The molecular structure of a cyclic oligothiophene, C10T, has been determined by single-crystal X-ray structure analysis. The exclusive syn-conformation of all thiophene units as confirmed in the solid state and the ring strain in this macrocycle result in its unusual and optoelectronic properties. This does not only apply to neutral C10T but also to its oxidized states, as demonstrated by absorption and ESR spectroscopy, supporting the formation of a polaron-pair structure upon oxidation of C10T to C10T2(·+) as has been discussed for linear oligothiophenes. To the best of our knowledge, C10T2(·+) represents an unambiguous example comprising a two-polaron structure (polaron- pair) of a thiophene-based conjugated macrocycle.
