INTERVALENCE TRANSITIONS IN MIXED-VALENCE BIS(TETRATHIAFULVALENE) COMPOUNDS

The series of new bis(tetrathiafulvalene) [bis(TTF)] compounds 10-12a,b have been synthetized with the TTF-like synthon 8a,b, previously used to prepare the pyrazine-fused bis(TTF) 7a,b. These compounds exhibit four reversible sequential one-electron oxidation steps, on the basis of cyclic voltammetry. For the cation radicals 7b(.+), 10(.+), and 12(.+), generated electrochemically, rather intense and broad bands in the near-IR region, specific of class II mixed valence compounds, were found. Theoretical calculations at the semiempirical AM1 level as well as at the ab-initio level indicated that these cation radicals are charge-distributions localized as related to mixed valence species. These calculations also allowed the low-energy absorption bands to be assigned to intervalence transitions; these transitions imply the existence of multiple low-lying excited states that are non-adiabatically coupled to the ground-state of these open-shell species.