Color thermochromic effect

The various sohd sulfur allotropes are all more or less yellow at 20 °C but the scale ranges from the deep orange-yellow of to the pale yellow of S. The color also depends on the particle size As finer the particles as lighter the color impression. Chemically produced Ss has in fact occasionally been obtained as a colorless material at room temperature which only slowly turns yellow as the initially tiny crystals grow [61]. The sample temperature is also of decisive influence (thermochromic effect) [10] (see above). 

In addition to the transparency another important feature of these hydrogels is that the thermochromic effect is not accompanied by volume changes. Therefore such hydrogels are applicable for smart windows, large area displays, and tunable color filters. 

A very interesting and potentially useful thermochromic effect occurs in the (undoped) alkyl-substituted polythiophenes, for alkyl groups larger than methyl [35]. The thermochromic effect is a (reversible) thermally-induced change in color. In order to characterize the observed thermochromic behavior of the P3ATs, a series of studies of the temperature-dependence of the electronic structure of P3HT have been carried out using optical absorption spectroscopy, UPS and XPS. The central results, as well as a model of the thermochromism which is consistent with all of the experimental data, are reviewed below. 

The effects of temperature on the color development of the porous film in chlorobenzene were shown in Table 6 [23]. The coloration was reversible thermochromism. The refractive index of the materials generally decreases as the temperature increases, and the temperature dependence of the liquid is greater than that of the solid. For example, the temperature dependence (A/id/°C) of PVA and chlorobenzene was found to be 3.0 x 10 and 4.5 x 10" at 589.3 nm. Consequently, it is interpreted that the wavelength of the crosspoint between the dispersion curves of PVA and chlorobenzene shifts from the long side to the short side with increasing tem-... 

Cu(II) Complexes with Diethylethylenediamine. The complexes [Cu(dieten)2]X2, where X is an anion and dieten is the bivalent ligand N, V-diethylethylendiamine, well illustrate the effect of the medium on the extent of cooperativity of the transformation. These systems present interesting properties, and they have been investigated extensively [498-503]. When X is BF4, CIO4, or NO3 the complexes have thermochromic behavior and the color changes from red, at low temperature, to blue in the high-temperature form [499, 504]. 

Solvatochromism and thermochromism are also characteristic of spirooxazines (Scheme 3) <1994RCB780>. The two forms 116 and 117 are in equilibrium in solution and more polar solvents shift the equilibrium more to the colored, acyclic form 117. Higher temperatures have the same effect for both solid spirooxazines and their solutions. A comprehensive review of spirooxazines <2002RCR893> has a collection of the absorption maxima for a large number of spirooxazines and their colored forms, which have their absorption maxima in the visible range at 480-670 nm. 

Thermochromic dyes change color with temperature (heat). The effect may be due to a single dye or a composite system, and the color change may be reversible or irreversible. Thermochromic dyes find use in direct thermal printing and as temperature sensors, as well as in clothing and novelties. 

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. 

Since other PDAs with urethane sidechains display thermochromism, the phenylurethane series was investigated for this effect. All were found to be thermochromic. Because of the remote location of the hot stage thermometer from the sample, an accurate transition temperature was not determined but a range could be defined. The color change was determined visually. 

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