Near-Infrared Dual-Emission of a Thiolate-Protected Au₄₂ Nanocluster: Excited States, Nonradiative Rates, and Mechanism

Both DFT and TD-DFT methods are used to elaborate on the excited-state properties and dual-emission mechanism of a thiolate-protected Au42 nanocluster. A three-state model (S-0, S-1, and T-1) is proposed with respect to the results. The intersystem crossing (ISC) process from S-1 to T-1 benefits from a small reorganization energy due to the similar geometric structures of S-1 and T-1. However, the ISC process is suppressed by relatively small spin-orbit coupling resulting from the similarity of the electronic structures of S-1 and T-1. As a result of the counterbalance, the ISC rate is comparable with the fluorescence emission rate. In the T-1 state, the phosphorescence emission prevails the reverse ISC process back to the S-1 state. Taken together, fluorescence and phosphorescence are achieved simultaneously. The present work provides deep mechanistic insights to aid the rational design of NIR dual-emissive metal nanoclusters.