Thermochromic effects

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

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. 

The various sohd sulfur allotropes are all more or less yellow at 20 °C but the scale ranges from the deep orange-yellow of to the pale yellow of S. The color also depends on the particle size As finer the particles as lighter the color impression. Chemically produced Ss has in fact occasionally been obtained as a colorless material at room temperature which only slowly turns yellow as the initially tiny crystals grow [61]. The sample temperature is also of decisive influence (thermochromic effect) [10] (see above). 

In these polymeric species, the M,AT2-1,2,4-triazole linkage is rigid, and allows an efficient transmission of cooperative effects. Consequently, abrupt ST with broad thermal hysteresis loops have been observed [26, 32-34]. The absorption spectra of these compounds show a broad band at 520 nm corresponding to the Aig Trg d-d transition in the LS state whereas no band is found in the visible region in the HS state, the 5T2g-5Eg transition being located around 850 nm [7a]. The ST is thus accompanied by a thermochromic effect, purple (LS) and white (HS). These characteristics make these compounds potential candidates for practical applications, e.g. thermal display devices [7, 8, 17]. Such behaviour has been observed, for example, in the compound [Fe(4-amino-l,2,4-triazole)3](NC>3)2 [32] whose SCO is associated with a hysteresis loop of width 35 K, centred above room temperature [8]. 

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. ... 

Although the Sb chains are qualitatively similar, there is a variation in the linearity of the chain and in the intermolecular distances. Thus, the chains are nearly linear (4 Sb—Sb—Sb 179.2°) and the Sb—Sb distances are short (3.68 A) for the sterically undemanding tetramethyldistibine (22), while the chains are distinctly zig-zag (4 Sb—Sb---Sb 165.8°) and the distances (3.99 A) are longer for the more bulky tetrakis(trimethylsilyl)dis-tibine (36). While short intermolecular contacts seem necessary for the thermochromic effect, the UV absorption maxima of the solid distibines do not correlate with the intermolecular Sb---Sb distances. Thus, the red compounds 22,36, and 40 have nearly identical absorption maxima despite a variation of more than 0.3 A in the intermolecular Sb contacts. On the other hand, the intermolecular Sb—Sb separations of 22 and 30 are nearly the same although the absorption maxima differ by 75 nm. 

The thermochromic effect of distibines has been treated in three papers by Hoffmann and colleagues using a tight bonding model based on extended Hiickel calculations (33,47,48). These calculations treated only unsaturated distibines, and major attention was focused on bistibole (47). The important orbitals of the stacked bistibole are derived from molecular orbitals of the SbC4H4 unit (see Fig. 6). The HOMO results from the in-phase mixing of the Sb(pz) and Sb(n ) orbitals and is largely localized on the Sb atoms. At the zone center (k = 0) this band has primarily lone pair... 

The electronic and geometric structures of poly[3-(4-octylphenyl)thiophene] (POPT) 109 (Figure 22) have been studied by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS, respectively). Thermochromic effects, and new charge-induced states generated by potassium doping, have been observed by direct UPS measurement <1996SM(76)263>. 

Static and dynamic light scattering measurements have been undertaken on dilute solutions of poly(3-dodecylthiophene) (PDDT) to evaluate its conformation over a range of temperature and to learn whether a reversible thermochromic effect is associated with any conformational change <1996MM933>. 

The thermochromic effect in solutions of poly(3-alkylthiophene)s has been attributed to an intramolecular conformational transition to an extended chain conformation, principally on the basis of an observed isosbestic point <1987PSB1071, 1992MM2141>, but evidence for supramolecular aggregates was noted <1987PSB1071>. 

The reversible thermochromic effect observed here and in the solid state is associated with enhanced order of the alkyl side chains with decreasing temperature, facilitating coplanar conformers in the polythiophene backbones, with the attendant enhancement in the 7t-7t -transition of the thiophene ring electronic absorption spectra and improvement in electronic conduction of the doped film. 

The colour of ICPs can also be sensitive to temperature. Substituted polythiophenes, in particular, show a marked blue shift of the highest wavelength absorption band when films or solutions are heated [7-9]. These reversible colour changes have been attributed to a twisting of the polymer backbone to a less ordered non-planar conformation. Less dramatic thermochromic effects have also been reported for polyanilines [3,10,11] where circular dichroism studies of... 

The thermochromic effect of heterocyclic bistiboles has been the subject of several theoretical publications " " with focus on the unsaturated compounds. Band structures have been calculated for the antimony chain of... 

At a given density, the data indicate that the solvent strength is essentially independent of temperature, consistent with the fact that the polarizability is also independent of temperature. The "thermochromic effect is minimal at constant density. However, it is significant at constant pressure in regions where density is a strong function of temperature. 

Poly(alkoxy-thiophene)s have been synthesized and studied for various properties by Leclerc et al. in Montreal [44,91,92]. This is an exceedingly interesting class of pol)4hiophenes, which will certainly be much studied in the years to come. Some of them are very steroregular and show pronounced thermochromic effect. The poly(methoxyethoxy)ethoxy-methyl-thio-phene) (PMEEMT) is an example [44], Interestingly, both the solid material and the tetrafuran solution show thermochromic behaviour. The behaviour of the solution, however, is reminiscent of the behaviour of PHT and POT in the marginal solvents dodecane and cyclohexanone, as discussed in Section 6.3, where the transition is between a suspended and a dissolved state [93]. 

The introduction of defects like /i-links alters the electronic structure substantially, in particular concerning the 7t-orbitals because of the limited delocalization. The 7t-bands in the MNDO simulated UPS spectrum of OL-ifia-i, e.g, is very similar to adding the spectra of two trimers and a monomer [123,124,133,134]. This is also underlined by studies of the thermochromic effects in polyalkylthiophene which can be explained by conformational changes which lead to segments with shorter conjugation length [91,129]. 

Giraud et al., 2002 Qiang, 2003 Wen-tao et al., 2005) anti-counterfeiting (Berger et al., 2004) polychromic and thermochromic effects (Aitken, 1996a,b R6 and Biscans, 1999 Oktem, 2003 Hatefi and Amsden, 2002) and technical textiles and specific medicinal applications (Gouin, 2004 Vasiliu et al., 2005). 

A number of standardised treatments were carried out to evaluate the durability of the thermochromic effect during the use of the fabric (fight fastness, mbbing) and... 

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