Poly , thermochromic behavior

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. 

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. 

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. 

The poly(dialkyl) stannanes, (Hex2Sn) and (Oct2Sn) exhibit a reversible thermochromic behavior upon warming above room temperature. The A. iax values of these polymers undergo a blue-shift from 384 to 369 nm for poly(di-n-octyl)stannane between 30 0 °C in toluene and from 392 to 382 nm in... 

A two-phase thermochromic behavior, as in PDDT and poly(3-methyl-4-octyloxythiophene), is related to the formation of delocalized conformational defects upon heating. These defects are possible due to the presence of sterically demanding substituents between each consecutive repeating unit [330]. In the solid state at room temperature, PDDT and poly(3-octyloxy-4-methyl-thiophene) have a coplanar conformation for the main chain. Heating (25 to 150°C) increases the repulsive intrachain steric interactions and introduces some conformational disorder in the side groups, forcing the polymer backbone to adopt a nonplanar conformation [331, 332]. Temperature dependent UV/vis absorption measurements of fluorinated PTs, e.g., poly(3 -perfluorohexyl-2,2 5, 2"-terthiophene), poly[3-(pentadecafluorooctyloxy)-4-methylthiophene] and poly[3-(tridecafluorononyl)thiophene], show a blue shift of the maximum... 

Thermochromism. Neutral poly(3-alkylthiophene)s and poly(3-alkoxy-4-metiiylthiophene)s exhibit strong chromic effect upon heating both in the solid state and in solution[25-2P]. Two types of thermochromic behavior can be observed and are correlated to the substitution pattern of the polymers. As an example of the first type (the two-phase behavior), the temperature dependence of the absorbance of a thin film of poly[3-oligo(oxyethylene)-4-methylthiophene] [57] is shown in Figure 4. 

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. 

Miteva, T., et al. 2000. Interplay of thermochromicity and liquid crystalline behavior in poly(p-phenyleneethynylenejs tt-tt interactions or planarization of the conjugated backbone Macromolecules 33 652. 

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