Mechanisms of thermochromism

The origin and mechanism of thermochromism in the polysilanes is an area of ongoing discussion, and it is likely that under different conditions, different mechanisms are operative, which could be usefully clarified by further study. 

Intensive investigations have been conducted to elucidate the nature of the mechanism of thermochromism of salicylidene Schiff bases. Different techniques or methods suitable for the study of the tautomeric equilibrium between the end form and the (Z)-keto form have been used, including X-ray diffraction, NMR, infrared (IR) and Raman spectroscopy, and theoretical calculations. In the anil-... 

Elmaci and Yurtsever used DPT to study the relationship between thermochromism and internal rotation in poly thiophenes [59b]. For head-to-tail methyl-substituted thiophenes, rotational potential maps of the dimer and trimer were calculated. Using a time-dependent DFT methodology, the UV-Vis absorption spectra of these oligomers were calculated as functions of the torsional angles. Theoretical temperature-dependent UV-Vis spectra were generated from the distribution of torsional angles. The results show that the internal rotation alone is not sufficient to explain the mechanism of thermochromism. 

B. Ya. Simkin, S. P. Makarov, V. I. Minkin, and V. A. Pichko, Photo- and thermochromic spirans. 18. Theoretical study of the mechanisms of the photocolorization and photodecolorization of 2H-pyrans and their structural analogs. Triplet state, Chem. Heterocycl. Cpds., 1991, 250-255. 

A better understanding of the mechanism of the thermochromic transition in PDHS is obtained by X-ray studies of fiber patterns. Comparison of such patterns from the same specimen at temperatures below or above the transition temperature (Figure 13) gives the following results ... 

K. Maeda and T. Hayashi, The mechanism of photochromism, thermochromism and piezochromism of dimers oftriarylimidazolyl, Bull. Chem. Soc. Jpn 43, 429-438 (1970). 

The synthesis of substituted polythiophenes for investigations of thermochromism has brought many examples of how the regioregular side chain substitution is central to the chromic phenomena. Some of these materials show two-phase behaviour with an isobestic point in the sequence of the spectra. As the optical absorption only reflects local properties, we have to understand what mechanism it is that will always keep the material in just two states, with no intermediate states. As the torsion of the main chain is the cause of the chromic behaviour, we have to understand more specifically what is the essential physics of a cooperative phase transition which takes the chain, or parts of the polymer chain, from one phase to the other. In many ways this might look like the falling of a row of dominoes, upon one fluctuation of one of the dominoes. In our case this would be... 

Scheme 11.4 Schematic representation of the mechanism of colour change in a microencapsulated leuco dye thermochromic system.

The picture that emerges from studies of thermochromism and solvatochromism is thus consistent with a torsional deformation of the polymer main chain via the disordering of the alkyl side chains. This mechanism is sufficient to account for all the features of the chromatic transitions. Another mechanism, however, invoking the interaction between the polymer and its surrounding medium through the coupling of polarizability and conformational structure, as proposed by Schweizer [72-74], has not yet been ruled out. It may indeed be another manner in which the transition can be explained. 

Mechanism of the Photochromism and Thermochromism of 2, 4,4 -Hexaphenyl-l,l -Biimidazole. Taro Hayashi, Kodo Maeda, and Midori Morinaga (Ochanomizu Univ., Tokyo). Bull. Chem. Soc. Japan 37 (10), 1563-4 (1964) (Eng.), cf. CA 54, 24674c, 23747d. The rate of decrease in absorbance at 347 nm which is produced by sunlight irradiation of solns. of the title compd. 

The Mechanism of the Photochromism and Thermochromism of 2,2, 4,4 -Hexa-Phenyl-1,1 -Biimidazoiyi in a Soiid State. Taro Hayashi Koko Maeda, and Takako Kanaji (Ochanomizu Univ., Tokyo). Bull. Chem. Soc. Japan 38 (5), 857 (1965) (Eng.). Pale yellow 2,2, 4,4, 5,5 -hexa-phenyl-l,l -biimidazolyl (I) becomes reddish purple upon irradiation or when heated in the solid state. On the basis of the E.S.R. and absorption spectrum measurements of a sample irradiated at 15°, the mechanism of photochromism of I, in the solid state, was concluded to be the radical dissociation of its N-N bond to triphenylimidazolyl radicals (II). The same mechanism can be applied to the thermochromism in the solid state. The photochromic color gradually reverted to its original pale yellow color at about 40°. The rate of recombination of II in the dark at 46 and 62.5° did not obey the rate expression of the 2nd-order reaction. 

Inorganic materials, usually involving transition metals, and organometallics exhibit a thermochromic response over a wide range of temperatures and by a variety of mechanisms. Examples include ... 

The pigment is coloured in the solid form of the material because in this state the colour former interacts with the developer, possibly via an ion-pair complex. Melting of the composite interferes with this interaction, leading to a negative thermochromic effect and a loss of colour. This is shown schematically in Figure 1.19. Possible mechanisms for the colonr change have been examined. ... 

Abstract We describe mechanochromic and thermochromic photoluminescent liquid crystals. In particular, mechanochromic photoluminescent liquid crystals found recently, which are new stimuli-responsive materials are reported. For example, photoluminescent liquid crystals having bulky dendritic moieties with long alkyl chains change their photoluminescent colors by mechanical stimuli associated with isothermal phase transitions. The photoluminescent properties of molecular assemblies depend on their assembled structures. Therefore, controlling the structures of molecular assemblies with external stimuli leads to the development of stimuli-responsive luminescent materials. Mechanochromic photoluminescent properties are also observed for a photoluminescent metallomesogen and a liquid-crystalline polymer. We also show thermochromic photoluminescent liquid crystals based on origo-(/ -phenylenevinylene) and anthracene moieties and a thermochromic photoluminescent metallocomplex. 

Valence and prototropic tautomeric reactions are among the most important mechanisms that govern transformations of a broad variety of photochromic organic systems.1,2 Until recently, no examples of photochromic compounds have been reported whose photochromic behavior was due to a combination of these two tautomeric reactions. Such a combination, which is characteristic of ring-chain tautomerism3 has been implemented in the photochromic and thermochromic rearrangements of a novel type of heterocyclic photochromes, derivatives of 2,3-dihydro-2-spiro-4 -(2, 6 -di-iert-butylcyclohexadien-2, 5 -one)perimidine la and its analogs.4 The occurrence of a proton transfer step is in accord with the fact that the AvV -dimethyl derivative of la exhibits no photochromic properties. 

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... 

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