Wavelength chiral nematics

Note 3 Chiral nematic mesophases exhibit Bragg scattering of circularly polarised light at a wavelength proportional to the pitch P (Xr = P, where is the mean refractive index). 

Crown ethers of the type discussed in this section have been used as sensors, membranes, or materials for chromatography. Shinkai used cholesterol-substituted crown ether 10 as a sensor for chirality in chiral ammonium compounds (Scheme 16). It was found that the pitch of the cholesteric phase exhibited by 10 was changed upon addition of the chiral salt. As the wavelength of reflection for incident light depends on the pitch, a color change was observed that was visible to the naked eye [45, 46]. Such chirality sensing systems were known before but chromophores had to be bound to the crown ether in order to observe color changes [47]. This problem could be overcome by 10, which uses intrinsic properties of the chiral nematic phase. 

Recently, true circular polarizers have been developed and are available from commercial sources. These are fashioned by sandwiching a chiral nematic liquid crystal between parallel glass windows. These devices are characterized by a high transmission and extinction ratios greater than 1000. They operate over a fairly narrow range of wavelengths about a central wavelength. 

Very large values of gCPPL may be more easily obtained via doping of LCs, e.g. the near maximum value of gCPPL = 1.8 for 0.2% achiral ter(fluorene) doped in chiral nematic LC film (35-pm thick) at the wavelength range of selective reflection for the LC [135],... 

Only a few solvents are known to dissolve cellulose completely, and solid cellulose decomposes before melting. Therefore, it is difficult to study the mesophase behavior of cellulose. Chanzy et al. [32] reported lyotropic mesophases of cellulose in a mixture of jV-methyl-morpholine-Af-oxide and water (20-50%), but were unable to determine the nature of the mesophase. Lyotropic cholesteric mesophase formation in highly concentrated mixtures of cellulose in trifluoroa-cetic acid + chlorinated-alkane solvent [33] and in ammonia/ammonium thiocyanate solutions [34] has been studied, and although poor textures were obtained in the polarizing microscope, high optical rotatory power has been measured in an optical rotation (ORD) experiment, which could be fitted to the de Vries equation [Eq. (3)] for selective reflection beyond the visible wavelength region and was taken as proof of a lyotropic chiral nematic phase. 

The optical properties of chiral nematic phases are closely related to their supermolecular Structures, as stated by the considerations of de Vries. In particular, the planar textures exhibit beautiful colors correlated to the pitch P of the helicoidal structures by Eq. (1), if the selective reflection wavelength lies in the visible range, and many examples are shown in Fig. 2. 

T-CNs film has a polydomain structure in which the helical axes of different chiral nematic domains point in different directions, with the pitch of each domain giving an average pitch. Because the pitch is related to the wavelength of maximum reflection of 2., by... 

The linear electro-optic effect in a cholesteric, i.e. a hard-twisted chiral nematic (the helical pitch must be less than the wavelength of visible light) was a very original proposal for using the flexoelectric effect in a new display, shutter or modulator device. The patent application by R.B. Meyer and J.S. Patel dates from 1987 and was granted in 1990. The physics was developed in a series of papers by these authors and later elaborated by others.It is now commonly called the flexoelectro-optic effect. 

Fig. 7.3. The deviation of the optic axis in a cholesteric (hard-twisted chiral nematic, p <, where p is the cholesteric pitch and A is the wavelength of light) when an electric field E is applied perpendicular to the helical axis. The cholesteric geometry allows a fiexoelectric polarization to be induced in the direction of E. The plane containing the director, which is perpendicular to the page in the middle figure and is shown in the lower figure, illustrates the splay-bend distortion and the corresponding polarization that arises. (After Rudquist, inspired by Meyer and Patel.

TLCs are optically active mixtures of organic molecules. The correct name for TLCs used for temperature measurements is chiral nematic or cholesteric Uquid crystals. TLCs are characterized by well-analyzed reflections of visible Ught (color play) within a definite bandwidth of temperature. A certain temperature leads to reflections of an explicit spectrum of wavelengths, with a local maximum and a narrow bandwidth. Below the start temperature of the color play, called red-start temperature, the TLCs are transparent, when applied in thin layers the bulk looks milky and white. In this state the molecules, which are elongated like a cigar, have a typical size of about 2-5 nm [1], are well ordered, and are close to each other like in a solid crystal (see Fig. 2a). 

Fig. 1. Schematic representation of (a) nematic, (b) smectic and (c) cholesteric (or chiral nematic) liquid crystalline phases. In the nematic phase only orientational correlations are present with a mean alignment in the direction of the director n. In the smectic phase there are additional layer-like correlations between the molecules in planes perpendicular to the director. The planes, drawn as broken lines, are in reality due to density variations in the direction of the director. The interplane separation then corresponds to the period of these density waves. In the cholesteric phase the molecules lie in planes (defined by broken lines) twisted with respect to each other. Since the molecules in one plane exhibit nematic-like order with a mean alignment defined by the director n, the director traces out a right- or left-handed helix on translation through the cholesteric medium in a direction perpendicular to the planes. When the period of this helix is of the order of the wavelength of light, the cholesteric phase exhibits bright Bragg-like reflections. In these illustrations the space between the molecules (drawn as ellipsoids for simplicity) will be filled with the alkyl chains, etc., to give a fairly high packing...

The pitch of a chiral nematic phase is the distance along the hehx over which the director rotates by 360°. It should be noted, however, that the stmcture repeats itself every half pitch due to the equivalency of and -ft Interesting optical effects occur when the wavelength of light in the liquid crystal is equal to the pitch. These will be described at length in Chapter 11. The pitch of a chiral nematic phase can be as short as 100 nm. Mixing the two optical isomers in various proportions allows the pitch to be increased from the pilch of either of the two pitre optical isomers. A racemic mixture (equal parts of each optical isomer) possesses an infinite pitch and is therefore nematic. Finally, the chiral nematic phase is often ealled the cholesteric phase, since many of the first compounds that possessed this phase were derivatives of cholesterol. 

There is a special case for propagation of light along the helical axis of a chiral nematic liquid crystal that is very important for display apphcations. This special case is when the pitch of the chiral nematic is much greater than the wavelength, P When this... 

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