Search

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.

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.

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.

The structural properties and binding motif of a strongly σ‐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., Served on a (golden) tray! A highly reactive and electron‐rich (alkyl)(amino)carbene (CAAC) derivative was deposited on a surface, and its binding mode and ordering were investigated for the first time. Mobile CAAC‐Au species were found that form islands on the surface. These results open the door for more on‐surface studies of the highly relevant class of CAACs.

Photoresponsive functional materials have gained increasing attention due to their externally tunable properties. Molecular switches embedded in these materials enable the control of phenomena at the atomic level by light. Metal–organic frameworks (MOFs) provide a versatile platform to immobilize these photoresponsive units within defined molecular environments to optimize the intended functionality. For the application of these photoresponsive 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 on the intermolecular interactions. 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., Light on interaction: Photoresponsive molecular switches embedded within metal–organic frameworks enable the modification of both the adsorption sites and the orientation of guest molecules. Atomistic simulations predict the high potential of these photoresponsive materials to change intermolecular interactions (see figure).

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.

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.

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., 34 pages, 26 figure

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.