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 polishe d and chemically modified glassy carbon (GC), to evaluate the effects of su rface structure on the heterogeneous transfer rate constant, k degrees, Met hyl 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. Me thylene blue and three other phenothiazines adsorbed to polished GC, but th e adsorption was suppressed by surface modification. For all four phenothia zines, chemisorbed or physisorbed monolayers of electroinactive species had minor effects on k degrees, with a compact nitrophenyl monolayer decreasin g k degrees by 50%. This minor change in k degrees was accompanied by a maj or decrease in adsorption, apparently due to inhibition of dipole-dipole or st-a interactions between the phenothiazine and GC. Chlorpromazine oxidati on 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 consi stent with a through-bond electron-tunneling mechanism for electron transfe r to all five redox systems studied. The tunneling constant for CPZ of 0.22 Angstrom(-1) is between that reported for electron tunneling through conju gated polyene spacers (0.14 A(-1)) and that reported for phenyl-methylene s pacers (0.57 A(-1)), on the basis of long-range electron transfer in rigid molecules.