Experimental determination of the flexoelectric coefficients of some nematic liquid crystals

Abstract

We report measurements of the temperature variation of the flexoelectric coefficient (e ₁–e ₃) of a number of nematic liquid crystals like phenylcyclohexanes, cyanobiphenyls, etc. We have also measured (e ₁+e ₃) of a few systems using appropriate methods of applying an electric field gradient to the sample. In most of the systems, (e ₁–e ₃)/k, where k is a curvature elastic constant, is found to be positive and independent of temperature, as expected. However, in 4-heptyl-1-(4-cyanocyclohexyl)cyclohexane and a few other compounds with relatively flexible parts, |(e ₁–e ₃)/k| increases with temperature. We discuss the possible molecular origin of the sign and temperature dependence of the flexoelectric coefficients of the systems studied.     

A phenomenological model for the undulating twist grain boundary-C* phase

We propose a simple phenomenological model which is able to account for the various twist grain boundary (TGB) phases, including the recently discovered undulating twist grain boundary-C* (UTGB_C*) phase. In the UTGB_C* phase, the smectic C* (SmC*)-like blocks and the grain boundaries separating them undulate to form a two-dimensional square lattice perpendicular to the TGB helix axis. We treat the grain boundaries separating adjacent smectic blocks as interfaces with an anisotropic interfacial tension. At moderate chiral strengths we find a TGB_A-TGB_C-SmC* sequence. As the chiral strength is increased this goes to the sequence TGB_A-UTGB_C*-SmC*. Such sequences have been observed experimentally.     

A simple molecular theory of a nematic–nematic phase transition in highly polar compounds

Abstract

We have extended an earlier molecular model which was developed to explain the double reentrant sequence in highly polar compounds to predict the possibility of a nematic (N₁)–nematic (N₂) phase transition in such compounds. At moderate densities the dipolar interactions would give rise to an antiparallel near neighbour arrangement of the polar molecules while at higher densities, the dipole–induced dipole and chain–chain dispersion interactions give rise to a parallel configuration. The N₁–N₂ transition corresponds to a jump in the relative concentration of the two species. Using the mean field approximation we have calculated the phase diagram. The weak first order transition disappears above a critical point as a function of an appropriate parameter. We have also calculated the specific heat anomaly around the transition region.     

Stereoselective synthesis of umuravumbolide

An efficient stereoselective total synthesis of umuravumbolide has been developed. The key features of the synthesis include Jacobsen resolution, Wadsworth Emmons olefination and silyl-tethered ring closing metathesis.     

Scaled Particle Theory of a System of Spherocylinders: Extension of Calculations to High Pressures

The viscosity coefficients of nematic substances for corresponding orientation geometry introduced and measured for PAA and PAP by the author in the thirties are discussed from the actual point of view.

The great universality of these coefficients of viscosity and their utility for describing several dynamical phenomena has been shown. There is a quantitative agreement between the experimental data of those investigations and continuum hydrodynamical theory of nematics with the numerical values of viscosities measured by the author.

As a new field of application of flow in definite orientation geometry, the properties of smectics and closely connected problems of phase transitions, pretransition effects and reentrant phase has been discussed.     

Molecular Iodine–Catalyzed,Mild, Effective,Ecofriendly, Microwave-Assisted,One-Pot Synthesis of 5-Arylmethylidene-2-phenyloxazol-4-ones (Azalactones) Under Solvent-Free Conditions

Fourteen azalactones have been synthesized in excellent yields under solvent-free conditions by heating an aldehyde, hippuric acid, and acetic anhydride in the presence of molecular iodine as a catalyst in a microwave oven. The short reaction time, cleaner reaction, and easy workup make this protocol practical and economically attractive.     

Glycine-Catalyzed Efficient Synthesis of Pyranopyrazoles via One-Pot Multicomponent Reaction

A facile and convenient protocol is developed for the fast (5–20 min) and high-yielding (85–95%) synthesis of fused pyranopyrazoles from ethyl acetoacetate, hydrazine hydrate, an aldehyde, and malononitrile in the presence of nontoxic, simple, and readily available organocatalyst glycine in aqueous medium at 25 °C.     

A simple and efficient stereoselective synthesis of (−)-cleistenolide

A simple and straightforward stereoselective synthesis of α,β-unsaturated δ-lactone, (?)-cleistenolide has been accomplished in 10 steps in an overall yield of 19%, starting from inexpensive and commercially available starting materials, respectively. This linear synthesis utilizes Sharpless asymmetric dihydroxylation, sulfur ylide mediated epoxide opening followed by ring-closing metathesis (RCM) for the formation of six-membered lactone ring as the key step sequence.     

Environment friendly protocol for the synthesis of nitriles from aldehydes

<正>A rapid and eco-friendly one-pot protocol for the synthesis of nitriles has been developed by treating various araldehydes bearing electron withdrawing as well as electron donating groups with hydroxylamine hydrochloride in the presence of non-toxic,non-corrosive and reusable zinc oxide(ZnO) as the catalyst under solvent-free microwave irradiation.The present approach offers the advantages of a clean reaction,simple methodology,employing readily available catalyst,short reaction duration(1 min),high selectivity;and high yield(90-98%).     

Escape configuration lattice near the nematic-isotropic transition: tilt analogue of blue phases

We predict the possible existence of a new phase of liquid crystals near the nematic-isotropic transition. This phase is an achiral, tilt analogue of the blue phase and is composed of a lattice of double-tilt, escape-configuration cylinders. We discuss the structure and the stability of this phase and provide an estimate of the lattice parameter.