Cocrystal

In materials science (specifically crystallography), cocrystals are "solids that are crystalline, single-phase materials composed of two or more different molecular or ionic compounds generally in a stoichiometric ratio which are neither solvates nor simple salts." A broader definition is that cocrystals "consist of two or more components that form a unique crystalline structure having unique properties." Several subclassifications of cocrystals exist. Cocrystals can encompass many types of compounds, including hydrates, solvates and clathrates, which represent the basic principle of host–guest chemistry. Hundreds of examples of cocrystallization are reported annually. The first reported cocrystal, quinhydrone, was studied by Friedrich Wöhler in 1844. Quinhydrone is a cocrystal of quinone and hydroquinone (known archaically as quinol). He found that this material was made up of a 1:1 molar combination of the components. Quinhydrone was analyzed by numerous groups over the next decade and several related cocrystals were made from halogenated quinones. Many cocrystals discovered in the late 1800s and early 1900s were reported in Organische Molekulverbindungen, published by Paul Pfeiffer in 1922. This book separated the cocrystals into two categories; those made of inorganic:organic components, and those made only of organic components. The inorganic:organic cocrystals include organic molecules cocrystallized with alkali and alkaline earth salts, mineral acids, and halogens as in the case of the halogenated quinones. A majority of the organic:organic cocrystals contained aromatic compounds, with a significant fraction containing di- or trinitro aromatic compounds. The existence of several cocrystals containing eucalyptol, a compound which has no aromatic groups, was an important finding which taught scientists that pi stacking is not necessary for the formation of cocrystals. Cocrystals continued to be discovered throughout the 1900s. Some were discovered by chance and others by screening techniques.