Studying Supramolecular Assemblies by ESEEM Spectroscopy: Inclusion Complexes of Cyclodextrins

Electron spin-echo envelope modulation (ESEEM) spectroscopy was used to investigate intramolecular and intermolecular complexes of cyclodextrins (CDs) with a nitroxide group. The interaction with solvent molecules (D2O) was followed through the H-2 modulation depth. Competition experiments with adamantane-type guests were used to confirm complexation. The shielding of the nitroxide group from solvent upon inclusion into a CD cavity made this technique more sensitive to complexation than cw EPR spectroscopy. ESEEM analysis of a series of CDs mono and bis spin-labeled on the primary rim of the cavity showed that only one compound formed a self-inclusion complex. This suggests that significant linker length/flexibility is required for formation of inclusion complexes in functionalized CDs. DEER (double electron-electron resonance) experiments confirmed that the self-inclusion complex of the spin-labeled CD was intra- rather than intermolecular.