Nitroxide Radicals as probes for Exploring the Properties of Supramolecular Complexes

Supramolecular chemistry refers to the area of chemistry that focuses on the noncovalent bonding interactions of molecules. Important concepts that have been demonstrated by supramolecular chemistry include host-guest chemistry, molecular self-assembly and mechanically-interlocked molecular architectures. Host-guest chemistry describes complexes that are composed of two or more molecules or ions held together in unique structural relationships by hydrogen bonding or by ion pairing or by Van der Waals forces other than those of full covalent bonds. Macrocycles like cyclodextrins, calixarenes, cucurbiturils are very useful in host-guest chemistry, as they provide whole cavities that can completely surround guest molecules and may be chemically modified to fine-tune their properties. Molecular self-assembly is a key concept in supramolecular chemistry since assembly of the molecules is directed through noncovalent interactions. Simple examples include the formation of micelle or monolayer-protected nanoparticles. More advanced examples of supramolecular assemblies demonstrate that a variety of different shapes and sizes can be obtained using molecular self-assembly. Mechanically-interlocked molecular architectures are connections of molecules not through traditional bonds, but instead as a consequence of their topology. On the molecular level the interlocked molecules cannot be separated without significant distortion of the covalent bonds that make up the conjoined molecules. Examples of mechanically-interlocked molecular architectures include catenanes and rotaxanes. This communication will focus on the above mentioned aspects of supramolecular chemistry involving the use of nitroxides. In particular, it will be shown that the combined use of nitroxide radicals and ESR spectroscopy provides valuable mechanistic information on the formation of host-guest complexes. Some example of complexed supramolecular nitroxidic structure obtained using molecular self-assembly will be reported. Finally, the synthesis and the characterization of mechanically-interlocked molecular architectures containing nitroxidic units showing enhanced persistency will also be presented.