Self-association of a naphthalene-capped-β-cyclodextrin through cooperative strong hydrophobic interactions

NMR, circular dichroism and fluorescence techniques were used to study the structure in solution of a new β-cyclodextrin derivate in which naphthalene chromophore group is bridged to O(2) and O(3) secondary positions of the same glucopyranose unit through a bidentate hinge. The results point to the formation of a very stable dimer in aqueous solution which dissociates in non-polar solvents. Dimerization was enthalpy and entropy favoured. The hydrophobic character of the naphthyl moiety plays a very important role in the entropy change sign. Molecular mechanics as well as molecular dynamics calculations indicated that the most stable dimers are head-to-head oriented. For these dimer structures the naphthyl moieties, relatively shielded from the solvent, are sufficiently close to each other to couple their transition moments, but without forming excimers.