A spectroscopic study of the inclusion of azulene by β- and γ-cyclodextrins

The inclusion of azulene (AZ) inside the cavities of β-cyclodextrin (β-CD) and γ-cyclodextrin (γ-CD) was studied using absorption, fluorescence and induced-circular dichroism spectroscopy. The inclusion of AZ into the cavity of β-CD has a stoichiometry of 1:1, whereas that of AZ/γ-CD complex is 1:2. The equilibrium constants for the formation of the two complexes were calculated to be 780 ± 150 M−1 for AZ:β-CD and (4.5 ± 0.86) × 105 M−2 for AZ:(γ-CD)2. The latter is due to a stepwise equilibrium mechanism in which a 1:1 complex is formed with a binding constant of 775 M−1, followed by the formation of a 1:2 complex with a binding constant of 580 M−1. The difference between the two binding constant values is slight, indicating an almost equal contribution from each of the γ-CD molecules to the overall binding in AZ:(γ-CD)2. From the induced-circular dichroism spectra, the inclusion of AZ was found to be axial in AZ:β-CD and nearly axial in AZ:(γ-CD)2.