Thermochromic transition

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

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). 

Among all Fe(II) spin crossover compounds known to date, the extensively studied polymeric [Fe(4-R-l,2,4-triazole)3](anion)2 systems (R=amino, alkyl, hydroxyalkyl) appear to have the greatest potential for technological applications, for example in molecular electronics [1, 24, 25] or as temperature sensors [24, 26]. This arises because of their near-ideal spin crossover characteristics pronounced thermochromism, transition temperatures near room temperature, and large thermal hysteresis [1, 24, 27]. 

The plot was then used to estimate the average dihedral angle for the 355 nm peak which becomes apparent in the spectra of PDBS and PDHS at their respective —36 and —31 °C thermochromic transitions.223 A value of ca. 175° was predicted, which is in the range also shown by a decameric di- -propylsilylene,15 although somewhat larger than the value of 165° suggested in other discussions.197... 

The molecular weight dependency of thermochromic transitions was also investigated by Sakurai and co-workers, who found that the transition temperature for PDHS and PDBS was proportional to the inverse of the number average molecular weight at low molecular weights.208... 

There has been a great deal of interest in the UV-visible spectroscopy of the polygermanes, particularly in comparison with the analogous chains that have silicon or tin backbones.41,42 Both conventional and Raman spectroscopy have been employed. One interesting observation is that the symmetrically disubstituted polyfdi- n - a I k y I g e n nanes) exhibit thermochromic transitions at temperatures below those of their polysilane analogues. Another is the conclusion that in poly(di-n-hexylgermane) the side chains adopt trans-planar conformations as in the polysilane counterpart. The two chains are also similar in that both backbones can, under certain circumstances, also adopt planar zig-zag conformations. 

UV spectra of stable germylenes, stannylenes and plumbylenes were recorded in solutions or in the solid phase under inert atmosphere in order to prevent access of oxygen and moisture. Thermochromic transitions have been revealed for many stable solid germylenes, stannylenes and plumbylenes. Their nature has not been studied. 

An opposite type of behavior, positive ionochromism, is shown by the polymer [C2H50CH2CH20(CH2)5]2Si n52- In this case, as shown in Figure 9, addition of lithium triflate to a thin film of the polymer promotes a bathochromic shift, increasing the midpoint temperature of the thermochromic transition from —45 to +50 °C Probably in this example the lithium ions, complexed to the ethylene oxide units, lock the polymer... 

Nearly identical spectral changes are observed when GAOX, prepared in a glassing solvent (e.g., 50% glycerol) that prevents formation of microcrystals and preserves the optical transparency of the sample, is cooled to cryogenic temperatures (Fig. 13B) (Whittaker and Whittaker, 1993 Whittaker et aL, 2000). A color change from blue (RT) to red (LT) reflects a thermochromic transition in the protein structure. The similarity of the optical spectrum of the low-temperature complex to the spectra observed for anion adducts suggests that the RT aquo complex (TyroN... 

Figure 8 The UV spectrum for (n-butyl2Si) solutions in hexane as a function of temperature, showing an abrupt thermochromic transition...

Absorption spectra displaying a thermochromic transition have also been reported for several of the symmetrically substituted polysilylenes in the solid state. One example is that of PDHS, as reported by Kuzmany et al. (9) and shown in Figure 6. At 45 °C, a single absorption at 317 nm is observed, which is very similar to the absorption maximum for this polymer in solution... 

As the sample is cooled, a second absorption band is observed in the range from 365 to 375 nm, which continues to grow upon further cooling. This behavior is completely reversible. This type of thermochromic transition is not observed in the solid-state absorption spectra of PDBS (iO) or PDPS 10,11). To understand this unusual absorption behavior of the polysilylenes in solution and in the solid state, a variety of studies have been directed toward the determination of the polymer chain conformation. 

PDHS Structures in Solution. The determination of the chain conformation of polysilylenes in solution, particularly the conformations at temperatures just above or below the low-temperature thermochromic transition, is of great interest. NMR spectroscopy is one of the most useful techniques for probing chain conformation in solution (2i), and NMR is especially effective because of the large sensitivity of the carbon chemical shift to bond conformation (22). Silicon nuclei are also very sensitive to chain conformation, but a good correlation between silicon chemical shift and bond conformation has not been established yet. Unfortunately, both of these nuclei suffer from low sensitivity, primarily because of their low natural abundance. In contrast, protons have an essentially 100% natural abundance, but compared with the carbon or silicon chemical shift, the proton chemical shift is not very sensitive to bond conformation. Efforts to use NMR to probe the low-temperature dilute-solution conformation of the polysilylenes have been unsuccessful thus far. The diflSculty is that PDBS and PDHS precipitate from solution in 20-30 min after cooling through the thermochromic tran-... 

Two temperatures for the thermochromic transition of PDHS in solution have been reported -24 (6) and -31 (7) °C. The difference between the two measurements is probably due to differences in material polydispersity and experimental procedures 10). If the light-scattering measurements were made at temperatures above the thermochromic transition and if the transition is first order, a change in s is not expected. Certainly, the dramatic increase in the scattering intensity observed at low temperatures is the result of the ordered phase precipitating from solution. However, whether the UV thermochromic transition occurs before the appearance of an ordered phase... 

IR Spectroscopy. The observation of a solid-solid phase transition in PDHS at the temperature of the UV thermochromic transition generated interest in the nature of the polymer chain conformation at temperatures above or below this critical temperature. A number of techniques have been used to study the solid-state structures of PDHS. Rabolt et al. (25) used IR spectroscopy to monitor the conformational behavior of the alkyl side chains of PDHS and Raman spectroscopy to follow that of the backbone. The IR spectrum of PDHS at +30 °C (Figure 9) consists of sharp, intense bands. When the film of PDHS is heated to +100 °C, the sharp deformation bands typical of a highly ordered hydrocarbon chain become broad and less intense, similar to the behavior observed in the IR spectra of n-alkanes at temperatures above the melting temperature. The data refiect conformational disorder in the side chains at temperatures above the +40 °C transition. After... 

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 ... 

Thermochromic Transition. The solid-state thermochromic transition observed in PDHS is associated with conformational disordering of the polymer backbone, with the all-frans backbone conformation giving rise to the absorption at 374 nm. The thermochromic transition observed for PDHS in solution is also thought to result from conformational ordering, with the long-wavelength absorption of 354 nm at -40 associated with an sl -trans... 

For PDBS (10) and PDFS (ii), such a dramatic thermochromic transition has not been observed in the solid state. The absorption spectrum of solid PDBS broadens as the film is heated, and shifts to the red region by about 10 nm (Figure 23a). The DSC data for PDBS (iO) indicate that the solid-solid phase transition is at +86 °C. The failure to observe a strong thermochromic transition in PDBS or PDFS is probably the result of the predominant non-trans conformations in the ordered phase I structures. These conformations prohibit the runs of trans conformations necessary to provide the conjugation responsible for the long-wavelength absorption observed in PDHS. In contrast, cooling of the PDBS solution results in a... 

Although much has been learned about the structures of polysilylenes, a tremendous amount of work remains before a full understanding of these materials is developed. The microstructure of the polymers can be studied directly by solution NMR spectroscopic techniques. The determination of the chain conformation in solution is diflScult, particularly at low temperature. Light-scattering techniques may be able to establish the solution dimensions of the polysilylenes through the low-temperature thermochromic transition. The chain conformation in the solid state can be established by X-ray and electron difiraction methods. Solid-state Si NMR spectroscopy can become... 

The scenario would envisage that the crystal strain increases in the phenylurethane series such that a blue shift, which is essentially that of the LT to HT transition, would result. The loss of detailed structure occurring on the thermochromic transition to the HT spectrum would result from an extreme increase in disorder which could be mainly attributed to dynamic processes, such as sidechain motions, which are intensified at higher temperatures. A theoretical approach not unlike this has been recently advanced by Schweizer (24). Both weak and strong disorder regimes are considered in that treatment. 

The DRS curves of a number of M [HgI4] complexes M = Pb, Cu, Hg, Ag, and Tl) are given in Figure 9.19. All the compounds exhibit rather sharp thermochromic transition, with the exception of Tl2[HgI4]. The latter compound is reported to have a transition at 116.5°C (9) however, it is not evident from the DRS curve. The change in reflectance of the compound appears to decrease linearly with temperature. 

Diffraction data for the alkoxy-substituted materials are scarce. For PMEEMT x-ray diffraction patterns were recorded [44] at two temperatures, room temperature and 150°C, showing in both cases three to four rather broad peaks on a diffuse background. The peaks correspond to interplanar distance d= 17.8 A, 7.0 A, 3.8 A and 2.4 A, the latter two being close to two values also found for PATs as the 6-axis parameter and a diffuse feature at 0 = 2.6 A related to the main chain periodicity. It is remarkable that the diffraction pattern survives heating to 150"C, which is above the thermochromic transition for PMEEMT. A recent diffraction study of poly methanol-thiophene) prepared electrochemically showed only broad amorphous scattering around 0= 1.4 A [94],... 

This result is in contrast to what was found for mixtures of PHT and POT, (PHT), - x (POT). Samples with x = 0, 0.25, 0.4, 0.5 and I were studied both for their temperature dependent optical absorption spectra and for their x-ray diffraction patterns at room temperature. In this case the crystallinity varies throughout the series and was found to be poorer for the mixtures than for the end members. The measured />axis parameter was slightly larger for the mixtures, but most importantly the < -axis parameter dependence on X was found to be strongly nonlinear. In fact a value of 20.1 A, very close indeed to that of the pure POT value (20.6 A) was found already at the molar fraction 0.5. The data are summarized in Figure 2.15. The thermochromic transition temperature, on the other hand, was also found to vary nonlinearly with the molar fraction, and when this temperature was plotted versus the observed a, an almost linear relation was found. 

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