posted on 2022-12-27, 22:13authored byTakuho Saito, Takashi Kajitani, Shiki Yagai
The amplification of molecular asymmetry
through self-assembly
is a phenomenon that not only comprehends the origin of homochirality
in nature but also produces chiroptically active functional materials
from molecules with minimal enantiomeric purity. Understanding how
molecular asymmetry can be transferred and amplified into higher-order
structures in a hierarchical self-assembly system is important but
still unexplored. Herein, we present an intriguing example of the
amplification of molecular asymmetry in hierarchically self-assembled
nanotubes that feature discrete and isolatable toroidal intermediates.
The hierarchical self-assembly is initiated via asymmetric intramolecular
folding of scissor-shaped azobenzene dyads furnished with chiral side
chains. When scalemic mixtures of the enantiomers are dissolved in
a non-polar solvent and cooled to 20 °C, single-handed nanotoroids
are formed, as confirmed using atomic force microscopy and circular
dichroism analyses. A strong majority-rules effect at the nanotoroid
level is observed and can be explained by a low mismatch penalty and
a high helix-reversal penalty. The single-handed nanotoroids stack
upon cooling to 0 °C to exclusively afford their respective single-handed
nanotubes. Thus, the same degree of amplification of molecular asymmetry
is realized at the nanotube level. The internal packing efficiency
of molecules within nanotubes prepared from the pure enantiomers or
their scalemic mixtures is likely different, as suggested by the absence
of higher-order structure (supercoil) formation in the latter. X-ray
diffraction analysis of the anisotropically oriented nanotube films
revealed looser molecular packing within the scalemic nanotubes, which
clearly reflects the lower enantiomeric purity of their internal chiral
side chains.