Terthiophene radical cations end-capped by bicyclo[2.2.2]octene units: formation of bent pi-dimers mutually attracted at the central position

A terthiophene fused with bicyclo[2.2.2]octene units only at both ends was newly synthesized. Since there is no steric hindrance at the central position, this terthiophene has a possibility to interact only at the central position. One-electron oxidation of this terthiophene afforded a highly stable radical-cation salt as deep blue crystals. The result of X-ray crystal structural analysis demonstrated a characteristically bent pi-dimereric structure, which is formed by mutual attraction of single radical-cation species at the central position to minimize the steric repulsion. Remarkably short intermolecular distances between the central thiophene rings of each unit of the dimeric pair, that is, 2.976(10) A for Cbeta-Cbeta, 3.091(10) A for Calpha-Calpha, and 3.779(3) A for S-S, are good indication of the existence of attracting interaction, which was confirmed by theoretical calculations. This interaction was experimentally demonstrated by the reversible formation of the pi-dimer in CH2Cl2 solution using ESR and UV-vis-NIR spectroscopy. The crystal of the pi-dimer is in its singlet state and ESR silent in the solid state at 300 K, but the signal of a triplet state of the pi-dimer was observed by heating the solid at 400 K. This indicates that this pi-dimer has a quite small triplet-singlet enegy gap and the triplet state is thermally accessible.