Recent Progress in Phenoxazine-Based Thermally Activated Delayed Fluorescent Compounds and Their Full-Color Organic Light-Emitting Diodes

Third-generation organic light-emitting diodes (OLEDs) based on metal-free thermally activated delayed fluorescent (TADF) materials have sparked tremendous interest in the last decade due to their nearly 100% exciton utilization efficiency, which can address the low-efficiency issue of the first-generation fluorescent emitters and the high-cost issue of the second-generation organometallic phosphorescent emitters. Construction of efficient and stable TADF-OLEDs requires utilizing TADF materials with a narrow singlet-triplet energy gap (Delta EST), high photoluminescence quantum yield (PLQY) and short TADF lifetime. A small Delta EST is necessary for an efficient reverse intersystem crossing (RISC) process, which can be achieved through the effective spatial separation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). TADF emitters have been generally designed as intramolecular charge transfer (ICT) molecules with highly twisted donor-acceptor (D-A) molecular architectures. A wide variety of combinations of electron donors and acceptors have been explored. In this review, we shall focus on recent progress in organic TADF molecules incorporating strong electron-donor phenoxazine moiety and their application as emitting layer (EML) in OLEDs.