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 trasfer 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 substituent 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 EQEmax of 6.7%, a current efficiency of 3.5 cd A⁻¹, and a CIEy of ~0.06. Importantly, PICz-CN, PICz-BP, PIP-CN and PIP-BP exceeded the conventional fluorescent emitters EQE limit ( 5%), underscoring the critical role of structure-function tailoring in regulating HLCT emisison in realizing efficient deep-blue OLEDs