Heisenberg spin exchange and relaxation dynamics in EPR oximetry: A photoinduced transition from non-adiabatic rapid-sweeps to the rapid-scan regime

Rapid-scan electron paramagnetic resonance spectroscopy (RS EPR) has emerged as a powerful tool to monitor light-induced processes accompanied by changes of the EPR lineshape and is frequently used for measuring local oxygen concentrations. Here we use LiPc (lithium phthalocyanine) and CTPO (3-Carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-oxyl) as spin probes and demonstrate control over the oxygen partial pressure through a photochemical reaction. The resulting changes in pO 2 manifest as characteristic distortions of the EPR lineshape and reveal how Heisenberg spin exchange influences relaxation dynamics. As a proof-of-concept, we investigate photopolymerization at reduced oxygen levels. These results highlight the potential of RS for studying oxidation processes in biochemical systems. • The oxygen partial pressure is controlled through a photochemical reaction. • The transition between non-adiabatic rapid-sweeps and the rapid-scan regime is monitored. • The approach for regulating the pO2 allows studying the interplay between Heisenberg spin-exchange and relaxation dynamics under carefully controlled experimental conditions.