Researchers at Kanazawa University report in ChemComm the visualization of polymers with helical structures. By means of atomic force microscopy, they were able to elucidate the precise difference between molecules that are structurally very similar but have totally different color and luminescent properties.
Optically active molecules, also known as enantiomers, are pairs of molecules that are each other’s mirror image — like a person’s left and right hand. Often, enantiomeric molecules feature helices, with the ‘left-’ or ‘right-handedness’ being related to how a helix coils. Visualizing the helical structures in complex organic molecules is important for understanding their chiral (‘handedness’-related) properties, and for the design of materials exploiting these. Now, Katsuhiro Maeda and colleagues from Kanazawa University have succeeded in visualizing the helical structure of an important set of polymers with chiral constituents.
The molecules investigated by Maeda and colleagues belong to the group of poly(diphenylacetylene)s (PDPAs), chemically and thermally stable polymers with excellent photoluminescent properties, making them promising functional materials. The researchers studied the helical structures in PDPAs bearing chiral amide pendants by means of high-resolution atomic-force microscopy (AFM).
AFM is a technique in which a very small tip, attached to a cantilever, is made to scan a sample’s surface. The tip’s response to height differences in the scanned surface provides structural information of the sample. For their AFM investigation of PDPAs, the research team put the molecules on a substrate known as highly oriented pyrolytic graphite (HOPG), a very pure and highly ordered form of synthetic graphite. The PDPA molecules self-assembled into a closely packed layer on the HOPG substrate, making it possible to observe not just one but a whole domain of molecules (Figure 1).
By using AFM, the researchers were able to observe structural differences between two particular PDPA diastereomers that were previously shown to have different colors (red and yellow) in solution. Specifically, they confirmed that the red polymer owns a contracted helix because of intramolecular hydrogen bonding (attraction between a hydrogen atom and another more electronegative atom within a molecule), whereas the yellow polymer possesses a stretched helix due to lack of intramolecular hydrogen bonding. The observations also confirm theoretical structure calculations.
The report of Maeda and colleagues signifies the first direct visualization of helical PDPA molecules, and holds promise for developing molecules with application potential. Quoting the scientists: “This study is expected to pave the way for the structural elucidation of the helical PDPAs, thus allowing the design of new polymeric structures with improved functions as a sensor, chiral stationary phase for high-performance liquid chromatography, circularly polarized luminescence source, and so on.”
Figure 1. High-resolution AFM images of PDPA molecules on a HOPG substrate (top left, middle, and bottom) and calculated structure of the individual molecule (top right).
An enantiomer (sometimes called optical isomer) is one of two molecules that are mirror images of each other, just like one’s left and right hands. The two members of a pair of enantiomers are also called enantiomorphs.
In symmetric environments, enantiomers have identical physical properties except for one: the ability to rotate a particular form of polarized light (so-called plane-polarized light) by equal amounts but in opposite directions. Enantiomers are therefore referred to as optically active.
Now, Katsuhiro Maeda and colleagues from Kanazawa University have revealed the precise structural difference between optically active polymers of a class of organic molecules known as poly(diphenylacetylene)s.
Atomic force microscopy
Atomic force microscopy (AFM) is an imaging technique in which the image is formed by scanning a surface with a very small tip attached to a small cantilever. Horizontal scanning motion of the tip is controlled via piezoelectric elements; the vertical position of the tip changes as it follows the sample’s height profile, generating a force on the cantilever that can be measured and back-converted into a measure of the height. The result is a height map of the sample’s surface. As the technique does not involve lenses, its resolution is not restricted by the so-called diffraction limit as in X-ray diffraction, for example.
Maeda and colleagues used high-speed AFM to study the structure of poly(diphenylacetylene)s with helical parts.
Ayhan Yurtsever, Sandip Das, Tatsuya Nishimura, Rafael Rodríguez, Daisuke Hirose, Kazuki Miyata, Ayumi Sumino, Takeshi Fukuma, and Katsuhiro Maeda.
Visualisation of helical structures of poly(diphenylacetylene)s bearing chiral amide pendants by atomic force microscopy, Chem. Commun. 57, 12266 (2021).
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