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%.

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.

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.

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.

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.

The luminescence of Sm²⁺-doped BaFBr has been measured as a function of temperature and pressure. The ⁷F_J 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 ⁵D_2,1,0 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 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.

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.

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.

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.

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.

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.

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 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.

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.

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.

A thorough characterization of the Ru−Hbpp (in,in-{[Ru^II(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 (Ce^IV) 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.

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.

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.

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.

The ground-state electronic structures of K₃V(ox)₃·3H₂O, Na₃V(ox)₃·5H₂O, and NaMgAl_1–xV_x(ox)₃·9H₂O (0 < x <= 1, ox = C₂O₄^2–) have been studied by Fourier–transform electronic absorption and inelastic neutron scattering spectroscopies. High-resolution absorption spectra of the ³Γ(t_2g²) → ¹Γ(t_2g²) 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 NaMgAl_1–xV_x(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.

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.

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.

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.

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.

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.

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.

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 αααα or αβαβ 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 αααα 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 d_x²-y²-type antibondingorbital level, to the restraints imposed by the straps, and to the presence of the axial chloride ligand.

The synthesis and structural characterization of the λ⁵-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.

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.

Deviations from statistical binding, that is cooperativity, in self-assembled polynuclear complexes partly result from intermetallic interactions ΔE^M,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 ΔE^M,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 (ΔE^Lu,Lu_{1-4 >} ΔE^Lu,Lu_1-2), ii) of larger magnitude when Zn²⁺ replaces Lu³⁺ (ΔE^Zn,Lu_1-2 > ΔE^Lu,Lu_1-2) and iii) attractive between two triply charged cations held at some specific distance (ΔE^Lu,Lu_1-3 < 0). The consequences of these trends are discussed for the design of polynuclear complexes in solution.

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.

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.

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 ³Ã‚₂ 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 ų 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 - ρ_B  ≥ 0, where ρ_B 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 - ρ_B 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 - ρ_B  ≥ 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 - ρ_B 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 v_emb[ρ_A,ρ_B]is evaluated for A taken from the previous iteration in theouter loop (j-index) and remains constant, whereas thev_KS[ρ_A] component is recalculated as A changes. (b) The conventional SCF scheme: Both v_KS[ρ_A] andv_emb[ρ_A,ρ_B] are recalculated as A changes.

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 (λ_ex = 350 nm, τ_resp = 300 ps) and fluorescence up-conversion (λ_ex = 266 nm, τ_resp = 300 fs) techniques. The acid-base equilibrium constants in the ground (pK_a) and the excited states (pK_a*), 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.

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).

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.

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.

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.

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.

Electronic absorption spectrum of 1 in DMF solution at room temperature, together with the calculated oscillator strengths.

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.

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 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.

Theoretically speaking: The mechanistic details associated with the generation and reaction of [CuO]⁺ species from Cu^I-α-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.

The results of a combined spectroscopic and computational study of lanthanide hydrides with the general formula MH_x(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 MH_x species formed in the ablation/deposition process and then analyzed the dihydrogen supercomplexes, MH_x(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 CeO_n and CeO_n⁺ (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(X³Σ_g^-) and Ce + O₂( a¹Δ_g), 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 (a¹Δ_g), a chemielectron band with higher kinetic energy than that recorded for Ce + O₂( X³Σ_g^-) is obtained. This is attributed to production of O( ¹D) from the reaction Ce + O₂( a¹Δ_g) → 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.

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.

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 ΔE_HL^el 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(ΔE_HL^el) = 2500 cm⁻¹. Using for [Fe(bpy)₃]²⁺ the CASPT2 value of ΔE_HL^el = 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 ΔE_HL^el = 6200 cm⁻¹.

The spin-transition (¹A₁↔⁵T₂) behaviour of a new mononuclear iron(II) compound [Fe^II(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.