Thermally Activated Delayed Fluorescence Mechanism of a Bicyclic "Carbene-Metal-Amide" Copper Compound: DFT/MRCI Studies and Roles of Excited-State Structure Relaxation.

Herein we investigated the luminescence mechanism of one "carbene-metal-amide" copper compound with thermally activated delayed fluorescence (TADF) using density functional theory (DFT)/multireference configuration interaction, DFT, and time-dependent DFT methods with the polarizable continuum model. The experimentally observed low-energy absorption and emission peaks are assigned to the S 1 state, which exhibits clear interligand and partial ligand-to-metal charge-transfer character. Moreover, it was found that a three-state (S 0 , S 1 , and T 1 ) model is sufficient to describe the TADF mechanism, and the T 2 state should play a negligible role. The calculated S 1 -T 1 energy gap of 0.10 eV and proper spin-orbit couplings facilitate the reverse intersystem crossing (rISC) from T 1 to S 1 . At 298 K, the rISC rate of T 1 → S 1 (∼10 6 s -1 ) is more than 3 orders of magnitude larger than the T 1 phosphorescence rate (∼10 3 s -1 ), thereby enabling TADF. However, it disappears at 77 K because of a very slow rISC rate (∼10 1 s -1 ). The calculated TADF rate, lifetime, and quantum yield agree very well with the experimental data. Methodologically, the present work shows that only considering excited-state information at the Franck-Condon point is insufficient for certain emitting systems and including excited-state structure relaxation is important.