Electrochemical gating of single osmium molecules tethered to Au surfaces

Abstract

The electrochemical study of electron transport between Au electrodes and the redox molecule Os[(bpy) 2 (PyCH 2 NH 2 CO-]ClO 4 tethered to molecular linkers of different length (1.3 to 2.9 nm) to Au surfaces has shown an exponential decay of the rate constant k ET 0 with a slope β = 0.53 consistent with through bond tunneling to the redox center. Electrochemical gating of single osmium molecules in an asymmetric tunneling nano-gap between a Au(111) substrate electrode modified with the redox molecules and a Pt-Ir tip of a scanning tunneling microscope was achieved by independent control of the reference electrode potential in the electrolyte, E ref − E s , and the tipsubstrate bias potential, E bias . Enhanced tunneling current at the osmium complex redox potential was observed as compared to the off resonance set point tunneling current with a linear dependence of the overpotential at maximum current vs. the E bias . This corresponds to a sequential two-step electron transfer with partial vibration relaxation from the substrate Au(111) to the redox molecule in the nano-gap and from this redox state to the Pt-Ir tip according to the model of Kuznetsov and Ulstrup (J Phys Chem A 104: 11531, 2000). Comparison of short and long linkers of the osmium complex has shown the same two-step ET (electron transfer) behavior due to the long time scale in the complete reduction-oxidation cycle in the electrochemical tunneling spectroscopy (EC-STS) experiment as compared to the time constants for electron transfer for all linker distances, k ET 0 .

Metal Nanoparticle Enhancement of Electron Transfer to Tethered Redox Centers through Self-Assembled Molecular Films

“…For Au/16-MHDA/Os, we obtained k ET 0 = 3.7 s –1 (Γ Os = 4.4 × 10 –13 mol cm –2 and C dl = 0.011 μF), and for Au/6-MHA/Os, k ET 0 = 5.2 × 10 4 s –1 (Γ Os = 2.8 × 10 –12 mol cm –2 and C dl = 0.017 μF). These values are similar to previously reported data for the same osmium complex tethered by alkanethiols of different lengths …”

“…These values are similar to previously reported data for the same osmium complex tethered by alkanethiols of different lengths. 20 A comparison of the fast electron transfer rate enhanced by AuNPs with that for C16 without AuNPs yields a remarkable enhancement factor of 1.9 × 10 4 . On the other hand, the electron transfer rate enhanced by AuNPs is very similar to that for C6 alkanethiol, 5.2 × 10 4 s −1 , which suggests that the limiting electron transfer rate in the two-step mechanism is the transfer from the AuNP to the osmium functionality by electron tunneling through the C6 α-lipoic acid linker.…”

“…This system is free of the diffusion limitations present in soluble redox systems as the redox centers are attached to the electrode. Because the osmium complex shows very fast ET kinetics, i.e., k ET 0 > 10 6 s –1 , electrochemical impedance over a wide frequency range was employed based on the method reported by Creager and Wooster . To determine the ET rate without AuNPs, the osmium complex was also covalently attached to C6 or C16 alkanethiol self-assembled monolayers on Au metal electrodes and studied by electrochemical impedance spectroscopy.…”

Gaussian Distribution of Electron Transfer Distance in Redox Terminated Self-Assembled Thiol Monolayers

“…In a previous communication, we studied an outer sphere osmium bipyridine complex tethered by alkanethiols of different lengths to Au surfaces, which exhibit linear plots of ln k ET vs the number of CH 2 units. However, frequency dispersion of the ET rate constants was observed which was more evident for the shortest alkanethiols . With β ≈ 1 Å –1 , very small differences in the tunneling distance due to different populations of redox molecules in slightly different configurations would result in significant variations of the ET rate constant, explaining the observed dispersion.…”

“…However, frequency dispersion of the ET rate constants was observed which was more evident for the shortest alkanethiols. 9 With β ≈ 1 Å −1 , very small differences in the tunneling distance due to different populations of redox molecules in slightly different config-urations would result in significant variations of the ET rate constant, explaining the observed dispersion.…”