Correlated backbone torsional potentials in poly(3-methylthiophene)

Poly(3-alkylthiophenes) (P3AT) have shown great promise as solar cell materials. Knowledge of the conformations and the conformational energies is a key step toward understanding the electronic properties in such systems that contain extended electronic conjugation. In this study, poly(3-methylthiophene) (P3MT) oligomers are selected as model systems for the torsional properties of P3AT’s. A 3-dimensional torsional energy map is computed for the P3MT tetramer by density functional methods. Single-layered and ONIOM calculations were performed at B3LYP/6-31++G(d,p) level. The intra-chain torsional potential, when expressed in terms of the inter-ring torsional angles, contains correlations attributable to extended electronic conjugation along the oligomer backbone. Cross terms in the 3-D torsional energy map were found to scale such that 11% of the total variation of the potential results from the relative orientations of second-nearest-neighbor rings, and more extended torsional interactions along the chain contribute less than 3%.