Relation between Nonlinear Optical Properties of Push–Pull Molecules and Metric of Charge Transfer Excitations


N. H. List, R. Zaleśny, N. A. Murugan, J. Kongsted, W. Bartkowiak, H. Ågren

J. Chem. Theory Comput. 2015, 11, 4182


We establish the relationships between the metric of charge transfer excitation (Δr) for the bright ππ* state and the two-photon absorption probability as well as the first hyperpolarizability for two families of push–pull π-conjugated systems. As previously demonstrated by Guido et al. (J. Chem. Theory Comput. 2013, 9, 3118–3126), Δr is a measure for the average hole–electron distance upon excitation and can be used to discriminate between short- and long-range electronic excitations. We indicate two new benefits from using this metric for the analyses of nonlinear optical properties of push–pull systems. First, the two-photon absorption probability and the first hyperpolarizability are found to be interrelated through Δr; if β ∼ (Δr)k, then roughly, δTPA ∼ (Δr)k+1. Second, a simple power relation between Δr and the molecular hyperpolarizabilities of push–pull systems offers the possibility of estimating properties for longer molecular chains without performing calculations of high-order response functions explicitly. We further demonstrate how to link the hyperpolarizabilities with the chain length of the push–pull π-conjugated systems through the metric of charge transfer.