The self-assembly of small molecular ‘building blocks’ into large and ordered structures, inspired by biological systems, is a highly attractive prospect because it does not involve complicated synthetic pathways or external manipulation. Despite intense research, however, the relation between the chemical nature of the building blocks and the structure that is ultimately formed remains challenging.
Now a team from Switzerland, Germany, and Canada have taken the idea of a structurally simple building block to a new level by developing a series of achiral molecules, consisting of a linear chain of three to five phenyl rings endcapped by carbonitril groups NC-Ph(3-5)-CN. In fact, the structural simplicity of these molecules is such that the team chose to refer to them as ‘molecular bricks’ [Schlickum et al., J. Am. Chem. Soc., doi: 10.1021/ja8028119].
After vapor-deposition of these molecules on a silver substrate and imaging by scanning tunneling microscopy (STM), the researchers found that long-range periodic lattices emerged for all three building blocks. Despite the structural similarity of the constituents, different structures were formed, increasing in complexity with the increasing length of the molecules. In particular, the NC-Ph3-CN molecules assembled into a chevron-layer structure, while the NC-Ph4-CN groups assembled into an open-rhombic network.
The most surprising finding, however, was observed for the NC-Ph5-CN molecules, which assembled into a complex structure known as a kagomé lattice. This periodic framework, characterized by a series of hexagonal pores surrounded by triangles, is one of only eleven general ways in which a plane can be filled by regular polyhedrons. Despite occurring naturally in some minerals, and having been studied extensively for their unusual magnetic properties, kagomé lattices have been observed only a few times in self-assembled networks so far. In contrast to these previous reports, however, the ordering in the present study is obtained based on the simple, low-symmetric linear bricks.
“Our results demonstrate that, with the help of self-assembly processes, it is possible to fabricate complex surface patterns even with simple linear molecular bricks, Uta Schlickum, tells Materials Today. He goes on to say “Such patterned surfaces are of high relevance for example applications such as surface refinement or data storage, and are the result of the interplay between molecule-molecule interactions, the substrate epitaxial fit and the conformational flexibility of the molecules. “Uta Schlickum continues, “Our results help to understand the mechanisms involved in self-assembly processes. A detailed understanding of these processes would lead to a strongly enhanced ability to fabricate any surface pattern needed in an easy, inexpensive way and with atomic precision.”