Porphyrin centered electron transfer processes

The best molecule mimicking multi-step electron-transfer processes in the photo synthetic reaction center so far reported is a ferrocene-meso, meso-linked porphyrin trimer-fullerene pentad [Fc-(ZnP)3-C60] in Fig. 13.16b, where the C60 and the ferrocene (Fc) are tethered at both the ends of (ZnP)3 (R = 46.9 A)... 

Iron-sulfur proteins, Fe-S-proteins a group of proteins found in all organisms. They contain iron-sulfur centers (iron-sulfur clusters) and take part in electron transfer processes. They are involved In Hj metabolism, nitrogen and carbon dioxide fixation, oxidative and photosynthetic phosphorylation, mitochondrial hydroxylation and nitrite and sulfite reduction. The iron in the active centers is coordinated with the sulfur atoms of cysteine residues. In addition, all Fe-S-proteins except for Rubredoxins (see) contain the same number of labile or inorganic sulfur atoms as iron atoms, and both are covalently bound in iron-sulfur clusters. Since the iron is not bound in a porphyrin ring, this group of proteins is included in the Non-... 

The formation of dimetal face-to-face macrocycles can effectively promote a 4-electron transfer pathway from oxygen to water. For example, such catalysts include dicobalt face-to-face porphyrins [15], and pillared dicobalt cofacial porphyrins [16], as well as other binuclear and polynuclear Co phthalocyanines [17]. In this catalyst, two Co centers can provide two adsorption sites for O2 to form a bridge adsorption, facilitating a 4-electron transfer process, as suggested by Anson et al. [18, 19] ... 

The variations in the exogenous ligand L in the mixed-metal hemoglobin hybrids, [MP, Fe(L)P] where M = Zn or Mg and L = H2O, imidazole, CN , F", and Nf, have a considerable effect on both the photoinitiated [ MFe(III)] and thermally activated [M <-Fe(II)] electron transfer processes. Electron transfer between zinc and iron porphyrins across a variety of aromatic spacers has been examined. The rate constants for charge separation displayed an inverse dependence on the center-center distance but no correlation with the orientation of the porphyrin rings. The charge recombination rate constants were independent of the separation distance up to 23 A. 

Elucidation of the Mn coordination sphere allows for further mechanistic insights into the Mn oxidation process specifically the electron transfer pathway from Mn11 to the acceptor (delocalized porphyrin % radical or the Fe center) may take place via the heme propionate ligand [52]. Upon further chelation a (neutral) Mnm complex may diffuse outside the enzyme and into the lignin polymer. 

An extension of this study revealed that following selective excitation of the Ir(III)-based unit at 355 nm, rather than that of the porphyrin units at 532 nm, i.e., when the Ir center acts as a photosensitizer, the type of ensuing photoinduced process can be switched in the case of array (a), Fig. 27, from the electron transfer discussed above, to energy transfer [87]. 

In making rotaxanes usable as parts of molecular devices and with the purpose of studying long range election transfer processes within large molecular systems of well controlled geometries, the introduction of photoactive and electroactive compounds has been a valuable development. Photoinduced electron transfer between porphyrin species has a particular relevance to the primary events occurring in bacterial photosynthetic reaction center complexes, and so is a well studied phenomenon. 

Another question is which one, the bis coordination of O2 to two CoP centers or the electronic effects induced by the peripherally coordinated Ru(III)Ru(III) Ru(II), would be responsible for the cobalt porphyrin activation in the multielectron-transfer process. In fact, considering that the Co(4-TCPyP) films are formed by stacks of supermolecules, it is not possible a priori to rule out the first hypothesis. This problem was solved by repeating the experiments using the electrostatic assembled Co(4-TCPyP)/ZnTPPS films in which the bis coordination of dioxygen to two cobalt porphyrin centers is not possible (176). The catalytic behavior is reproduced in the ion pair film, confirming that the activation is mainly due to electronic effects induced by the peripheral ruthenium clusters, in synergistic association with their normal role as intramolecular multielectron source. 

Nevertheless, other chromophores have been investigated and they have provided interesting insights, particularly porphyrin and Cso groups, since these serve as useful mimics of the cofactors present in the photosynthetic reaction center (Figure 37). Electron transfer involving porphyrins and fullerenes will be presented in more detail elsewhere in this Handbook Volume III, Part 2, Chapter 2 and Volume II, Part 1, Chapter 5 respectively), and so only a brief discussion is presented here. An excellent overview of photoinduced ET processes in Cso-based multichromophoric systems has been produced previously [116]. 

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