Electron injection from the transition metal complex Ru(dcbpy)(2)(NCS)(2) (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) into a titanium dioxide nanocrystalline film occurs on the femto- and picosecond time scales. Here we show that the dominating part of the electron transfer proceeds extremely rapidly from the initially populated, vibronically nonthermalized, singlet excited state, prior to electronic and nuclear relaxation of the molecule. The results are especially relevant to the understanding and design of molecular-based photovoltaic devices and artificial photosynthetic assemblies.

Time-resolved absorption spectroscopy was used to study the femtosecond and picosecond time scale electron injection from the excited singlet and triplet states of Ru(dcbpy) 2 (NCS) 2 (RuN3) into titanium dioxide (TiO 2 ) nanocrystalline particle film in acetonitrile. The fastest resolved time constant of ∼30 fs was shown to reflect a sum of two parallel ultrafast processes, nonergodic electron transfer (ET) from the initially excited singlet state of RuN3 to the conduction band of TiO 2 and intersystem crossing (ISC). The branching ratio of 1.5 between the two competing processes gives rate constants of 1/50 fs -1 for ET and 1/75 fs -1 for ISC. Following the ultrafast processes, a minor part of the electron injection (40%) occurs from the thermalized triplet state of RuN3 on the picosecond time scale. The kinetics of this slower phase of electron injection is nonexponential and can be fitted with time constants ranging from ∼1 to ∼60 ps.

Electron injection from the transition metal complex Ru(dcbpy) 2 (NCS) 2 (dcbpy ) 2,2′-bipyridine-4,4′dicarboxylate) into a titanium dioxide nanoparticle film occurs along two pathways. The dominating part of the electron injection proceeds from the initially excited singlet state of the sensitizer into the conduction band of the semiconductor on the sub-hundred-femtosecond time scale. The slower part of the injection occurs from the thermalized triplet excited state on the picosecond time scale in a nonexponential fashion, as was shown in a previous study et al. J. Am. Chem. Soc. 2002, 124, 489). Here we show that the slower channel of injection is the result of the excited state being localized on a ligand of the sensitizer that is not attached to the semiconductor; hence, the electron cannot be injected directly from such an excited state into the semiconductor. Before being injected, it has to be transferred from the non-surface-attached ligand to the attached one. The results show that the interligand electron-transfer time is on the picosecond time scale, depends on the relative energies of the two ligands, and controls the electron injection from the excited triplet state of the sensitizer. The findings provide information relevant to the design of molecularbased assemblies and devices.

In most of the previous ultrafast electron injection studies of Ru(dcbpy)2(NCS)2-sensitized nanocrystalline TiO2 films, experimental conditions and sample preparation have been different from study to study and no studies of how the differences affect the observed dynamics have been reported. In the present paper, we have investigated the influence of such modifications. Pump photon density, environment of the sensitized film (solvent and air), and parameters of the film preparation (crystallinity and quality of the film) were varied in a systematic way and the obtained dynamics were compared to that of a well-defined reference sample: Ru(dcbpy)2(NCS)2-TiO2 in acetonitrile. In some cases, the induced changes in the dynamics were uncorrelated to the electron injection process. High pump photon density (not in the linear response region) and exposure of the sensitized film to air altered the picosecond-time-scale kinetics considerably, and the changes were attributed mostly to degradation of the dye. In other cases, changes in the measured kinetics were related to the electron injection processes: reducing the firing temperature of the nanocrystalline film or making the film via electron beam evaporation (EBE) resulted in a decrease of the overall crystallinity of the film, and the electron injection slowed. In the sensitized EBE films, in addition to an increased contribution of triplet excited-state electron injection, a new electron transfer (ET) process with a time constant of 200 fs was observed.

Two different Li4Ti5O12 materials were investigated: smaller primary particle size forming large secondary particle aggregates (LTO-SP, surface area 22 m 2 /g) and larger primary particle size with less secondary particle aggregates (LTO-LP, surface area 7 m 2 /g). Both samples were synthesized using the same high temperature solid state synthesis but different end processing, resulting in same crystalline structure but different particle morphology. At 0.1C measured discharge capacities were close to the theoretical capacity of Li4Ti5O12 (175 mAh/g) and similar capacities were obtained at low C-rates and room temperature for both LTO-SP and LTO-LP. However, higher capacities were obtained with LTO-SP at high C-rates and -20 °C indicating beneficial effect of small particle size and large surface area. Shapes of the charge/discharge curves were different for LTO-SP and LTO-LP and this is attributed to the large surface area of LTO-SP which affects the electrochemical performance because of different reaction potentials at surface sites vs. bulk.

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