Rational Molecular Design via Cyanobenzene Integration for Constructing Efficient Yellow‐Orange Thermally Activated Delayed Fluorescence Emitters
Shantaram Kothavale, Junseop Lim, Rajendra Kumar Konidena, Jun Yeob Lee
Abstract
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.