Study of configuration differentia and highly efficient deep-red thermally activated delayed fluorescent organic light-emitting diodes based on phenanthro[4,5-]quinoxaline derivatives

The development of efficient thermally activated delayed fluorescent (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 an 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 ( Φ PL s) (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 Δ E ST and efficient reverse intersystem crossing to ensure the complete utilization of triplet excitons, indicating that the TADF characteristics are regulated by the co-acceptor of the 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 electroluminescence spectra at different voltages. The results present an efficient method to develop efficient red TADF emitters by isomer optimization strategy. A series of structural isomers are successfully obtained. Trans -PyCNTPA exhibits a smaller Δ E ST and 4-folded higher Φ PL than cis -PyCNTPA, and its doped OLED shows deep-red emission at 668 nm with the maximum EQE of 15.5%.