We present here the results of a CW-ESR investigation of a double spin labeled α-cyclodextrin-based [2]rotaxane that is characterized by the presence of nitroxide labels both at the wheel and at the dumbbell. This was accomplished by synthesizing a spin labeled α-CD (the wheel) that was mechanically blocked on a thread containing a nitroxide unit by a Cu(I) catalyzed azide-alkyne cycloaddition (CuAAC). Both ESI-MS analysis and NMR spectroscopy were used to provide evidence of the threading processes. Because of the unsymmetrical structure of both the wheel and the axle, two different geometrical isomers could be predicted on the basis of the orientation of the CD along the thread. By combining molecular dynamic calculations and information extracted from the CW-ESR spectra, we were able to determine the geometrical nature of the isomer that was isolated as the only species. The ESR spectra showed J-coupling between the two mechanically connected nitroxide units and were analyzed by a model assuming three main molecular states. The intramolecular noncovalent nature of spin exchange was confirmed by reversibly switching the magnetic interaction on-off by changing the pH of the solution in the presence of a competing macrocyclic host.

The first example of paramagnetic rotaxane containing cucurbit[6]urils has been reported and characterized both by ESR and NMR spectroscopy.

N-benzyl-tert-butyl nitroxide derivatives substituted at the aromatic ring form host-guest inclusion complexes with β-and γ-cyclodextrin. They were employed as probes to assess substituent effects on the geometry of the complex and on the kinetics of this complexation by combining EPR and (1)H NMR data.

Distance determination: The use of pulsed electron-electron double resonance (PELDOR/DEER) spectroscopy to determine the distances among the end units of a paramagnetic cucurbit[6]uril-based rotaxane is demonstrated.

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