Publications of the Department of Physical Chemistry


Nanomechanical and structural study of Au38 nanocluster Langmuir-Blodgett films using bimodal atomic force microscopy and X-ray reflectivity
Swierczewski, M.; Chenneviere, A.; Lee, L.-T.; Maroni, P.; Bürgi, T.
J. Colloid Interface Sci. 2023, 630, 28-36.


Langmuir-Blodgett (LB) technique allows the deposition of gold nanoparticles and nanoclusters (atomically precise nanoparticles below 2 nm in diameter) onto solid substrates with an unprecedented degree of control and high transfer ratios. Nanoclusters are expected to follow the crinkle folding mechanism, which promotes the formation of trilayers of nanoparticles but kinetically disfavors the formation of the fourth layer.


LB films of Au38(SC2H4Ph)24 nanocluster were prepared at a range of surface pressures in the bilayer/trilayer regime and their internal structure was analyzed with X-ray Reflectivity (XRR) and Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS). Bimodal atomic force microscopy (AFM) imaging was used to quantify the elastic modulus, which can be correlated with the topography at the same point on the surface.


Nanocluster bilayers and trilayers exhibited the elastic moduli of ca. 1.2 GPa and 0.9 GPa respectively. Films transferred in the 20–25 mN/m surface pressure regime displayed a particular propensity to form highly vertically organized trilayers. Further compression resulted in disorganization of the layers. Crucially, the use of two cantilevers of contrasting stiffness for bimodal AFM measurements has demonstrated a new approach to quantify the mechanical properties of ultrathin films without the use of deconvolution algorithms to remove the substrate contribution.

Analytical separation techniques: toward achieving atomic precision in nanomaterials science
Kumaranchira Ramankutty, K.; Buergi, T.
Nanoscale 2022, 14, 16415-16426.

The size- and shape-dependence of the properties are the most characteristic features of nanoscale matter. In many types of nanomaterials, there is a size regime wherein every atom counts. In order to fully realize the idea of ‘maneuvering things atom by atom’ envisioned by Richard Feynman, synthesis and separation of nanoscale matter with atomic precision are essential. It is therefore not surprising that analytical separation techniques have contributed tremendously toward understanding the size- as well as shape-dependent properties of nanomaterials. Fascinating properties of nanomaterials would not have been explored without the use of these techniques. Here we discuss the pivotal role of analytical separation techniques in the progress of nanomaterials science. We begin with a brief overview of some of the key analytical separation techniques that are of tremendous importance in nanomaterials research. Then we describe how each of these techniques has contributed to the advancements in nanomaterials science taking some of the nanosystems as examples. We discuss the limitations and challenges of these techniques and future perspectives.

Persistent Luminescence in Strontium Aluminate: A Roadmap to a Brighter Future
Van der Heggen, D.; Joos, J. J.; Feng, A.; Fritz, V.; Delgado, T.; Gartmann, N.; Walfort, B.; Rytz, D.; Hagemann, H.; Poelman, D.; Viana, B.; Smet, P. F.
Adv. Funct. Materials 2022, 2, 2208809.

Glow-in-the-dark materials have been around for a long time. While formerly materials had to be mixed with radioactive elements to achieve a sufficiently long and bright afterglow, these have now been replaced by much safer alternatives. Notably strontium aluminate, SrAl2O4, doped with europium and dysprosium, has been discovered over two decades ago and since then the phosphor has transcended its popular use in watch dials, safety signage, or toys with more niche applications such as stress sensing, photocatalysis, medical imaging, or flicker-free light-emitting diodes. A lot of research efforts are focused on further improving the storage capacity of SrAl2O4:Eu2+,Dy3+, including in nanosized particles, and on finding the underlying physical mechanism to fully explain the afterglow in this material and related compounds. Here an overview of the most important results from the research on SrAl2O4:Eu2+,Dy3+ is presented and different models and the underlying physics are discussed to explain the trapping mechanism at play in these materials.

Torsional disorder and planarization dynamics: 9,10-bis(phenylethynyl)anthracene as a case study
Fureraj, I.; Budkina, D. S.; Vauthey, E.
Phys. Chem. Chem. Phys. 2022, 24, archive unige:165071 pdf full text [free access]

Conjugated molecules with phenylethynyl building blocks are usually characterised by torsional disorder at room temperature. They are much more rigid in the electronic excited state due to conjugation. As a consequence, the electronic absorption and emission spectra do not present a mirror-image relationship. Here, we investigate how torsional disorder affects the excited state dynamics of 9,10-bis(phenylethynyl)anthracene in solvents of different viscosities and in polymers, using both stationary and ultrafast electronic spectroscopies. Temperature-dependent measurements reveal inhomogeneous broadening of the absorption spectrum at room temperature. This is confirmed by ultrafast spectroscopic measurements at different excitation wavelengths. Red-edge irradiation excites planar molecules that return to the ground state without significant structural dynamics. In this case, however, re-equilibration of the torsional disorder in the ground state can be observed. Higher-energy irradiation excites torsionally disordered molecules, which then planarise, leading to important spectral dynamics. The latter is found to occur partially via viscosity-independent inertial motion, whereas it is purely diffusive in the ground state. This dissimilarity is explained in terms of the steepness of the potential along the torsional coordinate.

Insight into the transient inactivation effect on Au/TiO2 catalyst by in-situ DRIFT and UV–vis spectroscopy
Wang, X.; Rosspeintner, A.; Ziarati, A.; Zhao, J.; Bürgi, T.
Nat. Commun. 2022, 13, 5458.

Au catalysts have drawn broad attention for catalytic CO oxidation. However, a molecular-level understanding of the reaction mechanism on a fast time-resolved scale is still lacking. Herein, we apply in situ DRIFTS and UV-Vis spectroscopy to monitor the rapid dynamic changes during CO oxidation over Au/TiO2. A pronounced transient inactivation effect likely due to a structural change of Au/TiO2 induced by the reactants (CO and O2) is observed at the beginning of the reaction. The transient inactivation effect is affected by the ratio of CO and O2 concentrations. More importantly, during the unstable state, the electronic properties of the Au particles change, as indicated by the shift of the CO stretching vibration. UV-Vis spectroscopy corroborates the structure change of Au/TiO2 surface induced by the reactants, which leads to a weakening of the Au catalyst’s ability to be oxidized (less O2 adsorption), resulting in the transient inactivation effect.

Efficient photoredox cycles to control perylenediimide self-assembly
Chen, C.; Valera, J. S.; Adachi, T. B. M.; Hermans, T.
Chem. Eur. J. 2022, in press.

Photoreduction of perylenediimide (PDI) derivatives has been widely studied for use in photocatalysis, hydrogen evolution, photo-responsive gels, and organic semiconductors. Upon light irradiation, the radical anion (PDI •– ) can readily be obtained, whereas further reduction to the dianion (PDI 2– ) is rare. Here we show that full 2-electron photoreduction can be achieved using UVC light: 1) in anaerobic conditions by ‘direct photoreduction’ of PDI aggregates, or 2) by ‘indirect photoreduction’ in aerobic conditions due to acetone ketyl radicals. The latter strategy is also efficient for other dyes, such as naphthalenediimide (NDI) and methylviologen (MV 2+ ). Efficient photoreduction on the minute time-scale using simple LED light in aerobic conditions is attractive for use in dissipative light-driven systems and materials.

Gold-doping effect on two-photon absorption and luminescence of atomically precise silver ligated nanoclusters
Pniakowska, A.; Ramankutty, K. K.; Obstarczyk, P.; Bakulić, M. P.; Maršić, . S.; Bonačić-Koutecký, V.; Bürgi, T.; Olesiak-Banska, J.
Angew. Chem. Int. Ed. 2022, 61, e202209645.

Noble metal nanoclusters allow for the atomically-precise control of their composition. However, to create nanoclusters with pre-defined optical properties, comprehensive description of their structure-property relation is required. Here, we report the gold atom doping impact on one-photon and two-photon absorption (TPA) and luminescence properties of ligated silver nanoclusters via combined experimental studies and time-dependent density functional theory simulations (TD-DFT). We synthesized a series of Ag25-xAux(DMBT)18 nanoclusters where x=0, 1 and 5-10. For Ag24Au1(DMBT)18 we demonstrate that the presence of the central Au dopant strongly influences linear and non-linear optical properties, increasing photoluminescence quantum yield and two-photon brightness, with respect to undoped silver nanoclusters. With improved TPA and luminescence, atomically-precise AuAg alloys presented in our work can serve as robust luminescent probes e.g. for bioimaging in the second biological window.

Reply to Liao and Wynne: The size of crystal nucleus remains an open question
Adachi, T. B. M.; Brazard, J.; Urquidi, O.
Proc. Natl. Acad. Sci. U.S.A. 2022, 119, 8.

The letter by Liao and Wynne criticizes the established knowledge in the field of optical trapping prior to our work. In this reply, we highlight some key literature along with the experiments we performed to reconfirm the preestablished facts.

Hazardous chemical elements in cleaning cloths: A potential source of microfibres
Filella, M.; Brazard, J.; Adachi, T. B. M.; Turner, A.
Sci. Total Environ. 2022, 846, archive unige:162584 pdf full text [free access]

Although potentially hazardous chemical elements (e.g., Cu, Cr, Pb, Sb, Ti, Zn) have been studied in clothing textiles, their presence in cleaning textiles is unknown. In this study, 48 cleaning cloth products (consisting of 81 individual samples) purchased in Europe, and consisting of synthetic (petroleum-based), semi-synthetic or natural fibres or combinations of these different types, have been analysed for 16 chemical elements by X-ray fluorescence (XRF) spectrometry. Titanium was detected in most cases (median and maximum concentrations ~3700 and 12,400?mg?kg−1, respectively) and Raman microspectroscopy revealed that TiO2 was present as anatase. Barium, Br, Cr, Cu, Fe and Zn were frequently detected over a range of concentrations, reflecting the presence of various additives, and Sb was present at concentrations up to about 200?mg?kg−1 in samples containing polyester as catalytic residue from the polymerisation process. Lead was detected as a contaminant in four samples and at concentrations below 10?mg?kg−1. Overall, the range of the chemical element profiles and concentrations was similar to those for clothing materials published in the literature, suggesting that broadly the same additives, materials and processes are employed to manufacture cloths and clothing textiles. The mechanisms by which potentially hazardous chemical elements are released into the environment with microfibres or mobilised into soluble or nano-particulate forms remain to be explored.

Combined in vitro and in vivo investigation of barite microcrystals in Spirogyra (Zygnematophyceae, Charophyta)
Barbosa, N.; Jaquet, J.-M.; Urquidi, O.; Adachi, T. B. M.; Filella, M.
J. Plant Physiol. 2022, 276, archive unige:162583 pdf full text [free access]

We have investigated the biomineralisation of barite ?a useful proxy for reconstructing paleoproductivity? in a freshwater alga, Spirogyra, by combining in vitro and in vivo approaches to unveil the nature of its barite microcrystals. Scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDXS) observations on simply dried samples revealed that the number and size of barite crystals were related to the barium concentration in the media. Additionally, their morphology showed a crystallographic face (011), which is not normally observed, suggesting the influence of organic molecules on the growth kinetics. The critical point drying method was used to preserve the internal and external structures of Spirogyra cells for SEM imaging. Crystals were found adjacent to the cytoplasmic membrane, near chloroplasts and fibrillary network. In vivo optical microscopy and Raman tweezer microspectroscopy in living cells showed that barite microcrystals are optically visible and follow cytoplasmic streaming. These results led us to propose that barite formation in Spirogyra occurs in the cytoplasm where barium and sulphate are both available: barium supplied non-selectively through the active transport of the divalent cations needed for actin polymerisation, and sulphate because necessary for amino acid biosynthesis in chloroplasts.

Complex-as-Ligand Strategy as a Tool for the Design of a Binuclear Nonsymmetrical Chromium(III) Assembly: Near-Infrared Double Emission and Intramolecular Energy Transfer
Doistau, B.; Jiménez, J.-R.; Lawson Daku, L. M.; Piguet, C.
Inorg. Chem. 2022, 61, 11023-11031.

The chromium(III) polypyridyl complexes are appealing for their long-lived near-infrared (NIR) emission reaching the millisecond range and for the strong circularly polarized luminescence of their isolated enantiomers. However, harnessing those properties in functional polynuclear CrIII devices remains mainly inaccessible because of the lack of synthetic methods for their design and functionalization. Even the preparation and investigation of most basic nonsymmetrical CrIII dyads exhibiting directional intramolecular intermetallic energy transfer remain unexplored. Taking advantage of the inertness of heteroleptic chromium(III) polypyridyl building blocks, we herein adapt the “complex-as-ligand” strategy, largely used with precious 4d and 5d metals, for the preparation of a binuclear nonsymmetrical CrIII complex (3d metal). The resulting [(phen)2Cr(L)Cr(tpy)]6+ dyad shows dual long-lived NIR emission and a directional intermetallic energy transfer that is controlled by the specific arrangements of the different coordination spheres. This strategy opens a route for building predetermined polynuclear assemblies with this earth-abundant metal.

Controlling the Defects of Cs2AgBiBr6 by Varied Precursor Compositions
Frebel, A.; Yoon, S.; Meles Neguse, S.; Jöckel, D. M.; Widenmeyer, M.; Lange, S.; Naumann, V.; Rosspeintner, A.; Ebbinghaus, S. G.; Balke, B.; Weidenkaff, A.
Adv. Photonics Res. 2022, 3, 2200061.

The amount and type of defects in Cs2AgBiBr6 are controlled by varying the Ag/Bi ratio of the precursor solutions in two different synthesis routes, that is, slow solution cooling crystallization and fast microwave-assisted hydrothermal synthesis. The correlation between the Ag/Bi ratio in the precursor solution and defect formation in the crystals is studied by band broadening analysis in Raman spectroscopy, the estimated Urbach energy in UV–vis spectroscopy, and thermogravimetric analysis. Ag-rich precursors are found to prevent the formation of the secondary-phase Cs3Bi2Br9, but at the same time induced the formation of Br vacancies and antisite defects. Time-resolved photoluminescence measurements reveal that the formation of beneficial defects such as Br vacancies causes to trap the charge carriers, thus avoiding the recombination of charge carriers and leading to a longer carrier lifetime. Herein, the findings provide a guidance to decrease the defect densities and can be applied to the fabrication of Pb-free solar cells based on Cs2AgBiBr6.

The Atomically Precise Gold/Captopril Nanocluster Au25(Capt)18 Gains Anticancer Activity by Inhibiting Mitochondrial Oxidative Phosphorylation
Bhattacharya, S. R.; Bhattacharya, K.; Xavier, V. J.; Ziarati, A.; Picard, D.; Bürgi, T.
ACS Appl. Mater. Interfaces 2022, 14, 29521-29536.

Atomically precise gold nanoclusters (AuNCs) are an emerging class of quantum-sized nanomaterials with well-defined molecular structures and unique biophysical properties, rendering them highly attractive for biological applications. We set out to study the impact of different ligand shells of atomically similar nanoclusters on cellular recognition and response. To understand the effects of atomically precise nanoclusters with identical composition on cells, we selected two different water-soluble gold nanoclusters protected with captopril (Capt) and glutathione (GSH): Au25(Capt)18 (CNC) and Au25(GSH)18 (GNC), respectively. We demonstrated that a change of the ligand of the cluster completely changes its biological functions. Whereas both nanoclusters are capable of internalization, only CNC exhibits remarkable cytotoxicity, more specifically on cancer cells. CNC shows enhanced cytotoxicity by inhibiting the OXPHOS of mitochondria, possibly by inhibiting the ATP synthase complex of the electron transport chain (ETC), and by initiating the leakage of electrons into the mitochondrial lumen. The resulting increase in both mitochondrial and total cellular ROS triggers cell death indicated by the appearance of cellular markers of apoptosis. Remarkably, this effect of nanoclusters is independent of any external light source excitation. Our findings point to the prevailing importance of the ligand shell for applications of atomically precise nanoclusters in biology and medicine.

Photoinduced Electron Transfer in a Porphyrin–Fullerene Dyad at a Liquid Interface
Sissaoui, J.; Efimov, A.; Kumpulainen, T.; Vauthey, E.
J. Phys. Chem. B 2022, 126, archive unige:161827 pdf full text [free access]

The excited-state properties of an amphiphilic porphyrin–fullerene dyad and of its porphyrin analogue adsorbed at the dodecane/water interface are investigated by using surface second-harmonic generation. Although the porphyrin is formally centrosymmetric, the second-harmonic spectra of both compounds are dominated by the intense Soret band of the porphyrin. Polarization-selective measurements and molecular dynamics simulations suggest an angle of about 45° between the donor–acceptor axis and the interfacial plane, with the porphyrin interacting mostly with the nonpolar phase. Time-resolved measurements reveal a marked concentration dependence of the dynamics of both compounds upon Q-band excitation, indicating the occurrence of intermolecular quenching processes. The significant differences in dynamics and spectra between the dyad and the porphyrin analogue are explained by a self-quenching of the excited dyad via an intermolecular electron transfer.

Probing luminescence of rare earth ions in natural pink fluorites using Raman microscopes
Hagemann, H.; Ayoubipour, S.; Delgado, T.; Schnyder, C.; Gnos, E.
J. Raman Spectrosc. 2022, 53, 1464-1470.

It is well-known that many natural fluorite crystals contain rare earth ions. In this work, a series of pink fluorites is studied to demonstrate that the use of Raman microscopes allows to obtain high resolution emission spectra of trivalent rare earth ions. These emission spectra allow in principle to distinguish differences of the local environment related to the charge compensation necessary when the divalent Ca ion is replaced by a trivalent rare earth ion. The pink fluorites from the Alps have grown under hydrothermal conditions, which results in the incorporation of oxygen into the fluorite. In one sample from Juchlistock (Bern, Switzerland), a trigonal Eu3+-oxygen center is clearly identified. Interestingly, a pink fluorite from China (Huanggang Mine, Inner Mongolia, China) shows a quite different emission spectrum from those of the Alps. In this sample, the emission bands have much lower relative intensity compared with the intensity of the CaF2 Raman peak, indicating a much lower content of Er3+ and Ho3+ which emit around 520–560 nm. Literature examples show that Raman measurements of differently colored fluorite crystals can also present sharp rare earth spectra. By extension, it is thus possible to observe and identify emission spectra of rare earth ions in other mineral crystals using Raman microspectroscopy and avoid misassignments of the corresponding observed Raman spectra.

Full relaxation dynamics recovery from ultrafast fluorescence experiments by means of the stochastic model: Does the solvent response dynamics depend on the fluorophore nature?
Nazarov, A. E.; Ivanov, A. I.; Rosspeintner, A.; Angulo, G.
J. Mol. Liq. 2022, 360, 119387.

An approach to analysing time-resolved fluorescence spectra has been developed for dyes with non-parabolic free energy curves of the excited and ground states. It incorporates the description of the solvent relaxation and redistribution/relaxation of intramolecular high-frequency vibrations proceeding in parallel with the pumping. The approach has been applied to simulate spectra of covalently linked perylene-dimethylaniline (PD) in a series of solvents of different polarity. The developed approach combined with the achieved temporal resolution of the modern fluorescence upconversion spectroscopy is shown to provide a possibility to disentangle the relaxation of high-frequency vibrations and the dynamics of the solvent. In this compound the reorganization of both intramolecular vibrations and the solvent strongly affects the absorption and fluorescence spectra so that their influence on spectral dynamics is highly entangled. The comparison of fluorescence spectral dynamics of two dyes, coumarin 153 and PD, shows that the solvation dynamics depend on the fluorophore. The fitting of simulated fluorescence spectral dynamics of PD to the experimental data enables us to determine the solvent relaxation function of hexane.

Molecule-like and lattice vibrations in metal clusters
Ramankutty, K. K.; Yang, H.; Baghdasaryan, A.; Teyssier, J.; Nicu, V. P.; Buergi, T.
Phys. Chem. Chem. Phys. 2022, 24, 13848-13859.

We report distinct molecule-like and lattice (breathing) vibrational signatures of atomically precise, ligand-protected metal clusters using low-temperature Raman spectroscopy. Our measurements provide fingerprint Raman spectra of a series of noble metal clusters, namely, Au25(SR)18, Ag25(SR)18, Ag24Au1(SR)18, Ag29(S2R)12 and Ag44(SR)30 (–SR = alkyl/arylthiolate, –S2R = dithiolate). Distinct, well-defined, low-frequency Raman bands of these clusters result from the vibrations of their metal cores whereas the higher-frequency bands reflect the structure of the metal–ligand interface. We observe a distinct breathing vibrational mode for each of these clusters. Detailed analyses of the bands are presented in the light of DFT calculations. These vibrational signatures change systematically when the metal atoms and/or the ligands are changed. Most importantly, our results show that the physical, lattice dynamics model alone cannot completely describe the vibrational properties of ligand-protected metal clusters. We show that low-frequency Raman spectroscopy is a powerful tool to understand the vibrational dynamics of atomically precise, molecule-like particles of other materials such as molecular nanocarbons, quantum dots, and perovskites.

Exploring Detailed Reaction Pathways for Hydrogen Storage with Borohydrides Using DFT Calculations
Johnson, S. I.; DeMaria, J. M.; Ginovska, B.; Edvenson, G. M.; Hagemann, H.; Autrey, S. T.
Energy Fuels 2022, 36, 5513-5527.

Borohydrides have seen renewed interest due to their ability to store and carry a high weight percentage of hydrogen, making them desirable materials for large-scale and long-duration energy storage applications. In this work, we present a systematic computational study of the thermochemical feasibility of a series of mechanistic pathways that can lead to the formation of the observed boron clusters. This allowed us to identify predominant pathways for buildup, following a complex network of pathways. We find that the preferred mechanisms are different for smaller and larger clusters. The primary mechanism for buildup in clusters up to ∼5 boron atoms is by addition of BH4/BH3 species, followed by loss of H2 or H. For larger clusters, fusion reactions dominate, again followed by the loss of H2 or H. A few species were identified as possible branching points, where the trends for favorability change; B2H7 as an initiation step and B4H7, B3H8, and B9H132– as minima on the potential energy surface are most notable. This work demonstrates that borohydride clusters feature a wealth of pathways for realizing the desired products (B10H102– or B12H122–), the redundancy of which may help avoid energetic sinks or kinetic bottlenecks. This robustness in reactivity has the potential to make them a versatile H2 storage material.

In situ optical spectroscopy of crystallization: One crystal nucleation at a time
Urquidi, O.; Brazard, J.; LeMessurier, N.; Simine, L.; Adachi, T. B. M.
Proc. Natl. Acad. Sci. U.S.A. 2022, 119, archive unige:162448 pdf full text [restricted access]

While crystallization is a ubiquitous and an important process, the microscopic picture of crystal nucleation is yet to be established. Recent studies suggest that the nucleation process can be more complex than the view offered by the classical nucleation theory. Here, we implement single crystal nucleation spectroscopy (SCNS) by combining Raman microspectroscopy and optical trapping induced crystallization to spectroscopically investigate one crystal nucleation at a time. Raman spectral evolution during a single glycine crystal nucleation from water, measured by SCNS and analyzed by a nonsupervised spectral decomposition technique, uncovered the Raman spectrum of prenucleation aggregates and their critical role as an intermediate species in the dynamics. The agreement between the spectral feature of prenucleation aggregates and our simulation suggests that their structural order emerges through the dynamic formation of linear hydrogen-bonded networks. The present work provides a strong impetus for accelerating the investigation of crystal nucleation by optical spectroscopy.

Preprint in ChemRxiv: 10.26434/chemrxiv.14317649.v2

Study of the Temperature- and Pressure-Dependent Structural Properties of Alkali Hydrido- closo -borate Compounds
Moury, R.; Łodziana, Z.; Remhof, A.; Duchêne, L.; Roedern, E.; Gigante, A.; Hagemann, H.
Inorg. Chem. 2022, 61, archive unige:160731 pdf full text [free access]

In this work, we report on the structural properties of alkali hydrido-closo-(car)borates, a promising class of solid-state electrolyte materials, using high-pressure and temperature-dependent X-ray diffraction experiments combined with density functional theory (DFT) calculations. The mechanical properties are determined via pressure-dependent diffraction studies and DFT calculations; the shear moduli appear to be very low for all studied compounds, revealing their high malleability (that can be beneficial for the manufacturing and stable cycling of all-solid-state batteries). The thermodiffraction experiments also reveal a high coefficient of thermal expansion for these materials. We discover a pressure-induced phase transition for K2B12H12 from Fm3? to Pnnm symmetry around 2 GPa. A temperature-induced phase transition for Li2B10H10 was also observed for the first time by thermodiffraction, and the crystal structure determined by combining experimental data and DFT calculations. Interestingly, all phases of the studied compounds (including newly discovered high-pressure and high-temperature phases) may be related via a group–subgroup relationship, with the notable exception of the room-temperature phase of Li2B10H10.

Beyond the Threshold: A Study of Chalcogenophene-Based Two-Photon Initiators
Lunzer, M.; Beckwith, J. S.; Chalupa-Gantner, F.; Rosspeintner, A.; Licari, G.; Steiger, W.; Hametner, C.; Liska, R.; Fröhlich, J.; Vauthey, E.; Ovsianikov, A.; Holzer, B.
Chem. Mater. 2022, 34, archive unige:160462 pdf full text [free access]

A series of nine soluble, symmetric chalcogenophenes bearing hexyl-substituted triphenylamines, indolocarbazoles, or phenylcarbazoles was designed and synthesized as potential two-photon absorption (2PA) initiators. A detailed photophysical analysis of these molecules revealed good 2PA properties of the series and, in particular, a strong influence of selenium on the 2PA cross sections, rendering these materials especially promising new 2PA photoinitiators. Structuring and threshold tests proved the efficiency and broad spectral versatility of two selenium-containing lead compounds as well as their applicability in an acrylate resin formulation. A comparison with commercial photoinitiators Irg369 and BAPO as well as sensitizer ITX showed that the newly designed selenium-based materials TPA-S and TPA-BBS outperform these traditional initiators by far both in terms of reactivity and dose. Moreover, by increasing the ultralow concentration of TPA-BBS, a further reduction of the polymerization threshold can be achieved, revealing the great potential of this series for application in two-photon polymerization (2PP) systems where only low laser power is available.

Orthogonal spin labeling and pulsed dipolar spectroscopy for protein studies
Galazzo, L.; Teucher, M.; Bordignon, E.
In Advances in Biomolecular EPR” Chapter Four, Britt, R. D. (Ed.), Collection “Methods in Enzymology” Volume 666, Academic Press 2022, p. 79-119.

Different types of spin labels are currently available for structural studies of biomolecules both in vitro and in cells using Electron Paramagnetic Resonance (EPR) and pulse dipolar spectroscopy (PDS). Each type of label has its own advantages and disadvantages, that will be addressed in this chapter. The spectroscopically distinct properties of the labels have fostered new applications of PDS aimed to simultaneously extract multiple inter-label distances on the same sample. In fact, combining different labels and choosing the optimal strategy to address their inter-label distances can increase the information content per sample, and this is pivotal to better characterize complex multi-component biomolecular systems. In this review, we provide a brief background of the spectroscopic properties of the four most common orthogonal spin labels for PDS measurements and focus on the various methods at disposal to extract homo- and hetero-label distances in proteins. We also devote a section to possible artifacts arising from channel crosstalk and provide few examples of applications in structural biology.

Neural networks in pulsed dipolar spectroscopy: A practical guide
Keeley, J.; Choudhury, T.; Galazzo, L.; Bordignon, E.; Feintuch, A.; Goldfarb, D.; Russell, H.; Taylor, M. J.; Lovett, J. E.; Eggeling, A.; Fábregas Ibáñez, L.; Keller, K.; Yulikov, M.; Jeschke, G.; Kuprov, I.
J. Magn. Reson. 2022, 338, 107186.

This is a methodological guide to the use of deep neural networks in the processing of pulsed dipolar spectroscopy (PDS) data encountered in structural biology, organic photovoltaics, photosynthesis research, and other domains featuring long-lived radical pairs and paramagnetic metal ions. PDS uses distance dependence of magnetic dipolar interactions; measuring a single well-defined distance is straightforward, but extracting distance distributions is a hard and mathematically ill-posed problem requiring careful regularisation and background fitting. Neural networks do this exceptionally well, but their “robust black box” reputation hides the complexity of their design and training – particularly when the training dataset is effectively infinite. The objective of this paper is to give insight into training against simulated databases, to discuss network architecture choices, to describe options for handling DEER (double electron-electron resonance) and RIDME (relaxation-induced dipolar modulation enhancement) experiments, and to provide a practical data processing flowchart.

Luminescence spectroscopy of CaAl12O19:Eu3+ and SrAl12O19:Eu3+ nanoparticles
Afshani, J.; Delgado, T.; Paveliuc, G.; Hagemann, H.
J. Lumin. 2022, 246, archive unige:159436 pdf full text [free access]

Development of red-emitting phosphors have gained enormous interest in recent year, due to their great potential for biological applications as well as display panels and solid-state lighting. Here, we present luminescence spectroscopy of deep-red-emitting CaAl12O19 (CA6) and SrAl12O19 (SA6) nanocrystals doped with Eu3+. CA6:Eu3+ and SA6:Eu3+ hexagonal nanoparticles (NPs) were prepared via a solvothermal reaction followed by a post-annealing process at 1150 °C. The luminescence spectra were obtained both at 298 and 5 K. Unaccustomed for Eu3+, the strongest emission was observed to originate from the 5D07F4 transition. Furthermore, emissions from the higher excited states (5D1-3) of the f-configuration of Eu3+ were also observed in these NP hosts at room temperature. In addition to the bands assigned to Eu3+ on the high symmetry D3h site, additional bands assigned to lower symmetry charge compensated sites have been identified. Crystal field (CF) parameters were obtained for the Eu3+ ion in SA6 compound, and the energy levels were estimated theoretically, which agreed well with the experimental values.

Watching Excited-State Symmetry Breaking in Multibranched Push–Pull Molecules
Vauthey, E.
J. Phys. Chem. Lett. 2022, 13, archive unige:159514 pdf full text [restricted access]

The emissive properties of symmetric molecules containing several donor–acceptor branches are often similar to those of the single-branched analogues. This is due to the at least partial localization of the excitation on one branch. Detailed understanding of this excited-state symmetry breaking (ES-SB) requires the ability to monitor this process in real time. Over the past few years, several spectroscopic approaches were shown to enable visualization of ES-SB and of its dynamics. They include the detection of new vibrational or electronic absorption bands associated with transitions that are forbidden in the symmetric excited state. Alternatively, ES-SB can be detected by observing transitions that become weaker or vanish upon localization of the excitation. Herein, we discuss these different approaches as well as their merits and weaknesses.

Elucidating the Mechanism of Bimolecular Photoinduced Electron Transfer Reactions
Vauthey, E.
J. Phys. Chem. B 2022, 126, archive unige:158624 pdf full text [restricted access]

The current developments in photoredox chemistry are stimulating a renewed interest for bimolecular photoinduced electron transfer reactions. Their investigation, initiated in the 1960s using conventional photochemical tools, resulted in a relatively simple reaction scheme. More recent studies, using not only spectroscopic techniques with better time resolution and extended spectral/temporal windows but also molecular dynamics simulations, reveal a more complex picture. This Perspective focuses on the results of these latest studies with neutral organic reactants, highlighting the time dependence of the quenching rate, the effect of mutual orientation of the reactants on the electronic coupling, and their consequence on the nature of the reaction product. Remaining questions, such as the occurrence of distant electron transfer in nonviscous liquids are also addressed, and possible directions toward their answer are proposed.

Evidence for stereoelectronic effects in ligand exchange reactions on Au25 nanoclusters
Wang, Y.; Bürgi, T.
Nanoscale 2022, 14, 2456-2464.

igand exchange reaction (LER) is an important post-synthesis strategy and has been studied widely. The mechanism of this dynamic process for gold nanoclusters proved to be associative (SN2). Many factors affect the LER of clusters, including stability, solubility, chirality, electronic properties and so on. Some of these factors are not well understood and need further exploration. Here, we use a chiral fluoro-substituted ligand (R)-5,5′,6,6′,7,7′,8,8′-octafluoro-[1,1′-binaphthalene]-2,2′-dithiol (8F-R-BINAS) to investigate the stereoelectronic and stereospecific effects during LER on achiral Au25 cluster. It is demonstrated that the fluorine-substituted BINAS significantly decreases the LER reactivity both at the molecule and the related cluster level. The stereoelectronic effect is global and can be transmitted to the cluster surface. In contrast, the stereospecific effect is marginal.

Quantifying Fluctuations of Average Solvent Environments for Embedding Calculations
González-Espinoza, C. E.; Rumble, C. A.; Borgis, D.; Wesolowski, T. A.
J. Chem. Theory Comput. 2022, 18, 1072-1088.

The viability and effectiveness of replacing an ensemble of embedded solute calculations by a single calculation using an average description of the solvent environment are evaluated. This work explores the fluctuations of the average description of the system obtained in two ways: from calculations on an ensemble of geometries and from an average environment constructed from the same ensemble. To this end, classical molecular dynamics simulations of a rigid acetone solute in SPCE water are performed in order to generate an ensemble of solvent environments. From this ensemble of solvent configurations, a number of different approaches for constructing an average solvent environment are employed. We perform a thorough numerical analysis of the fluctuations of the electrostatic potential experienced by the solute, as well as the resulting fluctuations of the solute’s electronic density, measured through its dipole moment and fitted atomic point charges. At the same time, we inspect the accuracy of the methods used to construct average environments. Finally, the proposed method for generating 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, which may be important in certain cases, the fluctuations in the shift due to the interaction with the solvent are evaluated using frozen-density-embedding theory. Our results demonstrate that, for normally distributed environments, the constructed average environment is a reasonably good representation of a fluctuating molecular solvent environment. We then provide guidance for future comparisons between these theoretical treatments of solute/solvent systems to experimental measurements.

Fundamental Loading-Curve Characteristics of the Persistent Phosphor SrAl2O4:Eu2+,Dy3+,B3+: The Effect of Temperature and Excitation Density
Delgado, T.; Gartmann, N.; Walfort, B.; LaMattina, F.; Pollnau, M.; Rosspeintner, A.; Afshani, J.; Olchowka, J.; Hagemann, H.
Adv. Photonics Res. 2022, 3, archive unige:160811 pdf full text [free access]

The compound SrAl2O4:Eu2+,Dy3+ is currently one of the best-performing persistent phosphors and numerous studies have been performed to understand the mechanisms involved in its afterglow process. One aspect which so far has received only limited attention is the dependence of loading curve characteristics on dopant concentrations. Herein, a detailed study of the loading curves of SrAl2O4:Eu2+,Dy3+ as a function of composition, sample temperature, and pump intensity is presented, completed by emission decay and quantum yield measurements. The observed emission decays can be described using the Inokuti−Hirayama equation for a Dexter energy transfer (ET) process. As the Dexter ET rate and the electron transfer rate have a similar radial dependence, the Inokuti−Hirayama equation can also describe the electron transfer process involved for the trapping process. These observations indicate that in this persistent phosphor, the trapping process is a local process; however, different types of traps appear to present different “Dexter” critical radii R0. This electron transfer is also temperature dependent, which requires further investigations.

The excited-state dynamics of the radical anions of cyanoanthracenes
Beckwith, J. S.; Aster, A.; Vauthey, E.
Phys. Chem. Chem. Phys. 2022, 24, archive unige:157670 pdf full text [free access]

The radical anion of 9,10-dicyanoanthracene (DCA) has been suggested to be a promising chromophore for photoredox chemistry, due to its nanosecond excited-state lifetime determined from indirect measurements. Here, we investigate the excited-state dynamics of the radical anion of three cyanoanthracenes, including DCA•−, produced by photoinduced electron transfer in liquid using both pump–probe and pump–pump probe transient electronic absorption spectroscopy. All three excited radical ions are characterised by a 3–5 ps lifetime, due to efficient non-radiative deactivation to the ground state. The decay pathway most probably involves D1/D0 conical intersection(s), whose presence is favoured by the enhanced flexibility of the radical anions relative to their neutral counterparts. The origin of the discrepancy with the nanosecond lifetime of DCA•−* reported previously is discussed. These very short lifetimes limit, but do not preclude, photochemical applications of the cyanoanthracene anions.

Metal nanoclusters as versatile building blocks for hierarchical structures
Bürgi, T.; Banach, E.
Helv. Chim. Acta 2022, 105, e202100186.

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.

Effect of excitation wavelength (blue vs near UV) and dopant concentrations on afterglow and fast decay of persistent phosphor SrAl2O4:Eu2+,Dy3+
Walfort, B.; Gartmann, N.; Afshani, J.; Rosspeintner, A.; Hagemann, H.
J. Rare Earths 2022, 40, 1022-1028.

The persistent phosphor SrAl2O4:Eu2+,Dy3+ is the subject of numerous investigations. One often neglected aspect is that in this phosphor, as well as in Sr4Al14O25:Eu2+,Dy3+, there are two different Sr2+ sites which can be occupied by the dopant Eu2+ ions. We first introduce a general scheme of possible energy transfers in these persistent phosphor materials including explicitly both europium ions. This scheme is used as a generic starting point to study experimentally specific pathways. We illustrate this application with the study of the effect of excitation wavelength (444 and 382 nm) on the afterglow of differently doped SrAl2O4:Eu2+,Dy3+ samples, as well as on the emission decay curves. With the same excitation intensity under 444 nm excitation, the resulting afterglow intensity is stronger than under near UV excitation. At 382 nm, Eu2+ ions on both Sr2+ sites in SrAl2O4 are excited, but at room temperature the blue emission is quenched, leading to a loss of photons. The observed effects can further be associated with the ratio of Eu2+ ions and trap states which are modulated by the concentrations of Eu2+ and Dy3+ in SrAl2O4, as well as by temperature. Increasing the nominal Dy3+ content from 0.1 mol% to 0.5 mol% with respect to Sr results in the doubling of the integrated afterglow intensity and confirms thus that Dy3+ ions are indeed involved in the trapping process. The concentration of trap states is much lower than the concentration of Eu2+ ions, as even with low excitation densities, a plateau of integrated afterglow intensity (corresponding to the total number of accessible traps) is reached. We postulate that an important fraction of excited Eu2+ ions can potentially transfer their energy to trap states. Once that all traps are filled or in a dynamical filling-depletion process under illumination (with thermal and/or optical depletion processes), for the remaining Eu2+ a “normal” steady-state emission is observed. The luminescence decay curves at 520 nm measured at 77 K show a mono-exponential decay with a common lifetime of about 1140 ns for all 5 samples under 437 nm excitation, while under 375 nm excitation, a feed process originating from the energy transfer between Eu2+ ions is demonstrated. Under 375 nm excitation, the non-exponential decay observed at 440 nm can be quantitatively associated to a Förster energy transfer process with R0 = 1.58 (8) nm. For the overall understanding of the afterglow processes, it appears that one has to consider the individual contributions of all active ions on different lattice sites.