Nematic esters

As mentioned previously, Schiff bases are chemically relatively unstable. This is especially true of d.c. driven cells incorporating components that favor liquid crystal decomposition. Using the glass solder technique in cell manufacture substantially improves the stability of Schiff bases. The stability of Azoxy-compounds and nematic ester, of which the latter has only been available more recently, is hardly affected by the cell components. 

Figure 22. Reflectance maxima as a function of temperature for a chiral nematic at normal incidence. The material is a mixture of chiral nematic esters from Merck and demonstrates the pitch divergence on approaching either the isotropic or smectic A phase from the N phase (see [46]).

The separation of Hquid crystals as the concentration of ceUulose increases above a critical value (30%) is mosdy because of the higher combinatorial entropy of mixing of the conformationaHy extended ceUulosic chains in the ordered phase. The critical concentration depends on solvent and temperature, and has been estimated from the polymer chain conformation using lattice and virial theories of nematic ordering (102—107). The side-chain substituents govern solubiHty, and if sufficiently bulky and flexible can yield a thermotropic mesophase in an accessible temperature range. AcetoxypropylceUulose [96420-45-8], prepared by acetylating HPC, was the first reported thermotropic ceUulosic (108), and numerous other heavily substituted esters and ethers of hydroxyalkyl ceUuloses also form equUibrium chiral nematic phases, even at ambient temperatures. 

In the ordered smectic or nematic phase, the rigid rods are arranged in parallel arrays that allow for close packing. The nematic phase is the most common type found with synthetic polymer molecules. The molecules long axes are parallel, but there is no layering. Aromatic polymer chains that have stiff ester or amide linkages are ideal. 

Derivatives of aliphatic alkynes (14 and 15) are more thermally unstable than 12, but they show SmA and N phases at low temperatures (below 130 °C). The type of phase and the mesophase stability depend on the length of both the terminal and the lateral chains. When both chains are elongated, the mesomorphism becomes metastable and compounds 14 display monotropic N and SmA transitions. Complexes IS, which contains an ester group with an opposite direction to that of complexes 14, display less stable nematic mesophases. 

One alternative approach is to use photoisomerisable chiral compounds where the E and Z isomers have different helical twisting powers, e.g. menthone derivatives. By incorporating co-polymers, prepared from menthone containing monomers and cyano esters (5.5), as dopants into nematic LC mixtures materials, e.g. a mixture of cyanobiphenyls and cyanoterphenyls (E7 available from Merck), colour change can be effected by irradiating with UV light (365 nm). The colour obtained is dependent... 

In the last few years disc-like molecules have been shown to form liquid crystals (Chandrasekhar, 1994). Typical of them are hexasubstituted esters of benzene (I) and certain porphyrin esters (II) (see below). In the liquid crystalline state, the disc-like molecules are stacked aperiodically in columns (liquid-like), the different columns packing in a two-dimensional array (crystal-like). The phases have translational periodicity in two dimensions but liquid-like disorder in the third. In addition to the columnar phase(D), the disc-like molecules also exhibit a nematic phase (Nj,). A transition between D and phases has been reported. 

The directors (long molecular axes) of the constituent molecules in nematic phases are parallel to one another on average. This is the only order present in nematic liquid crystals, which are the most fluid type of liquid-crystalline phase. Molecules that form cholesteric phases must be optically active or contain an optically active dopant. As the phase name implies, the constituent molecules are frequently steroids and most commonly are cholesteric esters or halides. A conceptual model of the cholesteric phase includes layers of molecules in nematic-like positions, each layer being twisted slightly with respect to the ones above and below it. When the phase consists only of optically active molecules, the angle of twist between layers is typically less than one degree. Several subclasses of discotic phases exist. In all, the molecular planes of the constituent molecules are parallel. However, the discs can pack in nematic-like arrangements (ND) or in columns that are internally ordered (D ) or disordered (Dd) and may be stacked vertically,... 

The methyl-substituted ester model compound, shown below, also shows both smectic and nematic phases, as well as a smectic C-to-nematic transitional phase (18) from 169 to 170°C. Figure 3 shows two of the textures observed in this model. 

Figure 2. DSC trace of methyl-substituted poly(ester) with pentamethylene spacer, which displayed both smectic and nematic textures.

Figure 3. Liquid-crystal textures of the methyl-substituted model ester viewed through crossed polarizers, a, Smectic C-to-nematic transitional phase and b, smectic mosaic texture at 160 °C. Original magnification, 320x.

It is also possible to attach chiral peripheral groups to benzo[15]crown-5 as demonstrated by Shinkai [28]. In 8 (Scheme 4), a cholesteryl subunit was attached using an ester group as linker and a chiral nematic phase was observed. 

Bayle presented liquid crystal 34 (Scheme 19) bearing four aromatic units linked by ester and azo functional groups [56]. Two butyloxy groups are attached at the ends of the molecule and the crown ether is bound at the side of the molecule. The nematic phase exhibited by 34 is quite broad (AT = 96 K). Upon complexation with LiBF4, the nematic range diminishes with increasing amounts of added salt and disappears completely at 0.5 equiv. of added LiBF4 which is most likely due to the formation of a 2 1 crown lithium complex. From 0.2 equiv. of salt, a smectic... 

After DMAP-catalyzed esterification of 42a with tri-benzoate esters, liquid crystals 55, 56 (Scheme 32) were obtained, which both displayed narrow nematic phases as identified by their typical Schlieren texture. The melting and clearing points are high and, in the case of 55, accompanied by decomposition. Compounds with shorter arms were also synthesized but found to be crystalline. 

Abstract It is well known that spontaneous deracemization or spontaneous chiral resolution occasionally occurs when racemic molecules are crystallized. However, it is not easy to believe such phenomenon will occur when forming liquid crystal phases. Spontaneous chiral domain formation is introduced, when molecules form particular liquid crystal phases. Such molecules possess no chiral carbon but may have axial chirality. However, the potential barrier between two chiral states is low enough to allow mutual transformation even at room temperature. Therefore the systems are essentially not racemic but nonchiral or achiral. First, enhanced chirality by doping chiral nematic liquid crystals with nonchiral molecules is described. Emphasis is made on ester molecules for their anomalous behavior. Second, spontaneous chiral resolution is discussed. Three examples with rod-, bent-, and diskshaped molecules are shown to give such phenomena. Particular attention will be paid to controlling enantiomeric excess (ee). Actually, almost 100% ee was obtained by applying some external chiral stimuli. This is very noteworthy in the sense that we can create chiral molecules (chiral field) without using any chiral species. 

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