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

The TiO2-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 TiO2-assisted photodegradations. The active oxygen species produced first when an aqueous TiO2 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 H2O2. The quantity of H2O2 increased during the photodegradation; though expected, no organoperoxides were detected. Of import, cleavage of the cyanine CC 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 TiO2-assisted photochemical transformation.