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) TiO 2 . 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 TiO 2 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 TiO 2 materials and are insensitive to an ∼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.