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 (V-on) 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 voltages of 2.39, 2.72, and 3.88 V for 100, 1000, and 10 000 cd m(-2) were achieved with simultaneously improved external quantum efficiency (eta(ext)) to give a high power efficiency, and the operating voltage for 100 cd m(-2) is already corresponding to the value of hv/e. Atomic force microscopy measurements reveal morphological fine structure of the thin film samples of ETMs with a pyrimidine core, which possibly contributes to the low driving voltages that were achieved for the devices that are based on these ETMs. These findings indicate that aside from the ultralow operating voltage well-balanced carriers can be also achieved with a finely synthesized hybrid heterocyle-containing ETM as a nondoped electron-transport layer, and the B3LYP calculation is an effective method to predict the regioselective cross-coupling reactions for fine design of such an ETM.