本工作借助第一性原理和动力学演化, 系统地研究了四个叔丁基-咔唑及吩噻嗪取代的硼-氮化合物(BCz-BN、2PTZ-BN、Cz-PTZ-BN和2Cz-PTZ-BN)的多共振热激活延迟荧光的高效发光机制. 结果表明上述分子T₁与T₂间的内转换速率远大于其它辐射与非辐射速率, 同时T₂到S₁的反向系间窜越速率也高于T₁到S₁的反向系间窜越速率, 因此其多共振热激活延迟荧光过程应遵循T₁→T₂→S₁→S₀的路径. 进一步动力学演化表明, T₁与T₂之间的内转换主要发生在演化初期, 随着时间的推移, 能量逐渐由T₂向S₁转移, 并最终在S₁完成荧光发射. 上述研究揭示了多共振延迟荧光的微观本质, 为未来设计及合成新的多共振热激活延迟荧光分子提供了理论依据.

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 (T₁) can convert into the lowest singlet excited state (S₁) through the reverse intersystem crossing (RISC), and then return to the ground state (S₀) by the radiative fluorescence emission. However, recent studies found that in MR-TADF molecules, the higher triplet excited state (T₂) 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 (T₁ and T₂) 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 T₁→S₁. Therefore, the TADF process of the above molecules should follow the T₁→T₂→S₁→S₀ 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 T₁ and T₂. With the time evolution, the energy gradually transferred from T₂ to S₁, and finally emitted fluorescence on S₁. However, if T₂ is not involved in the time evolution, the TADF process will be significantly slowed down, which could further confirm the importance of T₂ 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.