S. Grande

New variants of polymorphism in banana-shaped mesogens with cyano-substituted central core

A new series of achiral five-ring banana-shaped compounds is presented which exhibit a cyano substituent in the 4-position of the central core. It follows from X-ray investigations and from NMR, microscopical and electro-optical measurements that all members of the series do not only form the chiral B2 phase characteristic of a bent molecular shape, but also SmA and SmC (in one case also nematic) phases. The electro-optical studies on the B2 phase prove an antiferroelectric ground state which can be switched into ferroelectric states. On the base of the experimental data plausible structure models have been proposed.

Structure and properties of liquid crystalline phases formed by achiral banana-shaped molecules

Abstract Two new banana-shaped mesogens have been prepared and their mesophases have been studied. As indicated by calorimetry and polarizing microscopy both compounds exhibit two smectic mesophases. The structure of these phases have been studied by X-ray methods and NMR measurements. The electrooptical properties are discussed on the base of the structural data.

Chiral ordering in the nematic and an optically isotropic mesophase of bent-core mesogens with a halogen substituent at the central core

Two new homologous series of bent-core compounds have been synthesized. Their mesophase behaviour has been investigated by polarizing microscopy, differential scanning calorimetry, X-ray diffraction, NMR spectroscopy, and by dielectric and electro-optical measurements. It was found that, with one exception, all the chlorine-substituted compounds form a nematic phase and an optically isotropic ‘banana phase’. The latter phase shows spontaneously chiral domains of opposite handedness. This phase may be considered as a type of smectic blue phase. The mesophase behaviour of the homologous bromine-substituted compounds is more complicated. Depending on the chain length, B6, columnar, nematic or the isotropic ‘banana phase’ occur.

Ferroelectric and antiferroelectric “banana phases” of new fluorinated five-ring bent-core mesogens

Two homologous achiral five-ring mesogens with lateral fluorine substituents on the core and on the outer rings are presented. Both compounds exhibit two B5 phases and the B2 phase. The high-temperature B5 phases possess an antiferroelectric ground state. The low-temperature B5 phase was found to be ferroelectric, which is indicated by the bistable switching. The assignment and characterization of the mesophases is based on XRD measurements, 1H-, 19F-, 13C-NMR and electro-optical measurements.

Structural and conformational investigations in SmA and different SmC phases of new hockey stick-shaped compounds

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 SmC1 and SmC2. 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 SmC1 to SmC2 phase transition. It has also been shown, using NMR, that the SmC1 → SmC2 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 SmC2 phase. From dielectric measurements an increase by a factor of two was detected in the molecular mobility at the transition into the low temperature SmC2 phase. This finding supports a change in the packing as result of conformational changes.

Dimorphism SmA-B2 in bent-core mesogens with perfluorinated terminal chains

Several banana-shaped liquid crystal materials with perfluorinated terminal chains have been synthesized. A dimorphism SmA-B2 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.

Banana-shaped or rod-like mesogens? Molecular structure, crystal structure and mesophase behaviour of 4,6-dichloro-1,3-phenylene bis[4-(4-N-subst.-phenyliminomethyl) benzoates]

Abstract Terminally alkyl- and alkyloxysubstituted 4,6-dichloro-1,3-phenylene bis[4-(phenyliminomethyl)benzoates] have been synthesized and studied by optical microscopy, differential scanning calorimetry, NMR spectroscopy, X-ray diffraction and dielectric measurements. Surprisingly, nematic and smectic phases are observed, although comparable bisesters of resorcinol and monochlororesorcinol exhibit B phases typical for banana-shaped mesogens. It could be proved by NMR investigations in the liquid crystallinen state that the molecules have a nearly rod-like shape. Dielectric measurements indicate a rotation around the molecular long axis as known from calamitic mesogens, which is clearly distinguished from the behaviour of bent molecules. Obviously, substitution of the central phenyl ring of the bent 1,3-phenylene bis[4(4-n-alkyloxyphenyliminomethyl)benzoates] by two chlorine atomas causes a markedly change of the molecular shape resulting in nearly calamitic mesogens. These results are supported by singl...

Paraelectric–antiferroelectric transitions in the bent-core liquid-crystalline materials

Ferroelectric liquid crystalline phases formed by achiral bent-core mesogens have been known for about seven years. However, only few materials have been found to exhibit both conventional nematic/smectic mesophases and “banana” phases, where steric packing of bent mesogens results in ferroelectric properties. In this paper we report some studies of such compounds showing transitions between paraelectric smectic A, smectic C and antiferroelectric smectic CP phases. Structure characterization as well as studies of ordering in mesophases have been performed with the help of a variety of experimental techniques including X-ray diffraction, NMR and dielectric spectroscopy as well as electrooptical measurements.

Field-induced inversion of chirality in SmCPA phases of new achiral bent-core mesogens

A new series of achiral bent-core mesogens is presented. The mesophase behaviour has been investigated using differential scanning calorimetry, polarizing microscopy, X-ray diffraction, and dielectric and electro-optical measurements. Depending on the length of the terminal chains the homologues form nematic phases, conventional smectic phases (SmA, SmC) and polar smectic phases (SmCPA). It was found that the switching mechanism of the SmCPA phase depends on the temperature. At lower temperature the switching takes place in the usual way by rotation of the director around the tilt cone. At higher temperatures the switching is based on a collective rotation around the long axes which corresponds to a field-induced inversion of the layer chirality. Surprisingly the SmC phase also shows an unusual electro-optical response.

New banana-shaped mesogens with bromine-substituted central core

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 B2 phase.