Enhanced hyperpolarizability via electron correlations in donor-σ-acceptor systems

Ab initio calculations demonstrate that specific zwitterionic molecules which have a σ-bonded donor-acceptor system show an enhanced hyperpolarizability (β) value when the effects of electron correlations are considered. Ab initio second-order Møller-Plesset perturbation theory (MP2) through-space (TS)–through-bond (TB) interaction (TB denotes orbital interactions through directly bonded atoms in a molecule, and TS denotes all interactions other than TB interactions) analysis proposes that the σ conjugation between acceptor and donor through bonds causes the dependence of the perturbation term between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (and their vicinal molecular orbitals) in MP2 calculations on electric fields, making the hyperpolarizability (β) value remarkably large.