Influence of ligands on the optical properties of rod-shaped Au ₂₅ nanoclusters

In this study, we successfully synthesized rod-shaped [Au₂₅(PPh₃)₁₀(S-Adm)₅Cl₂]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 [Au₂₅(PPh₃)₁₀(PET)₅Cl₂]²⁺ clusters, both nanoclusters have an icosahedral composition of Au₁₃ linked by Au atoms that share a vertex, but [Au₂₅(PPh₃)₁₀(S-Adm)₅Cl₂]²⁺ clusters appear elongated due to the rigidity of adamantane. We conducted ultraviolet-visible spectrophotometry (UV-vis) measurements of [Au₂₅(PPh₃)₁₀(PET)₅Cl₂]²⁺ and [Au₂₅(PPh₃)₁₀(S-Adm)₅Cl₂]²⁺ in dichloromethane solvent to elucidate the modulation of the cluster properties of different ligands. The lowest energy absorption peak of [Au₂₅(PPh₃)₁₀(S-Adm)₅Cl₂]²⁺ shifted to lower energies compared to the [Au₂₅(PPh₃)₁₀(PET)₅Cl₂]²⁺ clusters in UV-vis measurements. Temperature-dependent absorption measurements revealed that [Au₂₅(PPh₃)₁₀(S-Adm)₅Cl₂]²⁺ clusters were less affected by temperature compared to [Au₂₅(PPh₃)₁₀(PET)₅Cl₂]²⁺. This result is attributed to the exciton phonon coupling of [Au₂₅(PPh₃)₁₀(S-Adm)₅Cl₂]²⁺ clusters being weaker than [Au₂₅(PPh₃)₁₀(PET)₅Cl₂]²⁺ clusters. Furthermore, the absorption spectra of [Au₂₅(PPh₃)₁₀(PET)₅Cl₂]²⁺ and [Au₂₅(PPh₃)₁₀(S-Adm)₅Cl₂]²⁺ clusters were measured using different types of solutions, and it was found that the lowest energy absorption peaks of [Au₂₅(PPh₃)₁₀(S-Adm)₅Cl₂]²⁺ were shifted and affected by the solution at room temperature, which suggested that the [Au₂₅(PPh₃)₁₀(S-Adm)₅Cl₂]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.