Electrochemical Investigation of Ruthenium and Osmium Oligothiophene Complexes: How Does Metal Binding Affect the Oligothiophene π-System?

We have studied the electrochemistry of a series of oligothiophene complexes with one or more "Cp * Ru⁺", "CpRu⁺", or "CpOs⁺" fragments (Cp = cyclopentadienyl; Cp* = pentamethyicyclopentadienyl) attached to the oligothiophene π-system. This series varies the metal (Ru or Os), ancillary ligand (Cp or Cp*), ring substituents (phenyl or methyl groups), and length of the oligothiophene (1-4 rings). The peak potentials for the oxidation of the free ohgothiophenes and their complexes indicate that the electron hole produced upon oxidation of the complexes is delocalized on the uncomplexed rings of the oligothiophene. Oxidation of the complexes results in conductive films on the electrode but the composition of the electrodeposited films is unclear. The electron added upon reduction of the complexes is localized on the [Cp/Cp*M(thiophene)]⁺ unit formed by complexation of the oligothiophene. We propose that complexation of a thiophene ring converts it into a [Cp/Cp*M(thiophene)]⁺ unit and removes it from conjugation with the remaining, uncomplexed rings. The unbound rings function as a shortened, metal-substituted oligothiophene unit. Complexation of oligothiophenes by "Cp*Ru⁺", "CpRu⁺", and "CpOs⁺" fragments is a rational method for controlling the properties of oligothiophenes.