Enhancing multiphoton absorption in atomically precise (AuAg) 13 clusters via 2-/4-mercaptopyridine ligand positional isomerism

Abstract

A Ligand isomerism can effectively modulate the properties of clusters. Herein, a series of alloy (AuAg)13 nanoclusters (NCs) capped by 2-or 4-mercaptopyridine were synthesized and well determined. Results showed that all four NCs exhibit pronounced multiphoton absorption under near-infrared (NIR) excitation, and 4-mercaptopyridine coordinated clusters display significantly enhanced multiphoton absorption coefficients and cross-sections compared with their 2mercaptopyridine coordinated analogues. Complementary density functional theory and time-dependent density functional theory calculations show that 4-mercaptopyridine ligation shifts the relative positions and densities of excited-state energy levels, alters the states contributing to low-energy absorption, and enhances ligand-to-metal charge transfer (LMCT), thereby accounting for the experimentally observed increase in nonlinear optical response. This work provides an intriguing ligand-engineering strategy and cluster platform for stepwise control of the nonlinear optical behaviour of (AuAg)13 alloy nanoclusters, and sheds light on the precise correlation between ligand structure and nonlinearity at the atomic level.