Vibrational Perturbations and Ligand–Layer Coupling in a Single Crystal of Au₁₄₄(SC₂H₄Ph)₆₀ Nanocluster

Abstract

We have determined vibrational signatures and optical gap of the Au₁₄₄(PET)₆₀ (PET: phenylethylthiol, SC₂H₄Ph) nanocluster solvated in deuterated dichloromethane (DCM-D2, CD₂Cl₂) and in a single crystal. For crystals, solid-state ¹³C NMR and X-ray diffraction were also measured. A revised value of 2200 cm^–1 (0.27 eV) was obtained for the optical gap in both phases. The vibrational spectra of solvated AU₁₄₄(PET)₆₀ closely resembles that of neat PET, while the crystalline-state spectrum exhibits significant inhomogeneous spectral broadening, frequency shifts, intensity transfer between vibrational modes, and an increase in the overtone and combination transition intensities. Spectral broadening was also observed in the ¹³C NMR spectrum. Changes in the intensity are explained due to vibrational coupling of the normal modes induced by the crystal packing, and the vibrational broadening is caused by ligand-environment inhomogeneity in the crystal. This indicates a pseudocrystalline state where the cluster cores are arranged in periodic fashion, while the ligand-layer molecules between the cores form amorphous structures