Photodegradation mechanism of the common non-steroid anti-inflammatory drug diclofenac and its carbazole photoproduct.

Diclofenac (DF) is a widely used non-steroid anti-inflammatory drug, associated with a range of side effects. The phototoxicity of DF is studied herein employing computational quantum chemistry at the B3LYP/6-31G(d,p) level of theory. The results show that the drug readily absorbs radiation from the UV-region. The deprotonated form spontaneously dechlorinates from its triplet state leading to ring closure and formation of an active photoproduct: chlorocarbazole acetic acid, CCA. The formed CCA is also photodegraded easily from its deprotonated triplet state. Photodegradation routes of deprotonated CCA are decarboxylation (barrier less than 4.5 kcal mol(-1)) and dechlorination (barrier around 6.2 kcal mol(-1)). The energy barrier required for dechlorination to take place from the neutral from is about 20 kcal mol(-1). The differences between the molecular orbitals of the neutral and the deprotonated forms of DF and CCA and spectra obtained using time-dependent density-functional theory (TD-DFT), in addition to the different radical and oxygenated intermediate species formed during the photodegradation mechanism, are discussed in more detail. The theoretical results obtained herein are in line with the experimental results available to date.