Electrochemical study of methylalkylviologens using an electrochemical quartz crystal microbalance

The electrochemical behavior of various 1-methyl-1′-alkylviologens (C₁C_nV^2-: n = 1, 7, 8, 9, 10, 12, 14, 16 and 18) was studied using an electrochemical quartz crystal microbalance (EQCM). Three different types of the frequency change-potential (Δf-E) curves were observed depending on the length of alkyl substituent during the cyclic voltammetry (CV) of the first redox step: for n = 1 and 7, Δf was less than 30 Hz (reversible); for n = 9, 10, 12 and 14. Δf = 200 Hz indicating electrodeposition of C₁C_nV^·+; for n = 16 and 18, Δf = 5000 Hz with two anodic peaks implying reorientation of the electrodeposited C₁C_nV^·+ occurring on the electrode surface. These results are consistent with our previous report where apparent dimerization of C₁C_nV^·+ was observed. The effect of cyclodextrins (CDs) on the frequency change-potential (Δf-E) curves was also investigated. Δf for 1 mM C₁C₁₆V²⁺ solution was 6000 Hz. while Δf was below 100 Hz in the presence of 13 mM of α-CD. However, Δf was 5000 Hz in the presence of 13 mM of β-CD. This implies that the complexation ability between C₁C₁₆V^·+/C₁C₁₆V²⁺ and α-CD is larger than that of β-CD through the tighter binding as per the report by Diaz et al. Comproportionation between C₁C-V^2- and C₁C₇V⁰ in the presence of α- and β-CD was also carried out. A negligible effect was observed in the CV or Δf-E curve by α-CD. With β-CD, the anodic stripping wave (C₁C₇V⁰-e^-1→C₁C-V^·+) disappeared, however, Δf was 9500 Hz (1100 Hz without CDs). It is clear that the complexation ability between C₁C-V⁰ and β-CD is larger than that of α-CD. This might be due to the limited solubility of β-CD upon complexation with C₁C₇V⁰. A similar result was obtained in a 1 mM C₁C₄V²⁺ with 30 mM γ-CD experiment. The mechanism of the cyclodextrin-induced comproportionation reaction as well as electrodeposition pathways are discussed in this paper based on EQCM and spectroelectrochemical experimental results.