Modulating the Resonance Structures Toward Highly Efficient Violet-Blue Organic Light-Emitting Diodes with Narrow Emission

Abstract

The spectral narrowing of organic fluorescence materials is a focus issue in the field of photochemistry and organic light-emitting diodes (OLEDs). Donor-acceptor emitters are usually susceptible to environmental interference, exhibiting significant spectral broadening. However, as the energy of the donor and acceptor is matched, a resonant energy form is formed in the molecule, resulting the spectrum narrower and stronger. The resonance structures of such emitters can be evaluated by the resonance parameter c². When the neutral state and zwitterionic state of dye molecules are in equilibrium, c²=0.5, and this state is called the cyanine limit state. Here, we introduce DOBNA-carbazole-based donor-acceptor emitters BO-2Cz and TB-tCz. The c² value of BO-2Cz (0.45) indicates that BO-2Cz has a more balanced contribution of resonance structures than TB-tCz (0.03). Although both exhibit short-wavelength emission with small full width at half maximum (FWHM) in n-hexane, TB-tCz shows significant spectral broadening in different solvents and the doped film, while BO-2Cz maintains narrow violet-blue emission with high environmental tolerance. As a result, the single-doped device with BO-2Cz achieves electroluminescence at 404 nm (FWHM=31 nm), CIE coordinates (0.162, 0.017), and a maximum external quantum efficiency of 7.3%, simultaneously realizing short-wavelength and narrow emission.