pi conjugation across the tetrathiafulvalene core: Synthesis of extended tetrathiafulvalene derivatives and theoretical analysis of their unusual electrochemical properties

A series of extended tetrathiafulvalene (TTF) derivatives bearing one or tw o 1,4-dithiafulven-6-yl substituents has been prepared. The new compounds p resent remarkable electrochemical singularities compared with other TTF der ivatives, which are strongly affected by the nature of the substitution on the lateral heterocycle(s). This unusual electrochemical behaviour follows a square-scheme sequence and is attributed to structural changes upon oxida tion of the pi-donating molecules. Digital simulations of the electrochemic al data have been used to reach the values of the kinetic and thermodynamic constants involved in the square scheme. Theoretical calculations establis h an important contribution of a highly delocalised resonant form involving a tetravalent sulphur in oxidised species, which could justify the occurre nce of an electrochemical behaviour distinct from that of TTF. Finally, thi rd-order susceptibilities chi(3) of two of these systems, for which electro n-donating and electron-withdrawing substituents coexist and are conjugated through the TTF pi system, are given.