Benzobisoxazole-based electron transporting materials with high T_g and ambipolar property: High efficiency deep-red phosphorescent OLEDs

Two new benzobisoxazole-based compounds, namely, 4,8-bis(3,5-di(pyridin-3-yl)phenyl)-2,6-dimethylbenzo[1,2-d:4,5-d′]bis(oxazole) (3Py-DBBO) and 4,8-bis(4-yl-triphenylphosphine oxide)-2,6-dimethylbenzo[1,2-d:4,5-d′]bis(oxazole) (TPO-DBBO), were synthesized and characterized. The steric effect of the ortho hydrogens on the phenyl ring at 4,8-positions of benzobisoxazole resulted in out-of-plane twisting, and consequently a decrease of the intermolecular π-π interaction. The two new compounds showed excellent thermal stabilities with high glass-transition temperatures (T_g) of 248 °C for 3Py-DBBO and 142 °C for TPO-DBBO. The two compounds exhibited ambipolar transport properties, with both electron and hole mobilities of 10^-6-10^-5 cm² V^-1 s^-1. Using the compounds as electron-transport materials, the deep-red phosphorescent organic light-emitting devices achieved a maximum external quantum efficiency up to 19.3%.