Porous Molecular Networks Research Articles

Two-Dimensional Molecular Network Built from Hierarchy Self-Assembly of Perylene Bisimide Derivatives.

The structures of two-dimensional (2D) self-assembled molecular networks formed by a series of perylene bisimide (PBI) derivatives were investigated by scanning tunneling microscopy (STM). By introducing different functional groups to the PBI rings, we successfully built a different self-assembled molecular network on the liquid-solid interface. When the substituent is propanol, PBI is aligned in lines. When we introduced either an ester group or an amide group to the PBI compounds, they tended to form dimers and trimers. Especially, the PBI with the amide groups can form a 2D porous molecular network by hierarchy self-assembly. The 2D porous molecular network has a great potential to be the host molecule for the accommodation of a guest molecule, coronene (COR), and the structure of the 2D porous molecular network can be tuned by varying the concentration. The density functional theory calculations were also performed to disclose the mechanisms involved.

Thermal-Driven Formation of 2D Nanoporous Networks on Metal Surfaces

Controlling the quantum confinement of (spin-dependent) electronic states by material design opens a unique avenue to accelerate the implementation of quantum technology in next generation photonic and spintronic applications. In the nanoworld, two-dimensional porous molecular networks have emerged as highly tunable material platform for the realization of exotic quantum phases for which the nanopores can be tuned by chemical functionalization of the size and shape of the molecules. Here, we demonstrate a new approach to control the periodicity, size, and barrier width in 2D porous molecular networks on surface by tuning the balance between intermolecular and molecule–surface interactions using temperature. At 106 K, the prototypical TPT molecules form nanoporous networks with different periodicity and barrier width depending on the surface reactivity. The network structures continuously transform into a close-packed molecular structure at room temperature. This reversible structural phase transition can ...

Reversing the Handedness of Self‐Assembled Porous Molecular Networks through the Number of Identical Chiral Centres

AbstractScanning tunnelling microscope observations at the 1‐phenyloctane/graphite interface reveal how chiral structural information at the molecular level is transferred and expressed structurally at the 2D supramolecular level for a porous system. The chirality of self‐assembled molecular networks formed by chiral dehydrobenzo[12]annulene (cDBA) derivatives having three chiral chains and three achiral chains, alternatingly, is compared with those of cDBAs having six chiral chains reported previously. While for all cDBAs homochiral surfaces are formed, their handedness is not simply a reflection of the absolute configuration of the stereogenic centres. Both the number of stereogenic centres as well as the length of the achiral chains determine the supramolecular handedness, providing a deep insight into the supramolecular chirality induction mechanisms at play. Moreover, these cDBAs act to induce chirality in porous networks formed by achiral DBAs.

Reversing the Handedness of Self-Assembled Porous Molecular Networks through the Number of Identical Chiral Centres.

Scanning tunnelling microscope observations at the 1-phenyloctane/graphite interface reveal how chiral structural information at the molecular level is transferred and expressed structurally at the 2D supramolecular level for a porous system. The chirality of self-assembled molecular networks formed by chiral dehydrobenzo[12]annulene (cDBA) derivatives having three chiral chains and three achiral chains, alternatingly, is compared with those of cDBAs having six chiral chains reported previously. While for all cDBAs homochiral surfaces are formed, their handedness is not simply a reflection of the absolute configuration of the stereogenic centres. Both the number of stereogenic centres as well as the length of the achiral chains determine the supramolecular handedness, providing a deep insight into the supramolecular chirality induction mechanisms at play. Moreover, these cDBAs act to induce chirality in porous networks formed by achiral DBAs.

Alkane–OH Hydrogen Bond Formation and Diffusion Energetics of n-Butane within UiO-66

Hydrocarbon diffusion and binding within porous molecular networks are critical to catalysis, separations, and purification technologies. Fundamental insight into n-butane uptake and mobility within a new class of materials for separations, metal–organic frameworks (MOFs), has been gained through in situ infrared spectroscopy. These ultrahigh vacuum (UHV) based measurements revealed that adsorption of n-butane within UiO-66 proceeds through the formation of hydrogen bonds between the alkane molecules and hydroxyl groups located at the inorganic node of UiO-66. Modeling the gas transport of n-butane with Fick’s second law yielded diffusion coefficients at several temperatures. The Arrhenius parameter for the activation energy of diffusion was found to be 21.0 ± 1.2 kJ/mol. These studies have further shown that the rate-determining step for diffusion is likely the dissociation of n-butane from a binding site located within the tetrahedral pores of UiO-66.

Host-Guest Chemistry in Integrated Porous Space Formed by Molecular Self-Assembly at Liquid-Solid Interfaces.

Host-guest chemistry in two-dimensional (2D) space, that is, physisorbed monolayers of a single atom or a single molecular thickness on surfaces, has become a subject of intense current interest because of perspectives for various applications in molecular-scale electronics, selective sensors, and tailored catalysis. Scanning tunneling microscopy has been used as a powerful tool for the visualization of molecules in real space on a conducting substrate surface. For more than a decade, we have been investigating the self-assembly of a series of triangle-shaped phenylene-ethynylene macrocycles called dehydrobenzo[12]annulenes (DBAs). These molecules are substituted with six alkyl chains and are capable of forming hexagonal porous 2D molecular networks via van der Waals interactions between interdigitated alkyl chains at the interface of organic solvents and graphite. The dimension of the nanoporous space or nanowell formed by the self-assembly of DBAs can be controlled from 1.6 to 4.7 nm by simply changing the alkyl chain length from C6 to C20. Single molecules as well as homoclusters and heteroclusters are capable of coadsorbing within the host matrix using shape- and size-complementarity principles. Moreover, on the basis of the versatility of the DBA molecules that allows chemical modification of the alkyl chain terminals, we were able to decorate the interior space of the nanoporous networks with functional groups such as azobenzenedicarboxylic acid for photoresponsive guest adsorption/desorption or fluoroalkanes and tetraethylene glycol groups for selective guest binding by electrostatic interactions and zinc-porphyrin units for complexation with a guest by charge-transfer interactions. In this Feature Article, we describe the general aspects of molecular self-assembly at liquid/solid interfaces, followed by the formation of programmed porous molecular networks using rationally designed molecular building blocks. We focus on our own work involving host-guest chemistry in integrated nanoporous space that is modified for specific purposes.

Chiral-Selective Formation of 1D Polymers Based on Ullmann-Type Coupling: The Role of the Metallic Substrate.

The chiral-selective formation of 1D polymers from a prochiral molecule, namely, 6,12-dibromochrysene in dependence of the type of metal surface is demonstrated by a combined scanning tunneling microscopy and density functional theory study. Deposition of the chosen molecule on Au(111) held at room temperature leads to the formation of a 2D porous molecular network. Upon annealing at 200 °C, an achiral covalently linked polymer is formed on Au(111). On the other hand, a chiral Cu-coordinated polymer is spontaneously formed upon deposition of the molecules on Cu(111) held at room temperature. Importantly, it is found that the chiral-selectivity determines the possibility of obtaining graphene nanoribbons (GNRs). On Au(111), upon annealing at 350 °C or higher cyclo-dehydrogenation occurs transforming the achiral polymer into a GNR. In contrast, the chiral coordination polymer on Cu(111) cannot be converted into a GNR.

Reticular synthesis of porous molecular 1D nanotubes and 3D networks.

Synthetic control over pore size and pore connectivity is the crowning achievement for porous metal-organic frameworks (MOFs). The same level of control has not been achieved for molecular crystals, which are not defined by strong, directional intermolecular coordination bonds. Hence, molecular crystallization is inherently less controllable than framework crystallization, and there are fewer examples of 'reticular synthesis', in which multiple building blocks can be assembled according to a common assembly motif. Here we apply a chiral recognition strategy to a new family of tubular covalent cages to create both 1D porous nanotubes and 3D diamondoid pillared porous networks. The diamondoid networks are analogous to MOFs prepared from tetrahedral metal nodes and linear ditopic organic linkers. The crystal structures can be rationalized by computational lattice-energy searches, which provide an in silico screening method to evaluate candidate molecular building blocks. These results are a blueprint for applying the 'node and strut' principles of reticular synthesis to molecular crystals.

Periodic Functionalization of Surface-Confined Pores in a Two-Dimensional Porous Network Using a Tailored Molecular Building Block

We present here the periodic functionalization of a two-dimensional (2D) porous molecular network using a tailored molecular building block. For this purpose, a dehydrobenzo[12]annulene (DBA) derivative, 1-isoDBA, having an isophthalic acid unit connected by an azobenzene linker to a C12 alkyl chain and five C14 chains, was designed and synthesized. After the optimization of monolayer preparation conditions at the 1,2,4-trichlorobezene (TCB)/graphite interface, scanning tunneling microscopy (STM) observation of the self-assembled monolayer of 1-isoDBA revealed the formation of extended domains of a porous honeycomb-type molecular network, which consists of periodically located nanowells each functionalized by a cyclic hexamer of hydrogen-bonded isophthalic acid units and those without functional groups. This result demonstrates that the present strategy based on precise molecular design is a viable route to site-specific functionalization of surface-confined nanowells. The nanowells of different size can be used for guest coadsorption of different guests, coronene COR and hexakis[4-(phenylethynyl)phenylethynyl]benzene HPEPEB, whose size and shape match the respective nanowells. STM observation of a ternary mixture (1-isoDBA/COR/HPEPEB) at the TCB/graphite interface revealed the site-selective immobilization of the two different guest molecules at the respective nanowells, producing a highly ordered three-component 2D structure.

Bicomponent H-bonded porous molecular networks at the liquid-solid interface: what is the influence of preorganization in solution?

Tailoring the architecture of porous two-dimensional networks formed by molecules is essential for developing functional materials with low dimensionality. Here we present bicomponent porous networks with tunable pore-sizes that were formed by self-assembly of hydrogen-bonding molecules at the liquid/graphite interface. Scanning tunneling microscopy investigations demonstrate the formation and coexistence of three polymorphs. It is found that the occurrence of these polymorphs depends critically on the surface coverage. Further on, atomic force microscopy measurements, spectroscopic studies, and dynamic light scattering investigations show the propensity of one of the two molecular components to form aggregates beyond the monolayer. We discuss how these preorganized aggregates in solution may affect the self-assembly at the interface.

Self-assembly of decoupled borazines on metal surfaces: the role of the peripheral groups.

Two borazine derivatives have been synthesised to investigate their self-assembly behaviour on Au(111) and Cu(111) surfaces by scanning tunnelling microscopy (STM) and theoretical simulations. Both borazines form extended 2D networks upon adsorption on both substrates at room temperature. Whereas the more compact triphenyl borazine 1 arranges into close-packed ordered molecular islands with an extremely low density of defects on both substrates, the tris(phenyl-4-phenylethynyl) derivative 2 assembles into porous molecular networks due to its longer lateral substituents. For both species, the steric hindrance between the phenyl and mesityl substituents results in an effective decoupling of the central borazine core from the surface. For borazine 1, this is enough to weaken the molecule–substrate interaction, so that the assemblies are only driven by attractive van der Waals intermolecular forces. For the longer and more flexible borazine 2, a stronger molecule–substrate interaction becomes possible through its peripheral substituents on the more reactive copper surface.

Porous molecular networks formed by the self-assembly of positively-charged trigonal building blocks at the liquid/solid interfaces.

Tris-(2-hydroxybenzylidene)triaminoguanidinium salts having six alkyl chains with proper spacing served as new molecular building blocks for the formation of porous honeycomb networks by van der Waals interaction between interdigitated alkyl chains at the liquid/graphite interfaces.

固液界面における多孔性二次元分子集合体の形成と制御

Two-dimensional (2D) architectures, particularly porous molecular networks, on solid surfaces formed via self-assembly have received a great deal of interest because of the prospective applications in optoelectronics and tailor-made catalysis. We found that porous 2D molecular networks were formed at liquid-solid interfaces, typically an organic solvent/graphite or Au(111), by triangle-shaped phenylene-ethynylene macrocycles, dehydrobenzo[12]annulenes (DBAs), via their self-assembly by van der Waals interactions between the interdigitated alkyl chains attached to the periphery of the macrocyclic core. Factors that led to the preferential formation of porous, honeycomb-shaped networks were elucidated including the size of the π-conjugated cores, alkyl chain length, solvent, concentration, temperature, and solid substrates. Not only homochiral molecular networks were achieved by self-assembly of DBAs with chiral side chains, but also network homochirality was induced by addition of a small amount of a chiral DBA into an achiral DBA as a chirality inducer. Co-adsorption of guest molecules in the pores occurred through recognition of the size and shape, leading to selective inclusion of homo-molecular as well as hetero-molecular clusters into the nanowells. Moreover, the interior of the pores was functionalized for tailored molecular recognition toward advanced use of 2D molecular networks on surfaces.

Derivatization and Functionalization of Molecular Matrix by Hydrogen Bond at Liquid–Solid Interface

Derivatization of porous molecular network formed by 1,3,5-tris (10-carboxydecyloxy) benzene (TCDB) via hydrogen bond interaction is demonstrated in this work. With the introduction of 4,4'-bipyridyl (Bpy) the TCDB network is observed to form a heteromolecular cluster, TCDB6-Bpy6, on highly oriented pyrolytic graphite (HOPG) surface under ambient conditions using scanning tunneling microscopy (STM). The derivatized network is further observed to accommodate single guest species such as coronene to form a well-ordered ternary two-dimensional (2D) structure.

Control and induction of surface-confined homochiral porous molecular networks

Homochirality is essential to many biological systems, and plays a pivotal role in various technological applications. The generation of homochirality and an understanding of its mechanism from the single-molecule to supramolecular level have received much attention. Two-dimensional chirality is a subject of intense interest due to the unique possibilities and consequences of confining molecular self-assembly to surfaces or interfaces. Here, we report the perfect generation of two-dimensional homochirality of porous molecular networks at the liquid-solid interface in two different ways: (i) by self-assembly of homochiral building blocks and (ii) by self-assembly of achiral building blocks in the presence of a chiral modifier via a hierarchical structural recognition process, as revealed by scanning tunnelling microscopy. The present results provide important impetus for the development of two-dimensional crystal engineering and may afford opportunities for the utilization of chiral nanowells in chiral recognition processes, as nanoreactors and as data storage systems.

Towards two-dimensional nanoporous networks: crystal engineering at the solid–liquid interface

Surface-confined porous molecular networks can be formed by self-assembly at the liquid–solid interface by controlling the subtle interplay between molecules, solvent and substrate. Typically, directional interactions such as hydrogen bonding or metal–ligand interactions are at play in defining the topology of the networks. However, porous molecular networks maintained only by van der Waals interactions are at the focus in this Highlight. The liquid–solid interface at atomically flat conductive substrates such as graphite is an excellent medium to support the self-assembly of alkylated molecules. Molecular symmetry defines the shape of the pores, ranging from diamond to honeycomb and Kagome. Solvent often plays an important role and sometimes several porous and non-porous structures are formed. The solute concentration is a key factor too, the formation of porous patterns is favoured at low concentrations. These pores, with diameters ranging from about three nanometres to more than seven nanometres, can be filled with guest molecules. In some cases, these porous topologies can only be formed upon addition of template molecules, which direct the structural transformation from a high-density to a low-density, i.e. porous, phase. The template molecules stabilize the pores. Following appropriate design principles, multicomponent networks can be formed, containing up to four different organic molecules.

菱形の形状を有する縮環型デヒドロベンゾ[12]アヌレンの多孔性２次元分子ネットワーク：構造とゲスト共吸着

菱形の形状を有する縮環型デヒドロベンゾ[12]アヌレンの多孔性２次元分子ネットワーク：構造とゲスト共吸着

Self-assembly and aggregation of melamine and melamine–uric/cyanuric acid investigated by STM and AFM on solid surfaces

The self-assembly and aggregation of melamine (M), and a mixture of melamine with uric/cyanuric acid (UA/CA) are investigated by performing scanning tunneling microscopy (STM) and atomic force microscopy (AFM) on graphite and mica surfaces at room temperature. Two-dimensional (2D) porous molecular networks and disordered adlayers are found, respectively, in the M and UA-M assembly on graphite. Uniformly dispersed nanoparticles of M and UA-M are found at various concentrations by AFM. Owing to complementary NH...O and NH...N hydrogen bonds, the CA-M mixture forms 2D densely packed adlayers and nanoscale crystals on graphite and aggregates into three-dimensional (3D) clusters on the surfaces. The size of the cluster is dependent on the molecular concentration. The results provide important information for the self-assembly of triazine compounds and are significant in the study of kidney disease related to melamine.

Two-dimensional rigid molecular network with elastic boundaries for constructing hybrid molecular assemblies

The star-shaped molecule, 2,6,10-trihexyloxy-3,7,11-tris[3,4,5-tris(dodecyloxy)benzoyloxy]triphenylene (sym) was used as the building unit for constructing porous molecular networks with large cavities. It has been identified that a variety of guest molecules (such as C60, phthalocyanine and substituted phthalocyanine) can be dispersed in this template to form supramolecular architectures, which were also studied by scanning tunneling microscopy (STM). It is interesting to observe that this template has elastic boundaries and rigid structures, which can accommodate different amounts of guest species by interacting with them, while keeping the size of cavities invariable.

固液界面におけるデヒドロベンゾ［１２］アヌレンの多孔性分子ネットワークによる２次元ホストゲスト化学

固液界面におけるデヒドロベンゾ［１２］アヌレンの多孔性分子ネットワークによる２次元ホストゲスト化学