Characterization of water-soluble cucurbit[7]uril in alcohol-water mixtures by high-pressure studies on the inclusion complexation with new methylene blue

The inclusion complexation of new methylene blue (NMB) with cucurbit[7]uril (CB[7]) was studied spectrophotometrically. It was found that CB[7] forms a 1:1 inclusion complex with NMB in alcohol-water mixtures and the structure of the inclusion complex has been established by ¹H-NMR measurements. We have determined the association constants of CB[7] with NMB under high pressures. The association constants of CB[7] increase as the external pressure and solvent polarity increase. Pressure dependence experiments enabled us to calculate the reaction volume for inclusion of CB[7] to range from -15.8 to -10.8 cm³ mol^-1. From analyses of the high-pressure results, the volume change caused by water and/or alcohol molecules repelled from the CB[7] cavity in the alcohol-water mixtures was evaluated, which is indicative of the presence of water and/or alcohol molecules in the CB[7] cavity in the alcohol-water mixtures. The volume of molecules repelled from the CB[7] cavity plays an important role in the volume change upon inclusion.