Nitroxides in Supramolecular Chemistry

With the advance of supramolecular chemistry, the need for diverse analytical methodologies as a tool to exploit basic principles for the rational design of the building blocks used to assembly supramolecular aggregates is continuously increasing. In this respect, electron paramagnetic resonance (EPR) and related methods offer particular opportunities to study supramolecular systems. The main advantages of EPR are the sensitivity of the method; the possibility of obtaining kinetic information in the submicrosecond time range; the ability to measure tumbling rates on the nanosecond timescale and distances between spin labels in the 0–100 Å range. The strength of EPR lies also in its possible application to heterogeneous and solid samples. Because of these EPR favourable features, in recent years spin probing and labelling methodologies have largely expanded beyond the classical context of biology and structural biochemistry and are now commonly used in the characterization of supramolecular assemblies. Both spin probe and spin label terminology refer to the methodology in which one (or more) paramagnetic species (in most cases a nitroxide radical) is used to report EPR information on the chemical environment experienced by the radical itself. Generally speaking, nitroxides are considered spin