Possible singlet oxygen generation from the photolysis of indigo dyes in methanol, DMSO, water, and ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate

Abstract

We suggest that singlet molecular oxygen [ 1O 2 ( 1Δ g)] is formed upon irradiation of indigo 1 [in air or O 2-saturated DMSO and DMSO (0.5% H 2SO 4)] and indigo carmine 2 [in air or O 2-saturated CH 3OH, D 2O, and 1-butyl-3-methylimidazolium tetrafluoroborate (BmIm–BF 4)]. The quantum yield for production of 1O 2 is estimated to be 0.6 for 1 and 0.3–0.5 for 2. The rates of reaction of 1O 2 with 1 and 2 were determined by monitoring the emission of 1O 2 at 1270 nm over time. Low molar absorptivities (at 532 nm) and rapid physical quenching caused by 1 and 2 limit their utility as 1O 2 photosensitizers in solution. Compounds 1 and 2 degrade slowly during the photolysis due to a self-sensitized (type I or II) photooxidation reaction. Oxidative cleavage of 1 by singlet oxygen and superoxide, and 2 by superoxide has been noted before (Kuramoto, N.; Kitao, T. J. Soc. Dyers Color. 1979, 95, 257–261; Kettle, A. J.; Clark, B. M.; Winterbourn, C. C. J. Biol. Chem. 2004, 279, 18521–18525).