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 this paper time-resolved EPR spectra are reported for porphyrin-quinone-quinone and porphyrin-porphyrin-quinone triads, obtained after laser photoexcitation in the nematic and soft glass phase of liquid crystals. Spin-polarized EPR spectra were observed for the triplet states of the porphyrin, created by spin-selective intersystem crossing (ISC) from the singlet excited state and those of the charge-separated radical pair states (RP) generated by electron transfer (ET) processes. The EPR polarization patterns of the RPs are discussed in terms of the favored decay channel of the photoexcited singlet state of the porphyrin donor. The decay pathway may either be singlet ET to the quinone(s) followed by singlet/triplet mixing to yield RPs with triplet character or triplet ET after ISC from the porphyrin singlet to the triplet state, or a superposition of both pathways. It is demonstrated that the nature of the linking bridge between donor and acceptor, i.e., aliphatic cyclohexylene or aromatic phenylene, significantly influences the ET mechanisms and thus the polarization patterns of the RP spectra. The polarization indicates, indeed, that in the former case the ISC/triplet channel is preferred, whereas the latter system the stronger electronic coupling between donor and acceptor via the conjugated phenylene bridge favors the singlet channel. This situation is related to the ET mechanisms occurring in the primary processes of native photosynthesis.
From the EPR spectra information about the orientation of the guest molecules in the LC matrix with respect to the long axes of the LC molecules can be obtained. Surprisingly, in some cases the long dipolar axis of the guest aggregates prefers an orientation perpendicular to the long axis of the host molecules. These findings are established by in-situ rotation of the sample tube at lower temperatures where reorientation of the nematic molecules is hampered. In the porphyrin-porphyrin-quinone triads the energy transfer and ET processes strongly depend on the type of metallation of the porphyrins, specifically, whether the (a) distal, (b) the vicinal or (c) both porphyrins bear a zinc atom. In situation (a) only the free base porphyrin triplet is observed. The center component obviously serves as a trap preventing any ET to occur. In case (b) shortly after the laser flash a superposition of both triplet spectra, namely that of the free base and zinc porphyrin are detected. After longer delay times after the flash a superposition of the free base porphyrin triplet and that of the charge-separated biradical state are observed. In case (c) the triplet state of the biradical is seen.
Finally, a novel model shall be discussed using anisotropic relaxation for the interpretation of the polarization pattern of the triplet species.

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