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

The widespread use of azo dyes in the textile industry and the difficulty in mineralizingthem under environmental conditions makes azo dyes a persistent pollution problem. This work reports the photodegradation of acid violet 3 in which the intrinsic azo dye photochemistry is harnessed to promote degradation. Two photodegradation pathways are proposed: an energy transfer and an electron transfer pathway.

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.

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