pH-Dependent Rectification in Redox Polymers: Characterization of Electrode-Confined Siloxane Polymers Containing Naphthoquinone and Benzylviologen Subunits

This paper describes the electrochemical characterization of electrode-confined siloxane polymers that contain both naphthoquinone (NQ) and benzylviologen (BV 2+ ) subunits. These “homopolymers,” abbreviated (NQ-BV 3+ ) n and (NQ-BV-BV 5+ ) 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-BV 3+ ) n and (NQ-BV-BV 5+ ) n undergo reversible 3e−/2H + and 4e−/2H + reduction, respectively, consistent with 2e−/2H + reduction of NQ subunits to the hydroquinone NQH 2 and 1e− reduction of BV 2+ subunits to the radical cation BV +• . Below pH 6, however, NQ/NQH 2 reduction becomes irreversible in both polymers, whereas BV 2+/+ reduction remains reversible. Electrochemical irreversibility of NQ/NQH 2 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/NQH 2, 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-BV 6+ ) n . 1 Charge in the form of reduced quinone is trapped in (BV-Q-BV 6+ ) n at low pH because there is essentially no direct charge transport through the Q/QH 2 system. Charge is also trapped in high coverages of (NQ-BV 3+ ) n and (NQ-BV-BV 5+ ) n, indicating no direct charge transport through the NQ/NQH 2 system. Unlike (BV-Q-BV 6+ ) n, however, monolayers comprised of 1a or 2a exhibit charge transport through the NQ/NQH 2 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-BV 3+ /siloxane) n and (NQ-BV-BV 5+ /siloxane) n, can be prepared by copolymerization of 1a or 2a with 1,2-bis(trimethoxysilyl)ethane. Charge trapped in (NQH 2 -BV 3+ /siloxane) n or (NQH 2 -BV-BV 5+ /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 I 3 - /I - will catalytically release the trapped charge when the potential of the electrode is brought close to E ° ‘(I 3 - /I - ). Surfaces modified with (NQ-BV 3+ /siloxane) n or (NQ-BV-BV 5+ /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-BV 3+ /siloxane) n or (NQ-BV-BV 5+ /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 (NQH 2 -BV 3+ /siloxane) n or (NQH 2 -BV-BV 5+ /siloxane) n . The irreversible production of H 2 O 2 upon oxidation of NQH 2 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/NQH 2 ) to a potential where BV 2+/+ can mediate the oxidation of NQH 2 to NQ and deliver charge to the electrode.