A. M. Levelut

Disc-Like Mesogen Polymorphism

Abstract Among the various mesogenic polyaromatic cores, only benzene, triphenylene, truxene and anthraquinone hexasubstituted derivatives exhibit one or several D columnar phases and/or a ND lenticular nematic phase. A simple scheme of nomenclature is suggested for the six different types of columnar phase requiring only two structural parameters: firstly the symmetry of the lattice (hexagonal: Dh, rectangular: Dr, oblique: Dob), and secondly the order or disorder within the column. Several examples of disc-like compounds which form phases Dho, Dhd, Drd(P21/a) , Drd(P2/a), Drd(C2/m), Dod.d are described and a discussion of the specificity of the corresponding optical textures is given. Then the complex polymorphism of these new mesogenic materials (normal, inverse, reentrant columnar-nematic sequences) is reviewed. Finally, a simple classification of the various types of lyotropic and thermotropic phase is proposed, based on the shape of the structural element (finite or infinite cylinder, disc, infinite...

Disc-Like Mesogens: A Classification

Abstract Five mesomorphic phases are described in several disc-like liquid-crystals. They are hexalkanoyloxy benzenes, triphenylenes or truxenes, hexa-alkoxy triphenylenes, hexa-benzoates of triphenylene. Several of these compounds exhibit a complex polymorphism. Informations on the structures of these phases have been obtained by means of X-Ray diffraction; except a Nd namatic phase all the others one are columnar phases. At the same time a systematic study has been performed by the well known method of isomorphy. All these results allow us to propose a general classification of these phases.

Tubular mesophases: liquid crystals consisting of macrocyclic molecules

The hexa-(p-n-dodecyloxybenzoyl) derivatives (1b), (2b), and (3b) of the macrocyclic polyamines [18]-N6, [24]-N6O2, and [27]-N6O3 display a liquid crystal phase in which the macrocyclic units are stacked, forming a tubular mesophase.

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.

X-rays investigations of the smectic A1 - smectic A2 transition

The existence in a binary system constituted of a three phenyl ring cyano compound mixed with TBBA (p=latm) of a smectic A-smectic A transition line has been reported [1]. By microscopic observation, it is impossible to detect this SA-SA transition because the two smectic A, called 1 and 2 are perfectly isomorphous. In fact, this line was first revealed by means of Differential Scanning Calorimetry and then by diamagnetic susceptibility measurements [2].

Structural features of the hexagonal phase of phasmidic liquid crystals

Abstract Eighteen compounds having the same molecular architecture, a rod-like aromatic core with four to six long aliphatic terminal groups, which all possess a hexagonal mesophase, have been studied by X-ray diffraction. From the lattice constants of these mesophases, we have deduced some information about the molecular organization. We discuss the curvature and the density of the interface between the aromatic and the paraffinic moieties, the orientational disorder and the packing of the rod-like central parts of the molecules. The occurrence of smectic, oblique or cubic mesophases in the same series underlines the similarities with lyotropic polymorphism and justifies a description of the molecular organization in terms of interfaces.

Long and Short Range Order in the Crystalline and Smectic B Phases of Terephthal-Bis-Butylaniline (TBBA)

Abstract We proposed recently a model for the structure of the smectic B modification of two liquid crystals (EBAC and TBBA). These two compounds only differ by the fact that the molecules are perpendicular to the smectic planes in EBAC and tilted in TBBA.

On Some New Series of Biforked Mesogens

Abstract Two series of biforked mesogens derived from 3,4-or 3,5-dialkoxybenzoic acids are described. The 3,5-derivatives exhibit only one hexagonal columnar mesophase, but the 3,4-derivatives display an interesting polymorphism such that phases with both columnar and lamellar mesomorphic order exist for the same sample. The relationship between this behaviour and the number and position of the substituents is discussed.

X-ray diffraction study of some mesogenic copper, nickel and vanadyl complexes

Abstract We present a comparative study of various metallomesogenic complexes, using X-ray diffraction methods. For a given ligand linked to different metal atoms (Cu, Ni, VO), the nature of this central atom influences mainly the magnetic susceptibilities of the mesophases. With different ligands, which keep the close neighbourhood of the metal atom unchanged, the apparent length of the mesogenic unit is longer for short ligands than for longer ones. This unexpected behaviour is qualitatively well explained by taking into account the global shape of the different complexes.

Columnar Phases from Covalent and Hydrogen-Bonded Liquid-Crystalline Ferrocene Derivatives

The synthesis and liquid-crystalline properties of tetracatenar covalent and H-bonded bis-ferrocene derivatives 1 and 2, respectively, are reported. Both compounds gave rise to enantiotropic columnar liquid- crystalline behavior with a hexagonal molecular organization. To explore the possibility to obtain also calamitic liquid-crystalline phases from H-bonded ferrocene-containing liquid crystals, a rod-shaped ferrocene mesogen 3 was synthesized, which gave rise to enantiotropic smectic C and smectic A phases. For the first time, a rational synthetic design at the ferrocene level led to ferrocene-based liquid-crystals with columnar behavior and to H- bonded metallomesogens. Introduction. - Current efforts oriented towards the development of ferrocene- based mesomorphic materials led to the synthesis of ferrocene-containing side-chain liquid-crystalline polymers (1), ferrocene-containing liquid-crystalline dendrimers (2), and a liquid-crystalline mixed (60)fullerene-ferrocene derivative (3). These structures are promising candidates to construct switchable anisotropic materials. We demon- strated that electron transfer can be used to generate mesomorphism in the ferrocene- ferrocenium redox system (4). To further explore the structure/supramolecular organization relationship in the case of ferrocene-containing mesomorphic molecular units, the study of novel structures is required. Mesomorphic ferrocenes exhibiting columnar liquid-crystalline phases would be of particular interest: No such examples have been reported (5 - 7) 1 ). Because of the bulky, three-dimensional structure of the ferrocene core, the primary influence of which is to reduce the intermolecular interactions (resulting in a lowering of the liquid-crystalline tendency in comparison with the ferrocene-free analogues) (5), the design of such compounds represents both a conceptual and synthetic challenge. Ultimately, owing to its redox activity, the incorporation of ferrocene units into columnar phases may open the doors to new supramolecular wires (9).