Polycatenar amphiphilic liquid crystals

In order to study the effect on mesophase organisation of incorporating groups (hydrocarbon and fluorocarbon) that normally phase separate into the same liquid-crystalline molecules, a series of tri- and tetra-catenar mesogens has been prepared containing various combinations of hydrocarbon (CnH2n+1–: n = 12 or 14) and semiperfluorocarbon (CmF2m+1(CH2)4–; m = 8 or 10) chains. In addition, many of the intermediate compounds prepared en route were studied for their liquid crystal properties. The semiperfluorocarbon chains were added using Williamson ether chemistry with CmF2m+1(CH2)4Br, itself prepared via a radical addition of CmF2m+1I to 3-buten-1-ol to give a secondary iodide which was deiodinated in a radical process. Conversion of the alcohol to the bromide used HBr under phase-transfer conditions. The properties of the new compounds were elucidated using optical microscopy, differential scanning calorimetry and low-angle X-ray diffraction.
The all-hydrocarbon compounds had been prepared previously and their mesomorphism was qualitatively identical to that of the all-semiperfluorocarbon analogues. However, when both hydrocarbon and semiperfluorocarbon chains were mixed in the same molecule, there was a significant change in behaviour as the mesogens organised themselves in order to maximise hydrocarbon-hydrocarbon and fluorocarbon-fluorocarbon interactions while minimising hydrocarbon-fluorocarbon interactions. For example, with tricatenar mesogens with a fluorocarbon chain at one end, this manifested itself as a doubling of the lamellar periodicity, while in unsymmetric tetracatenar mesogens, a reduction in phase symmetry from hexagonal (p6mm) to rectangular (p2gg) was seen along with a massive increase in lattice parameter.