Thermochromic changes

Solutions of the violet photoproduct (CLI) in ethyl benzoate show reversible thermochromic changes attributed to reversible changes (lactam-lactim tautomerism) (CLI CLII).251... 

Recently, however, Sanji and coworkers have pointed out that the thermochromic change and increase in absorption intensity is too abrupt to be the result of a Boltzmann distribution between anti and twisted units along the polymer chain23. They proposed instead that the transition is an interconversion between two well-ordered states a tightly-coiled helix, and all-anti . 

The poly(3-alkylthiophenes) exhibit a reversible thermochromic change that is due to a transition between low-temperature and high-temperature solid-state structures. The thermochromic mechanism involves the conformation of the alkyl group, which is dependent upon the temperature. At low temperatures the alkyl side chains adopt a fully extended, staggered conformation. As the temperature increases, the population of gauche conformations in the alkyl side chains increases... 

The nature of the interchain coupling of excitation has not been studied in detail. While exciton splitting has been reported where expected (11), additional investigation regarding the nature of the interchain coupling has not been pursued. This may be attributed to the phase and thermochromic changes in the PDAs as well as to difficulty in understanding the intramolecular transitions of the polymer. 

Both ETCD and TCDU have been previously reported to undergo thermochromic changes (1,3). With the former, the change is reversible which is not the case with the latter. 

Features Thermochromic changes from color to colorless as temp, rises color returns as temp, falls more sensitive to solv., UV light, pH, shear, and temp, than other pigment types Regulatory EINECS, DSLiNDSL listed... 

Fig. 25 Hand print on the Sway " ski wear. Palm warmness causes a thermochromic change.

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). 

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). 

Experimental results corroborate that shifts of 1.2 eV are always present if any of the variables acting on the electrochemical process are changed the solvent, the salt, or the temperature of work. We cannot attribute the observed shift to solvatochromic, counter-ion-chromic, or thermochromic effects taking place inside the film during oxidation-reduction processes. So, as predicted, these shifts are a consequence of the way the chains store or relax energy through conformational changes stimulated by electrochemical oxidation or reduction, respectively. 

In addition to chemical reactions, the isokinetic relationship can be applied to various physical processes accompanied by enthalpy change. Correlations of this kind were found between enthalpies and entropies of solution (20, 83-92), vaporization (86, 91), sublimation (93, 94), desorption (95), and diffusion (96, 97) and between the two parameters characterizing the temperature dependence of thermochromic transitions (98). A kind of isokinetic relationship was claimed even for enthalpy and entropy of pure substances when relative values referred to those at 298° K are used (99). Enthalpies and entropies of intermolecular interaction were correlated for solutions, pure liquids, and crystals (6). Quite generally, for any temperature-dependent physical quantity, the activation parameters can be computed in a formal way, and correlations between them have been observed for dielectric absorption (100) and resistance of semiconductors (101-105) or fluidity (40, 106). On the other hand, the isokinetic relationship seems to hold in reactions of widely different kinds, starting from elementary processes in the gas phase (107) and including recombination reactions in the solid phase (108), polymerization reactions (109), and inorganic complex formation (110-112), up to such biochemical reactions as denaturation of proteins (113) and even such biological processes as hemolysis of erythrocytes (114). 

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... 

The changes with steric hindrance and temperature must be due to conformational effects along the polymer chains. It is now generally agreed that the thermochromism results from an increase in the proportion of trans- to gauche conformations in the polymer chain as the temperature is decreased. Similarly, the introduction of sterically hindering substituents could increase the amount of trans junctions. Evidence in favor of this model is presented in several recent papers, (28i) as well as the chapter by Michl in this volume. 

The recent interest in substituted silane polymers has resulted in a number of theoretical (15-19) and spectroscopic (19-21) studies. Most of the theoretical studies have assumed an all-trans planar zig-zag backbone conformation for computational simplicity. However, early PES studies of a number of short chain silicon catenates strongly suggested that the electronic properties may also depend on the conformation of the silicon backbone (22). This was recently confirmed by spectroscopic studies of poly(di-n-hexylsilane) in the solid state (23-26). Complementary studies in solution have suggested that conformational changes in the polysilane backbone may also be responsible for the unusual thermochromic behavior of many derivatives (27,28). In order to avoid the additional complexities associated with this thermochromism and possible aggregation effects at low temperatures, we have limited this report to polymer solutions at room temperature. 

At room temperature, zinc oxide is white, but when heated it becomes yellow. A compound that changes color on heating is said to be thermochromic. 

The spin crossover characteristics of the corresponding Fe(II) compounds may be fine tuned by the systematic variation of the substituent at N-4 of the 1,2,4-triazole ring, as well as by changing the non-coordinated anionic groups. In this way, thermochromic Fe(II) materials showing a spin transition close to room temperature and accompanied by hysteresis have been obtained. As an example, the optical reflectivity measurements record-... 

Increasing the length of the alkyl spacer in such a way as to yield 1,4-bis(tetrazol-l-yl)butane (abbreviated as btzb) (Fig. 16), changes the dimensionality of the Fe(II) spin crossover material [89]. In fact, [Fe(btzb)3] (C104)2 is the first highly thermochromic Fe(II) spin crossover material with a supramolecular catenane structure consisting of three interlocked 3-D networks [89]. Unfortunately, only a tentative model of the 3-D structure of [Fe(btzb)3](Cl04)2 could be determined based on the x-ray data collected at 150 K (Fig. 20). 

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