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. The role of π- and σ-dimer formation in an electrochemical reaction of an endcapped quarterthiophene is investigated by in situ NMR spectroelectrochemistry.