An artificial photosynthetic reaction center consisting of a carotenoid (C), a dimesitylparphyrin (P), and a bis(heptafluoropropyl)-porphyrin (P F), 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 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-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-P Zn-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-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-PF and C-P Zn-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.
- Electron transfer
- Fluorescence spectroscopy
- Time-resolved spectroscopy
ASJC Scopus subject areas
- Atomic and Molecular Physics, and Optics
- Physical and Theoretical Chemistry