Photoluminescence Properties of [Core+exo]-Type Au6 Clusters: Insights into the Effect of Ligand Environments on the Excitation Dynamics

The use of multidentate chelating ligands enriches the geometric diversity of coordinated gold clusters, offering much knowledge to benefit the understanding of their unique electronic structures and optical properties. Herein we report different behaviors of [core+exo]-type Au6 clusters bearing C3- and C4-bridged diphosphines (1 and 2), highlighting profound effects of the steric constraints of the ligand environments on the excited-state structural dynamics. Although the Au6 geometries of 1 and 2 in the crystalline states were somewhat different, their absorption spectra in solution were almost similar. However, marked differences were found in the photoluminescence properties; phosphorescent-type emission was dominantly observed for 1, whereas 2 gave both fluorescence- and phosphorescence-type emissions. Theoretical calculations showed that the bridging chains influence the geometries of the Au6 unit in the excited states, leading to the observed differences in emission behaviors.