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 volt ammetry and double-potential-step chronoamperometry and then compared with the behavior of the corresponding pure terthiophenes. E degrees a nd lifetimes of the cation radicals have been measured. Ab initio and DFT calculations show that the cation radicals of thiophene-pyrrole-th iophene oligomers (N-alkyl-2,5-bis(thien-2-yl)pyrroles) are twisted in contrast to the terthiophene case where cation radicals are planar. T hese geometries explain the variations of the oxidation potentials wit h the substituents positions. When the alpha-positions are free, the c ation radical undergoes a fast coupling reaction in solution, leading to either a dimer or a polymer involving the coupling between two cati on radicals. The same cation radical-cation radical coupling reaction is involved when the alpha-terminal positions are substituted by bromi des. We propose that this reaction involves a hindered alpha-alpha' co upling followed by a nucleophilic attack of the protonated dimer and n ot 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.