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Abstract: The dielectric permittivity and refractive indices have been measured as a function of temperature, throughout the nematic and isotropic phases, for a newly synthesized liquid crystal sample. The measurements in nematic phase indicate positive dielectric and optical anisotropy for this sample. In order to characterize the anisotropic behavior of the investigated LC, Vuks' method was used to obtain the order parameter in different temperatures. The obtained characterization parameters are essential for W-1680 liquid crystal applications and improvement. [Copyright &y& Elsevier]

Characterisation of liquid crystals (LCs) as solvents is needed, to obtain the polarity and solvatochromic polarity parameters of these media. Polarity parameters demonstrate the effects of LC media on the photo-physical behaviour of solute molecules in an anisotropic medium. The practical limitations in determining solvent polarity scale parameters for LCs can overcome the overlapping absorption band of LCs and solvent-sensitive standard compounds or their insolubility in LCs. In this work, we report Kamlet–Abboud–Taft polarity functions of some nematic LCs in different temperatures and phases, isotropic and anisotropic, with the solvatochromic method, using the Reichardt's dye and 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)-phenolate standard probe. In addition, a new azo and coumarin dye were used as probes to obtain some solvatochromic polarity parameters. Finally, a new polarity parameter, the LC anisotropic matrix, is introduced. [ABSTRACT FROM AUTHOR]

Abstract: Three cyano-benzoate liquid crystals'' refractive indices have been measured throughout the nematic and isotropic phase in order to determine their order parameters by using Vuks methods and characterization of anisotropic behaviors. The temperature dependence of refractive indices, birefringence and order parameters in the anisotropic phase, has been investigated for these nematic liquid crystals. Furthermore, the alkyl substituent''s effects on the order parameters of cyano-benzoate liquid crystals were investigated. [Copyright &y& Elsevier]

Abstract: Absorption and emission spectra of two rhodamine dyes with same molecular skeletons and different functional groups have been studied in various isotropic and anisotropic media. The solute photo-physical behavior depends strongly on the solvent–solute interactions and solvent microenvironment as well as resonance structures of the rhodamine dyes. In order to understand the effect of intermolecular interactions on spectral behaviors of these dyes and to conceive nature and extend of solvent–solute interactions the spectral variations were analyzed by the linear solvation energy relationships concept. In addition, by means of solvatochromic method the dyes dipole moments variations, from ground to excited states, were investigated in isotropic and anisotropic solvents media. [Copyright &y& Elsevier]

Abstract: The ground state (μ g) and the excited state (μ e) dipole moments of two coumarin laser dyes, coumarin 440 and 460, were studied at room temperature in various solvents, viz., general solvents, alcohols and liquid crystals at 298K. In this work, we report dipole moment of laser dyes in different anisotropic (liquid crystal) and isotropic environments for understanding the effects of environments on the molecular dipole moment and comparing them. Ground and excited state dipole moments of coumarin dyes were evaluated by means of solvatochromic shift method. It was observed that dipole moment values of excited states (μ e) were higher than the corresponding ground state values (μ g) in all media. [Copyright &y& Elsevier]

Abstract: Four tolane-based liquid crystals, 4′-pentyl-, 4′-hexyl-, and 4′-heptyl-3-fluoro-4-isothiocyanatotolane and 4′-hexyloxy-3′-fluoro-4-isothiocyanatotolane have been investigated throughout the nematic and isotropic phase in order to determine their refractive indices and order parameters using the Vuks method. These factors characterize anisotropic behaviors of selected liquid crystals that are essential for their applications and improvement. The temperature dependence of refractive indices, birefringence and order parameters in anisotropic nematic phase, have been investigated for these four nematic liquid crystals. All the compounds have positive and large optical anisotropies that increase with decreasing temperature. [Copyright &y& Elsevier]

Abstract: The ground state (μ g) and the excited state (μ e) dipole moments of two coumarin laser dyes, C500 and C503, were studied at room temperature in various solvents, viz., aprotic solvents, alcohols and liquid crystals at 298K. We report dipole moment of laser dyes in different anisotropic (liquid crystals) and isotropic environments. The dipole moments values in different media help to investigate environment effects on the molecular dipole moment and provide a straightforward method for comparing their properties. Ground and excited state dipole moments of coumarin dyes were evaluated by means of solvatochromic shift method. It was observed that dipole moment values of excited states (μ e) were higher than corresponding ground state values (μ g), indicating a substantial redistribution of the π-electron densities in a more polar excited state for the dyes investigated. [ABSTRACT FROM AUTHOR]

Abstract: Absorption spectra of three disperse azo dyes with various molecular sizes and alkyl tails have been recorded in isotropic and anisotropic solvents in the range between from 200 to 700nm. The photo-physical behavior of a dissolved dye depends on the nature of its environment, i.e. the intensity, shape, and maximum absorption wavelength of the absorption band depends strongly on the solute–solvent interactions and solvent nature. The solvatochromic behaviors of disperse azo dyes and solute–solvent interactions can be analyzed by means of the linear solvation energy relationship concept proposed by Kamlet and Taft. Bathochromic shift of absorption maxima of these dyes, λ max, occurs in solvents with highest proton acceptor ability and dipolarity/polarizability. [Copyright &y& Elsevier]

The aggregation of rhodamine 6G tetrafluoroborate and rhodamine 6G chloride in liquid crystalline solution (anisotropic host) was studied using polarized spectroscopy (the electronic absorption spectroscopy with polarized light). The self-association of these two ionic dyes in the normal solvents (isotropic hosts) was studied using absorption spectroscopy in the visible region for comparison. Therefore, the influence of the [BF4]-1 anion on the molecular interaction of the guest molecules was investigated. The dimerization constants, Kd=cd/cm, of solutions containing rhodamine 6G tetrafluoroborate and rhodamine 6G chloride in dioxane were calculated to be 183 l mol-1 and 2363 l mol-1, respectively, indicating important role of anion–cation interaction in their solvation. In contrast, the similarities of absorption spectra of these two dyes in aqueous solutions suggest that the type of anion has a minor effect on their absorption spectra. The nature of the interacting pairs in these dyes was discussed using the exciton theory. [Copyright &y& Elsevier]

Dichroic ratios R and order parameters S of solutions of methylene blue, rhodamine 6G and rhodamine B in the nematic liquid crystalline solvent were investigated by the measuring of the intensity of the absorption bands in the visible region of parallel aligned samples. The molecular association of the dyes in a nematic solvent and also in an amphiphilic solution was studied using optical spectroscopy and in a guest–host system. The nature of the interacting pairs in these dyes was discussed using the exciton theory. [Copyright &y& Elsevier]

Photo-induced behavior of polymethacrylate polymer, with cyano azobenzene side group, was studied. The photoisomerization process occurs in cyano azo polymer, by illumination of a film with polarized and unpolarized light. The illumination of the polymer film with light results in color change, the color of film gets darker. This is in opposition to common azo polymers in which the result of illumination is a photo-bleaching. Study of spectrum changes of a dilute polymer solution shows that the color change under pump beam illumination is not due to interaction between dye molecules. Time evaluation of probe beam absorption induced by Ar + laser pump beam shows the fast change in population of isomers at higher light powers. Light induced birefringence (LIB) experiments with high power pump beams shows uncommon new features. A high long-term stability of LIB is demonstrated when the polymer film is kept in the dark. Not only a circular polarized light cannot erase the birefringence but it also induces anisotropy in polymer film. It appears that this is relevant of a phase transition in the polymer film. [ABSTRACT FROM AUTHOR]

Abstract: A quantitative study of the isotropic and anisotropic environment effects on the photophysical properties of a solvatochromic dye, Nile red, was carried out at room temperature. The absorption and emission spectra of the dye in liquid and liquid crystalline media were investigated. The solvatochromic method was used for determining the ground- and excited-state dipole moments. In addition, applicability of solvatochromic method for the dye solvation in anisotropic media was investigated. The nature and degree of the dye–solvent interactions were characterized using multi-parameter solvent polarity analyses. [Copyright &y& Elsevier]