Second Harmonic Generation Responses of Ion Pairs Forming Dimeric Aggregates

Abstract

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.20206048174826], 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 [ChemPhysChem201011495507]. 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.

First hyperpolarizability of water at the air–vapor interface: a QM/MM study questions standard experimental approximations

“…Moreover, one considers that the signal can be interpreted in terms of molecular hyperpolarizability. The decomposition of the non linear optical response of a cluster into a sum of molecular properties is not obvious 36 , since intermolecular interactions may strongly impact non linear optical properties (for a recent exemple see 37 ). In the special case of water, Maroulis 38 or Liang et al 39 compared the polarizabilities and/or hyperpolarizabilities of dimers to the sum of the properties of the individual molecules.…”

First hyperpolarizability of water in bulk liquid phase: long-range electrostatic effects included via the second hyperpolarizability

“…The second hyperpolarizability γ describes the variations of the first hyperpolarizability value under a static external electrostatic field e DC , present in the molecular frame. This approach permits to link the EFISHG macroscopic measurements for the gas phase to quantum calculations of the molecular first hyperpolarizability β [25][26][27][28][29][30] .…”

“…This approach permits to link the EFISHG macroscopic measurements for the gas phase to quantum calculations of the molecular first hyperpolarizability b. [25][26][27][28][29][30] The application of field expansions such as eqn (2) for condensed phases is very useful for the interpretation of S-SHG of aqueous solutions, and the values of b and g for water in liquid water become parameters of the equation. To obtain values of b and g for water in liquid water, one can use Quantum Mechanical/Molecular Mechanic (QM/MM) calculations.…”

“…This approach permits to link the EFISHG macroscopic measurements for the gas phase to quantum calculations of the molecular first hyperpolarizability β . 25–30…”

First hyperpolarizability of water in the bulk phase: long-range electrostatic effects included via the second hyperpolarizability

“…This approach permits to link the EFISHG macroscopic measurements for the gas phase to quantum calculations of the molecular first hyperpolarizability β [25][26][27][28][29][30] .…”