南开大学徐加良团队报道了柔性分子晶体中机械扭曲诱导的二阶光学非线性增强。相关研究成果发表在2025年1月7日出版的《美国化学会杂志》。

柔性分子晶体对于推进智能材料的发展至关重要，为下一代电子、制药和能源存储的应用提供了独特的功能和适应性。然而，柔性分子晶体的光学应用在很大程度上局限于线性光学，很少探索非线性光学（NLO）特性。

该文报道了柔性分子晶体的机械扭转在二阶非线性光学中的应用。通过模型化合物9-蒽醛（AA）的自组装形成的晶体具有固有的手性和非中心对称结构，表现出高效的二次谐波发生（SHG）和非线性圆二色性，这可以通过宏观机械扭曲大大增强。各向异性分子堆叠赋予AA晶体弹性弯曲和塑性扭曲相结合的机械柔性。

根据它们的M或P构型，等手性机械扭转可以将SHG强度大大提高一个数量级。同时，等手性扭曲晶体的SHG圆二色性因子gSHG-CD大大增加，实现了有机NLO材料中报道的最高NLO各向异性因子。这些增强的SHG强度和非线性手性响应的非线性光学性能，有望大大扩展柔性分子晶体的光子应用。

附：英文原文

Title: Mechanical Twisting-Induced Enhancement of Second-Order Optical Nonlinearity in a Flexible Molecular Crystal

Author: Zhihua Wang, Wenqing Han, Bona Fu, Hanwen Kang, Puxin Cheng, Junjie Guan, Yongshen Zheng, Rongchao Shi, Jialiang Xu, Xian-He Bu

Issue&Volume: January 7, 2025

Abstract: Flexible molecular crystals are essential for advancing smart materials, providing unique functionality and adaptability for applications in next-generation electronics, pharmaceuticals, and energy storage. However, the optical applications of flexible molecular crystals have been largely restricted to linear optics, with nonlinear optical (NLO) properties rarely explored. Herein, we report on the application of mechanical twisting of flexible molecular crystals for second-order nonlinear optics. The crystal formed through the self-assembly of the model compound 9-anthraldehyde (AA) features an intrinsic chiral and noncentrosymmetric structure, demonstrating high efficiency second harmonic generation (SHG) and NLO circular dichroism, which could be greatly enhanced by macroscopic mechanical twisting. The anisotropic molecular stacking imparts the AA crystal with mechanical flexibility of combined elastic bending and plastic twisting. The isochiral mechanical twisting could greatly enhance the SHG intensities by an order of magnitude depending on their M- or P-configuration. Meanwhile, the SHG circular dichroism factor gSHG-CD of the isochiral twisted crystal is greatly increased, achieving the highest reported NLO anisotropy factor among organic NLO materials. These boosted NLO performances of SHG intensity and nonlinear chiroptical response are expected to greatly expand the photonic applications of flexible molecular crystals.

DOI: 10.1021/jacs.4c15519

Source: https://pubs.acs.org/doi/abs/10.1021/jacs.4c15519