吉林省计算中心

In recent years, the multiple resonance thermally activated delayed fluorescence (MR-TADF) has become a hot topic due to the high fluorescence quantum yield and small Stokes shift. In traditional TADF molecules, the lowest triplet excited state (T-1) can convert into the lowest singlet excited state (S-1) through the reverse intersystem crossing (RISC), and then return to the ground state (S-0) by the radiative fluorescence emission. However, recent studies found that in MR-TADF molecules, the higher triplet excited state (T-2) might play an important role. Thus, the detailed mechanism for the luminescence in MR-TADF molecules, especially for the effect of higher triplet excited state, has still been an indefinite problem. In this work, the luminescence mechanism of four newly reported MR-TADF molecules (BCz-BN, 2PTZ-BN, CZ-PTZ-BN and 2Cz-PTZ-BN) which are substituted by tert-butyl-carbazole and phenothiazine groups has been systematically investigated by the first-principle calculation and kinetic time evolution. The results showed that the phenothiazine group can significantly increase the spin-orbit coupling (SOC) and increase the fluorescence quantum yield. More importantly, by computing the rate constants for the TADF process, we found that the internal conversion (IC) rate between two triplet excited states (T-1 and T-2) is much faster than the radiative and non-radiative rates, and meanwhile the rate of the reverse intersystem crossing of T2 -> S1 is also faster than the one of T-1 -> S-1. Therefore, the TADF process of the above molecules should follow the T-1 -> T-2 -> S-1 -> S-0 process. The above findings were also confirmed by the further kinetic time evolution. In the early stage, the system was mainly dominated by the internal conversion between T-1 and T-2. With the time evolution, the energy gradually transferred from T-2 to S-1, and finally emitted fluorescence on S-1. However, if T-2 is not involved in the time evolution, the TADF process will be significantly slowed down, which could further confirm the importance of T-2 in the TADF process. This work reveals the nature of the TADF process which could be of great importance for designing and synthesizing new TADF molecules.