New bent-core mesogens with carbon–carbon multiple linkages in the terminal chains

Abstract

New five-ring bent-core mesogens have been synthesized in which terminal alkynyl chains are connected with the terminal rings of the aromatic core by means of oxycarbonyl groups, cinnamic esters groups or oxyacetic ester moieties. The insertion of C–C triple bonds in different positions of the terminal chains can change the mesophase behaviour and can result in an increase or decrease of the clearing temperatures in comparison to corresponding compounds with saturated chains. In addition, lateral substituents are attached to different positions of the central ring. The mesophase behaviour of the new compounds has been studied by polarizing microscopy, differential scanning calorimetry, X-ray experiments and electro-optical measurements. On the base of the experimental findings new relationships between the molecular structure and the mesomorphic properties have been obtained. Different polar phases (SmCP, Col, B₇′) could be proved, in some of these the polar (ferro- or antiferroelectric) switching takes place through a collective rotation of the molecules around their long axes. Interestingly, three compounds with terminal alkynyl chains form a mesophase with all textural features of a B₇ phase although these phases possess obviously a simple layer structure. For the first time it could be observed that helical filaments formed on slow cooling the isotropic liquid are stable to low temperatures. Some of these spirals can serve as nuclei in the growing process of ribbon-like and oval domains. Surprisingly, the helical pitch of the spirals can be exactly recovered in the equidistant stripes of these domains. It is also remarkable that all compounds derived from 2-methylisophthalic acid exhibit unusually high clearing temperatures and that the SmC phases of these compounds show polar switching about 50 K below the SmA–SmC transition temperature.