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首页> 外文期刊>Crystal growth & design >The role of weak interactions in the phase transition and distinct mechanical behavior of two structurally similar caffeine co-crystal polymorphs studied by nanoindentation
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The role of weak interactions in the phase transition and distinct mechanical behavior of two structurally similar caffeine co-crystal polymorphs studied by nanoindentation

机译：通过纳米压痕研究，弱相互作用在两种结构相似的咖啡因共晶多晶型物的相变和不同的力学行为中的作用

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Although weak interactions, such as C-H?O and π-stacking, are generally considered to be insignificant, it is their reorganization that holds the key for many a solid-state phenomenon, such as phase transitions, plastic deformation, elastic flexibility, and mechanochromic luminescence in solid-state fluorophores. Despite this, the role of weak interactions in these dynamic phenomena is poorly understood. In this study, we investigate two co-crystal polymorphs of caffeine:4-chloro-3-nitrobenzoic acid, which have close structural similarity (2D layered structures), but surprisingly show distinct mechanical behavior. Form I is brittle, but shows shear-induced phase instability and, upon grinding, converts to Form II, which is soft and plastically shearable. This observation is in contrast to those reported in earlier studies on aspirin, wherein the metastable drug forms are softer and convert to stable and harder forms upon stressing. To establish a molecular-level understanding, we have investigated the two co-crystal polymorphs I and II by single-crystal X-ray diffraction, nanoindentation to quantify mechanical properties, and theoretical calculations. The lower hardness (from nanoindentation) and smooth potential surfaces (from theoretical studies) for shearing of layers in Form II allowed us to rationalize the role of stronger intralayer (sp~2)C-H?O and nonspecific interlayer π-stacking interactions in the structure of II. Although the Form I also possesses the same type of interactions, its strength is clearly opposite, that is, weaker intralayer (sp~3)C-H?O and specific interlayer π-stacking interactions. Hence, Form I is harder than Form II. Theoretical calculations and indentation on (111) of Form I suggested the low resistance of this face to mechanical stress; thus, Form I converts to II upon mechanical action. Hence, our approach demonstrates the usefulness of multiple techniques for establishing the role of weak noncovalent interactions in solid-state dynamic phenomena, such as stress-induced phase transformation, and hence is important in the context of solid-state pharmaceutical chemistry and crystal engineering.
机译：尽管通常认为微弱的相互作用（例如CH2O和π堆积）微不足道，但它们的重组是许多固态现象（例如相变，塑性变形，弹性柔韧性和机械变色）的关键。固态荧光团的发光。尽管如此，对这些动态现象中的弱相互作用的作用知之甚少。在这项研究中，我们研究了咖啡因的两种共晶多晶型物：4-氯-3-硝基苯甲酸，它们具有相似的结构相似性（二维分层结构），但出人意料地显示出不同的机械行为。形式I是脆性的，但是表现出剪切引起的相不稳定性，并且在研磨时转变成形式II，其是软的并且可塑性剪切的。该观察结果与较早的关于阿司匹林的研究报告的观察结果相反，后者的亚稳态药物形式较软，在压力作用下可转变为稳定和较硬的形式。为了建立分子水平的理解，我们通过单晶X射线衍射，纳米压痕量化机械性能和理论计算研究了两种共晶多晶型物I和II。较低的硬度（来自纳米压痕）和光滑的潜在表面（来自理论研究）用于剪切形式II中的层，这使我们能够合理地解释结构中较强的层内（sp〜2）CH2O和非特异性层间π堆积相互作用的作用II。尽管晶型I也具有相同类型的相互作用，但其强度明显相反，即较弱的层内（sp〜3）C-H2O和特定的层间π堆积相互作用。因此，表格I比表格II难。理论计算和形式I的（111）压痕表明该面对机械应力的抵抗力很低。因此，形式I在机械作用下会转变为II。因此，我们的方法证明了多种技术在建立弱非共价相互作用在固态动态现象（如应力诱导的相变）中的作用方面的有用性，因此在固态药物化学和晶体工程的背景下非常重要。

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