The addition to polycyclic aromatic precursors of singlet oxygen ( 1 O2) photochemically generated in situ upon irradiation of the reaction mixture containing a sub-stoichiometric amount of a photosensitizer (PS), is a most efficient way to form EPOs. This photooxygenation reaction is straightforward and typically results in final products bearing the target intramolecular O-O bridge. Selection and positioning of the substituents on the aromatic cores are crucial for the overall ability to selectively bind and release O2.