Structure-property investigation of two red TADF isomers with different D-A conjugation for superior exciton utilization

In this text, by linking a phenoxazine donor (D) into different positions of the 11,12-diphenyldipyrido[3,2-a:2′,3′-c]phenazine acceptor (A), Two novel red thermally activated delayed fluorescence (TADF) isomers, PPZBP and BPZPP, were designed and synthesized. The different connection modes bring different steric hindrance between D and A segments, which lead to tunable conjugation of the whole molecule, resulting in the change of locally excited triplet (3LE) state. Beneficial from the suitable conjugation, PPZBP shows better energy level alignments than BPZPP for superior triplet up conversion. PPZBP-based red organic light-emitting diode realizes a higher EQE of 22.0% than that (14.8%) of the BPZPP-based one. This work not only demonstrates the importance of controlling D-A conjugations on the 3LE states and the resultant influences on RISC rates, it also gives a deep insight into preferred design strategies for high-efficiency red TADF emitters.