TiO2-Assisted Photodegradation of Dyes. 9. Photooxidation of a Squarylium Cyanine Dye in Aqueous Dispersions under Visible Light Irradiation

Abstract

The TiO 2 -assisted photodegradation of a squarylium cyanine dye (SQ) has been examined under visible light irradiation (λ ≥ 430 nm) by UV−vis, proton-NMR, ESR, and GC-MS spectroscopies, by peroxide assays in the presence of peroxidase and catalase enzymes, and by chemical oxygen demand (COD) methods. Significant results were obtained relevant to the mechanism(s) of TiO 2 -assisted photodegradations. The active oxygen species produced first when an aqueous TiO 2 dispersion containing SQ is irradiated by visible light is the superoxide radical anion, which is stable in methanol solvent but very unstable in aqueous media, and is readily converted to • OH radicals via formation and subsequent reduction of H 2 O 2 . The quantity of H 2 O 2 increased during the photodegradation; though expected, no organoperoxides were detected. Of import, cleavage of the cyanine C C double bond in the SQ dye dominated over the whole degradation process yielding 1-sulfopropyl-3,3-dimethyl-5-bromoindolenium-2-one, the predominant component in the dispersion. This intermediate is not excited by visible light and degrades no further. A sequence of steps is proposed in the initiation of the TiO 2 -assisted photochemical transformation.