A Dualistic Arrangement of a Chiral [1]Rotaxane Based on the Assembly of Two Rings and Two Rods

We demonstrate the synthesis and chiroptical properties
of doubled
molecules of a chiral [1]rotaxane, based on the assembly of an achiral
ring of a phenylacetylene macrocycle (6PAM) and a p-phenylene ethynylene rod. Two molecules of [1]rotaxane constituted
the doubled molecule through the ring fusion of 6PAMs to a 10PAM,
which assured stationary occupation relative to each optically active
unit. The absorption properties of the 10PAM-based doubled molecule
and 6PAM-based original unit were consistently characterized by the
independent existence of m-phenylene ethynylene ring(s)
and p-phenylene ethynylene rod(s). Thus, molar circular
dichroism (CD) was directly compared between the doubled molecule
(n = 2) and the original unit (n = 1) to show that molar CD was increased more than expected by an
increase in the number of units, or by an increase in absorbance.
Due to the invariance of the configuration and the relative occupation
of two units arranged adjacent to each other in 10PAM, one more comparison
was available with an isomeric molecule of two rings and two rods
in a threaded-and-unthreaded form. The additional arrangement of an
optically inactive unit in an unthreaded form also led to an increase
in molar CD, compared to that of the original chiral unit in a threaded
form.