Size-Dependent Excited State Behavior of Glutathione-Capped Gold Clusters and Their Light-Harvesting Capacity

Glutathione-protected gold clusters exhibit size-dependent excited state and electron transfer properties. Larger-size clusters (e.g., Au(25)GSH(18)) with core-metal atoms display rapid (<1 ps) as well as slower relaxation (similar to 200 ns) while homoleptic clusters (e.g., Au(10-12)GSH(10-12)) exhibit only slower relaxation. These decay components have been identified as metal metal transition and ligand-to-metal charge transfer, respectively. The short lifetime relaxation component becomes less dominant as the size of the gold cluster decreases. The long-lived excited state and ability to participate in electron transfer are integral for these clusters to serve as light-harvesting antennae. A strong correlation between the ligand-to-metal charge-transfer excited state lifetime and photocatalytic activity was evidenced from the electron transfer to methyl viologen. The photoactivity of these metal clusters shows increasing photocatalytic reduction yield (0.05-0.14) with decreasing cluster size, Au-25 < Au-18 < Au-15 < Au10-12. Gold clusters, Au(18)GSH(14), were found to have the highest potential as a photosensitizer on the basis of the quantum yield of electron transfer and good visible light absorption properties.