Redox states and associated interchain processes of thienylenevinylene oligomers

The electronic structure of successive redox states of two series of thieny lenevinylene oligomers (nTVs, n=4, 5, 6, 8, and 12) that carry hexyl substi tuents at the alpha- or beta-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 d ication for the shorter oligomers (n less than or equal to 6) and up to the tetracation for the longer oligomers (n greater than or equal to 8). While the monocation radicals of the nTVs exhibit two dipole-allowed electronic transitions in the Vis/near-IR region, all the higher oxidized states invar iably show a single strong absorption in the near-IR region. The electronic transitions of the various oxidized states and those of their vibronic rep licas shift to lower energies with increasing conjugation length. The ESR s pectra, 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 sp in in even-charged cations. Variable-temperature UV/Vis/near-IR and ESR spe ctroscopy establish that the tendency of the nTV monocation radicals to for m spin-less pi dimers in solution strongly depends on the number of solubil izing hexyl groups. While the oligomers that carry two hexyl chains at the alpha-positions of the terminal thiophenes (alpha-nTVs) readily form pi dim ers at low temperature, the oligomers that carry hexyl groups on the beta- and beta'-positions of every thiophene ring (beta-nTVs) do not form pi dime rs. Low-temperature UV/Vis/near-IR and ESR experiments on solutions in whic h neutral and singly oxidized nTVs are simultaneously present, reveal the o ccurrence of interchain interactions between these two species, accompanied by a pronounced change in the existing disproportionation equilibrium.