R. Festag

ELECTROCHEMICAL GROWTH OF SUPERPARAMAGNETIC COBALT CLUSTERS

The magnetization of stabilized cobalt colloids in tetrahydrofuran has been studied by a superconducting quantum interference device (SQUID) and magnetic balance measurements in dependence of applied magnetic field and temperature. The colloids are generated by a newly developed electrochemical method which allows one to generate clusters containing about 1000 atoms with a narrow size distribution. The final size distribution of the clusters is examined by high resolution transmission electron microscopy and small angle x‐ray scattering. The magnetization curves have been determined with special emphasis on changes at the freezing point of the solution. The curves of the liquid phase can be reasonably described by the Langevin function and the magnetic moments of isolated cobalt clusters that have been recently measured by Stern–Gerlach experiments. Deviations that appear at the freezing point can be understood in terms of magnetic anisotropy effects. It is shown that the cluster sizes and the susceptibil...

Fluoroalkylated discotic liquid crystals

Abstract The concept of mesophase stabilization by partial fluorination of side chains has been extended to discotic systems. The mesophase structure is essentially unchanged, whereas the temperature range of the mesophase is strongly affected by the fluorinated side chains. Triphenylene substituted with only one partially fluorinated side chain exhibits a decrease of the clearing temperature, whereas for symmetrically substituted systems a broad range mesophase has been observed. This behaviour can be attributed to a segregation of the incompatible parts of the molecule giving rise to a stabilized columnar structure for the symmetrical substitution pattern.

NEW DISC-SHAPED TRIARYLAMINO-1,3,5-TRIAZINES WITH HETEROAROMATIC CENTRAL CORES

Abstract New disc-shaped triarylamino-1,3,5-triazines were prepared by reaction of cyanuric chloride with two-fold alkoxy substituted anilines. Each of the triarylmelamines exhibits an enanti-otropic mesophase. X-ray investigations reveal the formation of columnar liquid crystalline structures with either a hexagonal or a rectangular lattice. The lattice parameters, as well as the intracolumnar ordering, depend on the length of the flexible lateral side chains. The new melamines form Langmuir monolayers at the air/water interface due to their amphiphilic nature. The monolayer organization is characterized by a side-on arrangement of the molecules. The polar central molecular parts lie flat on the water, whereas the lateral alkyl chains are oriented perpendicular to the water surface.

Oligomeric 1,3,5-triazines containing disc-shaped penta-alkyne and triphenylene side groups

Abstract A new type of discotic oligomer systems has been realized with either penta-alkyne or triphenylene side groups attached to amino substituted 1,3,5-triazine units in the main chain. The synthesis was carried out by reaction of 2,4-dichloro-1,3,5-triazine modified penta-alkynes and triphenylenes, respectively, with hexamethylene diamine. The discotic oligomers form stable edge-on oriented monolayers at the air/water interface. The mesophase structure of the novel discotic oligomers is characterized by an unusual lamellar layer arrangement. The mesogenic layer structure of the oligomeric triphenylenes changes to a rectangular columnar arrangement by charge transfer interaction with 2,4,7-trinitrofluorenone (TNF). Dielectric investigations were performed on the oligomeric triphenylene systems to analyse the influence of flexible segments on the internal mobility within the condensed state.

Supermolecular self organization via molecular aggregation II. Electro-optical studies on structure formation processes in associated liquid-crystalline diols

Abstract Orientational correlations are investigated in the isotropic phase for mesogens formed by diols which exhibit a columnar discotic or smectic phase at lower temperatures. These compounds consist of a central part to which either three or only one aliphatic chains of various length (n = 4–14) are attached at one end and two hydroxy groups at the other. The electro-optical studies reveal that associations are formed in the isotropic phase due to hydrogen bonding and that the stability is controlled by the collective association of a number of molecules giving rise to disclike or layer-like aggregates. It seems that these aggregates are responsible for the formation of the columnar phase which is stable at a lower temperature as far as the three chain diols with long aliphatic chains are concerned. The same situation holds for the formation of a smectic phase for the single chain diols. Structure formation in this new class of compounds is thus a function of the shape of the aggregates rather than of...

A novel class of mesogens: liquid-crystalline derivatives of linear oligoalkylene amides

Different members of a novel class of liquid crystals, linear oligoalkylene amides with 3, 4-bis(alkoxy)benzoyl substituents, are described. We have synthesized relevant derivatives of diethylenetriamine, triethylenetetramine and tetraethylenepentamine. The flexibility of the central alkylene amidic part causes very unusual mesogenic properties. Lamellar, columnar and cubic mesophase structures are displayed, depending on the length of the linear unit and of the alkoxy side-chains.

Low-temperature plasma treatment of magnesium stearate Langmuir-Blodgett films

Abstract Ultrathin Langmuir-Blodgett (LB) multilayers have been the focus of interest for use as gas separation membranes because of their well-defined thickness and defect-free structure. A considerable drawback from a technological point of view is the low thermal, chemical and mechanical stability of these films. One solution to this problem is the conversion of an organic multilayer structure, containing inorganic ions, to an organo-ceramic ultrathin film by low-temperature plasma treatment. This article describes the interfacial behaviour and the deposition of magnesium stearate monolayers and their subsequent conversion to organo-ceramic structures. X-ray, FTIR and AFM measurements were used to characterize the Langmuir-Blodgett (LB) multilayer structures, chemical compositions and surface topographics respectively: The LB-films consist of highly ordered Y-type multilayer structures with a stepwise surface height profile and nearly all amphiphiles are complexed as the magnesium salt. The modification of these LB-films by both argon and oxygen plasma leads to a decrease in the content of organic material by physical ablation and chemical etching processes. Whilst an internal order is retained within the plasma-treated films, the surface roughness increases significantly. Two parameters of the plasma process, the radio frequency power and the exposure time, were varied in order to find suitable process conditions. It is shown that the non-uniform multilayer structure can be converted to a uniform grainy organo-ceramic structure by plasma treatment.

Interfacial behaviour and mesomorphic properties of triazine-modified triphenylene oligomers

Abstract The supramolecular structures of new functional discotic oligomers formed on the water surface, within Langmuir-Blodgett (LB) multilayers and in the mesomorphic bulk state are described. The novel discotic oligomers are composed of triphenylene side groups attached to amino-substituted 1,3,5-triazine units in the main chain. The oligomeric triphenylenes form stable monofilms at the air-water interface with an edge-on orientation of the hydrophobic triphenylene side groups. LB multilayers are characterized by an edge-on bilayer packing with interdigitation of discotic side groups of adjacent bilayers. An unusual lamellar layer arrangement due to hydrogen bonding between the backbones is found for the bulk phase.