Influence of spacer and terminal group lengths on the smectic ordering of cholesterol-containing dimer liquid crystals

The smectic properties of four series of dimer liquid crystals containing c holesteryl and biphenylyl groups were investigated by polarization microsco py, DSC and X-ray diffraction. Compounds that contain the strongly dipolar cyanobiphenylyl group exhibit a smectic layer spacing that is about 1.7 tim es the length of the molecule. Three series of alkoxybiphenylyl-containing dimers exhibit two other smectic modifications. For short spacers, a smecti c layer spacing is observed that is about the same length as the molecule a nd for long spacers the smectic layer spacing is about half the length of t he molecule. In the latter cases the entropy change at the SmA-N* transitio n is clearly larger than for the compounds with the other smectic modificat ions. One or two compounds in each alkoxybiphenylyl-containing series, that have similar spacer length and terminal group length and have an odd numbe r of flexible units in the spacer, exhibit only a chiral nematic phase, and no smectic phase.