Strategic modification of the quinoxaline acceptor to induce broad-range orange to red thermally activated delayed fluorescence

Abstract

Thermally activated delayed fluorescence (TADF) OLEDs offer high efficiency without rare metals, but efficient orange-red emitters remain limited due to difficulty in achieving high photoluminescence quantum yields (PLQYs) at longer wavelengths. In this study, we report the design, synthesis, and comprehensive characterization of three novel donor-π-acceptor (D-π-A) TADF emitters-Cz-PhQx4CN, Ac-PhQx4CN, and PXZ-PhQx4CN-based on a cyanophenyl-substituted quinoxaline acceptor. Strategic donor tuning and cyano-functionalized quinoxaline acceptor design enabled broad orange-to-deep red emission (603-700 nm) by enhancing intramolecular charge transfer and minimizing the singlet-triplet energy gap (ΔE ST ).Photophysical, electrochemical, and theoretical studies confirmed strong ICT character and efficient RISC across the series.Cz-PhQx4CN showed pronounced AIE, a high PLQY (37.8%), a small ΔE ST (34 meV), and the best device performance with an EQE of 9.9% in vacuum-deposited and 3.6% in solution-processed OLEDs. PXZ-PhQx4CN and Ac-PhQx4CN also exhibited TADF behavior with red-shifted emissions but lower efficiencies These results highlight how molecular rigidity, extended conjugation, and careful donor-acceptor design enable efficient long-wavelength TADF emission, offering valuable guidance for developing high-performance orange-red OLEDs.