Regio-isomer enabling efficient red TADF emitters based on pyridobenzoquinoxaline

Abstract

The development of efficient red thermally activated delayed fluorescence (TADF) emitters experiences considerable challenges in maintaining a high photoluminescent quantum yield (PLQY) and tuning the singlet–triplet energy gap simultaneously. In addition, the positional isomeric effect could significantly impact the charge transfer efficiency and eventually influence the propensity of TADF features. In this study, two pairs of regio-isomeric V- and T-shaped D–A–D molecules involving a new acceptor of pyridobenzoquinoxaline (PQ) are developed and systematically characterized. These molecules differ from each other with respect to different substitutions of dimethylacridine and phenoxazine donors at 3,6- and 2,7-positions of PQ, respectively. Good separation of the frontier molecular orbitals is observed in these compounds leading to obvious TADF behaviors. Compared with T-shaped molecules, V-shaped emitters exhibit higher PLQY due to less geometry relaxation and less vibration modes, as confirmed by both theoretical and experimental analyses. The device based on PQDMAC-V displays a higher maximum external quantum efficiency (EQE_max) of 12.9% with emission peaking at 604 nm whereas PQDMAC-T achieves an EQE_max of 8.5% with emission peaking at 612 nm. This study highlights the importance of enhancing radiative decay for developing efficient red TADF emitters and suggests the better substitution pattern of V-typed design.