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.