Fluorescence Suppression of 7-Methoxycoumarin upon Inclusion into Cyclodextrins

Abstract

The fluorescence intensity of 7-methoxycoumarin (7MC) in aqueoussolution is found to significantly decrease upon addition of variouscyclodextrins. This observed phenomenon is described asfluorescence suppression, to distinguish it from fluorescencereduction via bimolecular quenching. The decrease in fluorescenceof 7MC is proposed to be the result of the formation of ahost–guest inclusion complex with cyclodextrin. Since 7MC isa polarity-sensitive fluorophore, which is less fluorescent ina nonpolar environment, its fluorescence decreases uponinclusion into the relatively nonpolar internal cavity of thecyclodextrin. The same equation used for extracting the associationconstant in the case of 1 : 1 host–guest inclusion-induced fluorescenceenhancement is shown to be applicable to the case of fluorescencesuppression. In the case of β-cyclodextrin, the degree of fluorescencesuppression, as well as the value of the binding constant for formationof the inclusion complex, are found to be unaffected by modificationof the cyclodextrin rims, suggesting that the molecule iscompletely included within the β-cyclodextrin cavity. In the case ofγ-cyclodextrin, the degree of fluorescence suppression, butnot the value of the binding constant, is found to be significantlyaffected by modification of the cyclodextrin rims, suggesting thatthe molecule is experiencing a less polar environment in the modifiedγ-cyclodextrin cavity. The binding constant is three timeslarger in β- as compared to γ-cyclodextrin, indicatinga much better size match in the smaller β-cyclodextrin cavity.