Langmuir-Blodgett (LB) films formed of some discotic liquid crystals, namely 3,4,9,10-tetra-(n-alkoxy-carbonyl)- perylenes, have been studied. The absorption and fluorescence spectra were recorded. The results obtained have led to conclusions about formation of dimers both in ground and excited states at the air-solid substrate interface. Moreover, information about the molecular arrangement in LB films has been obtained.
Langmuir and Langmuir-Blodgett films of various dyes, namely derivatives of: 3,4,9,10-perylenetetracarboxylic acid, 3,4,9-perylenetricarboxylic acid, naphthalenebicarboxylic acid and naphthoylenebenzimidazole as well as of their mixtures with a liquid crystal were investigated. The dependence of the surface pressure -- on the mean molecular area for Langmuir films was measured, from which the molecular organization and the miscibility of components in the monomolecular layer at the air-water interface were determined. For Langmuir-Blodgett films the absorption and emission spectra were recorded and information about the spectral properties and molecular alignment of dyes and liquid crystals in ultrathin layers at solid substrates was obtained.
The orientational behavior of some liquid crystals with various molecular structure was studied by means of classical methods of optical spectroscopy: absorption of the polarized light in the visible region, fluorescence depolarization and Raman scattering depolarization. The dichroic dye dimethylamino-nitrostilbene was added to liquid crystals and acted as guest probe reflecting the host orientation in absorption and fluorescence measurements. The absorption, emission and Raman scattering spectra of the linearly polarized light were recorded as a function of temperature in the nematic phase. On the basis of these spectra the order parameters <P2> and <P4> as well as the molecular distribution function were determined. It was found that the molecular structure of liquid crystals affects the orientational order of molecules. Moreover, the effect of the dye presence on the orienting properties of liquid crystalline hosts was discussed.
Langmuir (L) and Langmuir-Blodgett (LB) films of perylene-like dye and its binary mixtures with 4-octyl-4'-cyanobiphenyl (8CB) and 4-pentyl-4"-cyano-p-terphenyl (5CT) have been investigated. On the basis of the surface pressure-mean molecular area isotherms, the molecular organization on the air-water interface has been determined. The spectroscopic study of the LB films has allowed conclusions to be drawn about the alignment of the molecules on the quartz slides. The absorption and fluorescence spectra of the LB films of the perylene-like dye have revealed the formation of self-aggregates.
Surface pressure-mean molecular area isotherms of Langmuir films of some liquid crystals with CN or --NCS terminal group as well as of their binary mixtures have been recorded. It was found that only the liquid crystals which molecules have CN group are able to form a stable monolayer at the air-water interface. Information about the molecular organization and intermolecular interaction in two dimensional layer has been obtained. Moreover, the miscibility or phase separation of two components in the mixtures of various compositions has been determined by using excess area criterion and surface phase rule.
Monocolor, bicolor and black-color liquid crystal displays utilizing guest-host effect were investigated. The polarized absorption spectra for these displays were recorded and the order parameter of the dyes dissolved in liquid crystal matrix has been estimated. Moreover, the tristimulus values of the displays colors in OFF and ON states have been calculated and the changes of the color parameters under the electric field applied have been determined by using 1976 CIELAB Color System.
The absorption spectra of some dichroic dyes: derivatives of naphthalenebicarboxylic acid and of naphthoylenobenzimidazole in nematogenic liquid crystalline mixture ZLI 1695 have been studied in details using polarized light. On the basis of these spectra the order parameters S and D have been determined by using the theoretical relation D equals f(S) and proper calculation procedure. Moreover, the absorption transition moment directions with respect to the long axis of the dye have been assigned.
Results of the thermodynamic study of the molecular interaction in Langmuir films of mixtures of 4-n-octyl-4'- cyanobiphenyl (8CB) and 4-pentyl-4'-cyanoterphenyl (5CT) are presented. Both mixed materials are thermotropic liquid crystals and as a single component material is able to form a monolayer at the air-water interface. We demonstrate the effect of interaction of two similar liquid crystal molecules and how the surface pressure is affected by mixing the components in different mole fractions. The surface pressure-area isotherms indicate that 8CB and 5CT are immiscible in whole mole fraction range.
The following spectroscopic methods used to study of liquid crystals have been discussed: infrared, visible and ultraviolet absorption spectroscopy, fluorescence depolarization. Raman scattering, nuclear magnetic resonance and electron paramagnetic resonance. It has been shown, that using these methods the long-range orientational order in uniaxial liquid crystalline phases as well as some direction-dependent spectroscopic molecular properties of the compounds under investigation can be determined.
The results of the color evaluation for some monocolor, bicolor and black-white liquid crystal displays utilizing guest-host effect are presented. The polarized absorption spectra for the dichroic dyes dissolved in the liquid crystalline mixture E18 (BDH Ltd.) have been recorded and the order parameter has been calculated. Moreover, the tristimulus values of the displays color in OFF and ON states have been calculated and the changes of the color parameters under the electric field applied have been determined by using 1976 CIELAB Color System. The results have been compared with those obtained for twisted nematic liquid crystal display.
Dielectric relaxation spectroscopy has been sued to study the molecular dynamics of three nematogens, derivatives of bicyclo (2,2,2) octane with -NCS terminal group, in the nematic and isotropic phases. It has been found that the dielectric spectra in the isotropic phase and in the nematic phase can be resolved into three components, described by the Debye functions. The possible molecular reorientational movements responsible for the relaxation processes related to these components are discussed.
The spectral properties of some fluorescence dichroic dyes in isotropic solvents and nematic liquid crystals have been studied. The order parameter of the dyes in the anisotropic matrices has been determined by means of the polarized absorption and fluorescence measurements. The influence of the dye addition on the clearing temperature of the nematic host has been investigated. The main purpose of the investigation was to get some information on the dyes in order to use them in colored liquid crystal displays.
The method of determining the orientational order in uniaxial phases of anisotropic matrices based on the polarized absorption and fluorescence spectra is described. The application of this method for three liquid crystalline side chain polymers in uniaxial smectic and nematic phases is presented. Based on the experimental data, the order parameters 〈P2〉 and 〈P4〉 and the molecular distribution function f(β) versus temperature are obtained.
Long-range orientational order of four nematogenic liquid crystals has been determined on the basis of the optical birefringence, the dielectric anisotropy, and the polarized absorption spectra of the dichroic dye dissolved in the liquid crystalline host. The values of the order parameters obtained by using various experimental methods have been compared with those obtained from the Maier-Saupe mean field theory.
The orientational order of some nematogenic low molecular liquid crystals and three polymers with mesogenic side groups has been studied using 'guest-host' effect. The polarized absorption and fluorescence spectra of the fluorescent guest probe oriented in the liquid crystalline matrices have been measured as a function of the temperature in the nematic phase. From the experimental data, the order parameters <P2> and <P4> as well as the molecular distribution function have been determined. The small differences between the molecular orientation in the low molecular liquid crystals and the side chain polymers have been observed. It has been also found that the orientational order depends strongly on the molecular structure of the mesogenic molecules.
The order parameters of some dichroic fluorescent dyes (naphthalene derivatives of bicarboxylic acid) in nematic liquid crystal matrices have been determined. The influence of the dye addition on the clearing temperature of the nematic host has been investigated. It has been found that the dyes studied can be utilized in the guest-host liquid crystal displays working both in passive and active modes.
The polarized absorption spectra of the naphthalene bicarboxylic acid derivatives dissolved in nematic liquid crystal mixture ZLI 1695 have been studied in the ultraviolet and visible spectral region. On the basis of these spectra the degree of anisotropy and the order parameters S and D have been determined as a function of temperature. The experimental data have been compared with theoretical calculations predicted by the mean field theory.
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