pH-Dependent Rectification in Redox Polymers:  Characterization of Electrode-Confined Siloxane Polymers Containing Naphthoquinone and Benzylviologen Subunits (1997)

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 BV2+/+ reduction remains reversible. Electrochemical irreversibility of NQ/NQH2 in these polymers occurs because its electrochemistry is mediated by the BV2+/+ couple. Between pH 10.0 and 7.0, BV2+/+ can mediate both reduction and oxidation of NQ/NQH2, whereas below pH 6, the thermodynamics are such that BV2+/+ 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)63-/4-, preventing mediated charge release by this reagent. The lack of electrostatic binding of Fe(CN)63-/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 BV2+/+ can mediate the oxidation of NQH2 to NQ and deliver charge to the electrode.