Crystal engineering

Crystal engineering is an interdisciplinary field that focuses on designing and synthesizing solid-state structures. Scientists aim to create materials with specific, desired properties. They achieve this through the deliberate control of intermolecular interactions. This field acts as a bridge between solid-state chemistry and supramolecular chemistry. By understanding how molecules organize, researchers can dictate how a bulk material behaves.

The core mechanism of this field is molecular self-assembly. This process relies on noncovalent bonding to organize molecules and ions in a solid state. Scientists use three primary engineering strategies to guide this organization. The first is hydrogen bonding, which often serves as a primary director for crystal organization. The second is halogen bonding, which provides additional control in crystal design. The third strategy is coordination bonding.

To manage these complex assemblies, researchers use specific building blocks. One key concept is the supramolecular synthon. A synthon is a pair of molecules that form relatively strong intermolecular interactions during the early phases of crystallization. These pairs act as the basic structural motifs found in a crystal lattice. Another important concept is the secondary building unit. These tools allow scientists to order specific molecular groups within a solid structure.

The history of crystal engineering spans several decades of discovery. The term was first introduced by R. Pepinsky in 1955. However, the field's practical starting point is often credited to Gerhard Schmidt in 1971. Schmidt worked with photodimerization reactions in crystalline cinnamic acids. In 1988, Gautam Desiraju provided a modern, useful definition. He described the field as the understanding of intermolecular interactions to design new solids with specific properties.

One major application of this science is in the pharmaceutical industry. Scientists design multi-component crystals known as pharmaceutical cocrystals. These crystals consist of one active pharmaceutical ingredient (API) and other substances considered safe by the WHO. By forming cocrystals, researchers can modulate the solubility, bioavailability, and permeability of an API. This process is vital for improving how medicines work in the human body.

Crystal engineering also explores the phenomenon of polymorphism. Polymorphism occurs when the same chemical compound exists in more than one crystal form. This happens due to a competition between kinetic and thermodynamic factors. Kinetic factors involve how crystals form, while thermodynamic factors involve how closely molecules pack. There are three main types of polymorphism in organic molecules. Packing polymorphism occurs when molecules pack in different ways. Conformational polymorphism happens in flexible molecules with multiple shapes. The rarest type is synthon polymorphism, which arises from differences in the primary synthon.

Modern scientists also use computational tools like Crystal Structure Prediction (CSP). CSP is a method used to generate energetically feasible crystal structures from a molecular structure. This is a difficult task because experimental structures are often kinetic structures. A major advance occurred in 2007 with a hybrid method. This method uses tailor-made force fields followed by dispersion-corrected density functional theory (DFT). This allows for the precise calculation of lattice energies.

Finally, crystal engineering is used to design specific mechanical and optical properties. Scientists can target properties like plasticity, elasticity, brittleness, and shear strength. They can also design non-linear optical materials with second harmonic generation (SHG) properties. They even use synthons to design supramolecular gels. By manipulating the interaction network, they can control the entire architecture of the material.