Anion-tuned supramolecular gels: a natural evolution from urea supramolecular chemistry

This tutorial review looks at the formation of low molecular weight gels from molecular principles using the well-explored supramolecular chemistry of ureas as an example. Synthesising lessons learned from classical urea inclusion chemistry, ureas in crystal engineering, ureas in self-assembly, urea functional groups in anion binding and sensing, and ureas as organocatalysts lead to the development and understanding of a new class of anion-tunable, urea-based soft materials. This review concludes with a look at emerging application areas for tunable gel-phase materials as controlled crystal growth media, both in templating metallic nanoparticles and in the growth and isolation of high quality crystals of molecular organic compounds, including polymorphic pharmaceuticals.