Spectroelectrochemical study on monomer / dimer equilibria of methylalkylviologen cation radicals with and without α-cyclodextrin

The thin-layer electrochemical technique was used to investigate monomer / dimer equilibria of 1-methyl-1′-alkylviologen (alkyl = methyl, hexyl, heptyl, octyl, nonyl, decyl) cation radicals. Absorption spectra of 1-e^- reduction products show that the extent of dimerization does not change significantly with the length of the alkyl substituent of the viologen up to octyl. However, marked preference of dimerization is observed with alkyl substituents larger than octyl. The enhanced dimerization was parallel to adsorption/ precipitation phenomena of the viologen cation radicals. Dimerization constants K_D were determined for the 1-methyl-1′-alkylviologen cation radicals. The effect of α-cyclodextrin on the dimerization of viologen cation radicals was also investigated for methyloctyl-and dibutylviologen. Dimerization of dibutylviologen cation radical was virtually completely suppressed at high concentrations of α-cyclodextrin (α-CD), whereas that of methyloctylviologen was suppressed to a limited extent. This was explained in terms of inability to form a dimer for the 1:1 complex of C₄C₄V^.+-X D due to steric hindrance.