Heterogeneous electron transfer of quinone–hydroquinone in alkaline solutions at gold electrode surfaces: Comparison of saturated and unsaturated bridges

We compare the electron transfer (ET) rates of hydroquinone self-assembled monolayers (SAMs) on gold electrodes containing either an alkane or pi-delocalized oligo(phenylene vinylene) (OPV) bridge at basic pH (between pH 8 and 12.6). The hydroquinone SAMs were diluted with octane-1-thiol. Cyclic voltammograms of the quinone/hydroquinone (Q/H2Q) couple allowed the peak-to-peak separation of the anodic and catholic peak potentials to be analyzed and apparent rate constants (kapp) to be determined using Laviron’s formalism. We found that a simple rate law containing one equilibrium constant (KAH) and two rate constants (kAH and kA) describes the dependence of kapp as a function of pH. From this model, the ratio of the rate constants for the protonated form H2Q, kAH (OPV)/kAH (alk) was ∼800, and the ratio of the de-protonated form, HQ−, kA(OPV)/kA(alk) was ∼2. The large ratio at pH < pKAH suggests that, for protonated H2Q, the ability of the OPV bridges to tunnel electrons significantly affects the overall rate. The small ratio at strongly basic pH (>pKAH) implies that, for the de-protonated form HQ−, the overall ET rate is less sensitive to the bridge.