Scanning tunnelling microscopy shows chrial molecules to self-assemble on metal surfaces
Two forms of a prism-shaped molecule can arrange themseleves into separate groups on a gold surface, depending on whether they have a slight twist to the left or the right.
Many molecules come in two forms that are not superimposable on their mirror images, just like left and right hands. This handedness—known as chirality—plays a crucial role in many biochemical processes, for example, nature almost exclusively uses left-handed amino acids to build proteins.
Chiral catalysts are often used to create new molecules with a particular handedness, such as pharmaceuticals. These catalysts are usually dissolved into reaction mixtures, where chemists have extensively studied chirality.
“But considerably less is known about chirality on surfaces,” says Li-Jun Wan of Beijing National Laboratory of Molecular Sciences at the Chinese Academy of Sciences. Wan and his colleagues1 working with researchers at the University of Utah, have now studied how a chiral molecular cluster arranges itself on a gold surface.
The cluster was made from a mixture of a platinum-based compound and a bulky, star-shaped molecule containing cobalt. Together, they produce a cluster with a triagonal prism shape—an equilateral triangle expanded into the third dimension. A slight twist in the structure means they can come in two chiral forms, or enantiomers.
The scientists coated a gold bead with the prisms and used a scanning tunnelling microcope (STM) to look at the surface in atomic resolution—a level of detail impossible with chiral molecules in solution.
The STM revealed that the prisms on the gold surface to be lying on their side, showing a face that was not quite rectangular—a parallelogram—measuring less than 2 nanometres across and exposing four ‘petals’ from the star-shaped molecule around a bright, central cobalt atom.
Crucially, left-handed prisms gathered into one area, while right-handed prisms collected separately into a different area (Fig.1). This rapid form of self-assembly—where an equal mixture of enantiomers automatically separates into two chiral surfaces—could be useful for developing new chrial catalysts or sensor devices, the scientists say.
References
Yuan, Q-H. et al. Scanning Tunneling Microscopy Investigation of a Supramolecular Self-Assembled Three-Dimensional Chiral Prism on a Au(111) Surface. J. Am. Chem. Soc. (2008) doi: 10.1021/ja801934w
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Molecular self-assembly: Twisted prisms. NPG Asia Mater (2008). https://doi.org/10.1038/asiamat.2008.116
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DOI: https://doi.org/10.1038/asiamat.2008.116