C. Destrade

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.

Invited Article: Phasmids and polycatenar mesogens

Several series of phasmids and biforked mesogens or more generally of polycatenar mesogens, i.e. rod-like core mesogens ending in six, five, four or three aliphatic chains (respectively hexa-, penta-, tetra- and tricatenars) are reviewed. According to the number and positions of the aliphatic chains, lamello-columnar, columnar, cubic, smectic and nematic phases appear.

Nematic disc-like liquid crystals

Abstract We present the first example of disc-like mesogens which present at high temperature a highly fluid mesophase with a nematic optical texture. These nematic disc-like compounds are pure simple component models to study the carbonaceous mesophase.

An Homologous Series of Disc-Like Mesogens with Nematic and Columnar Polymorphism

Abstract An homologous series of hexa alkoxybenzoates of triphenylene has been prepared with n = 4 to n = 12. Interphase transitions between solid, mesomorphic and isotropic phases were studied by hot stage microscopy and differential scanning calorimetry. Binary phase diagram studies, optical textures observations and X-ray measurements showed the existence of three different mesomorphic phases: a nematic disc-like phase N D, a tilted columnar phase D t and a rectangular disordered columnar phase D rd. Only two different phases were observed in each compound and the following mesophase sequences were found. This remarkable even-odd effect is discussed and is compared to that observed for classic liquid crystals. Several hexa alkybenzoates of triphenylene (n = C7 − C10) were also prepared and their mesomorphic properties studied. Only short range N D were observed.

Truxene derivatives: Temperature inverted nematic-columnar sequence in disc-like mesogens

Abstract New disc-like mesogens are presented which are three hexa-n-alkanoates of truxene. A fluid phase is observed below a columnar viscous phase with respect to temperature. Perhaps the first example of a reentrant nematic phase in disc-like compounds?

New TGBA series exhibiting a SC∗SASA∗N∗ phase sequence

Abstract A new series with a chiral tolane core has been synthesized. These materials belong to the optically active series: 3-fluoro-4-[(R) or (S)-1-methylheptyloxy]4′-(4″-alkyloxy-3″-fluorobenzoyloxy)tolanes (nFBTFO1M7). For the first time, the helical SA∗ phase or TGBA phase is found in all of the derivatives from heptyloxy to octadecyloxy. The SA-SA∗-N∗ phase sequence is observed in several compounds with short chains and the SC∗-SA-SA∗-N∗ phase sequence is obtained with the decyloxy derivative. The TGBA phase has filament or cholesteric textures. The helical pitch of the TGBA phase is short for the octadecyloxy derivative and is compared with that for different chain lengths as a function of temperature. The layer spacing in the TGBA phase shows that it is incommensurate with the fully elongated molecular length.

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.

Mesomorphic Polymorphism in Some Disc-Like Compounds

Abstract A complex mesomorphic polymorphism is shown in a hexa-n-dodecanoate of triphenylene. Both by calorimetric measurements and optical textures, evidence of three different mesophases are given. Two of these transitions are practically second order ones.

The effect of the position of lateral fluoro-substituents on the stability of the S∗C α and S∗C A phases

Abstract Four new chiral series with benzoate cores have been synthesized and characterized. The mesomorphic properties have been analysed by optical microscopy on the pure enantiomeric and the racemic compounds and on mixtures, by D.S.C. and by electro-optic measurements. Three of the series (hydrogenous, monofluoro-substituted ortho to the alkoxy chain, and difluoro-substituted) display a very rich polymorphism including SA, S∗α, S∗C, S∗FI and S∗C A phases, whereas the series monofluoro-substituted in the meta-position does not exhibit S∗FI and S∗C A phases any more except for the dodecyloxy derivative, but gives a large enantiotropic S∗C α, phase. A comparison of these four series leads to a discussion about the effect of the transverse dipole moment on the existence of the S∗C A phase.