Spectra and Reactivity of Methoxyoligothiophene Cation Radicals

A set of α-coupled oligothiophenes, dimer through hexamer, was studied. Each oligomer was terminated with two methyl groups and was substituted with two or four methoxy groups. Electrochemical studies showed that methoxy groups substituted at “inside” β-positions on the terminal rings (positions toward the center of the chain) gave stable cation radicals. The E0 values for the reversible couples were 0.24−0.52 V (SCE). The changes in E0 with structure were explained using PPP molecular orbital calculations. Cation radicals were generated using either chemical or electrochemical oxidation, and in several cases stable salts were isolated. The vis−near-IR spectra of the cation radicals were recorded. In CH2Cl2 solvent the cation radicals were monomeric and showed two π−π*-bands, which shifted to longer wavelength with oligomer length. In CH3CN solvent the cation radicals were primarily in the form of π-dimers, which showed two π−π*-bands and a charge transfer band at longer wavelength in the near-IR. All thr...