Modulation of the Quinoxaline Core for Solution-Processable and Efficient Long-Wavelength Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes

Long-wavelength (λem > 600 nm) thermally activated delayed fluorescence (TADF) emitters remain scarce due to the stringent molecular design requirements and the intrinsic constraints of the energy-gap law. To address this challenge, we employed a functionalized quinoxaline-6,7-dicarbonitrile scaffold (Qx4CN) as an acceptor and integrated it with tert-butyl carbazole (tCz) and acridine (Ac) donor units to construct two solution-processable emitters, tCz-Qx4CN and Ac-Qx4CN. These molecules exhibit absorption maxima at 472 and 492 nm with electroluminescence (EL) peaks at 602 and 636 nm, respectively, along with high thermal stability. The incorporation of cyanophenyl substituents at the 5,8-positions of quinoxaline proved crucial in achieving solubility enhancement and long-wavelength emission. Notably, a solution-processed organic light-emitting diode (OLED) based on the Ac-Qx4CN TADF emitter achieved an external quantum efficiency (EQE) of 7.4% at 636 nm, contributing to the development of solution-processable pure-red TADF OLEDs. This work highlights a viable design strategy for the efficient, solution-processable, long-wavelength TADF OLEDs.