Interfacial electron transfer in FeII(CN)64--sensitized TiO2 nanoparticles:: A study of direct charge injection by electroabsorption spectroscopy

Electroabsorption (Stark) spectroscopy has been used to study the dye sensitized interfacial electron transfer in an Fe-II(CN)(6)(4-) donor complex bound to a TiO2 nanoparticle. The average charge-transfer distance determined from the Stark spectra is 5.3 Angstrom. This value is similar to the estimated distance between the Fe-II center of the complex and the Ti-IV surface site coordinated to the nitrogen end of a bridging CN ligand in (CN)(5)Fe-II-CN-Ti-IV(particle). This finding suggests that the electron injection is to either an individual titanium surface site or a small number of Ti centers localized around the point of ferrocyanide coordination to the particle and not into a conduction band orbital delocalized over the nanoparticle. The polarizability change, Tr(Deltaalpha), between the ground and the excited states of the Fe-II(CN)(6)(4-) -TiO2(particle) system is similar to3 time larger than normally observed in mixed-valence dinuclear metal complexes. It is proposed that the large polarizability of the excited state increases the dipole-moment changes measured by Stark spectroscopy.