Models for photosynthesis: EPR studies of cyclohexylene-linked porphyrin quinones

Time-resolved EPR spectra are reported for photoexcited cyclohexylene bridged porphyrin-quinone diads and porphyrin-quinone-quinone triads in isotropic solutions and in the soft glass phase of liquid crystals. Spin-polarized EPR spectra were observed for the charge-separated biradical states. The EPR polarization patterns 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 electron transfer to the quinone(s) followed by singlet-triplet mixing to yield radical pairs (RP) with triplet character or, more likely, triplet electron transfer after spin-orbit intersystem crossing, or a superposition of both pathways. Using liquid crystals, information about the orientation of the guest molecules in the liquid crystal matrix with respect to the long axes of the liquid crystal molecules can be obtained. High field/high frequency (3.4 T, 95 GHz) EPR spectra of the transient porphyrin-quinone RP state are reported for the first time. By comparing the EPR measurements at different Zeeman fields, additional information about the polarization mechanism can be obtained.