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New hockey stick-shaped mesogens with a lateral methyl group inserted between the m-alkyloxy-chain and the azomethine connecting group, 4-(3-n-alkyloxy-2-methyl-phenyliminomethyl) phenyl-4-n-alkyloxycinnamates have been synthesised. The interesting feature of these hockey stick-shaped mesogens is the appearance of the nematic phase in addition to two polymorphic tilted smectic phases - the synclinic smectic C (SmCs) and the anticlinic SmCa phase. Physical characterisation of these mesogens has been carried out using polarising optical microscopy, differential scanning calorimetry, optical birefringence, X-ray diffraction (XRD), electro-optical and dielectric spectroscopy measurements. The SmCs-SmCa phase transition is accompanied by only a small calorimetric signal but causes a pronounced change in the optical textures. Orientational order parameters determined from the birefringence measurements have been compared with those estimated from XRD patterns. Dielectric measurements point to the formation of small 'soft' ferroelectric clusters responsible for a low frequency absorption range. [ABSTRACT FROM AUTHOR]

The isobaric phase diagram for a binary system of liquid crystals is presented. One mixing component is a bent-core mesogen forming a B7 phase and the other one is a nematogenic calamitic compound. The occurring mesophases have been identified by their optical textures as well as by X-ray investigations. The B7 phase exists down to about 60 mol% of the bent-core compound A / 40 mol% of the calamitic compound B. Two additional mesophases are induced in the mixed phase region. At medium concentrations (around 50 mol%) an oblique columnar phase appears which shows a similar X-ray pattern to the B7 phase but an utterly different nucleation on cooling the isotropic liquid and a completely dissimilar texture. At concentrations below 35 mol% A, a further phase is induced which possesses an irregularly modulated layer structure. This phase can be regarded as an intermediate state between the regularly modulated Colr (B1) phase and the intercalated B6 phase. It transforms on cooling into the phase with a regularly modulated structure. [ABSTRACT FROM AUTHOR]

Twelve new five-ring bent-core compounds have been synthesised which are derived from 2-nitroresorcinol and which contain only ester linkage groups. Their mesophase behaviour was investigated by polarising microscopy, X-ray diffraction and electro-optical measurements. It was found that most of the compounds studied form a B7 phase. For several compounds, a phase transition between a B7 high-temperature phase (B7H) and a B7 low-temperature phase (B7L) was clearly detected by calorimetry, by a small change of the texture and by X-ray diffraction measurements. For one compound, (A12), well-aligned samples were obtained which allowed a more detailed structural characterisation of the B7 phases. Interestingly, by fast cooling of the isotropic liquid, the same compound forms a metastable columnar phase. Furthermore, the binary system composed of the compound A12 and a member of the B7 parent series was investigated. X-ray diffraction measurements give evidence that the B7 phases of both mixing components are completely miscible, i.e. they belong to the same phase type. [ABSTRACT FROM AUTHOR]

Three new homologous bent-shaped mesogens with terminal octyloxy P8, decyloxy P10 and dodecyloxy P12 groups were synthesized. Calorimetric, X-ray and electro-optic investigations prove the existence of two mesophases of the SmCPA type for compound P10 but only one SmCPA phase for the homologues P8 and P12. The static dielectric permittivities of compound P12 and that of the low-temperature phase of substance P10 are in the order of 30 and agree with results on bent-core mesogens published before. Surprisingly, the dielectric permittivities of P8 and of the high-temperature phase of P10 are of about 1000. We explain this extremely high dielectric response with the soft reaction of ferroelectric units in the external electrical field. [ABSTRACT FROM AUTHOR]

The physical behaviour, especially the dielectric properties, of two series of isomeric compounds formed by banana-shaped molecules is reported. The feature of all liquid crystalline substances investigated is the formation of a ferroelectric short range order. In one layer of the SmCPA phase the spontaneous polarisation shows in one direction and in the following in the opposite resulting in an antiferroelectric behaviour. It is demonstrated that the molecular origin of the polarization is connected with the perpendicular dipole moments. From the two relaxation modes seen the first results from ferroelectric units and the second from the reorientation of the molecules about the long axes. The collective character of both motions is demonstrated. The same mechanisms appear in the columnar phase, but they behave in a different way due to the different structure. Strong changes of the dielectric properties are observed at the transition from the isotropic in the liquid crystalline phase, but astonishingly not between both mesophases. [ABSTRACT FROM AUTHOR]

A bent-core mesogenic compound was used for preparation of thin solid Langmuir films (25–100 nm) that were transferred onto glass substrates and supplied with Al electrodes evaporated in vacuum. The Langmuir-Blodgett technique allowed us studying the thin polar films of the banana-shaped liquid crystal in sandwich geometry that keep amazing integrity on heating, even up to high-temperature B2 phase, and show antiferroelectric and ferroelectric properties, which depend on film thickness. The magnitude of the measured switched polarization (400 nC/cm2) is similar to that of the bulk material. The strong confinement of film results in high coercive field which reaches the value of 108 V/m for the thinnest film. The investigations of dielectric properties are reported. [ABSTRACT FROM AUTHOR]

A new smectic type mesophase without in-plane order is presented which is formed by nitro substituted banana-shaped compounds. The structure of this phase is not yet known in detail. The growth of germs and also the textures point to a helicoidal structure. The helical structure is obviously the result of the chirality of the smectic layers which is caused by the tilt of the moleonles together with the polar arrangement of the angled molecules. [ABSTRACT FROM AUTHOR]

The article studies bent-core mesogens formed by achiral banana-shaped molecules. The study found out that the X-ray pattern showed that liquid crystal feature unusual textures. Researchers describe the finding analysis as banana phase, commonly called as B7 [3, 4]. It also found out that B7 phase demonstrates the transition of temperature as well as the reduction of permitive dynamics of the achiral structure.

Kojic acid was used to synthesize bent‐core mesogens. Antiferroelectric polar smectic mesophases, an oblique and a rectangular columnar mesophase were found. The mesophases were characterized by X‐ray scattering, electro‐optical measurements, polarizing microscopy and DSC. On the base of these measurements, models of the mesophases are proposed. [ABSTRACT FROM AUTHOR]

We prepared novel macromolecular architectures of main-chain polymers poly(1)–poly(6) by a ruthenium-catalyzed acyclic diene metathesis (ADMET) polymerization of banana diene monomers 1–6. Copolymerization with a calamitic diacrylate monomer 7 yielded a copolymer poly(1-alt-7) in which the banana and the calamitic units regularly alternated along the polymer main chain. We also started the structural characterization of the polymers and found a varied mesophase behavior depending on the nature of the banana mesogen incorporated. [ABSTRACT FROM AUTHOR]

Null transmission ellipsometry and depolarized reflected light microscopy have been performed on freestanding films of two achiral meta-substituted three-ring compounds. These compounds exhibit two different tilted smectic phases, SmC1 and SmC2. Our studies confirmed previous results obtained from another achiral meta-substituted three-ring compound that the SmC1 and SmC2 phases are SmC and SmCA, respectively. [ABSTRACT FROM AUTHOR]

Two new compounds containing bent-core mesogens are presented in which a flexible central methyleneoxycarbonyl unit acts as the bent fragment. The compounds form different two-dimensional columnar ('B 1 ') phases both with a rectangular lattice but different in the angle of the director of the molecules with respect to the a -axis of the lattice. Based on X-ray diffraction measurements structural models are proposed which cannot be described with the common model of p 2 mg symmetry for B 1 phases of bent-shaped molecules. The new models are of layer group symmetry ( p 11 n and pm 2 1 n ) and are discussed as a new common model for two-dimensional columnar phases of bent-shaped mesogens. [ABSTRACT FROM AUTHOR]

A new series of five-ring banana-shaped mesogens derived from isophthalaldehyde was synthesized and investigated. Some of the compounds exhibit a B 2 -B 4 dimorphism. The mesophases were characterized alternatively by optical methods, X-ray studies, and dielectric and electro-optical measurements. [ABSTRACT FROM AUTHOR]

New meta -substituted homologous three-ring mesogens, the 4-(3- n -decyloxyphenyliminomethyl) phenyl 4- n -alkyloxybenzoates, have been synthesized, which are non-linear due only to the attachment of one of the alkyloxy groups in a meta -position. The mesophases were studied by optical microscopy, differential scanning calorimetry, NMR spectroscopy, X-ray diffraction, and electro-optical and dielectric measurements. Unusual phase behaviour was observed on varying the length of the terminal chain. The most interesting finding is the occurrence of two polymorphic tilted smectic phases designated as SmC 1 and SmC 2 . The existence of these phases was revealed by calorimetric studies and also from the pronounced difference in optical textures. It was shown from NMR measurements that the molecular orientation changes from a synclinic to an anticlinic arrangement in the SmC 1 to SmC 2 phase transition. It has also been shown, using NMR, that the SmC 1 → SmC 2 phase transition in these compounds is accompanied by a conformational change in the molecular fragment containing the aromatic ring with the meta -substituted terminal alkyloxy chain. This conformational change is linked to a change in the shape of the molecules and leads to a different packing of the molecules within the layers of the SmC 2 phase. From dielectric measurements an increase by a factor of two was detected in the molecular mobility at the transition into the low temperature SmC 2 phase. This finding supports a change in the packing as result of conformational changes. [ABSTRACT FROM AUTHOR]

Dielectric investigations on a magnetically oriented sample forming N, SmA, SmC and B[sub 2] phases were carried out. The sample shows a negative dielectric anisotropy in the N state and a strong increase of the static dielectric constants perpendicular to the director in the ensuring 'classical' phases. This proves directly the strong and continuous increase of the positive dipole correlation in the lateral direction due to the steric interactions of the bent-shape molecules. From a dynamical point of view, the SmA and SmC phases of this sample show deviations from the classical phases of rod-like molecules. Crystallization did not allow us to extend the dielectric measurements into the B[sub 2] state. [ABSTRACT FROM AUTHOR]

We report an electro-optic investigation of a freely suspended LC film formed by bent-core ( banana-shaped ) mesogenic molecules. The B 2 phase investigated is antiferroelectric, however, a weak nonzero spontaneous polarization couples to the external in-plane electric field and can be used to reorient the anisotropic material. When the polarity of the external field is reversed, local 360° domain walls form the source of kinks which propagate into the unstable domains. The shape of these kinks and their propagation velocity is determined and compared with numerical calculations of the dynamic equations. Viscous and elastic properties of the material are derived from the comparison of theory and experiment. [ABSTRACT FROM AUTHOR]

The ground state of the novel liquid crystal phase formed by achiral banana-shaped nitro-molecules and known as B7 is not understood. While spirals and myelins spontaneously appear on cooling from the isotropic liquid, neither mature to form B7 monocrystals. We tried suppressing the flow associated with these growth transients by applying large electric fields to thin samples (1-4μm thick). While a field untwists some spirals and coarsens the less mobile myelin textures, it stimulates even more flow and a B7 texture with variable but exceedingly small length scales. [ABSTRACT FROM AUTHOR]

We have performed resonant X-ray scattering on two bent-core liquid crystal compounds exhibiting the B2 phase using three sample geometries: free-standing film and two others with free surface arrangements. The results conclusively demonstrate the two-layer orientational periodicity in this phase suggested by optical studies. We have performed the first resonant scattering experiments on liquid crystals at the chlorine K-edge, opening up a new class of compounds for resonant X-ray scattering studies. Furthermore, we have achieved an excellent alignment of the B2 phase with a free surface. [ABSTRACT FROM AUTHOR]

Three phase diagrams of binary systems are presented where one of the mixing components is a bent-core mesogen and the other is a calamitic mesogen. In two of these systems transitions between the 'banana phase' B[sub 2] and more usual smectic phases in the sequences B[sub 2]-SmA, B[sub 2]-SmC, B[sub 2]-SmC-SmA, B[sub 2]-SmC-SmA-N could be observed. The mesophases have been identified by their textures, by X-ray investigations and by electro-optical measurements. [ABSTRACT FROM AUTHOR]

A new achiral banana-shaped five-ring 2-methylresorcinol derivative fluorinated on the outer rings has been synthesized. This compound exhibits four antiferroelectric smectic phases characteristic for bent-core molecules (B[sub 2], B′[sub 2], B′′[sub 2], B[sub 5]). In addition, a highly viscous solid-like phase has been detected which also shows electro-optical switching. This is the first switchable highly ordered B phase. The characterization of the mesophases was made by X-ray diffraction, electro-optical measurements and dielectric spectroscopy. [ABSTRACT FROM AUTHOR]

A detailed study of the field induced polarization switching has been carried out over the whole temperature range of the antiferroelectric B2 phase of a banana-shaped compound, the tetradecyl homologue (n=14) of the series 4-chloro-1,3-phenylene bis[4-(4-n-alkylphenylimino)benzoate]s. The fine structure of the current response oscillograms was attributed to the field switching of two antiferroelectric subsystems, most probably the racemic and homogeneously chiral domains which, according to the Boulder group, appear as a result of the spontaneous breaking of mirror symmetry. The temperature and field dependences of polarization as well as switching times and the corresponding viscosity of the substance have been measured. [ABSTRACT FROM AUTHOR]

The first clear experimental evidence of a dielectric relaxation in the B[sub 7] phase is given. The associated molecular process is probably the reorientation of the bent-shaped molecules about the long molecular axis. It is also shown that the dielectric constant strongly increases at the phase transition I-B[sub 7] pointing to a co-operative motion of ferroelectric clusters. Furthermore, a phase transition B[sub 7]-B[sub X] is observed. The values of about 10 for the high frequency limit of the dielectric constants of the B[sub 7] and B[sub X] phases and the static dielectric constants of the isotropic phase are in agreement. Thus, in all the phases the main dipoles can reorient faster than 10[sup -8]s, i.e. the experimental limit. Only in the supercooled B[sub X] phase were the dynamics of the transversal dipoles measured. Using atomic force microscopy, focal-conic domains modulated by parallel lines have been observed at room temperature. [ABSTRACT FROM AUTHOR]

A homologous series of banana-shaped compounds (n =1-12) is described. The short-chain derivatives exhibit a B[sub 6] phase, while B[sub 6]/B[sub 1] dimorphism is found for the pentyloxy and hexyloxy homologues. The B[sub 1] phase of the compounds having longer alkyloxy chains is changed to a B[sub 2] 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 B[sub 1] phase was investigated also by high-pressure DTA. [ABSTRACT FROM AUTHOR]

Examines the polymorphic smectic A phases in perfluoroalkylated mesogenic dimers. Dependence of mesophase behavior on length and structure of spacer; Occurrence of smectic A phases in longer chains; Influence of high enthalpy on transition of dimorphic smectic A phases.

Banana-shaped five-ring mesogens derived from either isophthalic acid or 4-bromoisophthalic acid have been synthesized. It is shown that the melting and clearing points of the bromine-substituted compounds are markedly decreased in comparison with the analogous unsubstituted compounds. On the basis of X-ray diffraction studies and NMR measurements, as well as dielectric and electro-optical investigations, the mesophase of these compounds could be identified as a B[sub 2] phase. [ABSTRACT FROM AUTHOR]

D. Vorländer 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 Vorländer also synthesized some compounds with a bent molecular shape. In this paper we show that one of these compounds indeed forms a mesophase (B[sub 6]) which is characteristic for the banana-shaped molecular structure. [ABSTRACT FROM AUTHOR]

Dielectric measurements on a sample consisting of banana-shaped molecules were carried out in a frequency range between 0.1 Hz and 10 MHz. The sample exhibited B[sub 2] and B[sub 4] phases. As usual, two ranges of relaxation were detected in the B[sub 2] phase, the fast reorientation about the long axes of the molecules and a slow collective process. In the B[sub 4] phase, only one dielectric active process at low frequencies was found; this does not differ from the low frequency relaxation of the B[sub 2] modification. This relaxation is probably related to the dynamics of superstructures. Crystallization could be observed after keeping the sample for a longer time at higher temperature. Thus, it was possible to differentiate clearly between the crystalline and the B[sub 4] phases. AFM investigations prove the existence of focal-conic domains and periodic superstructures in the B[sub 4] phase; then do not appear in the crystalline state. For this reason the B[sub 4] phase is regarded as different from a classical crystalline phase. [ABSTRACT FROM AUTHOR]

Dielectric measurements on two samples formed from banana-shaped molecules with terminal alkyl or alkyloxy groups were carried out in a frequency range between 10[sup -3] Hz and 10 MHz. Both samples exhibit B[sub 2] and B[sub 3] phases; one of them has a B[sub 4] state as well. As usual, two ranges of relaxation were detected in the B[sub 2] phase, the fast reorientation about the long axes of the molecules and a slow collective process. Only one dielectrically active low frequency process could be separated in the B[sub 3] and B[sub 4] phases; this is probably related to the dynamics of superstructures. The high frequency limit of the dielectric constants gives a hint that neither phase is a classical solid. A quite strong increase in the conductivity at the transition into the B[sub 3] phase is critically discussed. [ABSTRACT FROM AUTHOR]

The formation of coils is common in nature when achiral symmetry breaking occurs. Here we describe spectacular examples of single, double and triple coils observed in smectic liquid crystal phases of achiral banana-shaped molecules. Such molecules form chiral smectic phases due to two symmetry-breaking instabilities: polar molecular packing, and molecular tilt. The appearance of helical filaments at the isotropic-smectic transition is therefore a direct indication of the achiral symmetry-breaking of the smectic structures. The number of observed left- and right-handed domains is equal, reflecting the achiral nature of the constituent molecules. Our studies indicate that the helical filaments consist of concentric smectic layers. The coiling stabilizes the growth process and suppresses the penetration of molecules from the isotropic phase, leading to moving of the tip with constant speed. [ABSTRACT FROM AUTHOR]

Materials consisting of asymmetric banana-shaped molecules may form a smectic C[sub G] phase having C[sub 1] symmetry. We have studied textural transformations in a smectic phase of an asymmetric chlorine-substituted banana-shaped material under electric and mechanical fields. We observed two novel features that have not been observed so far on corresponding materials with symmetric banana-shaped molecules. These observations, however, could be explained by the same arguments as were used for the symmetric molecules. Although our studies do not exclude the possibility that the material has C[sub 1] symmetry, we suggest that the chlorine molecules are positioned arbitrarily and the bulk has C[sub 2] symmetry. [ABSTRACT FROM AUTHOR]

A new achiral five-ring banana-shaped mesogen is presented which has a cyano substituent in the 4-position of the central core. Microscopical, X-ray, dielectric and electro-optical investigations give evidence for the unusual phase sequence B[sub 2], SmC, SmA. The B[sub 2] phase shows an antiferroelectric switching behaviour; the spontaneous polarization was found to be 330 nCcm[sup -2]. [ABSTRACT FROM AUTHOR]

Several banana-shaped liquid crystal materials with perfluorinated terminal chains have been synthesized. A dimorphism SmA-B[sub 2] was proved by X-ray studies and electro-optical measurements. The unusual ratio d/L (d = layer spacing, L = molecular length) and the unexpected behaviour of the layer spacing at the phase transition SmA –> B2 can be interpreted by a change of the packing of the bent molecules. [ABSTRACT FROM AUTHOR]

Seven homologous symmetric dimeric liquid crystals are presented where two three-ring mesogenic units are connected by a bis(carbonyloxy)alkylene spacer. For homologues with terminal heptyloxy, octyloxy and nonyloxy chains, the unusual phase sequence SmA-SmC with increasing temperature was detected by the study of the optical textures and by X-ray diffraction measurements. This unusual behaviour is discussed on the basis of the molecular structure of these compounds. [ABSTRACT FROM AUTHOR]

In five double-swallow-tailed compounds a weakly birefringent high temperature phase occurs which exhibits a characteristic optical texture. On the basis of X-ray diffraction measurements on well oriented monodomains, the structure of this phase could be described by a rhombohedral lattice of space group R3c or R32/c; the lattice parameters are not far from a cubic lattice. In spite of the three dimensional superstructure with long range character, the lateral packing of the molecules is liquid-like - similar to the well known cubic mesophases. [ABSTRACT FROM AUTHOR]

Two strongly polar 4-(4-cyanobenzyloxy)benzylidene 4- n -alkylanilines were synthesized for the first time and mixed with the hydrophilic aerosil A 300. The compounds, showing a rich polymorphism, and the mixtures were investigated by calorimetric and dielectric methods. Phase transitions of first and second order were detected. Neither the phase transition temperatures nor dielectric relaxation times are strongly influenced by addition of aerosil up to 20%. [ABSTRACT FROM AUTHOR]

Six members of a new homologous series of achiral banana-shaped molecules have been synthesized and studied by optical microscopy, differential scanning calorimetry, NMR spectroscopy and X-ray diffraction. According to X-ray diffraction measurements on oriented samples, four homologues form an XB2 phase behaviour. From electro-optical studies the spontaneous polarization and the tilt angle could be measured. An orientational order parameter of 0.8 was determined by 13C NMR and this is nearly independent of the temperature. NMR investigations also give information about the real conformation of the molecules in the XB2 phase. Dielectric measurements indicate that the rotation around the molecular long axis is clearly hindered because of the packing of the bent molecules within the smectic layers. which exhibits antiferroelectric switching. [ABSTRACT FROM AUTHOR]

A new smectic type mesophase without in-plane order is presented which is formed by nitro substituted banana-shaped compounds. The structure of this phase is not yet known in detail. The growth of germs and also the textures point to a helicoidal structure. The helical structure is obviously the result of the chirality of the smectic layers which is caused by the tilt of the molecules together with the polar arrangement of the angled molecules. [ABSTRACT FROM AUTHOR]

A group of 4-[4-(4-n-octyloxybenzoyloxy)benzoyloxy]benzylidenemalonic acid bis(perfluoralkyl) esters and 4-{4-[2,2-bis(n-octyloxycarbonyl)ethenyl]benzoyloxy}benzoic acid 4-perfluoro- alkyloxyphenyl esters with varied lengths of the perfluorinated chains were investigated by X-ray measurements. The liquid crystalline behaviour (SmA, SmC, Col) and the structures of the phases are described. The substances show the influence of the steric interaction caused by the wedge shape of the molecules and the repulsion caused by chemically incompatible segments (fluorophobic effect). In the first group of compounds investigated the steric interaction clearly dominates the structure, so that an antiparallel packing of the molecules in SmA and SmC phases results. In the second group, the influence of the incompatibility is more and more enhanced and a low temperature phase with a columnar structure arises in which the incompatible parts are separated on a microscopic scale. [ABSTRACT FROM AUTHOR]