Institut für Organische Chemie, Freie Universität Berlin, D-14195 Berlin.
Knowledge of the properties of the triplet excited states of porphyrins is important, because light-induced electron transfer between porphyrin donors and quinone acceptors, e.g., in biomimetic model compounds for photosynthetic reaction centres, may proceed via the triplet state.[1,2] The zero-field splitting parameters, notably the E parameter, depend on the electron distribution and differ in porphyrins and chlorins. In liquid-crystalline solutions, the triplet ESR spectra provide a sensitive probe of the alignment of the molecules. In covalently linked porphyrin-quinones, the distance between donor and acceptor affects the triplet lifetime.
We present the results of CW-ESR studies of the photoexcited triplets of a series of porphyrins and covalently linked porphyrin-quinones in isotropic and anisotropic (liquid-crystalline) frozen solutions. In frozen nematic solutions, the appearance of the spectra is strongly dependent on the orientation of the sample relative to the external magnetic field. This fact allows a differentiation between the z and x,y axes and hence a more accurate determination of the zero-field splitting parameters D and E. In the case of some porphyrin-quinones, steady-state triplet ESR signals cannot be observed which is ascribed to singlet electron transfer enhanced by folding. The doublet signal of porphyrin or chlorin cation radicals, which is also observed, appears in emission under certain conditions, indicative of the radical triplet pair mechanism. Even without light modulation, the triplet CW ESR spectra of porphyrin free bases may exhibit spin polarization.
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