Tilt order parameters, polarity and inversion phenomena in smectic liquid crystals

The order parameters for the phenomenological description of the smectic-{\it A} to smectic-{\it C} phase transition are formulated on the basis of molecular symmetry and structure. It is shown that, unless the long molecular axis is an axis of two-fold or higher rotational symmetry, the ordering of the molecules in the smectic-{\it C} phase gives rise to more than one tilt order parameter and to one or more polar order parameters. The latter describe the indigenous polarity of the smectic-{\it C} phase, which is not related to molecular chirality but underlies the appearance of spontaneous polarisation in chiral smectics. A phenomenological theory of the phase transition is formulated by means of a Landau expansion in two tilt order parameters (primary and secondary) and an indigenous polarity order parameter. The coupling among these order parameters determines the possibility of sign inversions in the temperature dependence of the spontaneous polarisation and of the helical pitch observed experimentally for some chiral smectic-{\it $C^{\ast}$} materials. The molecular interpretation of the inversion phenomena is examined in the light of the new formulation.Comment: 12 pages, 5 figures, RevTe

Molecular Twisting Power chiraler Azobenzole

The Molecular Twisting Power (MTP) of the trans- and cis-azocompounds 1 - 4, each substituted either with para- or orthostanding chiral side chains, of the azoxyanalogues 5, 6 and of the chiral nitrobenzenes 7, 8 dissolved in the nematic liquid crystal NP 1132 are investigated. For all various series, althoug there is a distinct change in the geometry of the aromatic part and the substitution pattern, the helical screw sense of the induced cholesteric phase alternates with the number of bonds between the chiral center and the aromatic ring system. For the orthosubstituted azobenzenes 3, 4 a reversible inversion of the helix during the cis-trans-isomerisation is observed. Most of the compounds with the chiral center in the a-position to the aromatic ring system show anomalous behaviour of the MTP.</jats:p