Characterization of Host-Guest Complexes of Supramolecular Self-Assembled Cages Using EPR Spectroscopy

Host-guest interactions between nitroxide stable radicals and supramolecular coordination cages were investigated using electron paramagnetic resonance (EPR) spectroscopy in water and acetonitrile. TEMPO showed negligible association with the cages in water, while 4-oxo-TEMPO bound with a strength comparable to that previously reported for related ketones. Carboxylic acid-functionalized nitroxides bound strongly to the acetonitrile-soluble coordination cages. In all cases, host-guest complex formation resulted in significant decreases in the molecular tumbling rate of the guests, with tumbling becoming strongly anisotropic. The polarity of the cage environment in both solvents was found to be intermediate between water and acetonitrile.