University of Namur, Theoretical Chemistry Lab, Unit of Theoretical and Structural Physical Chemistry, Namur Institute of Structured Matter, rue de Bruxelles, 61, B-5000 Namur, Belgium.
Université de Bordeaux, Institut des Sciences Moléculaires, UMR 5255 CNRS, cours de la Libération 351, F-33405 Talence Cedex, France.
J Phys Chem B. 2021 Apr 8;125(13):3386-3397. doi: 10.1021/acs.jpcb.1c00939. Epub 2021 Mar 26.
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 -toluenesulfonate. These complexes showed a strong self-aggregation behavior in molecular dynamics simulations within high-concentration conditions and formed stable dimeric aggregates, (AC), 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 [ 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 [ 2010, 11, 495-507]. Moreover, results have highlighted that (i) the second-order contribution, , dominates the global EFISHG response; (ii) the 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 ; (iv) the presence of a strong amino donor group in the cation enhances the 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, as on , while it reduces the one-dimensional character of β. This work constitutes a step forward for the modeling of the NLO responses of AC aggregates in solution.
已制定出一种顺序方法，将分子动力学和密度泛函理论计算相结合，以揭示形成二聚体聚集体时阴阳离子（AC）对的二次谐波产生响应，其中阳离子为芘并唑鎓衍生物，阴离子范围从小分子无机碘化物到中尺寸有机对甲苯磺酸盐。在高浓度条件下的分子动力学模拟中，这些配合物表现出强烈的自聚集行为，并形成稳定的二聚体聚集体（AC），它们可以采用不同的结构形状，从堆叠、Λ 到对头配置。这些不同的结构与不同的对称性相关联，这些对称性被证明可以调节二阶和三阶非线性光学（NLO）响应。通过将这项工作的 NLO 结果与之前为单个 AC 对获得的结果相结合[2020,60,4817-4826]，我们能够解释这些复合物作为浓度函数的电致二次谐波产生（EFISHG）响应的实验观察到的变化[2010,11,495-507]。此外，结果强调了(i)二阶贡献，γ，主导全局 EFISHG 响应；(ii)二聚体的响应大约是母体 AC 对计算出的响应的一半，而三阶贡献，γ，仅降低了 10%；(iii)这些不同的趋势归因于主要采用 Λ 和对头形状的二聚体的形成，与单体相比，增加了中心对称特性，其中二阶响应也抵消了并且影响了关于的偶极矩；(iv)阳离子中存在强氨基供体基团将响应增强了 1 个数量级，γ增强了约 2 倍；最后，(v)二聚体聚集对超瑞利散射响应β的影响与对γ的影响相同，同时降低了β的一维特性。这项工作是在溶液中 AC 聚集体的 NLO 响应建模方面迈出的一步。
