Conformational Excitations Thermochromism

A very interesting and potentially useful thermochromic effect occurs in the (undoped) alkyl-substituted polythiophenes, for alkyl groups larger than methyl [35]. The thermochromic effect is a (reversible) thermally-induced change in color. In order to characterize the observed thermochromic behavior of the P3ATs, a series of studies of the temperature-dependence of the electronic structure of P3HT have been carried out using optical absorption spectroscopy, UPS and XPS. The central results, as well as a model of the thermochromism which is consistent with all of the experimental data, are reviewed below. 

FIGURE 13. Optical absorption spectra of a thin solid film P3HT at temperatures between 20 and 200 C [67]. 

FIGURE 16. The chemical (geometric) structure of P3HT (Top), and the Dimer Model (bottom). 

FIGURE 17. Variable-temperature % NMR spectra of the thiophene proton of P30T. The concentration of P30T was Img/ml in CDQ3 acetone-d5 (9 1) [34]. 

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. 

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