Organic photoredox catalysts: tuning the operating mechanisms in the degradation of pollutants

Photocatalysis is an emerging area of chemistry that takes advantage of light as the primary source of energy to carry out chemical transformations. In this context, organic photocatalysts appear as an alternative that has proven to be efficient in treating polluted effluents. Although organic photocatalysts are not able to generate hydroxyl radical, their photoactivated excited states generated using visible light can act as strong oxidants in most cases. In fact, pollutant photooxidation can be produced from an initial electron transfer between an excited state of an organic photocatalyst and the contaminant, generating their respective radical anion and cation (Type I mechanism). However, as most of the organic photocatalysts are able to generate singlet oxygen, pollutant degradation can also be initiated from this oxidative species (Type II mechanism). Moreover, the heterogenization of the photocatalysts seems the straightforward step to boost photostability and facilitate recovery after the reaction. In the present review, we chronicle our research progress and how interestingly, it cannot be assumed that the main reaction pathways of a photocatalyst are the same under homogeneous conditions as in heterogeneous media. Herein we have selected Rose Bengal (RB), Riboflavin (RF), and a perylene diimide derivative (PDI) to illustrate the different modes of action of these organic photocatalysts under homogeneous/heterogeneous conditions.