Use of nitroxide radicals to investigate supramolecular entities

Supramolecular complexes can be studied by a variety of spectroscopic methods and substantial information obtained by optical methods, by NMR spectroscopy, and by other techniques, elucidating structure and their stability, are reported in the scientific literature. However, very few data can be found concerning the rates by which these aggregates are formed or dissociated into separate species. This is due to the fact that association/dissociation takes place in a frequency range that is not accessible (or it is accessible only in very peculiar situations) either to optical techniques or to NMR. Thus distinct signals from the free and complexed species are detected with optical spectroscopies, while spectra representing concentration-weighted averages are observed by NMR since the exchange between free and complexed species is usually too fast in the NMR time scale. The association/dissociation processes, taking place in the time range between 10(-5) and 10(-9) seconds, can instead be studied very conveniently by EPR spectroscopy. Thus, this technique has been found among those more suitable to investigate the kinetics of association and dissociation in supramolecular chemistry and has provided valuable mechanistic information on the dynamics of these processes. In this review the results of our EPR studies on the supramolecular species formed in aqueous solutions with the guest benzyl tert-butyl nitroxide or related nitroxides and different host systems will be reported.