Electrochemistry and polymerization mechanisms of thiophene-pyrrole-thiophene oligomers and terthiophenes. Experimental and theoretical modeling studies

The electrochemical properties of several substituted terheterocycles thiophene-pyrrole-thiophene oligomers have been studied by cyclic voltammetry and double-potential-step chronoamperometry and then compared with the behavior of the corresponding pure terthiophenes. E degrees and lifetimes of the cation radicals have been measured. Ab initio and DFT calculations show that the cation radicals of thiophene-pyrrole-thiophene oligomers (N-alkyl-2,5-bis(thien-2-yl)pyrroles) are twisted in contrast to the terthiophene case where cation radicals are planar. These geometries explain the variations of the oxidation potentials with the substituents positions. When the alpha-positions are free, the cation radical undergoes a fast coupling reaction in solution, leading to either a dimer or a polymer involving the coupling between two cation radicals. The same cation radical-cation radical coupling reaction is involved when the alpha-terminal positions are substituted by bromides. We propose that this reaction involves a hindered alpha-alpha' coupling followed by a nucleophilic attack of the protonated dimer and not the coupling with one of the beta-positions (or beta' or beta "). This view is supported by comparison with the electrochemical behavior of other oligothiophenes and oligopyrroles.