Smart Materials Lab, New York University Abu Dhabi, PO Box 129188, Abu Dhabi, United Arab Emirates.
Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, 600 S. Mathews Ave., Urbana, IL 61801, USA.
Chem Soc Rev. 2023 May 9;52(9):3098-3169. doi: 10.1039/d2cs00481j.
In the last century, molecular crystals functioned predominantly as a means for determining the molecular structures X-ray diffraction, albeit as the century came to a close the response of molecular crystals to electric, magnetic, and light fields revealed that the physical properties of molecular crystals were as rich as the diversity of molecules themselves. In this century, the mechanical properties of molecular crystals have continued to enhance our understanding of the colligative responses of weakly bound molecules to internal frustration and applied forces. Here, the authors review the main themes of research that have developed in recent decades, prefaced by an overview of the particular considerations that distinguish molecular crystals from traditional materials such as metals and ceramics. Many molecular crystals will deform themselves as they grow under some conditions. Whether they respond to intrinsic stress or external forces or interactions among the fields of growing crystals remains an open question. Photoreactivity in single crystals has been a leading theme in organic solid-state chemistry; however, the focus of research has been traditionally on reaction stereo- and regio-specificity. However, as light-induced chemistry builds stress in crystals anisotropically, all types of motions can be actuated. The correlation between photochemistry and the responses of single crystals-jumping, twisting, fracturing, delaminating, rocking, and rolling-has become a well-defined field of research in its own right: photomechanics. The advancement of our understanding requires theoretical and high-performance computations. Computational crystallography not only supports interpretations of mechanical responses, but predicts the responses itself. This requires the engagement of classical force-field based molecular dynamics simulations, density functional theory-based approaches, and the use of machine learning to divine patterns to which algorithms can be better suited than people. The integration of mechanics with the transport of electrons and photons is considered for practical applications in flexible organic electronics and photonics. Dynamic crystals that respond rapidly and reversibly to heat and light can function as switches and actuators. Progress in identifying efficient shape-shifting crystals is also discussed. Finally, the importance of mechanical properties to milling and tableting of pharmaceuticals in an industry still dominated by active ingredients composed of small molecule crystals is reviewed. A dearth of data on the strength, hardness, Young's modulus, and fracture toughness of molecular crystals underscores the need for refinement of measurement techniques and conceptual tools. The need for benchmark data is emphasized throughout.
在上个世纪，分子晶体主要作为确定分子结构的手段——X 射线衍射，尽管本世纪即将结束，但分子晶体对电场、磁场和光场的响应表明，分子晶体的物理性质与分子本身的多样性一样丰富。在本世纪，分子晶体的力学性能继续增强了我们对弱结合分子对内部干扰和外加力的集体响应的理解。在这里，作者回顾了近几十年来发展起来的主要研究主题，首先概述了将分子晶体与传统材料（如金属和陶瓷）区分开来的特殊考虑因素。在某些条件下，许多分子晶体在生长过程中会自行变形。它们是对内在应力还是外部力或晶体生长场之间的相互作用作出响应，仍然是一个悬而未决的问题。在有机固态化学中，单晶的光反应一直是一个主导主题；然而，研究的重点传统上一直是反应的立体和区域特异性。然而，由于光诱导化学在晶体中各向异性地产生应力，所有类型的运动都可以被驱动。光化学与单晶响应之间的相关性——跳跃、扭曲、断裂、分层、摇摆和滚动——本身已成为一个明确的研究领域：光机械。我们对这一领域的理解的推进需要理论和高性能计算。计算晶体学不仅支持对力学响应的解释，而且还可以预测响应本身。这需要采用基于经典力场的分子动力学模拟、基于密度泛函理论的方法以及使用机器学习来推断出算法比人类更适合的模式。力学与电子和光子输运的结合被认为是在柔性有机电子学和光子学中有实际应用的。对热和光快速而可逆响应的动态晶体可以用作开关和致动器。还讨论了识别高效形状变化晶体的进展。最后，还回顾了机械性能对制药行业中仍由小分子晶体组成的活性成分压片和片剂的重要性。缺乏关于分子晶体的强度、硬度、杨氏模量和断裂韧性的数据，突出了需要改进测量技术和概念工具。在整个过程中都强调了对基准数据的需求。
