Univ. Bordeaux, CNRS, Bordeaux INP, ISM, UMR 5255, F-33400Talence, France.
Donostia International Physics Center (DIPC), Manuel Lardizabal Ibilbidea 4, 20018Donostia, Euskadi, Spain.
Acc Chem Res. 2022 Dec 20;55(24):3716-3726. doi: 10.1021/acs.accounts.2c00616. Epub 2022 Dec 5.
The last 30 years have witnessed an ever-growing application of computational chemistry for rationalizing the nonlinear optical (NLO) responses of organic chromophores. More specifically, quantum chemical calculations proved highly helpful in gaining fundamental insights into the factors governing the magnitude and character of molecular first hyperpolarizabilities (β), be they either intrinsic to the chromophore molecular structure and arising from symmetry, chemical substitution, or π-electron delocalization, or induced by external contributions such as the laser probe or solvation and polarization effects. Most theoretical reports assumed a rigid picture of the investigated systems, the NLO responses being computed solely at the most stable geometry of the chromophores. Yet, recent developments combining classical molecular dynamics (MD) simulations and DFT calculations have evidenced the significant role of structural fluctuations, which may induce broad distributions of NLO responses, and even generate them in some instances.This Account presents recent case studies in which theoretical simulations have highlighted these effects. The discussion specifically focuses on the simulation of the second-order NLO properties that can be measured experimentally either from Hyper-Rayleigh Scattering (HRS) or Electric-Field Induced Second Harmonic Generation (EFISHG). More general but technical topics concerning several aspects of the calculations of hyperpolarizabilities are instead discussed in the Supporting Information.Selected examples include organic chromophores, photochromic systems, and ionic complexes in the liquid phase, for which the effects of explicit solvation, concentration, and chromophore aggregation are emphasized, as well as large flexible systems such as peptide chains and pyrimidine-based helical polymers, in which the relative variations of the responses were shown to be several times larger than their average values. The impact of geometrical fluctuations is also illustrated for supramolecular architectures with the examples of nanoparticles formed by organic dipolar dyes in water solution, whose soft nature allows for large shape variations translating into huge fluctuations in time of their NLO response, and of self-assembled monolayers (SAMs) based on indolino-oxazolidine or azobenzene switches, in which the geometrical distortions of the photochromic molecules, as well as their orientational and positional disorder within the SAMs, highly impact their NLO response and contrast upon switching. Finally, the effects of the rigidity and fluidity of the surrounding are evidenced for NLO dyes inserted in phospholipid bilayers.
过去的 30 年见证了计算化学在合理化有机发色团的非线性光学（NLO）响应方面的应用不断增加。更具体地说，量子化学计算在深入了解控制分子第一超极化率（β）大小和性质的因素方面提供了高度帮助，这些因素既可以是发色团分子结构固有的，也可以是由对称性、化学取代或π电子离域引起的，或者是由外部贡献（如激光探针或溶剂化和极化效应）引起的。大多数理论报告都假设所研究系统的刚性图像，NLO 响应仅在发色团最稳定的几何形状上进行计算。然而，结合经典分子动力学（MD）模拟和 DFT 计算的最新发展已经证明了结构波动的重要作用，这些波动可能会导致 NLO 响应的广泛分布，甚至在某些情况下产生它们。本报告介绍了最近的案例研究，其中理论模拟强调了这些效应。讨论特别侧重于可以从超瑞利散射（HRS）或电场感应二次谐波产生（EFISHG）实验测量的二阶 NLO 性质的模拟。相反，在支持信息中讨论了与极化率计算的几个方面相关的更一般但技术主题。选择的例子包括有机发色团、光致变色系统和液相中的离子配合物，其中强调了明确溶剂化、浓度和发色团聚集的影响，以及大的柔性系统，如肽链和嘧啶基螺旋聚合物，其中响应的相对变化被证明比它们的平均值大几倍。对于具有在水溶液中形成有机偶极染料的纳米粒子的超分子结构的例子，还说明了几何波动的影响，其柔软的性质允许大的形状变化，从而导致其 NLO 响应的时间上的巨大波动，以及基于吲哚啉-恶唑烷或偶氮苯开关的自组装单层（SAMs），其中光致变色分子的几何变形，以及它们在 SAMs 内的取向和位置无序，对它们的 NLO 响应及其在开关时的对比度有很大影响。最后，对于插入磷脂双层中的 NLO 染料，证明了周围环境的刚性和流动性的影响。
