Developing new electron acceptors with high rigidity and structural adjustability is significant for realizing efficient long-wavelength thermally activated delayed fluorescence (TADF) emitters. Herein, a new acceptor group, acenaphtho[1,2-b]quinoxaline-3,4-dicarbonitrile (AQCN), featuring a π-extended, rigid skeleton with high electron affinity, is developed for the construction of red TADF dyes. Two red light-emitting regioisomers, LB-Y and LB-T, featuring a Y-shape and T-shape D-A-D molecular configuration, respectively, were prepared by adjusting the relatively substituting position of two triphenylamine electron-rich moieties on AQCN. Through the isomeric engineering, it is found that LB-Y shows a high degree of molecular rigidity, matched excited-state energy level alignment, and decent interplay among singlet charge transfer excited state (1CT), triplet locally excited state (3LE) and triplet hybridized local and charge transfer excited state (3HLCT). Therefore, LB-Y has a higher radiative transition rate (8.36 × 107 s−1), reverse intersystem crossing rate (1.02 × 105 s−1), and better electroluminescent performances (λEL = 614 nm, EQEmax = 4.1%) than LB-T (λEL = 612 nm, EQEmax = 2.6%). This study reports a promising electron acceptor and provides an efficient approach to modulating the optoelectronic properties of red TADF dyes.
