Inclusion complexation of octaethyl-p-tert-butylcalix[8]arene octaacetate with methylene blue and control of its inclusion ability by alkali metal cations

The equilibrium constants for the inclusion complexation of octaethyl-p-tert-butylcalix[8]arene octaacetate (Calix-B8-EA) with methylene blue (MB) were determined spectrophotometrically. Calix-B8-EA, which has a flexible hydrophilic pseudo-cavity formed by polyfunctional esters on the lower rim, is an effective receptor for alkali and alkaline-earth cations, and its ester carbonyl group forms the complex with metal cations. We have examined the ability to include the organic molecule (methylene blue (MB)) into the upper main cavity of Calix-B8-EA formed the complex of alkali metal cations with ester carbonyl groups on the lower rim. It was found that Calix-B8-EA forms a 1:1 inclusion complex of MB with the upper main cavity and, in the presence of excess alkali metal cations, the association constants increase with an increase in the size of the metal cations complexed with the polyfunctional groups on the lower rim. Further, the structure of the inclusion complex of MB with cation-complexed Calix-B8-EA is characterized by 2D ROESY-NMR measurements. Based on the results, we have demonstrated the control of the inclusion ability by changing the portal size of the calixarene cavity.