Host-guest chemistry in two-dimensional supramolecular networks

Host-Guest Chemistry in Binary and Ternary Complexes Utilizing π-Conjugated Carbon Nanorings

The carbon nanorings, possessing a radial π system, have garnered significant attention primarily due to their size-dependent photophysical properties and the presence of a unique curved π-conjugated cavity. This is evidenced by the rapid proliferation of publications. Furthermore, the integration of building blocks into CPP skeletons can confer [n]CPPs with novel and exceptional photophysical and electronic characteristics, as well as chiral properties and host-guest interactions, thereby augmenting the diversity of [n]CPPs. Notably, the curved π surface structures and concave cavity of carbon nanorings enable them to host aromatic or non-aromatic guests with a complementarily curved surface, resulting in interesting binary or ternary complexes. This review provides a comprehensive treatment of literature reports on binary and ternary complexes, focusing on both their host-guest interactions and properties. It is important to note that the scope of this review is limited to host-guest chemistry in binary and ternary complexes based on π-conjugated carbon nanorings.

Influence of Surface Chemistry on Host/Guest Interactions: A Model Study on Redox-Sensitive β-Cyclodextrin/Ferrocene Complexes

While host/guest interactions are widely used to control molecular assembly on surfaces, quantitative information on the effect of surface chemistry on their efficiency is lacking. To address this question, we combined electrochemical characterization with quartz crystal microbalance with dissipation monitoring to study host/guest interactions between surface-attached ferrocene (Fc) guests and soluble β-cyclodextrin (β-CD) hosts. We identified several parameters that influence the redox response, β-CD complexation ability, and repellent properties of Fc monolayers, including the method of Fc grafting, the linker connecting Fc with the surface, and the diluting molecule used to tune Fc surface density. The study on monovalent β-CD/Fc complexation was completed by the characterization of multivalent interactions between Fc monolayers and β-CD-functionalized polymers, with new insights being obtained on the interplay between the surface chemistry, binding efficiency, and reversibility under electrochemical stimulus. These results should facilitate the design of well-defined functional interfaces and their implementation in stimuli-responsive materials and sensing devices.

Branched-Chain-Induced Host-Guest Assembly in Covalent-Organic Frameworks for Efficient Separation of No-Carrier-Added 177Lu

No-carrier-added (NCA) 177Lu is one of the most interesting nuclides for endoradiotherapy. With the dramatically rapid development of radiopharmaceutical and nuclear medicine, there is a sharp increase in the radionuclide supply of NCA 177Lu, which has formed a great challenge to current radiochemical separation constituted on classical materials. Hence, it is of vital importance to design and prepare new functional materials able of recovering 177Lu from an irradiated target with excellent efficacy. In this work, we proposed to apply noncovalent interactions to regulate the porous properties of covalent organic frameworks (COFs) by tuning the branched chain, rendering related covalent hosts different encapsulation abilities toward a flexible guest, 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (P507). More interestingly, we found that the noncovalent interaction has a great effect on the host-guest complexes, which can achieve efficient NCA 177Lu separation with high recovery (95.97%). A systematic mechanism combined with experimental and theoretical investigations has confirmed that the noncovalent interactions between COFs and P507 play a preeminent role in adjusting the macroscopic properties of the host-guest complexes. This work not only uncovers that noncovalent interactions can affect the basic properties of covalent organic bonded materials but also provides a strategy for the design and preparation of other new moieties with specific functionalities.

Highly Ordered Co-Assembly of Bisurea Functionalized Molecular Switches at the Solid-Liquid Interface

Immobilization of stimulus-responsive systems on solid surfaces is beneficial for controlled signal transmission and adaptive behavior while allowing the characterization of the functional interface with high sensitivity and high spatial resolution. Positioning of the stimuli-responsive units with nanometer-scale precision across the adaptive surface remains one of the bottlenecks in the extraction of cooperative function. Nanoscale organization, cooperativity, and amplification remain key challenges in bridging the molecular and the macroscopic worlds. Here we report on the design, synthesis, and scanning tunneling microscopy (STM) characterization of overcrowded alkene photoswitches merged in self-assembled networks physisorbed at the solid-liquid interface. A detailed anchoring strategy that ensures appropriate orientation of the switches with respect to the solid surface through the use of bis-urea groups is presented. We implement a co-assembly strategy that enables the merging of the photoswitches within physisorbed monolayers of structurally similar 'spacer' molecules. The self-assembly of the individual components and the co-assemblies was examined in detail using (sub)molecular resolution STM which confirms the robust immobilization and controlled orientation of the photoswitches within the spacer monolayers. The experimental STM data is supported by detailed molecular mechanics (MM) simulations. Different designs of the switches and the spacers were investigated which allowed us to formulate guidelines that enable the precise organization of the photoswitches in crystalline physisorbed self-assembled molecular networks.

Rational Design of 2D Supramolecular Networks Switchable by External Electric Fields

The reversible formation of hydrogen bonds is a ubiquitous mechanism for controlling molecular assembly in biological systems. However, achieving predictable reversibility in artificial two-dimensional (2D) materials remains a significant challenge. Here, we use an external electric field (EEF) at the solid/liquid interface to trigger the switching of H-bond-linked 2D networks using a scanning tunneling microscope. Assisted by density functional theory and molecular dynamics simulations, we systematically vary the molecule-to-molecule interactions, i.e., the hydrogen-bonding strength, as well as the molecule-to-substrate interactions to analyze the EEF switching effect. By tuning the building block's hydrogen-bonding ability (carboxylic acids vs aldehydes) and substrate nature and charge (graphite, graphene/Cu, graphene/SiO2), we induce or freeze the switching properties and control the final polymorphic output in the 2D network. Our results indicate that the switching ability is not inherent to any particular building block but instead relies on a synergistic combination of the relative adsorbate/adsorbate and absorbate/substrate energetic contributions under surface polarization. Furthermore, we describe the dynamics of the switching mechanism based on the rotation of carboxylic groups and proton exchange, which generate the polarizable species that are influenced by the EEF. This work provides insights into the design and control of reversible molecular assembly in 2D materials, with potential applications in a wide range of fields, including sensors and electronics.

Synthesis of Large-Scale High-Quality Metal-Organic Frameworks on Cu(100) via Hierarchical Dehydrogenation Reactions

Thermal stimulus has been considered as a promising strategy for controlling on-surface reactions, allowing the formation of diverse products on metal substrates. Here, we successfully achieve hierarchical dehydrogenation reactions of amino groups on a Cu(100) surface. By carefully adjusting the experimental parameters, we synthesize large-scale and low-defect density surface metal-organic frameworks on copper surfaces. Our work sheds light on a controllable route for the synthesis of high-quality metal-organic coordination supramolecular structures via on-surface chemistry.

Two-dimensional molecular networks at the solid/liquid interface and the role of alkyl chains in their building blocks

Nanoarchitectonics has attracted increasing attention owing to its potential applications in nanomachines, nanoelectronics, catalysis, and nanopatterning, which can contribute to overcoming global problems related to energy and environment, among others. However, the fabrication of ordered nanoarchitectures remains a challenge, even in two dimensions. Therefore, a deeper understanding of the self-assembly processes and substantial factors for building ordered structures is critical for tailoring flexible and desirable nanoarchitectures. Scanning tunneling microscopy is a powerful tool for revealing the molecular conformations, arrangements, and orientations of two-dimensional (2D) networks on surfaces. The fabrication of 2D assemblies involves non-covalent interactions that play a significant role in the molecular arrangement and orientation. Among the non-covalent interactions, dispersion interactions that derive from alkyl chain units are believed to be weak. However, alkyl chains play an important role in the adsorption onto substrates, as well as in the in-plane intermolecular interactions. In this review, we focus on the role of alkyl chains in the formation of ordered 2D assemblies at the solid/liquid interface. The alkyl chain effects on the 2D assemblies are introduced together with examples documented in the past decades.

Role of supramolecular polymers in photo-actuation of spiropyran hydrogels

Supramolecular-covalent hybrid polymers have been shown to be interesting systems to generate robotic functions in soft materials in response to external stimuli. In recent work supramolecular components were found to enhance the speed of reversible bending deformations and locomotion when exposed to light. The role of morphology in the supramolecular phases integrated into these hybrid materials remains unclear. We report here on supramolecular-covalent hybrid materials that incorporate either high-aspect-ratio peptide amphiphile (PA) ribbons and fibers, or low-aspect-ratio spherical peptide amphiphile micelles into photo-active spiropyran polymeric matrices. We found that the high-aspect-ratio morphologies not only play a significant role in providing mechanical reinforcement to the matrix but also enhance photo-actuation for both light driven volumetric contraction and expansion of spiropyran hydrogels. Molecular dynamics simulations indicate that water within the high-aspect-ratio supramolecular polymers exhibits a faster draining rate as compared to those in spherical micelles, which suggests that the high-aspect-ratio supramolecular polymers effectively facilitate the transport of trapped water molecules by functioning as channels and therefore enhancing actuation of the hybrid system. Our simulations provide a useful strategy for the design of new functional hybrid architectures and materials with the aim of accelerating response and enhancing actuation by facilitating water diffusion at the nanoscopic level.

A Paradigm Shift from 2D to 3D: Surface Supramolecular Assemblies and Their Electronic Properties Explored by Scanning Tunneling Microscopy and Spectroscopy

Exploring supramolecular architectures at surfaces plays an increasingly important role in contemporary science, especially for molecular electronics. A paradigm of research interest in this context is shifting from 2D to 3D that is expanding from monolayer, bilayers, to multilayers. Taking advantage of its high-resolution insight into monolayers and a few layers, scanning tunneling microscopy/spectroscopy (STM/STS) turns out a powerful tool for analyzing such thin films on a solid surface. This review summarizes the representative efforts of STM/STS studies of layered supramolecular assemblies and their unique electronic properties, especially at the liquid-solid interface. The superiority of the 3D molecular networks at surfaces is elucidated and an outlook on the challenges that still lie ahead is provided. This review not only highlights the profound progress in 3D supramolecular assemblies but also provides researchers with unusual concepts to design surface supramolecular structures with increasing complexity and desired functionality.

Supramolecular Host–Guest Assemblies of [M6Cl14]2–, M = Mo, W, Clusters with γ-Cyclodextrin for the Development of CLUSPOMs

Host–guest assemblies open up opportunities for developing novel functional CLUSPOM multicomponent systems based on transition metal clusters (CLUS), polyoxometalates (POMs) and macrocyclic organic ligands. In water–ethanol solution γ-cyclodextrin (γ-CD) interacts with halide metal clusters [M6Cl14]2– (M = Mo, W) to form sandwich-type structures. The supramolecular association between the clusters and CDs, however, remains weak in solution, and the interactions are not strong enough to prevent the hydrolysis of the inorganic guest. Although analysis of the resulting crystal structures reveals inclusion complexation, 1H NMR experiments in solution show no specific affinity between the two components. The luminescent properties of the host–guest compounds in comparison with the initial cluster complexes are also studied to evaluate the influence of CD.

Surface supramolecular assemblies tailored by chemical/physical and synergistic stimuli: a scanning tunneling microscopy study

Supramolecular self-assemblies formed by various non-covalent interactions can produce diverse functional networks on solid surfaces. These networks have recently attracted much interest from both fundamental and application points of view. Unlike covalent organic frameworks (COFs), the properties of the assemblies differ from each other depending on the constituent motifs. These various motifs may find diverse applications such as in crystal engineering, surface modification, and molecular electronics. Significantly, these interactions between/among the molecular tectonics are relatively weak and reversible, which makes them responsive to external stimuli. Moreover, for a liquid-solid-interface environment, the dynamic processes are amenable to in situ observation using scanning tunneling microscopy (STM). In the literature, most review articles focus on supramolecular self-assembly interactions. This review summarizes the recent literature in which stimulation sources, including chemical, physical, and their combined stimuli, cooperatively tailor supramolecular assemblies on surfaces. The appropriate design and synthesis of functional molecules that can be integrated on different surfaces permits the use of nanostructured materials and devices for bottom-up nanotechnology. Finally, we discuss synergic effect on materials science.

Pillar[n]arene-calix[m]arene hybrid macrocyclic structures

To reserve planar chirality, enhance molecular recognition, and build advanced self-assemblies, hybrid macrocyclic hosts containing rigid pillar[n]arene and flexible calix[m]arene were designed, prepared and investigated for interesting applications. This review summarizes and discusses different synthetic strategies for constructing hybrid macrocyclic structures. Pillar[n]arene dimer with rigid aromatic double bridges provided the possibility of introducing calix[m]arene cavities, where the planar chirality was reserved in the structure of pillararene. The capacity for molecular recognition was enhanced by hybrid macrocyclic cavities. Interestingly, the obtained pillar[n]arene-calix[m]arene could self-assemble into "channels" and "honeycomb" in both the solid state and solution phase as well as donate the molecular architecture as the wheel for the formation of mechanically interlocked molecules, such as rotaxane. In addition, the pillar[n]arene and calix[m]arene could also be coupled together to produce pillar[n]arene embeded 1,3-alternate and cone conformational calix[m]arene derivatives, which could catalyze the oxidative polymerization of aniline in aqueous solutions. Except for building hybrid cyclophanes by covalent bonds, weak supramolecular interactions were used to prepare pillar[n]arene-calix[m]arene analogous composites with other pillar-like pillar[n]pyridiniums and calix-like calix[m]pyrroles, exhibiting reasonable performances in enhancing molecular recognition and trapping solvent molecules.

A self-complementary macrocycle by a dual interaction system

Self-complementary assembly is one of the most promising phenomena for the formation of discrete assemblies, e.g., proteins and capsids. However, self-complementary assembly based on multiple host-guest systems has been scarcely reported due to the difficulty in controlling each assembly. Herein, we report a dual interaction system in which the key assembly direction is well regulated by both π-π stacking and hydrogen bonding to construct a self-complementary macrocycle. Continuous host-guest behavior of anthracene-based molecular tweezers during crystallization leads to successful construction of a cyclic hexamer, which is reminiscent of Kekulé’s monkey model. Furthermore, the cyclic hexamer in a tight and triple-layered fashion shows hierarchical assembly into cuboctahedron and rhombohedral assemblies in the presence of trifluoroacetic acid. Our findings would be potentially one of metal-free strategies for constructing anthracene-based supramolecular assemblies with higher-order structure.

In nature, HIV capsid consists of single class of protein unit by self-complementarity. Here, the authors find that a molecular tweezer forms a cyclic hexamer by its continuous host-guest behavior, and constructs a large cuboctahedron by hierarchical assembly.

Minor adjustments in the chemical structures of pyridine derivatives induced different co-assemblies by O-H⋯N hydrogen bonds

The co-adsorption behaviours of aromatic carboxylic acids with various pyridine derivatives were investigated with scanning tunneling microscopy and density functional theory. Surprisingly, minor adjustments in the chemical structures of the pyridine derivatives, such as the relative position of the nitrogen atom or the lengths of the side chains on the backbone would evidently affect the intermolecular O-H⋯N hydrogen bonds and further form various co-adsorption structures.

Flow-induced-crystallization: tailoring host-guest supramolecular co-assemblies at the liquid-solid interface

Here, we report that using the method of simply contacting a sample solution droplet with a piece of tissue paper can create a solvent flow (capillary force). During this process, the dynamics and solvent removal can promote the formation and stabilization of a meta-stable linear quasi-crystal composed of p-terphenyl-3,5,3',5'-tetracarboxylic acid (TPTC) molecules, which would otherwise pack into thermodynamically favored random tiling. The tailored quasi-crystal (linear) template allows atop it higher-efficiency accommodation of fullerene molecules (C60) from 40.1% to 97.5%, compared with that obtained in the random-tiling (porous) case. Overall, the result of this study presents an unusual yet remarkably simple strategy for tailoring complex host-guest supramolecular systems at the liquid-solid interface.

Induced Fit and Mobility of Cycloalkanes within Nanometer-Sized Confinements at 5 K

Host-guest architectures provide ideal systems for investigating site-specific physical and chemical effects. Condensation events in nanometer-sized confinements are particularly interesting for the investigation of intermolecular and molecule-surface interactions. They may be accompanied by conformational adjustments representing induced fit packing patterns. Here, we report that the symmetry of small clusters formed upon condensation, their registry with the substrate, their lateral packing, and their adsorption height are characteristically modified by the packing of cycloalkanes in confinements. While cyclopentane and cycloheptane display cooperativity upon filling of the hosting pores, cyclooctane and to a lesser degree cyclohexane diffusively redistribute to more favored adsorption sites. The dynamic behavior of cyclooctane is surprising at 5 K given the cycloalkane melting point of >0 °C. The site-specific modification of the interaction and behavior of adsorbates in confinements plays a crucial role in many applications of three-dimensional porous materials as gas storage agents or catalysts/biocatalysts.

Co-assembly Behaviors of Flavonol Derivatives Induced by a Pyridine Derivative on HOPG via Hydrogen Bonding and Van der Waals Forces

In this paper, two new flavonol derivatives, 2-(4-(dodecyloxy)phenyl)-3-hydroxyflavone (DHF) and 2-(3,5-bis(dodecyloxy)phenyl)-3-hydroxyflavone (BDHF), were synthesized to investigate the respective self-assembly behaviors at the liquid/solid interface by scanning tunneling microscopy. In addition, a linear pyridine derivative with acetylene groups called BisPy was added to regulate the assembly of DHF and BDHF, individually. However, only BDHF molecules successfully co-assembled into grid structures with BisPy molecules. Furthermore, the assembly and co-assembly behavior mechanism of flavonol derivatives and BisPy molecules were further studied by density functional theory calculations. This work will lay a foundation for investigating the self-assembly of flavonol derivatives and the co-assembly regulated by pyridine derivatives at the liquid-solid interface.

Water-Phase Lateral Interconnecting Quantum Dots as Free-Floating 2D Film Assembled by Hydrogen-Bonding Interactions to Acquire Excellent Electrocatalytic Activity

Supramolecular self-assembly of nanoparticles in two orthogonal directions would potentially allow one to fabricate nanomaterials with fascinating properties. In this study of a hydrothermal polycondensation of melamine/cyanuric acid, graphitic carbon nitride-based quantum dots (CNQD, ∼2 nm) are in situ arranged along two orthogonal directions through lateral hydrogen bonding, and free-floating two-dimensional hydrogen-bonded films of CNQD (2D CNQD) are built. On the basis of the universality of this hydrothermal in situ supramolecular self-assembly technique, 2D films linked by other quantum dots such as sulfur-doped graphitic carbon nitride and CdTe are also constructed. With the benefits of stimuli responsiveness and the reversibility of hydrogen bonds, controllable assembly/disassembly of the 2D CNQD film is feasibly achieved by external stimuli such as inletting CO₂/N₂, which endows the assembled 2D CNQD films optimal electrochemical superiorities of both 2D film and zero-dimensional (0D) quantum dots. Accordingly, the 2D CNQD film delivers a high bifunctional activity in both a nitrogen reduction reaction (NRR) and an oxygen evolution reaction (OER). Especially in NRR, it exhibits the high yield rate of NH₃ reaching 75.07 μg h^-1 mg^-1 at -0.85 V versus reversible hydrogen electrode at ambient condition. Strikingly, the power density of the rechargeable Zn-N₂ battery using 2D CNQD film as cathode reaches 31.94 mW cm^-2, outperforming the majority of Zn-N₂ batteries. Density functional theory calculations proved the promoted adsorption of N₂ and stabilized NRR intermediates on 2D CNQD cooperated by multiply hydrogen-bonding interactions are the main reasons for the excellent NRR electrocatalytic performances. This work hints that hydrothermal in situ supramolecular self-assembly is a feasible and direct way to integrate 0D quantum dots into 2D directional arrays, and the hydrogen bond that interlinks enables this free-floating 2D structure to maintain the electrochemical superiority of both 0D and 2D structures.

Visualizing the Entire Range of Noncovalent Interactions in Nanocrystalline Hybrid Materials Using 3D Electron Diffraction

Noncovalent interactions are essential in the formation and properties of a diverse range of hybrid materials. However, reliably identifying the noncovalent interactions in nanocrystalline materials remains challenging using conventional methods such as X-ray diffraction and spectroscopy. Here, we demonstrate that accurate atomic positions including hydrogen atoms can be determined using three-dimensional electron diffraction (3D ED), from which the entire range of noncovalent interactions in a nanocrystalline aluminophosphate hybrid material SCM-34 are directly visualized. The protonation states of both the inorganic and organic components in SCM-34 are determined from the hydrogen positions. All noncovalent interactions, including hydrogen-bonding, electrostatic, π–π stacking, and van der Waals interactions, are unambiguously identified, which provides detailed insights into the formation of the material. The 3D ED data also allow us to distinguish different types of covalent bonds based on their bond lengths and to identify an elongated terminal P=O π-bond caused by noncovalent interactions. Our results show that 3D ED can be a powerful tool for resolving detailed noncovalent interactions in nanocrystalline materials. This can improve our understanding of hybrid systems and guide the development of novel functional materials.

Supramolecular Self-Assembly in Living Cells

Supramolecular interactions rely on non-covalent forces, such as hydrophobic effects, hydrogen-bonding, and electrostatic interactions, which govern many intracellular biological pathways. In cellulo supramolecular self-assembly is mainly based on host-guest interactions, changes in pH, enzymes, and polymerization-induced self-assembly to accurately induce various unnatural reactions without disturbing natural biological processes. This process can produce synthetic biocompatible macromolecules to control cell properties and regulate biological functions, such as cell proliferation and differentiation. This Minireview focuses on the latest reports in the field of in cellulo supramolecular self-assembly and anticipates future advances regarding its activation in response to internal and external stimuli, such as pH changes, reactive oxygen species, and enzymes, as well as external light illumination.

Self-Assembly of Triphenylamine Macrocycles and Co-assembly with Guest Molecules at the Liquid-Solid Interface Studied by STM: Influence of Different Side Chains on Host-Guest Interaction

The side chains of macrocyclic molecules have a non-negligible effect on the two-dimensional (2D) supramolecular networks at the liquid-solid interface. In this study, we investigate the self-assembly behaviors of two conjugated triphenylamine macrocycles modified with different alkyl chains and construct the host-guest supramolecular nanopatterns on the highly oriented pyrolytic graphite with a scanning tunneling microscope. In combination with density functional theory calculations, how different side chains affect the host-guest interaction is discussed. This work provides insights into constructing a 2D host-guest dynamic co-assembly on the surface.

Thermally Stable Array of Discrete C60s on a Two-Dimensional Crystalline Adlayer of Macrocycles both in Vacuo and under Ambient Pressure

A periodic monolayer array of discrete C₆₀s was generated on an atomically flat Au(111) surface with the aid of a template adlayer. The template was a two-dimensional (2D) array of molecular pits prepared on an Au(111) surface through 2D crystallization of shape-persistent macrocycles composed of four carbazole and four salphens/Ni-salphens with a 1 nm hollow. Scanning tunneling microscopy imaging under ultra-high vacuum revealed that the square-shaped macrocycles, with 1.5 nm sides, were arranged with a periodic spacing of approximately 4.0 nm on the Au(111) surface, where the orientation and periodicity of the macrocycles were dependent on their chemical structures. After sublimation of C₆₀s onto the adlayer, a single C₆₀ molecule was entrapped in each pit, and an ordered molecular array of C₆₀s was attained with a pattern similar to that of the macrocycles. The periodic pattern of C₆₀s on the surface was thermally stable up to approximately 200 °C, even under ambient pressure. Scanning tunneling spectroscopy suggested the existence of an electronic interaction between the C₆₀s and the Au(111) surface that was influenced by the macrocycle template on the surface.

The long-awaited synthesis and self-assembly of a small rigid C3-symmetric trilactam

The challenging synthesis of a fused C₃-symmetric trilactam (1) was executed in racemic and enantiomerically pure form. The rigidity, symmetry and high density of hydrogen bonding motifs make 1 an attractive candidate for self-assembly study, which revealed different hydrogen bond patterns in the crystals of rac-1-d₃ and (+)-(SSS)-1.

Well-organised two-dimensional self-assembly controlled by in situ formation of a Cu(II)-coordinated rufigallol derivative: a scanning tunnelling microscopy study

The two-dimensional self-assembly of rufigallol derivatives and their metal coordination were studied by scanning tunnelling microscopy. Ex situ Cu(II)-coordinated rufigallol derivatives exhibited columnar structures with some defects, whereas regular and linear structures were formed upon in situ metal coordination at solid/liquid interfaces.

Biasing the Hierarchy Motifs of Nanotoroids: from 1D Nanotubes to 2D Porous Networks

Hierarchical organization of self-assembled structures into superstructures is omnipresent in Nature but has been rarely achieved in synthetic molecular assembly due to the absence of clear structural rules. We herein report on the self-assembly of scissor-shaped azobenzene dyads which form discrete nanotoroids that further organize into 2D porous networks. The steric demand of the peripheral aliphatic units diminishes the trend of the azobenzene dyad to constitute stackable nanotoroids in solution, thus affording isolated (unstackable) nanotoroids upon cooling. Upon drying, these nanotoroids organize at graphite surface to form well-defined 2D porous networks. The photoirradiation with UV and visible light enabled reversible dissociation and reconstruction of nanotoroids through the efficient trans↔cis isomerization of azobenzene moieties in solution.

Halogen bond-directed self-assembly in bicomponent blends at the solid/liquid interface: Effect of the alkyl chain substitution position

The fabrication of well-organised molecular assemblies on surfaces is fundamental for the creation of functional molecular systems applicable to nanoelectronics and molecular devices. In this study, we investigated the effect...

Strategic Design of a 2,6-Disubstituted Pyridine-based Probe having Hard-Soft Centers: Responsive Divergence from One Core

Alkyne is a versatile functional group in organic chemistry, and is able to undergo a wide variety of reactions and interactions. Featuring a reactive functional group, alkyne participates in many...

Subtle metal(ii) effects of 2D coordination networks on SCSC guest exchange

The multi-channel crystals consisting of 2-D networks G@[M(NO3)2L] are an unusually efficient, tolerant, and reproducible matrix offering M-dependent adsorption/desorption of various guest molecules in the single-crystal-to-single-crystal mode.

Construction of cucurbit[n]uril-based supramolecular frameworks via host-guest inclusion and functional properties thereof

Frameworks utilizing cucurbit[n]uril-based chemistry build on the rapid developments in the fields of metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and supramolecular organic frameworks (SOFs), and as porous materials have found...

Supramolecular Nanopatterns of Molecular Spoked Wheels with Orthogonal Pillars: The Observation of a Fullerene Haze

Molecular spoked wheels with intraannular functionalizable pillars are synthesized in a modular approach. The functionalities at their ends are variable, and a propargyl alcohol, a [6,6]-phenyl-C61-butyrate, and a perylene monoimide are investigated. All compounds form two-dimensional crystals on highly oriented pyrolytic graphite at the solid-liquid interface. As determined by submolecularly resolved scanning tunneling microscopy, the pillars adopt equilibrium distances of 6.0 nm. The fullerene has a residual mobility, limited by the length of the flexible connector unit. The experimental results are supported and rationalized by molecular dynamics simulations. These also show that, in contrast, the more rigidly attached perylene monoimide units remain oriented along the surface normal and maintain a smallest distance of 2 nm above the graphite substrate. The robust packing concept also holds for cocrystals with molecular hexagons that expand the pillar-pillar distances by 15 % and block unspecific intercalation.

Ex Vivo and In Vivo Evaluation of Dodecaborate-Based Clusters Encapsulated in Ferumoxytol Nanoparticles

Host-guest interactions represent a growing research area with recent work demonstrating the ability to chemically manipulate both host molecules as well as guest molecules to vary the type and strength of bonding. Much less is known about the interactions of the guest molecules and hybrid materials containing similar chemical features to typical macrocyclic hosts. This work uses in vitro and in vivo kinetic analyses to investigate the interaction of closo-dodecahydrododecaborate derivatives with ferumoxytol, an iron oxide nanoparticle with a carboxylated dextran coating. We find that several boron cluster derivatives can become encapsulated into ferumoxytol, and the lack of pH dependence in these interactions suggests that ion pairing, hydrophobic/hydrophilic interaction, and hydrogen bonding are not the driving force for encapsulation in this system. Biodistribution experiments in BALB/c mice show that this system is nontoxic at the reported dosage and demonstrate that encapsulation of dodecaborate-based clusters in ferumoxytol can alter the biodistribution of the guest molecules.

Mixing behavior of p-terphenyl-3,5,3',5'-tetracarboxylic acid with trimesic acid at the solid-liquid interface

The molecular self-assembly of carboxylic acid molecules on a solid surface plays an important role in understanding the nanoscale-precision construction of functional patterns. In this study, the mixing behavior of p-terphenyl-3,5,3',5'-tetracarboxylic acid (TPTC) and trimesic acid (TMA) on a highly oriented pyrolytic graphite surface was studied by scanning tunneling microscopy (STM). The STM images show how the presence of a small percentage of TPTC molecules adsorbed onto TMA molecules can drastically change the on-surface self-assembly behavior of aromatic tetracarboxylic acid by initiating the nucleation and growth of a different polymorph. Molecular mechanics and density functional theory simulations of the adsorption energy and the additional stabilizing energy, induced by hydrogen bonds during assembly formations, provide insights into the relative stability of different assembled structures. Moreover, STM-based "nanoshaving" was conducted to confirm that the template layer underneath the second layer is indeed a random network.

"Concentration-in-Control" self-assembly concept at the liquid-solid interface challenged

Self-assembled molecular networks (SAMNs) on surfaces evoke a lot of interest, both from a fundamental as well as application point of view. When formed at the liquid–solid interface, precise control over different polymorphs can be achieved by simply adjusting the concentration of molecular building blocks in solution. Significant influence of solute concentration on self-assembly behavior has been observed, whether the self-assembly behavior is controlled by either van der Waals forces or hydrogen bonding interactions. In both cases, high- and low-density supramolecular networks have been observed at high and low solute concentrations, respectively. In contrast to this “concentration-in-control” self-assembly concept here we report an atypical concentration dependent self-assembly behavior at a solution–solid interface. At the interface between heptanoic acid (HA) and highly oriented pyrolytic graphite (HOPG), we show, using scanning tunneling microscopy (STM), the formation of a low-density porous network at high solute concentrations, and a high-density compact network at low solute concentrations. This intriguing inverse concentration dependent self-assembly behavior has been attributed to the preaggregation of solute molecules in the heptanoic acid solution as revealed by UV-vis spectroscopy. The observed results have been correlated to the molecular density of self-assembled monolayers attained at the HA/HOPG interface.

Surprise! against expectations, increasing (decreasing) the solute concentration leads to the formation of a low-density (high-density) self-assembled molecular network at the liquid–solid interface.

Constructing and Transferring Two-Dimensional Tessellation Kagome Lattices via Chemical Reactions on Cu(111) Surface

Two-dimensional (2D) tessellation of organic species acquired increased interests recently because of their potential applications in physics, biology, and chemistry. 2D tessellations have been successfully constructed on surfaces via various intermolecular interactions. However, the transformation between 2D tessellation lattices has been rarely reported. Herein, we successfully fabricated two types of Kagome lattices on Cu(111). The former phase exhibits (3,6,3,6) Kagome lattices, which are stabilized via the intermolecular hydrogen bond interactions. The latter phase is formed through direct chemical transferring from the former one maintaining almost the same Kagome lattices, except for that the unit cell rotates for 4°. Detailed scanning tunneling microscopy and density functional calculation studies reveal that the chemical transformation is achieved by the formation of the N-Cu-N metal-organic bonds via dehydrogenation reactions of the amines.

Host-guest interactions for extracting antibiotics with a γ-cyclodextrin poly(glycidyl-co-ethylene dimethacrylate) hybrid sorbent

A procedure for the solid-phase extraction of antibiotics (enoxacin, ofloxacin, norfloxacin, ciprofloxacin, and sparfloxacin) in water has been developed. The sorbent used is based on a poly(glycidyl-co-ethylene dimethacrylate) network, whose previously modified surface has been functionalized with γ-cyclodextrin through a click-chemistry reaction. The architecture of the material has been characterized by thermogravimetric analysis, N₂ adsorption-desorption, Raman spectroscopy, confocal microscopy, and scanning electron microscopy, showing good capability to be used as a filler for extraction cartridges. The optimization of the extraction methodology shows good intra-day and inter-day repeatability of the extraction procedure, with coefficients of variation between 2.5 and 5.1% and the possibility of reusing the material at least five times. The detection limits of the method have been established at the μg L^-1 level, confirming the possibility of quantifying trace levels. To end, real groundwater samples have been analyzed and the results are comparable with those obtained with a reference method. The proposed material can be used for assessing the presence of antibiotics in aqueous environments through an extraction procedure taking advantage of the presence of γ-cyclodextrin on its structure.

On-surface photopolymerization of two-dimensional polymers ordered on the mesoscale

The use of solid supports and ultra-high vacuum conditions for the synthesis of two-dimensional polymers is attractive, as it can enable thorough characterization, often with submolecular resolution, and prevent contamination. However, most on-surface polymerizations are thermally activated, which often leads to high defect densities and relatively small domain sizes. Here, we have obtained a porous two-dimensional polymer that is ordered on the mesoscale by the two-staged topochemical photopolymerization of fluorinated anthracene triptycene (fantrip) monomers on alkane-passivated graphite surfaces under ultra-high vacuum. First, the fantrip monomers self-assemble into highly ordered monolayer structures, where all anthracene moieties adopt a suitable arrangement for photopolymerization. Irradiation with violet light then induces complete covalent crosslinking by [4+4] photocycloaddition to form a two-dimensional polymer, while fully preserving the long-range order of the self-assembled structure. The extent of the polymerization is confirmed by local infrared spectroscopy and scanning tunnelling microscopy characterization, in agreement with density functional theory calculations, which also gives mechanistic insights.

The kinetics of formation of resveratrol-β-cyclodextrin-NH2 and resveratrol analog-β-cyclodextrin-NH2 supramolecular complexes

The determination of the kinetics of inclusion processes is significant for the application of inclusion complexes as carriers for bioactive molecules. We determined the kinetic parameters of inclusion between modified β-cyclodextrin (β-CD-NH₂) and the polyphenols resveratrol (RES) and its structural analog (RESAn1), using the real-time analysis of surface plasmon resonance. The association and dissociation rate constants (k_a and k_d) showed that RESAn1 inclusion and its dissociation from β-CD-NH₂ were faster than a similar process for RES ( [Formula: see text]  = 3.10∙10⁴ ± 0.14 M^-1s^-1, [Formula: see text] =1.87∙10³ ± 0.11 M^-1s^-1; [Formula: see text] =0.39 ± 0.02 s^-1, [Formula: see text] =0.30 ± 0.02 s^-1, at 25 °C). The activated complex formation was also affected by the structural differences between the polyphenols, as showed by the activation energies of the association step ( [Formula: see text] 14.81 ± 0.64 kJ∙mol^-1, [Formula: see text] -15.01 ± 0.75 to 82.35 ± 4.47 kJ∙mol^-1). These effects of polyphenol structural differences are due to the desolvation process of interacting molecules. These results elucidate the role of small group to the dynamics of the molecular inclusion of β-CD.

Supramolecular Chemistry: Host-Guest Molecular Complexes

In recent times, researchers have emphasized practical approaches for capturing coordinated and selective guest entrap. The physisorbed nanoporous supramolecular complexes have been widely used to restrain various guest species on compact supporting surfaces. The host-guest (HG) interactions in two-dimensional (2D) permeable porous linkages are growing expeditiously due to their future applications in biocatalysis, separation technology, or nanoscale patterning. The different crystal-like nanoporous network has been acquired to enclose and trap guest molecules of various dimensions and contours. The host centers have been lumped together via noncovalent interactions (such as hydrogen bonds, van der Waals (vdW) interactions, or coordinate bonds). In this review article, we enlighten and elucidate recent progress in HG chemistry, explored via scanning tunneling microscopy (STM). We summarize the synthesis, design, and characterization of typical HG structural design examined on various substrates, under ambient surroundings at the liquid-solid (LS) interface, or during ultrahigh vacuum (UHV). We emphasize isoreticular complexes, vibrant HG coordination, or hosts functional cavities responsive to the applied stimulus. Finally, we critically discuss the significant challenges in advancing this developing electrochemical field.

Construction of Janus dendrimers through a self-assembly approach involving chiral discrimination at a focal point

A strategy to build Janus dendrimers via the chirality-directed self-assembly of heteroleptic Zn(ii) BOX complexes is reported. The method allows quantitative synthesis of Janus dendrimers in situ without the need for purifications. Each dendritic domain of the Janus dendrimers can be recycled upon disassembly at the focal point.

Host guest chemistry and supramolecular doping in triphenylamine-based covalent frameworks on Au(111)

The post-synthetic modification of covalent organic frameworks (COFs) via host-guest chemistry is an important method to tailor their electronic properties for applications. Due to the limited structural control in the assembly of two-dimensional surface-supported COFs, supramolecular networks are traditionally used at present for host-guest experiments on surfaces, which lack structural and thermal stability, however. Here, we present a combined scanning tunneling microscopy and density functional theory study to understand the host-guest interaction in triphenylamine-based covalently-linked macrocycles and networks on Au(111). These triphenylamine-based structures feature carbonyl and hydrogen functionalized pores that create preferred adsorption sites for trimesic acid (TMA) and halogen atoms. The binding of the TMA through optimized hydrogen-bond interactions is corroborated by selective adsorption positions within the pores. Band structure calculations reveal that the strong intermolecular charge transfer through the TMA bonding reduces the band gap in the triphenylamine COFs, demonstrating the concept of supramolecular doping by host-guest interactions in surface-supported COFs. Halogen atoms selectively adsorb between two carbonyl groups at Au hollow sites. The mainly dispersive interaction of the halogens with the triphenylamine COF leads to a small downshift of the bands. Most of the halogens change their adsorption position selectively upon annealing near the desorption temperature. In conclusion, we demonstrate evidence for supramolecular doping via post-synthetic modification and to track chemical reactions in confined space.

Large-Area Assembly of Metal-Organic Layered Ultrathin Films at the Liquid/Liquid Interface

Two-dimensional functional metal-organic frameworks and coordination polymers have attracted much attention and have been successfully prepared in solutions and at interfaces through the coordination of ligands to metal ions. However, the preparation of large-area ultrathin ordered films is still a challenge. Here, a modified liquid/liquid interfacial epitaxial growth method has been developed. A planar liquid/liquid interface between a chloroform solution of bipyridine derivatives and pure water was constructed first, and then an aqueous solution of Eu³⁺ or Cu²⁺ ions was added dropwise into the water phase. A layered ultrathin film with the size of several hundreds of square micrometers appeared at the liquid/liquid interface after a certain time. The monitoring results showed that the formation of ultrathin films was a result of continuous epitaxial growth of the adsorbed species due to the synergistic effects of hydrophobic effects of the alkyl chains, coordination bonds between the ligands and metal ions, π-π interactions between the ligands, and the restriction of the interface on the vertical growth. This offers a way to fabricate more large-area thin films of amphiphilic molecules.

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.