Structurally simple phenanthroimidazole-based bipolar hosts for high-performance green and red electroluminescent devices

With the aim of developing bipolar host materials, m-DPPI with two phenanthroimidazole groups meta-linked on a benzene ring and its donor–acceptor-type analogue, m-PPPI, with one of the phenanthroimidazole moieties replaced by an electron-withdrawing imidazophenanthroline group have been synthesized. The meta-linkage endows these compounds with relatively high triplet energies of ca. 2.6 eV for hosting both green and red phosphorescent emitters. The single-carrier devices indicate that both the neat and phosphor-doped films of these compounds have bipolar transporting properties, with relatively higher current densities for the films of m-DPPI. Efficient green (maximum efficiencies: 58.5 cd A−1, 67.1 lm W−1, external quantum efficiency (ηext) of 15.5%) and red (maximum efficiencies: 19.7 cd A−1, 26.9 lm W−1, ηext of 11.6%) PhOLEDs have been achieved using m-DPPI as the host. The performance of m-PPPI-based devices is also high, although a bit lower than that of m-DPPI-based devices. Moreover, all the devices exhibit low current-efficiency and ηext roll-off. The simple structures, easy syntheses and high device performance of these compounds are attractive regarding practical applications.