Structure and torsional potential of p-phenylthiophene: a theoretical comparative study

Quantum chemical calculations employing Hartree–Fock, MP2 and density functional (using distinct functionals) approaches were carried out for the p-phenylthiophene dimer. The fully optimized stationary points located on the potential energy surface were characterized as minima or transition state (TS) structures according to harmonic frequency analysis. A mixture of syn–gauche and anti–gauche conformers was predicted with a relative percentage of ca. 60% and 40%, respectively. A TS structure connecting the syn–gauche and anti–gauche minima was also determined, with the MP2 energy barrier being ca. 10 kJ mol−1. A six-term truncated Fourier series representation of the potential energy for internal rotation was obtained using a fitting procedure to the calculated HF/6-31G* and B3LYP/6-31G* partially optimized points. Additional fittings were performed with the MP2/6-31G*//HF/6-31G*, MP2/6-31G*//B3LYP/6-31G*, B3LYP/6-31G*//HF/6-31G*, BLYP/6-31G*//HF/6-31G*, B3P86/6-31G*//HF/6-31G* and SVWN/6-31G*//HF/6-31G* single energy points. The energy barriers obtained from the fitted curve were compared to the ones calculated from the energy differences between fully optimized minima and TS structures. The fitted Fourier potential is found to be adequate for the description of the internal rotation in the p-phenylthiophene dimer. The B3LYP/6-31G*//HF/6-31G* level of calculation seems sufficient for studying this class of compounds. The inclusion of the phenyl substituent group in the bithiophene, which makes it more easily processable, does not alter significantly the energy gap. Therefore, the p-phenylthiophene would be expected to exhibit similar conductivity to the parent non-substituted bithiophene compound.