SPECTRA AND REACTIVITY OF METHOXYOLIGOTHIOPHENE CATION RADICALS

A set of alpha-coupled oligothiophenes, dimer through hexamer, was stu died. Each oligomer was terminated with two methyl groups and was subs tituted with two or four methoxy groups. Electrochemical studies showe d that methoxy groups substituted at ''inside'' beta-positions on the terminal rings (positions toward the center of the chain) gave stable cation radicals. The E(0) values for the reversible couples were 0.24- 0.52 V (SCE). The changes in E(0) with structure were explained using PPP molecular orbital calculations. Cation radicals were generated usi ng either chemical or electrochemical oxidation, and in several cases stable salts were isolated. The vis-near-IR spectra of the cation radi cals were recorded. In CH2Cl2 solvent the cation radicals were monomer ic and showed two pi-pi-bands, which shifted to longer wavelength wit h oligomer length. In CH3CN solvent the cation radicals were primarily in the form of pi-dimers, which showed two pi-pi-bands and a charge transfer band at longer wavelength in the near-IR. All three bands shi fted to longer wavelength with oligomer length. The equilibrium consta nt for dimerization increased by a factor of 10(3) on going from a ter thiophene to a quinquethiophene. Spectra of oligomers terminated with hydrogens or dimethylphenylsilyl groups showed that steric effects of the small hydrogen and bulky silyl group on pi-dimerization were very small. Methyl-terminated oligomers substituted with ''outside'' methox y in the terminal rings gave rather positive oxidation potentials and unstable cation radicals. In one case, the cation radical decompositio n products were studied and it was shown that reactions had occurred o n a terminal methyl group.