Direct synchrotron x-ray scattering measurements of the orientational order parameter, S, corresponding to the siloxane and hydrocarbon parts of the molecule, smectic layer spacing, and director tilt angle with respect to the smectic-C (SmC) layer normal in the de Vries smectics-A (SmA) and SmC phases of two organosiloxane mesogens are reported. The results reveal that (i) the SmC (tilt) order parameter exponent β = 0.26 ± 0.01 for 2nd order SmA-SmC transition in excellent agreement with the tricritical behavior, (ii) the siloxane and hydrocarbon parts of the molecules are segregated and oriented parallel to the director with different degree of orientational order, and (iii) thermal evolution of the effective molecular length is different in the two phases contrary to the conventional wisdom.
Two guest dyes are used to change dramatically the torque exerted by DC and optical frequency electric fields on ferroelectric and nematic liquid crystal hosts. The particularity of the photoisomerization mechanisms of these dyes is responsible for the unusual behavior of the guest- host compounds. Corresponding molecular model is provided by describe the photoexcitation of dyes and its influence on the host material.
The chiral thioindigo dye (R,R)-5,5'-dinitro-6,6'-bis(2- octyloxy)thioindigo has been synthesized in optically active form and doped into a racemic S<SUB>C</SUB> liquid crystal host (x<SUB>d</SUB> equals 0.028) to induce a ferroelectric S<SUB>C</SUB>* phase. Irradiation of an aligned sample of this induced S<SUB>C</SUB>* phase at (lambda) greater than 450 nm produces a cis- enriched photostationary state of the dopant, and an increase in the spontaneous polarization of the S<SUB>C</SUB>* phase by a factor of 1.8. This modulation is reversible via thermal relaxation of the dopant to the trans-form.