π‑Extended Pyrene-Fused Double [7]Carbohelicene as a Chiral Polycyclic Aromatic Hydrocarbon

A π-extended
double [7]­carbohelicene 2 with
fused pyrene units was synthesized, revealing considerable intra-
and intermolecular π–π interactions as confirmed
with X-ray crystallography. As compared to the previous double [7]­carbohelicene 1, the π-extended homologue 2 demonstrated
considerably red-shifted absorption with an onset at 645 nm (1: 550 nm) corresponding to a smaller optical gap of 1.90
eV (1: 2.25 eV). A broad near-infrared emission from
600 to 900 nm with a large Stokes shift of ∼100 nm (2.3 ×
103 cm–1) was recorded for 2, implying formation of an intramolecular excimer upon excitation,
which was corroborated with femtosecond transient absorption spectroscopy.
Moreover, 2 revealed remarkable chiral stability with
a fairly high isomerization barrier of 46 kcal mol–1, according to density functional theory calculations, which allowed
optical resolution by chiral HPLC and suggests potential applications
in chiroptical devices.