Molecular engineering of hybrid π-conjugated oligomers combining 3,4-ethylenedioxythiophene (EDOT) and thiophene-S,S-dioxide units

A series of π-conjugated oligomers based on various combinations of thiophene, 3,4-ethylenedioxythiophene (EDOT) and thiophene-S,S-dioxide units have been synthesized. Theoretical calculations, optical and electrochemical data show that the relative positions of EDOT and S,S-dioxide units in the conjugated system exert strong influence on its electronic properties. The insertion of EDOT units inside the conjugated chain with the S,S-dioxide units located at the lateral positions leads to a planar conjugated system presenting moderate band gap. In contrast, substituted S,S-dioxide units located within the backbone produce a torsion of the conjugated chain. However, the twist angle is significantly reduced when the EDOT and S,S-dioxide moieties are separated by unsubstituted thiophene cycles. Introduction of S,S-dioxide unit in the median position of the chain has a stronger effect on the lowering of the LUMO level while insertion of EDOT units at the lateral positions limits the decrease of the HOMO level. Consequently, the corresponding oligomers present the smallest electrochemical gap while electrooxidation leads directly to the dication state through a bi-electronic process.