title: Photoinduced electron transfer within a zinc porphyrin-cyclobis(paraquat-p-phenylene) donor-acceptor dyad.
abstract: Understanding the mechanism of efficient photoinduced electron-transfer processes is essential for developing molecular systems for artificial photosynthesis. Towards this goal, we describe the synthesis of a donor-acceptor dyad comprising a zinc porphyrin donor and a tetracationic cyclobis(paraquat-p-phenylene) (CBPQT(4+) ) acceptor. The X-ray crystal structure of the dyad reveals the formation of a dimeric motif through the intermolecular coordination between the triazole nitrogen and the central Zn metal of two adjacent units of the dyad. Photoinduced electron transfer within the dyad in MeCN was investigated by femtosecond and nanosecond transient absorption spectroscopy, as well as by transient EPR spectroscopy. Photoexcitation of the dyad produced a weakly coupled ZnP(+.) -CBPQT(3+.) spin-correlated radical-ion pair having a τ=146 ns lifetime and a spin-spin exchange interaction of only 0.23 mT. The long radical-ion-pair lifetime results from weak donor-acceptor electronic coupling as a consequence of having nine bonds between the donor and the acceptor, and the reduction in reorganization energy for electron transfer caused by charge dispersal over both paraquat units within CBPQT(3+.) .
authors.affiliation: Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208 (USA), Fax: (+1) 847-491-1009; Present Address: Department of Chemistry Faculty of Science, Zagazig University, Zagazig (Egypt).
authors.forename: Maher
authors.initials: M
authors.lastname: Fathalla
authors.forename: Jonathan C
authors.initials: JC
authors.lastname: Barnes
authors.forename: Ryan M
authors.initials: RM
authors.lastname: Young
authors.forename: Karel J
authors.initials: KJ
authors.lastname: Hartlieb
authors.forename: Scott M
authors.initials: SM
authors.lastname: Dyar
authors.forename: Samuel W
authors.initials: SW
authors.lastname: Eaton
authors.forename: Amy A
authors.initials: AA
authors.lastname: Sarjeant
authors.forename: Dick T
authors.initials: DT
authors.lastname: Co
authors.forename: Michael R
authors.initials: MR
authors.lastname: Wasielewski
authors.forename: J Fraser
authors.initials: JF
authors.lastname: Stoddart
mesh_terms: D040761:Biomimetic Materials / Q000737:chemistry; D004579:Electron Transport; D008665:Metalloporphyrins / Q000737:chemistry*; D008958:Models, Molecular; D010269:Paraquat / Q000031:analogs & derivatives* / Q000737:chemistry; D055668:Photochemical Processes; D015032:Zinc / Q000737:chemistry*
chemical_list: D008665:Metalloporphyrins; D015032:Zinc; D010269:Paraquat
keywords: donor-acceptor systems; electron transfer; porphyrinoids; transient absorption spectroscopy