Achiral H-shaped liquid crystals exhibiting an electric-field-induced chiral nematic phase

Abstract

We prepared two achiral H-shaped liquid crystals, 4,4′-bis(7-{2-[4-(4-cyanophenyl)phenyloxycarbonyl]-5-octyloxyphenyloxycarbonyl}heptyloxy)biphenyl (I) and 4,4′-bis(7-{2-[4-(4-cyanophenyl)phenyloxycarbonyl]-5-octyloxyphenyloxycarbonyl}heptyloxy)octafluorobiphenyl (II), and investigated their phase transition properties using optical microscopy and differential scanning calorimetry. Both compounds exhibit a monotropic nematic (N) phase with compound I showing a higher Iso–N transition temperature than compound II. The phase transition behavior reveals that the molecules of both compounds form a rod-like shape instead of an H shape in the N phase. They exhibit monotropic blue phase III in their mixtures with a chiral compound. Furthermore, both compounds I and II are characterized by an electric-field-induced chiral nematic phase. When applying an AC electric field on homogeneously aligned sample cells of each compound in its nematic phase, the molecules reorient perpendicular to the substrate. Upon removing the electric field, the molecules align once again parallel to the substrate; however, this time their nematic texture changes to one consisting of domains with opposite helical handedness. We establish that coupling of the on/off switching of the electric field and the surface anchoring is responsible for breaking the mirror symmetry in the nematic phase of the flexible achiral liquid crystal oligomer.