pH-dependent rectification in redox polymers: Characterization of electrode-confined siloxane polymers containing naphthoquinone and benzylviologen subunits

Article abstract of DOI:10.1021/jp9626475

This paper describes the electrochemical characterization of electrode-confined siloxane polymers that contain both naphthoquinone (NQ) and benzylviologen (BV²⁺) subunits. These "homopolymers," abbreviated (NQ-BV³⁺)_n and (NQ-BV-BV⁵⁺)_n, are derived from monomers, 2-chloro-3-[[2-{dimethyl[[[[N′-[[4-(trimethoxysilyl)-phenyl]methyl]-4, 4′-bipyridiniumyl]methyl]phenyl]methyl]ammonium}ethyl]amino]-1,4- naphthoquinone, 1a, and 2-chloro-3-[[2-{dimethyl[[[[[[[[N′-[N′-[[4-(trimethoxysilyl)phenyl] methyl]-4,4′-bipyridiniumyl]methyl]-phenyl]methyl]-4,4′- bipyridiniumyl]methyl]phenyl]methyl]ammonium}ethyl]amino]-1,4-naphthoquinone, 2a, respectively. Particular to these types of surface-confined homopolymers is the ability to "trap" charge at low pH in the form of reduced quinone. Between pH 10.0 and 7.0, (NQ-BV³⁺)_n and (NQ-BV-BV⁵⁺)_n undergo reversible 3e-/2H⁺ and 4e-/2H⁺ reduction, respectively, consistent with 2e-/2H⁺ reduction of NQ subunits to the hydroquinone NQH₂ and 1e- reduction of BV²⁺ subunits to the radical cation BV^+·. Below pH 6, however, NQ/NQH₂ reduction becomes irreversible in both polymers, whereas BV^2+/+ reduction remains reversible. Electrochemical irreversibility of NQ/NQH₂ in these polymers occurs because its electrochemistry is mediated by the BV^2+/+ couple. Between pH 10.0 and 7.0, BV^2+/+ can mediate both reduction and oxidation of NQ/NQH₂, whereas below pH 6, the thermodynamics are such that BV^2+/+ can only mediate the reduction of NQ. This behavior is similar to that of a previously studied (benzylviologen)-benzoquinone-(benzylviologen) polymer, (BV-Q-BV⁶⁺)_n.¹ Charge in the form of reduced quinone is trapped in (BV-Q-BV⁶⁺)_n at low pH because there is essentially no direct charge transport through the Q/QH₂ system. Charge is also trapped in high coverages of (NQ-BV³⁺)_n and (NQ-BV-BV⁵⁺)_n, indicating no direct charge transport through the NQ/NQH₂ system. Unlike (BV-Q-BV⁶⁺)_n, however, monolayers comprised of 1a or 2a exhibit charge transport through the NQ/NQH₂ system. The flexibility of these monolayers apparently allows direct contact of the NQ subunit with the electrode surface. Less flexible and more robust surface-confined polymers, abbreviated (NQ-BV³⁺/siloxane)_n and (NQ-BV-BV⁵⁺/siloxane)_n, can be prepared by copolymerization of 1a or 2a with 1,2-bis(trimethoxysilyl)ethane. Charge trapped in (NQH₂-BV³⁺/siloxane)_n or (NQH₂-BV-BV⁵⁺/ siloxane)_n can be released and delivered to the surface of the electrode via chemical mediation or by an increase in solution pH. For example, the redox couple I₃^-/I^- will catalytically release the trapped charge when the potential of the electrode is brought close to E°′ (I₃^-/I^-). Surfaces modified with (NQ-BV³⁺/siloxane)_n or (NQ-BV-BV⁵⁺/siloxane)_n, however, are impermeable to the large anionic redox couple Fe(CN)₆^3-/4-, preventing mediated charge release by this reagent. The lack of electrostatic binding of Fe(CN)₆^3-/4- to electrodes modified with (NQ-BV³⁺/siloxane)_n or (NQ-BV-BV⁵⁺/siloxane)_n suggests a high degree of cross-linking in these polymers provided by 1,2-bis(trimethoxysilyl)ethane. At neutral pH, dioxygen will chemically induce the release of charge trapped in (NQH₂-BV³⁺/siloxane)_n or (NQH₂-BV-BV⁵⁺/siloxane)_n. The irreversible production of H₂O₂ upon oxidation of NQH₂ in water, however, prevents the return of charge to the electrode. Charge release is also demonstrated by pH jump experiments where an increase in pH shifts E°′ (NQ/NQH₂) to a potential where BV^2+/+ can mediate the oxidation of NQH₂ to NQ and deliver charge to the electrode.