Guest-host interactions in dichroic dye-liquid crystal mixtures in smectic A and nematic phases

Pretransitional effects in a mesogenic mixture under an electric field

Nematogenic liquids are materials that are in high demand for many application purposes. Field-induced nematic order in isotropic liquids can be observed by the enhancement of electric permittivity during dielectric spectroscopy measurements under the bias electric field. For this reason, dielectric measurements were done on a nematogenic mixture of highly polar compounds. This paper presents a discussion on the influence of the bias electric field on the dielectric response. We observed a decrease of electric permittivity in the isotropic phase under the bias electric field. This effect is most likely caused by the field-induced creation of some complex molecular objects. In the paper, relaxation parameters of the molecular modes were analyzed. The field-induced orientational order was determined using a dichroic ratio in a guest-host system. The measurements show the significant order parameter in the isotropic phase under an electric field.

Photo-physical and structural studies of some synthesized arylazoquinoline dyes

This study presents the spectral and structure characteristics of seven azoquinoline dyes with different substituents and their new methylated counterparts for the first time, where some compounds are newly synthesized. The solvatochromic, tautomeric, halochromic, and dichroic behavior of the compounds were studied by electronic spectroscopy in various media. The different types of media were ordinary, multifunctional, and ordered liquids. The experiments were extended to include under acidic or basic conditions. The orientational behavior of the azo dye-doped liquid crystals was studied, and it was established that the azo form is the main species in high polar anisotropic media. The multi-parameter polarity scales were used to correlate the spectral data. Influence of acid and base on the absorption spectra of the dyes was also examined. Ionization constants for these dyes were determined in ethanol-water media. As a result, at the high dye concentrations, the intermolecular hydrogen bonding is more stable than the intra-molecular hydrogen bond, and therefore, the azo form is the main species in concentrated solutions. In order to provide more details, time-dependent density functional theory (TD-DFT) calculations were carried out for the representative models.

Effects of fullerene addition on the thermo-optical properties of smectic liquid crystals at the vicinity of the nematic-smectic-A phase transition

The present work is devoted to the study of the thermo-optical properties of liquid crystals doped with traces of fullerene C(60) at the vicinity of the nematic-smectic-A phase transition. By using the time-resolved Z-scan technique, we measure the temperature dependence of the thermo-optical coefficient and the thermal diffusivity. Our results reveal that the critical behavior of the thermal diffusivity is strongly affected by the fullerene addition. We provide a detailed discussion under the light of the distinct mechanisms behind the thermal transport in liquid-crystal samples.

Tautomeric behavior of some azoquinoline dyes in liquid and liquid crystalline media

The absorption spectra of three azoquinolin-8-ol derivatives (o-, m-, p-cyano hydroxy azoquinolin dyes) were investigated in liquid and liquid crystalline solutions as a function of the solvent polarity. The spectral data of the dyes were compared in both ordinary liquid solvents and liquid crystalline media. Analysis of the spectral data was used to determine the azo and hydrazone forms in both the environments. The spectral shifts were correlated by Kamlet-Taft and Katritzky multi-parameter polarity scales. For the azoquinoline dyes, the azo form is almost entirely dominated in polar anisotropic hosts. In contrast, the compounds remain dominantly in hydrazone form in some polar solvents such as DMF. The polarized absorption spectra of the compounds in the anisotropic media were measured and their degree of anisotropies was determined.

Supramolecular order following binding of the dichroic birefringent sulfonic dye Ponceau SS to collagen fibers

The optical anisotropies (linear dichroism or LD and birefringence) of crystalline aggregates of the sulfonic azo-dye Ponceau SS and of dye complexed with chicken tendon collagen fibers were investigated in order to assess their polarizing properties and similarity to liquid crystals. In some experiments, the staining was preceded by treatment with picric acid. Crystalline fibrous aggregates of the dye had a negative LD, and their electronic transitions were oriented perpendicular to the filamentary structures. The binding of Ponceau SS molecules to the collagen fibers altered the LD signal, with variations in the fiber orientation affecting the resulting dichroic ratios. The long axis of the rod-like dye molecule was assumed to be bound in register, parallel to the collagen fiber. Picric acid did not affect the oriented binding of the azo dye to collagen fibers. There were differences in the optical anisotropy of Ponceau SS-stained tendons from 21-day-old and 41-day-old chickens, indicating that Ponceau SS was able to distinguish between different ordered states of macromolecular aggregation in chicken tendon collagen fibers. In the presence of dichroic rod-like azo-dye molecules such as Ponceau SS, collagen also formed structures with a much higher degree of orientation. The presence of LD in the Ponceau SS-collagen complex even in unpolarized light indicated that this complex can act as a polarizer.