Simple Boron–Nitrogen Covalent Bond Constructs Multi‐Resonance TADF Emitters: Ultra‐Narrowband Deep‐Blue Electroluminescence

High-efficiency, pure deep-blue emitters are critically needed to meet the rising demands of ultra-high-definition displays. Although high-order B/N-doped polycyclic aromatic hydrocarbons (PAHs) leveraging multi-resonance (MR) effects show promise, their complex syntheses and large molecular weights hinder practical application. Here, we report a compact MR framework featuring three nitrogen-linked boron centers, synthesized at the gram scale via a single-step, amine-directed borylation. This emitter displays deep-blue emission with an ultra-narrow full-width at half-maximum (FWHM) of 13 nm and achieves an order-of-magnitude increase in the reverse intersystem crossing rate constant (k_RISC) compared to previous BN-bond-based blue MR emitters. Theoretical studies reveal that its π-extended framework and partially distorted geometry synergistically minimize structural relaxation to reduce FWHM and enhance spin-orbit coupling to facilitate efficient spin-flip processes. As a result, the corresponding deep-blue organic light-emitting diodes exhibit an FWHM of 15 nm and a high maximum external quantum efficiency (η_EQE,max) approaching 30% at color coordinates of (0.155, 0.060), rivaling the leading performance of deep-blue OLEDs based on conventional B/N-doped frameworks.