Liquid crystalline and monotropic phase behaviors of 2,3,6,7,10,11-hexa(4′-octyloxybenzoyloxy)triphenylene discotic molecules

Multiple phase transitions and monotropic phase behavior are observed in a discotic liquid crystal, 2,3,6,7,10,11-hexa(4'-octyloxybenzoyloxy)triphenylene (HOBT-C8). Detailed phase structural analyses using wide-angle X-ray diffraction and electron diffraction show that, in addition to a discotic nematic (ND) phase at high temperatures, a rectangular columnar (Phi (R)) phase and an orthorhombic crystalline (K-I) phase developing sequentially upon cooling can be identified. A crystalline (K-II) phase with a higher melting temperature than that of the K-I phase forms during heating. Therefore, the K-I phase represents a metastable phase with respect to the K-II phase, and the more stable K-II phase can be bypassed during cooling at relatively fast cooling rates. The K-I phase is thus monotropic with respect to the K-II phase at relatively slow heating rates. The optical texture of each phase can be distinguished from polarized light microscopy. A relationship between Gibbs free energy and temperature for different phases can be constructed, which thermodynamically and kinetically represents these multiple phase transitions and the monotropic phase behavior of the K-I phase.