New discotic mesogens based on triphenylene-fused triazatruxenes: Synthesis, physical properties, and self-assembly

A new type of discotic mesogen based on triphenylene-fused triazatruxenes was prepared by microwave-assisted 6-fold Suzuki coupling reactions from hexabromotriazatruxene, followed by FeCl₃-mediated oxidative cyclodehydrogenation. These disklike molecules showed extended π-conjugation, compared with the triphenylene and triazatruxene themselves. More importantly, they possess desirable HOMO energy levels, which allow efficient charge injection from electrodes such as gold electrodes. Their thermal behavior and self-assembly were studied by different techniques such as thermogravimetric analysis, differential scanning calorimetry, polarizing optical microscopy, and variable-temperature X-ray diffraction. These new discotic mesogens have very good thermal stability and show thermotropic liquid crystalline behavior. Ordered columnar liquid crystalline phase and crystalline phase were observed in both compounds with tunable phase transition temperatures and mesophase widths. The charge carrier mobilities of these extended triazatruxene samples were determined using the space-charge limited-current (SCLC) technique and high hole mobilities of 0.03 and 0.8 cm² V^-1 s^-1) were obtained for TP-TATC6 and TP-TATC12, respectively. Interestingly, the long-aliphatic-chain-substituted TP-TATC12 can gelate several nonpolar hydrocarbon solvents or polar aliphatic alcohol and ester solvents, because of strong intermolecular interactions. All these properties qualify this new type of discotic liquid crystals as potential hole transporting materials for electronic devices such as field-effect transistors, light-emitting diodes, and solar cells.