Synthesis and Properties of Octithiophene Dication Sterically Segregated by Annelation with Bicyclo[2.2.2]octene Units

Octithiophene sterically segregated by annelation with bicyclo[2.2.2]octene (BCO) units was synthesized to study the unimolecular properties of longer oligothiophene dications. For the preparation of such longer oligomers, a new route for the synthesis of the monomer annelated with BCO unit at the 3,4-positions of thiophene ring was developed. Attempted synthesis of octithiophene 1(8T) fully annelated with BCO units was hampered by low solubility of the product, and octithiophene 2(8T) having octyl groups instead of the BCO units at the second rings from the both ends of 1(8T) was synthesized to solve the solubility problem. Neutral 2(8T) has a lower planarity due to the steric repulsion between the octyl substituents and the neighboring thiophene units. However, dication 2(8T)(2+), which was obtained as a stable salt by chemical two-electron oxidation with NO(+)SbF(6)(–), has a planar quinoid structure, as judged from a linear correlation between the inverse chain length and the absorption energy for 1(nT)(2+) (n = 3,4,6) and 2(8T)(2+). Based on the comparison with the calculated absorption spectra and the result of ESR inactive properties, 2(8T)(2+) appears to have a singlet ground state with open-shell biradical character rather than a closed-shell singlet structure.