The thermochromism of poly(3-alkylthiophene)s: the role of the side chains

A combined atomistic simulation involving static lattice and molecular dyna mics simulation methods investigates the structure of poly(3-alkylthiophene )s in order to study their thermochromism. Taking the shortest alkyl chain (butyl) that makes the polymer thermochromic, conformational changes are mo nitored at different temperatures. The simulations demonstrate that there i s a tendency for the side chain to twist between 40 and 70 degrees about th e C-C bond linking it to the thiophene ring, although the carbon atoms in t he side chain largely retain a Irans conformation to above room temperature . These results, and the prediction of a non-zero setting angle, are qualit atively in accord with lattice powder diffraction structures. There is a st ep-like increase in the mean angle between the thiophene rings as a functio n of temperature at T = 340 K, suggesting the occurence of an abrupt reduct ion in the planarity of the conjugated chain at this temperature that shoul d lead to an increased band width and thermochromism. This is shown to be c oncerted with similar changes in side chain torsions at the same temperatur e, on the basis of which a tentative mechanism for the thermochromism is pr oposed. (C) 2001 Elsevier Science B.V. All rights reserved.