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 ΔEST = 0.39 eV, a photoluminescence quantum yield (ΦPL) of 27%, and λPL = 415 nm in 10 wt % doped
mCP films. The shortened analogue AZB-TRZ possesses red-shifted
emission with a reduced singlet–triplet gap (ΔEST = 0.01 eV) and fast reverse intersystem crossing
(kRISC of 5 × 106 s–1) in mCP. Despite a moderate ΦPL 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 (EQEmax) 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.