Rigidity-Induced Delayed Fluorescence by Ortho Donor-Appended Triarylboron Compounds: Record-High Efficiency in Pure Blue Fluorescent Organic Light-Emitting Diodes

A synthetic approach to highly efficient thermally activated delayed fluorescence (TADF) is proposed that uses ortho donor (D)–acceptor (A) compounds (PXZ o B, DPA o B, and Cz o B), wherein the acceptor is based on triarylboron and the donor is phenoxazine (PXZ), diphenylamine (DPA), or carbazole (Cz). Combined with the ortho D–A connectivity, the bulky nature of the triarylboron endows the D–A dyads with inherent steric “locking” for a highly twisted arrangement, leading to a small energy difference between singlet and triplet excited states (Δ E ST ) and thus exhibiting very efficient TADF with microsecond-range lifetimes. In sharp contrast, the corresponding para D–A derivatives, DPA p B and Cz p B, only display short-lived, normal fluorescence. Organic light-emitting diodes (OLEDs) incorporating the proposed ortho D–A compounds as emitters display orange, greenish-blue, and pure blue emission and exhibit high external quantum efficiency (η EQE ). In particular, the pure blue OLEDs based on the proposed ortho D–A emitters with a carbazole donor (Cz o B) show a record-high η EQE of 22.6% with CIE color coordinates of (0.139, 0.150), well illustrating the validity of the proposed approach. Upon optical optimization, the η EQE is further improved to 24.1%.