ELECTROCHEMISTRY OF SURFACE-CONFINED MIXED MONOLAYERS OF 4-AMINOTHIOPHENOL AND THIOPHENOL ON AU

We report the electrochemistry of surface-confined monolayers of 4-ami nothiophenol (4-ATP) and mixed monolayers of 4-ATP and thiophenol (TP) on Au surfaces. Cyclic voltammograms of the 4-ATP monolayer in acidic aqueous perchlorate solutions are characterized by an irreversible ox idative wave at 0.730 V vs Ag/AgCl on the first scan and, upon scan re versal, by a persistent, reversible, surface-confined wave centered at approximately 0.500 V and a transient peak at about 0.300 V. We propo se an ECE mechanism to account for this electrochemistry. 4-ATP is fir st oxidized to the cation radical, followed by chemical coupling to fo rm an adsorbed dimer. The dimer is subsequently hydrolyzed in the pres ence of H2O to yield an adsorbed quinone species that is reversible el ectroactive. Grazing angle FTIR spectroscopy was used to identify the product. The transient peak is due to the coupling of desorbed molecul es and is consistent with the formation of a phenazine species. We the n show that mixed monolayers of 4-ATP and TP can be used to study the coverage dependence of surface-confined reactions. The chemical compos ition of the mixed systems was determined using two independent Auger electron spectroscopic techniques and grazing angle FTIR spectroscopy. Using 20 min assembly times, we find that the surface concentration o f 4-ATP is directly proportional to its mole fraction in solution. Int erestingly, TP does not participate in the 4-ATP electrochemistry and functions only to dilute the surface concentration of 4-ATP. We find t hat the efficiency of the conversion of 4-ATP to product is somewhat h igher at low mole fractions of 4-ATP.