Influence of ligands on the optical properties of rod-shaped Au₂₅ nanoclusters

In this study, we successfully synthesized rod-shaped [Au-25(PPh3)(10)(S-Adm)(5)Cl-2](2+) nanoclusters using kinetic controls. The complete molecular structure was determined by single-crystal X-ray crystallography and electrospray ionization mass spectrometry. In comparison with the previously reported [Au-25(PPh3)(10)(PET)(5)Cl-2](2+) clusters, both nanoclusters have an icosahedral composition of Au-13 linked by Au atoms that share a vertex, but [Au-25(PPh3)(10)(S-Adm)(5)Cl-2](2+) clusters appear elongated due to the rigidity of adamantane. We conducted ultraviolet-visible spectrophotometry (UV-vis) measurements of [Au-25(PPh3)(10)(PET)(5)Cl-2](2+) and [Au-25(PPh3)(10)(S-Adm)(5)Cl-2](2+) in dichloromethane solvent to elucidate the modulation of the cluster properties of different ligands. The lowest energy absorption peak of [Au-25(PPh3)(10)(S-Adm)(5)Cl-2](2+) shifted to lower energies compared to the [Au-25(PPh3)(10)(PET)(5)Cl-2](2+) clusters in UV-vis measurements. Temperature-dependent absorption measurements revealed that [Au-25(PPh3)(10)(S-Adm)(5)Cl-2](2+) clusters were less affected by temperature compared to [Au-25(PPh3)(10)(PET)(5)Cl-2](2+). This result is attributed to the exciton phonon coupling of [Au-25(PPh3)(10)(S-Adm)(5)Cl-2](2+) clusters being weaker than [Au-25(PPh3)(10)(PET)(5)Cl-2](2+) clusters. Furthermore, the absorption spectra of [Au-25(PPh3)(10)(PET)(5)Cl-2](2+) and [Au-25(PPh3)(10)(S-Adm)(5)Cl-2](2+) clusters were measured using different types of solutions, and it was found that the lowest energy absorption peaks of [Au-25(PPh3)(10)(S-Adm)(5)Cl-2](2+) were shifted and affected by the solution at room temperature, which suggested that the [Au-25(PPh3)(10)(S-Adm)(5)Cl-2](2+) clusters with solution hydrogen bonds also interacted strongly at room temperature. Theoretical calculations show that changes in ligands affect the differences in the molecular orbitals and structures of the clusters, which cause changes in the optical properties. The ligand can have a significant influence on the optical absorption spectra of rod-shaped Au25 nanoclusters.