Strongly coupled ruthenium-polypyridyl complexes for efficient electron injection in dye-sensitized semiconductor nanoparticles

Dynamics of interfacial electron transfer (ET) in the ruthenium-polypyridyl complex [{bis(2,2'-bpy)-(4-[2(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol)} ruthenium(II) hexafluorophosphate] (Ru-cat)-sensitized TiO2 nanoparticles has been investigated using femtosecond transient absorption spectroscopy detecting in the visible and near-infrared region. It has been observed that Ru-cat is coupled strongly with the TiO2 narroparticles through its pendant catechol moiety. Electron injection has been confirmed by direct detection of electrons in the conduction band, cation radical of the adsorbed dye, and a bleach of the dye in real time as monitored by transient absorption spectroscopy. A single-exponential and pulse width limited (< 100 fs) electron injection has been observed, and the origin of it might have been from the nonthermalized excited states of the Ru-cat molecule. The result gave a strong indication that the electron injection competes with the thermalization of the photoexcited states due to large coupling elements for the forward ET reaction. Back-ET dynamics has been determined by monitoring the decay kinetics of the cation radical and injected electron and also from recovery kinetics of the bleach of the adsorbed dye. It has been fit with a multiexponential function, where similar to 30% of the injected electrons are recombined with a time constant of < 2 ps, again indicating large coupling elements for the charge recombination reaction. However, our results have shown relatively long-lived charge separation in the Ru-cat/TiO2 system as compared to other organic dye-sensitized TiO2 nanoparticles with similar interactions.