Ligand Effects on Optical Properties of Small Gold Clusters: A TDDFT Study

Ligand influence on the excited state structure of small neutral gold clusters (Au₂ and Au₄) has been investigated using Time Dependent Density Functional Theory. We study in detail the absorption profile of bare and ligated small gold clusters in solution modeled with Polarizable Continuum Model. Performance of CAM-B3LYP and TPSS DFT functionals combined with TZVP basis set has been assessed. We found that ligands substantially modify the excited state structure of clusters by eliminating low-lying optically inactive excited states. Depending on the ligand environment, the cluster may gain significant fluorescence efficiency. Our results suggest that small gold clusters ligated with amines will have better fluorescence potential compared to those ligated with phosphine or thiol ligands, in agreement with preliminary experimental data. TPSS fails to describe excited state structure of ligated clusters due to spurious charge-transfer states, thus highlighting the necessity of choosing appropriate quantum-chemistry model for correct excited state description.