Mesogenic optically active

Chiral nematic Hquid crystals are sometimes referred to as spontaneously twisted nematics, and hence a special case of the nematic phase. The essential requirement for the chiral nematic stmcture is a chiral center that acts to bias the director of the Hquid crystal with a spontaneous cumulative twist. An ordinary nematic Hquid crystal can be converted into a chiral nematic by adding an optically active compound (4). In many cases the inverse of the pitch is directiy proportional to the molar concentration of the optically active compound. Racemic mixtures (1 1 mixtures of both isomers) of optically active mesogens form nematic rather than chiral nematic phases. Because of their twist encumbrance, chiral nematic Hquid crystals generally are more viscous than nematics (6). 

The first success was achieved when optically active (chiral) monomeric units were combined with a nematic LC polymer 105,123,143,144). The approach was based on the idea that a cholesteric mesophase may actually be realized as a helical nematic structure. Then by chemical binding of chiral and mesogenic units into a chain, accomplished by copolymerization or copolycondensation (in case of linear polymers) of nematogenic and optically active compounds, it was found feasible to twist a nematic mesophase and obtain copolymers of cholesteric type (Table 13). 

A number of liquid crystalline polyphosphazenes with mesogenic side groups have been prepared (48—50). Polymers with nonlinear optical activity have also been reported (51). Polyphosphazene membranes have been examined for gas, liquid, and metal ion separation, and for filtration (52—54). There is interest in phosphazene—organic copolymers, blends, and interpenetrating polymer networks (IPNs) (55—61) to take advantage of some of the special characteristics of phosphazenes such as flame retardance and low temperature flexibility. A large number of organic polymers with cydophosphazene substituents have been made (62). 

Chien and Cada [42] have prepared optically active and photoactive SCLC copolymers, 15, with the 4-alkoxyphenyl-4 -alkoxycinnamate chromophore, with the intention of creating LC polysiloxane networks that could be used to prepare macroscopically oriented organic ferroelectric polymers for electro-optical devices. Optical activity was introduced into the polymer by the use of a chiral spacer. Those copolymers which were mesogenic exhibited properties characteristic of a Sc. phase. UV-irradiation of thin films of the polymers in their mesomorphic states at 90°C, led to a loss of the IR absorption at 1635 cm-1 that is due to the cinnamate double bond, and to cross-linking. Long-term irradiation led to... 

Photoenantiomerization of different classes of organic molecules was investigated. However, the expectation that if a pure enantiomer can be racemized by light then photoderacemization of the racemate with cpl should lead to an optically active photostationary state, does not hold in all cases. Even when enantiomerization is fast, the Ae values are often too small. Molecules like 6 and 7 were extensively studied by Schuster and his group (e.g., [52,53]) in search of a molecule suited for an optical switch for mesogenic phase transitions. 

A large number of azobenzene-based amorphous and liquid crystalline polymers, particularly polyacrylates and polymethacrylates with chiral azobenzene pendants, have been prepared for the development of data storage and photonic devices [1-3,11-14]. For instance, the introduction of optically active mesogenic azobenzene residues into the side groups of the polymers produces chiral nematic and cholesteric phases, which are regulated by photoisomerization of the azobenzene units [10,14]. In most cases, however, the optical activity and chiroptical... 

In general, cholesteric liquid crystals are found in optically active (chiral) mesogenic materials. Nematic liquid crystals containing optically active compounds show cholesteric liquid crystalline behavior. Mixtures of right-handed and left-handed cholesteric liquid crystals at an adequate proportion give nematic liquid crystals. From these results cholesteric liquid crystals are sometimes classified into nematic liquid crystals as twisted nematics . On the other hand, cholesteric liquid crystals form batonnet and terrace-like droplets on cooling from isotropic liquids. These behaviors are characteristic of smectic liquid crystals. Furthermore, cholesteric liquid crystals correspond to optically negative mono-axial crystals, different from nematic... 

It is also known that in side-chain LC polymers the copolymerization of optically active monomers with mesogenic monomers, in the same manner as the mixing of optically active compounds with nematic low molecular weight compounds, can induce the formation of a cholesteric mesophase. Therefore, it is expected that inclusion of chiral spacers in main chain liquid crystal polymers, which would be nematic... 

Aromatic compounds have not only been of academic interest ever since organic chemistry became a scientific discipline in the first half of the nineteenth century but they are also important products in numerous hydrocarbon technologies, e.g. the catalytic hydrocracking of petroleum to produce gasoline, pyrolytic processes used in the formation of lower olefins and soot or the carbonization of coal in coke production [1]. The structures of benzene and polycyclic aromatic hydrocarbons (PAHs) can be found in many industrial products such as polymers [2], specialized dyes and luminescence materials [3], liquid crystals and other mesogenic materials [4]. Furthermore, the intrinsic (electronic) properties of aromatic compounds promoted their use in the design of organic conductors [5], solar cells [6],photo- and electroluminescent devices [3,7], optically active polymers [8], non-linear optical (NLO) materials [9], and in many other fields of research. 

A new series of liquid crystalline smectic C polymers has been successfully synthesized using anionic polymerizations of optically active mesogenic methaciylate... 

Abrikosov-like phases are also believed to be exhibited by some optically active side-chain liquid crystal polymethacrylates where the mesogenic side-group is derived from cholesterol [41]. The polymer, of structure 7, was reported by Freidzon et al. to exhibit a twisted layered structure in its smectic... 

Other metallomesogens containing the optically active butadiene iron tricarbonyl moiety, incorporated into chiral nematic mesogens, have recently been reported [116]. Both the diastereoisomers exhibited a chiral nematic and a monotropic smectic phase, the chiral nematic phase being monotropic for one diastereoisomer, and enantiotropic for the other. 

Usually, it is possible to observe chiral mesophases exhibiting ferroelec-tricity in mesogens made from optically active molecules, but in some particular cases, achiral bent-core molecules can form polar ordered smectic phases... 

In structure II in heptane both the main chain and the side chuns are hindered for PChMAA-11 Tjfc. is greater than 30 ns. The nuclei of the mesophase appear under conditions of structure 11, when the mobility of these mesogenic groups at the ends of hindered side chains decreases. Tteir formation changes the optical activity of the solution of cholesterol-containii polymers (F. 10) and greatly decreases the IMM of the main chain of PChMAA-11 increases up to 490 ns If the temper-... 

L. Chen, Y. Chen, K. Yao, W. Zhou, F. li, L. Chen, R. Hu, B.Z. Tang, A novel type of optically active helical liquid crystalline polymers synthesis and characterization of poly (p-phenylene)s containing teiphenyl mesogen with different terminal groups. J. Poly. ScL Part A Poly. Chem. 47, 4723-4735 (2009)... 

Januszko, A. Kaszynski, R Drzewinski, W. Ring effect on helical twisting power of optically active mesogenic esters derived from benzene, bicyclo[2.2.2]octane, and p-carborane carboxylic acids, J. Mater. Chem. 2006,16,452- 61. 

Figure 1. A smectic A phase extended on the left-hand side, and showing defect lines in the form of conics a smectic rodlet and small germs are floating in the isotropic phase. The mesogenic product, 4-cyano-4 -n-octylbiphenyl (8CB), was added together with a small amount of Canada balsam, and the mixture was observed between crossed polarizers at room temperature. Canada balsam is an isotropic but optically active and fluid resin, extracted from the conifer Abies balsamea, which facilitates the production of regular smectic textures in several thermotropic liquids. Collophony, or rosin, is a stabilized pine tree resin, used to rub violin bows Friedel used this substance not only for his violin, but also to obtain remarkable smectic textures. Scale bar 20 pm.

Replacement of the oxygen atom in the epoxy moiety by a sulfur atom to produce the analogous thiiranes [84] results not only in lower transition temperatures, but also significantly lower values for P, if the optically active unit is situated on the periphery of the molecule. The opposite is observed with respect to P, if the chiral entity is situated between two mesogenic units such as six-membered rings. A Walden inversion during the reaction produces an inversion of configuration at both asymmet-... 

In laterally alkyl substituted mesogens, each branch in a terminal or lateral substituent further decreases the mesophase stability dramatically (e.g. 2g, 2h, 3b, 6c, 6d, 7 and lOh). This fact is important in the synthesis of optically active compounds. 

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