Complexes of basic tricyclic dyes in their acid and basic forms with cucurbit[7]uril: Determination of pKa and association constants in the ground and singlet excited state

Cucurbituril having seven glycoluril units (CB[7]) forms strong host–guest complexes with tricyclic basic dyes in water. Depending on the pH of the aqueous solution, these dyes can be in their acidic or basic form. Typically, the basic form is present at pH values higher than 12. One part of this study deals with the difference in the association constants between the acid or basic forms of the dye with CB[7]. Other part of our study focus on the CB[7] complexation of the dyes in their excited states. Thus, for seven dyes (proflavine, acridine orange, pyronine, pyronine Y, oxonine, thionine and methylene blue) we have determined the association constants (KB) of their acid and basic forms with CB[7], both in the ground and in the excited state, as well as the pKa values of free and complexed dyes. As a general trend, we have observed that the pKa values of the complexed dyes in the ground are about two units higher than the pKa values of the free dyes. Also, we have found that the association constants in the excited state as well as those of the basic form are between one or two orders of magnitude smaller than the association constants of the ground state dye in the acid form.