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 S₁ and T₁ emitting states mainly involve HOMO and LUMO and show clear metal-ligand charge transfer and intra-ligand charge transfer characters. The good spatial separation of HOMO and LUMO minimizes the S₁−T₁ energy gap, which benefits the reverse intersystem crossing (rISC) from T₁ to S₁. At 300 K, the rISC rate is faster than the T₁ 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.