Mechanical properties of organic molecular crystals have been noted and studied over the years but the complexity of the subject and its relationship with diverse fields such as mechanochemistry, phase transformations, polymorphism, and chemical, mechanical, and materials engineering have slowed understanding. Any such understanding also needs conceptual advances—sophisticated instrumentation, computational modeling, and chemical insight—lack of such synergy has surely hindered progress in this important field. This Account describes our efforts at focusing down into this interesting subject from the viewpoint of crystal engineering, which is the synthesis and design of functional molecular solids. Mechanical properties of soft molecular crystals imply molecular movement within the solid; the type of property depends on the likelihood of such movement in relation to the applied stress, including the ability of molecules to restore themselves to their original positions when the stress is removed. Therefore, one is interested in properties such as elasticity, plasticity, and brittleness, which are linked to structural anisotropy and the degree to which a structure veers toward isotropic character. However, these matters are still by no means settled and are system dependent. While elasticity and brittleness are probably displayed by all molecular solids, the window of plasticity is perhaps the one that is most amenable to crystal engineering strategies and methods. In all this, one needs to note that mechanical properties have a kinetic component: a crystal that is elastic under slow stress application may become plastic or brittle if the same stress is applied quickly. In this context, nanoindentation studies have shown themselves to be of invaluable importance in understanding structural anisotropy. Several problems in solid state chemistry, including classical ones, such as the melting point alternation in aliphatic straight chain dicarboxylic acids and hardness modulation in solid solutions, have been understood more clearly with this technique. The way may even be open to picoindentation studies and the observation of molecular level movements. As in all types of crystal engineering, an understanding of the intermolecular interactions can lead to property oriented crystal design, and we present examples where complex properties may be deliberately turned on or off in organic crystals: one essentially fine-tunes the degree of isotropy/anisotropy by modulating interactions such as hydrogen bonding, halogen bonding, π···π interactions, and C–H···π interactions. The field is now wide open as is attested by the activities of several research groups working in the area. It is set to take off into the domains of smart materials, soft crystals, and superelasticity and a full understanding of solid state reactivity.

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从分子到相互作用再到晶体工程：有机固体的机械性能

多年来，人们已经注意到并研究了有机分子晶体的机械性能，但是该主题的复杂性及其与不同领域（如机械化学，相变，多态性以及化学，机械和材料工程）的关系使人们的理解减慢了。任何这样的理解还需要概念上的进步-精密的仪器，计算模型和化学见解-缺乏这种协同作用肯定会阻碍这一重要领域的进步。该帐户从晶体工程的角度描述了我们致力于集中精力研究这一有趣主题的努力，这是功能性分子固体的合成和设计。软分子晶体的机械特性暗示了固体中的分子运动；这意味着分子在固体中的运动。属性的类型取决于与施加的应力有关的这种运动的可能性，包括分子在消除应力后恢复自身原始位置的能力。因此，人们对诸如弹性，可塑性和脆性之类的特性感兴趣，这些特性与结构各向异性以及结构趋向各向同性的程度有关。但是，这些问题仍未解决，并且取决于系统。尽管所有分子固体都可能显示出弹性和脆性，但塑性窗口也许是最适合晶体工程策略和方法的窗口。在所有这一切中，需要注意的是机械性能具有动力学成分：如果快速施加相同的应力，在缓慢施加应力的情况下具有弹性的晶体可能会变成塑料或变脆。在这种情况下，纳米压痕研究表明，它们对于理解结构各向异性具有极其重要的意义。固态化学中的几个问题，包括经典问题，例如脂族直链二元羧酸的熔点变化和固溶体的硬度调节，已通过该技术得到了更清晰的了解。该方法甚至可能对皮下压痕研究和分子水平运动的观察开放。像在所有类型的晶体工程中一样，对分子间相互作用的理解可以导致面向特性的晶体设计，并且我们提供了一些示例，这些示例可以在有机晶体中有意地打开或关闭复杂的属性：通过调节相互作用（例如氢键，卤素键，π··π相互作用和C·H··π相互作用），本质上可以微调各向同性/各向异性的程度。正如该地区几个研究小组的活动所证明的那样，该领域现在是开放的。它准备进入智能材料，软晶体和超弹性领域，并全面了解固态反应性。