Highly Efficient, Red Delayed Fluorescent Emitters with Exothermic Reverse Intersystem Crossing via Hot Excited Triplet States

Three donor–acceptor–donor
molecular emitters have
been designed by taking triphenylamine or N-phenylcarbazole
as the donor and maleimide or phenyl maleimide as the acceptor, in
which the highest occupied molecular orbital interaction between two
donor units is maximized via the acceptor bridge. This is envisaged
to enable both strong fluorescence radiation and fast exoergic reverse
intersystem crossing via the second triplet state. Detailed photophysical
characterization and theoretical calculations confirm that all the
compounds have large oscillator strengths and short delayed fluorescence
lifetimes of ∼0.2 μs. The fabricated organic light-emitting
diodes (OLEDs) give red emission above 600 nm, luminance exceeding
6000 cd m–2, and external quantum efficiencies (EQEs)
of over 6%. In particular, the best device shows an emission at 645
nm and a maximum EQE of 10.4%. Moreover, the EQEs remain above 3%
at 1000 cd m–2 for all the emitters. This work provides
an effective method to develop organic emitters for highly efficient
OLEDs with low-efficiency roll-off.