Design and synthesis of artificial photosynthetic reaction centers

Triads consisting of a caroteniod (C), a bismesitylporphyrin (P or P Zn) and tris(heptafluoropropyl)porphyrin (PF) have been designed and synthesized. They are artificial reaction centers that use a fluorinated porphyrin as the primary electron acceptor. Photoinduced electron transfer from the first excited singlet state of P, or hole transfer from the first excited singlet state of PF, yielded C-P •+-PF•-. Hole transfer from C yields the final charge separated state C•+-P-P F•- with a quantum yield of 0.73 and a lifetime of 500 ns in tetrahydrofuran at room temperature. It decays primarily to a triplet-excited state localized on the carotenoid, 3C-P-P F, rather than the ground state. Amphiphilic triads bearing a carboxyphenyl-bis(heptafluropropyl)porphyrin (PF-CO2H), as the primary electron acceptor were also synthesized. Photoinduced electron transfer from the first excited singlet state of P results in similar photophysical behavior to that of the first two triads. Preliminary studies of one of the amphiphilic triads in the lipid bilayer of liposomes indicated the formation of the final charge separated state in this medium.;Additional triads consisting of a carotenoid (C), a purpurin (Pur) and a fullerene (C60) were synthesized to harvest light energy through the carotenoid moiety. After the energy transfer from the carotenoid to the purpurin a series of electron transfer steps follow to give C•+ -Pur-C60•-. The central purpurin moiety of each triad differs slightly in structure, but maintains essentially the same electronic properties. Investigations of corresponding model caroteno-purpurin dyads of each triad demonstrate that energy is transferred to the purpurin moiety from the carotenoid with a quantum yield of 0.56 and 0.74, respectively. The final charge separated state, C•+-Pur-C60•- , of each triad is formed rapidly and decays with a relatively short time constant, 750 ps and 650 ps, respectively, in benzonitrile.