Thermochromism

Two different types of thermochromism, which are correlated to the substitution pattern of the polymers, may be distinguished [330]  

A two-phase thermochromic behavior, as in PDDT and poly(3-methyl-4-octyloxythiophene), is related to the formation of delocalized conformational defects upon heating. These defects are possible due to the presence of sterically demanding substituents between each consecutive repeating unit [330]. In the solid state at room temperature, PDDT and poly(3-octyloxy-4-methyl-thiophene) have a coplanar conformation for the main chain. Heating (25 to 150°C) increases the repulsive intrachain steric interactions and introduces some conformational disorder in the side groups, forcing the polymer backbone to adopt a nonplanar conformation [331, 332]. Temperature dependent UV/vis absorption measurements of fluorinated PTs, e.g., poly(3 -perfluorohexyl-2,2 5, 2 -terthiophene), poly[3-(pentadecafluorooctyloxy)-4-methylthiophene] and poly[3-(tridecafluorononyl)thiophene], show a blue shift of the maximum 

In contrast to polymers having two distinct forms, poly(3-butoxy-3 -decyl-2,2 -bithiophene) and poly(3-dodecyl-2,2 -bithiophene) have no isosbestic point in the temperature-dependent UV/vis spectra. The absorption maximum shows a continuous and monotonic blue shift on heating [330, 334]. This phenomenon can be described as a continuous modification of the polymer backbone, where the rotation of a first thiophene unit will induce the twisting of the following units [334]. These polymers allow only the formation of localized conformational defects [330]. 

A mixture of PHT and POT is a single phase material which is structurally intermediate between PHT and POT. The thermochromic transition temperature and the longest interchain distance in the crystalline part of the polymer change non-linearly with composition, but in such a way that the thermochromic transition temperature and the interchain distance are in an approximately linear relationship to each other. This indicates that the side chains act primarily as spacers between main chains, rather than being directly involved in the thermochromic transition [345]. 

in his seminal work, defined thermochromism as an easily noticeable reversible colour change brought about by the boiling point of each liquid, the boiling point of the solvent in the case of a solution or the melting point for solids .  

Whilst this definition is academically accurate for many inorganic and organic materials, the label thermochromic has also been applied to important technical areas that involve other external influences as well as heat in the observed colour change, e.g. thermochromic pigments. 

A technically more appropriate approach is to separate reversible organic thermochromism into intrinsic systems, where heat is the sole cause of the colour change, from indirect systems, in which the colour variation involves changes in 

Colour formation reactions of this type are utilised in carbonless copy paper, which is based on the principle of colour formation on the copy as a result of pressure of writing or typing in the master sheet. In such systems, the underside of the master sheet contains the colour former, for example compound 243, encased in microcapsules, which are tiny spheres with a hard polymer outer shell. Pressure on the master sheet breaks the microcapsules and allows the colour former to come into contact with an acidic reagent coated on the copy sheet, thus causing an irreversible colour formation reaction. 

Colour formers such as compounds 243 and 245 are not inherently thermochromic. For example, they melt without any change in colour. However, they may be used to generate colour thermally, either irreversibly or reversibly, as composite materials. In thermally sensitive paper, the colour former and an acidic developer, usually a phenol, are dispersed as insoluble particles in a layer of film-forming material. When brought into contact with a thermal head at around 80-120 °C, the composite 

My nocturnal illumination is magnificent, indeed I prefer it to daylight. .. You must come sometime to see how the light sets off every object, how deep black shadows frame all the pictures and are thrown upon the beams. You can find that in almost all my still lifes, most of which I have painted at night... 

Mercury salts are poisonous. Protective gloves must be worn. 

200-mL beaker, two 100-mL beakers, glass rods, funnel with filter paper, spatula, protective gloves, safety glasses. 

Hg(N03)2, or HgCb, CuCl, AgN03, distilled water, four round filter papers, filter paper for drying. 

5 g Hg(N03)2 (or the same amount of HgCl2) are stirred with 150 mL of water in the 200-mL beaker the clear solution is separated from any precipitate which appears and treated with solid potassium iodide until the deep red precipitate initially formed dissolves to give a colorless solution. Half of this solution is placed in each of the 100-mL beakers. A 5 % AgN03 solution is added to the first half, a freshly prepared saturated CuCl solution to the second. The mixtures are allowed to stand for a while, decanted and filtered through filter paper. The precipitate from the first beaker is deep yellow, that from the second bright red. Both precipitates are dried by placing them between sheets of filter paper. 

The thermocliromism of dianthrone and its 2,2 -dicarboxylic acid45, which change from yellow through green to blue-green, has been explained as due to the existence of a distinct low triplet electronic level situated 3-5 kcal above the ground state. 

Like fluorescence, thermocliromism is influenced by steric effects. Thus, substitution of dixanthylene in the positions 1 and V which hinders the planarity of the molecule is detrimental to the development of thermo-chromic properties48. 

A case of negative thermocliromism has been observed49 during the study of the magnetic properties of the deep red-violet prisms, N-ethyl phenazyl radicals, which show strong bands at 80 °K but none at room temperature. 

1 Pringsheim, P. and Vogel, M., Luminescence of Liquids and Solids, Interscience, New York, 1943. 

34 Curie, M., Luminescence of Solid Bodies, University of France Press, 1934, Chapter I, 

One of the main arms of high added value products is the development of properties that directly affect the safety and comfort of users. One of the ways to accomplish this is by using microencapsulation processes to impart new functionalities and properties to textiles (Anon, 2008). Thermochromic microencapsulated pigments bear the potential for medical application in terms of registering elevated body temperature of patients, thus allowing easier monitoring and point of care diagnosis. 

A technically more appropriate approach would be to diversify the reversible organic thermochromism of intrinsic systems, in which heat is the only factor influencing the colour change, from the indirect systems in which the colour change is induced by the heat that affects the surroundings of the chromophore (Burkinshaw et al., 1999). 

1 Real organic thermochromism - reversibly intrinsic systems 

The main application area for these indirectly thermochromic, reversible systems is the manufacture of organic thermochromic pigments. 

Inorganic systems are of extremely limited application potential, but a use for them was found in dyes and chalks that indicate hot surfaces on industrial machinery (Day, 1963). Only LCDs and conjugated polymers found practical use in optical indicators and thermal imaging (Hirota et al., 1996). 

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Unlike many distibines and dibismuthines, diarsines do not appear to exhibit thermochromic effects (105). 

Table 1. Some Typical Thermochromic Compounds and Their Transitions...

Thermochromic material CAS Registry Number Thermochromic transition p,°c References... 

The sohd-state, transition-metal example in Table 1 of [(CH2)2NH2]3CuCl illustrates another form of thermochromism the color shifts gradually and continuously because of changes in bandwidth with either heating or cooling (6). It is not unique, as this behavior has been mentioned for the class of... 

Hahde complexes of Cu with nitrogen base ligands are known to exhibit another form of reversible spectral change known as fluorescence thermochromism. The example of Cu4l4(Py)4 from Table 1 is typical and shows red shifting ia the visible emission spectmm while the sample is both cooled and irradiated with a 364 nm ultraviolet source (7). 

The subject of thermochromism in organic and polymeric compounds has been reviewed in some depth previously (8,16,18), and these expansive overviews should be used by readers with deeper and more particular interest in the subject. Many more examples can be found in the reviews that further illustrate the pattern of association between thermochromism and molecular restmcturing of one kind or another. The specific assignment of stmctures is still Open to debate in many cases, and there are still not many actual commercial uses for these or any of the other thermally reversible materials discussed herein. Temperature indicators have been mentioned, though perhaps as much or more for irreversible materials. 

Some cyan dyes derived from both naphthols and phenols are reported to show thermochromism, a reversible shift in the dye hue as a function of temperature. This can occur in a negative while prints are being made (65). 

Crystalline S4N4 is thermochromic, being pale yellow below about -30 the colour deepens to orange ai room temperature and to a deep red at 100° (cf. sulfur, p. 656). 

Occasionally, equilibria between a quinoid and a diradicaloid form of tetraazafulvaleiies of type 77 have been discussed (66AG303 72NKK100 79JOC1241). Based on ESR measurements, only traces of radicals (0.1% at 200°C) could be observed and therefore 77 (Ar = Ph) exists at room temperature predominately in the quinoid structure. Other authors stated that the thermochromism of 77 mainly results from a change in intermolecular interaction, not from biradical formation (84MI1030). 

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-... 

Polythiophene [78] is a promising material for certain future electronic applications, due to its relatively high stability and processability in the substituted form [79-81]. Upon substitution, with e.g. alkyl side-chains [79, 80], polythiophene exhibit properties such as solvalochromism [82] and thermochromism [83]. Presently, a large variety of substituted polythiophenes with various band gaps exists (for example see Ref. [81 ]). 

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