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

We suggest that singlet molecular oxygen [O-1(2) ((1)Delta(g))] is formed upon irradiation of indigo 1 [in air or O-2-saturated DMSO and DMSO (0.5% H2SO4)l and indigo carmine 2 [in air or O-2-saturated CH3OH, D2O, and 1-butyl-3-methylimidazolium tetrafluoroborate (BmIm-BF4)]. The quantum yield for production Of 102 is estimated to be 0.6 for 1 and 0.3-0.5 for 2. The rates of reaction Of O-1(2) with 1 and 2 were determined by monitoring the emission Of O-1(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 O-1(2) photosensitizers in solution. Compounds I 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). (c) 2006 Elsevier Ltd. All rights reserved.