Intervalence transfer transitions

Table 4. EPR and Intervalence Transfer Transition Parameters for Half Met L Hemocyanlns...

Unlike the discrete molecular complexes described above, many compounds with bridging cyanide ligands are three-dimensional coordination polymers. The classic example of this type of compound is Pmssian blue, [Fe4 Fe(CN)6 3]-xH20 (13), whose intense color arises from an intervalence transfer transition see Intervalence Transfer Transition) from low-spin Fe + to high-spin Fe +... 

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. 

Ferrocenyl derivatives of 73, namely (Co3(CO)9 L (/i3-CFc)] (n = 0-3, L = phosphine or phosphite) undergo reversible reduction at the tricobalt center to detectable radical anions (n = 0), and oxidation at the iron center to give isolable cations (182). For the trisubstituted derivatives [ = 3, L = P(OMe)3 or P(OPh)3], the monocations appear to be mixed-valence species in which the two redox sites are weakly interacting low-energy absorption bands, assigned to intervalence transfer transitions, have been observed. The dications (Co3(CO)6 P(OR)3 3(/i3-CFc)] (R = Me or Ph) are also isolable, oxidation having occurred at both possible redox sites (183). 

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 photophysical pathway leading from chromophore absorption to luminophore emission is a-e. Processes f and g correspond to the intervalence transfer transitions detected in flash photolysis. 

As seen in Sections 4.1 and 4.2, a Ru(II) -> Cr(III) intervalence state is present in all the chromophore-luminophore systems. On the other hand, the study of the photophysics originating from this state is precluded by the presence of more intense, overlapping MLCT absorption bands of the chromophore. A system designed to allow selective intervalence transfer excitation is the trinuclear complex shown in Fig. 17 [88]. In this system, the Ru(II) center has similar oxidation potential as in the previously studied cases (and thus similar intervalence transfer transitions are expected), but the Ru(II)-based component is not chromophoric as it lacks of the polypyridine ligands responsible for MLCT absorption. Actually, a single prominent band, of intervalence transfer type, is present at 338 nm (aqueous... 

Levanda, C., Cowan, D. O., Bechgaard, K. Electronic and Stereoelectronic Effects on Intervalence Transfer Transition in Biferrocene Cations. J Am Chem Soc 1975, 97, 1980-1981. 

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