Electron Paramagnetic Resonance Detection of Singlet Oxygen: Opportunities and Pitfalls of Sterically Hindered Amine...

Singlet oxygen (¹O₂), a nonradical reactive oxygen species, has shown great potential for driving redox reactions. Electron paramagnetic resonance (EPR) spectroscopy has become a widely utilized tool for identifying the adducts formed by ¹O₂ with sterically hindered amines and indirectly determining the concentrations of ¹O₂. However, the characteristic 1:1:1 triplet EPR signal of ¹O₂ adduct can arise from other molecules and impurities. In this work, we demonstrate the presence of N-oxyl impurities in commercial 2,2,6,6-tetramethylpiperidine (TEMP) derivatives can lead to overestimation of ¹O₂ in redox systems. Additionally, the relatively low water solubility of TEMP may cause underestimation of ¹O₂ in aqueous media, while more soluble 4-substituted derivatives (4-amino, 4-oxo, and 4-hydroxy TEMP) undergo untargeted oxidation, forming products besides their corresponding N-oxyl derivatives. This study proposes vacuum distillation of TEMP to minimize paramagnetic impurities and recommend the combined use of EPR and mass spectrometry (MS) for accurate identification of N-oxyl adducts, particularly for 4-substituted TEMP in aqueous media. Our findings on the solubility and oxidation behavior of TEMP derivatives provide an improved and robust detection and quantification strategy of singlet oxygen (¹O₂) in aqueous environment.