Singlet Photochemistry in Model Photosynthesis: Identification of Charge Separated Intermediates by Fourier Transform and CW-EPR Spectroscopies

Intramolecular electron transfer from the photoexcited state of a carotenoid-porphyrin-diquinone tetrad was studied by selective laser excitation with both Fourier transform and CW EPR spectroscopies. It is shown that the electron transfer occurs from the singlet state of the porphyrin constituent to produce the terminal benzoquinone radical anion and the carotenoid radical cation. This tetrad molecule can maintain the charge-separated state for a substantial period of time (~1 μs), allowing the characterization of the short-lived radicals. The derivative-like spectrum of the quinone radical anion and its dependence on the turning angle of the microwave pulse indicates that electron transfer proceeds via the singlet state.