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.