Quantifying structural polarization by continuous regulation of lone-pair electron expression in molecular crystals

The rational design of structural polarization through the manipulation of stereochemical lone-pair electrons (LPEs) is crucial for engineering functional materials with tailored physical properties. However, a systematic understanding of the relationship between LPE behavior and structural polarization remains limited. Recently, a research team led by Dr. Xujie Lü from the Center for High Pressure Science and Technology Advanced Research (HPSTAR) and Prof. Tianyou Zhai from Huazhong University of Science and Technology has made significant strides in this area. Their research provides new insights into achieving precise control and quantification of structural polarization by tuning and quantifying LPE expression in molecular crystals. The study, titled “Quantifying Structural Polarization by Continuous Regulation of Lone-Pair Electron Expression in Molecular Crystals”, has been published in a recent issue of the Journal of the American Chemical Society.      

The control of structural polarization is of paramount importance in matter sciences, as it directly influences the physical properties and functionalities of materials. This research area has long been a vibrant and enduring hotspot, driving significant impacts on the realm of modern technologies including nonlinear optics, ferroelectricity, and quantum devices. Great progress has been made to introduce and tune the structural polarization, such as by introducing acentric structural units, manipulating structural strains, and constructing hydrogen bonding networks. Among these strategies, the introduction of stereochemically active LPEs has demonstrated to be one of the most effective ways to shape the structural polarization. These nonbonded electron pairs impel molecular configuration reformation and drive local structural distortion, which breaks inversion symmetry and results in structural polarization. Importantly, understanding the behavior of stereochemically active LPEs could offer insights into the formation and variation of local dipoles and, therefore, aid in engineering the desired polarization characteristics. However, no research has quantitatively elucidated the relationship between the dynamic expression of LPEs and structural polarization.

In this work, the researchers employed pressure to continuously regulate the expression of LPEs and introduce a structural parameter to quantify the polarization.  They selected the non-centrosymmetric molecular crystal SbI3·3S8 as a prototype, which possesses the structural polarization that is dominated by the LPE effect. By using pressure to continuously tune the LPE expression and introducing an I−Sb−I angle (ᾱ) that describes the degree of LPE expression, the researchers establish a quantitative relationship between ᾱ and dipole moments and further elucidate their correlation with second harmonic generation (SHG) property. “Decreasing ᾱ shapes LPE expression more localized, which repels the bonding pairs electrons and thus increases the dipole moments and improves SHG response.” explained by Dr. Kejun Bu from HPSTAR. The research team has extended this quantified relationship to a series of molecular crystals and demonstrates its applicability for the design of structural polarization by tailoring LPE expression.

Caption: Top panel: Schematic illustration of the regulation of LPE expression and polarization engineering. Dashed lines indicate LPE orbitals. Bottom panel: SHG response versus ᾱ for a series of non-centrosymmetric molecular crystals, showing a close correlation between LPE expression and SHG response.

This work not only provides a quantitative understanding of the relationship between LPE expression and structural polarization but also offers novel insights into the exploration of polarized materials through the manipulation of LPEs.

操控具有立体化学活性的孤对电子（LPEs）以实现结构极化的合理设计，对于功能材料性质和功能的定制至关重要。然而，目前我们对孤对电子的演变规律及其与结构极化特性之间的关系仍缺乏充分的理解。为此，北京高压科学研究中心（HPSTAR）吕旭杰研究员与华中科技大学翟天佑教授等展开合作研究，利用压力调控结合多种原位测试技术，揭示了分子晶体中孤对电子表达与结构极化之间的定量构效关系，并阐明了其对非线性光学性质的影响机制。相关成果以“Quantifying Structural Polarization by Continuous Regulation of Lone-Pair Electron Expression in Molecular Crystals”为题，发表于近期的《美国化学会志》（Journal of the American Chemical Society）上，论文第一作者为北京高压科学研究中心的卜克军博士。