Optical investigations in the various phases of an antiferroelectric liquid crystal.

Abstract

We report detailed results of optical rotatory power (ORP), slective reflection (SR), refractive index anisotropy (RIA), and anisotropic Rayleigh scattering (ARS) measurements in the antiferroelectric liquid crystal, 4-(1-methylheptyl-oxycarbonyl) phenyl-4'octyloxy phenyl-4-carboxylate (MHPOBC). The ORP in Sm-A and Sm-${\mathit{C}}_{\mathrm{\ensuremath{\alpha}}}^{\mathrm{*}}$ phases has been found to be zero or at least very weak compared to ordinary liquids with strongly chiral molecules. No helical pitch is observed in the Sm-A, Sm-${\mathit{C}}_{\mathrm{\ensuremath{\alpha}}}^{\mathrm{*}}$ and Sm-${\mathit{C}}_{\ensuremath{\gamma}}^{\mathrm{*}}$ phases in the range 200--2200 nm. The combined use of ORP and SR allows the precise determination of the tilt angle of the smectic-C phases. Obtained values are found to be in good agreement with both theoretical calculations and experimental results obtained from RIA. Moreover, ARS reveals the local biaxial character of the Sm-${\mathit{C}}_{\mathit{A}}^{\mathrm{*}}$ phase, while the absence of critical behavior of the Sm-A\ensuremath{\parallel}Sm-${\mathit{C}}_{\mathrm{\ensuremath{\alpha}}}^{\mathrm{*}}$ phase transition is a direct proof of the lack of biaxiality in the Sm-${\mathit{C}}_{\mathrm{\ensuremath{\alpha}}}^{\mathrm{*}}$ phase.