Molecular design for organic nonlinear optics: Polarizability and hyperpolarizabilities of furan homologues investigated by ab initio molecular orbital method

Static polarizabilities and hyperpolarizabilities have been calculated for the furan homologues including furan (C4H4O), thiophene (C4H4S). selenophene (C4H4Se), and tellurophene (C4H4Te), at the Hartree-Fock and various correlation levels by ab initio molecular orbital, methods. The influence of substitution with heavy atoms on the molecular properties of the homologues was examined theoretically. in the calculations for molecules with Se and Te atoms, the contribution of core electrons in the atoms was estimated using effective core potential (ECP) methods. With the calculation for the thiophene molecule, effectiveness of the use of the ECP method was examined, and the ECP method was found to give the (hyper)polarizability values similar to those obtained considering all electrons for the molecule. The calculated results show that substitution of the heteroatom with heavy atoms systematically increases the individual components of the polarizability, alpha, and second hyperpolarizability, gamma. On the other hand, this does not give systematic variations an the first hyperpolarizability, beta. The variation of these values depending on the homologues was examined in terms of the perturbation expansion formula and was explained by the energy levels of the electronic excited states in the valence states. The second hyperpolarizability values calculated, including electron correlation effect, are well correlated with the observed electronic components of nonlinear optical susceptibilities, chi((3)), Obtained using optical Kerr effect experiments, which have been done in our laboratory. However, some discrepancy is observed between the theoretical and the experimental values for the homologues with heavier heteroatoms. Possible causes for the discrepancy are discussed in detail.