Achieving Spectral Narrowing by Strengthening the Soliton‐Like Characteristics through Peripheral Group Substitution in Aromatic Heterocyclic Emitters

Abstract The effect of substituents on the spectral behavior of aromatic heterocyclic systems is a significant yet highly complex issue. Inspired by cyanine dyes, it was found that emission spectrum narrowing can be realized by regulating the terminal substituents to enhance the soliton‐like characteristics of emitters. Herein, compared to the parent compound 2AcPh (full‐width at half‐maximum, FWHM = 0.124 eV), the introduction of fluorobenzene‐group substituents in 2AcPh‐PhF narrows the emission spectral FWHM to 0.108 eV. Single crystal structures reveal more homogeneous carbon–carbon bond lengths in 2AcPh‐PhF compared to 2AcPh. Theoretical calculations show that the charge magnitudes on carbon atoms of 2AcPh‐PhF also become homogenized, but the charges on adjacent carbon atoms are opposite. These results suggest that the soliton‐like electronic structural characteristics in 2AcPh‐PhF are enhanced after introducing peripheral substituents, which is considered to be the reason for the narrower emission spectrum of 2AcPh‐PhF. The sky‐blue organic light‐emitting diode devices based on 2AcPh‐PhF demonstrated a maximum external quantum efficiency of 29.17% and a small electroluminescent spectral FWHM of 25 nm.