Search

The nature of the nematic-nematic phase transition in the liquid crystal dimer 1″,9″-bis(4-cyanobiphenyl-4'-yl) nonane (CB9CB) has been investigated using techniques of calorimetry, dynamic dielectric response measurements, and (2)H NMR spectroscopy. The experimental results for CB9CB show that, like the shorter homologue CB7CB, the studied material exhibits a normal nematic phase, which on cooling undergoes a transition to the twist-bend nematic phase (N(TB)), a uniaxial nematic phase, promoted by the average bent molecular shape, in which the director tilts and precesses describing a conical helix. Modulated differential scanning calorimetry has been used to analyze the nature of the N(TB)-N phase transition, which is found to be weakly first order, but close to tricritical. Additionally broadband dielectric spectroscopy and (2)H magnetic resonance studies have revealed information on the structural characteristics of the recently discovered twist-bend nematic phase. Analysis of the dynamic dielectric response in both nematic phases has provided an estimate of the conical angle of the heliconical structure for the N(TB) phase. Capacitance measurements of the electric-field realignment of the director in initially planar aligned cells have yielded values for the splay and bend elastic constants in the high temperature nematic phase. The bend elastic constant is small and decreases with decreasing temperature as the twist-bend phase is approached. This behavior is expected theoretically and has been observed in materials that form the twist-bend nematic phase. (2)H NMR measurements characterize the chiral helical twist identified in the twist-bend nematic phase and also allow the determination of the temperature dependence of the conical angle and the orientational order parameter with respect to the director.

Cycloalkenones are shown to be mesogens and can be synthesised in near quantitative yields by a convergent palladium(0)-catalysed cross-coupling strategy; a 2-methyl group induces a change of phase from smectic to nematic.

In this paper the director configurations and the free energies of a nematic droplet with a surface normal anchoring condition are calculated numerically. For this surface anchoring, a transition occurs between the radial and axial structures with respect to an applied field. In the calculation of the director configurations, the position of a disclination has been fixed. Comparing the free energies for different disclinations, the stable position which gives the minimum free energy is found. In calculating the free energy of a droplet, it is assumed that the free energy density of the nematic phase does not exceed the isotropic free energy density, so that the large distortion in the vicinity of the disclination causes a nematic-isotropic transition and the free energy density of the disclination core becomes equal to the isotropic free energy density. The director configuration in a droplet is calculated as a function of an applied field for different isotropic free energy densities, elastic constant ra...

A new approach to the synthesis of azulene liquid crystals is described based on Hafner’s procedure involving reaction of a pyridinium salt with a cyclopentadienide. The preparation of a variety of liquid crystalline materials using this method is described. Variants are reported with single substituents on the azulene ring in the 6-position and doubly substituted in the 2, 6- positions. The effect of the azulene ring as a core or dipolar terminal structural element is explored. 6-(5-Alkyl-1,3-dioxan-2-yl)azulenes are shown to exhibit smectic A phases and 2-cyclohexyl-6-(5-tridecyl-1,3-dioxan-2-yl)azulene show smectic A and B phases. Phase characterisation of the materials is recorded together with X-ray measurements on the smectic A phase of one representative compound. Results of dichroism studies on the azulene mesogens are also briefly reviewed.

This paper reports the synthesis of six chiral liquid crystalline 2- or 3-substituted-alkyl 4-(4′-dodecyloxybiphenyl-4-carbonyloxy)benzoates, their mesophase assignments and the phase transition temperatures. All six materials display S∗A followed by S∗C phases on cooling from their isotropic liquids. The magnitudes of switched tilt angle, spontaneous polarization, helical twist sense (L.H.) and direction of polarization (P s(−)) are given for the S∗C phases. Close inspection of the S∗C phases revealed that for the six compounds, two showed a ferrielectric (S∗C γ) and an antiferroelectric (S∗A) subphase, two showed only a ferrielectric subphase and one showed only an antiferroelectric subphase. The field-dependent behaviour of the switched tilt angle of the ferrielectric and antiferroelectric phases of some of the compounds is also presented.

Achiral ‘swallow-tailed’ liquid crystalline materials are known to give alternating-tilt smectic C phases (SCalt) which have structural similarities to the chiral antiferroelectric phases denoted as S∗CA. This paper describes the synthesis and characterization of three achiral branchedalkyl 4-(4′-dodecyloxybiphenyl-4-carbonyloxy)-3-fluorobenzoates. Optical microscopy and differential scanning calorimetry confirm that these materials show SCalt and overlying SA phases. The compounds were investigated as potential hosts which could be doped with a chiral ferroelectric liquid crystal so as to provide a viable- antiferroelectric mixture. These studies (microscopy and differential scanning calorimetry), to characterize the properties of the mixtures, show that antiferroelectric phases are induced. However, switching studies show that the antiferroelectric phases are extremely stable, a property which is almost certainly a consequence of the length of the lateral branching groups (ethyl, propyl and butyl).

Reaction of β-enaminoketones with lanthanide salts leads to mesomorphic complexes displaying an SA mesophase. The mesophase was characterized using X-ray scattering, EPR and magnetic susceptibility. A very high magnetic anisotropy was deduced for a complex containing dysprosium.

The synthesis of the novel chiral mesogen (R)-(–)-nonan-3-yl 4-(4′-dodecyloxybiphenyl-4-ylcarbonyloxy)-3-fluorobenzoate and a discussion of its various liquid-crystalline properties are reported. Initial optical microscopy studies revealed the presence of a smectic A* phase and what was at first thought to be a smectic C* phase; further examination of this compound as thin film preparations revealed the presence of s± 1/2 disclinations suggesting that the phase is an antiferroelectric subphase. Furthermore, these observations were supported by results from both differential scanning calorimetry and contact miscibility studies using known smectic C*A standards. The antiferroelectric nature of the phase was confirmed by obtaining sharp threshold characteristic curves during electrical switching studies from the antiferroelectric to an induced ferroelectric phase. Additionally, the chirality of the compound results in a reduction in the environmental symmetry for the smectic A* phase, and this allows electroclinic switching in the pretransitional region of the smectic A* phase close to the smectic C*A transition to take place under suitable conditions.

This paper describes an experimental investigation of dipole−dipole interactions between molecules in an orientationally ordered environment. Measurements are reported of the dielectric properties of a series of anisotropic solutions of two structurally similar dipolar solutes (1-cyano-2-fluoro-4-[trans-4-(trans-4-propylcyclohexyl)cyclohexyl]benzene (CP1) and 1,2-difluoro-4-[trans-4-(trans-4-propylcyclohexyl)cyclohexyl]benzene (CP3)) dissolved in a nonpolar nematic liquid crystal solvent (1-[trans-4-ethylcyclohexyl]-2-[4-ethyl-2-fluorobiphenyl]ethane (I22)). The solvent provides an orienting medium for the solute molecules, and by varying the temperature the degree of orientational order of the solutions is changed. The measurements have been made on aligned thin films of the liquid crystalline solutions for different concentrations and over a range of frequencies from 1 kHz to 10 MHz. All solutions exhibited a low-frequency relaxation associated with end-over-end reorientation of the polar solutes in the...

This communication was stimulated by a meeting of the Sonderforschungsbereich 335, ‘Anisotrope Fluide’ at the Technische Universitat Berlin, in December 1994. At this conference we realized that there are some very strong similarities between the melting processes of blue phases, twist grain boundary phases, and cubic D phases to the isotropic, amorphous liquid. In all three cases the melting appears to be a two stage process with a sharp melting enthalpy followed by a broad diffuse endotherm as the disorganized liquid is formed. In this article we wish to draw attention to similarities in the nature of these melting processes, to suggest a general descriptive model for the breakdown in molecular ordering, and to stimulate a theoretical debate in order to gain an understanding of the role of frustration phenomena in melting processes.

Optical rotation and circular dichroism measurements performed in the pretransitional isotropic region above the chiral smectic phases TGBA∗, TGBC∗, S∗C, and S∗A exhibited by 1-alkylalkyl 4′-(4″-n-alkoxyphenylpropioloyloxy)biphenyl-4-carboxylates are reported. These results revealed a high degree of increasing chiral ordering on cooling in the isotropic phase, particularly in the temperature region where a broad DSC peak appears. However, the behaviour of the pretransitional optical activity was unlike that commonly observed for chiral nematic or blue phase compounds and did not follow a simple Landau-de Gennes temperature dependence. Characteristic features of the pretransitional optical activity may aid in understanding the local structure of chiral smectic phases. Electric fields did not seem to influence the results significantly, but strong electric field-induced optical rotation behaviour was demonstrated for a mixture constituted of the laterally fluorinated propiolate ester component and ...

We have calculated the scattering properties of a single droplet with uniform and radial structures in PDLCs in the absence of a voltage for a non-collimated system such as a real PDLC projector. In these calculations, we have developed Zumer's result for a collimated system based on the anomalous diffraction approach. The scattering properties of PDLCs depend not only on droplet properties, e. g. refractive indices, radius and director configuration, but also on the film thickness and the number density of droplets. Assuming a constant concentration, we have shown that the scattering properties of PDLCs can be discussed in terms of an efficiency factor, which removes the dependence on thickness and number density. The differences between collimated and non-collimated systems are discussed as well as the effects of refractive indices, droplet radius and angular dependence of the scattering.

The molecular organisation in a binary mixture of a strongly polar mesogen (5CB) and a weakly-polar mesogen (ME50.5) is investigated using results of X-ray scattering and density studies. Analysis of the intensity distribution of X-ray scattering has yielded results for orientational order parameters in addition to smectic layer spacings and effective molecular lengths. The results are compared with a mean field theory of the smectic phase. Previously unexplained results for mixtures showing a minimum in the anisotropy of physical properties coincident with a maximum in the nematic/smectic A transition entropy are shown to be consistent with the theory. We conclude that favourable packing of molecules in the injected smectic A phase stabilises the layers but increases the orientational free volume, consistent with a lower orientational order parameter.

The preparation and characterisation of novel ionic polymer liquid crystal complexes of poly(ethylene oxide)–Na+ with mesogenic anions and their uncharged structural analogues in homogeneous mixtures are described. The systems discused most fully are the anion of the phenolic mesogen 4-n-hexyloxybenzylidene-4-hydroxy aniline (1) with its uncharged analogue 4-n-hexyloxybenzylidene aniline (1a) and the anion of 5-(4-n-octyloxy-2,3-dicyanophenyleneoxycarbonyl)benzimidazo-le (2) and its uncharged analogue 4-octyloxy-2,3-dicyanophenyloxybenzoyl (2). Differential scanning calorimetry, wide angle x-ray diffraction and polarised light microscopy show that PEO–Na1/1a and PEO–Na2/2a are homogeneous mesogenic phases when Na1 or Na2 are present at 50% or less of the stoichiometric composition in complexes (EO : salt = 3 : 1) and the shortfall is made up from 1a or 2a uncharged analogues of 1a and related systems substituted by methoxy rather than hydrogen do not form homogeneous mixtures.

The syntheses of four chiral laterally fluoro-substituted propiolate esters are described, along with transition temperatures, ferroelectric properties, phase diagrams and related data. The position of the fluoro-substituent was found to influence dramatically the formation of twist grain boundary (TGB A* and TGB C*) phases as well as the magnitudes of spontaneous polarization and optical tilt angle in the ferroelectric smectic C* mesophases. Differential scanning calorimetric studies revealed the presence of a diffuse liquid-liquid transition above the clearing point in both of the 3-fluoro enantiomers, but not in the racemate. Circular dichroism and optical rotation measurements, carried out over a temperature range in which the diffuse peak occurs, appear to confirm the presence of a degree of chiral organization within the isotropic liquid. It is suggested that this phenomenon may be due to the presence of cybotactic groups or a network of entangled screw dislocations occurring close to the clearing point.

Measurements are reported of the ferroelectric polarization P s induced in a non-chiral smectic C phase by a variety of chiral dopants having different molecular structural features. Using molecular calculations of contributing dipole moments, ferroelectric order parameters are determined from the experimental results. The relationships between the P s and various other molecular properties are discussed, and it is shown that restricted rotation of the molecule due to its shape and internal energy barriers to rotation can result in relatively high values of P s. In contrast dipolar groups flexibly attached to a chiral centre may have their contribution to P s greatly reduced through internal rotation.

The cyclohex-2-en-1-one unit flanked by one or more aryl rings is shown to provide a new system which can exhibit liquid crystal properties; these new mesogens contain a chiral centre which is both adjacent to a lateral dipolar carbonyl group and is also located in the central core.

Measurements of the orientational and translational order parameters for the nematic, smectic A and smectic C phases of a commercial liquid crystal material are reported. The order parameters have been obtained by analysis of the angular distribution of the intensity of X-rays scattered by a sample aligned by a magnetic field. Results are presented as a function of temperature, and it is found that the apparent orientational order parameter in the smectic C phase decreases with decrease in temperature. This is explained using a model of random tilt. The experimental order parameters are compared with those calculated from a mean field model.

A general and versatile route to the 2-alkyl-5-(4′-cyanophenyl)thiophenes was established, involving the condensation of an aryl vinyl ketone with an aldehyde in the presence of a thiazolium catalyst (Stetter procedure) to give a 1,4-diketone which with Lawesson's reagent undergoes ring-closure to give the corresponding arylthiophene. In the final step the exchange of a bromo substituent on the phenyl ring for a cyano group is accomplished by copper(I) cyanide in N,N-dimethylforamamide at reflux. The same sequence afforded 2-alkyl-5-(4″-cyanobiphenylyl)thiophenes. Several of the 1,4-diketones were obtained by conjugate addition of nitroalkanes to aryl vinyl ketones, and treatment of the γ-nitroketones with silica gel-supported potassium permanganate. Three alkyl 5-(4-cyanophenyl)thiophene-2-carboxylates were prepared by condensing β-chlorovinylaldehydes with thioglycolates. The cyanophenylthiophenes exhibited only monotropic phases, but incorporation of an additional phenyl ring provided cyanobip...

A novel series of low-melting metallomesogens (metal-containing liquid crystals) has been prepared and characterised. Reaction of Zeise's salt K[PtCl3(η2-CH2CH2)] with the 4-n-alkyloxy-4′-stilbazole ligands (L = CnH2n+1OC6H4CHCHC5H4N; n= 3–12) yielded trans-[PtCl2(η2-CH2CH2)L]1, which react with alk-1-enes (CH2CHCmH2m+1; m= 1–8, 10 or 12) to give trans-[PtCl2(CH2CHCmH2m+1)L]2. The complexes where (m + n)ca. 8 are generally liquid crystalline (smectic A mesophases); those with shorter tails [(m + n)= 8 to ca. 13] show monotropic mesophases, while those with longer tails [(m + n) > ca. 13] show enantiotropic behaviour. The melting temperatures are lower than would be anticipated for metallomesogens of this size, reflecting the non-centrosymmetric molecular structures which make the crystal packing less easy. A 70 : 30 mixture of the complexes 2(n= 12, m= 3; K → SA at 61 °C) and (n= 12, m= 6; K → SA at 70 °C) shows the transition into the mesophase, K → SA, at 45 °C, an unusually low temperature. The mesophase to isotropic (SA→ I) transition temperatures remain rather constant at 92 ± 4 °C for all compositions.

The cyclohex-2-en-1-one unit flanked by one or more aryl rings is shown to provide a new system which can exhibit liquid crystal properties; these new mesogens contain a chiral centre which is both adjacent to a lateral dipolar carbonyl group and is also located in the central core.

The syntheses of four chiral laterally fluoro-substituted propiolate esters are described, along with transition temperatures, ferroelectric properties, phase diagrams and related data. The position of the fluoro-substituent was found to influence dramatically the formation of twist grain boundary (TGB A* and TGB C*) phases as well as the magnitudes of spontaneous polarization and optical tilt angle in the ferroelectric smectic C* mesophases. Differential scanning calorimetric studies revealed the presence of a diffuse liquid-liquid transition above the clearing point in both of the 3-fluoro enantiomers, but not in the racemate. Circular dichroism and optical rotation measurements, carried out over a temperature range in which the diffuse peak occurs, appear to confirm the presence of a degree of chiral organization within the isotropic liquid. It is suggested that this phenomenon may be due to the presence of cybotactic groups or a network of entangled screw dislocations occurring close to the clearing point.

A general route to potentially mesogenic heterocycles, based on 1-(4-bromophenyl) and 1-(4'-bromobiphenyl)-1,4-dioxoalkanes is described and used to prepare pyrroles, thiophene 1,1-dioxides and pyridazines. No mesogenic compounds were obtained in the pyrrole and thiophene 1,1-dioxide series, but four new mesogenic pyridazines, 3-hexyl-6-(4-bromophenyl)pyridazine, 3-hexyl-6-(4'-bromobiphenyl)pyridazine, and the two corresponding cyano-derivatives (obtained by copper-catalysed displacement of bromine by cyanide) were prepared and their mesophase behaviour is described. The mesophase behaviour of some synthetic intermediates including 4'-bromobiphenyl vinyl ketone and several 1-(4'-bromobiphenyl)-1,4-dioxoalkanes is also described.

The weakly mesomorphic trans-4-alkyloxy-4′-stilbazoles(n-OPhVPy) are readily synthesised from alkyloxyiodobenzenes and 4-vinylpyridine in a palladium-catalysed Heck reaction. While the highly symmetric palladium and platinum complexes trans-[MCl2(n-OPhVPy)2] show mesophases only at very high temperatures, the lower symmetry platinum complexes trans-[PtCl2(η2-olefin) (n-OPhVPy)] melt below 100°C. Similarly, the complexes [MCl(CO)2(n-OPhVPy)] (M = Rh, Ir) are low melting. The silver complexes [Ag(n-OPhVPy)2][X] (X = C12H25OSO3, C8H17OSO3, CF3SO3) can be obtained by direct reaction of the stilbazole with AgX. These ionic materials show a rich thermotropic polymorphism, including nematic and cubic mesophases. Some fluorinated stilbazoles have also been synthesised and complexed to silver; the mesomorphism of these complexes is also reported.

A homologous series of 2-alkyl-5-(4′-cyanophenyl)thiophenes has been prepared and shown to have improved properties for use in liquid crystal displays. The thiophene ring is prepared from a 1-(4′-bromophenyl)-1,4-dioxoalkane, constructed using the Stetter procedure, by the action of Lawesson’s reagent, and the cyano-group is finally introduced using a copper(I)-mediated exchange reaction.

Reaction of AgX (X NO3 or CF3SO3) with two equivalents of 4-alkyloxystilbazole leads to the mesomorphic silver(I) complexes [Ag(n-OPhVPy)2] [X] in high yield. The long-chain homologues of each series show SA and SC mesophases, while for shorter chain lengths the nitrate salts show only an SA phase and the triflate salts only a nematic phase. The phase behaviour of these new complexes is discussed in relation to analogous complexes with different counter-anions or ligands.

A novel series of low-melting metallomesogens (metal-containing liquid crystals) has been prepared and characterised. Reaction of Zeise's salt K[PtCl3(η2-CH2CH2)] with the 4-n-alkyloxy-4′-stilbazole ligands (L = CnH2n+1OC6H4CHCHC5H4N; n= 3–12) yielded trans-[PtCl2(η2-CH2CH2)L]1, which react with alk-1-enes (CH2CHCmH2m+1; m= 1–8, 10 or 12) to give trans-[PtCl2(CH2CHCmH2m+1)L]2. The complexes where (m + n)ca. 8 are generally liquid crystalline (smectic A mesophases); those with shorter tails [(m + n)= 8 to ca. 13] show monotropic mesophases, while those with longer tails [(m + n) > ca. 13] show enantiotropic behaviour. The melting temperatures are lower than would be anticipated for metallomesogens of this size, reflecting the non-centrosymmetric molecular structures which make the crystal packing less easy. A 70 : 30 mixture of the complexes 2(n= 12, m= 3; K → SA at 61 °C) and (n= 12, m= 6; K → SA at 70 °C) shows the transition into the mesophase, K → SA, at 45 °C, an unusually low temperature. The mesophase to isotropic (SA→ I) transition temperatures remain rather constant at 92 ± 4 °C for all compositions.

Some alkali and alkaline earth metal salts of the amphiphilic anion [Fe(CN)4(H2O)(C12H25NH2)]2- have been synthesized by reaction of [Fe(CN)5NO]3- with C12H25NH2. Using optical microscopy, they have been shown to give a hexagonal (H1) mesophase in water.

Reaction of tran.s-4-alkyloxy-4′-stilbazoles (n-OPhVPy) with silver dodecylsulphate (AgDOS) led to the complexes [Ag(n-OPhVPy)2][DOS] which showed thermotropic mesomorphism. All homologues from met...

Measurements are reported of the permittivity as a function of temperature and composition for strongly dipolar cyano-substituted molecules dissolved in a non-polar nematic liquid crystal solvent. Solute molecules with dipole moments perpendicular to the molecular alignment axes exhibited strong parallel association of their dipoles, even at low concentrations, and the measurements can be interpreted in terms of a ‘strong dimer’ model. Results for a solute molecule with a large dipole moment co-linear with the long molecular axis gave the major dipole correlation factor equal to 1, consistent with little or no preferred dipole-dipole correlation.

Low angle X-ray scattering studies have been used to identify the mesophase of some calamitic lanthanide mesogens as smectic A, while magnetic birefringence studies have shown a huge magnetic anisotropy for these complexes.