The reaction monitoring could be achieved by UV-Vis absorption spectroscopy following the disappearing of the absorption bands of the aromatic substrates upon formation of the EPOs (Figure ). Since the formation of the O-O bridge breaks the aromaticity of the central ring of the anthracenes, their characteristic absorption bands in the region between 325 and 450 nm dicraese proportionally to the reaction extent. Upon exclusive, quantitative formation of the target EPOs, the final absorption spectra showed only the profile of the PS above 300 nm.