Topological effects in bridge-mediated electron transfer between redox molecules and metal electrodes

Electron-transfer rates between ferrocene and gold were measured fora serie s of alkanethiolate-based monolayers in which ferrocene groups were linked to gold via bridges that included phenyl rings with varying isomeric substi tution patterns attached directly to one of the cyclopentadiene rings on fe rrocene. Electron transfer was substantially faster (factor of 10) in monol ayers 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 el ectronic coupling is, therefore, much stronger in the former case, despite the fact that the directly bonded pathway between ferrocene and gold is sho rter in the latter case. This finding provides definitive evidence for a to pological effect (meaning an effect relating to the position of substitutio n on a bridge component) on the rate of bridge-mediated electron transfer b etween a redox molecule and a metal electrode fore bridge that includes bot h aromatic and aliphatic subunits linked together. The effect is somewhat m oderated when the chain length of the (non-ferrocene-containing) alkanethio l coadsorbate is changed. It is thought that in such cases the bridge no lo nger adopts a fully extended conformation and that the electronic coupling then includes contributions from nonbonded pathways involving alkanethiolat e chains adjacent to the ferrocene group in the monolayers.