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.