Structure-property relationships in discotic and elliptical mesogens

Liquid crystals are a promising class of materials used in or proposed for a wide variety of applications, ranging from liquid crystal displays and light emitting diodes to photovoltaic devices and field effect transistors. Many molecular level structural features are known to influence the properties of liquid crystalline materials, but the outcome of this influence is often poorly understood. A strategy was developed for the formation of elliptical mesogens having a shape intermediate between the well-known rods and discs by rigidly linking two disc-shaped molecules. These studies have led to investigations of structure-property relationships in discotic and elliptical mesogens, including not only the effect of shape and overall symmetry, but also the influence of the presence and position of a nitrogen atom in the core, the location of functional groups, and the length of alkoxy chains. Disc-shaped molecules were assembled through the condensation of 2,3,6,7-tetrakis(alkoxy)phenanthrene-9,10-diones with diamines, allowing the preparation of mesogens bearing a wide variety of functional groups. A simple and inexpensive capillary furnace was developed to improve and facilitate the study of liquid crystalline materials using variable temperature X-ray diffraction. Two of the systems studied appear to form elliptical mesogens through hydrogen bonding, and it was found that elongation of the mesogen shape from a disc to an ellipse promotes the formation of nematic and rectangular columnar phases, as well as supercooling of the columnar phases resulting in columnar ordering at room temperature. The formation of non-elliptical dimers of discs was attempted in two different ways, but in both cases, no rigidly-linked dimers were observed in the mesophases. However, one of these systems was found to result in solution self-assembly and in hexagonal columnar phases stable over remarkably broad temperature ranges, which appears to be the result of hydrogen bonding within the columns.