Conformations and torsional potentials of poly(3-hexylthiophene) oligomers: Density functional calculations up to the dodecamer

Fully optimized conformations of poly(3-hexylthiophene) (P3HT) oligomers up to the decamer and torsional potentials up to the dodecamer (302 atoms) are investigated by large-scale density functional calculations (B3LYP/6-31+G(d,p)). Fully relaxed P3HT oligomers were investigated at a variety of conformational minima, many with skeletal structures far from planarity. The lowest energy conformations found have each hexyl group approximate to 74 degrees out of plane and each backbone twist angle out of plane by approximate to 47 degrees The energies of these non-planar conformations are lower than that of the planar reference geometry by approximate to 30 meV per monomer. Backbone torsional potentials and hexyl torsional potentials converge with oligomer length by the octamer. A wide variety of oligomer conformations are sufficiently close in energy (similar to kT) that the actual conformations found in a condensed phase will be determined primarily by intermolecular interactions, and substantial conformational disorder might be expected in the heterogenous environment of practical solar devices. (C) 2012 Elsevier B.V. All rights reserved.