Cross-Conjugated Oligothiophenes Derived from the (C₂S)<i>_n</i> Helix: Asymmetric Synthesis and Structure of Carbon−Sulfur [11]Helicene

(−)-Sparteine-mediated asymmetric synthesis of di-n-octyl-substituted carbon−sulfur [11]helicene, a helical (C2S)n β-undecathiophene, is described. The atom-efficient routes rely on one-step tri-annelation or two-step di- and mono-annelation to provide enantiomeric excess of (+)- or (−)-[11]helicene, respectively. X-ray structures for homologous [11] and [7]helicenes indicate similar helical curvatures. The optical band gap, Eg ≈ 3.5 eV, is estimated for the (C2S)n helix polymer, with onset of electron localization at n ≤ 7.