Fluorescence Suppression of 7-Methoxycoumarin upon Inclusion into Cyclodextrins

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.

7-甲氧基香豆素（7-methoxycoumarin，简称7MC）在水溶液中的荧光强度，在加入多种环糊精后会出现显著降低。此观测现象被归类为荧光抑制效应，以区分于经由双分子淬灭机制产生的荧光衰减。现有研究提出，7MC的荧光强度降低源于其与环糊精形成主-客体包结络合物。由于7MC属于极性敏感性荧光团，在非极性环境中其荧光量子产率更低，因此当其被包埋进入环糊精相对非极性的内部空腔时，荧光强度便会下降。用于推导1:1主客体包结诱导荧光增强体系结合常数的同一方程，经证实同样适用于荧光抑制体系。就β-环糊精（β-cyclodextrin）而言，荧光抑制程度以及包结络合物形成的结合常数均不受环糊精边缘修饰的影响，这表明7MC分子完全被包埋于β-环糊精的空腔内部。而对于γ-环糊精（γ-cyclodextrin）而言，荧光抑制程度会受到环糊精边缘修饰的显著影响，但结合常数数值并未发生变化，这表明经修饰后的γ-环糊精空腔内，7MC分子所处的环境极性更低。β-环糊精体系的结合常数是γ-环糊精体系的三倍，这说明尺寸更小的β-环糊精空腔与7MC分子具有更优异的尺寸匹配性。