Revisiting aqueous redox process of alkyl-linked bis-viologen: Evaluation of redox potential inversion

Redox reactions of highly water-soluble bis-viologens were examined at Au electrodes using ultra-micro electrode steady state voltammetry and electroreflectance methods in addition to conventional voltammetry. A bulk UV–vis absorption spectral approach was also used. Alkane-1,n-diyl bis-viologens appeared as a class of prototypical electroactive molecule possessing two equivalent redox centers; they undergo two-consecutive one-electron transfer processes. When intramolecular dimerization takes place immediately after accepting the second electron to produce a form having two viologen radical cation (V+) sites in one molecule, the second reduction potential may be more positive than the first one. This redox potential inversion is caused by strong tendency to form a stable intramolecular π−π stacking between the two V+ sites. The redox potential inversion of a highly water-soluble bis-viologen with a butane-1,4-diyl linkage was quantitatively obtained to be 116 mV (E1 – E2 = −116 mV). In contrast, the bis-viologen with an ethane-1,2-diyl linkage exhibited no dimerization below 0.2 mM, and its second redox potential was found to at 63 mV more negative than the first (E1 – E2 = 63 mV), highlighting repulsive interaction. The value of E1 – E2 = −116 mV for the bis-viologen with a butane-1,4-diyl linkage corresponds to the attractive interaction energy in the redox process of 5.9 kBT of room temperature; this value is apparently the same as the dimerization Gibbs free energy of methyl viologen in water.