Symmetry Reconstruction Breaks the Trade‐Off Between Spectral Redshift and Broadening in Long‐Wavelength Multiple Resonance Emitters

ABSTRACT Long‐wavelength multiple resonance (MR) emitters are desirable for high‐color‐purity organic light‐emitting diodes (OLEDs), yet conventional redshifting strategies often enhance excited‐state delocalization and vibronic coupling, leading to spectral broadening. In this study, we present a symmetry reconstruction strategy to break the trade‐off between spectral redshift and broadening in MR emitters. Unilateral and bilateral naphthalene‐fused π‐extension afford SN‐BN and DN‐BN, respectively, providing a direct platform for correlating excited‐state symmetry, vibronic coupling, and emission bandwidth. SN‐BN lowers the excited‐state energy but disrupts the balanced excited‐state hole–electron distribution, inducing partial charge‐transfer character and enhanced vibronic coupling. In contrast, DN‐BN expands the π‐conjugated framework and reconstructs excited‐state symmetry, thereby preserving short‐range charge‐transfer character while suppressing structural relaxation and vibronic coupling. Consequently, DN‐BN exhibits yellow emission at 545 nm with an ultranarrow full‐width at half‐maximum (FWHM) of 16 nm/0.07 eV and a photoluminescence quantum yield of 90% in toluene. The corresponding OLED achieves yellow electroluminescence at 560 nm with an FWHM of 24 nm, a maximum external quantum efficiency of 23.3%, and efficiency roll‐offs of 33.5% and 43.8% at 1000 and 100 000 cd m −2 , respectively. This work establishes symmetry‐reconstructive π‐extension as a promising design strategy for high‐color‐purity, long‐wavelength MR emitters.