State of the art
- Photosensitizers are molecules that can be activated by light to generate excited states in organic molecules by energy transfer.
- Porphyrins, known as pigments of life, play a key role in energy and electron transfer processes in Nature, notably including photosynthesis, transport and storage of respiratory gases, methyl transfer, rearrangement reactions, etc.
- In chemical processes, after light absorption porphyrins are excited to the triplet state and in 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, in water splitting and few examples describing their use in C- C bond forming reactions were described.
- Porphyrins with their 18 π-electron aromatic macrocycle are perfectly suited for photosensitization because they a) absorb in a wide range of visible-light region, b) have high absorption coefficients, 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.
Scheme 5. Porphyrin-based photocatalysts for diazo compound activation
Progress beyond the state of the art
- Recently, the PAS group reported mechanistic studies on porphyrins as photocatalysts where varying the substituents and the state of metalation allows for adjusting their electrochemical potentials and changes mechanistic pathway of a model reaction (Scheme 5).
- Similarly, it is believed that substitution of the macrocycle by different functional groups or metal ion incorporation influences efficiency of intersystem crossing which is attributed to increase in generation of singlet oxygen.
- Simultaneously, we proved that chiral A4, A2B2, and AB3 porphyrins bearing chiral moieties at the meso-phenyl group are promising photosensitizers in enantioselective photooxidation.
- Comprehensive mechanistic studies on influence of porphyrin structure and state of metalation on photosensitization ability will be performed.
- Synthesis of at least two asymmetric porphyrins bearing various chiral moieties will be developed.
- Chiral porphyrins will be employed in enantioselective processes involving energy transfer including oxidations and activation of diazo reagents.
- Dual photocatalysis conditions using porphyrin sensitizers and nickel or iron co-catalysts will be explored