Structure-property relationships in Discotic Mesogens

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 ?-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.