Surface Engineering of Bimetallic Nanoclusters: Ligand–Solvent Interplay for Enhanced Charge Transfer and Optical Modulation

Bimetallic nanoclusters (BMNCs) exhibit unique quantum confinement effects and synergistic intermetallic interactions, providing enhanced electronic and optical properties over those of monometallic nanoclusters (MNCs). Their dual-metal composition offers superior tunability for applications in optoelectronics, catalysis, and biomedicine. Among BMNCs, Ag₃Cu₂ was selected for its distinct dual-emission behavior and well-characterized ligand-metal interactions, making it an ideal model for investigating solvent- and ligand-induced modifications. This study employs density functional theory (DFT) to investigate the electronic structures, adsorption behavior, and optical properties of Ag₃Cu₂ modified by triphenylphosphine (PPh₃), glutathione (GSH), and phenylacetylene under vacuum and solvent conditions (dichloromethane (DCM) and methanol). Ground- and excited-state geometries were optimized, and key parameters such as density of states (DOS), highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps, natural transition orbitals, and charge-transfer processes were analyzed. The results demonstrate that ligand adsorption induces significant shifts in DOS peaks and modulates the HOMO-LUMO gap, with solvent polarity further amplifying these effects. Distinct ligand-specific behaviors were observed, including enhanced metal-to-ligand charge transfer (MLCT) for GSH and PPh₃ and minimal solvent sensitivity for phenylacetylene. Furthermore, UV-vis absorption spectra revealed ligand-dependent redshifts and intensity variations, providing insight into optical tunability. This work emphasizes the critical role of ligand chemistry and solvation in tuning the electronic and optical properties of BMNCs. The findings highlight Ag₃Cu₂ as a valuable model system, taking advantage of its unique dual-emission property to bridge theoretical insights with practical design strategies. These findings provide insights into the design strategies for bimetallic nanoclusters with potential implications for applications in optoelectronics, catalysis, and nanomedicine.