Highly efficient deep-red to near-infrared thermally activated delayed fluorescence organic light-emitting diodes using a 2,3-bis(4-cyanophenyl)quinoxaline-6,7-dicarbonitrile acceptor

Abstract

The demand for deep-red (DR) or near-infrared (NIR) emitters has grown markedly owing to their essential roles in optical telecommunications, night vision technologies, flat panel displays, bioimaging, photodynamic therapy, and sensors, among other applications. However, achieving highly efficient DR/NIR organic light-emitting diodes (OLEDs) is challenging, primarily due to the inherent limitations of the energy gap law. In this study, we propose a novel molecular design for a thermally activated delayed fluorescence emitter, tBuTPA-Qx4CN, based on the 2,3-bis(4-cyanophenyl)quinoxaline-6,7-dicarbonitrile (Qx4CN) acceptor, shifting the emission wavelength toward the DR/NIR region. The tBuTPA-Qx4CN design involves a para-directed connection between the donor moieties and the Qx4CN acceptor unit. Unlike conventional peripheral attachment of donors, two identical 4-(tert-butyl)-N-(4-(tert-butyl)phenyl)-N-phenylaniline donor units were attached at the 5,8-position of the central Qx4CN unit. Using the advantageous H-bonding effect between the donor and acceptor units, the tBuTPA-Qx4CN emitter achieved a high oscillator strength, resulting in a substantial radiative rate. The strong Qx4CN acceptor unit facilitated a marked red shift in the emission wavelength of tBuTPA-Qx4CN toward the NIR region. OLEDs incorporating tBuTPA-Qx4CN as the TADF emitter achieved enhanced external quantum efficiency of up to 16.3%, peaking at 674 nm. A new synthetic route, starting from 4,5-diamino-3,6-dibromophthalonitrile, was developed for tBuTPA-Qx4CN synthesis. This route may be useful for future designs of highly efficient DR/NIR TADF emitters for OLED applications.