On optical polarization measurements in liquid crystals

Abstract

``Compensated'' nematic mixtures of cholestryl derivatives are very well suited as anisotropic solvents for optical polarization studies in the ultraviolet since these compounds are transparent to about 240 nm and can be homogeneously oriented by dc electric fields. Such mixtures have been prepared in a temperature range between 20 and 100 °C by only slight changes in the composition. Polarization experiments have been performed both in the liquid state above room temperature and in the ordered frozen state below −30 °C. The degree of absorption polarization has been calculated as function of the elements ${\epsilon }_{x_i}$ of the molar extinction coefficient tensor and the average orientations $(\mathrm{order\; parameters}{\mathrm{\hspace{0.17em}S}}_{x_i{\mathrm{\hspace{0.17em}x}}_i})$ of the solute molecular axes x_i. For symmetric molecules all elements ${\epsilon }_{x_i}\mathrm{\hspace{0.17em}(\lambda )}$ and order parameters can be determined. Results of our polarization studies of anthracene, chrysene and a charge transfer complex are presented. Moreover, it is shown that measurements of the circular dichroism, observed in the absorption region of solute molecules incorporated in a helical fashion in cholesteric phases, yields a very simple method to determine relative polarization directions. By comparing the absorption polarization and (or) the circular dichroism spectra with the fluorescence polarization spectrum the position of weak hidden bands can be detected with good accuracy. The long wavelength edge of the ¹L_b transition of anthracene has been located in this way at about 29 000 cm⁻¹.