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

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.