Fast Electron Transfer Exchange at Self-Assembled Monolayers of Organometallic Ruthenium(II) σ-Arylacetylide Complexes

A new series of ruthenium organometallic carbon-rich complexes, exhibiting fast electron transfer kinetics combined to a low oxidation potential, was synthesized for self-assembled monolayer (SAM) :formation on gold surfaces. The molecules consist of highly conjugated ruthenium(II) mono(sigma-arylacetylide) or bis(sigma-arylacetylide) complexes functionalized with different bridge units with specific (protected) anchoring groups that possess high affinity for gold, such as thiol, carbodithioate, and isocyanide. Single component and mixed SAMs were prepared and fully characterized by wettability studies, infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), and electrochemical analyses. By applying the Laviron's formalism, fast electron transfer kinetics (approximate to 10(4) s(-1)) were found at the derived self-assemblies while no significant effect could have been evidenced with variation of the bridging unit and of the anchoring moiety. Interestingly, a hexyl aliphatic spacer in the bridging unit with a thiol group and dilution with suitable nonelectroactive thiols lead to better SAM organization and packing, in comparison with undiluted complexes with shorter spacers. Such features make these compounds suitable alternatives to the widely used ferrocene center as redox-active building blocks for reversible charge storage devices.