Phase behaviour under pressure of a dichiral liquid crystal with an optically isotropic cubic phase: 2‐{4‐[(R)‐2‐fluorohexyloxy]phenyl}‐5‐{4‐[(S)‐2‐fluoro‐2‐methyldecanoyloxy]phenyl}pyrimidine

Abstract

The phase behaviour of a dichiral liquid crystalline compound with an optically isotropic cubic phase, 2‐{4‐[(R)‐2‐fluorohexyloxy]phenyl}‐5‐{4‐[(S)‐2‐fluoro‐2‐methyldecanoyloxy]phenyl}pyrimidine, (R,S)‐FPPY, was investigated under pressures up to 100 MPa using a polarizing optical microscope equipped with a high pressure optical cell and a high pressure differential thermal analyser. (R,S)‐FPPY exhibits the isotropic liquid (I)–chiral smectic C (SmC*)–optically isotropic cubic (Cub)–chiral smectic X (SmX*)–crystal (Cr) transition sequence on cooling at pressures up to about 13 MPa, in which the SmC*–Cub transition occurs with an associated endothermic peak. The heating process of the (R,S)‐FPPY crystals showed the Cr–SmX*–Cub–I and Cr–SmX*–SmC*–I phase sequences under pressures below and above about 50 MPa, respectively. The triple point for the SmX*, Cub and isotropic liquid phases was estimated to be about 52±1 MPa, 143.0±0.5°C, indicating the upper limit of pressure for the formation of the cubic phase. On the other hand, the phase behaviour of (R,S)‐FPPY in the cooling mode changed drastically at pressures above 13–14 MPa: after the I–SmC* transition, the SmC* phase remained without the SmC*–Cub transition on cooling, but the subsequent heating showed the SmC*–Cub–I phase sequence in the intermediate pressure region between 14 and about 50 MPa, and the reversible SmC*–I transition in the high pressure region. It is concluded that the cubic phase appears reversibly in the low pressure region and appears only on heating in the intermediate pressure region, but that it disappears completely in the high pressure region. The SmC* phase is thermodynamically metastable at low pressures below 50 MPa and is stable at higher pressures.