posted on 2021-03-26, 20:29authored byTárcius
N. Ramos, Frédéric Castet, Benoît Champagne
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.