Oligothiophene Cation Radicals. π-Dimers as Alternatives to Bipolarons in Oxidized Polythiophenes

Three oligomers of thiophene, 2, 5ʺ-dimethylterthiophene (3Th-Me), 2, 5ʹ-(dithiomethyl)-5, 2ʹ-bithienyl (2Th-SMe), and 2, 5ʺ-(dithiomethyl)-5, 2ʹ,5ʹ,2ʺ-terthienyl (3Th-SMe), were prepared to model polythiophene. In these compounds, the thiophene rings were α-linked and the terminal α-positions were blocked with methyl or thiomethyl substituents to prevent polymerization. Electrochemical methods were used to generate cationic solution analogues of these postulated in oxidized polythiophenes. UV-vis-near-IR spectroelectrochemistry, in situ ESR, and cyclic voltammetry (CV) provided a sufficiently short time scale to characterize one-and two-electron oxidation products without incursion of further, multielectron, chemistry. Variable-temperature spectroelectrochemistry at potentials corresponding to the first CV oxidation peaks of the oligomers revealed the formation of cation radicals and their diamagnetic π-dimers at equilibrium. At more positive potentials, dications were formed and characterized. These results are compared to previously characterized oligothiophene cations and oxidized polythiophene. It is suggested that the radical dimerization phenomena enucleated here may be common for oxidized oligo-and polythiophenes and can explain previous observations attributed to bipolarons.