Sofia I. Torgova

Banana-shaped 1,2,4-oxadiazoles

Abstract3,5-Disubstituted 1,2,4-oxadiazoles are a new type of liquid crystalline (LC) compounds with asymmetrical five-membered heterocycle as a central unit. They have a bent shape and are very convenient model-compounds for studying the dependence of the LC properties on the molecular design. We have also synthesized and investigated ‘banana-shaped’ 1,2,4-oxadiazoles using the ester groups as the linkage units. The new compounds exhibit spontaneous polarization in the smectic phase, even if there is no chiral group in the molecules. Preliminary experimental data suggest the presence of spontaneous polarization in the nematic phase as well. In order to study the structural properties of the LC phases, X-ray diffraction (XRD) measurements on powder samples have been carried out. Based on the XRD data, a model of the structural arrangement of the bent molecules in the smectic phase is provided, which accounts for the macroscopic spontaneous polarization as well as the ferroelectric switching behavior.

The oxadiazole as a structural fragment in liquid crystals

This paper examines in some detail the effect of introducing the oxadiazole fragment into the molecular core of liquid crystals on their physico-chemical properties. The results of this study are compared with those obtained for other molecular fragments, and are rationalized in terms of existing theories.

Influence of chemical structure on the mesomorphic behaviour of 3,5-disubstituted 1,2,4-oxadiazoles

The correlation between chemical structure and mesomorphic properties is one of the most important problems in liquid crystals science. 3,5-Disubstituted 1,2,4-oxadiazoles are very convenient model-compounds for studying the dependence of the LC properties on the molecular design. The transition temperatures and dielectric properties of 1,2,4-oxadiazoles depend significantly both on the position of the substituents with respect to the heterocycle and on their donor or acceptor features.

Banana-Shaped 1,2,4-Oxadiazole Analogues of 1,3,4- Oxadiazoles

Abstract In the last years bent banana-shaped mesomorphic molecules found an increasingly great interest among theoreticians and experimentalists due to the nature of this type of liquid crystals to exhibit a spontaneous polarization in the absence of any chiral group in the molecules. Similar property was found in mesomorphic 1,3,4-oxadiazoles containing as a central unit the five-membered heterocycle with the symmetric positions of heteroatoms and as the link units - the ester groups [1]. In the present work 1,2,4-oxadiazoles - asymmetric isomers of the liquid crystals mentioned above, keeping the same banana-shape, were synthesized and their properties were investigated.

Spontaneous chiral domains in the nematic phase of achiral trans-4-alkylcyclohexanecarboxylic acids

We have discovered a spontaneous chiral ordering in thermotropic nematic liquid crystals forming hydrogen bonds, namely in trans -4-alkylcyclohexanecarboxylic acids, which do not contain any chiral group in the molecular structure, and have suggested a possible explanation of this phenomenon. The investigation was performed by polarizing light microscopy, using cells with glass substrates treated to give a uniform planar alignment in the nematic phase. On cooling from the isotropic phase, two types of nematic domains with different alignment were found to grow: some possessing a unidirectional planar orientation as expected, and others with a spontaneous twisted molecular arrangement. The appearance of the twisted domains and the sense of their twist were random, i.e. they always emerged in various regions of the cell with different twist sense: thus, they cannot be regarded as induced by stable defects in the surface aligning film. With the coalescence of the nematic droplets, the number of the domains w...

THE TRANS-1,4-CYCLOHEXYLENE GROUP AS A STRUCTURAL FRAGMENT IN LIQUID CRYSTALS

In this review the effect on physico-chemical and electro-optical properties of the introduction of the trans-1,4-cyclohexylene fragment into the molecular core of liquid crystals is discussed, rationalized in terms of existent theories, and compared with the effect of other well-known molecular fragments.

New Chiral Dopant Possessing High Twisting Power

We have synthesized and investigated a new chiral dopant. It belongs to the derivatives of p-terphenyldicarboxylic acid and has twisting power in the smectic C* mixture much higher than any chiral compound of this series known before. The ferroelectric liquid crystal mixtures on the base of achiral SmC matrix and the new chiral dopant have been elaborated and there optical and electro-optical properties were investigated.

Dependence of Mesomorphic Properties of 3,5-Disubstituted 1,2,4-Oxadiazoles on Geometric and Electronic Factors

Abstract New three and four rings containing liquid crystalline 3,5-disubstituted 1,2,4-oxadiazoles with 4-substituted phenyl, cyclohexyl, biphenyl and phenylcyclohexyl substituents have been synthesized. It was shown that structure and nature of the cyclic fragments, position of the strong polar (NO2) or weak polar (Alk, AlkO, Hal) groups in respect to the heterocyclic unit significantly effect on the temperature and dielectric properties of the new mesogens.

Phase modulation and ellipticity of the light transmitted through a smectic C* layer with short helix pitch

A chiral ferroelectric smectic C* liquid crystal (FLC) with the helix pitch p 0 = 330 nm was developed to avoid any scattering of visible light when the helix is not unwound over a certain limit. Planar cells with different FLC layer thickness (16 and 44 μm) have been assembled with helix axis parallel to the glass plates and aligned along the rubbing direction. The ellipticity of the light passing through the cells vs. the electric field was investigated, and a method for evaluating the electrically controlled birefringence via ellipticity measurements has been established. We have found that the FLC cell is an optical retardation layer driven by the electric field, the effective birefringence being proportional to the square electric field. The physical origin of the electrically controlled phase shift of the light passing through the FLC layer has been analysed.

Hydrogen Bonding and its Role in the Liquid Crystal Formation and Properties

Abstract In this paper the different types of hydrogen bonds are described. The literature, concerning different kinds of mesogenic structures which were built up via intermolecular hydrogen bonds, is discussed. The role of hydrogen bonding in the liquid crystal formation is pointed out and its influence on the thermal and chiral properties of liquid crystals is shown as well.