EPR Studies of Porphyrin and Porphyrin-Quinone Triplet States in Frozen Nematic Solutions

B. Kirste, P. Tian, W. Kalisch and H. Kurreck

Institut für Organische Chemie, Freie Universität Berlin, Takustr. 3, 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 EPR 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 EPR 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 EPR signals cannot be observed which is ascribed to singlet electron transfer enhanced by folding of the quinone acceptor over the porphyrin donor.

The observed ZFS parameters are rationalized by means of MO and D tensor calculations. It turns out that substituents may have a considerable influence on the E parameter by lifting the degeneracy of LUMO and NLUMO.

[1] K. Hasharoni, H. Levanon, J. von Gersdorff, H. Kurreck and K. Möbius, J. Chem. Phys. 98, 2916 (1993).
[2] L. Sun, J. von Gersdorff, D. Niethammer, P. Tian and H. Kurreck, Angew. Chem. Int. Ed. Engl. 33, 2318 (1994).

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Burkhard Kirste, 1995/09/30