Diffusion-limited interfacial electron transfer with large apparent driving forces

The Re(I) coordination compounds fac-Re(deeb)(CO)(3)(X), where deeb is 4,4'-(COOEt)(2)-2,2'-bipyridine and X is I-, Br-, Cl-, or CN-, and [fac-Re(deeb)(CO)(3)(py)](OTf), where OTf- is triflate anion and py is pyridine, have been prepared, characterized, and anchored to nanocrystalline (anatase) TiO2. In regenerative solar cells with 0.5 M LiI-0.005 M I-2 acetonitrile electrolyte, the Re(I) compounds convert absorbed photons into electrons efficiently. The rate of interfacial charge separation could not be time resolved, k(cr) > 10(8) s(-1). Thermodynamically favorable recombination of the injected electron in TiO2 with the oxidized sensitizer requires milliseconds for completion. Charge recombination kinetics have been quantified on a 10(-7)-s and longer time scale and are insensitive to the Re sensitizer employed. The charge recombination kinetics have been contrasted with other sensitized TiO2 materials and are insensitive to an similar to 960-mV change in apparent driving force. The results suggest that charge recombination is rate limited by diffusional encounters of the injected electron with the oxidized sensitizer.