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, β_HRS, as on μβ_//, while it reduces the one-dimensional character of β_HRS. This work constitutes a step forward for the modeling of the NLO responses of AC aggregates in solution.