Nematic and Smectic Phases

The complex c/.v-IrCl(CO)2(OPhVpy), OPhVpy= /ra .v-4-alkoxy-4 -stilbazole), is mesomorphic and shows nematic and smectic phases below 140 °C.617,618... 

It could be concluded that there exists a more close structural analogy between nematic and smectic phases in LC polymers than for low-molecular liquid crystals. 

Most interesting are the effects of salt complexation on the mesomorphic behavior of liquid crystalline crown ethers and liquid crystalline crown ether polymers. Sodium triflate was added to poly(17) [34] and poly(25) (Scheme 14) [39]. The enantiotropic nematic and smectic phases of poly(17) were changed dramatically [40]. With increasing amounts of salt, the clearing temperatures are shifted to higher values while the melting transition increases only slightly. 

In these systems, the formation of both the nematic and smectic phases is attributed to a particular chain conformation implying that all the sub-units he parallel to each other to give an overall rod-hke shape, rather than a disk-hke or spherical shape. This is possible since the racemic AB2 monomer pos-... 

Molecules that contain a chiral center can form chiral liquid crystalline phases, where the orientation direction rotates in a helical fashion as one moves along the helical axis, which is perpendicular to the locally preferred direction of orientation. Both nematic and smectic phases can be chiral. In a chiral nematic phase, also known as a cholesteric, as one moves along the helical axis, the director rotates sinusoidally (see Fig. 10-31. Thus, if z is... 

Explain what a liquid crystal is, and state how nematic and smectic phases differ from ordinary liquids and crystalline solids (Section 23.3, Problems 11-12). 

Earlier, H-shaped orthometallated palladium(II) complexes were discussed which displayed nematic and smectic phases. However, Praefcke has reported related complexes where the number of chains on the periphery of the molecule is increased leading to columnar mesophases. The complex in Figure 91 displays a monotropic nematic discotic phase (No) and the structure has been confirmed as that in the diagram by single-crystal X-ray crystallography [170]. 

Figure 5. Arrhenius plots for triplet decay of 8a (a), 8b (b), and 8c (c) in the isotropic, nematic, and smectic phases of CCH-4 (O) and CCH-2 ( ). Filled arrows show the transition temperatures for CCH-4 and the open arrows show those for CCH-2 (Data from refs. 31a, 39).

Replace the terminal group by a chiral center and the chiral nematic and smectic phases can be obtained. 

Transient EMR has also been reported on the triplet state of retinal dissolved in liquid crystalline phase (Munzenmaier et al., 1992). The simulation of the transients with the stochastic Liouville equation provides the motional and order parameters of the pigment. The anisotropy ofmotional correlation times is high as expected for such an extended linear molecule and the correlation times couldbe followed with temperature over a range of two orders of magnitude in the nematic and smectic phase. 

N. Boden, Y. K. Levine, D. Lightowlers, and R. T. Squires, "NMR dipolar echoes in liquid crystals," Chem. Phys. Letters 31, 511 (1975) "Internal molecular disorder in the nematic and smectic phases of thermotropic liquid crystals studied by NMR SPDE experiments," ibid 34, 63-68 (1975). 

The true liquid-crystal mesophases of calamitic mesogens are divided into two classes—the nematic and smectic phases and these will be described in turn. There is, in addition, a series of crystal smectic phases that are not really liquid-crystal phases but which for many years were classified as such these will be mentioned briefly below. 

When a compound exhibits both nematic and smectic phases, then, as a rule, the nematic occurs at a higher temperature. Exceptions to the rule were discovered by Cladis in certain strongly polar materials. The first observations were on binary mixtures of two cyano compounds over a range of composition the sequence of transitions on cooling was as follows. 

Chemical modification of the polymer structure allows the obtention of nematic and smectic phases [4, 5]. If the side group and/or the chain are chiral, then cholesteric or chiral smectic C (SmC) phases can be obtained. These can also be obtained by mixing a chiral compound with the SCLCP. SmC SCLCPs are of particular interest and their behavior is described in Sec. 2 of this Chapter. 

This section will concentrate on rod-like complexes which typically form nematic and smectic phases. While the approach does not concentrate on design aspects, it may become apparent that in rod-like systems, design is not a simple matter and there are many factors to be taken into account to do with anisotropy, disposition of ligands and functional groups associated with the metal and introduced on complexation. Thus, while it has been said (albeit with tongue slightly in cheek) that for discotic systems it is possible to realize liquid crystal systems by taking a... 

These results show that the side chain polymers fit the model well. Furthermore they confirm the expected analogy of these spacer-group-containing polymers with conventional liquid crystals that nematic and smectic phases can be realized, depending on the para substituents of the mesogenic moieties. 

Figure 1. Schematic of Molecular Organization in Nematic and Smectic Phases,...

The perturbations to rod-like molecules created by functional groups can operate also on disk-like ones. When substituted with at least one flexible chain and at least one other (frequently polar) group, preferably in a para orientation, many molecules with benzene or cyclohexane as cores provide nematic and smectic phases [11,12]. Additional substituents can also lead to other mesomorphic aggregation schemes [105]. The shapes of these molecules can no longer be considered disk-like, and they owe their ability to be liquid crystalline to shape anisotropy and electronic factors alluded to previously [18,19]. 

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