Optical Electron Transfer Intervalence Transitions

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). 

The absorption spectrum of a supramolecular system can differ substantially from the sum of the spectra of the molecular components. Aside from those small shifts that can be dealt with in terms of perturbation of the spectra of the single components upon bridging, some totally new bands can be present in the spectrum of the supermolecule. These bands correspond to optical electron transfer transitions, commonly denominated charge-tranter and intervalence transfer transitions, in the organic [26] and inorganic [23-25] literature, respectively (process 7 in Fig. 4, eq 1). The factors that determine the spectroscopic characteristics of such bands... 

The main spectroscopic consequence of the combined action of electron transfer and vibronic interaction is the occurrence of the so-called electron transfer optical absorption (intervalence band), which is shown by the arrows in Fig. 10. The shape and intensity of the intervalence band in the PKS model is defined by the ratio t /(v /cu). In the case of weak transfer the Franck-Condon transitions are almost forbidden, and at the same time, the Stokes shift can be significant. Therefore the MV dimers of Class I are expected to exhibit weak and wide intervalence bands. On the contrary, in the Class III compounds the Franck-Condon transition is allowed, and the Stokes shift is zero. For this reason, intervalence optical bands in delocalized MV dimers are strong and narrow. When the extra electron jumps over the spin cores in a multielecton MV dimer d" — > 1) [85-87] we are dealing with... 

Figure 1 Hush diagram for intervalence transfer within a class II mixed-valence ion. The dotted lines correspond to diabatic potential energy surfaces. The solid lines are adiabatic potential energy surfaces. Electron transfer can occur either optically (vertical transition with energy, Eop, equaling A) or thermally by moving along the lower adiabatic surface. In the diabatic limit, the barrier height for thermal electron...

Spectroelectrochemistry provides a convenient avenue to assess whether this conclusion is correct by allowing the isolation and spectroscopic smdy of the mixed valence state. Oxidation of the first metal center results in the formation of the mixed valence Ru(II)Ru(III) ion reflected in the grow-in of a new optical transition centered at 1700 nm (s = 2250 dm mol cm ) and identified as an intervalence charge transfer transition (IVCT). The optical characteristics of such transitions can be analyzed according to the Hush theory (21) and used to estimate the extent of electronic coupling between two metals across the intervening bridge. The full width at half maximum (FWHM) (Vj j) of the... 

Optical Spectra. The optical properties of smectites have been studied by various workers (32,37-40), and involve several different types of electronic transitions. One important type of transition is the intervalence charge transfer (IT), which is observed in the optical spectra of minerals containing both Fe2 and Fe3 in their... 

The simplest model consists of two centres, one donor (D) and one acceptor (A), separated by a distance I and contains two electrons. Here we consider this simple system to illustrate some general relations between charge transfer, transition intensities and linear as well as non-linear optical polarizabilities. We will show below that the electro-optic parameters and the molecular polarizabilities may be described in terms of a single parameter, c, that is a measure of the extent of coupling between donor and acceptor. Conceptually, this approach is related to early computations on the behaviour of inorganic intervalence complexes (Robin and Day, 1967 Denning, 1995), Mulliken s model for molecular CT complexes (Mulliken and Pearson, 1969) and a two-form/two-state analysis of push-pull molecules (Blanchard-Desce and Barzoukas, 1998). 

Fig. 2.21. Polarized electronic (optical) absorption spectra of a vivianite crystal with zones (labeled 1, 2, 3) exhibiting three different degrees of oxidation. The arrows identify Fe + crystal-field bands at 8,300 and 11,400 cm , and the Fe + —> Fe intervalence charge-transfer transition at 15,800 cm (after Amthauer and Rossman, 1984 reproduced with the publisher s permission).

---