Artificial photosynthetic reaction centers: Mimicking sequential electron and triplet-energy transfer

An artificial photosynthetic reaction center consisting of a carotenoid (C), a dimesitylparphyrin (P), and a bis(heptafluoropropyl)-porphyrin (P _F), C-P-P_F, and the related triad in which the central porphyrin has been metalated to give C-P_Zn-P_F have been synthesized and characterized by transient spectroscopy. These triads are models for amphipathic triads having a carboxylate group attached to the P_F moiety; they are designed to carry out redox processes across lipid bilayers. Triad C-P-P_F 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 P _F yield the initial charge-separated state C-P^.+-P _F^.-. Subsequent hole transfer to the carotenoid moiety generates the final charge-separated state C^.+-P-P_F ^.-, which has a lifetime of 1.1 μs and is formed with a quantum yield of 0.24. In triad C-P_Zn-P_F energy transfer from the P_Zn excited singlet to the P_F moiety yields C-P _Zn-¹P_F. A series of electron-transfer reactions analogous to those observed in C-P-P_F generates C ^.+-P_Zn-P_F^.-, 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-P_Zn-P _F 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-P_F and C-P _Zn-P_F, 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 P_F 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.