Electroactive Coatings of Tris(bipyridyl)‐ and Tris(o‐phenanthroline)‐Ruthenium(II) Attached to Electrodes via Hydrosilylation or Electropolymerization of Vinyl Derivatives

Synthetic methods are given for the introduction of a vinyl group into αα‐bipyridine (bipy) and o‐phenanthroline (phen). These groups have been employed to attach and 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 with contact time, after an induction period, and also increased with increasing temperature. coatings prepared in this way showed reversible cyclic voltammograms up to about 20 layers. For thicker films the electrode kinetics became increasingly slow, and the amount of electroactive material actually decreased. Silanized coatings showed sluggish behavior even 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 Ru 2+ produced immediate loss of electroactivity. Holding the electrode at a potential sufficiently positive to produce Ru 3+ led to a slow loss of electroactivity, while continuous scanning of the wave led to much faster losses (time scale of minutes), the rate increasing with scan rate. A still more rapid loss was observed 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 the silanized film is permeable to counterions and solution redox species, but is also fragile, and susceptible to disruption by the ion fluxes. Electropolymerization gave a coating which was much more stable, showing very little loss upon continuous cycling. However the cathodic peak, but not the anodic peak, gradually became less reversible. A slow conformation change resulting in inhibited counterion efflux from the film upon reduction is suggested. When electropolymerized at a electrode, the film, in contact with an aqueous solution containing hydroquinone as mediator, supports a steady photocurrent which is stable for at least 15 hr.