SELF-ASSEMBLED MONOLAYERS IN ORGANIC-SOLVENTS - ELECTROCHEMISTRY AT ALKANETHIOLATE-COATED GOLD IN PROPYLENE CARBONATE

Alkanethiolate monolayers with excellent barrier properties can be formed and maintained on gold electrodes in the nonaqueous solvent propylene carbonate. The layers are most stable when alkanethiol is present in the propylene carbonate solution and when the potential is held between +0.50 and -0.70 V vs Ag/AgCl/saturated KCl. The ability to work in a nonaqueous solvent has enabled study of several redox-active probe molecules that have not previously been studied at alkanethiolate-coated electrodes. Specifically, studies on three ferrocene derivatives (ferrocene, decamethylferrocene, and [(trimethylamino)methyl]ferrocene) and three organic redox mediators (tetracyanoquinodimethane, N,N,N',N'-tetramethylphenylenediamine, and N,N'-dimethylviologen) have been pursued. Two complex metal ions (hexacyanoferrate(III) and hexaammineruthenium(III)) were also studied to facilitate comparison with earlier work in water. A weak correlation exists between the rate of interfacial electron transfer at the coated electrodes and the homogeneous electron self-exchange rates of the redox-active probe molecules. The correlation is strongest within the series of structurally homologous ferrocene derivatives and is weakest when comparing probe molecules of very different structure. There is no apparent correlation between electrochemical reactivity and molecular size, suggesting that size exclusion is not a determining factor for electrochemistry at alkanethiolate-coated electrodes. There is however a correlation with molecular charge, suggesting that electrostatic effects may be important.