Room Temperature Nematic Liquid Research Articles

Photorefractivity in Nematic Liquid Crystals Using a Donor−Acceptor Dyad with a Low-Lying Excited Singlet State for Charge Generation

The photorefractive effect was observed in room-temperature nematic liquid crystal composites consisting of a eutectic mixture of 65% 4‘-(n-pentyl)-4-cyanobiphenyl and 35% 4‘-(n-octyloxy)-4-cyanobiphenyl doped with a donor−acceptor dyad in which 9-(N-pyrrolidinyl)perylene-3,4-dicarboximide is directly linked to 1,8:4,5-naphthalenetetracarboxydiimide. This dyad has a broad, intense absorption band that extends from 500 to 750 nm. Measurements of two-beam asymmetric energy exchange (beam-coupling) and the diffraction efficiency of the refractive index grating produced in these liquid crystal thin films were carried out. A comparison is made between the covalent donor−acceptor dyad and the corresponding noncovalent donor−acceptor pair. In each case the covalent dyad outperforms the noncovalent donor−acceptor pair. The results show that aminoaryleneimides are promising new charge donating chromophores that are capable of extending photorefractive performance of nematic liquid crystal composites into the near-infrared region of the spectrum.

Elastic, dielectric and optical constants of 4'-pentyl-4-cyanobiphenyl

4'-n-Pentyl-4-cyanobiphenyl (5CB) is a room temperature nematic liquid crystal with a high positive dielectric anisotropy and a high chemical stability. Many experimental results concerning the elastic and dielectric constants of 5CB are available in the literature, although there is often no satisfactory agreement between the experimental data obtained by different groups, especially as far as the dielectric constants are concerned. Furthermore, no detailed investigation of the temperature dependence of the elastic and dielectric constants close to the nematic-isotropic transition temperature T NI has yet been reported. In this paper, we report the measurement of the elastic and dielectric constants of 5CB, and the temperature behaviour close to T NI has been investigated in detail. The experiment consists in the measurement of the director deformation induced by an electric field using simultaneously both a dielectric and an optical method. The simultaneous use of these two methods provides an indirect check on the reliability of the measurements. Special attention has been devoted to control possible sources of uncertainty. In particular, the effects of finite anchoring energy and of finite pretilt angle have been considered. The temperature dependence of the anisotropy of the refractive indices is also obtained in the experiment.

Liquid-Crystalline Physical Gels Formed by the Aggregation of Trans-(1R,2R)-Bis(Dodecanoylamino)Cyclohexane in a Thermotropic Nematic Liquid Crystal. Phase Behavior and Electro-Optic Properties

Liquid-crystalline physical gels have been obtained by the self-association of hydrogen-bonded molecules in a room-temperature nematic liquid crystal. Thermoreversible three states, isotopic liquid, normal (isotropic) gel, and anisotropic gel can be achieved for these mixtures. Electro-optic properties of the anisotropic gels have been examined with a twisted nematic cell.

Measurement and Assignment of Long-Range C−H Dipolar Couplings in Liquid Crystals by Two-Dimensional NMR Spectroscopy

We describe multidimensional NMR techniques to measure and assign 13C−1H dipolar couplings in nematic liquid crystals with high resolution. In particular, dipolar couplings between aromatic and aliphatic sites are extracted, providing valuable information on the structural correlations between these two components of thermotropic liquid crystal molecules. The NMR techniques are demonstrated on 4-pentyl-4‘-biphenylcarbonitrile (5CB), a well-characterized room-temperature nematic liquid crystal. Proton-detected local-field NMR spectroscopy is employed to obtain highly resolved C−H dipolar couplings that are separated according to the chemical shifts of the carbon sites. Each 13C cross section in the 2D spectra exhibits several doublet splittings, with the largest one resulting from the directly bonded C−H coupling. The smaller splittings originate from the long-range C−H dipolar couplings and can be assigned qualitatively by a chemical shift heteronuclear correlation (HETCOR) experiment. The HETCOR experiment incorporates a mixing period for proton spin diffusion to occur, so that maximal polarization transfer can be achieved between the unbonded 13C and 1H nuclei. To assign the long-range C−H couplings quantitatively, we combined these two techniques into a novel reduced-3D experiment, in which the 1H chemical shift-displaced C−H dipolar couplings are correlated with the 13C chemical shifts. The time domain of this experiment involves separate but synchronous incrementation of the evolution periods for the C−H dipolar couplings and the 1H chemical shifts, with a variable ratio of the respective dwell times to optimize the resolution and facilitate resonance assignment in the spectrum.

Fast write and erase surface resistance induced nematic liquid crystal display

A room-temperature nematic liquid crystal cell is prepared in the homeotropic geometry using dielectrically positive pentyl cyanobiphenyl. The indium tin oxide electrodes are treated to increase locally their surface resistance. At high frequencies (ω>ωc) the border of these regions behaves as capacitance edge, creating an oblique field which ‘‘writes.’’ At low frequencies (ω<ωc) the field is back normal to the electrodes and ‘‘erases’’ rapidly. A 50-μs erasing time has been achieved for E∼10 V/μm and ωc/2π=104 Hz. The practical smallest response time of the device is ∼(8ωc)−1. Sub-μs writing and erasing are possible.

Polarization absorption spectroscopy: Determination of the direction and degree of orientation of absorption transitions

In an ordered, directional medium, such as liquid crystals or biological membranes, it is often desirable to determine both the degree of ordering (i.e., order parameters) and the directions in space which most aptly characterize it. Here, a method is presented for determining these quantities relative to a laboratory frame. It consists in measuring changes in the light absorbed by a pleochroic dye dissolved in dilute quantity in a directionally ordered medium (e.g., nematic liquid crystal or lipid bilayer) as functions of polarization and angles of incidence. The results of testing this method on a model system of dilute mixtures of dyes in a room temperature nematic liquid crystal are presented. In principle, only six such measurements are necessary to make these determinations; however, some redundancy is desirable to obtain sufficiently accurate results. We show that an accuracy of 5% or better can be obtained from nine measurements. A detailed discussion of the errors that can be expected is given.

Polarized Fluorescence of Dyes Oriented in Room Temperature Nematic Liquid Crystals

Abstract Pleochroic absorption and polarized fluorescence of guest molecules oriented in nematic liquid crystals were observed in the nematic phase and at the nematic-isotropic phase transition. The order parameters calculated from fluorescence intensities were found to be smaller than those calculated from the absorption coefficients. For some guest-host combinations a change in the quantum yield of the fluorescence was observed at the nematic-isotropic transition point.

Low electrooptic threshold in new liquid crystals

New room temperature nematic liquid crystals with positive dielectric anisotropy were investigated in display cells with twisted nematic molecular ordering. Threshold voltages as low as 0.9 V and contrast ratios of 100:1 at 1.3 V are reported.

Anomalous Alignment and Domains in a Nematic Liquid Crystal

The relationship between anomalous alignment and domain formation is examined for the room temperature nematic liquid crystal, p-methoxybenzilidene p-n-butylaniline (MBBA). An increase in order with the long molecular axes preferring a direction perpendicular to an externally applied low-frequency electric field is observed for potential differences below the threshold voltage for anomalous alignment. Employing measurements of the dielectric loss at a microwave frequency of 24.4 GHz, the threshold voltage for anomalous alignment is examined as a function of the frequency of an externally applied electric field. This is then contrasted with the frequency dependence of the threshold voltage for domain formation as determined from visual observation. Comparing optical and dielectric data we show that the threshold is the same in each case. At very high electric field strengths, behavior indicating a tendency towards random molecular alignment is observed.

Anisotropic Ultrasonic Properties of a Nematic Liquid Crystal

Ultrasonic measurements (2 to 6 MHz) on a room-temperature nematic liquid crystal have shown the attenuation to vary strongly with the angle $\ensuremath{\theta}$ between the sound-wave propagation direction and the direction of an aligning magnetic field. The velocity change is quite small, amounting to some 0.1% between the $\ensuremath{\theta}={90}^{\ensuremath{\circ}}$ and the $\ensuremath{\theta}={0}^{\ensuremath{\circ}}$ case. The orientation-dependent attenuation is of the hysteresis type.

REVERSIBLE ULTRAVIOLET IMAGING WITH LIQUID CRYSTALS

Light-scattering images are recorded in nearly real-time in nematic liquid crystals. The material is activated by ultraviolet exposure of a photoconductive ZnS layer on one of the Nesa electrodes with which an electrical field is applied. In a room-temperature nematic liquid crystal, an incident ultraviolet exposure pulse of 0.1 mJ/cm2 produces an image in a risetime of 0.1 sec, and it decays in 2.5 sec. Long-lived images can be stored in nematic/cholesteric mixtures.

DYNAMIC SCATTERING IN A ROOM-TEMPERATURE NEMATIC LIQUID CRYSTAL

The dynamic scattering characteristic of a room-temperature nematic liquid crystal is described. This electro-optic effect has been observed in many high-temperature nematic liquid crystals. The material described here has a mesomorphic phase from 11 to 41 °C. A contrast ratio of 30 to 1 is observed with a field of 3.7×104V/cm.