Complex semiregular tessellations (right, microscopic image) are formed from building blocks (left, red contours) consisting of two organic molecules and a silver atom (blue). Image: Klappenberger and Zhang/TUM.Parquet floors, comprising a geometric mosaic of wood sections, are typically found in living rooms. But microstructured parquets, or rather tessellations, can occur in materials as well. Materials with such tessellations can possess some interesting properties, including outstanding electrical conductivity, special light reflection and extreme mechanical strength.
Up to now, selective generation of such structures has required large molecular building blocks that are generally not compatible with conventional manufacturing processes. In a paper in Nature Chemistry, researchers at Karlsruhe Institute of Technology (KIT) and the Technische Universität München (TUM), both in Germany, detail a new process for encouraging molecules to form complex tessellations through self-organization.
Only a few basic geometric shapes – triangles, rectangles and hexagons – lend themselves to covering a surface without overlaps or gaps using uniformly shaped tiles. A much greater variety of regular patterns, with much greater complexity, can be produced using two or more tile shapes; these are known as Archimedean tessellations or tilings.
Materials can also exhibit tiling characteristics, which can confer properties such as outstanding electrical conductivity, special light reflection and extreme mechanical strength. It is, however, difficult to manufacture such materials. Their production requires large molecular building blocks that are not compatible with traditional manufacturing processes.
An international team led by Florian Klappenberger and Johannes Barth at TUM and Mario Ruben at KIT have now made a breakthrough in a class of supramolecular networks. They have developed a way to get simple organic molecules to combine into larger building blocks that form complex semiregular patterns in a self-organized manner.
As a starting compound, they used ethynyl iodophenanthrene, an easy-to-handle organic molecule comprising three coupled carbon rings with an iodine and an alkyne end. On a silver substrate, this molecule forms a regular network with large hexagonal meshes. Heat treatment sets in motion a series of chemical processes that produce a novel, significantly larger building block, which then automatically forms a complex, self-organized layer with small hexagonal, rectangular and triangular pores. In the language of geometry, this pattern is referred to as a semiregular 3.4.6.4 tessellation.
The working group headed by Mario Ruben at KIT’s Institute of Nanotechnology was responsible for synthesizing and characterizing the multinuclear molecular complexes that serve as tessellation building blocks. “We have discovered a completely new approach to producing complex materials from simple organic building blocks,” explain Klappenberger and Ruben. “This is important for being able to synthesize materials with specific novel and extreme characteristics. These results also contribute to a better understanding of the spontaneous appearance (emergence) of complexity in chemical and biological systems.”
Scanning tunneling microscopy measurements conducted at TUM clearly show that the molecular reorganization involves reactions that would normally result in numerous by-products. In this new process, however, the by-products are recycled, providing an economical way to create the desired end-product. In further experiments, the researchers worked out how this happens.
Using X-ray spectroscopy measurements at the electron storage ring BESSY II of the Helmholtz-Zentrum Berlin, the researchers were able to decipher how iodine splits from the starting product, how hydrogen atoms move to new positions and how alkyne groups capture a silver atom. By means of the silver atom, two starting building blocks bind together to create a new, larger building block. These new building blocks then form the observed complex pore structure.
This story is adapted from material from KIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.