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

Recent advancements in on-surface synthesis have enabled the reliable and predictable preparation of atomically precise low-dimensional materials with remarkable properties, which are often unattainable through traditional wet chemistry. Among these materials, porphyrins stand out as a particularly intriguing class of molecules, extensively studied both in solution and on surfaces. Their appeal lies in the ability to fine-tune their unique chemical and physical properties through central metal exchange or peripheral functionalization. However, the synthesis of π-extended porphyrins featuring unsubstituted anthracenyl groups has remained elusive. Herein, we report an in vacuo temperature-controlled cyclodehydrogenation of bis- and tetraanthracenyl Zn(II) porphyrins on a gold(111) surface. By gradually increasing the temperature, sequential dehydrogenation leads to the formation of fused anthracenyl porphyrin products. Notably, at high molecular coverage, the formation of bowl-shaped porphyrins occurs, along with transmetalation of Zn with Au. These findings open the door to a variety of π-extended anthracenyl-containing porphyrin products via cyclodehydrogenation and transmetalation, offering significant potential in the fields of molecular (photo/electro)catalysis, (opto)electronics, and spintronics.

• Publication type
• Journal Article MeSH

Two-dimensional materials have unusual properties and promise applications in nanoelectronics, spintronics, photonics, (electro)catalysis, separations, and elsewhere. Most are inorganic and their properties are difficult to tune. Here we report the preparation of Zn porphene, a member of the previously only hypothetical organic metalloporphene family. Similar to graphene, these also are fully conjugated two-dimensional polymers, but are composed of fused metalloporphyrin rings. Zn porphene is synthesized on water surface by two-dimensional oxidative polymerization of a Langmuir layer of Zn porphyrin with K2IrCl6, reminiscent of known one-dimensional polymerization of pyrroles. It is transferable to other substrates and bridges μm-sized pits. Contrary to previous theoretical predictions of metallic conductivity, it is a p-type semiconductor due to a predicted Peierls distortion of its unit cell from square to rectangular, analogous to the appearance of bond-length alternation in antiaromatic molecules. The observed reversible insertion of various metal ions, possibly carrying a fifth or sixth ligand, promises tunability and even patterning of circuits on an atomic canvas without removing any π centers from conjugation.

• Publication type
• Journal Article MeSH