Heterogeneous electron transfer processes in self-assembled monolayers of amine terminated conjugated molecular wires

A versatile synthesis of triarylamine and phenothiazine end-capped oligo(phenyleneacetylene) molecular wires which are terminated by thiol functions is described. The repetitive synthesis allows the preparation of molecular wires with different chain length and different substituents attached to the wire backbone. These molecular wires were used to form dense self-assembled monolayers (SAM) on gold substrates as proved by cyclic voltammetry and quartz crystal microbalance measurements. The heterogeneous electron transfer rate constant of these SAMs was measured by impedance spectroscopy between 1 MHz and 0.1 Hz. The rate constants are somewhat larger for the triarylamine terminated systems than for the phenothiazine compound, due to the higher reorganization energy in the latter. While the molecular wires with electron withdrawing substituents display an electron transfer which is slow enough to be measurable with our impedance setup, we were unable to determine the rate of molecular wires with electron donating substituents.