Excited state electron transfer to nanocrystalline semiconductor thin films

Several factors that affect the efficiency for excited state electron transfer from ruthenium compounds to nanocrystalline (anatase) TiO 2 films have been explored in this dissertation work. Chapter 1 provides a background and results from selected studies. In Chapters 2 and 4, three ruthenium ammine compounds, Ru(NH3)5(eina)(PF6 )2 (1), Ru(NH3)4(deeb)(PF 6)2 (2) and Ru(en)2(deeb)(PF 6)2 (3), where eina is 4-CO2Et-pyridine, deeb is 4,4'-(CO2Et)2-2,2'-bipyridine and en is NH 2(CH2)2NH2, were synthesized and attached to optically transparent TiO2 thin films. The metal-to-ligand charge transfer (MLCT) absorptions were observed to shift in wavelength with solvent. All three compounds were found to be non-emissive with excited state lifetimes less than 10 ns under all conditions studied. Electron transfer from all three compounds to TiO2 films occurred with rate constants, kinj > 108 s-1. Electron recombination kinetics from TiO2 to the oxidized compounds were independent of the sensitizer identity, in spite of a ∼ 600 mV change in apparent driving force. Recombination required milliseconds to complete and was well modeled by a second-order equal-concentration kinetic model.; The injection quantum yields were determined by comparative actinometry and were found to be excitation wavelength dependent, consistent with injection from vibrationally "hot" excited states. Sluggish iodide oxidation resulted in inefficient incident-photon-to-current conversion efficiency (IPCE) in regenerative solar cells based on 1. The photocurrent action spectra of 2 and 3 provided evidence that injection from hot excited states also occurs in operational regenerative solar cells.; The injection quantum yields were found to be dependent on the deuteration of the ammine ligands in Chapter 3. A ∼ 30 % increase in injection quantum yield was measured after ammine deuteration. In contrast, the RuIII/II reduction potentials and the charge recombination rate constants were insensitive to deuteration. Control experiments with Ru(deeb)(bpy) 2(PF6)2 indicated that deuteration of the TiO 2 surface alone did not affect the injection or recombination processes. A model was proposed wherein electron injection occurred in competition with vibrational relaxation and/or intersystem crossing of the excited states. Exchange of hydrogen by deuterium, slowed vibrational relaxation and/or intersystem crossing resulting in higher injection quantum yields.; In Chapters 5 and 6, nine ruthenium polypyridyl compounds with a series of substituted phenanthroline and bipyridine ligands were synthesized and bound to TiO2 thin films. These compounds are of interest because they provide systematic variations in ground state reduction potentials as well as excited state energetics and dynamics. The results show that modest changes in ligand substitution may have dramatic effects on the molecules' photophysical properties and solar cell performances. (Abstract shortened by UMI.)...