Kinetic analysis of nitroxide radical formation under oxygenated photolysis: toward quantitative singlet oxygen topology

Abstract

Reaction kinetics for two sterically hindered secondary amines with singlet oxygen have been studied in detail. A water soluble porphyrin sensitizer, 5,10,15,20-tetrakis-(4-sulfunatophenyl)-21,23H-porphyrin (TPPS), was irradiated in oxygenated aqueous solutions containing either 2,2,6,6-tetramethylpiperidin-4-one (TMPD) or 4-[N,N,N-trimethyl-ammonium]-2,2,6,6-tetramethylpiperidinyl chloride (N-TMPCl). The resulting sensitization reaction produced singlet oxygen in high yield, ultimately leading to the formation of the corresponding nitroxide free radicals (R2NO) which were detected using steady-state electron paramagnetic resonance (EPR) spectroscopy. Careful actinometry and EPR calibration curves, coupled with a detailed kinetic analysis, led to a simple and compact expression relating the nitroxide quantum yield ΦR2NO (from the doubly-integrated EPR signal intensity) to the initial amine concentration [R2NH]i. With all other parameters held constant, a plot of ΦR2NOvs. [R2NH]i gave a straight line with a slope proportional to the rate constant for nitroxide formation, kR2NO. This establishment of a rigorous quantitative relationship between the EPR signal and the rate constant provides a mechanism for quantifying singlet oxygen production as a function of its topology in heterogeneous media. Implications for in vivo assessment of singlet oxygen topology are briefly discussed.

Do We Appropriately Detect and Understand Singlet Oxygen Possibly Generated in Advanced Oxidation Processes by Electron Paramagnetic Resonance Spectroscopy?

“…Moreover, it is notable that the formation amount of 4-OH-TEMPO in the UV/H 2 O 2 process (conducted at pH i = 9.0, Figure S8) is much higher than the Fe­(II)/H 2 O 2 process (conducted at pH i = 3.0, Figure a) under identical reactant dosage. This is because the hindered amine probe is more prone to be oxidized under alkaline conditions compared to acid conditions. , …”

“…This is because the hindered amine probe is more prone to be oxidized under alkaline conditions compared to acid conditions. 46,47 It should be noted that the triplet-line shape collected using 4-oxo-TEMP was different from that using 4-OH-TEMP in terms of linewidth (ΔH pp ) and nitrogen hyperfine coupling constant (α N ). The ΔH pp and α N of 4-OH-TEMPO and 4-oxo-TEMPO were determined to be 1.5 G and 17.3 G, 0.8 G and 16.1 G (Figure 1a,b), respectively, which are in line with those of the standard samples (Figure S10) and the reported values in literature.…”

CADRE-SS, an in Silico Tool for Predicting Skin Sensitization Potential Based on Modeling of Molecular Interactions

“…All three are tetramethylpiperidines, which can autoxidize to form stable nitroxide radicals; nitroxides can undergo 1e/2e reductions to hydroxylamines. 42 Both nitroxides and hydroxylamines can bind skin protein residues. 43 Lastly, octanethiol (compound 12) is flagged as a precursor for nucleophilic substitution.…”

“…Compounds 21 , 22 , and 82 (Figure ) are flagged as hydroxylamine precursors. All three are tetramethyl­piperidines, which can autoxidize to form stable nitroxide radicals; nitroxides can undergo 1e/2e reductions to hydroxylamines . Both nitroxides and hydroxylamines can bind skin protein residues .…”

Photoinduced Charge Transfer: From Photography to Solar Energy

“…The most useful is probably the ferrioxalate actinometer. 25,26 Photolysis of ferrioxalate, [Fe(III)(ox)3 3-], in solution generates ferrous ions in a high quantum yield 27 by a ligand to metal charge transfer (LMCT) transition. The amount of Fe(II) produced can be measured spectrophotometrically using phenanthroline (phen) which gives an intensely coloured red Fe(II)(phen)3 2+ complex.…”