EPR study of photochemical transformations of triarylmethane dyes

The production of reactive radical species upon irradiation (lambda > 300 nm) of six triarylmethane dyes, i.e. C.I. Basic Red 9 (42500; Basic Fuchsin), C.I. Basic Violet 3 (42555; Crystal Violet), C.I. Basic Green 4 (42000; Malachite Green Oxalate), C.I. Acid Green 16 (44025; Acid Green V), C.I. Basic Blue 11 (44040; Victoria Blue R), C.I. Acid Blue 93 (42780; Aniline Blue), as well as a phenothiazine dye C.I. Basic Blue 9 (52015; Methylene Blue) and a xanthene dye C.I. Acid Red 87 (45380; Eosin Y) in different solvents (water, ethanol, dimethylsulphoxide), and subsequently on paper substrate was investigated by means of EPR spectroscopy. The EPR experiments using spin trapping agents (5,5-dimethyl-1-pyrroline-N-oxide, DMPO and 2,3,5,6-tetramethyl nitrosobenzene, ND) showed evidence of the free radical formation in both deoxygenated and aerated systems during photoexcitation. The photooxidation processes in presence of oxygen resulted in the electron transfer generating super-oxide anion radicals (Type I process), simultaneously with energy transfer producing singlet oxygen (Type 11 process) confirmed by 2,2,6,6-tetramethyl-4-piperidinol (TMP). (C) 2003 Elsevier Ltd. All rights reserved.