Molecule-like Photodynamics of Au₁₀₂(<i>p</i>MBA)₄₄ Nanocluster

Photophysical properties of a water-soluble cluster Au₁₀₂(<i>p</i>MBA)₄₄ (<i>p</i>MBA = <i>para</i>-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