Mechanistic study of the TiO2-assisted photodegradation of squarylium cyanine dye in methanolic suspensions exposed to visible light

Details of the degradation pathway(s) of squarylium cyanine (SQ) dye in methanolic TiO2 dispersions under visible light irradiation were probed in an attempt to more clearly understand the photodegradation mechanism of dye under visible radiation. For SQ, degradation begins by cleavage of the C=C double bond to yield ultimately the final product 1-sulfopropyl-3,3-dimethyl-5-bromoindolenium-2-one. A considerable amount of H2O2 is also formed during degradation through reduction (but not through dismutation) of the superoxide radical anion. In comparison with the previous results in aqueous media, it is evidenced for the first time that the pathway for the TiO2-assisted photodegradation of SQ using visible light irradiation is initiated by a series of complex reactions involving the dye cation radical SQ(.+) with dissolved dioxygen and does not implicate any of the photogenerated active oxygen species such as (OH)-O-., O-2(.-) and (OOH)-O-. radicals. These results clarify the previous equivocal conceptions on the details of the reaction between dye cationic radical and oxygen species.