Controlling the Molecular Direction of Dinuclear Ruthenium Complexes on HOPG Surface through Noncovalent Bonding.

Abstract

We synthesized three types of binuclear Ru complexes (1-3) that contain pyrene anchors for the adsorption of 1-3 onto nanocarbon materials via noncovalent π-π interactions, in order to investigate their adsorption onto and their desorption from highly ordered pyrolytic graphite (HOPG). The adsorption saturation for 1 (6.22 pmol/cm2), 2 (2.83 pmol/cm2), and 3 (3.53 pmol/cm2) on HOPG was obtained from Langmuir isotherms. The desorption rate from HOPG electrodes decreased in the order 3 (2.4 × 10-5 s-1) > 2 (1.4 × 10-5 s-1) ≫ 1 (1.8 × 10-6 s-1). These results indicate that the number of pyrene anchors and their position of substitution in such complexes strongly affect the desorption behavior. However, neither the free energy of adsorption (ΔGads) nor the heterogeneous electron-transfer rate (kET) showed any significant differences among 1-3, albeit that the surface morphologies of the modified HOPG substrates showed domain structures that were characteristic for each Ru complex. In the case of 3, the average height changed from ∼2 to ∼4 nm upon increasing the concentration of the solution of 3 that was used for the surface modification. In contrast, the height for 1 and 2 remained constant (1.5-2 nm) upon increasing the concentration of the complexes in the corresponding solutions. While the molecular orientation of the Ru-Ru axis of 3 relative to the HOPG surface normal changed from parallel to perpendicular, the Ru-Ru axis in 1 and 2 remained parallel, which leads to an increased stability of 1 and 2.