A rare example of synthon polymorphism

Scientists in India have reported a rare example of synthon polymorphism in co-crystals of 4,4′-bipyridine and 4-hydroxybenzoic acid.

Graphical abstract: Synthon polymorphism and pseudopolymorphism in co-crystals. The 4,4′-bipyridine–4-hydroxybenzoic acid structural landscape

Polymorphism is defined as the ability of a material to exist in more than one form or crystal structure. It has important implications for the properties of such materials; for example in pharmaceuticals, the dissolution rate of a drug can be dependent on the polymorphic form. While this is a common phenomenon in single crystals it is much less common in co-crystals, systems where the structure has at least two distinct components.

Gautam Desiraju from the Indian Institute of Science, found that when 4,4′-bipyridine and 4-hydroxybenzoic acid were dissolved together in a solvent such as methanol they would co-crystallise to form two different polymorphs. They noticed that a third form, a pseudopolymorph, was also present.

If you would like to read more about Desiraju’s discovery then why not download his ChemComm article for free today? This article is also part the ChemComm web themed issue on Supramolecular Chemistry to mark the International Year of Chemistry 2011.

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