Cholesteric reflection wavelength

While the above process is irreversible, trans - cis conversion of azobenzene in the mixtures of cholesteryl chloride and its nonanoate causes reversible shift of the cholesteric reflection wavelength (21). When azobenzene is chemically bonded to cholesteryl moiety, the same phenomenon was observed (22). 

The CD reflection spectra are quite sharp at all temperatures, and the reflection wavelength, corresponding to the optical pitch of the TChLC phase, increased progressively with temperature from 500 nm at 70 °C to 1,000 nm at 140 °C. It was considered that the positive sign of the CD reflection band indicated M screw sense helicity of the cholesteric phase. Very recently, a smectic A-cholesteric phase transition was also observed for PDMBS.348... 

The reflective wavelength of cholesteric liquid crystals varies according to temperature. Such an effect has been made useful in thermography. It has been applied in the diagnosis of cancers by displaying the skin temperature distribution. It has also been applied to test faults in integrated circuits. The applications also include thermometers and temperature warning indicators and non-destructive detection. 

Figure 3. Schematic representation of the cholesteric liquid-crystalline structure of cellulosics P=Xjn where P represents the pitch, A the reflection wavelength, and n the mean refractive index of a sheet. P>0 for a right-handed twist, and P<0 for a left-handed twist.

Equations (1) and (2) contain the pitch P besides experimentally available quantities, i.e., the selective reflection wavelength A , the optical rotation 0, and the refractive indices Mo and of a nematic sheet. The usually determined cholesteric birefringence can be converted to the nematic one by A nem= 2Awci,. By fitting Eq. (2) to the actual measured data, the pitch is obtained and the validity of Eq. (2) tested. In the limit of A Eq. (2) leads to... 

Recently, Li et al. reported the thermally, photochemicaUy, and electrically switch-able reflection colors in an azo-based chiral bent-core LC 29 [103]. This compound exhibited the cholesteric phase in the temperature range of 50 °C to 109 °C. Figure 5.22 shows the dependence of the central reflection wavelength of this compound upon UV irradiation. These changes can be attributed to trans to cis isomerization... 

FIGURE 5.22 Change in the central reflection wavelength of the cholesteric phase of compound 29 at 75 °C using continuous UV irradiation (X ax = 365 nm, 5.0 mW cm ). Reprinted with permission from Reference 103. Copyright 2011 Royal Society of Chemistry. 

Figure 13.5 Selective reflection wavelengths of the cholesteric and blue phases [21].

Equation (1) describes the selective reflection from a cholesteric material for normal incidence only. As the illuminating radiation becomes off-axis, the optics of the system become highly complicated. However, it has been shown that the angular dependence of the selectively reflected wavelengths varies approximately as... 

FIGURE 2.34 A fish tankthermometer uses a reflective cholesteric film as the sensor. As the water temperature changes, the cholesteric helix varies in pitch, causing a change in the reflected wavelength of the material. 

Fig. 8.5. Change in the selective reflection wavelength (1) and optical transparency (2) of cholesteric copolymer DC in an electric field (the direction of the axis of the cholesteric helix is indicated by the arrow) [13].

The reported cholesteric polysiloxanes are characterized by typical reflection of incident light in accordance with the establishment of a right-handed helical structure. The pitch of the helix decreases with increasing temperature with a concurrent shift of the maximum of reflection wavelength from near-IR towards the visible range." ... 

The cholesteric phase maybe considered a modification of the nematic phase since its molecular stmcture is similar. The cholesteric phase is characterized by a continuous change in the direction of the long axes of the molecules in adjacent layers within the sample. This leads to a twist about an axis perpendicular to the long axes of the molecules. If the pitch of the heHcal stmcture is the same as a wavelength of visible light, selective reflection of monochromatic light can be observed in the form of iridescent colors. 

The thermochromic behaviour of cholesteric LCs results from the fact that the pitch length, and hence the wavelength of the reflected light, is dependent on the temperature. 

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