Cholesteric liquid crystals, optical

G. Chilaya, Cholesteric liquid crystals optics, electro-optics, and photo-optics. Chirality in liquid crystals, ed. H.-S. Kitzerow and C. Bahar (Springer, New York, 2001). 

Cholesteric Liquid Crystals Optics, Electro-optics, and Photo-optics... 

The unique properties of liquid crystals have also provided opportunity for study of novel nonlinear optical processes. An example involves the ability to modify the pitch of cholesteric liquid crystals. Because a pseudo-wave vector may be associated with the period of pitch, a number of interesting Umklapp type phasematching processes (processes in which wave vector conservation is relaxed to allow the vector addition to equal some combination of the material pseudo-wave vectors rather than zero) are possible in these pseudo-one-dimensional media. Shen and coworkers have investigated these employing optical third harmonic generation (5.) and four-wavemixing (6). 

Optically active polymerizable liquid-crystal monomers, (I), providing polymeric cholesteric liquid-crystal films were prepared by Seki et al. (2) and used in optical applications. 

Several unique optical properties arise from helical structures of cholesteric liquid crystals. 

These unique optical properties of cholesteric liquid crystals have been investigated... 

Electronic displays make use of the fact that the orientation of the molecules in liquid crystals changes in the presence of an electric field. This reorientation causes a change in their optical properties, making them opaque or transparent, and hence forming a pattern on a screen. Cholesteric liquid crystals are also of interest because the helical structure unwinds slightly as the temperature is changed. Because the twist of the helical structure affects the optical properties of the liquid crystal, these... 

This review deals with LC polymers containing mesogenic groups in the side chains of macromolecules. Having no pretence to cover the abundant literature related to thermotropic LC polymers, it seemed reasonable to deal with the most important topics associated with synthesis of nematic, smectic and cholesteric liquid crystals, the peculiarities of their structure and properties, and to discuss structural-optical transformations induced in these systems by electric and magnetic fields. Some aspects of this topic are also discussed in the reviews by Rehage and Finkelmann 27), and Hardy 28). Here we shall pay relatively more attention to the results of Soviet researchers working in the field. 

Belaykov, V. A., Sonin, A. S. Optica kholestericheskikh zidkih kristallov (Optics of cholesteric liquid Crystals), Moscow, Nauka, 1982... 

Organic materials with large optical rotations include cholesteric liquid crystals, molecules and polymers with chiral jt-conjugated systems, especially [n]helicenes [21, 31, 139]. The most important factor contributing to their large optical rotations is anomalous optical rotatory dispersion (ORD), which is associated with the presence of absorption (or reflection) with large rotational strength (Fig. 15.30). 

Examples for the many cases where the observed development of optical activity in a reaction could not be reproduced are the photoaddition of H2O2 to diethylfumarate [13] or the thermal decarboxylation of 2-phenyl-2-carboxylbu-tyric acid in cholesteric liquid crystals [14]. On the other hand, spurious optically active impurities may, especially in autocatalytic systems, cause considerable asymmetry effects. This exceptional case was demonstrated by Singleton and Vo... 

Cholesteric liquid crystals are optically active nematic phases as a result of their gradual twist in orientational alignment. Therefore, cholesteric liquid-crystalline solvents are expected to induce enantioselectivity in chemical reactions see reference [713] for a review on photoasymmetric induction by chiral mesophases. The existing results are not very promising. So far, the maximum photoasymmetric induction reported has... 

The theory of quadratic variations in optical activity with respect to the electric field strength was first formulated for macromolecules by Tinoco and Hammerle," and then developed by others." The earliest e qperi ments are due to Tinoco in solutions of poly-y-L-sJutamate in ethjdene dichloride of late, this experiment has been extended to transient optical rotation changes by Jennings and Baily." Also, electric field effects on the optical rotatory power of a compensated cholesteric liquid crystal have been stuped." ... 

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... 

Cholesteric liquid crystals are similar to smectic liquid crystals in that mesogenic molecules form layers. However, in the latter case molecules lie in two-dimensional layers with the long axes parallel to one another and perpendicular or at a uniform tilt angle to the plane of the layer. In the former molecules lie in a layer with one-dimensional nematic order and the direction of orientation of the molecules rotates by a small constant angle from one layer to the next. The displacement occurs about an axis of torsion, Z, which is normal to the planes. The distance between the two layers with molecular orientation differing by 360° is called the cholesteric pitch or simply the pitch. This model for the supermolecular structure in cholesteric liquid crystals was proposed by de Vries in 1951 long after cholesteric liquid crystals had been discovered. All of the optical features of the cholesteric liquid crystals can be explained with the structure proposed by de Vries and are described below. 

Other equally remarkable optical properties are associated with the selective reflection. At normal incidence, the reflected light is circularly polarized one circular component is totally reflected, while the other passes through unchanged. Also, quite contrary to what is found in normal substances, the reflected wave has the same sense of circular polarization as that of the incident wave. This is an important difference between the nature of the optical rotation of normal substances and of cholesteric liquid crystals. While the more familiar cases of optical rotation have their origin in the selective absorption of one circularly polarized component of the light, the form optical rotation of the twisted structure in cholesteric liquid crystals originates in the selective reflection of one circularly polarized component of the light. 

In Fig. 7 the optical rotatory dispersion (ORD) as well as the circular dichroism (CD) is shown for the right-handed cholesteric liquid crystal. A right-handed helical structure reflects right circularly polarized light and it shows positive optical rotation on the short wavelength side of the reflection band. 

Fig. 7. Optical properties of right-handed cholesteric liquid crystals...

The arrangement of chiral molecules in thermotropic liquid crystals is more complex, since entire volumes of space - rather than the bounded twisted ribbons discussed above - must be ed subject the constraint of a preferred twist between neighbouring molecules. The simplest examples of such mesophases are the cholesteric liquid crystals, discovered last century, (c/. section 5.1.8). This class of thermotropic liquid crystals derives its generic name from chiral cholesterol derivatives (shown below), which were found a century ago to exhibit peculiar optical changes as they were heated. 

Three-dimensional Cholesteric liquid crystal Multilayer Single crystal Optical notary pourcf Circular dichroism... 

Photocyclization using circular polarized light yielded dihydro[5]helicene in a small enantiomeric excess (ee = 3 %). Attempts were also made to enantioselectively synthesize helicenes using chiral solvents as well as cholesteric liquid crystals Excellent enantiomeric excesses (up to 98%) were obtained through temporary introduction of optically active residues like mandelic acid, lactic acid derivatives and (—)-menthyl esters... 

---