Boosting Linear and Nonlinear Optical Properties of Pt ₁ Ag ₁₈ Nanoclusters by Manipulating Ligand‐Shell Rigidity

ABSTRACT 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 Pt 1 Ag 18 nanoclusters through controlled tuning of ligand‐shell rigidity and electron–core interactions. we prepared a family of [Pt 1 Ag 18 (DPPP) 6 (SR) x Cl 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.