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The magic of non-covalent bonds: Do you know how to control crystallization using hydrogen bonds and halogen bonds?
Crystallization engineering is an important discipline involving the design and synthesis of solid-state structures, aiming to obtain desired material properties by controlling intermolecular interactions. Its core concept is to control the interactions of non-covalent bonds such as hydrogen bonds, halogen bonds, and coordination bonds, so that molecules can be arranged in an orderly manner. This discipline combines aspects of solid-state chemistry and supramolecular chemistry to allow scientists to meaningfully engineer solid materials with specific physical and chemical properties.
"Crystal engineering is the discipline of understanding intermolecular interactions in the context of crystal packing and using this understanding to design new solids."
Since R. Pepinsky first proposed the term "crystallization engineering" in 1955, this field has experienced tremendous changes and gradually formed a modern definition. Its main strategies include the regulation of non-covalent bonds, helping researchers understand how to obtain specific properties by controlling the arrangement of molecules.
Control of non-covalent bonds
The foundation of crystallization engineering lies in the concept of non-covalent bonds. Although initial research focused on hydrogen bonding applications, as research progressed, coordination bonds and halogen bonds provided more options. The self-assembly process of molecules is achieved through these interactions, and researchers often exploit complementary hydrogen bonding surfaces or interactions between metals and ligands.
"Supramolecular synthetic units are common building blocks in many structures, so they can be used to order specific groups of molecules in the solid state."
In the design of multi-component crystals, researchers tend to exploit strong heterologous molecular interactions to synthesize co-crystals. The pharmaceutical industry is particularly focused on this direction because there are opportunities to improve the quality of drugs by rationally adjusting properties such as solubility, bioavailability and permeability of active pharmaceutical ingredients.
Design of multi-component crystals
In the design of pharmaceutical crystals, the most important thing is to understand and utilize the interactions between drug molecules and other molecules. Such co-crystals typically consist of an active pharmaceutical ingredient (API) combined with other molecules recognized as safe by the World Health Organization. By optimizing these combinations, key properties of medicinal chemistry can be effectively improved.
Design of two-dimensional crystal structure
The design of two-dimensional architecture is another branch of crystallization engineering. Such structures typically involve the formation of adsorbed monolayers on solid surfaces that can exhibit spatially defined crystalline properties. The diverse shapes and properties of 2D crystals have further led to the emergence of so-called "supramolecular engineering".
"Supramolecular engineering refers to the design of molecular units in a way that predicts their structure."
The dynamic nature of supramolecular engineering has attracted considerable research interest, particularly in the visualization of two-dimensional structures using scanning probe microscopy.
The meaning of polymorphism
Polymorphism, the condition in which a compound exists in multiple crystal forms, is crucial to commerce because different polymorphs may enjoy independent patent protection. This phenomenon is mainly caused by the competition between kinetic and thermodynamic factors during the crystallization process, which requires researchers to continue to explore the scientific logic behind polymorphism.
Crystal structure prediction
Crystallographic Structure Prediction (CSP) is a computational method used to generate feasible crystal structures starting from a given molecular structure. Although this is a challenging task, recent research has shown that using customized force fields combined with density functional theory methods can effectively improve the accuracy of predictions. Not only does this allow structure prediction, it also provides an energy distribution map of the crystal structure, helping us understand polymorphs and the design of new structures.
Focus on performance design
Designing crystal structures with desired properties is the ultimate goal of crystallization engineering. These principles have been applied to the design of nonlinear optical materials, especially in second harmonic generation (SHG), and supramolecular gels designed through supramolecular synthesis units provide broad prospects for the development of new materials.
Discussion on mechanical properties
To design crystalline materials with predetermined properties, you first need to understand the impact of their molecular and crystalline characteristics on mechanical properties. Plasticity, elasticity, brittleness and shear strength are four important mechanical property indicators in the research of crystalline materials. Correct molecular design allows the performance changes of crystalline materials to be reasonably predicted and controlled.
"During the crystallization process, the strength of hydrogen bonds is usually the main driving force in organizing crystals."
When discussing crystal structure, structural defects of materials such as point defects and displacement are key factors that affect mechanical properties. How to accurately evaluate the impact of these defects on performance has become an important topic in current research.
Through experimental methods such as X-ray diffraction and electron microscopy, the structure and defects inside the crystal can be analyzed in detail, and the understanding of the crystallization process can be deepened. As technology advances, our understanding of materials will become deeper and deeper. However, with so many variables and interactions, can we fully grasp the magic of these non-covalent bonds?
