Atomic step-edges on metallic surfaces are highly active catalytic sites due to their reduced coordination and modified electronic structure. Yet, approaches to organic ligand functionalization on the single-molecule level have largely targeted flat terrace geometries, whereas site-specific step-edge functionalization remains unaddressed. This study shows that decorating the atomically defined step-edges of Au(788) with N-heterocyclic carbenes (NHCs) enhances their catalytic activity toward CO 2 ${\rm CO}_2$ reduction compared to undecorated metallic step-edges. Using high-resolution scanning probe microscopy, an upright-tilted adsorption geometry and a unified binding mode of three different NHCs at step-edges are revealed. The exceptional stability of these well-defined nanostructures allows the use of the single-crystalline samples as working electrodes in electrochemical experiments. Photoelectron spectroscopy and theoretical simulations correlate charge transfer and conformational details with their catalytic performance. By combining macroscopic electrochemical experiments with single-molecule microscopy, this study highlights NHC step-edge functionalization as an effective approach to design highly selective and efficient catalysts.

We report an implementation of projection-based quantum embedding that combines periodic density functional theory in the CP2K code with correlated wavefunction calculations in the Q-Chem program. Using this interface, correlated wavefunction methods can be applied to a subset of the molecular orbitals, selected in an automated manner based on a user-defined list of nuclei, wth an embedding potential that provides electronic coupling to the remaining orbitals that comprise the environment. Our implementation uses exact projection, without the need for any level-shift operator, and is “pseudoperiodic” in the sense that periodic boundary effects are implicit in the orbitals used in the (non-periodic) wavefunction calculation. Convergence tests demonstrate that computed properties are faithful to the corresponding periodic quantities, provided that the high-level subsystem is spatially localized and small, relative to the periodic simulation cell. Spectroscopic examples for aqueous chromophores demonstrate converged results using a very limited subset of the water molecules in the simulation cell, decoupling the choice of excited-state method and basis set from the functional that is used to propagate ground-state ab initio molecular dynamics. Solvatochromic shifts for aqueous uracil are converged using equation-of-motion coupled-cluster theory, without the need for solvent molecules in the wavefunction calculation, and the embedded calculation captures solvent effects that are absent in gas-phase “microhydration” studies.

Das Screening von Katalysatoren stellt eine anspruchsvolle Aufgabe für die computergestützte Chemie dar, da die große strukturelle Vielfalt von Oberflächen unter Operando‐Bedingungen mit hohen Anforderungen an die Genauigkeit der kinetischen Vorhersagen einhergeht. Einbettungsmethoden, die es ermöglichen den rechnerischen Aufwand auf die chemisch aktiven Bereiche zu konzentrieren, sind vielversprechende Werkzeuge, um ein Gleichgewicht zwischen Genauigkeit und Effizienz herzustellen. Für metallische Oberflächenkatalysatoren gestaltet sich die notwendige Trennung des Systems in einen aktiv behandelten Bereich und ein Umgebungssystem jedoch als technisch schwierig, da die Elektronen in der leitfähigen Oberfläche delokalisiert sind. Aus diesem Grund sind Studien, die das Potenzial von Einbettungsmethoden für das Screening heterogener (elektro‐)katalytischer Systeme untersuchen, bislang selten. In dieser Arbeit zeigen wir, dass einfache Einbettungsansätze zur Untersuchung metallischer Katalysatoren durchaus realisierbar sind, wenn i) der aktive Orbitalraum entlang der Reaktionskoordinaten konstant gehalten wird und ii) das zur Berechnung des Einbettungspotenzials verwendete nicht‐additive Austausch‐Korrelationsfunktional einen Anteil exakter Austauschwechselwirkung enthält, um Delokalisierungsfehler zu minimieren. Wir verifizieren diesen Ansatz anhand einer Auswahl offenschaliger und geschlossenschaliger Zwischenprodukte der CO 2 ‐Reduktionsreaktion, welche an verschiedenen Adsorptionsplätzen einer Cu(111)‐Oberfläche adsorbiert sind. Die Oberflächen sind durch Clustermodelle repräsentiert und zeigen, dass das Screening von Katalysatoren mithilfe von Einbettungsmethoden möglich ist.

Aromatic belts, ultrashort carbon nanotubes, and related structures, are emerging molecular entities in the fields of organic electronics and supramolecular chemistry owing to their structural rigidity, fully fused π-conjugation, and well-defined cavity. Synthesis of aromatic belts with embedded thiophene structures, which manifest significant optoelectronic and conductive properties, has not yet been achieved. Herein, we report the synthesis of thiophene-fused aromatic belts (thiophene belts) via one-step sulfur cross-linking reaction of partially fluorinated cycloparaphenylenes. Their structural features, including unidirectional columnar stacking with high dipole moment in crystals, two-dimensional layer assembly on metal surfaces, and photophysical properties, such as long-lifetime phosphorescence, are uncovered. These distinctive features of the thiophene belts should inspire a range of applications such as optoelectronic devices and polar materials. Synthesis of aromatic belts with embedded thiophene structures, which manifest significant optoelectronic and conductive properties, has not yet been achieved. Herein, the authors report the synthesis of thiophene-fused aromatic belts via one-step sulfur cross-linking reaction of partially fluorinated cycloparaphenylenes.

Catalyst screening is a demanding task for computational chemistry since the profound diversity of surface structures under operando conditions is accompanied by high demands on the accuracy to predict the relevant kinetics. Embedding approaches that allow researchers to focus the computational effort on the chemically active regions of interest are promising tools in the pursuit of balancing accuracy and efficiency. However, for metallic catalysts, the required separation of the system into an active part treated with highly accurate methods and an environment is technically hard to achieve due to the delocalization of electrons in the conducting surface. Therefore, studies analyzing the potential of embedding methods for heterogeneous (electro-)catalyst screening are scarce. In this contribution, we demonstrate that simple embedding approaches are indeed achievable for studying metallic catalysts if i) the active orbital space is held consistent over a reaction coordinate and ii) the nonadditive exchange-correlation functional used to calculate the embedding potential includes a fraction of exact exchange to mitigate delocalization errors. We verify the approach for a set of open- and closed-shell CO2 reduction reaction intermediates on different adsorption sites of a Cu(111) surface represented by cluster models to demonstrate that catalyst screening with embedding approaches is achievable.

The strongly electron-donating N-heterocyclic imines (NHIs) have been employed as excellent surface anchors for the thermodynamic stabilization of electron-deficient species due to their enhanced nucleophilicity. However, the binding mode and interfacial property of these new ligands are still unclear, representing a bottleneck for advanced applications in surface functionalization and catalysis. Here, NHIs with different side groups have been rationally designed, synthesized, and analyzed on various metal surfaces (Cu, Ag). Our results reveal different binding modes depending on the molecular structure and metal surface. The molecular design enables us to achieve a flat-lying or upright configuration and even a transition between these two binding modes depending on the coverage and time. Importantly, the two binding modes exhibit different degrees of interfacial charge transfer between the molecule and the surface. This study provides essential microscopic insight into the NHI adsorption geometry and interfacial charge transfer for the optimization of heterogeneous catalysts in coordination chemistry.

2-Acetonaphthones, which bear an alkenyl group tethered to its C1 carbon atom via an oxygen atom, were found to undergo an enantioselective intramolecular ortho photocycloaddition reaction. A chiral oxazaborolidine Lewis acid leads to a bathochromic absorption shift of the substrate and enables an efficient enantioface differentiation. Visible light irradiation (λ=450 nm) triggers the reaction which is tolerant of various groups at almost any position except carbon atom C8 (16 examples, 53–99 % yield, 80–97 % ee ). Consecutive reactions were explored including a sensitized rearrangement to tetrahydrobiphenylenes, which occurred with full retention of configuration. Evidence was collected that the catalytic photocycloaddition occurs via triplet intermediates, and the binding mode of the acetonaphthone to the chiral Lewis acid was elucidated by DFT calculations.

A widely used strategy to reduce the computational cost of quantum-chemical calculations is to partition the system into an active subsystem, which is the focus of the computational efforts, and an environment that is treated at a lower computational level. The system partitioning is mostly based on localized molecular orbitals. When reaction paths or energy differences are to be calculated, it is crucial to keep the orbital space consistent for all structures. Inconsistencies in orbital space can lead to unpredictable errors on the potential energy surface. While successful strategies to ensure this consistency have been established for organic and even metal-organic systems, these methods often fail for metal clusters or nanoparticles with a high density of near-degenerate and delocalized molecular orbitals. However, such systems are highly relevant for catalysis. Accurate yet feasible quantum-mechanical ab initio calculations are therefore highly desired. In this work, we present an approach based on the subsystem projected atomic orbital decomposition algorithm that allows us to ensure automated and consistent partitioning even for systems with delocalized and near-degenerate molecular orbitals and demonstrate the validity of this method for the binding energies of small molecules on transition-metal clusters.

Molecular machines enable external control of structural and dynamic phenomena at the atomic level. To efficiently transfer their tunable properties into designated functionalities, a detailed understanding of the impact of molecular embedding is needed. In particular, a comprehensive insight is fundamental to design hierarchical multifunctional systems that are inspired by biological cells. Here, we applied an on-the-fly trained force field to perform atomistic simulations of a systematically modified rotaxane functionalized metal-organic framework. Our atomistic studies reveal a symmetric and asymmetric interplay of the mechanically bonded rings (MBRs) within the framework depending on the local environment. As a result, their translational motion is modulated ranging from fast oscillatory behavior to cooperative and potentially directed shuttling. The derived picture of competitive interactions, which influence the operation mechanism of the MBRs embedded in these soft porous materials, promotes the development of responsive functional materials, which is a key step toward intelligent matter.

Mechanically interlocked molecules have gained significant attention because of their unique ability to perform well-defined motions originating from their entanglement, which is important for the design of artificial molecular machines. Atomistic simulations based on force fields (FFs) provide detailed insights into such architectures at the molecular level enabling one to predict the resulting functionalities. However, the development of reliable FFs is still challenging and time-consuming, in particular for highly dynamic and interlocked structures such as rotaxanes, which exhibit a large number of different conformers. In the present work, we present an on-the-fly training (OTFT) algorithm. By a guided and nonguided phase space sampling, relevant reference data are automatically and continuously generated and included for the on-the-fly parametrization of the FF based on a population swapping genetic algorithm (psGA). The OTFT approach provides a fast and automated FF parametrization scheme and tackles problems caused by missing phase space information or the need for big data. We demonstrate the high accuracy of the developed FF for flexible molecules with respect to equilibrium and out-of-equilibrium properties. Finally, by applying the ab initio parametrized FF, molecular dynamic simulations were performed up to experimentally relevant time scales (ca. 1 μs) enabling capture in detail of the structural evaluation and mapping out of the free-energy topology. The on-the-fly training approach thus provides a strong foundation toward automated FF developments and large-scale investigations of phenomena in and out of thermal equilibrium.

Photo-responsive molecular motors incorporated in soft porous materials enable the amplification of the motion of individual motor units by employing their collective and cooperative behavior. Metal-organic frameworks (MOFs) provide in this regard, due to their structural diversity and modular assembly, a unique matrix to construct well-defined and systematically tunable molecular environments for the embedding of molecular motors. However, despite advances in the development of such photo-responsive functional materials, a thorough understanding of the governing interactions at the atomic scale has been missing so far, limiting the possibility of predicting and fully exploring the potential of these assembled machineries. Here, we present a conformational study to unravel the collective structural behavior and elucidate the impact of motor-motor interactions on the local and global properties of the scaffold. In particular, our work highlights the impact of full conversion of the embedded molecular motors on the overall network topology of the MotorMOF and thus acts as a benchmark for future studies to further explore the correlation of responsive building units with the resulting functionality of these hierarchical systems.

Molecular spatial conformational evolution following the corresponding chemical reaction pathway at surfaces is important to understand and optimize chemical processes. Combining experimental and theoretical methods, the sequential N-H and C-H dehydrogenation of pyromellitic diimide (PMDI) on a Cu(111) surface are reported. STM experiments and atomistic modeling allow structural analysis at each well-defined reaction step. First, exclusively the aromatic N-H dehydrogenation of the imide group is observed. Subsequently, the C-H group at the benzene core of PMDI gets activated leading to a dehydrogenation reaction forming metalorganic species where Cu adatoms pronouncedly protruding from the surface are coordinated by one or two PMDI ligands at the surface. All reactions of PMDI induce conformational changes at the surface as confirmed by STM imaging and DFT simulations. Such conformational evolution in sequential N-H and C-H activation provides a detailed insight to understand molecular dehydrogenation processes at surfaces.

In response to external stimuli, molecular motors enable to control phenomena at the molecular scale with high precision. In order to utilize their unique properties and to gain designated functionalities, their molecular embedding is important. Despite the great progress in the development of corresponding functional materials, a detailed picture of how the structural and dynamic properties of these responsive molecular units are transferred to a macroscopic outcome is so-far missing. Here, we provide an atomistic insight into the solvation dynamics around a light-driven molecular motor. By performing molecular dynamic simulations based on an ab initio parametrized and validated force field, we elucidate in detail the intermolecular interactions depending on the state of the motor. Detailed analysis of the solvation shells revealed the impact on both the location of the primary interaction sites and the orientation of the solvent molecules with respect to the molecular motor. Furthermore, we studied the influence of structural modifications of the molecular motor on its local environment. By investigating the motor-solvent interaction, our results provide a strong foundation to decipher the ability of molecular machines to specifically alter molecular processes, which is fundamental to predict and tailor the resulting macroscopic functionality.

The structural properties and binding motif of a strongly sigma-electron-donating N-heterocyclic carbene have been investigated on different transition-metal surfaces. The examined cyclic (alkyl)(amino)carbene (CAAC) was found to be mobile on surfaces, and molecular islands with short-range order could be found at high coverage. A combination of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations highlights how CAACs bind to the surface, which is of tremendous importance to gain an understanding of heterogeneous catalysts bearing CAACs as ligands.

Tunable thiolate coordination networks were prepared on metal surfaces using benzenehexathiol (BHT) under ultrahigh vacuum conditions. The two-dimensional (2D) coordination networks achieved via thermal annealing give diverse structures on different surfaces. The structures of the networks on the copper surface could be well tuned by adjusting the thermal condition of the surface. Understanding the significant role of the metal surfaces will bring more information in the controllable synthesis of 2D coordination materials. More information can be found in the Communication by Wei Xu, Saeed Amirjalayer, Daoben Zhu, Harald Fuchs et al.

Thiolate coordination networks (TCNs) were synthesized on metal surfaces using benzenehexathiol (BHT) and analyzed by cryogenic scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) as well as density functional theory. Upon adsorption, the deprotonation of the thiol groups occurred on the metal surface with the formation of metal-sulfur coordination bonds. Increasing the surface temperature triggered the complete dehydrogenation of BHT and promoted the formation of TCNs. On Ag(111), the TCN of [Ag-3(C6S6)](n) was achieved and showed good thermal stability. On Cu(111), two TCNs ([Cu-6(C6S6)](n) and [Cu-8(C6S6)](n),) were identified. Interestingly, the construction of the two TCNs on Cu(111) can be precisely controlled by adjusting the temperature of the surface during deposition and thermal annealing. Our results reveal the significant influence of the metal surface on the formation of coordination networks.

AbstractDie strukturellen Eigenschaften und das Bindungsmotiv eines stark σ‐elektronenschiebenden N‐heterocyclischen Carbens werden an verschiedenen Übergangsmetalloberflächen erforscht. Das untersuchte cyclische (Alkyl)(amino)carben (CAAC) erweist sich auf Oberflächen als mobil, und bei hoher Bedeckung konnten molekulare Inseln mit Nahordnung gefunden werden. Durch die Kombination von Rastertunnelmikroskopie (STM), Röntgenphotoelektronenspektroskopie (XPS) und Dichtefunktionaltheorie (DFT) zeigt unsere Studie, wie CAACs an die Oberfläche binden, was für das Verständnis heterogener Katalysatoren, die CAACs als Liganden tragen, von enormer Bedeutung ist.

We present an extension of the tunneling theory for scanning tunneling microcopy (STM) to include different types of vibrational-electronic couplings responsible for inelastic contributions to the tunnel current in the strong-coupling limit. It allows for a better understanding of more complex scanning tunneling spectra of molecules on a metallic substrate in separating elastic and inelastic contributions. The starting point is the exact solution of the spectral functions for the electronic active local orbitals in the absence of the STM tip. This includes electron-phonon coupling in the coupled system comprising the molecule and the substrate to arbitrary order including the anti-adiabatic strong coupling regime as well as the Kondo effect on a free electron spin of the molecule. The tunneling current is derived in second order of the tunneling matrix element which is expanded in powers of the relevant vibrational displacements. We use the results of an ab-initio calculation for the single-particle electronic properties as an adapted material-specific input for a numerical renormalization group approach for accurately determining the electronic properties of a NTCDA molecule on Ag(111) as a challenging sample system for our theory. Our analysis shows that the mismatch between the ab-initio many-body calculation of the tunnel current in the absence of any electron-phonon coupling to the experiment scanning tunneling spectra can be resolved by including two mechanisms: (i) a strong unconventional Holstein term on the local substrate orbital leads to reduction of the Kondo temperature and (ii) a different electron-vibrational coupling to the tunneling matrix element is responsible for inelastic steps in the $dI/dV$ curve at finite frequencies.

Photo-responsive functional materials have gained increasing attention due to their externally tunable properties. Molecular switches embedded in these materials enable to control phenomena at the atomic level by light. Metal-Organic Frameworks (MOFs) provide a versatile platform to immobilize these photo-responsive units within defined molecular environments to optimize the intended functionality. For the application of these photo-responsive MOFs (pho-MOFs), it is crucial to understand the influence of the switching state on the host-guest interaction. Therefore, we present a detailed insight into the impact of molecular switching the intermolecular interaction. By performing atomistic simulations, we revealed that due to different interactions of the guest molecules with the two isomeric states of an azobenzene-functionalized MOF, both the adsorption sites and the orientation of the molecules within the pores are modulated. By shedding light on the host-guest interaction, our study highlights the unique potential of pho-MOFs to tailor molecular interaction by light.

Tuning the binding mode of N-heterocyclic carbenes on metal surfaces is crucial for the development of new functional materials. To understand the impact of alkyl side groups on the formation of NHC species at the Au(111) surface, we combined scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. We reveal two significantly different binding modes depending on the alkyl chain length. In the case of a short alkyl substituent, an up-standing configuration with one Au adatom is preferred, whereas the longer alkyl groups result exclusively in NHC-Au-NHC complexes lying flat on the surface. Our study highlights how well-defined structural modifications of NHCs allow for controlling the local binding motif on surfaces, which is important to design designated catalytic sites at interfaces.