Collisions of atoms and molecules are required for bond formation and are thus key to any chemical reaction. Their outcome is determined by the collision energy, relative orientation, and impact parameter of the reactants. Collision studies in the gas phase have shown that energies are easily controlled, whereas it is difficult to steer the molecular orientations and completely impossible to control the impact parameter. Here, we study the collision of individual molecules at a surface where each involved compound is imaged before and after collision by scanning tunneling microscopy, allowing precise control of orientations and impact parameter. We observe that the molecules must collide in a narrow cone of reaction and that surface atom displacements can enable a reaction even for disfavored pathways.

The light induced trans-cis isomerization of azobenzene allows studying and controlling the properties of photosensitive materials. Promising applications include optical switching, data storage, and light-driven nanomechanical devices. Understanding switching in molecular assemblies is essential to scale these photoinduced mechanisms for practical applications. In this study, we analyze the switching behavior of single azobenzene derivatives in large assemblies (n >> 103) with advanced computer vision techniques. This approach enables us to detect subtle variations in switching yields among densely packed, similarly oriented molecules. Our findings provide new insights into the switching of molecular assemblies.

We studied the influence of cobalt (Co) nanostructures on the photo-isomerization of meta-TBA molecules on a Au(111) surface by scanning tunneling microscopy (STM). Switching from trans to cis isomers of individual molecules was investigated by scanning tunneling microscopy for different molecule locations with respect to Co islands on the surface. Specifically, we studied the isomerization rates for molecules on top of cobalt islands, on clean Au(111) areas and on Au(111) areas that are close to Co islands. We observe that isomerization can be induced successfully on both, gold and cobalt areas, with similar rates. Comparing different gold areas, about the same number of switching events per molecule are found on cobalt-free Au(111) terraces and in Au(111) areas if a Co island is located nearby. This finding shows that no local field enhancement, caused by plasmonic excitations within Co nanostructures, is present, at least not in strength that would substantially affect the molecular photoisomerization process.

ABSTRACT Laws of motion are of fundamental importance for the understanding of dynamic systems. At the atomic scale, the kinetic energy of molecules controls chemical reactions. The velocity of individual molecules, however, has never been measured, because it is difficult to determine time–distance relationships. Here, we show how the real speed of a single molecule moving across a Ag(111) surface is measured at the microsecond time scale. We found that atomic defects extend the travelling time by orders of magnitude, even at distances of few nanometers from the molecule and with high sensitivity to their precise location. Statistical measurements, obtained by repetitive translation of a single molecule in an oscillating electric field, give insight into the time scales of molecular motion in a confined environment. The strong influence of the defects underscores the need for their investigation during chemical reactions or collision experiments at variable velocities and geometries on a surface.

Depositing intact organic molecules with large molecular weight under ultrahigh-vacuum conditions onto a solid surface remains an important challenge in surface science, as conventional sublimation methods are often incompatible with thermally sensitive, high-mass species. Here, we demonstrate the use of a modified electrospray deposition (ESD) setup, integrated into the load-lock of an UHV chamber, to deposit porphyrin derivatives and polymerized polyethylene glycol (PEG) chains in a clean manner onto a Au(100) surface. Our experiments show that, despite the technical simplicity and absence of ion optics, the ESD setup enables preparation of clean high-quality samples, which are suitable for imaging by scanning tunneling microscopy (STM) without the need of postdeposition annealing. Detailed STM imaging at low temperatures reveals intact PEG chains with complex conformations, including folded and looped structures, which highlight the nondestructive nature of the deposition. These examples demonstrate that simple ESD methods can serve as practical and efficient alternatives to more complex ion beam systems for UHV-compatible deposition of functional molecules with a large molecular weight.

The breaking of an interatomic bond is at the heart of chemistry yet remains a challenge to be investigated. Molecules on metal surfaces exhibit defined positions and orientations and can be characterized by scanning tunneling microscopy that moreover is able to trigger bond breaking. Until now, the bond dissociation dynamics has been studied in small molecules but not in large ones with various degrees of freedom. Here, we dissociate bromine atoms from single dibromo-terfluorene molecules on Ag(111), identifying not only the displacement but also the rotation of each fragment. It turns out that the molecular excitation that causes dissociation is not locally confined. Instead, it can propagate through the molecule, and the dynamics of the resulting fragments is uncorrelated. The fragment binds to the nearest silver atom after dissociation and dissipates its energy in rotational motion. Our findings could be useful for the precise engineering of chemical reactions with prearranged precursor molecules.

Nitrenes are known as key intermediates in various chemical reactions. Nitrene transfer reactions are particularly effective for synthesizing nitrogen-containing compounds, where metal catalysts play a crucial role in controlling nitrene reactivity and selectivity. In this study, we demonstrate the formation of a stable surface-supported dinitrene on Au(111) through UV irradiation of its diazide precursor, characterized by scanning probe techniques. The photoreaction mechanism is elucidated with wavelength-dependent experiments and time-dependent density functional theory calculations. Our findings present the first real-space visualization of a metal nitrene adsorbed on a surface, highlighting its potential in catalysis and surface functionalization.

We generated trinitreno-s-heptazine, a small molecule featuring three nitrene centers, by tip-induced chemistry from the precursor 2,5,8-triazido-s-heptazine on bilayer NaCl on Au(111). The precursor's azide groups were dissociated to form mono-, di- and trinitreno-s-heptazine, yielding molecules with one to three nitrene centers. The precursor and its products are characterized by atomic force microscopy and scanning tunnelling microscopy. Broken-symmetry DFT and configuration interaction calculations of inter-nitrene and intra-nitrene exchange couplings suggest a ferromagnetic coupling of the S = 1 nitrene centers, resulting in a high-spin septet ground state for neutral trinitreno-s-heptazine in the gas phase. On bilayer NaCl on Au(111), the combined results of experiments and theory suggest trinitreno-s-heptazine to be an anion with a sextet ground state.

Azobenzene derivatives, an important class of molecular switches that undergo isomerization between trans and cis states, have been studied on various coinage metal surfaces in the past. However, on reactive copper surfaces switching turned out to be difficult. Here, we show that the use of bulky 2,6-dimethylphenyl side groups enables switching on a Cu(111) surface as studied by scanning tunneling microscopy. By applying voltage pulses from the STM tip, irreversible trans → cis isomerization of individual molecules can be induced where exactly one molecule within each unit cell is isomerized. The absence of backward cis → trans switching illustrates the importance of the adsorption structure on the potential energy landscape of azobenzene. Isomerization, which is activated by inelastic scattering of single tunneling electrons, happens in a very localized fashion, thus allowing to ‘write’ patterns of individual cis isomers into the homogeneous molecular layer. Our study demonstrates how the bulky spacer groups direct adsorption and self-assembly into close-packed structures, which are similar to three-dimensional molecular crystals, on the surface. In addition, they modulate the electronic coupling to the underlying substrate to maintain successful switching behavior even on the most reactive coinage metal surfaces.

Light is a versatile tool to remotely activate molecules adsorbed on a surface, for example, to trigger their polymerization. Here, we explore the spatial distribution of light-induced chemical reactions on a Au(111) surface. Specifically, the covalent on-surface polymerization of an anthracene derivative in the submonolayer coverage range is studied. Using scanning tunneling microscopy and X-ray photoemission spectroscopy, we observe a substantial increase of the local molecular coverage with the sample illumination time at the center of the laser spot. We find that the interplay between thermally induced diffusion and the reduced mobility of reaction products steers the accumulation of material. Moreover, the debromination of the adsorbed species never progresses to completion within the experiment time, despite a long irradiation of many hours.

The breaking of a chemical bond is fundamental in most chemical reactions. To understand chemical processes in heterogeneous catalysis or on-surface polymerization the study of bond dissociation in molecules adsorbed on crystalline surfaces is advantageous. Single molecule studies of bond breaking can give details of the dissociation dynamics, which are challenging to obtain in mole-scale ensemble experiments. Bond breaking in single adsorbed molecules can be triggered using the energy of the tunnelling electrons in a scanning tunnelling microscope (STM) at selected positions to investigate the dissociation dynamics. Single bond dissociation dynamics has been deeply investigated only in small molecules, but not in larger molecules that exhibit distinct rotational degrees of freedom. Here, we use low temperature (7 K) STM to dissociate a single bromine atom from an elongated molecule (dibromo-terfluorene) adsorbed on a Ag(111) surface. This rod-like molecule allows to clearly identify not only displacement of the reaction fragments, but also their rotation. The results show that the molecular fragment binds to the nearest silver atom and only further rotation is allowed. Moreover, the excitation responsible for the bond breaking can propagate through the molecular backbone to dissociate a bromine atom that is not located at the pulse position. These results show the important role of the metal substrate in conditioning the bond dissociation dynamics. Our results might allow to improve the control of the synthesis of 2D materials and targeted engineering of molecular architectures.

Building nanostructures one-by-one requires precise control of single molecules over many manipulation steps. The ideal scenario for machine learning algorithms is complex, repetitive, and time-consuming. Here, we show a reinforcement learning algorithm that learns how to control a single dipolar molecule in the electric field of a scanning tunneling microscope. Using about 2250 iterations to train, the algorithm learned to manipulate the molecule toward specific positions on the surface. Simultaneously, it generates physical insights into the movement as well as orientation of the molecule, based on the position where the electric field is applied relative to the molecule. This reveals that molecular movement is strongly inhibited in some directions, and the torque is not symmetric around the dipole moment.

We have studied 22-oxahemiporphycene molecules by a combination of scanning tunneling microscopy at low temperatures and density functional theory calculations. In contrast to other molecular switches with typically two switching states, these molecules can in principle exist in three different tautomers, due to their asymmetry and three inequivalent binding positions of a hydrogen atom in their macrocycle. Different tautomers are identified from the typical appearance on the surface and tunneling electrons can be used to tautomerize single molecules in a controllable way with the highest rates if the STM tip is placed close to the hydrogen binding positions in the cavity. Characteristic switching processes are explained by the different energy pathways upon adsorption on the surface. Upon applying higher bias voltages, deprotonation occurs instead of tautomerization, which becomes evident in the molecular appearance.

Molecular motors have chemical properties that enable unidirectional motion, thus breaking microscopic reversibility. They are well studied in solution, but much less is known regarding their behavior on solid surfaces. Here, single motor molecules adsorbed on a Cu(111) surface are excited by voltages pulses from an STM tip, which leads to their rotation around a fixed pivot point. Comparison with calculations shows that this axis results from a chemical bond of a sulfur atom in the chemical structure and a metal atom of the surface. While statistics show approximately equal rotations in both directions, clockwise and anticlockwise, a detailed study reveals that these motions are enantiomer-specific. Hence, the rotation direction of each individual molecule depends on its chirality, which can be determined from STM images. At first glance, these dynamics could be assigned to the activation of the motor molecule, but our results show that this is unlikely as the molecule remains in the same conformation after rotation. Additionally, a control molecule, although it lacks unidirectional rotation in solution, also shows unidirectional rotation for each enantiomer. Hence, it seems that the unidirectional rotation is not specifically related to the motor property of the molecule. The calculated energy barriers for motion show that the propeller-like motor activity requires higher energy than the simple rotation of the molecule as a rigid object, which is therefore preferred.

Artificial molecular motors are designed to transform external energy into useful work in the form of unidirectional motion 1 . They have been studied mainly in solution 2 – 4 , but also on solid surfaces 5 , 6 , which provide fixed reference points, allowing for tracking of their movement. However, these molecules require sophisticated design and synthesis, because the motor function must be imprinted into the chemical structure, and show reduced functionality on surfaces compared with in solution 5 – 8 . DNA walkers 9 , 10 , on the other hand, impart high directionality as they include the surface as part of the motor function, but they require chemical surface patterning and sequential solvent modification for motor activation. Here we show how efficient motors can operate at much smaller length scales on a homogeneous metal surface without any liquid. This is realized by combining a surface with a simple molecule, which, by itself, does not contain any motor unit. The motion, which is tracked at the single-molecule level, is triggered by intramolecular proton transfer with a corresponding modulation of the potential energy surface. Each molecule moves with 100 percent unidirectionality along an atomically defined straight line. Proof of the motor performing meaningful work is shown by controlled transport of single carbon monoxide molecules. This simplistic concept could form the basis for the controlled bottom-up assembly of nanostructures at the atomic scale.

Two different metal-organic frameworks with either a honeycomb or Kagome structure were grown on Cu(111) using para-aminophenol molecules and native surface adatoms. Although both frameworks are made up from the same chemical species, they are structurally different emphasizing the critical role being played by the reaction conditions during their growth. This work highlights the importance of the balance between thermodynamics and kinetics in the final structure of surface-supported metal-organic networks.

Understanding the directed motion of a single molecule on surfaces is not only important in the well-established field of heterogeneous catalysis but also for the design of artificial nanoarchitectures and molecular machines. Here, we report how the tip of a scanning tunneling microscope (STM) can be used to control the translation direction of a single polar molecule. Through the interaction of the molecular dipole with the electric field of the STM junction, it was found that both translations and rotations of the molecule occur. By considering the location of the tip with respect to the axis of the dipole moment, we can deduce the order in which rotation and translation take place. While the molecule-tip interaction dominates, computational results suggest that the translation is influenced by the surface direction along which the motion takes place.

microRNAs represent promising drugs to treat and prevent several diseases, such as diabetes mellitus. microRNA delivery brings many obstacles to overcome, and one strategy to bypass them is the manufacturing of self-assembled microRNA protein nanoparticles. In this work, a microRNA was combined with the cell-penetrating peptide protamine, forming so-called proticles. Previous studies demonstrated a lack of microRNA dissociation from proticles. Therefore, the goal of this study was to show the success of functionalizing binary proticles with citric acid in order to reduce the binding strength between the microRNA and protamine and further enable sufficient dissociation. Thus, we outline the importance of the present protons provided by the acid in influencing colloidal stability, achieving a constant particle size, and monodispersing the particle size distribution. The use of citric acid also provoked an increase in drug loading. Against all expectations, the AFM investigations demonstrated that our nanoparticles were loose complexes mainly consisting of water, and the addition of citric acid led to a change in shape. Moreover, a successful reduction in binding affinity and nanoparticulate stability are highlighted. Low cellular toxicity and a constant cellular uptake are demonstrated, and as uptake routes, active and passive pathways are discussed.

Spatial control over molecular movement is typically limited because motion at the atomic scale follows stochastic processes. We used scanning tunneling microscopy to bring single molecules into a stable orientation of high translational mobility where they moved along precisely defined tracks. Single dibromoterfluorene molecules moved over large distances of 150 nanometers with extremely high spatial precision of 0.1 angstrom across a silver (111) surface. The electrostatic nature of the effect enabled the selective application of repulsive and attractive forces to send or receive single molecules. The high control allows us to precisely move an individual and specific molecular entity between two separate probes, opening avenues for velocity measurements and thus energy dissipation studies of single molecules in real time during diffusion and collision.

Molecular motors have been intensely studied in solution, but less commonly on solid surfaces that offer fixed points of reference for their motion and allow high-resolution single-molecule imaging by scanning probe microscopy. Surface adsorption of molecules can also alter the potential energy surface and consequently preferred intramolecular conformations, but it is unknown how this affects motor molecules. Here, we show how the different conformations of motor molecules are modified by surface adsorption using a combination of scanning tunneling microscopy and density functional theory. These results demonstrate how the contact of a motor molecule with a solid can affect the energetics of the molecular conformations.