Francesca Moresco

Azulene-based systems offer a robust platform for exploring molecular self-assembly and electronic properties on surfaces due to their non-benzenoid topology and dipolar structure. Here, we report the low-temperature scanning tunnelling microscopy investigation of two structurally related azulene-based molecules, namely cyclopenta[cd]azulene trimers ( CPAT ) and its phenyl extended analogue cyclopenta[cd]azulene trimers ( CPAT-Ph ). The latter has lateral extensions in the form of three phenyl rings, designed to investigate the influence of functionalization on supramolecular behaviour. We demonstrate that while CPAT self-assembles in homochiral domains on Au(111), CPAT-Ph is weakly adsorbed and mobile on Au(111) and on Cu(110), exhibiting no ordered domains. Spatially resolved scanning tunnelling spectroscopy (STS) reveals that lateral π-extension primarily modifies electronic level alignment through altered molecule–substrate coupling, while leaving intermolecular electronic interactions weak. Upon thermal activation, both molecules undergo disordered intermolecular linkages on Au(111). However, on Cu(110) enhanced interaction between the substrate and the CPAT-Ph molecules enables intramolecular cyclodehydrogenation through multiple competing ring-closure pathways. These findings indicate lateral π-extension as a design parameter for tuning adsorption-dominated electronic structure and surface reactivity in non-benzenoid π-systems.

For the development of single-molecule machines on surfaces, a vertical molecular geometry based on a simple, common platform is a promising design approach. This could allow decoupling of the active unit from the supporting surface and obtaining of a flexible modular system. An ideal platform for this purpose is subphthalocyanine with its bowl-shaped geometry and axial functionalization. We functionalized SubPcs with a series of vertical, axial ligands with varying conjugation lengths. Their adsorption on the Au(111) surface was studied by low-temperature scanning tunneling microscopy, supported by simulations. We found that increasing the conjugation length of the axial ligand induces a distinct transition in the adsorption geometry. Long ligands, such as azobenzene and naphthalene derivatives, adopt a reverse adsorption geometry with the ligand adsorbed flat on the surface and the SubPc platform pointing upward. These reverse molecules further interact, forming one-dimensional chains. The intermolecular arrangement and distances in the chains are determined by the orientation of the axial ligand on the surface. In contrast, the shortest ligand, which is formed by a single phenyl ring derivative, predominantly adsorbs with the SubPc platform on the surface and allows rotation by the STM tip. Our findings reveal a clear structure-adsorption relationship and offer a rational strategy to control the orientation and packing of SubPc-based single-molecule machines on surfaces through the design of the axial ligands.

N ‐Heterocyclische Carbene sind hocheffektive Liganden zur Verankerung funktioneller organischer Moleküle auf Metalloberflächen und Nanopartikeln, wodurch die Bildung selbstorganisierter Monoschichten erleichtert wird. Ihre Adsorption auf Oberflächen ist jedoch schwer vorherzusagen und zu kontrollieren, und die Geometrie von NHC‐Derivaten auf Goldoberflächen sowie die Rolle von Gold‐Adatomen stehen im Zentrum wissenschaftlicher Diskussionen. Hier präsentieren wir zwei Moleküle welche auf einem Benzimidazol‐NHC Motiv basieren, eines mit einem Thiophen‐Substituenten und das andere mit einem Br‐Atom. Mittels Tieftemperatur‐Rastertunnelmikroskopie (RTM) zeigen wir, dass beide Moleküle planar auf Au(111) adsorbieren und auf der Oberfläche chiral sind. Unsere Ergebnisse deuten darauf hin, dass in beiden Fällen ein Komplex zwischen NHC und einem Gold‐Adatom gebildet wird. Nach Spannungspulsen mit der RTM‐Spitze bewegen sich beide Komplexe durch inelastische Tunnel‐Elektronen. Für das Derivat mit Thiophen beobachten wir eine schrittweise 60°‐Rotation um das S‐Atom in eine Richtung. Die Drehrichtung wird sowohl durch die Chiralität als auch durch die Position des angewendeten Impulses bestimmt. Im Gegensatz dazu bewegt sich das NHC‐Derivat ohne Thiophen lateral auf der Oberfläche. Adsorption, Bindung an Goldatome und Bewegung werden durch Dichtefunktionaltheorie‐Berechnungen und Bildsimulationen unterstützt und diskutiert.

N ‐Heterocyclic carbenes are highly effective ligands for anchoring functional organic molecules to metal surfaces and nanoparticles, facilitating the formation of self‐assembled monolayers. However, their adsorption on surface is difficult to predict and control, and there is an ongoing debate on the geometry of NHC derivatives on gold surfaces and on the role of gold adatoms. We present two single molecules based on a benzimidazole NHC, one equipped with a thiophene substituent, and the other ending with a Br atom. By low temperature scanning tunneling microscopy we show that both molecules adsorb planar on Au(111) and are chiral on the surface. Our results indicate that in both cases a complex between NHC and a gold adatom is formed. Upon voltage pulses with the STM tip, both complexes move excited by inelastic tunneling electrons. For the derivative with thiophene, we observe a stepwise 60° unidirectional rotation around the S atom. The direction of rotation is determined by both the chirality and the position of the applied pulse. On the contrary, the NHC derivative without thiophene moves laterally on the surface. Adsorption, binding to gold atoms, and motion are discussed with the support of density functional theory calculations and image simulations.

The on‐surface synthesis of an isomer of undecacene, bearing two four‐membered rings and two para‐quinodimethane moieties, starting from a tetramethyl‐substituted diepoxy precursor, is presented. The transformation implies a thermal double deoxygenation followed by a stepwise double dehydrogenation reaction on the Au(111) surface, locally induced by inelastic tunneling electrons. This results in the transformation of para‐dimethylbenzene moieties into non‐aromatic para‐quinodimethanes. The structures and electronic properties of the intermediate and final products are investigated at the single molecule level with high spatial resolution, using both scanning tunneling microscopy/spectroscopy and non‐contact atomic force microscopy. The experimental results are supported by density functional theory calculations.

Non-planar aromatic molecules are interesting systems for organic electronics and optoelectronics applications due to their high stability and electronic properties. By using scanning tunneling microscopy and spectroscopy, we investigated thianthrene-based molecules adsorbed on Au(111), which are non-planar in the gas phase and the bulk solid state. Varying the molecular coverage leads to the formation of two different kinds of self-assembled structures: close-packed islands and quasi one-dimensional chains. We found that the molecules are non-planar within the close-packed islands, while the configuration is planar in the molecular chain and for single adsorbed molecules. Using vertical tip manipulation to isolate a molecule from the island, we demonstrate the conversion of a non-planar molecule to its planar configuration. We discuss the two different geometries and their electronic properties with the support of density functional theory calculations.

Exploring the limits of the microscopic reversibility principle, we investigated the interplay between thermal and electron tunneling excitations for the unidirectional rotation of a molecule-rotor on the Au(111) surface. We identified a range of moderate voltages and temperatures where heating the surface enhances the unidirectional rotational rate of a chemisorbed DMNI-P rotor. At higher voltage, inelastic tunneling effects dominate while at higher temperature the process becomes stochastic. At each electron transfer event during tunneling, the quantum mixing of ground and excited electronic states brings part of the surface thermal energy in the excited electronic states of the molecule-rotor. Thermal energy contributes therefore to the semi-classical unidirectional rotation without contradicting the microscopic reversibility principle.