Abstract
The photophysical properties and photobleaching of the xanthene dye merbromin (MR), and the production and quenching of singlet molecular oxygen, O 2( 1Δ g), were investigated in aqueous solutions, in comparison with the xanthene dyes eosin (Eos) and rose bengal (RB). MR photobleaches in the absence of O 2 in a process involving the solvent (H 2O or alcohols). No participation of O 2· − or O 2( 1Δ g) could be established in the presence of O 2, although O 2 was consumed. In aqueous and alcoholic solutions, MR forms ground state, charge transfer (CT) complexes with biologically relevant electron-donor amino acids, such as histidine and tryptophan, and other indolic derivatives. These complexes are formed with relatively high association constants (3×10 3−5×10 3) and are mainly responsible for the photobleaching of MR and the amino acids in these solutions by a mechanism involving O 2( 1Δ g) generated probably by an excited state of the CT complex. MR generates O 2( 1Δ g) with a relatively high yield (Φ Δ=0.23±0.04) and quenches O 2( 1Δ g) only physically with k q=(2.3±0.05)×10 8 M −1 s −1. This process does not affect the photosensitizing ability of MR under the low concentrations needed for its action. Non-donor compounds such as the amino acid methionine and linoleic acid methyl ester are photodegraded by a type II mechanism without implication of a dye CT complex.
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