Electrochemistry and Polymerization Mechanisms of Thiophene−Pyrrole−Thiophene Oligomers and Terthiophenes. Experimental and Theoretical Modeling Studies

Abstract

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° 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 α-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 α-terminal positions are substituted by bromides. We propose that this reaction involves a hindered α−α‘ coupling followed by a nucleophilic attack of the protonated dimer and not the coupling with one of the β-positions (or β‘ or β‘‘). This view is supported by comparison with the electrochemical behavior of other oligothiophenes and oligopyrroles.