Concentration-Dependent Supramolecular Engineering of Hydrogen-Bonded Nanostructures at Surfaces: Predicting Self-Assembly in 2D

Self-Adaptive Aromatic Cation-π Driven Dimensional Polymorphism in Supramolecular Polymers for the Photocatalytic Oxidation and Separation of Aromatic/Cyclic Aliphatic Compounds

The phenomenon of polymorphism is ubiquitous in nature, the controlled manipulation of which not only increases our ontological understanding of nature but also facilitates the conceptualization and realization of novel functional materials. However, achieving targeted polymorphism in supramolecular assemblies (SAs) remains a formidable challenge, largely because of the constraints inherent in controlling the specific binding motifs of noncovalent interactions. Herein, we propose self-adaptive aromatic cation-π binding motifs to construct polymorphic SAs in both the solid and solution states. Using distinct discrete cation-π-cation and long-range cation-π binding motifs enables control of the self-assembly directionality of a C2h-symmetric bifunctional monomer, resulting in the successful formation of both two-dimensional and three-dimensional crystalline SAs (2D-CSA and 3D-CSA). The differences in the molecular packing of 3D-CSA compared with that of 2D-CSA significantly improve the charge separation and carrier mobility, leading to enhanced photocatalytic activity for the aerobic oxidation of thioanisole to methyl phenyl sulfoxide (yield of 99 % vs 57 %). 2D-CSA, which has a vertical extended structure with favorable stronger interaction with toluene though face-to-face cation-π interactions than methylcyclohexane, shows higher toluene/methylcyclohexane separation efficiency than 3D-CSA (96.9 % for 2D-CSA vs 56.3 % for 3D-CSA).

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.

On the origin of cooperativity effects in the formation of self-assembled molecular networks at the liquid/solid interface

In this work we investigate the behaviour of molecules at the nanoscale using scanning tunnelling microscopy in order to explore the origin of the cooperativity in the formation of self-assembled molecular networks (SAMNs) at the liquid/solid interface. By studying concentration dependence of alkoxylated dimethylbenzene, a molecular analogue to 5-alkoxylated isophthalic derivatives, but without hydrogen bonding moieties, we show that the cooperativity effect can be experimentally evaluated even for low-interacting systems and that the cooperativity in SAMN formation is its fundamental trait. We conclude that cooperativity must be a local effect and use the nearest-neighbor Ising model to reproduce the coverage vs. concentration curves. The Ising model offers a direct link between statistical thermodynamics and experimental parameters, making it a valuable tool for assessing the thermodynamics of SAMN formation.

On-Surface Synthesis of a Radical 2D Supramolecular Organic Framework

The design of supramolecular organic radical cages and frameworks is one of the main challenges in supramolecular chemistry. Their interesting material properties and wide applications make them very promising for (photo)redox catalysis, sensors, or host-guest spin-spin interactions. However, the high reactivity of radical organic systems makes the design of such supramolecular radical assemblies challenging. Here, we report the on-surface synthesis of a purely organic supramolecular radical framework on Au(111), by combining supramolecular and on-surface chemistry. We employ a tripodal precursor, functionalized with 7-azaindole groups that, catalyzed by a single gold atom on the surface, forms a radical molecular product constituted by a π-extended fluoradene-based radical core. The radical products self-assemble through hydrogen bonding, leading to extended 2D domains ordered in a Kagome-honeycomb lattice. This approach demonstrates the potential of on-surface synthesis for developing 2D supramolecular radical organic chemistry.

Anisotropic functionalized platelets: percolation, porosity and network properties

Anisotropic functionalized platelets are able to model the assembly behaviour of molecular systems in two dimensions thanks to the unique combination of steric and bonding constraints. The assembly scenarios can vary from open to close-packed crystals, finite clusters and chains, according to the features of the imposed constraints. In this work, we focus on the assembly of equilibrium networks. These networks can be seen as disordered, porous monolayers and can be of interest for instance in nano-filtration and optical applications. We investigate the formation and properties of two dimensional networks from shape anisotropic colloids functionalized with four patches. We characterize the connectivity properties, the typical local bonding motives, as well as the geometric features of the emerging networks for a large variety of different systems. Our results show that networks of shape anisotropic colloids assemble into highly versatile network topologies, that may be utilized for applications at the nanoscale.

From One-Dimensional Disordered Racemate to Ordered Racemic Conglomerates through Metal-Coordination-Driven Self-Assembly at the Liquid-Solid Interface

In recent years, there has been significant focus on investigating and controlling chiral self-assembly, specifically in the context of enantiomeric separation. This study explores the self-assembly behavior of 4-dodecyl-3,6-di(2-pyridyl)pyridazine (DPP-C12) at the interface between heptanoic acid (HA) and highly oriented pyrolytic graphite (HOPG) using a combination of scanning tunneling microscopy (STM) and multiscale molecular modeling. The self-assembled monolayer structure formed by DPP-C12 is periodic in one direction, but aperiodic in the direction orthogonal to it. These structures resemble 1D disordered racemic compounds. Upon introducing palladium [Pd(II)] ions, complexing with DPP-C12, these 1D disordered racemic compounds spontaneously transform into 2D racemic conglomerates, which is rationalized with the assistance of force-field simulations. Our findings provide insights into the regulation of two-dimensional chirality.

Tensor renormalization group study of orientational ordering in simple models of adsorption monolayers

A simple lattice model of the orientational ordering in organic adsorption layers that considers the directionality of intermolecular interactions is proposed. The symmetry and the number of rotational states of the adsorbed molecule are the main parameters of the model. The model takes into account both the isotropic and directional contributions to the molecule-molecule interaction potential. Using several special cases of this model, we have shown that the tensor renormalization group (TRG) approach can be successfully used for the analysis of orientational ordering in organic adsorption layers with directed intermolecular interactions. Adsorption isotherms, potential energy, and entropy have been calculated for the model adsorption layers differing in the molecule symmetry and the number of rotational states. The calculated thermodynamic characteristics show that entropy effects play a significant role in the self-assembly of dense phases of the molecular layers. All the results obtained with the TRG have been verified by the standard Monte Carlo method. The proposed model reproduces the main features of the phase behavior of the real adsorption layers of benzoic, terephthalic, and trimesic acids on a homogeneous surface of metal single crystals and graphite.

Structure transformation from Sierpiński triangles to chains assisted by gas molecules

Reversible transformations between fractals and periodic structures are of fundamental importance for understanding the formation mechanism of fractals. Currently, it is still a challenge to controllably achieve such a transformation. We investigate the effect of CO and CO2 molecules on Sierpiński triangles (STs) assembled from Fe atoms and 4,4″-dicyano-1,1':3',1″-terphenyl (C3PC) molecules on Au surfaces. Using scanning tunneling microscopy, we discover that the gas molecules induce a transition from STs into 1D chains. Based on density functional theory modeling, we propose that the atomistic mechanism involves the transformation of a stable 3-fold coordination Fe(C3PC)3 motif to Fe(C3PC)4 with an axially bonded CO molecule. CO2 causes the structural transformation through a molecular catassembly process.

Polygonal tessellations as predictive models of molecular monolayers

Molecular self-assembly plays a very important role in various aspects of technology as well as in biological systems. Governed by covalent, hydrogen or van der Waals interactions-self-assembly of alike molecules results in a large variety of complex patterns even in two dimensions (2D). Prediction of pattern formation for 2D molecular networks is extremely important, though very challenging, and so far, relied on computationally involved approaches such as density functional theory, classical molecular dynamics, Monte Carlo, or machine learning. Such methods, however, do not guarantee that all possible patterns will be considered and often rely on intuition. Here, we introduce a much simpler, though rigorous, hierarchical geometric model founded on the mean-field theory of 2D polygonal tessellations to predict extended network patterns based on molecular-level information. Based on graph theory, this approach yields pattern classification and pattern prediction within well-defined ranges. When applied to existing experimental data, our model provides a different view of self-assembled molecular patterns, leading to interesting predictions on admissible patterns and potential additional phases. While developed for hydrogen-bonded systems, an extension to covalently bonded graphene-derived materials or 3D structures such as fullerenes is possible, significantly opening the range of potential future applications.

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.

Defect-engineered surfaces to investigate the formation of self-assembled molecular networks

Herein we report the impact of covalent modification (grafting), inducing lateral nanoconfinement conditions, on the self-assembly of a quinonoid zwitterion derivative into self-assembled molecular networks at the liquid/solid interface. At low concentrations where the compound does not show self-assembly behaviour on bare highly oriented pyrolytic graphite (HOPG), close-packed self-assembled structures are visualized by scanning tunneling microscopy on covalently modified HOPG. The size of the self-assembled domains decreases with increasing the density of grafted molecules, i.e. the molecules covalently bound to the surface. The dynamics of domains are captured with molecular resolution, revealing not only time-dependent growth and shrinkage processes but also the orientation conversion of assembled domains. Grafted pins play a key role in initiating the formation of on-surface molecular self-assembly and their stabilization, providing an elegant route to study various aspects of nucleation and growth processes of self-assembled molecular networks.

Study on the oriented self-assembly of cuprous oxide micro-nano cubes and its application as a non-enzymatic glucose sensor

Herein, cuprous oxide (Cu₂O) micro-nano cubes were successfully synthesized via a seed-medium process. It is worth noting that the microcubes were formed by oriented self-assembly of 2 × 2 × 2 nanocubes. The oriented self-assembly process can be effective controlled by simply adjusting the concentration of reactants. What's more, the obtained samples were applied for non-enzymatic glucose detection and exhibited excellent performance. The Cu₂O nanocubes obtained at the highest concentration exhibited the highest sensitivity (2864 μAmM^-1cm^-2), while the Cu₂O microcubes obtained at the lowest concentration shared the widest linear range (up to 10.65 mM) and lowest limit of detection (LOD, 0.87 μΜ). The acceptable anti-interference ability, excellent stability together with the practical application ability make our obtained electrodes a new strategy for monitoring glucose in biological and food samples.

Thermodynamics of self-assembled molecular layers of trimesic acid from fields-supported kinetic Monte Carlo simulation

A technique has been developed for calculating the thermodynamic characteristics of rigid self-assembled organic adsorption layers and the parameters of polymorphic transitions using two types of external fields and the kinetic Monte Carlo method.

"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.

Simple lattice model of self-assembling metal-organic layers of pyridyl-substituted porphyrins and copper on Au(111) surface

A simple lattice model of metal-organic adsorption layers self-assembling on a Au(111) surface and based on pyridyl-substituted porphyrins differing in the number of functional groups and their position has been proposed. The model has been parameterized using DFT methods. The ground state analysis of the considered model demonstrates the variety of surface-confined metal-organic networks (SMONs) containing square, linear, and discrete elements appearing in the adsorption layer depending on the partial pressure of the components. The SMONs comprising more symmetrical molecules with a greater number of pyridyl substituents in the porphyrin core exhibit more diverse phase behavior. Structures of the phase diagrams were verified at nonzero temperatures using Grand Canonical Monte Carlo simulations. It was found that the continuous SMONs have higher thermal stability at relatively low partial pressures of the organic component, while the linear and discrete SMONs are more thermally stable at high pressure. Depending on the partial pressure of the organic component, thermal destruction of continuous SMONs occur either through the formation of defects/islands having structures of the linear SMONs, or through the sublimation of individual structural elements. Melting of linear SMONs reveals the appearance of 2D pores or islands of a purely organic phase. The latter fact is confirmed by the experimentally observed coexistence of these phases.

Progress in self-assemblies of macrocycles at the liquid/solid interface

Macrocyclic self-assemblies have gained great interest for diversified structures and potential applications, such as catalysis, magnetism, photovoltaic devices, organic light-emitting diodes. Macrocycles can present regular assembly systems at the liquid/solid interface due to theπ-conjugated structures. Furthermore, suitable guest molecules can be selected for constructing multi-component supramolecular co-assemblies. This review mainly summarizes macrocyclic self-assembly structures with different shapes in recent years. All of the studies are completed with the assistance of scanning tunneling microscope at the liquid/solid interface.

Designing 2D covalent networks with lattice Monte Carlo simulations: precursor self-assembly

Organic synthesis reactions in the adsorbed phase have been recently an intensively studied topic in heterogeneous catalysis and material engineering. One of such processes is the Ullmann coupling in which halogenated organic monomers are transformed into covalently bonded polymeric structures. In this work, we use the lattice Monte Carlo simulation method to study the on-surface self-assembly of organometallic precursor architectures comprising tetrasubstituted naphthalene building blocks with differently distributed halogen atoms. In the coarse grained approach adopted herein the molecules and metal atoms were modeled by discrete segments, two connected and one, respectively, placed on a triangular lattice representing a (111) metallic surface. Our simulations focused on the influence of the intramolecular distribution of the substituents on the morphology of the resulting superstructures. Special attention was paid to the molecules that create porous networks characterized by long-range order. Moreover, the structural analysis of the assemblies comprising prochiral building blocks was made by running simulations for the corresponding enantiopure and racemic adsorbed systems. The obtained results demonstrated the possibility of directing the on-surface self-assembly towards networks with controllable pore shape and size. These findings can be helpful in designing covalently bonded 2D superstructures with predefined architecture and functions.

Application of Scanning Tunneling Microscopy in Electrocatalysis and Electrochemistry

Abstract

Scanning tunneling microscopy (STM) has gained increasing attention in the field of electrocatalysis due to its ability to reveal electrocatalyst surface structures down to the atomic level in either ultra-high-vacuum (UHV) or harsh electrochemical conditions. The detailed knowledge of surface structures, surface electronic structures, surface active sites as well as the interaction between surface adsorbates and electrocatalysts is highly beneficial in the study of electrocatalytic mechanisms and for the rational design of electrocatalysts. Based on this, this review will discuss the application of STM in the characterization of electrocatalyst surfaces and the investigation of electrochemical interfaces between electrocatalyst surfaces and reactants. Based on different operating conditions, UHV-STM and STM in electrochemical environments (EC-STM) are discussed separately. This review will also present emerging techniques including high-speed EC-STM, scanning noise microscopy and tip-enhanced Raman spectroscopy.

Graphic Abstract

Figure-eight arylene ethynylene macrocycles: facile synthesis and specific binding behavior toward Hg2+

Two figure-eight arylene ethynylene macrocycles (AEMs) were synthesized from non-helical precursors and the figure-eight shape was clearly imaged by STM.

Trapping a pentagonal molecule in a self-assembled molecular network: an alkoxylated isosceles triangular molecule does the job

We herein report a unique example of on-surface adaptive self-assembly. A pentagon-shaped macrocycle, cyclic [5]meta-phenyleneacetylene [5]CMPA, is trapped by the adaptive supramolecular network formed by an isosceles triangular molecule, alkoxy substituted dehydrobenzo[14]annulene [14]ISODBA at the liquid/graphite interface, leading to a highly ordered and large-area bicomponent self-assembled molecular network (SAMN), as revealed by scanning tunneling microscopy (STM).

Synthesis and photophysics of new pyridyl end-capped 3D-dithia[3.3]paracyclophane-based Janus tectons: surface-confined self-assembly of their model pedestal on HOPG

Once synthesized, these new tectons demonstrated both ionic and coordination bonding. Surprisingly, P forms a quasi-square self-assembly independently of the underlying HOPG lattice.

Design of Patchy Rhombi: From Close-Packed Tilings to Open Lattices

In the realm of functional materials, the production of two-dimensional structures with tunable porosity is of paramount relevance for many practical applications: surfaces with regular arrays of pores can be used for selective adsorption or immobilization of guest units that are complementary in shape and/or size to the pores, thus achieving, for instance, selective filtering or well-defined responses to external stimuli. The principles that govern the formation of such structures are valid at both the molecular and the colloidal scale. Here we provide simple design directions to combine the anisotropic shape of the building units-either molecules or colloids-and selective directional bonding. Using extensive computer simulations, we show that regular rhombic platelets decorated with attractive and repulsive interaction sites form specific tilings, going smoothly from close-packed arrangements to open lattices. The rationale behind the rich tiling scenario observed can be described in terms of steric incompatibilities, unsatisfied bonding geometries, and interplays between local and long-range order.

Temperature-induced molecular reorganization on Au(111) driven by oligomeric defects

The formation of ordered molecular structures on surfaces is determined by the balance between molecule-molecule and molecule-substrate interactions. Whether the aggregation process is guided by non-covalent forces or on-surface reactions, a deeper understanding of these interactions is pivotal to formulating a priori predictions of the final structural features and the development of bottom-up fabrication protocols. Theoretical models of molecular systems corroborate the information gathered through experimental observations and help explain the thermodynamic factors that underpin on-surface phase transitions. Here, we report a scanning tunneling microscopy investigation of a tribromo-substituted heterotriangulene on the Au(111) surface, which initially forms an extended close-packed ordered structure stabilized by BrBr halogen bonds when deposited at room temperature. X-ray photoelectron spectroscopy reveals that annealing the self-assembled layer induces a fraction of the molecular precursors to partially dehalogenate that in turn leads to the formation of a less stable BrO non-covalent network which coexists with the short oligomers. Density functional theory (DFT) and Monte Carlo (MC) simulations illustrate how dimer moieties act as defects whose steric hindrance prevents the retention of the more stable configuration. A small number of dimers is sufficient to drive the molecular reorganization into a lower cohesive energy phase. Our study shows the importance of a combined DFT - MC approach to understand the evolution of molecular systems on substrates.

2D Co-crystallization of molecular homologues promoted by size complementarity of the alkyl chains at the liquid/solid interface

Co-crystallization of organic molecules is an important strategy for the fabrication of molecular materials. In this contribution, we investigated the mixing behavior of 5-(benzyloxy)-isophthalic acid homologues (BIC-Cn, n = 6, 8, 10, 12, and 14) at the liquid/solid interface using a scanning tunneling microscope. Deposition of the single component of BIC-Cn always results in typical honeycomb networks, whereas co-deposition of two BIC-Cn homologues leads to hybrid double-walled honeycomb networks or phase separation depending on the difference in the length of their alkyl chains. 2D co-crystallization can only be realized for BIC-C6/BIC-C10 or BIC-C8/BIC-C12 which have a four-methyl unit difference in their alkyl chains. The size complementarity of the alkyl chains in the two components suggests that it is responsible for the 2D co-crystallization, though hydrogen bonding contributes a lot both to the pristine honeycomb network and to the hybrid co-crystal. This result is of importance for understanding the role of van der Waals interaction and its interplay with hydrogen bonding in 2D co-crystallization.

Surface-Confined Self-Assembly of Asymmetric Tetratopic Molecular Building Blocks

Surface-confined self-assembly of functional molecular building blocks has recently been widely used to create low-dimensional, also covalent, superstructures with tailorable geometry and physicochemical properties. In this contribution, using the lattice Monte Carlo simulation method, we demonstrate how the structure-property relation can be established for the 2D self-assembly of a model tetrapod molecule with reduced symmetry. To that end, a rigid functional unit comprising a few interconnected segments arranged in different tetrapod shapes was used and its self-assembly on a triangular lattice representing a (111) crystal surface was simulated. The results of our calculations show strong dependence of the structure formation on the molecular symmetry, in particular on the (pro)chiral nature of the building block. The simulations predicted the formation of unusual ordered racemic networks with unique aperiodic spatial distribution of the surface enantiomers. Molecular symmetry was also found to have significant influence on the enantiopure self-assembly which resulted in the Kagome and brickwall networks and other less ordered extended superstructures with parallelogram pores. The theoretical findings of this contribution can be relevant to designing and on-surface synthesis of molecular superstructures with predefined geometries and functions. In particular, the predicted molecular architectures can stimulate experimental efforts to fabricate and explore new nanostructures, for example graphitic, having the composition and geometry proposed in our study.

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.

Hexagonal array formation by intermolecular halogen bonding using a binary blend of linear building blocks: STM study

Hexagonal arrays were fabricated via intermolecular halogen bonding between two linear molecular building blocks in a bicomponent blend. The substitution position of the pyridine N atom involved in the halogen bond plays an important role in the formation of the hexagonal structures.

On-surface self-assembly of tetratopic molecular building blocks

Self-assembly of functional molecules on solid substrates has recently attracted special attention as a versatile method for the fabrication of low dimensional nanostructures with tailorable properties. In this contribution, using theoretical modeling, we demonstrate how the architecture of 2D molecular assemblies can be predicted based on the individual properties of elementary building blocks at play. To that end a model star-shaped tetratopic molecule is used and its self-assembly on a (111) surface is simulated using the lattice Monte Carlo method. Several test cases are studied in which the molecule bears terminal arm centers providing interactions with differently encoded directionality. Our theoretical results show that manipulation of the interaction directions can be an effective way to direct the self-assembly towards extended periodic superstructures (2D crystals) as well as to create assemblies characterized by a lower degree of order, including glassy overlayers and quasi one-dimensional molecular connections. The obtained structures are described and classified with respect to their main geometric parameters. A small library of the tetratopic molecules and the corresponding superstructures is provided to categorize the structure-property relationship in the modeled systems. The results of our simulations can be helpful to 2D crystal engineering and surface-confined polymerization techniques as they give hints on how to functionalize tetrapod organic building blocks which would be able to create superstructures with predefined spatial organization and range of order.

Concentration-dependent supramolecular patterns of C3 and C2 symmetric molecules at the solid/liquid interface

Here we report on a scanning tunnelling microscopy (STM) investigation on the self-assembly of C₃- and C₂-symmetric molecules at the solution/graphite interface. 1,3,5-tris((E)-2-(pyridin-4-yl)vinyl)benzene and 1,1,2,2-tetrakis(4-(pyridin-4-yl)phenyl)ethane are used as model systems. These molecules displayed a concentration dependent self-assembly behaviour on graphite, resulting in highly ordered supramolecular structures, which are stabilized jointly by van der Waals substrate-adsorbate interactions and in-plane intermolecular H-bonding. Denser packing is obtained when applying a relatively high concentration solution to the basal plane of the surface whereas a less dense porous network is observed upon lowering the concentration. We show that the molecular conformation does not influence the stability of the self-assembly and a twisted molecule can pack into dense and porous architectures under the concentration effect.

Self-assembly of hairy disks in two dimensions - insights from molecular simulations

We report the results of large scale molecular dynamics simulations conducted for sparsely grafted disks in two-dimensional systems. The main goal of this work is to show how the ligand mobility influences the self-assembly of particles decorated with short chains. We also analyze the impact of the chain length on the structure of dense phases. A crossover between the systems controlled by the core-core or by the segment-segment interactions is discussed. We prove that the ligand mobility determines the structure of the system. The particles with fixed tethers are found to order into different structures, an amorphous phase, hexagonal or honeycomb lattices, and a "spaghetti"-like phase containing single strings of cores, depending on the length of attached chains. The disks with mobile monomers assemble into a hexagonal structure, while the particles with longer mobile chains attached to them form a lamellar phase consisting of double strings of cores.

Harnessing complexity in molecular self-assembly using computer simulations

In molecular self-assembly, hundreds of thousands of freely-diffusing molecules associate to form ordered and functional architectures in the absence of an actuator. This intriguing phenomenon plays a critical role in biology and has become a powerful tool for the fabrication of advanced nanomaterials. Due to the limited spatial and temporal resolutions of current experimental techniques, computer simulations offer a complementary strategy to explore self-assembly with atomic resolution. Here, we review recent computational studies focusing on both thermodynamic and kinetic aspects. As we shall see, thermodynamic approaches based on modeling and statistical mechanics offer initial guidelines to design nanostructures with modest computational effort. Computationally more intensive analyses based on molecular dynamics simulations and kinetic network models (KNMs) reach beyond it, opening the door to the rational design of self-assembly pathways. Current limitations of these methodologies are discussed. We anticipate that the synergistic use of thermodynamic and kinetic analyses based on computer simulations will provide an important contribution to the de novo design of self-assembly.

Unidirectional molecular assembly alignment on graphene enabled by nanomechanical symmetry breaking

Precise fabrication of molecular assemblies on a solid surface has long been of central interest in surface science. Their perfectly oriented growth only along a desired in-plane direction, however, remains a challenge, because of the thermodynamical equivalence of multiple axis directions on a solid-surface lattice. Here we demonstrate the successful fabrication of an in-plane, unidirectional molecular assembly on graphene. Our methodology relies on nanomechanical symmetry breaking effects under atomic force microscopy tip scanning, which has never been used in molecular alignment. Individual one-dimensional (1D) molecular assemblies were aligned along a selected symmetry axis of the graphene lattice under finely-tuned scanning conditions after removing initially-adsorbed molecules. Experimental statistics and computational simulations suggest that the anisotropic tip scanning locally breaks the directional equivalence of the graphene surface, which enables nucleation of the unidirectional 1D assemblies. Our findings will open new opportunities in the molecular alignment control on various atomically flat surfaces.

Donor-Acceptor Conjugated Macrocycles: Synthesis and Host-Guest Coassembly with Fullerene toward Photovoltaic Application

Electron-rich (donor) and electron-deficient (acceptor) units to construct donor-acceptor (D-A) conjugated macrocycles were investigated to elucidate their interactions with electron-deficient fullerene. Triphenylamine and 4,7-bisthienyl-2,1,3-benzothiadiazole were alternately linked through acetylene, as the donor and acceptor units, respectively, for pentagonal 3B2A and hexagonal 4B2A macrocycles. As detected by scanning tunneling microscopy, both D-A macrocycles were found to form an interesting concentration-controlled nanoporous monolayer on highly oriented pyrolytic graphite, which could effectively capture fullerene. Significantly, the fullerene filling was cavity-size-dependent with only one C₇₀ or PC₇₁BM molecule accommodated by 3B2A, while two were accommodated by 4B2A. Density functional theory calculations were also utilized to gain insight into the host-guest systems and indicted that the S···π contact is responsible for stabilizing these host-guest systems. Owing to the ellipsoidal shape of C₇₀, C₇₀ molecules are standing or lying in molecular cavities depending on the energy optimization. For the 3B2A/PC₇₁BM blended film, PC₇₁BM was intercalated into the cavity formed by the macrocycle 3B2A and provided excellent power conversion efficiency despite the broad band gap (2.1 eV) of 3B2A. This study of D-A macrocycles incorporating fullerene provides insights into the interaction mechanism and electronic structure in the host-guest complexes. More importantly, this is a representative example using D-A macrocycles as a donor to match with the spherical fullerene acceptor for photovoltaic applications, which offer a good approach to achieve molecular scale p-n junctions for substantially enhanced efficiencies of organic solar cells through replacing linear polymer donors by cyclic conjugated oligomers.

Two-dimensional crystal engineering using halogen and hydrogen bonds: towards structural landscapes

Two-dimensional (2D) crystallization on solid surfaces is governed by a subtle balance of supramolecular and interfacial interactions. However, these subtle interactions often make the prediction of supramolecular structure from the molecular structure impossible. As a consequence, surface-based 2D crystallization has often been studied on a case-by-case basis, which hinders the identification of structure-determining relationships between different self-assembling systems. Here we begin the discussion on such structure-determining relationships by comparing the 2D crystallization of two identical building blocks based on a 1,3,5-tris(pyridine-4-ylethynyl)benzene unit at the solution-solid interface. The concepts of supramolecular synthons and structural landscapes are introduced in the context of 2D crystallization on surfaces to identify common structural elements. The systems are characterized using scanning tunneling microscopy (STM). This strategy involves carrying out minor structural modifications on the parent compound to access supramolecular patterns that are otherwise not obtained. We demonstrate that this chemical perturbation strategy translates equally well for 2D co-crystallization experiments with halogen bond donors yielding porous bi-component networks. The holistic approach described here represents a stepping stone towards gaining predictive power over the 2D crystallization of molecules on solid surfaces.