Highly Effective Binding of Methyl Viologen Dication and Its Radical Cation byp-Sulfonatocalix4,5arenes

The binding behaviors and thermodynamic origins of p-sulfonatocalix4arene (C4AS) and p-sulfonatocalix5arene (C5AS) with methyl viologen (MV2+) have been investigated by the methods of isothermal titration calorimetry, NMR, and cyclic voltammetry, showing that the binding abilities of C4AS and C5AS and their host selectivity are dramatically pH-controlled, which is closely discussed from the viewpoint of thermodynamics. Moreover, the radical form of MV+* can also be effectively included by C4AS and C5AS.