Bernd Kohne

News on Nematic-Biaxial Liquid Crystals1

Abstract Recent studies have established that not only some polymeric materials exhibit nematic biaxial phases, but that also low molecular weight compounds can do so if certain structural conditions are met. In this paper new developments in the field of thermotropic N, phases of low mass synthetic products are discussed regarding the relationship between structure and nematic biaxial property. Various syntheses of new structures of different geometries and some first results of investigations of such compounds are also presented. While thus far few N, phases have been known, their number should increase rapidly in the near future and stimulate the scientific research on this interesting topic. Biaxial nematic phases are of great importance for the system of phase types, the theory of liquid crystals, as well as for the understanding of the molecular dynamic of mesogens, and of the discovery or development of new possibilities of application of materials in this state of matter.

Charge transfer induced nematic columnar phase in low molecular weight disc-like systems [1]

Abstract Two large pentakis(phenylethynyl)phenyl alkyl ethers, without side chains as is usual for discotic systems, were synthesized in palladium catalysed one pot coupling reactions. These low molecular weight and non-liquid-crystalline electron donor compounds were doped with 2,4,7-trinitrofluorenone under the formation of charge transfer complexes. The induction of two types of enantiotropic mesophases (i.e. nematic and hexagonal columnar phases (Nc and Dho)) was observed. These charge transfer-induced mesophases were characterized by polarization microscopy, differential scanning calorimetry, and X-ray analysis; phase diagrams of the binary systems were also recorded as well as the hexagonal columnar to nematic columnar transition as a function of the length of the disturbing alkoxy group. It is considered possible now to study low molecular mass Nc phases by tailor-made two component mixtures on the basis of our easily accessible disc-shaped multiynes.

First carbohydrate liquid crystals of columnar structure

Abstract S, S-Dialkylacetals of aldoses, tripodal in structure and recently synthesized in large numbers by three groups, do not exhibit thermotropically a smectic, but a columnar hexagonal mesophase (Hx) as we have proved by X-ray diffraction. The molecular organization in this mesophase is comparable with a similar one known for phasmidic molecules. These various multiols, although different in stereo-chemistry, form only one type of hydrogen-bonded disc-shaped multimer. Its mesophase structure is made up of about five molecules placed in columns with a skeleton of hydrogen-bridged sugar parts surrounded by thioalkyl groups in the periphery. Since three 6-deoxy sugar dithioacetals were shown to be non-thermomesomorphic the terminal hydroxyl function is essential for this molecular arrangement which seems not to be true for missing ones in the inner part of a sugar chain, as we have found in one case.

Thermotropic biaxial nematic phases with negative optical character [1]

Abstract Two disc-shaped multialkynyl arene ethers (1 and 2) with unusual thermo-mesomorphic properties are presented. Conoscopic studies show that the nematic phases of these new low molecular weight liquid crystals are biaxial and that the sign of their biaxiality is negative. The diether 2 is the first discotic twin liquid crystal which exhibits a nematic phase.

Synthesen und ungewöhnliche Mesophasensequenzen von Kohlenwasserstoffen eines neuartigen Typs nicht-calamitischer Flüssigkristalle

Abstract Mehrere Vertreter einer neuen Familie thermotroper, nicht-calamitischer Flussigkristalle, die p-substituierten, biaxialen und zentrosymmetrischen 1,2,3,5,6,7-Hexakis(phenylethinyl)naphthaline 3a-f, wurden in einer Pd-kataly-sierten CC-Kupplungsreaktion aus dem entsprechenden Hexabromnaphthalin und verschiedenartig monosubstituiertem Acetylen (2a-f) synthetisiert. Diese Synthesemethode stellt ein vielversprechendes, ‘Lego-artiges’ Aufbauprinzip fur Mesogene mit flachendeckenden, hoch ungesattigten und konjugierten π-Systemen als neuen Typ von ‘Super-disc-Kernen’ dar. Die stabilen Mesophasenbereiche von 3a-f reichen von c. 42 bis 180 K. Mit einer Ausnahme (3a) zeigen alle neuen Verbindungen eine stabile, fluide ND-Phase. Wahrend ein Kohlenwasserstoff dieser Serie (3e) sowie der Hexaether 3f nur eine Mesophase ausbilden, zeigen die vier Kohlenwasserstoffe 3a-d Multimesomorphismus mit einer inversen Phasen-sequenz (C → ND → 2 D → I) bei den mittleren Homologen 3b-d. Beim schnellen Heizen der ND-Phase...

Further Low Mass Liquid Crystal Systems with Nematic Columnar (Nc) Phase [1]

Abstract A number of new disc-like pentaynes 1–4 with super-disc cores are presented. These new non-mesogenic electron donors doped with 2,4,7-trinitrofluorenone (5) form charge transfer complexes showing enantiotropic nematic and hexagonal columnar mesophases.

Inositol Liquid Crystals, X-Ray Diffraction Studies of Their Hydrogen-Bond Supported Supramolecular Mesophases1

Abstract Results of x-ray measurements on the various columnar mesophases of multi-ethers of the naturally occurring stereoisomers myo- and scyllo-inositol are presented. Depending on the number of alkyl substituents at their inositol cores such derivatives show interesting kinds of molecular organization in their liquid-crystalline states. Thus, to the best of our knowledge, the first thermotropic mesophase of tetragonal symmetry exhibited by a low-molecular weight system is described. Vicinal diols and a tetrol of this new group of liquid crystals are coupled within their mesophases by hydrogen bonding networks. In case of scyllo-inositol it could be shown that the number of functionalizations determines the size of this network or the degree of multimerization; columnar mesophases composed of dimers or pentamers were observed. A comparison of stereoisomeric inositol tetraethers (vicinal cis- and trans-diols) clearly revealed stereochemical influences on their liquid-crystalline properties.

Spectroscopic investigations : XVII. 77Se and 125Te NMR resonances of some selenol and tellurol esters☆

Abstract In this paper the first directly determined 77Se or 125Te NMR chemical shifts of Se- and Te-aryl esters of three aromatic carboxylic acids are presented and discussed.

Mesophase characterization of some long chain vicinal multihydroxy amphiphiles [1,2]

Abstract The mesophase characterization of three stereoisomeric multihydroxy compounds, the 1, 2(S), 3(R), 4(R)-, 1, 2(R), 3(S), 4(R)-, and 1, 2(S), 3(S), 4(R)-ico-sanetetrols, as well as of an octadecafuranose derivative has been carried out by means of polarizing microscopy, X-ray diffraction etc. The four multiols with their very long flexible alkyl chains are mono-thermomesomorphic; their single mesophases are miscible with each other, with a typical carbohydrate derivative, and are characterized by X-ray diffraction having a layer structure without order in it. The phase transition data of the three icosanetetrols are strongly dependent on their stereochemistry. The measurements of the refractive indices of one of the tetrols show a typical behaviour known for SA phases or for lamellar phases, i.e. the sign of the birefringence is positive.

Spektroskopische Untersuchungen : XVIII. Vergleichende 1H−und 13C-NMR-Studie isoelektronischer O-, S-, Se- und Te-arylester aromatischer Carbonsäuren

Abstract The 13 C resonances of the sp 2 hybridized carbon atoms of the isoelectronic substituted O -, S -, Se -, and Te -aryl esters of benzenecarboxylic acids (I–IV) and of the benzenedicarboxylic acid (IX) as well as of the diaryl dichalkogenides V–VII are assigned. The same is done for the aromatic 1 H NMR peaks of compounds III, IV, VI and IX. The influence of the chalkogen atoms on the positions of these NMR signals in all isoelectronic series of compounds is discussed. Furthermore parallels in the effects of the four chalkogenes O, S, Se, and Te in the compounds I–VII and IX as well as the four halogens F, Cl, Br, and I in the 4-halotoluenes VIII on the 1 H and 13 C resonances of the aryl rings directly attached to these elements are described and explained.