Bis(tetrathiafulvalenes) with aromatic bridges: electron delocalization in the oxidized species through EPR and theoretical studies

A series of bis(TTF) donors containing aromatic linkers between the two TTF units has been synthesized in order to investigate on the electronic structure of the oxidized species from an experimental and theoretical point of view. A mono(TTF)-pyridine compound has been also prepared and characterized by single-crystal X-ray diffraction analysis. Oxidation of a solution of 2,6-bis(TTF)-pyridine (TTF-Pyr-TTF) or of 1,3-bis(TTF)-benzene (TTF-Bz-TTF) in CH₂Cl₂ with less than 0.1 equivalent of [Cp₂Fe][PF₆] gives rise to a seven-line EPR spectrum consistent with the hyperfine structure calculated by DFT for the corresponding radical monocation. Increasing the proportion of oxidant leads to a four-line hyperfine structure, similar to the quartet pattern observed after oxidation of mono(TTF)-pyridine (Pyr-TTF) or mono(TTF)-benzene (Bz-TTF). In good accordance with the very weak value of J calculated by DFT for the dicationic biradicals these four-line spectra are attributed to [2,6-bis(TTF)-pyridine]²⁺ and [1,3-bis(TTF)-benzene]²⁺. Similar experimental results are obtained for 1,4-bis(TTF)-benzene. In this case, however, electrochemical oxidation leads to the monoradical at low potential and to the diradical at higher potential, while only the diradical could be observed by electrochemical oxidation of 2,6-bis(TTF)-pyridine or of 1,3-bis(TTF)-benzene.