Comparison of the mesomorphic properties between substituted Schiff's bases incorporating a linear hydrocarbon or fluorinated tail

We have developed new compounds within the perfluorinated series incorporating a Schiffs base as the connector between the aliphatic chain and the 4-biphenyl used as mesogen group. All members of the series are thermally stable and exhibit enantiotropic smectic behavior. A smectic polymorphism is observed only with F-butyl and F-hexyl tails. They show an oscillation in the smectic-isotropic transition temperature. The influence of the fluorinated chain and the shape of the connector on the stability or the reversibility of mesophases has been investigated by comparison with their hydrocarbon analogues and with a compound obtained by condensation of 2-(F-alkyl)ethylamine with 4-acetylbiphenyl. The fluorinated moiety strongly promotes enhanced smectic phases and the presence of a methyl group on the connector strongly decreases the stability of the mesophase. Exceptionally, it would seem that the fluorinated chain generates a liquid crystal behavior which can not be observed with the hydrocarbon homologous structures. The physical properties of these novel fluorinated materials are reported and the effect of fluorination on these properties is discussed. All these points are commented on and illustrated by the study realized from the differential thermal analysis and the light microscopy.