Iron complexes charge transfer transitions

In heterogeneous photoredox systems also a surface complex may act as the chromophore. This means that in this case not a bimolecular but a unimolecular photoredox reaction takes place, since electron transfer occurs within the lightabsorbing species, i.e. through a ligand-to-metal charge-transfer transition within the surface complex. This has been suggested for instance for the photochemical reductive dissolution of iron(III)(hydr)oxides (Waite and Morel, 1984 Siffert and Sulzberger, 1991). For continuous irradiation the quantum yield is then ... 

The complexity of the low temperature MCD spectra of the oxidized and reduced trinuclear cluster shows the multiplicity of the predominantly S — Fe charge transfer transitions that contribute to the absorption envelope. While MCD spectroscopy provides a method of resolving the electronic transitions, assignment cannot be attempted without detailed knowledge of the electronic structure. However, the complexity of the low temperature MCD spectra is useful in that it furnishes a discriminating method for determining the type and redox state of protein bound iron-sulfur clusters. Each well characterized type of iron-sulfur cluster, i.e. [2Fe-2S], [3Fe-4S], and [4Fe-4S], has been shown to have a characteristic low temperature MCD spectrum in each paramagnetic redox state (1)... 

Williams (28) suggested that the band found at about 16 kK in high-spin systems was a charge transfer transition, in which the excited state arises from a configuration in which an electron has been transferred from a porphyrin 71-orbital to a metal -orbital. Such transitions frequently occur in iron complexes. His alternative suggestion that the band could be an intra-metal d—d transition mixed with charge transfer was considered most unlikely on intensity grounds (88). 

Figure 1. Mulliken correlation of the charge-transfer transition energy ( cr = hvci) with the ionization potential [IP) of ferrocene and arene donors in 6w(durene)iron(II) complexes. The straight line is arbitrarily drawn with a slope of unity [124].

As discussed in the previous section, a ligand-to-metal charge-transfer transition of the surface complex (mechanism 1) and/or a Fe -O"11 charge-transfer of hematite (mechanism 2) are the oscillators involved in the surface photoredox reaction, leading to reductive dissolution of hematite in the presence of oxalate. The elementary steps and the derivations of the rate expressions of photochemical surface iron(II) formation of mechanism 1 and 2 are outlined in reactions 16-19, Eqs. 20-26, reactions 27-31, and Eqs. 32-37, respectively. 

The purple acid phosphatases can occur in two diferric forms—one as the tightly bound phosphate complex (characterized for the bovine and porcine enzymes) (45, 171, 203) and the other derived from peroxide or ferricyanide oxidation of the reduced enzyme (thus far accessible for only the porcine enzyme) (206). These oxidized forms are catalytically inactive. They are EPR silent because of antiferromagnetic coupling of the two Fe(IIl) ions and exhibit visible absorption maxima near 550-570 nm associated with the tyrosinate-to-Fe(III) charge-transfer transition. The unchanging value of the molar extinction coefficient between the oxidized and reduced enzymes indicates that the redox-active iron does not contribute to the visible chromophore and that tyrosine is coordinated only to the iron that remains ferric in agreement with the NMR spectrum of Uf, (45). 

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