Poly , thermochromic

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. 

Poly(arylether ketone), conducting, 7 524 Poly(aryl ether ketone)s, sulfonation reaction of, 23 717-718 Poly(di-n-alkylsilanes), thermochromic materials, 6 619... 

A number of poly(arylene vinylene) (PAV) derivatives have been prepared. Attachment of electron-donating substituents, such as dimethoxy groups (structure 19.3), acts to stabilize the doped cationic form and thus lower the ionization potential. These polymers exhibit both solvatochromism (color changes as solvent is changed) and thermochromism (color is temperature-dependent). 

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. 

Specific properties of polysilanes have been linked to the method of synthesis.35 For example, in the case of anionic polymerization of poly[l-(6-methoxy-hexyl)-l,2,3-trimethyldisilanylene] a new type of chromism was induced in the polysilane film by the difference in the surface properties of substrates and was termed a surface-mediated chromism. The polysilane exhibited thermochromism with an absorption maximum at 306 nm at 23°C, but <15°C a band at 328 nm began to appear. A monolayer of the polysilane was transferred onto both a clean hydrophilic quartz plate and a hydrophobic one treated with hexamethyldisilazane by the vertical dipping method. With the hydrophobic plate, a broad UV absorption at 306 nm is obtained, whereas the absorption on a hydrophilic plate shifts to 322 nm. The conformation of the polysilane is preserved by hydrogen bonding between the silica surface and the ether section of the substituent on the hydrophilic plate. The polysilane is attached to the hydrophobic surface only by van der Waals forces, and this weaker interaction would not sustain the thermodynamically unstable conformational state that is attained on the water surface. 

In solution, poly(dialkyl)germanes such as (n-IIexjGe),. (H-Pen2Gc)n and (n-Bu2Ge) show a distinct thermochromic behaviour31,55. The kmax value of (n-Hex2Ge) in pentane gradually shifts from kmax = 340 nm at room temperature to kmax = 350 nm at — 60 °C. 

Poly(dioctyl)stannane and poly(dihexyl)stannane exhibit a reversible thermochromic behaviour as a result of a phase transition at ca 40 °C. A discolouration of the polymers was observed upon slightly warming above room temperature and variable-temperature UV-vis spectrometry showed a reduction of the >.max values of (n-Oct2Sn) in toluene solution from 384 to 369 nm and of (n-Hex2Sn) from 398 to 382 nm82. On the other hand, however, poly(dibutyl)stannane and poly(diaryl)stannane do not exhibit thermochromic behaviour in the temperature range between —10 to 90 °C and —20 to 90 °C82 100, respectively. 

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... 

Regioregular poly(3-alkylthiophene)s have received a lot of attention, especially because of their high electrical conductivities in the doped state, and because of their unusual solvatochromic and thermochromic behavior . Hence, a lot of research has been focused on clarifying the structure of these materials, both in the solid state and in solution. Today, it is agreed that supramolecular aggregation of polythiophene chains plays an important role in their physical properties. 

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>. 

The DOUS (density of the valence states) obtained from the calculations was in good agreement with the direct measurements by UPS spectra. By studying temperature-induced changes in the UPS spectra, it can be deduced that the torsion angles of the POPT backbone decrease at elevated temperatures, leading to an increase of crystallinity. This conversion process was found to be irreversible, in contrast to the real thermochromism in poly(3-hexylthio-phene), which is reversible. 

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>. 

Order-Disorder Transitions. General Features, Experimental data are summarized in Table II, and representative thermochromic behaviors are shown in Figure 2. For the dialkyl-substituted polysilylenes the transition is very sharp, with a barely discernible coexistence region and an approximate isosbestic point. On the other hand, the asymmetrically substituted polymers, except poly(n-dodecylmethylsilylene), display very smooth behavior only in n-hexane solution and a broad but clearly discernible transition in dilute toluene solution. The transition width (ATc) in toluene solution was taken to be the interval between departure from the extrapolated, smooth, high-temperature behavior and the onset of peak absorption wavelength saturation at low temperature. The transition temperature (Tq) is defined arbitrarily as the midpoint of this region. 

Another class of soluble polysilylenes exhibits essentially no or very weak thermochromism. This class includes poly(cyclohexylmethyl- 15, 38), poly(phenylmethyl- 15, 38), (polytrimethylsilylmethyl- 15), and poly(diarylsilylenes) 46), all of which appear to be conformationally locked over a wide range of temperatures. In terms of our theoretical perspective, this behavior would arise from the steric effects of bulky substituents, which imply a large value of e and, hence, a small coupling constant Vj /e. For aryl-substituted polysilylenes, the proximity of an aromatic group to the backbone could also stabilize a highly ordered rodlike conformation via enhanced dispersion interactions. 

---