Artificial Photosynthetic Reaction Centers: Mimicking Sequential Electron and Triplet-Energy Transfer

An artificial photosynthetic reaction center consisting of a carotenoid (C), a dimesitylporphyrin (P), and a bis(heptafluoropropyl)porphyrin (PF), C‐P‐PF , and the related triad in which the central porphyrin has been metalated to give C‐PZn‐PF have been synthesized and characterized by transient spectroscopy. These triads are models for amphipathic triads having a carboxylate group attached to the PF moiety; they are designed to carry out redox processes across lipid bilayers. Triad C‐P‐PF undergoes rapid singlet–singlet energy transfer between the porphyrin moieties, so that their excited states are in equilibrium. In benzonitrile, photoinduced electron transfer from the first excited singlet state of P and hole transfer from the first excited singlet state of PF yield the initial charge‐separated state C‐P.+‐PF.−. Subsequent hole transfer to the carotenoid moiety generates the final charge‐separated state C.+‐P‐PF.−, which has a lifetime of 1.1 μs and is formed with a quantum yield of 0.24. In triad C‐PZn‐PF energy transfer from the PZn excited singlet to the PF moiety yields C‐PZn‐1PF . A series of electron‐transfer reactions analogous to those observed in C‐P‐PF generates C.+‐PZn‐PF.−, which has a lifetime of 750 ns and is formed with a quantum yield of 0.25. Flash photolysis experiments in liposomes containing an amphipathic version of C‐PZn‐PF demonstrate that the added driving force for photoinduced electron transfer in the metalated triad is useful for promoting electron transfer in the low‐dielectric environment of artificial biological membranes. In argon‐saturated toluene solutions of C‐P‐PF and C‐PZn‐PF , charge separation is not observed and a considerable yield of triplet species is generated upon excitation of the porphyrin moieties. In both triads triplet energy localized in the PF moiety is channeled to the carotenoid chromophore by a triplet energy‐transfer relay mechanism. Certain photophysical characteristics of these triads, including the sequential electron transfer and the triplet energy‐transfer relay mechanism, are reminiscent of those observed in natural reaction centers of photosynthetic bacteria. Carotenoid–diporphyrin triads such as that shown in the picture have been synthesized and studied spectroscopically. In benzonitrile solution, excitation of either porphyrin leads to photoinduced electron transfer involving the second porphyrin to generate a charge‐separated state. In toluene, charge separation does not occur. The ener