Synthesis and self-assembly of aroylhydrazone based polycatenars: A structure-property correlation

Abstract

Two series of aroylhydrazone based polycatenars which differ from each other in terms of the linking groups (semi flexible ester or flexible ether linkage) were synthesized. These hexacatenars were mesogenic and also showed gelation in organic solvents at very low concentration, which is assisted by the intermolecular hydrogen bonding. Both the series of hexacatenars were prepared by varying the peripheral chain length from n-hexyloxy to n-hexadecyloxy chain. In the case of ester based series on moving from n-decyloxy to n-dodecyloxy chain, a transition from columnar rectangular phase to columnar hexagonal phase was noticed. Such behavior was not seen in the case of ether-based hexacatenars, irrespective of the chain length. Representative hexacatenars from each series exhibited gelation at low concentration, but the critical gelation concentration of ester-based hexacatenar was almost half of that observed for ether-based hexacatenar. From these studies we can understand the importance of the linking group, which constitutes very small part of the molecule, but has a significant effect on the type of the self-assembly in bulk as well as in presence of solvents. The presence of semi-flexible ester group provides a bent conformation to the molecule, while the presence of flexible ether group provides a linear shape to the molecule, as evident from the molecular modeling studies. Further the presence of the ester group enhances the extent of intermolecular hydrogen bonding with the solvent that leads to a lower critical gelation concentration in comparison to the ether-based linking group. Such H-bonded liquid crystals have potential in the preparation of liquid crystalline physical gels and related applications.