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 ≈74° out of plane and each backbone twist angle out of plane by ≈47°. The energies of these non-planar conformations are lower than that of the planar reference geometry by ≈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 (∼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.