Encapsulation of Porphyrins in Polymer Microgels and Microbeads for Heterogeneous Photooxidation

Using porphyrin-mediated photocatalytic energy transfer to generate singlet oxygen is a premier strategy for green, selective chemical synthesis. However, such catalytic systems are mostly conducted in organic solvents. This work aims to explore the generation and utilization of singlet oxygen for photochemical oxidation in aqueous media. To this end, we prepared two porphyrins with reactive primary amines or charged quaternized ammonium groups for covalent and ionic crosslinking, producing TPP-loaded hydrophilic microgels and microbeads, respectively. We explored catalyst formulations to optimize singlet oxygen generation and the direct oxidation of various substrates. Both catalysts effectively facilitate the chemical conversion of thioethers, thiols, and aldehydes. In deuterium oxide, oxidation reactions of methionine, allyl sulfide, and benzaldehyde achieved high quantitative conversion due to a longer lifetime of singlet oxygen in deuterium oxide than that in water. Notably, the oxidation of thioethers stopped selectively at the sulfoxide stage, avoiding over-oxidation to sulfones. While this method offers an easy way to isolate the catalysts in aqueous media, the high cost of deuterium oxide and slower reaction rates in water suggest that future development of nanobeads via microfluidic devices could overcome molecular oxygen diffusion limitation and enhance reaction rates in water.