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

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.