Topological Effects in Bridge-Mediated Electron Transfer Between Redox Molecules and Metal Electrodes

Electron-transfer rates between ferrocene and gold were measured for a series of alkanethiolate-based monolayers in which ferrocene groups were linked to gold via bridges that included phenyl rings with varying isomeric substitution patterns attached directly to one of the cyclopentadiene rings on ferrocene. Electron transfer was substantially faster (factor of 10) in monolayers with a 1,4-substituted phenyl ring in the bridge relative to that in monolayers with a 1,3-substituted ring in the bridge. The ferrocene/gold electronic coupling is, therefore, much stronger in the former case, despite the fact that the directly bonded pathway between ferrocene and gold is shorter in the latter case. This finding provides definitive evidence for a topological effect (meaning an effect relating to the position of substitution on a bridge component) on the rate of bridge-mediated electron transfer between a redox molecule and a metal electrode for a bridge that includes both aromatic and aliphatic subunits linked together. The effect is somewhat moderated when the chain length of the (non-ferrocene-containing) alkanethiol coadsorbate is changed. It is thought that in such cases the bridge no longer adopts a fully extended conformation and that the electronic coupling then includes contributions from nonbonded pathways involving alkanethiolate chains adjacent to the ferrocene group in the monolayers.