Dependence of dimerization of electrogenerated methylalkylviologen radical cations on the length of alkyl chain in the presence of γ-cyclodextrin

The effect of γ-cyclodextrin (γ-CD) on the monomer-dimer equilibria of 1-methyl-1′-alkylviologen radical cations (C_lC_nV^·+: n = 1-10) has been studied by electrochemical and spectroelectrochemical techniques. The apparent dimerization is enhanced upon addition of γ-CD for n = 1-8 and the effect of γ-CD is a maximum for n = 4. This is attributed to the stabilization of dimers by the inclusion of the dimer in the cavity of γ-CD. However, the dimerization of C₁C₉V^·+ and C₁C₁₀V^·+, which exhibits strong tendencies of dimerization in the absence of γ-CD due to electrodeposition of the radical cations, is suppressed upon addition of γ-CD. From the dependence of the apparent dimerization constants on the concentration of γ-CD, the association constants of the dimer (K_C2) and monomer (K_C1) with γ-CD were determined for n = 1-8. Comparing the present results with our previous report with dialkylviologen radical cations (C_nC_nV^·+: n = 1-5), we show that the dimers with the longer alkyl substituents in the same and opposite sides have little difference in their stability in the absence of γ-CD, but the former type of dimer is included preferentially in the cavity of γ-CD.