The role of electron-transporting Benzo[f]quinoline unit as an electron acceptor of new bipolar hosts for green PHOLEDs

Abstract We prepared three new compounds [3,6-di(9H-carbazol-9-yl)phenanthrene ( 3,6-DCP ), 2,9-di(9H-carbazol-9-yl)benzo[ f ]quinoline ( 2,9-DCBQ ), and 3,9-di(9H-carbazol-9-yl)benzo[ f ]-quinoline ( 3,9-DCBQ )] containing phenanthrene or benzo[ f ]quinoline as an electron-withdrawing moiety and a carbazole as electron-donating moiety, respectively, as bipolar hosts for green phosphorescent organic light emitting diodes (PHOLEDs). We intentionally substituted nitrogen atom to the C-3 position of phenanthrene moiety to prepare benzo[ f ]quinolinegroup. And, we found that it allowed better electron transporting behavior than the phenanthrene moiety. Meanwhile, the benzo[ f ]quinoline/phenanthrene core moieties significantly improved the thermal stability of those host materials, which exhibited glass transition and decomposition temperatures of 132–139 and 395–427 °C, respectively. The green PHOLEDs which were fabricated with those host materials showed the lowest operating voltage of 4.7 V at 1000 cd/m 2 when we used 3,9-DCBQ . Very interestingly, it has an asymmetric structure with completely separated HOMO and LUMO in space. In contrast, 3,6-DCP having phenanthrene and carbazole moieties showed much higher operating voltage of 6.1 V which imply that replacing nitrogen at the C-3 position of phenanthrene improves carrier transport, that is, electron transporting behavior. As a result, the 3,9-DCBQ -based PHOLED showed the best overall performance, exhibiting current and power efficiencies of 48.5 cd/A and 20.6 lm/W, respectively.