Aromaticity‐Localized Square Tetraboron‐Extended Multiple‐Resonance Emitters With B‒N Covalent Bonds for Highly Efficient Narrowband Electroluminescence

Polycyclic aromatic hydrocarbons with multiple‐resonance (MR) effect are emerging as leading candidates for wide‐color‐gamut displays. However, it remains a significant challenge to red‐shift their emissions to long wavelengths without sacrificing the spectral full width at half maximum (FWHM). Herein, a synergistic strategy of aromaticity localization and tetraboron extension is newly proposed, in which a B‒N covalent‐bond‐fused MR core is fused with 5,10‐dihydro‐5,10‐diphenylphenazine (DPPA) to afford two yellow‐emitting narrowband tetraboron emitters. By integrating multiple local B–π–B motifs into a rigid square‐like tetraboron MR framework, DPA‐4B and DMAC‐4B achieve a pronounced red shift from violet‐blue DPA‐1B to yellow emission. The antiaromatic N 2 C 4 ring and surrounding nonaromatic 1,2‐BN/1,4‐BN heterocycles promote aromaticity localization and suppress excited‐state relaxation, affording narrow emissions at 544–559 nm with FWHMs of 20–21 nm. The DPA‐4B and DMAC‐4B‐based sensitized OLEDs further achieve narrowband yellow electroluminescence with maximum EQE values of 36.7% and 30.1%, respectively. These results demonstrate that the combination of aromaticity localization and tetraboron extension is a promising molecular design rule for simultaneously realizing emission red‐shifting, spectral narrowing, and device efficiency in B‒N covalent‐bond‐containing MR emitters.