Ionic liquids: the chemistry beyond the ion pair

Ionic liquid (ILs) offer a versatile platform for designing supramolecular architectures through the manipulation of covalent, ionic and hydrogen bonds (HBs), along with van der Waals interactions. This is especially true for ILs containing planar and rigid cations. The structural organization of these materials arises from the supramolecular association of contact ion pairs (IPs) through cooperative HBs and van der Waals interactions, forming well-defined supramolecular structures with predictable functions. This structural arrangement is observed across solid, liquid and gas phases and even in solution. These supramolecular structures adapt and are adaptable, allowing for the incorporation of molecular, ionic, macromolecular and inorganic materials to form inclusion compounds, aggregates, clusters, triple ions, contact IPs and, at high dilutions, solvent-separated IPs. These interactions occur with minimal changes to their supramolecular organization. This work examines the rich and complex chemistry of ILs, drawing on the vast data accumulated over the last 30 years. It considers various aspects beyond the basic interactions between cations and anions, demonstrating that the properties and applications of ILs extend far beyond their fundamental ionic nature. This article is part of the discussion meeting issue 'Ionic liquids and the future of soft materials'.