Band Gap of Carbon–Sulfur [<i>n</i>]Helicenes

Asymmetric synthesis of (−)-[9]helicene, as well as preparation of its lower homologues, completes the series of carbon–sulfur [5]-, [7]-, [9]-, and [11]helicenes. Spectroscopic and electrochemical studies of this series provide an absorption onset-based band gap, Eg = 3.40 eV, for a cross-conjugated (C2S)n helix; this value may be compared to Eg = 3.59 eV obtained from TD-DFT computed excitation energies for a series of dimethyl-substituted [n]helicenes (n ≤ 31).