Color chiral nematics

Liquid crystalline behavior occurs in the exocuticle of certain classes of beetles. The bright iridescent colors that are reflected from the surface of Scarabaeid beetles originates from a petrified chiral nematic stmctural arrangement of chitin crystaUites in the exocuticle (38). It is suggested that this chiral nematic texture forms spontaneously in a mobile, Hquid crystal phase that is present during the initial stages of the exocuticle growth cycle. 

Many cellulose derivatives form Hquid crystalline phases, both in solution (lyotropic mesophases) and in the melt (thermotropic mesophases). The first report (96) showed that aqueous solutions of 30% hydroxypropylceUulose [9004-64-2] (HPC) form lyotropic mesophases that display iridescent colors characteristic of the chiral nematic (cholesteric) state. The field has grown rapidly and has been reviewed from different perspectives (97—101). 

The history of liquid crystals started with the pioneer works of Reinitzer and Lehmann (the latter constructed a heating stage for his microscope) at the end of the nineteenth century. Reinitzer was studying cholesteryl benzoate and found that this compound has two different melting points and undergoes some unexpected color changes when it passes from one phase to another [1]. In fact, he was observing a chiral nematic liquid crystal. 

The first application described was as temperature sensors by using a chiral nematic liquid crystal, which displays different colors at different temperatures. It is also worth noting that many common fluids are in fact liquid crystals. Soap, for instance, is a liquid crystal, and forms a variety of liquid crystal phases depending on its concentration in water. 

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. 

Fig. 10 (a, b) Schematic mechanism demonstrated for a reflective color M-paper with magnetically controllable characteristics, (c, d) The intensity of magnetic field dependence on the reflection spectra of chiral nematic mixtures doped with magnetite nanoparticles that are surface modified with oleic acid and a chiral pyridine-based dopant, as well as photographs of both formulations before and after a magnetic field of 1,000 GS was applied (see photograph insets above) [364], (Copyright 2010, Taylor Francis)... 

Fig. 2 Fingerprint pattern for chiral nematic mesophase of 40% EC in m-cresol. (View this art in color at www.dekker.com.)...

Fig.48 The Grandjean plane texture of the chiral nematic phase of supermolecule 43. There is a blue iridescent color which is due to the selective reflection of light from the helical macrostructure (xlOO)...

Like the other side-chain LCPs described in the previous section, these materials also oflfer the possibility of locking the chiral nematic phase into the glassy state by rapid cooling to temperatures below Tg. This leads to a preservation of the structure and its reflected color. With suitable systems, the process can thus be used to produce stable and light-fast monochromatic films. 

The chiral nematic phases can show a planar Grandjean textnre, with oily streaks caused by defects, but they can also show strong reflection colors, depending on the pitch of the helical structure within the 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. 

Lee H K, Doi K, Harada H, Tsutsumi O, Kanazawa A, Shiono T, Ikeda T. 2000. Photochemical modulation of color and transmittance in chiral nematic liquid crystal containing an azobenzene as a photosensitive chromophore. J Phys Chem B 104(30) 7023 7028. 

M. Mathews, N. Tamaoki, Planar chiral azobenzenophanes as chiroptic switches for photon mode reversible reflection color control in induced chiral nematic liquid crystals. J. Am. Chem. Soc. 130, 11409-11416 (2008)... 

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). 

FIGURE 3.1 (See color insert following page 14-20.) Chiral nematic LC induced by the addition of chiral dopant into... 

An explanation of the nomenclature should be made here. First, chiral nematic molecules need not come from cholesteryl derivatives, so we use the term chiral nematic instead of cholesteric when referring to liquid crystal materials. The chiral nematic/cholesteric phase itself we will call helical. Second, blue phases got their name from their blue appearance in early investigations. Blue phases are not always blue, however we now know that they may reflect light of other colors, including near infrared. Finally, BPIII was known as the fog phase or the gray fog phase in early publications. Although these terms are descriptive of this phase s appearance, BPIII seems to have survived. 

Interference and angle-dependent color effects can also be achieved by layers or particles based on liquid crystal polymers (LCP) Such effects can, for example, be produced by small plate-like substances which consist of an LCP material itself, or by small platelets which are uniformly coated with a cross-linked liquid crystalline polymer in a chiral-nematic arrangement. ... 

The observed optical properties of chiral nematic films depend critically on the direction of the director at the surface interface and on how this propagates to the bulk material. If the director is oriented along the surface of the cell using suitable alignment agents, such as rubbed polyimide, PVA, or PTFE, then the helix axis direction (see Fig. 1) is perpendicular to the substrates, as shown in Fig. 2 a. In this case, an optically active transparent planar texture is obtained. It is this texture that is normally used to observe the bright iridescent reflection colors initially observed by Reinitzer and Leh-... 

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