QM/MM study on thermally activated delayed fluorescence mechanism of a three-coordinated Au(I) complex: Key roles of crystal environments

Three-coordinate Au(I) complexes with thermally activated delayed fluorescence (TADF) have recently gained experimental attention. However, its luminescence mechanism is elusive. Herein, we have employed density functional theory (DFT), time-dependent DFT (TD-DFT), and QM/MM methods to investigate the excited-state and emission properties of the Au(I) complex in both gas and crystal phases. In both environments, the S-1 and T-1 emitting states mainly involve HOMO and LUMO and show clear metal-ligand charge transfer and intraligand charge transfer characters. The good spatial separation of HOMO and LUMO minimizes the S-1-T-1 energy gap, which benefits the reverse intersystem crossing (rISC) from T-1 to S-1. At 300 K, the rISC rate is faster than the T-1 phosphorescence emission, which enables the TADF emission. However, at 77 K, such a rISC process is blocked and TADF disappears; instead, only phosphorescence is recorded experimentally. Importantly, this work highlights the importance of environments in regulating luminescence properties and contributes to understanding the TADF emission of organometallic complexes.