Structure Dependence of Hyperpolarizability in Octopolar Molecules.

Recent Hyper Rayleigh Scattering measurements report a significant increase of second-order hyperpolarizability upon introduction of positive charges at the pyridyl end groups in trispyridyl octopolar chromophores. We calculated the geometries, linear response, and first-order hyperpolarizabilities of a series of six trispyridyl molecules both in the neutral and protonated forms. The calculations were performed with ab initio and semiempirical methods. The results are in good agreement with the experimental values and a correlation between the first hyperpolarizability and two structural properties, the N-C bond elongation and the C-C bond length alternation, ⟨Δr⟩ was established. To test these effects we computed the hyperpolarizability for several constrained geometries and confirmed the importance of planarity on the hyperpolarizability values. However the ⟨Δr⟩ values alone seem to have little influence both on the hyperpolarizability and on the gap values. Replacing the triple C≡C bond by a double C=C bond in the conjugation bridge has no significant effect due to the strong hyperpolarizability dependence on the pyridyl-benzene dihedral angle.