Photophysical properties of a water-soluble cluster Au102(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: (1) 0.5-1.5 ps electronic relaxation, (2) 6.8 ps vibrational cooling, (3) 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 ∼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 Au144(SC2H4Ph)60 cluster, which shows relaxation typical for metallic particles. These results therefore establish that the transition between molecular and metallic behavior occurs between Au102 and Au144 species.
Keywords: electronic relaxation; femtosecond; gold nanocluster; transient absorption; vibrational spectroscopy.