Excited-State Electron Transfer from Ruthenium-Polypyridyl Compounds to Anatase TiO2 Nanocrystallites: Evidence for a Stark Effect

Abstract

Photophysical studies were performed with [Ru(dtb)(2)(dcb)](PF(6))(2) and cis-Ru(dcb)(dnb)(NCS)(2,) where dtb is 4,4'-(C(CH(3))(3))(2)-2,2'-bipyridine, dcb is 4,4'-(COOH)(2)-2,2'-bipyridine, and dnb is 4,4'-(CH(3)(CH(2))(8))(2)-2,2'-bipyridine), anchored to anatase TiO(2) particles (∼15 nm in diameter) interconnected in a mesoporous, thin film (∼10 μm thick) immersed in Li(+)-containing acetonitrile electrolytes. Pulsed-laser excitation resulted in rapid, nonquantitative excited-state injection into TiO(2) with a rate constant that could not be time-resolved, k(inj) > 10(8) s(-1), to yield an interfacial charge-separated state. Return of this state to ground-state products displayed observation-wavelength-dependent kinetics due to charge recombination and a second process. The second process occurred in parallel and was assigned to a transient Stark effect created by the electric field originating from the electrons in TiO(2) on ruthenium sensitizers that had not undergone excited-state injection. The kinetics for this processes were well modeled by a stretched exponential function. The impact of this field on the metal-to-ligand charge transfer excited-state of Ru(dtb)(2)(dcb)(2+) or the oxidized form of cis-Ru(dcb)(dnb)(NCS)(2) were also investigated. Unambiguous identification of a Stark effect on the excited-state sensitizers was accomplished through fluence-dependent measurements. The possible influence of the electric field on the oxidized sensitizers was at best speculative. The unique relative orientation of the electric field and sensitizer afforded by the nanocrystal geometry resulted in unidirectional shifts in the absorption and photoluminescence spectra of the Ru(II) coordination compounds. On the basis of the magnitude of the shift, it was estimated that a transient field as large as 2.7 MV/cm was generated upon excited-state injection of electrons in TiO(2) at concentrations relevant to an operational dye-sensitized solar cell.