Publications of the Department of Physical Chemistry


Chiral Functionalization of an Atomically Precise Noble Metal Cluster: Insights into the Origin of Chirality and Photoluminescence
Krishnadas, K. R.; Sementa, L.; Medves, M.; Fortunelli, A.; Stener, M.; Fürstenberg, A.; Longhi, G.; Burgi, T.
ACS Nano 2020, in press.

We probe the origin of photoluminescence of an atomically precise noble metal cluster, Ag24Au1(DMBT)18, (DMBT = 2,4-dimethylbenzenethiolate) and the origin of chirality in its chirally functionalized derivatives, Ag24Au1(R/S-BINAS)x(DMBT)18-2x, with x = 1-7 (R/S-BINAS = R/S-1,1’-[binaphthalene]-2,2’-dithiol), using chiroptical spectroscopic measurements and density functional theory (DFT) calculations. Combination of chiroptical and luminescence spectroscopies to understand the nature of electronic transitions has not been applied to such molecule-like metal clusters. In order to impart chirality to the achiral Ag24Au1(DMBT)18 cluster, the chiral ligand, R/S-BINAS, was incorporated into it. A series of clusters, Ag24Au1(R/S-BINAS)x(DMBT)18-2x, with x = 1-7 were synthesized. We demonstrate that the low energy electronic transitions undergo an unexpected achiral to chiral and back to achiral transition from pure Ag24Au1(DMBT)18 to Ag24Au1(R/S-BINAS)x(DMBT)18-2x, by increasing the number of BINAS ligands. The UV/Vis, luminescence, circular dichroism and circularly polarized luminescence spectroscopic measurements, in conjunction with DFT calculations suggest that the photoluminescence in Ag24Au1(DMBT)18 and its chirally functionalized derivatives is originated from the transitions involving the whole Ag24Au1S18 framework, not merely from the icosahedral Ag12Au1 core. These results suggest that the chiroptical signatures and photoluminescence in these cluster systems cannot be solely attributed to any one of the structural components, i.e., the metal core or the protecting metal-ligand oligomeric units, but rather to their interaction, and that the ligand shell plays a crucial role. Our work demonstrates that chiroptical spectroscopic techniques such as circular dichroism and circularly polarized luminescence represent useful tools to understand the nature of electronic transitions in ligand protected metal clusters, and that this approach can be utilized for gaining deeper insights into the structure-property relationships of the electronic transitions of such molecule-like clusters.

Broadband fluorescence reveals mechanistic differences in excited-state proton transfer to protic and aprotic solvents
Verma, P.; Rosspeintner, A.; Dereka, B.; Vauthey, E.; Kumpulainen, T.
Chem. Sci. 2020, in press.

Excited-state proton transfer (ESPT) to solvent is often explained according to the two-step Eigen–Weller model including a contact ion pair (CIP*) as an intermediate, but general applicability of the model has not been thoroughly examined. Furthermore, examples of the spectral identification of CIP* are scarce. Here, we report on a detailed investigation of ESPT to protic (H2O, D2O, MeOH and EtOH) and aprotic (DMSO) solvents utilizing a broadband fluorescence technique with sub-200 fs time resolution. The time-resolved spectra are decomposed into contributions from the protonated and deprotonated species and a clear signature of CIP* is identified in DMSO and MeOH. Interestingly, the CIP* intermediate is not observable in aqueous environment although the dynamics in all solvents are multi-exponential. Global analysis based on the Eigen–Weller model is satisfactory in all solvents, but the marked mechanistic differences between aqueous and organic solvents cast doubt on the physical validity of the rate constants obtained.

Fluorescent Membrane Tension Probes for Super-Resolution Microscopy:  Combining Mechanosensitive Cascade Switching with Dynamic-Covalent Ketone Chemistry
García-Calvo, J.; Maillard, J.; Fureraj, I.; Strakova, K.; Colom, A.; Mercier, V.; Roux, A.; Vauthey, E.; Sakai, N.; Fürstenberg, A.; Matile, S.
J. Am. Chem. Soc. 2020, 142, archive unige:138589 pdf full text [restricted access]

We report the design, synthesis, and evaluation of fluorescent flipper probes for single-molecule super-resolution imaging of membrane tension in living cells. Reversible switching from bright-state ketones to dark-state hydrates, hemiacetals, and hemithioacetals is demonstrated for twisted and planarized mechanophores in solution and membranes. Broadband femtosecond fluorescence up-conversion spectroscopy evinces ultrafast chalcogen-bonding cascade switching in the excited state in solution. According to fluorescence lifetime imaging microscopy, the new flippers image membrane tension in live cells with record red shifts and photostability. Single-molecule localization microscopy with the new tension probes resolves membranes well below the diffraction limit.

Bimolecular photoinduced electron transfer in non-polar solvents beyond the diffusion limit
Nançoz, C.; Rumble, C.; Rosspeintner, A.; Vauthey, E.
J. Chem. Phys. 2020, 152, archive unige:137602 pdf full text [free access]

Electron transfer (ET) quenching dynamics in non-polar solvents are investigated using ultrafast spectroscopy with a series of six fluorophore/quencher pairs, covering a driving force range of more than 1.3 eV. The intrinsic ET rate constants, k0, deduced from the quenching dynamics in the static regime, are of the order of 1012–1013 M−1 s−1, i.e., at least as large as in acetonitrile, and do not exhibit any marked dependence on the driving force. A combination of transient electronic and vibrational absorption spectroscopy measurements reveals that the primary product of static quenching is a strongly coupled exciplex that decays within a few picoseconds. More weakly coupled exciplexes with a longer lifetime are generated subsequently, during the dynamic, diffusion-controlled, stage of the quenching. The results suggest that static ET quenching in non-polar solvents should be viewed as an internal conversion from a locally excited state to a charge-transfer state of a supermolecule rather than as a non-adiabatic ET process.

Propyl acetate/butyronitrile mixture is ideally suited for investigating the effect of dielectric stabilization on (photo)chemical reactions
Verma, P.; Rosspeintner, A.; Kumpulainen, T.
RSC Adv. 2020, 10, 23682-23689.

Characterization of propyl acetate/butyronitrile (PA/BuCN) mixtures by various spectroscopic techniques is described. The neat solvents have identical viscosities and refractive indices but their dielectric constants differ significantly. Detailed solvatochromic and titration data show that the mixtures do not exhibit specific solute–solvent interactions or significant dielectric enrichment effects. Therefore, the mixtures are ideally suited for investigating the effect of dielectric stabilization on (photo)chemical reactions. Dynamic Stokes shift experiments performed on two push–pull probes demonstrate that the solvation dynamics are significantly decelerated in the mixtures as compared to the neat solvents. Therefore, the mixtures allow for varying both the extent and time scale of the dielectric stabilization in a predictable manner.

Synthesis, Characterization, and Crystal Structures of Two New Manganese Aceto EMIM Ionic Compounds with Chains of Mn2+ Ions Coordinated Exclusively by Acetate
Dera, P.; Bruffey, E.; Finkelstein, G. J.; Kelly, C.; Gigante, A.; Hagemann, H.; Severa, G.
ACS Omega 2020, 5, 15592-15600.

We synthesized and determined crystal structures of two manganese(II) aceto EMIM coordination compounds with simplified empirical formulas Mn4(OAc)10[EMIM]2 and Mn4(OAc)10[EMIM]2·2H2O. Both compounds feature extended chains of Mn2+ octahedrally coordinated exclusively by acetate anions, which has been observed for the first time. The EMIM moieties and water molecules participate in hydrogen bonding with acetate anions but do not directly interact with the metal cation. Both compounds have melting temperatures around 120 °C and can be considered as (non-room-temperature) ionic liquids. The structural arrangement represented by the two title compounds is robust in terms of accommodating other types of cations and allows for tuning of physical properties of the ionic liquid by means of cation substitution. Thermal analysis results obtained using TGA–DSC and VT IR suggest melting phase transitions around 120 °C, followed by structural rearrangement in the molten state taking place around 140–160 °C. Compounds I and II have a higher thermal stability range compared to [EMIM][OAc] ionic liquid, with an onset decomposition temperature above 260 °C.

Embedding-theory-based simulations using experimental electron densities for the environment
Ricardi, N.; Ernst, M.; Macchi, P.; Wesolowski, T. A.
Acta Crystallogr., Sect. A: Found. Adv. 2020, in press.

The basic idea of frozen-density embedding theory (FDET) is the constrained minimization of the Hohenberg–Kohn density functional EHK[ρ] performed using the auxiliary functional EFDETνABAB], where ΨA is the embedded NA-electron wavefunction and ρB(r) is a non-negative function in real space integrating to a given number of electrons NB. This choice of independent variables in the total energy EFDETνABAB] functional makes it possible to treat the corresponding two components of the total density using different methods in multi-level simulations. The application of FDET using ρB(r) reconstructed from X-ray diffraction data for a molecular crystal is demonstrated for the first time. For eight hydrogen-bonded clusters involving a chromophore (represented as ΨA) and the glycylglycine molecule [represented as ρB(r)], FDET is used to derive excitation energies. It is shown that experimental densities are suitable for use as ρB(r) in FDET-based simulations.

Chemistry on the electrodes: post-functionalization and stability enhancement of anchored dyes on mesoporous metal oxide photoelectrochemical cells with copper-free Huisgen cycloaddition reaction
Bentounsi, Y.; Seintis, K.; Ameline, D.; Diring, S.; Provost, D.; Blart, E.; Pellegrin, Y.; Cossement, D.; Vauthey, E.; Odobel, F.
J. Mater. Chem. A 2020, 8, 12633-12640.

Hybrid materials consisting of nanocrystalline metal oxide films coated with molecules have considerable implications for the development of optoelectronic devices. We report on a straightforward and versatile procedure to engineer stable layers made of molecules chemisorbed on the surface of mesoporous inorganic metal oxides. The procedure is based on a thermal copper-free Huisgen reaction, which is directly conducted on TiO2 or NiO nanocrystalline films, between already bound diketopyrrolopyrrole sensitizers substituted by two azido groups and a crosslinking agent consisting of a tetrapropiolate ester. The procedure is mild and simple and does not require a catalyst, since quantitative conversion is obtained by a simple heating of the photoelectrode into a solution of the crosslinking agent. The photoelectrodes were characterized by ToF-SIMS, femtosecond transient absorption spectroscopy, electrochemistry and were finally used to fabricate dye-sensitized solar cells with iodide/triiodide and cobalt trisbipyridine complexes as redox mediators. Important increased stability of the crosslinked films was demonstrated by desorption and cyclic voltammetry experiments. Transient absorption spectroscopy and photovoltaic measurements showed that the dyes keep their initial photoelectrochemical properties upon crosslinking. This bottom-up approach is certainly broadly applicable and opens the possibility to make “chemistry on the electrode” to functionalize and crosslink dyes with any component.

Lipid Nanotubes as an Organic Template for an Electrically-Conductive Gold Nanostructure Network
Jajcevic, K.; Sugihara, K.
J. Phys. Chem. B 2020, 124, 5761-5769.

We demonstrate an approach to fabricate a gold nanowire network that presents a macroscopic electrical conductivity based on a lipid nanotube (LNT) template with attached gold nanoparticles. The poor electrical conductivity that we have previously faced was overcome by centrifugation and resuspension of gold nanoparticle solution for removing stabilizing agents, which increased the density of gold nanoparticles on the LNTs. An additional electroless metal plating further enhanced their contacts at nanoscale. Thanks to these procedures the sheet resistance was improved by 11 orders of magnitude. As a proof of principle, transparent conductive films were fabricated with these gold nanowires, which exhibited sheet resistance of maximum 70 Ω/? and transmittance of 50-75% in visible light.

Self-Assembled Arrays of Gold Nanorod-Decorated Dielectric Microspheres with a Magnetic Dipole Response in the Visible Range for Perfect Lensing and Cloaking Applications
Grillo, R.; Beutel, D.; Cataldi, U.; Rockstuhl, C.; Bürgi, T.
ACS Appl. Nano Mater. 2020, 3, 6107-6117.

Photonic nanostructures made of a dielectric sphere covered with many metallic nanospheres fabricated by self-assembly constitute a basic building block for optical metamaterials with a magnetic response in the visible. However, they suffer from limited degrees of freedom to tune their response. Once the involved materials are chosen, the response is mostly determined. To overcome such limitation, we design, fabricate, and characterize here a bottom-up metamaterial where metallic nanorods are used instead of nanospheres. Nanorods offer the ability to tune the spectral position of the resonances by changing their aspect ratio. Building blocks consisting of dielectric spheres covered with metallic nanorods are fabricated and characterized. They are also deposited in densely pack arrays on a substrate using a blade coating deposition of the dielectric spheres first and a subsequent deposition of the metallic nanorods. Full-wave optical simulations support the spectroscopic characterization. These simulations also indicate a dominant magnetic dipolar response of the building blocks. These arranged core-shell structures are promising materials for applications such as perfect lensing and cloaking.

Change of Quadrupole Moment upon Excitation and  Symmetry Breaking in Multibranched Donor-Acceptor Dyes
Szakacs, Z.; Tasior, T.; Gryko, D.; Vauthey, E.
ChemPhysChem 2020, in press.

Upon photoexcitation, a majority of quadrupolar dyes, developed for large two-photon absorption, undergo excited-state symmetry breaking (ES-SB) and behave as dipolar molecules. We investigate how the change of quadrupole moment upon S 1  <- S 0  excitation, Δ Q , influences the propensity of a dye to undergo ES-SB using a series of molecules with a A -π- D -π- A motif where D is the exceptionally electron-rich pyrrolo[3,2-b]pyrrole and A are accepting groups. Tuning of Δ Q is achieved by appending a secondary acceptor group, A’ , on both sides of the D core and ES-SB is monitored using a combination of time-resolved IR and broadband fluorescence spectroscopy. The results reveal a clear correlation between Δ Q and the tendency to undergo ES-SB. When A is a stronger acceptor than A’ , ES-SB occurs already in non-dipolar but quadrupolar solvents. When A and A’ are identical, ES-SB is only partial even in highly dipolar solvents. When A is a weaker acceptor than A’ , the orientation of Δ Q changes, ES-SB is observed in dipolar solvents only and involves major redistribution of the excitation over the D -π- A  and D - A’ branches of the dye.

Frozen-Density Embedding Theory based simulations with experimental electron densities
Ricardi, N.; Ernst, M.; Macchi, P.; Wesolowski, T. A., e-Print Arch., Phys. 2020, arXiv:2005.13409.

The basic idea of Frozen-Density Embedding Theory (FDET) is the constrained minimisation of the Hohenberg-Kohn density functional EHK[ρ] performed using the auxiliary functional EF D ETvABA,ρB], where ΨA is the embedded NA-electron wave-function and ρB(r) a non-negative function in real space integrating to a given number of electrons NB. This choice of independent variables in the total energy functional EF D ETvABA,ρB] makes it possible to treat the corresponding two components of the total density using different methods in multi-level simulations. We demonstrate, for the first time, the applications of FDET using ρB(r) reconstructed from X-ray diffraction data on a molecular crystal. For eight hydrogen-bonded clusters involving a chromophore (represented with ΨA) and the glycylglycine molecule (represented as ρB(r)), FDET is used to derive excitation energies. It is shown that experimental densities are suitable to be used as ρB(r) in FDET based simulations.

Liquid flow and control without solid walls
Dunne#, P.; Adachi#, T.; Dev, A. A.; Sorrenti, A.; Giacchetti, L.; Bonnin, A.; Bourdon, C.; Mangin, P. H.; Coey, J.; Doudin, B.; Hermans, T. M.
Nature 2020, 581, 58-62.

When miniaturizing fluidic circuitry, the solid walls of the fluid channels becomeincreasingly important because they limit the flow rates achievable for a givenpressure drop, and they are prone to fouling. Approaches for reducing the wallinteractions include hydrophobic coatings, liquid-infused porous surfaces, nanoparticle surfactant jamming, changes to surface electronic structure, electrowetting, surface tension pinning and use of atomically flat channels. A better solution may be to avoid the solid walls altogether. Droplet microfluidics andsheath flow achieve this but require continuous flow of the central liquid and thesurrounding liquid. Here we demonstrate an approach in which aqueous liquidchannels are surrounded by an immiscible magnetic liquid, both of which arestabilized by a quadrupolar magnetic field. This creates self-healing, non-clogging,anti-fouling and near-frictionless liquid-in-liquid fluidic channels. Manipulation of thefield provides flow control, such as valving, splitting, merging and pumping. The latteris achieved by moving permanent magnets that have no physical contact with theliquid channel. We show that this magnetostaltic pumping method can be used totransport whole human blood with very little damage due to shear forces. Haemolysis (rupture of blood cells) is reduced by an order of magnitude compared withtraditional peristaltic pumping, in which blood is mechanically squeezed through aplastic tube. Our liquid-in-liquid approach provides new ways to transport delicateliquids, particularly when scaling channels down to the micrometre scale, with noneed for high pressures, and could also be used for microfluidic circuitry.

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Ligand engineering of immobilized nanoclusters on surfaces: ligand exchange reactions with supported Au11(PPh3)7Br3
Truttmann, V.; Herzig, C.; Illes, I.; Limbeck, A.; Pittenauer, E.; Stöger-Pollach, M.; Allmaier, G.; Bürgi, T.; Barrabés, N.; Rupprechter, G.
Nanoscale 2020, 12, 12809-12816.

The properties of gold nanoclusters, apart from being size-dependent, are strongly related to the nature of the protecting ligand. Ligand exchange on Au nanoclusters has been proven to be a powerful tool for tuning their properties, but has so far been limited to dissolved clusters in solution. By supporting the clusters previously functionalized in solution, it is uncertain that the functionality is still accessible once the cluster is on the surface. This may be overcome by introducing the desired functionality by ligand exchange after the cluster deposition on the support material. We herein report the first successful ligand exchange on supported (immobilized) Au11 nanoclusters. Dropcast films of Au11(PPh3)7Br3 on planar oxide surfaces were shown to react with thiol ligands, resulting in clusters with a mixed ligand shell, with both phosphines and thiolates being present. Laser ablation inductively coupled plasma mass spectrometry and infrared spectroscopy confirmed that the exchange just takes place on the cluster dropcast. Contrary to systems in solution, the size of the clusters did not increase during ligand exchange. Different structures/compounds were formed depending on the nature of the incoming ligand. The feasibility to extend ligand engineering to supported nanoclusters is proven and it may allow controlled nanocluster functionalization.

Solvent tuning of photochemistry upon excited-state symmetry breaking
Dereka, B.; Svechkarev, D.; Rosspeintner, A.; Aster, A.; Lunzer, M.; Liska, R.; Mohs, A. M.; Vauthey, E.
Nat. Commun. 2020, 11, archive unige:135135 pdf full text [free access]

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.

Antibacterial and ATP Synthesis Modulating Compounds from Salvia tingitana
Bisio, A.; Schito, A. M.; Pedrelli, F.; Danton, O.; Reinhardt, J. K.; Poli, G.; Tuccinardi, T.; Bürgi, T.; De Riccardis, F.; Giacomini, M.; Calzia, D.; Panfoli, I.; Schito, G. C.; Hamburger, M.; De Tommasi, N.
J. Nat. Prod. 2020, 83, 1027-1042.

A surface extract of the aerial parts of Salvia tingitana afforded a nor-sesterterpenoid (1) and eight new sesterterpenoids (29), along with five known sesterterpenoids, five labdane and one abietane diterpenoid, one sesquiterpenoid, and four flavonoids. The structures of the new compounds were established by 1D and 2D NMR spectroscopy, HRESIMS, and VCD data and Mosher’s esters analysis. The antimicrobial activity of compounds was evaluated against 30 human pathogens including 27 clinical strains and three isolates of marine origin for their possible implications on human health. The methyl ester of salvileucolide (10), salvileucolide-6,23-lactone (11), sclareol (15), and manool (17) were the most active against Gram-positive bacteria. The compounds were also tested for the inhibition of ATP production in purified mammalian rod outer segments. Terpenoids 101115, and 17 inhibited ATP production, while only 17 inhibited also ATP hydrolysis. Molecular modeling studies confirmed the capacity of 17 to interact with mammalian ATP synthase. A significant reduction of ATP production in the presence of 17 was observed in Enterococcus faecalis and E. faecium isolates.

The Enantiomers of Trinorbornane and Derivatives Thereof
Delarue Bizzini, L.; Bürgi, T.; Mayor, M.
Helv. Chim. Acta 2020, 103, e2000019.

Herein, we report the synthesis of the enantiomers of trinorbornane, a tetracyclic saturated hydrocarbon with the chemical formula C 11 H 16 . The preparation of these rigid carbon scaffolds was enabled by the successful chiral separation of its tricyclic precursor, thus allowing the enantiomers to be synthesized via a reductive radical cyclization rection. Assignment of the absolute conformation of the enantiomers was achieved via VCD experiments. Further we report an alternative cyclization procedure providing access to hydroxyl and phenyl sulfone functionalized trinorbornanes.

Visible Light CO2 Reduction to CH4 Using Hierarchical Yolk@shell TiO2-xHx Modified with Plasmonic Au-Pd Nanoparticles
Ziarati, A.; Badiei, A.; Luque, R.; Dadras, M.; Burgi, T.
ACS Sustainable Chem. Eng. 2020, 8, 3689-3696.

Engineering of advanced semiconductor photocatalysts for CO2 conversion to solar fuels is a promising strategy to solve the greenhouse effect and energy crisis. Herein, hierarchical urchin like yolk@shell TiO2-xHx decorated with core/shell Au-Pd plasmonic nanoparticles (HUY@S-TOH/AuPd) have been prepared using a multi-step process and employed as advanced visible light active photocatalyst in CO2 conversion to CH4 with rate of 47 µmol/gcat.h (up to 126 µmol/gcat after 7h). Different engineered sites in this structure for high gas adsorption, powerful visible light activation and intense electron transformation are responsible for the observed high photocatalytic CO2 conversion efficiency. The present smart designing process can produce a considerable cooperation, not only to disclose the architectural engineering to improvement of photo-conversion efficiency, but also as a viable and appropriate photocatalytic process to sustainable energy production.

Probing traps in the persistent phosphor SrAl2O4:Eu2+,Dy3+,B3+ - A wavelength, temperature and sample dependent thermoluminescence investigation
Bierwagen, J.; Delgado, T.; Jiranek, G.; Yoon, S.; Gartmann, N.; Walfort, B.; Pollnau, M.; Hagemann, H.
J. Lumin. 2020, 222, 117113.

Wavelength-dependent thermoluminescence (TL) experiments were performed on SrAl2O4:Eu, SrAl2O4:Eu,B, SrAl2O4:Eu,Dy and SrAl2O4:Eu,Dy,B polycrystalline samples. Excitation at 445 nm allows to selectively excite one of the two different Eu2+ ions substituting for Sr in the crystal, whereas excitation at 375 nm excites both Eu2+ ions. Incorporation of boron generates the deepest traps which contribute to the very long afterglow in this material, while dysprosium increases significantly (by a factor of about 4–8) the total number of traps involved in the afterglow of this persistent phosphor. Increasing the temperature at which the samples are irradiated (loaded) from 173 K to 248 K reveals that many new traps can only be occupied or activated at higher temperatures, leading to a strong increase of the integrated TL intensity, in particular for the Dy-containing samples. Boron does not appear to contribute to these thermally-activated traps significantly responsible for the long afterglow of SrAl2O4:Eu,Dy,B. The results of this study reveal that the diversity of traps leading to the long afterglow is much larger than previously reported in the literature. We propose that boron stabilizes F centers (which absorb in the far UV), while the presence of dysprosium induces an excitation-induced charge-transfer reaction Eu2+ + Dy3+ → Eu3+ + Dy2+. However, the principal traps responsible for the efficient afterglow are temperature-activated and appear to be associated with the green emitting Eu2+ ion on the Sr2 site coupled to a nearby dysprosium ion.

Thiolato Protected Copper Sulfide Cluster with the Tentative Composition Cu74S15(2-PET)45
Baghdasaryan, A.; Besnard, C.; Lawson Daku, L. M.; Delgado, T.; Burgi, T.
Inorg. Chem. 2020, 59, 2200-2208.

Ligand protected copper nanoclusters with precise compositions have attracted considerable attention due to their unique photoluminescent properties. However, the acquisition of structural information, knowledge of the factors affecting the stability, and high quantum yields are prerequisites for assessing their applications in biomedicine as fluorescent contrast agents, biosensors, and probes for cells. Despite all the effort, only finite examples of single crystal structures of CuNCs are reported. Herein, we report the phosphine-free synthesis and structure determination of 2-PET protected CuNCs. The structure analysis established by single crystal X-ray diffraction reveals the formation of binary Cu74S15(2-PET)45 sulfide cluster. A similar phenomenon has been observed for several other chalcogenide-bridged copper clusters. The synthesized cluster possesses a rod-like structure, protected with 45 thiol ligands on the surface. Fifteen independent bridged-sulfur atoms couple to the copper atoms inside the core. Calculations for both a neutral and negatively charged cluster showed no major differences in their geometrical structures. Further analysis of frontier MO levels of the closed-shell anion predicts the HOMO–LUMO transition to be intramolecular L7 → L1 charge transfer, where “L7” and “L1” abbreviations refer to the corresponding sulfur layer in the structure. For the neutral cluster, the calculated spin density is delocalized over the two moieties. On the basis of TDDFT+TB calculations, the onset of the measured absorption spectrum could be satisfactorily reproduced.

Modified Density Functional Dispersion Correction for Inorganic Layered MFX Compounds (M = Ca, Sr, Ba, Pb and X = Cl, Br, I)
Sethio, D.; Martins, J. B. L.; Lawson Daku, L. M.; Hagemann, H.; Kraka, E.
J. Phys. Chem. A 2020, 124, 1619-1633.

MFX (M = Ca, Ba, Sr, Pb and X = Cl, Br, I) compounds have received considerable attention due to their technological application as X-ray detectors, pressure sensors and optical data storage materials, when doped with rare-earth ions. MFX compounds belong to the class of layered materials with a tetragonal Matlockite crystal structure, characterized by weakly stacked double-halide layers along the crystallographic $c$-axis. These layers predominantly determine phase transitions, elastic and mechanical properties. However, the correct description of the lattice parameter $c$ is a challenge for most standard DFT functionals, which tend to overestimate the lattice parameter $c$. Because of the weak interactions between the halide layers dispersion corrected functionals seem to be a better choice. We investigated 11 different inorganic layered MFX compounds for which experimental data is available, with standard and dispersion-corrected functionals to assess their performance in reproducing the lattice parameter $c$, structural and vibrational properties of the MFX compounds. Our results revealed that these functionals do not describe the weak interactions between the halide layers in a balanced way. Therefore, we modified Grimme's popular DFT-D2 dispersion correction scheme in two different ways by (i) replacing the dispersion coefficients and van der Waals radii with those of noble gas atoms, or (ii) increasing the van der Waals radii of the MFX atoms up to 40 percent. Comparison with the available experimental data revealed that the latter approach applied to the PBE (Perdew-Burke-Ernzerhof)-D2 functional with 30 percent increased van der Waals radii, which we coined PBE-D2* (Stextsubscript{rvdw}1.30) is best suited to fine-tune the description of the weak interlayer interactions in MFX compounds, thus significantly improving the description of their structural and vibrational properties. Work is in progress applying this new, computationally inexpensive scheme to other inorganic layered compounds and periodic systems with weakly stacked layers.

Probing the local symmetry of Tb3+ in borohydrides using luminescence spectroscopy
Christmann, J.; Mansouri, A.; Grinderslev, J. B.; Jensen, T. R.; Hagemann, H.
J. Lumin. 2020, 221, 117065.

Rare-earth borohydrides have recently attracted a strong interest as potential hydrogen storage materials. While the local structure of the BH4 ion can be probed using vibrational spectroscopy, the local structure of the rare-earth ion can be studied using luminescence spectroscopy. This is reported here for Tb(BH4)3 and its solvate with S(CH3)2. The spectra clearly show smaller CF splittings for the unsolvated compound. These experimental data are compared with recent DFT+U calculations of the energy levels and CF splittings for Tb(BH4)3 and show that these calculations significantly overestimate the experimental results.

Modeling Infrared Spectral Dynamics upon Symmetry Breaking of a Photo-Excited Quadrupolar Dye
Nazarov, A. E.; Ivanov, A. I.; Vauthey, E.
J. Phys. Chem. C 2020, 124, 2357-2369.

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 undergosymmetry breaking (SB) upon optical excitation. During this process, the electronicexcitation, originally distributed evenly over the molecule, concentrates on one D-π-Abranch, and the molecule becomes dipolar. This process can be monitored by timeresolved infra-red (TRIR) spectroscopy and causes significant spectral dynamics. Atheoretical model of excited-state SB developed earlier ( Ivanov, A. I.J. Phys. Chem.C,2018,122, 29165-29172) is extended to account for the temporal changes takingplace in the IR spectrum upon SB. This model can reproduce the IR spectral dynamicsobserved in the−C≡C−stretching region with a D-π-A-π-D dye in two polar solventsusing a single set of molecular parameters. This approach allows estimating the degreeof asymmetry of the excited state in different solvents as well as its change during SB.Additionally, the relative contribution of the different mechanisms responsible for thesplitting of the symmetric and antisymmetric−C≡C−stretching bands, which areboth IR active upon SB, can be determined.

Monitoring Fe(II) Spin-State Equilibria via Eu(III) Luminescence in Molecular Complexes: Dream or Reality?
Lathion, T.; Fürstenberg, A.; Besnard, C.; Hauser, A.; Bousseksou, A.; Piguet, C.
Inorg. Chem. 2020, 59, archive unige:129611 pdf full text [restricted access]

The modulation of light emission by Fe(II) spin-crossover processes in multifunctional materials has recently attracted major interest for the indirect and noninvasive monitoring of magnetic information storage. In order to approach this goal at the molecular level, three segmental ligand strands, L4L6, were reacted with stoichiometric mixtures of divalent d-block cations (M(II) = Fe(II) or Zn(II)) and trivalent lanthanides (Ln(III) = La(III) or Eu(III)) in acetonitrile to give C3-symmetrical dinuclear triple-stranded helical [LnM(Lk)3]5+ cations, which can be crystallized with noncoordinating counter-anions. The divalent metal M(II) is six-coordinate in the pseudo-octahedral sites produced by the facial wrapping of the three didentate binding units, the ligand field of which induces variable Fe(II) spin-state properties in [LnFe(L4)3]5+ (strictly high-spin), [LnFe(L5)3]5+ (spin-crossover (SCO) around room temperature), and [LnFe(L6)3]5+ (SCO at very low temperature). The introduction of the photophysically active Eu(III) probe in [EuFe(Lk)3]5+ results in europium-centered luminescence modulated by variable intramolecular Eu(III) → Fe(II) energy-transfer processes. The kinetic analysis implies Eu(III) → Fe(II) quenching efficiencies close to 100% for the low-spin configuration and greater than 95% for the high-spin state. Consequently, the sensitivity of indirect luminescence detection of Fe(II) spin crossover is limited by the resulting weak Eu(III)-centered emission intensities, but the dependence of the luminescence on the temperature unambiguously demonstrates the potential of indirect lanthanide-based spin-state monitoring at the molecular scale.

Supporting Information (pdf, 1.5 MB)

Mechanosensitive Membrane Probes:  Push-Pull Papillons
Humeniuk, H. V.; Licari, G.; Vauthey, E.; Sakai, N.; Matile, S.
Supramol. Chem. 2020, 33, archive unige:131618 pdf full text [free access]

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%).

Supplemental material (pdf)

Direct Access to Chiral Secondary Amides by Copper-Catalyzed Borylative Carboxamidation of Vinylarenes with Isocyanates
Fiorito, D.; Liu, Y.; Besnard, C.; Mazet, C.
J. Am. Chem. Soc. 2020, 142, archive unige:128621 pdf full text [restricted access]

A Cu-catalyzed borylative carboxamidation reaction has been developed using vinylarenes and isocyanates. Alkynes, branched 1,3-dienes and bicyclic alkenes were also found to be competent coupling partners. Using a chiral phosphan-amine ligand, an enantioselective variant of this transformation was developed, affording a set of α-chiral amides with unprecedented levels of enantioselectivity. The synthetic utility of the method was demonstrated through a series of representative stereoretentive post-catalytic derivatizations.

Theory of fluorescence spectrum dynamics and its application to determining the relaxation characteristics of the solvent and intramolecular vibrations
Fedunov, R. G.; Yermolenko, I. P.; Nazarov, A. E.; Ivanov, A. I.; Rosspeintner, A.; Angulo, G.
J. Mol. Liq. 2020, 298, 112016.

A general analytical expression for the transient fluorescence spectrum is derived. The formation of a wave packet in the excited state of a fluorophore is described, assuming that the pump pulse has a Gaussian time-profile. The expression explicitly connects the relaxation characteristics of the medium with the spectral dynamics of a fluorophore. Fitting the expression to experimental spectral dynamics allows obtaining the solvent relaxation function. So far this approach was applicable for the analysis of experimental data when the pump pulse does not populate excited sublevels of intramolecular high-frequency vibrational modes. Here, the approach is generalized to include vibrational relaxation in the excited electronic state. In this case, fitting to the experimental spectral dynamics provides reliable information not only on the solvent relaxation, but also on the relaxation time constants of intramolecular high-frequency vibrational modes. This approach is applied to the excited state dynamics of coumarin 153 in multiple solvents, obtained from broadband fluorescence upconversion spectroscopy.

Untying the Photophysics of Quinolinium-Based Molecular Knots and Links
Caprice, K.; Aster, A.; Cougnon, F. B. L.; Kumpulainen, T.
Chem. Eur. J. 2020, 26, archive unige:129606 pdf full text [restricted access]

Today, complex molecular knots and links are still difficult to synthesize and the properties arising from their topology are mostly unknown. Here, we report on a comparative photophysical study carried out on a family of closely related quinolinium-based knots and links to determine the impact exerted by topology on the molecular backbone. Our results indicate that topology has a negligible influence on the behavior of loosely braided molecules, which mostly behave like their unbraided equivalents. On the other hand, tightly braided molecules display distinct features. Their higher packing density results in a pronounced ability to resist deformation, a significant reduction in the solvent-accessible surface area and favors close-range π-π interactions between the quinolinium-units and neighboring aromatics. Finally, the sharp alteration in behavior between loosely and tightly braided molecules sheds light upon the factors contributing to braiding tightness.

Supporting Information (pdf / 8 MB)