Synthesis and thermotropic liquid crystalline properties of calamitic molecules with laterally attached hydrophilic groups: Y-shaped three-block molecules which can form smectic and columnar mesophases

Abstract

The synthesis and the thermotropic liquid crystalline properties of calamitic mesogens (p-terphenyl derivatives, a biphenyl and a p-quintaphenyl derivative) with a laterally attached hydrophilic group (1,2-diol groups, primary and secondary amides, polyether chains, crown ether units, carbohydrate units, a hydrazide, a quaternary ammonium salt, a carboxylic acid and a sodium carboxylate) are reported. The compounds were investigated by means of polarizing microscopy and calorimetry. The influence of the type of the polar group, of the length of the rigid core and the position of the connection of the hydrophilic group with the rod-like rigid core have been investigated. Many of these amphiphilic molecules can form monolayer S _A phases. If a sufficient amount of hydrogen bonding is available their mesophase stability can be higher than that of related compounds with other lateral substituents. Rectangular columnar mesophases can be found for compounds with rather large and flexible polar lateral substituents (polyether chains) fixed to the center of the rigid terphenyl unit. These columnar phases should represent ribbon phases resulting from the collapse of the smectic layers (modulated smectic phases). The proposed model is also related to that suggested for supermolecular structures of triblock copolymers. Thus, these molecules can be regarded as low molecular weight block compounds consisting of three different and incompatible molecular parts.