Volkmar Vill

Physical properties of liquid crystals

Introduction and Historical Development. Guide to the Nomenclature and Classification of Liquid Crystals. Theory of the Liquid Crystalline State. Physical Properties.

Studies on liquid-crystalline glycosides

Abstract A number of α- and β-glycosides with long chain aliphatic alcohols as aglycones were prepared, and the liquid-crystalline properties of the various mono-, di, and trisaccharide series compared. Further, the first liquid-crystalline glycosides of glucuronic acid and of an amino sugar hydrochloride were detected. It may be concluded that the clearing points increase with (a) a larger number of hydrogen bonds, (b) a rope-like, space-filling structure, and (c) a balanced molecular weight ratio between the polar and apolar parts of the molecule. In all of the cases studied so far only smectic A phases were observed.

Carbohydrate liquid crystals: structure–property relationship of thermotropic and lyotropic glycolipids

Abstract Glycolipids have important biological functions in nature: as membrane formation, mediators of cell–cell-recognitions and various specialized functions. Thousands of very complex glycoconjugates have been described by the biochemistry research, whereas the liquid crystal research in the previous decades only recognizes the structure/property relationships of more simple alkylated sugars, e.g. most of the natural materials have never been tested for their mesomorphous properties and the relation between the physical state of matter and the biological functions. This is the challenge of the present research. The bicontinuous cubic phase seems to have a high importance for cell fusion, transportation processes and drug delivery. Glycolipids also have a high potential to form chiral nanostructures, e.g. nanotubes, fibers, gels, bicontinuous and discontinuous cubic phases, and lyotropic and thermotropic cholesteric phases, etc.

Thermotropic and lyotropic properties of long chain alkyl glycopyranosides. Part II. Disaccharide headgroups

We have investigated the thermotropic and lyotropic properties of some long chain alkyl glycosides with disaccharide headgroups. The thermotropism was measured with polarising microscopy and additionally the lyotropism with the contact preparation method, Fourier-transform Infrared (FTIR) spectroscopy, X-ray diffraction and small angle neutron scattering. A broad thermotropic as well as lyotropic polymorphism was found. The compounds displayed thermotropic S(A) (lamellar) and cubic phases, and the investigation of the lyotropic phase behaviour led to the observation of inverted bicontinuous cubic V(II) phases, lamellar L(alpha) phases, normal bicontinuous cubic V(I) phases, normal columnar H(I) phases, normal discontinuous cubic I(I) phases and lyotropic cholesteric phases. The phases are discussed with respect to the chemical structures that have been varied systematically to derive structure-property relationships.

Convergent Synthesis of Columnar Twins and Tetramers from Triphenylene Building Blocks – The First Example of a Columnar Spiro-Twin

The novel spiro-twins 3a–f were prepared in three steps by one-pot oxidative coupling of guaiacol (9) and 1,2-dialkoxybenzenes 10a–f, followed by demethylation of the triphenylene 11 and subsequent etherification with the tetrabromide 6. A related strategy gave the tetramers 7a–f in two steps. Whereas derivatives 3a,b and 7a,b with pentyloxy and hexyloxy chains showed isotropic melting behavior, compounds 3c–f and 7c–f with longer alkyl chains displayed columnar mesophases. DSC, polarizing microscopy, X-ray diffraction and molecular modeling studies were used to further characterize the type of mesophase. For the spiro-twin 3 either a rectangular or a hexagonal columnar phase were conceivable. In case of 7f a hexagonal columnar mesophase was assigned.

On the structure of the thermotropic cubic mesophases

Abstract Two new substances, glycosides, with an optically isotropic mesophase were investigated. Using measurements of the refractive index, the optical isotropy of the thermotropic phases was proved. X-ray diffraction experiments provided information about the structure of the cubic mesophase. Similarly to the lyotropic cubic phases, here too structures of space group la-3d are present.

Nematic and Smectic a Phases in Ortho-Hydroxy-Para-Hexadecanoyloxbenzylidene-Para-Substituted Anilines

Four Schiff base esters containing the different substituents of R=CN, OH, SH, and NO2 were synthesized, and the physical characterization was carried out along with spectroscopic techniques (FT-IR and NMR). Thermal properties were analyzed by using differential scanning calometry (DSC), and their liquid crystal textures were observed under polarized optical microscope. All compounds thus obtained exhibit mesomorphic properties of which the alignment of molecules are inclined to either nematic, N (R=SH), or smectic, SmA (for R=CN, OH, and NO2). The unidentifiable subphases in crystal region were recorded in all compounds except that with R=OH. The presence of a lateral hydroxyl group has been claimed to contribute to the enhanced molecular ordering, leading to a higher clearing temperature.

Synthesis, Properties and Conformations of Liquid Crystalline 1,4-Dioxanes and Comparison with the Corresponding 1,4-Dithiane, Cyclohexane, and Benzene Derivatives

Abstract The preparation and mesomorphic properties of a series of bis-(4-alkyl- or 4-alkoxy-benzoyloxy)-trans-1,4-dioxanes 1, trans-1,4-dithianes 2 and trans-1,4-cyclohexanes 3 are reported and compared to the corresponding hydroquinone bis-esters 4. The 1,4-dithianes 2 do not show liquid crystalline properties and the 1,4-dioxanes 1 in their trans diaxial conformation are less favorable for nematic phases than the corresponding cyclohexanes 3 which are predominantly in the diequatorial conformation. However, owing to the cross-dipole moments they seem to be of advantage in stabilizing smectic phases.

Synthesis and mesogenic properties of glycosyl diacylglycerols

We synthesised glycosyl diacylglycerols bearing unsaturated or chiral methyl branched fatty acid chains. The thermotropism was measured with polarising microscopy and additionally the lyotropism with the contact preparation method. The synthesised compounds displayed thermotropic S(A) (lamellar), cubic and columnar phases and investigation of the lyotropic phase behaviour led to the observation of inverted bicontinuous cubic V(II) phases, lamellar L(alpha) phases and normal bicontinuous cubic V(I) phases. The phases are discussed with respect to the chemical structures that have been varied systematically to derive structure--property relationships.

Investigation of the influence of carbohydrate amphiphiles on the complex catalysed asymmetric hydrogenation of (Z)-methyl α-acetamidocinnamate in water

Abstract A series of carbohydrate amphiphiles has been tested in the asymmetric hydrogenation of (Z)-methyl α-acetamidocinnamate using water as medium. Dependent on the chain length and the hydrophilicity of the head group an enhancement of activity and enantioselectivity of the [Rh(COD)2]BF4 + BPPM catalyst system could be observed. Sometimes the enantioselectivity exceeds that in methanol. The method used here is convenient and environmentally friendly because carbohydrates as amphiphiles are biologically degradable in water. Selected carbohydrate amphiphiles are able to transfer chirality onto the included substrate in a low manner (up to 6% ee). In these cases hydrogen bondings in the head group seem to be responsible for the effect of chiral induction.