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.

Columnar liquid crystals have emerged as a promising class of materials for light emitting diodes, photovoltaic devices and field effect transistors. In addition to their practical importance, the ability of disc-shaped molecules to spontaneously form columnar nanostructures represents a striking example of self-assembly driven largely by ∏-∏ interactions. Any factor that alters the strength of ∏-stacking between neighbouring molecules should therefore have a dramatic impact on the propensity of these molecules to form columnar mesophases. Studying the relationship between a molecule's structure and its tendency to self-assemble into columns can thus provide valuable insight into the nature and strength of noncovalent interactions between discotic mesogens in addition to facilitating the design of new liquid crystalline materials. A series of disc-shaped molecules was produced by the condensation of 1,2-diamines with 2,3,6,7-tetraalkoxy-phenanthrene-9,10-diones in order to systematically investigate the relationship between changes in molecular structure and the self-assembly of columnar liquid crystalline phases. Functional groups were found to have a pronounced effect on the tendency of these molecules to self-assemble. Moreover, the thermal stability of these columnar phases was very sensitive to the position of the substituents and their electron withdrawing ability, with the columnar-to-isotropic transition temperature strongly related to Hammett Ơ-parameters of the functional groups. The effect of core size were also investigated through the preparation of molecules containing 4-, 5- and 6-membered fused aromatic rings. The effects of heteroatoms in the aromatic core were also explored. Phase behaviour had a striking dependence on both the number and position of heteroatoms in the core. Furthermore, substituting hexaalkoxy-[a, c]dibenzophenazine with different lengths of pendant chains showed that changing molecular symmetry and shape had an effect on the phase behaviour of discotic mesogens.

The self-contained properties of discotic liquid crystals (DLCs) render them powerful functional materials for many semiconducting device applications and models for energy and charge migration in self-organized dynamic functional soft materials. The past three decades have seen tremendous interest in this area, fueled primarily by the possibility

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.

In this book we have collected a series of state-of-the art papers written by specialists in the field of ionic liquid crystals (ILCs) to address key questions concerning the synthesis, properties, and applications of ILCs. New compounds exhibiting ionic liquid crystalline phases are presented, both of calamitic as well as discotic type. Their dynamic and structural properties have been investigated with a series of experimental techniques including differential scanning calorimetry, polarized optical spectroscopy, X-ray scattering, and nuclear magnetic resonance, impedance spectroscopy to mention but a few. Moreover, computer simulations using both fully atomistic and highly coarse-grained force fields have been presented, offering an invaluable microscopic view of the structure and dynamics of these fascinating materials.

Both an introductory course to broadband dielectric spectroscopy and a monograph describing recent dielectric contributions to current topics, this book is the first to cover the topic and has been hotly awaited by the scientific community.