Second Harmonic Generation, Electrooptical Pockels Effect, and Static First-Order Hyperpolarizabilities of 2,2′-Bithiophene Conformers: An HF, MP2, and DFT Theoretical Investigation

摘要

The static and dynamic electronic (hyper)polarizabilities of the equilibrium conformations of 2,2′-bithiophene (anti-gaucheandsyn-gauche) were computed in the gas phase. The calculations were carried out using Hartree-Fock (HF), Møller-Plesset second-order perturbation theory (MP2), and density functional theory methods. The properties were evaluated for the second harmonic generation (SHG), and electrooptical Pockels effect (EOPE) nonlinear optical processes at the typicalλ=1064nm of the Nd:YAG laser. Theanti-gaucheform characterized by theS–C2–C2′–Sdihedral angle of 137° (MP2/6-311G**) is the global minimum on the potential energy surface, whereas thesyn-gaucherotamer (S–C2–C2′–S= 48°, MP2/6-311G**) lies ca. 0.5 kcal/mol above theanti-gaucheform. The structural properties of thegauchestructures are rather similar to each other. The MP2 electron correlation effects are dramatic for the first-order hyperpolarizabilities of the 2,2′-bithiophenes, decreasing the HF values by ca. a factor of three. When passing from theanti-gaucheto thesyn-gaucheconformer, the static and frequency-dependent first-order hyperpolarizabilities increase by ca. a factor of two. Differently, the electronic polarizabilities and second-order hyperpolarizabilities of these rotamers are rather close to each other. Thesyn-gauchestructure could be discriminated from theanti-gaucheone through its much more intense SHG and EOPE signals.