Comparative lyotropy study of homologous hexaesters of hexahydroxy-benzene and -cyclohexane (scyllitol) in linear and cyclic hydrocarbons: microsegregation and mesophase formation of discotics [1

Abstract

The investigation addressed in the title was carried out on a total of 17 members of the two disc-shaped series 1 or 2 of radial hexaesters differing only in their cyclic C6-core units. Their lyotropic behaviour appeared to be dependent on the intracolumnar order of stacking of the respective hexaester molecules. With linear alkanes in mixtures with the phenylene centred hexaesters (series 1 derived from hexahydroxybenzene) no lyomesomorphism occurs, whereas with most of the cyclohexane centred analogues (series 2, scyllitol derivatives) even two lyomesophases were observed: a hexagonal type and, in 49 cases, a nematic columnar (induced) phase. Here, the lyotropy demonstrates its dependence on the length of chains both of the alkanoyl groups of 2 and the solvent (the linear alkanes) used. In the case of the selected cyclic solvents, three factors proved to govern the lyotropic mesomorphism of all these disc-like hexaesters: (1) their degree of saturation, (2) their molecular size/volume, and (3) their stereostructure. Interestingly, with benzene and other (less unsaturated) monocyclic solvents, both series of hexaesters exhibit only a hexagonal type of mesophase which is induced in four cases. Saturated monocyclic hydrocarbons act on members of the aromatic centred series 1 in the same way; with cyclohexane however, two more cases of induction of a hexagonal type of lyomesophase have been observed. To our surprise, binary mixtures of most hexaesters of series 2 (scyllitol derivatives) with saturated monocyclic hydrocarbons show the same polymorphic (nematic and hexagonal) situation as that found here with linear hydrocarbons; moreover, in comparison with the results with linear alkanes, four more cases of induced phases appear with these cycloalkanes. These results, in particular the many cases of lyomesophase induction, relate to the first examples of lyotropic mesomorphism in binary systems of disc-like materials with saturated cyclic solvents/hydrocarbons. Finally, in some examples presented here it was found for the first time that bulkiness/space-filling or the stereostructure of the saturated cyclic solvents/hydrocarbons, e.g. cis- or trans-decalin, also play an important role in the lyotropic mesomorphism of their mixtures with the radial hexaesters of the two series 1 and 2. By means of models shown here for the first time in figures 4 to 6, ideas are put forward about peculiarities due to microsegregation in relation to the (formally) very similar members of both hexaester series 1 and 2 as a key to understanding the drastic difference in their thermotropic as well as their lyotropic states.