Synthesis and Structural Features of Quadruple Helicenes: Highly Distorted π Systems Enabled by Accumulation of Helical Repulsions

Quadruple helicenes, bearing dithia[6]­helicene
and [5]­helicene
substructures, were prepared by a well-controlled Scholl reaction.
The 4-fold helicity provides 9 stereoisomers including 4 pairs of
enantiomers and 1 meso isomer. Among them, differently distorted structures
of a propeller-shaped isomer (QH-A) and a saddle-shaped
isomer (QH-B) were unambiguously determined by X-ray
crystallography. Especially in the latter isomer, a proper accumulation
of repulsions on the helical substructures twisted the naphthalene
core to the limit (69.5°), the highest degree of twisting deformation
per benzene unit (35.3° at the most). Photophysical and electrochemical
studies showed a broadened HOMO–LUMO gap and a HOMO of QH-B lying lower compared to those of QH-A. These
results together with the density functional theory (DFT) calculations
have clearly demonstrated the electronic state dependency on the molecular
geometry. Additionally, kinetic studies of the isomerization between
these isomers using 1H NMR, circular dichroism, and DFT
calculations shed light on the interconversion pathways among the
stereoisomers. The height of barriers in the inversion of a certain
helical substructure may be affected by the neighboring helical substructures.