Self-Assembled Monolayers of Parent and Derivatized

Synthesis of the terminally disubstituted rigid rod molecules, [n]staffane-3,3("')-dithiols, n = 1-4, and their singly functionalized derivatives carrying an acetyl or a pentaammineruthenium(II), [ R u ( N H ~ ) ~ ] ~ + , substituent is described. Self-assembled monolayers of the neat [nlstaffane derivatives and of their mixtures with n-alkyl thiols were prepared on polycrystalline gold electrodes. Grazing incidence FTIR spectroscopy and comparison with polarized IR spectra of model compounds oriented in stretched polyethylene revealed that, in the films assembled directly from the dithiols, the average orientation of the [nlstaffane rods was halfway between perpendicular and parallel to the surface, whereas, in the films obtained from the singly functionalized derivatives, the rods were nearly perpendicular to the surface. The degree of ordering and packing was also investigated by contact angle measurements with deionized water. The electrode blocking properties of the films were investigated by three independent electrochemical techniques, cyclic voltammetry in a solution containing K,Fe(CN),, chronoamperometry in a solution of RU(NH,)~CI~, and cyclic voltammetry of the gold surface in sulfuric acid. The results from the three methods were in good agreement and showed that the films assembled from the dithiols were not as well organized as those obtained from the monoderivatized analogues. In situ hydrolytic removal of the protective acetyl groups and subsequent functionalization of the resultant film covered with free thiol groups with aquapentaammineruthenium(II), [RU(NH~)~(H~O)]*+, are possible without disturbing the structure of the film. Electrodes covered with monolayers carrying ruthenium pentaammine groups on their outer Surface exhibit oxidation-reduction waves at 0.51 V vs SCE attributable to the Ru(I1)-Ru(II1) couple, demonstrating electron transfer across the monolayer. The formal potential of the immobilized couple is about 60" more positive than that of the dissolved complex, [ [(NH3),Ru]-[n]staffanedithiol]2+, in 1 M HClO, solution. The surface concentration of Ru is the same in the films assembled from this complex and those produced by in situ functionalization of a film covered with free thiol groups and corresponds to a monolayer of (NH3)SRu. However, in the former type of fdm, the surface concentration of the staffane rods is only about half of that in the latter, implying that only about half of the surface thiol groups have been metalated.