Exchange experiments utilizing Ru(bpy) 2 (4,4‘-(CO 2 H) 2 bpy) 2+ and Os(bpy) 2 (4,4‘-(CO 2 H) 2 bpy) 2+ (4,4‘-(CO 2 H) 2 bpy is 4,4‘-dicarboxy-2,2‘-bipyridine) provide evidence for a distribution of binding sites on ITO electrodes by electrochemical monitoring and on optically transparent, nanoparticle TiO 2 films by optical monitoring. Using the Albery model of dispersed kinetics for the analysis of exchange, the results are consistent with dissociative loss from the surface with k 1 = 0.1 s - 1 for the dissociative step in CH 3 CN on ITO. Equilibrium binding data after exchange suggest that a site or distribution of sites is also present that undergoes exchange on a very slow time scale. Complete exchange takes place in EtOH, and the rate is greatly enhanced compared to that in CH 3 CN, suggesting that EtOH labilizes the surface to exchange. On TiO 2 the time scale for exchange is far longer than that on ITO, and in CH 3 CN it scales with film thickness.
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Trammell et al. (2003) studied this question.