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

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 (η 5 C 5 H 5 )Fe(η 5 C 5 H 4 )CO 2 (CH 2 ) 16 SH and CH 3 (CH 2 ) 15 SH. Rate constants from 1 per second (s -1 ) to 2 × 10 4 s -1 in 1 molar HClO 4 are reasonably fit with a reorganization energy of 0.85 eV and a prefactor for electron tunneling of 7 × 10 4 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.