The Characterization of a New Helical Liquid Crystal Phase

We report the discovery of a novel liquid crystal phase which exhibits two seemingly incongruous characteristic features; namely, a smectic-A like layered structure along with a macroscopic (1microns) helical structure with the helical axis parallel to the layer planes. The layered structure results in resolution-limited x-ray diffraction peaks while the helix makes the phase optically iridescent. The new phase is apparently miscible with the classical smectic-A phase, therefore we classify it as a chiral modification of the smectic-A or smectic-A*. A*-phase was observed in a variety of optically active R- and S-1 methylheptyl 4'-[(4"-n-alkoxyphenyl)propionoyloxy]-bipheny 1-4-carboxylates which show a high degree of chirality. It has thus for been observed when the A-phase is simultaneously in the vicinity of both a chiral smectic-C* phase as well as an isotropic phase. As the temperature range between these two phases extends beyond -6C, the classical A-phase appears instead. Moreover, in the non-chiral racemic mixtures of these compounds the A*-phase is replaced by a classical A-phase. We will argue that this behavior, along with the structural properties of the A*-phase, make it a good candidate for the "Twist Grain Boundry" (TGB) phase recently proposed by Renn and Lubensky sup 1.