Exciplex-Forming Co-Host Systems for Efficient TADF and Phosphorescent Organic Light-Emitting Diodes

In this study, a series of bipolar acceptor materials, DPQCN-oPhCz, DPQCN-pPhCz, and DPQCN-pPhCzt, featuring a cyano-substituted diphenylquinoxaline (DPQCN) core conjugated with various phenylcarbazole derivatives, were successfully synthesized and characterized. The molecular design introduces a strongly electron-withdrawing cyano substituent into the diphenylquinoxaline framework to lower the LUMO energy level and improve the bipolar charge-transport properties. Photophysical investigations and theoretical calculations revealed distinct intramolecular charge transfer characteristics, resulting from the effective spatial separation of the frontier molecular orbitals. Specifically, the introduction of tert-butyl groups and a twisted conformation in DPQCN-pPhCzt not only increased the steric bulk to suppress molecular aggregation but also facilitated the formation of an efficient exciplex when blended with the donor material TCTA. Thermodynamic analysis using the Rehm-Weller equation further confirmed the strong driving force for exciplex formation in these D-A systems. These results, characterized by high thermal stability, well-aligned energy levels, and minimal singlet-triplet energy gaps (ΔE_ST), underscore the significant potential of this DPQCN-based series as high-performance exciplex hosts for achieving OLEDs with high brightness and low turn-on voltages.