The dimerization and inclusion complexation equilibria of six phenothiazine (PN) dyes with cyclodextrins (R-, β-, and γ-CDs) in aqueous media have been studied using absorption and fluorescence spectroscopy. The PN dyes used in this study are thionine (TH), azure A (AZA), methylene blue (MB), toluidine blue (TB), new methylene blue (NMB), and 1,9-dimethylmethylene blue (DMMB). The dimerization constants (K D ) of the dyes having two methyl substituents at the phenothiazine ring (NMB and DMMB) are much greater than those of other dyes having unsubstituted rings, and the presence of methyl groups on the amine groups affects little the K D values. The positions of the monomer/dimer equilibria do not change with the presence of R-CD, while the addition of β-CD suppresses and γ-CD enhances the dimerization of the dyes except DMMB. The equilibrium constants for the inclusion complexation of the dye monomers and dimers with CDs are determined from the analysis of the dependence of the absorption spectra of the dye solutions on the concentrations of the CDs using a multiple-equilibrium scheme. The results indicated that, except DMMB, which has methyl groups at the 1-position of the fused phenothiazine ring, the dye monomers fit better to β-CD and the dimers fit snugly to γ-CD. The DMMB monomer is too large to fit in β-CD but forms stable complexes with γ-CD. It appears that the inclusion complexes of the dye monomers and dimers are formed by deep insertion of the phenothiazine rings into the cavities of the CDs, with the 2-methyl groups (in TB and NMB) and the amine groups protruding from the cavities of the CDs. Fluorescence spectroscopic studies indicate that the dye dimers are not fluorescent and inclusion of the monomer in β-CD results in a 3-5 times enhancement of fluorescence intensity. The determined equilibrium constants of the multipleequilibrium scheme of the dyes in CD media and fluorescent properties of the dyes can be used to control the dye aggregation and the photophysical and photochemical properties of the phenothiazine dyes for various applications.

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The effects of cyclodextrins (a-, ~3-, and 7-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 a-CD, while the addition of ~3-CD suppresses the dimerization. This is attributed to that the cavity of n-CD is large enough to stabilize dye monomers by the formation of inclusion complexes, while that of a-CD is too narrow to include the dye molecules. The apparent dimerization is enhanced upon addition of 'y-CD for MB and TH due to inclusion complexation of the dimers of dyes with 'y-CD. However, the suppression of the apparent dimerization by addition of 'y-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 'y-CD: the cavity size of 'y-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 103 M"' for MB, 3.2 x 103 M"' for TH, 1.6 x l05 M"' for DMMB. The association constants of dye monomers (Ku) 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 r-CD are larger than the corresponding K~1 values by two orders of magnitude. An opposite trend is found for binding with a-CD.