A synergistic study on the synthesis of juglone via photooxidation in a UV–Vis LED based photomicroreactor

Photochemical transformations are mobilized by traceless photons of narrow or broad visible light wavelength and have been proven as a green and sustainable synthesis route for organic compounds. However, most of such investigations were restricted to only parametric studies, lacking in examining a synergistic involvement of actinometric, kinetics and mass transfer mechanisms on product yield. To address this vital issue, we executed the synthesis of natural product juglone via the photooxidation of 1,5-dihydroxynapthalene (DHN) mediated through singlet oxygen, which was produced by triplet–triplet energy transfer from three porphyrin-based photosensitizers to triplet oxygen upon irradiation from UV–Vis LED. Compared to the other two photosensitizers, m-TcPP (meso-tetracarboxyphenylporphyrine) showed a higher molar extinction coefficient and remarkable energy transfer, resulting in higher DHN conversion and juglone yield. Full DHN conversion and 82% yield were obtained at the m-TcPP loading of 2.0 mol% and the residence time of 22.1 min. Actinometric investigation showed that the photon flux was significantly improved by rising m-TcPP and DHN concentrations. The maximum radiant power absorbed by the DHN photooxidation could reach 17.3% of the total input power of UV–Vis LED strip. The detailed reaction mechanism of the DHN photooxygenation was discussed through the Jablonski diagram and rate constants of intermediate steps were determined. A positive relationship between the photon flux and the apparent rate constant was found by rising m-TcPP concentration from 0.01 to 0.05 mM, while the rise in DHN concentration from 5.0 to 10.0 mM resulted in a negative correlation between these parameters. Consequently, a synergistic improvement of photon flux and apparent rate constant was necessitated to attain complete DHN conversion and high juglone yield.