Publications of the Department of Physical Chemistry


Metal nanoclusters as versatile building blocks for hierarchical structures
Bürgi, T.; Banach, E.
Helv. Chim. Acta 2021, in press.

Atomically precise monolayer-protected metal clusters (nanoclusters) are currently in the focus of interest due to their strongly size dependent properties, which gives rise to different applications, and the possibility to study the evolution of structure as a function of size. The use of truly monodisperse metal clusters as building blocks for the preparation hierarchical structures offers exciting possibilities for the design of new materials. In this overview article we describe the current state of the art in the field of assembly of nanoclusters in one, two and three dimensions. The nanoclusters show rich chemistry that can be used for their assembly. Several reports indicate that within the assemblies of clusters their photoluminescence is enhanced. Although there is already a considerable body of reports on the assembly of nanoclusters we believe that much more exciting properties of nanocluster assemblies will be discovered in the future.

Fe4(OAc)10[EMIM]2: Novel Iron-Based Acetate EMIM Ionic Compound
Severa, G.; Bruffey, E.; Nguyen, P. Q. H.; Gigante, A.; Leick, N.; Kelly, C.; Finkelstein, G. J.; Hagemann, H.; Gennett, T.; Rocheleau, R. E.; Dera, P.
ACS Omega 2021, 290, 663.

We synthesized and characterized a novel iron(II) aceto EMIM coordination compound, which has a simplified empirical formula Fe4(OAc)10[EMIM]2, in two different hydration forms: as anhydrous monoclinic compound and triclinic dihydrate Fe4(OAc)10[EMIM]2·2H2O. The dihydrate compound is isostructural with recently reported Mn4(OAc)10[EMIM]2·2H2O, while the anhydrate is a superstructure of the Mn counterpart, suggesting the existence of solid solutions. Both new Fe compounds contain chains of Fe2+ octahedrally coordinated exclusively by acetate groups. The EMIM moieties do not interact directly with the Fe2+ and contribute to the structural framework of the compound through van der Waals forces and C–H···O hydrogen bonds with the acetate anions. The compounds have a melting temperature of ∼94 °C; therefore, they can be considered metal-containing ionic liquids. Differential thermal analysis indicates three endothermic transitions associated with melting, structural rearrangement in the molten state at about 157 °C, and finally, thermal decomposition of the Fe4(OAc)10[EMIM]2. Thermogravimetric analyses indicate an ∼72 wt % mass loss during the decomposition at 280–325 °C. The Fe4(OAc)10[EMIM]2 compounds have higher thermal stability than their Mn counterparts and [EMIM][OAc] but lower compared to iron(II) acetate. Temperature-programmed desorption coupled with mass spectrometry shows that the decomposition pathway of the Fe4(OAc)10[EMIM]2 involves four distinct regimes with peak temperatures at 88, 200, 267, and 345 °C. The main species observed in the decomposition of the compound are CH3, H2O, N2, CO, OC–CH3, OH–CO, H3C–CO–CH3, and H3C–O–CO–CH3. Variable-temperature infrared vibrational spectroscopy indicates that the phase transition at 160–180 °C is associated with a reorientation of the acetate ions, which may lead to a lower interaction with the [EMIM]+ before the decomposition of the Fe4(OAc)10[EMIM]2 upon further heating. The Fe4(OAc)10[EMIM]2 compounds are porous, plausibly capable of accommodating other types of molecules.

Thermal and Electrochemical Interface Compatibility of a Hydroborate Solid Electrolyte with 3 V-Class Cathodes for All-Solid-State Sodium Batteries
Asakura, R.; Duchêne, L.; Payandeh, S.; Rentsch, D.; Hagemann, H.; Battaglia, C.; Remhof, A.
ACS Appl. Mater. Interfaces 2021, 13, 55319-55328.

Thermal stability of solid electrolytes and their compatibility with battery electrodes are key factors to ensure stable cycling and high operational safety of all-solid-state batteries. Here, we study the compatibility of a hydroborate solid electrolyte Na4(B12H12)(B10H10) with 3 V-class cathode active materials: NaCrO2, NaMnO2, and NaFeO2. Among these layered sodium transition metal oxide cathodes, NaCrO2 shows the highest thermal compatibility in contact with the hydroborate solid electrolyte up to 525 °C in the discharged state. Furthermore, the electrolyte remains intact upon the internal thermal decomposition of the charged, that is, desodiated, cathode (Na0.5CrO2) above 250 °C, demonstrating the potential for highly safe hydroborate-based all-solid-state batteries with a wide operating temperature range. The experimentally determined onset temperatures of thermal decomposition of Na4(B12H12)(B10H10) in contact with 3 V-class cathodes surpass those of sulfide and selenide solid electrolytes, exceeding previous thermodynamic calculations. Our results also highlight the need to identify relevant decomposition pathways of hydroborates to enable more valid theoretical predictions.

Quantifying Fluctuations of Average Environments for Embedding Calculations
Gonzalez, C.; Rumble, C. A.; Borgis, D.; Wesolowski, T. A.
ChemRxiv 2021, in press.

In the context of employing embedding methods to study spectroscopic properties, the viability and effectiveness of replacing an ensemble of calculations by a single calculation using an average description of the system of study are evaluated. This work aims to provide a baseline of the expected fluctuations in the average description of the system obtained in the two cases: from calculations of an ensemble of geometries, and from an average environment constructed with the same ensemble. To this end, the classical molecular dynamics simulation of a very simple system was used: a rigid molecule of acetone in a solution of rigid water. We perform a careful numerical analysis of the fluctuations of the electrostatic potential felt by the solute, as well as the fluctuations in the effect on its electronic density, measure through the dipole moment and the atomic charges derived from the corresponding potential. At the same time, we inspect the accuracy of the methods used to construct average environments. Finally, the proposed approach to generate the embedding potential from an average environment density is applied to estimate the solvatochromic shift of the first excitation of acetone. In order to account for quantum-confinement effects that may be important in certain cases, the fluctuations on the shift due to the interaction with the solvent are evaluated using Frozen-Density Embedding Theory. Our results demonstrate that, for normally-behaved environments, the constructed average environment is a reasonably good representation of a discrete solvent environment.

Embedding non-rigid solutes in an averaged environment: a case study on rhodopsins
Ricardi, N.; González-Espinoza, C. E.; Adam, S.; Church, J.; Schapiro, I.; Wesolowski, T. A.
ChemRxiv 2021, in press.

Many simulation methods concerning solvated molecules are based on the assumption that the solvated species and the solvent can be characterized by some representative structure of the solute and some embedding potential corresponding to this structure. This assumption is re-examined and generalized for conformationally flexible solutes. In the proposed and investigated generalization, the solute is characterized by a set of representative structures and the corresponding embedding potentials. The representative structures are identified by means of subdividing the statistical ensemble, which in this work is generated by a constant-temperature molecular dynamics simulation. The embedding potential defined in Frozen-Density Embedding Theory is used to characterize the average effect of the solvent in each subensemble. The numerical examples concern vertical excitation energies of protonated retinal Schiff bases in protein environments. It is comprehensively shown that subensemble averaging leads to huge computational savings compared to explicit averaging of the excitation energies in the whole ensemble while introducing only minor errors.

Reconstruction of Ice Surfaces upon Acetone Adsorption: An In Situ ATR-IR Modulation Excitation Spectroscopy Study
Wang, X.; Bürgi, T.
J. Phys. Chem. C 2021, 125, 25284-25289.

Ice surfaces are widely present in our environment and participate in many important chemical reactions. We study the adsorption of acetone, a typical polar small organic molecule, on ice by combining attenuated total reflection IR concentration modulation spectroscopy and phase-sensitive detection. Spectroscopic studies revealed that acetone adsorption on ice is significantly affected by temperature and acetone showed a strong interaction with an ice surface at a low temperature (e.g., −50 °C) where desorption is slow. At a higher temperature (e.g., −10 °C), both adsorption and desorption happened rapidly. A two-step process was observed during the adsorption and desorption on an ice surface. More importantly, the spectra provide clear evidence that the acetone adsorption results in the reconstruction of the ice surface, resulting in a liquid-like film.

Analysis of PDA Dose Curves for the Extraction of Antimicrobial Peptide Properties
Zhao, J.; Sugihara, K.
J. Phys. Chem. B 2021, 125, 12206-12213.

A mechanochromic polymer, polydiacetylene, changes color upon ligand binding, being a popular material in biosensing. However, whether it can also detect ligand functions in addition to binding is left understudied. In this work, we report that the polydiacetylene can be used to determine the net charges and the mode of actions (carpet model, toroidal pore model, etc.) of antimicrobial peptides and detergents via EC50 and Hill coefficients from the colorimetric response–dose curves. This opens a potential for high-throughput peptide screening by functions, which is difficult with the conventional methods.

Excited-state Dynamics of Radical Ions in Liquids
Grilj, J.; Beckwith, J. S.; Vauthey, E.
CHIMIA 2021, 75, 856-861.

Thomas Bally has acquired international recognition for his work on the photochemistry of reactive intermediates, which include radical ions. Here, we present a brief overview of our investigations of the excited-state dynamics of radical ions in liquids at room temperature, which are still poorly documented. A better understanding of these dynamics is most relevant, as open-shell ions in the excited state are being increasingly used in redox photochemistry and have been proposed to play a key role in highly exergonic photoinduced electron transfer reactions.

Dye-Sensitized Photoelectrosynthesis Cells for Benzyl Alcohol Oxidation Using a Zinc Porphyrin Sensitizer and TEMPO Catalyst
Nikoloudakis, E.; Pati, P. B.; Charalambidis, G.; Budkina, D. S.; Diring, S.; Planchat, A.; Jacquemin, D.; Vauthey, E.; Coutsolelos, A. G.; Odobel, F.
ACS Catal. 2021, 11, archive unige:154975 pdf full text [restricted access]

Exploring a catalytic reaction other than water oxidation at the photoanode of a photoelectrochemical cell is probably a key feature to more efficiently generate the electrons needed to produce solar fuels. In this framework, we describe herein the fabrication of a TiO2-based dye-sensitized photo-electrosynthesis cell (DSPEC) using a zinc porphyrin (ZnP) sensitizer and a 2,2,6,6-tetramethyl-1-piperidine N-oxyl (TEMPO) organo-catalyst that quite efficiently catalyzes light-driven oxidation of methoxybenzyl alcohol into aldehyde. Two dyads ZnP–TEMPO, differing by the anchoring group (carboxylic acid and hydroxamic acid) on ZnP, were prepared and their electrochemical, absorption, and emission properties were recorded and quantum chemical modeling was realized. The photovoltaic performances in dye-sensitized solar cells were first examined in order to optimize the dyeing conditions and compare the relative efficiencies of the compounds. The dyads substituted with TEMPO outperform the reference zinc porphyrin lacking TEMPO with a much higher Jsc and Voc. The photocatalytic properties after immobilization on TiO2 nanocrystalline films toward para-methoxy benzyl alcohol oxidation were explored in borate buffer and in an acetonitrile electrolyte. In borate buffer, the optimal pH was 8 and using dyad ZnP–TEMPO anchored with hydroxamic acid, para-methoxy benzaldehyde was selectively produced with an average photocurrent density of 200 μA/cm2, a Faradaic efficiency of 82%, a turnover number (TON) of 26, and a turnover frequency (TOF) of 47 h–1. In acetonitrile, in the presence of 0.1 M N-methyl-imidazole, the same dyad gives an average photocurrent density of about 100 μA/cm2, a Faradaic efficiency of 76%, a TON of 13, and a TOF of 24 h–1. The stability of the anchor is crucial in the acetonitrile electrolyte, where the dyad is quite soluble since only the dyad functionalized with hydroxamic acid is compatible with these organic solvent conditions. Overall, this study paves the way to the development of more efficient and probably more stable TiO2-based DSPECs for alcohol oxidation that could advantageously complement those devoted to water oxidation.

Molecular Dynamics Simulations of Bimolecular Electron Transfer: the Distance-Dependent Electronic Coupling
Rumble, C. A.; Vauthey, E.
J. Phys. Chem. B 2021, 125, archive unige:155082 pdf full text [restricted access]

Understanding the distance dependence of the parameters underpinning Marcus theory is imperative when interpreting the results of experiments on electron transfer (ET). Unfortunately, most of these parameters are difficult or impossible to access directly with experiments, necessitating the use of computer simulations to model them. In this work, we use molecular dynamics simulations in conjunction with constrained density functional theory calculations to study the distance dependence of the electronic coupling matrix element, |HRP|, for bimolecular ET. Contrary to what is typically assumed for such intermolecular reactions, we find that the magnitude of |HRP| does not decay exponentially with the center-of-mass separation of the reactants, rCOM. The addition of other simple measures of donor/acceptor (D/A) orientation did not improve the correlation of |HRP| with rCOM. Using the minimum distance separation, rmin, of the reactants as the structural descriptor allowed the system to be partitioned into high-coupling/close-contact and low-coupling/non-contact regimes, but large fluctuations of |HRP| were still found for the close-contact reactant pairs. Despite the persistent large fluctuations of |HRP|, its mean value was found to decay piecewise exponentially with increasing rmin, which was attributed to significant changes in the average D/A pair structure. The results herein advise one to use caution when interpreting the experimental results derived from spherical reactant models of bimolecular ET.

Predictive optical photoabsorption of Ag24 Au(DMBT)18 via efficient TDDFT simulations
Medves, M.; Sementa, L.; Toffoli, D.; Fronzoni, G.; Krishnadas, K. R.; Bürgi, T.; Bonacchi, S.; Dainese, T.; Maran, F.; Fortunelli, A.; Stener, M.
J. Chem. Phys. 2021, 155, 084103.

We report a computational study via time-dependent density-functional theory (TDDFT) methods of the photo-absorption spectrum of an atomically precise monolayer-protected cluster (MPC), the Ag24Au(DMBT)18 single negative anion, where DMBT is the 2,4-dimethylbenzenethiolate ligand. The use of efficient simulation algorithms, i.e., the complex polarizability polTDDFT approach and the hybrid-diagonal approximation, allows us to employ a variety of exchange-correlation (xc-) functionals at an affordable computational cost. We are thus able to show, first, how the optical response of this prototypical compound, especially but not exclusively in the absorption threshold (low-energy) region, is sensitive to (1) the choice of the xc-functionals employed in the Kohn–Sham equations and the TDDFT kernel and (2) the choice of the MPC geometry. By comparing simulated spectra with precise experimental photoabsorption data obtained from room temperature down to low temperatures, we then demonstrate how a hybrid xc-functional in both the Kohn–Sham equations and the diagonal TDDFT kernel at the crystallographically determined experimental geometry is able to provide a consistent agreement between simulated and measured spectra across the entire optical region. Single-particle decomposition analysis tools finally allow us to understand the physical reason for the failure of non-hybrid approaches.

Raman Spectroscopic Fingerprints of Atomically Precise Ligand Protected Noble Metal Clusters: Au38(PET)24 and Au38−xAgx(PET)24
Krishnadas, K. R.; Baghdasaryan, A.; Kazan, R.; Banach, E.; Teyssier, J.; Nicu, V. P.; Buergi, T.
Small 2021, 17, 2101855.

Distinct Raman spectroscopic signatures of the metal core of atomically precise, ligand-protected noble metal nanoclusters are reported using Au38(PET)24 and Au38−xAgx(PET)24 (PET = 2-phenylethanethiolate, -SC2H4C6H5) as model systems. The fingerprint Raman features (occurring −1) of these clusters arise due to the vibrations involving metal atoms of their Au23 or Au23−xAgx cores. A distinct core breathing vibrational mode of the Au23 core has been observed at 90 cm−1. Whereas the breathing mode shifts to higher frequencies with increasing Ag content of the cluster, the vibrational signatures due to the outer metal-ligand staple motifs (between 200 and 500 cm−1) do not shift significantly. DFT calculations furthermore reveal weak Raman bands at higher frequencies compared to the breathing mode, which are associated mostly with the rattling of two central gold atoms of the bi-icosahedral Au23 core. These vibrations are also observed in the experimental spectrum. The study indicates that low-frequency Raman spectra are a characteristic fingerprint of atomically precise clusters, just as electronic absorption spectroscopy, in contrast to the spectrum associated with the ligand shell, which is observed at higher frequencies.

Intramolecular Charge-Transfer Dynamics in Benzodifuran-Based Triads
Keller, S.; Hankache, J.; Yushchenko, O.; Lawson Daku, L. M.; Sun, Q.; Ding, J.; Decurtins, S.; Vauthey, E.; Häner, R.; Hauser, A.; Liu, S.-X.
Helv. Chim. Acta 2021, 104, archive unige:154011 pdf full text [free access]

A facile and efficient approach for the synthesis of new conjugated donor-π-acceptor (D-π-A) chromophores has been developed, in which benzodifuran (BDF) and/or triphenyl amine (TPA) units are the donor moieties, linked by ethylenic bridges to electron-deficient anthraquinone (AQ) and 11,11,12,12-tetracyano-9,10-anthraquinodimethane (TCAQ) as the acceptor moieties. The resultant triads either with a symmetric A–D–A or an asymmetric D′–D–A structure show intense absorption bands in the visible spectral region due to efficient intramolecular charge transfer (ICT) from the HOMO localized on the BDF core to the LUMO localized on the AQ or the TCAQ unit. Electronic interactions between these redox-active components were studied by a combination of cyclic voltammetry, spectroelectrochemistry, UV-visible and ultrafast transient absorption spectroscopy. Analysis of the femtosecond excited-state dynamics reveal that all triads undergo a rapid charge recombination process which occurs within a few picoseconds, indicating that ethylenic linkers can facilitate electron delocalization among BDF and AQ/TCAQ units and thus impart effective electronic interactions between them.

Structural and dynamic studies of Pr(11BH4)3
Gigante, A.; Payandeh, S.; Grinderslev, J. B.; Heere, M.; Embs, J. P.; Jensen, T. R.; Burankova, T.; Remhof, A.; Hagemann, H.
Int. J. Hydrogen Energy 2021, 46, 32126-32134.

Rare earth borohydrides RE (BH4)3 are studied in the context of energy storage, luminescence and magnetic applications. We have investigated the structural behavior of praseodymium borohydride Pr (11BH4)3 containing 11B isotope because of the previously reported negative thermal expansion. Differential scanning calorimetry (DSC), in-situ variable temperature synchrotron radiation powder X-ray diffraction (SR-PXD) and infrared studies reveal that Pr (11BH4)3 undergoes to a volume contraction during the phase transition from alpha α-Pr (11BH4)3 to rhombohedral r-Pr (11BH4)3 phase upon heating to 493 K. Surprisingly, the phase transition persists upon cooling at room temperature. Vibrational analysis also shows that the stretching frequency of BH4 anion does not change upon heating which indicates that the B–H bond length remains constant during the structural phase transition from α-Pr (11BH4)3 to r-Pr (11BH4)3 phase. Additionally, the energy barrier of reorientation motion of the BH4 anion in the α-phase was estimated to be ca 23 kJ/mol by quasi-elastic neutron scattering (QENS) and Raman spectroscopy.

Observation of multiple sites for trivalent europium ions in SrAl2O4
Bierwagen, J.; Delgado, T.; Afshani, J.; Yoon, S.; Gartmann, N.; Walfort, B.; Hagemann, H.
J. Lumin. 2021, 239, archive unige:154223 pdf full text [free access]

We have measured room temperature and low temperature luminescence spectra of Eu3+ in SrAl2O4:Eu3+/2+. The powder samples were synthesized in reducing atmosphere above 1200 °C, cooled down to room temperature in the same atmosphere, and heated again to 750 °C in air. The analysis of the emission spectra of Eu3+ showed the presence of multiple sites for this ion, revealing that many different charge compensating defects are located in close vicinity to the europium ions. These defects can be positioned in each of the two void sites identified in the crystal structure of SrAl2O4. Lifetime measurements reveal the presence of very short-lived excited Eu3+ species with lifetimes around 0.7 μs, in addition to species with lifetimes around 1.6 ms. These very short lifetimes are postulated to be related to intervalence charge transfer with few remaining divalent Eu2+ ions which had not been oxidized by the thermal treatment in air. This type of intervalence charge transfer between divalent to the trivalent rare earth ion could be crucially associated to processes involved in the afterglow of the persistent phosphor SrAl2O4:Eu2+,Dy3+.

Excited-state symmetry breaking in 9,10-dicyanoanthracene-based quadrupolar molecules: the effect of donor–acceptor branch length
Szakács, Z.; Glöcklhofer, F.; Plasser, F.; Vauthey, E.
Phys. Chem. Chem. Phys. 2021, 23, archive unige:153776 pdf full text [free access]

Excited-state symmetry breaking is investigated in a series of symmetric 9,10-dicyanoanthracenes linked to electron-donating groups on the 2 and 6 positions via different spacers, allowing for a tuning of the length of the donor–acceptor branches. The excited-state properties of these compounds are compared with their dipolar single-branch analogues. The changes in electronic structure upon their optical excitation are monitored by transient electronic spectroscopy in the visible and near-infrared regions as well as by transient vibrational spectroscopy in the mid-infrared. Our results reveal that, with the shortest branches, electronic excitation remains distributed almost symmetrically over the molecule even in polar environments. Upon increasing the donor–acceptor distance, excitation becomes unevenly distributed and, with the longest one, it fully localises on one branch in polar solvents. The influence of the branch length on the propensity of quadrupolar dyes to undergo excited-state symmetry breaking is rationalised in terms of the balance between interbranch coupling and solvation energy.

Benchmark of the Extension of Frozen-Density Embedding Theory to Nonvariational Correlated Methods: The Embedded-MP2 Case
Sen, R.; González-Espinoza, C. E.; Zech, A.; Dreuw, A.; Wesolowski, T. A.
J. Chem. Theory Comput. 2021, 17, 4049-4062.

The extension of the frozen-density embedding theory for nonvariational methods [J. Chem. Theory Comput.2020,16, 6880] was utilized to evaluate intermolecular interaction energies for complexes in the Zhao–Truhlar basis set. In the applied method (FDET-MP2-FAT-LDA), the same auxiliary system is used to evaluate the correlation energy by means of the second-order Møller–Plesset perturbation theory (MP2), as in our previous work [J. Chem. Phys.2019,150, 121101]. Local density approximation is used for ExcTnadAB] in both cases. Additionally, the contribution to the energy due to the neglected correlation potential was evaluated and analyzed. The domain of applicability of the local density approximation for ExcTnadAB] was determined based on deviations from the interaction energies from the conventional MP2 calculations. The local density approximation for ExcTnadAB] performs well for hydrogen- or dipole-bound complexes. The relative errors in the interaction energy lie within 3–30%. While for charge-transfer complexes, this approximation fails consistently, and for other types of complexes, the performance of this approximation is not systematic. The sources of error are discussed in detail.

Absolute configuration retention of a configurationally labile ligand during dynamic processes of thiolate protected gold clusters
Wang, Y.; Makkonen, E.; Chen, X.; Bürgi, T.
Chem. Sci. 2021, 12, 9413-9419.

Monolayer protected metal clusters are dynamic nanoscale objects. For example, the chiral Au38(2-PET)24 cluster (2-PET: 2-phenylethylthiolate) racemizes at moderate temperature. In addition, ligands and metal atoms can easily exchange between clusters. Such processes are important for applications of monolayer protected metal clusters; however, the mechanistic study of such processes turns out to be challenging. Here we use a configurationally labile, axially chiral ligand, biphenyl-2,2′-dithiol (R/S-BiDi), as a probe to study dynamic cluster processes. It is shown that the ligand exchange of free R/S-BiDi on a chiral Au38(2-PET)24 cluster is diastereospecific. Using chiral chromatography, isolated single diastereomers of the type anticlockwise/clockwise-Au38(2-PET)22(R/S-BiDi)1 could be isolated. Upon heating, the cluster framework racemizes, while the R/S-BiDi ligand does not. These findings demonstrate that during cluster racemization and/or ligand exchange between clusters, the R/S-BiDi ligand is sufficiently confined, thus preventing its racemization, and exclude the possibility that the ligand desorbs from the cluster surface.

Molecular engineering for optical properties of 5-substituted-1,10-phenanthroline-based Ru( ii ) complexes
Rousset, E.; Mongin, O.; Moreau, J.; Lawson-Daku, L. M.; Beley, M.; Gros, P. C.; Chevreux, S.; Blanchard-Desce, M.; Lemercier, G.
Dalton Trans. 2021, 50, 10119-10132.

A series of homo- and heteroleptic Ru(II) complexes [Ru(phen)3−n(phen-X)n](PF6)2 (n = 0–3, X = CN, epoxy, H, NH2) were prepared and characterized. The influence of electron-withdrawing or electron-releasing substituents of the 1,10-phenanthroline ligands on the photo-physical properties was evaluated. It reveals fundamental interests in the fine tuning of redox potentials and photo-physical characteristics, depending both on the nature of the substitution of the ligand, and on the symmetry of the related homo- or heteroleptic complex. These complexes exhibit linear absorption and two-photon absorption (2PA) cross-sections over a broad range of wavelength (700–900 nm) due to absorption in the intra-ligand charge transfer (ILCT) and the metal-to-ligand charge transfer (MLCT) bands. These 2PA properties were more particularly investigated in the 700–1000 spectral range for a family of complexes bearing electro-donating ligands (phen-NH2).

The Challenge of Accurate Computation of Two-Photon Absorption Properties of Organic Chromophores in the Condensed Phase
Fu, M.; Wesolowski, T. A.
J. Chem. Theory Comput. 2021, 17, 3652-3665.

Two strategies are applied to evaluate the effect of the environment on the two-photon absorption (TPA) cross sections for two characteristic excited states of C2H4 upon complexation with H2O. The supermolecular strategy provides the reference complexation-induced shifts and uses either the EOM-CCSD or ADC(2) method. The embedding strategy is based on frozen-density-embedding theory (FDET) and uses only fundamental constants. The TPA cross sections from high-level supermolecular calculations are extremely basis-set-sensitive. Literature data and the present study indicate that accuracy of the absolute TPA cross sections below 100 atomic units and their shifts below 10 atomic units remains a challenge. The obtained FDET results show a similar basis-set behavior. For the largest basis set (d-aug-cc-pVQZ), TPA cross sections obtained from these two strategies are in excellent agreement. The complexation-induced shifts have the correct sign of the effect and a small (12–33%) relative error in magnitude. The deviations of the FDET-derived shifts from the reference are of similar magnitude as the reliability threshold of the reference shifts.

Excited-State Symmetry Breaking and the Laporte Rule
Szakács, Z.; Vauthey, E.
J. Phys. Chem. Lett. 2021, 12, archive unige:151219 pdf full text [restricted access]

Excited-state symmetry breaking (ES-SB) is common to a large number of multibranched electron donor–acceptor (DA) molecules in polar environments. During this process, the electronic excitation, originally evenly distributed over the molecule, localizes, at least partially, on one branch. Due to the absence of an unambiguous spectroscopic signature in the UV–vis region, electronic transient absorption (TA) has not been the method of choice for real-time observation of this phenomenon. Herein, we demonstrate that the Laporte rule, which states that one-photon transitions conserving parity are forbidden in centrosymmetric molecules, provides such clear signature of ES-SB in electronic TA spectroscopy. Using a dicyanoanthracene-based D–A–D dye, we show that transitions from the S1 state of this molecule, which are initially Laporte forbidden, become allowed upon ES-SB. This leads to the rise of new TA bands, whose intensity provides a direct measure of the extent of asymmetry in the excited state.

Combined spectroscopic studies on post-functionalized Au25 cluster as an ATR-FTIR sensor for cations
Baghdasaryan, A.; Brun, E.; Wang, Y.; Salassa, G.; Lacour, J.; Bürgi, T.
Chem. Sci. 2021, 12, archive unige:152187 pdf full text [free access]

Recently, significant research activity has been devoted to thiolate-protected gold clusters due to their attractive optical and electronic properties. These properties as well as solubility and stability can be controlled by post-synthetic modification strategies. Herein, the ligand exchange reaction between Au25(2-PET)18 cluster (where 2-PET is 2-phenylethanethiol) and di-thiolated crown ether (t-CE) ligands bearing two chromophores was studied. The post-functionalization aimed to endow the cluster with ion binding properties. The exchange reaction was followed in situ by UV-vis, 1H NMR and HPLC. MALDI mass analysis revealed the incorporation of up to 5 t-CE ligands into the ligand shell. Once functionalized MALDI furthermore showed complexation of sodium ions to the cluster. ATR-FTIR spectroscopic studies using aqueous solutions of K+, Ba2+, Gd3+ and Eu3+ showed noticeable spectral shifts of the C-O stretching band around 1100 cm-1 upon complexation. Further spectral changes point towards a conformational change of the two chromophores that are attached to the crown ether. Density functional theory calculations indicate that the di-thiol ligand bridges two staple units on the cluster. The calculations furthermore reproduce the spectral shift of the C-O strechning vibrations upon complex formation and reveal a conformational change that involves the two chromophores attached to the crown ether. The functionalized clusters have therefore attractive ion sensing properties due to the combination of binding properties, mainly due to the crown ether, and the possibility for signal transduction via an induced conformational change involving chromophore units.

Ligand exchange reactions on thiolate-protected gold nanoclusters
Wang, Y.; Bürgi, T.
Nanoscale Adv. 2021, 3, 2710-2727.

As a versatile post-synthesis modification method, ligand exchange reaction exhibits great potential to extend the space of accessible nanoclusters. In this review, we summarized this process for thiolate-protected gold nanoclusters. In order to better understand this reaction we will first provide the necessary background on the synthesis and structure of various gold clusters, such as Au25(SR)18, Au38(SR)24, and Au102(SR)44. The previous investigations illustrated that ligand exchange is enabled by the chemical properties and flexible gold–sulfur interface of nanoclusters. It is generally believed that ligand exchange follows a SN2-like mechanism, which is supported both by experiments and calculations. More interesting, several studies show that ligand exchange takes place at preferred sites, i.e. thiolate groups –SR, on the ligand shell of nanoclusters. With the help of ligand exchange reactions many functionalities could be imparted to gold nanoclusters including the introduced of chirality to achiral nanoclusters, size transformation and phase transfer of nanoclusters, and the addition of fluorescence or biological labels. Ligand exchange was also used to amplify the enantiomeric excess of an intrinsically chiral cluster. Ligand exchange reaction accelerates the prosperity of the nanocluster field, and also extends the diversity of precise nanoclusters.

Lipid nanotubes as an organic template for the fabrication of carbon nanostructures by pyrolysis
Jajcevic, K.; Sequeira, A. M.; Kalbacova, J.; Zahn, D. R. T.; Sugihara, K.
Nanoscale 2021, 13, 6927-6933.

We demonstrate the fabrication of carbon nanoribbons with a width of 40 nm based on fixation and pyrolysis of an organic template, lipid nanotubes. To our best knowledge, this is the smallest feature size achieved by pyrolysis of surface-patterned organic templates. Such a pyrolytic carbon nanostructure can be used for electronics and sensing applications in future.

Thermal Conversion of Unsolvated Mg(B3H8)2 to BH4 in the Presence of MgH2
Gigante, A.; Leick, N.; Lipton, A. S.; Tran, B.; Strange, N. A.; Bowden, M.; Martinez, M. B.; Moury, R.; Gennett, T.; Hagemann, H.; Autrey, T. S.
ACS Appl. Energy Mater. 2021, 4, archive unige:151382 pdf full text [restricted access]

In the search for energy storage materials, metal octahydrotriborates, M(B3H8)n, n = 1 and 2, are promising candidates for applications such as stationary hydrogen storage and all-solid-state batteries. Therefore, we studied the thermal conversion of unsolvated Mg(B3H8)2 to BH4 as-synthesized and in the presence of MgH2. The conversion of our unsolvated Mg(B3H8)2 starts at ∼100 °C and yields ∼22 wt % of BH4 along with the formation of (closo-hydro)borates and volatile boranes. This loss of boron (B) is a sign of poor cyclability of the system. However, the addition of activated MgH2 to unsolvated Mg(B3H8)2 drastically increases the thermal conversion to 85–88 wt % of BH4 while simultaneously decreasing the amounts of B-losses. Our results strongly indicate that the presence of activated MgH2 substantially decreases the formation of (closo-hydro)borates and provides the necessary H2 for the B3H8-to-BH4 conversion. This is the first report of a metal octahydrotriborate system to selectively convert to BH4 under moderate conditions of temperature (200 °C) in less than 1 h, making the MgB3H8-MgH2 system very promising for energy storage applications.

Optical Spectroscopy of Crystal Nucleation One Nucleus at a Time
Urquidi, O.; Brazard, J.; LeMessurier, N.; Simine, L.; Adachi, T. B. M.
ChemRxiv 2021, Preprint.

Crystallization is an important process in a wide range of disciplines from fundamental science to industrial application. Despite the importance of controlling the crystallization and its morphology (e.g. polymorphism), the lack of microscopic description of crystal nucleation often limits the rational approach to its engineering and control. The biggest challenge to experimentally track the nucleus formation is the stochastic and heterogeneous nature of the nucleation occurring at nanometer scale. To overcome this challenge, we developed a method we call “Single Nucleus Spectroscopy” or SNS and use it to follow the formation of single crystal glycine nucleus by Raman spectroscopy at 46 ms time resolution. The spectral evolution was analyzed by non-supervised spectral decomposition algorithm which unraveled the Raman spectrum of prenucleation aggregates. In order to gain microscopic insights into the structure of these aggregates we have established a direct comparison between the experiments and theoretical works. The outcome of our analysis is a new hypothesis of glycine crystal nucleation mechanism.

Porous shape-persistent rylene imine cages with tunable optoelectronic properties and delayed fluorescence
Huang, H.-H.; Song, K. S.; Prescimone, A.; Aster, A.; Cohen, G.; Mannancherry, R.; Vauthey, E.; Coskun, A.; Šolomek, T.
Chem. Sci. 2021, 12, archive unige:150993 pdf full text [free access]

A simultaneous combination of porosity and tunable optoelectronic properties, common in covalent organic frameworks, is rare in shape-persistent organic cages. Yet, organic cages offer important molecular advantages such as solubility and modularity. Herein, we report the synthesis of a series of chiral imine organic cages with three built-in rylene units by means of dynamic imine chemistry and we investigate their textural and optoelectronic properties. Thereby we demonstrate that the synthesized rylene cages can be reversibly reduced at accessible potentials, absorb from UV up to green light, are porous, and preferentially adsorb CO2 over N2 and CH4 with a good selectivity. In addition, we discovered that the cage incorporating three perylene-3,4:9,10-bis(dicarboximide) units displays an efficient delayed fluorescence. Time-correlated single photon counting and transient absorption spectroscopy measurements suggest that the delayed fluorescence is likely a consequence of a reversible intracage charge-separation event. Rylene cages thus offer a promising platform that allows combining the porosity of processable materials and photochemical phenomena useful in diverse applications such as photocatalysis or energy storage.

Click Chemistry on NiO Photocathode to Postfunctionalize a Diketopyrrolopyrrole Sensitizer by Naphthalene Diimide Electron Acceptor
Bentounsi, Y.; Seintis, K.; Diring, S.; Vauthey, E.; Odobel, F.
ACS Appl. Energy Mater. 2021, 4, archive unige:150679 pdf full text [restricted access]

This study addresses a practical aspect of hybrid dye-sensitized photoelectrochemical cells by exploring a simple method to prepare multicomponent systems. Building on a previously reported methodology based on a copper-free click chemistry dipolar cycloaddition of azide with activated alkyne, a naphthalene diimide (NDI) derivative substituted with two propiolic esters was clicked on a NiO photocathode already coated with a diketopyrrolopyrrole (DPP) dye bearing two azido groups. A detailed photophysical study by transient absorption spectroscopy demonstrates that optical excitation of DPP dye leads to an effective electron transfer chain from the NiO valence band to the NDI passing via the DPP dye, resulting in a long-lived charge-separated state (hole in NiO/NDI radical anion) of 170 μs. The p-type dye-sensitized solar cells were also fabricated with the above molecular components and confirm the occurrence of the electron transfer as the performances of the solar cells were improved in terms of Voc and Jsc compared to the DPP dye lacking the NDI unit. The above-clicked system was also compared to a covalently linked DPP–NDI dyad, whose performances are 30% superior to the clicked system probably due to longer mean distance between the NiO surface and the NDI with the dyad. This finding paves the way for the design of multicomponent hybrid dye-sensitized photoelectrochemical cells by chemistry on the electrode.

Copper nanoclusters: designed synthesis, structural diversity, and multiplatform applications
Baghdasaryan, A.; Bürgi, T.
Nanoscale 2021, 13, archive unige:153797 pdf full text [free access]

Atomically precise metal nanoclusters (MNCs) have gained tremendous research interest in recent years due to their extraordinary properties. The molecular-like properties that originate from the quantized electronic states provide novel opportunities for the construction of unique nanomaterials possessing rich molecular-like absorption, luminescence, and magnetic properties. The field of monolayer-protected metal nanoclusters, especially copper, with well-defined molecular structures and compositions, is relatively new, about two to three decades old. Nevertheless, the massive progress in the field illustrates the importance of such nanoobjects as promising materials for various applications. In this respect, nanocluster-based catalysts have become very popular, showing high efficiencies and activities for the catalytic conversion of chemical compounds. Biomedical applications of clusters are an active research field aimed at finding better fluorescent contrast agents, therapeutic pharmaceuticals for the treatment and prevention of diseases, the early diagnosis of cancers and other potent diseases, especially at early stages. A huge library of structures and the compositions of copper nanoclusters (CuNCs) with atomic precisions have already been discovered during last few decades; however, there are many concerns to be addressed and questions to be answered. Hopefully, in future, with the combined efforts of material scientists, inorganic chemists, and computational scientists, a thorough understanding of the unique molecular-like properties of metal nanoclusters will be achieved. This, on the other hand, will allow the interdisciplinary researchers to design novel catalysts, biosensors, or therapeutic agents using highly structured, atomically precise, and stable CuNCs. Thus, we hope this review will guide the reader through the field of CuNCs, while discussing the main achievements and improvements, along with challenges and drawbacks that one needs to face and overcome.

Long-Lived Triplet Charge-Separated State in Naphthalenediimide Based Donor–Acceptor Systems
Aster, A.; Rumble, C.; Bornhof, A.-B.; Huang, H.-H.; Sakai, N.; Solomek, T.; Matile, S.; Vauthey, E.
Chem. Sci. 2021, 12, archive unige:150871 pdf full text [free access]

1,4,5,8-Naphthalenediimides (NDIs) are widely used motifs to design multichromophoric architectures due to their ease of functionalisation, their high oxidative power and the stability of their radical anion. The NDI building block can be incorporated in supramolecular systems by either core or imide functionalization. We report on the charge-transfer dynamics of a series of electron donor–acceptor dyads consisting of a NDI chromophore with one or two donors linked at the axial, imide position. Photo-population of the core-centred π–π* state is followed by ultrafast electron transfer from the electron donor to the NDI. Due to a solvent dependent singlet–triplet equilibrium inherent to the NDI core, both singlet and triplet charge-separated states are populated. We demonstrate that long-lived charge separation in the triplet state can be achieved by controlling the mutual orientation of the donor–acceptor sub-units. By extending this study to a supramolecular NDI-based cage, we also show that the triplet charge-separation yield can be increased by tuning the environment.

Two RuII Linkage Isomers with Distinctly Different Charge Transfer Photophysics
Tisaun, J.; Laramée-Milette, B.; Beckwith, J. S.; Bierwagen, J.; Hanan, G. S.; Reber, C.; Hauser, A.; Moucheron, C.
Inorg. Chem. 2021, 60, 3677-3689.

The ligand PHEHAT (PHEHAT = 1,10-phenanthrolino[5,6-b]1,4,5,8,9,12-hexaazatriphenylene) presents a structural asymmetry that has a dramatic influence on the photophysical properties depending on the chelation site of the metal ion in the linkage isomers. While [RuII(phen)2HATPHE]2+ behaves classically, like [RuII(bpy)3]2+, [RuII(phen)2PHEHAT]2+ exhibits an unusual behavior. It appears that this complex has two 3MLCT bright states, the lower one being weakly emissive or nonemissive depending on the solvent and temperature. Different photophysical techniques involving a wide range of various temperatures and timescales are essential to analyze this difference. A full photophysical scheme is proposed based on experimental data and density functional theory calculations. While previous studies focused on high temperatures and longer timescale emission, we explore the complexes at very low temperatures and very short times in order to obtain a more complete picture of the intriguing photophysical behavior of these complexes.

Frontiers in Multiscale Modeling of Photoreceptor Proteins
Mroginski, M.-A.; Adam, S.; Amoyal, G. S.; Barnoy, A.; Bondar, A.-N.; Borin, V. A.; Church, J. R.; Domratcheva, T.; Ensing, B.; Fanelli, F.; Ferré, N.; Filiba, O.; Pedraza-González, L.; González, R.; González-Espinoza, C. E.; Kar, R. K.; Kemmler, L.; Kim, S. S.; Kongsted, J.; Krylov, A. I.; Lahav, Y.; Lazaratos, M.; NasserEddin, Q.; Navizet, I.; Nemukhin, A.; Olivucci, M.; Olsen, J. M. H.; Pérez de Alba Ortíz, A.; Pieri, E.; Rao, A. G.; Rhee, Y. M.; Ricardi, N.; Sen, S.; Solov'yov, I. A.; De Vico, L.; Wesolowski, T. A.; Wiebeler, C.; Yang, X.; Schapiro, I.
Photochem. Photobiol. 2021, 97, archive unige:151231 pdf full text [free access]

This perspective article highlights the challenges in the theoretical description of photoreceptor proteins using multiscale modeling, as discussed at the CECAM workshop in Tel Aviv, Israel. The participants have identified grand challenges and discussed the development of new tools to address them. Recent progress in understanding representative proteins such as green fluorescent protein, photoactive yellow protein, phytochrome, and rhodopsin is presented, along with methodological developments.

Deposition of Extended Ordered Ultrathin Films of Au38(SC2H4Ph)24 Nanocluster using Langmuir–Blodgett Technique
Swierczewski, M.; Maroni, P.; Chenneviere, A.; Dadras, M. M.; Lee, L.-T.; Bürgi, T.
Small 2021, 17, 2005954.

Langmuir–Blodgett technique is utilized to deposit ultrathin films of Au38(SC2H4Ph)24 nanocluster onto solid surfaces such as mica and silicon. The morphologies of the films transferred at various surface pressures within the mono/bi/trilayer regime are studied by atomic force microscopy (AFM). The time spent on the water surface before the deposition has a decisive effect on the final ordering of nanoclusters within the network and is studied by fast AFM, X-ray reflectivity, and grazing-incidence wide-angle X-ray scattering.

Singlet Fission in a Flexible Bichromophore with Structural and Dynamic Control
Aster, A.; Zinna, F.; Rumble, C.; Lacour, J.; Vauthey, E.
J. Am. Chem. Soc. 2021, 143, archive unige:148899 pdf full text [restricted access]

Singlet fission (SF), i.e., the splitting of a high-energy exciton into two lower-energy triplet excitons, has the potential to increase the efficiency for harvesting spectrally broad light. The path from the photopopulated singlet state to free triplets is complicated by competing processes that decrease the overall SF efficiency. A detailed understanding of the whole cascade and the nature of the photoexcited singlet state is still a major challenge. Here, we introduce a pentacene dimer with a flexible crown ether spacer enabling a control of the interchromophore coupling upon solvent-induced self-aggregation as well as cation binding. The systematic change of solvent polarity and viscosity and excitation wavelength, as well as the available conformational phase space, allows us to draw a coherent picture of the whole SF cascade from the femtosecond to microsecond time scales. High coupling leads to ultrafast SF (

Lifetime Broadening and Impulsive Generation of Vibrational Coherence Triggered by Ultrafast Electron Transfer
Aster, A.; Bornhof, A.-B.; Sakai, N.; Matile, S.; Vauthey, E.
J. Phys. Chem. Lett. 2021, 12, archive unige:147895 pdf full text [restricted access]

The absorption band shape of chromophores in liquid solution at room temperature is usually dominated by pure electronic dephasing dynamics, which occurs on the sub-100 fs time scale. Herein, we report on a series of dyads consisting of a naphthalenediimide (NDI) electron acceptor with one or two phenyl-based donors for which photoinduced intramolecular electron transfer is fast enough to be competitive with pure electronic dephasing. As a consequence, the absorption band of the π–π* transition of these dyads is broader than that of the NDI alone to an extent that scales with the electron transfer rate. Additionally, this reaction is so fast that it leads to the impulsive excitation of a low-frequency vibrational mode of the charge-separated product. Quantum-chemical calculations suggest that this vibration involves the C–N donor–acceptor bond, which shortens considerably upon electron transfer.

Experimental Confirmation of a Topological Isomer of the Ubiquitous Au25(SR)18 Cluster in the Gas Phase
Kalenius, E.; Malola, S.; Matus, M. F.; Kazan, R.; Bürgi, T.; Häkkinen, H.
J. Am. Chem. Soc. 2021, 143, 1273-1277.

High-resolution electrospray ionization ion mobility mass spectrometry has revealed a gas-phase isomer of the ubiquitous, extremely well-studied Au25(SR)18 cluster both in anionic and cationic form. The relative abundance of the isomeric structures can be controlled by in-source activation. The measured collision cross section of the new isomer agrees extremely well with a recent theoretical prediction (Matus, M. F.; et al. Chem. Commun. 2020, 56, 8087) corresponding to a Au25(SR)18 isomer that is energetically close and topologically connected to the known ground-state structure via a simple rotation of the gold core without breaking any Au–S bonds. The results imply that the structural dynamics leading to isomerization of thiolate-protected gold clusters may play an important role in their gas-phase reactions and that isomerization could be controlled by external stimuli.

Energy transfer between different Eu2+ ions in the white phosphor Ba7F12Cl2:Eu2
Hasler, C.; Hauser, A.; Olchowka, J.; Hagemann, H.
J. Lumin. 2021, 233, archive unige:154224 pdf full text [free access]

We have studied in detail the emission spectra of the white phosphor Ba7F12Cl2:Eu2+ as a function of Europium content, excitation wavelength and temperature. The change of the emission spectrum with excitation wavelength shows a systematic shift in the CIE chromaticity diagram from warm white upon excitation in the near UV (370 nm) to cold white upon excitation at shorter wavelengths. The observed intensity changes with europium concentration confirm that energy transfer takes place, which is both concentration and temperature dependent. Temperature and sample dependent lifetime studies show that the observed lifetimes do not change within experimental error between dilute and concentrated Eu-doped samples, and they remain constant between 5 K and room temperature. The second observation confirms the previous results that the thermal quenching of the white emission occurs at high temperature (200 °C). The combination of all observations suggests that the energy transfer takes place first between the normal 4f65 d1 states of the Eu ions located on the 3 different crystallographic sites of Ba, and is followed by subsequent relaxation to anomalous emission states whose emission lifetimes remain constant with sample concentration and temperature from 5 K to 300 K.

Observation of Carbonic Acid Formation from Interaction between Carbon Dioxide and Ice by Using in situ Modulation Excitation IR Spectroscopy
Wang, X.; Bürgi, T.
Angew. Chem. Int. Ed. 2021, 60, 7860-7865.

Carbonic acid, H2CO3, is of fundamental importance in nature both in living and non-living systems. A lot of research has been devoted to this molecule and it has been  characterized  in  solution,  in  the  solid  state  and  in  gas  phase,  however  its existence under certain conditions and its properties remain controversial. Providing direct spectroscopic evidence for carbonic acid formation is a challenge. Furthermore, the methods for carbonic acid generation are indirect e.g. high-energy irradiation or protonation of carbonate and bicarbonate ions by strong acids. Here we provide clear evidence by in situ  attenuated total reflection infrared spectroscopy combined with modulation excitation spectroscopy and phase sensitive detection that CO2 adsorption on ice surfaces is accompanied by carbonic acid formation. Hence we demonstrate that  carbonic  acid  can  be  formed  from  CO2  on  ice  in  the  absence  of  high-energy irradiation and without protonation by strong acids. The formation of carbonic acid is favored at low temperature, whereas at high temperature it rapidly dissociates to form bicarbonate (HCO3-) and  carbonate  (CO32-). The  direct formation of  carbonic acid from adsorption of CO2 on ice could play a role in the upper troposphere in cirrus clouds, where all the necessary ingredients to form carbonic acid, i.e. low temperature, CO2 gas and ice, are present.

Palladium(0)-Catalyzed Enantioselective Intramolecular Arylation of Enantiotopic Secondary C−H Bonds
Melot, R.; Zuccarello, M.; Cavalli, D.; Niggli, N.; Devereux, M.; Bürgi, T.; Baudoin, O.
Angew. Chem. Int. Ed. 2021, 60, 7245-7250.

The enantioselective functionalization of nonactivated enantiotopic secondary C–H bonds is one of the greatest challenges in transition-metal-catalyzed C–H activation proceeding via an inner-sphere mechanism. Notably, such reactions have remained elusive within the realm of palladium(0)-catalysis. Here we report that  N -heterocyclic carbene ligands from the IBiox family display a unique reactivity profile in the Pd 0 -catalyzed intramolecular arylation of such nonactivated secondary C–H bonds. Chiral C 2 -symmetric IBiox ligands allowed to achieve high enantioselectivities for a broad range of valuable indane products containig a tertiary stereocenter. Similar reaction conditions were applicable to the arylation of secondary C–H bonds adjacent to amides. Depending on the amide substituents and upon control of reaction time, indanes containing a labile tertiary stereocenters were also obtained with high enantioselectivities. Analysis of the steric maps of the IBiox ligands indicated that the level of enantioselectivity correlates with the difference between the two most occupied and the two less occupied space quadrants, and provided a blueprint for the design of even more efficient ligands.

Role of Intercluster and Interligand Dynamics of [Ag25(DMBT)18] Nanoclusters by Multinuclear Magnetic Resonance Spectroscopy
Salassa, G.; Krishnadas, K. R.; Pupier, M.; Viger-Gravel, J.; Bürgi, T.
J. Phys. Chem. C 2021, 125, archive unige:148583 pdf full text [restricted access]

Even though gold and silver belong to the same group of the periodic table, they provide significantly different nanocluster (NC) structures in terms of both nuclearities and ligand coordination motifs. Until today, only one isostructural gold analogue has been found for silver nanoclusters, [Ag25(DMBT)18], and it is only obtained by using 2,4-dimethylbenzenethiol (DMBT) as the ligand. Our study of the dynamics of DMBT ligands in metal NCs using multinuclear magnetic resonance spectroscopy demonstrates that DMBT favors the formation of two types of interligand interactions, i.e., H−π and π–π. These interactions stabilize the entire nanocluster, yet we observe that thermal and chemical stimuli have the capability to weaken the [Ag25(DMBT)18] structure triggering irreversible decomposition. Moreover, employing 2D-NMR spectroscopy we demonstrate the intercluster exchange of DMBT ligands and their temperature dependence.

Quantitative and Anisotropic Mechanochromism of Polydiacetylene at Nanoscale
Juhasz, L.; Ortuso, R. D.; Sugihara, K.
Nano Lett. 2021, 21, 543-549.

Quantitative and anisotropic mechanochromism of polydiacetylene over nanoscale distances remains unaddressed even after 50 years of extensive research. This is because its anisotropic structure on substrates necessitates the application of both vertical and lateral forces (shear forces) to characterize it, whereas atomic force microscopy, which is the usual technique used to investigate nanoscale forces, is only capable of quantifying vertical forces. In this study, we address this lacuna by utilizing quantitative friction force microscopy that measures lateral forces. Our data confirm that polydiacetylene reacts only to lateral forces, F//, and disprove the previously claimed hypothesis that the edges of the polymer crystals exhibit higher force sensitivity than the rest of the crystal. In addition, we report a correlation between mechanochromism and thermochromism, which can be attributed to the fact that both work and heat are different means of providing the same transition energy.

Universal quenching of common fluorescent probes by water and alcohols
Maillard, J.; Klehs, K.; Rumble, C.; Vauthey, E.; Heilemann, M.; Fürstenberg, A.
Chem. Sci. 2021, 12, archive unige:148739 pdf full text [free access]

Although biological imaging is mostly performed in aqueous media, it is hardly ever considered that water acts as a classic fluorescence quencher for organic fluorophores. By investigating the fluorescence properties of 42 common organic fluorophores recommended for biological labelling, we demonstrate that H2O reduces their fluorescence quantum yield and lifetime by up to threefold and uncover the underlying fluorescence quenching mechanism. We show that the quenching efficiency is significantly larger for red-emitting probes and follows an energy gap law. The fluorescence quenching finds its origin in high-energy vibrations of the solvent (OH groups), as methanol and other linear alcohols are also found to quench the emission, whereas it is restored in deuterated solvents. Our observations are consistent with a mechanism by which the electronic excitation of the fluorophore is resonantly transferred to overtones and combination transitions of high-frequency vibrational stretching modes of the solvent through space and not through hydrogen bonds. Insight into this solvent-assisted quenching mechanism opens the door to the rational design of brighter fluorescent probes by offering a justification for protecting organic fluorophores from the solvent via encapsulation.