Efficient red delayed fluorescence molecule based on cyano-substituted 11,12-diphenyldibenzo[a,c]phenazine acceptor for high-performance organic light-emitting diodes

Developing red luminescent materials with high photoluminescence quantum yields and good exciton harvesting ability is of high importance for organic light-emitting diodes (OLEDs) but remains challenging due to the obstacle of energy gap law. Herein, a design strategy of introducing cyano (CN) groups to electron acceptor is proposed for red luminescent materials, and two tailored molecules (2CNDPDBP-TPA and 4CNDPDBP-TPA) consisting of CN-substituted 11,12-diphenyldibenzo[a,c]phenazine acceptor and triphenylamine donor are prepared. They enjoy high thermal and electrochemical stabilities, exhibit strong thermally activated delayed fluorescence and prefer horizontal dipole orientation in doped films. It is demonstrated that introducing CN groups to the acceptor can not only strengthen the charge transfer effect to move emission peaks to the red region but also effectively modulate energy levels and transition characteristics of the excited state to accelerate the reverse intersystem crossing process. The large quasi-planar configuration with extended conjugation by CN groups endow the molecules with enhanced horizontal dipole ratio. High-performance red OLEDs are achieved by employing 4CNDPDBP-TPA as an emitter at varied doping concentrations, providing the state-of-the-art external quantum efficiencies of 35.2% at 612 nm, 33.7% at 628 nm and 29.8% at 640 nm, validating the proposed strategy is promising for exploring robust red luminescent materials for OLEDs.