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

Abstract 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 ( t CzQx2CN and t CzQx4CN ). 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 t CzQx2CN , t CzQx4CN 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, t CzQx4CN exhibits reduced singlet‐triplet energy splitting and improves reverse intersystem crossing. An organic light–emitting diode (OLED) of t CzQx4CN manifestes bright orange‐red emission ( λ EL ≈581 nm), a superior external quantum efficiency (EQE) of ≈23.7% (compared with that of t CzQx2CN) , and a satisfactory operational lifetime. Furthermore, a t CzQx4CN‐ 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.