Study On The Synthesis And Structure-properties Relationship Of Triphenylene-based Discotic Unsymmetrical Dimers

Three series of triphenylene-based dimers T34 Dm, T45 Dm, T35 Dm were synthesized which combined any two of the three differently substituted triphenylene rings, each of which bears five β-OCnH2n+1(n=3,4,5) substituents,which are linked through the remaining β positions by a flexible O(CH2)mO chain(m=3-12). The structure-properties relationship of the dimers influenced by spacer lengths and peripheral side chains was investigated by differential scanning calorimetry(DSC), polarizing optical microscopy(POM), and wide-angle X-ray diffraction(WAXD). Compounds in all of the three series for which the spacer length was less than 5 were crystals. For the compounds which the spacer length was 5, T35D5 did not form liquid crystal while T45D5 and T34D5 formed rectangle columnar plastic phase(Colrp). When the spacer length is more than 5, dimers of T34 Dm showed hexagonal columnar plastic phase(Colhp) and some of them(m=6, 11 and 12) exhibited a transition from Colhp phase to Colh phase,dimers of T45 Dm showed polymorphism with a transition from Colhp phase to Colh phase and some of them(m=6, 7, 11 and 12)exhibited a single Colh phase,dimers of T35 Dm formed hexagonal columnar phase(Colh). The variation of isotropization temperatures, the corresponding enthalpy changes, core-core distance and charge carrier mobilities dependent on spacer length in the three series of dimers were compared. It is concluded that dimers whose spacer length were twice of side chains showed the most ordered mesophase structure. 2D WAXD pattern which showed beautiful superlattice structure can explain the complex structure of highly ordered columnar plastic phase. The introduction of unsymmetry makes the dimers much ordered and much easier formed superlattice structure.