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.