Tuning photosensitized singlet oxygen production from microgels synthesized by polymerization in aqueous dispersed media

Abstract

Novel sub-micronic photoactive polymer colloids grafted with Rose Bengal (RB) photosensitizer were designed to promote singlet oxygen production from a supported organic photosensitizer. Photooxygenation of fine chemicals under visible light irradiation is considered as a green process. To enhance the overall process sustainability, stable colloidal particles were synthesized by polymerization in aqueous dispersed media with the ability to be transferred into ethanol, recycled by a centrifugation step and reused with no significant decrease of the quantum yield of singlet oxygen production. The microgels were synthesized for the first time by miniemulsion copolymerization of vinyl acetate (VAc), N-vinyl caprolactam (VCL), polymerizable vinyl benzyl Rose Bengal (VBRB) monomers and divinyl adipate (DVA) crosslinker. The microgels were characterized by UV-visible spectroscopy and compared with the homologue non-crosslinked polymer in order to discriminate the effect of RB grafted onto the linear polymer from its grafting inside crosslinked microgels. The quantum yields of singlet oxygen production were almost null in water but interestingly in the range of 0.27–0.47 in ethanol. The singlet oxygen quantum yield of these polymer materials is tuned by the aggregation state of VBRB units, hence producing an ON/OFF photosensitizing colloidal system. The absorption and emission spectra of the VBRB containing microgels in water were characteristic of strongly aggregated VBRB, while no evidence of aggregation was observed from the spectra in ethanol. The highest singlet oxygen quantum yield of the linear polymer was correlated with a less aggregated state of RB units compared with the crosslinked microgels. The present RB-based microgels were 20% more resistant to photobleaching than free RB.