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Dimerlike smectic-A and -C phases in highly fluorinated thermotropic liquid crystals.

X-ray scattering studies on a homologous series of thermotropic liquid crystals with one tail perfluorinated reveal that the smectic layer thickness depends only on the length of the fluorocarbon tail. Density measurements in combination with the x-ray results show that the average cross-sectional area per molecule, parallel to the smectic layers, depends only on the length of the hydrocarbon tail. These experimental results lead to a model in which steric interactions drive antiparallel alignment of nearest neighbors in the smectic-{ital A} and -{ital C} phases. These unique dimerlike phases have a layer spacing comparable to the length of the molecules, demonstrate registration of nearest neighbors along their lengths, and exhibit ferroelectric switching in the chiral smectic-{ital C} phase.