Quantified chirality, molecular similarity, and helical twisting power in lyotropic chiral nematic guest/host systems

Lyotropic liquid crystals can exhibit phase chirality. The mechanism behind the transfer of chirality between a chiral dopant and a liquid crystalline host phase is still under discussion. Our own recent results and proposals are the following. Lyotropic phase chirality can exist even at very low concentrations of chiral dopants, with less than 1 chiral dopant per 50 micelles. There is evidence for an intramicellar double twist which could be due to the induction of chiral conformations in the achiral surfactant chains. The chirality of arbitrary molecules can be quantified by means of the 'Hausdorff distance'. Increasing chirality of a dopant does not necessarily imply increasing helical twisting power, and molecular similarity between chiral guest and achiral host is essential for effective chirality transfer.     

Solute-solvent interactions and chiral induction in liquid crystals

The induction of a cholesteric phase by doping an achiral nematic liquid crystal with an enantiopure solute is a phenomenon that, as in all general supramolecular phenomena of chiral amplification, depends in a subtle way on intermolecular interactions. The micrometric helical deformation of the phase director in the cholesteric phase is generated by the interplay of anisotropy and chirality of probe-medium interactions. In the case of a flexible chiral dopant, the solvent can influence the twisting power in two ways, difficult to disentangle: it is responsible for the solute orientational order, an essential ingredient for the emergence of phase chirality; but also it can affect the dopant conformational distribution and then the chirality of the structures present in the solution. In this work we have investigated methyl phenyl sulfoxide, a flexible, chiral molecule that, when dissolved in different nematics, can produce cholesteric phases of opposite handedness. This peculiar, intriguing sensitivity to the environment makes MPS a suitable probe for a thorough investigation of the effects of solute-solvent interactions on chiral induction in liquid crystals. NMR experiments in various nematic solvents have been performed in addition to twisting power measurements. From the analysis of partially averaged 1H-1H and 13C-1H dipolar couplings, the effects of solvent on solute conformation and orientational order are disentangled, and this information is combined with the modeling of the chirality of intermolecular interactions, within a molecular field theory. The integration of different techniques allows an unprecedented insight into the role of solvent in mediating the chirality transfer from molecule to phase.     

Sugar Amphiphiles as Revealing Dopants for Induced Chiral Nematic Lyotropic Liquid Crystals

The existence of phase chirality in lyotropic liquid crystals still raises questions. The mechanisms behind the transfer of chirality throughout the long-range orientational order are not yet obvious. Guest/host systems with chiral dopants in achiral host phases offer the capability of systematic investigations. We demonstrate that the large amount of accessible sugar amphiphiles exhibits remarkable structure/property relations. Their helical twisting power HTP increases strongly with the number of sugar units of a dopant molecule. The spatial range of the chirality information reaching from a chirally doped micelle to adjacent aggregates is essential for the development of phase chirality. The induced twist of the lyotropic nematic host phase is highly sensitive to small changes of the sugar type (e.g., galacto- to glucopyranose). Depending on the nature of the host phase, either the alpha- or the beta-linkage of the sugar to the hydrophobic moiety of the sugar dopant results in larger HTP values. We propose that our amphiphilic sugar derivatives act like antennae to transfer chirality information. Their effectiveness as chiral dopants is due to a hydrophobic anchoring within the micelles and an extension of their chiral moiety far into the intermicellar region. The chirality transfer works especially well if the hydrophilic and chiral sugar moieties are oriented toward a neighboring micelle in the direction of the helix axis. Copyright 2001 Academic Press.     

Correlation between molecular structure and helicity of induced chiral nematics in terms of short-range and electrostatic-induction interactions. The case of chiral biphenyls

The helical structure of the chiral nematic phases induced by chiral dopants in nematic solvents provides a macroscopic image of the molecular chirality of the dopant promoted by the orientational order. Chiral biphenyls are challenging systems because their twisting ability shows a strong dependence on the molecular structure, which does not conform to empirical correlation rules. This points out the need for adequate interpretative tools, able to establish a link between molecular properties and macroscopic response. In this paper the twisting ability of chiral biphenyls is reviewed, by reporting examples taken from the literature together with some new experimental results. The microscopic origin of the observed behavior is explained in terms of chirality and anisotropy of short-range and electrostatic-induction interactions. These are described, respectively, by a shape model and a reaction field method, having the common characteristics of a realistic representation of the structure and properties of the chiral dopants in terms of molecular surface, atom charges, and distributed polarizabilities.     

The influence of pretransitional phenomena on blue phase range

Abstract

The particular phase in which a liquid crystal system will exist is that which has the lowest free energy at a certain temperature. The free energy may depend on variables such as temperature, pressure, chirality, etc. One way in which the stability of a particular thermodynamic phase, relative to its neighbours, would be manifest is in its temperature range. The effect of chirality, in particular, on the temperature range or stability of blue phases has been well-studied both experimentally and theoretically. To date these studies assume that chirality is the only parameter which will influence the existence of blue phases. However, blue phases with relatively low chirality and broad range, which should in principle only show very narrow blue phases, have been reported. This suggests that factors other than chirality are involved in blue phase stability. In this paper we investigate the phase stability of various blue phase mixtures containing equal amounts of a chiral dopant, via their blue phase temperature range. Correlation between blue phase stability and the elastic constants k ₂ and k ₂₂, molecular length of the nematic host, and the order parameter at the blue phase to cholesteric transition is discussed. We have confirmed that for our mixtures the total blue phase temperature range may be related equally to the elastic constant k ₂₂ and the chirality. We also present the first data displaying an odd-even effect in blue phases. Finally, we have found an apparent correlation between the stability of the blue phases and the magnitude of the orientational order parameter of the cholesteric phase at the cholesteric to blue phase transition.     

Enhancement of twisting power in the chiral nematic phase by introducing achiral banana-shaped molecules

Achiral banana-shaped molecules with dodecyloxy tail groups, P-12-O-PIMB, N-12-O-PIMB, and S-12-O-PIMB, have exhibited unusual smectic phases which possess chiral and helical structures. In this work, we mixed these banana-shaped molecules with the chiral molecule forming a chiral nematic liquid crystal and found an exclusive effect of the achiral dopant that the twisting power of the chiral nematic phase in the mixtures is significantly increased with the increase of the content of achiral banana-shaped molecules. This characteristic effect in the chiral nematic field seems to offer the rational evidence for the twist conformation of such banana-shaped molecules, since the chirality should be included intrinsically within each chain. The asymmetric twist conformation in the ester linkage group connecting the central core with the side wings is likely to be the origin of enhanced twisting power.     

The phase behaviour and optical properties of a nematic/chiral dopant liquid crystalline mixture system

The phase behaviour and aggregation states of a binary mixture of a nematic liquid crystal and a chiral dopant have been investigated. The nematic liquid crystal E7 was miscible with the chiral dopant S811 over their entire concentration range. Binary E7/S811 mixtures formed the N* phase for S811 contents under 20%, and the SmA* phase for S811 contents between 40% and 90%. BP and TGBA* frustrated phases were found during cooling, for S811 contents between 25% and 35%. The helical pitches of the binary mixtures decreased with increasing chiral dopant content. From XRD profiles, the orientational ordering of the binary composites was found to increase with increasing chiral dopant content.     

Enlargement of blue-phase stability for rod-like low-molecular-weight chiral nematic liquid crystal mixtures

In this study, we investigated the enlargement of liquid crystal (LC) blue-phase (BP) temperature range using the rod-like low-molecular-weight cyano phenyl-type chiral nematic LC with various core group and chiral dopant concentrations. Also, the electro-optic response time was investigated for them. We found that the BP temperature range was strongly dependent upon the core structure and the chiral dopant concentration for the chiral nematic LC mixtures having the same terminal group. Also, we found a stable BP with a wide temperature range (more than 6 K), including a BP-isotropic coexistence state over 13.5 K upon heating and cooling processes and very fast response time (less than 1 ms), by using the cyano phenyl-type chiral nematic LC mixture with a high molecular aspect ratio and a high chiral dopant concentration.     

Thermal switching characteristics based on smectic-A↔chiral nematic phase transitions of (liquid crystals/chiral dopant) composites,with and without side chain-type liquid crystalline polymer

The light switching characteristics induced by a thermal smectic A (SmA) ? chiral nematic (N*) phase transition were studied for homeotropically aligned [smectic A liquid crystal (SmA-LC)/nematic liquid crystal (N-LC)/chiral dopant] and [side chain type smectic A liquid crystalline polymer (SmA-LCP)/N-LC/chiral dopant] composites. A drastic change from a transparent SmA phase to a light-scattering N* phase occurred in both composites upon heating. In the case of the heat-induced N* phase for the (SmA-LC/N-LC/chiral dopant) composite, the N* phase exhibited weak light scattering due to formation of a scroll texture. On the other hand, in the case of the heat-induced N* phase for the (SmA-LCP/N-LC/chiral dopant) composite, the N* phase showed strong light scattering due to formation of a focalconic texture. The existence of a SmA-LCP was responsible for a higher contrast ratio between the transparent SmA phase and the light scattering N* phase for the (SmA-LCP/ N-LC/chiral dopant) composite than for the (SA-LCN/N-LC/chiral dopant) composite.     

Amplification of chirality in liquid crystals

The amplification of molecular chirality by liquid crystalline systems is widely applied in investigations towards enantioselective solvent-solute interactions, chiral supramolecular assemblies, smart materials, and the development of liquid crystal displays. Here we present an overview of recent achievements in the development of new chiral dopant systems for the generation of cholesteric liquid crystalline phases. Based on a distinction between shape-persistent and bistable dopants, several dopant classes will be discussed.     

Homochiral helices of oligonaphthalenes inducing opposite-handed cholesteric phases

The helical structure of the chiral nematic phases (cholesterics) obtained by doping nematic solvents with chiral non-racemic compounds is a macroscopic proof of the solute chirality. Oligonaphthalene (tetra-, hexa-, octa-) derivatives linked at the 1,4-positions have been used as chiral dopants: When the chirality axes are configurationally homogeneous (that is, all-S), the molecular structures correspond to right-handed helices. Yet, we have found series of derivatives with the surprising property that the handedness of the induced cholesteric phase alternates from positive to negative and to positive again, on passing from tetra- to hexa- and to octanaphthalene. A comparison with oligonapthalene derivatives, which do not exhibit this twisting ability, points to the importance of the substitution pattern. Both the possibility of inducing oppositely-handed cholesteric phases by homochiral helices of different length, and the role played of substituents, are confirmed by calculations performed with the surface chirality model.     

Lattice structure in liquid-crystal blue phase with various chiral concentrations

Lattice structures, including reflection lattice planes and lattice constant, of liquid-crystal blue phase I (BPI) are studied via the measurements on reflection spectrum and Kossel diagram as concentration of a chiral dopant is changed. Peaks of the reflection wavelength in BPI are mainly dominated by the lattice plane and the lattice constant, which are affected by the chiral concentration. In the chiral nematic state, as decreasing the chiral concentration the reflection peak will shift to a longer wavelength because the helical pitch linearly depends on the chiral concentration and becomes longer. However, this dependence of the chiral concentration and reflection wavelength is broken in the BPI. The reflection peak of BPI moves to a short wavelength when the chiral concentration is less due to the contraction of the lattice constant as well as helical pitch. Moreover, when the concentration of the chiral dopant increases over a certain value, a discontinuous shift in reflection peak occurs due to the production of the different lattice planes. It means that the relationship between the chiral concentration and the helical pitch in BPI is not the same as it in the chiral nematic phase and should be reconsidered.     

Light‐Directed Dynamic Chirality Inversion in Functional Self‐Organized Helical Superstructures

Helical superstructures are widely observed in nature, in synthetic polymers, and in supramolecular assemblies. Controlling the chirality (the handedness) of dynamic helical superstructures of molecular and macromolecular systems by external stimuli is a challenging task, but is of great fundamental significance with appealing morphology‐dependent applications. Light‐driven chirality inversion in self‐organized helical superstructures (i.e. cholesteric, chiral nematic liquid crystals) is currently in the limelight because inversion of the handedness alters the chirality of the circularly polarized light that they selectively reflect, which has wide potential for application. Here we discuss the recent developments toward inversion of the handedness of cholesteric liquid crystals enabled by photoisomerizable chiral molecular switches or motors. Different classes of chiral photoresponsive dopants (guests) capable of conferring light‐driven reversible chirality inversion of helical superstructures fabricated from different nematic hosts are discussed. Rational molecular designs of chiral molecular switches toward endowing handedness inversion to the induced helical superstructures of cholesteric liquid crystals are highlighted. This Review is concluded by throwing light on the challenges and opportunities in this emerging frontier, and it is expected to provide useful guidelines toward the development of self‐organized soft materials with stimuli‐directed chirality inversion capability and multifunctional host–guest systems.     

The measurement of weak chirality using nematic liquid crystals

ABSTRACT

Two recently reported methods for measuring weak chirality using nematic liquid crystals are described. These are between one and two orders of magnitude more sensitive than the standard Grandjean-Cano wedge. Pitch lengths of 50 mm have been measured with prospects for extending this beyond 100 mm; this is equivalent to measuring the presence of 1 ppm of a strong chiral dopant. Results for a number of standard chiral dopants and nematic liquid crystal hosts are reported.     

Mesomorphic Characteristics of Induced Chiral Nematic Phase of [Smectic LCP,PS(4BC/DM)/Nematic LC,E7/Chiral Dopant,CB-15]-Ternary Composite System

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Photo-optical properties of polymer composites based on stretched porous polyethylene filled with photoactive cholesteric liquid crystal

A new type of polymer-liquid crystal composite with photovariable dichroism and birefringence is described. Porous stretched polyethylene films were used as polymer matrices. To induce a cholesteric phase in a commercial nematic host, a chiral photochromic dopant based on sorbide and cinnamic acid capable of E-Z isomerization under UV irradiation was used. A merocianine-type substance was selected as a dichroic dye. Introduction of a dye-doped cholesteric mixture with a helical pitch higher than ∼300 nm to polymer film led to an almost complete transition from a cholesteric to an oriented nematic phase, as well as to an increase in birefringence and the appearance of dichroism. Decrease of the helical pitch by increasing in the chiral dopant concentration in the liquid crystal-polymer composite results in a reduction of the dichroism values. UV irradiation of polymer composite leading to an isomerization of the chiral dopant and helix untwisting induces a noticeable gradual growth of dichroism and birefringence. These new composites can be considered as promising materials for optical applications.     

Photo‐optical properties of polymer composites based on stretched porous polyethylene filled with photoactive cholesteric liquid crystal

A new type of polymer–liquid crystal composite with photovariable dichroism and birefringence is described. Porous stretched polyethylene films were used as polymer matrices. To induce a cholesteric phase in a commercial nematic host, a chiral photochromic dopant based on sorbide and cinnamic acid capable of E–Z isomerization under UV irradiation was used. A merocianine‐type substance was selected as a dichroic dye. Introduction of a dye‐doped cholesteric mixture with a helical pitch higher than ～300 nm to polymer film led to an almost complete transition from a cholesteric to an oriented nematic phase, as well as to an increase in birefringence and the appearance of dichroism. Decrease of the helical pitch by increasing in the chiral dopant concentration in the liquid crystal–polymer composite results in a reduction of the dichroism values. UV irradiation of polymer composite leading to an isomerization of the chiral dopant and helix untwisting induces a noticeable gradual growth of dichroism and birefringence. These new composites can be considered as promising materials for optical applications.     

The Chiral Twist-Bend Nematic Phase (N*TB)

The twist-bend nematic, N_TB, phase has been observed for chiral materials in which chirality is introduced through a branched 2-methylbutyl terminal tail. The chiral twist-bend nematic phase, N*_TB, is completely miscible with the N_TB phase of the standard achiral material, CB6OCB. The N*_TB phase exhibits optical textures with lower birefringence than those observed for the achiral N_TB phase, suggesting an additional mechanism of averaging molecular orientations. The N*−N*_TB transition temperatures for the chiral materials are higher than the N_TB−N transition temperatures seen for the corresponding racemic materials. This suggests the double degeneracy of helical twist sense in the phase is removed by the intrinsic molecular chirality. A square lattice pattern is observed in the N* phase over a temperature range of several degrees above the N*_TB–N phase transition, which may be attributed to a non-monotonic dependence of the bend elastic constant.     

Light-induced Freedericksz transition in a nematic liquid crystal with chiral dopant

The influence of phototransformed molecules with chiralproperties changing on the absorption of light field on the light-induced Freedericksz transition threshold in a homeotropically oriented nematic cell is considered. It is shown that the appearance of the light-induced chiral molecules can decrease or increase the Freedericksz threshold value depending on the chirality sign of the phototransformed molecules and of the initial chiral dopant. Expressions for the threshold are obtained for circular and linear polarization of the incident light. The dependence of the threshold on the periodicity of the spatially modulated light intensity is estimated for large periods of modulation. The dependence of dopant threshold chirality on the director anchoring energy has been found.     

Cholesteric mesophase of the hydrogen-bonded blends of liquid crystalline ionogenic copolymers with a low molecular weight chiral dopant

A family of a new hydrogen-bonded complexes based on comb-shaped LC copolymers containing the monomer units of cyanobiphenyl derivative and n-alkyloxy-4-oxybenzoic acid with a chiral dopant on the base of 4-pyridinecarboxylic acid and L -menthol, was prepared. At concentrations of chiral groups 1–25 mol %, the induction of cholesteric phase was observed. Temperature dependences of selective light reflection wavelengths were studied, and helix twisting power was calculated. Depending on the type of copolymer nematic matrix, this value is changed in the range from 12.1 to 19.6 µm⁻¹. It was shown that an increase of a distance between the chiral dopant and the main polymer chain results in a lower values of helix twisting power. With respect to optical properties, the chiral nematic phase in the hydrogen-bonded complexes is comparable to classical cholesteric copolymers, in which the chiral group is covalently bound to polymer chain. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3215–3225, 1999