X-RAY-DIFFRACTION STUDY OF SMECTIC-A LAYERING IN TERMINALLY FLUORINATED LIQUID-CRYSTAL MATERIALS

We report an X-ray study of smectic A layering for mesogenic compounds with fluorinated substituents in terminal positions. The measurements were carried out using diffractometers with one- and two-coordinate p roportional chambers. It was found that in contrast to -CN or -NO2 ter minated smectics, the polar -OCF3 compounds form only a monolayer smec tic A1 phase. The ratio of the intensity of the second harmonic to the first in smectic A phases formed by molecules with lengthy perfluorin ated chains was found to be two orders of magnitude higher than is com monly reported for low molar mass thermotropic mesogens, indicating de viations of the density distribution function rho(z) from a pure sinus oidal form. The layer periodicity d for these mesogens exceeds the mol ecular length L: d/L congruent-to 1.1, which corresponds to a smectic A(d) phase consisting of parallel or antiparallel dimers. We have obse rved that lateral fluorine substitution in the benzene ring adjacent t o the perfluorinated chain leads to the disturbance of uniform smectic A layering and to the formation of a defect-modulated smectic A struc ture of a chequer-board type. For polyphilic compounds containing both hydrogenous and perfluorinated units in the terminal chain, the in-pl ane structure factor displays double-peaked liquid-like profiles indic ating the existence of nearest-neighbour molecular stacking at differe nt distances. The peculiarities of smectic A layering in fluorinated m esogens are discussed in terms of steric coupling and enhanced conform ational rigidity of fluorine containing moieties.