Strong electron correlation effects on first- And second-order hyperpolarizabilities in zwitterionic σ-conjugated systems: Its dependence on substituents, conformations, spacer size, and basis sets

Nonlinear optical properties of zwitterionic σ-conjugated systems were theoretically investigated with relation to the electron correlation effects at the ab initio molecular orbital level. We examined the strong electron correlation effects on the first- and second-order hyperpolarizabilities in the specific systems with effective "π-σ-π" and (or) "π-σ-n" interactions. The electron correlation effects on the hyperpolarizabilities strongly depend on the type of substituents, conformations, spacer size, and basis sets. It was found that the Hartree-Fock level calculations qualitatively predict the behavior of the hyperpolarizabilities after considering the correlation effects. Through-space/-bond interaction analysis quantitatively revealed that the electron correlation effects on the hyperpolarizabilities were induced mainly by the σ-conjugations on the spacer unit in the zwitterionic σ-systems.