Phase behaviour of thermotropic banana-shaped compounds under pressure

The phase behaviour of two achiral bent core banana-shaped compounds, the hexyloxy (compound I) and decyloxy (compound II) members of the 1,3-phenylene bis[ N -(2-hydroxy-4- n -alkoxybenzylidene)-4'-aminobenzoate] series was investigated under hydrostatic pressures up to 300 MPa using high pressure differential thermal analysis and light transmission methods. The reversible transition sequence crystal (Cr-1 )-B-1 phase-isotropic liquid (I), observed at room pressure for compound I, remains in the pressure region up to c 70 MPa. At higher pressures a pressure-induced crystalline phase (Cr-i ) appears between the Cr-1 and B-1 phases, its temperature region becoming wider with increasing pressure. The temperature vs. pressure phase diagram shows a triple point of 72.9 MPa and 160.3degreesC for the Cr-1 , Cr-i and B-1 phases, indicating the lower limit of pressure for the Cr-i phase. In compound II the reversible transition sequence crystal (Cr-1 )-B-2 phase-I is seen over the whole pressure region, and the temperature range of the B-2 phase remains unaltered. It is concluded that both the B-1 and B-2 banana phases are stable over the whole pressure region studied.