Stepwise Toward Pure Blue Organic Light‐Emitting Diodes by Synergetically Locking and Shielding Carbonyl/Nitrogen‐Based MR‐TADF Emitters

Abstract Deep‐blue multi‐resonance (MR) emitters with stable and narrow full‐width‐at‐half‐maximum (FWHM) are of great importance for widening the color gamut of organic light‐emitting diodes (OLEDs). However, most planar MR emitters are vulnerable to intermolecular interactions from both the host and guest, causing spectral broadening and exciton quenching in thin films. Their emission in the solid state is environmentally sensitive, and the color purity is often inferior to that in solutions. Herein, a molecular design strategy is presented that simultaneously narrows the FWHM and suppresses intermolecular interactions by combining intramolecular locking and peripheral shielding within a carbonyl/nitrogen‐based MR core. Intramolecularly locking carbonyl/nitrogen‐based bears narrower emission of 2,10‐dimethyl‐12,12‐diphenyl‐4H‐benzo[9,1]quinolizino[3,4,5,6,7‐defg]acridine‐4,8(12H)‐dione in solution and further with peripheral‐shielding groups, deep‐blue emitter (12,12‐diphenyl‐2,10‐bis(9‐phenyl‐9H‐fluoren‐9‐yl)−4H‐benzo[9,1]quinolizino[3,4,5,6,7‐defg]acridine‐4,8(12H)‐dione, DPQAO‐F) exhibits ultra‐pure emission with narrow FWHM (c.a., 24 nm) with minimal variations (∆FWHM ≤ 3 nm) from solution to thin films over a wide doping range. An OLED based on DPQAO‐F presents a maximum external quantum efficiency (EQEmax) of 19.9% and color index of (0.134, 0.118). Furthermore, the hyper‐device of DPQAO‐F exhibits a record‐high EQEmax of 32.7% in the deep‐blue region, representing the first example of carbonyl/nitrogen‐based OLED that can concurrently achieve narrow bandwidth in the deep‐blue region and a high electroluminescent efficiency surpassing 30%.