TADF emitters based on a tri-spiral acridine donor and a spiro-<i>B</i>-heterotriangulene acceptor with high horizontal dipole orientation ratios and high efficiencies in deep-blue OLEDs

Abstract

Developing thermally activated delayed fluorescence (TADF) emitters showing high horizontal transition dipole orientation and molecular rigidity is crucial for enhancing the color purity and performance of deep-blue organic light-emitting diodes (OLEDs). Here, we report two linearly expanded TADF emitters, O-tsAC-BAsBP (1) and S-tsAC-BAsBP (2), based on a tri-spiral acridine donor and a spiro-fluorenyl B-heterotriangulene acceptor. These emitters exhibit deep-blue emissions, with peaks centered at 458-467 nm for 1 and 462-469 nm for 2, respectively, in the host films, with high photoluminescence quantum yields, small singlet-triplet energy splitting (Delta E-ST < 0.05 eV), and short delayed fluorescence lifetimes (tau(d) < 2 mu s). Theoretical studies demonstrate that effective spin-orbit coupling between the charge transfer singlet ((CT)-C-1) and acceptor-centered local triplet ((LE)-L-3) excited states accelerates the reverse intersystem crossing (RISC) process, resulting in a high RISC rate constant of similar to 10(6) s(-1). Notably, both emitters exhibit very high horizontal dipole orientation ratios (Theta(parallel to)) of similar to 93% in their doped host films. Owing to the outstanding TADF characteristics and high Theta(parallel to) values, TADF-OLEDs incorporating emitters 1 and 2 achieve high maximum external quantum efficiencies of 27.4% and 31.5%, respectively, in the deep-blue region.