A facile strategy for enhancing reverse intersystem crossing of red thermally activated delayed fluorescence emitters

Realizing highly efficient reverse intersystem crossing (RISC) process for red thermally activated delayed fluorescence (TADF) emitters remains a formidable challenge. Herein we demonstrate that charge transfer (CT) energy levels are gradually reduced with enhancing electron-withdrawing abilities of the secondary acceptor (A) attachments. Meanwhile, their local excited triplet ((3)LEA) state levels are well maintained due to the conjugations between the main framework and the attachments are all similar for these A segments. The optimized molecular energy level alignments are obtained in the case of mDPBPZ-DPXZ, in which 3CT and the 3LEA states are almost degenerate, and gives rise to a small singlet-triplet energy difference (Delta EST) of 0.02 eV, a fast RISC rate of 1.54 x 10(6) s(-1) and a high photoluminescence quantum yield of 93%, simultaneously. And the corresponding device achieves an external quantum efficiency of 21.6%, significantly higher than the other counterparts. This work demonstrates that the energy alignment between the CT and 3LEA state can be easily managed by introducing secondary A attachments on original A framework, which shifts the 1CT and 3CT state to lower energies without affecting the 3LEA energy. It provides a facile design strategy to promote the RISC process, which is particular useful in designing highly efficient red TADF emitters.