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 (PXZoB, DPAoB, and CzoB), 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 (ΔEST) and thus exhibiting very efficient TADF with microsecond-range lifetimes. In sharp contrast, the corresponding para D-A derivatives, DPApB and CzpB, 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 (ηEQE). In particular, the pure blue OLEDs based on the proposed ortho D-A emitters with a carbazole donor (CzoB) show a record-high ηEQE of 22.6% with CIE color coordinates of (0.139, 0.150), well illustrating the validity of the proposed approach. Upon optical optimization, the ηEQE is further improved to 24.1%.