A Boron, Nitrogen, and Oxygen Doped π-Extended Helical Pure Blue Multiresonant Ther-mally Activated Delayed Fluorescent Emitter for Organic Light Emitting Diodes That Shows Fast kRISC Without the Use of Heavy Atoms

Narrowband emissive multiresonant thermally activated delayed fluorescence (MR-TADF) emit-ters are a promising solution to achieve the current industry targeted color standard, BT.2020, for blue color without using optical filters, aiming for high efficiency organic light-emitting di-odes (OLEDs). However, their long triplet lifetimes, largely affected by their slow reverse inter-system crossing rates, adversely affect device stability. In this study, a helical MR-TADF emitter (f-DOABNA) has been designed and synthesized. Because of its -delocalized structure, f-DOABNA possesses a small singlet-triplet gap, EST, and displays simultaneously an exception-ally faster reverse intersystem crossing rate constant, kRISC, of up to 2 × 106 s1 and a very high photoluminescence quantum yield, PL, of over 90% in both solution and doped films. The OLED with f-DOABNA as the emitter achieved a narrow deep-blue emission at 445 nm (full width at half-maximum of 24 nm) associated with CIE coordinates of 0.150, 0.041, and showed a high maximum external quantum efficiency, EQEmax, of ~20%.