Cholesteric selective reflection

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. 

Thermotropic cholesterics have several practical applications, some of which are very widespread. Most of the liquid crystal displays produced use either the twisted nematic (see Figure 7.3) or the supertwisted nematic electrooptical effects.6 The liquid crystal materials used in these cells contain a chiral component (effectively a cholesteric phase) which determines the twisting direction. Cholesteric LCs can also be used for storage displays utilizing the dynamic scattering mode.7 Short-pitch cholesterics with temperature-dependent selective reflection in the visible region show different colors at different temperatures and are used for popular digital thermometers.8... 

Selective reflection of light Thermochromic cholesteric devices, gel displays... 

Cholesteric LCs can act as hosts for dyes to produce coloured displays (see section 5.2.2.1 below) " their temperature dependent colour change has found applications in thermochromic inks, °" and as pigments and copy safe colours their selective reflecting capabilities have been applied in colours and filters for reflective displays and projection systems, " reflective polarisers and their electrical field induced switching in displays and smart reflectors, in colour patterning for full-colour recording. ... 

The selective reflection of circularly polarized light on radiation of normal light is also exhibited by the cholesteric polymers. Like the 1-l.c. systems, the Grandjean-texture is formed spontaneously, if the polymer is sandwiched between glass plates as shown in Fig. 23. The measurements of indicate no difference to low molar mass systems. In Fig. 24 VHT) is shown for the induced cholesteric polymers, whose birefringence was discussed above (refer to Table 8, copolymers No. 4, m = 6). The different curves refer to different mole fractions of the chiral comonomer. With... 

Liquid-crystalline solutions and melts of cellulosic polymers are often colored due to the selective reflection of visible fight, originating from the cholesteric helical periodicity. As a typical example, hydroxypropyl cellulose (HPC) is known to exhibit this optical property in aqueous solutions at polymer concentrations of 50-70 wt%. The aqueous solution system is also known to show an LCST-type of phase diagram and therefore becomes turbid at an elevated temperature [184]. 

For example, the selective reflection of a cholesteric mixture of derivative 1 ( 45 wt %)/monomer 2 (diethylene glycol dimethacrylate) covered the whole visible spectrum in a narrow temperature range i.e., the mixture was bluish at 20 °C, greenish at 30 °C, and reddish at 50 °C. Therefore, by successive photo-crosslinking of the liquid-crystalline sample at different temperatures through a mask, a colored picture or pattern could be fixed onto the resulting composite film. 

Figure 4.6-9 Induced cholesteric solutions Schematic outline of experiment and evaluation of the optical rotation p(A) related to the selective reflection band (reflection Cotton effect, RCE, centred at the wavelength A/ ) in order to characterize the chirality of the solute molecules by the helical twisting power.

Experimentally, the cholesteric structure parameters, i.e., pitch and handedness, can be derived from the optical properties of the phase and very specially from its so-called selective reflection. This most striking phenomenon is the reflection of one component of circular polarized radiation in a spectral interval around that wavelength which within the medium matches the cholesteric pitch, i.e. XrIh = i when n denotes the... 

Because of its correlation with the selective reflection we refer to this feature as reflection Cotton effect or, in short, RCE (Korte and Schrader, 1981). All together, a positive RCE indicates a left-handed cholesteric helical structure also called M helix and, vice versa,a negative RCE indicates a right-handed P helix. The evaluation is summarized in Fig. 4.6-9. 

As early as in 1951 de Vries showed that a twisted stack of birefringent layers is an adequate model for a cholesteric structure in order to reproduce a principally correct spectral dependence of the optical rotation also around the selective reflection band as it was recorded in a different way for Fig. 4.6-8. Even if the layer thickness is formally reduced to zero the optical rotation and its spectral dependence is preserved. Several other approaches were reported to describe particular effects of the cholesteric structure such as the selective reflectance or the rotatory anomaly (e.g. Chandrasekhar and Prasad, 1971 Chandrasekhar and Ranganath, 1974 SchSnhofer et al., 1983 Eidner et al., 1989). 

The transfer of the nomenclature of isotropic optical activity is surely acceptable where it evidently yields a descriptive picture of the experimental observations. However, precautions are necessary as to apparently self-evident implications, this is all the more important since the anisotropic nature of the sample is by no means obvious when observing parallel to the optical and thus helical axis. An unbiased and complete record of how a cholesteric sample acts on the measuring radiation can be obtained by el-lipsometry. This method (compare Sec. 6.4) yields a comprehensive description of the state of polarization including the degree of polarization (Rbseler, 1990). An adequate simulation can be based on the Berreman formalism (1972) rendering possible a study of particularities observed, such as pronounced depolarization related to the selective reflection band (Reins et al., 1994). 

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. 

Three types of copoly(Y-n-alkyl L-glutamate)s with the combination of methyl-hexyl (MH), methyl-octyl (MO) and propyl-octyl (PO) were prepared by alcoholysis of PMLG in ethylene dichloride using p-toluenesulfonic acid as the catalyst at 60 °C. The thermotropic mesophase was detected by the iridescent cholesteric color which appeared by annealing at temperatures in the region 110 190 C. Also the circular dichroism due to the selective reflection was measured at room temperature for the quenched films. Only the films with the copolymer composition of about 50/50... 

Starting from these polymers it is possible to introduce the chiral acids known from low molar mass liquid crystals (12) and to obtain the chiral homopolymers presented in Scheme III and Table III. These polymers show a high spontaneous polarization in the chiral smectic C phase (14) (see polymer 7, Table III) and selective reflection of visible light in the cholesteric phase (see polymer 9, Table III) (13). 

Hibert and Solladie obtained 0.43% ee of ( + )-hexahelicene(21). More importantly, they found a measurable ee when the solvent pitch approaches infinity. These results indicate chat cholesteric order does influence the motions which lead from reactant to product (probably by affecting the equilibrium between the reactant conformations) but that oven nematic-like order, when combined with molecular asymmetry, can alter the balance between enantiomeric rate processes. Additionally, a part of the ee may arise from the partial circular polarization of the excitation light which results from selective reflection of one light component at the cholesteric surface. 

In this liquid crystal phase, the molecules have non-symmetrical carbon atoms and thus lose mirror symmetry. Otherwise optically active molecules are doped into host nematogenic molecules to induce the chiral liquid crystals. The liquid crystals consisting of such molecules show a helical structure. The most important chiral liquid crystal is the cholesteric liquid crystals. As discussed in Section 1.2, the cholesteric liquid crystal was the first discovered liquid crystal and is an important member of the liquid crystal family. In some of the literature, it is denoted as the N phase, the chiral nematic liquid crystal. As a convention, the asterisk is used in the nomenclature of liquid crystals to mean the chiral phase. Cholesteric liquid crystals have beautiful and interesting optical properties, e.g., the selective reflection of circularly polarized light, significant optical rotation, circular dichroism, etc. 

As mentioned in Chapter 1, cholesteric liquid crystals have a helical structure. The helical structure results in unique optical properties, such as selective reflection, circular dichroism, drastic optical activity, and electro-optic effects. 

A sheet of cholesteric liquid crystals is sandwiched between two glass plates separated by a gap of tens microns. The cholesteric liquid crystals on two glass plates are homogeneously aligned to form the planar texture. The cell displays a bright color. The color varies according to the view angle and temperature. This is an important characteristic of cholesteric liquid crystals—selective reflection. 

The pitch P is the most important parameter of cholesteric liquid crystals. The physical properties of cholesteric liquid crystals are associated with P, such as selective reflection, optical rotation dispersion, circular dichroism, etc. The helical pitch is sensitive to the temperature and external field, for example, electric and magnetic field, chemical environment, pressure or radiation, etc. 

The cholesteric-like structure has been found in many biological bodies. The report on these discoveries is scattered, mainly based on optical rotation observations and chiral structure. For example, the wings and shells of some insects can rotate incident light and selectively reflect one circular polarized light (Shimamura et al., 1981). Very few experiments on these materials are available because of the difficulty in extracting and separating the effective materials from biological bodies. 

---