Multinuclear Metal Complex Photosensitizers Covalently-Coupled to Cobalt-based Catalysts for Hydrogen Evolution

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In the last decades, tremendous research has been focused on the conversion of solar energy into chemical energy. In this regard, photocatalytic processes play a crucial role to produce so-called “solar fuels”. Oxygen and hydrogen are two important solar fuels and can be generated by the catalytic oxidation of water and reduction of protons, respectively. The two main components of a photocatalytic device are the photosensitizer and the catalyst. Recent developments could significantly improve the performance of both components. For example, multinuclear ruthenium(II) polypyridyl complexes as photosensitizer have been employed for the photocatalytic oxygen evolution reaction to increase the sunlight absorption and they are capable to absorb lower-energetic solar light. However, multinuclear complexes are rarely described in literature for the similar hydrogen evolution reaction, usually mononuclear complexes are used. On the other hand, a new family of molecular cobalt(II) polypyridyl complexes as catalysts for hydrogen evolution has recently emerged. These catalysts proved to be more robust than other systems and display higher activities for electrocatalysis in pure water. For this reason, the research plan is a combination of these two recent developments in photocatalytic systems. To further increase efficiency of the designed systems, another aim is the covalent linkage of photosensitizer and catalyst. By this approach, the excess of photosensitizer to catalyst can be avoided and the electron-transfer process from the photosensitizer to the catalyst will be facilitated. The designed systems should possess distinctly higher photocatalytic activity in aqueous solutions than previously reported systems. Moreover, it is expected to afford an efficient photocatalytic system, which utilize a wide range of the visible to near-infrared solar spectrum. This could be a major impact for the development of new systems, because photocatalysis in aqueous solutions with low-energetic light is still challenging.