TY - JOUR
T1 - Charge-transport properties of an electrode-confined redox polymer derived from a monomer consisting of a quinone flanked by two benzylviologen subunits
AU - Smith, Diane K.
AU - Lane, Gregg A.
AU - Wrighton, Mark S.
PY - 1988
Y1 - 1988
N2 - Electrochemical and optical spectroelectrochemical characterization of an electrode-confined siloxane-based polymer having two benzylviologen subunits flanking a benzoquinone subunit is reported. The polymer, (BV-Q-BV6+)n, is formed by hydrolysis of the -Si(OMe)3 groups of the monomer I, 1,1″-[2,5-dichloro-3,6-dioxo-1,4-cyclohexadiene-1,4-diyl)bis[imino-2,1- ethanediyl(dimethyliminio)methylene-1,4-phenylenemethylene]]bis[1′-[[4- (trimethoxysilyl)phenyl]methyl]-4,4′-bipyridinium. Further, electrochemical and optical properties of the solution analogue of I are reported. The (BV-Q-BV6+) unit has two fixed positive charges, one in each of the BV-Q links. The (BV-Q-BV6+) unit is reducible by a 6e-/2H+ process in a manner roughly consistent with the behavior of BV2+ (E°′(BV2+/+) ≈-0.5 V vs SCE; E°′(BV+/0) ≈-0.9 V vs SCE) and Q (E°′(Q/QH2) ≈ -0.3 V vs SCE at pH 7) studied separately in solution and as siloxane-based polymers confined to electrodes. The important finding is that the (BV-Q-BV6+)n system shows pH-dependent rectification in the sense that at low pH, less than ∼6, the reduction of the Q centers to QH2 cannot be electrochemically reversed, even at electrode potentials significantly positive of E°′(Q/QH2). Thus, reducing equivalents can be kinetically and thermodynamically trapped in the form of QH2. The rectification observed is established to be due to a combination of kinetic, thermodynamic, and structural factors all consistent with the conclusion that reduction of all Q centers to QH2 occurs via reducing equivalents delivered through the BV2+/+ system and that the reverse process is possible only above a certain pH, ≥6. Studies of conventional electrode-confined bilayers (BV2+)n/(Q)n and electrode-confined (Q)n are reported and reveal factors influencing the rectification in (BV-Q-BV6+)n: charge transport in (Q)n via Q/QH2 self-exchange is very sluggish but reduction of Q to QH2 via BV+ can be fast. Previous studies of (BV2+/+)n show that charge transport via BV2+/+ self-exchange can be fast and is pH-independent, but steady-state charge transport via BV2+/+ self-exchange in (BV-Q-BV6+)n is significantly slower and is modestly pH dependent in a manner consistent with cross exchange between the BV2+/+ and Q/QH2 contributing to charge transport at pH ≥6 but not for pH <6. The temperature dependence of steady-state charge transport in (BV-Q-BV6+)n shows an Arrhenius activation energy of 50-60 kJ/mol.
AB - Electrochemical and optical spectroelectrochemical characterization of an electrode-confined siloxane-based polymer having two benzylviologen subunits flanking a benzoquinone subunit is reported. The polymer, (BV-Q-BV6+)n, is formed by hydrolysis of the -Si(OMe)3 groups of the monomer I, 1,1″-[2,5-dichloro-3,6-dioxo-1,4-cyclohexadiene-1,4-diyl)bis[imino-2,1- ethanediyl(dimethyliminio)methylene-1,4-phenylenemethylene]]bis[1′-[[4- (trimethoxysilyl)phenyl]methyl]-4,4′-bipyridinium. Further, electrochemical and optical properties of the solution analogue of I are reported. The (BV-Q-BV6+) unit has two fixed positive charges, one in each of the BV-Q links. The (BV-Q-BV6+) unit is reducible by a 6e-/2H+ process in a manner roughly consistent with the behavior of BV2+ (E°′(BV2+/+) ≈-0.5 V vs SCE; E°′(BV+/0) ≈-0.9 V vs SCE) and Q (E°′(Q/QH2) ≈ -0.3 V vs SCE at pH 7) studied separately in solution and as siloxane-based polymers confined to electrodes. The important finding is that the (BV-Q-BV6+)n system shows pH-dependent rectification in the sense that at low pH, less than ∼6, the reduction of the Q centers to QH2 cannot be electrochemically reversed, even at electrode potentials significantly positive of E°′(Q/QH2). Thus, reducing equivalents can be kinetically and thermodynamically trapped in the form of QH2. The rectification observed is established to be due to a combination of kinetic, thermodynamic, and structural factors all consistent with the conclusion that reduction of all Q centers to QH2 occurs via reducing equivalents delivered through the BV2+/+ system and that the reverse process is possible only above a certain pH, ≥6. Studies of conventional electrode-confined bilayers (BV2+)n/(Q)n and electrode-confined (Q)n are reported and reveal factors influencing the rectification in (BV-Q-BV6+)n: charge transport in (Q)n via Q/QH2 self-exchange is very sluggish but reduction of Q to QH2 via BV+ can be fast. Previous studies of (BV2+/+)n show that charge transport via BV2+/+ self-exchange can be fast and is pH-independent, but steady-state charge transport via BV2+/+ self-exchange in (BV-Q-BV6+)n is significantly slower and is modestly pH dependent in a manner consistent with cross exchange between the BV2+/+ and Q/QH2 contributing to charge transport at pH ≥6 but not for pH <6. The temperature dependence of steady-state charge transport in (BV-Q-BV6+)n shows an Arrhenius activation energy of 50-60 kJ/mol.
UR - https://www.scopus.com/pages/publications/0010218234
U2 - 10.1021/j100320a044
DO - 10.1021/j100320a044
M3 - Article
AN - SCOPUS:0010218234
SN - 0022-3654
VL - 92
SP - 2616
EP - 2628
JO - Journal of Physical Chemistry
JF - Journal of Physical Chemistry
IS - 9
ER -