Cofacial porphyrin dimers

Fig. 21. The two types of cofacial porphyrin dimers A and B and the chemical structures of the coordination cages 29 and 30 (133).

Kahta D, Morisue M, Kobuke Y (2006) Synthesis and electrochemical properties of shpped-cofacial porphyrin dimers of ferrocene-functionahzed Zn-imidazolyl-poiphyrins as potential terminal electron donors in photosynihetic models. New J Chem 30(l) 77-92... 

Le Mest Y, L Her M, Hendricks NH, Kim K, Collman JP. 1992. Electrochemical and spectroscopic properties of dimeric cofacial porphyrins with nonelectroactive metal centers. Delocalization processes in the porphyrin rr-cation-radical systems. Inorg Chem 31 835... 

Fujita and coworkers have also reported the encapsulation of multimeric porphyrin assemblies in the box-shaped cavities of ternary Pd6 coordination cages. Two types of cofacial porphine dimers A and B could be stabilized (133). In the smaller [Pd6(L14)2(L15)6]12+ cage 29, whose diameter is 10.4 A (Fig. 21), two porphyrin molecules can be stackedo directly on top of each other with an interplane distance of 3.4 A. In the larger cage 30, an additional molecule of L14 is intercalated between the two porphyrin bases. All complexes were found to be water-soluble in contrast to other 7i-stacked porphyrin dimers. The encapsulation... 

One such reaction that has been studied is the electrocatalytic reduction of oxygen directly to water.25,27 The electrocatalysts for this process are often based on metal porphyrins and phthalo-cyanins. Thus a graphite electrode whose surface was modified by the irreversible adsorption of a cofacial dicobalt porphyrin dimer was able to reduce oxygen under conditions where the reduction did not occur on the bare electrode itself. Similarly, a catalytic chemically modified electrode for the oxidation of chloride to chlorine has been prepared28 where the active catalyst was reported to be a ruthenium dimer, [(bipy)2(OH)RulvORuvO(bipy)2]4+, which was reduced to the corresponding [Rum-RuIV] dimer during the reaction. 

A related series of mixed-metal face to face porphyrin dimers (192) has been studied by Collman et al.506 A motivation for obtaining these species has been their potential use as redox catalysts for such reactions as the four-electron reduction of 02 to H20 via H202. It was hoped that the orientation of two cofacial metalloporphyrins in a manner which permits the concerted interaction of both metals with dioxygen may promote the above redox reaction. Such a result was obtained for the Co11 /Co" dimer which is an effective catalyst for the reduction of dioxygen electrochemic-ally.507 However for most of the mixed-metal dimers, including a Con/Mnn species, the second metal was found to be catalytically inert with the redox behaviour of the dimer being similar to that of the monomeric cobalt porphyrin. However the nature of the second metal ion has some influence on the potential at which the cobalt centre is reduced. 

The energy transfer process can be conveniently monitored by ultrafast (femtosecond) spectroscopy, using 555-nm excitation pulses so as to achieve substantial excitation of the Zn-porphyrin chromophore. The spectral changes obtained for the slipped cofacial hetero-dimer lOZni/2 are shown in Fig. 30. 

Funatsu, K., T. Imamura, A. Ichimura, and Y. Sasaki (1998). Novel cofacial ruthe-nium(II) porphyrin dimers and tetramers. Inorg. Chem. 37, 4986 995. 

Bolze, R, C.P. Gros, P.D. Harvey, and R. Guilard (2001). Luminescence properties of a cofacial dipalladium porphyrin dimer under argon and in the presence of dioxygen. J. Porphyrins Phthalocyanines 5(7), 569-574. 

A series of cofacial porphyrins connected with covalent bonds have been reported on dimer [658-662,673,689-692], trimer [693], and oligomer [694] (9-12). 

A series of slipped cofacial zinc(II)porphyrin dimers formed through axial coordination of a wieso-methyl-imidazolyl substituent and possessing an appended ferrocenyl as electron donor groups were realized as photosynthetic models (Fig. 9b) [52]. Directly bond and methyl-bridged ferrocene derivatives show good electronic coupling whereas phenylene-ethylene and phenylene-ethynylene spacers decreased the electronic communication. Those electronic effects were reflected on the absorption and fluorescence properties, as well as on the porphyrin ring redox properties. 

Several strategies based on porphyrin derivatives have been developed to reach this objective. The first approach involves the utilization of dimeric cofacial metallic porphyrins adsorbed on the surface of an edge-plane graphite (EPG) electrode.18 An 02 molecule was expected to be coordinated to form a fi-02 bridge between the two metal centers allowing subsequent scission of the O O bond by reductive activation, while the dimeric structure acts as a four-electron reservoir. 

A similar catalytic activity with a monomeric porphyrin of iridium has been observed when adsorbed on a graphite electrode.381-383 It is believed that the active catalyst on the surface is a dimeric species formed by electrochemical oxidation at the beginning of the cathodic scan, since cofacial bisporphyrins of iridium are known to be efficient electrocatalysts for the tetraelectronic reduction of 02. In addition, some polymeric porphyrin coatings on electrode surfaces have been also reported to be active electroactive catalysts for H20 production, especially with adequately thick films or with a polypyrrole matrix.384-387... 

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