Fourier Transform‐EPR Spectroscopy of Electron Transfer from Excited State of Chlorophyll and Porphyrin to Duroquinone

Electron transfer reactions between the photoexcited triplets of several porphyrin and chlorophyll precursors, 3P (electron donors), and neutral duroquinone, DQ (electron acceptor), were studied by selective laser excitation‐Fourier transform (FT)‐EPR spectroscopy. Four different P, which are related to the photosynthetic apparatus, were examined in ethanol solutions at −243 K: pyrochlorophyll a (pChla), zinc‐pyrochlorophyll a (ZnpChla), zinc‐tetraphenylporphyrin (ZnTPP), and magnesium‐tetraphenylporphyrin (MgTPP). This highly time‐resolved EPR spectroscopy (∼10 ns delay time, ẗd, between the laser excitation and the detection pulse) permits differentiation between the various CIDEP mechanisms, i.e., triplet mechanism (TM) and radical‐pair mechanism (RPM), associated with the electron transfer process. The spectra of the DQ.− generated by the different triplet precursors strongly depend on two inherent parameters characteristic of the molecular system: (1) the zero‐field splitting (ZFS) and the selective singlet‐triplet (Ti l̊ S1) population rates, Ai (i = x, y, z); (2) the difference in g‐values and hyperfine components between the donor and acceptor radicals. Since the differences in g‐values and hyperfines are very nearly the same in all four P.+‐DQ.− systems examined, the different results, particularly in the time range 10 ns < ẗ<d < 300 ns, are attributed to triplet spin memory transfer via TM. A quantitative spectral analysis is presented to describe the time evolution FT‐EPR spectra. This method is used to determine the following dynamic processes: (1) spin lattice relaxation rates of the triplet precursors in solution; (2) donor‐acceptor electron transfer reaction rates; (3) spin lattice relaxation rates of the acceptor radical.