Visible light plays a vital role in Nature, making it a cutting edge discipline in 21st century Science. The use of solar radiation as an energy source in chemical industry could be a response to worldwide issues such as energy shortage and environmental pollution. Solar light is not only free but it is an inexhaustible energy source. Therefore, implementation into green technologies should eliminate or at least reduce the use and generation of hazardous substances. The following project focuses on the development of new organic, photoredox catalysts. I envisaged that porphyrins with their 18 pi-electron aromatic macrocycle are perfectly suited for this role because they a) absorb visible-light, b) have high absorption coefficient, c) exhibit a small singlet-triplet splitting, d) have high quantum yield for intersystem crossing, e) and possess longer lifetime of the triplet state in comparison to the singlet state, not to mention straightforward synthesis. After light absorption porphyrins are excited to the triplet state and at this state they are able to transfer energy (photosensitization) or electrons (photoredox catalysis). These properties have been broadly used in the generation of singlet oxygen, conversion of solar energy, and in water splitting but there are only scarce examples describing their use in C-C bond forming reactions through the porphyrin ring oxidation and reduction to ion radicals. Therefore, the main objective of this proposal is to establish a solid background for porphyrin’s photoredox catalysts especially in C-C bond forming reactions. In the first instance the influence of the porphyrin structure, reaction conditions and light on a model reaction (arylation of heteroarenes) will be developed (Scheme 1). Subsequently, the methodology will be extended to other radical reactions - a useful tool in organic synthesis. Importantly a basic scientific knowledge about photoredox catalysis and porphyrins, in general, will be further increased.

The use of a porphyrin as a photocatalyst will be fully investigated in an arbitrarily selected, model photoredox arylation of heteroarenes (Scheme 1). First, the mechanism of the reaction will be elaborated via, Scheme 1. Model reaction to be studied an in-depth analysis of all aspects of the reaction. These include synthesis of different porphyrins of determination electrochemical properties in excited state- in collaboration with prof. Karl Kadish from the University of Houston, Department of Chemistry, EPR, NMR and MS studies. Following a full breakdown of the mechanism of the model reaction, the established methodology will serve as a background for the development of other radical C-C bond forming reactions such as arylation of aldehydes in order to further confirm the suitability of porphyrins as catalysts for dual-photoredox- and organocatalysis. The combined work will substantially expand our knowledge of photoredox catalysis and will open new opportunities for the application of porphyrins in organic synthesis. During my studies, it is planned to follow the reactions’ progress by chromatographic (GC, HPLC, HPLC/MS) and spectroscopic methods (1H, 13C NMR). Each new compound will be fully characterized using well-established techniques available at the Institute of Organic Chemistry, 1H and 13C nuclear magnetic resonance techniques including 2D techniques (COSY, HSQC, HMBC, ROESY) if required for full interpretation, mass spectrometry (HR-MS, DFT techniques), UV-Vis spectroscopy, elemental analysis.

Implementation of this research proposal will provide useful methodology for photocatalytic functionalization of organic compounds which, in due course, will be used to complement existing methodologies. Furthermore, new reactivity of porphyrins will open opportunities for discoveries of new radical C-C bond forming reactions. Thus it will be not only of scientific importance but of broader interest. Such green methodologies should and will in time be implemented by chemical industry.