Twisted Hybrid‐Quadruple‐Borylated MR‐TADF Emitter Bridged by B─N─B Bond With Sub‐10 nm Full‐Width at Half‐Maximum and Fast Reverse Intersystem Crossing

ABSTRACT Multiple‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters with narrowband emission have significantly advanced organic light‐emitting diodes (OLEDs). However, achieving a sub‐10 nm full‐width at half‐maximum (FWHM) in MR‐TADF systems has remained elusive, let alone combining a fast reverse intersystem crossing (RISC) concurrently. Herein, we present an innovative twisted hybrid‐quadruple‐borylated MR‐TADF emitter, constructed via an amine‐directed B─N─B bond‐bridging strategy. The symmetric electron‐withdrawing nature of the B─N─B motif preserves the strong electron‐accepting ability of the boron atoms, significantly enhancing atom‐scale orbital separation and yielding an ultra‐narrow photoluminescence with a FWHM of only 9 nm (44 meV) in toluene—making it the first reported B,N‐doped emitter with sub‐10 nm bandwidth across all emission colors. Furthermore, the convergent characteristic of the B─N─B motif renders a twisted helical molecule configuration for the enhanced spin–orbit couplings, affording a high RISC rate of 1.37 × 10 6 s −1 , nearly an order of magnitude greater than those of previous emitters containing a B─N covalent bond. The corresponding device exhibits green electroluminescence at 505 nm with an ultranarrow electroluminescence FWHM of 12 nm (58 meV) and a maximum external quantum efficiency of 31.5%, which could be further boosted to 42.1% with exceptionally low efficiency roll‐off (36.8% at 10 000 cd m −2 ) in a sensitized architecture.