Theoretical study on the photophysical properties of hexapyrrolidine C60 adducts with Th, D3, and S6 symmetries

The equilibrium geometries of three isomeric hexapyrrolidine C(60) adducts with T(h), D(3), and S(6) symmetries are optimized by means of the B3LYP method at the 6-31G basis sets in this paper. On the basis of the optimized structures, the excited state and third-order nonlinear optical properties, such as third-harmonic generation (THG), electric-field-induced second-harmonic generation (EFISHG), and degenerate four-wave mixing (DFWM), and two-photon absorption (TPA) cross sections, delta, are calculated by using the TDB3LYP model based on the 6-31G level coupled with the sum-over-states (SOS) method. The computational results show that the transition energies from S(0) to S(1) of the T(h) hexaadduct and the D(3) hexaadduct have a remarkable blue shift by comparison with that of the C(60) parent. These results are in agreement with experimental ones. However, the first singlet excitation energy of the S(6) hexaadduct has a red shift compared with that of the C(60) parent. Accordingly, we predict that different positions located by six addends may result in the different spectrum properties. Finally, the two-photon absorption cross sections indicate that the largest average value of resonant TPA, delta, of the D(3) hexaadduct has a red shift compared with those of the T(h) and S(6) hexaadducts.