Redox properties spectra

The electrochemistry of S-N and Se-N heterocycles has been reviewed comprehensively. The emphasis is on the information that electrochemical studies provide about the redox properties of potential neutral conductors. To be useful as a molecular conductor the 4-1, 0, and -1 redox states should be accessible and the neutral radical should lie close to the centre of the redox spectrum. The chalcogen-nitrogen heterocycles that have been studied in most detail from this viewpoint... 

The redox properties elicited for Rh(bpy)3 + and its congeners are thus entirely consistent with the description of these species as bound-ligand radicals. On the other hand, the disproportionation reactions eq 2-6 are not known to be characteristic of ligand-centered radicals, but are consistent with behavior expected for rhodium(II). Furthermore the substitution lability deduced for Rh(bpy)3 + and Rh(bpy)2 +> while consistent with that expected for Rh(II), is orders of magnitude too great for Rh(lII). Finally the spectrum observed for the intermediate Rh(bpy)3 + is not that expected for [RhIII(bpy)2(bpy")]2+. The spectrum measured has an absorption maximum at 350 nm with e 4 x 10 M 1 cm l and a broad maximum at 500 nm with e = 1 x 1()3 M 1 cm l. The spectra of free and bound bpy radical anions are quite distinctive (23.35-38) very intense absorption maxima (e 1 x 10 to 4 x 10 M - cm l) are found at 350-390 nm and are accompanied by less intense maxima (e 5 x 10 cm ) at 400 to 600 nm. While the Rh(bpy)3 +... 

The preparation and characterization of K3[0s(CN)5(NH3)] 2H20 have been described. This complex is a useful starting material for the synthesis of other [Os (CN)5L] species (e.g., L = py, pyz) and [0s(CN)5(H20)] can be obtained by the controlled aquation of [Os(Cf 5(NH3)f The kinetics of these ligand displacements have been investigated and mechanisms for the processes have been discussed. The UV-vis spectrum of each complex in the series [Os"(CN)5L] in which L is py, pyz, Mepz, or derivatives thereof, exhibits an intense, asymmetric MLCT absorption, split by spin-orbit coupling, and the effects of the electronic properties of L on the spectra have been examined. Redox properties of the complexes and the kinetics of the dissociation of pyz from [Os(CN)s(pyz)] have also been reported. ... 

Quinone Imines. - These display similar chemical properties to quinones, differing slightly due to the difference in electronegativity between the -C = O and -C=NH functions. Alberti et al. have examined the redox properties of the anti-cancer agent 5//-pyridophenoxazin-5-one. 18 Electrochemical reduction under anaerobic conditions gave the spectrum of the radical anion 02 was trapped in the presence of 02. 

The redox activity of Cyt c bound to the galleries of a-ZrP was found to be similar to that of the native protein. Dithionite or ascorbate can reduce Fe(IIII)Cyt c to the ferrous form, which can be oxidized by ferricyanide to the Fe(III) form. Retention of this redox property of Cyt c-a-ZrP is indicated by the absorption spectrum of Cyt c-a-ZrP. Although ascorbate and a-ZrP surface are both negatively charged, the addition of ascorbate to Cyt c-a-ZrP rapidly yields the reduced form of Cyt c (Fig. 49A). The absorption at 550 nm, due to Fe(II) form, grows with time and reaches a maximum value after 20 min. The addition of ferricyanide to Fe(II)Cyt c oxidizes it back to the Fe(III) form and restores the original absorption spectrum. These observations highlight the accessibility of bound protein to external small molecules and these immobilized proteins... 

Reaction of Cytochrome cIinn with Bis(ferrozine)copper(II) Knowledge of the redox properties of cytochrome c was an encouragement to initiate a kinetics investigation of the reduction of an unusual copper(II) complex species by cyt c11. Ferrozine (5,6-bis(4-sulphonatophenyl)-3-(2-pyridyl)-1,2.4-triazine)286 (see Scheme 7.1), a ligand that had come to prominence as a sensitive spectrophotometric probe for the presence of aqua-Fe(II),19c,287 forms a bis complex with Cu(II) that is square pyramidal, with a water molecule in a fifth axial position, whereas the bis-ferrozine complex of Cu(I) is tetrahedral.286 These geometries are based primarily upon analysis of the UV/visible spectrum. Both complexes are anionic, as for the tris-oxalato complex of cobalt in reaction with cytochrome c (Section 7.3.3.4), the expectation is that the two partners will bind sufficiently strongly in the precursor complex to allow separation of the precursor formation constant from the electron transfer rate constant, from the empirical kinetic data. 

The redox properties of Qg are also unlike those of plastoquinone in the pool. The semiquinone form, Qb . is tightly bound to a protein of the reaction centre and is thus stabilized. Qb is much more stable than Qa . since forward electron transfer does not take place from Qb - The lifetime of Qb, like that of Qa in the presence of DCMU, is determined by the stability and availability of positive charges on the donor side. For example, Qb recombination occurs with S2 or S3 (Ref. 115, and see section 3.5) with a ty2 of approximately 30 s [116] but when Qb is present in centres where the stable S states, S,) and Sj, are present, Qb is stable for hours. This probably explains why a certain proportion of Qg is present even in PS II which has been dark-adapted for long periods. A number of measurements have indicated the involvement of proton uptake when Qb is reduced to semiquinone form [117], although the optical spectrum is more compatible with Qb being the unprotonated anion. This can be explained by the protonation of a group on the protein close to Qg, as first proposed in purple bacterial reaction centres to explain similar phenomena [118]. 

---