Multinuclear Metal Complex Photosensitizers Covalently-Coupled to Cobalt-based Catalysts for Hydrogen Evolution
Inorganic Molecular Chemistry - Synthesis and Characterisation
Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Final Report Abstract
Globally accessible, inexhaustible sunlight is one of the few alternatives that could serve as source for energy supply in the future. In the last decades, tremendous research has been focused on the conversion of solar energy into electricity (i.e., photovoltaics) and chemical energy (i.e., photocatalysis). Among various photocatalytic systems, the reduction of protons into hydrogen by sunlight is extensively investigated. In this study molecular systems made of photosensitizers (PS) and catalysts (Cat) for the photocatalytic hydrogen evolution reaction are investigated. A crucial process in such systems is the light driven electron transfer (ET) between the PS and the Cat. Both compounds have to come in close proximity to enable an efficient ET process. Usually, a high concentration of the photosensitizer is used to increase more efficient close contact with the Cat. In this research covalent-connections between the PS and Cat were incorporated to prevent the molecules from drawing apart by diffusion. By this approach less amount of the PS is used and the concentration can stay as low as possible. PSs based on bipyridine (bpy) ruthenium(II) complexes and Cats based on pentapyridine (PY5) cobalt(II)-based were prepared in a modular synthetic route. Surprisingly, the nonfunctionalized bpy/PY5 unit in a nonconnected PS/Cat system resulted in the highest amount of hydrogen formed. It was found that the type functionalization on both moieties is important for their redox potentials and the driving force for the ET process. Since the connected PS-Cat systems cannot compete with the separate, nonfunctionalized system, a variety functionalized bpy/PY5 units for a nonconnected PS/Cat systems were prepared. It could be shown that almost the double amount of hydrogen is formed in the connected PS-Cat system when compared with these compounds as a reference system. Another part of this study was the development of multinuclear metal complexes as PS for enhanced light absorption especially for the lower energetic sunlight. It was possible to synthesize trinuclear ruthenium(II) complexes and introduce functional groups for a potential covalent bonding. UV-vis absorption spectra revealed visible absorption up to 800 nm and a more than doubled molar absorptivity compared to the mononuclear complex. Future studies will show whether this new type of PS can compete with the high photoactivity of the mononuclear complex and if a covalent-connection facilitates the ET process.
Publications
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Wissenschaftsforum Chemie 2019 (GDCh) Aachen, Germany, 2019/09/15 − 2019/09/18. “A Ruthenium(II) Complex Photosensitizer Covalently-Coupled to a Cobalt(II)-based Catalyst for Hydrogen Evolution”
Kevin Barthelmes
