The influence of the medium on the quantum yields of singlet oxygen (O2(1Δ(g))) production (Φ(Δ)) by a series of photosensitizing nonsteroidal anti-inflammatory drugs (NSAID) derived from 2-arylpropionic acid (APA) has been investigated. Four-component oil-in-water and water-in-oil microemulsions, based on anionic and cationic surfactants, have been employed as the simplest models to mimic more complex biological environments. Φ(Δ) values have been determined by monitoring the singlet oxygen (1O2) luminescence at 1270 nm upon continuous excitation of the drugs under air-equilibrated conditions. Results indicate that Φ(Δ) values are highly affected by the medium, being higher in microheterogeneous systems than in (homogeneous) solution. Some of the anti-inflammatory derivatives are very efficient1O2sensitizers: e.g., values of apparent Φ(Δ) as high as 0.86 (± 0.04) and 0.70 (± 0.03) have been found for tiaprofenic acid and suprofen, respectively. The location of the drugs in the interfacial region of the microemulsions combined with their high Φ(Δ) values suggest that type II reactions may play a significant role in the overall photodynamic process in more complex organized media, such as biological membranes.
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