Photophysics of a novel Ru-4H-imidazole sensitizer for TiO2-based DSSCs

Ruthenium(II) dyes which coordinate a 4H-imidazole ligand (see figure) show intense absorption over a wide range of the visible spectrum, which is one prerequisite for potential applications in artificial light-harvesting systems. The present study investigates the impact of introducing an anchoring group (COO-) to the 4H-imidazole-ligand on the excited-state dynamics of the dye in solution. Particular emphasis is put on the effect of the anchoring group on the excited-state planarization – a process that was previously found to dominate the photophysics associated with the 4H-imidazole ligand. Aside from solution studies we report on the photophysics of the dye attached to a TiO2 surface. In this situation photoinduced electron injection into the conduction band of the semiconductor occurs from thermally non-relaxed metal-to-ligand charge transfer (3MLCT) states. Electron injection from the subsequently populated thermalized 3MLCT state of the dye does not take place, which limits the overall injection efficiency.