Selective inclusion of methylviologen by β-cyclodextrin: effect of cyclodextrins on the electrochemistry of methylviologen

Abstract

A study of the electrochemistry of the methylviologen dication (MV²⁺)in aqueous solution on glassy carbon electrodes in the presence of α -, β - and γ -cyclodextrins (CD) has been carried out. The cyclic voltammograms of MV²⁺ obtained at low scan rates show irreversible redox behaviour for the MV⁺/MV⁰ couple and the peak current ratios are not near to one. The presence of α- and γ -CDs do not greatly affect the redox behaviour. However, on the addition of β -CD the anodic wave of the second reduction starts to develop and the MV⁺/MV⁰ redox becomes reversible in the presence of equimolar β -CD. The plot of the anodic peak current of both reductions vs. concentration of β -CD shows a break, revealing the selective formation of a 1:1 inclusion complex of methylviologen with β -CD. From the values of the ratio of equilibrium binding constants it is inferred that MV⁺ binds more strongly to β -CD than either MV²⁺ or MV⁰.