Study of configuration differentia and highly efficient deep-red thermally activated delayed fluorescent organic light-emitting diodes based on phenanthro[4,5-fgh]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 (phi(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 Delta 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.

Automated summary

This study successfully obtained a series of structural isomers in TADF materials by attaching a TPA group to different positions of acceptor units, and found that the trans-isomers exhibited higher photoluminescence quantum yields. It was demonstrated that the TADF characteristics were regulated by the co-acceptor of the CN unit, enabling the development of efficient red TADF emitters.