Distance dependence of electron transfer rates in bilayers of a ferrocene Langmuir-Blodgett monolayer and a self-assembled monolayer on gold

Abstract

Bilayers consisting of a self-assembled alkanethiol monolayer of variable chain length and Langmuir-Blodgett monolayers of 16-ferrocenylhexadecanoic acid were fabricated on Au electrodes. The hydrophobic ferrocene groups reside at the monolayer-monolayer interface and are variably spaced from the underlying electrode. Interfacial electron transfer rates k” for ferrocene electron transfer across the alkanethiol spacer monolayer were measured cyclic voltammetrically. The rates were found to fall off exponentially with increasing alkanethiol chain length, giving a decay constant of -0.96 per methylene. The electron transfer rate with a dodecanethiol spacer layer (k” = 12 s-l), which involves a non-bonded donor-acceptor couple, is almost 2 orders of magnitude lower than those for the analogous ferrocenes chemically bound on Au through alkane thiols of similar bond length. The higher rate in the latter may be attributed to a through-bond mechanism, as opposed to the former in which there exists a single “break” in the chemical bond network. The results have their implications on the electron transfer studies of biological systems. We have sought to probe long-distance interfacial electron transfer’ (ET) by using designed electrochemical interfaces based on self-assembled monolayer2 (SAM) and LangmuirBlodgett (LB) films.3 In particular, we have fabricated on gold surfaces bilayer structures composed of an inner spacer monolayer of long-chain alkanethiols (CnH2,+1SH, n 2 10) and an outer electroactive LB monolayer, with electrochemically reactive head groups (ferrocene) positioned at the monolayermonolayer interface (Scheme la). The distance dependence of the heterogeneous ET rate constants can be investigated by varying the alkanethiol chain length. Furthermore, a comparison of these rates with those obtained at gold electrodes modified with ferrocene-terminated alkanethiol monolayers2c.f (Scheme lb) can address the question of the relative efficiency of “through-bond” versus “through-space’’ ET pathways. This is one of the key issues currently under investigation4 in the field of long range electron transfer, a reaction that is ubiquitous in biological systems and is involved in such important processes as oxidative phosphorylation and photosynthesis. In a biological electron transfer reaction, the electron donor and acceptor are separated by polypeptides organized into defined threedimensional structures mostly through noncovalent interactions such as hydrogen bonds and van der Waals contacts. The modulation of these noncovalent interactions on the overall electron tunneling efficiency through the polypeptide matrix is therefore well worth investigating. Experimental measurements5 as well as theoretical calculations6 have been carried out using metal-derivatized proteins and physiological protein-protein complexes to examine the effect of biological medium on electron transfer kinetics, although the ability to control the structural details of these systems has not been achieved yet. With the rapid advancement in the field of molecular selfassembly, we are now in a position to fabricate highly ordered monomolecular and multilayer films on solid surfaces with molecular level control over the structures and employ them in