Electrochemical gating of single osmium molecules tethered to Au surfaces

The electrochemical study of electron transport between Au electrodes and the redox molecule Os[(bpY)(2)(PYCH2 NH2CO-]ClO4 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 kET(0) with a slope beta = 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 tip substrate 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 Ebias. This corresponds to a sequential two-step electron transfer with partial vibration relaxation from the substrate Au(I11) 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(FT)(0).