Terthiophene Radical Cations End-Capped by Bicyclo[2.2.2]octene Units:  Formation of Bent π-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 π-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) Å for C_β−C_β, 3.091(10) Å for C_α−C_α, and 3.779(3) Å 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 π-dimer in CH₂Cl₂ solution using ESR and UV−vis−NIR spectroscopy. The crystal of the π-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 π-dimer was observed by heating the solid at 400 K. This indicates that this π-dimer has a quite small triplet−singlet enegy gap and the triplet state is thermally accessible.