Chiral periodic mesoporous organosilica in a smectic-A liquid crystal: source of the electrooptic response

Chiral periodic mesoporous organosilica (PMO) materials have been shown to deracemise a configurationally achiral, but conformationally racemic liquid crystal in which the PMO is embedded. In particular, application of an electric field E in the liquid crystal’s smectic-A phase results in a rotation of the liquid-crystal director by an angle proportional to E, which is detected optically – this is the so-called ‘electroclinic’ effect. Here we present results from electroclinic measurements as a function of frequency and temperature, which allow us to distinguish the component of optical signal that arises from liquid-crystal chirality induced within the PMO’s chiral pores from that induced just outside the silica colloids. Our central result is that the overwhelming source of our electrooptic signal emanates from outside the PMO, and that the contribution from the liquid crystal embedded in the chiral pores is much smaller and below the noise level.     

A simple electro-optic technique for studying the electroclinic effect in the smectic A* phase

The critical behaviour of the electroclinic response in the chiral smectic A* phase in the vicinity of the second-order smectic A* to smectic C* phase-transition temperature has been investigated using a new electro-optic technique. The temperature variation of the electroclinic coefficient, the relaxation frequency and the coefficient of the quartic term in the tilt angle in the Landau free energy expansion have been studied. The electroclinic coefficient diverges with decreasing temperature as the smectic A* to smectic C* phase-transition temperature is approached with a critical exponent, as predicted in the mean field Landau theory. The measured quartic coefficient varies strongly with temperature, contrary to the usual assumptions of the mean field Landau theory.     

Study of the mesogenic behaviour of hydrogen-bonded complexes prepared from aminophenylazopyridines and benzoic acid derivatives

Complexes have been formed between N, N-disubstituted aminophenylazopyridines and a series of alkoxybenzoic acids by intermolecular hydrogen bonding; liquid crystalline phases of the resulting complexes are induced thereby.     

Synthesis and characterisation of novel isoxazole-based banana liquid crystals from naturally occurring curcumin

Isoxazole-based bent-core liquid crystals (LCs) derived from naturally occurring curcumin were synthesised and their LC properties were investigated by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction techniques. Five compounds, including a branched alkyl chain derivative, were prepared and characterised. These derivatives exhibit enantiotropic mesophases. While lower homologues display wide-temperature-range nematic phase, a longer-chain derivative 3d shows smectic C phase in addition to the nematic phase. The bent angle in these compounds is in between calamitic LCs and banana LCs. Therefore, the molecules escape from polar order packing observed in typical bent-core LCs. Increasing the length of alkyl chain reduces both melting and isotropic temperatures in the series. However, the compound with branched alkyl chains exhibits significant reduction in the nematic-isotropic temperature only. Detailed XRD experiments confirm the presence of the N phase in the lower homologues and SmC phase in a higher homologue.     

Synthesis and liquid crystalline investigation of chiral 6-alkoxy-2-naphathoic acid derivatives

A two new series of materials with a chiral fragment derived from ((S)-(─)-2-methyl-1-butanol and 6-alkoxy-2-naphathoic acid as the molecular core was synthesised and investigated. All the homologues exhibited enantiotropic mesomorphism. Chiral smectic C (SmC*), smectic A (SmA) and chiral nematic (N*) phases were observed in different homologues. All the compounds were characterised by spectroscopic and elemental analysis. Thermal investigations and mesophase characterisations for all the compounds were carried out by the combination of DSC and POM analysis. The effects of the central linkage and various terminal normal alkyl chains with its structurally related compounds have been discussed.     

Synthesis and characterisation of imidazolium-based ionic liquid crystals bearing a cholesteryl mesogenic group

A series of cholesteryl-containing imidazolium chlorides and imidazolium tetrachloroaluminates were synthesised, and the chemical structure, liquid crystalline behaviour and ionic conductivity were characterised by several technical methods. Whereas the imidazolium chlorides show chiral smectic A (SmA*) phase on heating and cooling cycles, the imidazolium tetrachloroaluminates display chiral nematic (N*) phase, which is uncommon for ionic liquid crystals (ILCs). The imidazolium chlorides display similar phase transition temperature and entropy, indicating the cholesteryl component influence predominately on the phase transition rather than the different alkyl substituent groups. The imidazolium tetrachloroaluminates show lower melting point temperatures and lower clear point temperature than the imidazolium chlorides. The mesophases exist at rather moderate temperatures. Non-mesomorphic imidazolium tetrachloroaluminate(III) salts with short alkyl substituents have been known for a long time, and the synthesised imidazolium tetrachloroaluminates are the first examples of tetrahalogenoaluminate(III)-containing ILCs. For the imidazolium tetrachloroaluminates, imidazolium cations combine loosely with AlCl₄^? ions because AlCl₄^? ions are large and occupy more space in spite of the hydrogen bond and electrostatic attraction interaction, indicating that the layer structure can be destroyed easily to form N* phase on heating.     

Study of the electrocaloric effect in ferroelectric liquid crystals

ABSTRACT

Electrocaloric effect (ECE) in two ferroelectric liquid crystalline (FLC) materials has been evaluated by mean of two indirect characterization methods: the photopyroelectric (PPE) technique and the polarization current reversal one. The obtained results show a good correspondence of the adiabatic temperature change associated with the ECE evaluated from both methods. This validates the possibility to use the PPE technique to investigate the ECE in FLCs. This study also demonstrates that FLCs can be used as electrocaloric material. More particularly, it shows that as for their solid homologous, liquid crystals displays more pronounced ECE in the vicinity of a first order transition than that measured near to second-order one.     

Synthesis,characterisation and liquid crystalline behaviour of some lanthanides complexes containing two azobenzene Schiff base

Schiff base ligand N,N′-di-(4-butylsalicylidene)-1′,3′-diaminopropane [H₂L] was synthesised by the reaction of substituted azobenzene and 1,3-diaminopropane in 2:1 molar ratio. Four mononuclear lanthanide complexes of the type [Ln(H₂L)LCl] (Ln = LaIII, CeIII, SmIII and GdIII) were synthesised and characterised by ¹H,¹³C NMR, fourier transform infrared (FT-IR) spectroscopy, elemental analysis (CHNO), gas chromotography-mass, magnetic susceptibility and molar conductivity. Thermal properties of the title compounds were studied using the thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) and optical polarising microscopy (OPM). The ligand and coordination compounds exhibit liquid-crystalline properties (smectic A).     

Remarkable effect of pre‐organization on the self assembly in chiral liquid crystals

In this article, liquid crystal phases possessing a helical molecular assembly, including frustrated three dimensional (3D) structures, are overviewed. Then, the chirality‐originated superstructures in liquid crystals studied by the author are reviewed. The importance of the concept of “pre‐organization” is highlighted, thus, molecular design producing a strong chiral effect has been proposed. Dichiral twin materials have been prepared systematically based on this concept, and correlation between molecular architectures and resulting frustrated liquid crustal phases, such as smectic blue, cubic, tetragonal smectic Q, and sponge phases, has been investigated. An electrically induced anisotropic birefringent structure in the chiral isotropic phase and a photoinduced 3D‐3D phase transition in the smectic Q phase are introduced as possible application on the basis of the frustrated chiral 3D structured liquid crystal phases. A new type of chiral effect inducing the structural anisotropy in the 3D cubic structure of soft material is also described. © 2010 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 9: 340–355; 2009: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.200900029     

Synthesis of novel (-)-menthol-based azobenzene chiral liquid crystals and effect of terminal alkyl chains on mesomorphic behaviour

ABSTRACT

A series of novel azobenzene chiral liquid crystals (Mt4AZOnB) containing (-)-menthyl were successfully synthesised. Terminal alkyl chains of the molecules varied from methyl to hexyl. The chemical structures were characterised by Fourier transform infrared and ¹H-NMR spectra. Liquid crystalline behaviour of the chiral liquid crystals was characterised by differential scanning calorimetry and polarising optical microscopy. The effect of terminal alkyl chains on mesomorphic and photoisomerisation behaviour of azobenzene-based liquid crystals is discussed. The lower members (n = 1, 2, 3, 4) are monotropic and only the higher members (n = 5, 6) show enantiotropic behaviour. These compounds show a distinct odd-even effect for the phase transition temperature when the parity of the terminal chain is varied.     

The effect of ethanol on the phase behaviour and micro-rheology of liquid crystals

The effect of ethanol on the phase behaviour and micro-rheology of lyotropic liquid crystals (LC) has been studied using a binary mixture of monoglyceride (MG) and aqueous ethanol. The phase behaviour study reveals the structural modulation of surfactant aggregates with increasing ethanol concentration, namely a bicontinuous cubic phase (Ia3d) transitions to the lamellar phase (L_α), at a fixed MG concentration. This behaviour is explained by considering the critical packing parameter (CPP) of the surfactant molecule. Because ethanol dehydrates the surfactant head group (a _s), the CPP values increase (decreasing a _s) and thus the formation of larger CPP aggregates is favoured (i.e., the Ia3d–L_α transition occurs). Cross-polarised images and X-ray scattering data support this conclusion. The structural modulation of the LC has further been investigated using a diffusing wave spectroscopy technique. The correlation and relaxation times, determined from the intersection point at short and long time scales of the mean square displacement (MSD), decrease with increasing concentrations of ethanol, indicating structural modulation of the LC. The micro-viscoelastic moduli (G′ and G′′) derived from the Laplace transformation of the MSD decrease with increasing ethanol concentrations, due to the LC modulation. The thermal effects on the micro-rheology of the LC have also been studied.     

Effect of counter-ion on the thermotropic liquid crystal behaviour of bis(alkyl)-tris(imidazolium salt) compounds

Recently, new thermotropic ionic liquid crystals (LCs) with a hexyl-linked tris(imidazolium bromide) core and two terminal alkyl chains were synthesised and characterised. To explore the effect of different counter-ions on the LC behaviour of this system, derivatives with BF₄^? and Tf₂N^? counter-ions were prepared and analysed. Five of the BF₄^? derivatives were found to exhibit thermotropic LC behaviour. The 12-, 14- and 16-carbon tail BF₄^? compounds form SmA phases. The 18- and 20-carbon tail homologues form what appears to be a smectic phase but are weakly mesogenic and harder to characterise. Only two of the Tf₂N^? derivatives exhibited mesogenic behaviour. The 18-carbon tail Tf₂N^? compound forms an as-yet unidentified, highly periodic smectic phase with positional order while the 20-carbon tail homologue forms a periodic SmA phase. The Tf₂N^? mesogens have much lower clearing points even though their LC phases have more order than the Br^? and BF₄^? mesogens. X-ray diffraction showed that these mesogens have different amounts of tail interdigitation between the smectic layers depending on the counter-ion present. Atomistic molecular dynamics simulations indicated that counter-ion size plays an important role in defining the density of the ionic region, which in turn affects the amount of interdigitation in the smectic phases.     

The effect of voltage controlled orientation order of liquid crystals in non-acrylic polymer dispersed liquid crystals films

The nematic liquid crystals (LCs) are randomly dispersed material with random orientation order in polymer dispersed liquid crystal (PDLC) films. The LCs change their orientation from random to vertical as electric field is applied. This transformation of orientation order of nematic liquid crystals in the PDLC films is controlled by many factors operating simultaneously. For instance, some factors like the internal forces of attractions among the neighboring LC molecules, anchoring with polymeric matrix, ITO glass boundaries, and chemical structures of the materials are less studied. The learning of extent of vertical orientation of liquid crystal droplets in an electric field is essential to attain optimum electro optical properties of PDLCs. In this finding, bipolar and radial LCs droplets with random orientation have been observed in non-acrylic polymeric media. It is learned that with small increase of contents of external material, the extent of vertical orientation has been varied intensely. The extent of vertical orientation of LCs molecules increases as the contents of external non-acrylic polymeric material decreased. For this study, the orientations of LCs with respect to material type/contents, external applied force, and restoration of electric filed as hysteresis have been studied in details.     

Photoinduced alignment of polymer liquid crystals containing azobenzene moieties in the side group VII. On He-Ne laser beam irradiation

Photoinduced alignment in a polymer liquid crystal prepared from 6-{1-[4-(2-cyano-4-nitrophenylazo)phenyl]piperazino}hexyl acrylate and 4'-[6-(methacryloyloxy)hexyloxy]-4-cyanobiphenyl was investigated for the first time on irradiation with a polarized He-Ne laser beam at 633 nm. The azobenzene moieties as well as the inert cyanobiphenyl mesogenic units were aligned with the molecular long axis perpendicular to the polarization direction of the irradiation light. Alignment induced on short irradiation was reversible, while that induced under prolonged irradiation was irreversible due to the occurrence of crosslinking which might be caused by photoinduced decomposition of the azobenzene moieties during the photoirradiation process.     

The effect of molecular weight of polymer matrix on properties of polymer-dispersed liquid crystals

The electro-optical properties and memory effects are important characters of polymer-dispersed liquid crystal (PDLC) devices. Molecular weight of polymer matrix influences the morphology of liquid crystal droplets in PDLC film and the performance of PDLC devices. In this letter, PDLC films were doped with a small amount of chain transfer agent (CTA), in order to control the molecular weight of polymer matrix. It is observed that the addition of CTA induces a reduction of threshold voltage. In addition, the effect of molecular weight on memory effects of PDLC devices was also discussed. It is found that the entanglement between polymer and liquid crystal molecule increased with the molecular weight, which reduces the memory effect.     

Selected X-ray diffraction and differential scanning calorimetry measurements on nonaqueous liquid crystals and gel phases in the K-myristinate/glycerol binary system

Samples from the three selected concentrations x _KC14 = 0.25,0.37, and 0.50 of the K-myristinate/glycerol (KC₁₄/Gl) binary system have been investigated by means of small- and wide-angle X-ray diffraction measurements as well as differential scanning calorimetry (DSC) measurements as a function of temperature. The results are – The G₁ gel phase, contained in the preliminary phase diagram according to Dörfler and Senst [(1993) Colloid Polym Sci 271: 173], is nonexistent. – The same applies of the isotropic phase in the preliminary phase diagram according to Dörfler and Senst. Initially an isotropic or a cubic phase was assumed based on polarized microscopy texture observations. X-ray diffraction and DSC measurements provided no indication for their existence. – Due to the nonexistence of the G₁ and isotropic phases the preliminary phase diagram of the KC₁₄/Gl binary system had to be corrected. The region of the lamellar phase extends over a wider region. – X-ray diffraction and DSC measurements provided concordant results, which were further confirmed by electron microscopic investigation. Differences in phase-transition points from DSC data obtained for rising and falling temperatures have been observed. – The crystalline-to-gel phase transition correlates with a sharp shift in the d value of the first small-angle reflex. – The gel phase is accompanied by a distinct splitting of the first small-angle reflexes. The wide-angle reflexes show rearrangement and reduced intensity. – Similar to the crystalline-to-gel phase transition, the gel-to-lamellar phase transition is accompanied by a sharp change in the d values.     

Preorganised effects of a tetramesogenic supermolecule on supramolecular assembly in the liquid crystalline phases

After preparing a homologous series of tetrameric mesogenic compounds in which two U-shaped molecules were connected via a rigid benzene derivative or a flexible alkyl chain, we investigated their phase transition behaviour using optical microscopy, differential scanning calorimetry and X-ray diffraction analysis. The compounds possessing an alkyl spacer as the central group exhibited nematic and smectic A phases just as the corresponding U-shaped molecule did. The compound possessing a 1,2-benzene unit as the connecting group showed nematic and smectic A phases, although the compound possessing a 1,3-benzene unit exhibited only an anticlinic smectic C phase. Structure–property relations of the liquid crystalline tetramers are interpreted in terms of preorganised effects of the four mesogenic units.     

High-pressure studies of the hydrogen bond networks in molecular crystals

This article gives some examples of using high-pressure techniques to study hydrogen bond networks in molecular crystals.     

Doping effects of fluorinated chiral dopant in blue phase liquid crystals and its electro-optical behavior

ABSTRACT

We reported a fluorine-containing binaphthyl derivative, denoted as BD-F, which showed a low helical twisted power. The blue phase liquid crystal (BPLC) doped with BD-F achieved a lower operating voltage than that only used traditional chiral dopant, resulting from the larger dielectric property of fluorinated BD-F. In addition, the BPLC composite exhibited a wider temperature range and an enhanced Kerr effect with the decreased frequency. In terms of the applications, the devices revealed microsecond response time, thereby providing broad prospects for photonics and displays.     

The influence of flexible spacer and the chirality of the core on the formation of chiral nematic phase of symmetric dimers containing trifluoromethyl terminal

Seven symmetric liquid crystal (LC) dimers containing different chiral cores and LC arms have been synthesised, termed EPDA-(R,S)PD, TFBDA-(R,S)PD, TFBA-(R,S)PD, TFBDA-(R)PD, TFBA-(R)PD, TFBDA-IB and TFBA-IB, respectively. TFBDA-(R,S)PD, TFBA-(R,S)PD, TFBDA-(R)PD and TFBDA-IB displayed chiral smectic A (SmA*) phase, while EPDA-(R,S)PD, TFBA-(R)PD and TFBA-IB exhibited chiral nematic (N*) phase. The effects of flexible spacer, structural type of LC arms and the chirality of the cores on the thermal properties of the dimers and the formation of N* phase have been studied. The results indicated that the chiral core was prone to induce the N* phase in LC dimer that contained nematic arms although the chirality of the core is very weak, while for the smectic LC arms containing CF3 terminal, the removal of the flexible spacer between chiral core and rigid LC units was conducive to the formation of N* phase. For example, TFBDA-(R)PD and TFBDA-IB displayed SmA* phase, while TFBA-(R)PD and TFBA-IB exhibited N* phase. However TFBA-(R,S)PD did not display N* phase, which reminded us that the chirality of the core and the conformation of the dimer also played an influence in the formation of the LC phase.