Klaus Praefcke

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.

Lyotropic phase behaviour of disc-shaped tetra-palladium organyls in apolar organic solvents [1]

Abstract The lyomesomorphic behaviour in apolar organic solvents of members of two series 1 and 2 of disc-shaped palladium organyls—the syntheses of four of them are new (1d, f and 2a, e) and are described here—each carrying twelve long lipophilic chains have been studied. Their lyomesomorphism appeared to be complex and has been observed here for the first time with metal organyls. As can be seen by polarizing microscopy, most of the binary mixtures investigated here exhibit nematic properties. For two members of series 1—the chloro-bridged 1b and its bromo-analogue 1c—there is even evidence for the existence of two different nematic lyomesophases side by side showing reversible transitions between them. In ternary systems, i.e. after the addition of 2,4,7-trinitrofluorenone (TNF, a strong electron acceptor), the nematic phases are stabilized, suggesting that their mesophase structures are made up by columns with intercalated TNF molecules (charge transfer complex formation). Influences of structural fea...

Schlieren textures in free-standing nematic films: evidence of biaxiality

Conoscopic studies reveal that the nematic phase of 4,4-(p-terphenyl)-bis\[2,3,4-tri(dodecyloxy)benzal]imine is biaxial, the biaxial angle increasing with schlieren textures exhibited by the nematic phase have been between glass plates, as well as free-standing consisting entirely of s or 2-brush disclinations, confirming an earlier suggestion that the absence of 4-brush disclinations is evidence of biaxiality. decreasing temperature. The examined. Films sandwiched films, exhibit a nematic schlieren texture

New disc-shaped mesogens based on pentakis(phenylethynyl)benzene derivatives

Abstract First examples of amphiphilic alkyl pentakis(phenylethynyl)benzene ethers containing functional groups at the terminal position of their alkoxy chains were synthesized by etherification of pentabromophenol with 11-bromoundecan-1-ol and 11-bromoundecanoic acid ethyl ester, respectively, and subsequent palladium catalyzed, fivefold coupling reactions with the substituted phenylacetylene. Two undecanoic acid derivatives with different pentakis(phenylethynyl)phenoxy substituents in the ω-position were prepared by ester cleavage from the corresponding ethyl esters. The thermal behaviours of five penta-alkynes in their pure states, as well as of their charge transfer complexes formed from mixtures with 2,4,7-tri-nitrofluorenone, were examined.

Inclusions between large flat organic molecules; the induction of columns and mesophases

We describe here an overview on the formation of mesophases by the interaction between large flat aromatic cored molecules and relatively small organic electron acceptors which form stacked, sandwich-like columnar π-π electron donor-acceptor complexes. The inclusion of suitable donor and acceptor components may be used to induce columnar mesophases in mixtures of compounds which can be either mesomorphic or non-mesomorphic in the pure state. With judicious choice of donor and acceptor partners, induced mesophases of remarkable range and thermal stability can be prepared.

Multi-Nuclear Palladium Organyls-Novel Structures and Properties of Disc-Like Metallomesogens1

Abstract The first non-calamitic/disc-shaped metal organyl series 3 and 6 containing two or even four palladium atoms and eight or twelve flexible side chains presented here have been synthesized via the novel roof-shaped/bent dimers 2 or (the thermomesomorphic) 5, respectively. In addition, from the chemical point of view the dodecaethers 5 and 6a-c are the first derivatives of four novel, uniquely structured tetrapallado-macroheterocycles. Whereas the type of mesophase of 5 is still not distinctly clear yet those ones of 6a-c are enantiotropic discotic hexagonal columnar (Dh) over a wide range of temperature. On the other hand, the dinuclear palladocompounds 3a-d represent the first (monotropic) cases of metallomesogens exhibiting the nematic-discotic (ND) phase. This thermotropic nematic type of phase clearly gets stabilized by doping 3c, d with a strong electron acceptor; under such circumstances mesophase conversions/inductions (ND→Dh) are observed in the cases of 3a-c. Finally, it is important to em...

Core-halogenated, helical-chiral triphenylene-based columnar liquid crystals

It is shown that chlorination and even bromination of the mesogenic hexahexyloxytriphenylene 1 can take place at the sterically congested tetracyclic core delivering the respective monohalogenated, sevenfold substituted enantiotropic mesogens 3 and 4 in 32 and 17% yields, respectively, giving mesophases over greater temperature ranges. Miscibility tests among all mesogenic compounds of this study prove formation of the columnar hexagonal type of phase. The steric and electronic effects of those substituents, different from the six equal alkoxy groups, on the mesophase are discussed in detail. In our hands and to our surprise, the sterically hindered acceptor (nitro or halogeno) substituents of 2, 3, or 4 could not be replaced by an alkylthio group in nucleophilic aromatic substitution reactions using the aprotic polar solvent DMEU. Semi-empirical calculations on numerous hexaethers with one or two substituents, e.g. halogens or others, in one of the three bay regions of the triphenylene core reveal that t...