The thermal behaviors of ligand-protected metal clusters, [Au9(PPh3)8]3+ and [MAu8(PPh3)8]2+ (M = Pd, Pt) with a crown-motif structure, were investigated to determine the effects of the gas composition, single-atom doping, and counter anions on the thermal stability of these clusters. We successfully synthesized crown-motif [PdAu8(PPh3)8][HPMo12O40] (PdAu8–PMo12) and [PtAu8(PPh3)8][HPMo12O40] (PtAu8–PMo12) salts with a cesium-chloride-type structure, which is the same as the [Au9(PPh3)8][PMo12O40] (Au9–PMo12) structure. Thermogravimetry-differential thermal analysis/mass spectrometry analysis revealed that the crown-motif structure of Au9–PMo12 was decomposed at ∼475 K without weight loss to form Au nanoparticles. After structural decomposition, the ligands were desorbed from the sample. The ligand desorption temperature of Au9–PMo12 increased under 20% O2 conditions because of the formation of Au nanoparticles and stronger interaction of the formed O=PPh3 than PPh3. The Pd and Pt single-atom doping improved the thermal stability of the clusters. This improvement was due to the formation of a large bonding index of M–Au and a change in Au–PPh3 bonding energy by heteroatom doping. Moreover, we found that the ligand desorption temperatures were also affected by the type of counter anions, whose charge and size influence the localized Coulomb interaction and cluster packing between the cationic ligand-protected metal clusters and counter anions.

Gold (Au) clusters with well-defined geometrical structures have attracted substantial attention due to their unique optical and catalytic properties, which are drastically changed by the ligands, compositions, and geometric structures. Here, we investigated the effect of ligand on the electronic state of Au in [Au 9 (PPh 3 ) 8 ] 3+ (Au9) and [Au 25 (SC 2 H 4 Ph) 18 ] − (Au25) by X-ray absorption spectroscopy using high energy resolution fluorescence detection (HERFD) and theoretical calculations. Au L 3 -edge X-ray absorption near-edge structure (XANES) spectra revealed that the white-line intensity of Au9 was comparable to that of Au25, while the white-line peak of Au25 was 3 eV lower than that of Au9. The total area of the white line of Au9 corresponded to that of Au25, which is explained by the natural bond orbital analysis, showing that the occupancy of Au 5d orbitals of Au9 was close to that of Au25. The simulated XANES spectra using finite difference method near-edge structure software resembled the experimental XANES spectra. The projected density of state profiles and molecular orbitals indicated that the unoccupied 5d orbitals of the surface Au in Au9 and of surface and oligomer Au in Au25 interacted with P/S 3s+3p orbitals. The difference in peak locations in Au L 3 -edge XANES between phosphine-and thiolate-protected gold clusters was ascribed to the energy shift of unoccupied Au 5d orbitals, which are modulated by the Au 5d and P/S 3s+3p interaction.

Many plants, including fruits and vegetables, release biogenic gases containing various volatile organic compounds such as ethylene (C2H4), which is a gaseous phytohormone. Non-destructive and in-situ gas sampling technology to detect trace C2H4 released from plants in real time would be attractive for visualising the ageing, ripening, and defence reactions of plants. In this study, we developed a C2H4 detection system with a detection limit of 0.8 ppb (3σ) using laser absorption spectroscopy. The C2H4 detection system consists of a mid-infrared quantum cascade laser oscillated at 10.5 µm, a multi-pass gas cell, a mid-IR photodetector, and a gas sampling system. Using non-destructive and in-situ gas sampling, while maintaining the internal pressure of the multi-pass gas cell at low pressure, the change in trace C2H4 concentration released from apples (Malus domestica Borkh.) can be observed in real time. We succeeded in observing C2H4 concentration changes with a time resolution of 1 s, while changing the atmospheric gas and surface temperature of apples from the ‘Fuji’ cultivar. This technique allows the visualisation of detailed C2H4 dynamics in plant environmental response, which may be promising for further progress in plant physiology, agriculture, and food science.

Controlling the geometric structures of metal clusters through structural isomerization allows for tuning of their electronic state. In this study, we successfully synthesized butterfly-motif [PdAu8(PPh3)8]2+ (PdAu8-B, B means butterfly-motif) and [PtAu8(PPh3)8]2+ (PtAu8-B) by the structural isomerization from crown-motif [PdAu8(PPh3)8]2+ (PdAu8-C, C means crown-motif) and [PtAu8(PPh3)8]2+ (PtAu8-C), induced by association with anionic polyoxometalate, [Mo6O19]2– (Mo6) respectively, whereas their structural isomerization was suppressed by the use of [NO3]– and [PMo12O40]3– as counter anions. DR-UV-vis-NIR and XAFS analyses and density functional theory calculations revealed that the synthesized [PdAu8(PPh3)8][Mo6O19] (PdAu8-Mo6) and [PtAu8(PPh3)8][Mo6O19] (PtAu8-Mo6) had PdAu8-B and PtAu8-B respectively because PdAu8-Mo6 and PtAu8-Mo6 had bands in optical absorption at the longer wavelength region and different structural parameters characteristic of the butterfly-motif structure obtained by XAFS analysis. Single-crystal and powder X-ray diffraction analyses revealed that PdAu8-B and PtAu8-B were surrounded by six Mo6 with rock salt-type packing, which stabilizes the semi-stable butterfly-motif structure to overcome high activation energy for structural isomerization.