An electrochemical study of the kinetic and thermodynamic properties of 6-(diferrocenylhexanylcarbonyl)pentanethiol self-assembled monolayers /

Abstract

Monolayers of 6-(diferrocenylhexanylcarbonyl)pentanethiol (DFHC6SH) were formed on gold electrodes and the kinetic and thermodynamic parameters associated with electron transfer were investigated via cyclic voltammetry (CV). The voltammograms in 1.0 M HCI04 (aq) displayed two peaks centered at approximately 90 mV and 390 mV vs. Ag/AgCI, associated with oxidation of the outer and inner ferrocene groups, respectively. The measured formal potentials were characteristic of carbonyl and akyl substituted ferrocenes. Both of the oxidations were found to be chemically and electrochemically reversible on the voltammetric time scale. In addition, FWHM values for both electron transfers were determined to be significantly larger than the theoretical value of 90/n mY, indicating that the monolayers were somewhat disordered. This conclusion was also supported by the inability to measure a heterogeneous electron transfer rate constant for the outer ferrocene. The surface coverage for the inner ferrocene group was determined to be approximately 7 x 10-11 mole/cm2 , a value which is typical for a monolayer containing an electroactive terminal group which occupies a volume as large as diferrocenylhexane. The surface coverage for the outer ferrocene moiety was found to be approximately 40-50% of that for the inner ferrocene, indicating that many of the outer ferrocene sites are electrochemically inaccessible. This is of great interest because it is the opposite of what was observed by Mirken e1. aI., where an outer ferrocene group was fully accessible and an inner azobenzene group was totally inaccessible.27 IV This unusual phenomenon was explored by comparing the voltammetry in 1.0 M HCI04 (aq) with that obtained in 0.10 M TBAPITHF electrolyte. Use of the less polar, aprotic solvent caused the surface coverages for both ferrocenes to decrease by approximately 40-60% due to desorption. The ratio of the surface coverages for the inner and outer ferrocenes, however, was found to be the same as that obtained in aqueous electrolytes, indicating that the electrochemical inaccessibility of the outer ferrocenes was probably not due to them folding into the non-polar hexane bridge. lon-pairing thermodynamics were also probed by obtaining cyclic voltammograms in 1.0 M H2S04 (aq) solutions containing varying amounts of perchloric acid. 1o The formal potentials for both ferrocene groups were observed to shift anodically with decreasing perchloric acid concentrations. Plots of the formal potentials vs. log10 [HCI04] were used to obtain the ion-pairing constants for the two ferrocenes. The values of Keff for the inner and outer ferrocenes were determined to be 13.06 M-1and 3.915 M-1, respectively. The values were much smaller than those reported by Creager et. aI., indicating that DFHC6SH monolayers are more disordered than the mixed monolayers employed by Creager. 10 In addition, the larger K eff measured for the inner ferrocenes reflects the increased "alkane-like" environment caused by the bridging hexane group.