Planar Expansion Enhances Horizontal Orientation of Multi‐Resonance TADF Emitter Toward Highly Efficient Narrowband Blue OLEDs

Multi‐resonance thermally activated delayed fluorescence (MR‐TADF) materials are of vital importance for ultra‐high‐definition displays. However, developing molecular design tactics for blue MR‐TADF emitters that simultaneously achieve good TADF properties, narrowband emission, and ideal molecular orientation remains challenging. Here, an effective approach to enhance the electroluminescent (EL) performance of MR‐TADF emitter by enlarging the molecular planar through a peripheral decoration strategy is presented. The resulting proof‐of‐concept emitter, 3QCzBN, not only exhibits a blue emission with a narrow full‐width at half maximum (FWHM) of only 22 nm, but also a remarkable photoluminescent quantum yield (PLQY) of 98%. Together with the accelerated reverse intersystem crossing (RISC) processes and nearly perfect horizontal ratios of emitting dipole orientation (Θ // ) of 98% compared to its parent MR molecule, the organic light‐emitting diodes (OLEDs) based on 3QCzBN demonstrate blue emission with good color purity and an outstanding maximum external quantum efficiency (EQE max ) of 34.8% with low efficiency roll‐off.