Intrazeolite Photochemistry. 22. Acid−Base Properties of Coumarin 6. Characterization in Solution, the Solid State, and Incorporated into Supramolecular Systems

The spectroscopic properties of the dye coumarin 6 in solution are reported. Relatively large shifts are seen in both the absorption and fluorescence spectra upon protonation. The very large Stokes shift that is seen for the dye in its solid state and as the hydrochloride salt reveals that the dye molecules are forming aggregates, which is a phenomenon that is also seen in very high concentrations of coumarin 6 in solution. The acid−base properties of coumarin 6 in heterogeneous systems such as zeolites and clays are also presented. We have found that this dye molecule is extremely sensitive to the presence of Lewis and Brønsted sites and were able to detect acidity in the faujasite zeolite NaY which is usually considered to be nonacidic. Two series of faujasite zeolites, one in which the Brønsted acidity increases and the other in which the Lewis acidity increases, were also studied. The dication of the dye was detected in both HY100 and CBV 740, the zeolites with the highest Brønsted and highest Lewis acidity, respectively, used in the present study. Due to the large shifts seen for coumarin 6 in its neutral, monocation, and dication forms, both the absorption and fluorescence spectra resulted in detection of two species of the dye in those zeolites with intermediate acidity. The ability of coumarin 6 to sense small amounts of Lewis and Brønsted sites makes it an attractive molecule to study other systems as well.