Paweł Szabelski

Concentration-Dependent Supramolecular Engineering of Hydrogen-Bonded Nanostructures at Surfaces: Predicting Self-Assembly in 2D

We report a joint computational and experimental study on the concentration-dependent self-assembly of a flat C3-symmetric molecule at surfaces. As a model system we have chosen a rigid molecular module, 1,3,5-tris(pyridine-4-ylethynyl)benzene, which can undergo self-association via hydrogen bonding (H-bonding) to form ordered 2D nanostructures. In particular, the lattice Monte Carlo method, combined with density functional calculations, was employed to explore the spontaneous supramolecular organization of this tripod-shaped molecule under surface confinement. We analyzed the stability of different weak H-bonded patterns and the influence of the concentration of the starting molecule on the 2D supramolecular packing. We found that ordered, densely packed monolayers and 2D porous networks are obtained at high and low concentrations, respectively. A concentration-dependent scanning tunneling microscopy investigation of the molecular self-assembly at a graphite-solution interface revealed supramolecular motifs, which are in perfect agreement with those obtained by simulations. Therefore, our computational approach represents a step forward toward the deterministic prediction of molecular self-assembly at surfaces and interfaces.

On-surface Ullmann coupling: the influence of kinetic reaction parameters on the morphology and quality of covalent networks.

On-surface Ullmann coupling is a versatile and appropriate approach for the bottom-up fabrication of covalent organic nanostructures. In two-dimensional networks, however, the kinetically controlled and irreversible coupling leads to high defect densities and a lack of long-range order. To derive general guidelines for optimizing reaction parameters, the structural quality of 2D porous covalent networks was evaluated for different preparation protocols. For this purpose, polymerization of an iodine- and bromine-functionalized precursor was studied on Au(111) by scanning tunneling microscopy under ultrahigh vacuum conditions. By taking advantage of the vastly different temperature thresholds for C-Br and C-I cleavage, two different polymerization routes were compared - hierarchical and direct polymerization. The structural quality of the covalent networks was evaluated for different reaction parameters, such as surface temperatures, heating rates, and deposition rates by statistical analysis of STM data. Experimental results are compared to Monte Carlo simulations.

One building block, two different nanoporous self-assembled monolayers: a combined STM and Monte Carlo study.

With the use of a single building block, two nanoporous patterns with nearly equal packing density can be formed upon self-assembly at a liquid-solid interface. Moreover, the formation of both of these porous networks can be selectively and homogenously induced by changing external parameters like solvent, concentration, and temperature. Finally, their porous properties are exploited to host up to three different guest molecules in a spatially resolved way.

Supramolecular assembly of interfacial nanoporous networks with simultaneous expression of metal-organic and organic-bonding motifs

The formation of 2D surface-confined supramolecular porous networks is scientifically and technologically appealing, notably for hosting guest species and confinement phenomena. In this study, we report a scanning tunneling microscopy (STM) study of the self-assembly of a tripod molecule specifically equipped with pyridyl functional groups to steer a simultaneous expression of lateral pyridyl-pyridyl interactions and Cu-pyridyl coordination bonds. The assembly protocols yield a new class of porous open assemblies, the formation of which is driven by multiple interactions. The tripod forms a purely porous organic network on Ag(111), phase α, in which the presence of the pyridyl groups is crucial for porosity, as confirmed by molecular dynamics and Monte Carlo simulations. Additional deposition of Cu dramatically alters this scenario. For submonolayer coverage, three different porous phases coexist (i.e., β, γ, and δ). Phases β and γ are chiral and exhibit a simultaneous expression of lateral pyridyl-pyridyl interactions and twofold Cu-pyridyl linkages, whereas phase δ is just stabilized by twofold Cu-pyridyl bonds. An increase in the lateral molecular coverage results in a rise in molecular pressure, which leads to the formation of a new porous phase (ε), only coexisting with phase α and stabilized by a simultaneous expression of lateral pyridyl-pyridyl interactions and threefold Cu-pyridyl bonds. Our results will open new avenues to create complex porous networks on surfaces by exploiting components specifically designed for molecular recognition through multiple interactions.

Mixing Behavior of Alkoxylated Dehydrobenzo[12]annulenes at the Solid-Liquid Interface: Scanning Tunneling Microscopy and Monte Carlo Simulations

We present a systematic scanning tunneling microscopic study on the mixing behavior of molecules (DBAs) with different alkyl substituents at the solid-liquid interface to reveal the phase behavior of complex systems. The phase behavior of binary mixtures of alkylated DBAs at the solid-liquid interface can be predicted by the 2D isomorphism coefficient. In addition, we also investigated the influence of coadsorption of template molecules on the phase behavior of DBA mixtures. Coadsorption of these molecules significantly promotes mixing of DBAs, possibly by affecting the recognition between alkyl chains. Monte Carlo simulations prove that the 2D isomorphism coefficient can predict the phase behavior at the interface. These results are helpful for the understanding of phase behavior of complex assembling systems and also for the design of programmable porous networks and hierarchical architectures at the solid-liquid interface.

Computer Simulation of Chiral Nanoporous Networks on Solid Surfaces

A lattice Monte Carlo (MC) model was proposed with the aim of understanding the factors affecting the chiral self-assembly of tripod-shaped molecules in two dimensions. To that end a system of flat symmetric molecules adsorbed on a triangular lattice was simulated by using the canonical ensemble method. Special attention was paid to the influence of size and composition of the building block on the morphology of the adsorbed overlayer. The obtained results demonstrated a spontaneous self-assembly into extended chiral networks with hexagonal cavities, highlighting the ability of the model to reproduce basic structural features of the corresponding experimental systems. The simulated assemblies were analyzed with respect to their structural and energetic properties resulting in quantitative estimates of the unit cell parameters and mean potential energy of the adsorbed layer. The predictive potential of the model was additionally illustrated by comparison of the obtained superstructures with the recent STM images that have been recorded for different organic tripod-shaped molecules adsorbed at the liquid/pyrolytic graphite interface.

Chiral separation on a model adsorbent with periodic surface heterogeneity.

Optimization of enantioselectivity in heterogeneous catalysis and chiral chromatography is a challenging task for the production of enantiopure chemicals. Enantioselective adsorbents usually consist of a surface with chiral receptors being either chiral molecules linked to the surface or chiral pockets formed by molecular templating of the surface. In both cases, the enantioselectivity is controlled mainly by the strength of the receptor-enantiomer interaction, such that one-to-one correspondence is usually preserved. The authors use Monte Carlo calculations to show that this steric requirement is not a necessary condition for the effective separation of chiral molecules. In particular, they propose a way in which a chiral surface can be constructed by a suitable spatial distribution of active sites for which the classical concept of a chiral receptor is no longer useful. Their calculations indicate that the effectiveness of the separation is affected mainly by the difference in shape of the adsorption energy distribution functions corresponding to the enantiomers.

Formation of a non-crystalline bimolecular porous network at a liquid/solid interface.

An equimolar mixture of two structurally related molecular building blocks self-assembles into a 2D non-crystalline bimolecular porous pattern at a liquid-solid interface as revealed by scanning tunneling microscopy.

Extended Surface Chirality for Enantiospecific Adsorption

A rapid development of nanotechnology opens up a way for the fabrication of solid surfaces containing unique adsorption properties. In this article, we present the concept of a chiral nanostructured surface as a potential environment for the separation of chiral molecules. In particular, we focus on the effect of size and shape of the adsorbing molecules on the effectiveness of their separation on a surface with a special distribution of active sites. The Monte Carlo simulation method was used to study enantiospecific adsorption of model chiral molecules that differ in molecular footprint and adsorption energy. It was demonstrated that manipulating the footprint offers many possibilities for tuning the preference of the surface for adsorption of a selected enantiomer. One interesting finding was that subtle differences in the interaction pattern of the molecule with the chiral surface can lead to a reversal of enantioselectivity. The results of this work highlight the role of extended surface chirality in enantiospecific adsorption of enantiomers. They also suggest that the proposed mechanism of chiral selection can be a realistic alternative to those inherent in conventional enantioselective adsorbents.

Study of proton adsorption at heterogeneous oxide/electrolyte interface. Prediction of the surface potential using Monte Carlo simulations and 1‐pK approach

Adsorption of protons on a heterogeneous solid surface is modeled using the Monte Carlo (MC) simulation method. The surface of an oxide is assumed to consist of adsorption sites with pK assigned according to a quasi‐Gaussian distribution. The influence of the electrostatic interactions combined with the energetic heterogeneity of the surface is examined and the MC results are compared with the predictions of the analytical 1‐pK approach. The surface potential behavior is examined using both “experimental” MC results and “theoretical” results obtained from the application of 1‐pK model. The results are compared qualitatively with experimental determination of the surface potential of metal oxide surfaces. They confirm that the relation between the surface potential and the pH of bulk solution should not be described by the Nernst equation but by the equation with the parameter linearly reducing Nerstian potential. The values of this parameter are examined with respect to degree of surface energetic heterogeneity and site density of the surface.