Second Harmonic Generation, Electrooptical Pockels Effect, and Static First-Order Hyperpolarizabilities of 2,2′-Bithiophene Conformers: An HF, MP2, and DFT Theoretical Investigation
The static and dynamic electronic (hyper)polarizabilities of the equilibrium conformations of 2,2′-bithiophene (anti-gauche and syn-gauche) were computed in the gas phase. The calculations were carried out using Hartree-Fock (HF), M？ller-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 typical nm of the Nd:YAG laser. The anti-gauche form characterized by the dihedral angle of 137° (MP2/6-311G**) is the global minimum on the potential energy surface, whereas the syn-gauche rotamer ( = 48°, MP2/6-311G**) lies ca. 0.5？kcal/mol above the anti-gauche form. The structural properties of the gauche structures 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 the anti-gauche to the syn-gauche conformer, 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. The syn-gauche structure could be discriminated from the anti-gauche one through its much more intense SHG and EOPE signals. 1. Introduction Thiophene-based oligomers and polymers are an interesting class of π-conjugated materials for the development and construction of conductive and nonlinear optical (NLO) devices [1–5]. Electronic properties of -conjugated polymeric systems are significantly affected by the twisting degree of the backbone and extension of the electron delocalization although molecular structure and physicochemical properties of extended oligomers and polymers are usually modelled through smaller oligomeric chains [6, 7]. The Cα–Cα’ bonded bithiophene oligomer, 2,2′-bithiophene (Figure 1), is the principal building block of polythiophene chains, extensively characterized by experimental and theoretical studies. In the solid state, 2,2′-bithiophene predominantly exists as a planar anti-structure together with a nonnegligible fraction of planar syn-conformation (ca. 15%) [8]. A slightly different picture occurs in the gas phase: two nonplanar minimum-energy structures are observed to coexist (Figure 1), which are characterized by S–C2–C2’–S dihedral angles of 148° ± 3° (anti-gauche) and 36° ± 5° (syn-gauche) [9]. On the basis of electron diffraction data [9] and
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