Adsorption dynamics and electrochemical and photophysical properties of thiolated ruthenium 2,2'-bypyridine monolayers

A new complex [Ru(bpy)₂(bpySH)](PF₆)₂, RuBpySH, has been prepared bearing two anchoring groups for surface attachment, where bpy is 2,2‘-bipyridyl and bpySH is 5,5‘-bis(mercaptomethyl)-2,2‘-bipyridine. Monolayers of RuBpySH have been formed on micro and macro platinum electrodes by spontaneous adsorption from micromolar solutions of the complex in 50:50 v/v water/acetone. The monolayers can be reversibly switched between the Ru²⁺ and the Ru³⁺ forms. Cyclic voltammetry is well-defined with a peak-to-peak splitting of 30 ± 5 mV and a full width at half-maximum of 110 ± 10 mV being observed for scan rates up to 5 V s^-1 where the supporting electrolyte is 0.1 M tetrabutylammonium tetrafluoroborate in acetonitrile. Adsorption is irreversible in this system, and the saturation coverage obtained is 8.1 ± 0.4 × 10^-11 mol cm^-2 when the complex concentration in the deposition solution is between 10 μM and 1.0 mM. The dynamics of adsorption depend markedly on the bulk concentration and are described in terms of irreversible adsorption. Dry monolayers display luminescence properties similar to those of powder samples of the complex, indicating that the monolayer has characteristics of the solid-state sample rather than the solution sample of the complex. Significantly, efficient electrochemiluminescence is generated using tripropylamine as the coreactant. The rate of electron transfer across the electrode/monolayer interface has been probed using high scan rate cyclic voltammetry. The standard heterogeneous electron-transfer rate constant, k°, is 0.9 ± 0.1 × 10⁴ s^-1, and there is weak adsorbate−electrode electronic communication.