γ-cyclodextrin-enhanced dimerization of viologen radicals

The effects of cyclodextrins (CDs) on the monomer-dimer equilibria of 1,1′ -dialkylviologen radical cations (C_n C_n V^.+ : n = 1-5) and one-electron reduced 1,1′-bis(3-sulfopropyl)viologen (SPV) were investigated spectroelectrochemically. The dimerization is enhanced upon addition of γ-CD, while the addition of α-CD and β-CD suppresses the dimerization. This is attributed to the stabilization of dimers by the inclusion of the dimer in the cavity of γ-CD. From the dependence of the apparent dimerization constants (K′_D) on the concentration of γ-CD, the association constants of the monomer (K_C1) and dimer (K_C2) with γ-CD were determined. The K_C1 value increases slightly, from 5 M^-1 for n = 1 to 35 M^-1 for n = 5, as the length of the alkyl substituents of viologen increases. The K_C2 value depends strongly on the alkyl substituents giving the smallest value of 85 M^-1 for (SPV^.-)2 and the largest one of 3500 M^-1 for (C₄C₄V^.+)2. These results, together with the difference in UV-vis absorption spectra obtained in the absence and in the presence of γ-CD, suggest that the alkyl substituents and part of the bipyridine ring of viologen radicals are included in the γ-CD cavity forming dimers in which the bipyridine rings are stacked at an oblique angle.