Supramolecular self-assembled cages are interesting and useful structures that have a variety of different potential applications including catalysis, reaction control, and chemical transport and storage. It is therefore important to understand in detail how guest molecules interact with these cage structures and determine the characteristics of the host-guest complexes formed in order to develop these cages and their uses further. Previous studies into supramolecular assemblies such as cucurbiturils, cyclodextrins and resorcinarene capsules have demonstrated that EPR spectroscopy is particularly suitable for characterisation of the host-guest complexes of these structures with paramagnetic radical probes as guests, and this work aimed to extend these studies to cage structures.
In this thesis, a number of different radical probes were investigated qualitatively with host cage structures of the form [M8L12][X]16, M = Cd or Co, L = C28H22N6 and X = ClO4 or Cl, comparing EPR spectra of radical in neat solvent to radical+cage solution to identify if binding had occurred. Competing guest molecules were used to confirm binding effects by displacing the radical probes and observing the effect on the EPR spectrum.
Three nitroxide radical guests were identified for detailed binding studies: 4-oxo-TEMPO, 4-carboxy-TEMPO and 3-carboxy-PROXYL, and titrations of solutions of guest with cage solution were carried out. Simulations of the EPR spectra revealed the contributions of two components, bound and unbound radical, to the radical@cage complex spectra, allowing characteristics of the complex to be determined. The ratio between bound and unbound component allowed calculation of association constants for the complexes, whilst comparison between bound and unbound rotational diffusion rates showed that binding of radical led to restricted motion of the probe, and hence slower tumbling rates. Finally, changes in the hyperfine values were used to determine changes in the polarity of the environment of the radical.
