Showing papers in "Liquid Crystals in 1993"
TL;DR: In this article, the first observation of cholesteric blue phases in chiral anisotropic polymer networks was reported, in which two-component mixtures of a chiral and a non-chiral diacrylate were photopolymerized at constant temperature.
Abstract: We report the first observation of cholesteric blue phases in chiral anisotropic polymer networks. In two-component mixtures of a chiral and a non-chiral diacrylate, we observed typical textures of BPI, BPII and BPIII phases. By photopolymerization of these materials at constant temperature we obtained blue phase networks. After polymerization, the blue phases were stored, which enabled us to further study them without any temperature control.
1,008 citations
TL;DR: A range of nematogenic materials which incorporate a 2,6-disubstituted naphthyl moiety and a terminal cyano-substituent have been synthesized by using palladium-catalysed cross-coupling procedures involving arylboronic acids and alkynylzinc reagents with aryls iodides, bromides and trifluoromethanesulphonates (triflates).
Abstract: A range of nematogenic materials which incorporate a 2,6-disubstituted naphthyl moiety and a terminal cyano-substituent have been synthesized by using palladium-catalysed cross-coupling procedures involving arylboronic acids and alkynylzinc reagents with aryl iodides, bromides and trifluoromethanesulphonates (triflates). The compounds have very high nematic phase stability, but their melting points are also quite high. The birefringences were measured using an extrapolation technique and the values were found to be between 0·26 and 0·42.
196 citations
TL;DR: In this paper, the authors report the synthesis of a range of novel materials by the strategic use of palladium-catalysed cross-coupling methods, in addition to the traditional use of bromide and iodide leaving groups, invaluable use of the triflate leaving group and the importance of selective crosscoupled methods using both arylboronic acids and alkynylzinc chloride derivatives.
Abstract: Liquid crystal materials of high optical anisotropy consist of moieties of high electron density in conjugation with each other along the molecular length. Such structures are conducive to convergent synthesis methods. Here we report the synthesis of a range of novel materials by the strategic use of palladium-catalysed cross-coupling methods. In addition to the traditional use of bromide and iodide leaving groups, invaluable use of the triflate leaving group and the importance of selective cross-coupling methods using both arylboronic acids and alkynylzinc chloride derivatives is illustrated. This systematic methodology allows the separate synthesis of the appropriate sub-units that can be efficiently coupled together to provide high overall product yields.
188 citations
TL;DR: In this paper, a modified procedure using iron III chloride instead of chloranil yields hexa-alkoxytriphenylenes in a single step, which can be performed on a large scale and also provides a better route than those currently available for making the unsymmetrically substituted triphenylenes required in the synthesis of polymeric discotic liquid crystals.
Abstract: The synthesis of discotic liquid crystal materials based on the triphenylene core normally involves the preparation of hexahydroxytriphenylene. This has traditionally been accomplished by a chloranil promoted trimerization of 1,2-dimethoxy-benzene followed by demethylation of the hexamethoxytriphenylene produced. Hexahydroxytriphenylene is subsequently alkylated to give the required derivative. The overall yield of this three step procedure is low and it is difficult to perform on a large scale. It is herein shown that a modified procedure using iron III chloride instead of chloranil yields hexa-alkoxytriphenylenes in a single step. The reaction is high yielding and can be performed on a large scale. This also provides a better route than those currently available for making the unsymmetrically substituted tri-phenylenes required in the synthesis of polymeric discotic liquid crystals. Some related elaborations of the triphenylene nucleus are also discussed.
173 citations
TL;DR: In this paper, the authors summarized the progress in the field of discotic liquid crystals, with emphasis on experimental results rather than theoretical developments, including metallo-mesogens and discotic polymers.
Abstract: Progress in the field of discotic liquid crystals is summarized, with emphasis on experimental results rather than theoretical developments. Examples are given of discotic mesogens (including metallo-mesogens) and discotic polymers, and the structures of the mesophases identified to date are described.
167 citations
TL;DR: Several series of phasmids and biforked mesogens or more generally of polycatenar mesogens, i.e. rod-like core mesogens ending in six, five, four or three aliphatic chains (respectively hexa-, penta-, tetra-and tricatenars) are reviewed in this paper.
Abstract: Several series of phasmids and biforked mesogens or more generally of polycatenar mesogens, i.e. rod-like core mesogens ending in six, five, four or three aliphatic chains (respectively hexa-, penta-, tetra- and tricatenars) are reviewed. According to the number and positions of the aliphatic chains, lamello-columnar, columnar, cubic, smectic and nematic phases appear.
164 citations
TL;DR: The odd-even behavior observed for liquid crystal dimers has been predicted to be a sensitive function of the geometry of the link joining the mesogenic groups to the flexible spacer as discussed by the authors.
Abstract: The odd-even behaviour observed for liquid crystal dimers has been predicted to be a sensitive function of the geometry of the link joining the mesogenic groups to the flexible spacer. Here we report the synthesis of two cyanobiphenyl dimers with methylene links, together with a determination of their transitional properties. In particular we have probed their orientational order via the major and biaxial orientational order parameters of a solute molecule, anthracene-d 10, using NMR spectroscopy. For comparison we have also determined the corresponding quantities for the cyanobiphenyl dimers with ether links. The enhanced odd-even effect observed for the methylene linked dimers relative to those with ether links is in good accord with theory.
144 citations
TL;DR: In this article, the stability of a hydrogen-bonded complex built through inter-molecular hydrogen bonding between carboxylic acid and pyridine fragments has been examined using infrared spectroscopy.
Abstract: The stability of a hydrogen-bonded complex built through inter-molecular hydrogen bonding between carboxylic acid and pyridine fragments has been examined using infrared spectroscopy. Infrared spectra as a function of temperature have been recorded for the 1:1 complex of 4-hexyloxybenzoic acid and trans-4-propoxy-4′-stilbazole from the crystalline state to the isotropic state. A dependence of the stability of the hydrogen bond on molecular orientation is observed clearly in the infrared spectra. The spectra also suggest that the hydrogen bond is an unionized type with a double minimum potential energy.
124 citations
TL;DR: In this article, the structural, material and electro-optical properties of halogenated nematic liquid crystals are correlated, and a recently presented, efficient liquid crystal colour projection concept, whose functional elements, i.e. polarizers, filters and modulators, consist entirely of liquid crystal devices, is reviewed.
Abstract: The structural, material and electro-optical properties of novel, halogenated nematic liquid crystals which contain quite different functional groups are correlated. Synergisms which lead to broad mesophases, low viscosities and large dielectric anisotropies further improve the performance of actively and passively addressed, high information content liquid crystal displays. Some recent developments, such as operation of supertwisted nematic displays with not only linear, but also circularly polarized light, are included. A recently presented, efficient liquid crystal colour projection concept, whose functional elements, i.e. polarizers, filters and modulators, consist entirely of liquid crystal devices, is reviewed. Its circular polarizers and filters are made up of novel, negative dielectric anisotropic cholesteric liquid crystals designed such that, dislocation-free, optically uniform, planar textures result from electric field alignment. Novel, non-linear optical ferroelectric liquid crystals...
98 citations
TL;DR: In this article, a sequence of transitions with increasing Ericksen number analogous to that observed in small molecule tumbling nematics was observed in liquid crystalline poly(γ-benzyl-glutamate) solutions.
Abstract: Samples of liquid crystalline poly(γ-benzyl-glutamate) solutions are sheared between glass surfaces with gaps, d = 10–500 μ, and shearing velocities, V = 0·05–10 000 μs−1 so that the Ericksen number E [tbnd] Vdγ 1/K is varied over a large range, E ≈ 1–107. Here γ1 is the rotational viscosity and K 1 is the Frank splay constant, with γ1/K 1 estimated to be approximately 1 s μ−2 for our samples. We observe by polarizing microscopy a sequence of transitions with increasing Ericksen number analogous to that observed in small molecule tumbling nematics: namely rotation of the director out of the shearing plane and into the vorticity direction at Vd ≈ 25 μ2 s−1, and formation of roll cells at Vd ≈ 50 μ2 s−1. The roll cells become finer with increased Vd in accord with predictions of linear stability theory using the Leslie-Ericksen equations, and at Vd ≳ 500 μ2 s−1, the cells become very irregular, producing director turbulence. The turbulence becomes finer in scale as Vd increases, reaching sub-micron...
96 citations
TL;DR: In this article, a model is proposed to rationalize the structural modifications induced by varying the nature of the connecting link between the macrocycle and the side chains (alkyl, alkoxymethyl, alkoxy).
Abstract: Phthalocyanines R, PcM (R = alkyl, M = H, Ni, Cu) substituted with alkyl chains have been synthesized and their liquid crystalline properties studied by differential scanning calorimetry and X-ray diffraction at small angles. The influence of the chain length on the mesomorphic properties has been determined. A model is proposed to rationalize the structural modifications induced by varying the nature of the connecting link between the macrocycle and the side chains (alkyl, alkoxymethyl, alkoxy).
TL;DR: In this paper, DSC and X-ray studies on a series of 1,2-phenylene bis[4-alkoxyphenyliminomethyl)benzoates are presented.
Abstract: DSC and X-ray studies on a series of 1,2-phenylene bis[4-(4-alkoxyphenyliminomethyl)benzoates] are presented. The phases were shown to be classical nematic and/or smectic by their continuous miscibilities with reference mesophases. The layer spacings in the smectic A phase measured as a function of the alkyl chain length revealed that the mean tilt angle of the linear three-ring part of the molecule to the layer normal is about 37° and that the molecular core is V-shaped with an angle of about 70° between the two out-stretched benzoate legs. This molecular model accounts for the effects of a methyl group introduced into the 3- or 4-position of the 1,2-phenylene moiety on the mesomorphic properties. Both the nematic and smectic A properties are markedly promoted by replacement of the 1,2-phenylene group by the 2,3-naphthylene group. The phases exhibited by the less-elongated series of 2,3-naphthylene bis(4-alkoxybenzoates) are also nematic and/or smectic A, but are now monotropic. The formation of...
TL;DR: In this paper, a series of dimeric compounds, called di-BB-n, have been investigated and the most interesting effect among them was observed for a type of smectic phase.
Abstract: In this study, we prepared a homologous series of dimeric compounds, di-BB-n These compounds have two mesogenic groups linked by a flexible methylene spacer and so can be regarded as model compounds for main chain liquid crystalline BB-n polyesters. In this dimeric system, we observed distinct odd-even effects in which the smectic mesophase properties and structures are significantly affected by the number of carbon atoms in the flexible spacer. The most interesting effect among them was observed for a type of smectic phase. From detailed optical microscopy and X-ray observations, we found that the even-membered di-BB-n materials form a smectic A while the odd-membered ones form a smectic C2 phase. Such a distinct odd-even effect was concluded to result from the confined spatial orientation of mesogenic groups which is controlled by the conformation of the flexible spacer similar to the BB-n polymeric system.
TL;DR: The biphenylyl esters of the 4-n-alkoxyphenylpropiolic acids are a unique family of liquid-crystalline materials.
Abstract: The biphenylyl esters of the 4-n-alkoxyphenylpropiolic acids are a unique family of liquid-crystalline materials. In particular, when the biphenyl moiety of the compounds carries a chiral end-group, many optically active mesophases are created which exhibit unusual structures and physical properties. For instance, when the chiral group attached to the biphenyl moiety is 1-methylheptyl then Abrikosov, twist grain boundary smectic A* and antiferroelectric smectic C* phases are observed. The wide variety of chiral phases and electrochiral properties exhibited by this family of materials makes them ideal candidates for exploring chirality in the liquid-crystalline state. These investigations allow us to contrast and compare chirality dependent phenomena in liquid crystals, thereby producing a broader view of the concept of chirality in organized fluids than is traditionally presented.
TL;DR: In this paper, an unsophisticated model is proposed for evaluating the activation energies of the cross-linking reaction of liquid-crystalline epoxy resins, which is used in characterizing the isothermal cure.
Abstract: By endcapping mesogenic rigid rod molecules with reactive epoxy groups a novel class of liquid-crystalline thermoset has been obtained. In fact is has been shown that the nematic molecular arrangement is sustained over the crosslinking reaction of liquid-crystalline epoxy resins when the curing reaction is carried out in the thermal stability range of the liquid-crystalline phase. Calorimetric analysis was used in characterizing the isothermal cure. An unsophisticated model is proposed for evaluating the activation energies of the crosslinking reaction. For liquid-crystalline epoxy resins lower activation energies result with respect to the cure reactions for non liquid-crystalline epoxy resins.
TL;DR: In this article, the static properties of a simple phenomenological model of antiferroelectric liquid crystals are analyzed, capable of explaining some different sequences of phase transitions found experimentally.
Abstract: We present the analysis of static properties of a simple phenomenological model of antiferroelectric liquid crystals, capable of explaining some different sequences of phase transitions found experimentally. Stability analysis shows the existence of three simply modulated phases. Results are summed up in phase diagrams with dependence on the parameters of the model. The existence of doubly modulated incommensurate phases is predicted in parts of the phase diagrams.
TL;DR: In this article, achiral phases were observed for mixtures with racemic MBTA as a proton donor, and the transition temperatures and enthalpies were similar for the chiral and Achiral systems.
Abstract: Hydrogen-bonded dimers of 4-alkoxy-4′-stilbazole homologues (C n OSB, proton acceptors) and the mono-(2-methylbutyl)ester of terephthalic acid (MBTA, proton donor) behaved like conventional thermotropic mesogens. For chiral MBTA mixtures, chiral nematic and blue phases were observed with n = 1 and 2, and chiral smectic C phases were observed with n ≥ 5. Achiral phases were observed for mixtures with racemic MBTA as proton donor. Transition temperatures and enthalpies are similar for the chiral and achiral systems.
TL;DR: In this paper, the formation of optical anisotropy in stretched, thin polymer dispersed liquid crystal (PDLC) films with micron sized nematic droplets have been studied experimentally and the results analyzed in terms of a proposed theoretical model.
Abstract: The basic mechanisms determining the formation of optical anisotropy in stretched, thin polymer dispersed liquid crystal (PDLC) films with micron sized nematic droplets have been studied experimentally and the results analysed in terms of a proposed theoretical model. The experiments were performed on PDLC films with the bipolar nematic director configuration in the droplets, where the film transmittance, microscopic structure, and birefringence of the polymer matrix were studied. It is shown that the orientational ordering of bipolar nematic droplets, introducing the main contribution to the ability of stretched PDLC film to polarize the transmitted light, is strongly dependent upon initial droplet shape and the elastic properties of the polymer matrix. The ‘anomalous’ nematic director orientation is also observed in a portion of elongated droplets where the axes of bipolar configurations do not coincide with the major axes of the droplet cavities due to the presence of inclusions at the cavity ...
TL;DR: In this paper, a series of N, N-di-n-alkyl-N,N-dimethylammonium bromides was synthesized and characterized, and its thermotropic liquid-crystalline behaviour was studied by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction.
Abstract: A series of N, N-di-n-alkyl-N,N-dimethylammonium bromides was synthesized and characterized. Its thermotropic liquid-crystalline behaviour was studied by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. A novel smectic mesophase was thus identified, in which the lateral packing of the molecules within the layers is ordered and tetragonal in symmetry.
TL;DR: In this paper, the mesophases were assigned as discotic lamellar (DL) phases by X-ray diffraction studies of 5,10,15,20,20-tetrakis(4n-alkylphenyl)porphyrins.
Abstract: Studies of 5,10,15,20-tetrakis(4-n-alkylphenyl)porphyrins revealed that the homologues with longer alkyl chains than hexyl were mesomorphic, in contrast to the non-mesomorphic alkoxy derivatives. Metal complexes (Co, Ni, Cu, Zn, and Pd) of the dodecyl derivative were also shown to exhibit mesophases. These mesophases were assigned as discotic lamellar (DL) phases by X-ray diffraction studies.
TL;DR: In this paper, a ferroelectric liquid crystal with 5-amino-2-nitrophenyl groups attached close to the chiral centers was proposed for frequency doublers and optical modulators.
Abstract: From our research for novel non-linear optical (NLO) materials for frequency doublers and optical modulators we report on new ferroelectric liquid crystals, which for the first time, exhibit second order NLO coefficients (for example d 22 = 5 pm V−1, which are comparable to those of state of the art inorganic NLO materials. The novel compounds contain 5-amino-2-nitrophenyl groups attached close to the chiral centres. The switching behaviour of the new compounds, their spontaneous polarization, as well as their frequency doubling of Nd:YAG laser pulses in the S*c and in the glass state, are reported. Moreover their waveguiding properties are presented.
TL;DR: In this article, a series of mesogens with three rings in the main structure and a lateral aromatic branch was synthesized, and three new homologous mesogens containing three rings with three different types of mesogen substituents were synthesized.
Abstract: Three new homologous series of mesogens containing three rings in the main structure and a lateral aromatic branch have been synthesized. When the lateral aromatic branch is attached to the middle ring, monotropic phases are observed, and there is a strong tendency to form the smectic C phase. When the lateral aromatic branch is attached to an outer ring, the compounds possess purely enantiotropic nematic phases. The terminal alkoxy chain in the outer ring which has a lateral aromatic substituent can be replaced by a rigid group without disturbing the large nematic range. Selective introduction of a particular substitutent may be useful for the construction of devices possessing desirable characteristics such as non-linear optical properties.
TL;DR: In this article, the electrooptic properties of the smectic O and O* phases of m7tac (1-methylheptyl terephthalylidene-bis-4-aminocinnamate) were investigated.
Abstract: We present the results of our investigations on the electrooptic properties of the smectic O and O* phases of m7tac (1-methylheptyl terephthalylidene-bis-4-aminocinnamate). At low voltages, we observed striking electrochromic effects and, in O* at high voltages, a field induced phase transition. Freely suspended thick films of m7tac in the O and O* phases are weakly biaxial. A framework is proposed to account for our observations.
TL;DR: In this paper, a new low molar mass chiral organosiloxane mesogen and its racemic analogue have been synthesized and their mesomorphic and ferroelectric properties investigated.
Abstract: A new low molar mass chiral organosiloxane mesogen and its racemic analogue have been synthesized and their mesomorphic and ferroelectric properties investigated. The chiral derivative, denoted A∗B, exhibits one tilted enantiotropic ferroelectric smectic C mesophase over a broad temperature range, with very high tilt angles and moderate spontaneous polarization (36° and 19 nC cm−2 at 20°C). The achiral siloxane derivative, denoted A∗B, exhibits one broad enantiotropic smectic C phase. Preliminary electro-optic measurements indicate that the spontaneous polarization is weakly dependent on temperature between 10°C and 50°C, the latter being the S∗c to isotropic phase transition. The tilt angle and layer spacing are temperature independent, and current response times of less than 200 μs were measured at 25°C for fields of 10 V μ−1. These results are discussed in comparison with those for side chain polymer liquid crystal structures and other low molar mass ferroelectric materials.
TL;DR: In this paper, the Cano-Grandjean method, optical rotation dispersion and UV-VIS spectroscopy were used to reveal a cholesteric phase with temperature induced reversal of the helical twist.
Abstract: Investigations of 4-[(S, S)-2, 3 epoxyhexyloxy]-phenyl-4-(decyloxy)-benzoate by polarizing microscopy, the Cano-Grandjean method, optical rotation dispersion and UV-VIS spectroscopy reveal a cholesteric phase with temperature induced reversal of the helical twist. Switching time experiments in the Sc* phase show that the intrinsic helix can be unwound reversibly and irreveribly by application of electric fields of different strengths.
TL;DR: In this article, the authors compare viscoelastic data with diffusion data by calculating the time τ that it takes a polymer to diffuse a distance equal to its coil size R (τ=R2/D).
Abstract: Small amplitude oscillatory shear has been used to study thermotropic liquid-crystalline polymers that have mesogenic groups pendant to flexible backbones. The polymers studied form nematic and smectic glasses, enabling viscoelastic response to be studied over a wide range of frequencies using time-temperature superposition. In contrast to main chain liquid-crystalline polymers, the nematic side chain polymers exhibit linear viscoelastic response over a wide range of strain amplitudes that is independent of thermal and shear histories. Viscoelastic response is very sensitive to smectic-nematic and smectic-isotropic transitions, but insensitive to the nematic-isotropic transition, as time-temperature superposition applies across this transition. We compare viscoelastic data with diffusion data by calculating the time τ that it takes a polymer to diffuse a distance equal to its coil size R (τ=R2/D). At frequencies lower than 1/τ side chain polymers in their nematic show the terminal response charac...
TL;DR: In this article, four discotic liquid crystals (LCs) from the hexa-alkoxytriphenylene series (R = C n H2n + 1, n = 5, 7, 9, 11) classified as Dho phases were investigated with respect to their photoconducting properties.
Abstract: Four discotic liquid crystals (LCs) from the hexa-alkoxytriphenylene series (R = C n H2n + 1, n = 5, 7, 9, 11) classified as Dho phases were investigated with respect to their photoconducting properties. Under irradiation, all samples show photoconductivity within the mesophase, whereas in the isotropic phase the photocurrent drops to zero. All of the observed effects, the phase dependence of the photocurrent, the increasing values of the photocurrent with decreasing length of the side chains, and the higher photoconductivity during the cooling process, can be explained in terms of the transition temperatures, the intercolumnar distances and the orientation behaviour. Comparison of the photocurrents of the investigated homologues at a reduced temperature leads to an activation energy of 0·67 eV for the charge transport.
TL;DR: The dependence of mesomorphism of 1,3-diketonate vanadium-oxo (vanadyl) complexes on the number of side chains was investigated in this paper.
Abstract: The dependence of mesomorphism of 1,3-diketonate vanadium-oxo (vanadyl) complexes on the number of side chains was investigated. These complexes have a large dipole normal to the disc plane and are under investigation in an effort to generate discotic phases with polar order. Relatively complex phase behaviour is observed when the vanadyl 1,3-diketonate complexes are appended with four phenyl groups with two octyloxy or decyloxy side chains each for a total of eight side chains. These complexities are most likely related to the order/disorder associated with the polar vanadyl group. Vanadyl 1,3-diketonate complexes with four side chains were prepared by replacement of two of the phenyl groups with methyl or trifluoromethyl groups, and these complexes displayed only crystal phases. Complexes with two trialkyloxy phenyls and two dialkyloxy phenyls were synthesized and found to display a very stable Dhd phase. The presence of additional alkyloxy groups was found also to promote a linear chain struct...
TL;DR: In this article, the results of computer simulation studies for a bulk system composed of chiral particles interacting via the Gay-Berne potential and an additive chiral potential were reported.
Abstract: We report the results of computer simulation studies for a bulk system composed of chiral particles interacting via the Gay-Berne potential and an additive chiral potential. Using Monte Carlo (MC) simulations in the NVT ensemble, the chirality-temperature plane of the phase diagram was studied at different points by a variation of the chirality parameter c describing the strength of the chiral potential. Additionally to the well-known isotropic, nematic and smectic phases of the Gay-Berne fluid, we localized regions of cholesteric phase. For large values of the chirality parameter we also observed blue phases. Furthermore, when starting from a cholesteric phase and decreasing the temperature at constant c, we obtained a phase region showing characteristics of the recently discovered helical smectic A∗ phase. All phases have been characterized by correlation functions, order parameters, and visual representations of selected configurations. All results of the simulation are limited by the small sy...
TL;DR: In this article, the first examples of amphiphilic alkyl pentakis(phenylethynyl)benzene ethers containing functional groups at the terminal position of their alkoxy chains were synthesized by etherification of pentabromophenol with 11-bromoundecan-1-ol and 11-binormoundecanoic acid ethyl ester, respectively, and subsequent palladium catalyzed fivefold coupling reactions with the substituted phenylacetylene.
Abstract: First examples of amphiphilic alkyl pentakis(phenylethynyl)benzene ethers containing functional groups at the terminal position of their alkoxy chains were synthesized by etherification of pentabromophenol with 11-bromoundecan-1-ol and 11-bromoundecanoic acid ethyl ester, respectively, and subsequent palladium catalyzed, fivefold coupling reactions with the substituted phenylacetylene. Two undecanoic acid derivatives with different pentakis(phenylethynyl)phenoxy substituents in the ω-position were prepared by ester cleavage from the corresponding ethyl esters. The thermal behaviours of five penta-alkynes in their pure states, as well as of their charge transfer complexes formed from mixtures with 2,4,7-tri-nitrofluorenone, were examined.