Charge separation and recombination in dye-sensitized nanocrystalline titanium dioxide films

Nanocrystalline (anatase), mesoporous TiO₂ thin films were derivatized with [Ru(bpy)₂(deebq)](PF₆)₂ or [Os(bpy)₂(deebq)](PF₆)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/cm2. Electrochemical measurements show that the first reduction of these compounds (−0.60 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 compound, [MII(deebq−)(bpy) ₂]+/TiO₂. Neither the photoluminescent excited states nor the reduced forms of these compounds inject electrons efficiently into TiO₂.; The process of ligand-localized trapping was more extensively studied by employing the above Ru and Os compounds along with three other compounds, [Ru(bq)₂deeb]2+, [Ru(bpy)(deebq)₂] 2+, and [Ru(bpy)(deebq)(NCS)₂]. First, “hot” injection from upper vibrational MLCT excited states was probed using nanosecond transient absorption spectroscopy. The excitation wavelength dependence of the MIII/M+ quantum yield was further studied using comparative actinometry and the results are consistent with previous quantum yield determinations.; Four tripodal sensitizers, Ru(bpy)₂(Ad-tripod-phen) 2+ (1), Ru(bpy)₂(Ad-tripod-bpy)2+ (2), Ru(bpy)₂(C-tripod-phen)2+ ( 3), and Ru(bpy)₂(C-tripod-bpy)2+ ( 4) (where bpy is 2,2′-bipyridine, phen is 1,10-phenanthroline, and Ad-tripod-bpy (phen) and C-tripod-bpy (phen) are tripod-shaped bpy (phen) ligands based on 1,3,5,7-tetraphenyladamantane and tetraphenylmethane, respectively), have been synthesized and characterized.; The above tripodal compounds were further studied along with longer versions and a new rigid-rod series of Ru(II) compounds. The quantum yields for injection were determined using comparative actinometry for the compounds on pH = 1 pretreated TiO₂ and pH = 11 pretreated TiO₂ in the presence of lithium cations (pH = 11/Li+). The yields were larger on the acidic surfaces. A fraction of the electron injection process could be time resolved (k_inj ∼ 107 s −1) for the tripodal/rigid-rod compounds under pH = 11/Li + conditions. The absorbed photon-to-current efficiencies (APCE) for the tripodal/rigid-rod compounds were observed to be consistently lower than that for the model compound and the well-studied N3 dye, [Ru(dcb)2(NCS) ₂].; Two new organic rigid-rod linkers were used to couple pyrene, dimethyl 5-(1-pyrenylethynyl)isophthalate and dimethyl 5-(4-(1-pyrenylethynyl)phenylethynyl)isophthalate, to the surface of TiO₂ (anatase) and ZrO₂ nanoparticle thin films. The rigid-rods increase the extinction coefficient and shift to the red the long wavelength absorbance of the pyrene chromophore.; The growth of ZnO nanoparticles from zinc acetate in propanol proceeds by coarsening. The driving force for this process is the increase in equilibrium solubility with decreasing particle size, described by the Gibbs-Thompson equation. We show that the growth rate, and hence particle size, can be controlled by injection of capping ligands that adsorb onto the particle surface. Both thiols and phosphonic acids are shown to slow the growth rate of ZnO na...