Heterogeneous Electron-Transfer Kinetics for Ruthenium and Ferrocene Redox Moieties through Alkanethiol Monolayers on Gold

Abstract

The standard heterogeneous electron-transfer rate constants between substrate gold electrodes and either ferrocene or pentaaminepyridine ruthenium redox couples attached to the electrode surface by various lengths of an alkanethiol bridge as a constituent of a mixed self-assembled monolayer were measured as a function of temperature. The ferrocene was either directly attached to the alkanethiol bridge or attached through an ester (CO(2)) linkage. For long bridge lengths (containing more than 11 methylene groups) the rate constants were measured using either chronoamperometry or cyclic voltammetry; for the shorter bridges, the indirect laser induced temperature jump technique was employed to measure the rate constants. Analysis of the distance (bridge length) dependence of the preexponential factors obtained from an Arrhenius analysis of the rate constant versus temperature data demonstrates a clear limiting behavior at a surprisingly small value of this preexponential factor (much lower than would be expected on the basis of aqueous solvent dynamics). This limit is independent of both the identity of the redox couple and the nature of the linkage of the couple to the bridge, and it is definitely different (smaller) from the limit derived from an equivalent analysis of the rate constant (versus temperature) data for the interfacial electron-transfer reaction through oligophenylenevinylene bridges between gold electrodes and ferrocene. There are a number of possible explanations for this behavior including, for example, the possible effects of bridge conformational flexibility upon the electron-transfer kinetics. Nevertheless, conventional ideas regarding electronic coupling through alkane bridges and solvent dynamics are insufficient to explain the results reported here.

Long-range electron transfer

“…The driving force-optimized azurin tunneling timetable reveals a nearly perfect exponential distance dependence, with a decay constant (␤) of 1.1 Å Ϫ1 , and an intercept at close contact (r o ϭ 3 Å) of 10 13 s Ϫ1 . This decay constant is quite similar to that found for superexchange-mediated tunneling across saturated alkane bridges (␤ Ϸ1.0 Å Ϫ1 ) (12,35), strongly indicating that a similar coupling mechanism is operative in the polypeptide (Fig. 1).…”

Fundamental signatures of short- and long-range electron transfer for the blue copper protein azurin at Au/SAM junctions

“…Adjacent water is believed to contribute strongly to λ o , especially to its entropic part (11), and one might expect such contributions to be negligible for hydrophobically adsorbed Az, and lead to ΔS Ã a ≈ 0. Importantly, a similar conclusion is deducible for ET within the comparable Au/SAM systems with covalently attached nonbiological metal complexes acting as redox probes (46,50).…”

Rate of Interfacial Electron Transfer through the 1,2,3-Triazole Linkage

“…This method reveals any multiphasic character of the electron-transfer kinetics and allows measurement of the electron-transfer rate not only at the standard potential, E 0 , of the couple but also as a function of the standard overpotential, E − E 0.2 Equation 1 shows the expected exponential decay of the redox current, i redox (t), for a kinetically homogeneous population of one-electron redox sites on the surface of an electrode: 2 (1) where e is the electronic charge, A is the area of the electrode, ΔΓ ox is the change in coverage (number per unit area) of the oxidized form of the redox species during the course of the measurement, and the first-order electron-transfer relaxation rate constant, k et (E), is the sum of the first-order reduction rate constant, k red (E), and the first-order oxidation rate constant, k ox (E): (2) Equations 1 and 2 are derived from the first-order rate law for reversible single-electron transfer, dΓ ox (t)/dt = k ox (E)Γ red (t) − k red (E)Γ ox (t), and conservation of the redox species at the monolayer surface, Γ red (t) + Γ ox (t) = constant. Figure 3a shows an example of the current transient for a mixed monolayer formed from 0.015 mM N 3 (CH 2 ) 16 SH and 0.09 mM CH 3 (CH 2 ) 15 SH in ethanol with overnight exchange in 100 mM CH 3 (CH 2 ) 15 SH and then click coupled to ferrocene acetylene. The potential was stepped from 10 mV negative of the ferrocene standard potential to the standard potential of the ferrocene redox couple.…”

“…[2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] One of the major limits to the routine use of this method is the difficulty of forming the desired assemblies of electrode, monolayer, and redox species. Typical assembly strategies rely on appending a spacer or electron-transfer "bridge" terminated with a thiol group to the redox species of interest and then coadsorbing this molecule with a more plentiful unmodified or "diluent" thiol.…”