Engineering Localized Aromaticity in Amine‐Embedded Polycyclic Aromatic Hydrocarbons for Narrowband Fluorescence Emitter

The development of narrowband fluorescent emitters remains a long‐standing challenge in organic optoelectronics. Herein, we present an aromaticity engineering approach based on a perylene core to construct highly efficient narrowband fluorescent emitters. Replacement of one naphthalene unit with a carbazole moiety enhances the localization of aromaticity, while extension of the π–conjugation further attenuates the aromaticity of the remaining naphthalene ring. This dual modulation effectively suppresses emission shoulder bands, yielding spectrally pure fluorescence. Consequently, the centrosymmetric c‐NaDTCz and the axially symmetric a‐NaDTCz exhibit sharp emissions at 536 and 600 nm, with narrow full widths at half‐maximum (FWHM) of 17 and 30 nm, respectively. When applied in OLEDs, the c‐NaDTCz‐based device displays sharp yellow emission peaking at 548 nm with a high external quantum efficiency (EQE) of 26.1%, while a‐NaDTCz delivers narrowband red electroluminescence at 610 nm, achieving a record‐high EQE of 27.8% for conventional red fluorescent emitters.