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

Singlet oxygen ( 1 O 2 ), 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 1 O 2 with sterically hindered amines and indirectly determining the concentrations of 1 O 2 . However, the characteristic 1:1:1 triplet EPR signal of 1 O 2 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 1 O 2 in redox systems. Additionally, the relatively low water solubility of TEMP may cause underestimation of 1 O 2 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 ( 1 O 2 ) in aqueous environment.