Electrochemical study of methylalkylviologens using an electrochemical quartz crystal microbalance

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 (Delta f-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, Delta f was less than 30 Hz (reversible): for n = 9, 10, 12 and 14, Delta f = 200 Hz indicating electrodeposition of C1CnV.+; for n = 16 and 18, Delta f = 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 (Delta f-E) curves was also investigated. Delta f for 1 mM C1C16V2+ solution was 6000 Hz, while Delta f was below 100 Hz in the presence of 13 mM of alpha-CD. However, Delta f was 5000 Hz in the presence of 13 mM of beta-CD. This implies that the complexation ability between C1C16V.+/C1C16V2+ and alpha-CD is larger than that of beta-CD through the tighter binding as per the report by Diaz et al. (J. Phys. Chem., 92 (1988) 3537). Comproportionation between C1C7V2+ and C1C7V0 in the presence of alpha- and beta-CD was also carried out. A negligible effect was observed in the CV or Delta f-E curve by alpha-CD. With beta-CD, the anodic stripping wave (C1C7V0 - e(-) --> C1C7V.+) disappeared, however, Delta f was 9500 Hz (1100 Hz without CDs). It is clear that the complexation ability between C1C7V0 and beta-CD is larger than that of alpha-CD. This might be due to the limited solubility of beta-CD upon complexation with C1C7V0. A similar result was obtained in a 1 mM C1C4V2+ with 30 mM gamma-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. (C) 1999 Elsevier Science S.A. All rights reserved.