Reversible Local and Global Switching in Multicomponent Supramolecular Networks: Controlled Guest Release and Capture at the Solution/Solid Interface

Deciphering the factors influencing electric field mediated polymerization and depolymerization at the solution-solid interface

Strong and oriented electric fields are known to influence structure as well as reactivity. The strong electric field (EF) between the tip of a scanning tunneling microscope (STM) and graphite has been used to modulate two-dimensional (2D) polymerization of aryl boronic acids where switching the polarity of the substrate bias enabled reversible transition between self-assembled molecular networks of monomers and crystalline 2D polymer (2DP) domains. Here, we untangle the different factors influencing the EF-mediated (de)polymerization of a boroxine-based 2DP on graphite. The influence of the solvent was systematically studied by varying the nature from polar protic to polar aprotic to non-polar. The effect of monomer concentration was also investigated in detail with a special focus on the time-dependence of the transition. Our experimental observations indicate that while the nucleation of 2DP domains is not initiated by the applied electric field, their depolymerization and subsequent desorption, are a consequence of the change in the polarity of the substrate bias within the area scanned by the STM tip. We conclude that the reversible transition is intimately linked to the bias-induced adsorption and desorption of the monomers, which, in turn, could drive changes in the local concentration of the monomers.

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.

Manipulating Molecular Self-Assembly Process at the Solid-Liquid Interface Probed by Scanning Tunneling Microscopy

The phenomenon of ordered self-assembly on solid substrates is a topic of interest in both fundamental surface science research and its applications in nanotechnology. The regulation and control of two-dimensional (2D) self-assembled supra-molecular structures on surfaces have been realized through applying external stimuli. By utilizing scanning tunneling microscopy (STM), researchers can investigate the detailed phase transition process of self-assembled monolayers (SAMs), providing insight into the interplay between intermolecular weak interactions and substrate-molecule interactions, which govern the formation of molecular self-assembly. This review will discuss the structural transition of self-assembly probed by STM in response to external stimuli and provide state-of-the-art methods such as tip-induced confinement for the alignment of SAM domains and selective chirality. Finally, we discuss the challenges and opportunities in the field of self-assembly and STM.

Influence of core size on self-assembled molecular networks composed of C3h-symmetric building blocks through hydrogen bonding interactions: structural features and chirality

The effect of the core size on the structure and chirality of self-assembled molecular networks was investigated using two aromatic carboxylic acid derivatives with frameworks displaying C3h symmetry, triphenylene derivative H3TTCA and dehydrobenzo[12]annulene (DBA) derivative DBACOOH, each having three carboxy groups per molecule. Scanning tunneling microscopy observations at the 1-heptanoic acid/graphite interface revealed H3TTCA exclusively forming a chiral honeycomb structure, and DBACOOH forming three structures (type I, II, and III structures) depending on its concentration and whether the system is subjected to annealing treatment. Hydrogen bonding interaction patterns and chirality were carefully analyzed based on a modeling study using molecular mechanics simulations. Moreover, DBACOOH forms chiral honeycomb structures through the co-adsorption of guest molecules. Structural diversity observed for DBACOOH is attributed to its relatively large core size, with this feature modulating the balance between molecule-molecule and molecule-substrate interactions.

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.

STM Study of the Self-Assembly of Biphenyl-3,3',5,5'-Tetracarboxylic Acid and Its Mixing Behavior with Coronene at the Liquid-Solid Interface

We report a scanning tunneling microscopy (STM) study of the molecular self-assembly of biphenyl-3,3',5,5'-tetracarboxylic acid (BPTC) at the octanoic acid/graphite interface. STM revealed that the BPTC molecules generated stable bilayers and monolayers under high and low sample concentrations, respectively. Besides hydrogen bonds, the bilayers were stabilized by molecular π-stacking, whereas the monolayers were maintained by solvent co-adsorption. A thermodynamically stable Kagomé structure was obtained upon mixing BPTC with coronene (COR), while kinetic trapping of COR in the co-crystal structure was found by the subsequent deposition of COR onto a preformed BPTC bilayer on the surface. Force field calculation was conducted to compare the binding energies of different phases, which helped to provide plausible explanations for the structural stability formed via kinetic and thermodynamic pathways.

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.

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.

Resonant Electron Tunneling Induces Isomerization of π-Expanded Oligothiophene Macrocycles in a 2D Crystal

Macrocyclic oligothiophenes and their π-expanded derivatives constitute versatile building blocks for the design of (supra)molecularly engineered active interfaces, owing to their structural, chemical, and optoelectronic properties. Here, it is demonstrated how resonant tunneling effect induces single molecular isomerization in a 2D crystal, self-assembled at solid-liquid interfaces under ambient conditions. Monolayers of a series of four π-expanded oligothiophene macrocycles are investigated by means of scanning tunneling microscopy and scanning tunneling spectroscopy (STS) at the interface between their octanoic acid solutions and the basal plane of highly oriented pyrolytic graphite. Current-voltage characteristics confirm the donor-type character of the macrocycles, with the highest occupied molecular orbital and the lowest unoccupied molecular orbital (LUMO) positions consistent with time-dependent density functional theory calculations. Cyclic STS measurements show the redox isomerization from Z,Z-8T6A to its isomer E,E-8T6A occurring in the 2D crystal, due to the formation of a negatively charged species when the tunneling current is in resonance with the LUMO of the macrocycle.

Bidirectional Phase Transformation of Supramolecular Networks Using Two Molecular Signals

Reversible control of molecular self-assembly is omnipresent in adaptive biological systems, yet its realization in artificial systems remains a major challenge. Using scanning tunneling microscopy and density functional theory calculations, we show that a 2D supramolecular network formed by terthienobenzenetricarboxylic acid (TTBTA) can undergo a reversible structural transition between a porous and dense phase in response to different molecular signals (trimethyltripyrazolotriazine (TMTPT) and C₆₀). TMTPT molecules can induce a phase transition from the TTBTA honeycomb to the dense phase, whereas a reverse transition can be triggered by introducing C₆₀ molecules. This response stems from the selective association between signal molecules and TTBTA polymorphs. The successful realization of reversible molecular transformation represents important progress in controlling supramolecular surface nanostructures and could be potentially applicable in various areas of nanotechnology, including phase control, molecular sensing, and "smart" switchable surfaces.

"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.

Hydrogen and Halogen Bonding in Homogeneous External Electric Fields: Modulating the Bond Strengths

Recent years have witnessed various fascinating phenomena arising from the interactions of noncovalent bonds with homogeneous external electric fields (EEFs). Here we performed a computational study to interpret the sensitivity of intrinsic bond strengths to EEFs in terms of steric effect and orbital interactions. The block-localized wavefunction (BLW) method, which combines the advantages of both ab initio valence bond (VB) theory and molecular orbital (MO) theory, and the subsequent energy decomposition (BLW-ED) approach were adopted. The sensitivity was monitored and analyzed using the induced energy term, which is the variation in each energy component along the EEF strength. Systems with single or multiple hydrogen (H) or halogen (X) bond(s) were also examined. It was found that the X-bond strength change to EEFs mainly stems from the covalency change, while generally the steric effect rules the response of H-bonds to EEFs. Furthermore, X-bonds are more sensitive to EEFs, with the key difference between H- and X-bonds lying in the charge transfer interaction. Since phenylboronic acid has been experimentally used as a smart linker in EEFs, switchable sensitivity was scrutinized with the example of the phenylboronic acid dimer, which exhibits two conformations with either antiparallel or parallel H-bonds, thereby, opposite or consistent responses to EEFs. Among the studied systems, the quadruple X-bonds in molecular capsules exhibit remarkable sensitivity, with its interaction energy increased by -95.2 kJ mol^-1 at the EEF strength 0.005 a.u.

Detection and Stabilization of a Previously Unknown Two-Dimensional (Pseudo)polymorph using Lateral Nanoconfinement

We report on the detection and stabilization of a previously unknown two-dimensional (2D) pseudopolymorph of an alkoxy isophthalic acid using lateral nanoconfinement. The self-assembled molecular networks formed by the isophthalic acid derivative were studied at the interface between covalently modified graphite and an organic solvent. When self-assembled on graphite with moderate surface coverage of covalently bound aryl groups, a previously unknown metastable pseudopolymorph was detected. This pseudopolymorph, which was presumably "trapped" in between the surface bound aryl groups, underwent a time-dependent phase transition to the stable polymorph typically observed on pristine graphite. The stabilization of the pseudopolymorph was then achieved by using an alternative nanoconfinement strategy, where the domains of the pseudopolymorph could be formed and stabilized by restricting the self-assembly in nanometer-sized shallow compartments produced by STM-based nanolithography carried out on a graphite surface with a high density of covalently bound aryl groups. These experimental results are supported by molecular mechanics and molecular dynamics simulations, which not only provide important insight into the relative stabilities of the different structures, but also shed light onto the mechanism of the formation and stabilization of the pseudopolymorph under nanoscopic lateral confinement.

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.

2D cocrystal engineering: switching the robust carboxylic acid–pyridine supramolecular heterosynthon via an oriented external electric field

The response of 2D cocrystals with the carboxylic acid–pyridine supramolecular heterosynthon to the oriented external electric field is identified on solid surfaces.

Two-dimensional co-crystallization of two carboxylic acid derivatives having dissimilar symmetries at the liquid/solid interface

By the co-assembly of two carboxylic acids with distinct symmetries and different numbers of carboxyl groups, we obtained two novel cocrystal structures at the n-octanoic acid/HOPG interface, one of which was sustained by unoptimized R22(8) hydrogen bonding. Benefiting from the bias-sensitivity of the BTB (1,3,5-tris(4-carboxyphenyl)benzene) molecule, a structure transition between the cocrystal network and a denser BTB lamella is achieved.

Recent progress in hybrid perovskite solar cells through scanning tunneling microscopy and spectroscopy

Currently, sustainable renewable energy sources are urgently required to fulfill the cumulative energy needs of the world's 7.8 billion population, since the conventional coal and fossil fuels will be exhausted soon. Photovoltaic devices are a direct and efficient means to produce a huge amount of energy to meet these energy targets. In particular, hybrid-perovskite-based photovoltaic devices merit special attention not only due to their exceptional efficiency for generating appreciable energy but also their tunable band gaps and the ease of device fabrication. However, the commercialization of such devices suffers from the instability of the compositional materials. The cause of instability is the perovskite's structure and its morphology at the sub-molecular level; thereby revealing and eliminating these instabilities are a striking challenge. To address this issue, scanning tunneling microscopy/spectroscopy (STM/STS) presents a comprehensive method to allow the visualization of the morphology and electronic structure of materials at atomic-level resolution. Here, we review the recent developments of perovskite-based solar cells (PSCs), the STM/STS analysis of photoactive halide/hybrid and oxide materials, and the real-time STM/STS investigation of electronic structures with defects and traps that are believed to mainly affect device performances. The detailed STM/STS analysis can facilitate a better understanding of the properties of materials at the nanoscale. This informative study may hold great promise to advance the development of stable PSCs under atmospheric conditions.

Nanoscale tailoring of supramolecular crystals via an oriented external electric field

The oriented external electric field of a scanning tunneling microscope (STM) has recently been adapted for controlling the chemical reaction and supramolecular phase transition at surfaces with molecular precision. However, to date, advance controls using such electric-fields for crystal engineering have not been achieved yet. Here, we present how the directional electric-field of an STM can be utilized to harness supramolecular crystallization on a solid surface. We show that a glass-like random-tiling assembly composed of p-terphenyl-3,5,3',5'-tetracarboxylic acid can transform into close-packed periodic assemblies under positive substrate bias conditions at the liquid/solid interface. Importantly, the nucleation and subsequent crystal growth for such field-induced products can be artificially tailored at the early stage in a real-time fashion. Through this method, we were able to produce a two-dimensional supramolecular single crystal. The as-prepared crystals with apparent brightness are ascribed to a spectroscopic feature linked to the electron density of states, which is thus strongly STM bias dependent.

Dynamic Supramolecular Template: Multiple Stimuli-Controlled Size Adjustment of Porous Networks

Dynamically switchable porous networks offer exciting potential in functionalizing surfaces. The structure and morphology of the networks can be controlled by applying external stimuli. Here, a dynamic supramolecular template assembled by 1,3,5-tris(4-carboxyphenyl)benzene (BTB) is successfully achieved at the liquid-solid interface by applying two external stimuli simultaneously. Upon varying the concentration of BTB solution together with switching the polarity of the sample bias, self-assembled monolayers (SAMs) undergo phase transitions twice: an immediate transition from a compact structure to a macroporous (honeycomb) structure as a response to the change in the electric field and a fast-changing transition from the macroporous to a microporous (oblique) structure. With saturated BTB solution, however, the initial compact structure can only transform into the oblique structure after switching the polarity of the sample bias without the appearance of a honeycomb structure. The different phase transitions suggest that the dynamic supramolecular template can only survive at a specific concentration range and is obtainable by performing multiple stimuli simultaneously. Interestingly, introducing a guest molecule to the system can adjust the phase transition process and effectively stabilize the honeycomb structure of BTB. The flexibility associated with the porous networks renders it a dynamic supramolecular template for guest binding.

Dynamic surface-assisted assembly behaviours mediated by external stimuli

Supramolecular self-assembly behaviors on solid substrates have been widely investigated in the last few decades. Owing to the complexity of interfacial assembly systems, the precise regulation of supramolecular nanostructures is still challenging and waits to be solved. The supramolecular nanostructures are governed by non-covalent bonds, so they can be disrupted and influenced by an external environment. In this review, the dynamic supramolecular nanostructures that are mediated by external stimuli containing guest species, light irradiation, temperature and electric field are discussed in detail. The research studies mentioned in this article are all accomplished by STM, and the effects of these external stimuli on the assembled nanostructures have been elucidated exhaustively here.

Electric-field-induced supramolecular phase transitions at the liquid/solid interface: cat-assembly from solvent additives

The electrically triggered phase transformations of trimesic acid can be efficiently promoted to occur in an environment where trace levels of a highly polar solvent additive are present at the liquid/solid interface, as revealed by STM and DFT simulations.

On-Surface Crystallization Behaviors of H-Bond Donor-Acceptor Complexes at Liquid/Solid Interfaces

Two-dimensional (2D) crystallization behaviors of A-TPC _n ( n = 4, 6, 10), T3C₄, and hydrogen-bonded complexes T3C₄@TPC _n ( n = 4, 6, 10) are investigated by means of scanning tunneling microscope (STM) observations and density functional theory (DFT) calculations. The STM observations reveal that A-TPC₄, A-TPC₁₀, and T3C₄ self-organize into dumbbell-shaped structures, well-ordered bright arrays, and zigzag structures, respectively. Interestingly, T3C₄@TPC₁₀ fails to form the cage-ball structure, whereas T3C₄@TPC₄ and T3C₄@TPC₆ co-assemble into cage-ball structures with the same lattice parameters. The filling rates of the balls of these two kinds of cage-ball structures depend heavily on the deposition sequence. As a result, the filling rates of the cages in T3C₄/A-TPC _n ( n = 4, 6) with deposition of T3C₄ anterior to A-TPC _n are higher than those in A-TPC _n/T3C₄ ( n = 4, 6) with the opposite deposition sequence. Furthermore, lattice defects formed by T3C₄ coexist with the cage-ball structures. Moreover, the similar energy per unit area of lattice defects (-0.101 kcal mol^-1 Å^-2) and the two cage-ball networks (-0.194 and -0.208 kcal mol^-1 Å^-2, respectively), illustrating the similar stabilities of lattice defects and cage-ball networks, demonstrates the rationality of lattice defects. Combining STM investigations and DFT calculations, this work could provide a useful approach to investigate the 2D crystallization mechanisms of supramolecular liquid crystals on surfaces.

Tri-Stable Structural Switching in 2D Molecular Assembly at the Liquid/Solid Interface Triggered by External Electric Field

A tri-stable structural switching between different polymorphisms is presented in the 2D molecular assembly of a 5-(benzyloxy)isophthalic acid derivative (BIC-C12) at the liquid/solid interface. The assembled structure of BIC-C12 is sensitive to the applied voltage between the STM tip and the sample surface. A compact lamellar structure is exclusively observed at positive sample bias, while a porous honeycomb structure or a quadrangular structure is preferred at negative sample bias. Selective switching between the lamellar structure and the honeycomb structure or the quadrangular structure is realized by controlling the polarity and magnitude of the sample bias. The transition between the honeycomb structure and the quadrangular structure is, however, absent in the assembly. This tri-stable structural switching is closely related to the molecular concentration in the liquid phase. This result provides insights into the effect of external electric field on molecular assembly and benefits the design and construction of switchable molecular architectures on surfaces.

Synergic Effect: Temperature-Assisted Electric-Field-Induced Supramolecular Phase Transitions at the Liquid/Solid Interface

Using trimesic acid (TMA) as a model system by means of scanning tunneling microscope (STM) equipped with a temperature controller, here, we report a temperature-assisted method to cooperatively control electric-field-induced supramolecular phase transitions at the liquid/solid interface. Octanoic acid is used as a solvent due to its good solubility for TMA and its less complicated pattern formed under negative STM bias (e.g., only chicken-wire polymorphs existing). At positive substrate bias, STM revealed that TMA assembly based on temperature modulations underwent phase transitions from a porous (22 °C) to a flower (45 °C) and further to a zigzag (68 °C) structure. The transitions are ascribed to the partial deprotonation of the carboxyl groups of TMA. Both the temperature and electrical polarity of the substrate are crucial, i.e., the transitions only take place at positive substrate bias and elevated temperatures. Molecular mechanics simulations were carried out to calculate the temperature and electric field dependence of the adsorption enthalpy and free energy of the chicken-wire assembly of TMA on the two layers of graphene surface. The calculated decrease in adsorption enthalpy with the increase of temperature and electric field values that causes the TMA chicken-wire assembly to be less stable is proposed to promote the occurrence of the phase transition observed by STM. This study paves the way toward program-controlled supramolecular phase switching via the synergic effect of electrical and thermal stimuli.

Dependence of the photo-response behavior of self-assembled 2D Azo-derivatives on the functional groups on a solid surface

Here, by means of scanning tunneling microscopy, we found that 2D self-assembled monolayers of four azobenzene derivatives exhibited different isomerization behaviors when taken from dark to irradiation conditions.

Two-dimensional self-assembled nanostructures of nucleobases and their related derivatives on Au(111)

The construction of two-dimensional (2D) self-assembled nanostructures has been one of the considerably interesting areas of on-surface chemistry in the past few decades, and has benefited from the rapid development and improvement of scanning probe microscopy techniques. In this research field, many attempts have been made in the controllable fabrication of well-ordered and multifunctional surface nanostructures, which attracted interest because of the prospect for artificial design of functional molecular nanodevices. DNA and RNA are considered to be programmable self-assembly systems and it is possible to use their base sequences to encode instructions for assembly in a predetermined fashion at the nanometer scale. As important constituents of nucleic acids, nucleobases, with intrinsic functional groups for hydrogen bonding, coordination bonding, and electrostatic interactions, can be employed as a potential system for the versatile construction of various biomolecular nanostructures, which may be used to structure the self-assembly of DNA-based artificial molecular constructions and play an important role in novel biosensors based on surface functionalization. In this article, we will review the recent progress of on-surface self-assembly of nucleobases and their derivatives together with different reactants (e.g., metals, halogens, salts and water), and as a result, various 2D surface nanostructures are summarized.

Bias-induced conformational switching of supramolecular networks of trimesic acid at the solid-liquid interface

Using the tip of a scanning tunneling microscope, an electric field-induced reversible phase transition between two planar porous structures ("chickenwire" and "flower") of trimesic acid was accomplished at the nonanoic acid/highly oriented pyrolytic graphite interface. The chickenwire structure was exclusively observed for negative sample bias, while for positive sample bias only the more densely packed flower structure was found. We suggest that the slightly negatively charged carboxyl groups of the trimesic acid molecule are the determining factor for this observation: their adsorption behavior varies with the sample bias and is thus responsible for the switching behavior.

Electrostatically Driven Guest Binding in a Self-Assembled Porous Network at the Liquid/Solid Interface

We present here the construction of a self-assembled two-dimensional (2D) porous monolayer bearing a highly polar 2D space to study guest co-adsorption through electrostatic interactions at the liquid/solid interface. For this purpose, a dehydrobenzo[12]annulene (DBA) derivative, DBA-TeEG, having tetraethylene glycol (TeEG) groups at the end of the three alternating alkoxy chains connected by p-phenylene linkers was synthesized. As a reference host molecule, DBA-C10, having nonpolar C₁₀ alkyl chains at three alternating terminals, was employed. As guest molecules, hexagonal phenylene-ethynylene macrocycles (PEMs) attached by triethylene glycol (TEG) ester and hexyl ester groups, PEM-TEG and PEM-C6, respectively, at each vertex of the macrocyclic periphery were used. Scanning tunneling microscopy observations at the 1,2,4-trichlorobenzene/highly oriented pyrolytic graphite interface revealed that PEM-TEG was immobilized in the pores formed by DBA-TeEG at higher probability because of electrostatic interactions such as dipole-dipole and hydrogen bonding interactions between oligoether units of the host and guest, in comparison to PEM-C6 with nonpolar groups. These observations are discussed based on molecular mechanics simulations to investigate the role of the polar functional groups. When a nonpolar host matrix formed by DBA-C10 was used, however, only phase separation and preferential adsorption were observed; virtually no host-guest complexation was discernible. This is ascribed to the strong affinity between the guest molecules which form by themselves densely packed van der Waals networks on the surface.

Template-Assisted Proximity for Oligomerization of Fullerenes

Demonstrated herein is an unprecedented porous template-assisted reaction at the solid-liquid interface involving bond formation, which is typically collision-driven and occurs in the solution and gas phases. The template is a TMA (trimesic acid) monolayer with two-dimensional pores that host fullerenes, which otherwise exhibit an insignificant affinity to an undecorated graphite substrate. The confinement of C₈₄ units in the TMA pores formulates a proximity that is ideal for bond formation. The oligomerization of C₈₄ is triggered by an electric pulse via a scanning tunneling microscope tip. The spacing between C₈₄ moieties becomes 1.4 nm, which is larger than the edge-to-edge diameter of 1.1-1.2 nm of C₈₄ due to the formation of intermolecular single bonds. In addition, the characteristic mass-to-charge ratios of dimers and trimers are observed by mass spectrometry. The experimental findings shed light on the active role of spatially tailored templates in facilitating the chemical activity of guest molecules.

Host-guest chemistry in two-dimensional supramolecular networks

Nanoporous supramolecular networks physisorbed on solid surfaces have been extensively used to immobilize a variety of guest molecules. Host-guest chemistry in such two-dimensional (2D) porous networks is a rapidly expanding field due to potential applications in separation technology, catalysis and nanoscale patterning. Diverse structural topologies with high crystallinity have been obtained to capture molecular guests of different sizes and shapes. A range of non-covalent forces such as hydrogen bonds, van der Waals interactions, coordinate bonds have been employed to assemble the host networks. Recent years have witnessed a surge in the activity in this field with the implementation of rational design strategies for realizing controlled and selective guest capture. In this feature article, we review the development in the field of surface-supported host-guest chemistry as studied by scanning tunneling microscopy (STM). Typical host-guest architectures studied on solid surfaces, both under ambient conditions at the solution-solid interface as well as those formed at the ultrahigh vacuum (UHV)-solid interface, are described. We focus on isoreticular host networks, hosts functionalized pores and dynamic host-guest systems that respond to external stimuli.

Superstructure manipulation and electronic measurement of monolayers comprising discotic liquid crystals with intrinsic dipole moment using STM/STS

In this work, we studied the discotic liquid crystals (DLCs) of dibenzo[a,c]phenazine at the liquid–solid interface using scanning tunnelling microscopy/spectroscopy, by which we show how to tailor the DLC assemblies and in turn their electron-transfer efficiency.

Phase separation and selective guest–host binding in multi-component supramolecular self-assembly on Au(111)

We find a phase separation and selective guest–host inclusion in the self-assembly of trimesic acid, benzenetribenzoic acid and coronene on Au(111).

Nanoscale Control over the Mixing Behavior of Surface-Confined Bicomponent Supramolecular Networks Using an Oriented External Electric Field

Strong electric fields are known to influence the properties of molecules as well as materials. Here we show that by changing the orientation of an externally applied electric field, one can locally control the mixing behavior of two molecules physisorbed on a solid surface. Whether the starting two-component network evolves into an ordered two-dimensional (2D) cocrystal, yields an amorphous network where the two components phase separate, or shows preferential adsorption of only one component depends on the solution stoichiometry. The experiments are carried out by changing the orientation of the strong electric field that exists between the tip of a scanning tunneling microscope and a solid substrate. The structure of the two-component network typically changes from open porous at negative substrate bias to relatively compact when the polarity of the applied bias is reversed. The electric-field-induced mixing behavior is reversible, and the supramolecular system exhibits excellent stability and good response efficiency. When molecular guests are adsorbed in the porous networks, the field-induced switching behavior was found to be completely different. Plausible reasons behind the field-induced mixing behavior are discussed.

Air/Liquid Interfacial Nanoassembly of Molecular Building Blocks into Preferentially Oriented Porous Organic Nanosheet Crystals via Hydrogen Bonding

Nanosheets with highly regulated nanopores are ultimately thin functional materials for diverse applications including molecular separation and detection, catalysis, and energy conversion and storage. However, their availability has hitherto been restricted to layered parent materials, covalently bonded sheets, which are layered via relatively weak electrostatic interactions. Here, we report a rational bottom-up methodology that enables nanosheet creation beyond the layered systems. We employ the air/liquid interface to assemble a triphenylbenzene derivative into perfectly oriented highly crystalline noncovalent-bonded organic nanosheets under ambient conditions. Each molecular building unit connects laterally by hydrogen bonding, endowing the nanosheets with size- and position-regulated permanent nanoporosity, as established by in situ synchrotron X-ray surface crystallography and gas sorption measurements. Notably, the nanosheets are constructed specifically by interfacial synthesis, which suppresses the intrinsic complex interpenetrated structure of the bulk crystal. Moreover, they possess exceptional long-term and thermal stability and are easily transferrable to numerous substrates without loss of structural integrity. Our work shows the power of interfacial synthesis using a suitably chosen molecular component to create two-dimensional (2D) nanoassemblies not accessible by conventional bulk crystal exfoliation techniques.

Sequential nested assembly at the liquid/solid interface

Studying the stepwise assembly of a four component hybrid structure on Au(111)/mica, the pores of a hydrogen bonded bimolecular network of 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) and 1,3,5-triazine-2,4,6-triamine (melamine) were partitioned by three and four-armed molecules based on oligo([biphenyl]-4-ylethynyl)benzene, followed by the templated adsorption of either C₆₀ fullerene or adamantane thiol molecules. The characterisation by ambient scanning tunneling microscopy (STM) reveals that the pore modifiers exhibit dynamics which pronouncedly depend on the molecular structure. The three-armed molecule 1,3,5-tris([1,1'-biphenyl]-4-ylethynyl)benzene (3BPEB) switches between two symmetry equivalent configurations on a time scale fast compared to the temporal resolution of the STM. Derivatisation of 3BPEB by hydroxyl groups substantially reduces the switching rate. For the four-armed molecule configurational changes are observed only occasionally. The observation of isolated fullerenes and small clusters of adamantane thiol molecules, which are arranged in a characteristic fashion, reveals the templating effect of the trimolecular supramolecular network. However, the fraction of compartments filled by guest molecules is significantly below one for both the thermodynamically controlled adsorption of C₆₀ and the kinetically controlled adsorption of the thiol with the latter causing partial removal of the pore modifier. The experiments, on the one hand, demonstrate the feasibility of templating by nested assembly but, on the other hand, also pinpoint the requirement for the energy landscape to be tolerant to variations in the assembly process.

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.

2D self-assembly of phenylene-vinylene tectons at the liquid-highly oriented pyrolytic graphite interface: from chain length effects to anisotropic guest-host dynamics

Here we report the synthesis and characterization of a series of new phenylene-vinylene tectons. The study by scanning tunneling microscopy of their supramolecular self-assembly at the interface between a phenyloctane solution and highly oriented pyrolytic graphite  demonstrates that variation of concentration and length of alkyl chains led to the formation of different networks, a compact one and a nanoporous one, with a fine control of the lattice parameters. The study of guest-host properties of the nanoporous network revealed a selectivity toward guest compounds according to their shape and size. Moreover, the statistical analysis of pore-to-pore guest dynamics evidences an anisotropic diffusion process.

Physical and chemical model of ion stability and movement within the dynamic and voltage-gated STM tip–surface tunneling junction

The interaction and mobility of ions in complex systems are fundamental to processes throughout chemistry, biology, and physics. However, nanoscale characterization of ion stability and migration remains poorly understood. Here, we examine ion movements to and from physisorbed molecular receptors at solution–graphite interfaces by developing a theoretical model alongside experimental scanning tunneling microscopy (STM) results. The model includes van der Waals forces and electrostatic interactions originating from the surface, tip, and physisorbed receptors, as well as a tip–surface electric field arising from the STM bias voltage (V_b). Our model reveals how both the electric field and tip–surface distance, d_tip, can influence anion stability at the receptor binding sites on the surface or at the STM tip, as well as the size of the barrier for anion transitions between those locations. These predictions agree well with prior and new STM results from the interactions of anions with aryl-triazole receptors that order into functional monolayers on graphite. Scanning produces clear resolution at large magnitude negative surface biases (−0.8 V) while resolution degrades at small negative surface biases (−0.4 V). The loss in resolution arises from frequent tip retractions assigned to anion migration within the tip–surface tunneling region. This experimental evidence in combination with support from the model demonstrates a local voltage gating of anions with the STM tip inside physisorbed receptors. This generalized model and experimental evidence may help to provide a basis to understand the nanoscale details of related chemical transformations and their underlying thermodynamic and kinetic preferences.

The STM bias voltage-dependent polymorphism of a binary supramolecular network

We control complex multicomponent switches by tuning the local electric field at the liquid/solid interface.

Controllable Orientation of Ester-Group-Induced Intermolecular Halogen Bonding in a 2D Self-Assembly

Halogen bonding with high specificity and directionality in the geometry has proven to be an important type of noncovalent interaction to fabricate and control 2D molecular architectures on surfaces. Herein, we first report how the orientation of the ester substituent for thienophenanthrene derivatives (5,10-DBTD and 5,10-DITD) affects positive charge distribution of halogens by density functional theory, thus determining the formation of an intermolecular halogen bond and different self-assembled patterns by scanning tunneling microscopy. The system presented here mainly includes heterohalogen X···O═C and X···S halogen bonds, H···Br and H···O hydrogen bonds, and I···I interaction, where the directionality and strength of such weak bonds determine the molecular arrangement by varying the halogen substituent. This study provides a detailed understanding of the role of ester orientation, concentration, and solvent effects on the formation of halogen bonds and proves relevant for identification of multiple halogen bonding in supramolecular chemistry.

1,3,5-Benzenetribenzoic Acid on Cu(111) and Graphene/Cu(111): A Comparative STM Study

The self-assembly of 1,3,5-benzenetribenzoic acid (BTB) molecules on both Cu(111) and epitaxial graphene grown on Cu(111) were studied by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) under ultrahigh vacuum conditions. On Cu(111), the BTB molecules were found to mainly arrange in close-packed structures through H-bonding between the (partially) deprotonated carboxylic acid groups. In addition, porous structures formed by intact BTB molecules-and also based on H-bonding-were observed. On graphene grown on Cu(111) the BTB molecules mainly form porous structures accompanied by small patches of disordered close-packed structures. Upon annealing, BTB adsorbed on Cu(111) is fully deprotonated and arranges in the close-packed structure while in contrast on graphene/Cu(111) the porous network is exclusively formed. This shows that the molecular self-assembly behavior is highly dependent on the first substrate layer: one graphene layer is sufficient to considerably alter the interplay of molecule substrate and intermolecular interactions in favor of the latter interactions.