Synthesis, Structures, and Properties of π‑Extended Double Helicene: A Combination of Planar and Nonplanar π‑Systems

The
synthesis, structures, and properties of a π-extended
double helicene 1 are described. This double helicene 1 was synthesized by a four-fold oxidative C–H biphenylation
of naphthalene followed by the Scholl reaction or via five steps including
the Suzuki–Miyaura cross-coupling reaction and the Scholl reaction.
Due to the two helical substructures, 1 has three isomers,
i.e., two enantiomers in a twisted form [(P,P) and (M,M)] and one
diastereoisomer in a meso form. X-ray crystallographic analysis of
the twisted isomers (twisted-1) revealed
a tightly offset packing pattern of (P,P)- and (M,M)-twisted isomers, affording
a three-dimensional lamellar stacking structure. A high isomerization
barrier (43.5 kcal mol–1) and the relative thermal
stability of twisted-1 isomer over meso-1 by 0.9 kcal mol–1 were
estimated by DFT calculations. The three isomers were successfully
separated by chiral HPLC and characterized by circular dichroism spectroscopy
as well as by TD-DFT studies. Electronic state variation resulting
from the molecular geometry difference between the two diastereoisomers
(twisted-1 and meso-1) was observed by UV–vis absorption and fluorescence
spectra.