Switching between TADF and HLCT emissions by connecting different donors based on theoretical calculations

Abstract

Thermally activated delayed fluorescence (TADF) and hybrid local and charge transfer (HLCT) emitters with high exciton utilization and high fluorescence efficiency have attracted great interest in the field of optoelectronics. In this work, two donor–acceptor (D–A) types of structural isomers were designed, where benzofuran derivatives (DBF) acted as the acceptor unit and carbazole (Cz), dimethylacridine (DMAC), and dihydrophenazine (DHPZ) acted as donor units; these isomers were named DBFo-Cz, DBFo-DMAC, DBFo-DHPZ, DBFp-Cz, DBFp-DMAC and DBFp-DHPZ. Based on density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, DBFo-Cz, DBFo-DMAC and DBFo-DHPZ exhibited greater oscillator strengths, leading to faster radiation rates. We adjusted the electron-donating ability of the donor unit to reduce the singlet–triplet energy gap (ΔE_ST), thereby enhancing the reverse intersystem crossing (RISC) process and fluorescence radiative rates (k_r). Significantly, the designed D–A molecules were able to switch flexibly between HLCT and TADF emission characteristics by introducing different donor fragments. DBFo-Cz showed the feature of the high-lying RISC process from the T_n to the S₁ states. With small ΔE_S1T5 (−0.01 eV) and large k_r (2.57 × 10⁸ s⁻¹) values, DBFo-Cz exhibited the potential to be an efficient HLCT molecule. Interestingly, the small ΔE_S1T1 (0.10 eV) value and large k_r (2.38 × 10⁷ s⁻¹) value of DBFo-DMAC made it a promising candidate for an efficient TADF molecule. This design strategy offers new ideas and avenues for regulating and optimizing the performances of TADF and HLCT emitters.