Host–guest interaction between water-soluble calix[6]arene hexasulfonate and p-nitrophenol

The inclusion complexation of calix[6]arene hexasulfonate with p-nitrophenol has been studied by photoluminescence (PL), differential scanning calorimetry (DSC) and quantum-chemical methods in aqueous media. The results indicate 1:1 complex stoichiometry. The directly measured molar enthalpy of inclusion shows strong interaction between the host and the guest, however the entropy change of the complex formation is negative and quite high. Therefore, the Gibbs free enthalpy change of the complex formation is small resulting in a relatively low complex stability. This well-known enthalpy–entropy-compensation effect is probably due to the increased freedom of guest molecules relative to the host calixarenes and also due to the increased disorder of solvent molecules after the complex has been dissociated. The good correlation between the van’t Hoff enthalpy determined by PL studies and the calorimetric enthalpy reflects the two-state character of complexation. Quantum-chemical investigation suggests π–π interaction between the host and the guest in agreement with earlier results.