On-surface synthesis is a promising strategy for the preparation of molecules that are not achievable otherwise. Understanding the mechanism of on-surface reactions requires knowledge of the molecular structure and possible organization of reactants into supramolecular assemblies during the reaction. Scanning probe techniques are essential for the unambiguous identification of the products and for determining their electronic and magnetic properties. However, these are generally not capable of imaging the surface at reaction conditions and, therefore, answering some of the key questions about the reaction mechanism. Here, we show that real-time low-energy electron microscopy (LEEM) can monitor the surface processes in real time and provide the necessary complementary mechanistic insights into on-surface reactions. We monitor the intramolecular ring-closure reaction of 1,3,5-tris(7-methyl-α-carbolin-6-yl)benzene on the Au(111) surface and show that it takes place in the 2D molecular gas phase at elevated temperatures. Products condense into separate islands upon cooling, enabling fast and efficient assessment of product yields. This makes LEEM an efficient tool for studying intramolecular chemical reactions.

• Keywords
• Chirality, Low-Energy Electron Microscopy, On-Surface Synthesis, Scanning Probe Microscopy,
• Publication type
• Journal Article MeSH

Here, we report a new on-surface synthetic strategy to precisely introduce five-membered units into conjugated polymers from specifically designed precursor molecules that give rise to low-bandgap fulvalene-bridged bisanthene polymers. The selective formation of non-benzenoid units is finely controlled by the annealing parameters, which govern the initiation of atomic rearrangements that efficiently transform previously formed diethynyl bridges into fulvalene moieties. The atomically precise structures and electronic properties have been unmistakably characterized by STM, nc-AFM, and STS and the results are supported by DFT theoretical calculations. Interestingly, the fulvalene-bridged bisanthene polymers exhibit experimental narrow frontier electronic gaps of 1.2 eV on Au(111) with fully conjugated units. This on-surface synthetic strategy can potentially be extended to other conjugated polymers to tune their optoelectronic properties by integrating five-membered rings at precise sites.

• Publication type
• Journal Article MeSH

The synthesis of nanographenes (NGs) with open-shell ground states have recently attained increasing attention in view of their interesting physicochemical properties and great prospects in manifold applications as suitable materials within the rising field of carbon-based magnetism. A potential route to induce magnetism in NGs is the introduction of structural defects, for instance non-benzenoid rings, in their honeycomb lattice. Here, we report the on-surface synthesis of three open-shell non-benzenoid NGs (A1, A2 and A3) on the Au(111) surface. A1 and A2 contain two five- and one seven-membered rings within their benzenoid backbone, while A3 incorporates one five-membered ring. Their structures and electronic properties have been investigated by means of scanning tunneling microscopy, noncontact atomic force microscopy and scanning tunneling spectroscopy complemented with theoretical calculations. Our results provide access to open-shell NGs with a combination of non-benzenoid topologies previously precluded by conventional synthetic procedures.

• Keywords
• STM, nanographenes, nanomagnetism, nc-AFM, on-surface synthesis, open-shell character, polycyclic aromatic hydrocarbons,
• Publication type
• Journal Article MeSH