Diffusion-Limited Interfacial Electron Transfer with Large Apparent Driving Forces

Abstract

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 I2 acetonitrile electrolyte, the Re(I) compounds convert absorbed photons into electrons efficiently. The rate of interfacial charge separation could not be time resolved, kcr > 108 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 ∼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.