Synthesis and mesomorphic properties of perfluorinated rod-like liquid crystals with sulphur-containing spacers

In order to determine the influence of 'sulphur-containing' spacers on the formation of mesophases in low molecular mass compounds, we have examined t he mesomorphic behaviour of molecules which consists of a 4-biphenyl unit l inked to an unbranched fluorinated chain via a short spacer including at le ast a sulphur atom. The synthesis of these compounds has been carried out f rom 2-F-butylethyl iodide or from the 2-F-alkylethyl mercaptans in the case of the F-hexyl and F-octyl tails. The mesomorphic properties have been cha racterized by polarized light microscopy and by differential thermal analys is showing the peculiar contribution of each of the spacers. The influence of the fluorinated chain and the shape of the connector on the stability of the mesophases has been investigated. The compounds with a thioether or a hemithioacetal spacer showed no mesomorphic properties, whereas the structu res with a thioester spacer showed a very interesting enantiotropic behavio ur of the smectic A type over a wide temperature range. Furthermore from th e series exhibiting liquid crystalline behaviour, increasing the number of fluoromethylene units simultaneously increases both the melting and the cle aring temperature. These mesomorphic properties within the fluorinated seri es are compared with those of their monocatenar hydrocarbon homologues.