Intervalence charge transfer , redox

By applying an outside potential, many metal oxide films are switchable to a nonstoichiometric redox state, creating an intervalence charge transfer with an... 

In the nonstochiometric redox state, intervalence charge transfer between adjacent WVI and Wv sites leads to the observed intense blue color. Apparently, if the ratio of WVI to Wv is increased further, the ion insertion becomes irreversible, resulting in a permanent color change to red or golden [34,35]. It should be noted that in spite of the apparent progress in explaining the electrochromism on a fundamental level, questions remain and alternate mechanisms have been proposed which suggest the involvement of other oxidation states [36]. 

The charge transfers in polynuclear complexes with metal centres ready to undergo redox reactions are called intervalence charge transfer (IT). These compounds are often characterized by low-energy M-M transitions and thereby have intense colour (eg Prussian blue and its analogues). 

Figure 1.46). ° The monocation 97 can be considered as a push-pull chromophore and exhibits a / i.o6 (HRS) value twice that of its neutral precursor 97 and the fully oxidised compound 97. Complex 97 is a class II mixed-valence complex, retaining charge localisation, but with enough electronic coupling between the redox centres to permit intervalence charge transfer transitions. As the redox couples 97/97" and... 

The interesting redox chemistry of such conjugated polymeric metallocene systems has been reviewed [121]. For example, oligo(l,ll -dihexylferrocenylenes) 61 show an intervalence charge transfer band (IVCT) on oxidation. For the singly oxidized chains, the IVCT band zmax decreases as the number of ferrocene units increases, as expected. Also, as expected the energy of the IVCT band increases as the oxidation level increases. 

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