Enhanced nonlinear optical response in zwitterionic molecules: A computational study on the role of orbital interactions through σ bonds

Ab initio coupled perturbated Hartree-Fock calculations, using 6-31G** basis sets, on a heterocyclic zwitterionic molecule with a σ-spacer between the donor and the acceptor ring shows a static first hyperpolarizability (β₀) of around 240 × 10^-30 esu. Substitution of electron withdrawing functional groups, such as NO₂, on the acceptor ring of this molecule enhances the β₀ value to around 3960 × 10^-30 esu. Studies on various such zwitterionic molecules demonstrate the importance of π-σ orbital mixing (through bond interaction) between the π-aromatic rings and the σ-spacers in enhancing the nonlinear optical (NLO) response. Analysis of the transitions reveals low oscillator strengths, large changes in the dipole moments, and very low energy charge transfers that take place in the excited states of the molecule, while in the ground state they are stabilized in a charge separated resonance form. This is mainly responsible for the large NLO response. MP2 calculations on small molecules with dominant through bond interactions show that the inclusion of electron correlation further enhances the β₀ value. The molecules, which have a strong IR absorption, have potential applications in filters, polarizers, optical recording, etc.