The effect of molecular shape and symmetry on discotic mesogens

In the field of material science, structure/property relationships can allow a more efficient design of materials exhibiting the desired properties. Self-assembled super-structures possess several advantages that can be exploited. The formation of complex architecture is dictated by the properties of the small sub-units. The formation of liquid crystalline phases requires molecular anisotropy, which is often achieved by using flat rigid rod or disc-shaped cores decorated with aliphatic chains. Other structural morphologies have also proven to allow the formation of these ordered fluid phases: plate, bent-core, bowl etc. Our research group is interested in disc-shaped molecules that have the ability to form liquid crystalline phases constituted of columns distributed in a two-dimensional lattice. The discs within a column are stacked on top of each other that could be used to conduct either energy or charges. The research projects presented in this thesis were concerned in establishing the effect of molecular symmetry and shape on the mesogenic properties of compounds based on a dibenz[a,c]phenazine core decorated with alkoxy chains (four and six). Series of structural isomers have been synthesized and their mesogenic behaviour analyzed. Reducing the molecular symmetry of mesogens lowers the melting temperature and, to a smaller extent, the clearing temperature leading to broader liquid temperature ranges of mesogenic behaviour. To study the effect of molecular shape, compounds with chains of different length and different motifs were prepared. This type of substitution leads to different mesogenic behaviour. Comparisons between structural isomers showed that this new substitution pattern lowered the clearing temperatures for these mesophases. The molecular shape of these compounds does have an effect on the mesophase and the results could allow more efficient engineering of systems used in various devices.