Molecular Engineering Accelerating Reverse Intersystem Crossing Endowed by Confining Donor to Ensure Low Efficiency Roll-Off OLEDs

To achieve stable organic light emitting diodes (OLEDs), great efforts are devoted to accelerating the reverse intersystem crossing (RISC) process of efficient thermally activated delayed fluorescence (TADF). Here, we focus on spin–orbit coupling engineering to increase the rate constant of RISC and the photoluminescence quantum yield (PLQY). Three TADF emitters consisting of a carbonly carbazole core as the initially donor–acceptor system plus diphenylamine as the π-extended group were developed. We show that this design strategy realizes the fine adjustment of excited states to effect the spin–orbit coupling (SOC) matrix element between triplet and singlet states, resulting in accelerating k_RISC while maintaining high PLQYs and small ΔE_ST. OLEDs achieved excellent electroluminescence performance with a maximum external quantum efficiency of 23.8% and low efficiency roll-off, demonstrating great potential in efficient OLEDs.