Elucidating Dual Pathways for Light-Driven Degradation Using Intrinsic Azo Dye Photochemistry

In this work, the photochemical properties of Acid violet 3 (AV3), along with a sacrificial oxidant, MV²⁺, are used to promote its own degradation in aqueous solutions. Irradiation of light at 375 nm produces excited-state AV3 (*AV3) that is able to be oxidized by MV²⁺, as monitored through UV–visible spectroscopy. MV²⁺ is added in various concentrations, showing an increased degradation rate with increasing MV²⁺ concentrations. Degradation of AV3 is still observed in control experiments in which AV3 is illuminated without the presence of MV²⁺. Photodegradation experiments are also performed in deuterated water, showing a five times increased rate of degradation, providing evidence of an inverse kinetic isotope effect. Based on these results, two different degradation pathways are proposed: an energy transfer pathway and an electron transfer pathway. In the electron transfer pathway, *AV3 is oxidized by MV²⁺, which produces MV^•+. MV^•+ interacts with dissolved oxygen to produce reactive oxygen species, likely superoxide radicals (O₂^•−), that are highly reactive and further attack AV3 until it is degraded. In the energy transfer pathway, *AV3 populates a triplet state that is energetically able to sensitize triplet oxygen (³O₂) to singlet oxygen (¹O₂), which can break down AV3.