Influence of Donor–Acceptor\nConformation on\nThermally Activated Delayed Fluorescence Properties of Two-Coordinate\nCarbene–Au–Amide Complexes

Monovalent coinage metal complexes with a carbene–metal–amide\n(CMA) structure have emerged as an appealing class of thermally activated\ndelayed fluorescence (TADF) emitters. The relative donor–acceptor\norientation has been identified as a crucial factor that determines\nthe TADF properties. In this work, two Au(I)-based CMA complexes with\nsandwich structures, namely, <b>Au-DPh</b> and <b>Au-DDPXZ</b>, were designed and synthesized. The benzene (Ph) and O-bridged triphenylamine\n(DPXZ) having different electron-donating capabilities were introduced\nto the carbazole (Cz) fragment. Theoretical and photophysical studies\nwere performed to investigate the electronic structures and photophysical\nproperties of the CMA complexes. Both complexes show tiny Δ<i>E</i><sub>ST</sub> values (0.01 and 0.02 eV) and thus faster\nreverse intersystem crossing rates owing to the good separation of\nfrontier molecular orbitals. However, the photoluminescence quantum\nyields are lower for the present sandwich-type complexes (58% for <b>Au-DDPXZ</b> and 71% for <b>Au-DPh</b>) than for the CMA\nanalogues with a coplanar or less twisted structure. Together with\ntheir lifetimes of 1.14 and 1.69 μs, the radiative decay rates\nwere determined to be 3.43–6.23 × 10<sup>5</sup> s<sup>–1</sup>. The theoretical calculations manifest a decreased\noscillator strength for the S<sub>1</sub> radiative decay arising\nfrom the separation of frontier orbitals. This work reveals that the\nTADF properties of CMA complexes are affected by various parameters\nand that a suitable donor–acceptor torsion angle is necessary\nfor optimizing the TADF efficiency.