Electron transfer intervalence

All compounds were measured at room temperature in air between Au electrodes inside a Faraday cage. IVT = maximum of the optical InterValence Transfer or intramolecular charge transfer band. RR Rectification Ratio. Survives cycling means RR does not decrease with cycling of IV measurements. AR or anti-AR indicates whether electron flow is from A to D (AR) or from D to A (anti-AR)... 

Where thermal electron transfer can occur, there is also always a corresponding optical transfer, which I have termed intervalence transfer (19-21). The principle is illustrated in a single coordinate representation in Figure 2. Clearly, it will be possible to relate thermal to optical transfer probabilities, and, hence, to predict the rate of one process from parameters obtained for the others. The connection is, in fact, a very close one, as indicated schematically in Figure 3. 

Many of these complexes are deeply colored and their electronic spectra contain very strong intervalence transfer bands in the visible, due to electron transfer from B to A (Robin and Day, 1967). They also show magnetic coupling and they have semiconducting properties. They are stable in air and in acid solutions, but they dissociate in alkali. They can be reversibly oxidized, whereby B gives up an electron and goes to a higher oxidation state, and the intense color disappears. 

An increase in the electronic coupling interaction results in the disappearance of the ET barrier and complete delocalization of the transferred electron between the donor and the acceptor. Such effects have been extensively studied for intramolecular ET in bridged intervalence compounds [57]. As regards intermolecular systems, the only spectrally and structurally characterized system has been that of NO+/arene complexes [28]. 

For a series of similar class II mixed-valence complexes that differ only in the nature of the bridging ligand, MMCT band energy will increase with increasing distance between the metal ions. This dependence has its origin in the magnitude of x as described by Eq. (19) and has been the subject of study (136,137). The relationship between Eop and the barrier to thermal electron transfer [Eq. (1)] was nicely demonstrated by a correlation between electron-transfer rate constants and intervalence-transfer energies (138) and is shown in Fig. 12. 

Intervalence charge transfer Electron transfer (thermal or photoinduced) between two metal sites differing only in oxidation state. Quite often such electron transfer reverses the oxidation states of the sites. The term is frequently extended to the case of metal-to-metal charge transfer between non-equivalent metal centers. 

For class I systems, 0 = 0 and the mixed valence absorption bands observed generally fall at frequencies larger than 27 000 cm , except when the system contains a colored ion as a constituent. Intervalence interactions (see Intervalence Transfer Transition) can provide a source for intensification of such ligand field transitions, however, in class I systems. The mixed valence electronic transition is given by equation (2), where has the same meaning for the excited state as a does for the ground state. Hence a = /J = 0 for true class I behavior. 

The copper(I) alkynyls displayed rich photochemistry and particularly strong photoreducing properties. The transient absorption difference spectrum of [Cu3(dppm)3(/X3-) -C=CPh)2]+ and the electron acceptor 4-(methoxycarbonyl)-A-methylpyridinium ion showed an intense characteristic pyridinyl radical absorption band at ca. 400 nm. An additional broad near-infrared absorption band was also observed and it was assigned as an intervalence-transfer transition of the mixed-valence transient species [Cu Cu Cu (dppm)3(/x3- -C=CPh)2] +. The interesting photophysical and photochemical properties of other copper(I) alkynyl complexes such as [Cu(BTA)(hfac)], 2 [Cui6(hfac)8(C=C Bu)8], and [Cn2o(hfac)8(CsCCH2Ph)i2] have also been studied. 

Optical electron transfer (often designated intervalence transfer, IT, in the case of binuclear mixed-valence transition metal complexes [6, 12]), occurring vertically from the equilibrium configuration of the initial state (Figure 2a). 

All three classes of intramolecular electron transfer are examples of scheme I ( 12.2.2.2.1) kinetics in which the precursor complex is sufficiently stable to observe under some condition. These classes differ in the components that make up the precursor complex. In one, two metal ions with similar coordination spheres are involved. The classic example is [(NH3)jRu(pyz)Ru(NH3)5] where pyz is pyrazine there are other examples with Ru Cu(l) and Cu(II), ferrocene and cyanide complexes of Fe. This field began with Prussian Blue (1704 est.). In these species the phenomenon of intervalence transfer is exhibited. Species of this sort have a low extinction coeffi-... 

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