Tetraborylated Multiple Resonance Emitter Incorporating B─O Bond‐Embedded π‐Extension for Ultra‐Narrowband and High‐Efficiency Blue Devices

Ultra‐narrowband multiple resonance (MR) thermally activated delayed fluorescence (MR‐TADF) emitters are crucial for next‐generation ultra‐high‐definition full‐color displays. However, current blue MR‐TADF emitters still face significant challenges in simultaneously achieving ultra‐narrowband emission, high quantum efficiency, and a fast reverse intersystem crossing (RISC) rate ( k RISC ). Herein, we propose a boron–oxygen (B─O) bond‐embedded design strategy and develop a nanographene‐based MR‐TADF molecule containing 17‐fused rings. This tetraboron compound, 4B4N2O, exhibits ultra‐narrow blue emission in n ‐hexane solution with an emission peak at 461 nm and a full width at half maximum (FWHM) of only 11 nm. Benefiting from its high photoluminescence quantum yield (PLQY = 99%), high horizontal dipole ratio ( Θ // = 93%), and fast k RISC (5.32 × 10 5 s −1 ), the corresponding device achieves an excellent external quantum efficiency (EQE) of 40.3% and low efficiency roll‐off (EQE 1000 = 31.4%). Moreover, the device exhibits an ultra‐narrow electroluminescence (EL) FWHM of 16 nm and CIE coordinates of (0.11, 0.18). The ultra‐narrowband tetraboron emitter presented herein holds great promise for enabling next‐generation energy‐saving ultra‐high‐definition displays and AR/VR technologies.