The synthesis and characterization of a novel oligothiophene, in which the thiophene rings are annelated into a helicene with cross-conjugated π-system, are described. Such helicenes may be viewed as fragments of the unprecedented carbon-sulfur (C2S)n helix, possessing sulfur-rich molecular periphery. Racemic synthesis of helicene is based upon iterative alternation of two steps: C-C bond homocouplings between the β-positions of thiophenes and annelation between the α-positions of thiophenes. Asymmetric synthesis is carried out using (-)-sparteine-mediated annelation of the axially chiral bis(aryllithium) with electrophilic sulfur equivalent. Alternatively, enantiomers of the helicene are obtained via resolution using menthol-based chiral siloxanes. Racemic helicene and four other macrocyclic products of the annelation are characterized by X-ray crystallography. One of the solvent polymorphs of the helicene possesses π-stacked columns of opposite enantiomers and multiple short intermolecular contacts, including both homochiral and heterochiral short S⋯S contacts, suggesting an effective intermolecular electronic coupling in two-dimensions. The helicene is configurationally stable at room temperature and racemizes at 199 °C with a half-time of about 11 h. Selected physicochemical studies (UV-vis absorption, CD, optical rotation, and cyclic voltammetry) of the helicene are described.
ASJC Scopus subject areas
- Colloid and Surface Chemistry