CN-Containing donor-acceptor-type small-molecule materials for thermally activated delayed fluorescence OLEDs

Very recently, the design of thermally activated delayed fluorescence (TADF) emitters has drawn significant attention in the production of low-cost and highly efficient organic light-emitting diodes (OLEDs). Several types of TADF materials have been successfully developed such as pure organic small molecules, polymers, metal complexes, and exciplexes. Among them, donor-acceptor (D-A)-structured small molecules have been regarded as most popular due to their twisted structure with large dihedral angle between the electron-donating and accepting subunits that permits small singlet-triplet energy gaps (ΔE_ST) for efficient reverse intersystem crossing (RISC). Various electron-withdrawing functional groups have been introduced for D-A-type TADF materials. The conjugated cyano (CN) moiety has been mostly demonstrated through rational molecular design based on the number of electron-accepting CN molecules and the relative linking position with electron-donating π-conjugated units. Herein, we reviewed the recent development of CN-containing D-A-type small molecular TADF materials according to different CN containing acceptors, such as mono- or multi-cyanobenzene, mono- or dicyano based N-heterocyclic aromatic rings and acrylonitrile derivatives. Moreover, structure-property relationships have been specially emphasized.