Theoretical studying of basic photophysical processes in a thermally activated delayed fluorescence copper(I) complex: Determination of reverse intersystem crossing and radiative rate constants

We found that the intersystem crossing (ISC) from the first excited singlet state (S1) to the triplet state (T1) is forbidden by direct spin-orbit coupling at the first-order perturbation, but becomes allowed through combined with vibronic coupling. The reverse intersystem crossing (RISC) proceeds at a rate of KRISC = 3.98 × 108 s-1 at room temperature 300 K, which is about 6 order of magnitude larger than the mean phosphorescence rate, KP, av = 7.3 × 102 s-1. At the same time, the ISC rate KISC = 3.06 × 109 s-1 is again about 3 order of magnitude larger than the fluorescence rate KF = 6.47 × 106 s-1. This implies that the S1 state can be populated from the T1 state, TADF should be observed and TADF decay time is τ(300 K) = 2.32 μs by fitting calculation.152