Thermochromic

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

When apphcable, expressed as a change upon heating various colors have been reported, often quaUtatively. 

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 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

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). 

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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...

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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