Electrochemical Rectification at a Monolayer-Modified Electrode

Abstract

An electrochemical current rectifier based on a ferrocene-monolayer-modified electrode in contact with an electrolyte solution containing sodium hexacyanoferrate(II) (sodium ferrocyanide) was constructed and evaluated. Rectification was accomplished via mediated ferrocyanide oxidation by electrogenerated ferricenium in monolayers of (10-mercapto-N-decyl)ferrocenecarboxamide with dodecanethiol and (7-mercapto-N-heptyl)-ferrocenecarboxamide with nonanethiol on gold electrodes. The reverse reaction of ferricyanide reduction by immobilized ferrocene is thermodynamically disfavored, thereby providing the basis for rectification. A model proposed to describe the rectification process is reminiscent of earlier treatments of electron-transfer mediation at electrodes modified with redox polymers, with the important difference that, in the present system, long-range electron transfer across the monolayer replaces electron hopping through the polymer layer as a possible rate-limiting step. Steady-state current-voltage curves corresponding to mediated ferrocyanide oxidation were recorded and analyzed using the proposed model to give values for k 0 , the standard rate constant for electrochemical oxidation/reduction of immobilized ferrocene groups, and k cross , the rate constant for reaction of ferricenium ions in the monolayer with ferrocyanide ions in solution. Mean k 0 values of 2700 ( 1000 s -1 for the (10-mercapto-N-decyl)ferrocenecarboxamide/dodecanethiol system and 90 000 ( 60 000 s -1 for the (7-mercapto-N-heptyl)ferrocenecarboxamide/nonanethiol system obtained by analysis of steady-state current-voltage data are to be compared with values of 1400 and 51 000 s -1 for the same systems obtained by the fast-scan cyclic voltammetry method. The k cross value of 1.1 × 10 8 M -1 s -1 obtained from analysis of steady-state current-voltage data for the (10-mercapto-N-decyl)ferrocenecarboxamide/dodecanethiol system is in good agreement with a value calculated using the Marcus expression with literature values for the electron self-exchange rate constants for ferrocene/ferricenium and ferrocyanide/ferricyanide, indicating that the reactivity of ferricenium ions in the monolayer is similar to that expected for ferricenium in free solution.

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

“…Indeed, if the initial mixed monolayer of azideterminated and diluent thiol is not incubated in the solution of pure diluent thiol prior to the coupling, then the value of k et (E) is larger near E 0 and the potential dependence is even more shallow. If, for the defect pathway, the self-exchange is rate-limiting rather than the electron transfer between the electrode and redox species at the defective site, then one would obtain a potential independent contribution due to the self-exchange, here labeled k se , in parallel with the electrochemical rate constant: (5) The ferrocene data in Figure 4 are indeed well fit by this equation. The fit parameters suggest that about 40% of the electrons are transferred by the defective pathway.…”

Self-assembly of mono- and multilayers of polyconjugated conducting polymers

“…7 the absorbance is plotted versus the number of dipping cycles. The relationship is linear with slope of 65 N 10 -3 bilayer -1 up to at least 10 bilayers, formulated as (26/ PAB) 10 . Given an extinction coefficient of 6 N 10 7 mol -1 N cm 2 for sexithiophene 76) , the absorbance increase per bilayer corresponds to 11 N 10 -10 mol N cm -2 bilayer This coverage is identical to that reported for tetrathiophene diphosphonate multilayered with zirconium ions 60) .…”

“…The interesting electron transfer behaviour 9) of self-assembled monolayers (SAMs) also led to the development of new strategies for modifying electrode surfaces 10) aiming at the control of their electrochemical reactivity. Electrons can be transferred across an alkylthiol monolayer by a tunnelling mechanism 11) , which operates up to ca.…”

Demonstration of unimolecular electrical rectification in hexadecylquinolinium tricyanoquinodimethanide

“…13,14 The scanning tunnelling microscopy (STM) current through a porphyrin covalently attached to a carboxylated highly oriented pyrolytic graphite (HOPG) surface is asymmetric. 15 Unsymmetrical STM tunnelling currents were seen through an alkylated hexabenzocoronene deposited on graphite 16 (salt formation?)…”