra furthermore points out the failure of single-reference correlation methods to obtain predictive IR signatures in some cases, as is unambigously illustrated in the case of the HSiN species.
The liquid Mg - Bi system exhibits strong compound formation at the `octet' composition . We present results of first-principles molecular dynamics simulations of this alloy system at different compositions: the pure Mg and Bi liquid components, the stoichiometric liquid, and a Mg-rich composition . For the pure liquids, our results are in excellent agreement with experimental diffraction data. For , a significant modification of the characteristics of the local ordering is found w.r.t. the crystalline -phase: the ordering in the liquid is much more ionic. This structural modification is consistent with the structure of the superionic -phase, that was reported recently by Barnes et al 1994 J. Phys.: Condens. Matter 6 L467. Our simulations cannot reproduce the `reverse' metal - nonmetal transition observed upon melting, the computed conductivity being much larger than found in experiments. Instead, for the Mg-rich alloy, the calculated conductivity approaches closely to the experimental value.
  • Electron affinity of hydrogen peroxide and the [H2,O2].- potential energy surface : a comparative DFT and ab initio study
    Jan Hrusak, Heike Friedrichs, Helmut Schwarz, Holy Razafinjanahary and Henry Chermette
    Journal of Physical Chemistry, 100 (1) , 1996, p100-110
    DOI:10.1021/jp9519934 | unige:2816 | Abstract | Article HTML | Article PDF
Standard ab initio and density functional calculations have been applied to the reaction O•- + H2O → [H2O2]•- → OH- + OH•. While two intermediates are found as minima on the anionic potential energy surface, neither of them is directly related to the structure of neutral hydrogen peroxide. The results of different combinations of exchange and correlation functionals are systematically compared to each other and to MP2, MP4SDTQ, and CCSD(T) calculated data. The role of the basis set and the Hartree−Fock exchange in the hybrid DFT scheme is discussed. While for the two minima a reasonable agreement between all the methods was found, the geometries of the located transition structures strongly depend on the method and basis set used.
  • The electroaffinity of O2 by DFT and coupled MCSCF/perturbation approaches : a computational experiment
    O. Parisel, Y. Ellinger and C. Giessner-Prettre
    Chemical physics letters, 250 (2) , 1996, p178-186
    DOI:10.1016/0009-2614(96)00016-4 | unige:2825 | Abstract | Article PDF
The electroaffinity of the O2 molecule is revisited using density functional theory (DFT) and perturbation treatments built on a MCSCF wavefunction that includes most of the non-dynamic correlation effects (MC/P approach). Using a standard 6–31 + G* basis set, DFT treatments based on BLYP or B3LYP functionals provide electroaffinities of the order of +0.6 eV that compare favorably to experiment. Coupled MCSCF/perturbation treatments using an Epstein-Nesbet partition of the molecular Hamiltonian give a more accurate value of +0.492 eV in excellent agreement with the most recent experimental data (+0.431 eV) as well as with highest-level purely variational ab initio treatments which are far less tractable for larger systems. The analysis of the results in terms of differential correlation effects made it possible to identify the failure of the previous MCSCF-limited treatments as arising from the dynamic correlation of the electron pair describing the σO---:O bond.
Some properties of small Lin clusters (n up to 20) are theoretically investigated, within the density functional theory formalism. The structural properties are examined at the so-called local level of approximation. For very small clusters (n<=8), the Lin conformations which are well known from ab initio calculations are found at very low computational cost. For n>8, optimal starting geometries are generated from two growth patterns, based on the increase of the number of pentagonal subunits in the clusters by adsorption of one or two Li atoms. Several new stable structures are proposed, for which the corresponding vibrational analysis is performed for n up to 18. The study of energetic properties and stability requires the use of gradient-approximated functionals. Such functionals are used for the determination of the relative stability of these clusters. For example, we show that the icosahedral structure is the most favorable geometry for Li13, whereas this is not the case for Na13. Ionization potentials and binding energies are also investigated in regard to the size and the geometry of the clusters. Comparison with experimental results and other theoretical approaches (such as nonspherical jellium model) suggests that some combinations of gradient-corrected functionals are more adapted than others to describe Lin energetic and structural properties.
We present density functional calculations of the bonding structures and diffusion barriers for a Cl adatom on Si(001)-(2×1). Besides the stable adsorption site at the dangling bond (DB), a metastable bridge-bonded state breaking a surface dimer bond and ~1.1 eV higher in energy than the DB is found. The calculated properties of this state agree with recent ESDIAD and HREELS observations. This bridge-bonded site is not along the Cl intradimer diffusion pathway of lowest energy. For this path a transition state also having a bridging structure (but not breaking the dimer bond) and rather low in energy ( ~0.6 eV with respect to the DB) is determined. The low intradimer barrier is consistent with the facile switching of Cl recently observed in scanning tunneling microscopy experiments.
The recently developed frozen density functional theory (FDFT) is extended to ab initio free energy calculations of chemical reactions in solution. This method treats the solute−solvent system as a supermolecule but constrains the electron density of the solvent molecules. Unlike hybrid quantum mechanical/molecular mechanics techniques, FDFT represents the solvent quantum mechanically. The quality of the solute−solvent interaction potential is examined by generating clusters of a reacting system and several solvent molecules and comparing the supermolecule DFT energies to the corresponding FDFT energies. The FDFT potential surfaces for solute−solvent systems provide a good approximation of the supermolecule DFT surfaces and require, in some cases, several orders of magnitude less computation time (in particular if one treats many solvent molecules quantum mechanically). The ab initio free energy surface for the F- + HF → FH + F- proton transfer reaction in solution is calculated using the corresponding “classical” empirical valence bond (EVB) potential surface as a reference potential. The encouraging results indicate that FDFT can be used to study chemical reactions in solution, capturing the quantum mechanical aspects of the solvent, which is not possible using hybrid quantum mechanical/molecular mechanics approaches. Furthermore, the use of EVB as a reference potential is found to be an extremely effective way of obtaining ab initio free energies for chemical processes in solution or in clusters.
  • How much correlation can we expect to account for in density functional calculations ? Case studies of electrostatic properties of small molecules
    J. Weber, P. Jaber, P. Gulbinat and
    in "Strategies and Applications in Quantum Chemistry" (Y. Ellinger and M. Defranceschi, eds.), Kluwer, Dordrecht, 1996, p219
From our combined experimental and computer modeling study we found a structurally and kinetically well-defined second coordination shell around chromium(III) ions in aqueous solution. Strong hydrogen binding due to polarization of first coordination sphere water molecules leads to a mean coordination number of 12.94 water molecules in the second shell and to short first shell hydrogen−second shell oxygen distances of about 1.4 Å. The experimentally measured exchange rate constant of = (7.8 ± 0.2) × 109 s-1 (ΔH = 21.3 ± 1.1 kJ mol-1, ΔS = +16.2 ± 3.7 J K-1 mol-1) corresponds to a lifetime of 128 ps for one water molecule in the second coordination shell and compares very well with a lifetime of 144 ps as observed from molecular dynamics simulation of a [Cr(H2O)6]3+ complex in aqueous solution. The geometry and the partial atomic charges of [Cr(H2O)6]3+ were determined by density functional theory (DFT) calculations. Water exchange from the second coordination shell to the bulk of the solution proceeds between a H2O sitting in the second shell and an adjacent one which just entered this shell from the bulk. By a small rotation of the first coordination shell water molecule, one of its two hydrogen bonds jumps to the entered water molecule and the one which lost its hydrogen bond leaves the second shell of the [Cr(H2O)6]3+. This associative reaction mode is a model for water exchange between water molecules which are bound by strong hydrogen bonds, as in the case for strongly polarizing 3+ ions such as Al3+ or Rh3+. Furthermore, the exchange phenomenon between second sphere and bulk water involving only two adjacent water molecules is strongly localized and independent of other water molecules of the second shell. In this respect it may be considered as a starting point for a study of water exchange on a protonated metal oxide surface.
  • Accuracy of approximate kinetic energy functionals in the model of Kohn-Sham equations with constrained electron density : the FH···NCH complex as a test case
    T.A. Wesolowski, H. Chermette and J. Weber
    Journal of Chemical Physics, 105 (20) , 1996, p9182-9190
    DOI:10.1063/1.472823 | unige:2829 | Abstract | Article PDF | Article PS (gzipped)
Ground‐state properties of a linear hydrogen‐bonded FH...NCH complex are studied by means of the ‘‘freeze‐and‐thaw’’ cycle of Kohn–Sham Equations with constrained electron density (KSCED) [T. A. Wesolowski and J. Weber, Chem. Phys. Lett. 248, 71, (1996)]. For several geometries of the complex, the electron density and the total energy are compared to the ones obtained by means of the standard Kohn–Sham calculations. The comparisons are made to assess the accuracy of several gradient dependent approximate kinetic energy functionals applied in the KSCED equations. It was found that the closest results to the Kohn–Sham ones were obtained with the functional whose analytical form was proposed by Perdew and Wang for exchange energy [J. P. Perdew and Y. Wang in Electronic Structure of Solids ’91, edited by P. Ziesche and H. Eschrig (Academie Verlag, Berlin, 1991), p. 11] and parametrized by Lembarki and Chermette for kinetic energy [A. Lembarki and H. Chermette, Phys. Rev. A 50, 5328 (1994)]. Around the interaction energy minimum as well as for larger intermolecular distances, the ‘‘freeze‐and‐thaw’’ cycle of KSCED leads to very similar potential energy surface as the standard supermolecule Kohn–Sham calculations. 
The ground and excited state properties of the Cr3+ ion doped into the cubic host lattices Cs2NaYCl6 and Cs2NaYBr6 have been studied using density functional theory. A new symmetry based technique was employed to calculate the energies of the multiplets 4A2g, 4T2g, 2Eg, and 4T1g. The geometry of the CrX3 - 6 cluster was optimized in the ground and excited states. A Madelung correction was introduced to take account of the electrostatic effects of the lattice. The experimental Cr–X distance in the ground state can be reproduced to within 0.01 Å for both chloride and bromide systems. The calculated d–d excitation energies are typically 2000–3000 cm–1 too low. An energy lowering is obtained in the first 4T2g excited state when the octahedral symmetry of CrX3 - 6 is relaxed along the eg Jahn–Teller coordinate. The geometry corresponding to the energy minimum is in excellent agreement with the 4T2g geometry derived from high-resolution optical spectroscopy of Cs2NaYCl6:Cr3+. It corresponds to an axially compressed and equatorially elongated CrX3 - 6 octahedron.
  • Kohn-Sham equations with constrained electron density : an iterative evaluation of the ground-state electron density of interacting molecules
    T.A. Wesolowski and J. Weber
    Chemical Physics Letters, 248 (1-2) , 1996, p71-76
    DOI:10.1016/0009-2614(95)01281-8 | unige:2831 | Abstract | Article PDF
A new method for calculating the ground state electron density of interacting molecules is presented. The supermolecule electron density is obtained using an iterative procedure. At each step the electron density of one molecule is calculated using previously introduced Kohn-Sham equations with constrained electron density. These equations contain terms representing the coupling between constrained and non-constrained electron densities. The coupling terms also involve a new functional, namely the non-additive kinetic energy functional that is not present in the original Kohn-Sham method. Its first-principles analytical form in not yet known. We examine the analytical form of this functional derived from Thomas-Fermi theory. The electron density obtained is compared with that calculated using the original Kohn-Sham method applied to the supermolecule. Good agreement has been found for a broad range of electron density overlaps.
  • Resonant and phonon-assisted excitation energy transfer in the R1 line of [Cr(ox)3]3-
    M.E. Von Arx, A. Hauser, H. Riesen, R. Pellaux and S. Decurtins
    Physical Review B, 54 (22) , 1996, p15800-15807
    DOI:10.1103/PhysRevB.54.15800 | unige:2832 | Abstract | Article PDF
In resonant fluorescence line narrowing (FLN) experiments in the R1 transition of the [Cr(ox)3]3- chromophore in [Ru(bpy)3][NaAl:Cr(1%)(ox)3] and [Rh(bpy)3][NaCr(ox)3]ClO4 multiline spectra are observed at 1.8 K, (ox=oxalate, bpy=2,2’-bipyridine). For [Rh(bpy)3][NaCr(ox)3]ClO4 the number of lines and their relative intensities depend critically upon the excitation wavelength within the inhomogeneous distribution, and in time-resolved FLN experiments additionally upon the delay. This behavior is clear evidence for a resonant energy-transfer process. At 4.2 K the more common phonon-assisted process becomes dominant, manifesting itself as spectral diffusion.
Chemical variation and combination of metal ions of different valencies in the oxalate backbone as well as in the tris-bpy cation of the three-dimensional network structures of the type [MII2(ox)3][MII(bpy)3] (bpy = 2,2'-bipyridine, ox = C2O42-), [MIMIII(ox)3][MII(bpy)3] and [MIMIII(ox)3][MIII(bpy)3]ClO4 offer unique opportunities for studying a large variety of photophysical processes. Depending upon the relative energies of the excited states of the chromophores, excitation energy transfer either from the tris-bipyridine cation to the oxalate backbone or vice versa is observed, as for instance from [Ru(bpy)3]2+ as photo-sensitiser to [Cr(ox)3]3- as energy acceptor in the combination [NaCr(ox)3][Ru(bpy)3], or from [Cr(ox)3]3- to [Cr(bpy)3]3+ in [NaCr(ox)3][Cr(bpy)3]ClO4. In addition efficient energy migration within the oxalate backbone is observed. Furthermore, depending upon the excited state redox potentials, light-induced electron transfer processes may be envisaged.
  • Chiral three-dimensional supramolecular compounds: Homo and bimetallic oxalate and 1,2-dithiooxalate-bridged networks. A structural and photophysical study.
    S. Decurtins, H.W. Schmalle, R. Pellaux, P. Schneuwly and A. Hauser
    Inorganic Chemistry, 35 (6) , 1996, p1451-1460
    DOI:10.1021/ic950791j | unige:2973 | Abstract | Article HTML | Article PDF
In analogy to the [MII(bpy)3]2+ cations, where MII is a divalent transition-metal and bpy is 2,2‘-bipyridine, the tris-chelated [MIII(bpy)3]3+ cations, where MIII is CrIII or CoIII, induce the crystallization of chiral, anionic three-dimensional (3D) coordination polymers of oxalate-bridged (μ-ox) metal complexes with stoichiometries [MII2(ox)3]n2n- or [MIMIII(ox)3]n2n-. The tripositive charge is partially compensated by inclusion of additional complex anions like ClO4-, BF4-, or PF6- which are encapsulated in cubic shaped cavities formed by the bipyridine ligands of the cations. Thus, an elaborate structure of cationic and anionic species within a polymeric anionic network is realized. The compounds isolated and structurally characterized include [CrIII(bpy)3][ClO4] [NaCrIII(ox)3] (1), [CrIII(bpy)3][ClO4][MnII2(ox)3] (2), [CrIII(bpy)3][BF4] [MnII2(ox)3] (3), [CoIII(bpy)3][PF6][NaCrIII(ox)3] (4). Crystal data:  1, cubic, P213, a = 15.523(4) Å, Z = 4; 2, cubic, P4132, a = 15.564(3) Å, Z = 4; 3, cubic, P4132, a = 15.553(3) Å, Z = 4; 4, cubic, P213, a= 15.515(3) Å, Z = 4. Furthermore, it seemed likely that 1,2-dithiooxalate (dto) could act as an alternative to the oxalate bridging ligand, and as a result the compound [NiII(phen)3][NaCoIII(dto)3]·C3H6O (5) has successfully been isolated and structurally characterized. Crystal data:  5, orthorhombic, P212121, a = 16.238(4) Å, b = 16.225(4) Å, c = 18.371(5) Å, Z = 4. In addition, the photophysical properties of compound 1 have been investigated in detail. In single crystal absorption spectra of [CrIII(bpy)3][ClO4][NaCrIII(ox)3] (1), the spin−flip transitions of both the [Cr(bpy)3]3+ and the [Cr(ox)3]3- chromophores are observed and can be clearly distinguished. Irradiating into the spin-allowed 4A2 → 4T2absorption band of [Cr(ox)3]3- results in intense luminescence from the 2E state of [Cr(bpy)3]3+as a result of rapid energy transfer processes.
  • Intersystem Crossing Dynamics in the Iron(III) Spin-Crossover Compounds [Fe(acpa)2]PF6 and [Fe(Sal2tr)]PF6
    S. Schenker, A. Hauser and R.M. Dyson
    Inorganic Chemistry, 35 (16) , 1996, p4676-4682
    DOI:10.1021/ic960010u | unige:2974 | Abstract | Article HTML | Article PDF
The high-spin → low-spin relaxation dynamics of the Fe(III) spin-crossover complexes [Fe(Sal2tr)]PF6 (H2Sal2tr = Bis(salicylaldimino)triethylenetetramine) and [Fe(acpa)2]PF6(Hacpa = N-(1-acetyl-2-propylidene)-2-pyridylmethylamine) are discussed within the theory of nonadiabatic multiphonon relaxation. A Huang−Rhys factor S of ≈25, estimated on the basis of average metal−ligand bond length differences ΔrHL of ≈ 0.12 Å, explains the observed low-temperature tunneling rate constants kHL(T→0) of ≈ 102 s-1 as well as the thermally activated process at T > ≈100 K semiquantitatively. The results obtained for the Fe(III) compounds are compared to those for Fe(II) spin-crossover compounds.
  • The HS -> LS Relaxation under External Pressure in the Fe(II) Spin Crossover System [Zn1-xFex(ptz)6](BF4)2 (ptz = 1-propyltetrazole, x = 0.1)
    J. Jeftic and A. Hauser
    Chemical Physics Letters, 248 (5-6) , 1996, p458-463
    DOI:10.1016/0009-2614(95)01297-4 | unige:2975 | Abstract | Article PDF
At low temperatures an external pressure of 1 kbar accelerates the high-spin → low-spin relaxation in the [Zn1−xFex(ptz)6](BF4)2, X = 0.1, spin-crossover system by one order of magnitude. This is due to the large difference in volume between the high-spin and low-spin states of 26 Å3/molecule. The relative vertical and horizontal shifts of the potential wells of the two states as a function of pressure are estimated to be 130 cm−1/kbar and 10−3 Å/kbar, respectively.
  • Interplay between the spin transition and the crystallographic phase transition in the Fe(II) spin-crossover system [Zn1-xFex(ptz)6](BF4)2 (x = 0.1, 1; ptz = 1-propyltetrazole)
    J. Jeftic, H. Romstedt and A. Hauser
    Journal of Physics and Chemistry of Solids, 57 (11) , 1996, p1743-1750
    Keywords: inorganic compounds; high pressure; phase transitions; thermodynamic properties
    DOI:10.1016/0022-3697(96)00033-9 | unige:2976 | Abstract | Article PDF
In the [Fe(ptz)6](BF4)2 (ptz = 1-propyltetrazole) spin-crossover system, the thermal spin transition is accompanied by a first order crystallographic phase transition (Tc↓ = 128 K and Tc↑ = 135 K) from R3 above Tc↓ to P1 at low temperatures (Wiehl L., Acta Cryst. B49, 289 (1993)). The high-symmetry phase can be super-cooled, in which case the spin transition is still complete and quite steep (T1/2 = 125 ± 2 K) but now without a hysteresis. The corresponding interaction constant Γ is 170 cm−1. In the diluted system [Zn1−xFex(ptz)6](BF4)2, X = 0.1, the spin transition is gradual with T1/2 = 95 ± 2 K. From the shift of T1/2 towards high temperatures with external pressure a value for ΔVHL0 of 26 Å3 molecule−1 is obtained. Pressures above 250 bar induce a crystallographic phase transition even in the diluted system, as a result of which the spin transition is discontinuous. The interplay between the thermal spin transition and the crystallographic phase transition in the neat and the diluted system is discussed consistently.
  • The [Fe(etz)6](BF4)2 Spin-Crossover System - Part Two: Hysteresis in the LIESST Regime
    R. Hinek, H. Spiering, P. GĂźtlich and A. Hauser
    Chemistry - A European Journal, 2 (11) , 1996, p1435-1439
    Keywords: hysteresis; iron complexes; LIESST; spin crossover; tetrazoles
    DOI:10.1002/chem.19960021116 | unige:2977 | Abstract | Article PDF
In the [Fe(etz)6](BF4)2 spincrossover system the iron(II) complexes occupy two nonequivalent lattice sites, sites A and B. Complexes on site A show a thermal high-spin (HS) → low-spin (LS) transition at 105 K, whereas complexes on site B remain in the HS state down to 10 K. Complexes on both sites exhibit light-induced spin state conversions (LIESST) at 20 K: LS → HS on site A with λ = 514.5 nm, and HS → LS on site B with λ = 820 nm. The relaxation processes subsequent to the HS → LS conversion on site B reveal a light-induced HS→LS bistability for the complexes on site B at 70 K. The bistability as well as the absence of a thermal spin transition on site B are attributed to a thermal hysteresis for the B-site complexes with a critical temperature T↑c K on heating. This hysteresis can be interpreted in terms of strong cooperative effects of elastic origin, which, in addition, cause characteristic deviations of the relaxation on site B from first-order kinetics (self-acceleration). In contrast, the HS → LS relaxation at 60 K on site A after irradiation with λ = 514.5 nm shows an unusual self-retardation.
  • The [Fe(etz)6](BF4)2 Spin-Crossover System - Part One: HS-LS Transition on Two Lattice Sites
    R. Hinek, H. Spiering, D. Schollmeyer, P. GĂźtlich and A. Hauser
    Chemistry - A European Journal, 2 (11) , 1996, p1427-1434
    Keywords: iron complexes; LIESST; spin crossover; tetrazoles
    DOI:10.1002/chem.19960021115 | unige:2978 | Abstract | Article PDF
The [Fe(etz),](BF,), spin-cross-over system (etz = 1-ethyl-1 H-tetrazole) crystallizes in space group P1, with the following lattice constants at 298 K: a10.419(3), b=15.709(1), c = 18.890(2) Å, α = 71.223(9), β =77.986(10), and γ = 84.62(1)° V = 2862.0(9) Å3 and Z = 3. Two nonequivalent lattice sites, one without (site A) and one with (site B) inversion symmetry, are observed. The population of the two sites nA:nB is 2:l. Iron(II) on site A undergoes a thermal low-spin (LS) → high-spin (HS) transition with T1/2I, = 105 K. whereas that on site B remains in the high-spin state down to cryogenic temperatures. Application of external pressure of up to 1200 bar between 200 and 60 K does not cause formation of the low-spin state on site B. On site A the high-spin state can be populated as a metastable state at 20 K by irradiating the sample with λ = 514.5 nm; on site B a light-induced population of the low-spin state can be achieved with λ = 820 nm.
  • Acid-catalysed backbone rearrangement of cholesta-6,8(14)-diene
    Liu Rong-Ming, X.Fr.D. Chillier, P. Kamalaprija, U. Burger and F.O. GĂźlaçar
    Helvetica Chimica Acta, 79 (4) , 1996, p989-998
    DOI:10.1002/hlca.19960790407 | unige:2833 | Abstract | Article PDF
Rearrangement of 5α- and 5β-cholesta-6,8(14)-dienes (13a and 13b, resp.) in the presence of anhydrous toluene-4-sulfonic acid in acetic acid leads to 5α- and 5β-12(13 → 14)-abeo-cholesta-8,13(17)-dienes (15a and 15b, resp.) via 5α- and 5β-cholesta-8,14-dienes (14a and 14b, resp.), respectively. Epimerization at C(20) of the spirosteradienes 15a and 15b occurs with increasing reaction time. Molecular-mechanics calculation of the relative stabilities of these compounds and of congeners thereof is in agreement with the observed reaction pathway.
To determine the limitations of electrospray mass spectrometry for the study of condensed-phase chemistry, it is important to understand the origin of cases for which the electrospray mass spectra, which are a measure of the relative abundances of gas-phase ions, do not reflect the equilibrium ion abundances in the solution electrosprayed. One such divergent case is that of free-base octaethylporphyrin. Under conditions for which this porphyrin is present in solution predominantly as the doubly charged, diprotonated molecule, the predominant ionic species observed in the electrospray mass spectrum is the singly charged, monoprotonated molecule. In this paper, direct optical spectroscopic measurements of the ions in solution (absorption spectra) and in the electrospray plume (fluorescence excitation spectra) are correlated with the ion distribution observed in the gas-phase (as reflected in the electrospray mass spectra) to determine at what point in the electrospray process and by what mechanism(s) the transformation from dication to monocation occurs. The data indicate that the major portion of the doubly protonated porphyrin species originally present in solution are converted to singly protonated species relatively late in the electrospray process, during the latter stages of droplet desolvation in the atmospheric/vacuum interface of the mass spectrometer, via the loss of a charged solvent molecule/cluster.
A study of the energetics of electron transfer (e.t.) quenching of 9-cyanoanthracene fluorescence by aromatic (π) and aliphatic (n) donors in n-hexane using picosecond transient thermal phase grating spectroscopy is reported. The results show that the enthalpy of exciplex formation is well correlated with the redox potentials of the partners and independent of the nature of the donor, as long as exciton interaction is small. The main difference between exciplexes with donors of the two classes is the larger ground-state repulsion energy with n-donors. However, if the adiabatic ionization potentials are considered, the enthalpy of exciplex formation with n-donors is larger than expected by about 0.4 eV. These two contradictory results reflect an inconsistency between the values for adiabatic ionization potentials and oxidation potentials of n-donors. The occurrence of two distinct Rehm−Weller plots for e.t. quenching with π- and n-donors shifted by about 0.8 eV on the horizontal axis seems to originate to some extent from the use of adiabatic ionization potentials.
  • Photoinduced Electron Transfer between Triethylamine and Aromatic Carbonyl Compounds: the Role of the Nature of the Lowest Triplet State
    Patrizio Aspari, Nagwa Ghoneim, Edwin Haselbach, Markus Von Raumer, Paul Suppan and Eric Vauthey
    Journal of the Chemical Society. Faraday transactions, 92 (10) , 1996, p1689-1691
    DOI:10.1039/FT9969201689 | unige:2980 | Abstract | Article PDF
The rate constants of quenching of the triplet state T1 of four aromatic ketones by triethylamine in acetonitrile show no correlation with the Gibbs energy of the reaction, but depend on the nature of T1, i.e. n–π*, π–π* or charge transfer (CT) type. For example, the ratio of rate constants between two systems of n–π* and CT type, with the same driving force, is over 108. It is concluded that these reactions are kinetically controlled, the decisive factor for the activation energy being the electrostatic charge distribution of the carbonyl group in the triplet state. 
  • Picosecond Polarization Grating Study of the Effect of Excess Excitation Energy on the Rotational Dynamics of Rhodamine 6G in Different Electronic States
    J.-C. Gumy and E. Vauthey
    Journal of Physical Chemistry, 100 (21) , 1996, p8628-8632
    DOI:10.1021/jp953729g | unige:2982 | Abstract | Article HTML | Article PDF
The effect of excess excitation energy on the rotational dynamics of rhodamine 6G in the ground and the first singlet excited state has been investigated in series of n-alcohols and alkanenitriles using the picosecond polarization grating technique. In nitriles, the reorientation times are the same for excitation at the S1 ← S0 and S2 ← S0 transitions, and no state dependence could be detected. In alcohols, the rotational dynamics of rhodamine 6G in the excited state is about 25% faster when formed with 1.15 eV excess excitation energy. This effect is ascribed to a decrease of the hydrodynamic volume due to dissociation of solute/solvent hydrogen bond following intramolecular vibrational redistribution. An accompanying perturbation of the solvent shell structure caused by the fast local temperature jump is not excluded.
  • Picosecond transient Grating Spectroscopy: the Nature of the Diffracted Spectrum
    C. Hoegemann, M. Pauchard and E. Vauthey
    Review of Scientific Instruments, 67 (10) , 1996, p3449-3453
    Keywords: optical spectrometers | visible radiation | time resolution | ps range | absorption spectra | diffraction | time delay | aromatics | solutes | comparative evaluations
    DOI:10.1063/1.1147157 | unige:2983 | Abstract | Article PDF | Article PS (gzipped)
A ps transient grating setup using white light continuum for probing is presented. Measurements on an aromatic molecule in solution have been carried out with this system. The diffracted spectrum is analyzed using Kogelnik’s coupled wave theory. At short time delay after excitation, the diffracted spectrum is strongly dominated by absorption and in this case transient grating spectroscopy is equivalent but more sensitive to transient absorption spectroscopy. If some of the excitation energy is dissipated as heat, the diffracted spectrum is essentially the same as the dispersion spectrum of the transient species at time delays approaching half the acoustic period. The performances of this technique and of transient absorption spectroscopy are compared.
  • 1,2-Bis[2,4,6-tri-tert-butylphenyl]phosphanediylmethyl]benzene, L : synthesis and structure of L, of the chelated complex [PdLCl2] and of a derived cyclometallated chiral complex
    A. Jouaiti, M. Geoffroy and G. Bernardinelli
    ChemComm, (3) , 1996, p437-438
    DOI:10.1039/CC9960000437 | unige:2838 | Abstract | Article PDF
The synthesis and crystal structure of 1,2-bis[2-(2,4,6-tri-tert-butylphenyl)phosphanediylmethyl]benzene, L, are reported as well as the preparation and conformation of the novel seven-membered ring complex [PdLCl2]; this complex reacts with alcohols (MeOH, EtOH) to give a chiral cyclometallated complex [rac(R)P, (R)C; (S)P, (S)C] where the metal is bound to both a phosphaalkene and a phosphite phosphorus atom.
  • 9-substituted triptycene as a probe for the study of internal rotation around a C-PH bond in the solid state : a single crystal EPR study at variable temperature
    G. Ramakrishnan, A. Jouaiti, M. Geoffroy and G. Bernardinelli
    Journal of Physical Chemistry, 100 (26) , 1996, p10861-10868
    DOI:10.1021/jp960836h | unige:2835 | Abstract | Article HTML | Article PDF
To measure the rotation barrier around an R3C−•PH bond in the solid state, 9-phosphinotriptycene50 2 has been synthesized and its crystal structure has been determined. It is shown, by EPR, that the radiogenic radical 3, which results from a homolytic scission of a P−H bond, can indeed be trapped in the crystal matrix. Its g-tensor together with its 31P and 1H hyperfine coupling have been measured at 300 and 77 K. These tensors show that free rotation around the C−P bond occurs at room temperature but is blocked at liquid nitrogen temperature. The temperature dependence of the EPR spectra has been analyzed using the density matrix formalism and has led to a rotation barrier of about 2.5 kcal·mol-1 . This result and the various hyperfine couplings have been compared with the values predicted by ab initio methods for two isolated model radicals: the tert-butylphosphinyl radical 4 and the barrelenophosphinyl radical 5.
  • Jet spectra of aromatic molecules in hydrogen-bonded microsolvent clusters
    A.G. Taylor, T. BĂźrgi and S. Leutwyler
    in "Jet Spectroscopy and Molecular Dynamics" J. M. Hollas and D. Phillips, Blackie academic & professional, London, 1995
    Abstract
  • An ab initio derived torsional potential energy surface for (H2O)3. Analytical representation and stationary points
    Thomas BĂźrgi, Stephan Graf, Samuel Leutwyler and Wim Klopper
    Journal of Chemical Physics, 103 (3) , 1995, p1077-8
    DOI:10.1063/1.469818 | unige:14645 | Abstract | Article PDF
An intermolecular potential energy surface was derived for the hydrogen‐bonded water trimer as a function of the three torsional angles ω1, ω2, ω3, for energies up to 1300 cm−1 (3.7 kcal/mol) above the global minimum. The O...O distances and the intramolecular geometry of the H2O molecules are held fixed. This surface is based on the ab initio calculations presented in a companion paper [W. Klopper et al., J. Chem. Phys. 103, 1085 (1995)], which involve very large basis sets and the most extensive treatment of correlation energy for calculations of (H2O)3 so far. The 70 ab initio interaction energies, multiplied by six due to the S6 symmetry of the surface, were fitted using an analytical potential function, with an average error of ≊11 cm−1. This potential provides a rapidly computable analytical expression for use in calculations of torsional eigenfunctions and ‐values and other properties of this cluster. Also given is a classification of the low‐lying torsional wave functions according to nodal properties.
  • Intermolecular vibrations of phenol·(H2O)3 and d1-phenol·(D2O)3 in the S0 and S1 states
    Thomas BĂźrgi, Martin SchĂźtz and Samuel Leutwyler
    Journal of Chemical Physics, 103 (15) , 1995, p6350-12
    DOI:10.1063/1.470416 | unige:14647 | Abstract | Article PDF
We report a combined spectroscopic and theoretical investigation of the intermolecular vibrations of supersonic jet‐cooled phenol⋅(H2O)3 and d1‐phenol⋅(D2O)3 in the S0 and S1 electronic states. Two‐color resonant two‐photon ionization combined with time‐of‐flight mass spectrometry and dispersed fluorescence emission spectroscopy provided mass‐selective vibronic spectra of both isotopomers in both electronic states. In the S0 state, eleven low‐frequency intermolecular modes were observed for phenol⋅(H2O)3, and seven for the D isotopomer. For the S1 state, several intermolecular vibrational excitations were observed in addition to those previously reported. Ab initio calculations of the cyclic homodromic isomer of phenol⋅(H2O)3 were performed at the Hartree–Fock level. Calculations for the eight possible conformers differing in the position of the ‘‘free’’ O–H bonds with respect to the almost planar H‐bonded ring predict that the ‘‘up–down–up–down’’ conformer is differentially most stable. The calculated structure, rotational constants, normal‐mode eigenvectors, and harmonic frequencies are reported. Combination of theory and experiment allowed an analysis and interpretation of the experimental S0 state vibrational frequencies and isotope shifts.
  • Van der Waals binding energies and intermolecular vibrations of carbazole·R R=Ne, Ar, Kr, Xe)
    Thierry Droz, Thomas BĂźrgi and Samuel Leutwyler
    Journal of Chemical Physics, 103 (10) , 1995, p4035
    DOI:10.1063/1.469589 | unige:14650 | Abstract | Article PDF
Mass‐selective ground‐state vibrational spectra of jet‐cooled carbazole⋅R (R=Ne, Ar, Kr, and Xe) van der Waals complexes were obtained by populating ground‐state intra‐ and intermolecular levels via stimulated emission pumping, followed by time delayed resonant two‐photon ionization of the vibrationally hot complex. By tuning the dump laser frequency, S0 state vibrational modes were accessed from ≊200 cm−1 up to the dissociation energy D0. Upon dumping to ground‐state levels above D0, efficient vibrational predissociation of the complexes occurred, allowing us to determine the S0 state van der Waals binding energies very accurately. The D0(S0) values are <214.5±0.5 cm−1 (R=Ne), 530.4±1.5 cm−1 (R=Ar), 687.9±4.0 cm−1 (R=Kr), and 890.8±1.6 cm−1 (R=Xe). In the S1 state, the corresponding binding energies are larger by 9% to 12%, being <222.9±1.0 cm−1, 576.3±1.6 cm−1, 756.4±4.5 cm−1, and 995.8±2.5 cm−1, respectively.
  • Binding energies of carbazole·S van der Waals complexes (S=N2, CO and CH4)
    Thomas BĂźrgi, Thierry Droz and Samuel Leutwyler
    Journal of Chemical Physics, 103 (17) , 1995, p7228-12
    DOI:10.1063/1.470298 | unige:14644 | Abstract | Article PDF
Mass‐selective ground‐state vibronic spectra of molecular van der Waals complexes carbazole⋅S, S=N2, CO, and CH4, were measured by stimulated emission pumping followed by resonant two‐photon ionization of the vibrationally hot complexes. S0‐state vibrational modes were accessed from ≊200 cm−1 up to the ground‐state dissociation limit D0(S0) of the van der Waals bond. Above D0, efficient vibrational predissociation of the complexes occurs, allowing accurate determination of the van der Waals dissociation energies as 627.2±7.9 cm−1 for N2, 716.5±29.8 cm−1 for CO, and 668.6±15.1 cm−1 for CH4. In the S1 excited state, the van der Waals binding energies increase to 678.5±8.0, 879.2±29.9, and 753.8±15.2 cm−1, respectively. The relative increases upon electronic excitation are about 8% and 13% for N2 and CH4, similar to the analogous rare gases Ar and Kr. For CO, the relative increase of van der Waals binding energy is 23%. The differences are primarily due to electrostatic interactions.
Accurate hydrogen-bond dissociation energies were determined for gas-phase hydrogen-bonded complexes between 1-naphthol or 1-naphthol-d3 and H2O, CH3OH, NH3 and ND3, using the stimulated emission pumping-resonant two-photon ionization spectroscopy technique in supersonic jets. The hydrogen-bond dissociation energies obtained for the electronic ground state are D0 = 2035 ± 69 cm−1 for 1-naphthol · H2O, 2645 ± 136 cm−1 for 1-naphthol · CH3OH, 2680 ± 5cm −1 for 1-naphthol · NH3 and 2801 ± 14 cm−1 for 1-naphthol-d3 · ND3, respectively. Upon electronic excitation to the S1 state the binding energies increase by approximately 8%.
A coupled three-dimensional model calculation of the low-frequency large-amplitude intermolecular torsional states in (H2O)3 and (D2O)3 is presented, based on the analytical modEPEN intermolecular potential surface and a three-dimensional discrete variable representation approach. The lowest torsional levels of both (H2O)3 and (D2O)3 lie above the sixfold (upd) torsional barrier. The first eight (eleven) torsions of (H2O)3 ((D2O)3) are pseudorotational states. The ‘radial’ and ‘polar’ torsional fundamental frequencies are predicted at 151 and 160 cm−1 for (D2O)3, and for (H2O)3 at 185.0 and 185.3 cm−1, respectively. Each of these in turn support a ladder of pseudorotational levels.
  • Functional Activity of a Biotinylated Human Neurokinin 1 Receptor Fusion Expressed in the Semliki Forest Virus System
    K. Lundstrom, A. Vargas and B. Allet
    Biochemical and Biophysical Research Communications, 208 (1) , 1995, p260-266
    Keywords: HAMSTER OVARY CELLS; ESCHERICHIA-COLI; SUBSTANCE-P; PEPTIDES; BINDING; PROTEIN; VECTORS
    DOI:10.1006/bbrc.1995.1332 | unige:2992 | Abstract | Article PDF
The 1.3 S biotinylatable subunit of Proprionibacterium shermanii transcarboxylase complex was fused to the C-terminus of the human neurokinin 1 receptor gene and introduced into the Semliki Forest virus expression vector pSFV1. RNA transcribed from pSFV1-NK1-biot and pSFV-Helper2 was coelectroporated into BHK cells permitting in vivo packaging of recombinant virus. Infection of BHK and CHO cells with SFV-NK1-biot virus yielded high level of the fusion receptor as detected by metabolic labeling, immunoblotting with streptavidin alkaline phosphatase and binding to substance P. Like native receptor, the biotinylated receptor fusion was able to stimulate Ca2+ mobilization in infected CHO cells,indicating functional coupling to guanine-nucleotide-binding proteins.
The spectroscopic properties of several Ag+ activated strontium fluoride crystals—originating from different batches—were examined. Absorption, emission and excitation experiments including luminescence dynamics and polarization studies were performed at various temperatures. Four Ag+ related photoluminescent centers were found. Their properties are described. The predominant species in the asgrown crystals emits in the UV (at 315 nm) and was shown to be the single Ag+ ion in cubic surroundings. Two other centers emitting in the violet (400 nm) and in the yellow (550 nm), respectively, were attributed to (Ag+)2pairs with different geometries. Models of these pseudo-molecular clusters are proposed. The nature of the system which produces a weak green (500 nm) luminescence remains open to questions.
We performed CAS-CI calculations on Li2 using a set of molecular orbitals (MO) optimized with a procedure that, in the case of highly symmetric molecules, permits extraction of a small set of MO out of a large set of atomic orbitals (AO). The dimension of the CAS-CI space was of about 12 million symmetry-adapted determinants. We determined some spectroscopic constants of Li2 with three different atomic basis sets of increasing quality. The values obtained with the largest atomic basis set are very close to the experimental results.
  • Solvation Free Energies of Amides and Amines: Disagreement between Free Energy Calculations and Experiment
    and Peter A. Kollman
    Journal of the American Chemical Society, 117 (22) , 1995, p6057-6063
    DOI:10.1021/ja00127a019 | unige:2988 | Abstract | Article PDF
We present molecular dynamidfree energy calculations on the molecules acetamide, N-methylacetamide, N,N-dimethylacetamide, ammonia, methylamine, dimethylamine, and trimethylamine. Unlike the experimental data, which suggest a very non-additive solvation free energy (N-methylacetamide and methylamine having the most negative free energy of solvation), the calculations all find that the free energy of solvation monotonically increases as a function of methyl addition. The disagreement with experiment is surprising, given the very good agreement (within 0.5 kcal/mol) with experiment for calculation of the solvation free energy of methane, ethane, propane, water, methanol, and dimethyl ether.
  • Reconstructions and phase transitions at semiconductor surfaces: Ge(111)
    A. Selloni, Noboru Takeuchi and E. Tosatti
    Surface Science, 331-333 (2) , 1995, p995-1001
    DOI:10.1016/0039-6028(95)00102-6 | unige:2989 | Abstract | Article PDF
First principles molecular dynamics studies of the low, intermediate, and high temperature phases of Ge(111) are reviewed. The atomic structure and electronic properties of the c(2 × 8) reconstruction, the diffusion of Ge adatoms at the c(2 × 8) → (1 × 1) disordering transition at T ~ 300°C, and the behavior of Ge(111) close to the bulk melting temperature are discussed.
  • X-ray Absorption Near-Edge Structures of Chloroferrates [FeIICl4]2-, [FeIIICl4]-, and [FeIIICl6]3-: Experimental and MS-LSD Computational Studies
    C. Mehadji, H. Chermette, C. Cartier and M. Verdaguer
    Journal of Physical Chemistry, 99 (15) , 1995, p5568-5574
    DOI:10.1021/j100015a046 | unige:2840 | Abstract | Article PDF
The influence of electronic and structural effects in FeCL and FeCl6 entities has been investigated through X-ray absorption spectroscopy and theoretical calculations of electronic transition energies using the MSLSD method. The relative importance of the formal oxidation degree of the metal, the metal-ligand distance, and the symmetry of the site on the energy of near-edge structures are studied. The principal effect is the stabilization of the iron 1s orbital in the ground state when the oxidation degree increases. The accuracy of the theoretical determination regarding the experimental spectra is discussed. The incidence of the same electronic parameters on the intensities of near-edge structures is also investigated through a transition cross section calculation.
Thermally stimulated depolarization current (TSDC) measurements of CaF2:Cu+ samples show three different dipolar relaxation bands, with temperature peaks at 52, 144 and 187 K. The two higher temperature bands are more intense and reveal the presence of Cu+ in probably two different lattice sites. The band at 144 K suggests it is related to the Cu+ off-center effect, observed through an optical absorption at 328 nm, and it is likely temperature independent. To this Cu+ off-center effect it is also associated an impurity-vacancy defect, that result from needed charge compensation in CaF2. The observed TSDC band at 187 K is attributed to Cu+i—Fi- pair, both ions in interstitial positions. The weakest TSDC at 52 K is assumed to be from a low un-intentional Mn2+ doping, sitting in an off-center substitutional position. It is proposed a model to explain the several positions available for the substitutional Cu+ that response for the TSDC result.
  • Teaching computational chemistry using computers
    Jacques Weber and
    Chimia, 49 (3) , 1995, p77-83
    unige:2842 | Abstract | Article PDF
  • Synthesis of 3-aza-8-oxabicyclo[3.3.0]octane and 3-aza-9-oxabicyclo[4.3.0]nonane derivatives from D-glucose via retro-cope eliminations
    Jean M.J. Tronchet, Martina ZsĂŠly, Rami Nadra Yazji, Françoise Barbalat-Rey and Michel Geoffroy
    Carbohydrate letters, 1 , 1995, p343-348
    unige:2846
  • Structure and Stability of Zeolite Offretite under Si4+/(Al3+, M+) Substitution (M = Na, K): A First Principles Molecular Dynamics Study
    L. Campana, A. Selloni, J. Weber and A. Goursot
    Journal of Physical Chemistry, 99 (44) , 1995, p16351-16356
    DOI:10.1021/j100044a023 | unige:2845 | Abstract | Article PDF
A local density functional study of the structure and energetics of offretite, with a Si4+ ion substituted by (A13+, M+) (M = Na, K), is presented. The calculations have been performed within the framework of the first principles molecular dynamics method using a periodically repeated unit cell with 55 atoms. It is found that the preferred site for K+ cations lies inside the cancrinite cage, in agreement with experiment. This site is related to A1 substituting a Si atom at a TI site, which is one of the two inequivalent tetrahedral sites of offretite, and belongs to the hexagonal prism, Site selectivity is quite pronounced in the case of K+, as the large size of this cation gives rise to important steric effects in the offretite framework. For Na+, instead, the lowest energy site is located in the channel, near a window of the cancrinite cage, and corresponds to an A1 in Tz. For this cation the spread in the relative substitution energies of the different binding sites is very small, the four sites of lower energies being in a range of -2.5 kcal/mol.
Neoglycolipids bearing a paramagnetic probe in their lipophilic aglycon have been prepared. All belong to the Image -glucose series, both anomers for the glucoside representatives, respectively β and ι anomers in the S- and C-glucosyl series. Two different types of radical sites have been used, a relatively short-lived imino N-oxyl group for glucosides and a more stable N-acylamino N-oxyl moiety in the other cases. EPR spectra of these radical species afforded information on the conformation of the lipophilic chain in the vicinity of the paramagnetic probe.
  • Spin labelled C-glycoside analogs : derivatives of 1,4-anhydro-4-deoxy-2,3-O-cyclopentylidene-1,4-N-hydroxyimino-DL-erythrofuranose
    Jean M.J. Tronchet, Mirna Balkadjian, Guido Zosimo-Landolfo, Françoise Barbalat-Rey, Patrick Lichtle, Alessandra Ricca, Istvan Komaromi, GĂŠrald Bernardinelli and Michel Geoffroy
    Journal of Carbohydrate Chemistry, 14 (1) , 1995, p17-34
    DOI:10.1080/07328309508006434 | unige:2844 | Abstract | Article PDF
A series of 2,3-O-cyclopentylidene C-glycoside analogs in which the furanose ring has been replaced with a N-hydroxypyrrolidine have been prepared. A structural study of these tricyclic compounds and the aminoxyl radicals thereof has been carried out using variable temperature 1H NMR, X-ray diffraction, molecular dynamics and EPR spectroscopy. Both types of compounds, N-hydroxypyrrolidines and pyrrolidine N-oxyls, fundamentally prefer - in solutions- N-endo conformations over the alternate, N-exo forms found by X-ray diffraction studies and computed to be more stable by molecular dynamics.
  • Samarium doped alkaline earth halide thin films as spectrally selective materials for hole burning ?
    A. Monnier, M. Schnieper, R. Jaaniso and H. Bill
    Radiation effects and defects in solids, 135 (1-4) , 1995, p253-256
    Keywords: Calciumfluoride: Samarium; thin films; MBE; hole burning
    DOI:10.1080/10420159508229846 | unige:2856 | Abstract | Article PDF
Optical hole burning, a potential technique for spectrally selective recording, was demonstrated in Sm-doped MBE-grown thin films of CaF2/Si(111). The inhomogeneous broadening of the corresponding Sm2+ 5d(T1u) ↞ 4f(7F0, A1g) transition (690 nm) was investigated as a function of substrate temperature and film thickness. The MBE apparatus is briefly described as well as the thin film growth procedure.
  • Quantum chemistry and drug design
    Hans Ulrich Suter, Djordje M. Maric, Jacques Weber and Colin Thomson
    Chimia, 49 (5) , 1995, p125-125
    unige:2857 | Abstract | Article PDF
The state-of-the-art computational chemistry software, visualization tools and high performance computing infrastructure are employed to assist the design of new and more effective drugs. At present a project focuses on the quantitative structure activity relationships of antimalarial drugs.
Bis(N-methylimidazolidinethi-2-one)copper(I) chloride has been synthesized and its crystal structure determined. X-Irradiation of a single crystal of this compound leads to the formation of a CuII complex which was studied by EPR: it was shown that this species results from the addition of a radiogenic Cl atom on the CuI precursor. The structural changes induced by this reaction are revealed by the g-tensor and by the hyperfine tensors of one copper and two chlorine nuclei. The structure of this S2Cl2Cu type complex was compared with other sulfur- or chlorine-containing CuII complexes.
The fundamental aspects of N2 and 0 2 adsorption in zeolites have been investigated by density functional calculations on different models. Simple systems where these molecules interact with a positive point charge or with isolated Li+ and Na+ cations have led to a qualitative explanation for the N2/02 separation process. A classical description involving electrostatic and induction energies is adequate to explain the basic reason for a stronger N2 adsorption. At short distances (bonding interaction), the electronic structure of the cation has to be taken into account. The presence of core electrons in large cations limits the stabilizing contribution of the electrostatic and induction terms to the total energy, implying that Li+ is more efficient than Na+ in the adsorption process. The presence of zeolite clusters decreases the binding energies for both N2 and 0 2 , but the main trends remain valid. Moreover, due to a larger screening of 0 2 adsorption, it improves the efficiency of Li+ with respect to Na+ for the N2/02 separation.
  • Gradient-corrected exchange potential with the correct asymptotic behavior and the corresponding exchange-energy functional obtained from the virial theorem
    A. Lembarki, F. Rogemond and H. Chermette
    Physical Review A, 52 (5) , 1995, p3704-3710
    DOI:10.1103/PhysRevA.52.3704 | unige:2847 | Abstract | Article PDF
In density-functional theory (DFT), Perdew, Parr, Levy, and Balduz [Phys. Rev. Lett. 49, 1691 (1982)] have shown that for all the electronic systems, the energy of the highest occupied molecular orbital (HOMO) is equal to the negative of the ionization potential. This equality is not recovered within the different approximations of the exchange-correlation functional proposed in the literature. The main reason is that the exchange-correlation potentials of various functionals used in DFT calculations decay rapidly to zero whereas they should exhibit a Coulombic asymptotic -1/r behavior. In this work we propose a gradient-corrected (GC) exchange potential with a correct asymptotic -1/r form for large values of r. The energy of the HOMO calculated with this potential is improved compared to the local-density approximation (LDA) and to the GC functionals widely used in the DFT. Our HOMO eigenvalues are compared to the optimized-potential-model eigenvalues which are the exact values for the exchange-only potential. Using the fact that the LDA satisfies the virial theorem, the exchange energy corresponding to this GC exchange potential can be calculated under a simple assumption.
  • Crystallochemical and optical study of mixed alkaline earth-samarium fluorohalides of the lead fluorine chloride type
    F. Kubel, H. Hagemann and H. Bill
    Materials Research Bulletin, 30 (4) , 1995, p405-412
    Keywords: alkaline earths; lanthanide; fluorohalides; luminescent materials
    DOI:10.1016/0025-5408(95)00014-3 | unige:2984 | Abstract | Article PDF
Single crystals X-ray diffraction was performed on five crystals with nominal compositions Sr1/2Sm1/2FCl, Sr1/2Ca1/2FCl, Sr2/3Ba1/3FCl1/3Br2/3, Sr1/2Sm1/2FCl1/2Br1/2 and Sr0.6Sm0.1Ca 0.3FCl. Population refinements confirm the presence of correlations between lattice constants and composition. A new correlation between the heavy halogen z value and the unit cell volume is found. Luminescence spectra of bivalent samarium in these crystals contribute to the structural characterization of these compounds.
  • Computation of oscillator strengths from Kohn-Sham wave functions : the example of small lithium clusters
    Georges Gardet, François Rogemond and Henry Chermette
    Theoretica Chimica Acta, 91 (3-4) , 1995, p249-266
    DOI:10.1007/BF01114991 | unige:2848 | Abstract | Article PDF
Density functional calculations of ground and excited states of Li n (nle8) clusters have been performed, within two different approaches. Using a set of Kohn-Sham orbitals to construct wave functions, the calculation of the oscillator strengths of the electric dipole transitions is performed. Our results have been tested at two levels: first the necessary comparison with the experimental data, second the comparison with high level CI (MRD-CI) calculations. This last point is not a trivial challenge, because such anab initio method leads for small clusters to a highly accurate description of the electronic structure and optical absorption spectra. Finally, this is also a new test for the capability of using Kohn-Sham orbitals to construct physically meaningful wave functions. Transition energies, oscillator strengths and finally optical absorption spectra presented here are in general in reasonable agreement with the experimental data and the MRD-CI calculations. That is very promising for bigger systems, with regard to the modest computational effort (CPU time and memory size) of density functional calculations.
  • The benzodiphosphaalkene ligand and its Pd-II and Pt-II complexes : their synthesis, structure and an ESR study of their reduction products
    Abdelaziz Jouaiti, Michel Geoffroy, Gustavo Terron and GĂŠrald Bernardinelli
    Journal of the American Chemical Society, 117 (8) , 1995, p2251-2258
    DOI:10.1021/ja00113a014 | unige:2849 | Abstract | Article PDF
The new diphosphaalkene 1,3-bis[2-(2,4,6-tri-tert-butylphenyl)phosphanediylmethyl]benzenLe,, h as been synthesized. Due to the presence of two P=C bonds three isomers (EE, EZ, ZZ) were observed by 31P NMR, and the crystal structures of two of them could be determined (EE, ZZ). The electrochemical behavior of L has been studied by cyclic voltametry: a quasi-reversible reduction occurs at -1.89 V/SCE and corresponds to the formation of a radical anion which has been studied by ESR at variable temperature. The experimental 31P and 'H hyperfine constants are consistent with free rotation about the P=C and Cphosphaalkene-Cbenzene bonds at room temperature and agree with ab initio predictions. One of the isomers of L forms complexes with palladium(I1) and platinum(I1) ions. The crystal structures show that L is orthometalated and acts as a terdentate ligand by coordinating the metal with each phosphorus atom. These complexes are electrochemically reduced between -0.92 and - 1.29 V, and the resulting paramagnetic species are studied by ESR in liquid and frozen solutions. This reduction process was shown to be a ligand-centered process, an appreciable part of the unpaired electron is localized on each of the phosphaalkene carbons (20%) and phosphorus atoms (5%).
The possibilities and limits of the molecular orbital theory to deal with the problem of determining electronic structure of solids have been explored. A cluster model based on the charge neutrality in the solid has been used in test calculations on some III-V semiconductors and have given quite satisfactory results. Recommendations are given to widen the field of applications of this procedure.
Oxygen-free strontium fluoride crystals containing single monovalent silver ions in a cubic site were grown. Our experiments showed that the Ag+ ion remained chemically stable upon optical irradiation at 222 nm (KrCl excimer). The ion exhibits a strong UV luminescence which presents no thermal quenching up to RT. At this temperature, the emitting level time-constant is 12 Îźs. An explanation is proposed for the silver photostability by relating it to the large electronic bandgap of the host (11.4 ev). The 222 nm absorbing level lies below the conduction band in a way such that photoionization of Ag+ is avoided, as well as other optically-induced opacity phenomena. A minimum source intensity at threshold is estimated at some 276 kW, when using a Fabry-Perot cavity. This power can normally be achieved with the excimer laser.
  • Radical reactions in a single crystal of phosphaalkene: EPR and ab initio calculations of phosphoniumyl radical cations
    Shrinivasa N. Bhat, Theo Berclaz, Michel Geoffroy and Abdelaziz Jouaiti
    Journal of Physical Chemistry, 99 (43) , 1995, p15864-15869
    DOI:10.1021/j100043a026 | unige:2853 | Abstract | Article PDF
Two radiogenic radicals trapped in a single crystal of 1-[2,4,6-tri-tert-butylphenyl]-2-phenylphosphaethene have been studied by EPR and have been identified, from their 31P   hyperfine tensors, as being phosphoniumyl radical cations. The spectra modifications caused by 13C or 2D enrichment of the phosphaalkene moiety show that these species result from an intramolecular cyclization which can lead to two possible conformations of the radical. The experimental 31P, 13C, and 1H hyperfine tensors are compared with those predicted by ab initio calculations on model phosphoniumyl radical cations. These calculations show that these interactions are very sensitive to the geometry of the radical and that their measurement can yield precise structural information.
Basic properties relevant to spectral hole burning (homogeneous and inhomogeneous spectral broadening, hole burning and filling mechanisms) are investigated in MeIyMeII1 − yFXIxXII1 − x: Sm2+ (Me = Ca,Sr,Ba; X = Cl,Br,I) single crystals. The relations between the spectral characteristics of 5D0,1-7F0 transitions and the material structure are described. Hole stability is investigated up to 430 K and is shown to be determined by ionic diffusion.
The 63Cu NQR spectra of five dicoordinated complex cations of Cu(I) with substituted imidazoles as ligands and six analogous complexes with substituted pyrazoles as ligands are reported. The structures of four of these complexes have been previously determined and the relationship of their 63Cu resonance frequency to the average Cu---N bond length is compared to that of the analogous lutidine or collidine complexes. It is concluded that there are probably significant differences between the electronic structures of the pyridine complexes and those of the pyrazole or imidazole series.
  • Enantioselective hydrogenation of &alpha; -ketoesters : a molecular view on the enantio-differentiation
    A. Baiker, T. Mallat, B. Minder, O. Schwalm, K.E. Simons and J. Weber
    in "Chiral Reactions in Heterogeneous Catalysis" ( G. Jannes and V. Dubois eds.), Plenum, New York, 1995, p95
    unige:4049
  • Comparison of the performance of various gradient corrected exchange and correlation functionals in density functional theory. Case studies of CO and N2O molecules.
    H. Chermette, A. Lembarki, P. Gulbinat and J. Weber
    International Journal of Quantum Chemistry, 56 (6) , 1995, p753-762
    DOI:10.1002/qua.560560611 | unige:2860 | Abstract | Article PDF
An investigation of the influence of various gradient-corrected exchange and correlation functionals on the bond lengths and dipole moments of CO and N2O has been carried out using density functional theory. It is shown that whereas some functionals are found to be more sensitive to the basis set quality than are others, the more commonly used gradient-corrected functionals lead to properties in very good agreement with experiment provided that a sufficiently large basis set is employed.
  • Intersystem Crossing in Iron(II) Coordination Compounds: A Model Process between Classical and Quantum Mechanical Behaviour.
    Andreas Hauser
    Comments on Inorganic Chemistry, 17 (1) , 1995, p17-40
    Keywords: intersystem crossing; spin-crossover; iron(II) coordination compounds; nonadiabatic multiphonon relaxation; strong vibronic coupling limit; tunnelling
    DOI:10.1080/02603599508035780 | unige:2985 | Abstract | Article PDF
Intersystem crossing is the crucial first step determining the quantum efficiency of very many photochemical and photophysical processes. Spin-crossover compounds of first-row transition metal ions, in particular of Fe(II), provide model systems for studying it in detail. Because in these compounds there are no competing relaxation processes, intersystem crossing rate constants can be determined over a large temperature interval. The characteristic features are tunnelling at temperatures below ∟80 K and a thermally activated process above ∟ 100 K. This, as well as the twelve order of magnitude increase of the low-temperature tunnelling rate constant on going from a spin-crossover compound with a small zero-point energy difference to a low-spin compound with a substantially larger one, can be understood on the basis of a nonadiabatic multiphonon process in the strong vibronic coupling limit.
  • Potential of cutin hydroxyacids as palaeoenvironmental molecular indicators
    Olivier Blum, S. Dong, A. Buchs and F.O. GĂźlaçar
    in "Organic Geochemistry: Developments and applications to energy, climate, environment and human history. Selected papers from the 17th Int. Meet. Org. Geochemistry, San Sebastian, 1995." (J.O. Grimalt & C. Dorronsoro, eds.), A.I.O.G.A., San Sebastian, 1995, p875-877
    unige:3789
  • Picosecond Transient Thermal Phase Grating Study of a Photoinduced Electron Transfer Reaction in Solution
    E. Vauthey and A. Henseler
    Journal of Physical Chemistry, 99 (21) , 1995, p8652-8660
    DOI:10.1021/j100021a033 | unige:2986 | Abstract | Article PDF
A study of the dynamics and thermodynamics of the photoinduced electron transfer reaction between benzophenone and 1 ,Cdiazabicycl0[2.2.2]octane in acetonitrile using picosecond transient thermal phase grating spectroscopy is reported. Two heat releases were observed within the time window of the experiment (0-4 ns): a fast one corresponding to the formation of a benzophenone lower triplet state and a slow one due to the electron transfer reaction. The observed dynamics is in agreement with earlier studies using transient absorption spectroscopy. The quenching of benzophenone in the first singlet excited state at high concentration of quencher and the very rapid decay of the resulting singlet geminate ion pair are confirmed. The enthalpy of formation of the geminate ion pair was determined by comparing the amounts of heat released in the fast and slow processes. The electron transfer is more exergonic by 0.23 eV than calculated from the Rehm- Weller equation, assuming spherical ions with point charges in their centers. The difference is tentatively ascribed to the electrostatic interaction within the ion pair.
  • Isomerisation Dynamics of a Thiacarbocyanine Dye in Different Electronic States and in Different Classes of Solvents
    E. Vauthey
    Chemical Physics, 196 (3) , 1995, p569-582
    DOI:10.1016/0301-0104(95)00135-B | unige:2990 | Abstract | Article PDF
A study of the isomerisation dynamics of 3,3′-diethylthiacarbocyanine in the excited state in alkanenitriles as well as in the ground state in both alkanenitriles and n-alcohols, using ps transient grating spectroscopy and flash photolysis is reported. The intrinsic activation energy for isomerisation has been determined from isoviscosity plots. In the ground state, the activation energy amounts to 12.9 kcal/mol in both nitriles and alcohols, while it is equal to 3.0 kcal/mol in the excited state. The reduced isomerisation rate constant shows a fractional dependence on the solvent viscosity, with a fractional exponent α of 0.20 and 0.38 for the ground state isomerisation in alcohols and nitriles and of 0.65 for the excited state isomerisation. The data have been analysed within the framework of the Kramers theory using different models for friction. The best agreement was obtained with the Grote-Hynes model, which takes into account the frequency dependence of the friction. The solvent dependence of α for the ground state isomerisation is accounted for by a different average value of the infinite frequency shear modulus in alcohols and nitriles. The barrier height dependence of α is explained by a decrease of the barrier's top frequency by going from the ground to the excited state.
An application of transient thermal phase grating spectroscopy with near-infrared probing to the determination of free ion yield in photoinduced electron transfer reactions is presented. The model system is 9,10-dicyanoanthracene with various electron donors in acetonitrile. The measured yields are in good agreement with those obtained from photoconductivity, when the latter are correctly calibrated. The method is explained and its advantages and limitations are discussed.
  • Comparative modelling studies on 3,6-substituted 1,2,4-trioxan-5-ones
    C.W. Jefford, G. Bernardinelli, M.C. Josso, and J. Weber
    Journal of Molecular Structure: THEOCHEM, 337 (1) , 1995, p31-37
    DOI:10.1016/0166-1280(94)04110-E | unige:2861 | Abstract | Article PDF
The main geometric elements of three 3,6-substituted 1,2,4-trioxan-5-ones have been calculated by using molecular mechanics (MM2), and semiempirical (AM1, PM3) methods. The results are compared with those obtained by X-ray analysis.
The structure of the ground and lowest two excited states of H2NO have been determined in large scale configuration interaction calculations using a multiconfiguration self-consistent description of the molecular orbitals. These treatments are based on a systematic building of the correlation contribution which has been designed to account for the characteristics of the nitroxide group. This approach shows that the aminoxyl functional group is more than a three electron group shared by two atoms, but, in fact, a nine electron entity. Our best estimate of the geometry of the ground electronic state, obtained after second-order configuration interaction using a large basis of atomic natural orbitals, is pyramidal. However, since the potential depth between 0° and 40° is lower or of the same order of magnitude as the estimated inversion frequency, the conclusion that this molecule behaves like a planar system is totally justified. The structure of the excited (η−π*) and (π−π*) states have been determined and the transitions energies are in accordance with the experimental results on the highly substituted stable nitroxide radicals.
  • Density-functional investigation of the excited state properties and the Jahn-Teller effect in [CrX6]3- (X=Cl,Br)
    Khalid Bellafrouh, Claude Daul, Hans U. GĂźdel, François Gilardoni and Jacques Weber
    Theoretica Chimica Acta, 91 (3-4) , 1995, p215-224
    DOI:10.1007/BF01114988 | unige:2863 | Abstract | Article PDF
The luminescence of [CrX6]3– X=Br–, Cl– has been studied through density functional theory (DFT) using both deMon and ADF codes. Multiplet energies4A2,2E,4T2, and4T1 have been expressed as energies of non-redundant single determinants and calculated as in Ref. [1]. The influence of the metal ligand distance on the multiplet energies has been investigated. Of particular interest to this work is the Jahn-Teller effect distortion. We found that the system moves to a more stable geometry when the axial bond length is compressed and the equatorial one elongated in agreement with the experimental value.
  • Density functional calculations of the structures and bond energies of Cr(CO)6 and (η6-C6H6)Cr(CO)2(CX) (X=O,S) complexes
    E. Furet and J. Weber
    Theoretica Chimica Acta, 91 (3-4) , 1995, p157-167
    DOI:10.1007/BF01114983 | unige:2864 | Abstract | Article PDF
Quantum chemical calculations based on density functional theory have been performed on Cr(CO)6, (eegr6-C6H6)Cr(CO)3 and (eegr6-C6H6)Cr(CO)2(CS) at the local and nonlocal level of theory using different functionals. Good agreement is obtained with experiment for both optimized geometries and metal-ligand binding energies. In particular, a comparison of metal-arene bond energies calculated for the (eegr6-C6H6)Cr(CO)3 and (eegr6-C6H6)Cr(CO)2(CS) complexes correlates well with kinetic data demonstrating that substitution of one CO group by CS leads to an important labilizing effect of this bond, which may be primarily attributed to a larger pgr-backbonding charge transfer to the CS ligand as compared with CO.
The enantioselective hydrogenation of Îą-ketoesters to Îą-hydroxyesters over Pt/Al2O3 catalysts modified by cinchona alkaloids is an interesting model reaction for the investigation of heterogeneous catalysis capable of producing optically active products. The aim of the present theoretical study is to rationalize the interaction between protonated cinchona alkaloids (modifiers) and methyl pyruvate (substrate) by investigating the possible weak complexes formed by these two species. For this purpose we use molecular mechanics and the AM1 semiempirical method. The optimization leads to two stable forms of the complexes, where the substrate is bound to the modifier via hydrogen bonding between the oxygen of the Îą-carbonyl of pyruvate and the quinuclidine nitrogen of the alkaloid. In such complexes the methyl pyruvate is transformed into a half-hydrogenated species which can be adsorbed on the platinum surface and, after hydrogenation, leads to methyl lactate product. The results show that adsorption of the complex leading to (R)-methyl lactate is more favorable than that of the corresponding system yielding (S)-methyl lactate, which may be the key for the enantio-differentiation.
The parametrization of the EHMO-ASED method we have recently suggested for organometallics is shown to be also applicable, in principle without any modification, to derive the major structural parameters of second-row transition metal systems such as carbonyls or metallocenes. Furthermore, this model leads to satisfactory results when used to calculate the structure of compounds as large as (N-methylindole)tricarbonylchromium(0) or (phenylo.xazoline) tricarbonylchromlum(0) with full geometry optimization of the ligands.
  
  • High-temperature spectral hole burning on Samarium(II) in single crystals of the lead fluorohalide structure family and thin films of calcium fluoride
    H. Bill, R. Jaaniso, H. Hagemann, D. Lovy, A. Monnier and M. Schnieper
    Optical Engineering, 34 (8) , 1995, p2333-2338
    DOI:10.1117/12.201813 | unige:3537 | Abstract | Article PDF
When modern spectral hole burning applications for high-density information storage under noncryogenic temperatures are envisioned, it is necessary to develop new frequency-selective photoactive materials for this purpose. Mixed compounds of the PbFCl family, doped with samarium(II) ions, exhibit promising and true room-temperature hole burning capabilities. We investigate this class of systems (and related ones) by combining material synthesis and high-resolution spectroscopy. Whole groups of isomorphous crystals were synthesized with varying degrees of halide anion and/or cation substitutions. Thin films of fluoride-based materials were made in a laboratory-built molecular beam epitaxy system. An extended x-ray study, differential thermal analysis, luminescence, and Raman measurements allowed the characterization of the materials. Formal models were developed for both the inhomogeneous zero-phonon optical line shapes of the samarium(II) and the time evolution of the hole burning.
  • ENDOR and transferred hyperfine interaction of impurity rare-earth ions with nearest diamagnetic ions in crystals
    M.L. Falin, M.V. Eremin, H. Bill and D. Lovy
    Applied Magnetic Resonance, 9 (3) , 1995, p329-354
    DOI:10.1007/BF03161957 | unige:2867 | Article PDF
The tetragonal Er3+ ion associated with the interstitial F− ion along the [100] axis in CaF2 is studied using ENDOR. The parameters of the transferred hyperfine interaction and of the nuclear Zeeman interaction of the surrounding fluorine ions are determined. Anomalously large values of the pseudo-nuclear Zeeman effect on the F− nuclei are found. The theoretical analysis of these parameters is carried out in a frame of operator techniques in the theory of transferred hyperfine interactions. A number of useful relations for practical calculations of the values of the local field at ligand nuclei are reported.
  • Raman Optical Activity of (β)-citronellene
    Hans Hagemann, Anne Lucken, Dominique Lovy and Hans Bill
    in "Proc. XIV th Conference on Raman Spectroscopy" Eds. N.T. Yu, X.Y. Ku, J. Wiley and sons, Singapore, 14 , 1994, p1072-1073
  • Low- and high-temperature phases of a Pb monolayer on Ge(111) from ab initio molecular dynamics
    Francesco Ancilotto, Annabella Selloni and Roberto Car
    Physical Review B, 50 (20) , 1994, p15158-15165
    DOI:10.1103/PhysRevB.50.15158 | unige:79092 | Abstract | Article PDF
We present an ab initio molecular dynamics study of the structure and dynamics of a close-packed monolayer of Pb on the Ge(111) surface, with coverage FTHETA=4/3, at different temperatures. The room-temperature (√3 × √3 ) structure is characterized by large in-plane fluctuations of the Pb adatoms, and agrees well with recent x-ray standing wave data. At T∼800 K we observe a (1×1) disordered structure showing in-plane diffusion of the Pb atoms. Disordering is confined to the plane of the overlayer. A strong correlation with the solid Ge substrate is present, leading to preferential residence sites and diffusion paths of the Pb atoms on the surface. The calculated local density of states for the high-temperature phase is found to agree with recent scanning tunnel microscope observations, which show a simply Pb-terminated (1×1) surface with an apparent coverageFTHETA=1.
  • Absorption and luminescence spectroscopy of ferrate (VI) doped into crystals of K2MO4 (M = S, Se, Cr, Mo)
    Thomas C. Brunold, Andreas Hauser and Hans U. GĂźdel
    Journal of Luminescence, 59 (5) , 1994, p321-332
    DOI:10.1016/0022-2313(94)90059-0 | Abstract | Article PDF
The absorption spectra of the ferrate (VI) ion (FeO2-4) in K2MO4 (M = S, Se, Cr, Mo) host lattices consist of a series of relatively weak bands at low energy, which can be assigned to transitions within the partially filled 3d shell and some intense bands at higher energy, which are assigned to ligand-to-metal charge transfer transitions (LMCT). In the near-infrared (NIR) region sharp lines are observed belonging to the spin-forbidden spin-flip transitions 3A2→ 1E and 3A2  → 1A1. The lowest excited state is the 1E state, serving as initial state for 1E → 3A2 sharp-line luminescence at around 6200 cm-1. Another luminescence is observed centered at 9000 cm-1, which is assigned to the 3T2 → 3A2 transition. It is rather broad and three orders of magnitude weaker than the 1E luminescence at 30K as a result of efficient non-radiative relaxation processes to the 1E state. The temperature dependence of the total intensity and the lifetime of the 1E → 3A2 luminescence is understood within a complex scheme of radiative and non-radiative processes.
A spectroscopic study of supersonic jet‐cooled catechol (1,2‐dihydroxybenzene) and its d1‐ and d2‐isotopomers, deuterated at the hydroxy groups, was performed by resonant two‐photon ionization (R2PI) and fluorescence emission techniques, and supplemented by molecular‐beam hole‐burning experiments. The latter prove that one single rotamer of catechol is dominant under molecular beam conditions. The complicated vibrational structure in the S0→S1 spectrum from the 000 band to 400 cm−1 above is not due to three different rotamers, as previously thought, but is due to the excitation of a vibrational progression associated mainly with the torsion of the hydroxy groups. The torsional bands are very prominent in the R2PI spectra, but are weak in the emission spectra. Detailed analysis of the torsional bands was based on a fit to the S1 and S0 state frequencies and the Franck–Condon factors in absorption and emission, using a double‐minimum potential for the S1 state and a harmonic potential for the S0 state. In the S1 state one of the two –O–H torsional mode frequencies is lowered from τ2≊250 to ≊50 cm−1, and the molecule is only quasiplanar with respect to the –O–H torsional coordinates.
  • Ground state binding energy and vibrations of the carbazole·Ar van der Waals complex by pump/dump-R2PI measurements
    Thomas BĂźrgi, Thierry Droz and Samuel Leutwyler
    Chemical Physics Letters, 225 (4-6) , 1994, p351-358
    DOI:10.1016/0009-2614%2894%2987093-4 | unige:14643 | Article PDF
Mass-selective ground state vibrational spectroscopy of the jet-cooled carbazole¡Ar complex was performed by populating ground-state levels via a pump/dump laser pulse sequence, followed by selective resonant two-photon ionization of the vibrationally relaxed complexes. Intra- and inter-molecular van der Waals modes in the S0 state are measurable with good signal/noise. The ground-state binding energy can be determined by detecting the negative signals resulting from loss of ground-state population via vibrational predissociation of the complex.
  • Structures, dynamics and vibrations of cyclic (H2O)3 and its phenyl and naphthyl derivatives
    Samuel Leutwyler, Thomas BĂźrgi, Martin SchĂźtz and Alan Taylor
    Faraday Discussions, 97 (97) , 1994, p285-297
    DOI:10.1039/FD9949700285 | unige:14662 | Abstract | Article PDF
The cyclic water trimer shows a fascinating complexity of its intermolecular potential-energy surface as a function of the three intermolecular torsional coordinates: there are six isometric but permutationally distinct minimum-energy structures of C1 symmetry, which can interconvert by torsional motions via six isometric transition states, also of C1 symmetry. A second type of interconversion can occur through different torsional motions via two C3 symmetric transition structures, and a third interconversion type via a planar C3h symmetric transition structure. The equivalence of the six minima is broken if the ‘free’ H atom of one H2O molecule in the cluster is chemically substituted, yielding three distinct conformers, which occur in enantiomeric pairs. Not all three conformers are necessarily locally stable minima; this depends on the substituent. The phenol–(H2O)2, p-cyanophenol–(H2O)2, 1-naphthol–(H2O)2 and 2-naphthol–(H2O)2 clusters, which are the phenyl, p-cyanophenyl and naphthyl derivatives of (H2O)3, were examined by resonant two-photon ionization spectroscopy in supersonic beams. These clusters exhibit S0→ S1 vibronic spectra with very different characteristics. These reflect the number of cluster structures formed, their low-frequency intermolecular vibrations and indirectly give information about the cluster fluxionality.
  • A polymeric two-dimensional mixed-metal network. Crystal structure and magnetic properties of {[P(Ph)4][MnCr(ox)3]}n
    S. Decurtins, H.W. Schmalle, H.R. Oswald, A. Linden, J. Ensling, P. GĂźtlich and A. Hauser
    Inorganica Chimica Acta, 216 (1-2) , 1994, p65-73
    Keywords: crystal structures, magnetism, manganese complexes, chromium complexes, dinuclear complexes
    DOI:10.1016/0020-1693(93)03711-I | unige:3005 | Abstract | Article PDF
The mixed-metal ferromagnet {[P(Ph)4][MnCr(ox)3]}n, where Ph is phenyl and ox is oxalate, has been prepared and a two-dimensional network structure, extended by Mn(II)-ox-Cr(III) bridges, has been determined from single crystal X-ray data. Crystal data: space group R3c, a=b=18.783(3), c=57.283(24) Å, α=β=90, γ=120°, Z=24 (C30H20O12PCrMn). The magnetic susceptibility data obey the Curie-Weiss law in the temperature range 260–20 K with a positive Weiss constant of 10.5 K. The temperature dependence of the molar magnetization exhibits a magnetic phase transition at Tc=5.9 K. The structure is discussed in relation to the strategy for preparing molecular based ferromagnets and, in addition, it is a solution to the question of the dimensionality of the [MM'(ox)3]n network, which in principle can extend two- or three-dimensionally to the crystal lattice. The optical absorption spectra of the single crystals are assigned to the ‘CrO6' chromophores. Their polarization patterns reflect the electric dipole selection rules for D3 symmetry. A strong site selective luminescence from the chromium(III) 2E states is observed at low temperature and the system may be suitable for studying energy transfer mechanisms.
  • Radical intermediates in radiation chemistry of As, Sb and Bi compounds
    M. Geoffroy and T Berclaz
    in "Chem. Org. Arsenic, Antimony Bismuth Compd." Patai, Saul. (Wiley, Chichester), 1994, p511-525
  • Chemical reactivity and molecular shape interactions
    Jacques Weber, Dominique Stussi and
    in "New data challenges in our information age" CODATA, Ann Arbor, 1994
  • The challenge of visualizing microscopic molecular worlds in chemical education
    J. Weber, A. Deloff, O. Schwalm, and P. FlĂźkiger
    in "University education uses of visualization in scientific computing" North-Holland, Amsterdam, 1994, p9-24
  • Characterization of chlorins within a natural chlorin mixture using electrospray/ion trap mass spectrometry
    Xavier Fr.D. Chillier, Gary J. Van Berkel, Fazil O. GĂźlaçar and Armand Buchs
    Organic Mass Spectrometry, 29 (11) , 1994, p672-678
  • Visualization in computational chemistry
    Jacques Weber, AndrĂŠ Deloff and Peter FlĂźkiger
    SPEEDUP, 8 (1) , 1994, p63-70
  • N-alkylimidazolidine-2-thione dihalogenocuprate anions : preparation, structure and nuclear quadrupole resonance spectra
    Sundara Ramaprabhu, Nahid Amstutz, Edwin A.C. Lucken and GĂŠrald Bernardinelli
    Journal of Chemical Research, Synopses, 1994, p368-369
 
We have developed a model to describe the inhomogeneous broadening of optical spectra in the substitutionally disordered crystals. The comparison with the experimentalf–f fluorescence spectra of SrFClxBr1−x:Sm2+ (0≤x≤1) allowed to establish, in a very detailed manner, the relationship between the inhomogeneous spectral distribution and the crystal structure around the Sm2+ impurity.
Single crystals of the alkaline earth fluorides doped with silver were grown successfully. This paper presents details of the methodology. The as-grown crystals consist of colorless transparent and yellowish regions. The former were shown to contain the Ag+ ion and the latter also silver pairs, small clusters and probably colloidal aggregates. Complex optical absorption bands were observed in the samples of the former parts after they had been X-irradiated. The samples were subsequently exposed to extended series of physico-chemical treatments with the aim to obtain information regarding the electronic structures involved. The evolution was monitored with the aid of optical absorption experiments. Factor analysis technique is presented and was applied to uncover mutually independent contributions to these absorption spectra. We identified the Jahn-Teller Ag+ [Bill H. et al., Solid State Commun.70, 511 (1989)] and several centers which formally involve an Ag− ion.
The spectroscopic properties of Ag+-doped strontium fluoride crystals were investigated at various temperatures, using absorption and fluorescence spectroscopies. The system exhibits a strong ultraviolet emission upon excitation into the two principal absorption bands. The azimuthal dependence of the degree of polarization of this luminescence is analysed, as well as its dynamics. The monovalent silver ions are shown to substitute for a host cation, with cubic symmetry. This is the first reported example of a cubic coordination for the Ag+ ions in an insulator. This cubic field, together with the strong ionic character of the framework, confers rather original spectroscopic properties to this system. The luminescence mechanisms are interpreted on the basis of the measured decay times and with the aid of energy diagram calculations. Two closed thermalized spin-orbit levels, with symmetry A2g and T2g respectively, are involved in the luminescence processes. The pure spin triplet A2g only emits at low temperatures (T<15 K), whereas the T2g level ( approximately 2% spin singlet character) emits in turn upon warming the crystal. One-dimensional configuration coordinate diagrams are proposed to interpret the peculiar temperature dependence of the emission band maximum.
  • The Nuclear Quadrupole Resonance of the 63,65Cu nuclei in Copper(I) complexes
    E.A.C. Lucken
    Zeitschrift fĂźr Naturforschung, Special Issue of the XII International Symposium on NQR, Zurich, July 19-23, 49a , 1994, p155-166
Transition metal chemistry contains a class of complex compounds for which the spin state of the central atom changes from high spin to low spin when the temperature is lowered. This is accompanied by changes of the magnetic and optical properties that make the thermally induced spin transition (also called spin crossover) easy to follow. The phenomenon is found in the solid state as well as in solution. Amongst this class, iron(II) spin crossover compounds are distinguished for their great variety of spin transition behavior; it can be anything from gradual to abrupt, stepwise, or with hysteresis effects. Many examples have been thoroughly studied by MĂśssbauer and optical spectroscopy, measurements of the magnetic susceptibilities and the heat capacities, as well as crystal structure analysis. Cooperative interactions between the complex molecules can be satisfactorily explained from changes in the elastic properties during the spin transition, that is, from changes in molecular structure and volume. Our investigations of iron(II) spin crossover compounds have shown that green light will switch the low spin state to the high spin state, which then can have a virtually unlimited lifetime at low temperatures (this phenomenom is termed light-induced excited spin state trapping - acronym: LIESST). Red light will switch the metastable high spin state back to the low spin state. We have elucidated the mechanism of the LIESST effect and studied the deactivation kinetics in detail. It is now well understood within the theoretical context of radiationless transitions. Applications of the LIESST effect in optical information technology can be envisaged.
  • Spincrossover und LIESST: thermisch und optisch schaltbare Eisen(II)-Komplexmoleküle
    P. GĂźtlich, A. Hauser and H. Spiering
    Angewandte Chemie, 106 (20) , 1994, p2109-2141
    DOI:10.1002/ange.19941062006 | unige:2998 | Abstract | Article PDF
In der Übergangsmetallchemie gibt es eine Klasse von Komplexverbindungen, bei denen eine Temperaturerniedrigung einen Wechsel im Spinzustand des Zentralatoms vom High-Spin- in den Low-Spin-Zustand bewirkt. Dabei ändern sich die magnetischen und optischen Eigenschaften, über die der thermische Spinübergang (auch Spincrossover genannt) sehr gut verfolgt werden kann. Dieses Phänomen tritt sowohl in flüssiger Phase als auch im Festkörper auf. Eine herausragende Stellung nehmen Eisen(II) - Spincrossover - Verbindungen ein, in denen der Spinübergang im Festkörper auf sehr unterschiedliche Weise - graduell, abrupt, mit Hysterese oder stufenweise - verlaufen kann und mit Mößbauer- und optischer Spektroskopie, mit magnetischen Suszeptibilitäts- und Wärmekapazitätsmessungen sowie durch Kristallstrukturanalyse intensiv untersucht worden ist. Die kooperative Wechselwirkung zwischen den einzelnen Komplexmolekülen kann befriedigend durch elastische Eigenschaften und durch die Änderung von Gestalt und Volumen der Komplexmoleküle beim Spinübergang erklärt werden. Bei Untersuchungen an Eisen(II)-Spincrossover-Verbindungen konnte man beobachten, daß sich der Low-Spin-Zustand mit grünem Licht in den High-Spin-Zustand umschalten läßt, der bei tiefen Temperaturen eine nahezu unendlich lange Lebensdauer haben kann (LIESST = Light-Induced Excited Spin State Trapping). Mit rotem Licht läßt sich der metastabile High-Spin- wieder in den Low-Spin-Zustand zurückschalten. Der Mechanismus des LIESST-Effekts ist aufgeklärt, die Zerfallskinetik im Detail untersucht und im Rahmen der Theorie strahlungsloser Übergänge verstanden. Anwendungen des LIESST-Effekts in der optischen Informationstechnik sind denkbar.
  • Spectral Hole-Burning and Stark Effect: Frequency Dependence of the Induced Dipole Moment of a Squaraine Dye in Polymers
    E. Vauthey, J. Voss, C. De Caro, A. Renn and U.P. Wild
    Chemical Physics, 184 , 1994, p347-356
    DOI:10.1016/0301-0104(94)00078-6 | unige:2999 | Abstract | Article PDF
A study of the effect of an external electric field on spectral holes burnt at different frequencies in the inhomogeneous absorption band of a centrosymmetric squaraine dye, bis [4-(diethylamino)-2-hydroxyphenyl] squaraine (DEAH), in polymers of different polarity is presented. Average matrix induced dipole moment differences of about 1 D and 0.37 D were measured in the directions parallel and perpendicular to the long axis of DEAH. In all polymers investigated, the induced dipole moment difference decreased from the higher to the lower frequencies. Solvatochromic shift measurements were performed in order to elucidate the origin of this effect. The matrix field inducing the dipole moment is also partially responsible for the frequency shift of the absorption of DEAH. With increasing matrix field, the absorptiion frequency is shifted to the blue due to electrostatic interaction with the local dipoles of DEAH. The contribution of the electrostatic interactions to the frequency shift is smaller than the dispersion interactions by two orders of magnitude in polystyrene, but increases slightly in more polar polymers.
The free ion yield (R) resulting from the fluorescence quenching of 9,10-dicyanoanthracene (DCA) by various electron donors in acetonitrile has been studied using ns laser photoconductivity. The influence of the chemical nature of the doors is established in a general manner. For a given oxidation potential Eox(D/D+), the rate constant of geminate ion recombination, kbac, decreases significantly as the electronic delocalization of the donor increases. As a consequence multiple Marcus plots are observed in the inverted region. These plots show decreasing curvature when going from stilbenes to amines as donors. This fan effect is tentatively explained by considering the detailed roles of the parameters V, and h in the Marcus model.
  • Picosecond Transient Grating Spectroscopy: An Application of Holography for Investigating Ultrafast Photoinduced Processes
    E. Vauthey, Y. Pariat and A. Henseler
    Chimia, 48 (9) , 1994, p397-400
    unige:3001 | Abstract | Article PDF
The principles of ps transient grating spectroscopy are presented. The capabilities of this technique for the study of photoinduced processes are illustrated by several new results. These include the observation of an anomalous effect in the reorientation dynamics of two ionic dyes in long-chain alkanenitriles, the measurement of the local viscosity in the interior of reverse micelles containing methanol, and the determination of the rate constant of separation of a geminate radical pair formed by photoinduced electron-transfer reaction.
The rotational diffusion time of ruthenium (II)-bis(2,2'-bipyridine) (2,2'-biquinoleine) has been measured in polar solvents of different viscosity. The rotational dynamics can be explained in terms of the Stokes—Einstein—Debye hydrodynamics theory under stick boundary condition by considering the rotating body as a prolate spheroid enclosing the complex. This overestimates the intrinsic molecular volume by a factor of 1.5. The difference can be accounted for by solvent molecules intercalated in the interligand space and stabilized by electrostatic interaction with the charge of the metal atom.
  • Time Resolved Resonance Raman Study of the Rate of Separation of a Geminate Ion Pair into Free Ions in a Medium Polarity Solvent
    E. Vauthey, A.W. Parker, B. Nohova and D. Phillips
    Journal of the American Chemical Society, 116 (20) , 1994, p9182-9186
    DOI:10.1021/ja00099a037 | unige:3003 | Abstract | Article PDF
The rate of separation into free ions of a geminate ion pair generated by photoinduced electron transfer between 9,lO-anthraquinone excited to the lowest triplet state and 1,2,4-trimethoxybenzenein 1,1,2,2-tetrachloroethane has been measured at different temperatures by nanosecond time-resolved resonance Raman spectroscopy (TR3). The intrinsic activation energy for the separation amounts to 0.04 eV, suggesting that the center-to-center interionic distance in the geminate ion pair is about 7.5 A. The activation barrier is due to a loss of electrostatic stabilization upon separation to a distance of about 9.5 A, where a solvent molecule or part of it can interpenetrate to increase the solvation energy. This suggests that the geminate ion pair is a loose ion pair but is not truly solvent separated.
  • Enantioselective hydrogenation of a-ketoesters over Pt/alumina modified with cinchonidine : theoretical investigation of the substrate-modifier interaction
    O. Schwalm, B. Minder, J. Weber and A. Baiker
    Catalysis Letters, 23 (3-4) , 1994, p271-279
    DOI:10.1007/BF00811362 | Abstract | Article PDF
  • Thermolysis and chemiluminescence of monocyclic 1,2,4-trioxan-5-ones
    C.W. Jefford, M.C. Josso, M. Da Graça H. Vicente, H. Hagemann, D. Lovy and H. Bill
    Helvetica Chimica Acta, 77 (7) , 1994, p1851-1860
    DOI:10.1002/hlca.19940770716 | unige:2888 | Abstract | Article PDF
The 3,6-substituted 1,2,4-trioxan-5-ones 11-14, on heating to 170-200°, underwent unimolecular thermolysis to generate electronically excited singlet ketones with an efficiency of ca. 0.2%. The chemiluminescence quantum yields (ΌoSCL) depended on the nature of the 6-substitutents and increased linearly with temperature. The Arrhenius activation energies were obtained by measuring the rate of decay of luminescence and determined as 22.9, 30.4, 35.6, and 34.2 kcal/mol for 11-14, respectively. Step analysis of the chemiluminescence of 14 afforded an average activation energy of 44.3 kcal/mol. This latter result is explicable in terms of two decomposition paths, higher and lower in energy, leading to excited and 'dark' products, respectively. The thermolysis of trioxanones 12-14 lacking a H-atom at the 6-position is interpreted as involving successive rupture of the peroxide bond, excision of ketone at the 3-substituted end, and loss of CO2, to finally produce ketone originating from the 6-position (see Scheme 4).
  • Intermolecular bonding and vibrations of phenol·H2O (D2O)
    Martin SchĂźtz, Thomas BĂźrgi, Samuel Leutwyler and Thomas Fischer
    Journal of Chemical Physics, 98 (5) , 1993, p3763-14
    DOI:10.1063/1.464055 | unige:14671 | Abstract | Article PDF
Extensive ab initio calculations of the phenol⋅H2O complex were performed at the Hartree–Fock level, using the 6‐31G(d,p) and 6‐311++G(d,p) basis sets. Fully energy‐minimized geometries were obtained for (a) the equilibrium structure, which has a translinear H bond and the H2O plane orthogonal to the phenol plane, similar to (H2O)2; (b) the lowest‐energy transition state structure, which is nonplanar (C1 symmetry) and has the H2O moiety rotated by ±90°. The calculated MP2/6‐311G++(d,p) binding energy including basis set superposition error corrections is 6.08 kcal/mol; the barrier for internal rotation around the H bond is only 0.4 kcal/mol. Intra‐ and intermolecular harmonic vibrational frequencies were calculated for a number of different isotopomers of phenol⋅H2O. Anharmonic intermolecular vibrational frequencies were computed for several intermolecular vibrations; anharmonic corrections are very large for the β2 intermolecular wag. Furthermore, the H2O torsion τ around the H‐bond axis, and the β2 mode are strongly anharmonically coupled, and a two‐dimensional τ/β2 potential energy surface was explored. The role of tunneling splitting due to the torsional mode is discussed and tunnel splittings are estimated for the calculated range of barriers. The theoretical studies were complemented by a detailed spectroscopic study of h‐phenol⋅H2O and d‐phenol⋅D2O employing two‐color resonance‐two‐photon ionization and dispersed fluorescence emission techniques, which extends earlier spectroscopic studies of this system. The β1 and β2 wags of both isotopomers in the S0 and S1 electronic states are newly assigned, as well as several other weaker transitions. Tunneling splittings due to the torsional mode may be important in the S0 state in conjunction with the excitation of the intermolecular σ and β2 modes.
  • Intermolecular bonding and vibrations of 2-naphthol·H2O (D2O)
    Martin SchĂźtz, Thomas BĂźrgi, Samuel Leutwyler and Thomas Fischer
    Journal of Chemical Physics, 99 (3) , 1993, p1469-13
    DOI:10.1063/1.465316 | unige:14670 | Abstract | Article PDF
A combined experimental and theoretical study of the 2‐naphthol⋅H2O/D2O system was performed. Two different rotamers of 2‐naphthol (2‐hydroxynaphthalene, 2HN) exist with the O–H bond in cis‐ and trans‐position relative to the naphthalene frame. Using Hartree–Fock (HF) calculations with the 6‐31G(d,p) basis set, fully energy‐minimized geometries were computed for both cis‐ and trans‐2HN⋅H2O of (a) the equilibrium structures with trans‐linear H‐bond arrangement and Cs symmetry and (b) the lowest‐energy transition states for H atom exchange on the H2O subunit, which have a nonplanar C1 symmetry. Both equilibrium and transition state structures are similar to the corresponding phenol⋅H2O geometries. The H‐bond stabilization energies with zero point energy corrections included are ≊5.7 kcal/mol for both rotamers, ≊2.3 kcal/mol stronger than for the water dimer, and correspond closely to the binding energy calculated for phenol⋅H2O at the same level of theory. Extension of the aromatic π‐system therefore hardly affects the H‐bonding conditions. The barrier height to internal rotation around the H‐bond only amounts to 0.5 kcal/mol. Harmonic vibrational analysis was carried out at these stationary points on the HF/6‐31G(d,p) potential energy surface with focus on the six intermolecular modes. The potential energy distributions and M‐matrices reflect considerable mode scrambling for the deuterated isotopomers. For the a′ intermolecular modes anharmonic corrections to the harmonic frequencies were evaluated. The β2 wag mode shows the largest anharmonic contributions. For the torsional mode τ (H2O H‐atom exchange coordinate) the vibrational level structure in an appropriate periodic potential was calculated. On the experimental side resonant‐two‐photon ionization and dispersed fluorescence emission spectra of 2HN⋅H2O and d‐2HN⋅D2O were measured. A detailed assignment of the bands in the intermolecular frequency range is given, based on the calculations. The predicted and measured vibrational frequencies are compared and differences discussed.
  • Fluxionality and low-lying transition structures of the water trimer
    Martin SchĂźtz, Thomas BĂźrgi, Samuel Leutwyler and Hans Beat BĂźrgi
    Journal of Chemical Physics, 99 (7) , 1993, p5228-11
    DOI:10.1063/1.465991 | unige:14669 | Abstract | Article PDF
The minimum energy structure of the cyclic water trimer, its stationary points, and rearrangement processes at energies <1 kcal/mol above the global minimum are examined by ab initio molecular orbital theory. Structures corresponding to stationary points are fully optimized at the Hartree–Fock and second‐order Møller–Plesset levels, using the 6‐311++G(d,p) basis; each stationary point is characterized by harmonic vibrational analyses. The lowest energy conformation has two free O–H bonds on one and the third O–H bond on the other side of an approximately equilateral hydrogen‐bonded O...O...O (O3) triangle. The lowest energy rearrangement pathway corresponds to the flipping of one of the two free O–H bonds which are on the same side of the plane across this plane via a transition structure with this O–H bond almost within the O3 plane. Six distinguishable, but isometric transition structures of this type connect six isometric minimum energy structures along a cyclic vibrational‐tunneling path; neighboring minima correspond to enantiomers. The potential energy along this path has C6 symmetry and a very low barrier V6=0.1±0.1 kcal/mol. This implies nearly free pseudorotational interconversion of the six equilibrium structures. The corresponding anharmonic level structure was modeled using an internal rotation Hamiltonian. Two further low‐energy saddle points on the surface are of second and third order; they correspond to crown‐type and planar geometries with C3 and C3h symmetries, respectively. Interconversion tunneling vibrations via these stationary points are also important for the water trimer dynamics. A unified and symmetry‐adapted description of the intermolecular potential energy surface is given in terms of the three flipping coordinates of the O–H bonds. Implications of these results for the interpretation of spectroscopic data are discussed.
  • Evidence for strong mixing between the LC and MLCT excited states in bis(2-phenylpyridinato-C2,N')(2,2'-bipyridine)iridium(III)
    Mirco G. Colombo, Andreas Hauser and Hans U. Guedel
    Inorganic Chemistry, 32 (14) , 1993, p3088-3092
    DOI:10.1021/ic00066a020 | unige:6515 | Abstract | Article PDF
The well-resolved absorption, excitation, and luminescence spectra of [Ir(ppy)2bpy]+ (ppyH = 2-phenylpyridine, bpy = 2,2'-bipyridine) in different media at cryogenic temperatures are presented. In solutions and glasses at ambient temperature the lowest energy excited state corresponds to an Ir - bpy charge-transfer excitation whereas in the crystalline host lattice [Rh(ppy)2bpy]PF6 the lowest excited state at 21 450 cm-1 is assigned to a 37r-r* excitation localized on the cyclometalating ppy- ligands. The next higher excited Ir - bpy charge-transfer state has shifted to 21 820 cm-', only 300 cm-I above the 3LC excited state. The close proximity of the 3LC and 3MLCT excited states and the large spin-orbit coupling constant of Ir3+ induce a strong mixing of charge-transfer character into the 3LC lowest excited states, resulting in increased oscillator strengths, reduced lifetimes, short axis polarized transitions, and a large zero-field splitting of 10-15 cm-1.
  
  • Cooperative Formation of Inorganic-Organic Interfaces in the Synthesis of Silicate Mesostructures
    A. Monnier, F. SchĂźth, Q. Huo, D. Kumar, D. Margolese, R.S. Maxwell, G.D. Stucky, M. Krishnamurty, P. Petroff, A. Firouzi, M. Janicke and B.F. Chmelka
    Science, 261 (5126) , 1993, p1299-1303
    DOI:10.1126/science.261.5126.1299 | Abstract | Article PDF
A model is presented to explain the formation and morphologies of surfactant-silicate mesostructures. Three processes are identified: multidentate binding of silicate oligomers to the cationic surfactant, preferential silicate polymerization in the interface region, and charge density matching between the surfactant and the silicate. The model explains present experimental data, including the transformation between lamellar and hexagonal mesophases, and provides a guide for predicting conditions that favor the formation of lamellar, hexagonal, or cubic mesostructures. Model Q230 proposed by Mariani and his co-workers satisfactorily fits the x-ray data collected on the cubic mesostructure material. This model suggests that the silicate polymer forms a unique infinite silicate sheet sitting on the gyroid minimal surface and separating the surfactant molecules into two disconnected volumes.
  • The Jahn-Teller systems Ag2+ and Ca2+ in RbCdF3. An EPR investigation
    E. Minner and H. Bill
    in "Proc. XIIth Int. Conf. on Defects in Insulating Materials" G. Kanert, J.M. Spaeth, 1993, p571-579
  • Cu+ Luminescent centers in CaF2
    C. Pedrini, B. Moine and H. Bill
    in "Proc. XIIth Int. Conf. on Defects in Insulating Materials" G. Kanert, J.M. Spaeth, 1993, p593-595
  • Two-photon Spectroscopy of Sm2+ in SrF2: Polarization dependances of 7F0 – 5DJ Transition
    J.C. Gâcon, G.W. Burdick, B. Moine and H. Bill
    in "Proc. XIIth Int. Conf. on Defects in Insulating Materials" G. Kanert, J.M. Spaeth, 1993, p1306-1308
  • Inhomogeneous broadening and spectral hole burning in SryBa1-yFClxBr1-x
    R. Jaaniso, H. Hagemann and H. Bill
    Technical Digest Opt. Soc. America, 1993
    unige:3015
  • Computer-aided design of organometallic compounds : a semi-empirical approach of their structure and reactivity
    Jacques Weber, François Savary, Dominique Stussi and
    in "Computers and applied chemistry" Science Press, Beijing, 1993, p36-43
Making use of the phenomenon of light-induced spin-crossover in the [Zn1−xFex(ptz)6](BF4)2 spin-crossover system, very high diffraction efficiencies η can be achieved in non-degenerate four-wave-mixing. In the mixed crystal with x=0.1 and at 76 K, i.e. at a temperature where the system is predominantly in the low-spin state, a value for η of 43% was obtained. This is attributed to a phase grating due to the large difference in metal---ligand bondlength between the low-spin ground state and the light-induced high-spin state. The rate constant for the decay of the laser-induced grating as a function of temperature is found to be exactly twice the one of the high-spin→low-spin relaxation, as expected for a dilute system in the absence of cooperative effects.
  • Exchange interactions in mixed Yb3+-Cr3+ and Yb3+-Ho3+ dimers: An inelastic-neutron-scattering investigation of Cs3Yb1.8Cr0.2Br9 and Cs3Yb1.8Ho0.2Br9
    M.A. Aebersold, H.U. GĂźdel, A. Hauser, A. Furrer, H. Blank and R. Kahn
    Physical Review B, 48 , 1993, p12723-12731
    DOI:10.1103/PhysRevB.48.12723 | unige:3021 | Abstract | Article PDF
The two title compounds were synthesized and investigated with the inelastic-neutron-scattering (INS) technique. They contain mixed YbMBr93- (M=Cr3+, Ho3+) dimers as discrete units, and the magnetic excitations of mixed Yb3+-Cr3+ and Yb3+-Ho3+ dimers could thus be observed. The Yb3+-Cr3+ dimer has three INS transitions, for which anisotropic exchange, as well as zero-field splitting of Cr3+, has to be included in the exchange Hamiltonian. For the Yb3+-Ho3+ dimer the effect of the exchange interaction manifests itself as a broadening and a splitting of the crystal-electric-field levels of the isolated Ho3+ ion. Taking into account the full (2J + 1) ground-state multiplet of Ho3+, as well as anisotropic exchange, gives a satisfactory description of this dimer.
We present the results of a crystallographic and optical study realized on the system SryBa1-yFClxBr1-x where x and y vary from zero to one. All mixed crystals studied were of tetragonal symmetry with the parent PbFCl structure. Complete structure determinations were performed for two single crystals with y=0.3 and x=1 respectively 0.7 (nominal composition). This system forms complete solid solutions, in contrast to previously published results.
  • A Theoretical Study of the Vibrational Dependence of the Nuclear Quadrupole Coupling Constants of some Diatomic Molecules
    E.A.C. Lucken
    Zeitschrift fĂźr Naturforschung, Special Issue of the XII International Symposium on NQR, Zurich, July 19-23, 49a , 1993, p133-136
  • Stark effect and Spectral Hole-Burning: Solvation of Organic Dyes in Polymers
    Eric Vauthey, Keith Holliday, Changjiang Wei, Alois Renn and Urs P. Wild
    Chemical Physics, 171 , 1993, p253-263
    DOI:10.1016/0301-0104(93)85148-2 | unige:3016 | Abstract | Article PDF
Spectral hole-burning studies of nile red and cresyl violet in polyvinylbutyral and polyvinylformal films have been performed. From the shape of spectral holes under the influence of an electric field, the dipole moment difference between the ground and excited state of both dyes has been determined. The Stark effect was investigated at different positions in the inhomogeneously broadened absorption band of the guest molecules. The observed dipole moment difference decreases with increasing wavelength. This variation is caused by the matrix induced dipole moment. For nile red, which is a neutral and polar molecule, the distribution of induced dipole moments is strongly correlated with the orientation of its ground state dipole moment. In the case of cresyl violet perchlorate, which is a salt, this distribution is anisotropic for guests absorbing in the blue part of the inhomogeneous band but becomes more isotropic as the absorption wavelength increases. The wavelength dependence of the observed dipole moment is much stronger and is ascribed to the existence of the cresyl violet perchlorate salt in different states of solvation.
  • Spectral Hole Burning and Holography in an Y2SiO5:Pr3+ Crystal
    Keith Holliday, Mauro Croci, Eric Vauthey and Urs P. Wild
    Physical Review B, 47 (22) , 1993, p14741-14752
    DOI:10.1103/PhysRevB.47.14741 | unige:3018 | Abstract | Article PDF
Holographic detection of spectral holes is demonstrated in a crystalline host material with signal-to-noise ratios of up to 104. Hole burning occurs in two Pr3+ sites in the Y2SiO5 lattice, in both cases due to population redistribution between the ground-state quadrupole levels. The signal contains contributions due to a resonant hole and several side holes and antiholes, a phenomenon not previously observed using the holographic technique. The diffracted spectrum is modeled in two ways. In the first case the transmission spectrum is used to determine the population gratings and thus the diffraction efficiency. In the second case the transition probabilities between ground- and excited-state Kramer's doublets are used to model the population gratings. The technique is applied to pseudo-Stark-effect measurements from which the crystallographic sites as determined by x-ray analysis are matched to the spectroscopic data presented here. The time decay of the diffracted signal is used to study nuclear spin-lattice relaxation. It is shown that at 1.6 K temperature-dependent phonon-induced processes make no contribution to this decay. The nonexponential time decay of the population upon radio-frequency irradiation resonant with a ground-state quadrupole splitting is attributed to Pr-Pr cross relaxation
  • The Reliability of Free Ion Yield in Photoinduced Electron Transfer Reactions. The Model System 9,10-Dicyanoanthracene/Biphenyl in acetonitrile
    E. Vauthey, D. Pilloud, E. Haselbach, P. Suppan and P. Jacques
    Chemical Physics Letters, 215 (1-3) , 1993, p264-268
    DOI:10.1016/0009-2614(93)89298-V | unige:3019 | Abstract | Article PDF
A detailed study of the separation efficiency in the photoinduced electron transfer reaction between 9,10-dicyanoanthracene and biphenyl in acetonitrile is presented. Both transient absorption and photoconductivity indicate a separation efficiency of about 0.4. This value is in discrepancy with two of three previously reported efficiencies. The problems arising with too large donor concentrations and with the use of a secondary donor to determine the separation efficiency are discussed.
A study of the hole-burning mechanisms of bis[4-(diethylamino)-2-hydroxyphenyl]squaraine (DEAH) and bis[4-(disethylamino)-phenyl]squaraine (DEA) in hydrogen-bonding and non-hydrogen-bonding polymers is presented. Intramolecular H-bonding is only possible for DEAH. In all systems, the spectral holes are not persistent and decay with a distribution of rates ranging from 10-5s-1 to about 1 s-1, the time resolution of the experiment. In H-donating matrices, this distribution varies with the burning wavelength. From the hole-burning efficiencies and the kineticsof the hole refilling, four different types of nonphotochemical hole-burning mechanisms are postulated. The efficiency of these mechanisms depends mainly on the occurrence of processes slowing down the relaxation to the initial product state.
The rotational dynamics of nile red has been studied in polar protic, polar aprotic and non-polar solvents. In the non-polar and the aprotic solvents, with the exception of long alkanenitriles, the rotation dynamics is consistent with the prediction of the Stokes—Einstein—Debye hydrodynamics theory for slip and close to the stick boundary condition, respectively. However in protic solvents, the rotation dynamics can be explained in terms of the Stokes—Einstein—Debye hydrodynamics theory under stick boundary condition only if solvent attachment via hydrogen bonding is assumed. The anomalous behaviour observed in longer alkanenitriles has been assigned to the formation of a reverse micelle-like solvation layer around nile red.
  • Molecular Conformation and Excited-State Dipole Moments of Di- and Tetramethylaminobenzonitrile (DMABN and TMABN)
    W. Rettig, D. Braun, P. Suppan, E. Vauthey, K. Rotkiewicz, R. Luboradzki and K. Suwinska
    Journal of Physical Chemistry, 97 (51) , 1993, p13500-13507
    DOI:10.1021/j100153a014 | unige:3026 | Abstract | Article PDF
The conformational analysis of TMABN by three different methods X-ray analysis, photoelectron spectroscopy, and UV molar absorption coefficient yields a twist angle of the dimethylamino group of 60-70° in the ground state, whereas DMABN is not far from planar in qualitative agreement with the predictions from force field calculations (QCFF/PI and MM3). Dipole moment determinations by the thermochromic method agree with those from other methods (solvatochromism, electrochromism and time resolved microwave absorption) in that the excited state dipole moment of TMABN is very large, as well as that of the TICT state of DMABN. Its value increases somewhat with solvent polarity. This is explained by a nuclear polarizability model. The force field calculations are used to predict twist angle values for various sterically hindered DMABN derivatives
Quantum chemical calculations based on density functional theory have been performed on ruthenocene. Excellent agreement is obtained with ground‐ and excited‐state properties derived from optical spectroscopy. In particular, the energies of the first d–d excitations, the unusually large Stokes shift, the structural expansion of Ru(cp)2 and the substantial reduction of the Ru‐cp force constant in the first triplet excited state are almost quantitatively reproduced. The lowest‐energy excitation is found to have substantial charge transfer character.
  • Theoretical investigation of the electronic structure of the mixed-sandwich complex (h5-cyclopentadienyl)(h6-benzene)iron and its cation
    A. Le Beuze, R. Lissillour and J. Weber
    Organometallics, 12 (1) , 1993, p47-55
    DOI:10.1021/om00025a013 | Abstract | Article PDF
Copper and silver, respectively, were introduced into single crystals of CsCdF3. Our detailed electron paramagnetic resonance (EPR) study showed that both elements enter the Cd lattice site—copper as Cu2+, silver as Ag+, which then was converted into Ag2+ by x raying the corresponding samples. Cu2+ and Ag2+ were shown to present in their ground state a pseudostatic Jahn–Teller effect. Motional effects were observed in the respective EPR spectra and studied in some detail for Cu2+ as they are seen over a wide temperature range. Predictions of a stochastic Kubo model [J. Phys. Soc. Jpn. 9, 935 (1954)] were compared with the temperature dependent linewidths of the motionally averaged EPR spectrum. A power law (Tn with n≂1.9) was determined for the temperature dependence of the reorientation frequency between 30 and 90 K.
Ab initio electronic structure calculations for phenol and the hydrogen-bonded complexes phenol ¡ H2O and d-phenol ¡ D2O were performed at the Hartree-Fock 4-31G and 6-31G** levels. Both phenol and phenol ¡ H2O were fully structure optimized. Based on the minimumenergy structures so obtained, full normal coordinate analyses were carried out. The resulting harmonic frequencies were scaled and compared to available experimental data. The agreement is satisfactory and allows for an assignment of a majority of the bands observed in the experimental spectra. Comparison with previous calculations on (H2O)2 reveals a considerable increase in the strength of the hydrogen bond on going from (H2O)2 to phenol ¡ H2O.
  • Quantum-mechanical calculations of solvation free energies. A combined ab initio pseudopotential free-energy perturbation approach
    Nagarajan Vaidehi, Tomasz Adam Wesolowski and Arieh Warshel
    Journal of Chemical Physics, 97 (6) , 1992, p4264-4271
    DOI:10.1063/1.463928 | Abstract | Article PDF | Article PS (gzipped)
A practical ab initio quantum-mechanical approach for calculations of free energies of molecules in solutions is developed. This approach treats the solute molecules by an explicit ab initio self-consistent-field approach while representing the solvent molecules by a pseudopotential. The solvation energies are evaluated by a free-energy perturbation approach that uses the distribution function associated with a classical force field as a reference state for the quantum-mechanical calculations. The performance of the method is examined by evaluating the solvation energy of an Li + ion. It is found that the calculation times are not much longer than that of the corresponding classical free-energy perturbation calculations.
  • Synthesis, structure and ligand-centred reduction of an orthometallated complex of palladium containing two phosphaalkene groups
    Abdelaziz Jouaiti, Michel Geoffroy, Gustavo Terron and GĂŠrald Bernardinelli
    ChemComm, (2) , 1992, p155-156
    DOI:10.1039/C39920000155 | Abstract | Article PDF
  • Structure and energetics of hydrogen clusters : structure of H1+1 and H1+3 : vibrational frequencies and infrared intensities of the H2+n+1 clusters (n=2-6)
    Michel Farizon, Henry Chermette and Bernadette Farizon-Mazuy
    Journal of chemical physics, 96 (2) , 1992, p1325-1332
    DOI:10.1063/1.462168 | Abstract | Article PDF | Article PS (gzipped)
Ab initio self‐consistent‐field (SCF) Hartree–Fock and configuration interaction (CI) calculations have been carried out for H+ 2n+1 (n=1–6) clusters using a triple‐zeta plus polarization basis set. Fully optimized structures and energies of H+ 11 and H+ 13 are presented. These structures can be thought as the addition of H2 molecules to a deformed H+9. Dissociation energies as a function of cluster size follow the pattern established experimentally by Hiraoka and Mori. Nevertheless, our energy results on the biggest clusters suffer from the lack of size consistency of CI with single and double substitutions (CISD) calculations. Analytic gradient techniques have been used to locate stationary point geometries and to predict harmonic vibrational frequencies and infrared intensities at the two levels of theory SCF (n=1–6) and CISD (n=1–4) both with triple‐zeta polarizationbasis sets. Of special interest are the new vibrational modes of H+ 11 and H+ 13, which have no counterpart in the H+9 cluster. Our predicted frequencies compare fairly well with the experimental results of Okumura, Yeh, and Lee.
  • Radiation damage in carnidazole : a single crystal EPR study
    ThĂŠo Berclaz, GĂŠrald Bernardinelli, Michel Geoffroy and Natarajan Rajalakshmi
    Chimia, 46 (4) , 1992, p130-132
    Abstract | Article PDF
  • Nuclear quadrupole resonance and crystallographic studies of three-co-ordinated copper(I) cations with alkylpyridine ligands
    Augustin Habiyakare, Edwin A.C. Lucken and GĂŠrald Bernardinelli
    Dalton Transactions, (17) , 1992, p2591-2599
    DOI:10.1039/DT9920002591 | Abstract | Article PDF
  • NQR studies of inclusion compounds
    Edwin A.C. Lucken
    in "Spectroscopic and computational studies of supramolecular systems" ed. by J.E.D. Davies. Dordrecht, 1992, p61-82
  • An NQR and calorimetric study of a phase transition in tetrakis(?-phenylethylamine)-bis(isothiocyanato)nickel(II)/ortho-dichlorobenzene clathrate
    Li Pang, E.A.C. Lucken and P. Tissot
    Zeitschrift fĂźr Naturforschung A, 47 (1-2) , 1992, p251-253
  • Molecular graphics and chemistry
    Jacques Weber, Peter FlĂźkiger and
    Educational media international, 29 (4) , 1992, p247-253
  • Members of the PbFCl-type family : possible candidates for room-temperature photochemical hole burning
    Raivo Jaaniso, Hans Hagemann, Frank Kubel and Hans Bill
    Chimia, 46 (4) , 1992, p133-137
    unige:2908 | Abstract | Article PDF
We report on crystal growth and about physico-chemical studies on SryBa1−yFClxBr1−x (y = 0, 0.5, and 1) compounds doped with Sm. Persistent spectral hole burning at 300 K is further reported on Sr0.5Ba0.5FCl0.5Br0.5:Sm single crystals.
  • An EPR and ab initio study of a phosphaalkene radical anion and comparison with other phosphorus-containing radical ions
    M. Geoffroy, G. Terron, A. Jouaiti, P. Tordo and Y. Ellinger
    Bulletin of Magnetic Resonance, 14 (1-4) , 1992, p229-233
  • Development and applications of an extended-HĂźckel-based reactivity index for organometallic complexes
    Jacques Weber, Dominique Stussi, Peter FlĂźkiger, and E.Peter KĂźndig
    Comments on inorganic chemistry, 14 (1) , 1992, p27-62
    DOI:10.1080/02603599208048656 | Abstract | Article PDF
The high-spin to low-spin (HS→LS) relaxation in the [Fe(ptz)6](BF4)2 spin-crossover system deviates strongly from first-order kinetics because of cooperative effects of elastic origin. The shift in horizontal and vertical displacement of the potential wells of the initial and final state relative to each other due to the build-up of an "internal" pressure is estimated from spectroscopic measurements. The HS→LS relaxation as such is described by the theory of nonadiabatic multiphonon relaxation in the strong-coupling limit, with a Huang—Rhys factor S ≈ 45 which is much larger than the reduced energy gap p. The sigmoidal relaxation curves in [Fe(ptz)6](BF4)2 result when a change in S of ≈ −1 and in p of 1 during the relaxation is taken into account.
  • Detection of Specifically Oriented Impurities in an Amorphous Host Through Interference between Holograms Stored as Spectral Holes
    Keith Holliday, Eric Vauthey, Mauro Croci, Alois Renn and Urs P. Wild
    Journal of the Optical Society of America B, 9 (6) , 1992, p982-986
    DOI:10.1364/JOSAB.9.000982 | unige:3029 | Abstract | Article PDF
The constructive interference between two Stark-effect-broadened holograms produced by spectral hole burning is discussed. The holograms are burned at the same frequency but at different external electric-field values. The phase difference is selected to be zero so that constructive interference between the waves diffracted by each grating occurs. Experimentally it is found that a dip in the hologram efficiency that is not predicted by previous theory occurs for all reconstruction external electric-field values in the region between the original burn values. This dip is interpreted as being due to the time nonlinearity of the hologram burn process. The dip corresponds to those molecules, oriented in a specific direction with respect to the electric field, for which no Stark shift occurs and that are therefore resonant with the laser during the production of both holograms. The width of the anomalous feature is close to that of the hologram when the hologram is reconstructed at the original burn external electric-field strength. Other molecular orientations may be selected by burning pairs of holographic gratings at other combinations of the frequency and the electric field.
  • Time-Resolved Resonance Raman Studies of Triplet Geminate Ion Pair
    E. Vauthey, D. Phillips and A.W. Parker
    Journal of Physical Chemistry, 96 (18) , 1992, p7356-7360
    DOI:10.1021/j100197a041 | unige:3030 | Article PDF
The electron transfer reaction between 9,lO-anthraquinone (AQ) excited to the lower triplet state and 1,2,4trimethoxybenzene (TMB) in solvents of different polarity has been studied by nanosecond time-resolved resonance Raman spectroscopy. In no solvent was there evidence for the formation of a triplet exciplex. All the observed vibrations were characterized as AQ* or TMB" modes. The frequency of the band assigned to the C-C stretch of TMB" has been found to be very dependent upon the environment and this effect has been used to observe the separation of the geminate ion pair into free ions in polar solvents. The data strongly suggest that the ions are at van der Waals contact in the geminate ion pair and not separated by solvent molecules.
  • Oxidation reaction in X-irradiated bis(ethylenediamine)platinum(II) bis(hydrogen squarate) : a single crystal EPR study of a platinum(III) complex
    G. Bernardinelli, P. Castan, H. Chermette, D. Deguenon, M. Geoffroy, S. Nour, J. Weber and M. Wermeille
    Inorganic Chemistry, 31 (24) , 1992, p5056-5060
    DOI:10.1021/ic00050a025 | Abstract | Article PDF
  • Computer modeling of the activation processes of the aromatic nucleophilic substitution mechanism
    , P. FlĂźkiger and J. Weber
    Journal de Chimie Physique et de Physico-Chimie Biologique, 89 , 1992, p1723
  • Experimental and theoretical investigation of asymmetric induction in the synthesis of disubstituted cyclohexadienes via chiral benzene chromium complexes
    G. Bernardinelli, A.F. Cunningham, C. DuprĂŠ, E.P. KĂźndig, D. Stussi and J. Weber
    Chimia, 46 (4) , 1992, p126-129
    unige:2932 | Abstract
A series of [Cr(benzene)(CO)2L] complexes with L = PPh3, P(OMe)3, PPh2 ((−)-menthyl), P(OPh)2(O-(−)-menthyl), P(O-(−)-menthyl)3 were subjected to a nueleophile addition/acylation sequence to give trans-5,6-disubstituted cyelohexadienes. Low-to-moderate asymmetric induction was observed with the chira] ligands. Experimental and theoretical evidence for an alkylation at the metal center trans to the P ligand is presented, and a crystal structure determination of a [Cr(η5-cyclohexadienyl)(P(OMe)3)(CO)2SnPh3] complex is included.
  • Molecular graphics investigation of the addition of nucleophiles to (η4:butadiene) M(CO)3 complexes (M=Fe,Co+)
    J. Weber, and O. Eisenstein
    Journal of Molecular Structure: THEOCHEM, 254 , 1992, p343-357
    DOI:10.1016/0166-1280(92)80079-2 | Abstract | Article PDF
The ion Ag2+ introduced into NaF shows a tetragonal electron paramagnetic resonance spectrum at 4.2 K which dynamically averages above ≂40 K. Uniaxial stress is used to show that the ground state is a strongly coupled E⊗ϵ Jahn–Teller state. The well‐resolved superhyperfine structure due to the F− neighbors is analyzed with a linear combination of atomic orbitals picture. Optical absorption of as‐grown and treated crystals is further presented. The former ones show peaks at 202, 213, 219 nm due to Ag+. The latter ones present complex absorption spectra related to silver.
The 5T2(HS)1A1(LS) intersystem crossing rates have been determined for a number of Fe(II) coordination compounds between 10 and 270 K using time-dependent optical spectroscopy. Strong deviations from Arrhenius kinetics with nearly temperature independent tunneling at low temperatures and a thermally activated behavior at elevated temperatures with apparent activation energies smaller than the classical energy barrier were found. The tunneling rates range from ~10−6 s−1 for the doped spin crossover system [Zn1−xFex(ptz)6](BF4)2 to ~106 s−1 for the doped low-spin (LS) system [Zn1−xFex(bipy)3](PF6)2. The large range of 12 orders of magnitude in the low temperature tunneling rates as well as the activated region can be understood in terms of nonadiabatic multiphonon relaxation. Values for the Huang–Rhys parameter S of 40–50 and for the reduced energy gap p of 1–12 are estimated for the present series of compounds. The validity of an inverse energy gap law in the strong vibronic coupling limit with Sp is borne out by experiment.
  • Estimation of free energy systematic errors of globular proteins surrounded by finite water clusters
    T.A. Wesolowski
    Molecular Simulation, 6 , 1991, p175
The temperature dependent Raman spectra of the title compound confirm the presence of some rotational disorder of the NH3 end groups below 112 K. The central carbon-carbon stretching mode around 865 cm-1 is coupled to the order parameter of the incommensurate phase transition at 168 K. No other clear evidence of the incommensurate modulation appears in the Raman spectra between 112 and 168 K. In the Abma phase (above 168 K) a dynamic conformational gauche-trans equilibrium is observed. The corresponding enthalpy difference is estimated to be 18.4+or-6.5 kJ mol-1.
  • Room temperature persistent spectral hole burning in Sm-dopedSrFCl1/2Br1/2 mixed crystals
    R. Jaaniso and H Bill
    Europhysics Letters, 16 (6) , 1991, p569-574
    Keywords: optical hole burning; samarium; spectral line breadth; strontium compounds; two-photon spectra
    DOI:10.1209/0295-5075/16/6/010 | Abstract | Article PDF
Due to the fact that for d6systems there are a number of low-lying ligand field (LF) states the relaxation from excited states of Fe(II) coordination compounds is, in general, a very fast and radiationless process. In Fe(II) spin-crossover systems, however, the zero point energy difference between the two lowest states, namely the low-spin (LS) 1A1 and the high-spin (HS) 5T2 state, is of the order of kBT, and some systems can be converted quantitatively to the HS state well below the thermal transition temperature by irradiating either into MLCT or LF absorption bands of the LS species, with HS→LS relaxation rates as small as 10−6 s−1 at XXX10 K. It is also possible to achieve a light-induced transient population of a HS state in Fe(II) LS compounds, but in this case the HS→LS relaxation rates can be larger than 106 s−1 even at low temperatures. The HS→LS relaxation rates show strong deviations from Arrhenius kinetics with nearly temperature independent tunnelling below ˜70 K and a thermally activated behaviour above ˜100 K. The range of 12 orders of magnitude in the low temperature tunnelling rate can be understood in terms of nonadiabatic multiphonon relaxation, where in the strong coupling limit, with the Huang-Rhys parameter S much larger than the reduced energy gap p, an inverse energy gap law holds.
[Fe(ptz)6](BF4)2 (ptz=1-propyltetrazole) is an Fe(II) spin crossover system, which shows a light-induced low-spin (1A1)-->high-spin (5T2) conversion below ~50 K by irradiating into the spin allowed 1A1-->1T1 d–d absorption band. This phenomenon, known as light-induced excited spin state trapping (LIESST), is reversible, and a subsequent irradiation into the 5T2-->5E band results in a light-induced 5T2-->1A1 conversion (reverse LIESST). Single crystal absorption spectra of the title compound in the region of d–d transitions are reported. In addition to the well-established spin allowed 1A1-->1T1 and 1A1-->1T2 transitions of the low-spin species and the 5T2-->5E transition of the high-spin species two weak bands in the NIR are assigned to the spin forbidden 1A1-->3T1 and 1A1-->3T2 transitions. Direct irradiation into the 1A1-->3T1 absorption band at 20 K results in a quantitative 1A1-->5T2 conversion, proving that this low lying triplet state plays an important role in the mechanism of LIESST. A full kinetic scheme for LIESST and reverse-LIESST with the 3T1 state as intermediate state is developed, and the quantum efficiencies for the various intersystem crossing steps involved are given: they are of the order of unity for the first step from the initially excited 1T1 and 5E states to the intermediate 3T1 state, respectively. The branching ratio from the 3T1 state to the 1A1 and the 5T2 states is 1:4.
The observatiqn of photon-gated hologram formation in a boric acid glass doped with triphenylene is reported. The first photon excites triphenylene to its first singlet excited state and, through intersystem-crossing, populates the first triplet stateTI. The second photon excites TI to T,, where autoionization occurs, leading to the formation of a radical cation. The gatinglight populating TI via SI is spatially uniform, while the light exciting TI to T, is spatially modulated. The long lifetime of the first triplet state allows Recording with low light intensities (mW/cm2). The spatially modulated excitation light forms three gratings (educt, intermediate state, and product). The extent of the interaction between these gratings depends on the overlap between educt, intermediate, and product absorption and refraction spectra as well as on the reading wavelength. The holograms were read at 363.8 and 632.8 nm. When the gating light is blocked, the holographic efficiency stays constant when read at 632.8 nm but increases substantially when read at 363.8 nm.
  • The Quenching of Triplet Benzophenone by 1,4-Diazabicyclo[2,2,2]octane in acetonitrile revisited
    E. Haselbach, P. Jacques, P. Suppan, D. Pilloud and E. Vauthey
    Journal of Physical Chemistry, 95 (19) , 1991, p7115-7117
    DOI:10.1021/j100172a001 | unige:3035 | Abstract | Article PDF
In conditions of laser flash photolysis, the kinetics of decay of the absorption of the benzophenone radical anion show that free, solvated ions are formed after electron transfer between the title compounds in neat, dry acetonitrile. Furthermore, it is shown that the opposite conclusion claimed by Devadoss and Fessenden (J. Phys. Chem., 1990, 94,4540), Le., no ion pair dissociation, results from a misinterpretation of the transient decay rate.
  • Base hydrolysis of acidato pentaamine complexes with inert metal centers: electronic structure of the intermediates, requirements for their formation, and the unique reactivity of the complexes of cobalt(III)
    F.P. Rotzinger, J. Weber and C. Daul
    Helvetica Chimica Acta, 74 (6) , 1991, p1247-1263
    DOI:10.1002/hlca.19910740613 | Abstract | Article PDF
  • Recent developments in molecular graphics : visualization of chemical structures and properties
    J. Weber, , P. FlĂźkiger and A. Goursot
    The Visual Computer, 7 (2-3) , 1991, p158-169
    DOI:10.1007/BF01901186 | Abstract | Article PDF
  • Calculation and visualization of a reactivity index for organometallics based on the extended HĂźckel model
    J. Weber, P. FlĂźkiger, D. Stussi and
    Journal of Molecular Structure: THEOCHEM, 227 , 1991, p175-185
    DOI:10.1016/0166-1280(91)85282-C | Abstract | Article PDF
  • Determination of the angular dependence of Hc2 in high Tc single crystals by a microwave technique
    D. Shaltiel, H. Bill, A. Grayevsky, A. Junod, D. Lovy, S. Sadowski and E. Walker
    Superconductor Science and Technology, 4 (1S) , 1991, p85-87
    DOI:10.1088/0953-2048/4/1S/014 | Abstract | Article PDF
It is shown that using an ESR spectrometer with magnetic field modulation and sweeping the temperature across Tc (at a constant and a very low magnetic field), is equivalent to temperature modulation. The signal intensity obtained when crossing Tc is proportional to 1/( delta Hc2/ delta T) at T=Tc. Using the WHH relation Hc2(T=0)=0.7 Tc( delta Hc2/ delta T)T=Tc enabled the measurement of the relative angular variation of Hc2 in single crystals of YBaCuO with Tc approximately 85 K. The data fit the Ginzberg-Landau theory. This very sensitive technique can be used to characterize properties of high Tc superconducting materials. Results on thin films is also be presented.
  • Microwave absorption across Tc: Determination of the angular dependance Hc2(θ) / Hc2
    D. Shaltiel, H. Bill, A. Grayevsky, A. Junod, D. Lovy, W. Sadowski and E. Walker
    Physical Review B, 43 (16) , 1991, p13594-13597
    DOI:10.1103/PhysRevB.43.13594 | Abstract | Article PDF
  
It is shown that measuring microwave absorption in high-Tc superconductors at constant and very low magnetic fields, using magnetic-field modulation, is, under some conditions, equivalent to temperature modulation when sweeping the temperature across Tc. Using an ESR spectrometer, the derivative of microwave absorption is measured close to Tc. This allows a determination of the relative angular variation of dHc2/dT at T=Tc in single crystals of Y-Ba-Cu-O. The data fit the Ginzburg-Landau theory on the relative angular variation of Hc2. The ratio (dHc2/dT)T=Tc parallel and perpendicular to the Cu planes was found to be 2.7 and 5.3 for two Y-Ba-Cu-O single crystals with Tc=89 and 86 K, respectively. These values obtained at 1010 Hz are close to the values obtained by conventional dc methods.
Recently, we have discovered a fascinating photophysical effect in spin crossover complexes of iron(II) : Light-Induced Excited Spin State Trapping (LIESST). At sufficiently low temperatures, the low spin state (1A1) can be converted quantitatively to the high spin state (5T2) by irradiating the sample into the 1A1 → 1T1 d-d absorption band (—540 nm). The resulting metastable HS state has a very long lifetime at low temperatures, in some cases it does not decay noticeably over a period of several days at 10 K. Only at temperature above some critical temperature does thermal relaxation back to the LS state set in. The sample can also be reconverted to the LS state by irradiating into the 5T2 → 5E absorption band (∞50 nm). The system thus behaves like an optical switch. The relative positioning - horizontally and vertically - of the potential wells of the two spin states is crucial for the lifetime of the metastable HS state.
  • Modeling of organometallic reactivity using a combination of extended Hueckel and molecular graphics techniques
    , Peter FlĂźkiger, Jacques Weber and E.Peter KĂźndig
    Studies in Physical and Theoretical Chemistry, 71 , 1990, p313-321
Polarized Raman measurements on single crystals of CuO yield the symmetry assignments of the three predicted Raman active lattice modes : 297cm−1 (Ag, 344cm−1 (Bg) and 629cm−1 (Bg). These results are compared to literature data, including IR spectra. Our measurements confirm at low temperature the appearance of an additional Raman band around 240 cm−1. The temperature dependence of the linewidth of the Ag mode presents an anomalous behavior near the magnetic phase transition, suggesting the possible presence of magnon-phonon couplings in the antiferromagnetic phase.
  • Growth of single crystals, thermal dependency of lattice parameters and Raman scattering in the Nd2-xCexCuO4-δ system
    W. Sadowski, H. Hagemann, M. François, H. Bill, M. Peter, E. Walker and K. Yvon
    Physica C: Superconductivity, 170 (1-2) , 1990, p103-111
    DOI:10.1016/0921-4534(90)90235-7 | unige:3054 | Abstract | Article PDF
We report on the growth of Nd2-xCexCuO4-δ single crystals (0<x<0.2) from Cu2O flux. Free separated crystals with maximum size of 5x8x0.15 nm3 have been obtained. Magnetic AC susceptibility measurements show a sharp superconducting transition at temperatures up to 23 K. The temperature dependence of the lattice parameters has been measured by means of X-ray powder diffraction between 10 K (a=3.9413(3) Å, c=12.0290(18) Å) and 290 K (a=3.9482(3) Å, c=12.0590(18) Å). Room temperature Raman spectra reveal a new band at 320 cm-1 which is not observed in Nd2CuO4. Raman spectra of crystals withTc ranging from 7 to 22 K show a systematic intensity change of the broad band at 590 cm-1.
The polarized Raman spectra of four different beryl crystals were studied at room temperature in the range from 30 to 4000 cm-1. The spectra show significant differences between the samples studied, and corrections are proposed for the reference Raman spectra of beryl previously reported by Adams and Gardner (1974). Type II water is observed in two crystals; the corresponding symmetric Raman stretching band at 3595 cm-1 is extremely strong for an impurity (about 20% of the strongest beryl lattice mode). Another, sharper, band of similar intensity at 3605 cm-1 could possibly originate from a hydroxyl stretching mode. Additional weaker bands are observed around 1600 cm-1 and 3600–3750 cm-1. The first polarized Raman spectra of bazzite are presented and discussed.
  • Extraction of biomarkers from sediments with supercritical carbon dioxide: A comparative study with solvent extraction and thermodesorption methods
    GĂŠrard Hopfgartner, Jean-Luc Veuthey, Fazil O. GĂźlaçar and Armand Buchs
    Organic Geochemistry, 15 (4) , 1990, p397-402
    DOI:10.1016/0146-6380(90)90166-W | Abstract | Article PDF
A study of viscosity and temperature effects on the rate of back electron transfer (BET) within an exciplex (9,10-dicyanoanthracene/N,N-dimethylaniline) with a strong charge transfer character in six non-polar solvents is reported. The extent of charge transfer has been estimated from the solvatochromic and thermochromic shifts of the fluorescence. Conformational changes are a prerequisite to the BET. In non-viscous solvents, where they are much faster than the ET step itself, the observed rate can be explained within the theory of non-adiabatic ET reactions, while in more viscous solvents, a time-dependent electronic coupling constant V has been introduced. In decalin and butylbenzene, a transition from a "solvent independent" to a "solvent controlled" non-adiabatic regime is observed.
  
The construction of a new high-resolution multichannel low-cost pulse generator is reported. It is fully computer controlled through a simple RS232 serial interface. Its features are 1 ns resolution within any time, pulse delays up to 16 ms and pulse lengths up to 65 mu s, and pulse sequence repetition rate from 66 Hz up to 250 kHz. It has fully programmable sequencing, including step increments for any pulse delay or length. It governs a pulsed ESR spectrometer, which is also described, but it could be used in a very wide range of experimental set-ups. A few examples of spin-echo detected ESR and ESEEM of some paramagnetic centres are shown.
  • Animation: a useful tool for protein molecular dynamicists, applied to hydrogen bonds in the active site of elastase
    Stanley M. Swanson, Tomasz Wesolowski, Maciej Geller and Edgar F. Meyer
    Journal of Molecular Graphics, 7 (4) , 1989, p240-242
    DOI:10.1016/0263-7855(89)80009-8 | Abstract | Article PDF
Massive amounts of coordinate data result from molecular dynamics calculations. The animation program MDKINO is a simple but powerful tool for previewing or reviewing the results. In recent simulations of elastase, we have examined hydrogen bonding patterns, conformational changes involving shifts in ring positions and rotations of amino acid side chains, electric fields in interatomic space, and electric forces acting on chosen nuclei. Animation is also useful for checking on the stability of calculations in progress. Simple programming techniques achieve acceptable levels of animation with readily available hardware (PS330 or PS390 display with a serial interface to a laboratory VAX). In about half an hour, it is possible to make and watch a color stereo “movie” of a selected subsystem of a simulation (up to 1 000 frames of about 100 atoms each).
  • Electronic structure of Cr3+ in Cs2NaYCl6 and Cs2NaYBr6 lattices. Electron-paramagnetic resonance and electron-nuclear double resonance measurements and multiple scattering X α calculations
    A. Monnier, D. Chambaz, H. Bill, H.U. GĂźdel and J. Weber
    Journal of Chemical Physics, 91 (11) , 1989, p6650-6656
    DOI:10.1063/1.457383 | Abstract | Article PDF | Article PS (gzipped)
The electronic ground state of isolated Cr3 + introduced into the title compounds has been investigated with electron spin resonance and electron‐nuclear double resonance spectroscopy. Simultaneously a multiple scattering(MS) Xα study of the (CrCl6)3 −cluster has been performed. The experimental results agree with a cubic Cr site. They further show evidence for strong quadrupoleinteraction at the anion neighbor nuclei and for observably different covalency in the two hosts. Rather good agreement is found between the predictions of the MS Xα model and the experimental superhyperfine interaction constants but not with the Cr‐hyperfine structure constant. It is suspected that the second neighboring Cs play a non‐negligible role in the electronic structure of the cluster.
Temperature-dependent Raman measurements of n-propylammonium chloride solutions in water, concentrated hydrochloric acid and methanol yield experimental gauche-trans enthalpy differences of 0.21 ± 0.13, 0.51 ± 0.26 and 0.37 ± 0.11 kcal mol−1, respectively. Raman spectra in different aqueous solutions show that the gauche-trans equilibrium in solution is affected by the total chloride concentration. The gauche-trans enthalpy difference for CH3CH2CH2NH3+ is found by 4-31G ab initio calculations to be 0.26 kcal mol−1.
  • Observation of ESR in the Bi high-Tc superconductors
    D. Shaltiel, H. Bill, M. Descroux, H. Hagemann, A. Junod, M. Peter, Y. Ravi Sekhar, G. Triscone, E. Walker, Y.F. Yan and Z.X. Zhao
    Physica C: Superconductivity, 157 (2) , 1989, p240-246
    DOI:10.1016/0921-4534(89)90008-7 | unige:3067 | Abstract | Article PDF
Electron spin resonance of a high-Tc Bi superconductor sample is reported. The d.c. susceptibility, d.c. resistivity and a.c. susceptibility show two superconducting transitions at 105 K and 75 K. The ESR spectra show a main resonance line whose temperature dependence is studied in detail. The g value shows a maximum of 2.24 at 230 K and decreases to 2.12 at 100 K. The line width also shows a maximum of 520 G at the same temperature and drops to 200 G at 100 K. An unusual behaviour is observed in the decrease of the integrated intensity from a maximum at 230 K to below noise at level 100 K. It is possible that the origin of this signal is due to impurity phases. However, the unusual behaviour of its intensity (disappearance of the signal below Tc) may indicate that it arises from pair formation much above Tc.
  • Single crystal ESR studies on tetragonal YBa2Cu3O6+x
    D. Shaltiel, H. Bill, P. Fischer, M. François, H. Hagemann, M. Peter, Y. Ravi Sekhar, W. Sadowski, H.J. Scheel, G. Triscone, E. Walker and K. Yvon
    Physica C: Superconductivity, 158 (3) , 1989, p424-432
    DOI:10.1016/0921-4534(89)90239-6 | unige:3068 | Abstract | Article PDF
We have characterized as-grown and thermally treated YBaCuO single crystals by ESR, Raman spectroscopy, magnetic susceptibility, X-ray and neutron diffraction measurements. The as-grown crystals are tetragonal and are superconducting with an onset temperature of 30 K. They show an ESR signal which behaves as a localized Cu2+ ion with tetragonal symmetry and presumably originates from copper chain atoms that are octahedrally coordinated by six oxygen neighbors. The temperature dependence of the ESR between 150 and 270 K shows paramagnetic behavior and also dynamical features. Below 90 K, the ESR signal disappears reversibly. A likely explanation is that the onset of local superconductivity frustrates the spins responsible for the ESR signal.
  • Quantitative infrared methods for the measurement of crystallinity and its temperature dependence: polyethylene
    H. Hagemann, R.G. Snyder, A.J. Peacock and L. Mandelkern
    Macromolecules, 22 (9) , 1989, p3600-3606
    DOI:10.1021/ma00199a017 | unige:3069 | Abstract | Article PDF
Vibrational spectroscopic methods are widely used to characterize semicrystalline polymers in terms of crystallinity. The temperature coefficient of crystallinity, an important and fundamental quantity, is seldom determined for lack of a sensitive method. In this paper, we describe an infrared approach to the measurement of the temperature coefficient of crystallinity. We start from the well-known observation that the integrated intensities of the bands in the spectrum of a semicrystalline polymer change with temperature. It is also known, though less appreciated, that only part of the change is due to changes in crystallinity, the remaining part being due to changes in the intrinsic intensity of the bands. We outline a method for separating these overlapping effects. The method has been applied to a variety of semicrystalline polyethylene samples. The temperature coefficients are found to be highly dependent both on the temperature and on the morphology of the sample. In addition we report crystallinity measurements on a solution crystallized low molecular weight (A& = 13 600) sample, discuss the origin of an apparent anomalous temperature dependence of band intensity cited in the literature, and offer quantitative evidence that the temperature dependence of specific volume is, at temperatures above 0 "C, largely determined by partial melting.
The energy balance of a photoinduced electron transfer reaction is given by the Rehm-Weller equation which combines the oxidation potential Eox(D) of the electron donor, the reduction potential Ered(A) of the electron acceptor, an electrostatic correction term C and the excited state energy of the light-absorbing species: It is shown that if light carries a thermodynamic entropy the excitation energy term must be given by ΡE*, Ρ being the efficiency of the conversion of the energy of light into chemical free energy. Measurements of fluorescence quenching through electron transfer at very low light intensities show that the Rehm-Weller equation remains valid in spite of its implied assumption that Ρ = 1; it is concluded that contrary to much current thinking light is a form of high grade energy which can be converted in principle entirely into chemical free energy and electrical energy.
  • Temperature Effect on Back Electron-Transfer Reactions within a Geminate Radical Pair: the Influence of the Solvent on the Adiabaticity of the Process
    E. Vauthey and P. Suppan
    Chemical Physics, 139 (2-3) , 1989, p381-390
    DOI:10.1016/0301-0104(89)80150-8 | unige:3064 | Abstract | Article PDF
A study of the temperature dependence (from 233 to 353 K) of the rate of back electron-transfer reactions within geminate radical pairs by measurement of the free radical yield is reported. The radical pair is generated by photoinduced electron transfer with rhodamine 6G and oxazine 118 cations as electron acceptors and aromatic amines and methoxy-benzene derivatives as electron donors in acetonitrile, methanol and ethanol. In acetonitrile, the back electron transfer is non-adiabatic and apparent negative activation energies are observed for barrierless reactions. In alcohol solvents, an anomalously large temperature dependence is observed, which is attributed to a solvent-controlled adiabatic behaviour.
The effect of externally applied stresses of eg and t2g symmetry on the cubic Eg(X)eg Jahn-Teller (JT) system Y2+ in SrCl2 is studied. Coupling constants were obtained with the aid of EPR and by relying on a Ham-type cluster JT Hamiltonian, in conjunction with a random strain distribution. This is established from a critical review of the contributing strain sources (including possible percolation effects due to the isotopic composition of natural SrCl2). The stress of t2g symmetry shows strong effects. The authors conclude that the cubic Ham factor q is nearly one and that the experiments allow, as a function of this latter stress, one to pass gradually towards a trigonal JT effect. The optical absorption spectrum assigned to Y2+ is given in addition.
  • Unusual behavior of the Gd ESR in single crystals of GdyY1-yBa2Cu3O6+x with x=0.1-0.8 and y=0.03-0.06: evidence for a magnetic interaction in the superconductors
    D. Shaltiel, S.E. Barnes, H. Bill, M. François, H. Hagemann, J. Jegondaz, D. Lovy, P. Monod, M. Peter, A. Revcolevschi, W. Sadowski and E. Walker
    Physica C: Superconductivity, 161 (1) , 1989, p13-20
    DOI:10.1016/0921-4534(89)90036-1 | unige:3071 | Abstract | Article PDF
The ESR of small concentration of Gd 0.03<y<0.06 substituting for Y in single crystals of GdyY1−yBa2Cu3O6−x has been measured. In the insulating compound, with x ~ 0.1, and the superdconducting materials with 30 K < Tc < 80 K, the measurements were performed at X-band, 9.3 GHz, and Kα-band, 36 GHz, over a large temperature range above Tc. Angular dependence measurements exhibit a spectrum which is fully resolved in certain directions, but only partially resolved, because of exchange narrowing, in other directions. Comparisons between the spectra in the insulating and superconducting compounds shows similar angular dependent behavior. This seems to indicate that the origin of the exchange narrowing is the same in both compounds. Since this narrowing in the insulating compound arises from interaction with, or via, the Cu magnetic system, it is implied that there is a similar, perhaps fluctuating, system in the superconducting state. Preliminary measurements of the temperature dependence of the line widths may indicate the presence of spin pairing at about 110 K, above the actual Tc of 70 K. The crystal field parameters are D = 3B02 = 1307 MHz, B04 = 3.014 MHz and B44 = -11.43 MHz, for the semiconducting sample. The g-value is 1.989 ± 0.005. These values change only slightly in the superconducting crystals.
ESR results are reported on the Ag2+ ion introduced into alkaline earth fluoride crystals. In SrF2 (as in CaF2) a trigonal centre is present which tunnels between four equivalent C3 directions as shown by ESR under uniaxial stress. A T2g ⊗ t2g Jahn-Teller model describes adequately the paramagnetic and stress effects when the strong coupling case is considered. Ag2+ in BaF2 forms a static tetragonal cluster similar to Ag2+ and Cu2+ in SrCl2.
  • Low temperature Raman study of the thermochromic crystal (C2H5NH3)2CuCl4(EACuC)
    Hans Hagemann and Hans Bill
    in "Proc. Int. Conf. Raman Spectroscopy XI" Eds. R.J.H. Clark and D.A. Long, 11 , 1988, p451-452
  • Calculations of the conformational properties of acyclonucleosides: Part 2. Comparison of active and nonactive compounds
    T.A. Wesolowski, A. Godzik and M. Geller
    Journal of Molecular Structure: THEOCHEM, 180 (1) , 1988, p105-112
    DOI:10.1016/0166-1280(88)80082-4 | Article PDF
Two related molecules from the acyclic nucleoside family 9-(1,3-dihydroxy-2-propoxymethyl) guanine (DHPG) and 9-(1,5-dihydro-4-hydroxymethyl-3-oxapentyl-2- [R])guanine (2′, 3′- secoG) have been compared by means of force field conformational analysis. They are respectively active and nonactive analogs of the antivirial compound 9-((2-hydroxyethoxy)methyl)guanine (ACG). As in the case of ACG many local minima are found for both molecules, indicating their great flexibility. For all three molecules conformations similar to those occurring in cyclic nucleosides have energies from 3 to 7 kcal mol−1 above the most stable minima.
  • Light-induced excited spin state trapping (LIESST) in [Fe(2-mephen)3]2+ embedded in polymer matrices
    Andreas Hauser, JĂźrgen Adler and Philipp GĂźtlich
    Chemical Physics Letters, 152 (6) , 1988, p468-472
    DOI:10.1016/0009-2614(88)80443-3 | unige:14660 | Abstract | Article PDF
It is known that [Fe (2-mephen)3] (ClO4)2 in the solid state is an iron(II) spin-crossover system which shows light-induced excited spin state trapping (LIESST). The thermal spin-crossover behaviour of the complex [Fe(2-mephen)3]2+ embedded in various polymer matrices is similar to the solid-state behaviour, and a light-induced long-lived excited state is observed at temperatures below 50 K. Relaxation curves show that polymer matrices are not very homogeneous media.
  • EPR evidence of a pressure-induced phase transition in LiKSO4
    Y. Ravi Sekhar and H. Bill
    Physical Review B, 38 (16) , 1988, p11802-11804
    DOI:10.1103/PhysRevB.38.11802 | Abstract | Article PDF
A new structural phase transition is induced in LiKSO4 crystals by the application of a uniaxial pressure at 77 K. EPR spectra resulting from a MoO43- probe undergo reversible changes as the pressure is increased to about 51 MPa. From a quantitative analysis of the new spectrum above 60 MPa, a space-group change is inferred. These results are corroborated by the evidence of a similar phase transition at 64 K and ambient pressure.
  • Structure and analysis of the binding energy of the copper carbonyl ion CuCO+ complex: an ab initio study
    and Jacques Weber
    Journal of Molecular Structure: THEOCHEM, 166 , 1988, p247-252
    DOI:10.1016/0166-1280(88)80444-5 | Abstract | Article PDF
  • The modeling of nucleophilic and electrophilic additions to organometallic complexes using molecular graphics techniques
    Jacques Weber, Peter FlĂźkiger, , Olivier Schaad, Annick Goursot and Claude Daul
    Journal of Computer-Aided Molecular Design, 2 (3) , 1988, p235-253
    DOI:10.1007/BF01531997 | Abstract | Article PDF
  • Theoretical and ESR/ENDOR single-crystal study of an azaallyl radical
    Malledi V.V.S. Reddy, Alice Celalyan-Berthier, Michel Geoffroy, , Jacques Weber and Gerald Bernardinelli
    Journal of the American Chemical Society, 110 (9) , 1988, p2748-2753
    DOI:10.1021/ja00217a009 | Abstract | Article PDF
  • A vibrational study of some 1,2,4-Trioxanes
    K.Martin Mohnhaupt, Hans Hagemann, Jean-Pascal Perler, Hans Bill, John Boukouvalas, Jean-Claude Rossier and Charles W. Jefford
    Helvetica Chimica Acta, 71 (5) , 1988, p992-999
    DOI:10.1002/hlca.19880710508 | Abstract | Article PDF
The vibrational spectra of some 1,2,4-trioxanes present two characteristic bands at 790 and 880 cm−1. On the basis of 18O-isotopic substitution and comparison with analogous compounds, these bands have been assigned to coupled C—O and O—O stretching modes of the C—O—O element.
  • Stereospecific and unspecific pathways for aromatization of 1,4 cyclohexadienes
    P. MĂźller, N. Pautex and H. Hagemann
    Chimia, 42 , 1988, p414
    unige:3080
  • Free Energy Dependence of the Ion Yield Of Photoinduced Electron-Transfer Reactions in Solution
    E. Vauthey, P. Suppan and E. Haselbach
    Helvetica Chimica Acta, 71 (1) , 1988, p93-99
    DOI:10.1002/hlca.19880710111 | unige:3073 | Abstract | Article PDF
A study of photo-induced electron-transfer reactions in MeCN with 9,10-dicyanoanthracene as acceptor and 21 electron donors with transient photoconductivity measurements is reported. The free-ion yield and the rate constant of back electron transfer are determined. For exergonic reactions, the Marcus-inverted region is observed. The fit with the theory is best, when a nearly solvent-independent Coulomb term is used in the calculation of the energy balance.
  • Comparison of Photoinduced Electron-Transfer Reactions of Aromatic Carbonyl vs. Cyano-Compounds with Electron Donors in Condensed Phase: the Importance of the Spin State of the Geminate Ion Pair for Obtaining High Ion Yield
    E. Haselbach, E. Vauthey and P. Suppan
    Tetrahedron, 44 (24) , 1988, p7335-7344
    DOI:10.1016/S0040-4020(01)86228-1 | unige:3074 | Abstract
Photoinduced electron transfer reactions in acetonitrile with bensopheneone, anthraquinone, 9-cyanoanthracene and 9,10-dicyanoanthracene as electron acceptors, and with 1,4-diasabicyclo[2,2,2]octane and N,N-dimethylaniline as electron donors have been studied with ns-laser flash photolysis and fluorescence quenching measurements. For these systems the resulting free ion yield depends on the spin state of the geminate ion pair: its separation is very efficient if formed in a triplet state (carbonyl compounds/donors), while it is very inefficient if formed in a singlet state (cyanoanthracenes/donors). In the triplet systems, geminate back electron transfer is limited by the rate of spin flip.
A new phase transition is observed in the KLiSO4 crystals below 77 K using MoO43- as a paramagnetic probe. The EPR spectra of this molecular ion, which substitutes for the host SO42- ion, show triclinic symmetry at 77 K due to the joint action of a trigonal crystal field and a trigonal Jahn-Teller effect of E(X)e type. Above 77 K, motional averaging in the a-b plane occurs; thereby an axial spectrum is observed. Below 64 K, the two sulphate sites are no longer equivalent resulting in the appearance of two distinct EPR spectra of the molybdate ion. The possible space groups of the crystal in the vicinity of the phase transition are discussed. The authors' results indicate that one needs to distinguish carefully between the local dynamics and the structural changes in this crystal.
Single crystals of MoO3 have been grown under an oxygen atmosphere using a horizontal tubular furnace. The temperature and pressure necessary to grow satisfactory crystals in oxygen along [100] and [010] directions are determined.
  • Vibrational spectra of MnO4- isolated in KBr crystals: an illustration of pure Td symmetry in the solid state
    H. Hagemann, Y. Ravi Sekhar and H. Bill
    Journal of Chemical Education, 64 (5) , 1987, p456-458
    DOI:10.1021/ed064p456 | unige:3089 | Abstract | Article PDF
A solid state example that can clearly illustrate the rigorous application of IR and Raman selection rules in a pure Td symmetry.
  • Structure and crystallization of n-C21H44, n-C36H74 and low molecular weight polyethylene glasses
    H. Hagemann, H.L. Strauss and R.G. Snyder
    Macromolecules, 20 (11) , 1987, p2810-2819
    DOI:10.1021/ma00177a029 | unige:3090 | Abstract | Article PDF
Glassy films of n-CzlH,, n-C36H74a, nd low molecular weight polyethylene were prepared by vacuum sublimation onto a CsI window held at 7 K and were studied by infrared spectroscopy. The conformational disorder achieved for the glass was comparable to that of the liquid near the sublimation temperature. The chain-organizing processes were monitored for Czl and polyethylene as the sample was warmed to 300 K. A number of separate steps are involved. Each step occurs over a more or less broad temperature region, and sometimes the steps overlap. In the case of CZ1, the first step involves a conformational ordering of the chains to their extended all-trans form. The extended chains initially pack in a monoclinic subcell, but they are not in longitudinal register; that is, the end methyl groups do not lie in parallel planes. At a higher temperature, the monoclinic subcell is converted to an orthorhombic subcell, but the chains are still not in register. In the last annealing step, which occurs over a narrow temperature range, the chains come into register so that the Czl finally assumes its stable orthorhombic crystal structure. The annealing of the polyethylene glass proceeds in exactly the same way except that there is no chain-registering step. In general, the transition temperatures are higher and the temperature range over which the transitions occur is broader for the polyethylene sample.
Naphthalene and anthracene undergo a monophotonic ionization process in MeCN to produce the radical cations in low quantum yields (around 0.06 for anthracene). This reaction originates from the relaxed singlet excited state S1, and it is not due to traces of H2O in the solvent.
  • The identification and structure of FSO2 radical trapped in potassium sulphate crystals
    Y. Ravi Sekhar, H. Bill and D. Lovy
    Chemical Physics Letters, 136 (1) , 1987, p57-61
    DOI:10.1016/0009-2614(87)87298-6 | Abstract | Article PDF
ESR and ENDOR results on irradiated K2SO4:BF4− crystals are presented. Based on the ESR spectra and their symmetry at the X and Ka bands, a new paramagnetic radical trapped at the sulphate sites is identified. The ENDOR spectra are consistent with the involvement of a fluorine nucleus. The spectra are assigned to the symmetric FSO2 radical.
Experimental results on KBr:ClO−4 single crystals, before and after x irradiation at 77 K, are reported. Infrared and polarized Raman spectra show clearly that the perchlorate molecules are isolated in the KBr matrix and possess Td  symmetry. EPR spectra, at 9.5 and 36 GHz, of irradiated crystals at 4.2 K indicate the formation of ClO2− 4 in trigonal symmetry. The g and 35Cl hyperfinetensors are found to be strictly axial and collinear. Temperature variation of the EPRspectra presented no motional averaging while uniaxial stress applied to the crystals yielded negative results. The structure of the ClO2−4 molecule is discussed in terms of a trigonal Jahn–Teller effect and a probable dissociation into (ClO−3 – O−) complex.
Fe(ptz)6(BF4)2 (ptz = 1-propyltetrazole) is an iron(II) spin-crossover system which shows light-induced excited spin state trapping. In this paper we show that (a) the same phenomenon can also be observed in Zn1−xFex(ptz)6(BF4)2 (x ≈ 0.1) and is therefore basically a single-ion property, and (b) that the phenomenon is reversible. The efficiency of the light-induced spin crossover is of the order of 0.5% in the forward direction and 0.1% in the reverse direction.
  • High-spin -> low-spin relaxation kinetics and cooperative effects in the hexakis(1-propyltetrazole)iron bis(tetrafluoroborate) and [Zn1-xFex(ptz)6](BF4)2 (ptz = 1-propyltetrazole) spin-crossover systems
    Andreas Hauser, Philipp GĂźtlich and Hartmut Spiering
    Inorganic Chemistry, 25 (23) , 1986, p4245-4248
    DOI:10.1021/ic00243a036 | unige:3095 | Abstract | Article PDF
[Fe(ptz)6l(BF4)2 (ptz= 1-propyltetrazole) and the mixed crystals [Znl,Fe,(ptz)6] (BF4)2 are Fe(I1) spin-crossover compoundsthat exhibit light-induced excited-spin-state trapping. It is shown that (a) for x I 0.1 a single-ion treatment of both the spinequilibrium ( M H L = 510 (12) cm-', ASHL = 5.1 (2) cm-'/K at T = 100 K) and the relaxation from the excited high-spin state (.Eao = 810 (30) cm-I, A - 105/s) is appropriate and (b) for 0.2 I x I 1 cooperative effects observed in the relaxation from the high-spin state are of long-range nature and therefore of elastic rather than of electronic origin.
The construction of a simple high temperature Raman furnace is presented. This furnace has been operated efficiently in the temperature range from 320 to 700K.
Laser-flash-photolysis experiments show that, in MeCN at 20°, perylene (P) undergoes three distinct electron-transfer reactions: These processes originate probably from the thermally relaxed excited states of P.
The authors show that CaF2:Y crystals that have been X-irradiated at low temperatures contain Y2+ ions associated with a trigonally located interstitial F- ion. Experimental results are presented that support this identification and that show that a trigonal Jahn-Teller effect (with warping) is involved. An estimate for the Jahn-Teller energy of EJT approximately=490 cm-1 is obtained.
  • Luminescence and absorption properties of vanadium dichloride (VCl2), magnesium vanadium chloride (Mg1-xVxCl2) and cadmium vanadium chloride (Cd1-xVxCl2) crystals
    Brigitte Gaelli, Andreas Hauser and Hans U. Guedel
    Inorganic Chemistry, 24 (14) , 1985, p2271-2275
    DOI:10.1021/ic00208a029 | unige:6521 | Abstract | Article PDF
Low-temperature luminescence and absorption spectra were recorded of VClz doped into MgCl, and CdCl, as well as of the pure compound. There is evidence for excitation energy transfer in Velz down to 5 K. In the diluted materials the luminescence remains unquenched up to 200 K (Cd,,V,Cl2) and 250 K (MgI-,V,CI2). The broad-band 4T2, - 4Azs luminescence transition is highly structured in the diluted samples. There is multiple evidence for a Jahn-Teller effect in the 4Tz, state with an estimated Jahn-Teller energy of the order of 200 cm-I. Polarized absorption and Zeeman meausurements were used to assign the 4Az, -,E,, *TI, transitions.
Electron paramagnetic resonance of CrO2 − 4doped K2SO crystals has been studied after x‐irradiation at 77 K. The resulting species, a CrO4 3 − center, has the principal g values g x=1.8743, g y =1.8507, and g z =1.9386. The EPR data indicate a small distortion of CrO4 3 −tetrahedra. A temperature variation study of the spectra in the range 1.6–77 K shows that they broaden rapidly above 10 K and no motionally averaged spectrum is observed. The experimental results are discussed in terms of several effects including a possible Jahn–Teller effect.
  • EPR of MnO42- in KBr under uniaxial stress. A case of intermediate to weak Jahn-Teller effect
    Y. Ravi Sekhar and H. Bill
    Molecular Physics, 54 , 1985, p757-760
    DOI:10.1080/00268978500100601 | Abstract | Article PDF
E.P.R. of MnO2- 4 centres in KBr single crystals has been investigated at 9¡5 and 35 GHz in the temperature range 1¡6-77 K. At 4¡2 K, the spectrum exhibits a more nearly cubic anisotropy. A temperature variation study indicates that the spectra disappear reversibly above about 8 K. Uniaxial stress measurements at X-band have shown the importance of random strain.
We have obtained the room temperature Raman spectra of the three salts n-PrNH3X (X = Cl, Br and I). The concentration of gaucheisomers increases when passing from the chloride to the iodide. We have evaluated experimentally the lattice contribution to the energy difference between gauche and anti conformers. A force field calculation yielded an energy difference of 0.85 kcal mol−1 between the two conformations of the propyl-ammonium moiety.
We have obtained the room temperature Raman spectra of liquid 1,4-cyclohexadiene and its isotopic analogs: C6H7D,gem-C6H6D2, trans-C6H6D2 and C6H4D4. Additionally, we have measured the Raman spectra of 1,4-cyclohexadiene as a function of temperature from 5 to 300 K. The conformation of 1,4-cyclohexadiene is found to be very likely planar in the solid state. The solid—solid phase transition at 192 K reported by calorimetric measurements has been observed and is discussed qualitatively.
The authors have studied the Raman spectra of the crystals (EtNH3)2MCl4 (M=Cd and Mn) as functions of temperature from 5 to 300K. The external vibrations have been assigned. The space group of the Cd crystal below 114K is found to be P22/b and the phase transition P21/b-Pbca is described as a displacive first-order transition. The order-disorder transitions Pbca-Abma in (EtNH3)2MCl4 for M=Cd and M=Mn are compared. Linewidth measurements performed on the carbon-carbon stretching mode at about 870 cm-1 suggest the possibility of a second-order transition in the Mn crystal, while the first-order behaviour of the transition in the Cd crystal is confirmed.
An EPR study has been carried out on a Cu2+ center in NaF single crystals (with 2 % mole of copper) from 1.6 k – 150 K. The symmetry of the EPR spectra is triclinic at 1.6 K, monoclinic at 4.2 K, tetragonal at T ≥ 40 K. No hyperfine structure has been observed. Models which include both the Jahn-Teller effect and a low symmetry crystal field are proposed.
  • EPR studies on ReO4 in alkali halide crystals: intramolecular E ⊗ e type static Jahn-Teller effect
    Yellepedi Ravi Sekhar and Hans Bill
    Pproc. XXiith Ampère Congress Zßrich, 22 , 1984, p198-201
  • EPR of Cu2+ in NaF and KZnF3 
    Georges Magne and Hans Bill
    Proc. XXIIth Ampère Congress Zßrich, 22 , 1984, p661-664
  • Observation of the Jahn-Teller effect with electron paramagnetic resonance
    H. Bill
    in "The dynamical Jahn-Teller effect in localized systems" Yu E. Perlin, M. Wagner, North Holland, Amsterdam, 1984
A room temperature Raman study has been performed on the following compounds: EtNH3X (X = Cl,Br), CD3CH2NH3Cl, CD3CH2ND3Cl, CH3CD2NH3Cl, CH3CD2ND3Cl, and EtND3Cl. The complete vibrational spectra are discussed in terms of external and internal modes. The external modes of the halogens have been identified on the basis of the isotopic substitutions. The internal vibrations are divided into three frequency ranges. Several modes are clearly identified through the observed isotopic shifts, while other modes are found to interact strongly. The Fermi resonances of the carbon-hydrogen stretching modes with the carbon-hydrogen deformational modes are analyzed.
  • Supergiant variability: theoretical pulsation periods and comparison with observations
    D. Lovy, A. Maeder, A. NoĂŤls and M. Gabriel
    Astronomy and Astrophysics, 133 , 1984, p307-312
    Abstract | Article PDF
  
Attention is given to the theoretical period-luminosity-color relations of stars evolving with mass loss that cover the upper part of the HRD, and pulsation periods are determined for the fundamental radial mode and first and second overtones. It is noted that the pulsation Q-terms present a minimum near T(eff) of 20,000 K, and that the Q-terms increase both for hot main sequence stars and cool and luminous red supergiants. A comparison of models with observations indicates that about 40 percent of the supergiants have periods which are in agreement with the fundamental mode of radial pulsation.
  • (ReO4)2- ion in KCI: Experimental evidence for a multimode intramolecular Jahn-Teller effect
    H. Bill and Y. Ravi Sekhar
    Physical Review B, 28 (5) , 1983, p2352-2355
    DOI:10.1103/PhysRevB.28.2352 | Abstract | Article PDF
For the first time, experimental results have been obtained on a transition-metal tetroxy anion stabilized without any associated defects in a cubic crystal: the (ReO4)2- ion in KC1. Our EPR study, including experiments under uniaxial stress and static electric fields applied to the crystal, shows that a strong and essentially intramolecular Jahn-Teller effect occurs but not exclusively of the E⊗e type. Appropriate linear combinations of the T2 vibrations involving a pseudo-Jahn-Teller effect and spin-orbit coupling must play an important part.
This paper demonstrates that the color of natural yellow fluorites is due to an O− 3 molecule ion incorporated into the lattice. The center has several equilibrium positions. A detailed EPR and ENDOR study of this center is presented and the motional effects observed in the EPR spectrum are analyzed.
  • Etude par des techniques de partition d'ĂŠnergie des facteurs qui conditionnent l'ĂŠquilibre conformationnel de propènes disubstituĂŠs-1,3 de configuration E
    Tho Nguyen-Xuan, Jean M.J. Tronchet and Hans Bill
    Helvetica Chimica Acta, 64 (6) , 1981, p1949-1958
    DOI:10.1002/hlca.19810640628 | Abstract | Article PDF
This experimental study reports EPR and ENDOR results of an O2- molecule ion in natural yellow fluorites. The O2- is associated with a sodium neighbor and a calcium neighbor. This cluster performs a rapid switching movement between energetically equivalent positions. The resulting dynamical features in the EPR spectrum are investigated with the aid of the stochastic Liouville equation.
  • EPR investigation of the 4d1 ion Y2+ in CaF2 crystals
    Claude Balestra and Hans Bill
    in "Proc. XVIIIth Colloque Ampère" Notthingham, 18 , 1974, p155-156
  • Measurements of the magnetoelectric effect in Ni-Cl boracite
    J.P. Rivera, H. Schmid, J.M. Moret and H. Bill
    International Journal of Magnetism, 6 , 1974, p1
  • EPR Study of the Ag2+ ion in SrCl2 
    Hans Bill, C. Milleret and Roger Lacroix
    in "Proc. XVIIth Colloque Ampère" Ed. V. Hovi, 17 , 1973, p233-236
  • Resolved Electron Phonon Interaction observed in the optical absorption spectrum of a colour centre in CaF2
    Hans Bill
    in "Proc. XVIth Colloque Ampère" Ed. I. Ursu, 16 , 1971, p929-932
  • A Vk centre in SrFCl Crystals
    Hans Bill, Jean Marie Moret and Roger Lacroix
    in "Proc. XVth Colloque Ampère" Ed. R. Averbuch, 15 , 1969, p323-325
  • Investigations on colour centres in alcaline earth fluorides
    H. Bill
    Helvetica Physica Acta, 42 , 1969, p771
  • Investigation on different centres in CaF2 crystal
    Hans Bill and Roger Lacroix
    in "Proc. XIVth Colloque Ampère" Editor R. Blinc, 14 , 1967, p262-268

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Adler, Peter Aebersold, M.A. Aebischer, Annina Aebischer, Nicolas Afonina, S. 
Afshar, Majid Ghahraman Agapaki, E. Aharon, Tal Ahmed, Saleh A. Ajami, Aliasghar 
Akbarimoosavi, M. Akiba, Etsuo Al Badri, Abdelouahed Alameddine, Bassam Albinsson, Bo 
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Aliev, Almaz Allamandola, Louis J. Allet, B. Allonas, X. Alonso Doval, David 
Altamura, Emiliano Altmann, P. Altorfer, F. Alvarez, S. Amacher, Anneliese 
Ambrus, Christina Ameline, D. Ameline, Jean-Claude Amstutz, Nahid Ancilotto, Francesco 
Anderson, I. Andrews, Lester Andruh, Marius AndrĂŠ, N. Angeloni, Silvia 
Angelova, P. Angulo, Gonzalo Anson, Christopher E. Antonello, Sabrina Aquilante, Francesco 
Aramburu, J.A. Ardizzone, Antonio Armitage, N.P. Arrico, L. Arrizabalaga, Philippe 
Asensio, Xavier Aspari, Patrizio Assies, L. Aster, Alexander Atamny, Fachri 
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Aubert, Emmanuel Auffray, E. Augustynski, Jan Avarmaa, Tea Avarvari, Narcis 
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BarrabĂŠs, Noelia BarrabĂŠs Rabanal, NoĂŤlia Barras, Jean-Pierre Barriuso, M.T. Bart, Suzanne C. 
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Colom, Adai Colombo, Mirco G. Comba, Peter Comins, J.D. Corminboeuf, ClĂŠmence 
Coronado, Eugenio Coskun, A. Cossement, D. Costantino, Lorenzo Cotts, B. 
Couronne, O. Coutsolelos, A.G. Covington, Aaron M. Cowan, Sarah Cramariuc, Oana 
Cramer, Christopher J. Crassous, Jeanne Crespo, GastĂłn A. Croci, Mauro Crottaz, Olivier 
Cuetos, Alejandro Cunningham, Alastair Cunningham, Allan F. Curlik, Ivan Cusanovich, Michael 
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Daher, Sawsan Dainese, Tiziano Daku, L.M.Lawson Dalla Favera, Natalia Daly, Scott R. 
Danila, Ion Das, Gopal Dass, Amala Daul, Claude A. Davidson, R.S. 
Davis, Erik D’Anna, Vincenza De Angelis, F. De Caro, C. De Saint Laumer, Jean-Yves 
De Siebenthal, Jean Marie De Silva, Piotr De Sio, Luciano De Villedon de Naide, Fabienne De Vita, A. 
De Vito, David A. De Wijs, G.A. Dechamps, Damien Decurtins, Silvio Deguenon, D. 
Dekhtyar, Marina L. Delahaye, Sandra Delaire, Jacques A. Delgado, Irene Hernandez Delgado, Teresa 
Deligeorgiev, Todor G. Deloff, AndrĂŠ Demont, Philippe Denning, R.G. Deree, Y. 
Dereka, Bogdan Derivery, Emmanuel Deschamps, Eliane Deschenaux, Robert Descroux, M. 
Desplanches, CĂŠdric Desroches, Cedric Devarajan, A. Devaux, AndrĂŠ Devillers, M. 
Deyhimi, Farzad Dhar, S.K. Dharmaratne, Asantha C. Di Bari, Lorenzo Di Vara, N. 
Dias, Marylène Didelot, Emilie Dimitrov, Vladimir Ding, Jie Diring, S. 
Dixon, J.M. Dmitriev, Vladimir Dobkowski, Jacek Dobrikov, Georgi Doelker, Eric 
Dogadkin, Denis Doistau, Benjamin Dolamic, Igor Dolatshahi, N. Dolder, Stefan 
Dolotova, Elena Dominik, Janusz Dong, S. Donnio, Bertrand Doppelt, P. 
Dordevic, Nikola Dorizon-Duval, Delphine Dornheim, Martin Doux, Marjolaine Dovgaliuk, I. 
Dreier, F. Dreuw, Andreas Dreyer, Jens Driscoll, Paul C. Droz, Thierry 
Duboc, Carole DuchĂŞne, LĂŠo Dujardin, C. Dulak, Marcin Dunklemann, Daniel L. 
Dunsch, Lothar Dupont, Nathalie DuprĂŠ, C. Durand, Etienne Dutan, Cosmina 
Duvanel, Guillaume Duwald, Romain Dworkin, Jason P. Dyke, John M. Dyson, R.M. 
Dzubak, Allison L. DĂŠchamps-Olivier, Isabelle DĂśhrer, D. Eakin, Paul A. Ebbinghaus, Stefan G. 
Ecolivet, Claude Edelstein, Norman M. Eder, S. Efimov, A. Egger, LĂŠo 
Eisenstein, O. El Aroussi, Badr El-Ghayoury, Abdelkrim El Helou, Z. El Nahhas, Amal 
Eliseeva, Svetlana V. Ellinger, Yves Elstner, M. Emamzadah, Soheila Embs, Jan P. 
Emery, Daniel Emmons, Erik D. Enachescu, Cristian Engeli, Laurine Ensling, J. 
Eremin, M.V. Eriksson, M.A.L. Erim, F.B. Ernst, Karl-Heinz Ernst, M. 
Ernsting, Nikolaus P. Ertem, Mehmed Z. Escher, Werner Espinosa, Enrique Evangelisti, Stefano 
Evans, William J. Extermann, J. Fabrizioli, Patrizia Fajula, François Fakis, Mihalis 
Falin, M.L. Fallas, Juan C. Farinola, Gianluca M. Farizon, Michel Farizon-Mazuy, Bernadette 
Farrag, Mostafa Fasel, A. Faure, Christèle Favarger, Pierre-Yves Fedin, Vladimir P. 
Fedorov, Vladimir E. Fedoseeva, Marina Fedunov, Roman G. Felt-Baeyens, Olivia Fernandez, Samuel 
Fernandez-Teran, R.J. Ferrante, Francesco Ferrer, Roger Domènech Ferretti, Rachel Ferri, Davide 
Feskov, Serguei V. Fessner, Nico D. Feurer, T. Fichtner, M. Fieber, Wolfgang 
Filinchuk, Yaroslav Filipek, S.M. Filso, Uffe Filter, R. Fin, Andrea 
Finck, Yvan Finke, Richard G. Firouzi, A. Fischer, Peter Fischer, Thomas 
Fita, Piotr FlodĂŠn, Nils J. Floriani, C. FlĂźkiger, Peter Foglia, F. 
Fois, Ettore Fontrodona, Xavier Forrest, William P. Forster, Paul M. Fotiadu, FrĂŠdĂŠric 
Fouqueau, Antony Four, MickaĂŤl FourmiguĂŠ, Marc Fowler, Patrick W. Fradelos, Georgios 
Francotte, Eric Frank, Walter Franz, Patrick François, M. Frath, Denis 
Frauchiger, Simon Frein, StĂŠphane Fretzen, Angelika Frey, U. Frick, Bernhard 
Friedrichs, Heike Fringeli, U.P. Frioud, M. Frisch, B. Fritzsche, Stephan 
Fromm, K.M. Frontera, Antonio FrĂśhlich, Johannes FrĂźhmann, B. Fu, Yongchun 
Fujisawa, Kaori Fureraj, I. Furet, Eric Furlan, Alan Furrer, A. 
FĂźrstenberg, Alexandre Gabay, Cem Gabriel, M. Gadjev, Nikolai I. Gaelli, Brigitte 
Gaenko, A. Gagliardi, Laura Gajewska, A. Galet, Ana Gallis, K.W. 
Galtier, M. Gamelin, Daniel R. Ganesan, K. Gans, Pierre Ganster, Beate 
Garavelli, Marco Garbacz, Piotr Garcia-Lastra, Juan Maria GarcĂ­a, Ana M. GarcĂ­a-Calvo, J. 
Gardet, Georges Gartmann, Nando Garzon, Ignacio L GascĂłn, JosĂŠ A. Gatineau, David 
Gaul, Michael Gaumont, Annie-Claude Gautier, Cyrille Gawelda, Wojciech Geller, Maciej 
Geng, Y. Geoffroy, Michel Gerard, David Gerasimov, K.I. Gerber, A. 
Gerecke, Mario Gescheidt, Georg Gherman, Benjamin F. Ghigo, Giovanni Ghoneim, Nagwa 
Ghosh, Harekrishna Giannini, Enrico Gibson, John K. Gibson Jr., Everett K. Gidron, O. 
Giessner-Prettre, C. Gigante, Angelina Gil Bardaji, E. Gilardoni, François Gillon, B. 
Gingl, F. Giordano, Laurent Giovannini, R. Girard-Reydet, Clement Girerd, Jean-Jacques 
Girolami, Gregory S. Gisler, Andreas Gladiali, Serafino Gladkikh, Vladislav GlĂścklhofer, F. 
Godzik, A. Goesmann, H. Goeta, AndrĂŠs E. Gogick, Kristy Alexa Golesorkhi, Bahman 
Gomes, S. Gomez-Casado, Alberto Gonzalez-Gaitan, Marcos Gonzalez Hevia, Miguel GonzĂĄlez, Leticia 
GonzĂĄlez-Lafont, Angels Gonçalves, M.-H. Goodwin, Harold A. Gorelik, Vitaly Gorin, Georges 
Gorteau, Virginie Gossauer, Albert Gostein, M. Gosztola, David Gouin, JĂŠrĂ´me 
Goujon, A. Gour, Jeffrey R. Goursot, Annick Gouverd, Cyril Govind, C. 
Goze, Christine Gradinaru, Julieta Graf, Stephan Graff, Roland Grampp, GĂźnter 
Gramsch, Stephen A. Grand, E. Grandjean, Fernande Grass, StĂŠphane Grassi, David 
Grayevsky, A. Green, Jennifer C. Gremaud, R. Grenthe, Ingmar Griesser, Markus 
Grigorov, Martin G. Grilj, Jakob Grimaldi, Natascia Gritsan, Nina P. Grivin, Vjacheslav P. 
Grolimund, Daniel Gros, A. Grosshans, Philippe Gruber, Peter Gruenberg, Jean 
Grunwaldt, Jan-Dierk Gryko, Daniel T. Grzybowski, Marek GrĂźtzmacher, HansjĂśrg Guagliardi, A. 
Guberman-Pfeffer, Matthew J. Gudipati, Murthy S. Guedel, Hans U. Gueneau, E.D. Guglieri, Clara 
Guignard, F. Guillon, Daniel Guillon, Thomas Guin, Joyram Gulbinat, P. 
Guldi, Dirk M. Gumy, Jean-Claude Gundacker, S. Gurny, Robert Gusarov, Sergey 
Gutfleisch, Oliver Gutlich, Philipp Gutmann, Michael Guzelturk, B. GuĂŠgano, Xavier 
GuĂŠnĂŠe, Laure Gysler-Sanz, J. Gâcon, J.C. GĂŠrard, David GĂźdel, Hans Ulrich 
GĂźlaçar, Fazil Osman GĂźnther, Detlef GĂźntherĂśdt, H.-J. GĂźtlich, Philipp Haas, Marco 
Haase, DĂśrthe Habiyakare, Augustin Hagberg, Daniel Hagemann, Hans-Rudolf Hagen, Collin M. 
Hagihara, Shinya Hajnal, Z. Halazonetis, Thanos D. Hall, Michael B. Hamacek, Josef 
Hametner, Christian Hammarson, Martin Handy, Nicholas C. Hangarge, Rahul V. Hankache, Jihane 
Hanson, Jonathan C. Hanss, David Hanus, M. Harrison, Andrew Harthong, Steven 
Haselbach, Edwin Hauback, Bjørn C. Haug, K.L. Hauke, Verena Hauser, Andreas 
Hauser, JĂźrg Hauser, Peter C. Hawcock, Anthony B. Hazemann, Jean-Louis He, Bice 
He, Liqing Hearns, Nigel G.R. Heaven, Michael C. Hecht, B. Heeger, Alan J. 
Hegetschweiler, Kaspar Heilemann, M. Heine, Thomas Heiniger, Leo-Philipp Heinz, Luisa G. 
Heinzelmann, Harry Heiz, Ueli Helbing, Jan Heller, Jorge Hellou, Nora 
Helm, L. Hemley, Russell J. Hennig, Andreas Henseler, A. Herbst, E. 
Herrmann, Werner Herse, Christelle Herzog, William Hess, Reto Hillemanns, H. 
Hinek, Roland Hino, Satoshi Hirao, Kimihiko Hitchcock, Peter B. Ho, Gabriel M.-W. 
Hoang, Thi Nhu Y. Hochstrasser, G. Hoegemann, C. Hoffmann, Stefan Hofmann, Annette 
Holliday, Keith Hollingsworth, G.J. Holzer, Brigitte Homberg, Alexandre Hong, Gongyi 
Hong, Sungjun Hopfgartner, GĂŠrard Horkel, Ernst Hostettler, Marc Houot, FrĂŠdĂŠric 
Howard, Judith A.K. Hoyt, Jordan M. Hrdina, Radim HrobĂĄrik, Peter Hrudka, Jeremy J. 
Hrusak, Jan Hu, Kuo-Juei Hu, Xuefeng Huang, H.-H. Hubbell, Jeffrey A. 
Huber, Stefan M. Huck, W.-R. Huet, Tiphaine Hulliger, JĂźrg Humair, Arnaud 
Humbert-Droz, Marie Humeniuk, Heorhii V. Huo, Q. Husinsky, Wolfgang Huskens, Jurriaan 
Hutin, Marie Hwang, Son-Jong Häkkinen, Hannu J. Häner, R. Hävecker, M. 
Igimbayeva, Dilyara Imbert, Daniel Infante, Ivan Ingold, Gerhard Ioannou, Andrew G. 
Ionkin, Vladimir N. Iqbal, Z. Ivanov, Anatoly I. Iveson, Peter Iznaden, Mohammed 
Jaaniso, Raivo Jaber, P. Jacob, Christoph R. Jacquemin, Denis Jacques, Patrice 
Jaggi, Michael Jajcevic, Kristina Jani, Chintan H. Janicke, M. Jansen, M. 
JarolĂ­movĂĄ, Zdenka Jarron, P. Jay, Ashley N. Jeanneau, Erwann Jeannerat, Damien 
Jedrak, Jakub Jefford, Charles W. Jeftic, Jelena Jegondaz, J. Jennings, Guy 
Jenny, Titus A. Jensen, Craig M. Jensen, Torben Rene R. Jepsen, Lars Haahr Jia, Chunyang 
Jia, Hong-Peng Jimenez Estrada, Manuel Johns, Robert W. Johnson, Steven L. Joho, Felix 
Joly, D. Jones, Cameron Jonkheijm, Pascal Jorand, Chantal Jordanov, Jeanne 
Josso, M.C. Jouaiti, Abdelaziz Judge, R.H. Julliard, Marc D. Julve, Miguel 
Jungwirth, Pavel Junod, A. Jupally, Vijay Jurczak, J. Juryiska, R. 
Jäger, Christof M. Jørgensen, Christian K. Kadria-Vili, Yara Kahn, R. Kahnt, Axel 
Kaila, Ville R.I. Kaim, Wolfgang Kairouani, SĂŠlim Kaiser, Maik Kalenius, Elina 
Kalyanaraman, S. Kamalaprija, P. Kaminski, Jakub W. Kamisetty, Vamsi K. Kanibolotsky, Alexander L. 
Kaptein, R. Karakostas, Nikolaos Karlen, Thomas KarlstrĂśm, Gunnar Karmazin-Brelot, Lydia 
Karpfen, Alfred Karvonen, Lassi Katayev, Dmitry Kattnig, Daniel R. Kautny, Paul 
Kavran Belin, Gamze Kazakov, B.N. Kazan, Rania Kaziz, Sameh Kedracki, Dawid 
Kel, Oksana Keller, Lukas Kelson, Melissa Keniley, Lawrence K. Kerbe, W. 
Keresszegi, Csilla Kerscher, Marion Kevorkyants, Ruslan Khajenhouri, Fereidoun Khrouz, Lhoussain 
Kieffer, Isabelle Kijak, M. Kilin, Vasyl Kim, Ki Chul Kinal, Armagan 
Kindler, Ueli King, R.B. Kinsinger, Christopher R. Kiselev, D. Kishore, Ravuri S.K. 
Kittilstved, Kevin R. Kiwi-Minsker, Lioubov Klappert, Rolf Klehs, K. Klein, GĂŠrard 
Klink, Georges Kloeti, Werner Klopper, Wim Knop-Gericke, A. Knoppe, Stefan 
Knowles, Elisabeth S. Koch, Marius Kohlhaas, T. Kolesov, Boris A. Kollman, Peter A. 
Kolodezny, Dmitry Kolodieznyi, Dmytro Komaromi, Istvan Konings, Rudy J.M. Kopylova, Lyudmila V. 
Korchowiec, Jacek Korolev, Valerii V. Kostova, K. Kothalawala, Nuwan Koufaki, M. 
Kovacs, Attila Kovalenko, Andriy Kovalenko, M.V. Kovnir, Kirill Kozina, M. 
Kozintsev, Alexander Kraft, Steven J. Kratzert, Daniel Krause, Andreas M. Krausz, Elmars 
Krebs, Frederik C. Krishnakumar, V. Krishnamurty, M. Krivokapic, Itana Krogh, Jesper Wisborg 
Krzeszewski, Maciej KrähenbĂźhl, Stephan Krämer, Karl Kubel, Franck Kuchukova, N. 
Kulkarni, Ruta Kumar, D. Kumara, Chanaka Kumpulainen, Tatu Kurdyukov, Vladimir V. 
Kurtz, Charles A. Kurylyak, Yuriy Kusz, J. Kuzmenko, A.B. Kuzmin, Michael 
Kwon, Oh-Hoon Kyrychenko, Alexander KĂśster, A.M.