Synthesis and characterization of heteroatom substituted carbazole derivatives: potential host materials for phosphorescent organic light-emitting diodes

A series of carbazole derivatives based on 9-ethyl-carbazole substituted at the 3-position with diphenylamine, carbazole, phenoxazine, phenothiazine, and phenothiazine-S,S-dioxide units were successfully developed, and a comparative study of their thermal and photoelectrical properties was carried out by differential scanning calorimetric measurements, thermogravimetric analysis, UV-vis absorption spectroscopy, photoluminescence spectroscopy, cyclic voltammetry, and phosphorescent spectroscopy. All the synthesized compounds were found to have high thermal decomposition temperature in the range of 304–344 °C (except for C-SO, 145 °C) and showed a remarkable improvement when compared with 9-ethyl-carbazole (180 °C). The emission properties can be effectively tuned by systematically changing the substituent units. The highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), and triplet energy (ET) of the synthesized compounds were determined both theoretically by quantum chemical calculations and experimentally by examination. Their energy levels can also be tuned by the change of the moieties connected to the 3-position through coupling reaction. The experimental values of ionization potentials range from 5.37 to 5.46 eV. Some of these compounds exhibit high ET, and C-SO achieves the highest ET value of 2.79 eV, which exhibits a marked improvement over that of 4,4′-bis(9-carbazolyl)-biphenyl (2.6 eV). Green phosphorescent devices using tris(2-phenylpyridine)iridium as a guest and C-O as a host show excellent performances. Remarkably, a device with C-O as host exhibits the best performance with a maximum luminance of 10 270 cd m−2, a maximum current efficiency of 34.8 cd A−1, and a maximum power efficiency of 26.0 lm W−1. These results illustrate that the introduction of heteroatoms into carbazole can result in a significant change in their optoelectronic characteristics. The low LUMO, high HOMO, suitable ET and thermal stability of the carbazole derivatives endow them with the potential to be green, red, and even blue host materials for phosphorescent organic light-emitting diodes.