Katalin Fodor-Csorba

Structure and stability of crystalline C60 · n-pentane clathrate

Abstract Clathrate type single crystals with the composition of C60(n-C5H12)0.88(C7H8)0.05 were grown from C60-toluene- petroleum ether solution. The structure of the crystals, determined by X-ray diffraction is b-face centered orthorhombic. No phase transition, corresponding to the orientational ordering is observed at 250 K by differential scanning calorimetry. Around 420 K the solvent molecules are released from the clathrate as determined by thermogravimetry-mass spectrometry. An irreversible phase transition is observed simultaneously with the evolution of solvents, resulting in the face centered cubic structure of pure C60.

Structural characterization of the new polymorphic mesophases formed by bent-core molecules

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 (B2, B′2, B″2, B5). 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.

Viscosities of a bent-core nematic liquid crystal

Viscosity measurements are reported for a bent‐core nematic liquid crystal, 4‐ chloro‐1,3‐phenylenebis{4‐[4′‐(9‐decenyloxy)benzoyloxy]} benzoate (ClPbis10BB). The rotational viscosity was measured by analysing the dynamics of director rotation in pulsed magnetic fields, and the flow viscosities were determined by employing a new electro‐rotation technique. The results show that whereas the rotational viscosity is more than ten times larger than for calamitic liquid crystals, the flow viscosity is more than 100 times larger. Even more striking is the difference between the ratio of the flow and rotational viscosities, which for calamitic nematics is typically 0.1, whereas in this bent‐core material it is ∼50. This suggests that the large shear viscosity is primarily due not so much to the molecular size, but rather the shape. A model is discussed that may explain the observations.

Rheological properties of bent-core liquid crystals

We show that bent-core liquid crystalline materials exhibit non-Newtonian flow in their optically isotropic liquid phase. We conjecture that this behavior is due to the existence of nanostructured, fluctuating clusters composed of a few smectic-like layers. Shear alignment of these clusters explains the shear thinning observed in bent-core liquid crystals having either a nematic phase or non-modulated smectic phase. By contrast, smectogens having a modulated smectic phase do not shear thin at low shear rates, but even show a slight shear thickening which may be due to entanglements of wormlike and/or helical clusters.

Flexoelectric effect in a bent-core mesogen

The flexoelectricity in a bent-core liquid crystal was evaluated quantitatively using two independent electrooptic methods. The absolute value of the bend flexoelectric coefficient e 3 was measured to be about 15.8 pC m−1. Our results revealed that there is little difference between the flexoelectricity of bent-core mesogens and that of conventional calamitic mesogens, i.e. in the same pC m−1 order. The principles of the methods used are reviewed in detail.

Ester type banana-shaped liquid crystalline monomers: synthesis and physical properties

New ester-type banana (bent-shaped) monomers, 1,3-phenylene bis[4′-(alken-1-yloxy)-biphenyl-4-carboxylate]s Ia–Vb, with different substituent(s) on the central phenyl ring (H, CH3, Cl or NO2) and alkenyl tails in the side arms (decenyl or undecenyl) were prepared. The analogues IIIa–IVb with 4-chloro- or 4,6-dichloro-substituents exhibited a nematic phase, while Va,Vb with 2-nitro-substituent showed a B7 phase at relatively low temperature. All the compounds were stable, no degradation or polymerization was observed under applied electric fields or heat treatments. Electro-hydrodynamic instabilities were observed in the nematic phase of each sample. In the B7 phase of Vb there was an electro-optical switching in the range 112–114.5 °C with a switching time of about 150 µs at 20 V µm−1 field. At lower temperatures no electro-optical switching occurred, but a ferroelectric-type spontaneous polarization of 22 nC cm−2 could be detected. The bend angle was calculated by the semiempirical CAChe/MOPAC/PM5 method.

Microwave-assisted deuterium exchange reactions for the preparation of reactive intermediates

Deuterium labelling of a number of ketones was achieved on a preparative scale by a microwave-assisted deuterium exchange reaction with D2O/CF3COOD. The reaction was rapid (<15 min) and highly isotope-efficient (ca. 100%), even though deuterium incorporation can be decreased during work-up (85–90%). No exchange reaction was observed for aryl protons of aromatic ketones.

Chromatographic methods for the determination of pesticides in foods

Chromatography is the most important technique available to the analyst dealing with the determination of pesticide residues in food, feed and environmental samples. Numerous methods for pesticide residues in foods have been developed in the past few years, and this paper reviews some of the most important procedures. A great variety of chromatographic methods, such as solid-phase extractions, column chromatographic clean-up methods, thin-layer, gas, high-performance liquid and supercritical fluid chromatography, and their coupling with sensitive and selective detection methods are surveyed.

2H-NMR and SAXS of a ferroelectric liquid crystal : unwinding of the ferroelectric chiral helix by high magnetic fields

Abstract 2 H-NMR in three different magnetic fields and small angle X-ray scattering (SAXS) are employed to investigate the orientational order, molecular organisation and phase transitions of the chiral smectic liquid crystal (−)-(S)-[4-(2-methylbutyloxycarbonyl)phenyl] 4- n -heptyl-biphenyl-carboxylate (MBHB) suitably deuterated for 2 H-NMR measurements. Unwinding of the ferroelectric chiral helix by high magnetic fields is observed and discussed. Tilt angle values are obtained with the two different techniques and compared. The results show the importance of molecular conformation for the structural properties of this mesogen.

A chiral side-chain polymer with layered structure

Abstract A chiral side-chain polymer was synthesized and its chemical structure was proved by infrared spectroscopy. According to the X-ray diffraction studies the polymer exhibited smectic type ordering however it did not show any birefringence macroscopically.