The photocatalytic reduction of water to form hydrogen gas is a promising approach to collect, convert, and store solar energy. Typically, ruthenium tris-bipyridine and its numerous derivatives are used as photosensitizers (PSs) in a variety of photocatalytic conditions. The bis-terpyridine analogs, however, have only recently gained attention for this application due to their poor photophysical properties. Yet, by introducing electron donating or withdrawing groups on the terpyridine ligands, the photophysical and electrochemical properties can be improved significantly.
In this study, a series of 2,6-di(pyridin-2-yl)-pyrimidine ligands with peripheral pyridine substituents has been prepared and used to prepare ruthenium(II) complexes. The presence of the pyrimidine ring stabilizes the lowest unoccupied molecular orbital (LUMO), leading to a red-shifted absorption and emission and prolonged excited-state lifetimes as well as higher luminescence quantum yields compared to analogous terpyridine complexes. Furthermore, all complexes are easier to reduce than previously reported bis-terpyridine complexes used as PS. An interesting correlation between substitution pattern and properties of the complexes was observed and further investigated using TD-DFT. In hydrogen evolution experiments under blue and red light irradiation, all investigated complexes exhibit a much higher activity compared to previously reported ruthenium(II) bis-terpyridine complexes but none of the complexes is as stable as the literature compounds, presumably due to an additional decomposition pathway of the reduced PS competing with the electron transfer from the reduced PS to the catalyst.