Regulating Linear and Nonlinear Optical Properties of Copper Nanoclusters by Capping Ligands: A Case Study of Cysteine Derivatives

Metal nanoclusters (MNCs) have recently attracted attention for their unique linear and nonlinear optical properties and potential applications in optoelectronics, photovoltaics, sensing, bioimaging, and catalysis. Copper nanoclusters (NCs) are difficult to stabilize due to their high reactivity. However, their low toxicity, availability, and cost render them appealing. Capping ligands not only help to stabilize Cu NCs but also regulate their optical properties. Three derivatives of cysteine ( l -cysteine, Seleno- l -cysteine, and N-acetyl- l -cysteine) have been used here as capping ligands for Cu NCs, each with different electron-donating abilities, which affect the photoluminescence spectrum (from blue to cyan) and photoluminescence quantum yield (PLQY). The trend shows an increase in PLQY from N-acetyl- l -cysteine to Seleno- l -cysteine to l -cysteine, reaching a maximum PLQY of 11.1% with Cu@ l -cysteine. The presence of free amino and carboxylate groups in cysteine allows interaction with the Cu atoms, enhancing emission properties. A strong H-bond network helps protect against solvent exposure, favoring the ligands’ effectiveness in improving nonlinear optical responses, particularly in two-photon excited fluorescence (with Cu@ l -cysteine reaching a two-photon excited fluorescence cross-section of 0.05 ± 0.03 GM). Similar trends are observed for the first hyperpolarizability β values (with Cu@ l -cysteine reaching a β value of [63 ± 5] × 10 –30 esu), where the charge transfer character of excitations in Cu NCs is the driving factor. This report provides insights into the structures and tunable nonlinear optical properties of Cu NCs, paving the way for the design of promising applications.