Liquid crystals, thermochromism

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

KEYWORDS liquid crystal, thermochromism, nickel complex, structure analysis... 

Thermochromic Liquid Crystal Thermochromic fiquid crystals have the property that their dominant reflected wavelength is dependent on the temperature, and they have been employed for full-field mapping of... 

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]. 

Reliable micro-scale measurement and control of the temperature are required in developing thermal micro-devices. Available measurement techniques can be largely classified into contact and non-contact groups. While the resistance thermometer, thermocouples, thermodiodes, and thermotransistors measure temperature at specific points in contact with them, infrared thermography, thermochromic liquid crystals (TLC), and temperature-sensitive fluorescent dyes cover the whole temperature field (Yoo 2006). 

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). 

The liquid crystal thermographs method has been used for measuring microtube surface temperature with uncertainties of lower than 0.4 K by Lin and Yang (2007). The average outside diameter micro-tubes was 250 pm and 1,260 pm, respectively. The surface was coated with thermochromic liquid crystal (TLC). The diameters of encapsulated TLC were ranging from 5 to 15 pm. The TLC was painted on the tested tubes surface with thickness of approximately 30 pm. 

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. 

Thermochromic photoluminescent liquid crystals have been reported recently as well as the mechanochromic photoluminescent liquid crystals. Weder and coworkers reported on oligo(p-phenylene vinylene) derivative 6 showing thermochromic... 

McDonnell, D. G. Thermochromic Cholesteric Liquid Crystals, in Thermotropic Liquid Crystals, Gray, G. W. (Ed.), Wiley, Chichester, 1987, Chapter 5, pp. 120-144. 

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). 

I. Cabrera, V. Krongauz, and H. Ringsdorf, Photo- and thermochromic liquid crystal polysilox-anes, Angew, Chem. Int. Ed. Engl. 26, 1178-1180 (1987). 

This chapter is mainly devoted to organic compounds for which the observed reversible color changes (coloring and fading reactions) are due to the effect of temperature, exclusively. The thermochromic behavior of inorganics, organo-metallics, macromolecular systems (such as polythiophenes), or supramolecular systems (such as liquid crystals) and of molecular systems for which the observed thermochromism is due to external intervention (association with another species like a metal ion or a proton) or to modification of the medium by a thermal effect (thermosolvatochromism, for instance) are not reviewed in this chapter. 

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... 

K.-H. Platzer, C. Hirsch, D. E. Metzger, and S. Wittig, Computer-Based Areal Surface Temperature and Local Heat Transfer Measurements with Thermochromic Liquid Crystals (TLC), Experiments in Fluids, 13, pp. 26-32,1992. 

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