Synthesis and characterization of novel topology-varied compounds based on fluorene and carbazole: Potential host materials for phosphorescent organic light-emitting diodes

In order to investigate the explicit effect of molecular structures with sterically perpendicular configuration on the general optoelectronic properties of fluorene and carbazole derivatives, we designed and synthesized several topology-varied compounds. The photophysical properties, thermal stability, energy levels of the compounds and device applications were further investigated to correlate the relationship between the chemical structure and the performance of host materials for phosphorescent organic light-emitting diodes. All the compounds are found to have high thermal decomposition temperature in a range of 276–386 °C and show a remarkable improvement when compared with sublimation temperature (180 °C) of 9-ethyl-carbazole. The introduction of spiro[fluorene-9,9′-xanthene] can increase the lowest unoccupied molecular orbital, while keeping the highest occupied molecular orbital practically unchanged. Phosphorescent organic green light-emitting diodes of one compound doped with 10 wt% tris(2-phenylpyridine) iridium achieves a maximum current efficiency of 21.0 cd A−1, a maximum power efficiency of 11.9 lm W−1, a turn-on voltage of 3.5 V and a maximum luminance of 49,000 cd m−2, which are higher than those of the blue light-emitting diodes based on the same compound doped with bis[(4,6-difluorophenyl)-pyridinato-N,C2]. In this regard, another compound with relatively higher thermal stability and triplet energy gap has potential to be a better host material for phosphorescent organic light-emitting diodes.