Azaborine as a Versatile Weak Donor for Thermally Activated Delayed Fluorescence

Extensive research has been devoted to the development of thermally activated delayed fluorescence emitters, especially those showing pure-blue emission for use in lighting and full-color display applications. Toward that goal, herein we report a novel weak donor, 1,4-azaborine (AZB), with complementary electronic and structural properties compared to the widely used dimethylacridan (DMAC) or carbazole (Cz) donors. Coupled with a triazine acceptor, AZB-Ph-TRZ is the direct structural analogue of the high-performance and well-studied green TADF emitter DMAC-TRZ and has ΔE_ST = 0.39 eV, a photoluminescence quantum yield (Φ_PL) of 27%, and λ_PL = 415 nm in 10 wt % doped mCP films. The shortened analogue AZB-TRZ possesses red-shifted emission with a reduced singlet-triplet gap (ΔE_ST = 0.01 eV) and fast reverse intersystem crossing (k_RISC of 5 × 10⁶ s^-1) in mCP. Despite a moderate Φ_PL of 34%, OLEDs with AZB-TRZ in mCP showed sky-blue emission with CIE₁₉₃₁(x,y) of (0.22,0.39) and a maximum external quantum efficiency (EQE_max) of 10.5%. Expanding the chemist's toolkit for the design of blue donor-acceptor TADF materials will enable yet further advances in the future, as AZB is paired with a wider range of acceptor groups.