Facile access to isocoumarin-based D-A-D triad: A thermally activated delayed-fluorescence host for efficient red phosphorescent OLEDs

Isocoumarin (ICO) is a natural occurring and well-documented π-conjugated structure due to its electron-withdrawing property and thermal stability. Herein, a novel donor-acceptor-donor (D-A-D) triad type of TADF host using ICO as the acceptor has been developed for the first time via a facile Ru(II) catalytic self-annulation of α-benzoyl sulfoxonium ylides. The high electron-deficiency and planar geometry of ICO structure may also be in favour of compact π-π stacking, thus providing fast transporting pathways for electrons, while the lone pair electrons of oxygen atom in ICO structure would offer a favourable condition to form intramolecular C–H⋯O hydrogen bonding, thus potentially reducing exciton quenching due to the inhibited structural relaxation. The resulting D-A-D triad (PXZ-ICO) exhibits significant thermally activated delayed fluorescence and well-balanced bipolar transporting property. The OLED devices utilizing this triad as a host exhibit outstanding electroluminescence with low driving voltage, high efficiency and small efficiency roll-off. This work would probably give us helpful inspiration for discovering organic functional materials using ICO derivatives.