Effect of structural factor on the electropolymerization of bithiophenic precursors containing a 3,4-ethylenedisulfanylthiophene unit

A series of bithiophenes based on bis(3,4-alkylenedisulfanylthiophene) and hybrid systems associating 3,4-ethylenedisulfanylthiophene (EDST) with thiophene or 3,4-ethylenedioxythiophene (EDOT) moieties have been synthesized. The molecular structures of these systems are characterized by X-ray diffraction and theoretical geometric optimization. Although bis(alkylenesulfanylthiophenes) present a large dihedral angle between the thiophene cycles, for the hybrid systems the steric hindrance to coplanarity is counterbalanced by the development of intramolecular (SS)-S-... and (SO)-O-... interactions, Dramatic structural change from twisted to planar conformation of bis(3,4-ethylenedisulfanylthiophene) is observed when the dimer is associated with TCNQ. The electronic properties of the dimers, analyzed by UV-vis spectroscopy, cyclic voltammetry, and theoretical calculations, are discussed in relation with the molecular structure of the bithiophenes. Finally, the analysis of the electrogenerated polymers confirm the key role of the electronic and self-structuring effects for enhancing the planarity or the conjugated systems and for controlling the gap of the polymers. Thus, the precursor combining EDST and EDOT units shows the lowest oxidation potential and leads to a polymer presenting a lower band gap than those of the two polymers derived from the pure EDOT or EDST dimers.