Light-Induced Increase of the Local Molecular Coverage on a Surface

Light is a versatile tool to remotely activate molecules adsorbed on a surface, for example, to trigger their polymerization. Here, we explore the spatial distribution of light-induced chemical reactions on a Au(111) surface. Specifically, the covalent on-surface polymerization of an anthracene derivative in the submonolayer coverage range is studied. Using scanning tunneling microscopy and X-ray photoemission spectroscopy, we observe a substantial increase of the local molecular coverage with the sample illumination time at the center of the laser spot. We find that the interplay between thermally induced diffusion and the reduced mobility of reaction products steers the accumulation of material. Moreover, the debromination of the adsorbed species never progresses to completion within the experiment time, despite a long irradiation of many hours.

Coexistence of Crumpling and Flat Sheet Conformations in Two-Dimensional Polymer Networks: An Understanding of Aggrecan Self-Assembly

We investigate the conformational properties of self-avoiding two-dimensional (2D) ideal polymer networks with tunable mesh sizes as a model of self-assembled structures formed by aggrecan. Polymer networks having few branching points and large enough mesh tend to crumple, resulting in a fractal dimension of d_{f}≈2.7. The flat sheet behavior (d_{f}=2) emerges in 2D polymer networks having more branching points at large length scales; however, it coexists with crumpling conformations at intermediate length scales, a feature found in scattering profiles of aggrecan solutions. Our findings bridge the long-standing gap between theories and simulations of polymer sheets.

Room Temperature On-Surface Synthesis of a Vinylene-Linked Single Layer Covalent Organic Framework

Conjugated single-layered two-dimensional covalent organic frameworks are flat and extended polymer networks with a unique combination of material properties, giving rise to potential applications in sensing, optoelectronics, and photonics. Despite their great potential, thus far only a few reactions to access such extended conjugated 2D polymers have been reported. Here, the on-surface polymerization of the first vinylene-linked single layered two-dimensional covalent organic framework using reversible Knoevenagel polycondensation under solvothermal conditions is described. Self-assembly of the two monomer building blocks at the solid-liquid interface led to the formation of extended covalent networks at room temperature without the need of additional catalysts or reagents. The described approach grants access to extended conjugated 2D polymers under unprecedentedly mild conditions and paves the way to new hybrid material systems.

Synthesis, Hole Doping, and Electrical Properties of a Semiconducting Azatriangulene-Based Covalent Organic Framework

Two-dimensional covalent organic frameworks (2D COFs) containing heterotriangulenes have been theoretically identified as semiconductors with tunable, Dirac-cone-like band structures, which are expected to afford high charge-carrier mobilities ideal for next-generation flexible electronics. However, few bulk syntheses of these materials have been reported, and existing synthetic methods provide limited control of network purity and morphology. Here, we report transimination reactions between benzophenone-imine-protected azatriangulenes (OTPA) and benzodithiophene dialdehydes (BDT), which afforded a new semiconducting COF network, OTPA-BDT. The COFs were prepared as both polycrystalline powders and thin films with controlled crystallite orientation. The azatriangulene nodes are readily oxidized to stable radical cations upon exposure to an appropriate p-type dopant, tris(4-bromophenyl)ammoniumyl hexachloroantimonate, after which the network's crystallinity and orientation are maintained. Oriented, hole-doped OTPA-BDT COF films exhibit electrical conductivities of up to 1.2 × 10^-1 S cm^-1, which are among the highest reported for imine-linked 2D COFs to date.

Unveiling the formation mechanism of the biphenylene network

We have computationally studied the formation mechanism of the biphenylene network via the intermolecular HF zipping, as well as identified key intermediates experimentally, on the Au(111) surface. We elucidate that the zipping process consists of a series of defluorinations, dehydrogenations, and C-C coupling reactions. The Au substrate not only serves as the active site for defluorination and dehydrogenation, but also forms C-Au bonds that stabilize the defluorinated and dehydrogenated phenylene radicals, leading to "standing" benzyne groups. Despite that the C-C coupling between the "standing" benzyne groups is identified as the rate-limiting step, the limiting barrier can be reduced by the adjacent chemisorbed benzyne groups. The theoretically proposed mechanism is further supported by scanning tunneling microscopy experiments, in which the key intermediate state containing chemisorbed benzyne groups can be observed. This study provides a comprehensive understanding towards the on-surface intermolecular HF zipping, anticipated to be instructive for its future applications.

Novel quadrilateral-pore 2D-COFs as visible-light driven catalysts evaluated by the descriptor of integrated pz-orbital population

In the past few decades, strategies for designing new two-dimensional covalent organic framework (2D-COF) structures have been limited to the shape of positive hexagonal pores, and the underlying relationship between their structure and electronic properties still remains unclear. Herein, novel 2D-COFs with C, N and H elements confined to the quadrilateral-pore skeleton based on first-principles calculations and the topological assembly of different benzene-based building blocks were designed and studied. These 2D-COFs enriched the topology types and can offer an ideal platform for band engineering aimed at spontaneously driving the hydrogen evolution reaction (HER) under visible light irradiation. The approach for regulating pore structures on nodes, linkers and linkages can effectively tune band gaps, and thus the 2D-COF, consisting of benzene building blocks and imine linkages, has the optimal activity for the photocatalytic HER under common visible light conditions. Furthermore, the integrated p_z-orbital population was found to evaluate the photocatalytic activity efficiently. We demonstrate that the p_z-orbital population is in linear relationship with the intensity of H⁺ adsorption, indicating that the total contribution of the p_z-orbital electrons can be an efficient descriptor for screening suitable 2D-COF structures for use as photocatalysts for the HER. Therefore, this work presents a new strategy for designing novel quadrilateral-pore 2D-COFs as visible-light photocatalysts and provides an important insight into the relationship between catalytic activity and the population of activated electrons.

Intra- and Inter-Self-Assembly of Identical Supramolecules on Silver Surfaces

Self-assembly of identical organometallic supramolecules into ordered superstructures is of great interest in both chemical science and nanotechnology due to its potential to generate neoteric properties through collective effects. In this work, we demonstrate that large-scale self-organization of atomically precise organometallic supramolecules can be achieved through cascaded on-surface chemical reactions, by the combination of intra- and inter-supramolecular interactions. Supramolecules with defined size and shape are first built through intramolecular reaction and intermolecular metal coordination, followed by the formation of well-ordered two-dimensional arrays with the assistance of Br atoms by -C-H···Br interactions. The mechanism of this process has been investigated from the perspectives of thermodynamics and kinetics.

One-pot synthesis of dimerized arenes and heteroarenes under mild conditions using Co(i) as an active catalyst

A cheap and robust methodology for dimerization of arenes and heteroarenes with new Co(i) as an active catalyst at room temperature in a shorter time.

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

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

Nano-sheets of two-dimensional polymers with dinuclear (arene)ruthenium nodes, synthesised at a liquid/liquid interface

We developed a new class of mono- or few-layered two-dimensional polymers based on dinuclear (arene)ruthenium nodes, obtained by combining the imine condensation with an interfacial chemistry process, and use a modified Langmuir-Schaefer method to transfer them onto solid surfaces. Robust nano-sheets of two-dimensional polymers including dinuclear complexes of heavy ruthenium atoms as nodes were synthesised. These nano-sheets, whose thickness is of a few tens of nanometers, were suspended onto solid porous membranes. Then, they were thoroughly characterised with a combination of local probes, including Raman spectroscopy, Fourier transform infrared spectroscopy and transmission electron microscopy in imaging and diffraction mode.

Tessellation strategy for the interfacial synthesis of an anthracene-based 2D polymer via [4+4]-photocycloaddition

Inspired by the tessellation or tiling process in daily life, a rigid triangular macrocyclic molecule containing anthracene as a photo-active moiety was synthesized to realize pre-organization through π-π interactions. The successful preparation of a 2D polymer monolayer at the air/water interface was achieved through [4+4]-photocycloaddition.

Recent Advances on Conductive 2D Covalent Organic Frameworks

As a burgeoning family of crystalline porous copolymers, covalent organic frameworks (COFs) allow precise atomic insertion of organic components in the topology construction to form periodic networks and ordered nanopores. Their 2D networks bear great similarities to graphene analogs, and therefore are essential additions to the 2D family. Here, the electronic properties of conductive 2D-COFs are reviewed and their bonding strategies and structural characteristics are examined in detail. The controlling approaches toward the morphologies of conductive 2D-COFs are further explored, followed by a discussion of their applications in field-effect transistors, photodetectors, sensors, catalysis, and energy storage. Finally, research challenges and forthcoming developments are projected. The resulting survey reveals that the extended porous 2D organic networks with conductive properties will provide great opportunities and essential innovations in various electronics and energy-related fields.

Materials Design and Optimization for Next-Generation Solar Cell and Light-Emitting Technologies

We review some of the most potent directions in the design of materials for next-generation solar cell and light-emitting technologies that go beyond traditional solid-state inorganic semiconductor-based devices, from both the experimental and computational standpoints. We focus on selected recent conceptual advances in tackling issues which are expected to significantly impact applied literature in the coming years. Specifically, we consider solution processability, design of dopant-free charge transport materials, two-dimensional conjugated polymeric semiconductors, and colloidal quantum dot assemblies in the fields of experimental synthesis, characterization, and device fabrication. Key modeling issues that we consider are calculations of optical properties and of effects of aggregation, including recent advances in methods beyond linear-response time-dependent density functional theory and recent insights into the effects of correlation when going beyond the single-particle ansatz as well as in the context of modeling of thermally activated fluorescence.

Electrochemical and Spectroelectrochemical Studies on the Reactivity of Perimidine-Carbazole-Thiophene Monomers towards the Formation of Multidimensional Macromolecules versus Stable π-Dimeric States

During research on cross-linked conducting polymers, double-functionalized monomers were synthesized. Two subunits potentially able to undergo oxidative coupling were used—perimidine and, respectively, carbazole, 3,6-di(hexylthiophene)carbazole or 3,6-di(decyloxythiophene)carbazole; alkyl and alkoxy chains as groups supporting molecular ordering and 14H-benzo[4,5]isoquinone[2,1-a]perimidin-14-one segment promoting CH⋯O interactions and π–π stacking. Electrochemical, spectroelectrochemical, and density functional theory (DFT) studies have shown that potential-controlled oxidation enables polarization of a specific monomer subunit, thus allowing for simultaneous coupling via perimidine and/or carbazole, but mainly leading to dimer formation. The reason for this was the considerable stability of the dicationic and tetracationic π-dimers over covalent bonding. In the case of perimidine-3,6-di(hexylthiophene)carbazole, the polymer was not obtained due to the steric hindrance of the alkyl substituents preventing the coupling of the monomer radical cations. The only linear π-conjugated polymer was obtained through di(decyloxythiophene)carbazole segment from perimidine-di(decyloxythiophene)-carbazole precursor. Due to the significant difference in potentials between subsequent oxidation states of monomer, it was impossible to polarize the entire molecule, so that both directions of coupling could be equally favored. Subsequent oxidation of this polymer to polarize the side perimidine groups did not allow further crosslinking, because rather the π–π interactions between these perimidine segments dominate in the solid product.

On-Surface Self-Assembled Structural Transformation Induced by Schiff Base Reaction and Hydrogen bonds

By utilizing scanning tunneling microscopy (STM), the self-assembled nanostructures of three characteristic aldehydes have been examined at the solution-solid interface. By introducing the active reactant 5-aminoisophthalic acid (5-AIPA), we succeeded in changing the self-assembled molecular structures through the condensation reaction and obtained the information on structural transformation in real time. The corresponding carboxyl conjugated derivatives were formed in situ and developed into the closely packed and ordered molecular architectures via hydrogen bonds at the solution-solid surface. The relevant simulations have been utilized to interpret the mechanisms of forming the nanostructures. The corresponding theoretical calculation is used to explain the reaction mechanism. Compared with the traditional ways, the on-surface condensation reaction in situ could not only provide a more convenient method for regulating the self-assembled architectures but also offer a promising strategy for building functional nanostructures and devices.

Electrically Conductive Coordination Polymers for Electronic and Optoelectronic Device Applications

Electrically conductive coordination polymers (generally known as metal-organic frameworks, MOFs) are a class of crystalline hybrid materials produced by the reasonable self-assembly of metal nodes and organic linkers. The unique and intriguing combination of inorganic and organic components endows coordination polymers with superior optical and electrical properties, which have recently aroused much attention in several electronic and optoelectronic technological applications. However, there are many challenging obstacles and issues that need to be addressed in this burgeoning field. In this Perspective, we first provide a fundamental understanding about the electronic design strategies that provide better guidance for realizing high conductivities and good mobilities in coordination polymers. We then examine the current established synthetic approaches to construct high-quality working samples of electrically conductive coordination polymers for device integration. This is followed by a discussion of the current state-of-the-art progress toward the preliminary achievements in (opto)electronic devices spanning chemiresistive sensors, field-effect transistors, organic photovoltaics, photodetectors, etc. Finally, we conclude this Perspective with the existing hurdles and limitations in this area, along with the critical directions and opportunities for future research.

Electrochemical Synthesis of Large Area Two-Dimensional Metal-Organic Framework Films on Copper Anodes

Owing to their excellent physical and electrical properties, metal-organic framework (MOF) materials with well-defined supramolecular structures have received extensive research attention. However, the fabrication of large-area two-dimensional (2D) MOF films is still a significant challenge. Herein, we propose a novel electrochemical (EC) synthesis method for the preparation of large-area Cu₃ (HHTP)₂ MOF film on single-crystal Cu (100) anode. The surface reaction was achieved via charge-induced molecular assembly. The synthesized MOF film exhibited a high crystalline quality with an electrical conductivity of approximately 0.087 S cm^-1 , which was around 1000 times larger than the previously reported values for the same material prepared by the interface method. In addition, Cu₂ (MTCP), Cu₃ (BTPA)₂ , and Cu₃ (TPTC)₂ MOF films were synthesized on Cu foil with the same strategy, which confirmed the universality of the proposed method. This controllable EC method can be effectively applied to the industrial-scale production of 2D MOF films on Cu foil.

C-C Coupling Reactions for the Synthesis of Two-Dimensional Conjugated Polymers

Extension of conjugated polymers from 1D to 2D can not only significantly enhance the dissociation of charge and excitons, but also induce other advantages, such as high in-plane mechanical strength, large specific surface area and porosity, and more active centers. 2D conjugated polymers can be divided into C-C bonded 2D polymers based on C-C coupling reactions, and heteroatomic bonded 2D polymers based on reversible heteroatom coupling reactions. C-C bonded 2D polymers are generally more stable than heteroatomic bonded 2D polymers as the latter bonds are easily hydrolyzed. This Review mainly summarizes C-C coupling reactions that are suitable for synthesizing 2D conjugated polymers, and the properties of these 2D conjugated polymers are also introduced.

Electrochromic two-dimensional covalent organic framework with a reversible dark-to-transparent switch

Electrochromic (EC) materials with a dark-to-transmissive switch have great applications in optical communications, infrared wavelength detectors for spacecraft, and infrared camouflage coatings. However, such electroactive materials with high stability and cyclability are rare. Considering the advantages of the donor-acceptor approach (wide-range tuneable band position) and porous two-dimensional (2D) covalent organic framework (COF, well-ordered crystalline framework with stable structure and high surface area), in this work we constructed an extended delocalised π-electron layered dark purple EC-COF-1 by reacting the donor N,N,N',N'-tetrakis(p-aminophenyl)-p-benzenediamine (TPBD) with the acceptor 2,1,3-benzothiadiazole-4,7-dicarboxaldehyde (BTDD). A sandwiched device made of EC-COF-1 exhibits the two-band bleaching (370 nm and 574 nm) in the visible region and becomes transparent under the applied potential with an induced absorption centring at 1400 nm. This discovery of a stable dark-to-transmissive switch in COF might open another door for their application in many EC devices for various purposes.

Oxygen-Induced 1D to 2D Transformation of On-Surface Organometallic Structures

While surface-confined Ullmann-type coupling has been widely investigated for its potential to produce π-conjugated polymers with unique properties, the pathway of this reaction in the presence of adsorbed oxygen has yet to be explored. Here, the effect of oxygen adsorption between different steps of the polymerization reaction is studied, revealing an unexpected transformation of the 1D organometallic (OM) chains to 2D OM networks by annealing, rather than the 1D polymer obtained on pristine surfaces. Characterization by scanning tunneling microscopy and X-ray photoelectron spectroscopy indicates that the networks consist of OM segments stabilized by chemisorbed oxygen at the vertices of the segments, as supported by density functional theory calculations. Hexagonal 2D OM networks with different sizes on Cu(111) can be created using precursors with different length, either 4,4″-dibromo-p-terphenyl or 1,4-dibromobenzene (dBB), and square networks are obtained from dBB on Cu(100). The control over size and symmetry illustrates a versatile surface patterning technique, with potential applications in confined reactions and host-guest chemistry.

Synthesis of mesoscale ordered two-dimensional π-conjugated polymers with semiconducting properties

Two-dimensional materials with high charge carrier mobility and tunable band gaps have attracted intense research effort for their potential use in nanoelectronics. Two-dimensional π-conjugated polymers constitute a promising subclass because the band structure can be manipulated by varying the molecular building blocks while preserving key features such as Dirac cones and high charge mobility. The major barriers to the application of two-dimensional π-conjugated polymers have been the small domain size and high defect density attained in the syntheses explored so far. Here, we demonstrate the fabrication of mesoscale ordered two-dimensional π-conjugated polymer kagome lattices with semiconducting properties, Dirac cone structures and flat bands on Au(111). This material has been obtained by combining a rigid azatriangulene precursor and a hot dosing approach, which favours molecular diffusion and eliminates voids in the network. These results open opportunities for the synthesis of two-dimensional π-conjugated polymer Dirac cone materials and their integration into devices.

Discrete boronate ester ladders from the dynamic covalent self-assembly of oligo(phenylene ethynylene) derivatives and phenylenebis(boronic acid)

Reversible boronate ester chemistry enables the controlled, dynamic self-assembly of olig(phenylene ethynylene)s into highly conjugated ladder frameworks.

Wafer-scale synthesis of monolayer two-dimensional porphyrin polymers for hybrid superlattices

The large-scale synthesis of high-quality thin films with extensive tunability derived from molecular building blocks will advance the development of artificial solids with designed functionalities. We report the synthesis of two-dimensional (2D) porphyrin polymer films with wafer-scale homogeneity in the ultimate limit of monolayer thickness by growing films at a sharp pentane/water interface, which allows the fabrication of their hybrid superlattices. Laminar assembly polymerization of porphyrin monomers could form monolayers of metal-organic frameworks with Cu2+ linkers or covalent organic frameworks with terephthalaldehyde linkers. Both the lattice structures and optical properties of these 2D films were directly controlled by the molecular monomers and polymerization chemistries. The 2D polymers were used to fabricate arrays of hybrid superlattices with molybdenum disulfide that could be used in electrical capacitors.

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.

Exploring Approaches for the Synthesis of Few-Layered Graphdiyne

Graphdiyne (GDY) is an emerging carbon allotrope in the graphyne (GY) family, demonstrating extensive potential applications in the fields of electronic devices, catalysis, electrochemical energy storage, and nonlinear optics. Synthesis of few-layered GDY is especially important for both electronic applications and structural characterization. This work critically summarizes the state-of-art of GDY and focuses on exploring approaches for few-layered GDY synthesis. The obstacles and challenges of GDY synthesis are also analyzed in detail. Recently developed synthetic methods are discussed such as i) the copper substrate-based method, ii) the chemical vapor deposition (CVD) method, iii) the interfacial construction method, and iv) the graphene-templated method. Throughout the discussion, the superiorities and limitations of different methods are analyzed comprehensively. These synthetic methods have provided considerable inspiration approaching synthesis of few-layered or single-layered GDY film. The work concludes with a perspective on promising research directions and remaining barriers for layer-controlled and morphology-controlled synthesis of GDY with higher crystalline quality.

One-Dimensional Double Wires and Two-Dimensional Mobile Grids: Cobalt/Bipyridine Coordination Networks at the Solid/Liquid Interface

Various architectures have been generated and observed by STM at a solid/liquid interface resulting from an in situ chemical reaction between the bipyridine terminal groups of a ditopic ligand and Co(II) ions. Large monodomains of one-dimensional (1D) double wires are formed by Co(II)/ligand coordination, with polymer lengths as long as 150 nm. The polymers are organized as parallel wires 8 nm apart, and the voids between wires are occupied by solvent molecules. Two-dimensional (2D) grids, showing high surface mobility, coexist with the wires. The wires are formed from linear chain motifs where each cobalt center is bonded to two bipyridines. 2D grids are generated from a bifurcation node where one cobalt bonds to three bipyridines. Surface reconstruction of the grids and of the 1D wires was observed under the STM tip. As an exciting result, analysis of these movements strongly indicates surface reactions at the solid/liquid interface.

Two-dimensional semiconducting covalent organic frameworks via condensation at arylmethyl carbon atoms

Construction of organic semiconducting materials with in-plane π-conjugated structures and robustness through carbon-carbon bond linkages, alternatively as organic graphene analogs, is extremely desired for powerfully optoelectrical conversion. However, the poor reversibility for sp² carbon bond forming reactions makes them unavailable for building high crystalline well-defined organic structures through a self-healing process, such as covalent organic frameworks (COFs). Here we report a scalable solution-processing approach to synthesize a family of two-dimensional (2D) COFs with trans-disubstituted C = C linkages via condensation reaction at arylmethyl carbon atoms on the basis of 3,5-dicyano-2,4,6-trimethylpyridine and linear/trigonal aldehyde (i.e., 4,4″-diformyl-p-terphenyl, 4,4'-diformyl-1,1'-biphenyl, or 1,3,5-tris(4-formylphenyl)benzene) monomers. Such sp² carbon-jointed-pyridinyl frameworks, featuring crystalline honeycomb-like structures with high surface areas, enable driving two half-reactions of water splitting separately under visible light irradiation, comparable to graphitic carbon nitride (g-C₃N₄) derivatives.

An unexpected organometallic intermediate in surface-confined Ullmann coupling

Ullmann coupling or, more generally, dehalogenative aryl-aryl coupling, is one of the most widely exploited chemical reactions to obtain one- and two-dimensional polymers on metal surfaces. It is generally described as a two-step reaction: (i) dehalogenation, resulting in the formation of a stable intermediate organometallic phase and subsequent (ii) C-C coupling. The topology of the resulting polymer depends on the number and positions of the halogen atoms in the haloaromatic precursor, although its orientation and order are determined by the structure of the intermediate phase. Hitherto, only one intermediate structure, identified as an organometallic (OM) phase, has been reported for such a reaction. Here we demonstrate the formation of two distinct OM phases during the temperature-induced growth of poly(para-phenylene) from 1,4-dibromobenzene precursors on Cu(110). Beyond the already known linear-OM chains, we show that a phase reorganization to a chessboard-like 2D-OM can be activated in a well-defined temperature range. This new intermediate phase, revealed only when the reaction is carried out at low molecular coverages, was characterized by X-ray photoelectron spectroscopy, scanning tunneling microscopy and near-edge X-ray absorption fine structure spectroscopy, and modeled by density functional theory calculations. Our data show that the 2D-OM remains stable after cooling down the sample and is stabilized by four-Cu clusters at each node. The observation of such unexpected intermediate phase shows the complexity of the mechanisms underlying on-surface synthesis and broadens the understanding of Ullmann coupling, which continues to be astonishing despite its extensive use.

Functional π-Conjugated Two-Dimensional Covalent Organic Frameworks

Fingerprints of π-conjugated compounds are ubiquitous in nature and play a crucial part in human existence. For instance, cis-retinal, an endogenous π-conjugated molecule present in the eye, performs a vital role in the function of visual perception. π-Conjugated molecules have also received a great deal of attention owing to their intriguing optical properties and created a surge in optoelectronics. Varieties of π-conjugated molecules/oligomers have been developed and explored for a number of applications such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), organic photovoltaics (OPVs), and sensors, among others. While the extended π-delocalization in one-dimensional (1D) polymers versus oligomers produce superior optical and electronic properties, further extension of π-delocalization to the second dimension (2D) is expected to give rise even more intriguing properties as revealed by theoretical studies. As a matter of fact, graphene is the best example of 2D-conjugated polymers, but its zero-band-gap behavior is a major impediment for semiconducting applications. In contrast, it was challenging to prepare 2D crystalline polymers until the discovery of boroxine/boronate ester linked covalent organic frameworks (COFs) by Yaghi and co-workers. COFs are a new class of porous crystalline polymers in which organic building blocks are held together by covalent bonds. These polymers exhibit potential applications in gas storage, energy storage, photocatalyst, heterogeneous catalysis, sensors, etc. However, the first π-conjugated COF was realized in 2009 via the introduction of imine linker (-C═N-) between the building blocks. Since then, wide varieties of COFs with various π-delocalization promoting spacers have been developed and explored their electronic and optical properties and pertinent applications. In this review, we will highlight the importance of 2D π-conjugated COFs and their achievements in developing novel functionalities.

Recent Advances of Hierarchical and Sequential Growth of Macromolecular Organic Structures on Surface

The fabrication of macromolecular organic structures on surfaces is one major concern in materials science. Nanoribbons, linear polymers, and porous nanostructures have gained a lot of interest due to their possible applications ranging from nanotemplates, catalysis, optoelectronics, sensors, or data storage. During decades, supramolecular chemistry has constituted an unavoidable approach for the design of well-organized structures on surfaces displaying a long-range order. Following these initial works, an important milestone has been established with the formation of covalent bonds between molecules. Resulting from this unprecedented approach, various nanostructures of improved thermal and chemical stability compared to those obtained by supramolecular chemistry and displaying unique and unprecedented properties have been developed. However, a major challenge exists: the growth control is very delicate and a thorough understanding of the complex mechanisms governing the on-surface chemistry is still needed. Recently, a new approach consisting in elaborating macromolecular structures by combining consecutive steps has been identified as a promising strategy to elaborate organic structures on surface. By designing precursors with a preprogrammed sequence of reactivity, a hierarchical or a sequential growth of 1D and 2D structures can be realized. In this review, the different reaction combinations used for the design of 1D and 2D structures are reported. To date, eight different sequences of reactions have been examined since 2008, evidencing the intense research activity existing in this field.

The role of halogens in on-surface Ullmann polymerization

Ullmann coupling is the most common approach to form surface-confined one- and two-dimensional conjugated structures from haloaryl derivatives. The dimensions of the formed nanostructures can be controlled by the number and location of halogens within the molecular precursors. Our study illustrates that the type of halogen plays an essential role in the design, orientation, and extent of the surface-confined organometallic and polymeric nanostructures. We performed a comparative analysis of five 1,4-dihalobenzene molecules containing chlorine, bromine, and iodine on Cu(110) using scanning tunneling microscopy, fast-X-ray photoelectron and near edge X-ray absorption fine structure spectroscopies. Our experimental data identify different molecular structures, reaction temperatures and kinetics depending on the halogen type. Climbing image nudged elastic band simulations further clarify these observations by providing distinct diffusion paths for each halogen species. We show that in addition to the structure of the building blocks, the halogen type has a direct influence on the morphology of surface-confined polymeric structures based on Ullmann coupling.

Rational design of two-dimensional covalent tilings using a C6-symmetric building block via on-surface Schiff base reaction

Three types of well-ordered covalent two-dimensional tilings including triangular, rhombille and semi-regular Archimedean tilings were successfully constructed via on-surface Schiff base reaction. Among them, the covalent organic framework (COF) constructed from a C6 symmetry monomer and C3 symmetry monomer is the first reported COF with kgd (rhombille tiling) topology.

An experimental and theoretical study of metallorganic coordination networks of tetrahydroxyquinone on Cu(111)

DFT modeling of STM and XAS evidences investigated the adsorption of THQ@Cu(111) that generates different ordered configurations at different temperatures.

Sequential, Ultrafast Energy Transfer and Electron Transfer in a Fused Zinc Phthalocyanine-free-base Porphyrin-C60 Supramolecular Triad

A supramolecular triad composed of a fused zinc phthalocyanine-free-base porphyrin dyad (ZnPc-H₂ P) coordinated to phenylimidazole functionalized C₆₀ via metal-ligand axial coordination was assembled, as a photosynthetic antenna-reaction centre mimic. The process of self-assembly resulting into the formation of C₆₀ Im:ZnPc-H₂ P supramolecular triad was probed by proton NMR, UV-Visible and fluorescence experiments at ambient temperature. The geometry and electronic structures were deduced from DFT calculations performed at the B3LYP/6-31G(dp) level. Electrochemical studies revealed ZnPc to be a better electron donor compared to H₂ P, and C₆₀ to be the terminal electron acceptor. Fluorescence studies of the ZnPc-H₂ P dyad revealed excitation energy transfer from ¹ H₂ P* to ZnPc within the fused dyad and was confirmed by femtosecond transient absorption studies. Similar to that reported earlier for the fused ZnPc-ZnP dyad, the energy transfer rate constant, k_ENT was in the order of 10¹²  s^-1 in the ZnPc-H₂ P dyad indicating an efficient process as a consequence of direct fusion of the two π-systems. In the presence of C₆₀ Im bound to ZnPc, photoinduced electron transfer leading to H₂ P-ZnPc^.+ :ImC₆₀ ^.- charge separated state was observed either by selective excitation of ZnPc or H₂ P. The latter excitation involved an energy transfer followed by electron transfer mechanism. Nanosecond transient absorption studies revealed that the lifetime of charge separated state persists for about 120 ns indicating charge stabilization in the triad.