Molecule-like Photodynamics of Au102(pMBA)44 Nano cluster

Photophysical properties of a water-soluble cluster Au-102(pMBA)(44) (pMBA = para-mercaptobenzoic acid) are studied by ultrafast time-resolved mid-IR spectroscopy and density functional theory calculations in order to distinguish between molecular and metallic behavior. In the mid-IR transient absorption studies, visible or near-infrared light is used to electronically excite the sample, and the subsequent relaxation is monitored by studying the transient absorption of a vibrational mode in the ligands. Based on these studies, a complete picture of energy relaxation dynamics-is obtained: 11) 0.5-1.5 ps electronic relaxation, (2) 6.8 ps vibrational cooling,13) intersystem crossing from the lowest triplet state-to the-ground state with a time constant 84 ps, and (4) internal conversion to-the ground state with a time constant Of similar to 3.5 ns. A remarkable finding based on this work is that a large cluster containing 102 metal atoms behaves like a small molecule in a striking contrast. to a previously studied slightly larger Au-144(SC2H4Ph)(60) cluster, which shows relaxation typical for metallic particles. These results therefore establish that the transition between molecular and metallic behavior occurs between Au-102 and Au-144 species.