Fluorescence suppression of 7-methoxycoumarin upon inclusion into cyclodextrins

The fluorescence intensity of 7- methoxycoumarin (7MC) in aqueous solution is found to significantly decrease upon addition of various cyclodextrins. This observed phenomenon is described as fluorescence suppression, to distinguish it from fluorescence reduction via bimolecular quenching. The decrease in fluorescence of 7MC is proposed to be the result of the formation of a host-guest inclusion complex with cyclodextrin. Since 7MC is a polarity-sensitive fluorophore, which is less fluorescent in a nonpolar environment, its fluorescence decreases upon inclusion into the relatively nonpolar internal cavity of the cyclodextrin. The same equation used for extracting the association constant in the case of 1:1 host-guest inclusion-induced fluorescence enhancement is shown to be applicable to the case of fluorescence suppression. In the case of beta-cyclodextrin, the degree of fluorescence suppression, as well as the value of the binding constant for formation of the inclusion complex, are found to be unaffected by modification of the cyclodextrin rims, suggesting that the molecule is completely included within the beta-cyclodextrin cavity. In the case of gamma-cyclodextrin, the degree of fluorescence suppression, but not the value of the binding constant, is found to be significantly affected by modification of the cyclodextrin rims, suggesting that the molecule is experiencing a less polar environment in the modified gamma-cyclodextrin cavity. The binding constant is three times larger in beta- as compared to gamma-cyclodextrin, indicating a much better size match in the smaller beta-cyclodextrin cavity.