248955-02-4Relevant articles and documents
Toward exceeding the Shockley-Queisser limit: Photoinduced interfacial charge transfer processes that store energy in excess of the equilibrated excited state
Hoertz, Paul G.,Staniszewski, Aaron,Marton, Andras,Higgins, Gerard T.,Incarvito, Christopher D.,Rheingold, Arnold L.,Meyer, Gerald J.
, p. 8234 - 8245 (2006)
Nanocrystalline (anatase), mesoporous TiO2 thin films were functionalized with [Ru(bpy)2(deebq)]-(PF6)2, [Ru(bq)2(deeb)](PF6)2, [Ru(deebq) 2(bpy)](PF6)2, [Ru(bpy)(deebq)(NCS) 2], or [Os(bpy)2(deebq)](PF6)2, where bpy is 2,2′-bipyridine, bq is 2,2′-biquinoline, and deeb and deebq are 4,4′-diethylester derivatives. These compounds bind to the nanocrystalline TiO2 films in their carboxylate forms with limiting surface coverages of 8 (± 2) × 10-8 mol/cm2. Electrochemical measurements show that the first reduction of these compounds (-0.70 V vs SCE) occurs prior to TiO2 reduction. Steady state illumination in the presence of the sacrificial electron donor triethylamine leads to the appearance of the reduced sensitizer. The thermally equilibrated metal-to-ligand charge-transfer excited state and the reduced form of these compounds do not inject electrons into TiO2. Nanosecond transient absorption measurements demonstrate the formation of an extremely long-lived charge separated state based on equal concentrations of the reduced and oxidized compounds. The results are consistent with a mechanism of ultrafast excited-state injection into TiO2 followed by interfacial electron transfer to a ground-state compound. The quantum yield for this process was found to increase with excitation energy, a behavior attributed to stronger overlap between the excited sensitizer and the semiconductor acceptor states. For example, the quantum yields for [Os(bpy)2(dcbq)]/TiO2 were φ(417 nm) = 0.18 ± 0.02, φ(532.5 nm) = 0.08 ± 0.02, and φ(683 nm) = 0.05 ± 0.01. Electron transfer to yield ground-state products occurs by lateral intermolecular charge transfer. The driving force for charge recombination was in excess of that stored in the photoluminescent excited state. Chronoabsorption measurements indicate that ligand-based intermolecular electron transfer was an order of magnitude faster than metal-centered intermolecular hole transfer. Charge recombination was quantified with the Kohlrausch-Williams-Watts model.
