Influence of Electron Donating Ability on Reverse Intersystem Crossing Rate for One Kind of Thermally Activated Delayed Fluorescence Molecules

First-principles calculations are applied for investigating influence of electron donating ability of donor groups in eight thermally activated delayed fluorescence (TADF) molecules on their geometrical structures and transition properties as well as reverse intersystem crossing (RISC) processes. Results show that the diphenylamine substitution in the donor part can slightly change the bond angle but decrease bond length between donor and acceptor unit except for the lowest triplet state (T_1) of carbazole-xanthone molecule. As the electron donating ability of donor groups is increased, the overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is decreased. As the diphenylamine groups are added in donor part, the delocalization of HOMO is enlarged, which brings a decreased energy gap (\Delta E_\textS_1\text-T_1) between the lowest singlet excited state (S_1) and T_1 state. Furthermore, with the calculated spin-orbit coupling coefficient (H_\textso), one finds that the larger value of \displaystyle\frac\langle S_1|\hatH_\textso|T_1\rangle^2\Delta E_\textS_1\text-T_1^2 is, the faster the RISC is. The results show that all investigated molecules are promising candidates as TADF molecules. Overall, a wise molecular design strategy for TADF molecules, in which a small \Delta E_\textS_1\text-T_1 can be achieved by enlarging the delocalization of frontier molecular orbitals with large separation between HOMO and LUMO, is proposed.