Rational Molecular Design via Cyanobenzene Integration for Constructing Efficient Yellow-Orange Thermally Activated Delayed Fluorescence Emitters

Developing efficient long-wavelength thermally activated delayed fluorescence (TADF) emitters is a challenging issue due to inherent limitations of the energy-gap law. In this contribution, a new molecular design of cyanobenzene-decorated quinoxaline acceptor, combining two or four cyanobenzene acceptors and a rigid carbazole donor, is successfully utilized to construct long-wavelength TADF emitters (tCzQx2CN and tCzQx4CN). The two additional cyanobenzene units at the 5- and 8-positions of the quinoxaline acceptor are presumed to interlock the molecular rotations and improve the acceptor strength. Compared with tCzQx2CN, tCzQx4CN exhibits orange-red emission with a considerable bathochromic shift (>35 nm), as expected. Owing to its enhanced charge transfer and well-separated highest-occupied and lowest-unoccupied molecular orbitals, tCzQx4CN exhibits reduced singlet-triplet energy splitting and improves reverse intersystem crossing. An organic light-emitting diode (OLED) of tCzQx4CN manifestes bright orange-red emission (lambda(EL) approximate to 581 nm), a superior external quantum efficiency (EQE) of approximate to 23.7% (compared with that of tCzQx2CN), and a satisfactory operational lifetime. Furthermore, a tCzQx4CN-sensitized red-hyperfluorescent OLED device exhibits excellent performance with an EQE of 16.0% and a CIE(x,y) of (0.62, 0.37). This design would potentially pave the way for constructing red/deep-red TADF emitters by using a diverse combination of donor/acceptor units.