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Several new homologous series of cyano substituted aryl p-alkoxycinnamates have been synthesized. The reentrant behavior of these series is compared with those of the p-alkoxybenzoates series. The influence of the length of the conjugated system on the reentrant phenomenon is studied.

A homologous series of hexa-n-alkanoyloxy trithiatruxenes (2,3,7,8,12,13-hexa-n-alkanoyloxybenzo-[1, 2-b : 3, 4-b': 5,6-b”]trisbenzothiophenes) (Cn HATXS) has been prepared with n = 9 to n = 13 in order to study the influence of the substitution of the three methylenes of the truxene core by three sulphur atoms on the mesomorphic properties. Phase transitions were studied by hot stage microscopy and DSC. This series shows another example of an inverted sequence (crystal-ND nematic-D columnar-isotropic) and a new polymorphism with a columnar phase between Dh and Drd phases.

Parmi les differents motifs polyaromatiques mesogenes connus, seuls les derives hexasubstitues du benzene, du triphenylene, du truxene et de l’anthraqulnone presentent une (ou plusieurs) mesophase (s) colonnaire(s) D et/ou une phase nematique lenticulaire ND. Apres avoir propose une reponse simple aux problemes que pose la nomenclature des phases colonnaires : deux parametres cristallographiques — symetrie du reseau bidimensionnel (hexagonal = Dh , rectangle = Dr , oblique = Dob ) ; ordre ou desordre dans les colonnes — suffisent a differencier ces phases. Quelques exemples de substances presentant ce type de mesophase sont donnes. On discute la specificite des textures optiques correspondantes (Dho , Dhd , Drd(pmg) , Drd(pmg) , Drd(cm) , Dob.d ) en mentionnant l’influence de la chiralite sur la phase ND et sur certaines phases colonnaires ([math], [math]). Le polymesomorphisme complexe de ces nouveaux materiaux est passe en revue : sequences “normales”, “inverses”, rentrantes, en discutant l’apport de la methode de l’isomorphie. On propose enfin, un classement simple des phases thermotropes et lyotropes base sur la forme de l’element structural (cylindre fini, disque, feuillet indefini, cylindre indefini, globule).

Dielectric properties are reported of new pure substances exhibiting reentrant nematic and reentrant smectic phases. The quasistatic dielectric constants (e′‖, e′⊥, e′is) and the dispersion of e*‖, e*⊥ and e*is is the radiofrequency range (5Hz-1GHz) have been investigated. The current experimental dielectric behaviours are examined and discussed using our experimental data.

An homologous series of mesomorphic compounds, the I-(4-alkoxy or alkyl-benzoyloxyphenyl)-2-(4′-pentylphenyl)-ethanes, has been synthesized. The compounds with R = alkyl have low-melting points and wide mesomorphic ranges, for example from 33.5°C to 106.5C. The compounds with R = alkoxy exhibit a smectic polymorphism; for example a compound of this series is successively smectic B, F and C. The influence of the central linkage when the ethane group is replaced by an ester, single bond, ethylene, azomethine, azo, methylenecetone or cetone groups, is discussed.

Les molecules en forme de bâtonnets fortement polaires presentent un polymorphisme inattendu et souvent complexe. Nous proposons une etude des relations entre la structure moleculaire et les diverses sequences de phases observees jusqu’a present. Les influences des groupements polaires terminaux, de la longueur et de la nature de la chaine, du sens des moments dipolaires des groupements centraux, de la taille et de l’encombrement du cœur sont prises en consideration dans la formation des phases rentrantes, des differentes phases smectiques A ou smectiques C et des nouvelles phases fluides bidimensionnelles [math] et [math]. Dans cet article le phenomene rentrant, encore rare il y a peu d’annees, perd presque tout son caractere pathologique ; d’autre part, un nombre grandissant de composes dont les structures moleculaires sont tres voisines, se revelent susceptibles de fournir des transitions entre phases de meme symetrie apparente : leur etude experimentale et theorique — deja entamee cf. parties I et II — devrait etre fructueuse.

The X-ray diffraction patterns of powder samples of the fluid mesophase of the hexaalkoxybenzoates of triphenylene which is a disc-like molecule are similar to the diffraction patterns of non orientated nematic phase of rod-like molecules. Furthermore, we give evidence for a uniaxial character of this anisotropic medium with a negative diamagnetic anisotropy, the flat molecules are orientated parallel each other by a rotating magnetic field. In addition, the X-ray patterns of orientated samples of the hexyloxy compound revealed pretransitional effects in this nematic phase of disc-like molecules (ND). This short-range translational order, sort of skewed cybotactic groups originates from the existence of a low temperature columnar phase with an original tilted packing here described.

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.

The polymorphism of the 4 cyanobiphenyl-4′-n-alkoxy benzoate esters is investigated and a reentrant nematic phase is observed for the n = 8 and n = 9 homologues. X-Ray studies confirm this fact and show that in the reentrant phase the cybotactic groups are SA-like at high temperature near the SA phase while they appear SC-like at lower temperatures.

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.

Some biforked mesogens belonging to five series were prepared and studied. They exhibit an interesting polymorphism in which both columnar and lamellar mesomorphic orders coexist at different temperatures. The short chains lead to the classical nematic phase and/or the smectic C phase. The long ones give evidence of a hexagonal columnar mesophase similar to the φ h phasmidic phase. One cyclohexane derivative shows a body centered cubic phase and a hexagonal columnar phase with a large lattice parameter corresponding to twice the molecular length Description de quelques mesogenes bifourchus presentant un polymorphisme original ou coexistent phases colonnaires et phases lamellaires a des temperatures differentes

Dielectric study of the SA-ferroelectric Sc * phase transitio n is of double interest: knowledge of dielectric constants a r e useful for applications 1, experimental data can be compared with results obtained using theoretical models 2,3. We have carried out large frequency range dielectric studies of a pure compound. The chemical formulae and the phase sequence of this compound are given below 4

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...

A homologous series of 4-(4′-cyanobenzoyloxy) benzylidene-4-alkoxyaniline have been synthesized. Four derivatives with a long alkoxy chain from C8H17O-to CllH23O- exhibit a smectic phase made of ribbons (S∼C) recently discovered.

A homologous series of hexa-n-alkanoyloxytrioxatruxenes has been prepared with n=7 t o n=15 in order to check the influence upon the mesomorphic properties exerted by the substitution of the three ...

In this paper, we will report on the study, in a large frequency range, of some new thermotropic liquid crystals whose phase sequence is made up of S Ad -S A2 -type transitions. At this transition, we observe a dielectric behaviour appreciably different from one sample to another: thre is variation in the dielectric static anisotropy and variation int he activation energy of the «flip-flop» rotation in the parallel measurement direction. Our results confirm those obtained elsewhere by means of other investigation techniques like microscopic observations, DSC experiments and X-ray data Etude experimentale des permittivites statique et dynamique sur une large bande de frequences, pour plusieurs cristaux liquides presentant ce polymorphisme des phases smectiques. Suivant les composes, observation de la variation l'anisotropie dielectrique statique ou de l'energie d'activation du mouvement de basculement dans la direction parallele de mesure, lors de la transformation de phase

From microscopic observations, DSC experiments and X-ray data the Smectic A d -Smectic A 2 transition is studied in seven compounds of the 4-n alkoxy phenyl-4' cyanobenzoyloxy benzoate series (DB n O). In particular the excess enthalpy associated with this transition is found to depend on the width of the S Ad d range. These results suggest either tricritical or critical behaviour in a (T, molecular length) phase diagram Etude a partir d'observations microscopiques, d'analyses enthalpiques differentielles et de mesures de rayons X de la transition S Ad d-S Ad 2 de sept composes de la serie cyanobenzoyloxy-4 benzoate d'alcoxyphenyle. Influence de la plage de temperature d'existence de la phase S Ad d sur l'enthalpie de transition

Some derivatives of an homologous series of 4-aIkoxy-benzoyloxy-4′-cyanotolanes are synthesized. Two of them exhibit a stable nematic reentrant phase at atmospheric pressure.

A binary mixture of two disc-like liquid crystals 2, 3, 7, 8, 12, 13-hexa-n-tetradecanoyloxy and 2, 3, 7, 8, 12, 13-hexa (4-n-dodecyloxybenzoyloxy) truxenes exhibits an isotropic phase above and below the temperature existence range of a columnar mesophase.

Three new series of compounds exhibiting nematic and smectic A reentrant mesophases at atmospheric pressure are presented: 4-alcanoyloxybenzoyloxy-4′-cyanobiphenyl series, 4-alcanoyloxybenzoyloxy-4′-cyanostilbene series and 4-alcanoyloxybenzoyloxy-4′-cyanoazobenzene series. The influence of the alkyl, alkoxy and alcanoyloxy terminal substituents on the reentrant phenomenon is discussed.

A homologous series of 4′-alkoxyphenyl 4″-cyanophenyl, 1,4-cyclohexanedicarboxylates in which the alkoxy group is C1 → C12 has been prepared. Interphase transitions between solid, mesomorphic and isotropic phases were studied by hot stage microscopy. The nonyloxy compound is the first cyclohexane derivative to exhibit the reentrant phenomenon with the sequence K-Scre-Nre-Sc-SA-N-I. A reentrant nematic phase is also observed in the binary mixture of the octyloxy and decyloxy derivatives. A plot of the nematic-isotropic transition temperatures against the number of carbon atoms in the alkoxy chain shows the usual odd-even effect. The behaviour of this new series is compared with that of the corresponding benzenic homologues.

Induced cholesteric systems with reentrant mesomorphism were observed by doping reentrant nematic mixtures with optically active tygogenyn and l-menthol derivatives. Helix unwinding was observed on approaching T Ch -S A in both cholesteric phases. Influence of the dopant concentration upon the stability of the intermediate smectic phase and helical pitch is discussed Des systemes cholesteriques presentant un mesomorphisme rentrant ont ete obtenus en dopant un melange possedant une phase nematique rentrante avec des substances optiquement actives comme des derives de la tygogenyne et du l-menthol. Le deroulement de l'helice cholesterique a ete observe a l'approche de la transition T Ch -S A dans les phases cholesteriques haute et basse temperature. L'influence de la concentration du dopant sur la stabilite de la phase smectique intermediaire et sur la valeur du pas cholesterique est discutee

Four series of 4-alkoxyphenyl 4'-(4'-cyano- or nitro-benzoyloxy)-2'- or -3'-fluorobenzoates have been synthesized and the corresponding mesophases identified. These series show a rich variety of smectic modifications including several kinds of smectic C phases. A comparison of these series with related strongly polar mesogens is given.

Four series of 4-alkoxyphenyl 4′cyano or nitrobenzoyloxy 2″ or 3″-chlorobenzoates have been synthesized and various mesophases have been identified. A discussion of the relationships between the general molecular structure (relative direction of the different permanent dipoles) and the polymorphism is given. These series provide some interesting sequences including the first example of a SA2–SAd transition directly observable through a microscope.

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) ].

Two hexa-alkoxy derivatives (7a and 7b) of terephthal-bis-[4-benzoyloxyaniline] are described. They exhibit two new types of mesophase for which the term phasmidic is proposed: one has an hexagonal 2D lattice and the second an oblique 2D lattice Description de deux derives hexaalcoxy de la terephtal-bis-[benzoyloxy-4-aniline], presentent deux nouveaux types de mesophase. L'une de ces phases possede un reseau bidimensionnel hexagonal, l'autre un reseau bidimensionnel oblique

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.

Two homologous series of diarylethanes: 1-(4-alkoxy or alkyl-benzoyloxyphenyl)-2-(4′-bromo or cyanophenyl) ethanes (series I) and I-(4-alkoxy 3-bromo or cyanobenzoyloxyphenyl)-2-(4′-pentylphenyl) ethanes (series II) have been synthesized. The influence of the bromo or cyano group, in the meta or para position, on the mesomorphic range and the clearing point, is studied. In the series I, the bromo group induces a smectic polymorphism, whereas the cyano group induces nematic mesophases. In the series II, the larger the meta substituant, the lower the clearing point.

A new series of polar mesogens exhibits both a smectic B phase and a ribbon phase S cpâ . In this series, an incommensurate hexatic B phase has also been found. In this phase, a long wavelength modulation of the dipolar orientation is superimposed over the in-layer 2D hexagonal array. The structural features of this new phase are compared to those of the normal hexatic B and S cpâ phases Une nouvelle serie de mesogenes polaires presente a la fois des phases smectiques B et des phases en rubans S cpâ . Mise en evidence d'une phase hexatique B incommensurable dans laquelle une modulation de l'orientation des dipoles de grande periode se superpose a l'ordre hexagonal de chaque couche smectique. Comparaison des caracteres structuraux de cette phase a ceux des phases hexatiques et S cpâ

A systematic comparative study of several series of chemically neighbouring compounds apt to give a reentrant mesomorphism is proposed. Experimental data concerning the various reentrant phases are summed up. From these observations various factors favourable to the occurence of the reentrance are revealed. Thus, the chemical composition, the sense and the location of dipoles in the molecules, the reduced temperatures, the length of the rigid core are discussed as relevant parameters.

The polymorphism of the di- (4 - alkoxyphenyl) and di- (4 - alkanoyloxyphenyl) tetrathiafulvalenes (TTF) is investigated. In the dialkoxy series, the short chain derivative exhibits a smectic G phase and a nematic phase. The latter phase disappears in the long chain derivative. The dialkanoyloxy derivatives exhibit a smectic G phase (for short chains) and a smectic C phase (for long chains). A plot of the mesormorphic-isotropic transition temperatures against the number of carbon atoms of the substituent shows the usual odd-even effect. Some tetra (alkoxyphenyl) tetrathiafulvalenes are described, unfortunately they have no mesomorphic properties.

Six homologues of the new series 4–[4′-nitrobenzyloxy] benzylidene-4′-alkoxyaniline are presented. The reentrant sequence I N SA Nre SC was found for the first time in the pure compound : 4–[4′-nitrobenzyloxy]ben-zylidene-4′-undecyloxyaniline, while the dodecyloxy derivative presents the inverted sequence ISANSCS1.

Several homologous benzoate and cinnamate series which exhibit ferroelectric smectic CA phase have been synthesized. The chiral chains were prepared from commercially available α-amino acids. Some of these series give rise to compounds or mixtures which display large spontaneous polarization.

Several dissymmetrical hexasubstituted triphenylenes have been prepared. They exhibit a low temperature thermotropic mesophase. It is shown that it corresponds to a columnar mesophase (an hexagonal arrangement of columns of discs). The magnetic properties of these new disc-like liquid crystals are compared with those of the corresponding symmetrical ones.

Another example of Nre SA SC multicritical point is evidenced here with a new topology in a (x, T) binary diagram. In particular a second order Nre-SC line is confirmed and the tilt angle seems to be a pertinent orientational order parameter for this nematic-smectic change.

“Rod like” molecules with a cyano head group seldom exhibit a polymorphism with more than a nematic and a smectic A phase. The compound here reported presents a low temperature ordered mesophase which is shown to be of orthogonal smectic B type by the way of miscibi-lity observations in agreement with a recent X-ray characterization. This preliminary structural study also suggests some stimulating remarks about possible incommensurate periodicities in the three mesophases.

The first high-resolution specific-heat measurement on octylphenyl-2-chloro-4-(p-cyano benzoyloxy) benzoate (${\mathrm{DB}}_{8}$ClCN) is reported. At 390.2 K, the material undergoes a second-order phase transition from a bilayer smectic-A phase (Sm-${A}_{2}$) to a bilayer smectic-C phase (Sm-${C}_{2}$). On cooling, the specific heat of ${\mathrm{DB}}_{8}$ClCN is independent of temperature in the Sm-${A}_{2}$ phase and increases at ${T}_{c}$ in a steplike fashion. In the Sm-${C}_{2}$ phase, the specific heat again becomes temperature independent. This is in striking contrast to previously reported Sm-A\char21{}Sm-C transition data, all of which conform to a Landau model with an unusually large sixth-order term.