Thomas Bürgi

We present a high-pressure single-crystal X-ray diffraction study of the atomically precise Au25(PET)18q cluster (q = −1, 0) up to 10 GPa under strictly hydrostatic conditions. Our crystallographic analysis provides direct evidence of the pressure-induced phase transitions previously suggested by spectroscopic studies. Structural refinements reveal that the cluster accommodates compression through the reorganization of the flexible ligand shell and secondary distortions of the staple motifs, while the Au13 icosahedral core remains intact. Notably, the internal Au–Au distances exhibit a monotonic contraction that quantitatively mirrors the compressibility of bulk gold. This invariant rigidity at the sub-nanometer scale demonstrates that the fundamental stiffness of the metallic bond is preserved regardless of size. Our findings reconcile previous contradictions in the elasticity of metal nanostructures by isolating the intrinsic mechanical response of the Au kernel from extrinsic structural and experimental artifacts.

Designing two‐dimensional (2D) materials from atomically precise nanocluster building blocks is a challenging task, yet crucial for next‐generation electronic devices. By tuning cluster nuclearity and ligand environment, well‐defined assemblies can be designed, enabling control over optoelectronic properties at the atomic level. Herein, we report the bottom‐up synthesis of two structurally distinct 2D nanocluster‐assembled materials constructed from Ag ₁₂ (2D Ag ₁₂ ‐CAM) and Ag ₁₁ (2D Ag ₁₁ ‐CAM) nanocluster building blocks. The change in cluster nuclearity arises solely from ligand tuning; benzenethiol yields the Ag ₁₂ ‐based framework, whereas 3,5‐bis(trifluoromethyl)benzenethiol (BTFMBT) directs formation of the Ag ₁₁ analogue. Lateral memristive devices fabricated from these materials (D‐1: 2D Ag ₁₂ ‐CAM; D‐2: 2D Ag ₁₁ ‐CAM) exhibit coexistence of negative differential resistance (NDR) and robust resistive switching with endurance stable over 4000 switching cycles. Device D‐2, based on 2D Ag ₁₁ ‐CAM, shows a markedly enhanced on‐off ( I _on / I _off ) ratio (∼ 642) compared to device D‐1 (∼ 200), and more pronounced NDR. Furthermore, the 2D Ag ₁₁ ‐CAM (D‐2)‐based device was evaluated for neuromorphic functionality, mimicking the adaptive behavior of biological synapses. It exhibits analogue potentiation and depression under pulse stimulation, while experimentally extracted synaptic weight updates enable an artificial neural network to achieve a test accuracy of 84.2%, approaching that of a machine learning (ML)‐trained network (93.2%).

Monolayer-protected metal clusters in the size range between about 10 to 300 metal atoms are a special class of materials with strongly size-dependent behaviour, which is mainly ascribed to discrete electronic energy levels. It was observed that certain sizes of metal clusters show exceptional stability, whereas others are only formed under special conditions, as minor species or not at all. The field has made tremendous progress in the last two decades both in terms of the discovery of new clusters and the understanding of their properties. The focus of this article is on the chirality of these nano-objects and the use of chiroptical spectroscopy to probe their properties.

Stereochemistry controls oligourethane folding through backbone torsions, yielding helices and extended zig-zag structures formed by isotactic and syndiotactic motifs, respectively.

We report striking photophysical behaviour of Au25(PET)18 (PET = 2-phenylethanethiol) nanoclusters in both anionic and neutral forms. Using broad spectral coverage and tuneable excitation, steady-state, and time-resolved measurements reveal low-energy absorption and emission features, weak photoluminescence, and multiexponential decay dynamics. Notably, the neutral species shows excitation-wavelength-dependent emissions and a pronounced mismatch between excitation and absorption spectra, violating Kasha’s and Vavilov’s rules. These results point towards complex excited-state behaviour of these systems and the crucial role of instrumentation design in their accurate characterisation.

Chirality in atomically precise gold nanoclusters emerges from the interplay among the metal core, the metal–ligand interface, and the ligand shell, yet how these hierarchical elements influence ligand conformation remains unclear. This work investigates chirality transfer from the nanocluster framework to the ligand sphere by using vibrational circular dichroism (VCD) spectroscopy combined with density functional theory and SCC-DFTB calculations. Three enantiopure thiolate-protected gold nanoclusters─Au₂₅(2-MeBuS)₁₈, Au₃₈(2-MeBuS)₂₄, and Au₁₄₄(2-MeBuS)₆₀─were selected as model systems, representing one, two, and three hierarchical levels of chirality, respectively. VCD analysis reveals a progressive restriction and reorganization of the ligand conformations with increasing structural chirality. While the intrinsically achiral Au₂₅ induces only modest spectral changes, the chiral arrangement of the staple units in the Au₃₈ induces significant changes in both IR and VCD spectra. In Au₁₄₄, the additional chiral gold core leads to a strong amplification of the VCD response. Computational analysis shows that ligand conformations are governed by cluster geometry, with binding favoring the most stable free ligand conformer. These results provide direct insight into chirality transfer mechanisms in gold nanoclusters, leading to a better understanding of the design of chiral nanomaterials.

2,5-Diketopiperazines (DKPs) are the smallest cyclic peptides whose six-membered ring can pucker and thus generate conformational chirality in symmetric scaffolds. Here, we investigate how non-covalent interactions (NCIs) govern the emergence of chiral vs achiral conformations and the associated racemization pathways in gas-phase DKPs with identical residues. We combine conformational sampling with geometry optimizations at the density functional theory level, NCI analysis, and simulated spectroscopic features, such as infrared and vibrational circular dichroism (VCD), to determine structure–property relationships in aliphatic and aromatic derivatives. Chirality is quantified using a mirror-optimal root-mean-square deviation (moRMSD), which compares a structure to its mirror image. moRMSD is built on a graph-theoretical atom mapping that identifies each atom by its connectivity and local environment, providing a continuous, chemically informed measure of conformational chirality. When the residues have opposite absolute configuration, our results show that strong intramolecular hydrogen bonds systematically stabilize achiral minima. In contrast, weaker NCIs, such as CH⋯π and NH⋯π contacts, together with steric effects from bulky side chains, promote chiral puckered geometries. Racemization barriers increase with side-chain size, and transition states connecting enantiomers are characterized by (near) planar, centrosymmetric DKP rings. Complementary vibrational chiroptical spectroscopy, specifically VCD, provides insight into conformational preferences: for the most stable geometries in each system, the sign of the VCD νCO couplet tracks the puckering direction, offering a robust spectroscopic handle on ring chirality. These results clarify how specific non-covalent motifs encode puckering-induced conformational chirality in DKP scaffolds.

2,5-Diketopiperazines (DKPs) are the smallest cyclic peptides whose six-membered ring can pucker and thus generate conformational chirality in symmetric scaffolds. Here, we investigate how non-covalent interactions (NCIs) govern the emergence of chiral versus achiral conformations and the associated racemization pathways in gas-phase DKPs with identical residues. We combine conformational sampling with geometry optimizations at the density functional theory (DFT) level, NCI analysis, and simulated spectroscopic features, such as infrared (IR) and vibrational circular dichroism (VCD), to determine structure–property relationships in aliphatic and aromatic derivatives. Chirality is quantified using a mirror-optimal root-mean-square deviation (moRMSD), which compares a structure to its mirror image. moRMSD is built on a graph-theoretical atom mapping that identifies each atom by its connectivity and local environment, providing a continuous, chemically informed measure of conformational chirality. When the residues have opposite absolute configuration, our results show that strong intramolecular hydrogen bonds (HBs) systematically stabilize achiral minima. In contrast, weaker NCIs, such as CH...pi and NH...pi contacts together with steric effects from bulky side chains, promote chiral puckered geometries. Racemization barriers increase with side-chain size, and transition states connecting enantiomers are characterized by (near) planar, centrosymmetric DKP rings. Complementary vibrational chiroptical spectroscopy, specifically VCD, provides insight into conformational preferences: for the most stable geometries in each system, the sign of the VCD vCO couplet tracks the puckering direction, offering a robust spectroscopic handle on ring chirality. These results clarify how specific non-covalent motifs encode puckering-induced conformational chirality in DKP scaffolds.

Atomically precise nanoclusters find multiple applications in, for example, bioimaging or as catalysts due to their remarkable molecule-like properties which are tightly bound to their strictly defined structures. However, their physicochemical characterization and broad utilization in the aforementioned areas are often challenging due to the limited solubility. Herein, we report the synthesis of Au₂₅ nanoclusters (NCs) capped with 2-mercaptomethyl-12-crown-4 ether, imparting amphiphilic properties that confer solubility in both polar and nonpolar solvents. UV-vis spectroscopy confirmed the stability of the Au₂₅ framework in polar protic and apolar aprotic solvents. Moreover, FTIR analysis suggests that our supramolecular ligand responds to solvent polarity and protic/aprotic conditions, adjusting its conformation on the cluster surface. Furthermore, accompanying variations in photoluminescence underscore their potential utility as environmentally responsive polarity-sensitive probes. These findings establish a versatile Au₂₅ nanocluster platform for investigating structure-property relationships in diverse chemical environments without requiring additional chemical modifications to the cluster structure.

Chiral tetra-heterosubstituted methanes (i.e., tetraoxa and azatrioxa carbon spiro stereocenters) are synthesized under CpRu catalysis, using cyclic carbonates and carbamates as substrates and α-diazo-β-ketoesters as reagents. Single enantiomers, isolated by chiral stationary phase chromatography, display chiroptical properties, from g_abs ∼10^-5 to ∼10^-4, which, together with TD-DFT calculations, provide robust absolute configuration assignments. Crystalline spiro diastereomers were also obtained, confirming further the structural and configurational assignments. Using enantioselective dynamic chromatography, remarkable enantiomerization barriers were determined for the ortho-carbonates and ortho-carbamates, with values of up to 27.6 and 34.6 kcal/mol (half-lives 227 days and >84,000 years at 25 °C, respectively). DFT further elucidates the origin of this large difference pointing toward preferred C-O or C-N bond cleavages in the rate-determining step of the S_N1-like mechanism.