Tailoring long-lived third-order nonlinear optical responses in metal oxide nanoclusters by photoactive ligand

Designing nonlinear optical (NLO) materials that integrate high stability with broad tunability is essential for photonic technologies such as laser protection and dynamic light control. Herein, we utilize photoactive ligand coumarin to deliver photogenerated electrons and stabilize the reduced state in the vanadium-based nanocluster (C3), significantly enhancing its nonlinear refraction and prolonging the response stability. Density functional theory reveals that coumarin narrows the band gap (from 0.43 to 0.40 eV), promotes charge transfer, and stabilizes reduced vanadium centers (the average Mulliken charge decreases from 1.127 to 1.112), enabling C3 to retain more than 85% of original NLO efficiency after 24 h. Moreover, C3 exhibits self-initiated polymerization behavior, forming solid-state films that sustain strong nonlinearity for hours and allow multiple UV reactivation cycles. Therefore, this work provides a general design strategy for achieving highly tunable and robust NLO materials by leveraging photoactive organic ligands to transfer electrons and stabilize reduced electronic states in metal oxide nanoclusters.