Mesogenic substituent

Kinoshita, I. Kijima, M. Shirakawa, H. Liquid crystalline behavior of poly(m-phcnylcncbutadiynylene)s having a mesogenic substituent. Mol. Cryst. Liq. Cryst. 2001, 365, 1193-1202. 

Several 1,2-disubstituted Fc derivatives 48 ( = 6, 12) were prepared, but none of the compounds showed liquid-crystalline properties. " From our systematic investigations on disubstituted ferrocenes, we could clearly establish the following liquid-crystal tendency 1,3-> , -> 1,2-isomeric structures. This sequence can be explained in terms of structural features (i) the 1,3-disubstituted ferrocenes possess the highest molecular anisotropy among the isomeric structures, (ii) the substituents located at the 1,1 -positions generate a step in the structure (S- or U-shape), which results in a reduction of the molecular anisotropy and a loss of colinearity of the mesogenic substituents, and (iii) the substituents located at the 1,2-positions give rise to a hairpin structure which lacks molecular anisotropy. 

The properties described in the previous section apply to networks exhibiting helical structures. This type of mesophase is obtained by introducing side mesogenic substituents with chiral end groups on the backbone [94, 138-141, 145-147], or by doping an achiral network with a chiral molecule [144,145]. As a result of the interaction between orientational (liquid-crystalline) and translational (network) degrees of freedom, these noncentrosymmetric systems exhibit rich electromechanical effects, such as piezoelectricity. 

Figure 15. Snapshot showing the molecule order of a third generation carbosileme dendrimer with laterally bonded mesogenic substituents in a nematic solvent.

FIGURE 14.20 Topology of a metallocene-based mesogen (a) and tricarbollide derivative 66L (b). The gray element represents the plane of the it system that is coordinated to the metal, and the white element is a mesogenic substituent. 

Compounds with shorter spacers showed poor or no liquid crystalline properties. From this and the results of X-ray investigations it is concluded that the long mesogenic substituents are bent at the spacers, yielding a more elongated molecular shape despite the tetrahedral symmetry of the central carbon atom. 

The covalent attachment of mesogenic substituents on the framework of PT is another potentially powerful method for controlling the long-range order and orientation of the polymer chains. Until now this interesting approach has been considered in only a few papers, A seminal work reported the synthesis of thiophenes 60-62 [123]. Monomers 61 and 62 were claimed to exhibit mesogenic properties, but no detailed characterization was reported. 

Aromatic polyesters constitute an important class of main-chain liquid-crystalline polymers, but present the inconvenience of their reduced solubility and very high transition temperatures (sometimes not detected before the degradation of the sample). Their processability can be improved in several ways [2,3], e.g., reduction of the rigidity of the mesogen, lengthening of the spacer, or introduction of lateral substituents. 

Fig. 5a,b. Tolane compounda a melts at 39 °C and is not mesogenic . b differs only in one tail substituent melts to a nematic phase at 61 °C which persists until 89 °C... 

A common feature of many mesogenic molecules is the presence of polar substituents and aromatic cores [3]. The electrostatic interactions between such groups can be incorporated into a molecular potential with the addition of dipolar and quadrupolar terms, respectively. Rather than represent these permanent electrostatic interactions by using a model in which a charge distribution is scattered over the surface of the molecule, it is very common to use one (or more) point multipoles [2,29]. Thus for an electrostatic Gay-Berne model, the pair potential is given by the sum... 

Coco, S Espinet, P Marb n-Alvarez, J.M. and Levelut, A.M. (1997) Effects of Isonitrile Substituents on Mesogenic Properties of Halogoldiisonitrile Complexes Calamitic and Discotic Liquid Crystals. Journal of Materials Chemistry, 7, 19-23. 

Vemula, P.K., Mallia, V.A., Bizati, K. and John, G. (2007) Cholesterol Phenoxy Hexanoate Mesogens Effect of meta Substituents on Their Liquid Crystalline Behavior and in Situ Metal Nanoparticle Smtesis. Chemistry of Materials, 19, 5203-5206. 

Figure 6. General structure for phosphazenes with mesogenic side groups. Example is a mixed substituent polymer (VII) where R represents the trifluoroethoxy group and the mesogen with flexible spacer is represented by the curlicue and rectangular box.

Apart from the parent compound 1 and its very simple alkyl derivatives, 1,3,4-oxadiazoles are solids. Solid oxadiazoles containing biphenyl or triphenyl substituents exhibit interesting properties upon heating. The symmetric 2,5-bisbiphenyl-4-yl-l,3,4-oxadiazole 38 melts into an isotropic phase showing small monotropic mesophase. By contrast, the asymmetric (hockey stick-shaped) mesogen 2-terphenyl-4-yl-5-phenyl-l,3,4-oxadiazole 39 exhibits a more stable enantiotropic liquid crystalline phase (a smectic phase as well as a nematic phase) <2001PCB8845>. 

ID and 2D 13C NMR were carried out in a series of novel nematogens by Bayle and co-workers to study the effects on the conformation and order due to the addition of lateral and/or terminal substituents.249 251 For example, lateral flexible substituents are found to adopt a mean conformation more or less parallel to the mesogenic core. As a result, lateral chains are less disordered than terminal chains. Nematogens containing polyoxyethylene ether (POE) chain as a lateral crown ether and terminal chain(s) have been... 

Note See Fig. 35. The mesogenic side-groups can be attached either as lateral substituents to the backbone mesogenic moieties that are connected to each other either (a) directly or (b) by spacers or (c) they can be attached to the spacer incorporated into the main-chain. 

For device purposes, molecules used should be stable to heat and light. The LC phases have to be stable over a wide temperature range around room-temperature. This can be achieved by using mixtures of mesogens. A mixture of cyanobiphenyls, one with alkyl substituent (CjH,) and the other with alkoxy substituent (CgHi70), has proved to be most useful. 

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