Heptagonal intramolecular-lock strategy enables high-performance thermally activated delayed fluorescence emitters

The development of highly efficient thermally activated delayed fluorescence (TADF) emitters is persistently pursued for the application of organic light-emitting diodes (OLED) in full-colour display and solid-state lighting. Herein, we present a heptagonal intramolecular-lock strategy to design high-performance TADF emitters. As a proof-of-concept, a new type of tribenzotropone (TBP) acceptor has been designed and synthesized by a cascade decarboxylative cyclization of aryl oxoacetic acid derivative with biphenyl boronic acid. Compared with the unlocked benzophenone (BP) acceptor, the TBP acceptor has a highly twisted heptagonal geometry with moderate rigidity and flexibility, which enables a high-performance TADF emitter with a small single-triplet energy gap(ΔEST) of 0.04 eV, a high photoluminescence quantum yield (ΦPL) of 99% and a large horizontal orientation factor (Θ//) of 84.0%. Consequently, highly efficient OLEDs with an external quantum efficiency as high as 33.8% are assembled, which is significantly higher than those of DPAC-BP with a highly rotatable BP acceptor (23.8%) as well as DPAC-FO with a rigid fluorenone (FO) acceptor (6.9%).