Cation valences from absorption spectra

An important point is that the electrochemically driven charge transport in these polymeric materials is not dependent on the presence of mixed valence interactions which are well known to give rise to electronic conductivity — in a number of cation radical crystalline salts. This is clearly seen from the absorption spectrum of the electrochemically oxidized pyrazoline films (Figure 8) which show no evidence for the mixed valence states that are the structural electronic prerequisites for electrical conductivity in the crystalline salts. A more definitive confirmation of this point is provided by the absorption spectrum (Figure 10) of electrochemically oxidized TTF polymer films which shows... 

The valence and coordination symmetry of a transition metal ion in a crystal structure govern the relative energies and energy separations of its 3d orbitals and, hence, influence the positions of absorption bands in a crystal field spectrum. The intensities of the absorption bands depend on the valences and spin states of each cation, the centrosymmetric properties of the coordination sites, the covalency of cation-anion bonds, and next-nearest-neighbour interactions with adjacent cations. These factors may produce characteristic spectra for most transition metal ions, particularly when the cation occurs alone in a simple oxide structure. Conversely, it is sometimes possible to identify the valence of a transition metal ion and the symmetry of its coordination site from the absorption spectrum of a mineral. 

Interestingly, an unusual mixed-valence state of a stacked tetrathiafulvalene (TTF) dimer was observed within the coordination host 1 at ambient temperature in an aqueous solution [10], When excess TTF was added to an aqueous solution of 1, the colorless solution became dark green in color because of the formation of a 1 D (TTF)2 complex (Fig. 4.3a). Electrochemical studies revealed that an initial one-electron oxidation occurred at -150 mV that led to the mixed-valence dimer, whereas a second one-electron oxidation at -300 mV afforded the cation radical dimer. The mixed-valence state was also indicated by the appearance of a broad absorption band in the near-infrared region ( max = -2000 nm) of the UV-vis spectrum (Fig. 4.3b). The host framework effectively forced the two molecules of TTF into close proximity in the cavity. As a result, the labile mixed-valence dimer was protected from oxygen and solvent molecules. 

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