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

Abstract

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 intraligand 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.