多共振热激活延迟荧光过程的理论研究

Theoretical Study on the Multiple Resonance Thermally Activated Delayed Fluorescence Process

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T_(1)与T_(2)间的内转换速率远大于其它辐射与非辐射速率,同时T_(2)到S_(1)的反向系间窜越速率也高于T_(1)到S_(1)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T_(1)→T_(2)→S_(1)→S_(0)的路径.进一步动力学演化表明,T_(1)与T_(2)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T_(2)向S_(1)转移,并最终在S_(1)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(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 T_(2)→S_(1)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.