TADF emitters based on a tri-spiral acridine donor and a spiro-B-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 (ΔE_ST < 0.05 eV), and short delayed fluorescence lifetimes (τ_d < 2 μs). Theoretical studies demonstrate that effective spin–orbit coupling between the charge transfer singlet (¹CT) and acceptor-centered local triplet (³LE) excited states accelerates the reverse intersystem crossing (RISC) process, resulting in a high RISC rate constant of ∼10⁶ s⁻¹. Notably, both emitters exhibit very high horizontal dipole orientation ratios (Θ_‖) of ∼93% in their doped host films. Owing to the outstanding TADF characteristics and high Θ_‖ 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.