Picosecond photochemistry of a cofacial diporphyrin containing iron(III) and zinc(II): Mimicking electron transfer between cytochrome c and the primary electron donor in reaction centers of photosynthetic bacteria

Abstract

driven ETs in Zn-Fe(III)Cl is intended to provide additional data for modeling the cytochrome c oxidation.EXPERIMENTAL PROCEDURE The synthesis of Zn-Fe(III)Cl (structure I) will be reported elsewhere.Recent picosecond spectroscopic experiments on cofacial diporphyrins (1, 2) provide substantial evidence to support the conclusion that electron transfer (ET) reactions occur from singlet excited states (S) in these molecules in <6 ps. Although the primary electron donor in bacterial photosynthesis is thought to be a bacteriochlorophyll.dimer, (BChl)2, (3, 4) the work on diporphyrins shows that a dimeric electron donor is not required to produce rapid (>101" sol) ET from the SI state of porphyrin-type molecules. Indeed, a Mg-H2 diporphyrin provides a reasonable model for the primary electron donor-acceptor couple .(5, 6) in the reaction center (RC) of photosystem II (PS II) of green plants,In this paper we describe the photochemical properties of a Zn-Fe(III)Cl diporphyrin that has the same carbon skeleton as Mg-H2. However, this time one subunit has a central Fe(III) metal atom with.a Cl-ligand and the other a Zn(II) metal atom. The SI state of Mg-H2 is a ir-ir* type, whereas the Zn-Fe(III)Cl state of comparable energy has a mixed ir-wr* and ligand-to-metal charge transfer (LMCT) character. Thus, comparing the ET reactions in these diporphyrins may reveal differences due to the different types ofexcited states in the two cases. Also, the reverse ET in Zn-Fe(III)Cl to reform the. initial ground state,

Ethyne-Bridged (Porphinato)Zinc(II)−(Porphinato)Iron(III) Complexes: Phenomenological Dependence of Excited-State Dynamics upon (Porphinato)Iron Electronic Structure

“…Mechanistic studies of electron transfer, as well as excitation delocalization and excitation migration processes in mixed-metal multiporphyrin systems have typically examined assemblies that feature spectroscopically identifiable chromophoric entities that function as energy donors and acceptors. For multi(porphinato)metal systems that feature d 1 − d 9 metal centers, the metal d manifold typically affords a multiplicity of states lower in energy than the (porphinato)zinc(II) S 1 state. − In weakly coupled (porphinato)zinc(II)−spacer−(porphinato)iron(III) (PZn−Sp−PFe) systems, both energy- and electron-transfer migration to the PFe(III) chromophore from the PZn-localized singlet excited state have been observed. ,− Such PZn−Sp−PFe assemblies have utilized hydrogen-bonded interfaces, as well as rigid phenyl, − ,, and (phenylethynyl)-arene 73,85,88,89 bridging moieties, to connect the porphyrin-based pigments; for such linkage motifs, excitation delocalization is negligible, as these Sp structures afford only weak electronic coupling between the two (porphinato)metal units.…”

Stacked Zinc Triporphyrin as a Photoinduced Molecular Wire in Bilayers

“…In order to model energy conversion in photosynthesis, molecules have been synthesized in the recent years. They are able to mimic the fundamental functions of the photosynthetic reaction centers, such as antenna function, stabilization of charge separation, sequential electron transfer (Boxer and Bucks, 1979;Wasielewski et al, 1982;Fujita et at., 1982;Calcaterra et at., 1983;Moore et al, 1984). Porphyrins or phthalocyanins, included in diads or triads, colinked or piled up, have been designed (Sakata et al, 1985;Seta et al, 1985;Abdalmuhdi and Chang, 1985;Dubowchik and Hamilton, 1986;Gaspard et al, 1986).…”