Visible and 13C nuclear magnetic resonance spectra of azo dyes and their complexes with cyclomalto-oligosaccharides

Inclusion complexes of α-, β-, and γ-cyclomalto-oligosaccharides (CDs), heptakis(2,6-di-O-methyl)- and heptakis(2,3,6-tri-O-methyl)-βCDs (Me2-βCD and Me3-βCD) with methyl orange sodium 4-[4-(dimethylamino)phenylazo]benzenesulfonate (3). tropaeolin OO sodium 4-[[4-(phenylamino)phenyl]azo]benzenesulfonate (1), metanil yellow sodium 3-[[4-(phenylamino)phenyl]azo]benzenesulfonate (2), and orange II sodium 4-[(2-hydroxy-l-napthalenyl)azo]benzenesulfonate (4) were measured by UV-vis spectrophotometry. Complex formation constants (K) were calculated by a non-linear least-squares method. The order of K in the 1:1 complexes is Mc2-βCD > Me3-βCD > αCD > βCD. Thus lengthening of the hydrophobic torus by substituting the CD with methyl groups exhibits a greater effect than the sizes of the inner diameter of the cavity. Since the K values correlate with the bulkiness of the hydrophobic parts of the azo dyes, these numbers suggest a direction of inclusion of the azo dye molecules (i.e., hydrophobic groups of the guest molecules associated with hydrophobic groups of the CDs) into the CD cavities. Complex formation induces a hypsochromic shift to the λmax that very closely resembles the shift induced by dioxane, suggesting that the inside of the cavity resembles this solvent. As the order of the shift is 3 > 1, > 2 > 4, γCD > βCD series > αCD, and the shift scarcely appears in the complexes of 4, the shift may be mainly derived by the inclusion of an anilino group in 1, 2, and 3. NMR chemical shifts, relaxation times, and rotational correlation times from the 13C NMR studies all support the above conclusions.