Effects of surface monolayers on the electron-transfer kinetics and adsorption of methyl viologen and phenothiazine derivatives on glassy carbon electrodes

Five organic redox systems were examined in aqueous electrolytes on polished and chemically modified glassy carbon (GC), to evaluate the effects of surface structure on the heterogeneous transfer rate constant, k degrees, Methyl viologen reduction to its cation radical exhibited a voltammetric peak potential difference which was insensitive to surface modification, with k degrees decreasing by only 50% when a chemisorbed monolayer was present. Methylene blue and three other phenothiazines adsorbed to polished GC, but the adsorption was suppressed by surface modification. For all four phenothiazines, chemisorbed or physisorbed monolayers of electroinactive species had minor effects on k degrees, with a compact nitrophenyl monolayer decreasing k degrees by 50%. This minor change in k degrees was accompanied by a major decrease in adsorption, apparently due to inhibition of dipole-dipole or st-a interactions between the phenothiazine and GC. Chlorpromazine oxidation to its cation radical was studied in more detail, under conditions where adsorption was suppressed. A plot of the natural log of the observed rate constant vs the monolayer thickness for a variety of chemisorbed monolayers was linear, with a slope of -0.22 Angstrom(-1). The observations are consistent with a through-bond electron-tunneling mechanism for electron transfer to all five redox systems studied. The tunneling constant for CPZ of 0.22 Angstrom(-1) is between that reported for electron tunneling through conjugated polyene spacers (0.14 A(-1)) and that reported for phenyl-methylene spacers (0.57 A(-1)), on the basis of long-range electron transfer in rigid molecules.