Models for Photosynthetic Reaction Center: Steady State and Time Resolved EPR Spectroscopy

AG Kurreck

Institut für Organische Chemie der FU Berlin, Takustr. 3, 14195 Berlin

This poster was presented at the meeting "Chemie in Berlin und Potsdam" which was held at the University of Potsdam, 1996-12-04

Porphyrin Quinones

The search for an understanding of the factors controlling electron transfer (ET) reactions in the primary events of photosynthesis has led to the development of biomimetic model systems consisting of porphyrins covalently linked to quinones (P-Qs). In order to achieve a long-lived charge-separated state, syntheses of aggregates with more than two redox active components, triads, tetrads and even pentads have been reported in the literature. One example of a promising target molecule and its synthetic route is pictured on the left-hand side. In these model compounds the porphyrin is photoexcited to the singlet state from which, at room temperature, intramolecular ET occurs to the quinone, yielding the charge-separated radical pair in its singlet state. Alternatively, ET via spin selective spin-orbit intersystem crossing (ISC) creates the porphyrin triplet state from which ET may then occur to the quinone with spin conservation. Occurrence of triplet states allows observation of the ET processes by time-resolved EPR spectroscopy. Photoredox processes in anisotropic media, such as vesicles, micelles and liquid crystals (LC) have attracted attention as mimicking photosynthetic electron transfer (ET) across biological membranes. In this paper we present photochemical investigations of P-Qs (a) in isotropic solutions, (b) embedded in reversed micelles and (c) in nematic phases of liquid crystals.

(a) Isotropic Solutions: In situ laser irradiation of a porphyrin quinone triad yields time-resolved EPR spectra of the charge-separated biradical state with an antiphase a/e/a/e spin polarization (a: absorption; e: emission). It is currently under discussion whether this biradical is born via the singlet or the triplet ET channel.

(b) Reversed Micelles: Using steady state in situ lamp irradiation through resonator slits, various P-Qs can be converted to paramagnetic derivatives. The species, generated via intra- and intermolecular ET processes, can be identified by EPR spectroscopy. Interestingly, emissive signals can be observed which are indicative of electron spin polarization (ESP). The ESP effects can be interpreted in terms of the radical/triplet pair mechanism. A prerequisite for this mechanism to occur is (1) the photoreduction of the quinone to generate porphyrin hydroquinone which allows for a long-lived photoexcited triplet state of the porphyrin (the ET decay channel is blocked), and (2) the generation of a doublet state, either porphyrin radical cation or semiquinone radical anion. Interaction between the two porphyrin triplet electron spins and the doublet radical spin creates the emissive doublet signal via a quartet/doublet equilibrium.

(c) Liquid Crystals: Steady state illumination of guest P-Qs in LCs, frozen after alignment in an external magnetic field, yields orientation dependent EPR triplet spectra of the porphyrin moiety. In frozen solutions ET is blocked and the excited porphyrin states are no longer quenched by the quinone. Time-resolved EPR studies performed on photoexcited, oriented P-Q molecules in fluid nematic phases of liquid crystals exhibit well-resolved, spin-polarized EPR spectra of the porphyrin triplet and of the charge-separated biradical state, generated by ET. The time resolved EPR spectrum of the triad, pictured bottom, center, exhibits an e/e/a/a polarization and in addition two broad bumps in e/a on the low and high field side of the spectrum. By comparison of this spectrum with that of the diad precursor (QB protected, see formula bottom, right), it is obvious that the spectrum is a superposition of the spectra of two charge-separated states. The outer e/a line pair indicates the presence of the biradical with one unpaired electron on the porphyrin, the other on QA; the inner e/a line pair has to be assigned to the biradical with the second electron on QB. Investigation of the orientation dependence of the spectral features, achieved by sample tube rotation, yields information about the energetically favored orientation of the guest molecules with respect to the LC host solvent.

Acknowledgments: Fruitful collaboration with the groups of K. Möbius (G. Elger, M. Fuhs, E. Johnen), Free University Berlin, B. Röder, Humboldt University Berlin, and H. Levanon, Hebrew University, Jerusalem, is gratefully acknowledged. This paper was supported by Stiftung Volkswagenwerk. H. K. thanks the Fonds der Chemischen Industrie for financial support.

Selected References: H. Kurreck, M. Huber, Angew. Chem. Int. Ed. Engl., 1995, 34, 849; L. Sun, J. von Gersdorff, D. Niethammer, P. Tian, H. Kurreck, Angew. Chem. Int. Ed. Engl., 1994, 33, 2318; S. N. Batchelor, L. Sun, K. Möbius, H. Kurreck, Magn. Reson. Chem., 1995, 33, 28; C.W.M. Kay, H. Kurreck, S.N. Batchelor, P. Tian, J. Schlüpmann, K. Möbius, Appl. Magn. Reson., 1995, 9, 459; B. Kirste, P. Tian, W. Kalisch, H. Kurreck, J. Chem. Soc. Perkin Trans. 2 1995, 2147; M. Fuchs, J. von Gersdorff, H. Dieks, H. Kurreck, K. Möbius, T. Prisner, J. Chem. Soc. Faraday Trans. 1996, 92, 949; J. Fajer, K. M. Barkigia, D. Melamed, R. M. Sweet, H. Kurreck, J. von Gersdorff, M. Plato, H.-C. Rohland, G. Elger, K. Möbius, J. Phys. Chem., 1996, 100, 14239.

Group of Prof. Kurreck
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