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 ultrahigh‐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 ultranarrow full‐width at half‐maximum (FWHM) of 13 nm and achieves an order‐of‐magnitude increase in the reverse intersystem crossing rate constant (kRISC) 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 exhibits a FWHM of 15 nm, and a high maximum external quantum efficiency (ηEQE,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.