Theoretical Investigation on the Selective Hydroxyl Radical–Induced Decolorization of Methylene-Blue-Dyed Polymer Films

Abstract

On the basis of the decolorization caused by the reaction of active oxygen species (AOSs) with methylene blue (MB), our group recently developed colorimetric indicators for hydroxyl radical (OH radical) by embedding MB in polymer thin films made of water-soluble pullulan or sodium alginate. In the present work, to elucidate the reason for the selective decolorization induced by the OH radical compared with other AOSs, such as ozone (O3) and hydrogen peroxide (H2O2), density-functional-theory calculations were performed at the B3LYP/6-31G(d) level for these AOSs and MB and its complexes with pullulan or sodium alginate model molecules. A frontier orbital analysis revealed that the π orbital of MB tends to delocalize on the whole molecule upon complexing with pullulan and sodium alginate, while the energy level is lower than the lowest unoccupied molecular orbital levels of O3 and H2O2 but higher than the singly occupied molecular orbital level of the OH radical. The results support the observation that only the OH radical, as the strongest oxidant, can react with MB in the polymer matrices. The selective decolorization of MB-dyed polymer films by the OH radical is due to not only the steric hindrance in the polymer matrix but also the perturbation of the π orbital of MB through the interaction with the polymer molecules.