Surface-Mediated Ring-Opening and Porphyrin Deconstruction via Conformational Distortion.

The breakdown of macrocyclic compounds is of utmost importance in manifold biological and chemical processes, usually proceeding via oxygenation-induced ring-opening reactions. Here, we introduce a surface chemical route to selectively break a prototypical porphyrin species, cleaving off one pyrrole unit and affording a tripyrrin derivative. This pathway, operational in an ultrahigh vacuum environment at moderate temperature is enabled by a distinct molecular conformation achieved via the specific interaction between the porphyrin and its copper support. We provide an atomic-level characterization of the surface-anchored tripyrrin, its reaction intermediates, and byproducts by bond-resolved atomic force microscopy, unequivocally identifying the molecular skeletons. The ring-opening is rationalized by the distortion reducing the macrocycle's stability. Our findings open a route to steer ring-opening reactions by conformational design and to study intriguing tetrapyrrole catabolite analogues on surfaces.

Author Correction: Polycyclic aromatic chains on metals and insulating layers by repetitive [3+2] cycloadditions.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Polycyclic aromatic chains on metals and insulating layers by repetitive [3+2] cycloadditions.

The vast potential of organic materials for electronic, optoelectronic and spintronic devices entails substantial interest in the fabrication of π-conjugated systems with tailored functionality directly at insulating interfaces. On-surface fabrication of such materials on non-metal surfaces remains to be demonstrated with high yield and selectivity. Here we present the synthesis of polyaromatic chains on metallic substrates, insulating layers, and in the solid state. Scanning probe microscopy shows the formation of azaullazine repeating units on Au(111), Ag(111), and h-BN/Cu(111), stemming from intermolecular homo-coupling via cycloaddition reactions of CN-substituted polycyclic aromatic azomethine ylide (PAMY) intermediates followed by subsequent dehydrogenation. Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry demonstrates that the reaction also takes place in the solid state in the absence of any catalyst. Such intermolecular cycloaddition reactions are promising methods for direct synthesis of regioregular polyaromatic polymers on arbitrary insulating surfaces.

Layered Insulator/Molecule/Metal Heterostructures with Molecular Functionality through Porphyrin Intercalation.

Intercalation of molecules into layered materials is actively researched in materials science, chemistry, and nanotechnology, holding promise for the synthesis of van der Waals heterostructures and encapsulated nanoreactors. However, the intercalation of organic molecules that exhibit physical or chemical functionality remains a key challenge to date. In this work, we present the synthesis of heterostructures consisting of porphines sandwiched between a Cu(111) substrate and an insulating hexagonal boron nitride ( h-BN) monolayer. We investigated the energetics of the intercalation, as well as the influence of the capping h-BN layer on the behavior of the intercalated molecules using scanning probe microscopy and density functional theory calculations. While the self-assembly of the molecules is altered upon intercalation, we show that the intrinsic functionalities, such as switching between different porphine tautomers, are preserved. Such insulator/molecule/metal structures provide opportunities to protect organic materials from deleterious effects of atmospheric environment, can be used to control chemical reactions through spatial confinement, and give access to layered materials based on the ample availability of synthesis protocols provided by organic chemistry.

Quantitative determination of a model organic/insulator/metal interface structure.

By combining X-ray photoelectron spectroscopy, X-ray standing waves and scanning tunneling microscopy, we investigate the geometric and electronic structure of a prototypical organic/insulator/metal interface, namely cobalt porphine on monolayer hexagonal boron nitride (h-BN) on Cu(111). Specifically, we determine the adsorption height of the organic molecule and show that the original planar molecular conformation is preserved in contrast to the adsorption on Cu(111). In addition, we highlight the electronic decoupling provided by the h-BN spacer layer and find that the h-BN-metal separation is not significantly modified by the molecular adsorption. Finally, we find indication of a temperature dependence of the adsorption height, which might be a signature of strongly-anisotropic thermal vibrations of the weakly bonded molecules.

Fusing tetrapyrroles to graphene edges by surface-assisted covalent coupling.

Surface-assisted covalent linking of precursor molecules enables the fabrication of low-dimensional nanostructures, which include graphene nanoribbons. One approach to building functional multicomponent systems involves the lateral anchoring of organic heteromolecules to graphene. Here we demonstrate the dehydrogenative coupling of single porphines to graphene edges on the same metal substrate as used for graphene synthesis. The covalent linkages are visualized by scanning probe techniques with submolecular resolution, which directly reveals bonding motifs and electronic features. Distinct configurations are identified that can be steered towards entities predominantly fused to graphene edges through two pyrrole rings by thermal annealing. Furthermore, we succeeded in the concomitant metallation of the macrocycle with substrate atoms and the axial ligation of adducts. Such processes combined with graphene-nanostructure synthesis has the potential to create complex materials systems with tunable functionalities.

Corrugation in the Weakly Interacting Hexagonal-BN/Cu(111) System: Structure Determination by Combining Noncontact Atomic Force Microscopy and X-ray Standing Waves.

Atomically thin hexagonal boron nitride (h-BN) layers on metallic supports represent a promising platform for the selective adsorption of atoms, clusters, and molecular nanostructures. Specifically, scanning tunneling microscopy (STM) studies revealed an electronic corrugation of h-BN/Cu(111), guiding the self-assembly of molecules and their energy level alignment. A detailed characterization of the h-BN/Cu(111) interface including the spacing between the h-BN sheet and its support-elusive to STM measurements-is crucial to rationalize the interfacial interactions within these systems. To this end, we employ complementary techniques including high-resolution noncontact atomic force microscopy, STM, low-energy electron diffraction, X-ray photoelectron spectroscopy, the X-ray standing wave method, and density functional theory. Our multimethod study yields a comprehensive, quantitative structure determination including the adsorption height and the corrugation of the sp2 bonded h-BN layer on Cu(111). Based on the atomic contrast in atomic force microscopy measurements, we derive a measurable-hitherto unrecognized-geometric corrugation of the h-BN monolayer. This experimental approach allows us to spatially resolve minute height variations in low-dimensional nanostructures, thus providing a benchmark for theoretical modeling. Regarding potential applications, e.g., as a template or catalytically active support, the recognition of h-BN on Cu(111) as a weakly bonded and moderately corrugated overlayer is highly relevant.