DFTMaterials & EngineeringTransition Metals

Complex supramolecular tesselation from simple organic precursor

Left: An STM image of a supramolecular tessellation featuring the ( semiregular Archimedean tiling. Right: The upper panel shows an STM imaging of on-surface reaction intermediates, which can chemically transform to the final building blocks for the tessellation via further reactions. Employing gas-phase DFT modeling using ORCA, the structures of on-surface reaction intermediates can be rationalized (lower panel).

Complex, supramolecular structures provide intriguing physical and chemical properties. Of particular interest in this regard are semi-regular Archimedean tilings and quasicrystalline structures. Through interfacial supramolecular engineering, such two-dimensional structures have been obtained as architectures formed by molecular tectons through specific interactions between functional groups or metal-directed assembly. This approach has so far been limited to directly accessible structures, i.e. the formation of the two-dimensional structure relies solely on the interaction between the primary constituents, not on multiple chemical transformations of these constituents.

Researchers from the Technical University Munich, the Institute of Nanotechnology in Karlsruhe, the University of Strasbourg as well as the Ecole Normale Superieure in Paris have discovered a convergent multi-step mechanism for the formation of semi-regular Archimedean tilings on Ag(111) through deposition of a simple organic precursor (ethynyliodophenanthrene, EIP). To unravel the underlying multi-step reaction mechanism, researchers combined scanning tunneling microscopy and X-ray spectroscopy with computational modelling.

After deposition onto a cold Ag(111) surface, intact EIP self-assemble into a porous network on the flat surface. Network formation is driven by a three-fold chiral organization for the terminal alkynes and the iodine groups, implying weak C-H-...π interaction and halogen bonding. Through annealing, irregular arrangements start to evolve in a temperature range between 200 K and 250 K which at an annealing temperature of 300-350 K transform into an ordered complex architecture. This novel phase is constructed from interwoven 12-segmented rings, creating pores that host further species.The crossings of these rings represent the four-fold vertices of the architecture consisting of three types of polygon: triangles, tetragons and hexagons. The packing geometry of these polygons is closely related to a semi-regular Archimedean tiling. A bis(phenanthren-2-yleythnyl)silver (BPE–Ag) organometallic complex turns out to be the essential and only building block of the tesselation.

Extensive DFT calculations have been carried out to understand the reaction mechanism and the stability of the resulting Archimedean tiling. DFT calculations on the extended system were carried out using CP2K. Cluster calculations were carried out using ORCA.

The researchers conclude that two main characteristics are involved in the reactions leading to the formation of the semi-regular Archimedean tiling: The first characteristic is a C-H activation of terminal alkynes taking place at temperatures as low as 180 K, as observed by scanning tunneling microscopy. The second notable characteristic is a 92% conversion of EIP to BPE-Ag after 300 K annealing - regardless of the diversity of the intermediate products appearing in a broad temperature range.

Based on experimental observations and modelling insights, the researchers could formulate an exchange reaction scenario that coherently explains these two key features along with other experimental observations.