B. K. Sadashiva

Liquid Crystals of Disc-like Molecules

Thermotropic mesomorphism has been observed in pure compounds consisting of simple disc-like molecules, viz., benzene-hexa-n-alkanoates. Thermodynamic, optical and x-ray studies indicate that the mesophase is a highly ordered lamellar type of liquid crystal. Based on the x-ray data, a structure is proposed in which the discs are stacked one on top of the other in columns that constitute a hexagonal arrangement, but the spacing between the discs in each column is irregular. Thus the structure has translational periodicity in two dimensions and liquid-like disorder in the third.

Stable amorphous blue phase of bent-core nematic liquid crystals doped with a chiral material

We report an induction of the blue phase III (BPIII) at a relatively low and wide (over 20 °C) temperature range in nematogenic achiral bent-core liquid crystals doped with a high twisting power chiral material. The pitch decreases with increasing chiral dopant ratio, and easily reaches the ultraviolet wavelength, so that completely dark texture is obtained under crossed polarizers. Electrooptical switching was achieved in a time range of a few to a few tens of milliseconds. We propose for the stabilization of BPIII that broad-temperature range smectic nano-clusters inhibit the long-range order of the double twisted helical structures, and also inhibit possible separation of chiral dopants from the mixture.

Influence of fluorine substituent on the mesomorphic properties of five-ring ester banana-shaped molecules

The synthesis and characterization of more than a hundred achiral compounds composed of banana-shaped molecules and belonging to eleven different homologous series are reported. All these symmetrical five-ring compounds are esters and are derived from resorcinol. The compounds have been systematically substituted by fluorine on the phenyl rings of the arms of these banana-shaped molecules. The mesophases exhibited by these compounds have been characterized using a combination of polarized light optical microscopy, differential scanning calorimetry and X-ray diffraction studies. Electro-optical investigations have been carried out on several compounds and both ferroelectric and antiferroelectric properties are observed. The influence of the position of the fluorine substituent on the nature of the mesophase obtained has been examined.The synthesis and characterization of more than a hundred achiral compounds composed of banana-shaped molecules and belonging to eleven different homologous series are reported. All these symmetrical five-ring compounds are esters and are derived from resorcinol. The compounds have been systematically substituted by fluorine on the phenyl rings of the arms of these banana-shaped molecules. The mesophases exhibited by these compounds have been characterized using a combination of polarized light optical microscopy, differential scanning calorimetry and X-ray diffraction studies. Electro-optical investigations have been carried out on several compounds and both ferroelectric and antiferroelectric properties are observed. The influence of the position of the fluorine substituent on the nature of the mesophase obtained has been examined. Table 7. Transition temperatures (°C) and associated enthalpy changes (kJ mol−1) (in italics) for series VII. For key see table 1. Table 8. Transition temperatures (°C) and associated enthalpy changes (kJ mol−1) (in italics) for series VIII. For key see table 1. Table 9. Transition temperatures (°C) and associated enthalpy changes (kJ mol−1) (in italics) for series IX. For key see table 1.

A Thermotropic Biaxial Nematic Liquid Crystal

Abstract Optical studies have been carried out on a nematogenic copper complex, which incorporates the features of both rod-like and disk-like molecules. Conoscopic figures are presented demonstrating (i) the occurrence of a biaxial nematic phase in the pure complex, (ii) the uniaxial-biaxial (Na-Nb) transition in binary mixtures, and (iii) the temperature variation of the biaxiality near this transition. The I-Na-Nb phase diagram has been studied for the binary system.

Banana-shaped mesogens: effect of lateral substituents on seven-ring esters containing a biphenyl moiety

The synthesis and characterization of five homologous series of symmetrical compounds composed of banana-shaped molecules containing a biphenyl moiety are reported. All these compounds are non-Schiffs bases and are esters. The effects of lateral substituents such as fluoro, methyl and ethyl in the side arms of these molecules are examined. These substituents have a strong influence in reducing the clearing temperatures. Banana phases such as B1, B2 and B6 were observed in the above series of compounds. The mesophases were characterized by a combination of polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optic studies.

Synthesis and mesomorphic properties of banana-shaped compounds derived from 2,7-dihydroxynaphthalene

Thirty eight compounds composed of achiral banana-shaped molecules belonging to three new homologous series have been synthesized and their mesomorphic properties investigated. The mesophases have been characterized using a combination of polarized light microscopy, differential scanning calorimetry, miscibility methods and X-ray diffraction measurements. All the compounds reported exhibit mesomorphic properties. Most of the homologues in the three series show B1 and B2 phases and the transition temperatures of these are lower in the fluoro substituted compounds. Some of the lower homologues of the fluoro substituted derivatives also show the nematic phase.

Direct transition from a nematic phase to a polar biaxial smectic A phase in a homologous series of unsymmetrically substituted bent-core compounds

The synthesis and characterization of twenty seven new compounds belonging to three homologous series of compounds derived from 3-hydroxybenzoic acid are reported. One of the terminal positions of the constituent molecules of these compounds is substituted with a strongly polar cyano group while the other end contains an n-alkyl chain attached to a biphenyl moiety. Twenty two of these compounds show the polar partial bilayer biaxial smectic A phase which is antiferroelectric in nature. In addition, a direct transition from the nematic phase to the polar partial bilayer biaxial smectic A phase which has not been seen before is also reported. The mesophases have been characterized using polarized light optical microscopy, differential scanning calorimetry, X-ray diffraction studies and electro-optical experiments.

Evidence of columnar structure in compounds composed of banana-shaped molecules

Abstract The synthesis and mesomorphic properties of several achiral compounds composed of banana-shaped molecules are described. These compounds belong to three different homologous series. They have been examined by polarized light microscopy, differential scanning calorimetry, x-ray diffraction studies and electro-optic investigations. In the homologous series containing a lateral methyl group, three different mesophases viz. B6, B1 and B2 have been observed. In the compounds containing a chloro group, the B1 phase predominates, while the fluoro substituted compounds show the B2 phase.

Biaxial smectic A phase in homologous series of compounds composed of highly polar unsymmetrically substituted bent-core molecules

Several compounds belonging to three new series of five-ring banana-shaped esters, which are unsymmetrically substituted with respect to the central phenyl ring, have been synthesised. One of the arms of these bent-core molecules has been terminally substituted with a highly polar cyano group while the other end contains an alkoxy chain. The mesophases exhibited by these compounds have been characterised using a combination of optical polarising microscopy, differential scanning calorimetry and X-ray diffraction studies. These studies indicate that most of the compounds show two smectic A mesophases which have a partial bilayer ordering. Conoscopic experiments clearly reveal that the lower temperature smectic A phase is biaxial in nature. A structural model has been proposed for the lower temperature biaxial smectic A phase. It is argued that quartets of molecules, which are conducive to the formation of the biaxial phase, can form in the layers. The in-layer birefringence of the biaxial smectic A phase has also been measured as a function of temperature for one of the compounds.

A biaxial nematic liquid crystal

We report a biaxial nematic liquid crystal in a simple thermotropic system.