Electrochemical synthesis of a pyridinium-conjugated assembly based on nucleophilic substitution of oligothiophene π-radical cation

Abstract

A series of pyridinium-conjugated oligothiophenes is synthesized electrochemically by nucleophilic substitution of the oligothiophene π-radical cation. The nucleophilic reaction is designed on the basis of high reactivity of the π-radical cation of oligothiophene with various pyridine derivatives as nucleophiles. While electrochemical oxidation of oligothiophene gave a conducting polythiophene film on an electrode, the addition of pyridine depressed formation of the polythiophene film but the corresponding pyridinium-conjugated oligothiophene was exclusively formed as the product of nucleophilic substitution of the oligothiophene π-radical cation. In prolonged electrolysis at a higher potential, successive (two-step) substitution occurred to yield both mono- and di-(1-pyridinio)oligothiophenes. The nucleophilic substitution reaction was optimized with respect to the electrolytic conditions, and the nucleophilic attack toward the oligothiophene π-radical cation is discussed. The UV-visible absorption spectra and redox activities of the resulting pyridinium-conjugated oligothiophene were measured. Negative solvatochromism due to intramolecular charge transfer (IMCT) and redox behavior resulting from oligothiophene and pyridinium moieties are demonstrated.