Iron porphyrins, photoinduced electron transfer

A chromophore such as the quinone, ruthenium complex, C(,o. or viologen is covalently introduced at the terminal of the heme-propionate side chain(s) (94-97). For example, Hamachi et al. (98) appended Ru2+(bpy)3 (bpy = 2,2 -bipyridine) at one of the terminals of the heme-propionate (Fig. 26) and monitored the photoinduced electron transfer from the photoexcited ruthenium complex to the heme-iron in the protein. The reduction of the heme-iron was monitored by the formation of oxyferrous species under aerobic conditions, while the Ru(III) complex was reductively quenched by EDTA as a sacrificial reagent. In addition, when [Co(NH3)5Cl]2+ was added to the system instead of EDTA, the photoexcited ruthenium complex was oxidatively quenched by the cobalt complex, and then one electron is abstracted from the heme-iron(III) to reduce the ruthenium complex (99). As a result, the oxoferryl species was detected due to the deprotonation of the hydroxyiron(III)-porphyrin cation radical species. An extension of this work was the assembly of the Ru2+(bpy)3 complex with a catenane moiety including the cyclic bis(viologen)(100). In the supramolecular system, vectorial electron transfer was achieved with a long-lived charge separation species (f > 2 ms). 

Figure 2.5 Molecular structures of zinc-iron porphyrin complexes across (a) hydrogen, (b) aliphatic and (c) aromatic bridges, and the corresponding rates of photoinduced electron transfer for each species, as reported by Rege et al. [12]...

The insertion into porphyrins of central metals capable of easy oxidation or reduction can shift the site of redox chemistry from the macrocycle n-electron system to the metal. While this substitution allows large changes in the redox properties of a porphyrin, it may introduce changes in the photophysics if a transition metal is employed. For example, iron (III) porphyrins are particularly good electron acceptor moieties, and have been used as components of porphyrin dyad and more complex systems that show photoinduced electron transfer behavior [11, 18, 19, 26, 31, 32, 34, 43, 44]. McLendon and coworkers used this strategy with dyad 3, in... 

Photoinduced electron-transfer in the opposite direction was demonstrated upon irradiation of the Ru(bpy)3 +-Mb system in the presence of Co +(NH3)5Cl as a sacrificial electron acceptor (Figure 44B) [244]. The photochemical reaction results in the formation of ferryl species (i.e., Fe(IV)-heme), with the intermediate formation of the porphyrin cation radical (as demonstrated using laser flash photolysis [237]). The electron-transfer cascade includes the primary oxidative quenching of the excited chromophore, Ru(bpy)3"+, by Co +(NH3)5Cl to yield Ru(bpy)3 + [E° = +1.01 V vs. SCE). The resulting oxidant efficiently takes an electron from the porphyrin ring (fcet = 8.5 x 10 s ) and the porphyrin cation radical produced further oxidizes the central iron atom, converting it from the Fe(III) state to the Fe(IV) state (/cet = 4.0 x 10 s at pH 7.5). 

Irradiation of iron and cobalt porphyrins (13 Fe(TPP), 14 Co(TPP), H2TPP = 5,10,15,20-tetraphenyl-21/f,23/f-porphyrin) in the presence of triethylamine (TEA) using > 320-nm tight caused the photocatalytic reduction of CO2 [15, 16, 27-31]. When 13 was used as a photocatalyst, CO was detected with TNco = 70 after 180-h irradiation [15]. Formic acid was the main product when 14 was employed as a photocatalyst [16]. The reaction mechanism proposed on the basis of UV-vis absorption changes during photolysis and radiolysis, and electrochemical measurements are shown in Scheme 3. M (TPP) is reduced to M (TPP) by photoinduced electron transfer from TEA, which subsequently disproportionates to M°(TPP), the proposed catalyticaUy-active species. 

Typically, those ferrocenyl-porphyrins have been explored in photoinduced electron transfer and charge separation processes mimicking the activity of the photosynthetic system, as well as in the development of catalysts for multielectron transfer reactions. Ferrocenyl substituents were shown to enhance the electrocat-alytic activity of cobalt [41, 42], iron [43], and copper [44—46] porphyrins for tetraelectronic reduction of dioxygen to water. 

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