Search

The design of open-shell carbon-based nanomaterials is at the vanguard of materials science, steered by their beneficial magnetic properties like weaker spin-orbit coupling than that of transition metal atoms and larger spin delocalization, which are of potential relevance for future spintronics and quantum technologies. A key parameter in magnetic materials is the magnetic exchange coupling (MEC) between unpaired spins, which should be large enough to allow device operation at practical temperatures. In this work, we theoretically and experimentally explore three distinct families of nanographenes (NGs) (A, B, and C) featuring majority zigzag peripheries. Through many-body calculations, we identify a transition from a closed-shell ground state to an open-shell ground state upon an increase of the molecular size. Our predictions indicate that the largest MEC for open-shell NGs occurs in proximity to the transition between closed-shell and open-shell states. Such predictions are corroborated by the on-surface syntheses and structural, electronic, and magnetic characterizations of three NGs (A[3,5], B[4,5], and C[4,3]), which are the smallest open-shell systems in their respective chemical families and are thus located the closest to the transition boundary. Notably, two of the NGs (B[4,5] and C[4,3]) feature record values of MEC (close to 200 meV) measured on the Au(111) surface. Our strategy for maximizing the MEC provides perspectives for designing carbon nanomaterials with robust magnetic ground states.

Creating a good contact between electrodes and graphene nanoribbons (GNRs) has been a long-standing challenge in searching for the next GNR-based nanoelectronics. This quest requires the controlled fabrication of sub-20 nm metallic gaps, a clean GNR transfer minimizing damage and organic contamination during the device fabrication, as well as work function matching to minimize the contact resistance. Here, we transfer 9-atom-wide armchair-edged GNRs (9-AGNRs) grown on Au(111)/mica substrates to pre-patterned platinum electrodes, yielding polymer-free 9-AGNR field-effect transistor devices. Our devices have a resistance in the range of 106-108 Ω in the low-bias regime, which is 2-4 orders of magnitude lower than previous reports. Density functional theory calculations combined with the non-equilibrium Green's function method explain the observed p-type electrical characteristics and further demonstrate that platinum gives strong coupling and higher transmission in comparison to other materials, such as graphene., Applied Physics Letters, 122 (17), ISSN:0003-6951, ISSN:1077-3118

Dehydrogenation reactions are key steps in many metal-catalyzed chemical processes and in the on-surface synthesis of atomically precise nanomaterials. The principal role of the metal substrate in these reactions is undisputed, but the role of metal adatoms remains, to a large extent, unanswered, particularly on gold substrates. Here, we discuss their importance by studying the surface-assisted cyclodehydrogenation on Au(111) as an ideal model case. We choose a polymer theoretically predicted to give one of two cyclization products depending on the presence or absence of gold adatoms. Scanning probe microscopy experiments observe only the product associated with adatoms. We challenge the prevalent understanding of surface-assisted cyclodehydrogenation, unveiling the catalytic role of adatoms and their effect on regioselectivity. The study adds new perspectives to the understanding of metal catalysis and the design of on-surface synthesis protocols for novel carbon nanomaterials. Funding Agencies|Alexander von Humboldt foundation; Swedish Research Council; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [200900971]; MCIN/AEI [PID2020-113142RB-C21, IJCI-2014-19291]; ERDF A way of making Europe [MAT2017-85089-C2-1-R]; ESF Investing in your future [RYC2018-024364-I]; Swiss National Science Foundation [200020_182015, 51NF40-205602]; Knut and Alice Wallenberg (KAW) Foundation; Max Planck Society

Graphene nanoribbons (GNRs) have attracted great research interest because of their widely tunable and unique electronic properties. The required atomic precision of GNRs can be realized via on-surface synthesis method. In this work, through a surface assisted reaction we have longitudinally fused the pyrene-based graphene nanoribbons (pGNR) of different lengths by a pentagon ring junction, and built a molecular junction structure on Au (111). The electronic properties of the structure are studied by scanning tunneling spectroscopy (STS) combined with tight binding (TB) calculations. The pentagon ring junction shows a weak electronic coupling effect on graphene nanoribbons, which makes the electronic properties of the two different graphene nanoribbons connected by a pentagon ring junction analogous to type I semiconductor heterojunctions.

A recently developed bottom-up synthesis strategy enables the fabrication of graphene nanoribbons with well-defined width and non-trivial edge structures from dedicated molecular precursors. Here we discuss the synthesis and properties of zigzag nanoribbons (ZGNRs) modified with periodic cove-cape-cove units along their edges. Contrary to pristine ZGNRs, which show antiferromagnetic correlation of their edge states, the edge-modified ZGNRs exhibit a finite single particle band gap without localized edge states. We report the on-surface synthesis of such edge-modified ZGNRs and discuss tunneling conductance dI/dV spectra and dI/dV spatial maps that reveal a noticeable localization of electronic states at the cape units and the opening of a band gap without presence of edge states of magnetic origin. A thorough ab initio investigation of the electronic structure identifies the conditions under which antiferromagnetically coupled, edge-localized states reappear in the electronic structure. Further modifications of the ribbon structure are proposed that lead to an enhancement of such features, which could find application in nanoelectronics and spintronics.

The on-surface synthesis of graphene nanoribbons (GNRs) allows for the fabrication of atomically precise narrow GNRs. Despite their exceptional properties which can be tuned by ribbon width and edge structure, significant challenges remain for GNR processing and characterization. Herein, Raman spectroscopy is used to characterize different types of GNRs on their growth substrate and track their quality upon substrate transfer. A Raman-optimized (RO) device substrate and an optimized mapping approach are presented that allow for the acquisition of high-resolution Raman spectra, achieving enhancement factors as high as 120 with respect to signals measured on standard SiO2/Si substrates. This approach is well suited to routinely monitor the geometry-dependent low-frequency modes of GNRs. In particular, the radial breathing-like mode (RBLM) and the shear-like mode (SLM) for 5-, 7-, and 9-atom-wide armchair GNRs (AGNRs) are tracked and their frequencies are compared with first-principles calculations.

Organic open-shell compounds are extraordinarily attractive materials for their use in molecular spintronics thanks to their long spin-relaxation times and structural flexibility. Porphyrins (Pors) have widely been used as molecular platforms to craft persistent open-shell structures through solution-based redox chemistry. However, very few examples of inherently open-shell Pors have been reported, which are typically obtained through the fusion of non-Kekulé polyaromatic hydrocarbon moieties to the Por core. The inherent instability and low solubility of these radical species, however, requires the use of bulky substituents and multistep synthetic approaches. On-surface synthesis has emerged as a powerful tool to overcome such limitations, giving access to structures that cannot be obtained through classical methods. Herein, we present a simple and straightforward method for the on-surface synthesis of phenalenyl-fused Pors using readily available molecular precursors. In a systematic study, we examine the structural and electronic properties of three surface-supported Pors, bearing zero, two (PorA2), and four (PorA4) meso-fused phenalenyl moieties. Through atomically resolved real-space imaging by scanning probe microscopy and high-resolution scanning tunneling spectroscopy combined with density functional theory calculations, we unambiguously demonstrate a triplet ground state for PorA2 and a charge-transfer-induced open-shell character for the intrinsically closed-shell PorA4., This work was supported by the Swiss National Science Foundation under Grant No. 200020_182015. Financial support from the Spanish MICINN (CTQ2017-85393-P) andthe ERA-NET/European Commission/MINECO (UNIQUE, SOLAR-ERA.NET Cofund 2 No. 008/PCI2019-111889-2) are acknowledged. IMDEA Nanociencia acknowledges support from the “Severo Ochoa” Programme for Centres of Excellence in R&D (MINECO, Grant SEV2016-0686). R.R. and N.L. are grateful for funding from the EU-FET Open H2020 Mechanics with Molecules project (grant 766864).

Carbon-based magnetic structures promise significantly longer coherence times than traditional magnetic materials, which is of fundamental importance for spintronic applications. An elegant way of achieving carbon-based magnetic moments is the design of graphene nanostructures with an imbalanced occupation of the two sublattices forming the carbon honeycomb lattice. According to Lieb's theorem, this induces local magnetic moments that are proportional to the sublattice imbalance. Exact positioning of sublattice imbalanced nanostructures in graphene nanomaterials hence offers a route to control interactions between induced local magnetic moments and to obtain graphene nanomaterials with magnetically non-trivial ground states. Here, we show that such sublattice imbalanced nanostructures can be incorporated along a large band gap armchair graphene nanoribbon on the basis of asymmetric zigzag edge extensions, which is achieved by incorporating specifically designed precursor monomers during the bottom-up fabrication of the graphene nanoribbons. Scanning tunneling spectroscopy of an isolated and electronically decoupled zigzag edge extension reveals Hubbard-split states in accordance with theoretical predictions. Investigation of pairs of such zigzag edge extensions reveals ferromagnetic, antiferromagnetic or quenching of the magnetic interactions depending on the relative alignment of the asymmetric edge extensions. Moreover, a ferromagnetic spin chain is demonstrated for a periodic pattern of zigzag edge extensions along the nanoribbon axis. This work opens a route towards the design and fabrication of graphene nanoribbon-based spin chains with complex magnetic ground states.

Coronoids, polycyclic aromatic hydrocarbons with geometrically defined cavities, are promising model structures of porous graphene. Here, we report the on-surface synthesis of C168 and C140 coronoids, referred to as [6]- and [5]coronoid, respectively, using 5,9-dibromo-14-phenylbenzo[m]tetraphene as the precursor. These coronoids entail large cavities (>1 nm) with inner zigzag edges, distinct from their outer armchair edges. While [6]coronoid is planar, [5]coronoid is not. Low-temperature scanning tunneling microscopy/spectroscopy and noncontact atomic force microscopy unveil structural and electronic properties in accordance with those obtained from density functional theory calculations. Detailed analysis of ring current effects identifies the rings with the highest aromaticity of these coronoids, whose pattern matches their Clar structure. The pores of the obtained coronoids offer intriguing possibilities of further functionalization toward advanced host-guest applications.

Multiple fused pentagon-heptagon pairs are frequently found as defects at the grain boundaries of the hexagonal graphene lattice and are suggested to have a fundamental influence on graphene-related materials. However, the construction of sp2-carbon skeletons with multiple regularly fused pentagon-heptagon pairs is challenging. In this work, we found that the pentagon-heptagon skeleton of azulene was rearranged during the thermal reaction of an azulene-incorporated organometallic polymer on Au(111). The resulting sp2-carbon frameworks were characterized by high-resolution scanning probe microscopy techniques and feature novel polycyclic architectures composed of multiple regularly fused pentagon-heptagon pairs. Moreover, the calculated analysis of its aromaticity revealed a peculiar polar electronic structure.

We report the optical imaging and absorption spectroscopy on atomically precise armchair graphene nanoribbons (GNRs) on insulating fused silica substrates. This is achieved by controlling light polarization on macroscopically aligned GNRs which greatly enhances the optical contrast of the submonolayer GNRs on the insulating substrates. We measure the linear absorption spectra of 7-armchair and 9-armchair GNRs in this study, and the experimental data agree qualitatively with ab inito calculation results. The polarization spectroscopy technique enables an unambiguous optical identification of GNRs and provides a rapid tool to characterize the transferred film over a large area.

Zethrenes are model diradicaloids with potential applications in spintronics and optoelectronics. Despite a rich chemistry in solution, on-surface synthesis of zethrenes has never been demonstrated. We report the on-surface synthesis of super-heptazethrene on Au(111). Scanning tunneling spectroscopy investigations reveal that super-heptazethrene exhibits an exceedingly low HOMO-LUMO gap of 230 meV and, in contrast to its open-shell singlet ground state in the solution phase and in the solid-state, likely adopts a closed-shell ground state on Au(111).

The electronic, optical, and magnetic properties of graphene nanoribbons (GNRs) can be engineered by controlling their edge structure and width with atomic precision through bottom-up fabrication based on molecular precursors. This approach offers a unique platform for all-carbon electronic devices but requires careful optimization of the growth conditions to match structural requirements for successful device integration, with GNR length being the most critical parameter. In this work, the growth, characterization, and device integration of 5-atom wide armchair GNRs (5-AGNRs) are studied, which are expected to have an optimal bandgap as active material in switching devices. 5-AGNRs are obtained via on-surface synthesis under ultrahigh vacuum conditions from Br- and I-substituted precursors. It is shown that the use of I-substituted precursors and the optimization of the initial precursor coverage quintupled the average 5-AGNR length. This significant length increase allowed the integration of 5-AGNRs into devices and the realization of the first field-effect transistor based on narrow bandgap AGNRs that shows switching behavior at room temperature. The study highlights that the optimized growth protocols can successfully bridge between the sub-nanometer scale, where atomic precision is needed to control the electronic properties, and the scale of tens of nanometers relevant for successful device integration of GNRs.

The precise introduction of nonplanar pores in the backbone of graphene nanoribbon represents a great challenge. Here, we explore a synthetic strategy toward the preparation of nonplanar porous graphene nanoribbon from a predesigned dibromohexabenzotetracene monomer bearing four cove-edges. Successive thermal annealing steps of the monomers indicate that the dehalogenative aryl-aryl homocoupling yields a twisted polymer precursor on a gold surface and the subsequent cyclodehydrogenation leads to a defective porous graphene nanoribbon containing nonplanar [14]annulene pores and five-membered rings as characterized by scanning tunneling microscopy and noncontact atomic force microscopy. Although the C–C bonds producing [14]annulene pores are not achieved with high yield, our results provide new synthetic perspectives for the on-surface growth of nonplanar porous graphene nanoribbons.

Immobilization of organic molecules on metal surfaces and their coupling via thermally induced C–C bond formation is an important technique in organic and polymer synthesis. Using this approach, insoluble and reactive carbon nanostructures can be synthesized and the reactions monitored in situ using scanning probe microscopy methods. The diversity of conceivable products, however, is limited by the number and variety of known on-surface reactions. Here, we introduce the on-surface synthesis of polyarylenes by intermolecular oxidative coupling of isopropyl substituents of arenes. This [3+3] dimerization reaction forms a new phenylene ring and can be regarded as a formal cycloaromatization. The synthetic value of this reaction is proved by the synthesis of polyarylenes and co-polyarylenes, which we demonstrate by synthesizing poly(2,7-pyrenylene-1,4-phenylene). Scanning tunnelling microscopy and non-contact atomic force microscopy studies, complemented by density functional theory calculations, offer mechanistic insight into the on-surface cycloaromatization reaction.

Stacking two-dimensional layered materials such as graphene and transitional metal dichalcogenides with nonzero interlayer twist angles has recently become attractive because of the emergence of novel physical properties. Stacking of one-dimensional nanomaterials offers the lateral stacking offset as an additional parameter for modulating the resulting material properties. Here, we report that the edge states of twisted bilayer zigzag graphene nanoribbons (TBZGNRs) can be tuned with both the twist angle and the stacking offset. Strong edge state variations in the stacking region are first revealed by density functional theory (DFT) calculations. We construct and characterize twisted bilayer zigzag graphene nanoribbon (TBZGNR) systems on a Au(111) surface using scanning tunneling microscopy. A detailed analysis of three prototypical orthogonal TBZGNR junctions exhibiting different stacking offsets by means of scanning tunneling spectroscopy reveals emergent near-zero-energy states. From a comparison with DFT calculations, we conclude that the emergent edge states originate from the formation of flat bands whose energy and spin degeneracy are highly tunable with the stacking offset. Our work highlights fundamental differences between 2D and 1D twistronics and spurs further investigation of twisted one-dimensional systems., Comment: 42 pages, 18 figures

Correction to: Nature https://doi.org/10.1038/s41586-021-03842-3 Published online 13 October 2021.In the version of this Article initially published, there was an error in Equation (1). Further, a placeholder reference was used for ref. 42.

Fractionalization is a phenomenon in which strong interactions in a quantum system drive the emergence of excitations with quantum numbers that are absent in the building blocks. Outstanding examples are excitations with charge e/3 in the fractional quantum Hall effect, solitons in one-dimensional conducting polymers and Majorana states in topological superconductors. Fractionalization is also predicted to manifest itself in low-dimensional quantum magnets, such as one-dimensional antiferromagnetic S = 1 chains. The fundamental features of this system are gapped excitations in the bulk and, remarkably, S = 1/2 edge states at the chain termini, leading to a four-fold degenerate ground state that reflects the underlying symmetry-protected topological order. Here, we use on-surface synthesis to fabricate one-dimensional spin chains that contain the S = 1 polycyclic aromatic hydrocarbon triangulene as the building block. Using scanning tunneling microscopy and spectroscopy at 4.5 K, we probe length-dependent magnetic excitations at the atomic scale in both open-ended and cyclic spin chains, and directly observe gapped spin excitations and fractional edge states therein. Exact diagonalization calculations provide conclusive evidence that the spin chains are described by the S = 1 bilinear-biquadratic Hamiltonian in the Haldane symmetry-protected topological phase. Our results open a bottom-up approach to study strongly correlated quantum spin liquid phases in purely organic materials, with the potential for the realization of measurement-based quantum computation., 23 pages, 4 main figures and 7 extended data figures; supplementary information containing additional data is included

Topological theory has been recently applied in graphene nanoribbons (GNRs) and predicts the existence of topological quantum states in junctions connecting GNRs of different topological classes. Through the periodic alignment of the topological states along a GNR backbone, frontier GNR electronic bands with tunable band gaps and band widths could be generated. In this work, we demonstrate the evolution of the topological band by fabricating GNR structures hosting a single topological junction, dimerized junctions, and multiple coupled junctions with on-surface synthesis, which guarantees the atomic precision of these nanostructures. Their structural and electronic properties are investigated by scanning tunneling microscopy and spectroscopy supported by tight-binding theory. The 1D superlattice of the topological junction states can be described by an effective two-band tight-binding Su-Schrieffer-Heeger (SSH) type model considering two alternating coupling motifs.

We report the on-surface synthesis and gas-phase theoretical studies of two novel nanographenes, namely, dibenzohexacenohexacene and dibenzopentaphenoheptaphene, using 8,8′-dibromo-5,5′-bibenzo[rst]pentaphene as a precursor. These nanographenes display a combination of armchair and zigzag edges, as shown by noncontact atomic force microscopy (nc-AFM), and their electronic properties are elucidated by density functional theory (DFT) calculations which reveal relatively low HOMO-LUMO energy gaps of about 1.75 eV.

In this work, we demonstrate the bottom-up on-surface synthesis of poly( para-dibenzo[ bc, kl]-coronenylene) (PPDBC), a zigzag edge-encased analog of poly( para-phenylene) (PPP), and its lateral fusion into zigzag edge-extended graphene nanoribbons (zeeGNRs). Toward this end, we designed a dihalogenated di( meta-xylyl)anthracene monomer displaying strategic methyl groups at the substituted phenyl ring and investigated its applicability as precursor in the thermally induced surface-assisted polymerization and cyclodehydrogenation. The structure of the resulting zigzag edge-rich (70%) polymer PPDBC was unambiguously confirmed by scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM). Remarkably, by further thermal treatment at 450 °C two and three aligned PPDBC chains can be laterally fused into expanded zeeGNRs, with a ribbon width of nine ( N = 9) up to 17 ( N = 17) carbon atoms. Moreover, the resulting zeeGNRs exhibit a high ratio of zigzag (67%) vs armchair (25%) edge segments and feature electronic band gaps as low as 0.9 eV according to gaps quasiparticle calculations.

Porphyrin nanotapes (Por NTs) have attracted vast interest as potential molecular wires thanks to their exceptional electronic properties. Recently, Por NTs have been synthesized by solution-based methods, demonstrating high versatility and great potential for technological applications. However, their synthesis is tedious and their characterization limited by low solubility and stability. Here, we report the first example of meso-meso triply-fused Por NTs, which are prepared from a readily available Por precursor through a two-step synthesis on Au(111). The structural and electronic properties of individual Por NTs are addressed, both on Au(111) and on a thin insulating NaCl layer, by high-resolution scanning probe microscopy/spectroscopy complemented by density functional theory calculations. Remarkably, the triply-linked Por NTs carry one unpaired electron at each end, which leads to magnetic end states. Our study provides an alternative and versatile route to the fabrication of Por NTs and the atomic-scale characterizations of their structural and electronic properties.

Triangulene and its π-extended homologues constitute non-Kekule polyradical frameworks with high-spin ground states, and are anticipated to be key components of organic spintronic devices. We report a combined in-solution and on-surface synthesis of the hitherto largest triangulene homologue, [7]triangulene (C78H24), consisting of twenty-eight benzenoid rings fused in a triangular fashion. We employ low-temperature scanning tunneling microscopy to confirm the chemical structure of individual molecules adsorbed on a Cu(111) surface. While neutral [7]triangulene in the gas phase is predicted to have an open-shell septet ground state; our scanning tunneling spectroscopy measurements, in combination with density functional theory calculations, reveal chemisorption of [7]triangulene on Cu(111) together with considerable charge transfer, resulting in a closed-shell state. Furthermore, substantial hybridization between the molecular orbitals of [7]triangulene is observed.

Atomically precise electronics operating at optical frequencies require tools that can characterize them on their intrinsic length and time scales to guide device design. Lightwave-driven scanning tunnelling microscopy is a promising technique towards this purpose. It achieves simultaneous sub-ångström and sub-picosecond spatio-temporal resolution through ultrafast coherent control by single-cycle field transients that are coupled to the scanning probe tip from free space. Here, we utilize lightwave-driven terahertz scanning tunnelling microscopy and spectroscopy to investigate atomically precise seven-atom-wide armchair graphene nanoribbons on a gold surface at ultralow tip heights, unveiling highly localized wavefunctions that are inaccessible by conventional scanning tunnelling microscopy. Tomographic imaging of their electron densities reveals vertical decays that depend sensitively on wavefunction and lateral position. Lightwave-driven scanning tunnelling spectroscopy on the ångström scale paves the way for ultrafast measurements of wavefunction dynamics in atomically precise nanostructures and future optoelectronic devices based on locally tailored electronic properties., Here, the authors perform lightwave-driven terahertz scanning tunnelling microscopy and spectroscopy of graphene nanoribbons with atomic resolution in three dimensions, revealing localized wavefunctions that are inaccessible by conventional scanning tunnelling microscopy.

Atomically precise graphene nanoribbons (GNRs) are a promising emerging class of designer quantum materials with electronic properties that are tunable by chemical design. However, many challenges remain in the device integration of these materials, especially regarding contacting strategies. We report on the device integration of uniaxially aligned and non-aligned 9-atom wide armchair graphene nanoribbons (9-AGNRs) in a field-effect transistor geometry using electron beam lithography-defined graphene electrodes. This approach yields controlled electrode geometries and enables higher fabrication throughput compared to previous approaches using an electrical breakdown technique. Thermal annealing is found to be a crucial step for successful device operation resulting in electronic transport characteristics showing a strong gate dependence. Raman spectroscopy confirms the integrity of the graphene electrodes after patterning and of the GNRs after device integration. Our results demonstrate the importance of the GNR-graphene electrode interface and pave the way for GNR device integration with structurally well-defined electrodes.

Herein, we describe the first on-surface preparation of porphyrin nanotapes (Por NTs) on a gold surface. Structural and electronic characterization reveals that the Por NTs carry one unpaired electron at each end, which leads to magnetic end states. This study provides an alternative and versatile route to the fabrication of Por NTs and a detailed atomic-scale characterization of their structural and electronic properties., Porphyrin nanotapes (Por NTs) are promising structures for their use as molecular wires thanks to a high degree of π-conjugation, low HOMO—LUMO gaps, and exceptional conductance. Such structures have been prepared in solution, but their on-surface synthesis remains unreported. Here, meso–meso triply fused Por NTs have been prepared through a two-step synthesis on Au(111). The diradical character of the on-surface formed building block PorA2, a phenalenyl π-extended ZnIIPor, facilitates intermolecular homocoupling and allows for the formation of laterally π-extended tapes. The structural and electronic properties of individual Por NTs are addressed, both on Au(111) and on a thin insulating NaCl layer, by high-resolution scanning probe microscopy/spectroscopy complemented by DFT calculations. These Por NTs carry one unpaired electron at each end, which leads to magnetic end states. Our study provides a versatile route towards Por NTs and the atomic-scale characterization of such tapes., This work was supported by the Swiss National Science Foundation under Grant No. 200020_182015, the European Union's Horizon 2020 research and innovation programme under grant agreement number 785219 (Graphene Flagship Core 2), the Office of Naval Research (N00014-18-1-2708), MINECO, CTQ2017-85393-P (Phthalophoto, T.T.), and PID2020-116490GB-I00 (Porphyrinoids, T.T., G.B.). IMDEA Nanociencia also acknowledges support from the “Severo Ochoa” Programme for Centres of Excellence in R&D (MINECO, Grant SEV-2016-0686). R.R. and N.L. are grateful for funding from the EU-FET Open H2020 Mechanics with Molecules project (grant 766864).

Beginning with the early work of Clar et al. in 1955, zethrenes and their laterally extended homologues, super-zethrenes, have been intensively studied in the solution phase and widely investigated as optical and charge transport materials. Superzethrenes are also considered to exhibit an open-shell ground state and may thus serve as model compounds to investigate nanoscale ��-magnetism. However, their synthesis is extremely challenging due to their high reactivity. We report here the on-surface synthesis of the hitherto largest zethrene homologue - super-nonazethrene - on Au(111). Using single-molecule scanning tunneling microscopy and spectroscopy, we show that super-nonazethrene exhibits an open-shell singlet ground state featuring a large spin polarization-driven electronic gap of 1 eV. Consistent with the emergence of an open-shell ground state, high-resolution tunneling spectroscopy reveals singlet���triplet spin excitations in super-nonazethrene, characterized by a strong intramolecular magnetic exchange coupling of 51 meV. Given the paucity of zethrene chemistry on surfaces, our results therefore provide unprecedented access to large, open-shell zethrene compounds amenable to scanning probe measurements, with potential application in molecular spintronics.

On-surface synthesis provides a powerful approach toward the atomically precise fabrication of π-conjugated ladder polymers (CLPs). We report herein the surface-assisted synthesis of nonbenzenoid CLPs from cyclopenta-annulated anthracene monomers on Au(111) under ultrahigh vacuum conditions. Successive thermal annealing steps reveal the dehalogenative homocoupling to yield an intermediate 1D polymer and the subsequent cyclodehydrogenation to form the fully conjugated ladder polymer. Notably, neighbouring monomers may fuse in two different ways, resulting in six- and five-membered rings, respectively. The structure and electronic properties of the reaction products have been investigated via low-temperature scanning tunneling microscopy and spectroscopy, complemented by density-functional theory calculations. Our results provide perspectives for the on-surface synthesis of nonbenzenoid CLPs with the potential to be used for organic electronic devices., On-surface synthesis provides a powerful approach toward the fabrication of π-conjugated ladder polymers (CLPs). The synthesis of nonbenzenoid CLPs is achieved following two activation steps, including the formation of an intermediate 1D polymer.