Scientists from Kanazawa University reported in the Journal of the American Chemical Society that they prepared helical ladder polymers using a well-defined cyclic repeating unit and one-handed helical geometry.
It is very difficult to synthesize ladder polymers—molecules consisting of adjacent rings sharing two or more atoms—as they need highly selective, quantitative reactions to prevent the formation of branching structures or of interruptions in the ring sequence in the polymer chain. Also, a majority of the current approaches for the ladder polymer synthesis have serious limitations with respect to quantitativity and selectivity.
Another significant kind of molecules are molecules with a helical structure (for example, proteins and DNA), which play a key role in catalysis and molecular recognition. Hence, the production of molecules that possess both a ladder structure and a helical structure could pave the way for new applications of polymeric materials.
Tomoyuki Ikai, Timothy M. Swager, and coworkers from an international collaboration used triptycene as a starting material, which is an aromatic hydrocarbon and an achiral molecule. Chiral derivatives can be obtained from this molecule by asymmetrically introducing substituents into the benzene rings. Triptycenes are optically active and find practical uses as chiral materials, such as for circularly polarized luminescent materials and chiral separation.
Later, the scientists efficiently produced single-handed helical ladder polymers by electrophilic aromatic substitution using the chiral triptycenes as a framework. Steric repulsion in the system caused the formation of one-handed twisted ladder units. The reactions were regioselective (which means that there is a preferred direction for chemical bonding) and quantitative, enabling the synthesis of optically active ladder polymers with well-defined helical geometry. There was no detection of byproducts.
Many methods, including microscopy and spectroscopy techniques, were used to characterize the products of the reaction during synthesis, and molecular dynamics simulations were used to gain the understanding of the structure of the resulting molecules, which confirms the right-handed helical ladder geometry. The optical activity of the molecules was also measured by the scientists.
The path of the newly reported synthesis paves the way for the synthesis of optically active chiral materials and nanoscale helical ladder architectures.
The authors of the study remarked, “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.”
This research was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI through a Grant-in-Aid for Scientific Research (C) (Grant No. 17K05875). Research at MIT was supported in part by the Air Force Office of Scientific Research (Grant No. 17RT0904).