Photoinduced decarboxylation of 9-oxo-6,9-dihydro[1,2,5]selenadiazolo[3,4-f]quinoline-8-carboxylic acid

Photoinduced reactions of 9‐oxo‐6,9‐dihydro[1,2,5]selenadiazolo[3,4‐f]quinoline‐8‐carboxylic acid (SeQCA) were investigated in alkaline media (aqueous NaOH solutions) by electron paramagnetic resonance (EPR) spectroscopy, following the in situ formation of paramagnetic species. According to UV–Vis and nuclear magnetic resonance investigations, protonation (pH ≈ 11) and deprotonation (pH ≈ 13) of the imino hydrogen of the 4‐pyridone moiety has to be considered, reflected also in the different EPR spectra observed upon irradiation. Photoinduced generation of radicals was found only for carboxylate substituted SeQCA; other studied selenadiazoloquinolone derivatives, together with those substituted at the C(8) position (R = H, COOCH2CH3, COOCH3, COCH3 or CN), did not generate paramagnetic species during exposure. Consequently, photodecarboxylation was suggested as the decisive step, accompanied by the decomposition of the selenadiazole ring, resulting in the formation of ortho‐hydroxylate anions. EPR parameters elucidated from experimental EPR spectra obtained at pH ≈ 11 and pH ≈ 13 indicate the formation of oxygen‐centered radicals at the decarboxylated 4‐pyridone ring. EPR spin trapping experiments with nitromethane confirmed a very effective photoinduced electron transfer from all the selenadiazoloquinolones investigated. Copyright © 2011 John Wiley & Sons, Ltd. Photoexcitation of 9‐oxo‐6,9‐dihydro[1,2,5]selenadiazolo[3,4‐f]quinoline‐8‐carboxylic acid in alkaline aqueous solutions results in the generation of two different radicals as monitored by in situ electron paramagnetic resonance spectroscopy, reflecting the protonation (pH ≈ 11) and deprotonation (pH ≈ 13) at the imino hydrogen of the 4‐pyridone moiety. The reaction mechanism proposes photodecarboxylation, conversion of selenadiazole ring in ortho‐hydroxylate substructure, as was evidenced in 17O enriched water, and formation of oxygen‐centered radicals on decarboxylated 4‐pyridone ring.