Charge-Transfer Rates in Thin Polyelectrolyte Films Incorporating a Redox Molecule with Two Separate Electroactive Centers Having Different Redox Potentials. An Alizarine Red S–Cationic Perfluoropolymer Film System

Abstract The diffusion coefficients for the charge-transport processes through 9,10-dihydro-3,4-dihydroxy-9,10-dioxo-2-anthracenesulfonate (Alizarine Red S, 1)-incorporating cationic perfluoropolymer (CPFP) polyelectrolyte films on electrodes undergoing oxidation or reduction of two separate electroactive centers with different redox potentials, i.e., the two-electron reduction of 1 to 3,4,9,10-tetrahydroxy-2-anthracenesulfonate (2) and the two-electron oxidations of 2 to 1 and 1 to 3,4,9,10-tetrahydro-3,4,9,10-tetraoxo-2-anthracenesulfonate ( 3) were measured by potential-step chronoamperometry and chronocoulometry. The apparent diffusion coefficients for 1 to 2, 2 to 1, and 1 to 3 processes where the first two processes are reversible and the 1 to 3 process is irreversible, represented as Dapprev(1→2), Dapprev(2→1) and Dappirrrev (1→3), respectively, decreased with increasing the concentration (Cfilm) of 1 (or 2) in the CPFP film, and for a given Cfilm they were found to be in the following order: Ds′>>Dapprev (1→2)∼Dapprev(2→1)>Dappirrev (1→3), where Ds′ represents the diffusion coefficients corresponding to the diffusion of supporting electrolyte ions coupled to electron transfer and molecular motion of reactant itself. It became apparent that the difference in Dapprev(1→2) (or Dapprev(2→1)) and Dappirrev (1→3) can be attributed to different contributions of electron exchange between electroactive centers to the overall charge-transport rates. The results demonstrate that the contributions from molecular motion of reactant itself and electron exchange between reactants to the overall charge-transport rates can be estimated depending upon which redox center is employed in an electrochemical measurement of diffusional rates.