Optimization and modification of molecular systems for light-driven water oxidation

Abstract

Climate change undoubtedly is among the greatest challenges for humanity in the 21st century. In order to overcome this challenge the switch from fossile fuels to sustainable energy sources is inevitable. This change however should happen in a way that provides easy and fair access to energy for everyone. Among the potential sustainable energy sources, the energy provided by the sun is by far the most abundant around the globe. In order to harness the power of the sun in a convenient way, storage of solar energy is key. Storage in chemical bonds by forming so called solar fuels, presents a option with great potential in overcoming this difficulty. In nature this approach is taken in the form of natural photosynthesis by cyanobacteria, algea and green plants. During this process water, carbon dioxide and light serve as abundant feedstock to generate the energy dense compounds adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH), which in turn are used for the synthesis of higher hydrocarbons like sugars. Inspired by this blueprint, artificial photosynthesis aims to mimic this strategy, mainly by splitting water into molecular hydrogen and oxygen via photocatalytic reactions. The initial oxidation of water is widely regarded as particularly challanging, as it involves four proton-coupled electron transfer (PCET) steps, the formation of an O-O bond and the hash conditions that might lead to degradation of the catalytic system. In this thesis, a general introduction of the fundamental mechanisms of natural photosynthesis and how these translate to concepts in artifical photosynthesis is given. Since photophysical processes are integral part in photocatalysis, photophysical fundamentals important for the development of photocatalysts are outlined, mostly using the example of ruthenium polypyridine complexes. Since the present work is concerned with the investigation of molecular systems for the light-driven oxidation of water, the historic development and fundamental aspects of such systems are discussed in detail on landmark literature examples. As water oxidation is such an important step in artificial photosynthesis, it is important to reliably quantify oxygen evolution. To this end, the implementation of an in-operando measurement strategy, which relies on oxygen sensitive optical sensor spots, is discussed and used to optimize a literature known system, resulting in an increase of oxygen yield by an factor of approximately 25. This method is also used in flow-through reactor setup in order to get insights into the impact of technical parameters (e.g. light intensity and flow-rate) on the performance of the catalytic system. In doing so, the catalytic performance could be further enhanced by optimizing the technical operating conditions, not only with regards to turnover numbers (TON, 10-fold increade) but also with regards to external energetic and photonic efficiency (21-fold and 24-fold increase respectively). These investigations also give first insights into the complex interplay between processes on the molecular and macroscopic level. In addition, structural modifications of a photocatalyst and photosensitizer are described from the synthetic approach and structural characterization to photophysical and electrochemical investigations, ending in preliminary photocatalytic experiments if applicable. Since understanding of structure-property correlations are essential for a knowledge driven development of photocatalytic systems, the changes that are caused by the modifications are studied in detail and compared to their respective parent compounds. At the end, a final conclusion of the general results is drawn and a short outlook is given on how the our understanding of photocatalytic processes could be further increased by using in-operando methods. Further, it is outlined how the discussed modified systems might be used in the further development of photocatalytic systems and how they might provide building blocks in future applications.