Synergistic molecular design-triggered hybridized local and charge-transfer emission for efficient (>6%) deep-blue electroluminescence

Abstract

Rational molecular design coupled with strategic structure–property optimization is crucial for surpassing the theoretical EQE limit (>5%) of deep-blue fluorescent organic emitters in OLEDs. In this work, we present a series of phenanthro[9,10-d]imidazole (PI)-based deep-blue emitters, systematically modified at the N1- and C2-positions with carbazole, phenyl, or cyano functionalities. This molecular library enabled precise control over π-conjugation, excited state dynamics and charge-transfer (CT) characteristics, effectively confining emission within the deep-blue region (<420 nm) with narrow full width at half-maximum (FWHM < 60 nm). Cyano-substituted derivatives displayed red-shifted emission and pronounced positive solvatochromism in emission, attributed to photoinduced intramolecular charge transfer in the excited state. Combined experimental and computational studies confirmed hybridized local and charge-transfer (HLCT) behavior in compounds, PICz-BP, PICz-CN, PIP-CN and PIP-BP, ascribed to the regulation of high-lying excited states by virtue of functional group alteration. The carbazole-functionalized emitters exhibited low oxidation potential and enhanced thermal stability. In the series, PIP-BP demonstrated the best OLED performance, achieving an EQE_max of 6.7%, a current efficiency of 3.5 cd A⁻¹, and a CIE_y of ∼0.06. Importantly, PICz-CN, PICz-BP, PIP-CN and PIP-BP exceeded the EQE limit (5%) of conventional fluorescent emitters, underscoring the critical role of structure–function tailoring in regulating HLCT emission for realizing efficient deep-blue OLEDs.