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

Abstract

Thermally activated delayed fluorescence (TADF) organic light emitting diodes (OLEDs) offer high external quantum efficiencies 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 unit. Strategic donor modulation and cyano-functionalized quinoxaline acceptor design enabled broad range 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 behaviour 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.