Hybrid Heterocycle-Containing Electron-Transport Materials Synthesized by Regioselective Suzuki Cross-Coupling Reactions for Highly Efficient Phosphorescent OLEDs with Unprecedented Low Operating Voltage

A series of hybrid heterocyle-containing electron-transport materials (ETMs) with different pyridine substitution positions in a single molecule were successfully synthesized by regioselective sequential palladium-catalyzed Suzuki cross-coupling reactions. B3LYP calculations show that the experimental regioselectivity is consistent with the trends in calculated bond dissociation energies of the carbon–halogen (C–X) bonds of the reactants and intermediates. The two carbon species associated with C–N and C–C can be detected by X-ray photoelectron spectroscopy. Extremely low turn-on voltages (Von) of 2.1 V for electroluminescence, which are readily 0.2–0.3 V lower than the minimum value of the emitted photon energy (hv)/e, were experimentally achieved by utilizing the developed ETMs as an electron-transport and hole/exciton-block layer for the classical fac-tris(2-phenylpyridine) iridium (Ir(PPy)3)-based green phosphorescent organic light-emitting devices (OLEDs). In addition, hitherto the lowest operating v...