Elucidating the optical spectra of [Au₂₅(SR)₁₈]^q nanoclusters

Ligand-protected Au nanoclusters have attracted tremendous interest due to their atomically precise structural determination and controllable optical properties. However, the origin of their optical features is not well understood. Herein, we address the effects of charge state and type of ligands on the structural, electronic, and optical properties of the Au₂₅(SR)₁₈ nanocluster, using first principles calculations. In particular, we analyze in detail the optical absorption spectra of phenylethanethiol-capped [Au₂₅(SCH₂CH₂Ph)₁₈]^q nanocluster (q = -1, 0, and 1) and demonstrate that the nanocluster undergoes geometric changes when increasing its charge state, yielding the development of additional photoabsorption features. Moreover, by comparing the properties of the anionic [Au₂₅(SR)₁₈]^1- nanoclusters (with SR = -SCH₂CH₂Ph, -SCH₃ and -SH) we demonstrate that accounting for full (-SCH₂CH₂Ph) compared to simplified (-SCH₃ and -SH) ligands leads to significant modifications in the shape of the main absorption features, such as the emergence of step-like absorption shoulders. In addition, we show that the energy of the main absorption peaks of the [Au₂₅(SR)₁₈]^1- significantly blue-shifts when using -SH as model ligands. Overall, our computational work aids the experimental characterization of optical properties of ligand-protected nanoclusters and highlights the importance of accounting for full ligands in optical spectra calculations.