KANAZAWA, Japan, May 16, 2019 /PRNewswire/ -- Researchers at
Kanazawa University synthesized helical ladder polymers with a
well-defined cyclic repeating unit and one-handed helical geometry,
as they reported in the Journal of the American Chemical
Society.
Ladder polymers — molecules made of adjacent rings sharing two
or more atoms — are challenging to synthesize, because they require
highly selective, quantitative reactions to avoid the formation of
branching structures or of interruptions in the ring sequence in
the polymer chain. Moreover, most existing strategies for the
synthesis of ladder polymers suffer from severe limitations in
terms of selectivity and quantitativity. Another important type of
molecules are molecules with a helical structure (such as DNA and
proteins), which play an important role in molecular recognition
and catalysis. Thus, the fabrication of molecules that possess both
a ladder and a helical structure could open up new applications of
polymeric materials.
Tomoyuki Ikai, Timothy M. Swager and colleagues from an
international collaboration started from triptycene, an aromatic
hydrocarbon that is an achiral molecule, but from which chiral
derivatives can be obtained by introducing substituents in the
benzene rings in an asymmetric manner. Optically active triptycenes
have practical uses as chiral materials, for example for chiral
separation and circularly polarized luminescent materials. The
researchers then used the chiral triptycenes as a framework to
efficiently form single-handed helical ladder polymers using
electrophilic aromatic substitution. Steric repulsion in the system
resulted in the formation of one-handed twisted ladder units. The
reactions were quantitative and regioselective (that is, there is a
preferred direction of chemical bonding), which enabled the
synthesis of optically active ladder polymers with well-defined
helical geometry. No byproducts were detected.
Several techniques, including spectroscopy and microscopy
techniques, were used to characterize the reaction products during
synthesis, and molecular dynamics simulations were employed to
understand the structure of the resulting molecules, confirming the
right-handed helical ladder geometry. The researchers also measured
the optical activity of the molecules.
The newly reported synthesis route will open up the synthesis of
nanoscale helical ladder architectures and optically active chiral
materials. "We believe that these ladder polymers, which can fall
into a new category of helical polymers, represent a promising
class of advanced materials for use as nanochannels for
molecular/ion transport, organic electronics, specific reaction
fields, and functional hosts through further modification of the
backbone and pendant units," commented the authors in the
paper.
Background
Chirality
A chiral system is an asymmetric system that cannot be
superimposed on its mirror image (the word comes from the Greek for
hands, because hands are a good example of a chiral system). Most
biomolecules and molecules used in pharmaceutical compounds are
chiral. Two molecules with opposite chirality have the same
composition and structure, but mirror shapes, and they have
different properties when they interact with other chiral
molecules.
Electrophilic aromatic substitution
Electrophilic aromatic substitution is an organic reaction in
which one atom attached to an aromatic system is replaced by an
atom that is an electron acceptor (an electrophile). It is an
important class of reactions, usually involving a benzene ring.
Steric repulsion
Steric repulsion is an effect that results from repulsive forces
kicking in when atoms get too close to each other, so that their
electron clouds overlap.
Reference
Tomoyuki Ikai, Takumu Yoshida, Ken-ichi
Shinohara, Tsuyoshi Taniguchi, Yuya Wada,
and Timothy M. Swager, Triptycene-Based Ladder Polymers with
One-Handed Helical Geometry. J. Am. Chem.
Soc. 2019 141 (11), 4696-4703.
DOI: 10.1021/jacs.8b13865
About Kanazawa University
As the leading comprehensive university on the Sea of
Japan coast, Kanazawa University
has contributed greatly to higher education and academic research
in Japan since it was founded in
1949. The University has three colleges and 17 schools offering
courses in subjects that include medicine, computer engineering,
and humanities.
The University is located on the coast of the Sea of
Japan in Kanazawa – a city rich in
history and culture. The city of Kanazawa has a highly respected
intellectual profile since the time of the fiefdom (1598-1867).
Kanazawa University is divided into two main campuses: Kakuma and
Takaramachi for its approximately 10,200 students including 600
from overseas.
Kanazawa University website: http://www.kanazawa-u.ac.jp/e/
Further information
Kanazawa University
Kakuma, Kanazawa, Ishikawa 920-1192, JAPAN
E-mail: intl.pr@adm.kanazawa-u.ac.jp
Tel: +81-(76)-264-5963
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