Medium‐Ring Strategy Enables Multiple Resonance Emitters with Twisted Geometry and Fast Spin‐Flip to Suppress Efficiency Roll‐Off

Abstract Suppressing aggregation‐caused quenching (ACQ) effect and reducing device efficiency roll‐off are both crucial yet challenging for multi‐resonance (MR) emitters. Herein, we put forward a medium‐ring strategy to design efficient MR emitters that feature heptagonal tribenzo[ b , d , f ]azepine (TBA) donors. The highly twisted conformation enlarges the intermolecular distances between the MR‐emitting cores, and thus suppresses ACQ effect. Meanwhile, the introduction of heptagonal donors enhances spin‐orbital coupling, so as to accelerate reverse intersystem crossing (RISC) process. This medium‐ring strategy gives rise to the first example of blue MR emitter that simultaneously possesses radiative decay rate as fast as 10 8 s −1 and RISC rate as fast as 10 6 s −1 . Accordingly, DTBA‐B2N3 enables to assemble high‐performance blue organic light‐emitting diodes (OLEDs) with maximum external quantum efficiency (EQE max ) of 30.9 % and alleviated efficiency roll‐off (EQE 1000 : 20.5 %).