Molecular Crystals and Liquid Crystals, Vol.515, 16-38, 2009
pi-Conjugated Triphenylene Twins Exhibiting Polymesomorphism Including the Nematic Phase
Novel discotic liquid crystals possessing two triphenylene (TP) mesogenic cores covalently connected via a rigid spacer, i.e., -conjugated ethynyl- or butadiynyl-bridge, (TP twins) are reported. The TP twins are categorized by the lengths of -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 -substituted and peripheral chains (R1=R2=R3=R4), B) equivalent TP entities having a -substituted chain (R2=R3) and peripheral chains (R1=R4) of different length from the substitution, and C) TP twins with different TP entities having the same -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 -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.