Interfacial electron transfer in metal cyanide-sensitized TiO2 nanoparticles

Electroabsorption (Stark) spectroscopy has been used to study the charge-transfer absorption from a transition-metal-cyanide complex to a TiO2 nanoparticle. Transition-metal cyanide/TiO2(particle) systems were synthesized using Fe-II(CN)(6)(4-), Ru-II(CN)(6)(4-), Mo-IV(CN)(8)(4-), and W-IV(CN)(8)(4-). On formation of the M(CN)(n)(4-)/TiO2(particle) system, a new metal-to-particle charge-transfer (MPCT) absorption band is observed in the 390-480 nm region. Analysis of the absorption spectra suggests that the TiO2 level involved in the MPCT transition resides at significantly higher energy than the bottom of the conduction band and that the electronic coupling between the two metal centers is the dominant factor determining the position of the MPCT band maximum. The average charge-transfer distances determined by Stark spectra range from 4.1-4.7 A. The observation of relatively short charge-transfer distances leads to the conclusion that the MPCT absorption is from the transition-metal cyanide center to a level that is localized on the Ti atom bound to a nitrogen end of the [O2Ti-N-C-M(CN)(x)] system. The electronic coupling, H-ab, calculated for a two state model is similar to values observed in dinuclear metal complexes.