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 photo-energy 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 excited-state lifetime of TPP-CF3/Au11 was longer than that of TPP-F/Au11. This study clarifies how groups modulate the photophysical properties of AuCs and provides insights for designing gold-cluster-based materials for photo-energy conversion applications.