Phase behaviour of the discotic mesogen 2,3,6,7,10,11-hexahexyl thiotriphenylene (HHTT) under pressure

Abstract

The phase behaviour of the discotic mesogen 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT) was investigated under hydrostatic pressures up to 300 MPa using wide-angle X-ray diffraction equipped with a high-pressure vessel. The reversible transitions of HHTT, i.e., crystal (Cr) - helical phase (H) - hexagonal columnar phase (Col h ) - isotropic liquid (I), were observed under pressures up to about 30 MPa. The transition sequence changed significantly depending on the applied pressure. The H phase appeared monotropically in the pressure region between 50 and 180 MPa, indicating the Cr→Col h→I on heating and I→Col h→H→Cr transition sequences on cooling. Applying higher pressures altered further the phase behaviour: the enantiotropic Col h phase changed to monotropic one under high pressures above 180 MPa, indicating the Cr→ I on heating and I→ Col h→Cr transition sequences on cooling. The X-ray diffraction pattern of the H phase exhibited a sharp low-angle and a wide-angle reflections corresponding to the intercolumnar and the core-core distances, respectively. This feature is indicative of the three-dimensional ordering present in the H phase. On the other hand, the Col h phase showed only a low-angle reflection. It was found that the intercolumnar distance in the Col h phase decreases with increasing temperature at high pressures. This indicates that the negative coefficient of thermal expansion for the column packing in the Col h phase is held under pressure.