Photocatalytic hydrogen evolution with ruthenium polypyridine sensitizers: unveiling the key factors to improve efficiencies

Photochemical hydrogen evolution studies aimed at evaluating new molecular catalysts have usually exploited Ru(bpy)(3)(2+) (where bpy = 2,2'-bipyridine) as the reference photosensitizer, thanks to its suitable optical and redox properties. In principle, an additional improvement of the photocatalytic performances can be achieved also by a careful adjustment of the photophysical and/or electrochemical characteristics of the ruthenium-based sensitizer. Herein we describe homogeneous molecular systems for photocatalytic hydrogen evolution composed of a series of ruthenium polypyridine complexes as the photosensitizers (Ru1-4), a cobaloxime catalyst, and ascorbic acid as the sacrificial electron donor. Suitable functionalizations of the 4,4' positions of bipyridine ligands have been addressed in order to modify the redox properties of the chromophores rather than their optical ones. A careful and detailed kinetic characterization of the relevant processes at the basis of hydrogen evolving photocatalysis has been addressed to rationalize the observed behavior. The results show that the ruthenium complex involving two 2,2'-bipyridines and one 4,4'-dimethyl-2,2'-bipyridine (Ru2) may outperform the standard Ru(bpy)(3)(2+) (Ru1), combining the right balance of structural and redox properties, thus posing as an alternative benchmark photosensitizer for the study of new hydrogen evolving catalysts.