Showing papers in "Liquid Crystals in 2001"
TL;DR: The role of hydrogen-bonding interactions in the formation and/or stabilization of liquid crystalline phases has been recognized in recent years and significant work has been conducted as discussed by the authors, with the aim of establishing, in the first place, the diversity of organic compounds that can be used as building elements in the process of liquid crystal formation.
Abstract: The role of hydrogen-bonding interactions in the formation and/or stabilization of liquid crystalline phases has been recognized in recent years and significant work has been conducted. Following the first and well-established examples of liquid crystal formation through the dimerization of aromatic carboxylic acids, several classes of compounds have been prepared by the interaction of complementary molecules, the liquid crystalline behaviour of which is crucially dependent on the structure of the resulting supramolecular systems. In this review the main classes of liquid crystals prepared through hydrogen-bonding interactions are presented, with the aim of establishing, in the first place, the diversity of organic compounds that can be used as building elements in the process of liquid crystal formation. Rigid-rod anisotropic or amphiphilic-type molecules, appropriately functionalized with recognizable moieties, interact in the melt or in solution and lead to the formation of supramolecular complexes tha...
347 citations
TL;DR: A range of symmetric liquid crystal dimers which differ in the nature of the link, either ether or methylene, between the spacer and mesogenic units has been prepared and their transitional properties characterized as discussed by the authors.
Abstract: A range of symmetric liquid crystal dimers which differ in the nature of the link, either ether or methylene, between the spacer and mesogenic units has been prepared and their transitional properties characterized. The nematic-isotropic transition temperature, T NI, and the associated entropy change, ΔS NI/R, are sensitive to the chemical nature of this link. Specifically, T NI falls on replacing ether links with methylene links for both odd and even members although this reduction is more pronounced for odd members. In comparison, ΔS NI/R increases on changing ether links for methylene links for even dimers, but decreases for odd-membered dimers. These observations are completely in accord with the predictions of a model developed by Luckhurst and co-workers in which the difference between the ether-linked and methylenelinked dimers rests exclusively in their shapes. Furthermore, the highly non-linear pentamethylenelinked dimers show a greater tendency to exhibit smectic behaviour; this is interpreted i...
157 citations
TL;DR: In this article, the measurement of the elastic and dielectric constants of 4'-n-pentyl-4-cyanobiphenyl (5CB) has been investigated in detail.
Abstract: 4'-n-Pentyl-4-cyanobiphenyl (5CB) is a room temperature nematic liquid crystal with a high positive dielectric anisotropy and a high chemical stability. Many experimental results concerning the elastic and dielectric constants of 5CB are available in the literature, although there is often no satisfactory agreement between the experimental data obtained by different groups, especially as far as the dielectric constants are concerned. Furthermore, no detailed investigation of the temperature dependence of the elastic and dielectric constants close to the nematic-isotropic transition temperature T NI has yet been reported. In this paper, we report the measurement of the elastic and dielectric constants of 5CB, and the temperature behaviour close to T NI has been investigated in detail. The experiment consists in the measurement of the director deformation induced by an electric field using simultaneously both a dielectric and an optical method. The simultaneous use of these two methods provides an indirect ...
116 citations
TL;DR: In this paper, a fluid smectic phase of a chiral bent-core liquid crystal was found to have a ground state structure that is anticlinic in tilt and ferroelectric in polar order, SmCAPF*.
Abstract: A fluid smectic phase of a chiral bent-core liquid crystal was found to have a ground state structure that is anticlinic in tilt and ferroelectric in polar order, SmCAPF*. The layer chirality of this structure alternates from layer to layer despite their being composed of chiral mesogens. Observations of the optical second harmonic generation signal from well-aligned domains confirm that the ground state of this phase is bistable ferroelectric. In addition to the ground state two types of metastable domains are also observed.
82 citations
TL;DR: The synthesis and mesomorphic properties of a series of 1,2-phenylene bis[4-(4-alkyloxyphenylazo)benzoates] are reported in this article.
Abstract: The synthesis and mesomorphic properties of a series of 1,2-phenylene bis[4-(4-alkyloxyphenylazo)benzoates] are reported The mesophases exhibited by these bent-shaped azo compounds are identified as nematic, smectic A and crystal E phases All these compounds have fairly low transition temperatures, and so are suitable for physical studies It has been concluded that the -CH N- linkage can be more conducive to mesomorphism compared with the -N N- linkage
70 citations
TL;DR: In this paper, a series of liquid crystals with a chiral chain containing two asymmetric carbon atoms in two lactate groups have been synthesized and characterized by DSC, dielectric, and X-ray studies.
Abstract: New series of liquid crystals with a chiral chain containing two asymmetric carbon atoms in two lactate groups have been synthesized. Some compounds have a third asymmetric centre in an (S)-2-methylbutyl alkyl chain. All the new compounds have SmA, SmC* and hexatic phases. Two of the homologues with three chiral centres also exhibit very wide SmCA phases and one of them a re-entrant SmC* phase. The phases are characterized by DSC, dielectric, and X-ray studies. Spontaneous polarization and tilt angle are determined for the polar phases.
69 citations
TL;DR: Achiral compounds with banana-shaped molecules have been synthesized and studied by the classical techniques (optical microscopy, differential scanning calorimetry, X-ray diffraction, miscibility studies and electro-optic investigations) as mentioned in this paper.
Abstract: A new series of achiral compounds with banana-shaped molecules ('E n ') has been synthesized and studied by the classical techniques (optical microscopy, differential scanning calorimetry, X-ray diffraction, miscibility studies and electro-optic investigations). The short homologues (E7-E10) give switchable smectic mesophases. Nevertheless their textures clearly differ from those of the B2 mesophase. In addition, the E10 compound displays a smectic polymorphism involving a new bilayered structure.
64 citations
TL;DR: In this article, it was shown that one of these compounds indeed forms a mesophase (B6) which is characteristic for the banana-shaped molecular structure of the compound.
Abstract: D. Vorlander was the leading scientist in liquid crystal chemistry in the first half of the 20th century who discovered many of the building principles for liquid crystalline compounds. It was known that Vorlander also synthesized some compounds with a bent molecular shape. In this paper we show that one of these compounds indeed forms a mesophase (B6) which is characteristic for the banana-shaped molecular structure.
61 citations
TL;DR: In this paper, the synthesis of new monodentate heterocyclic ligands 5-(4-pyridyl)-2-alkyltetrazole (L1a,b) and 4-[5-(2-alketrazoles]-aryl-4'-pyridinecarboxylate (L2a, b,c) containing two or three aromatic or heter-cyclic rings (tetrazide, pyridine and benzene) and preparation of their corresponding silver(I) and palladium(II) complexes (I
Abstract: The synthesis of new monodentate heterocyclic ligands 5-(4-pyridyl)-2-alkyltetrazole (L1a,b) and 4-[5-(2-alkyltetrazole)]aryl-4'-pyridinecarboxylate (L2a,b,c) containing two or three aromatic or heterocyclic rings (tetrazole, pyridine and benzene) and preparation of their corresponding silver(I) and palladium(II) complexes (Ia,b,c and IIa,b,c) are described. The thermal behaviour of the ligands and complexes was characterized by polarizing optical microscopy. The ligands and the complexes Ia,b,c and IIc showed no liquid crystalline phase. The complexes IIa,b showed mesomorphic behaviour, exhibiting smectic A enantiotropic mesomorphism X-ray diffraction measurements for complex Ia showed monodentate coordination of N-pyridine, and no coordination on the nitrogen atoms of the tetrazole ring.
57 citations
TL;DR: In this paper, the physical properties of thiophenylacetylene and benzothiazolylacetylene derivatives were evaluated and compared with analogous phenylacetyl-based compounds.
Abstract: We have synthesized and evaluated the physical properties of thiophenylacetylene and benzothiazolylacetylene derivatives in order to obtain higher Δn liquid crystals as compared with analogous phenylacetylene-based compounds. These new materials had poorer liquid crystallinity than the phenylacetylene derivatives. Exchanging the phenyl ring for a benzothiazole ring was effective in increasing Δn, whereas a thiophene ring decreased Δn because of the associated reduction in the anisotropic polarizability. These conjugated hetero rings shifted the UV absorption spectra into the visible region such that they are yellow or brownish in colour.
57 citations
TL;DR: In this article, a chiral non-symmetric dimeric liquid crystal consisting of a cholesteryl ester moiety as chiral entity and a biphenyl aromatic core, interconnected through n-butyl (C4) or n-pentyl parity alkylene spacers, have been synthesized and investigated for their liquid crystalline properties.
Abstract: Chiral non-symmetric dimeric liquid crystals consisting of a cholesteryl ester moiety as chiral entity and a biphenyl aromatic core, interconnected through n-butyl (C4) or n-pentyl (C5) parity alkylene spacers, have been synthesized and investigated for their liquid crystalline properties. All the dimers exhibit enantiotropic mesophases. The first member of the dimers having the C4 central spacer exhibit only the chiral nematic (N*) mesophase, while the higher homologues also show smectic A (SmA) and twist grain boundary (TGB) mesophases. The dimers of the other series containing the C5 central spacer also have stable SmA, TGB and N* mesophases, except for the first which does not show the TGB phase. Both series of compounds show a weak odd-even effect with terminal alkyl chain substitution, while the spacer length has a marked influence on the phase transition temperatures.
TL;DR: In this article, the pendant drop method was used to measure the surface tension of alkylcyanobiphenyls (nCBs) in the nematic liquid crystal and isotropic phases.
Abstract: The temperature dependence of the surface tension was measured by the pendant drop method for four compounds from the homologous series of alkylcyanobiphenyls (nCB), in the nematic liquid crystal and isotropic phases. For 8CB (octylcyanobiphenyl) the temperature dependence was also measured in the smectic range. Not very close to the isotropic transition temperature, and with the exception of 8CB, the surface tension decreases with increase in temperature in the nematic range. A downward jump at the transition temperature was observed for all liquid crystals studied. The shape of the drop in the smectic A phase of 8CB gives indications of stratification in a system of terraces.
TL;DR: In this article, a 3-ring phenylacetylene liquid crystal with high birefringence values (Δn) was synthesized and investigated to improve the nematic temperature range while retaining a high optical anisotropes.
Abstract: We have synthesized and investigated new 3-ring phenylacetylene liquid crystals with high birefringence values (Δn) to improve the nematic temperature range while retaining a high optical anisotrop...
TL;DR: In this paper, a set of trimesogens consisting of a cholesteryl ester, a biphenyl and an azobenzene mesogenic segment joined together through either odd-even, even-even or odd-odd central alkylene spacers has been synthesized and the liquid crystalline properties investigated.
Abstract: A new set of trimesogens consisting of a cholesteryl ester, a biphenyl and an azobenzene mesogenic segment joined together through either odd-even, even-even or odd-odd central alkylene spacers has been synthesized and the liquid crystalline properties investigated. The trimesogens having even-even and odd-odd central alkylene spacers exhibit the undulated twist grain boundary phase (UTGBC*) mesophase over a temperature range of ~40-50°C.
TL;DR: In this article, a procedure for the extension of the aromatic core by Suzuki coupling is described, which minimizes competitive reductive dehalogenation, and new discogens based on polyphenylated triphenylene and hexabenztrinaphthylene are synthesized.
Abstract: We have synthesized new discogens based on polyphenylated triphenylene, polyphenylated azatriphenylene and hexabenztrinaphthylene. A procedure for the extension of the aromatic core by Suzuki coupling is described, which minimizes competitive reductive dehalogenation.
TL;DR: In this paper, the smectic properties of four series of dimer liquid crystals containing cholesteryl and biphenylyl groups were investigated by polarization microscopy, DSC and X-ray diffraction.
Abstract: The smectic properties of four series of dimer liquid crystals containing cholesteryl and biphenylyl groups were investigated by polarization microscopy, DSC and X-ray diffraction. Compounds that contain the strongly dipolar cyanobiphenylyl group exhibit a smectic layer spacing that is about 1.7 times the length of the molecule. Three series of alkoxybiphenylylcontaining dimers exhibit two other smectic modifications. For short spacers, a smectic layer spacing is observed that is about the same length as the molecule and for long spacers the smectic layer spacing is about half the length of the molecule. In the latter cases the entropy change at the SmA-N* transition is clearly larger than for the compounds with the other smectic modifications. One or two compounds in each alkoxybiphenylyl-containing series, that have similar spacer length and terminal group length and have an odd number of flexible units in the spacer, exhibit only a chiral nematic phase, and no smectic phase.
TL;DR: In this paper, the dielectric properties of nine members of the nBT (4n-alkyl-4thiocyanatobiphenyl, n= 2-10) homologous series in the crystal E (E) and isotropic (I) phases are presented.
Abstract: Results of the dielectric studies of nine members of the nBT (4-n-alkyl-4'-thiocyanatobiphenyl, n= 2-10) homologous series in the crystal E (E) and isotropic (I) phases are presented. The dependence of the static permittivity ∊s in the isotropic phase, the longitudinal relaxation times τis and τ∥, and the activation enthalpies ΔH is and ΔH ∥ on the number of carbon atoms in the alkyl chain n are analysed. A considerable increase in the retardation factor g= τ∥/τis with decreasing n is observed. The results are compared with those obtained for similar two-ring homologous series. The parameters characterizing the molecular rotations around the short axis in the E phase (τ∥ and ΔH ∥) indicate a hardening of this solid-like phase with shortening of the alkyl chain.
TL;DR: In this paper, the absorption spectra of triphenylene, perylene, and pyrene are investigated both experimentally and theoretically, and the spectra calculated using the density functional tight binding (DFTB) theory are in good agreement with experimental results.
Abstract: New derivatives of triphenylene, perylene, and pyrene are described, some of which form columnar mesophases. The absorption spectra are investigated both experimentally and theoretically. The spectra calculated using the density functional tight binding (DFTB) theory are in good agreement with experimental results. The investigated compounds show photoluminescence of violet-blue (triphenylene), yellow-green (pyrene), and orange-red (perylene) colours. In addition, electroluminescence is observed in thin films of these compounds between a positively charged ITO electrode and a negatively charged aluminium electrode. The brightness of the electroluminescence decreases in the order perylene > pyrene > triphenylene. Threshold voltages below 20 V and a luminance up to 100 cd m-2 were observed.
TL;DR: In this paper, a homologous series of banana-shaped compounds (n = 1-12) is described, where the short-chain derivatives exhibit a B6 phase, while B6/B1 dimorphism is found for the pentyloxy and hexyloxy homologues.
Abstract: A homologous series of banana-shaped compounds (n =1-12) is described. The short-chain derivatives exhibit a B6 phase, while B6/B1 dimorphism is found for the pentyloxy and hexyloxy homologues. The B1 phase of the compounds having longer alkyloxy chains is changed to a B2 phase for the compounds with n =11 and 12. The phase characterization was performed using their optical textures and by means of X-ray investigations. Transition enthalpies were measured using DSC. In addition, density measurements were performed on one selected homologue. The compound n =8 exhibiting a B1 phase was investigated also by high-pressure DTA.
TL;DR: In this article, the surface tension of smectic liquid crystals was determined based on the curvature pressure of the surface of the material drawn on communicating vessels, where the curvatures of the surfaces of communicating vessels were used to measure surface tension.
Abstract: We determine surface tensions σ of smectic liquid crystals from the curvature pressure of smectic films. A new technique is introduced for the comparison of surface tensions of different smectic materials. The method is based on the relation of curvatures of smectic films drawn on communicating vessels. The measurement of the temperature dependence of σ reveals anomalies in the vicinity of phase transitions to low temperature smectic modifications. This anomalous slope dσ/dT can be related to the surface excess entropy of the material in the corresponding temperature range. The surface tension values determined for a number of mesogens fit well into the classification proposed by Mach et al.
TL;DR: In this paper, it was shown that the dielectric anisotropy of conductivity in cells composed of nematic liquid crystal E7 (NLC-E7) is related to the fact that the diffusion constant (D) is greater in a cell with homeotropic alignment than in one with homogeneous alignment.
Abstract: This paper shows that the dielectric anisotropy of conductivity in cells composed of nematic liquid crystal E7 (NLC-E7) is related to the fact that the diffusion constant (D) is greater in a cell with homeotropic alignment than in one with homogeneous alignment (D ∥ > D ⊥). This behaviour can be understood by the study of the dielectric properties of the NLC based on the ionic hopping behaviour and on the analysis of the electrical conductivity in relation to the voltage applied.
TL;DR: In this article, the authors report optical, X-ray, polarization and response time measurements on a new "bananas-haped" mesogen, which has a salicylaldimine segment in the linear part of the core.
Abstract: We report optical, X-ray, polarization and response time measurements on a new 'bananas-haped' mesogen. The material has a salicylaldimine segment in the linear part of the core, but is otherwise structurally similar to the mesogen first reported to show a banana mesophase. Textural and X-ray studies show that this mesophase has all the characteristics of the recently discovered B7 phase. It is observed that the voltage required to obtain electrical switching is quite low compared with values for the few other materials exhibiting the B7 phase.
TL;DR: In this article, a series of intermolecular hydrogen bonding complexes, 2,2'-bipyridine: p-n-alkoxybenzoic acids (BP : nABA) was isolated from liquid crystalline P-nalkoxy benzoic acid (nABA), where n represents alkoxy carbon numbers, 3 to 10 and 12).
Abstract: A series of intermolecular hydrogen bonding complexes, 2,2'-bipyridine: p- n-alkoxybenzoic acids (BP : nABA) was isolated from liquid crystalline p-n-alkoxybenzoic acids (nABA) (where n represents alkoxy carbon numbers, 3 to 10 and 12) and a non-mesogen, 2,2'-bipyridine (BP). The thermal and phase behaviour of these complexes was studied by thermal microscopy and differential scanning calorimetry (DSC). These studies revealed the induction of tilted smectic F and crystal G phases. The structural elucidation pertaining to the formation of intermolecular hydrogen bonding was carried out by a detailed IR spectral investigation. Comparative crystallization kinectic studies using DSC, performed on two representative compounds, showed different forms of the crystallization process. The magnitudes of the Avrami exponent n suggests two different mechanisms operate for individual members leading to sporadic three-dimensional growth. Nevertheless, an overall unique mechanism is predicted to operate at each crystall...
TL;DR: In this paper, four mesogenic homologous series have been synthesized by fixing a rigid 4-substituted phenylazo group to a resorcinol moiety.
Abstract: Four mesogenic homologous series have been synthesized by fixing a rigid 4-substituted phenylazo group to a resorcinol moiety. In series I and II, one phenolic -OH group is esterified by 4-n-alkoxybenzoyl groups; in series III and IV both the phenolic -OH groups are esterified by 4-n-alkoxybenzoyl groups. All the homologues of series III and IV exhibit low melting smectic C phases. Monoesters of series I and II, having free lateral hydroxy group with strong hydrogen bonding, exhibit high temperature nematic phases. The effect of different substituents on mesomorphic properties is discussed. One homologue of series III was doped with a chiral dopant and its spontaneous polarization evaluated.
TL;DR: The synthesis and liquid crystalline properties of new Schiff's bases (series 3a-f) and azo compounds (4a,f) incorporating pyridine and 1,3,4thiadiazole rings are reported in this article.
Abstract: The synthesis and liquid crystalline properties of new Schiff 's bases (series 3a-f) and azo compounds (4a-f) incorporating pyridine and 1,3,4-thiadiazole rings are reported. The first homologues in the series of Schiff's bases (3a-c) show a monotropic mesophase and the homologues 3d-f display an enantiotropic SmA phase. The only azo compounds to exhibit liquid crystalline properties are the homologues 4e, f which show a monotropic nematic phase. These series are compared with Schiff's bases and azo compounds analogues (series 5 and 6), previously reported by us and an AM1 study of the structure/mesomorphic property relationship is described.
TL;DR: In this article, a new type of non-symmetric dimesogen has been built through self-assembly via intermolecular hydrogen bonding between appropriately designed H-bond donor (3-cholesteryloxycarbonylpentanoic acid) and acceptor (aromatic mesogen with a pyridyl group).
Abstract: Non-symmetric dimesogens are composed of two different mesogenic units linked via a flexible spacer. In this study, a new type of non-symmetric dimesogen has been built through the self-assembly via intermolecular hydrogen bonding between appropriately designed H-bond donor (3-cholesteryloxycarbonylpentanoic acid) and acceptor (aromatic mesogen with a pyridyl group) moieties. As for covalently linked dimesogens, several types of smectic periodicities are observed for these H-bonded cholesteryl compounds depending on the length of the terminal chain of the acceptor moiety: a smectic periodicity resulting from associated dimesogens is observed for long terminal chains, while short chain homologues display an intercalated structure corresponding to half the molecular length. The competition between these two incommensurate lengths can induce an incommensurate smectic phase where the two smectic periodicities coexist at long range.
TL;DR: In this paper, the existence of smectic C and nematic mesophases of 4-n-alkoxycinnamic acids with 4-4'-bipyridine (BPy) was demonstrated by a combination of polarized optical microscopy and X-ray diffraction (XRD).
Abstract: The hydrogen-bonded liquid crystalline complexes of 4-n-alkoxycinnamic acids (nCNA: n=4-8, 10, 12, 16, 18, where n is the number of carbons in the alkoxy chain) with 4,4'-bipyridine (BPy) have been prepared and characterized. The existence of smectic C, smectic A, and nematic mesophases of these complexes was demonstrated by a combination of polarized optical microscopy and X-ray diffraction (XRD). In this H-bonded mesogenic structure, nonmesogenic BPy functions as the core unit of the mesogen through the H-bond, as confirmed by infrared spectroscopy and XRD. A general comparison of nCNABPy with the benzoic acid analogues (nOBA)2-BPy, showed that the elongated nCNABPy mesogen behaves as a rod unit and increases the transition temperature. Smectic phases in nCNAs (5≤ n≤ 12) were induced on complexation with BPy. The favouring of smectic phases in these complexes is believed to originate from the increment of polarity of the mesogen by intermolecular H-bonding. With increasing chain length of the nCNABPy co...
TL;DR: In this article, new terthiophene derivatives containing an alkylamide group, N,N'-distearyl-2,2' : 5',2"-terthiopane-5,5"-dicarboxamide (DNC183T), were designed and synthesized, and their thermal behaviour examined.
Abstract: With the aim of developing new oligothiophene-based liquid crystals involving hydrogen bonding, new terthiophene derivatives containing an alkylamide group, N,N'-distearyl-2,2' : 5',2"-terthiophene-5,5"-dicarboxamide (DNC183T) and N,N'-dialkyl-5,5"-dibromo-2,2' : 5',2"-terthiophene-4,4"-dicarboxamides (DNCnDBr3T, n = 5, 8, 16, 18), were designed and synthesized, and their thermal behaviour examined. While DNC183T did not exhibit liquid crystallinity, DNCnDBr3T compounds with n = 8, 16, 18 were found to form smectic A phases. In addition to liquid crystal behaviour, crystal polymorphism was also observed for DNC16DBr3T. It is shown that both the position of the amido group and the alkyl chain length greatly affect liquid crystal phase formation. The absence of liquid crystallinity in the corresponding ester derivatives suggests that intermolecular hydrogen bonding also plays a role in the formation of liquid crystal phases in the DNCnDBr3T system.
TL;DR: In this article, the dynamics of ferroelectric liquid crystals confined in porous nitrocellulose membrane (Synpor) are investigated by broadband dielectric spectroscopy, and a characterization of the mesomorphism is achieved by DSC and X-ray scattering.
Abstract: The dynamics of ferroelectric liquid crystals confined in porous nitro-cellulose membrane (Synpor) are investigated by broadband dielectric spectroscopy, and a characterization of the mesomorphism is achieved by DSC and X-ray scattering. The complex inner membrane structure of random interconnected voids can be treated as a frozen random defect disorder in the mesophases. Its influence on the molecular dynamics of the confined mesogens is negligible. However, Goldstone and soft modes of DOBAMBC (2-methylbutyl 4-(4-decyloxybenzylideneamino)cinnamate) are suppressed by the confinement and not detected dielectrically. In the short pitch and high spontaneous polarization material C7, 4-(3-methyl-2-chloropentanoyloxy)-4'-heptyloxybiphenyl, confinement has a minor influence on the soft mode, and the Goldstone mode is clearly present, although with reduced strength, broadened with respect to the bulk, and shifted to higher frequencies by more than one decade. The results are interpreted as effects of random conf...
TL;DR: In this article, the effect of different lateral substituents on the type of mesophase formed is examined and a comparison made with unsubstituted compounds is made with different compounds composed of banana-shaped molecules.
Abstract: Several compounds composed of banana-shaped molecules and exhibiting mesophases are reported. The effect of different lateral substituents on the type of mesophase formed is examined and a comparison made with unsubstituted compounds. The mesophases have been characterized using techniques such as polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction. The B2 phase shows antiferroelectric switching behaviour.