Order Parameters of Guest/Host Systems from UV Spectra – A Test of a New Method and its Application to Rigid Guest Molecules –

Abstract

For a determination of the order parameters S*, D* of nematic guest/host phases from polarized UV and sometimes IR spectroscopy, the experimental information is often not sufficient because of overlapping absorption bands. For molecules with only one purely polarized band, the temperature dependence of the degree of anisotropy and a theoretical relation D* = f(S*) can be used to overcome this lack of information. A method introduced earlier [7] is examined with respect to its range of applicability. It can be shown that in the region of overlapping bands an amount of about 30% of the band with the proper polarization direction is sufficient to determine S* and D* with an error of at most 2 to 5% and 5 to 10%, respectively. The temperature dependence of S*, D* for the rigid molecules azulene, phenanthrene, anthracene, 9,10‐dichloro‐, 9,10‐dibromo‐, 9,10‐dimethylanthracene, phenazine, and acridine in ZLI 1695 has been determined. Three different types of structural dependence can be derived from the experimental data.

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

“…The molecular decadic absorption coefficients for light polarized parallel (ε 1 ) and perpendicular (ε 2 ) to the optical axis of a uniaxial sample are given by , are the absorption coefficients for light beams polarized linearly parallel to the axes in a completely oriented system. The axes are the principal axes of the absorption tensor ε i j . − a i j are the elements of the orthogonal matrix which transforms the coordinates into the coordinates. The coordinates refer to the common principal axes of the order tensors g i j k k ( k = 1, 2, 3) and the following convention for numbering the axes is used: ≥ ≥ with as the “orientation axis”.…”

“…In order to obtain information about the order parameters and the directions of the transition moments responsible for the given absorption band from the linear dichroism, we used the method described in detail previously. ,,, This method was elaborated for the case where one purely polarized band (at ν̄ 1 ) and another band (at ν̄ 2 ) with mixed polarization exist in the absorption spectrum, for molecules with and coordinate systems coinciding by symmetry. Thus, it is not directly applicable to molecules with, e.g., the point symmetry group C 2 .…”

“…In the method presented previously, only one further piece of information was needed in addition to the polarized absorption spectra ε 1 (ν̄) and ε 2 (ν̄). This was obtained from the temperature dependence of the degree of anisotropy. ,,, In the present paper, the above-mentioned method is extended to molecules of low symmetry and is applied to a case where only one principal axis is fixed by symmetry. It can also be applied to the case of two bands of mixed polarization with all axes being symmetry-fixed.…”

Guest-Host Interactions in Nematic p-Cyano-p'-Alkylphenylcyclohexanes

“…In practice, this effect can be utilized to the construction of the colour and black-white electrooptical displays [3][4][5][6][7], which in comparison with the conventional liquid crystal displays have such advantages as the wider viewing angle and the higher image brightness. From scientific point of view dye-liquid crystal mixtures are very interesting, because they can be used in the optical spectroscopy for determination of the absorption and emission bands polarization of the dye molecules [8][9][10][11][12][13] as well as to obtain information about the molecular interactions between guest molecules and their liquid crystal environment [14][15][16][17][18].…”

Linear optical parameters of a dyed nematic mixture in the presence of an ac voltage

“…This phenomena was first observed by Heilmeier et al [1,2], and is known as ''guesthost'' effect. The guest-host effect has found applications in obtaining information concerning the electronic spectra of the dyes [3][4][5] and in the construction of electro optical displays [6][7][8]. Oriented NLCs are excellently suited as anisotropic solvents for polarized spectroscopy [9][10][11].…”