New dimeric liquid crystals with chiral flexible spacers

In order to obtain chiral compounds of low molecular weight expected to exhibit mesogenic and ferroelectric properties, the asymmetric carbon was introduced into the central part of the molecule. The series consists of nine chiral dimers in which the chiral flexible spacer derives from S-2-chlorosuccinic acid. The mesomorphic properties of the compounds were studied and all of them were found to be mesogenic. From n = 6 onwards the mesophases were chiral smectic C. Some analogous compounds derived from S-2-chloroglutaric acid were also prepared; these were not found to be mesogenic. The thermal instability of the 2-chlorosuccinic acid derivatives prevented the study of their ferroelectric properties. We attempted to perform this study for several mixtures involving 3-methyladipic acid derivatives and both 2-chlorosuccinic and 2-chloroglutaric acids derivatives. All the mixtures studied show chiral smectic C phases. The mixtures containing 2-chlorosuccinoyl derivatives show thermal instability; however those containing 2-chloroglutaroyl derivatives are stable, but do not show significant values of spontaneous polarization.     

New chiral dopants for ferroelectric liquid crystals

New chiral dopants for application in ferroelectric liquid crystal mixtures are presented. They derive from chirally modified biphenylcyclohexylcarbonitriles, lactates and optically active 2-cyano-2-methylalkanoates. Synthetic routes to the new compounds are shown and the physical properties of the new compounds in a non-chiral smectic C host are reported.     

Novel ferroelectric non-symmetric dimeric liquid crystals

The synthesis, characterization, mesomorphism and ferroelectric behaviour of ten new chiral dimeric compounds are reported. These compounds are non-symmetric dimers consisting of two mesogenic units joined by two different flexible spacers derived from S(-)-lactic acid. The mesogenic units contain ester and thioester groups. The effects of the length of the flexible spacer and the structure of the mesogenic units on the mesomorphism of the compounds are investigated. These compounds are ferroelectric and the spontaneous polarizations of some were measured, giving values of c. 10nC cm^-2.     

New antiferroelectric liquid crystals with a trifluoromethyl group at the chiral centre

New antiferroelectric liquid crystals (AFLCs) with a trifluoromethyl group at the chiral centre were synthesized using optically active 3-(4-methoxyphenyl)-4,4,4-trifluorobutanoic acid. The new AFLCs showed the phase sequence on cooling: isotropic liquid (I)-smectic A (S_a) phase-antiferroelectric (AF) phase-crystal (Cr). In the antiferroelectric phase, a d.c.-threshold, a double hysteresis loop, a small dielectric constant value and two sharp switching-current peaks were observed. These are the first examples that are AFLCs which do not have a carbonyl moiety in the molecule.     

U-Shaped dimeric liquid crystals derived from phthalic acid

Details of the transitional properties of four homologous series of U-shaped liquid crystal materials, the benzene-1,2-di-(4-carboxyalkoxybenzylidene-4-n-alkylanilines), are presented. The spacers incorporate 3 to 6 methylene units, whilst terminal aliphatic chain lengths are varied from 1 to 12 units. Members of the two homologous series with an odd number of methylene units in the spacer form nematic and smectic phases as a function of terminal chain length. Materials with an even number of methylene units in the spacer are purely smectogenic, and in both series the first two homologues form only smectic B phases. For all four series the higher homologues show the phase sequence SF/I-SC-SA-I. X-ray diffraction studies have shown that these smectic phases are composed of molecules arranged in bilayers.     

Phase transition studies in symmetric dimeric liquid crystals

As a part of the systematic studies on symmetric liquid crystal dimer homologous series, α,ω-bis-(4-n-alkylaniline benzylidene-4′-oxy) alkanes, (referred to as m.OnO.m with m = 3, 4, and 5; and n = 8, 9, and 10), we present in this article the nature of phase transitions across isotropic–nematic and nematic–smectic-A (N–SmA) phases exhibited by the just mentioned compounds. The methods employed are differential scanning calorimetry and dilatometry. The compounds studied were 3.O8O.3, 4.O8O.4, and 5.O8O.5; and 3.O9.03, 5.O9O.5, 3.O10O.3, 4.O10O.4, and 5.O10O.5. Different from the case of their corresponding monomers, all these compounds exhibit a nematic phase only with the exception of 5.O8O.5 which exhibits a SA phase in addition to the nematic phase. The phase transitions viz., isotropic–nematic transitions studied in all these compounds were confirmed to be of first-order nature, whereas the N–SmA transition exhibited by the compound 5.O8O.5 only was found to be of second-order nature. We also report in this article the calculated density jumps, thermal expansion coefficient maxima, and pressure dependence of transition temperatures which are analyzed in the light of the available literature data.     

Diffusion measurements in chiral liquid crystals

Abstract

We present results of diffusion investigations in twisted nematics (nematic-cholesteric mixtures). The dependence of the diffusion coefficient along the helix on the concentration of the cholesteric compound, the sample temperature and the diffusion time are measured by a new nuclear magnetic resonance method. The diffusion coefficient was found to decrease with diffusion time in the studied range of about 0·5–50 ms. The results are consistent with data from ¹³C-N.M.R. experiments, and with field gradient measurements by other authors.     

Synthesis and mesomorphic properties of perfluorinated rod-like liquid crystals with sulphur-containing spacers

In order to determine the influence of 'sulphur-containing' spacers on the formation of mesophases in low molecular mass compounds, we have examined the mesomorphic behaviour of molecules which consists of a 4-biphenyl unit linked to an unbranched fluorinated chain via a short spacer including at least a sulphur atom. The synthesis of these compounds has been carried out from 2-F-butylethyl iodide or from the 2-F-alkylethyl mercaptans in the case of the F-hexyl and F-octyl tails. The mesomorphic properties have been characterized by polarized light microscopy and by differential thermal analysis showing the peculiar contribution of each of the spacers. The influence of the fluorinated chain and the shape of the connector on the stability of the mesophases has been investigated. The compounds with a thioether or a hemithioacetal spacer showed no mesomorphic properties, whereas the structures with a thioester spacer showed a very interesting enantiotropic behaviour of the smectic A type over a wide temperature range. Furthermore from the series exhibiting liquid crystalline behaviour, increasing the number of fluoromethylene units simultaneously increases both the melting and the clearing temperature. These mesomorphic properties within the fluorinated series are compared with those of their monocatenar hydrocarbon homologues.     

Synthesis and mesomorphic properties of perfluorinated rod-like liquid crystals with sulphur-containing spacers

In order to determine the influence of 'sulphur-containing' spacers on the formation of mesophases in low molecular mass compounds, we have examined the mesomorphic behaviour of molecules which consists of a 4-biphenyl unit linked to an unbranched fluorinated chain via a short spacer including at least a sulphur atom. The synthesis of these compounds has been carried out from 2-F-butylethyl iodide or from the 2-F-alkylethyl mercaptans in the case of the F-hexyl and F-octyl tails. The mesomorphic properties have been characterized by polarized light microscopy and by differential thermal analysis showing the peculiar contribution of each of the spacers. The influence of the fluorinated chain and the shape of the connector on the stability of the mesophases has been investigated. The compounds with a thioether or a hemithioacetal spacer showed no mesomorphic properties, whereas the structures with a thioester spacer showed a very interesting enantiotropic behaviour of the smectic A type over a wide temperature range. Furthermore from the series exhibiting liquid crystalline behaviour, increasing the number of fluoromethylene units simultaneously increases both the melting and the clearing temperature. These mesomorphic properties within the fluorinated series are compared with those of their monocatenar hydrocarbon homologues.     

Nematic conformation of oxyethylene spacers involved in main-chain liquid crystals

²HNMR measurements were performed on main-chain dimer and polymer liquid crystals (LC) having oxyethylene (OE) spacers -(OCH₂CH₂)_xO-(x=2, 3). The orientational as well as conformational characteristics of these molecules have been investigated in bulk and in a nematic solution. The OE spacer was found to take spatial arrangements characteristics of the nematic phase. The nematic conformation of the spacer remains nearly invariant over a wide range of temperature and concentration. In these analyses, the ratio of the deuterium quadrupolar splittings Δν/Δν^R (Δν: spacer and Δν^R: mesogenic unit) provided an important information regarding the spatial configuration of molecules in the LC state. The results obtained in this study are consistent with our previous conclusion drawn on a series of main-chain LC oligomers and polymers comprising n-alkane spacers -O(CH₂)_nO- (n=9, 10).     

Enantiodiscrimination in deuterium NMR spectra of flexible chiral molecules with average axial symmetry dissolved in chiral liquid crystals: the case of tridioxyethylenetriphenylene

Flexible chiral molecules undergoing fast interconversion (on the NMR time scale) between different conformational enantiomers may yield "average" axial species with enantiotopically related sites. Contrary to the situation observed for rigid axial molecules, signals from these enantiotopic sites in NMR spectra recorded in chiral liquid-crystalline solvents can be resolved. In the present work, we studied the deuterium NMR spectra of tridioxyethylenetriphenylene (compound 4) statistically deuterated to 10% in the flexible side chains and dissolved in chiral and achiral lyotropic liquid crystals based on poly(gamma-benzylglutamate). The fast chair-chair flipping of the side chains in 4 on average renders the molecule axially symmetric ( D 3 h ) with pairs of enantiotopic ethylene deuterons. These deuterons exhibit unusually large enantiodiscrimination. To explain this observation, we first describe how the average symmetry of flexible molecules can be derived from the symmetry of the "frozen" conformers and the nature of the averaging process. The procedure is then applied to 4 and used to analyze the NMR results. It is shown that the large enantiodiscrimination in the present case reflects a large difference in the orientational ordering of the conformational enantiomers participating in the interconversion processes as well as a large geometrical factor due to the special shape of the dioxyethylene side groups. (1)H and (13)C NMR spectra of 4 in the same lyotropic liquid crystalline solvent are analyzed to determine its ordering characteristics. Several related cases are also discussed.     

Cholesterol-based dimeric liquid crystals: synthesis and mesomorphic behaviour

Chiral non-symmetric dimeric liquid crystals consisting of a cholesteryl ester moiety as chiral entity and a biphenyl aromatic core, interconnected through n-butyl (C₄) or n-pentyl (C₅) parity alkylene spacers, have been synthesized and investigated for their liquid crystalline properties. All the dimers exhibit enantiotropic mesophases. The first member of the dimers having the C₄ central spacer exhibit only the chiral nematic (N*) mesophase, while the higher homologues also show smectic A (SmA) and twist grain boundary (TGB) mesophases. The dimers of the other series containing the C₅ central spacer also have stable SmA, TGB and N* mesophases, except for the first which does not show the TGB phase. Both series of compounds show a weak odd-even effect with terminal alkyl chain substitution, while the spacer length has a marked influence on the phase transition temperatures.     

Cholesterol-based dimeric liquid crystals: synthesis and mesomorphic behaviour

Chiral non-symmetric dimeric liquid crystals consisting of a cholesteryl ester moiety as chiral entity and a biphenyl aromatic core, interconnected through n-butyl (C₄) or n-pentyl (C₅) parity alkylene spacers, have been synthesized and investigated for their liquid crystalline properties. All the dimers exhibit enantiotropic mesophases. The first member of the dimers having the C₄ central spacer exhibit only the chiral nematic (N*) mesophase, while the higher homologues also show smectic A (SmA) and twist grain boundary (TGB) mesophases. The dimers of the other series containing the C₅ central spacer also have stable SmA, TGB and N* mesophases, except for the first which does not show the TGB phase. Both series of compounds show a weak odd-even effect with terminal alkyl chain substitution, while the spacer length has a marked influence on the phase transition temperatures.     

The mesomorphic and photophysical properties of perylene liquid crystals with different bay-rigid spacers

Three novel perylene derivatives were prepared by introducing two phenyl (biphenyl or naphthyl)-bridging cholesterol units on bay positions and six alkyl chains on imides positions. The influences of different bay-bridging spacers on mesomorphic properties and photophysical properties were investigated. They presented the ordered hexagonal columnar liquid crystalline behaviours, although the large aromatic spacers were bridged on the bay positions. The mesomorphic and photophysical properties could be tuned effectively by the structures of spacers on bay positions. The larger and more rigid aromatic spacer on bay positions resulted in higher phase transition temperatures and stronger fluorescence emission.     

Light-induced structural changes in chiral liquid crystals

Novel light-sensitive chiral dopants are studied as a light-sensitive component in chiral liquid crystals which may be used in tunable optical devices. Light-induced cis-trans- isomerization of chiral dopants results in changes of helical twisting power which translates into variations of helical pitch. Due to the light absorption in the liquid crystal cell the pitch variation is non-uniform across the cell, which leads, at first, to a deformation of cholesteric layers, and then to the formation of cholesteric bubbles. The sequence of structural changes has a distinct visual pattern and occurs at the surface close to the UV light source. Small deformations of cholesteric layers and bubbles are unstable and disappear after removing UV irradiation. The increasing size of the cholesteric bubbles results in better stability; large bubbles do not disappear after removing UV light. A theoretical model is suggested to describe the undulations of cholesteric layers.     

Synthesis and characterization of liquid–crystalline polyesters with chiral and achiral twin spacers

A series of novel thermotropic main-chain chiral liquid–crystalline random copolyesters consisting of spacers of two different types—chiral and achiral—was synthesized. Polyesters (BmTa) with tartaric acid as the chiral spacer (Ta), aliphatic diols (with ‘m’ = 2–10 methylene groups) as the achiral spacers, and 4,4′-dihydroxy biphenyl (B) as the mesogen were synthesized via condensation polymerization in solution after duly protecting the 2,3-dihydroxy groups of tartaric acid by acetylation. The copolymers were characterized by Fourier transform infrared spectroscopy, ¹H and ¹³C NMR spectra, gel permeation chromatography, and thermogravimetric analysis. Transition temperatures for phase changes recorded by DSC were corroborated with the textures observed by a hot-stage optical polarizing microscope. The wide-angle X-ray diffraction (WAXD) profiles indicated a SmE phase at room temperature. The lower angle region at 2θ = 0.5–2.45 covered by WAXD indicated a layer of thickness of 161 Å, less than the molecular length for B0Ta. The [α_D] values were recorded on a digital polarimeter. The birefringence was lost at higher mesophase temperatures in lower members with m < 5, a behavior found in certain chiral systems, and the higher members with m > 5 showed a lesser number of phase transitions. On cooling, the polyesters produced a texture with the formation of transition bars. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1743–1752, 2001     

Twist-grain-boundary phase diagrams in chiral liquid crystals

The experimental phase diagrams of several chiral systems are compared with a theoretical diagram based on the chiral Chen-Lubensky model, which predicts at least two kinds of twist-grain-boundary phase, TGBA and TGBC. Also shown for comparison are typical nonchiral phase diagrams that exhibit a nematic-smectic A-smectic C multicritical point. Several aspects of experiment and theory agree, but there appear to be common experimental features that differ from those predicted by current theory.     

Photoswitching of holographic polymer-dispersed liquid crystals doped with chiral dopant

Chiral dopants were added to the formulation of holographic polymer-dispersed liquid crystals and the effects studied in terms of grating formation dynamics, morphology, diffraction efficiency, contrast ratio and electro-optical properties of the films. A gradual increase of real-time diffraction efficiency, decrease of droplet size and increase of diffraction efficiency of the composite film were obtained with the addition and increasing content of chiral dopant, due to the increased viscosity of the liquid crystal (LC) doped with the chiral dopant leading to decreased droplet coalescence. The contrast ratio decreased with increasing content of chiral dopant due to the difficult orientation of LC molecules caused by the formation of a helical structure. Addition of a small amount of the chiral dopant increased the driving voltage slightly, whereas the decay time is decreased significantly as a result of the high twisting of the helical structure.     

Synthesis and characterization of new ferroelectric liquid crystals containing oligomethylene spacers

Two series of ferroelectric liquid crystals containing a phenyl biphenyl carboxylate mesogenic group and oligomethylene spacers (n 3, 4, 6, 8) were synthesized and characterized. The chiral moieties are the (S)-2-methylbutoxycarbonyl and (S)-1-methylheptoxycarbonyl groups. The mesomorphic behaviours of the compounds were characterized using differential scanning calorimetry and optical polarized microscopy. Spontaneous polarization (P) of the liquid crystals was measured by an automated polarization tester over a wide temperature range. The mesomorphic properties are discussed as a function of spacer length and position of the chiral centre. It is found that the phase transition temperature decreases with increasing number of oligomethylene units. The phase behaviours of the LC with (S)-1-methylheptoxycarbonyl are more complex than those with (S)-2-methylbutoxycarbonyl, due to the flexible tail in the former. Moreover, the spontaneous polarization depends mainly on the position of the chiral centre, with Ps 15 nC cm 2 and Ps 60 123 nC cm 2 for the series s with (S)-2-methylbutoxycarbonyl and (S)-1-methylheptoxycarbonyl groups, respectively.