Bio-inspired Multielectron Transfer Photosensitizers

The search for efficient multielectron transfer reagents is a topic of prime interest in the fields of homogeneous catalysis, biomimentic chemistry and solar energy conversion with a very large impact ranging from environmentally benign proceses to potential fuel cell applications. Despite the fact, that these molecules are key- components in several frontier research fields, there is currently a lack of suitable catalysts. New, more stable and efficient redox mediators, which are capable of performing this task are therefore highly desirable compounds. This project deals with the systematic development of novel light-sensitive coordination compounds, which are capable of catalyzing overall multielectron transfer substrate transformations. The photocatalytic reactions to be studied will be triggered or driven by visible-light, and will be strongly inspired by the redox chemistry of natural catalytic systems. An important task of this project is the construction of functional copies of biological substrate transformation process based on synthetic low-molecular-weight catalysts. Therefore, this work has the potential to settle a foundation for more elaborate applications of multielectron transfer sensitizers in the fields of green chemistry, artificial enzyme catalysis and solar fuel production from renewable and abundant raw materials.

The coupled transfer of multiple electrons and protons plays a crucial role for numerous biological and chemical processes. In the course of evolution, the essential processes of life such as biocatalysis and photosynthesis have been optimized and refined to a perfect level. In contrast, any less refined reaction pathway involving free radical intermediates (instable compounds with unpaired electrons) requires much higher activation energies and also leads to uncontrolled side processes. This usually will result in reduced efficiency, selectivity and unsatisfactory long-term stability of such systems. Even though the central role of controlled multielectron transfer processes for natural energy- conversion and substrate-transformation reactions had been recognized for a long time, the strategies for photoinduced and catalytic accumulation and transfer of more than one electron in molecular systems have hardly been studied systematically up to now. Following promising new concepts developed in our own working group, the FWF project Bioinspired Multielectron Transfer Photosensitizers was therefore launched to provide novel materials, light-harvesting dyes and tailor-made catalyst-systems for driving clean and efficient substrate conversion reactions as observed in the corresponding natural processes. In particular, photochemical redox reactions powered by irradiation with visible light or direct utilization of sunlight were investigated in order to provide the sustainable synthesis and accumulation of permanent products. A crucial motivation of these studies was to develop and fully characterize synthetic light-driven model compounds (artificial enzymes) for mimicking the functional properties of native biocatalysts such as metalloenzymes of the oxidoreductase-type. Following our novel approach, it could be successfully demonstrated in this project that even very complex biocatalytic functions can be fully replaced step-by-step with suitable multielectron transfer photosensitizers. These findings open many new possibilities in photocatalysis, which certainly will have strong implications on the fields of sustainable chemistry and solar energy conversion. Within the framework of the research project, we already could achieve a coupling of several different substrate conversion processes in a one-pot reaction cascade. This allows to drive and fully control a biomimetic light-mediated sequence of either two-electron or four-electron oxidation reactions of organic compounds with excellent product selectivity. Based on some of the research results obtained this FWF- project, in the mean time we furthermore were able to realize an unprecedented example of a fully operating functional counterpart of a natural photosynthetic reaction center (PS I) for artificial photosynthetic fuel production with very promising stability and efficiency.