Analysis of time-resolved CW-EPR spectra of short-lived radicals at different times after laser excitation

Laser excitation combined with time-resolved CW-EPR spectroscopy of a carotenoid-porphyrin-diquinone tetrad (1), observes polarized spectra of the end quinone radical anion, Q·_B^-. These spectra strongly depend on the time interval between the laser pulse and the position of the detection window of the transient magnetization profile, and may lead to misinterpretation of the radical intermediates participating in the reaction route. Spectral distortions arise from small hyperfine splittings and narrow line widths, resulting in the appearance of Torrey wiggles at off-resonance fields. The correct spectrum, however, and its line shape appear at times above 1 μs after the laser pulse, and is in complete agreement with the Fourier transform EPR spectrum. Analysis and reconstruction of the transient spectra is carried out in Fourier conjugate space. The experimental spectra are shown to be the Fourier transform of the free-induction decay (FID) multiplied by an appropriate windowing function. The changes in the spectra taken at t<1 μs are due to line distortions and not to chemical processes. This treatment is of general applicability in the spectral analysis of time-resolved CW-EPR spectra.