Small organic molecules : building blocks of functional materials

The potential of phosphorescent organic light emitting diodes (phOLEDs) in full-color flat-panel display solid state lighting devices is fueling the interest in high triplet energy materials. 4,4′-Bis(9-carbazolyl)–biphenyl is one of the most widely used host material in phOLEDs. Because of its triplet energy of 2.56 eV, it is a suitable hole transporting material for green phosphorescent emitters, such as tris(2-phenylpyridine)iridium(III). However, the commonly used dye bis[(4,6-difluorophenyl)-pyridinato-N,C2]-picolinateiridium(III) in blue phOLEDs exhibits a triplet energy of 2.65 eV. Therefore, new high-triplet energy host materials are required to allow for efficient OLED-devices.
The design of blue pixel host materials is based on the idea of reducing the level of conjugation between the carbazole subunit and the biphenyl backbone. Therefore, a series of CBP derivatives was synthesized and studied with spatially restricted degrees of freedom in their biphenyl-N-carbazole junctions by introducing spatial demanding methyl moieties either in positions 1 and 8 of the carbazole unit or in positions 3,5,3’,5’ of the biphenyl backbone. Moreover, the electronic features of the carbazole synthons were investigated by attaching electron-withdrawing or electron-donating groups in the positions 3 and 6 or positions 2 and 7 of the carbazole subunits.