Dimerization of Tri(4-bromophenyl)benzene by Aryl−Aryl Coupling from Solution on a Gold Surface

Substrate-Selective Intermolecular Interaction and the Molecular Self-Assemblies: 1,3,5-Tris(4-bromophenyl)benzene Molecules on the Ag(111) and Si(111) (√3 × √3)-Ag Surfaces

We report the formation processes of the self-assembled layer of 1,3,5-tris(4-bromophenyl)benzene (TBB) molecules on the Ag(111) and Si(111) (√3 × √3)-Ag surfaces by STM measurements and density functional theory (DFT) calculations. The self-assembled layers on the surfaces show characteristic structures controlled by the interplay between the intermolecular interaction and the molecule-substrate interaction. Through the cooperative interplay between the molecule-substrate interaction and the intermolecular halogen bond (XB), the periodic arrangement of TBB molecules appears on the Ag(111) surface. On the other hand, the two types of TBB arrangement appear on the Si(111) (√3 × √3)-Ag surface (phases 1 and 2). Phase 1 is the periodic arrangement of the TBB molecules and is derived from the cooperative interplay between the molecule-substrate interaction and the intermolecular van der Waals (vdW) interaction and the hydrogen bond (HB), and phase 2 is a random arrangement and is derived from the competitive interplay between the molecule-substrate interaction and the intermolecular XB and HB. Our present study specifies the role of the substrate in the molecular self-assembly of the substrate. Although the structure of the molecular self-assembly is controlled by the choice of the substrate, the cooperative interplay between the molecule-substrate interaction and the intermolecular interaction is necessary to realize the ideal periodic arrangement.

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.

Bulk COFs and COF nanosheets for electrochemical energy storage and conversion

The current advances, structure-property relationship and future perspectives in covalent organic frameworks (COFs) and their nanosheets for electrochemical energy storage (EES) and conversion (EEC) are summarized.

Photochemistry Highlights on On-Surface Synthesis

On-surface chemistry is a promising way to achieve the bottom-up construction of covalently-bonded molecular precursors into extended atomically-precise polymers adsorbed on surfaces. These polymers exhibit unprecedented physical or chemical properties which are of great interest for various potential applications. These nanostructures were mainly obtained in ultra-high vacuum (UHV) on noble metal single-crystal surfaces by thermal annealing as stimulus to provoke the polymerization with a catalytic role of the surface adatoms. Nevertheless, photons are also a powerful source of energy to induce the formation of covalent architectures, even if it is less-used on surfaces than in solution. In this minireview, we discuss the photo-induced on-surface polymerization from the basic mechanisms of photochemistry to the formation of extended polymers on different kinds of surfaces, which are characterized by scanning probe microscopies.

On-surface synthesis of one-type pore single-crystal porous covalent organic frameworks

Employing a 1,3,5-tris(4-bromophenyl)benzene precursor as a building block, we successfully fabricate large-scale, non-multihole and single-layer pCOFs on the Ag(111) surface in a controllable manner via the on-surface reaction. We reveal that two main factors, the heating rate and growth temperature, have a strong impact on the size and quality of the pCOFs by STM. Furthermore, the band gap of the pCOFs has been further measured to be approximately 3.01 eV.

Reversibility and intermediate steps as key tools for the growth of extended ordered polymers via on-surface synthesis

Surface-confined polymerization is a bottom-up strategy to create one- and two-dimensional covalent organic nanostructures with a π-conjugated backbone, which are suitable to be employed in real-life electronic devices, due to their high mechanical resistance and electronic charge transport efficiency. This strategy makes it possible to change the properties of the final nanostructures by a careful choice of the monomer architecture (i.e. of its constituent atoms and their spatial arrangement). Several chemical reactions have been proven to form low-dimensional polymers on surfaces, exploiting a variety of precursors in combination with metal (e.g. Cu, Ag, Au) and insulating (e.g. NaCl, CaCO₃) surfaces. One of the main challenges of such an approach is to obtain nanostructures with long-range order, to boost the conductance performances of these materials. Most of the exploited chemical reactions use irreversible coupling between the monomers and, as a consequence, the resulting structures often suffer from poor order and high defect density. This review focuses on the state-of-the-art surface-confined polymerization reactions, with particular attention paid to reversible coupling pathways and irreversible processes including intermediate states, which are key aspects to control to increase the order of the final nanostructure.

Halogen bonding versus hydrogen bonding induced 2D self-assembled nanostructures at the liquid-solid interface revealed by STM

We design a bifunctional molecule (5-bromo-2-hexadecyloxy-benzoic acid, 5-BHBA) with a bromine atom and a carboxyl group and its two-dimensional self-assembly is experimentally and theoretically investigated by using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The supramolecular self-organization of 5-BHBA in two different solvents (1-octanoic acid and n-hexadecane) at the liquid-solid interface at different solution concentrations is obviously different due to the cooperative and competitive intermolecular halogen and hydrogen bonds. Three kinds of nanoarchitectures composed of dimers, trimers and tetramers are formed at the 1-octanoic acid/graphite interface based on -COOHHOOC-, triangular C[double bond, length as m-dash]OBrH-C, -BrO(H), BrBr, and OH interactions. Furthermore, by using n-hexadecane as the solvent, two kinds of self-assembled linear patterns can be observed due to the coadsorption, in which the dimers are formed by intermolecular -COOHHOOC- hydrogen bonds. The molecule-solvent and solvent-solvent van der Waals force and intermolecular hydrogen bonds dominate the formation of coadsorbed patterns. We propose that the cooperative and competitive halogen and hydrogen bonds are related to the polarity of the solvent and the type of molecule-solvent interaction. The intermolecular binding energy of different dimers and their stability are supported by theoretical calculations. The result provides a new and innovative insight to induce the 2D self-assembled nanostructures by halogen and hydrogen bonds at the liquid-solid interface.

Role of halogen⋯halogen interactions in the 2D crystallization of n-semiconductors at the liquid–solid interface

The impact of X⋯X interactions on the 2D crystallization of perylene-based n-semiconductors at the liquid–solid interface was investigated.

Surface-assisted Ullmann coupling

Surface-assisted Ullmann coupling is both drosophila and workhorse of on-surface synthesis. The fabrication of novel covalent low-dimensional organic nanostructures is accompanied by fundamental studies of surface chemistry.

Gold-Organic Hybrids: On-Surface Synthesis and Perspectives

Gold-organic hybrids can be prepared on gold substrates by on-surface dehalogenation of molecular precursors with multiple halogen substituents. Various contact geometries of covalent arylAu bonds are achieved by changing the halogen substituents in the bay or peri regions. Scanning tunneling microscopy/spectroscopy (STM/STS) investigations allow a better understanding of the structure/property relationships in various gold-aryl contacts. Recent progress on the synthesis, large-scale alignment, and STS measurement of gold-organic hybrids is described, ending with an emphasis on potential future applications, e.g., as precursors (intermediates) for the synthesis of graphene nanoribbons (GNRs) on insulating surfaces, and as a model system to investigate the role of covalent arylAu bonds in electron transport through gold-GNR contacts.

On-surface synthesis of two-dimensional imine polymers with a tunable band gap: a combined STM, DFT and Monte Carlo investigation

Two-dimensional polymers are of great interest for many potential applications in nanotechnology. The preparation of crystalline 2D polymers with a tunable band gap is critical for their applications in nano-electronics and optoelectronics. In this work, we try to tune the band gap of 2D imine polymers by expanding the conjugation of the backbone of aromatic diamines both laterally and longitudinally. STM characterization reveals that the regularity of the 2D polymers can be affected by the existence of lateral bulky groups. Density functional theory (DFT) simulations discovered a significant narrowing of the band gap of imine 2D polymers upon the expansion of the conjugation of the monomer backbone, which has been confirmed experimentally by UV absorption measurements. Monte Carlo simulations help us to gain further insight into the controlling factors of the formation of regular 2D polymers, which demonstrated that based on the all rigid assumption, the coexistence of different conformations of the imine moiety has a significant effect on the regularity of the imine 2D polymers.

Effects of the position and number of bromine substituents on the concentration-mediated 2D self-assembly of phenanthrene derivatives

The effects of the position and number of bromine substituents on the self-assembled patterns of phenanthrene derivatives by changing multiple weak intermolecular interactions were investigated at the 1-octanoic acid/graphite interface at different concentrations by scanning tunneling microscopy. Two Br substituted DBHP molecules (2,7-DBHP, 3,6-DBHP) and BHP without a Br group formed a linear lamellar pattern by the van der Waals interactions between the alkoxyl chains in each lamella at high concentrations, which forces the phenanthrene derivatives to self-organize in a π-π stacked edge-on conformation. On decreasing the solution concentration, owing to the molecule-molecule van der Waals force and BrBr halogen bonds or the molecule-solvent cooperative BrO (C[double bond, length as m-dash]O) hydrogen and BrHO-hydrogen bonds, 2,7-DBHP molecules were found to form two kinds of network structures, whereas 3,6-DBHP molecules formed only a zigzag pattern due to the intermolecular BrBr van der Waals type interactions. One bromine substituted phenanthrene derivative (3-DBHP) formed a dislocated linear pattern by two C-HBr hydrogen bonds in each dimer. These observations revealed that an important modification of the position and number of halogen substituents might dramatically change the self-assembly behaviors by different intermolecular interactions including BrBr and BrO halogen bonding, BrBr van der Waals type interactions, and HBr hydrogen bonding. DFT calculations were explored to unravel how slightly tuning the molecular structure defines the geometry of a 2D self-assembled nanoarchitecture through the different elementary structural units having BrBr and BrH interactions.

Organisation and ordering of 1D porphyrin polymers synthesised by on-surface Glaser coupling

One-dimensional porphyrin polymer chains formed via on-surface Glaser coupling exhibit ordering and conformational flexibility.

Tuning Porphyrin Assembly and Electrochemical Catalytic Activity with Halogen Substituents

The adlayers of three metalloporphyrins, 5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin cobalt(II) (CoTMePP), 5,10,15,20-tetrakis(4-bromophenyl)porphyrin cobalt(II) (CoTBrPP), and 5,10,15,20-tetrakis(4-iodophenyl)porphyrin cobalt(II) (CoTIPP), on Au(111) were investigated at the solid-liquid interface under electrochemical conditions. In situ scanning tunneling microscopy (STM) was employed to investigate the adlayer structures of CoTMePP, CoTBrPP, and CoTIPP in HClO4 solution. Highly ordered adlayers of the three metalloporphyrins were formed on the Au(111) electrode surface by simple immersion into benzene solutions containing each compound. The adlayer structure of the three cobalt porphyrin derivatives was influenced by the functional group on the phenyl moieties. In particular, a characteristic molecular assembly of CoTIPP molecules was found on Au(111) as a result of the I···I interactions between CoTIPP molecules. The adlattice constants increased in the order -OCH3 < -Br < -I in the phenyl groups. The in situ STM observations showed that the CoTMePP adlayer changed during positive potential manipulation in 0.1 M HClO4, whereas these adlayers were stable in the potential range from 0.90 to 0 V versus the reversible hydrogen electrode. A dependence upon the functional groups among the three CoTPP derivatives was clearly found in the adlattice constants and O2 reduction potentials, revealing that the two-dimensional (2D) molecular assembly and electrochemical activity for dioxygen reduction of the tetraphenylporphyrin derivatives can be tuned by introducing functional groups at the 4 positions of the phenyl moieties, especially iodine substituents.

Surface-Activated Coupling Reactions Confined on a Surface

Chemical reactions may take place in a pure phase of gas or liquid or at the interface of two phases (gas-solid or liquid-solid). Recently, the emerging field of "surface-confined coupling reactions" has attracted intensive attention. In this process, reactants, intermediates, and products of a coupling reaction are adsorbed on a solid-vacuum or a solid-liquid interface. The solid surface restricts all reaction steps on the interface, in other words, the reaction takes place within a lower-dimensional, for example, two-dimensional, space. Surface atoms that are fixed in the surface and adatoms that move on the surface often activate the surface-confined coupling reactions. The synergy of surface morphology and activity allow some reactions that are inefficient or prohibited in the gas or liquid phase to proceed efficiently when the reactions are confined on a surface. Over the past decade, dozens of well-known "textbook" coupling reactions have been shown to proceed as surface-confined coupling reactions. In most cases, the surface-confined coupling reactions were discovered by trial and error, and the reaction pathways are largely unknown. It is thus highly desirable to unravel the mechanisms, mechanisms of surface activation in particular, of the surface-confined coupling reactions. Because the reactions take place on surfaces, advanced surface science techniques can be applied to study the surface-confined coupling reactions. Among them, scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) are the two most extensively used experimental tools. The former resolves submolecular structures of individual reactants, intermediates, and products in real space, while the latter monitors the chemical states during the reactions in real time. Combination of the two methods provides unprecedented spatial and temporal information on the reaction pathways. The experimental findings are complemented by theoretical modeling. In particular, density-functional theory (DFT) transition-state calculations have been used to shed light on reaction mechanisms and to unravel the trends of different surface materials. In this Account, we discuss recent progress made in two widely studied surface-confined coupling reactions, aryl-aryl (Ullmann-type) coupling and alkyne-alkyne (Glaser-type) coupling, and focus on surface activation effects. Combined experimental and theoretical studies on the same reactions taking place on different metal surfaces have clearly demonstrated that different surfaces not only reduce the reaction barrier differently and render different reaction pathways but also control the morphology of the reaction products and, to some degree, select the reaction products. We end the Account with a list of questions to be addressed in the future. Satisfactorily answering these questions may lead to using the surface-confined coupling reactions to synthesize predefined products with high yield.

Adsorption and Coupling of 4-aminophenol on Pt(111) surfaces

We have deposited 4-aminophenol on Pt(111) surfaces in ultra-high vacuum and studied the strength of its adsorption through a combination of STM, LEED, XPS and ab initio calculations. Although an ordered (2√3×2√3)R30° phase appears, we have observed that molecule-substrate interaction dominates the adsorption geometry and properties of the system. At RT the high catalytic activity of Pt induces aminophenol to lose the H atom from the hydroxyl group, and a proportion of the molecules lose the complete hydroxyl group. After annealing above 420K, all deposited aminophenol molecules have lost the OH moiety and some hydrogen atoms from the amino groups. At this temperature, short single-molecule oligomer chains can be observed. These chains are the product of a new reaction that proceeds via the coupling of radical species that is favoured by surface diffusion.

Side-functionalized two-dimensional polymers synthesized via on-surface Schiff-base coupling

An imine-based 2D polymer side-functionalized with o-hydroxyl group was designed in regard to its potential ability to serve as a chelating agent and synthesized on a highly oriented pyrolytic graphite surface with a relatively low annealing temperature. When annealed to a higher temperature the o-hydroxyl group reacts further with the imine group, leading to the formation of oxazoline, which causes significant distortion to the network. The formation of oxazoline was further confirmed by ATR-FTIR.

Covalent coupling via dehalogenation on Ni(111) supported boron nitride and graphene

Polymerization of 1,3,5-tris(4-bromophenyl)benzene on graphene and hexagonal boron nitride is investigated by scanning tunnelling microscopy and density functional theory.

On-surface Ullmann coupling: the influence of kinetic reaction parameters on the morphology and quality of covalent networks

On-surface Ullmann coupling is a versatile and appropriate approach for the bottom-up fabrication of covalent organic nanostructures. In two-dimensional networks, however, the kinetically controlled and irreversible coupling leads to high defect densities and a lack of long-range order. To derive general guidelines for optimizing reaction parameters, the structural quality of 2D porous covalent networks was evaluated for different preparation protocols. For this purpose, polymerization of an iodine- and bromine-functionalized precursor was studied on Au(111) by scanning tunneling microscopy under ultrahigh vacuum conditions. By taking advantage of the vastly different temperature thresholds for C-Br and C-I cleavage, two different polymerization routes were compared - hierarchical and direct polymerization. The structural quality of the covalent networks was evaluated for different reaction parameters, such as surface temperatures, heating rates, and deposition rates by statistical analysis of STM data. Experimental results are compared to Monte Carlo simulations.

Fullerenes as adhesive layers for mechanical peeling of metallic, molecular and polymer thin films

We show that thin films of C60 with a thickness ranging from 10 to 100 nm can promote adhesion between a Au thin film deposited on mica and a solution-deposited layer of the elastomer polymethyldisolaxane (PDMS). This molecular adhesion facilitates the removal of the gold film from the mica support by peeling and provides a new approach to template stripping which avoids the use of conventional adhesive layers. The fullerene adhesion layers may also be used to remove organic monolayers and thin films as well as two-dimensional polymers which are pre-formed on the gold surface and have monolayer thickness. Following the removal from the mica support the monolayers may be isolated and transferred to a dielectric surface by etching of the gold thin film, mechanical transfer and removal of the fullerene layer by annealing/dissolution. The use of this molecular adhesive layer provides a new route to transfer polymeric films from metal substrates to other surfaces as we demonstrate for an assembly of covalently-coupled porphyrins.

Thermodynamics of halogen bonded monolayer self-assembly at the liquid–solid interface

The overall enthalpy change associated with hexabromotriphenylene monolayer self-assembly at the heptanoic acid–graphite interface was assessed by an adapted Born–Haber cycle.

Temperature-induced structural phase transitions in a two-dimensional self-assembled network

Two-dimensional (2D) supramolecular self-assembly at liquid-solid interfaces is a thermodynamically complex process producing a variety of structures. The formation of multiple network morphologies from the same molecular building blocks is a common occurrence. We use scanning tunnelling microscopy (STM) to investigate a structural phase transition between a densely packed and a porous phase of an alkylated dehydrobenzo[12]annulene (DBA) derivative physisorbed at a solvent-graphite interface. The influence of temperature and concentration are studied and the results combined using a thermodynamic model to measure enthalpy and entropy changes associated with the transition. These experimental results are compared to corresponding values obtained from simulations and theoretical calculations. This comparison highlights the importance of considering the solvent when modeling porous self-assembled networks. The results also demonstrate the power of using structural phase transitions to study the thermodynamics of these systems and will have implications for the development of predictive models for 2D self-assembly.

Solution preparation of two-dimensional covalently linked networks by polymerization of 1,3,5-Tri(4-iodophenyl)benzene on Au(111)

The polymerization of 1,3,5-tri(4-iodophenyl)benzene (TIPB) on Au(111) through covalent aryl-aryl coupling is accomplished using a solution-based approach and investigated by scanning tunneling microscopy. Drop-casting of the TIPB monomer onto Au(111) at room temperature results in poorly ordered noncovalent arrangements of molecules and partial dehalogenation. However, drop-casting on a preheated Au(111) substrate yields various topologically distinct covalent aggregates and networks. Interestingly, some of these covalent nanostructures do not adsorb directly on the Au(111) surface, but are loosely bound to a disordered layer of a mixture of chemisorbed iodine and molecules, a conclusion that is drawn from STM data and supported by X-ray photoelectron spectroscopy. We argue that the gold surface becomes covered by a strongly chemisorbed iodine monolayer which eventually inhibits further polymerization.

Supramolecular Cl⋅⋅⋅H and O⋅⋅⋅H interactions in self-assembled 1,5-dichloroanthraquinone layers on Au(111)

The role of halogen bonds in self-assembled networks for systems with Br and I ligands has recently been studied with scanning tunneling microscopy (STM), which provides physical insight at the atomic scale. Here, we study the supramolecular interactions of 1,5-dichloroanthraquinone molecules on Au(111), including Cl ligands, by using STM. Two different molecular structures of chevron and square networks are observed, and their molecular models are proposed. Both molecular structures are stabilized by intermolecular Cl⋅⋅⋅H and O⋅⋅⋅H hydrogen bonds with marginal contributions from Cl-related halogen bonds, as revealed by density functional theory calculations. Our study shows that, in contrast to Br- and I-related halogen bonds, Cl-related halogen bonds weakly contribute to the molecular structure due to a modest positive potential (σ hole) of the Cl ligands.

The structure and formation of hydrogen-bonded molecular networks on Au(111) surfaces revealed by scanning tunnelling and torsional-tapping atomic force microscopy

A comprehensive scanning probe microscopy study has been carried out to characterise 3,4,9,10-Perylenetetracarboxylic diimide (PTCDI)-melamine hydrogen-bonded networks deposited on Au(111)-surfaces. Both scanning tunnelling and atomic force microscopy were utilized. Such complementary analysis revealed a multilayered structure of the networks on the Au(111)-surface as opposed to a widely reported monolayer structure. Details of the network formation mechanism are presented. We have also demonstrated that despite the apparent network stability in ambient conditions it is unstable in aqueous solutions of pH 4.5 and 7.1.

Halogen bonds in 2D supramolecular self-assembly of organic semiconductors

Weak interactions between bromine, sulphur, and hydrogen are shown to stabilize 2D supramolecular monolayers at the liquid-solid interface. Three different thiophene-based semiconducting organic molecules assemble into close-packed ultrathin ordered layers. A combination of scanning tunneling microscopy (STM) and density functional theory (DFT) elucidates the interactions within the monolayer. Electrostatic interactions are identified as the driving force for intermolecular Br···Br and Br···H bonding. We find that the SS interactions of the 2D supramolecular layers correlate with the hole mobilities of thin film transistors of the same materials.

Synthesis of well-ordered COF monolayers: surface growth of nanocrystalline precursors versus direct on-surface polycondensation

Two different straightforward synthetic approaches are presented to fabricate long-range-ordered monolayers of a covalent organic framework (COF) on an inert, catalytically inactive graphite surface. Boronic acid condensation (dehydration) is employed as the polymerization reaction. In the first approach, the monomer is prepolymerized by a mere thermal treatment into nanocrystalline precursor COFs. The precursors are then deposited by drop-casting onto a graphite substrate and characterized by scanning tunneling microscopy (STM). While in the precursors monomers are already covalently interlinked into the final COF structure, the resulting domain size is still rather small. We show that a thermal treatment under reversible reaction conditions facilitates on-surface ripening associated with a striking increase of the domain size. Although this first approach allows studying different stages of the polymerization, the direct polymerization, that is, without the necessity of preceding reaction steps, is desirable. We demonstrate that even for a comparatively small diboronic acid monomer a direct thermally activated polymerization into extended COF monolayers is achievable.

Polymorphic porous supramolecular networks mediated by halogen bonds on Ag(111)

Intermolecular structures of porous two-dimensional supramolecular networks are studied using scanning tunnelling microscopy combined with density functional theory calculations. The local configurations of halogen bonds in polymorphic porous supramolecular networks are directly visualized in support of previous bulk crystal studies.