Molecular Geometry Directed Kagomé and Honeycomb Networks: Toward Two-Dimensional Crystal Engineering

Single molecule studies of cyclic peptides using molecular matrix at liquid/solid interface by scanning tunneling microscopy

We report in this work the single molecule studies of cyclic peptide, cyclosporine A (CsA), using a molecular network formed by star-shaped oligofluorene (StOF-COOH(3)) at the liquid/solid interface by scanning tunneling microscopy (STM). Individual cyclosporine A can be identified and resolved in the molecular network, and the high-resolution STM images of CsA show polygon-like characteristics with a diameter of approximately 1.7 nm. Furthermore, the complex of CsA and Mg(2+) has also been observed to adsorb inside of the molecular matrix. The STM results reveal two adsorption characteristics for the CsA-Mg(2+) complex, which is suggestive of asymmetrical configurations of the complex. The difference in binding energy between the two observed adsorption configurations is estimated to be 1.88 kJ·mol(-1). These results help set the stage for studying the fine structures and functions of various cyclic peptides at the liquid/solid interface.

Supramolecular surface-confined architectures created by self-assembly of triangular phenylene-ethynylene macrocycles via van der Waals interaction

At the liquid/graphite interface triangular and rhombic phenylene-ethynylene macrocycles substituted by alkyl chains self-assemble to form porous two-dimensional (2D) molecular networks of honeycomb and Kagomé types, respectively, or close-packed non-porous structures via alkyl chain interdigitation as the directional intermolecular linkages. Factors that affect the formation of the 2D molecular networks, such as alkyl chain length, solvent, solute concentration, and co-adsorption of guest molecules, were elucidated through a systematic study. For the porous networks, various molecules and molecular clusters were adsorbed in the pores reflecting the size and shape complementarity, exploring a new field of 2D host-guest chemistry.

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.

2D networks of rhombic-shaped fused dehydrobenzo[12]annulenes: structural variations under concentration control

A series of alkyl- and alkoxy-substituted rhombic-shaped bisDBA derivatives 1a-d, 2a, and 2b were synthesized for the purpose of the formation of porous networks at the 1,2,4-trichlorobenzene (TCB)/graphite interface. Depending on the alkyl-chain length and the solute concentration, bisDBAs exhibit five network structures, three porous structures (porous A, B, and C), and two nonporous structures (nonporous D and E), which are attributed to their rhombic core shape and the position of the substituents. BisDBAs 1a and 1b with the shorter alkyl chains favorably form a porous structure, whereas bisDBAs 1c and 1d with the longer alkyl chains are prone to form nonporous structures. However, upon dilution, nonporous structures are typically transformed into porous ones, a trend that can be understood by the effect of surface coverage, molecular density, and intermolecular interactions on the system's enthalpy. Furthermore, porous structures are stabilized by the coadsorption of solvent molecules. The most intriguing porous structure, the Kagome pattern, was formed for all compounds at least to some extent, and the size of its triangular and hexagonal pores could be tuned by the alkyl-chain length. The present study proves that the concentration control is a powerful and general tool for the construction of porous networks at the liquid-solid interface.

Supramolecular chemistry at interfaces: molecular recognition on nanopatterned porous surfaces

Through the illustration of key examples that have recently appeared in the literature, the intention of this review is to provide a perspective of current advances on the molecular recognition at the interfaces aimed at the engineering of multifunctional organic-based materials. The great interest in such systems has been motivated by the need to fabricate smaller and smaller components in order to improve, for example, the information storage capabilities of classical silicon-based devices. Although great progress has been achieved on the exploitation of "top-down" approaches, strong hope is now put on the development of hybrid devices in which the elementary components are replaced with single organic molecules. Nevertheless, the drive towards such devices is restricted by both their stability and difficulties to precisely control and manipulate the structural organisation at the molecular level. To overcome these restrictions, the use of nanotemplated surfaces featuring porous domains in which responsive functional molecules can be precisely accommodated at the single-molecule level is one of the most promising approaches. In the first part of this manuscript, we therefore illustrate the main engineering strategies [1) through non-covalent interactions, 2) surface-confined covalent reactions and 3) assembly of pre-organised cavities such as synthetic macrocycles] currently in use to create two-dimensional (2D) patterned surfaces displaying porous structures at the nanoscale level. Such networks, featuring periodic hollow domains (controllable both in shape and size), are of particular significance as their cavities can be used as receptors for the recognition of remotely controllable functional molecules. In the second part, the confinement of molecular guests within the cavities is discussed, emphasising the selectivity and dynamics of key assemblies, with a particular focus on the biomolecular recognition and post-assembly covalent functionalisation, which could provide the opportunity to fabricate devices currently beyond our reach on an unprecedented precision and efficiency. All the examples will be discussed in terms of structural organisation as studied by scanning tunnelling microscopy (STM) techniques.

Carboxylic acids: versatile building blocks and mediators for two-dimensional supramolecular self-assembly

Two-dimensional (2D) supramolecular self-assembly of various organic molecules at the liquid-solid interface is presented and discussed with a focus on compounds that are primarily functionalized by carboxylic groups. The main analytical tool utilized is scanning tunneling microscopy (STM), a high-resolution real-space technique capable of readily providing full crystallographic information (i.e., not only lattice parameters but also number, type, and orientation of molecules within the unit cell). Carboxylic groups are of particular interest because their combined donor and acceptor character with regard to hydrogen bonds provides reliable intermolecular cross-linking, thereby facilitating the self-assembly of well-ordered, stable monolayers. By means of various homomeric (monomolecular) and heteromeric (here, bimolecular) examples, this feature article illustrates the influence of both molecular structure and external conditions (type of solvent, concentration, etc.) on monolayer self-assembly at the liquid-solid interface. A very intriguing aspect of interfacial self-assembly is that many systems are thermodynamically controlled (i.e., adsorbed molecules at the surface are in equilibrium with molecules dissolved in the supernatant liquid phase). This offers the unique possibility not only to steer the system reliably by intensive thermodynamic parameters such as temperature and concentration but also to gain fundamental knowledge about decisive processes and steps in supramolecular self-assembly.

Chiral kagomé lattice from simple ditopic molecular bricks

Self-assembly techniques allow for the fabrication of highly organized architectures with atomic-level precision. Here, we report on molecular-level scanning tunneling microscopy observations demonstrating the supramolecular engineering of complex, regular, and long-range ordered periodic networks on a surface atomic lattice using simple linear molecular bricks. The length variation of the employed de novo synthesized linear dicarbonitrile polyphenyl molecules translates to distinct changes of the bonding motifs that lead to hierarchic order phenomena and unexpected changes of the surface tessellations. The achieved 2D organic networks range from a close-packed chevron pattern via a rhombic network to a hitherto unobserved supramolecular chiral kagomé lattice.

Giant molecular spoked wheels in giant voids: two-dimensional molecular self-assembly goes big

We present here the formation of giant pores in surface-confined molecular networks of a triangular-shaped dehydrobenzo-[12]annulene derivative: the diameter of the pores reaches over 7 nm and the giant pores are used as templates to accommodate a giant molecular spoked wheel, which allows us to observe rotational and adsorption-desorption dynamics of single guest molecules.

Directing two-dimensional molecular crystallization using guest templates

The use of a coronene guest template directs the formation of a 2D Kagomé network in preference to alternative close packed and parallel hydrogen-bonded structures of tetracarboxylic acid tectons self-assembled from solution on a graphite surface.