Fusion of Heterocyclic Aromatic Hydrocarbons With Multiple Resonance: Balancing Spectral Redshifts and Broadening

Abstract Efforts to fabricate ideal narrowband, long‐wavelength (>520 nm) multiple resonance (MR) emitters are often hampered by the mutual obstacles of effective spectral redshift and maintaining a small full width at half maximum (FWHM). This review focuses on a sophisticated molecular design methodology called “fusing heterocyclic aromatic hydrocarbons (HAHs) with MR” to overcome the aforementioned challenges. Here, HAH serves as a rigid structural fragment with a modest reorganization energy, which mainly plays the role of reducing ground‐excited state structural displacement and spectral redshift, while the MR fragment mainly plays the role of restricting low‐frequency vibrational coupling and narrowband emission. The synergy of these two advantages theoretically allows the significant redshift of the emission peak while further narrowing the spectrum. Utilizing the ingeniously designed emitters, the corresponding MR devices can achieve superior external quantum efficiencies of over 20% while maintaining remarkably small FWHMs of <25 nm in the green to red regions. These discoveries will spur the development of narrowband, full‐color, high‐efficiency MR emitters and usher in a transformative shift in the organic light‐emitting diode industry.