Hierarchical assembly of two-dimensional homochiral nanocavity arrays

Using Achiral Monomers to Synthesize Organometallic Chiral Copolymers on an Achiral Surface

Preparing chiral polymers by using achiral building blocks in an achiral reaction environment is very demanding because this strategy avoids the use of expensive chiral reagents. Here, a surface-confined and sequence-controlled chiral copolymer is synthesized by the organometallic reactions between the achiral precursors 4,4″-dibromo-1,1':3',1''-terphenyl and 1-ethynyl-4-[2-(4-ethynylphenyl)ethynyl]benzene on an achiral Ag(111) surface. Combined scanning tunneling microscopy and density functional theory explorations show that the terminal alkyne and aromatic bromide precursors undergo dehydrogenated and debrominated metalations with surface Ag adatoms to yield the organometallic polymeric products. Despite the achiral precursors and substrate surface employed, the [phenyl-Ag-alkynyl-Ag-phenyl-Ag]n copolymer with a chiral adsorption configuration stands out from various achiral competitors to become the dominant copolymeric product. Mechanistic analysis reveals that such a selectivity originates from the precise control of both sequence and adsorption configuration of the polymeric product during its growth, where the interpolymer interactions play a pivotal role. These interpolymer interactions also drive the enantioselective assembly of the copolymers to form the chiral domains, which achieves chirality transfer from the single-molecular to supramolecular level. These findings demonstrate the power of surface chemistry in creating chiral nanostructures from achiral precursors, which may help achieve chiral surface functionalization.

Engineering Two-Dimensional Multilevel Supramolecular Assemblies from a Bifunctional Ligand on Au(111)

Herein, we demonstrate the supramolecular assemblies from a bifunctional ligand on Au(111), towards engineering two-dimensional (metal-) organic multilevel nanostructures. The bifunctional ligand employed, including two Br atoms and one carboxylic terminal, offers multiple bonding motifs with different configurations and binding energies. These bonding motifs are highly self-selective and self-recognizable, and thus afford the formation of subunits that contribute to engineering multilevel self-assemblies. Our scanning tunneling microscopy experiments, in combination with the density functional theory calculations, revealed various hydrogen, halogen and alkali-carboxylate bonding motifs dictating the different levels of the assemblies. The multilevel assembly protocol based on a judicious choice of multiple bonding motifs guarantees a deliberate control of surface-confined (metal-) organic nanostructures. Our findings may present new opportunities for the fabrication of complex two-dimensional (metal-) organic nanostructures with potential in applications of functionally diverse nanomaterials.

Successive Deprotonation Steering the Structural Evolution of Supramolecular Assemblies on Ag(111)

In this study, we demonstrate the structural evolution of a two-dimensional (2D) supramolecular assembly system, which is steered by the thermally activated deprotonation of the primary organic building blocks on a Ag(111) surface. Scanning tunneling microscopy revealed that a variety of structures, featuring distinct structural, chiral, and intermolecular bonding characters, emerged with the gradual thermal treatments. According to our structural analysis, in combination with density function theory calculations, the structural evolution can be attributed to the successive deprotonation of the organic building blocks due to the inductive effect. Our finding offers a facile strategy towards controlling the supramolecular assembly pathways and provides a comprehensive understanding of the 2D crystal engineering on surfaces.

Surface self-assembly involving the interaction between S and N atoms

Regulation of the self-assembly nanostructures by recruiting the electrostatic interaction between S and N atoms.

Atomic-Scale Visualization of Stepwise Growth Mechanism of Metal-Alkynyl Networks on Surfaces

One of the most appealing topics in the study of metal-organic networks is the growth mechanism. However, its study is still considered a significant challenge. Herein, using scanning tunneling microscopy, the growth mechanisms of metal-alkynyl networks on Ag(111) and Au(111) surfaces were investigated at the atomic scale. During the reaction of 1,3,5-tris(chloroethynyl)benzene on Ag(111), honeycomb Ag-alkynyl networks formed at 393 K, and only short chain intermediates were observed. By contrast, the same precursor formed honeycomb Au-alkynyl networks on Au(111) at 503 K. Progression annealing led to a stepwise evolution process, in which the sequential activation of three Cl-alkynyl bonds led to the formation of dimers, zigzag chains, and novel chiral networks as the intermediates. Moreover, density functional theory calculations indicate that chlorine atoms are crucial in assisting the breakage of metal-alkynyl bonds to form Cl-metal-alkynyl, which guarantees the reversibility of the break/formation equilibration as the key to forming regular large-scale organometallic networks.

On-Surface Cascade Reaction Based on Successive Debromination via Metal-Organic Coordination Template

Precise control over on-surface covalent reaction pathways is crucial for engineering organic nanostructures with the single-atom precision. Herein, we demonstrate a step-by-step control of an on-surface cascade covalent reaction based on a successive debromination templated by noncovalent metal-organic coordination motifs. The molecular precursor is predesigned with different reactive sites and functional ligands, allowing for both chemical and structural tuning during on-surface reactions. Through the Fe-terpyridine template effect, we are able to direct the reaction to proceed in a three-step cascade pathway and finally to achieve a porous polyarylene nanoribbon structure. The approach opens new opportunities for construction of on-surface organic nanostructures in a predictable manner.

Patterning Porous Networks through Self-Assembly of Programmed Biomacromolecules

Two-dimensional (2D) porous networks are of great interest for the fabrication of complex organized functional materials for potential applications in nanotechnologies and nanoelectronics. This review aims at providing an overview of bottom-up approaches towards the engineering of 2D porous networks by using biomacromolecules, with a particular focus on nucleic acids and proteins. The first part illustrates how the advancements in DNA nanotechnology allowed for the attainment of complex ordered porous two-dimensional DNA nanostructures, thanks to a biomimetic approach based on DNA molecules self-assembly through specific hydrogen-bond base pairing. The second part focuses the attention on how polypeptides and proteins structural properties could be used to engineer organized networks templating the formation of multifunctional materials. The structural organization of all examples is discussed as revealed by scanning probe microscopy or transmission electron microscopy imaging techniques.

Probing Molecular Nanostructures of Aromatic Terephthalic Acids Triggered by Intermolecular Hydrogen Bonds and Electrochemical Potential

Self-assembly provides unique routes to create supramolecular nanostructures at well-defined surfaces. In the present work, we employed scanning tunneling microscopy (STM) in combination with electrochemical techniques to explore the adsorption and phase formation of a series of aromatic carboxylic acids (ACAs) at Au(111)/0.1 M HClO₄. Specific goals are to elucidate the roles of electrochemical potential and directional hydrogen-bonding on the structures and orientation of individual ACAs that form nanoarchitectures. ACAs are prototype materials for supramolecular self-assemblies via stereospecific hydrogen bonds between neighboring molecules. In this study, we mainly focus on a special ACA, terephthalic acid (TPA), which is almost insoluble in water, making the assembly of this molecule from aqueous solution challenging. Depending on the applied electric field, TPA molecules form distinctly different, highly ordered adlayers on Au(111) triggered by directional intermolecular hydrogen bonds. At low electrochemical potentials, TPA molecules are planar oriented, forming a potentially infinite hydrogen-bonded adlayer without any observed domain boundaries. The increase of the electrode potential triggers the deprotonation of one carboxylic acid functional group of TPA; additionally, this is accompanied by an orientation change of molecules from planar to perpendicular. In contrast, structural "defects" and multiple domain boundaries were found at this positively charged surface. The assembled nanostructures of TPA are compared with other ACAs (trimesic acid, benzoic acid, and isophthalic acid), and corresponding adsorption models were built for all molecular adlayers, showing that intermolecular hydrogen-bonding plays a determining role in the formation of two-dimensional ACA nanostructures.

The Accelerating World of Graphdiynes

Graphdiyne (GDY), a 2D allotrope of graphene, is first synthesized in 2010 and has attracted attention as a new low-dimensional carbon material. This work surveys the literature on GDYs. The history of GDYs is summarized, including their relationship with 2D graphyne carbons and yearly publication trends. GDY is a molecule-based nanosheet woven from a molecular monomer, hexaethynylbenzene; thus, it is synthesized by bottom-up approaches, which allow rich variation via monomer design. The GDY family and the synthetic procedures are also described. Highly developed π-conjugated electronic structures are common important features in GDY and graphene; however, the coexistence of sp and sp² carbons differentiates GDY from graphene. This difference gives rise to unique physical properties, such as high conductivity and large carrier mobility. Next, the theoretical and experimental studies of these properties are described in detail. A wide variety of applications are proposed for GDYs, including electrocatalysts and energy devices, which exploit the carbon-rich nature, porous framework, and expanded π-electron system of these compounds. Finally, potential uses are discussed.

One-Dimensional Double Wires and Two-Dimensional Mobile Grids: Cobalt/Bipyridine Coordination Networks at the Solid/Liquid Interface

Various architectures have been generated and observed by STM at a solid/liquid interface resulting from an in situ chemical reaction between the bipyridine terminal groups of a ditopic ligand and Co(II) ions. Large monodomains of one-dimensional (1D) double wires are formed by Co(II)/ligand coordination, with polymer lengths as long as 150 nm. The polymers are organized as parallel wires 8 nm apart, and the voids between wires are occupied by solvent molecules. Two-dimensional (2D) grids, showing high surface mobility, coexist with the wires. The wires are formed from linear chain motifs where each cobalt center is bonded to two bipyridines. 2D grids are generated from a bifurcation node where one cobalt bonds to three bipyridines. Surface reconstruction of the grids and of the 1D wires was observed under the STM tip. As an exciting result, analysis of these movements strongly indicates surface reactions at the solid/liquid interface.

Linear array of cesium atoms assisted by uracil molecules on Au(111)

A series of metal–organic U + Cs structures are achieved in which the Cs cations tend to form linear arrays.

Improving the productivity of monodisperse polyhedral cages by the rational design of kinetic self-assembly pathways

Hollow polyhedral cages hold great potential for application in nanotechnological and biomedical fields. Understanding the formation mechanism of these self-assembled structures could provide guidance for the rational design of the desired polyhedral cages. Here, by constructing kinetic network models from extensive coarse-grained molecular dynamics simulations, we elucidated the formation mechanism of the dodecahedral cage, which is formed by the self-assembly of patchy particles. We found that the dodecahedral cage is formed through increasing the aggregate size followed by structure rearrangement. Based on this mechanistic understanding, we improved the productivity of the dodecahedral cage through the rational design of the patch arrangement of patchy particles, which promotes the structural rearrangement process. Our results demonstrate that it should be a feasible strategy to achieve the rational design of the desired nanostructures via the kinetic analysis. We anticipate that this methodology could be extended to other self-assembly systems for the fabrication of functional nanomaterials.

Coordination nanosheets (CONASHs): strategies, structures and functions

Nanosheets, which are two-dimensional polymeric materials, remain among the most actively researched areas of chemistry and physics this decade. Generally, nanosheets are inorganic materials created from bulk crystalline layered materials and have fascinating properties and functionalities. An emerging alternative is molecule-based nanosheets containing organic molecular components. Molecule-based nanosheets offer great diversity because their molecular, ionic, and atomic constituents can be selected and combined to produce a wide variety of nanosheets. The present article focuses on coordination nanosheets (CONASHs), a class of molecule-based nanosheets comprising organic ligand molecules and metal ions/atoms in a framework linked with coordination bonds. Following the Introduction, Section 2 describes CONASHs, including their definition, design, synthetic procedures, and characterisation techniques. Section 3 introduces various examples of CONASHs, and Section 4 explores their functionality and possible applications. Section 5 describes an outlook for the research field of CONASHs.

Imaging on-surface hierarchical assembly of chiral supramolecular networks

The bottom-up assembly of chiral structures usually relies on a cascade of molecular recognition interactions. A thorough description of these complex stereochemical mechanisms requires the capability of imaging multilevel coordination in real-time. Here we report the first direct observation of hierarchical expression of supramolecular chirality at work, for 10,10'-dibromo-9,9'-bianthryl (DBBA) on Cu(111). Molecular recognition first steers the growth of chiral organometallic chains and then leads to the formation of enantiopure islands. The structure of the networks was determined by noncontact atomic force microscopy (nc-AFM), while high-speed scanning tunnelling microscopy (STM) revealed details of the assembly mechanisms at the ms time-scale. The direct observation of the chirality transfer pathways allowed us to evaluate the enantioselectivity of the interchain coupling.

Hierarchical formation of Fe-9eG supramolecular networks via flexible coordination bonds

From the interplay between high-resolution scanning tunneling microscopy imaging/manipulations and density functional theory calculations, we display the hierarchical formation of supramolecular networks by codeposition of 9eG molecules and Fe atoms on Au(111) based on the flexible coordination bonds (the adaptability and versatility in the coordination modes). In the first step, homochiral islands composed of homochiral G₄Fe₂ motifs are formed; and then in the second step, thermal treatment results in the transformation into the porous networks composed of heterochiral G₄Fe₂ motifs with the ratio of the components being constant. In situ STM manipulations and the coexistence of some other heterochiral G₄Fe₂ motifs and clusters also show the flexibility of the coordination bonds involved. These studies may provide a fundamental understanding of the regulations of multilevel supramolecular structures and shed light on the formation of designed supramolecular nanostructures.

Diverse supramolecular structures self-assembled by a simple aryl chloride on Ag(111) and Cu(111)

Diverse self-assembled structures were obtained on Cu(111) and Ag(111) surfaces by using a simple and small 4,4′′-dichloro-1,1′:4′,1′′-terphenyl molecule.

Two-dimensional metal–organic framework nanosheets: synthesis and applications

Synthesis and applications of two-dimensional metal–organic framework nanosheets and their composites are summarized.

13-cis-Retinoic acid on coinage metals: hierarchical self-assembly and spin generation

Hierarchical self-assembly of 13-cis-retinoic acid on Au(111) and Ag(111) was investigated using low-temperature scanning tunnelling microscopy. On both surfaces, molecules form dimers by hydrogen bonds and the dimers arrange into ordered two-dimensional arrays through van der Waals forces. Three packing modes are observed on Au(111) and only one on Ag(111). We tentatively attribute the different patterns on the two surfaces to a stronger molecule-substrate interaction on Ag(111) and site-dependent molecular adsorption on different atomic lattices. In addition, 13-cis-ReA on Au(111) can be made to carry a localized spin.

Liquid/Liquid Interfacial Synthesis of a Click Nanosheet

A liquid/liquid interfacial synthesis is employed, for the first time, to synthesize a covalent two-dimensional polymer nanosheet. Copper-catalyzed azide-alkyne cycloaddition (CuAAC) between a three-way terminal alkyne and azide at a water/dichloromethane interface generates a 1,2,3-triazole-linked nanosheet. The resultant nanosheet, with a flat and smooth texture, has a maximum domain size of 20 μm and minimum thickness of 5.3 nm. The starting monomers in the organic phase and the copper catalyst in the aqueous phase can only meet at the liquid/liquid interface as a two-dimensional reaction space; this allows them to form the two-dimensional polymer. The robust triazole linkage generated by irreversible covalent-bond formation allows the nanosheet to resist hydrolysis under both acidic and alkaline conditions, and to endure pyrolysis up to more than 300 °C. The coordination ability of the triazolyl group enables the nanosheet to act as a reservoir for metal ions, with an affinity order of Pd²⁺ >Au³⁺ >Cu²⁺ .

Kinetically controlled hierarchical self-assemblies of all-trans-retinoic acid on Au(111)

Kinetically controlled hierarchical self-assemblies of all-trans-retinoic acid on Au(111) were investigated via low-temperature scanning tunneling microscopy in ultra-high vacuum. The dominant molecular hierarchical superstructure could be selectively controlled to dimer, tetramer, or pentamer patterns, which were stabilized by hydrogen bonds and van der Waals interactions.

Generalized lattice-gas model for adsorption of functional organic molecules in terms of pair directional interactions

A generalized lattice-gas model that takes into account the directional character of pair interactions between the lattice sites is proposed. It is demonstrated that the proposed model can be successfully used to deeply understand the self-assembly process in adsorption monolayers of functional organic molecules driven by specified directional interactions between such molecules (e.g., hydrogen bonding). To illustrate the idea, representative cases of the general model with different numbers of identical functional groups in the chemical structure of the adsorbed molecule are investigated with Monte Carlo and the transfer-matrix methods. The model reveals that the phase behavior of the adsorption systems considered can be characterized as a hierarchical self-assembly process. It is predicted that in real adsorption systems of this type, the energy of hydrogen bonding sufficiently depends on the mutual orientation of the adsorbed molecules.

Nanostring-cluster hierarchical structured Bi2O3: synthesis, evolution and application in biosensing

A new Bi₂O₃ with a nanostring-cluster hierarchical structure was employed for glucose enzyme immobilization and was designed for biosensors.

Surface-Supported Robust 2D Lanthanide-Carboxylate Coordination Networks

Lanthanide-based metal-organic compounds and architectures are promising systems for sensing, heterogeneous catalysis, photoluminescence, and magnetism. Herein, the fabrication of interfacial 2D lanthanide-carboxylate networks is introduced. This study combines low- and variable-temperature scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS) experiments, and density functional theory (DFT) calculations addressing their design and electronic properties. The bonding of ditopic linear linkers to Gd centers on a Cu(111) surface gives rise to extended nanoporous grids, comprising mononuclear nodes featuring eightfold lateral coordination. XPS and DFT elucidate the nature of the bond, indicating ionic characteristics, which is also manifest in appreciable thermal stability. This study introduces a new generation of robust low-dimensional metallosupramolecular systems incorporating the functionalities of the f-block elements.

Supramolecular nesting of cyclic polymers

Advances in template-directed synthesis make it possible to create artificial molecules with protein-like dimensions, directly from simple components. These synthetic macromolecules have a proclivity for self-organization that is reminiscent of biopolymers. Here, we report the synthesis of monodisperse cyclic porphyrin polymers, with diameters of up to 21 nm (750 C–C bonds). The ratio of the intrinsic viscosities for cyclic and linear topologies is 0.72, indicating that these polymers behave as almost ideal flexible chains in solution. When deposited on gold surfaces, the cyclic polymers display a new mode of two-dimensional supramolecular organization, combining encapsulation and nesting; one nanoring adopts a near-circular conformation, thus allowing a second nanoring to be captured within its perimeter, in a tightly folded conformation. Scanning tunnelling microscopy reveals that nesting occurs in combination with stacking when nanorings are deposited under vacuum, whereas when they are deposited directly from solution under ambient conditions there is stacking or nesting, but not a combination of both.

Two-dimensional chiral molecular assembly on solid surfaces: formation and regulation

The expression of chirality in 2D molecular assemblies on solid surfaces has unique features compared to the analogous process in 1D and 3D supramolecular assemblies. Understanding the formation of chiral molecular assemblies on surfaces not only provides insight into the origin and transfer of chirality in many enantioselective processes, but also aids rational design and construction of chiral architectures and materials. This present contribution reviews recent studies on how chirality is induced and expressed on the surface at different levels, both from intrinsically chiral and achiral molecules. Furthermore, we discuss the regulation effect of some pivotal factors, for example, the chemical structure, the chiral auxiliary molecules, and the assembled environments, on the expression of chirality in molecular assembly.

Dimeric packing of molecular clips induced by interactions between π-systems

We report the first observation synthesis and X-ray structures of seven glycoluril clips that feature extended aromatic sidewalls; compounds 1 and 2 are the first examples of the out–out dimeric motif.

Heat-induced formation of one-dimensional coordination polymers on Au(111): an STM study

Upon annealing, H-bonded nanoribbons are transformed into 1D coordination polymers on Au(111) governed by an unusual threefold coordination bonding motif.

Solution phase post-modification of a trimesic acid network on Au(111) with Zn2+ ions

Post-modification of a TMA network of crown-like hexamers with Zn²⁺ ions transforms it into a metal–organic network of TMA–Zn²⁺ coordinated chevron-pairs.

Selective anion-induced crystal switching and binding in surface monolayers modulated by electric fields from scanning probes

Anion-selective (Br(-) and I(-)) and voltage-driven crystal switching between two differently packed phases (α ⇆ β) was observed in 2D crystalline monolayers of aryl-triazole receptors ordered at solution-graphite interfaces. Addition of Br(-) and I(-) was found to stimulate the α → β phase transformation and to produce ion binding to the β phase assembly, while Cl(-) and BF4(-) addition retained the α phase. Unlike all other surface assemblies of either charged molecules or ion-templated 2D crystallization of metal-ligand or receptor-based adsorbates, the polarity of the electric field between the localized scanning tip and the graphite substrate was found to correlate with phase switching: β → α is driven at -1.5 V, while α → β occurs at +1.1 V. Ion-pairing between the countercations and the guest anions was also observed. These observations are supported by control studies including variation of anion species, relative anion concentration, surface temperature, tip voltage, and scanning time.

Self-assembly and chemical modifications of bisphenol a on Cu(111): interplay between ordering and thermally activated stepwise deprotonation

Bisphenol A (BPA) is a chemical widely used in the synthesis pathway of polycarbonates for the production of many daily used products. Besides other adverse health effects, medical studies have shown that BPA can cause DNA hypomethylation and therefore alters the epigenetic code. In the present work, the reactivity and self-assembly of the molecule was investigated under ultra-high-vacuum conditions on a Cu(111) surface. We show that the surface-confined molecule goes through a series of thermally activated chemical transitions. Scanning tunneling microscopy investigations showed multiple distinct molecular arrangements dependent on the temperature treatment and the formation of polymer-like molecular strings for temperatures above 470 K. X-ray photoelectron spectroscopy measurements revealed the stepwise deprotonation of the hydroxy groups, which allows the molecules to interact strongly with the underlying substrate as well as their neighboring molecules and therefore drive the organization into distinct structural arrangements. On the basis of the combined experimental evidence in conjunction with density functional theory calculations, structural models for the self-assemblies after the thermal treatment were elaborated.

Self-Assembled Two-Dimensional Heteromolecular Nanoporous Molecular Arrays on Epitaxial Graphene

The development of graphene functionalization strategies that simultaneously achieve two-dimensional (2D) spatial periodicity and substrate registry is of critical importance for graphene-based nanoelectronics and related technologies. Here, we demonstrate the generation of a hydrogen-bonded molecularly thin organic heteromolecular nanoporous network on epitaxial graphene on SiC(0001) using room-temperature ultrahigh vacuum scanning tunneling microscopy. In particular, perylenetetracarboxylic diimide (PTCDI) and melamine are intermixed to form a spatially periodic 2D nanoporous network architecture with hexagonal symmetry and a lattice parameter of 3.45 ± 0.10 nm. The resulting adlayer is in registry with the underlying graphene substrate and possesses a characteristic domain size of 40-50 nm. This molecularly defined nanoporous network holds promise as a template for 2D ordered chemical modification of graphene at lengths scales relevant for graphene band structure engineering.

Surface tectonics of nanoporous networks of melamine-capped molecular building blocks formed through interface Schiff-base reactions

Control over the assembly of molecules on a surface is of great importance for the fabrication of molecule-based miniature devices. Melamine (MA) and molecules with terminal MA units are promising candidates for supramolecular interfacial packing patterning, owing to their multiple hydrogen-bonding sites. Herein, we report the formation of self-assembled structures of MA-capped molecules through a simple on-surface synthetic route. MA terminal groups were successfully fabricated onto rigid molecular cores with 2-fold and 3-fold symmetry through interfacial Schiff-base reactions between MA and aldehyde groups. Sub-molecular scanning tunneling microscopy (STM) imaging of the resultant adlayer revealed the formation of nanoporous networks. Detailed structural analysis indicated that strong hydrogen-bonding interactions between the MA groups persistently drove the formation of nanoporous networks. Herein, we demonstrate that functional groups with strong hydrogen-bond-formation ability are promising building blocks for the guided assembly of nanoporous networks and other hierarchical 2D assemblies.