Controlling the conjugation extension inside acceptors for enhancing reverse intersystem crossing of red thermally activated delayed fluorescence emitters

Increasing the reverse intersystem crossing (RISC) rates remains a challenge for red thermally activated delayed fluorescence (TADF) emitters, and energy level alignments play an important role. In this work, we demonstrate that locally excited triplet energy levels from acceptor segments (3LEA) can be well modulated by changing the conjugation extension inside acceptors. Our newly designed compounds pDPAPQ-PXZ and pDPBPZ-PXZ show nearly identical charge-transfer (CT) properties because of similar donor and acceptor strengths. On the other hand, the 3LEA state of pDPBPZ-PXZ was significantly lower than that of pDPAPQ-PXZ only because of its greater pi-delocalization. In the case of pDPBPZ-PXZ, ideal molecular energy level alignments are obtained with neardegenerate CT featuring singlet and triplet states (1CT, 3CT) and 3LEA. As a result, pDPBPZ-PXZ simultaneously achieves an approximately 10-fold improved kRISC and a dramatically improved photoluminescence quantum yield of 85.6% compared with pDPAPQ-PXZ. The corresponding OLED successfully doubles the external quantum efficiency to 20.3%. This work demonstrates that the conjugation extensions of acceptors play a fatal role in determining 3LEA levels and that they can be isolated from controlling the acceptor strengths. This provides a promising approach toward enhancing RISC efficiency and developing highly efficient red TADF emitters.