Origin of Discrete Optical Absorption Spectra of M₂₅(SH)₁₈⁻ Nanoparticles (M = Au, Ag)

Time-dependent density functional theory calculations at the SAOP/TZP level of theory are employed to study the optical absorption spectra of pure metal nanoparticles (Au 25 (SH) 18 − and Ag 25 (SH) 18 − ) and mixed metal “core-shell” systems (Au 13 Ag 12 (SH) 18 − and Ag 13 Au 12 (SH) 18 − ). All four systems exhibit discrete absorption spectra. The splitting between the LUMO and LUMO+1 and between the HOMO and HOMO−1 varies markedly with the metal used in the core and in the oligomeric thiolate ligands. The optical absorption spectra are not separable into core and ligand contributions; geometric and electronic interactions between the two fragments are responsible for complex absorption spectra.