Influence of Donor–Acceptor Conformation on Thermally Activated Delayed Fluorescence Properties of Two-Coordinate Carbene–Au–Amide Complexes

Monovalent coinage metal complexes with a carbene–metal–amide (CMA) structure have emerged as an appealing class of thermally activated delayed fluorescence (TADF) emitters. The relative donor–acceptor orientation has been identified as a crucial factor that determines the TADF properties. In this work, two Au(I)-based CMA complexes with sandwich structures, namely, Au-DPh and Au-DDPXZ, were designed and synthesized. The benzene (Ph) and O-bridged triphenylamine (DPXZ) having different electron-donating capabilities were introduced to the carbazole (Cz) fragment. Theoretical and photophysical studies were performed to investigate the electronic structures and photophysical properties of the CMA complexes. Both complexes show tiny ΔEST values (0.01 and 0.02 eV) and thus faster reverse intersystem crossing rates owing to the good separation of frontier molecular orbitals. However, the photoluminescence quantum yields are lower for the present sandwich-type complexes (58% for Au-DDPXZ and 71% for Au-DPh) than for the CMA analogues with a coplanar or less twisted structure. Together with their lifetimes of 1.14 and 1.69 μs, the radiative decay rates were determined to be 3.43–6.23 × 105 s–1. The theoretical calculations manifest a decreased oscillator strength for the S1 radiative decay arising from the separation of frontier orbitals. This work reveals that the TADF properties of CMA complexes are affected by various parameters and that a suitable donor–acceptor torsion angle is necessary for optimizing the TADF efficiency.