Time-resolved near infrared studies on singlet oxygen production by the photosensitizing 2-arylpropionic acids

Time-resolved near infrared emission studies have been carried out on a series of nonsteroidal anti inflammatory 2-arylpropionic acids, in order to assess their ability to photosensitize the production of singlet molecular oxygen O₂(¹Δ(g)). Benoxaprofen, naproxen, tiaprofenic acid, suprafen, ketoprofen and carprofen showed quantum yields for this process (Φ(Δ)) in the range 0.2-0.7 in acetonitrile. Deprotonation of the propionic acid moiety in mixtures of acetonitrile-ethanol (4:1) basified with KOH produced a systematic decrease in Φ(Δ) for all compounds with the exception of naproxen. This effect was even more dramatic in D₂O-based phosphate buffer, where no singlet oxygen could be detected for benoxaprofen, ketoprofen or carprofen. The above results are consistent with the occurrence of competing photochemical reactions of 2-arylpropionic acids in different media and particularly with the enhanced photodecarboxylation from the carboxylate anions.