Dimesogenic compounds consisting of cholesterol and fluorinated azobenzene moieties: dependence of liquid crystal properties on spacer length and fluorination of the terminal tail

The liquid crystalline (LC) properties of two series of non-symmetric dimesogenic compounds consisting of cholesterol and ring-fluorinated azobenzene mesogenic moieties interconnected by v-oxyalkanoyl spacers of varying length are compared; one series (F-AOC-n) has the octyloxy tail attached to the fluorinated azobenzene mesogen unit while the other (F-AOCF-n) has the perfluoroheptylmethyloxy tail. In general, compounds with the fluorinated alkoxy tail exhibited a mesophase over a much wider temperature range than those with the alkoxy tail. On the other hand, the F-AOC-n series exhibited a greater variety of mesophases: TGB (twisted grain boundary), chiral smectic C (S-C*), antiphase smectic A (S-(A) over tilde), cholesteric (N*) and solid state smectic (S-X) phases were observed depending on the length of the central spacer, whereas the F-AOCF-n series favored the formation of only the SA phase with the N* phase completely suppressed. Both series showed an odd-even dependence of the isotropization temperature on the spacer length.