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 × 10⁶ 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.