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Charge transfer redox species

We recognize redox reactions by noting whether electrons have migrated from one species to another. The loss or gain of electrons is easy to identify for monatomic ions, because we can monitor the charges of the species. Thus, when Br ions are converted into bromine atoms (which go on to form Br2 molecules), we know that each Br ion must have lost an electron and hence that it has been oxidized. When 02 forms oxide ions, 02-, we know that each oxygen atom must have gained two electrons and therefore that it has been reduced. The difficulty arises when the transfer of electrons is accompanied by the transfer of atoms. For example, is chlorine gas, Cl2, oxidized or reduced when it is converted into hypochlorite ions, CIO" ... [Pg.103]

DCE interface in the presence of TPBCl [43,82]. The accumulation of products of the redox reactions were followed by spectrophotometry in situ, and quantitative relationships were obtained between the accumulation of products and the charge transfer across the interface. These results confirmed the higher stability of this anion in comparison to TPB . It was also reported that the redox potential of TPBCP is 0.51V more positive than (see Fig. 3). However, the redox stability of the chlorinated derivative of tetra-phenylborate is not sufficient in the presence of highly reactive species such as photoex-cited water-soluble porphyrins. Fermin et al. have shown that TPBCP can be oxidized by adsorbed zinc tetrakis-(carboxyphenyl)porphyrin at the water-DCE interface under illumination [50]. Under these conditions, the fully fluorinated derivative TPFB has proved to be extremely stable and consequently ideal for photoinduced ET studies [49,83]. Another anion which exhibits high redox stability is PFg- however, its solubility in the water phase restricts the positive end of the ideally polarizable window to < —0.2V [85]. [Pg.200]

A discussion of the charge transfer reaction on the polymer-modified electrode should consider not only the interaction of the mediator with the electrode and a solution species (as with chemically modified electrodes), but also the transport processes across the film. Let us assume that a solution species S reacts with the mediator Red/Ox couple as depicted in Fig. 5.32. Besides the simple charge transfer reaction with the mediator at the interface film/solution, we have also to include diffusion of species S in the polymer film (the diffusion coefficient DSp, which is usually much lower than in solution), and also charge propagation via immobilized redox centres in the film. This can formally be described by a diffusion coefficient Dp which is dependent on the concentration of the redox sites and their mutual distance (cf. Eq. (2.6.33). [Pg.332]

The concentration of the intermediate species after the first electron charge transfer at the three redox ratios can be represented as C°R]9 C%, and C°Rl, respectively. The relationship between them can be given as... [Pg.250]

If the concentrations of the intermediate species vary at the five redox ratios as C°Rll, C n, C jj, and C n after the first electron charge transfer and C°Rl, C°Rl, C, , C ", and C°Rl after the second electron charge transfer, then the relationships... [Pg.255]

T.W. Hamann, F. Gstrein, B.S. Brunschwig, N.S. Lewis, Measurement of the dependence of interfacial charge-transfer rate constants on the reorganization energy of redox species at ZnO/H20 interfaces,/. Am. Chem. Soc. 127 (2005) 7815-7824. [Pg.383]

The chemical association of the exciplex results from an attraction between the excited-state molecule and the ground-state molecule, brought about by a transfer of electronic charge between the molecules. Thus exciplexes are polar species, whereas excimers are nonpolar. Evidence for the charge-transfer nature of exciplexes in nonpolar solvents is provided by the strong linear correlation between the energy of the photons involved in exciplex emission and the redox potentials of the components. [Pg.95]

Photochemical ET reactions can be classified in at least three categories (which can co-exist), namely (i) simple homolysis of bonds of neutral molecules to give radicals of low redox reactivity (ii) excitation of a species D to produce an excited state D which initiates a second-order ET reaction involving another component of acceptor type, A, with formation of the radical pair D + A (iii) direct excitation of a charge transfer (CT) complex formed between two reaction components D and A to form the same radical pair D + A -. The first case is obviously an ideal situation if it can be realized, but this is seldom the case. The incursion or predominance of situations (ii) and/or (iii) in almost any system is possible, and precautions must be taken to avoid these complications. Much can be done by controlling the wavelength of the light source, but it is also possible to affect the chemistry in a predictable manner. [Pg.119]

Such reactions are commonly found as a result of the decomposition of charge transfer excited states. For example, while excitation of the MC bands of Co(NH3)5X (X = Cl, Br, I) leads to photosolvation and the formation of Co(NH3)5(0112) and Co(NH3)4(OH2)X, shorter wavelengths yield the LMCT state which decomposes into Cc II) ions and halogen atoms. The quantum yield for the reaction is found to depend on the excitation energy, indicating a role for the initially formed radical pair (Eq. 5). This may reform the starting complex (Eq. 6) or decompose to the redox products stabilised by the solvent or some other species (Eq. 7). The Co(II) complexes eventually decomposes (Eq. 8). [Pg.32]

From equation (3.4.31), if the ratio n/nso is unity there will be no net current flow across the interface this condition is depicted in Fig. 3.13(a) for an n-type semiconductor. Under this equilibrium state surface electrons can undergo isoenergetic electron transfers to the redox species due to a built-in potential, equal to the difference of potential between Ecb and Eredox- Equilibrium can be perturbed, with a resulting observable transient current flow, by varying the concentrations of the redox species. The surface electron concentration ng is related to the bulk concentration no by the potential difference of the space charge layer as follows ... [Pg.145]

The study of Li28 + DMF solutions [60] also allowed to characterize the electrochemical properties of polysulfides only redox couples of the type 8 /8 are involved. The chemical reactions coupled to charge transfers are classical dissociation and disproportionation equilibria no complex rearrangement reaction or transient species has been necessary. Redox potentials and charge-transfer coefficients of the redox couples involved in sulfur and polysulfide solutions are summarized in Table 2. [Pg.263]

Similar photovoltaic cells can be made of semiconductor/liquid junctions. For example, the system could consist of an n-type semiconductor and an inert metal counterelectrode, in contact with an electrolyte solution containing a suitable reversible redox couple. At equilibrium, the electrochemical potential of the redox system in solution is aligned with the Fermi level of the semiconductor. Upon light excitation, the generated holes move toward the Si surface and are consumed for the oxidation of the red species. The charge transfer at the Si/electrolyte interface should account for the width of occupied states in the semiconductor and the range of the energy states in the redox system as represented in Fig. 1. [Pg.330]

Jeveral aspects of the photolytic behavior of aqueous complex ions have been studied in this laboratory over the past few years. One continually interesting question has been the extent to which the photochemistry of a complex depends on the absorption band irradiated. In the case of Co(III) acidopentamines, such as Co(NH3)5Br+2, we found that irradiation of Ajg —> g) bands showing appreciable charge transfer led to redox and aquation reactions which were competitive. It was reasonable to suppose that the common precursor was the species formed by a prompt heterolytic bond fission (7). The ( Aig —> Tig) band was far less photoactive, and in model cases, irradiation led only to aquation. Each excited state or excited state manifold thus tended to show a distinct photochemistry, which meant that conversion from one excited state to another was not important. [Pg.236]


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See also in sourсe #XX -- [ Pg.3 , Pg.7 ]




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