Local Polarity-Induced Assembly of Second-Order Nonlinear Optical Materials

ConspectusSecond-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.