Thermal phase diagrams of columnar liquid crystals

In order to understand the possible sequence of transitions from the disordered columnar phase to the helical phase in hexa(hexylthio)triphenylene (HHTT), we study a three-dimensional planar model with octupolar interactions inscribed on a triangular lattice of columns. We obtain thermal phase diagrams using a mean-field approximation and Monte Carlo simulations. These two approaches give similar results, namely, in the quasi one-dimensional regime, as the temperature is lowered, the columns order with a linear polarization, whereas helical phases develop at lower temperatures. The helicity patterns of the helical phases are determined by the exact nature of the frustration in the system, itself related to the octupolar nature of the molecules.Comment: 12 pages, 9 figures, ReVTe

Molecular arrangement of mesogenic disc-like compounds : the hexa-n-alkoxy and hexa-n-alkanoyloxy triphenylenes

A theoretical study of homologous series of triphenylene hexaesters and hexaethers, disc-like mesogenic molecules, has been undertaken with semi-empirical methods. Preferential conformations of an isolated molecule, a pair of molecules and a set of molecules have been established For butyl and further homologues, hexaesters give irregular columns, resulting in piled pairs, with distances d and d' equal to 3.75 and 4.45 Å between neighbouring aromatic cores. On the other hand hexaethers give regular columns (d ≃ 3.28 Å) of helicoidal type with a pitch equal to 14 Å. These results are in agreement with data from X-ray diffraction on the mesophase.L'étude théorique de séries homologues d'hexaesters et d'hexaéthers du triphénylène, molécules discoïdes mésomorphogènes a été effectuée par des méthodes semi-empiriques. Les conformations préférentielles d'une molécule isolée, d'une paire de molécules et d'un ensemble de molécules ont été établies. Au-delà du terme butylique, les hexaesters forment des colonnes irrégulières, résultant de l'empilement de paires, avec des distances d et d' de 3,75 et 4,45 Å entre plateaux aromatiques parallèles successifs. Les hexaéthers, par contre, forment des colonnes régulières (d ≃ 3,28 Å) de type hélicoïdal de pas égal à 14 Å. Les résultats sont en bon accord avec ceux déduits de la diffraction des rayons X par la mésophase

Molecular conformation and arrangement of a disc like compound by semi-empirical calculations

The 2, 3, 6, 7, 10, 11 hexahydroxy triphenylene hexapropylester, a disc like molecule belonging to a series of compounds, some of which show a new mesomorphic and thermotropic phase has been studied by semi-empirical conformational analysis. In the preferential conformation, propyl ester chains are alternatively situated about and under the triphenylene core, the ester group making an angle of 56° with this latter. Molecules are associated in very stable pairs, with a rather specific configuration. A molecular arrangement in columns is proposed.L'hexapropylester du 2, 3, 6, 7, 10, 11 hexahydroxytriphénylène, molécule discoide appartenant à une série de composés, dont certains présentent une phase thermotrope nouvelle a fait l'objet d'une étude conformationnelle par des méthodes semi-empiriques. La conformation préférentielle correspond à des chaînes alternativement situées au-dessus et au-dessous du noyau triphénylène, le groupement ester faisant un angle de 56° avec ce dernier. Les molécules s'associent par paires très stables correspondant à une configuration bien définie. Un arrangement des molécules en colonnes est proposé

Crystalline arrangement of some disc like compounds

Three triphenylene esters, disc-like molecules, belonging to a series of compounds, some of which show a new mesomorphic and thermotropic phase have been studied by X-ray diffraction. The crystal structure of one of these esters has been established. Molecules are associated in pairs with their triphenylene cores parallel and 3.43 Å. These dimers give columns arranged in hexagonal lattice.Trois esters du triphénylène, molécules de type discoïde appartenant à une série de composés, dont certains présentent une phase mésomorphe thermotrope nouvelle, ont été étudiés par diffraction des rayons X. La structure d'un de ces esters a été établie. Les molécules s'associent en donnant des paires, leurs noyaux triphénylène étant parallèles et à 3,43 Å l'un de l'autre. Ces dimères forment des colonnes arrangées suivant un réseau hexagonal

Steric hindrance influence of a diazo link upon mesogenic properties of some ligands and related copper complexes

Some new polymethylated compounds containing three rings and four rings have been synthetized. The crystalline structures of 2,3-dimethyl-4-ethoxy benzoyloxy- 4'-(4-methoxysalicylaldimine)-azobenzene (DIM) and 2,3,6-trimethyl-4-methylben zoyloxy-4'-butylazobenzene (TRIM) are described. Due to larger steric interactions, the diazo linkage makes a larger dihedral angle in TRIM than in DIM. The effect of steric hindrance of the diazo linkage upon the mesophase stability is discussed by the use of isotropic Carbon-13 chemical shifts, molecular modelling and transition temperatures, TNI. For the three rings core, the thermal stability of the mesophase decreases with substitution pattern dissymetry. The effect of the conjugation loss is evidenced by the decrease of TNI versus the carbon number within the chain. For the four rings core, the effect on the transition temperatures of polymethylated ligands is nearly independent of the length of the mesogenic core and depends only on the lateral substituent positions within the core. For the related copper complexes, the TNI transition temperatures exhibit a small dependence upon the methyl substitution

Steric Hindrance Influence of a Diazo Link upon Mesogenic Properties of some Ligands and Related Copper Complexes

International audienceSome new polymethylated compounds containing three rings and four rings have been synthetized. The crystalline structures of 2,3-dimethyl-4-ethyloxbenzyloxy-4'-(4-methoxysalicylaldimine)-azobenzene (DIM ) and 2,3,6-trimerhyl-4-metÉylbènzoyloxy-4'-butylazobenzene (TRIM) are described. Due to large steric interactions, the diazo linkage make sa larger dihedral angle in TRIM than in DIM.The effect of steric hindrance of the diazo linkage upon the mesophase stability is discussed by the use of isotropic carbon-13C chemical shifts, molecular rmodeliing and transition temperatures Ty. For the three rngs core, the thermal stability of the mesophase decreases with substitution pattern dissymetry.The effect of the conjugation loss is evidenced by the decrease of TN versus the carbon number within the chain. For the four rings core, the effect on the transition temperatures of polymethylatedl igands is nearly independent of the length of the mesogenicc ore and dependso nly on the lateral substituentp ositionsw ithin the core. For the related copper complexes, the Tm transition temperatures exhibit a small dependence upon the methyl substitution