Negatively Curved Nanographene with Heptagonal and [5]Helicene Units

Negatively
curved nanographene (NG) 4, 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 3 and NG 4. The favorability of rearrangement over helicene formation
following radical cation or arenium cation mechanisms is supported
by theoretical calculations. NG 4 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 4-(P) and 4-(M) were measured and showed moderate Cotton effects at 350 nm (|Δε|
= 148 M–1 cm–1).