It is established that the electronic character, position and size of substituents have a direct influence on the kinetics of formation, regiochemistry and stability of aromatic EPOs. In particular, considering a concerted mechanism of the [4+2] reaction between the aromatic substrates and the 1 O2 generated in situ, their readiness to bind and release O2 can be directly related to the dissociation energy (Ediss) calculated using the density functional theory (DFT). The computed Ediss of the EPOs of NABA and PNBA resulted to be -3.49 kcal•mol -1 and -4.31 kcal•mol -1 , respectively, while in the case of PABA and ANBA it was found to be 10.44 kcal•mol -1 and 12.16 kcal•mol -1 , respectively (Figure , Table ). The negative values obtained for NABA and PNBA confirmed the lack of any stabilizing effect observed experimentally as the formation of the O-O bridge could not be detected. The opposite behaviour of PABA and ANBA with large positive Ediss values is in line with the straightforward formation of EPOs in solution and, above all, their high stability. In fact, only heating at 90°C for several hours afforded the clean release of O2 from PABA-O2 with similar performances in organic solvent and water, upon complexation with a polymeric matrix. for correlating the numbers to the structures).