Liquid crystals, thermochromic materials

Liquid crystal, thermochromic material Fiber itself Lithium niobate Gallium arsenide, various phosphors... 

Who among us does not enjoy the vision of a rainbow The spectral colors spread out across the sky inspire awe and pleasure no matter how many times we see them. For this reason, we have attempted to replicate rainbows artificially. We can go to any toyshop and find rainbow glasses, any rock shop and find rainbow quartz made by plasma ionization techniques. Jewelry and craft shops sell rainbow jewelry made by electroplating titanium or niobium oxide. Teachers use rainbow tubes with a universal indicator to demonstrate pH changes and acid-base reactions. Dye mixtures can be separated by rainbow electrophoresis and rainbow chromatography. Rainbow thermometers and toys are made from liquid crystals. Thermochromic materials can be heated to produce rainbow colors. Every toy shop is awash with rainbow colors, a phenomenon not available prior to the many inventions of the twentieth century [39]. 

In the thermochromic liquid crystal (TLC) the dominant reflected wavelength is temperature-dependent and it has been employed for full-field mapping of temperature fields for over three decades. Although it is non-intrusive and cost effective, there are some problems in applying it to micro-scale measurements, because of size (typically tens of micrometers) and time response (from a few milliseconds to several hundred milliseconds depending on the material and the form). Examples of application are micro-fabricated systems (Chaudhari et al. 1998 Liu et al. 2002) and electronic components (Azar et al. 1991). 

Although, in theory, intrinsically thermochromic compounds should be the materials of choice for many apphcations, in practice, with the major exception of thermotropic liquid crystals (see Chapter 5, section 5.2.2), they often require quite high temperatures to effect the change, and this has limited their use. Additionally, there is also a problem with synthesising compounds to cover the desired colour gamut across the visible spectrum. Consequently, indirect systems, in which the chro-mophoric material reacts to changes in its environment brought about by heat, have... 

Abstract We describe mechanochromic and thermochromic photoluminescent liquid crystals. In particular, mechanochromic photoluminescent liquid crystals found recently, which are new stimuli-responsive materials are reported. For example, photoluminescent liquid crystals having bulky dendritic moieties with long alkyl chains change their photoluminescent colors by mechanical stimuli associated with isothermal phase transitions. The photoluminescent properties of molecular assemblies depend on their assembled structures. Therefore, controlling the structures of molecular assemblies with external stimuli leads to the development of stimuli-responsive luminescent materials. Mechanochromic photoluminescent properties are also observed for a photoluminescent metallomesogen and a liquid-crystalline polymer. We also show thermochromic photoluminescent liquid crystals based on origo-(/ -phenylenevinylene) and anthracene moieties and a thermochromic photoluminescent metallocomplex. 

F. P. Shvartsman and V. A. Krongauz, Quasi-liquid crystals of thermochromic spiropyrans. A material intermediate between supercooled liquids and mesophases, J. Phys. Chem., 88, 6448-6453 (1984). 

As noted earlier, the incorporation of chiral groups in the liquid crystal moieties can have the effect of inducing non-linear properties, which include thermochromism, ferroelectricity, antiferroelectricity, electrostriction, and flexoelectricity. In a now classical study, Hult [82] demonstrated that it was possible for supermolecular material 34 to exhibit two-state ferroelectric switching. The remarkable material he investigated, shown in Fig. 30, was found to exhibit two hitherto unclassified mesophases between the smectic... 

Materials of this type may find application in thermochromic or liquid crystal devices. 

Major Appiications Fuel cell power generation system, liquid crystal displays, solor cells, sensors, thermochromic materials,coloring wood,n detergents, assessment of tobacco smoke, cosmetics,14 detect bacterial infections,i multidrug resistance inhibitors, treatment of bums, endodontic, diabetes, obesity, 5 cancer,2o age-related macular degeneration, viral diseases Safety/Toxicity Acute toxicity, combustion toxicity, 4 cytotoxicity, genotoxicity, mutagenicity, nephrotoxicity, phototoxicity, soil toxicity ... 

A huge number of other applications exist, where the chemical and physical requirements are totally different. These applications include reflectors, temperature measurement with thermochromic materials, nonlinear optics, polymer materials, SAMs (self-assembled mono-layers) and LB (Langmuir-Blodgett) Aims, the use of liquid crystals in template synthesis of porous materials, drug delivery, and many more. 

The commercial thermochromic products include multi-components, namely the colour former, an acidic catalyst and a non-polar co-solvent medium. The organic thermochromic materials are easy to obtain and cheaper than thermochromic liquid crystal, but they are not sensitive and their colour change range is narrow. These materials can be used on some thermochromic printing without high quality demands. 

Certain chiral nematic liquid crystals (or mixtures) are found to exhibit thermo-chromism, that is, temperature dependent selective reflection here the pitch length of such materials or mixtures is often found to have an inverse relationship with temperature and can depend on the nature of the material s mesomorphism. Thermochrom-ism is found to be at its most spectacular in materials which display chiral nematic phases and underlying smectic phases this is exemplified for a material with an I-N -SmA sequence (on cooling) and is shown schematically in Fig. 2. 

Industrial Applications Liquid crystal displays fuel cells photovoltaic cells solar cells photochro-mic materials sensors thermochromic materials detergents wood ... 

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