title: Artificial photosynthetic reaction centers: mimicking sequential electron and triplet-energy transfer.
abstract: An artificial photosynthetic reaction center consisting of a carotenoid (C), a dimesitylporphyrin (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 mus 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)-(1)P(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.
authors.affiliation: Department of Chemistry and Biochemistry, Center for the Study of Early Events in Photosynthesis, Arizona State University, Tempe, AZ 85287-1604, USA.
authors.forename: Rodrigo E
authors.initials: RE
authors.lastname: Palacios
authors.forename: Gerdenis
authors.initials: G
authors.lastname: Kodis
authors.forename: Stephanie L
authors.initials: SL
authors.lastname: Gould
authors.forename: Linda
authors.initials: L
authors.lastname: de la Garza
authors.forename: Alicia
authors.initials: A
authors.lastname: Brune
authors.forename: Devens
authors.initials: D
authors.lastname: Gust
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Moore
authors.forename: Ana L
authors.initials: AL
authors.lastname: Moore
mesh_terms: D002338:Carotenoids / Q000737:chemistry*; D004563:Electrochemistry; D004579:Electron Transport; D004735:Energy Transfer; D008081:Liposomes / Q000737:chemistry; D010777:Photochemistry; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D011166:Porphyrins / Q000737:chemistry*; D013050:Spectrometry, Fluorescence
chemical_list: D008081:Liposomes; D045322:Photosynthetic Reaction Center Complex Proteins; D011166:Porphyrins; D002338:Carotenoids