Cathodic and Photocatalytic Reduction of Nitroquinolones Investigated by In Situ EPR/UV-Vis Spectroelectrochemistry and EPR spectroscopy

The generation of paramagnetic intermediates upon photoinduced reduction of substituted nitroquinolones 1–6 in dimethylsulfoxide/methanol titania suspensions was investigated by in situ EPR spectroscopy. The assignment of the primary photogenerated paramagnetic signals was based on the results of cyclic voltammetry, amperostatic in situ spectroelectrochemistry and in situ EPR/UV-Vis spectroelectrochemistry in aprotic dimethylsulfoxide and dimethylsulfoxide/methanol mixed solvent. The primary reduction step in the cathodically- or in the photocatalytically-induced electron transfer process represents the formation of radical monoanion, the stability of which is crucially influenced by the 1-ethyl substitution at the nitrogen of the 4-pyridone ring of quinolone. 1-Ethyl 6-nitroquinolones typically form stable radical anions with well-resolved EPR spectra, with detailed interpretation of hyperfine coupling constants (hfcc) supported by theoretical calculations. On the other hand, the radical anions of nitroquinolones with amino hydrogen at nitrogen of the enaminone system (N–C=C–C=O) convert rapidly to diamagnetic s-dimer dianions, reduced in the second reversible reduction step to paramagnetic s-dimer radical trianions. The EPR spectra obtained upon prolonged irradiation of 1-ethyl nitroquinolones in titania suspensions were assigned to the R-NO•H intermediates produced via nitro group reduction. Experiments with deuterated methanol unambiguously confirmed the photoinduced reduction of the nitro group, including the interaction with hydrogen from the hydroxyl group of methanol. The generation of reactive radicals formed via methanol and dimethylsulfoxide oxidation in irradiated titania suspensions was investigated by EPR spin trapping technique. TiO2.