J. Malthete

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.

Mesomorphic Derivatives Of Ferrocene

Abstract The first series of ferrocene derivatives exhibiting mesomorphic properties (substituted N-(p-benzoyloxybenzylidene) anilines la and 1b) has been synthesized for application in Mossbauer spectroscopy.

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?

On a “cholesteric” phase in disc-like mesogens

Abstract Several chiral disc-like compounds are described. They exhibit nonchiral D columnar phases. But it is shown that, for example, chiral triphenylenes are able to twist the ND nematic phase of a hexabenzoate of triphenylene. That is, probably, the first evidence of a “cholesteric” phase in disc-like mesogens.

One-dimensional singlet energy migration in the columnar liquid crystal of a triphenylene derivative

The fluorescence properties of 2,3,6,7,10,11-hexa-n-hexyloxytriphenylene have been studied in solution and in its crystalline and columnar liquid-crystalline phases by steady-state and time-resolved spectroscopy. When an energy acceptor, perylene, is added to the organized phases, photosensitized fluorescence is observed. The triphenylene fluorescence decay curves obtained with the columnar phase are well described by a random walk of the excitation on a linear chain containing traps. The kinetic analysis permits the determination of the exciton hopping time (32 fs), the exciton diffusion coefficient (2.0 × 10–2 cm2 s–1) and the intrinsic trap molar fraction (10–3).

One-dimensional energy migration in crystalline and columnar liquid-crystalline phases of 2,3,6,7,10,11-hexa-n-hexyloxytriphenylene

Thermotropic columnar mesophases capable of giving highly oriented fibres are considered as interesting systems for the study of one-dimensional energy migration. Steady-state and time-resolved luminescence experiments carried out with homogeneous solutions, crystalline and liquid crystalline samples of a disc-like compound, 2,3,6,7,10,11-hexa-n-hexyloxytriphenylene, are reported. Normal fluorescence, phosphorescence and P-type delayed fluorescence related to energy migration have been detected in the pure compound phases.

Reentrant Mesophases in Disc-Like Liquid Crystals

Abstract The first case of reentrant phenomenon in disc-like mesogens was observed in hexaal-kanoyloxytruxenes (Cn HATX). In this series of ten compounds (n = 6 to 15) a few of them, the long chain ones, displayed a reentrant columnar phase. On the other hand, a reentrant ND nematic series was prepared: truxene P-alkoxybenzoates (Cn HBTX) with n = 6 to 14. Phase transitions between solid, mesomorphic and isotropic phases were studied by hot-stage microscopy and differential scanning calorimetry. Binary phase diagram studies, optical textural observations and X-ray measurements showed a rich polymorphism in these two series: a nematic disc-like phase (ND ), a hexagonal ordered columnar phase (Dho ), a hexagonal disordered columnar phase (Dhd ), a rectangular disordered columnar phase [Drd(P21/a) ] and a strongly tilted rectangular columnar phase [Drd(C2/m) ].

Reentrant Phenomenon in Disc-Like Liquid Crystal

Abstract Several Hexa-n-alkanoates of truxene with long alkyl chains exhibit, above and below a ND nematic phase, an hexagonal columnar phase. It is probably the first example of a reentrant phenomenon for pure disc-like liquid crystals at atmospheric pressure.

A Pure Disc-Like Molecule with Cholesteric Properties

Abstract A cholesterogenic disc-like derivative of triphenylene is described. The N*D phase exhibits oily streaks from 192.5 to 246.5°C. That is the first evidence of a cholesteric phase in a pure disc-like molecule.