Systematical Study On The Relationship Between Molecular Structure And Properties Of Triphenylene-based Discotic Liquid Crystalline Dimers

Two series of discotic dimers T3 Dn based on hexapropoxytriphenylene(HAT3) and T5D5 based on hexapentyloxytriphenylene(HAT5) have been synthesized, which contain two triphenylene monomers, and are linked through flexible O(CH2)nO polymethylene chains(n=3-12). Their chemical structures were confirmed by proton nuclear magnetic resonance spectrum(1H NMR), fourier transform infrared spectroscopy(FT-IR), high resolution mass spectrum(HRSM) and elemental analysis(EA). Their mesomorphism was investigated by differential scanning calorimetry(DSC), polarizing optical microscopy(POM), and X-ray diffraction(XRD). The charge carrier mobilities of these dimers with liquid crystal phase were measured by using time-of-flight technique(TOF). It was found that dimers T3D5 based on HAT3 formed a highly ordered rectangular columnar plastic phase and T3D6-T3D12(6-12)formed a highly ordered hexagonal columnar plastic phase. Dimers T5D12 based on HAT5 exhibited a rectangular columnar phase and T5D6-T5D11(n=6-11) formed a hexagonal columnar phase. In our previous work of dimers T4 Dn based on hexabutyloxytriphenylene(HAT4), dimers T4Dn(n=6, 7, 11,12) showed a highly ordered colhp phase in the relatively low temperature range and a colh phase in the higher temperature range. Dimers T4Dn(n=8, 9,10) formed a single colhp phase. Compared to the mesomorphism of these dimers, dimers in T3 Dn, T4 Dn, T5 Dn exhibited a transition from colhp phase to a colh phase, and dimers T4D6, T4D7, T4D11, T4D12 in T4 Dn showed a phase competition between colh phase and colhp phase, which was induced by linkages. Comparing with core-core distances, enthalpies of phase transition from mesophase to isotropic, charge carrier mobilities in each series of dimers T3 Dn,T4Dn and T5 Dn, it was found that those compounds T3D6, T4D8,T5D10, in which linkage lengths were twice those of side chains showed thesmallest intracolumnar spacings, the largest enthalpies and the highest charger carrier mobilities among their homologous, respectively, which implied that they formed the most highly ordered phase among their homologous. The charge carrier mobilities of T3D6, T4D8 and T5D10 were slightly larger than those of their corresponding monomers HAT3, HAT4 and HAT5, which indicated that the separated monomers in dimers were limited by the linkages,and T3D6, T4D8, T5D10 owned a higher order degree compared to their corresponding monomers.