Efficient blue thermally activated delayed fluorescence emitters showing very fast reverse intersystem crossing

We report a highly efficient blue thermally activated delayed fluorescence (TADF) emitter, namely, MCz-TXO. MCz-TXO exhibits close energy level matching of the three states: charge transfer (CT) and locally excited triplet states, and a CT singlet state. Owing to the good energy level matching and heavy atom effect of sulfur in the molecular structure, MCz-TXO shows very large experimental rate constant of reverse intersystem crossing (k (RISC)) of similar to 2 x 10(8) s(-1). This is one of the largest k (RISC) values among all reported pure organic TADF emitters. The MCz-TXO based organic light-emitting diodes exhibit blue emission with maximum external quantum efficiency of 17.4%.