Role of side groups in pyridine and bipyridine ruthenium dye complexes for modulated surface photovoltage in nanoporous TiO2

The influence of molecular side groups on the electron backtransfer into ruthenium-dye molecules adsorbed on ultra-thin nanoporous TiO2 layers has been investigated by modulated surface photovoltage spectroscopy in argon atmosphere. All ruthenium-dye molecules contained one bipyridine (bpy) ligand with two carboxylic groups for anchoring at the TiO2 surface. Ruthenium (II) complexes with a second bpy ligand with branching and non-branching side groups containing C and H only and ruthenium (II) complexes with a second pyridine (py) ligand with side groups containing different numbers of –C–O–C– sequences were synthesized. The electron backtransfer was practically not affected by branching or non-branching side groups in the bpy–bpy complexes. In contrast, electron backtransfer was much less for bpy–py complexes in comparison to bpy–bpy complexes and increased strongly with increasing number of –C–O–C– sequences in the side groups.