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The effects of cyclodextrins (α-, β-, and γ-CDs) on the monomer-dimer equilibria of phenothiazine dyes (methylene blue (MB), thionine (TH), and 1,9-dimethyl-methylene blue (DMMB)) have been studied using absorption spectroscopy. The positions of the monomer-dimer equilibria do not change by the presence of α-CD, while the addition of β-CD suppresses the dimerization. This is attributed to that the cavity of β-CD is large enough to stabilize dye monomers by the formation of inclusion complexes, while that of α-CD is too narrow to include the dye molecules. The apparent dimerization is enhanced upon addition of γ-CD for MB and TH due to inclusion complexation of the dimers of dyes with γ-CD. However, the suppression of the apparent dimerization by addition of γ-CD is observed for DMMB. It can be explained in terms of size-selective formation of inclusion complexes of phenothiazine dye monomers and dimers with γ-CD: the cavity size of γ-CD is not large enough to accommodate dimers of DMMB though it accommodate the dimers of MB or TH. The dimerization constants (KD) were determined as 9.6 x 10 3 M -1 for MB, 3.2 x 10 3 M -1 for TH, 1.6 x 10 5 M -1 for DMMB. The association constants of dye monomers (K c1 ) and dimers (K c2 ) with CDs are determined from the dependence of the apparent dimerization constants (K D 5 ) on concentrations of CDs. The K C2 values obtained for MB and TH with γ-CD are larger than the corresponding K C1 values by two orders of magnitude. An opposite trend is found for binding with β-CD.

Size-selective Inclusion Complexes of Phenothiazine Dye Monomers and Dimers with Cyclodextrins.