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 kRISC while maintaining high PLQYs and small ΔEST. 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.