The chemist’s conventional tool box to create new molecules is a large number of reactions for which the result can be usually well predicted. A single molecule possesses a structure on the nano-scale which is conventionally determined by covalent bonds between atoms. Most functional artificial or biological materials, however, work in the condensed phase, that is in assemblies of molecules arranged without strong covalent bonds. Weak reversible interactions are responsible for the self-organisation of molecules on surfaces, in thin films or bulk material (crystals, liquid crystals) and for the recognition of guest molecules by hosts. Such interactions are also crucial for the existence of life. Almost all biological nano-machines are complex self-assemblies of more than one large molecule and their performance is based on the propensity to reversibly bind guests or change conformations.

This lecture will introduce a list of different weak interactions, e.g. electrostatic multipole interactions, Van-der-Waals interactions, π-π interactions, coordinative bonds, hydrogen bonds, halogen bonds, hydrophopic and fluorophopic effects. They are almost all electrostatic, but are characterised by their directional nature, by the participating atoms, the distance dependence and the molecular size and packing. Examples of guests binding to hosts by multiple complementary weak interactions are discussed. Eventually, it is shown that these strategies can be applied to design new nano-aggregates and to link nanoparticles reversibly to clusters or to surfaces.