Chen Chenhao, Zhang Shaodong
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China.
Acc Chem Res. 2025 Feb 18;58(4):583-598. doi: 10.1021/acs.accounts.4c00754. Epub 2025 Jan 28.
ConspectusSymmetry is a pervasive phenomenon spanning diverse fields, from art and architecture to mathematics and science. In the scientific realms, symmetry reveals fundamental laws, while symmetry breaking─the collapse of certain symmetry─is the underlying cause of phenomena. Research on symmetry and symmetry breaking consistently provides valuable insights across disciplines, from parity violation in physics to the origin of homochirality in biology. Chemistry is particularly rich in symmetry breaking studies, encompassing areas such as asymmetric synthesis, chiral resolution, chiral structure assembly, and so on. Across different disciplines, a well-defined methodology is fundamental and necessary to analyze the symmetry or symmetry breaking nature behind the phenomenon, enabling researchers to uncover the underlying principles and mechanisms. Basically, three key points underpin symmetry-related research: the scale-dependency of symmetry/symmetry breaking, the driving force behind symmetry breaking phenomena, and the properties arising from symmetry breaking.This Account will focus on the three aforementioned key points elucidated with organic cages as proof-of-concept models, as organic cages exhibit shape-persistent 3D molecular frameworks, well-defined molecular motion, and a high propensity for crystallization.First, we examine racemization processes of organic cages with dynamic molecular motions to illustrate that symmetry and symmetry breaking are time-scale-dependent. Specifically, the racemization, driven by molecular motion, is influenced by hydrogen bonding and the rigidity of the cage framework, which may or may not be observable within the experimental temporal scale. This determines whether the enantiomeric excess system, namely, the symmetry broken system, can be detected experimentally. We also investigate the hierarchical structures self-assembled by racemic organic cages, demonstrating that symmetry and asymmetry manifest differently across spatial scales, from molecular to supramolecular and macroscopic levels. Second, we discuss the driving force behind spontaneous chiral resolution─a classic symmetry-breaking event during crystallization─from a thermodynamic perspective. We suggest that racemic compounds, compared to conglomerates, are more entropy-favored, explaining their greater prevalence in nature. Spontaneous chiral resolution can take place only when a favorable enthalpy compensates for unfavorable entropy. In conglomerates composed of organic cages, strong intermolecular interactions along the screw axes provide the necessary compensation. Finally, we explore the unique properties that emerge from symmetry-broken molecular packing within crystals of cage racemates, such as second-harmonic generation and piezoelectricity. It turns out that the symmetry operation in molecular packing plays a critical role in determining material properties. By comprehensively analyzing symmetry and symmetry-breaking in organic cage racemates, this Account provides insights into symmetry-related phenomena across scientific disciplines. It also paves the way for designing novel materials with tailored properties for applications in optics, electronics, and beyond.
综述
对称性是一种普遍存在的现象，涵盖了从艺术、建筑到数学和科学等多个领域。在科学领域，对称性揭示了基本规律，而对称性破缺——某些对称性的瓦解——则是各种现象的根本原因。对对称性和对称性破缺的研究不断为各学科提供有价值的见解，从物理学中的宇称不守恒到生物学中同手性的起源。化学在对称性破缺研究方面尤其丰富，包括不对称合成、手性拆分、手性结构组装等领域。在不同学科中，一种明确的方法对于分析现象背后的对称性或对称性破缺本质至关重要且必不可少，它能使研究人员揭示潜在的原理和机制。基本上，与对称性相关的研究有三个关键点：对称性/对称性破缺的尺度依赖性、对称性破缺现象背后的驱动力以及对称性破缺所产生的性质。
本综述将重点关注上述三个关键点，并以有机笼状化合物作为概念验证模型进行阐述，因为有机笼状化合物具有形状持久的三维分子框架、明确的分子运动以及较高的结晶倾向。
首先，我们研究具有动态分子运动的有机笼状化合物的外消旋化过程，以说明对称性和对称性破缺是时间尺度依赖的。具体而言，由分子运动驱动的外消旋化受到氢键和笼状框架刚性的影响，在实验时间尺度内可能观察到，也可能观察不到。这决定了是否能通过实验检测到对映体过量体系，即对称性破缺体系。我们还研究了外消旋有机笼状化合物自组装形成的层次结构，表明从分子到超分子以及宏观层面，对称性和不对称性在不同空间尺度上表现各异。
其次，我们从热力学角度讨论自发手性拆分——结晶过程中的一个经典对称性破缺事件——背后的驱动力。我们认为，与聚集体相比，外消旋化合物在熵方面更占优势，这解释了它们在自然界中更为普遍的原因。只有当有利的焓补偿不利的熵时，自发手性拆分才会发生。在由有机笼状化合物组成的聚集体中，沿螺旋轴的强分子间相互作用提供了必要的补偿。
最后，我们探索笼状外消旋体晶体中对称性破缺分子堆积所产生 的独特性质，如二次谐波产生和压电性。事实证明，分子堆积中的对称操作在决定材料性质方面起着关键作用。通过全面分析有机笼状外消旋体中的对称性和对称性破缺，本综述为跨学科的对称性相关现象提供了见解。它还为设计具有定制性质的新型材料以应用于光学、电子学及其他领域铺平了道路。
