Free Energy and Temperature Dependence of Electron Transfer at the Metal-Electrolyte Interface

Science  22 Feb 1991:
Vol. 251, Issue 4996, pp. 919-922
DOI: 10.1126/science.251.4996.919


The rate constant of the electron-transfer reaction between a gold electrode and an electroactive ferrocene group has been measured at a structurally well-defined metal-electrolyte interface at temperatures from 1° to 47°C and reaction free energies from -1.0 to +0.8 electron volts (eV). The ferrocene group was positioned a fixed distance from the gold surface by the self-assembly of a mixed thiol monolayer of (η5C5H5)Fe(η5C5H4)CO2(CH2)16SH and CH3(CH2)15SH. Rate constants from 1 per second (s–1) to 2 x 104 s–1 in 1 molar HClO4 are reasonably fit with a reorganization energy of 0.85 eV and a prefactor for electron tunneling of 7 x 104 s–1 eV–1. Such self-assembled monolayers can be used to systematically probe the dependence of electron-transfer rates on distance, medium, and spacer structure, and to provide an empirical basis for the construction of interfacial devices such as sensors and transducers that utilize macroscopically directional electron-transfer reactions.