Electrochemical and spectroelectrochemical probing of the ionic channel in Nafion films using the redox of perfluoroalkyl viologen

Abstract

We explored the electrochemistry of a perfluoroalkyl viologen, N,N′-di-(1H,1H,2H,2H-perfluorobutyl)-4,4′-bipyridinium dichloride (FC4VFC4) in a Nafion film on an Au electrode. The capability of FC4VFC4 as a redox-active probe for micro-environments of the ionic channels in the Nafion film was highlighted by clarifying the difference of its electrochemical behavior from that of its alkyl analog, dibutyl viologen (C4VC4). The time course of cyclic voltammograms (CVs) was tracked after immersion of a Nafion film-coated Au electrode in viologen solutions of various concentrations, Cs. The strongly Cs-dependent time change revealed the formation of an aggregate of FC4VFC4 in the ionic channels in the Nafion film. The aggregate blocks its own redox reaction at Cs ≥ 10 μM. At Cs = 5 μM, FC4VFC4 exhibited a quasi-reversible response. In contrast, C4VC4 showed a quasi-reversible response in the Cs range from 5 μM to 500 μM. We also used the time dependent change of electroreflectance (ER) spectra to track the state of viologens in the close proximity of the electrode|Nafion interface. After the immersion in 50 μM FC4VFC4 solution, the ER signals of the redox of FC4VFC4 molecules, being not in direct contact with the Au surface, rapidly increased in line with the change of CVs. What followed was a steep decrease of ER signal while the CV redox current was still increasing. It was also found that the FC4VFC4 aggregates in the Nafion film block the electrode reactions of [Ru(NH3)6]2+/3+, methylviologen, and C4VC4, which otherwise show reversible or quasi-reversible responses in the absence of FC4VFC4. All the results, especially the sharp contrast of the behavior of FC4VFC4 to that of C4VC4, revealed that either or both intermolecular perfluoro chain-chain interaction and perfuorinated FC4VFC4 side chain-Nafion perfluoroether side chain interaction are the key to determine the chemical micro-environment in the ionic channels.