Spectroscopy and Dynamics of Phosphonate-Derivatized Ruthenium Complexes on TiO₂

Knowledge of electronic structures and transport mechanisms in dye-sensitized semiconductors is motivated by their ubiquity in photoelectrochemical cells. In this work, optical spectroscopies are used to uncover the elementary dynamics initiated by light absorption at such molecule–semiconductor interfaces (e.g., electron transfer and nuclear relaxation). These processes are explored in a family of ruthenium bipyridyl complexes in aqueous solutions, wherein phosphonate groups are used to bind the molecules to TiO₂ nanocrystalline films. The complexes differ in (i) the number of phosphonate groups and (ii) the presence (or absence) of a methylene bridge between the molecule and the TiO₂ surface. A resonance Raman intensity analysis suggests that the electronic excitations possess very little charge transfer character for all complexes. That is, the electronic orbitals involved in light absorption are essentially localized to the molecules. Because the electronic resonances are molecular in character, the photophysics are most appropriately viewed as sequences in which light absorption precedes electron transfer. Transient absorption measurements conducted on the dye-sensitized films show that electron injection processes initiating directly from the photoexcited singlet states of the molecules occur in 100 fs or less. In contrast, the electron transfer rates slow down by at least a factor of 10 when intersystem crossing in the molecule precedes electron injection into TiO₂. For ruthenium complexes linked to TiO₂ with methylene bridges, intersystem crossing is more efficient than singlet electron injection because of attenuated molecule–TiO₂ couplings; electron transfer primarily initiates in triplet states for these systems. Overall, the fundamental connections drawn in this work between molecular structure and photophysical behavior contribute to the general understanding of photoelectrochemical cells based on related molecule–semiconductor systems