Electroactive coatings of tris(bipyridyl)- and tris(o-phenanthroline)-ruthenium(II) attached to electrodes via hydrosilylation or electropolymerization of vinyl derivatives

Abstract

Synthetic methods are given for the introduction of a vinyl group into αα-bipyridine (bipy) and o-phenanthroline (phen). These groups have beenemployed to attach Ru(bipy)₃²⁺ and Ru(phen)₃²⁺ to Pt electrodes by (i)hydrosilylation followed by polycondensation with the electrode, or (ii) cycling the electrode to negative potentials to induce anionic polymerization at the surface. The thickness of silanized films increased linearly withcontact time, after an induction period, and also increased with increasingtemperature. Ru(bipy)₃²⁺ coatings prepared in this way showed reversiblecyclic voltammograms up to about 20 layers. For thicker films the electrodekinetics became increasingly slow, and the amount of electroactive materialactually decreased. Silanized Ru(phen)₃²⁺ coatings showed sluggish behavioreven at lower coverages. The films were stable upon anaerobic storage (exposure to air reduced stability), but showed electrochemical instabilities.Scanning to potentials sufficiently negative to reduce Ru²⁺ produced immediate loss of electroactivity. Holding the electrode at a potential sufficiently positive to produce Ru³⁺ led to a slow loss of electroactivity, whilecontinuous scanning of the Ru^{-3+ /2+} wave led to much faster losses (time scaleof minutes), the rate increasing with scan rate. A still more rapid loss wasobserved after scanning the redox wave of ferrocene dissolved in the electrolyte, although the amplitude of the ferrocene wave was nearly undiminished with respect to that observed at an uncoated electrode. Thus thesilanized film is permeable to counterions and solution redox species, but isalso fragile, and susceptible to disruption by the ion fluxes. Electropolymerization gave a Ru(phen)₃²⁺ coating which was much more stable, showing verylittle loss upon continuous cycling. However the cathodic peak, but not theanodic peak, gradually became less reversible. A slow conformation changeresulting in inhibited counterion efflux from the film upon reduction is suggested. When electropolymerized at a TiO₂ electrode, the Ru(phen)₃²⁺ film,in contact with an aqueous solution containing hydroquinone as mediator,supports a steady photocurrent which is stable for at least 15 hr.