DISTANCE DEPENDENCE OF ELECTRON-TRANSFER RATES IN BILAYERS OF A FERROCENE LANGMUIR-BLODGETT MONOLAYER AND A SELF-ASSEMBLED MONOLAYER ON GOLD

Bilayers consisting of a self-assembled alkanethiol monolayer of varia ble chain length and Langmuir-Blodgett monolayers of 16-ferrocenylhexa decanoic acid were fabricated on Au electrodes. The hydrophobic ferroc ene groups reside at the monolayer-monolayer interface and are variabl y spaced from the underlying electrode. Interfacial electron transfer rates k(o) for ferrocene electron transfer across the alkanethiol spac er monolayer were measured cyclic voltammetrically. The rates were fou nd 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(o) = 12 s(-1)), which invol ves a non-bonded donor-acceptor couple, is almost 2 orders of magnitud e 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 th e 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.