Theoretical investigation of gas-phase torsion potentials along conjugated polymer backbones: Polyacetylene, polydiacetylene, and polythiophene

We present ab initio quantum-chemical calculations on the gas-phase torsion potentials of oligomers representative of polyacetylene, polydiacetylene, and polythiophene. Our goal is to obtain reliable values for the total energies involved in the torsions of fully conjugated chains, in order to assess the flexibility of such chains in solution and to provide the basis for meaningful discussions of persistence length and conjugation length models. Rotation around a single bond leads to barrier heights on the order of a few kilocalories per mole in polyacetylene and polythiophene (about 6 and 3 kcal/mol, respectively) but lower than 1 kcal/mol in polydiacetylene.