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)triphenyl ene (HOBT-C8). Detailed phase structural analyses using wide-angle X-ray di ffraction and electron diffraction show that, in addition to a discotic nem atic (ND) phase at high temperatures, a rectangular columnar (Phi (R)) phas e 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, th e K-I phase represents a metastable phase with respect to the K-II phase, a nd 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 phas e can be distinguished from polarized light microscopy. A relationship betw een Gibbs free energy and temperature for different phases can be construct ed, which thermodynamically and kinetically represents these multiple phase transitions and the monotropic phase behavior of the K-I phase.