Rose Bengal@MSNs as heterogeneous photocatalysts in photooxygenation reactions

The functionalization of mesoporous silica nanoparticles (MSNs) with photosensitizers e.g. Rose Bengal (RB) has attracted great research attention during last years to design efficient nanoplatforms to be used in nanomedicine as drug delivery systems or in targeted photodynamic cancer therapy (PDT) owing to their biocompatibility, high loading capacity and ease of surface functionalization. Additionally, its use as catalyst support in photooxygenation reactions via singlet oxygen (1O2) can produce high-value added organic molecules in continuous-flow microreactors decreasing the environmental impact due to the possibility to recover the solid catalyst from the reactor effluent and reutilize the photosensitizer. Various model organic substrates were tested as previously reported for homogeneous photooxygenation reactions including bio-sourced methionine [1] and α-terpinene as well as 2-furoic acid, triphenylphosphine, citronellol and cyclopentadiene. The immobilization of the dye inside mesoporous silica nanoparticles plays a strategic role in preserving the chemical integrity of the photosensitizer as demonstrated by UV-Vis spectroscopy. The mechanism of photooxygenation by 1O2 could be influenced by the state of the dye, avoiding the ground state complexation between the amino acid and the photosensitizer as occurs in homogeneous systems. Moreover, heterogeneous photooxygenation of several substrates in order to obtain high-value added organic molecules. High conversions were achieved with methionine, α-terpinene in 72 s and triphenylphosphine in 300 s. However, the heterogeneous 1O2 photooxygenation of cyclopentadiene into cis-2-cyclopentadiene-1,4-diol provided a complete conversion in 51 s of residence time. Our results show that heterogenization of PS for 1O2 production in microreactors is possible by grafting RB into MSNs by a covalent bond. This attachment is able to avoid the photobleaching of the dye and allows separating the solid support with the PS from the reactor effluent and reutilizing the PS in different cycles. The potential of these NPs to be used as heterogeneous catalyst for photooxygenation of several organic substrates has been demonstrated.