Linear and circular dichroism spectroscopic study of beta,beta-dimethylmesobrilirubin-XIIIa oriented in a nematic liquid crystal

(beta R,beta'R)-dimethylmesobilirubin-XIII alpha, an optically active synthetic analogue of bilirubin, the yellow pigment of jaundice, and xanthobilirubinat, a dipyrrinone analogue for one-half of a rubin pigment, dissolved in the nematic liquid crystal ZLI 1695, have been studied by means of UV-vis polarized absorption and circular dichroism (CD) spectroscopy, The order parameters have been evaluated from the temperature dependence of the degree of anisotropy, The method of ''vanishing spectral features'' has also been taken into consideration, The reduced absorption spectra have been obtained. It has been found that the orientational properties as well as the polarization of the absorption bands of the two compounds are quite different. Moreover, the UV-vis absorption and CD spectra of beta,beta'-dimethylmesobilirubin-XIII alpha have been interpreted on the basis of the exciton coupling model, assuming the point symmetry group C-2. The orientation of the principal axes of the orientational distribution tensor (order tensor) with respect to the molecular frame has been determined, That the anisotropic CD (ACD) spectra are not very different from the CD spectra can be understood from the spectroscopic analysis taking into account the orientational order.