Second Harmonic Generation Responses of Ion Pairs Forming Dimeric Aggregates.

A sequential approach combining molecular dynamics and density functional theory calculations has been worked out to unravel the second harmonic generation responses of anion-cation (AC) pairs when they form dimeric aggregates, where the cation is a stilbazolium derivative and the anions range from small inorganic iodide to medium-size organic p-toluenesulfonate. These complexes showed a strong self-aggregation behavior in molecular dynamics simulations within high-concentration conditions and formed stable dimeric aggregates, (AC)2, which can adopt different structural shapes from stacked, Λ, to head-to-head configurations. These various structures are associated with different symmetries, which are shown to modulate the second- and third-order nonlinear optical (NLO) responses. By consolidating the NLO results of this work with those previously obtained for single AC pairs [ J. Chem. Inf. Model. 2020, 60, 4817-4826], we have been able to explain the experimentally observed variations of the electrical-field-induced second harmonic generation (EFISHG) responses of these complexes as a function of concentration [ ChemPhysChem 2010, 11, 495-507]. Moreover, results have highlighted that (i) the second-order contribution, μβ//, dominates the global EFISHG response; (ii) the μβ// responses of dimers are about half of those computed for the parent AC pairs, while the third-order contributions, γ//, are reduced by only 10%; (iii) these distinct trends are ascribed to the formation of dimers adopting mainly Λ and head-to-head shapes, increasing the centrosymmetric character, in comparison to the monomers, a situation in which the second-order response cancels out as well as influences the dipole moment on μβ//; (iv) the presence of a strong amino donor group in the cation enhances the μβ// response by 1 order of magnitude and γ// by about a factor of 2; and finally, (v) dimeric aggregation has similar effects on the hyper-Rayleigh scattering response, βHRS, as on μβ//, while it reduces the one-dimensional character of βHRS. This work constitutes a step forward for the modeling of the NLO responses of AC aggregates in solution.