Helical Sense and Pitch of Cholesteric Liquid Crystals

The rotatory sense of a circularly polarized light transmitted through a cholesteric liquid crystal cell was studied by measuring the retardation of a birefringent plate placed upon the cell. Furthermore, the dependence of helical pitch on composition is described for various binary cholesteric and nematic-cholesteric mixtures. The helical sense of eleven cholesterics was determined by the experimental results. As a result, it is shown, for example, that cholesteryl chloride has a right-handed helical structure and cholesteryl propionate has a left-handed helical structure.     

Flexible Structures Based on a Short Pitch Cholesteric Liquid Crystals

We report on the realization and characterization of electro-responsive and pressure sensitive polydimethylsiloxane (PDMS) conductive photonic structures combined with the reconfigurable properties of short pitch cholesteric liquid crystals (aligned in Grandjean configuration). By combining ion-implantation process and surface chemistry functionalization, we have overcome the insulating properties of PDMS and induced long range organization of cholesteric liquid crystals, thus controlling both diffraction and selective Bragg reflection of light by means of external perturbations (electric field, pressure). We have characterized our devices in terms of morphological, optical and electro-optical properties.     

Circular Dichroism in Cholesteric Liquid Crystals

Crystallography Reports - The circular dichroism in cholesteric liquid crystals (CLCs) has been studied in the absorption bands of dissolved dichroic dyes possessing both the positive and negative...     

The Orientational Order in Cholesteric Liquid Crystals

The investigation of the orientational order in the mesophase of steroidal cholesterogens has been all but neglected by experimentalists, possibly because of the lack of an appropriate technique. However electron resonance spectroscopy can be used to determine the order parameter of a spin probe dissolved in a cholesteric mesophase. In addition the structure of the spin probe may be made to resemble that of the cholesteric solvent. Here this technique is employed to determine the orientational order for cholesteryl and cholestanyl benzoate, as a function of temperature. The results of these experiments yield order parameters which are comparable, or slightly smaller, than those typically observed for nematics. A comparison of these values with mean field theories suggests that the low order parameters of steroidal cholesterogens stems from deviation of the constituent molecules from cylindrical symmetry.     

Behaviour of Ultrasonic Velocity in Cholesteric Liquid Crystals

Ultrasonic velocities (V) have been determined, employing a fixed path double crystal interferometer, in three cholesteric liquid crystals, namely cholesteryl propionate, cholesteryl laurate and cholesteryl myristate in their isotropic and anisotropic phases including the region of the phase transition. The variation of specific volume (v) is also studied in the same temperature range by a special dilatometer constructed for this purpose. Anomalous behaviour of ultrasonic velocity is observed near the isotropic-cholesteric phase transition in all three compounds. In the two polymesomorphic liquid crystals, namely cholesteryl laurate and cholesteryl myristate, contrary to the ultrasonic behaviour of cholesteryl stearate, prominent velocity dips are observed at cholesteric-smectic transition temperatures. The parameters adiabatic compressibility (β_ad) and molar sound velocity (R) are estimated and they are found to exhibit sudden jumps at cholesteric-smectic and isotropic-cholesteric transitions. The thermal expansion and temperature co-efficient of compressibility are found to show abnormal increase near the phase transition indicating the existence of large-magnitude pre-transitional effects near the phase transition. A comparative study of the ultrasonic behaviour of six aliphatic esters of cholesterol has shown that the magnitude of the velocity dip observed at the isotropic-cholesteric transition increased with increase of molecular weight and only cholesteryl acetate shows deviation.     

The Failure of Asymmetric Synthesis in Cholesteric Liquid Crystals

A set of selected bimolecular reactions leading to asymmetric and dissymmetric products was carried out in cholesteric solvents with the aim of obtaining asymmetric syntheses.

In all cases, the products did not show significant optical rotation. The reasons for the lack of asymmetric induction are briefly discussed.     

Infrared Spectroscopic Studies of Homologous Cholesteric Liquid Crystals

Abstract

Infrared (500–2500 cm^?1) spectra of cholesteryl alkyl carbonates (methyl, ethyl and oleyl) have been examined in the solid (cholesteryl methyl carbonate and cholesteryl ethyl carbonate) and liquid crystalline (cholesteryl oleyl carbonate) states. An attempt has been made to correlate the variation in the intensities and frequency shifts with the molecular structures. The shift in the carbonyl frequency in the case of cholesteryl oleyl carbonate may be correlated with the affect on the coplanarity with benzene ring.     

Optical Properties of Cholesteric Liquid Crystals at Oblique Incidence

The optical properties of a periodic structure characterized by a uniform rotation of the dielectric tensor about a given axis are theoretically analyzed. The electromagnetic wave is described as a superposition of elementary modes having the form of Bloch waves. Each elementary mode is represented by a sum of plane waves elliptically polarized, whose wavevectors are the solutions of a characteristic equation. This equation, presented in a preceding paper, is furtherly analyzed, in order to obtain the wave vectors in terms of a power series of a small parameter δ, representing the anisotropy of the dielectric tensor. The coefficients of the series up to terms containing δ⁶ are explicitly given, and the corresponding truncation errors computed. The spectral composition and the polarization states of the Bloch waves are also analyzed and discussed for different values of the incidence angle in the frequency range containing the lower reflection bands. In particular it is shown that in the regions between the reflection bands both the wave functions and the wavevectors can be evaluated with very good approximation by simple analytic expressions, while for an accurate evaluation of the reflection properties of the structure more involved expressions are needed.     

Electric-Field Enhancement of Fluorescence Intensity of Cholesteric Liquid Crystals Containing Aromatic Chromophores

Fluorescence from cholesteric liquid-crystalline mixture containing cholesteryl 3-(l-pyrenyl)propanoate was measured under dc and ac electric field applied parallel to the helix axis. The excimer fluorescence of the pyrenyl chromophore increased and decreased its intensity reversibly upon turning the electric field on and off. Similar electric field effects were observed for such chromophores as tetracene, perylene, and 9,10-diphenylanthracene doped in the cholesteric mixture. The electric-field effect was observed only for fluorescence transitions which show left-handed circularly polarized fluorescence, indicating that only transitions which are polarized perpendicular to the optic axis in a quasi-nematic layer exhibit the effect. The experimental results were found to be intimately connected to the onset of the Helfrich periodic distortion of a planar structure.     

Influence of the Orientational Order on the Optical Properties of Cholesteric Liquid Crystals

An orientational order parameter for cholesteric liquid crystals is defined in analogy to nematic mesophases. The influence of this parameter on the selective reflexion and the optical rotatory dispersion is described. Its relative change with temperature is evaluated from ORD measurements for an induced cholesteric soIution and is found to be similar to that known for nematic liquid crystals.     

Reflectance,Transmittance and Color Gamut of Superimposed Layers of Cholesteric Liquid Crystals

The spectral reflectance curves of superimposed layers of cholesteric liquid crystals (CLC) having the same handedness add as there is little absorption. The peak reflectance of the sum cannot exceed 0.5 and the transmittance cannot be less than 0.5, which is the theoretical limit. The shape of the reflection and transmission band resembles rectangles and thus can be compared with the spectral curves of optimum colors. The chromaticity loci of CLC colors and its luminous reflectance equals 0.5 of the value of the luminous reflectance attached to the optimum colors. The color gamut thus produced can be shown to be greater than that which has been obtained with pigments.

A significant improvement can be obtained using superimposed layers of CLC having opposite handedness. Their spectral curves add and the peak reflectance is not limited by the value of 0.5 but approaches 1.0. Use of CLC's of opposite handedness is not always practical as they have different chemical and physical properties. A similar result can be obtained using two CLC layers of the same handedness and a half-wave retardation plate interposed between them. This plate converts the transmitted through the upper layer right-handed (left-handed) component into a left-handed (right-handed) one, which is then reflected from the lower layer and contributes to the reflected left-handed (right-handed) component from the upper layer. The chromaticity loci and color gamut of such superimposed CLC pairs of coatings now approaches that of optimum colors of the same luminous reflectance.     

Local Field Anisotropy in Nematic Liquid Crystals

The anisotropy of the local electric field in the interior of ordered fluids provides a measure of the anisotropy of the molecular distribution. In nematics, both the orientational order parameter and the anisotropy of the local field can be calculated from refractive index and density measurements; results are presented for the liquid crystal butyl p-(p-ethoxyphenoxycarbonyl) phenyl carbonate. It is found that the anisotropy of the local field is very nearly proportional to the orientational order parameter. A simple model incorporating steric intermolecular repulsion predicts this behaviour. The ratio of the local field anisotropy to the uniaxial and biaxial order parameters is simply related to the length-to-breadth ratio of the effective molecular hard core.     

Phase Transition Studies in Mixed Polymesomorphic Liquid Crystals with Cholesteric and Smectic A Phase

Cholesteric mesomorphism was induced in a polymesomorphic liquid crystal with nematic and smectic A phases by mixing it with a low pitch cholesteric. A variety of mixtures for a wide range of concentrations were studied. It was observed that the addition of cholesteric liquid crystal disrupts the smectic phase of the parent polymesomorphic liquid crystal. The measured valuses of ultrasound velocity and specific volume and their related parameters were found to exhibit anomalies near each of the phase transitions. The mean interaction parameter $ \bar \alpha $ was estimated for different mixtures and was found to decrease with increasing concentration of cholesteric liquid crystal in the mixture. It was observed that smectic A-cholesteric transition becomes second-order at a particular concentration in agreement with the theoretical predictions.     

Lasing Spectra of Dye Doped Cholesteric Liquid Crystals Under Reversible Phototuning of the Frequency

We investigate the lasing frequency phototuning in a distributed feedback (DFB) laser based on cholesteric liquid crystals (CLC). To improve the CLC planar texture quality, we use new materials such as nematic ZhK-654 and an analog of cholesteryl oleate as a twisting additive. The selective excitation of the cis-form molecules of the azoxy-nematic is carried out with the use of a combination of an interference filter with maximum transmission at 436 nm and a cut-off filter. Using the orienting substrates with SnO₂ allows us to reduce the threshold pump power by more than an order of magnitude. The proposed combination of filters for the selective excitation enables us to align the ranges of lasing wavelengths in both directions.     

Magnetic Field Induced Deformations in Nematic Liquid Crystals

Abstract

Unlike most organic materials, liquid crystals respond readily to magnetic fields. This response originates in the anisotropic magnetic susceptibility of nematics, and the relative ease with which they undergo orientational deformations. Because of the large optical birefringence of liquid crystals, these orientational deformations are easy to observe. An interesting demonstration is to place a small rare earth magnet on a nematic cell between crossed polarizers which is illuminated from below. As the magnetic field reorients the liquid crystal, beautiful interference colours appear, indicating director deformations.     

The Internal Field Parameters for Liquid Crystals

This paper presents calculations of the internal field constants γ_e for four homologous liquid crystal materials in their nematic and crystalline phases. We find that for the nematic phase, the γ_e ～ S ₂ curve is nearly a straight line for each compound. When one extrapolates the straight line to S ₂ = 1, the corresponding γ_e value is nearly equal to γ∥, the crystalline internal field constant. The birefringence data used are those reported by Somashekar et al.⁴ The four compounds studied are:

(1) p(p'-ethoxyphenylazo)phenyl valerate;

(2) p(p'-ethoxyphenylazo)phenyl hexanote;

(3) p(p'-ethoxyphenylazo)phenyl heptanoate;

(4) p(p'-ethoxyphenylazo)phenyl undecylenate.     

Cholesteric Pitch and Blue Phases in a Chiral-Racemic Mixture

The cholesteric and “blue” phases (BP) of a mixture of the chiral and racemic forms of the same compound were studied as a function of composition and temperature. It was found that the clearing point and the cholesteric-S_A transition temperature was independent of composition. The pitch of the cholesteric at a given temperature was inversely proportional to the fraction of chiral material, as was the lattice parameter of BPII. The lattice parameter of BPI did not follow the above rule. No second-order BPI-BPIIb transition was seen.

It is concluded that this mixture is thermodynamically ideal^(6,7) in that the intermolecular interactions between two molecules are independent of their relative chirality. Thus, this system comes close to the theorist's ideal of one in which only the chirality is varied.     

Determination of Pitch in a Cholesteric DSCG-Water Lyomesophase by NMR Techniques

Nuclear Magnetic Resonance (NMR) techniques are used to determine the pitch in a cholesteric lyomesophase prepared by adding of l-alanine to a solution of disodiumcromoglycate (DSCG) in water. The method, which is particularly suitable for the study of bulk samples, consists of a combination of two independent types of measurements: (i) The effective reorientational correlation time of the director, due to molecular translational diffusion, is determined by a lineshape analysis of the deuterium resonances of the solvent D₂O and/or deuterated l-alanine. (ii) The translational self-diffusion coefficients of both these constitutents is determined by a pulsed field-gradient spin-echo experiment. From the results of these measurements, the pitch and its dependence on the chiral dopant concentration is determined.     

Comments on Some Recent Developments in the Field of Liquid Crystals

Three areas of research into liquid crystals which have seen considerable development since the last International Liquid Crystal Conference at Bordeaux in 1978 have been selected for a brief over-view.

These areas concern: 1. New Liquid Crystal Materials containing non-aromatic ring systems and the developments which have followed from the earlier studies of esters of trans-4-n-alkylcyclohexane-1-carboxylic acids and the trans-4-n-alkyl-l-(4′-cyanopheny1)cyclohexanes (PCH's).

2. Agreement over a Nomenclature Problem Concerning Smectic Polymorphs and some interesting features of the smectic phase sequences for the 4-n-alkoxybenzylidene-4′-n-alkylanilines.

3. Discotic Phases—the discovery that the historically well known mesophase of di-isobutyl-silandiol is a discotic liquid crystal phase and the progress being made in studies of other discotic systems.