Spectroscopic study on the interaction of cyclodextrins with naphthyl groups attached to poly(acrylamide) backbone

Abstract

The interaction of β-cyclodextrin (β-CD) with (1-naphthyl)methyl (1Np) and (2-naphthyl)methyl (2Np) groups attached to the poly(acrylamide) backbone was investigated by several techniques, including steady-state fluorescence and circular dichroism (cd) spectroscopies. When β-CD was added to polymer aqueous solutions, the fluorescence intensity increased remarkably, indicative of the formation of inclusion complexes of β-CD with polymer-carrying Np groups. Comparison of these fluorescence data with those for the guest model compounds, (1-naphthyl)methanol and (2-naphthyl)methanol (2NpMeOH), indicated that, upon complex formation, the polymer main chain effectively protects Np groups of the singlet excited state from the contact of water molecules. Using steady-state fluorescence data at varying concentrations of β-CD, association constants ( K) were determined to be (1.1 ± 0.3) × 10 2 and (2.0 ± 0.4) × 10 2 M −1 for the complex formation of β-CD with polymer-carrying 1Np and 2Np groups, respectively. The structures of inclusion complexes of β-CD with Np groups were investigated by cd spectroscopy for the polymers and the model compounds in the presence of β-CD and by X-ray crystallographic analysis for the inclusion complex of β-CD with 2NpMeOH. These data indicated that β-CD included polymer-carrying 1Np groups shallowly, but it included polymer-carrying 2Np groups deeply to form inclusion complexes, in which the longer axis of the 2Np group is rather parallel to the rotation axis of β-CD. On the basis of these data, plausible structures of the inclusion complex of β-CD with polymer-carrying 2Np groups were proposed.