Influence of different spacers on the mesomorphic properties of 4-biphenyl derivatives within the F-alkylated series

In order to determine the influence of modification of the shape of the connector, within the fluorinated series, on the formation of mesophases in low molecular mass compounds, we decided to examine the mesomorphic behaviour of molecules which consist of a 4-biphenyl unit linked to a fluorinated tail via different spacers. The synthesis and the physical properties of these new materials are reported. The mesomorphic properties were characterized by polarized light microscopy and by differential thermal analysis, thereby showing the peculiar contribution of each of the spacers. The data collected from the 4-biphenyl derivatives synthesized allow us not only to confirm the enhancement of the smectic properties owing to the fluorinated alkyl chains, but also to display the impact of the spacer on these properties. Actually, some spacers like the amide or thioether type, as well as that of the hemithioacetal derivatives give suppression of the smectic character. Some others like that in the esters with an OC(O) spacer lead to mesophase or a short range SmA phase, and others give rise to broad enantiotropic behaviour like that of the esters with a C(O)O spacer, and of the ether or thioester types. These last mentioned give mesomorphic behaviour over a wide temperature range.