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

Abstract

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 S1 state, which exhibits clear interligand and partial ligand-to-metal charge-transfer character. Moreover, it was found that a three-state (S0, S1, and T1) model is sufficient to describe the TADF mechanism, and the T2 state should play a negligible role. The calculated S1–T1 energy gap of 0.10 eV and proper spin–orbit couplings facilitate the reverse intersystem crossing (rISC) from T1 to S1. At 298 K, the rISC rate of T1 → S1 (∼106 s–1) is more than 3 orders of magnitude larger than the T1 phosphorescence rate (∼103 s–1), thereby enabling TADF. However, it disappears at 77 K because of a very slow rISC rate (∼101 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