THE KINETICS OF ELECTRON-TRANSFER THROUGH FERROCENE-TERMINATED ALKANETHIOL MONOLAYERS ON GOLD

The kinetics of electron transfer between a substrate gold electrode and a self-assembled monolayer formed from CH3(CH2)(n-1)SH and (eta(5)-C5H5)Fe(eta(5)-C5H4)CO2(CH2)(n)SH were studied as a function of n, the number of methylenes in the alkyl chain tethering the ferrocene moiety to the electrode, using the indirect laser-induced temperature jump method (ILIT). For 5 less than or equal to n less than or equal to 9 the standard electron-transfer rate constants vary according to k(Gamma,s,n=0) exp[-beta(n)n] where k(Gamma,s,n=0) is the (extrapolated) rate constant for the electron transfer at n = 0. At T = 25 degrees C, k(Gamma,s,n=0) similar or equal to 6 x 10(8) s(-1) and beta(n) = 1.21 +/- 0.05. The ILIT method allows rates to be measured that are too fast to be,measured by conventional chronoamperometry at a macroelectrode, which is limited to rate constants of less than or similar to 10(4) s(-1). Using a Marcus formalism, the reorganization energy, lambda, for the electron-transfer process at a given n was determined from the slope of an Arrhenius plot over the temperature range 15-55 degrees C. Values of lambda determined from Arrhenius slopes for n = 8 and 9 using ILIT are in reasonable agreement with the value of lambda previously deduced from the potential dependence of the rate constant for n = 16. For n less than or equal to 8 the ILIT data show a decrease in the value of lambda as n decreases; the decrease is too large to be explained by image charge interactions. The data also suggest that \V-n\, the n-dependent electronic coupling term, does not increase as rapidly as expected with decreasing n for n < 8.