Study of Configuration Differentia and Highly Efficient Deep-Red Thermally Activated Delayed Fluorescence Organic Light-Emitting Diode Based on Phenanthro[4,5-fgh]quinoxaline Derivatives

The development of efficient thermally activated delayed fluorescence (TADF) materials with a deep-red emission (emission wavelength beyond 620 nm) remains a great challenge in the field of organic light-emitting diodes (OLEDs). In this work, a series of structural isomers are successfully obtained by attaching electron-donor triphenylamine (TPA) group to the various positions of two types of planar acceptor units, PQP and PQCN. Interestingly, the trans-PyCNTPA and trans-PyPTPA all exhibit much higher photoluminescence quantum yields (ΦPLs) (87% and 81%) than their respective cis-isomers, cis-PyCNTPA (19%) and cis-PyPTPA (12%). The theoretical calculations and dynamic studies indicate the effective suppression of the nonradiative decay processes in trans-isomers. The trans-PyCNTPA and cis-PyCNTPA show relatively small ΔEST and efficient reverse intersystem crossing process to ensure the fully utilization of triplet excitons, indicating the TADF characteristics are regulated by the co-acceptor of CN unit owing to its strong electron-withdrawing property. As a result, a deep-red TADF-OLED based on trans-PyCNTPA is achieved with a maximum external quantum efficiency (EQE) of 15.5% at 668 nm, CIE coordinates of (0.66, 0.35) and stable electroluminescent spectra at different voltages. The results present an efficient method to develop efficient red TADF emitter by isomer optimization strategy.