Thioxanthone-based thermally activated delayed fluorescence emitters showing fast reverse intersystem crossing for efficient organic light-emitting diodes with small efficiency roll-off

Abstract

Thermally activated delayed fluorescence-based organic light-emitting diodes (TADF OLEDs) usually suffer from severe efficiency roll-off at high brightness which is considered to originate from slow reverse intersystem crossing (RISC) and resulting long-lived triplet excitons. The development of TADF molecules with very fast RISC is an effective approach to overcome this issue. Here, we report two TADF molecules (MCz-TXO, DMCz-TXO) having thioxanthone as an acceptor unit to introduce heavy atom effect. Theoretical calculations predict that both molecules will achieve close energy level matching of the charge-transfer and locally excited triplet states (3CT and 3LE, respectively), together with a small energy gap between 3CT and the lowest excited singlet state. The newly designed molecule, MCz-TXO, showed an extremely large rate constant of RISC (kRISC) of 6.4×107 s− 1, one of the largest kRISC values among all reported pure organic TADF emitters. Moreover, DMCz-TXO showed not only a large kRISC but also a large rate constant of radiative decay both exceeding 107 s− 1, offering a sub-microsecond-scale delayed lifetime (~ 0.8 µs). These thioxanthone-based emitters exhibited great device performances with suppressed efficiency roll-offs at high luminance when applied to OLEDs.