Boosting Linear and Nonlinear Optical Properties of Pt₁Ag₁₈ Nanoclusters by Manipulating Ligand-Shell Rigidity

Ligand-protected metal nanoclusters exhibit novel molecular-like optical properties, yet how surface ligands regulate nonlinear optical behavior remains insufficiently understood. Here, we demonstrate a ligand-engineering strategy to modulate both linear and nonlinear optical responses of Pt₁Ag₁₈ nanoclusters through controlled tuning of ligand-shell rigidity and electron-core interactions. we prepared a family of [Pt₁Ag₁₈(DPPP)₆(SR)_xCl_8-x]²⁺ nanoclusters (SR = 1-adamantanethiol or 2-fluorothiophenol; x = 0-8), enabling progressive substitution of flexible 3D cage 1-adamantanethiol ligands with planar electron-donating 2-fluorothiophenol ligands. Increasing fluorinated thiolate content strengthens ligand-metal coupling and rigidifies the ligand environment, leading to suppressed nonradiative decay and markedly enhanced one-photon absorption, photoluminescence quantum yield, and two-photon absorption/two-photon excited photoluminescence cross-sections. Nonlinear optical measurements (700-1000 nm femtosecond excitation) reveal that resonance effects and ligand rigidity jointly govern multiphoton excitation efficiency. Furthermore, introducing bulky counterions induces additional rigidification, achieving substantial amplification of both one- and two-photon luminescence. Supported by TD-DFT calculations, this work establishes a structure-property relationship linking ligand geometry and charge-transfer character to nonlinear optical performance. These findings outline a generalizable ligand-shell design strategy for tuning optical responses in atomically precise metal nanoclusters and offer promising candidates for multiphoton bioimaging and photonic applications.