Phase behaviour of three homologues of the discotic hexa‐n‐alkoxyanthraquinones under pressure

Abstract

The phase transition behaviour of three homologous discotic mesogens, the hexa‐n‐alkoxyanthraquinones HOAQ(n), n indicating the number of carbon atoms in the alkoxy group, was investigated under hydrostatic pressures up to 500 MPa using a high pressure differential thermal analyser. The T vs. P phase diagrams of HOAQ(6), HOAQ(8) and HOAQ(9) were constructed for solution‐ (Cr0) and melt‐crystallized (Cr1) samples of the compounds. HOAQ(6) shows the reversible Cr0–rectangular columnar phase (Colr)–hexagonal columnar phase (Colh)–isotropic liquid (I) phase sequence at atmospheric pressure. The stable Colr phase of HOAQ(6) has a decreased temperature range with increasing pressure and then the Colr phase disappears under pressures above about 350 MPa; instead the Cr0–Colh–I phase sequence is exhibited. For HOAQ(8), the solution‐grown sample exhibits the stable Cr0–Colh–I phase sequence at atmospheric pressure. Applying pressure to the solution‐grown sample induces the formation of the stable Colr phase in the pressure region between 10 and 350 MPa, leading to the Cr0–Colr–Colh–I phase sequence. The pressure‐induced Colr phase disappears under higher pressures. The melt‐cooled sample of HOAQ(8) shows the formation of the metastable crystal (Cr1), unknown mesophase (X) and Colr phases at lower temperatures under atmospheric pressure, and exhibits the reversible Cr1–X–Colr–Colh–I phase sequence on subsequent thermal cycles. The metastable phase sequence was observed under pressures up to 100 MPa, but the phase transitions were too small to be detected under higher pressures. In HOAQ(9) the stable Cr0–Colh–I phase sequence is observed at all pressures, while the melt‐cooled sample shows the metastable Cr1–Colr–Colh–I phase sequence under pressures up to 300 MPa. The metastable Colr phase disappears under higher pressures.