Preparation, Spectroscopic Characterization and Theoretical Study of a Three-Dimensional Conjugated 70 π‑Electron Thiophene 6‑mer Radical Cation π‑Dimer

A radical cation,
generated from an extended π-conjugated
thiophene 6-mer composed of four ethynylene-thienylene and two vinylene-thienylene
units, was observed to form a stable three-dimensional π-dimer
containing 70 π-electrons. The π-dimer prepared in solution
was investigated by using magnetic circular dichroism (MCD), ESR spectroscopy,
and UV–vis–NIR absorption spectroscopy. Probing the
individual NIR absorption bands showed that the MCD signals can be
assigned to the pseudo Faraday A term, indicating
that the absorption bands are comprised of nearly degenerate electronic
transitions. X-ray crystallographic analysis revealed that the π-dimer
has a three-dimensional face-to-face and continuous π-conjugated
donutlike structure. Analysis of the UV–vis–NIR and
ESR spectra of the π-dimer in the solid state confirmed that
it possesses the dimer structure. The prediction made by using TD-DFT
calculations that the dimer would have a 70 π-electron diatropic
nature was confirmed by using solid state 1H NMR spectroscopy.