Intermolecular interaction effects on second hyperpolarizabilities of clusters including charged species

Abstract

We investigate the intermolecular-interaction effects on the longitudinal second hyperpolarizabilities (γ) for two types of $\mathit{\pi }\text{-}\mathit{\pi }$ stacking clusters: ${\mathrm{C}}_5{\mathrm{H}}_7^{+}$ dimer and π-conjugated main chains ( ${\mathrm{C}}_{\mathit{n}}{\mathrm{H}}_{\mathit{n}+2}$ , $6⩽\mathit{n}⩽16$ ) interacting in both end regions with two allyl cations ( ${\mathrm{C}}_3{\mathrm{H}}_5^{+}\mathrm{)}$ . The γ values are calculated by the finite-field approach using several ab initio molecular orbital and density functional methods. It is found that in the first model the $\mathit{\pi }\text{-}\mathit{\pi }$ interaction between the upper and lower π-orbitals significantly reduces the magnitude of effective γ per monomer, while the second model exhibits a remarkable enhancement of γ values as compared to those of isolated main chains. The analysis using the hyperpolarizability density and orbital interaction elucidates that the reduction of longitudinal γ value in the first model originates in the reduction of longitudinal π-conjugated electron distributions due to the perpendicular $\mathit{\pi }\text{-}\mathit{\pi }$ orbital interactions, whereas the enhancement of longitudinal γ in the second model is caused by the intermolecular charge transfer between both end perturbing molecules via the main chain. On the basis of these results, we discuss a novel guideline of controlling third-order nonlinear optical properties of clusters.