Publications of the Department of Physical Chemistry


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, in press.

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, in press.

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, 4067-4071.

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, 3737-3747.

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, 5275-5285.

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, 6283-6340.

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.

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, 117866.

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.