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., Au25GSH18) with core-metal atoms display rapid (<1 ps) as well as slower relaxation (~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, Au25 < Au18 < Au15 < Au(10-12). Gold clusters, Au18GSH14, 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.