Rotational potential functions for oligomethylthiophenes

Abstract

The bulk structure of polythiophenes is mostly determined by the rotational degrees of freedom as tyre are various packing possibilities within the bulk structure. The optical and electronic properties are also closely associated with the rotational motion around monomer-monomer bonds. Here we present accurate Density-Functional-Theory (DFT) calculations for head-to-tail methyl substituted oligothiophenes for generating a rotational force field The rotational potential function of the dieter is computed on a very fine grid and expanded in terms of the cosines of the torsional angle. The torsional space of the regular thiophene trimers and tetramers is fully spanned These energy values are compared to those obtained from additive potentials. The errors coming from this approach and several coupling possibilities to reduce the errors are discussed. It is noted that as the chain grows, the near-planar structures become more stable than the dieter calculations suggest.