Interfacial Electron Transfer in Fe^(II)(CN)_6^(4-)−Sensitized TiO_2 Nanoparticles: A Study of Direct Charge Injection by Electroabsorption Spectroscopy

Abstract

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 TiO_2 nanoparticle. The average charge-transfer distance determined from the Stark spectra is 5.3 Å. 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)_5Fe^(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(Δα), between the ground and the excited states of the Fe^(II)(CN)_6^(4-)−TiO_2(particle) system is ∼3 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.

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