Redox states and associated interchain processes of thienylenevinylene oligomers

The electronic structure of successive redox states of two series of thienylenevinylene oligomers (nTVs, n=4, 5, 6, 8, and 12) that carry hexyl substituents at the - or -positions in order to increase the solubility was investigated in detail by means of UV/Vis/near-IR and ESR spectroscopy in solution. The nTV redox states have been fully characterized up to the dication for the shorter oligomers (n6) and up to the tetracation for the longer oligomers (n8). While the monocation radicals of the nTVs exhibit two dipole-allowed electronic transitions in the Vis/near-IR region, all the higher oxidized states invariably show a single strong absorption in the near-IR region. The electronic transitions of the various oxidized states and those of their vibronic replicas shift to lower energies with increasing conjugation length. The ESR spectra, recorded as a function of the degree of oxidation, provide evidence for the presence of electron spin in the odd-charged and the absence of spin in even-charged cations. Variable-temperature UV/Vis/near-IR and ESR spectroscopy establish that the tendency of the nTV monocation radicals to form spin-less dimers in solution strongly depends on the number of solubilizing hexyl groups. While the oligomers that carry two hexyl chains at the -positions of the terminal thiophenes (-nTVs) readily form dimers at low temperature, the oligomers that carry hexyl groups on the - and -positions of every thiophene ring (-nTVs) do not form dimers. Low-temperature UV/Vis/near-IR and ESR experiments on solutions in which neutral and singly oxidized nTVs are simultaneously present, reveal the occurrence of interchain interactions between these two species, accompanied by a pronounced change in the existing disproportionation equilibrium.