X-ray studies on the columnar structures of discotic liquid crystals

Since the discovery of discotic liquid crystals in 1977 [1] the field has grown rapidly with an estimated 3000 compounds belonging to this category. Generally these compounds have flat or nearly flat cores and surrounded by four to eight long chain substituents. A vast majority of these compounds exhibit the columnar mesophase. This phase is characterized by a structure in which the disc shaped molecules are stacked one on top of the other to form columns, the different columns constituting a two-dimensional lattice. Several variants of the columnar structure have been identified-upright columns, tilted columns, hexagonal lattice, rectangular lattice, etc. The stacking of the discs within the column could be on the one extreme, liquid-like or on the other have a true long range order. Although X-ray measurements on quite a few compounds exhibiting these different structures have been reported, the temperature dependence of the various structural parameters does not seem to have received much attention. We have used an image plate detector to carry out detailed X-ray studies of several compounds exhibiting the columnar phase. Analysis of the X-ray patterns yields, apart from information on the structural symmetry in terms of the planar space groups, quantitative data on (i) the ordering of the chains and the cores of the mesogens, (ii) the core-core correlation length along the column axis and (iii) the intercolumnar spacing. In particular, it is seen that the extent of ordering of the discs within the columns has significant temperature dependence. Data collected as a function of temperature will be presented and questions regarding ordering within the columns addressed, particularly in two compounds, one of which exhibits a columnar to a plastic phase transition and the other a columnar to a helical phase transition.