Intramolecular CH/π interactions in alkylaromatics: Monomer conformations for poly(3-alkylthiophene) atomistic models

In existing poly(3-alkylthiophenes) atomistic models, an extended conformation of the side chain is usually assumed. We report a first principle study of the side-chain energetics of 3-hexylthiophene, with the constraint of compatibility with crystal packing requirements. The first two torsion angles of the side chain closest to the ring were considered. Electron correlation is shown to be of great relevance in the assessment of the relative stability of folded conformers against extended ones. The roles of local charge-transfer, rehybridization, steric repulsion, and basis set superposition error, were all considered in the rationalization of our results. We extend our analysis to the thiophene/methane complex in order to elucidate the main differences between intermolecular and intramolecular CH/π phenomena. While in the noncovalent complex a single C—H bond mediates the interaction, folded arrangements of 3-alkylthiophenes require the collective effort of several aliphatic bonds.