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Quantum chemical calculations have been used to model dye-sensitized nanostructured titanium dioxide systems that can be used in solar cells for solar energy to electricity conversion. Structural, electronic and spectral properties of isolated dyes and both bare and dye-sensitized TiO₂ have been calculated with density functional theory, providing detailed information about both the separate parts and the dye-TiO₂ interface.

The connection between the geometry, the ligand field splitting and the lifetime of the triplet metal-to-ligand charge transfer (MLCT) excited state has been explored for a series of ruthenium polypyridyl dyes. Moreover, the relative energetics of MLCT and metal centered triplet excited states have been studied for a number of such systems. It was found that small alterations of the polypyridyl ligands can result in significant changes in ligand field splitting and in the energetics of the triplet states.

Attachment of the dyes to the TiO₂ surface is achieved via anchor and spacer groups. The influence of such groups on various properties of the dye and their ability to act as mediators of photo-induced surface electron transfer has been studied. Delocalization of the lowest unoccupied dye orbital onto the spacer and/or anchor group indicates that certain unsaturated groups can mediate electron transfer.

With a combination of methods that enables efficient computations and a scheme for construction of metal oxide clusters, chemical models for bare TiO₂ nanocrystals in the 1-2 nm size range have been developed. The electronic structures show well-developed band structures with essentially no electronic band gap defect states.

Atomistic models of the interface between TiO₂ nanocrystals and Ru(II)-bis-terpyridine dyes, the so-called N3 dye as well as perylene dyes are reported. Electronic coupling strengths, which provide estimates for the electron injection times, are extracted from the interfacial electronic structure and the lowest electronic excitations are calculated.

Interfaces between semiconductors and adsorbed molecules form a central area of research in surface science, occurring in many different contexts. One such application is the so-called Dye-Sensitized Solar Cell (DSSC) where the nanostructured dye-semiconductor interface is of special interest, as this is where the most important ultrafast electron transfer process takes place. In this thesis, structural and electronic aspects of these interfaces have been studied theoretically using quantum chemical computations applied to realistic dye-semiconductor systems. Periodic boundary conditions and large cluster models have been employed together with hybrid HF-DFT functionals in the modeling of nanostructured titanium dioxide.

A study of the adsorption of a pyridine molecule via phosphonic and carboxylic acid anchor groups to an anatase (101) surface showed that the choice of anchor group affects the strength of the bindings as well as the electronic interaction at the dye-TiO₂ interface. The calculated interfacial electronic coupling was found to be stronger for carboxylic acid than for phosphonic acid, while phosphonic acid binds significantly stronger than carboxylic acid to the TiO₂ surface.

Atomistic and electronic structure of realistic dye-semiconductor interfaces were reported for Ru^II-bis-terpyridine dyes on a large anatase TiO₂ cluster and perylene dyes on a periodic rutile (110) TiO₂ surface. The results show strong influence of anchor and inserted spacer groups on adsorption and electronic properties. Also in these cases, the phosphonic acid anchor group was found to bind the dyes significantly stronger to the surface than the carboxylic acid anchor, while the interfacial electronic coupling was stronger for the carboxylic anchor. The estimated electron injection times were twice as fast for the carboxylic anchor compared to the phosphonic anchor. Moreover, the electronic coupling was affected by the choice of spacer group, where unsaturated spacer groups were found to mediate electron transfer more efficiently than saturated ones.