Rational Design of Chelated Aluminum Complexes toward Highly Efficient and Thermally Stable Electron-Transporting Materials

Two novel chelated aluminum complexes, bis(2-methyl-8-quinolinato)(2,6-di(pyridin-3-yl)phenolato)Al(III) (BPyAlq) and bis(2-methyl-8-quinolinato)(2,4,6-tri(pyridin-3-yl)phenolato)Al(III) (TPyAlq), have been designed and synthesized by introducing the electron-withdrawing pyridine moieties into the ancillary ligand. Their single-crystal and molecular structures, thermal and photophysical properties, and electron-transporting (ET) capabilities were systematically investigated in comparison with the parent complex bis(2-methyl-8-quinolinato)(4-phenyl-phenolato)Al(III) (BAlq). The introduction of the pyridine moieties helps to not only greatly lower the LUMO and HOMO energy levels but also bring about extra strong intra- and intermolecular interactions, thus significantly improving the electron-injection (EI) and ET capabilities of the involved materials. BPyAlq and TPyAlq exhibited significantly higher glass-transition temperatures (106 and 141 °C) than BAlq (92 °C). The electron-only devices based on BPyAlq or TPyAlq showed more than 2 orders of magnitude higher current density than that of BAlq, indicating much higher ET/EI capabilities. Their excellent ET/EI properties were investigated via the phosphorescent OLEDs using fac -tris(2-phenylpyridine)iridium as the green emitter. The devices with BPyAlq and TPyAlq as an electron-transporting layer (ETL) demonstrated superior performance compared to those using BAlq, remarkably lowering the drive voltage and improving efficiencies. In particular, the green PhOLEDs with BPyAlq as an ETL exhibited the maximum current and external quantum efficiency of 59.8 cd A –1, 17.3% with small efficiency roll-off.