π-Conjugated Triphenylene Twins Exhibiting Polymesomorphism Including the Nematic Phase

Abstract

Novel discotic liquid crystals possessing two triphenylene (TP) mesogenic cores covalently connected via a rigid spacer, i.e., π-conjugated ethynyl- or butadiynyl-bridge, (TP twins) are reported. The TP twins are categorized by the lengths of β-substituted chains (R2 and R3) and peripheral alkoxy or branched aliphatic chains (R1 and R4). Three varieties of TP twins are prepared: A) TP twins having identical β-substituted and peripheral chains (R1 = R2 = R3 = R4), B) equivalent TP entities having a β-substituted chain (R2 = R3) and peripheral chains (R1 = R4) of different length from the β substitution, and C) TP twins with different TP entities having the same β-substituted and peripheral alkoxy chains (R1 = R2, R3 = R4). The phase behavior is characterized by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD) studies. These TP twins exhibit a discotic nematic (N) and/or columnar rectangular (Colr) mesophases over a wide temperature range. We found that the relative length of rigid spacer and β-substituted alkoxy chains play an important role in the creation of the molecular self-assembly and formation of the mesophases. To the best of our knowledge, this is the first report of TP twins exhibiting polymesomorphism.