Hexa-peri-hexabenzo[7]helicene: Homogeneously pi-Extended Helicene as a Primary Substructure of Helically Twisted Chiral Graphenes

Helically twisted graphenes can be considered as a promising candidate for the nanometer-sized molecular inductors in molecular electronics and molecular spring materials in nanomechanics. Here, we report the synthesis of hexa-peri-hexabenzo[7]helicene, which represents a primary substructure of the helical graphenes. The helically twisted polycyclic aromatic hydrocarbon was synthesized by a tetrasubstituted alkene formation using McMurry coupling followed by stepwise photocyclodehydrogenation and aromatization reactions. The pi-extended helicoid structure with a noticeable intramolecular pi-pi interaction was unambiguously determined by X-ray crystallography. The primary helical nanographene molecule has a small HOMO-LUMO band gap evidenced by the absorption edge that appeared at ca. 800 nm, which exhibits an excellent chiroptical property with a dissymmetry factor of circular dichroism of vertical bar g(CD)vertical bar = 0.016 at 680 nm. The femtosecond transient absorption spectroscopy revealed the ultrafast excited-state dynamics of the helical nanographene molecule, with a lifetime of only few picoseconds in the lowest-energy excited (S-1) state.