Negatively Curved Nanographene with Heptagonal and [5]Helicene Units.

Negatively curved nanographene (NG) , having two heptagons and a [5]helicene, was unexpectedly obtained by aryl rearrangement and stepwise cyclodehydrogenations. X-ray crystallography confirmed the saddle-shaped structures of intermediate and NG . The favorability of rearrangement over helicene formation following radical cation or arenium cation mechanisms is supported by theoretical calculations. NG demonstrates a reversible mechanochromic color change and solid-state emission, presumably benefiting from its loose crystal packing. After resolution by chiral high-performance liquid chromatography, the circular dichroism spectra of enantiomers -() and -() were measured and showed moderate Cotton effects at 350 nm (|Δε| = 148 M cm).