The electrochemical behavior of various 1-methyl-1′-alkylviologens (C1CnV2-: 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 C1CnV·+; for n = 16 and 18, Δf = 5000 Hz with two anodic peaks implying reorientation of the electrodeposited C1CnV·+ occurring on the electrode surface. These results are consistent with our previous report where apparent dimerization of C1CnV·+ 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-V2- 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.
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We would like to acknowledge very helpful discussions with Professor Joon Woo Park. We also thank S.Y. Bang and H.K. Park for their assistance in obtaining experimental results and artwork, respectively. The support of this research by the Korea Science and Engineering Foundation (KOSEF 96-0501-05-01-3 for CL and KOSEF through the Center for molecular catalysis at Seoul National University for ICJ, respectively) and the Ministry of Education of the Republic of Korea through the Basic Science Research Institute Program (BSRI-97-3427 for CL and BSRI-97-3430 for ICJ, respectively) is gratefully acknowledged.