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 × 103 M-1 for MB, 3.2 × 103 M-1 for TH, 1.6 × 105 M-1 for DMMB. The association constants of dye monomers (KC1) and dimers (KC2) with CDs are determined from the dependence of the apparent dimerization constants (KD') on concentrations of CDs. The KC2 values obtained for MB and TH with γ-CD are larger than the corresponding KC1 values by two orders of magnitude. An opposite trend is found for binding with β-CD.
- Association constants
- Inclusion complexes