Unexpected effect of charge density of the aromatic guests on the stability of calix[6]arene-phenol host-guest complexes.

The pi-pi interaction-based inclusion complexation of calix[6]arene hexasulfonate as host with neutral aromatic guest molecules was studied in aqueous media. To vary the electron density on the guest's aromatic rings, the phenol parent compound was functionalized in the para-position with different electron-withdrawing groups, such as NO2 and Cl, as well as H and CH3 groups. To study the interaction between calixarene and the guests, PL, DSC, and quantum-chemical methods were used. The results indicate 1:1 stoichiometry for all examined host-guest complexes. Although the enthalpy change predicts strong interaction between the host and the guest, the Gibbs free energy change of the complex formation is small, resulting in a relatively low complex stability. This property is due to the high and negative entropy change during the complex formation. Comparing the thermodynamic parameters observed on the series of the guests, we observed a decrease of the enthalpy change when the electron density on the guest's aromatic ring increased. However, the Gibbs free energy and therefore, the stability of the complexes increased when the enthalpy change lowered. These unexpected results are based on the enthalpy-entropy compensation effect and probably due to the quite different entropy change related to the high and low electron density on the aromatic rings of different guest molecules. Using molecular dynamic calculations, a redistribution of the electron density of calixarene rings, followed by the reordering of the solvent molecules, was identified as a background of this unexpected entropy change at molecular level.