Evelin Jähne

Charge stabilisation by dimer formation of an endcapped thiophene tetramer—an in situ NMR spectroelectrochemical study

The role of dimers in the charge stabilisation of 5,5′′′-dicyano-3,3′′-dibutyl 2,2′:5′,2′′:5′′,2′′′-quaterthiophene (DCNDBQT) via an electrochemical reaction is studied in detail by in situ NMR spectroelectrochemistry. Among the two oxidation steps the first was found to be reversible while the second is completely irreversible. By in situ NMR spectroelectrochemistry we are able to demonstrate the formation of a π-dimer via the cation generated in the first electron transfer as well as the formation of a σ-dimer by a follow-up reaction of the dication generated in the second oxidation step. As the thiophene oligomer is blocked in the 2- and 5-positions by the cyanide group the monomer undergoes a dimerisation in the 4′-position. In situ NMR spectroelectrochemistry delivers proof of the coupling reaction of the thiophene rings in the electrochemistry of α,ω-endcapped oligothiophenes although such endcapped structures are expected to be prevented from chemical follow-up reactions. Furthermore, substitution by cyano groups opens the route to a cathodic reduction of the structure. The dicyanodibutylquaterthiophene gives a single reduction step which turns out to be quasi-reversible but the anionic structure formed in the reduction was not detectable by in situ NMR spectroelectrochemistry. This and the follow-up reactions are discussed in detail.

Novel Modified Pyrrole Monomers, 1: Synthesis and Characterization

Different 1-substituted-(pyrrole alkyl)phosphonic acid derivatives have been designed as adhesion promoters to graft polypyrrole layers on several substrates. These compounds contain two reactive, terminal moieties: the phosphonic acid group as anchor to the oxide surface and the pyrrolyl group, which functions as starting unit for surface polymerisation. Both groups are connected by an alkyl spacer ranging from 4 to 12 methylene units. The short- and long-chain derivatives were synthesized for the first time to study their self-assembling properties and their effectiveness to be used as an adhesion promoter for grafting covalently bound polypyrrole layers on oxide substrates. Surfactant-like properties of the long chain derivatives and their behaviour in water for pyrrole polymerization were also investigated, as will be discussed in future papers. In this first paper of the series, the synthesis and characterization of the monomers are presented. The structure of the molecules was proven by 1H-NMR, 31P-NMR and FT-IR spectroscopy. Thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC) revealed that the stability of the compound depended on their chain length: the longer the alkyl chain, the more stable the monomer. On the other hand, reactivity decreased with increasing chain length, as proven by cyclic voltammetry measurements (CV).