An Efficient Ultra‐Narrowband Yellow Emitter Based on a Double‐Boron‐Embedded Tetraazacyclophane

Ultra‐narrowband and highly modifiable multiple resonance thermally activated delayed fluorescence (MR‐TADF) materials are crucial for realizing high‐performance wide‐color‐gamut display applications. Despite progress, most MR‐TADF emitters remain confined to blue and green wavelengths, with difficulties extending into longer wavelengths without significant spectral broadening, which compromises color purity in full‐color organic light‐emitting diode (OLED) displays. In this work, we present a novel tetraazacyclophane‐based architecture embedding dual boron atoms to remarkedly enhance intramolecular charge transfer through the strategic positioning of boron and nitrogen atoms. This arrangement induces a substantial redshift while maintaining structural rigidity and molecular orbital symmetry, with a hole‐electron central distance of 0 Å, allowing for ultra‐narrowband emission. The resulting MR‐TADF material, HBN, delivers yellow emission peaking at 572 nm (2.168 eV) with an impressively narrow full‐width at half‐maximum (FWHM) of 17 nm (0.064 eV) in dilute toluene. Moreover, the corresponding phosphorescent‐sensitized fluorescence OLED achieves yellow emission maximum at 581 nm, with a narrow FWHM of 25 nm, a high maximum external quantum efficiency of 36.1%, and a luminance exceeding 40,000 cd m‐2. These outstanding photoluminescent and electroluminescent performances validate the superiority of our molecular design strategy, highlighting its significant potential for cutting‐edge optoelectronic applications.