Abstract 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-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 norbornadiene-quadricyclane photoswitch is a promising example of molecular solar thermal systems capable of energy conversion, storage and release. We present a scanning tunneling microscopy and spectroscopy study of a symmetric NBD-COOMe derivative adsorbed on the Au(1 1 1) surface, supported by density functional theory simulations. We observe the formation of ordered self-assembled islands of NBD-COOMe on the gold surface, driven by intermolecular van der Waals interaction, which become disordered and gradually desorb upon increasing the surface temperature. After annealing at 200 °C, a few adsorbed molecules show a different appearance and electronic structure suggesting the formation of single metalorganic complexes of NBD-COOMe with gold adatoms.

This study presents three different types of molecular switches studied on an Au(111) surface. For each case, the switching is induced via tip-based manipulations through inelastic tunnelling electrons. However, the mechanism responsible for the switching is different in each molecule. The first molecule is a well-known dipolar switch, wherein the chemical reaction required to switch between the two conformers is usually thermally activated. We show how a unidirectional switching event can be triggered through voltage pulses. The second molecule demonstrates that tuning intermolecular reactions can change its planarity, switching from a non-planar to planar geometry. The third molecule is a reversible switch that can be transformed from one conformation to another by causing an intramolecular change. We find that this is possible by addressing high-energy excited states through tip electrons.

The norbornadiene-quadricyclane photoswitch is a promising example of molecular solar thermal systems capable of energy conversion, storage and release. We present a scanning tunneling microscopy and spectroscopy study of a symmetric NBD-COOMe derivative adsorbed on the Au(1 1 1) surface, supported by density functional theory simulations. We observe the formation of ordered self-assembled islands of NBD-COOMe on the gold surface, driven by intermolecular van der Waals interaction, which become disordered and gradually desorb upon increasing the surface temperature. After annealing at 200 degrees C, a few adsorbed molecules show a different appearance and electronic structure suggesting the formation of single metalorganic complexes of NBD-COOMe with gold adatoms.

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.

This study investigates the unidirectional rotation of single moleculerotors, crucial for nanoscale work. We compared two DMBI-based molecule-rotors on an Au(111) surface, exploring their one-way rotation induced by tunneling electrons and thermally-induced random rotation rates. By optimizing the parameters of temperature and tunneling current, we identified a narrow voltage range that enhances unidirectional rotation without inelastic tunneling effects. By the quantum mixing of the ground and excited states occurring at each tunneling electron transfer event through the molecule-rotor, the thermal energy can be projected in part on the excited states of the molecule-rotor to enhance the one-way rotation effect in a bias voltage range where inelastic tunneling effects are nearly absent.

Nonplanar 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 nonplanar 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 nonplanar 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 nonplanar molecule to its planar configuration. We discuss the two different geometries and their electronic properties with the support of density functional theory calculations.

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.

Depending on its adsorption conformation on the Au(111) surface, a zwitterionic single-molecule machine works in two different ways under bias voltage pulses. It is a unidirectional rotor while anchored on the surface. It is a fast-drivable molecule-vehicle (nanocar) while physisorbed. By tuning the surface coverage, the conformation of the molecule can be selected to be either rotor or nanocar. The inelastic tunneling excitation producing the movement is investigated in the same experimental conditions for both the unidirectional rotation of the rotor and the directed movement of the nanocar.

Exploring the limits of the microscopic reversibilityprinciple,we investigated the interplay between thermal and electron tunnelingexcitations for the unidirectional rotation of a molecule-rotor onthe Au(111) surface. We identified a range of moderate voltages andtemperatures where heating the surface enhances the unidirectionalrotational 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 duringtunneling, the quantum mixing of ground and excited electronic statesbrings part of the surface thermal energy in the excited electronicstates of the molecule-rotor. Thermal energy contributes thereforeto the semiclassical unidirectional rotation without contradictingthe microscopic reversibility principle.

We present a low temperature scanning tunneling microscope investigation of a prochiral thiophene-based molecule that self-assembles forming islands with different domains on the Au(111) surface. In the domains, two different conformations of the single molecule are observed, depending on a slight rotation of two adjacent bromothiophene groups. Using voltage pulses from the tip, single molecules can be switched between the two conformations. The electronic states have been measured with scanning tunneling spectroscopy, showing that the electronic resonances are mainly localized at the same positions in both conformations. Density-functional theory calculations support the experimental results. Furthermore, we observe that on Ag(111), only one configuration is present and therefore the switching effect is suppressed.

The precise design of Nanocars and single-molecule machines requires understanding the mechanisms of inducing and controlling molecular motion and hence allowing the conversion of electric power into controlled movement. We present a series of azulene-based Nanocars specifically designed to uncover the influence of several chemical groups on the movement performance. While the azulene core is characterized by a high dipole moment, the rational choice of lateral groups can further enhance the dipole moment and influence the interaction of the molecules with the Au(111) surface and with each other. First, we investigated the high electric dipole moment molecule di-tert-butyl 2-isocyanoazulene-1,3-dicarboxylate (BCA). While single molecules were immobile, a complex structure consisting of cleaved molecules and gold adatoms moved under STM voltage pulses. This structure could be driven along an arbitrarily chosen path. To investigate the influence of the different functional chemical groups and the impact of the manifold physical effects taking place, two modified molecules were investigated. The results of these experiments demonstrate the efficiency of the functional chemical groups attached to the azulene core and shed new light on design strategies for fast and controllably moving Nanocars in general.

Dihydroazulene/vinylheptafulvene pairs are known as molecular dipole switches that undergo a ring-opening/-closure reaction by UV irradiation or thermal excitation. Herein, we show that the ring-closure reaction of a single vinylheptafulvene adsorbed on the Au(111) surface can be induced by voltage pulses from the tip of a scanning tunneling microscope. This cyclization is accompanied by the elimination of HCN, as confirmed by simulations. When inducing lateral movements by applying voltage pulses with the STM tip, we observe that the response of the single molecules changes with the ring closing reaction. This behaviour is discussed by comparing the dipole moment and the charge distribution of the open and closed forms on the surface.

Due to the low corrugation of the Au(111) surface, 1,4-bis(phenylethynyl)-2,5-bis(ethoxy)benzene (PEEB) molecules can form quasi interlocked lateral patterns, which are observed in scanning tunneling microscopy experiments at low temperatures. We demonstrate a multi-dimensional clustering approach to quantify the anisotropic pair-wise interaction of molecules and explain these patterns. We perform high-throughput calculations to evaluate an energy function, which incorporates the adsorption energy of single PEEB molecules on the metal surface and the intermolecular interaction energy of a pair of PEEB molecules. The analysis of the energy function reveals, that, depending on coverage density, specific types of pattern are preferred which can potentially be exploited to form one-dimensional molecular wires on Au(111).

We present the chemical anchoring of a DMBI-P molecule-rotor to the Au(111) surface after a dissociation reaction. At the temperature of 5 K, the anchored rotor shows a sequential unidirectional rotational motion through six defined stations induced by tunneling electrons. A typical voltage pulse of 400 mV applied on a specific location of the molecule causes a unidirectional rotation of 60° with a probability higher than 95%. When the temperature of the substrate increases above 20 K, the anchoring is maintained and the rotation stops being unidirectional and randomly explores the same six stations. Density functional theory simulations confirm the anchoring reaction. Experimentally, the rotation shows a clear threshold at the onset of the C-H stretch manifold, showing that the molecule is first vibrationally excited and later it decays into the rotational degrees of freedom.

The Cover Feature shows the evolution of scanning tunneling microscopy images during the cyclodehydrogenation of a polyaromatic hydrocarbon precursor sublimated onto an Au(111) surface under ultra-high vacuum. This strategy allows the on-surface preparation and study of planar large and dissymmetric starphenes that could not be prepared in solution, and may give access to families of molecular logic gates that are of interest in single-molecule electronics. More information can be found in the Full Paper by A. Jancarik, A. Gourdon, F. Moresco, and co-workers.

A large dissymmetric starphene molecule, the tetrabenzo[a,c,u,w]naphtho[2,3-l]nonaphene, was obtained by first preparing a soluble precursor which was then sublimated on a Au(111) surface in an ultra-high vacuum. In a second step, controlled annealings from 200 °C to 275 °C initiated two successive cyclodehydrogenation steps with the formation of 3 new carbon-carbon bonds. A second conformer was also stable enough during the annealing step to give another compound in similar yield, the benzodibenzo[7,8,9,10]naphthaceno[2,1-h]phenanthro[9,10-p]hexaphene. The formation of this more-hindered species stresses the importance of strong molecule-surface interactions during the cyclodehydrogenations steps of these large polyaromatic hydrocarbons.

The rapid development of on-surface synthesis provides a unique approach toward the formation of carbon-based nanostructures with designed properties. Herein, we present the on-surface formation of CN-substituted phenylene vinylene chains on the Au(111) surface, thermally induced by annealing the substrate stepwise at temperatures between 220 degrees C and 240 degrees C. The reaction is investigated by scanning tunneling microscopy and density functional theory. Supported by the calculated reaction pathway, we assign the observed chain formation to a Knoevenagel condensation between an aldehyde and a methylene nitrile substituent.