Linear and Circular Dichroism Spectroscopic Study of β,β‘-Dimethylmesobilirubin-XIIIα Oriented in a Nematic Liquid Crystal

Abstract

(βR,β‘R)-dimethylmesobilirubin-XIIIα, 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 β,β‘-dimethylmesobilirubin-XIIIα 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.

Chirality and circular dichroism of oriented molecules and anisotropic phases

“…The long wavelength ␣ transition (A → B) possesses a transition moment direction in the plane perpendicular to the C 2 symmetry axis with an angle of about 45°with respect to the naphthyl-naphthyl bond, whereas the transition moment direction of the ␤ transition (A → A) is parallel to the C 2 symmetry axis. 28,29 With the order parameters and the order tensors of R-6 and R-7, respectively, the polarized UV spectra can be explained in coincidence with the ratio ⌬(A → B)/⌬(A → A) that the orientation axis is rotated from being parallel to the naphthyl-naphthyl bond in R-6 to being parallel to the C 2 axis in R-7.…”

Linear dichroism of perylene derivatives in a nematic liquid crystal

“…In general, there are 6 unknown q* ii parameters (i 5 1, 2, 3 for n Å 1 and n Å 2 ). The number of these parameters can, however, be reduced in the case of molecular symmetry, as was shown earlier [ 2- 4,12 ] . Moreover, three criteria should be taken into account: (1) the positions of the experimental values of the degree of anisotropy in the R(n Å 1 ), R(n Å 2 ) plane; (2) the slope of the curve R(n Å 1 , T ) 5 W [ R(n Å 2 , T ) ] ; and (3) its curvature [ 3 ] .…”

“…Previously, we had used this equation (2). The additional pieces of information needed procedure in order to assign the absorption spectrum of are the absorption tensor e ij (n Å q ) for diOE erent positions n Å q b,b¾ -dimethylmesobilirubi n XIIIa [ 12 ] (C 2 point symin the spectrum. For these diOE erent wavenumbers n Å q , the metry group), because we had been able to use additional e 1 (n Å q ) have to be measured; here the wavenumbers n Å q relations derived from exciton theory.…”

“…S and 3,4,9-perylenetricarboxyli c acid and of 3,4,9,10-perylene -D will be determined by a procedure developed earlier tetracarboxylic acid can be used as dichroic dyes in [ 2,12 ] using standard equations. However, for molecules guest-host liquid crystal displays.…”

Fluorophenyl Bilirubins: Synthesis and Stereochemistry

“…3 [4,17,23,24]. The component dipyrrinone chromophores of 1±4 have strongly-allowed longwavelength electronic transitions (Table 4), making them excellent candidates for transition dipole±dipole interaction (exciton coupling) [24,25] (Fig. 4).…”