Liquid crystalline derivatives of galactose and galactitol: dependence of thermotropic mesomorphism on carbohydrate form

An almost complete homologous series of the acyclic, open-chain 6-O-n-alkyl-D-galactitols has been prepared. Most homologues in this series exhibit an enantiotropic smectic A* phase. The liquid crystal transition temperatures of these open-chain carbohydrates are compared with those of the corresponding homologues in the cyclic form, i.e., with those of the 6-O-n-alkyl-alpha-D-galactopyranoses. In general the carbohydrates in the open-chain form exhibit higher clearing points than those of the analogous cyclic pyranoses. However, the melting point of the galactitols is generally significantly lower than that of the corresponding galactopyranoses. This leads to a much wider temperature range of the smectic A* phase for the carbohydrates in the pyranose form. The lowest clearing point was found for related galactoses in the furanose form. Alternation was found for the clearing point of the 6-O-n-alkyl-alpha-D-galactopyranoses and the 6-O-n-alkyl-D-galactitols. Powder X-ray diffraction studies indicate a non-intercalated structure for the crystalline state and an interdigitated arrangement in the smectic A* phase with a degree of chain melting for both series.