Considerable change of fluorescence properties upon multiple binding of coralyne to 4-sulfonatocalixarenes.

The interaction of coralyne, an analogue of natural protoberberine alkaloids, with 4-sulfonatocalixarenes (SCXn) was studied in aqueous solution at pH 2 to reveal the major factors determining the stability, stoichiometry, and fluorescent properties of the species formed. Addition of SCXn to coralyne solution brought about remarkable fluorescence intensity diminution and hypochromism in the 300-440 nm absorption domain. SCXn hosts were capable of binding as many coralyne molecules as the number of their hydroxybenzenesulfonate units. The SCXn-promoted interaction among coralyne molecules was evidenced by the appearance of a long-lived fluorescence component. In dilute alkaloid solution, 1:1 and 1:2 coralyne/4-sulfonatocalix[4]arene complexes were formed, but only 1:1 association occurred with 4-sulfonatocalix[8]arene. Time-resolved fluorescence measurements demonstrated that photoinduced electron transfer from a hydroxybenzenesulfonate moiety to the singlet-excited coralyne can compete efficiently with the other deactivation processes.