Ligand Effects on the Ultrafast Excited-State Dynamics of Triphenylphosphine-Derivative-Protected Au11 Clusters

Organic ligand-protected gold clusters (AuCs) consist of a metal core surrounded by organic ligands and exhibit unique properties that depend on their chemical compositions. The physical properties of AuCs, important for applications in photoenergy conversion, are strongly influenced by organic ligands because of their high surface-to-volume atomic ratio compared with gold nanoparticles. However, the influence of ligands on ultrafast carrier dynamics remains unclear. In this study, we synthesized triphenylphosphine (TPP)-derivatized Au11 clusters with different substituent groups and analyzed their excited-state dynamics using femtosecond transient absorption spectroscopy and large-scale hybrid density functional theory calculations without simplifying the AuC structures. Although the Au11 core structure remained nearly identical, the excited-state dynamics varied. The excited-state lifetime of TPP-H/Au11 was longer than that of TPP-F/Au11, attributed to the reduced orbital overlap between the highest occupied and lowest unoccupied molecular orbitals. The trifluoromethyl group exhibits a stronger electron-withdrawing effect than the fluorine group. Because the groups on the ligands influence the gold core through a resonance effect rather than inductive effects, the S1 state lifetime of TPP-CF3/Au11 was longer than that of TPP-F/Au11. This study clarifies how the presence of different groups modulates the photophysical properties of AuCs and provides insights into the design of gold-cluster-based materials for photoenergy conversion applications.