Second-order nonlinear optical (NLO) materials, producing coherent light by the cascaded frequency conversion, are current hot topics in material chemistry and optics, as they play an important role in diverse fields, such as laser technology, precision measurements, and quantum information as well as future space propulsion. The prerequisite for a second-order NLO material is that it must crystallize in a noncentrosymmetric space group, but it tends to be centrosymmetric in terms of thermodynamical stability. Furthermore, it should simultaneously satisfy strict requirements on second-order NLO coefficients, birefringence, optical transmittance windows, crystal growth, and so on. Therefore, it is still an urgent challenge to rationally assemble high-performance second-order NLO materials. In this Account, we first review the significance and background of second-order NLO materials and some strategies to design new second-order NLO materials controllably. After that, we mainly present our recent studies on the rational assembly of second-order NLO materials by the so-called strategy of “local polarity-induced assembly”, including: (1) making use of the coordination habit between specific cations like Li⁺ or Mg²⁺ and tetrahedral (PO₄)^3–, (P₂O₇)^4–, and (SO₄)^2– units to generate the local polarity, and further inducing the assembly of second-order NLO materials of superb optical properties; (2) by virtue of self-condensation of weakly polarizable units, condensation of different weakly polarizable units and partial substitution of O atoms by F or S atoms in weakly polarizable units of tetrahedral MO₄ or octahedral MO₆ configuration, enhancing the local polarity to induce the assembly of second-order NLO materials; and (3) introducing strongly polarizable units of stereoactive lone pair electrons into π-conjugated systems to afford the local polarity and inducing second-order NLO materials with enhanced NLO responses. Based on this strategy, we successfully assembled a variety of excellent second-order NLO materials and expanded new sources of second-order NLO materials, like fluorooxosilicophosphates, thiosulfates, and borosulfates. Finally, we will conclude the topic and give some prospects for exploring new excellent second-order NLO materials. We believe that the “local polarity-induced assembly” strategy will not only be useful for understanding the structure–property relationships of second-order NLO materials but also provide researchers with insights into obtaining noncentrosymmetric structures that are essential to other functional materials in piezoelectricity, ferroelectricity, and pyroelectricity, etc.

中文翻译：

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二阶非线性光学材料的局部极性诱导组装

二阶非线性光学 (NLO) 材料通过级联频率转换产生相干光，是当前材料化学和光学领域的热门话题，因为它们在激光技术、精密测量和量子信息等多个领域发挥着重要作用以及未来的太空推进。二阶非线性光学材料的先决条件是它必须在非中心对称空间群中结晶，但在热力学稳定性方面倾向于中心对称。此外，它还应同时满足二阶非线性光学系数、双折射、透光窗口、晶体生长等方面的严格要求。因此，合理组装高性能二阶非线性光学材料仍然是一个紧迫的挑战。在这个帐户中，我们首先回顾了二阶非线性光学材料的意义和背景以及可控地设计新型二阶非线性光学材料的一些策略。之后，我们主要介绍了我们最近通过所谓的“局部极性诱导组装”策略合理组装二阶非线性光学材料的研究，包括：（1）利用特定阳离子（如 Li）之间的配位习性⁺或Mg ²⁺和四面体(PO ₄ ) ^3– , (P ₂ O ₇ ) ^4– , 和 (SO ₄ ) ^2–单元产生局域极性，进一步诱导二阶非线性光学材料的组装光学特性；_{(2)四面体MO 4}或八面体MO ₆弱极化单元中的弱极化单元自缩合、不同弱极化单元缩合以及O原子被F或S原子部分取代配置，增强局部极性以诱导二阶非线性光学材料的组装；(3) 将立体活性孤对电子的强极化单元引入 π 共轭系统中以提供局部极性并诱导具有增强的非线性光学响应的​​二阶非线性光学材料。基于这一策略，我们成功地组装了多种优异的二阶非线性光学材料，并拓展了氟氧硅磷酸盐、硫代硫酸盐和硼硫酸盐等二阶非线性光学材料的新来源。最后，我们将总结这个话题，并给出一些探索新的优秀二阶非线性光学材料的展望。