Nanocrystalline (anatase), mesoporous TiO
2 thin films were derivatized with [Ru(bpy)
2(deebq)](PF
6)
2 or [Os(bpy)
2(deebq)](PF
6)
2, where bpy is 2,2'-bipyridine and deebq is 4,4'-diethylester-2,2'-biquinoline. Both compounds bind to the nanocrystalline TiO
2 films with typical limiting surface coverages of 7 (±2) × 10
-8 mol/cm
2. Electrochemical
measurements show that the first reduction of these compounds (-0.60 V vs SCE) occurs prior to TiO
2 reduction. Steady-
state illumination in the presence of the sacrificial electron donor triethylamine leads to the appearance of the reduced compound, M
II(deebq
-)(bpy)
2+/TiO
2. Neither the photoluminescent excited
states or the reduced forms of these compounds inject electrons efficiently into TiO
2. Transient absorption
measurements after a ~10-ns laser pulse, reveal greater than 80% MLCT excited
states and a smaller fraction of extremely long-lived charge-separated
state intermediates assigned to equal concentrations of M
II(deebq
-)(bpy)
2+/TiO
2 and M
III(deebq)(bpy)
23+/TiO
2. The results are consistent with a mechanism of ultrafast electron injection followed by ligand-localized trapping on a second compound. The quantum yield for formation of the charge-separated
states (
CSS) is excitation wavelength dependent. With 417 nm excitation,
CSS(417) = 0.14 ± 0.03, and this decreases with 532.5 nm excitation,
CSS(532.5) = 0.08 ± 0.03, and 683 nm excitation for M = Os,
CSS(683) = 0.05 ± 0.01. Electron transfer to yield ground-
state products, M
II(deebq
-)(bpy)
2+/TiO
2 + M
III(deebq)(bpy)
23+/TiO
2 2 M
II(deebq)(bpy)
22+/TiO
2, occurs with a driving force of 2.05 eV for Ru/TiO
2 and 1.64 eV for Os/TiO
2. The dynamics of this process were quantified on a millisecond time scale and were found to follow second-order kinetics. The intermediates are sufficiently long-lived that continued pulsed excitation at 10 Hz leads to high concentrations and the formation of transient images on the semiconductor surface that are easily observed by the naked
eye.