Hierarchical on-surface synthesis and electronic structure of carbonyl-functionalized one- and two-dimensional covalent nanoarchitectures

On-surface synthesis of organometallic nanorings linked by unconventional intermediates of the Ullmann reaction

Ullmann coupling has been one of the most important organic reactions for the formation of an aryl-aryl bond, which is of great significance in medicinal chemistry, natural product synthesis, and optoelectronic material fabrication. However, the associated reaction mechanism has not been determined with certainty and has mostly relied on theoretical calculations, since the identification of reaction intermediates lacked experimental evidence. Herein, we report the visualization of an unprecedented C-Cu-Br-Cu-C bonded intermediate state of Ullmann coupling by means of on-surface synthesis. These intermediates tend to form nanorings on a Cu(111) surface, as thermodynamically stable structures. Advanced techniques, including scanning tunneling microscopy, non-contact atomic force microscopy, and synchrotron radiation photoemission spectroscopy, together with density functional theory calculations, were used to scrutinize the structural assignments and intermediate transition process at the sub-molecular level. The C-Cu-Br-Cu-C structure is confirmed to be the precursor state of the conventional C-Cu-C intermediate during an on-surface Ullmann reaction, since their coexistence and transformation were observed experimentally. Our findings offer insights into revisiting and understanding the reaction mechanism of Ullmann coupling.

Mechanistical Study on Substrate-Controlled Highly Selective [2+2] and [2+3] Cycloaddition Reactions

Polycyclic conjugated hydrocarbons have acquired increased interests recently because of their potential applications in electronic devices. On metal surfaces, the selective synthesis of four- and five-membered carbon rings remains challenging due to the presence of diverse reaction pathways. Here, utilizing the same precursor molecule, we successfully achieved substrate-controlled highly selective cycloaddition reactions towards four- and five-membered carbon rings. A 97 % yield for four-membered carbon rings on Au(111), while a 96 % yield towards five-membered carbon rings is achieved on Ag(111). The detailed topological structures of the reaction products are carefully examined by bond-resolving scanning tunneling microscopy (BR-STM) imaging with a CO functionalized tip. The underlying mechanism of the novel surface-directed reaction selectivity is elucidated by extensive density functional theory (DFT) calculations. Our study paves the way for high selective synthesis of polycyclic conjugated hydrocarbons with non-benzenoid rings.

On-Surface Synthesis and Characterization of Radical Spins in Kagome Graphene

Flat bands in Kagome graphene might host strong electron correlations and frustrated magnetism upon electronic doping. However, the porous nature of Kagome graphene opens a semiconducting gap due to quantum confinement, preventing its fine-tuning by electrostatic gates. Here we induce zero-energy states into a semiconducting Kagome graphene by inserting π-radicals at selected locations. We utilize the on-surface reaction of tribromotrioxoazatriangulene molecules to synthesize carbonyl-functionalized Kagome graphene on Au(111), thereafter modified in situ by exposure to atomic hydrogen. Atomic force microscopy and tunneling spectroscopy unveil the stepwise chemical transformation of the carbonyl groups into radicals, which creates local magnetic defects of spin state S = 1/2 and zero-energy states as confirmed by density functional theory. The ability to imprint local magnetic moments opens up prospects to study the interplay between topology, magnetism, and electron correlation in Kagome graphene.

Multiple Redox-Active Centers in An Azatriangulenetrione-Based Covalent Organic Framework for High-Capacity, High-Rate and Ultra-Stable Sodium-Ion Batteries

Sodium-ion batteries (SIBs) suffer from sluggish kinetics, large volume change, and limited specific capacity due to the large radius of Na+. These issues can be solved through using covalent organic frameworks (COFs) as electrodes. Herein, an azatriangulenetrione-containing COF (denoted as CityU-33) was designed and synthesized as an electrode material for SIBs. Due to its inherent abundance of multiple redox-active sites and fast intercalation kinetics, CityU-33 delivered a high discharge capacity of 410.4 mAh g-1 at 0.1 A g-1 and showed remarkable long-term cycling stability, where a discharge capacity of 288 mAh g-1 at 0.2 A g-1 with 97 % retention over 2000 cycles was achieved, making it the top COF electrode material for SIBs.

On-Surface Synthesis of Two-Dimensional Carbon-Based Networks via Hierarchical Ullmann Coupling Reactions

The recent developed bottom-up on-surface synthesis offers unprecedent opportunities for the fabrication of two-dimensional (2D) carbon-based networks with atomic precision. Hierarchical coupling approach has been proposed as an efficient strategy for improving the corresponding reaction selectivity and quality of target structures. Herein, we report the synthesis of a nitrogen-doped carbon-based network on Ag(100) utilizing a hierarchical Ullmann coupling strategy. The accurate identification of reaction intermediates and products by scanning tunneling microscopy allows us to unravel the reaction mechanism. The synthetic process of 2D carbon-based networks is kinetics-driven, relying on the competition between dechlorination and C-C coupling. We expect that our discussion on the mechanism of hierarchical coupling may shed light on the rational design and precise synthesis of 2D carbon-based networks on surfaces.

Physics and Chemistry of Two-Dimensional Triangulene-Based Lattices

ConspectusTriangulene (TRI) and its heterotriangulene (HT) derivatives are planar, triangle-shaped molecules that, via suitable coupling reactions, can form extended organic two-dimensional (2D) crystal (O2DC) structures. While TRI is a diradical, HTs are either closed-shell molecules or monoradicals which can be stabilized in their cationic form.Triangulene-based O2DCs have a characteristic honeycomb-kagome lattice. This structure gives rise to four characteristic electronic bands: two of them form Dirac points, while the other two are flat and sandwich the Dirac bands. Functionalization and heteroatoms are suitable means to engineer this band structure. Heteroatoms like boron and nitrogen shift the Fermi level upward and downward, respectively, while bridging groups and functionalized triangulene edges can introduce a dispersion to the flat bands.The stable backbone architecture makes 2D HT-polymers ideal for photoelectrochemical applications: (i) bridge functionalization can tune the band gap and maximize absorption, (ii) the choice of the center atom (B or N) controls the band occupation and shifts the Fermi level with respect to vacuum, allowing in some cases for overpotential-free photon-driven surface reactions, and (iii) the large surface area allows for a high flux of educts and products.The spin polarization in TRI and in open-shell HTs is maintained when linking them to dimers or extended frameworks with direct coupling or more elaborate bridging groups (acetylene, diacetylene, and phenyl). The dimers have a high spin-polarization energy and some of them are strongly magnetically coupled, resulting in stable high-spin or broken-symmetry (BS) low-spin systems. As O2DCs, some systems become antiferromagnetic Mott insulators with large band gaps, while others show Stoner ferromagnetism, maintaining the characteristic honeycomb-kagome bands but shifting the opposite spin-polarized bands to different energies. For O2DCs based on aza- and boratriangulene (monoradicals as building blocks), the Fermi level is shifted to a spin-polarized Dirac point, and the systems have a Curie temperature of about 250 K. For half-filled (all-carbon) systems, the Ovchinnikov rule or, equivalently, Lieb's theorem, is sufficient to predict the magnetic ordering of the systems, while the non-half-filled systems (i.e., those with heteroatoms) obey the more involved Goodenough-Kanamori rule to interpret the magnetism on the grounds of fundamental electronic interactions.There remain challenges in experiment and in theory to advance the field of triangulene-based O2DCs: Coupling reactions beyond surface chemistry have to be developed to allow for highly ordered, extended crystals. Multilayer structures, which are unexplored to date, will be inevitable in alternative synthesis approaches. The predictive power of density-functional theory (DFT) within state-of-the-art functionals is limited for the description of magnetic couplings in these systems due to the apparent multireference character and the large spatial extension of the spin centers.

Redox-active, photoluminescent porous polymers based on spirofluorene-bridged N-heterotriangulenes and their feasibility as organic cathode materials

Novel microporous polymers were synthesized through Yamamoto polymerization of selectively brominated spirofluorene-bridged N-heterotriangulenes. Extensive characterization, including combustion analysis, ToF-SIMS, IR, and Raman spectroscopy, confirmed the elemental composition and integrity of the polymers. The amorphous polymers, observed by scanning electron microscopy as globular particles aggregating into larger structures, exhibited remarkable thermal stability (decomposition temperatures > 400 °C) and BET surface areas up to 690 m2 g-1. Dispersions of the tert-butyl-substituted polymer in different solvents displayed bathochromically shifted emission with remarkable solvatochromism. The polymer is reversibly oxidized at +3.81 V (vs. Li/Li+) in composite electrodes with carbon black and reaches specific capacities up to 26 mA h g-1 and excellent cycling stability when implemented as cathode material in lithium-ion batteries. Our results highlight the potential of spirofluorene-bridged N-heterotriangulenes as versatile building blocks for the development of functional redox-active porous polymers.

Recent progress in on-surface synthesis of nanoporous graphene materials

Nanoporous graphene (NPG) materials are generated by removing internal degree-3 vertices from graphene and introducing nanopores with specific topological structures, which have been widely explored and exploited for applications in electronic devices, membranes, and energy storage. The inherent properties of NPGs, such as the band structures, field effect mobilities and topological properties, are crucially determined by the geometric structure of nanopores. On-surface synthesis is an emerging strategy to fabricate low-dimensional carbon nanostructures with atomic precision. In this review, we introduce the progress of on-surface synthesis of atomically precise NPGs, and classify NPGs from the aspects of element types, topological structures, pore shapes, and synthesis strategies. We aim to provide a comprehensive overview of the recent advancements, promoting interdisciplinary collaboration to further advance the synthesis and applications of NPGs.

Chiral Honeycomb Lattices of Nonplanar π-Conjugated Supramolecules with Protected Dirac and Flat Bands

The honeycomb lattice is a fundamental two-dimensional (2D) network that gives rise to surprisingly rich electronic properties. While its expansion to 2D supramolecular assembly is conceptually appealing, its realization is not straightforward because of weak intermolecular coupling and the strong influence of a supporting substrate. Here, we show that the application of a triptycene derivative with phenazine moieties, Trip-Phz, solves this problem due to its strong intermolecular π-π pancake bonding and nonplanar geometry. Our scanning tunneling microscopy (STM) measurements demonstrate that Trip-Phz molecules self-assemble on a Ag(111) surface to form chiral and commensurate honeycomb lattices. Electronically, the network can be viewed as a hybrid of honeycomb and kagome lattices. The Dirac and flat bands predicted by a simple tight-binding model are reproduced by total density functional theory (DFT) calculations, highlighting the protection of the molecular bands from the Ag(111) substrate. The present work offers a rational route for creating chiral 2D supramolecules that can simultaneously accommodate pristine Dirac and flat bands.

On-Surface Synthesis of Organolanthanide Sandwich Complexes

The synthesis of lanthanide-based organometallic sandwich compounds is very appealing regarding their potential for single-molecule magnetism. Here, it is exploited by on-surface synthesis to design unprecedented lanthanide-directed organometallic sandwich complexes on Au(111). The reported compounds consist of Dy or Er atoms sandwiched between partially deprotonated hexahydroxybenzene molecules, thus introducing a distinct family of homoleptic organometallic sandwiches based on six-membered ring ligands. Their structural, electronic, and magnetic properties are investigated by scanning tunneling microscopy and spectroscopy, X-ray absorption spectroscopy, X-ray linear and circular magnetic dichroism, and X-ray photoelectron spectroscopy, complemented by density functional theory-based calculations. Both lanthanide complexes self-assemble in close-packed islands featuring a hexagonal lattice. It is unveiled that, despite exhibiting analogous self-assembly, the erbium-based species is magnetically isotropic, whereas the dysprosium-based compound features an in-plane magnetization.

A high density nanopore 3-triangulene kagome lattice

Nanopore-containing two-dimensional materials have been explored for a wide range of applications including filtration, sensing, catalysis, energy storage and conversion. Triangulenes have recently been experimentally synthesized in a variety of sizes. In this regard, using these systems as building blocks, we theoretically examined 3-triangulene kagome crystals with inherent holes of ∼12 Å diameter and a greater density array of nanopores (≥1013 cm-2) compared to conventional 2D systems. The energetic, electronic, and transport properties of pristine and B/N-doped 3-triangulene kagome crystals were evaluated through a combination of density functional theory and non-equilibrium Green's function method. The simulated scanning tunneling microscopy images clearly capture electronic perturbation around the doped sites, which can be used to distinguish the pristine system from the doped systems. The viability of precisely controlling the band structure and transport properties by changing the type and concentration of doping atoms is demonstrated. The findings presented herein can potentially widen the applicability of these systems that combine unique electronic properties and intrinsically high-density pores, which can pave the way for the next generation of nanopore-based devices.

Gradient oxygen doping triggered a microscale built-in electric field in CdIn2S4 for photoelectrochemical water splitting

Construction of a built-in electric field has been identified as an attractive improvement strategy for photoelectrochemical (PEC) water splitting by facilitating the carrier extraction from the inside to the surface. However, the promotion effect of the electric field is still restrained by the confined built-in area. Herein, we construct a microscale built-in electric field via gradient oxygen doping. The octahedral configuration of the synthesized CdIn2S4 (CIS) provides a structural basis, which enables the subsequent oxygen doping to reach a depth of ∼100 nm. Accordingly, the oxygen-doped CIS (OCIS) photoanode exhibits a microscale built-in electric field with band bending. Excellent PEC catalytic activity with a photocurrent density of 3.69 mA cm-2 at 1.23 V vs. RHE is achieved by OCIS, which is 3.1 times higher than that of CIS. Combining the results of thorough characterization and theoretical calculations, accelerating migration and separation of charge carriers have been determined as the reasons for the improvement. Meanwhile, the recombination risk at the doping centers has also been reduced to the minimum via optimal experiments. This work provides a new-generation idea for constructing a built-in electric field from the view point of bulky configuration towards PEC water splitting.

Tailoring giant quantum transport anisotropy in nanoporous graphenes under electrostatic disorder

During the last 15 years bottom-up on-surface synthesis has been demonstrated as an efficient way to synthesize carbon nanostructures with atomic precision, opening the door to unprecedented electronic control at the nanoscale. Nanoporous graphenes (NPGs) fabricated as two-dimensional arrays of graphene nanoribbons (GNRs) represent one of the key recent breakthroughs in the field. NPGs interestingly display in-plane transport anisotropy of charge carriers, and such anisotropy was shown to be tunable by modulating quantum interference. Herein, using large-scale quantum transport simulations, we show that electrical anisotropy in NPGs is not only resilient to disorder but can further be massively enhanced by its presence. This outcome paves the way to systematic engineering of quantum transport in NPGs as a novel concept for efficient quantum devices and architectures.

Electronic Band Engineering of Two-Dimensional Kagomé Polymers

Two-dimensional conjugated polymers (2DCPs) are an emerging class of materials that exhibit properties similar to graphene yet do not have the limitation of zero bandgap. On-surface synthesis provides exceptional control on the polymerization reaction, allowing tailoring properties by choosing suitable monomers. Heteroatom-substituted triangulene 2DCPs constitute a playing ground for such a design and are predicted to exhibit graphene-like band structures with high charge mobility and characteristic Dirac cones in conduction or valence states. However, measuring these properties experimentally is challenging and requires long-range-ordered polymers, preferably with an epitaxial relationship with the substrate. Here, we investigate the electronic properties of a mesoscale-ordered carbonyl-bridged triphenylamine 2DCP (P2TANGO) and demonstrate the presence of a Dirac cone by combining angle-resolved photoemission spectroscopy (ARPES) with density functional theory (DFT) calculations. Moreover, we measure the absolute energy position of the Dirac cone with respect to the vacuum level. We show that the bridging functionality of the triangulene (ether vs carbonyl) does not significantly perturb the band structure but strongly affects the positioning of the bands with respect to the Au(111) states and allows control of the ionization energy of the polymer. Our results provide proof of the controllable electronic properties of 2DCPs and bring us closer to their use in practical applications.

On-Surface Two-Dimensional Polymerization: Advances, Challenges, and Prospects

Two-dimensional polymers (2DPs) are molecularly thin networks consisting of monomers covalently linked in at least two directions in the molecular plane. Because of the unique structural features and emergent physicochemical properties, 2DPs promise application potentials in catalysis, chemical sensing, and organic electronic devices. On-surface synthesis is of great interest to fabricate 2DPs with atomic precision, and the properties of the 2DPs can be characterized in situ through scanning probe techniques. In this Perspective, we first introduce the recent developments of on-surface 2D polymerization, including the design principle, the synthetic reactions, and the factors affecting the synthesis of 2DPs on surface. Then, we summarize some major challenges in this field, including the fabrication of high-quality 2DPs and the study of the intrinsic electronic properties of 2DPs, and we discuss some of the available solutions to address these issues.

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.

Tailoring On-Surface Molecular Reactions and Assembly through Hydrogen-Modified Synthesis: From Triarylamine Monomer to 2D Covalent Organic Framework

Relative to conventional wet-chemical synthesis techniques, on-surface synthesis of organic networks in ultrahigh vacuum has few control parameters. The molecular deposition rate and substrate temperature are typically the only synthesis variables to be adjusted dynamically. Here we demonstrate that reducing conditions in the vacuum environment can be created and controlled without dedicated sources─relying only on backfilled hydrogen gas and ion gauge filaments─and can dramatically influence the Ullmann-like on-surface reaction used for synthesizing two-dimensional covalent organic frameworks (2D COFs). Using tribromo dimethylmethylene-bridged triphenylamine ((Br₃)DTPA) as monomer precursors, we find that atomic hydrogen (H^•) blocks aryl-aryl bond formation to such an extent that we suspect this reaction may be a factor in limiting the ultimate size of 2D COFs created through on-surface synthesis. Conversely, we show that control of the relative monomer and hydrogen fluxes can be used to produce large self-assembled islands of monomers, dimers, or macrocycle hexamers, which are of interest in their own right. On-surface synthesis of oligomers, from a single precursor, circumvents potential challenges with their protracted wet-chemical synthesis and with multiple deposition sources. Using scanning tunneling microscopy and spectroscopy (STM/STS), we show that changes in the electronic states through this oligomer sequence provide an insightful view of the 2D COF (synthesized in the absence of atomic hydrogen) as the end point in an evolution of electronic structures from the monomer.

Bottom-Up Synthesis of Metalated Carbyne Ribbons via Elimination Reactions

Elimination reactions are one of the most important reactions in organic synthesis, especially in the formation of alkenes and alkynes. Herein, based on scanning tunneling microscopy, we report the bottom-up synthesis of one-dimensional carbyne-like nanostructures, metalated carbyne ribbons with the incorporation of Cu or Ag atoms, through α- and β-elimination reactions of tetrabromomethane and hexabromoethane on surfaces. Density functional theory calculations demonstrate a width-dependent band gap modulation within these ribbon structures, which is affected by interchain interactions. Moreover, mechanistic insights into the on-surface elimination reactions have also been provided in this study.

Tailoring Dense, Orientation-Tunable, and Interleavedly Structured Carbon-Based Heat Dissipation Plates

The controllability of the microstructure of a compressed hierarchical building block is essential for optimizing a variety of performance parameters, such as thermal management. However, owing to the strong orientation effect during compression molding, optimizing the alignment of materials perpendicular to the direction of pressure is challenging. Herein, to illustrate the effect of the ordered microstructure on heat dissipation, thermally conductive carbon-based materials are fabricated by tailoring dense, orientation-tunable, and interleaved structures. Vertically aligned carbon nanotube arrays (VACNTs) interconnected with graphene films (GF) are prepared as a 3D core-ordered material to fabricate compressed building blocks of O-VA-GF and S-VA-GF. Leveraging the densified interleaved structure offered by VACNTs, the hierarchical O-VA-GF achieves excellent through-plane (41.7 W m^-1 K^-1 ) and in-plane (397.9 W m^-1 K^-1 ) thermal conductivities, outperforming similar composites of S-VA-GF (through-plane: 10.3 W m^-1 K^-1 and in-plane: 240.9 W m^-1 K^-1 ) with horizontally collapsed carbon nanotubes. As heat dissipation plates, these orderly assembled composites yield a 144% and 44% enhancement in the cooling coefficient compared with conventional Si₃ N₄ for cooling high-power light-emitting diode chips.

Chirality variation from self-assembly on Ullmann coupling for the DBCh adsorbate on Au(111) and Ag(111)

On-surface Ullmann coupling has been considered an appealing approach for the precise fabrication of carbon-based covalent nanostructures under solution-free conditions. However, chirality has seldom been discussed in Ullmann reactions. In this report, self-assembled two-dimensional chiral networks are initially constructed in a large area on Au(111) and Ag(111) after adsorption of the prochiral precursor, 6,12-dibromochrysene (DBCh). Self-assembled phases are then transformed into organometallic (OM) oligomers after debromination, preserving the chirality; in particular, the formation of scarcely reported OM species on Au(111) is discovered herein. With the aryl-aryl bonding induced after intensive annealing, covalent chains are fabricated via the cyclodehydrogenation between chrysene blocks, resulting in the formation of 8-armchair graphene nanoribbons with staggered valleys on both sides. Before chiral polymer chains are constructed by chrysene blocks, the high structural flexibility of OM intermediates on Ag(111) is also revealed during reactions, which is derived from the twofold coordination of Ag atoms and conformationally flexible metal-carbon bonding. Our report not only provides solid evidence of atomically precise fabrication of covalent nanostructures with a feasible bottom-up approach but also sheds insights into the comprehensive investigation of chirality variation from monomers to artificial architectures via surface coupling reactions.

Carbon-Carbon Linked Organic Frameworks: An Explicit Summary and Analysis

Organic frameworks with carbon-carbon (CC) linkage are an important class of materials owing to their outstanding chemical stability and extended π-electron delocalization resulting in unique optoelectronic properties. In the first part of this review article, the design principles for the bottom-up synthesis of 2D and 3D sp/sp² CC linked organic frameworks are summarized. Representative reaction methodologies, such as Knoevenagel condensation, Aldol condensation, Horner-Wadsworth-Emmons reaction, Wittig reaction, and coupling reactions (Ullmann, Suzuki, Heck, Yamamoto, etc.) are included. This is discussed in the context of their reaction mechanism, reaction dynamics, and whether and why resulting in an amorphous or crystalline product. This is followed by a discussion of different state-of-the art bottom-up synthesis methodologies, like solvothermal, interfacial, and solid-state synthesis. In the second part, the structure-property relationships in CC linked organic frameworks with representative examples of organocatalysis, photo(electro)catalysis, energy storage and conversion, magnetism, and molecular storage and separation are analyzed. The importance of linkage type, building blocks, topology, and crystallinity of the framework material in connection with the structure-property relationship is highlighted. Finally, brief concluding remarks are presented based on the key development of bottom-up synthetic methods and provide perspectives for future development in this field.

On-surface synthesis and spontaneous segregation of conjugated tetraphenylethylene macrocycles

Creating conjugated macrocycles has attracted extensive research interest because their unique chemical and physical properties, such as conformational flexibility, intrinsic inner cavities and aromaticity/antiaromaticity, make these systems appealing building blocks for functional supramolecular materials. Here, we report the synthesis of four-, six- and eight-membered tetraphenylethylene (TPE)-based macrocycles on Ag(111) via on-surface Ullmann coupling reactions. The as-synthesized macrocycles are spontaneously segregated on the surface and self-assemble as large-area two-dimensional mono-component supramolecular crystals, as characterized by scanning tunneling microscopy (STM). We propose that the synthesis benefits from the conformational flexibility of the TPE backbone in distinctive multi-step reaction pathways. This study opens up opportunities for exploring the photophysical properties of TPE-based macrocycles.

Organic Higher-Order Topological Insulators: Heterotriangulene-Based Covalent Organic Frameworks

The ability to design and control the chemical characteristics of covalent organic frameworks (COFs) offers a new avenue for the development of functional materials, especially with respect to topological properties. Based on density functional theory calculations, by varying the core units through the choice of bridging groups [O, C═O, CH₂, or C(CH₃)₂] and the linker units [acetylene, diacetylene, or benzene], we have designed heterotriangulene-based COFs that are predicted to be two-dimensional higher-order topological insulators (TIs). The higher-order TI characteristics of these COFs are identified via their topological invariants and the presence of in-gap topological corner modes and gapped edge states. The frontier molecular orbital energies of the building moieties play an important role in determining the size of the higher-order TI gap, which we find to be highly dependent on linker units. We also examined the deposition of the COFs on a boron nitride substrate to assess the feasibility of experimental observation of a higher-order TI phase in the organic layer. This work thus provides new insights into heterotriangulene-based COFs and guidance for the exploration of purely organic topological materials.

On-Surface Synthesis toward Two-Dimensional Polymers

Two-dimensional (2D) polymers have garnered widespread interest because of their intriguing physicochemical properties. Envisaged applications in fields including nanodevices, solid-state chemistry, physical organic chemistry, and condensed matter physics, however, demand high-quality and large-scale production. In this perspective, we first introduce exotic band structures of organic frameworks holding honeycomb, kagome, and Lieb lattices. We further discuss how mesoscale ordered 2D polymers can be synthesized by means of choosing suitable monomers and optimizing growth conditions. We describe successful polymerization strategies to introducing a non-benzenoid subunit into a π-conjugated carbon lattice via delicately designed monomer precursors. Also, to obviate transfer and restore the intrinsic properties of π-conjugated polymers, new paradigms of aryl-aryl coupling on inert surfaces are discussed. Recent achievements in the photopolymerization demonstrate the need for monomer design. We conclude the potential applications of these organic networks and project the future possibilities in providing new insights into on-surface polymerization.

Low-Dimensional Porous Carbon Networks Using Single-/Triple-Coupling Polycyclic Hydrocarbon Precursors

Polycyclic hydrocarbons (PHs) share the same hexagonal structure of sp² carbons as graphene but possess an energy gap due to quantum confinement effect. PHs can be synthesized by a bottom-up strategy starting from small building blocks covalently bonded into large 2D organic sheets. Further investigation of the role of the covalent bonding/coupling ways on their electronic properties is needed. Here, we demonstrate a surface-mediated synthesis of hexa-peri-hexabenzocoronene (HBC) and its extended HBC oligomers (dimers, trimers, and tetramers) via single- and triple-coupling ways and reveal the implication of different covalent bonding on their electronic properties. High-resolution low-temperature scanning tunneling microscopy and noncontact atomic force microscopy are employed to in situ determine the atomic structures of as-synthesized HBC oligomers. Scanning tunneling spectroscopy measurements show that the length extension of HBC oligomers narrows the energy gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Furthermore, the energy gaps of triple-coupling HBC oligomers are smaller and decrease more significantly than that of the single-coupling ones. We hypothesize that the triple coupling promotes a more effective delocalization of π-electrons than the single coupling, according to density functional theory calculations. We also demonstrate that the HBC oligomers can further extend across the substrate steps to achieve conjugated polymers and large-area porous carbon networks.

Two-dimensional heterotriangulene-based manganese organic frameworks: bipolar magnetic and half semiconductors with perpendicular magnetocrystalline anisotropy

Two-dimensional (2D) organic intrinsic magnetic semiconductors have potential applications in low-dimensional organic spintronic devices due to their remarkable physical properties. However, 2D metal-organic frameworks with magnetic and semiconducting properties are rare. In this work, the electronic and magnetic properties of 2D heterotriangulene-based manganese organic frameworks including triphenylamine (TPA) and triphenylborane (TPB) organic ligands with methylene (M), carbonyl (C) or oxygen (O) coordination groups were studied by first-principles calculations. XTPA-Mn (X = M and O) is a bipolar magnetic semiconductor with a large spin-flip band gap. CTPA-Mn and XTPB-Mn (X = M, C and O) are half semiconductors with perpendicular magnetocrystalline anisotropy. The electronic properties of materials ranging from half semiconductors to bipolar magnetic semiconductors appear in CTPA-Mn and XTPB-Mn (X = M and C) at biaxial strains. XTPA-Mn and XTPB-Mn with a frustrated antiferromagnetic configuration are semiconductors with good ductility and stability. These results enrich the diversity of 2D organic intrinsic magnetic semiconductors, which have potential applications in spintronic devices.

Steric hindrance in the on-surface synthesis of diethynyl-linked anthracene polymers

Hybrid sp-sp² structures can be efficiently obtained on metal substrates via on-surface synthesis. The choice of both the precursor and the substrate impacts on the effectiveness of the process and the stability of the formed structures. Here we demonstrate that using anthracene-based precursor molecules on Au(111) the formation of polymers hosting sp carbon chains is affected by the steric hindrance between aromatic groups. In particular, by scanning tunneling microscopy experiments and density functional theory simulations we show that the de-metalation of organometallic structures induces a lateral separation of adjacent polymers that prevents the formation of ordered domains. This study contributes to the understanding of the mechanisms driving the on-surface synthesis processes, a fundamental step toward the realization of novel carbon-based nanostructures with perspective applications in nanocatalysis, photoconversion, and nano-electronics.

Chemical bond imaging using torsional and flexural higher eigenmodes of qPlus sensors

Non-contact atomic force microscopy (AFM) with CO-functionalized tips allows visualization of the chemical structure of adsorbed molecules and identify individual inter- and intramolecular bonds. This technique enables in-depth studies of on-surface reactions and self-assembly processes. Herein, we analyze the suitability of qPlus sensors, which are commonly used for such studies, for the application of modern multifrequency AFM techniques. Two different qPlus sensors were tested for submolecular resolution imaging via actuating torsional and flexural higher eigenmodes and via bimodal AFM. The torsional eigenmode of one of our sensors is perfectly suited for performing lateral force microscopy (LFM) with single bond resolution. The obtained LFM images agree well with images from the literature, which were scanned with customized qPlus sensors that were specifically designed for LFM. The advantage of using a torsional eigenmode is that the same molecule can be imaged either with a vertically or laterally oscillating tip without replacing the sensor simply by actuating a different eigenmode. Submolecular resolution is also achieved by actuating the 2^nd flexural eigenmode of our second sensor. In this case, we observe particular contrast features that only appear in the AFM images of the 2^nd flexural eigenmode but not for the fundamental eigenmode. With complementary laser Doppler vibrometry measurements and AFM simulations we can rationalize that these contrast features are caused by a diagonal (i.e. in-phase vertical and lateral) oscillation of the AFM tip.

Aza-Triangulene: On-Surface Synthesis and Electronic and Magnetic Properties

Nitrogen heteroatom doping into a triangulene molecule allows tuning its magnetic state. However, the synthesis of the nitrogen-doped triangulene (aza-triangulene) has been challenging. Herein, we report the successful synthesis of aza-triangulene on the Au(111) and Ag(111) surfaces, along with their characterizations by scanning tunneling microscopy and spectroscopy in combination with density functional theory (DFT) calculations. Aza-triangulenes were obtained by reducing ketone-substituted precursors. Exposure to atomic hydrogen followed by thermal annealing and, when necessary, manipulations with the scanning probe afforded the target product. We demonstrate that on Au(111), aza-triangulene donates an electron to the substrate and exhibits an open-shell triplet ground state. This is derived from the different Kondo resonances of the final aza-triangulene product and a series of intermediates on Au(111). Experimentally mapped molecular orbitals match with DFT-calculated counterparts for a positively charged aza-triangulene. In contrast, aza-triangulene on Ag(111) receives an extra electron from the substrate and displays a closed-shell character. Our study reveals the electronic properties of aza-triangulene on different metal surfaces and offers an approach for the fabrication of new hydrocarbon structures, including reactive open-shell molecules.

Theoretical modeling of the metal-organic precursors of anthracene-based covalent networks on surfaces

Surface-assisted fabrication of molecular network architectures has been a promising route to low-dimensional materials with unique physicochemical properties and functionalities. One versatile way in this field is the Ullmann coupling reaction of halogenated organic monomers on catalytically active metallic surfaces. In this work, using the coarse grained Monte Carlo simulations, we studied the on-surface self-assembly of metal-organic precursors preceding the covalent Ullman-type linkage of tetrahalogenated anthracene building blocks. To that end a series of positional isomers was examined and classified with respect to their ability of creation of extended network structures. Our simulations focused on the identification of basic types of self-assembly scenarios distinguishing enantiopure and racemic systems and producing periodic and aperiodic networks. The calculations carried out for selected tectons demonstrated wide possibilities of controlling porosity (e.g. pore size, shape, periodicity, chirality, heterogeneity) of the networks by suitable functionalization of the monomeric unit. The findings reported here can be helpful in rational designing of 2D polymeric networks with predefined structures and properties.

Directing on-surface polymerization via a substrate-directed molecular template

Using a template to control the on-surface polymerization process is valuable for building functional molecular nanostructures. Here, the role of the symmetric matching between a halogen-ligand component (H₂TBrPP) and the substrate for the fabrication of a regular metal-organic structure on Cu(111) and Cu(100) surfaces was studied using scanning tunnelling microscopy (STM). Considering the formation of short-range order polymers on the Au(111) surface via the process of debromination due to the weak directing effect from the substrate to the precursors, a bilayer of ordered assembled structure of H₂TBrPP/Au(111) has been fabricated and the molecules in the top layer are guided by the first-layer molecules. Owing to the steering effect of the substrate-directed molecular template, the H₂TBrPP components in the top layer were polymerized into ordered molecular chain arrays along the given direction that is determined by the initial close-packed assembled structure of H₂TBrPP components during the post-annealing treatment.

Asymmetric Elimination Reaction on Chiral Metal Surfaces

The production of enantiopure materials and molecules is of uttermost relevance in research and industry in numerous contexts, ranging from nonlinear optics to asymmetric synthesis. In the context of the latter, dehalogenation, which is an essential reaction step for a broad class of chemical reactions, is investigated; specifically, dehalogenation of prochiral 5-bromo-7-methylbenz(a)anthracene (BMA) on prototypical, chiral, intermetallic PdGa{111} surfaces under ultrahigh vacuum conditions. Asymmetric halogen elimination is demonstrated by combining temperature-programmed X-ray photoelectron spectroscopy, scanning probe microscopy, and density functional theory. On the PdGa{111} surfaces, the difference in debromination temperatures for the two BMA surface enantiomers amounts up to an unprecedented 46 K. The significant dependence of the dehalogenation temperature of the BMA surface enantiomers on the atomic termination of the PdGa{111} surfaces implies that the ensemble effect is pronounced in this reaction step. These findings evidence enantiospecific control and hence promote intrinsically chiral crystals for asymmetric on-surface synthesis.

Two-Dimensional Polymers and Polymerizations

Synthetic chemists have developed robust methods to synthesize discrete molecules, linear and branched polymers, and disordered cross-linked networks. However, two-dimensional polymers (2DPs) prepared from designed monomers have been long missing from these capabilities, both as objects of chemical synthesis and in nature. Recently, new polymerization strategies and characterization methods have enabled the unambiguous realization of covalently linked macromolecular sheets. Here we review 2DPs and 2D polymerization methods. Three predominant 2D polymerization strategies have emerged to date, which produce 2DPs either as monolayers or multilayer assemblies. We discuss the fundamental understanding and scope of each of these approaches, including: the bond-forming reactions used, the synthetic diversity of 2DPs prepared, their multilayer stacking behaviors, nanoscale and mesoscale structures, and macroscale morphologies. Additionally, we describe the analytical tools currently available to characterize 2DPs in their various isolated forms. Finally, we review emergent 2DP properties and the potential applications of planar macromolecules. Throughout, we highlight achievements in 2D polymerization and identify opportunities for continued study.

Visualizing designer quantum states in stable macrocycle quantum corrals

Creating atomically precise quantum architectures with high digital fidelity and desired quantum states is an important goal in a new era of quantum technology. The strategy of creating these quantum nanostructures mainly relies on atom-by-atom, molecule-by-molecule manipulation or molecular assembly through non-covalent interactions, which thus lack sufficient chemical robustness required for on-chip quantum device operation at elevated temperature. Here, we report a bottom-up synthesis of covalently linked organic quantum corrals (OQCs) with atomic precision to induce the formation of topology-controlled quantum resonance states, arising from a collective interference of scattered electron waves inside the quantum nanocavities. Individual OQCs host a series of atomic orbital-like resonance states whose orbital hybridization into artificial homo-diatomic and hetero-diatomic molecular-like resonance states can be constructed in Cassini oval-shaped OQCs with desired topologies corroborated by joint ab initio and analytic calculations. Our studies open up a new avenue to fabricate covalently linked large-sized OQCs with atomic precision to engineer desired quantum states with high chemical robustness and digital fidelity for future practical applications.

Identification of Topotactic Surface-Confined Ullmann-Polymerization

On-surface Ullmann coupling is an established method for the synthesis of 1D and 2D organic structures. A key limitation to obtaining ordered polymers is the uncertainty in the final structure for coupling via random diffusion of reactants over the substrate, which leads to polymorphism and defects. Here, a topotactic polymerization on Cu(110) in a series of differently-halogenated para-phenylenes is identified, where the self-assembled organometallic (OM) reactants of diiodobenzene couple directly into a single, deterministic product, whereas the other precursors follow a diffusion driven reaction. The topotactic mechanism is the result of the structure of the iodine on Cu(110), which controls the orientation of the OM reactants and intermediates to be the same as the final polymer chains. Temperature-programmed X-ray photoelectron spectroscopy and kinetic modeling reflect the differences in the polymerization regimes, and the effects of the OM chain alignments and halogens are disentangled by Nudged Elastic Band calculations. It is found that the repulsion or attraction between chains and halogens drive the polymerization to be either diffusive or topotactic. These results provide detailed insights into on-surface reaction mechanisms and prove the possibility of harnessing topotactic reactions in surface-confined Ullmann polymerization.

On-Surface Synthesis of Variable Bandgap Nanoporous Graphene

Tuning the bandgap of nanoporous graphene is desirable for applications such as the charge transport layer in organic-hybrid devices. The holy grail in the field is the ability to synthesize 2D nanoporous graphene with variable pore sizes, and hence tunable band gaps. Herein, the on-surface synthesis of nanoporous graphene with variable bandgaps is demonstrated. Two types of nanoporous graphene are synthesized via hierarchical CC coupling, and are verified by low-temperature scanning tunneling microscopy and non-contact atomic force microscopy. Nanoporous graphene-1 is non-planar, and nanoporous graphene-2 is a single-atom thick planar sheet. Scanning tunneling spectroscopy measurements reveal that nanoporous graphene-2 has a bandgap of 3.8 eV, while nanoporous graphene-1 has a larger bandgap of 5.0 eV. Corroborated by first-principles calculations, it is proposed that the large bandgap opening is governed by the confinement of π-electrons induced by pore generation and the non-planar structure. The finding shows that by introducing nanopores or a twisted structure, semi metallic graphene is converted into semiconducting nanoporous graphene-2 or insulating wide-bandgap nanoporous graphene-1.

Rational Design of Two-Dimensional Binary Polymers from Heterotriangulenes for Photocatalytic Water Splitting

On the basis of first-principles calculations, we report the design of three two-dimensional (2D) binary honeycomb-kagome polymers composed of B- and N-centered heterotriangulenes with a periodically alternate arrangement as in hexagonal boron nitride. The 2D binary polymers with donor-acceptor characteristics are semiconductors with a direct band gap of 1.98-2.28 eV. The enhanced in-plane electron conjugation contributes to high charge carrier mobilities for both electrons and holes, about 6.70 and 0.24 × 10³ cm² V^-1 s^-1, respectively, for the 2D binary polymer with carbonyl bridges (2D CTPAB). With appropriate band edge alignment to match the water redox potentials and pronounced light adsorption for the ultraviolet and visible range of spectra, 2D CTPAB is predicted to be an effective photocatalyst/photoelectrocatalyst to promote overall water splitting.

Surface-assisted fabrication of low-dimensional carbon-based nanoarchitectures

On-surface synthesis, as an alternative to traditional in-solution synthesis, has become an emerging research field and attracted extensive attention over the past decade due to its ability to fabricate nanoarchitectures with exotic properties. Compared to wet chemistry, the on-surface synthesis conducted on atomically flat solid surfaces under ultrahigh vacuum exhibits unprecedented characteristics and advantages, opening novel reaction pathways for chemical synthesis. Various low-dimensional nanostructures have been fabricated on solid surfaces (mostly metal surfaces) based on this newly developed approach. This paper reviews the classic and latest works regarding carbon-based low-dimensional nanostructures since the arrival of on-surface synthesis era. These nanostructures are categorized into zero-, one- and two-dimensional classes and each class is composed of numerous sub-nanostructures. For certain specific nanostructures, comprehensive reports are given, including precursor design, substrate choice, synthetic strategies and so forth. We hope that our review will shed light on the fabrication of some significant nanostructures in this young and promising scientific area.

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

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

Quasiparticle electronic structure of two-dimensional heterotriangulene-based covalent organic frameworks adsorbed on Au(111)

The modular nature and unique electronic properties of two-dimensional (2D) covalent organic frameworks (COFs) make them an attractive option for applications in catalysis, optoelectronics, and spintronics. The fabrications of such devices often involve interfaces formed between COFs and substrates. In this work, we employ the first-principlesGWapproach to accurately determine the quasiparticle electronic structure of three 2D carbonyl bridged heterotriangulene-based COFs featuring honeycomb-kagome lattice, with their properties ranging from a semi-metal to a wide-gap semiconductor. Moreover, we study the adsorption of these COFs on Au(111) surface and characterize the quasiparticle electronic structure at the heterogeneous COF/Au(111) interfaces. To reduce the computational cost, we apply the recently developed dielectric embeddingGWapproach and show that our results agree with existing experimental measurement on the interfacial energy level alignment. Our calculations illustrate how the many-body dielectric screening at the interface modulates the energies and shapes of the Dirac bands, the effective masses of semiconducting COFs, as well as the Fermi velocity of the semi-metallic COF.

2D Honeycomb-Kagome Polymer Tandem as Effective Metal-Free Photocatalysts for Water Splitting

On the basis of first-principles calculations, the potential of applying 2D honeycomb-kagome polymers made of heteroatom-centered triangulene derivatives to photocatalyze water splitting is explored. The designed 2D polymers possess indirect bandgaps in the range of 1.80-2.84 eV and show pronounced light absorption in the ultraviolet and visible region of the solar spectrum. With suitable band edge alignment, the examined N- and B-center polymers can generate sufficient photon-excited electrons and holes to activate the hydrogen and oxygen evolution reactions, respectively. The combination of lattice-inherent band features (flat bands) with chemical functionalization (potential shift due to heteroatoms) makes it possible to construct tandem cells with suppressed electron/hole recombination for effective overall water splitting. In addition, there is a potential difference between the half-electrodes that can be utlized to power​ auxiliary components in self-sufficient photocatalyzers.

From starphenes to non-benzenoid linear conjugated polymers by substrate templating

Combining on-surface synthetic methods with the power of scanning tunneling microscopy to characterize novel materials at the single molecule level, we show how to steer the reactivity of one anthracene-based precursor towards different product nanostructures. Whereas using a Au(111) surface with three-fold symmetry results in the dominant formation of a starphene derivative, the two-fold symmetry of a reconstructed Au(110) surface allows the selective growth of non-benzenoid linear conjugated polymers. We further assess the electronic properties of each of the observed product structures via tunneling spectroscopy and DFT calculations, altogether advancing the synthesis and characterization of molecular structures of notable scientific interest that have been only scarcely investigated to date, as applies both to starphenes and to non-benzenoid conjugated polymers.

Searching for kagome multi-bands and edge states in a predicted organic topological insulator

Recently, mixed honeycomb-kagome lattices featuring metal-organic networks have been theoretically proposed as topological insulator materials capable of hosting nontrivial edge states. This new family of so-called "organic topological insulators" are purely two-dimensional and combine polyaromatic-flat molecules with metal adatoms. However, their experimental validation is still pending given the generalized absence of edge states. Here, we generate one such proposed network on a Cu(111) substrate and study its morphology and electronic structure with the purpose of confirming its topological properties. The structural techniques reveal a practically flawless network that results in a kagome network multi-band observed by angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy. However, at the network island borders we notice the absence of edge states. Bond-resolved imaging of the network exhibits an unexpected structural symmetry alteration that explains such disappearance. This collective lifting of the network symmetry could be more general than initially expected and provide a simple explanation for the recurrent experimental absence of edge states in predicted organic topological insulators.

2D Materials and Heterostructures at Extreme Pressure

2D materials possess wide-tuning properties ranging from semiconducting and metallization to superconducting, etc., which are determined by their structure, empowering them to be appealing in optoelectronic and photovoltaic applications. Pressure is an effective and clean tool that allows modifications of the electronic structure, crystal structure, morphologies, and compositions of 2D materials through van der Waals (vdW) interaction engineering. This enables an insightful understanding of the variable vdW interaction induced structural changes, structure-property relations as well as contributes to the versatile implications of 2D materials. Here, the recent progress of high-pressure research toward 2D materials and heterostructures, involving graphene, boron nitride, transition metal dichalcogenides, 2D perovskites, black phosphorene, MXene, and covalent-organic frameworks, using diamond anvil cell is summarized. A detailed analysis of pressurized structure, phonon dynamics, superconducting, metallization, doping together with optical property is performed. Further, the pressure-induced optimized properties and potential applications as well as the vision of engineering the vdW interactions in heterostructures are highlighted. Finally, conclusions and outlook are presented on the way forward.

Atomic-Level Electronic Properties of Carbon Nitride Monolayers

Heteroatom-doped carbon-based materials are of significance for clean energy conversion and storage because of their fascinating electronic properties, low cost, high durability, and environmental friendliness. Atomically precise fabrication of carbon-based materials with well-defined heteroatom-dopant positions and atomic-scale understanding of their atomic-level electronic properties is a challenge. Herein, we demonstrate the bottom-up on-surface synthesis of 1D and 2D monolayer carbon nitride nanostructures with precise control of the nitrogen-atom doping sites and pore sizes. We also observe an electronic band offset at the C-N heterojunction. Using high-resolution scanning tunneling microscopy, the atomic structure of the as-prepared carbon nitride nanoporous monolayers are revealed, indicating successful and precise control of the structures and N atom doping sites. Furthermore, corroborated by theoretical calculations, scanning tunneling spectroscopy measurements reveal a valence band shift of 140 meV that results in an electric field of 2.9 × 10⁸ V m^-1 at the C-N heterojunction, indicating efficient separation of the electron-hole pair at the N doping site. Our finding offers direct atomic-level insights into the local electronic structure of the heteroatom-doped carbon-based materials.

Tailoring π-conjugation and vibrational modes to steer on-surface synthesis of pentalene-bridged ladder polymers

The development of synthetic strategies to engineer π-conjugated polymers is of paramount importance in modern chemistry and materials science. Here we introduce a synthetic protocol based on the search for specific vibrational modes through an appropriate tailoring of the π-conjugation of the precursors, in order to increase the attempt frequency of a chemical reaction. First, we design a 1D π-conjugated polymer on Au(111), which is based on bisanthene monomers linked by cumulene bridges that tune specific vibrational modes. In a second step, upon further annealing, such vibrational modes steer the twofold cyclization reaction between adjacent bisanthene moieties, which gives rise to a long pentalene-bridged conjugated ladder polymer featuring a low bandgap. In addition, high resolution atomic force microscopy allows us to identify by atomistic insights the resonance form of the polymer, thus confirming the validity of the Glidewell and Lloyd´s rules for aromaticity. This on-surface synthetic strategy may stimulate exploiting previously precluded reactions towards π-conjugated polymers with specific structures and properties.