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

The effect of gamma-cyclodextrin (gamma-CD) on the monomer-dimer equilibria of 1-methyl-1'-alkylviologen radical cations (C1CnV.+: n = 1-10) has been studied by electrochemical and spectroelectrochemical techniques. The apparent dimerization is enhanced upon addition of gamma-CD for n = 1-8 and the effect of gamma-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 gamma-CD. However, the dimerization of C1C9V.+ and C1C10V.+, which exhibits strong tendencies of dimerization in the absence of gamma-CD due to electrodeposition of the radical cations, is suppressed upon addition of gamma-CD. From the dependence of the apparent dimerization constants on the concentration of gamma-CD, the association constants of the dimer (K-C2) and monomer (K-Cl) with gamma-CD were determined for n = 1-8. Comparing the present results with our previous report with dialkylviologen radical cations (CnCnV.+: 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 gamma-CD, but the former type of dimer is included preferentially in the cavity of gamma-CD. (C) 1997 Elsevier Science S.A.