Thermochromic phenomenon

The thermochromic phenomenon is easily seen with polymer films, and the first studies [14,15,18-20] were only concerned with the optical spectroscopy of films and solutions. Among the first results was that the change of optical absorption occurs over a wide temperature range. The optical spectra taken at increasing temperatures show a blue-shift. The sequence of spectra may sometimes, but not always, have an isobestic point where all spectra share one common point. This shows that the sequence can be de-convoluted into two spectra, the low temperature and the high-temperature spectra, and that all intermediate... 

The thermochromism in Scheme 1-54 represents valence tautomerism in molybdenum complexes and, at the same time, the inherent redox activity of a dithoilate ligand. This phenomenon may be significant with respect to the biological activity of various molybdenum enzymes. 

In this chapter, the first section will deal with the complex question of conformations of polysilanes. This will be followed by sections on the chromotropism of polysilanes, beginning with their UV thermochromism. Despite intensive study this phenomenon is still not well understood, and is the subject of important recent findings. Solvatochromism, ionochromism and other types of chromotropic behavior of polysilanes will then be considered. A final brief section will deal with the electrical conductivity of polysilanes, a topic which has not previously been reviewed. 

When a substrate A is photo-isomerized to substrate B by UV irradiation, the absorption spectrum of A changes to that of B. Furthermore, substrate B may be transformed back to substrate A reversibly by irradiation at a different wavelength or through a thermal process (Scheme 1). Such a phenomenon is designated as photochromism. A change of colour by thermal excitation is called thermochromism. 

With this in mind, extensive investigations were carried out by Cohen and Schmidt,56,57 who found that photochromism and thermochromism were mutually exclusive properties of this scries of compounds and suggested that this phenomenon is related to the crystal structure of the compounds and not to the chemical nature of the ring substituents. For instance, salicylidene-2-chloroaniline is photochromic whereas salicylidene-4-chloroaniline is thermochromic. 

Since the original description of thermochromism of bianthrones at the beginning of the century, this phenomenon has evoked considerable interest and has been the subject of numerous reviews, the most recent being that published by Muszkatin 1988.96... 

The colour of a thermochromic compound is temperature-dependent the phenomenon is called thermochromism. 

Thermochromism, the change of colour upon heating, is treated in detail in Chapter 3 of this Handbook. The substituted polythiophenes exhibit this interesting phenomenon, which is manifest in pronounced shifts of the optical absorption spectra, and thus of the band gap, upon temperature variations (Figure 2.17). The... 

The phenomenon is found with many, but not all substituted polythiophenes. Studies of 3,3 -di-alkyl-substituted polythiophenes revealed no temperature-induced colour change [17]. But many other structures will give thermochromism and solvatochromism. Polythiophenes substituted both with alkoxy and alkyl side chains may show it both purely amorphous and partially crystalline polythiophenes show it short and long-chain polymers may show it both regular and irregular polymers show it (see Figure 15.1),... 

The reversible nature of thermochromism phenomena distinguishes them from the irreversible phenomena which recently have been reported, for instance in a highly regular poly(3-octylphenyl thiophene) showing an ineversible change of optical absorption when heating the virgin spin-coated film of the polymer [22,23]. While this phenomenon may have the same structural basis [24,25] as reversible thermochromism, we will not dwell further on these phenomena. 

An interesting property of SALAI and its substituted derivatives is their ability to colour markedly under UV irradiation. This photochromism has been attributed to the possible formation of a geometric isomer of the quinoid tautomer. On the other hand, these compounds also exhibit thermochromiSm, which consists of a thermally induced change in colour which increases with an increase of the temperature. This phenomenon has been attributed to intramolecular hydrogen transfer, such as that depicted in Scheme 1. Both processes are reversible and mutually exclusive for the same compound in a given crystalline form. However, since the same anil may exhibit polymorphism, it may be thermochromic in one crystalline form and photochromic in the other. Therefore, these processes should be correlated with differences in the crystal structure rather than with inherent properties of the molecule. All the crystallographic results, which will be reported in Section 20.1.2.3, confirm the earlier hypothesis that molecules which exhibit thermo-chromism are planar, while the others are non-planar, as shown in Figure 1. 

Very much like the photochromic systems described previously in this chapter, organic thermochromic composites are prone to degradation caused by various external stimuli. To evaluate the extent of the damage and to determine the conditions under which the microcapsules may be used without the fear of lost functionality or complete loss of the colour-changing phenomenon, the following investigation was done. 

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