Reactivity of oxidized oligothiophenes substituted with methoxy and thioalkyl groups

Abstract α-Oligothiophenes with one to five rings, substituted with β-methoxy groups and terminated with either (2-hydroxyethyl)thio ( 1–5 ) or (2-acetoxyethyl)thio groups ( 1a–5a ) were synthesized. Cyclic voltammetry, coulometry and vis spectroscopy were used to investigate the reactivity of the cation radicals and dications. The cation radicals, even the ‘monothiophene cation radical’ 3-methoxy-2,5-bis(hydroxyethylthio)thiophene ( 1 1+ ), were somewhat stable in acetonitrile. The dications of the acetoxy derivatives 2a–5a were also stable on this time scale. Dications 1a 2+ and 1 2+ – 5 2+ were not stable. The instability of 2 2+ – 5 2+ is attributed to an intramolecular reaction between the terminal hydroxy groups and the oxidized thiophene units. This reaction led to the isolation of an unusual spiro product from a preparative scale oxidation of bithiophene 2 . Several of the cation radicals precipitated on the electrode surface. The unusual shape of the voltammograms which resulted was investigated, and evidence for the formation of two different salts, one fully oxidized and one mixed valence, was obtained.