Molecular assembly at surfaces: progress and challenges

Molecules provide versatile building blocks, with a vast palette of functionalities and an ability to assemble via supramolecular and covalent bonding to generate remarkably diverse macromolecular systems. This is abundantly displayed by natural systems that have evolved on Earth, which exploit both supramolecular and covalent protocols to create the machinery of life. Importantly, these molecular assemblies deliver functions that are reproducible, adaptable, finessed and responsive. There is now a real need to translate complex molecular systems to surfaces and interfaces in order to engineer 21st century nanotechnology. ‘Top-down’ and ‘bottom-up’ approaches, and utilisation of supramolecular and covalent assembly, are currently being used to create a range of molecular architectures and functionalities at surfaces. In parallel, advanced tools developed for interrogating surfaces and interfaces have been deployed to capture the complexities of molecular behaviour at interfaces from the nanoscale to the macroscale, while advances in theoretical modelling are delivering insights into the balance of interactions that determine system behaviour. A few examples are provided here that outline molecular behaviour at surfaces, and the level of complexity that is inherent in such systems.

Simple rules and the emergence of complexity in surface chirality

Surface chirality arising from self-organized molecular monolayers may manifest both a handedness and footedness, leading to a dual level of chiral expression.

Role of molecular orientational anisotropy in the chiral resolution of enantiomers in adsorbed overlayers

Separation of chiral molecules using achiral inputs is an interesting alternative to traditional techniques based on the chiral recognition mechanism. In this article we propose a lattice gas Monte Carlo model of two-dimensional chiral segregation induced by breaking of molecular orientational symmetry. Simulations were performed on a square lattice for rigid chain molecules composed of four and five identical segments. Mirror-image flat chain conformations resulting in different enantiomeric pairs were considered for each probe molecule. The enantiomers were assumed to interact via short-ranged segment-segment interaction potential limited to nearest neighbors on the lattice. We considered two qualitatively different situations in which (1) the molecules were allowed to rotate on the surface and adopt any of the four planar orientations and (2) the rotation was blocked, so that only one planar orientation was possible. The results obtained for the racemic overlayers showed clearly that the orientational symmetry breaking can induce spontaneous segregation of the enantiomers into large enantiopure domains. However, this effect was observed only for molecules with sufficiently long linear fragment. In the case of kinked bulky molecules a mixed assembly was formed, demonstrating the role of molecular shape in the orientationally biased segregation of enantiomers in adsorbed films. The insights from this study can be useful in developing strategies for 2D chiral separations in which external directional fields are used.

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.

Monte Carlo modeling of chiral adsorption on nanostructured chiral surfaces and slit pores

Adsorptive separation of chiral molecules is a powerful technique that has long been used in the chemical and pharmaceutical industries. An important challenge in this field is to design and optimize new adsorbents to provide selective discrimination of enantiomers. In this article, we introduce an off-lattice model of chiral adsorption on nanostructured surfaces and slit pores with the aim of predicting their enantioslective properties. The concept presented here involves finding the optimal chiral pattern of active sites on the pore walls that maximizes the difference between the binding energies of the enantiomers. Our initial effort focuses on chiral molecules that do not have specific interactions with the pore surface. One candidate meeting this requirement is 1,2-dimethylcyclopropane (DMCP), a chiral hydrocarbon whose interaction with a model pore surface was described using the Lennard-Jones potential. To model the adsorption of DMCP, we used the Monte Carlo simulation method. It was demonstrated that the separation of the enantiomers of DMCP is hardly obtainable because of the smoothness of the potential energy surface for molecules physisorbed in the pore. However, the simulated results allowed the identification of key factors that influence the binding of the enantiomers of DMCP to the pore walls with a special distribution of active sites. This information will be useful in future considerations of the adsorption of more complex chiral molecules.