Comparison of the liquid crystalline properties of dimesogenic compounds bearing alkoxy and perfluoroalkoxy tails

The liquid crystalline properties of two series of nonsymmetric dimesogenic compounds consisting of cholesterol and Schiff base, azobenzene or stilbene mesogens interlinked by a pentamethylene spacer are compared. Compounds in the first series bear the fluorinated OCH2(CF2)(6)CF3 tail, whereas those in the second series bear the corresponding alkoxy tail. In general, compounds with the fluorinated alkoxy tail exhibited mesophases over a much wider temperature range than those with the alkoxy tail. Moreover, the former favored the formation of smectic phases (either chiral smectic C (Sc*) and S-A phases, or S-A phase) and suppressed the formation of the cholesteric phase. All of the three compounds of the second series were able to form the cholesteric phase. It was also observed that the layer spacing in compounds with the fluorinated tail was greater by 3 Angstrom than those with the alkoxy tail. The compounds that form the Sc* phases exhibited very high spontaneous polarization, 400-500 nC cm(-2). The compound with the fluorinated tail has a much shorter response time, 80 mus, than those with the alkoxy tail, 400-500 mus.