Highly Selective On-Surface Ring-Opening of Aromatic Azulene Moiety.

Controllable ring-opening of polycyclic aromatic hydrocarbons plays a crucial role in various chemical and biological processes. However, breaking down aromatic covalent C-C bonds is exceptionally challenging due to their high stability and strong aromaticity. This study presents a seminal report on the precise and highly selective on-surface ring-opening of the seven-membered ring within the aromatic azulene moieties under mild conditions. The chemical structures of the resulting products were identified using bond-resolved scanning probe microscopy. Furthermore, through density functional theory calculations, we uncovered the mechanism behind the ring-opening process and elucidated its chemical driving force. The key to achieving this ring-opening process lies in manipulating the local aromaticity of the aromatic azulene moiety through strain-induced internal ring rearrangement and cyclodehydrogenation. By precisely controlling these factors, we successfully triggered the desired ring-opening reaction. Our findings not only provide valuable insights into the ring-opening process of polycyclic aromatic hydrocarbons but also open up new possibilities for the manipulation and reconstruction of these important chemical structures.

Geminal-atom catalysis for cross-coupling.

Single-atom catalysts (SACs) have well-defined active sites, making them of potential interest for organic synthesis1-4. However, the architecture of these mononuclear metal species stabilized on solid supports may not be optimal for catalysing complex molecular transformations owing to restricted spatial environment and electronic quantum states5,6. Here we report a class of heterogeneous geminal-atom catalysts (GACs), which pair single-atom sites in specific coordination and spatial proximity. Regularly separated nitrogen anchoring groups with delocalized π-bonding nature in a polymeric carbon nitride (PCN) host7 permit the coordination of Cu geminal sites with a ground-state separation of about 4 Å at high metal density8. The adaptable coordination of individual Cu sites in GACs enables a cooperative bridge-coupling pathway through dynamic Cu-Cu bonding for diverse C-X (X = C, N, O, S) cross-couplings with a low activation barrier. In situ characterization and quantum-theoretical studies show that such a dynamic process for cross-coupling is triggered by the adsorption of two different reactants at geminal metal sites, rendering homo-coupling unfeasible. These intrinsic advantages of GACs enable the assembly of heterocycles with several coordination sites, sterically congested scaffolds and pharmaceuticals with highly specific and stable activity. Scale-up experiments and translation to continuous flow suggest broad applicability for the manufacturing of fine chemicals.

Aromatic Ring Fusion to Benzoporphyrin via γ-ortho Cyclodehydrogenation on a Ag(111) Surface.

Aromatic ring fusion to porphyrins and their derivatives represents an attractive route to tune the molecular conjugation and thus expand their functionalities. Here, we report the expansion of the aromatic π-system of palladium tetraphenyltetrabenzoporphyrins (Pd-TPTBP) via surface-assisted γ-ortho cyclodehydrogenation on Ag(111). The chemical transformation of Pd-TPTBP into different products at an elevated temperature of 600 K was revealed at the single-molecule level using bond-resolved scanning tunneling microscopy with a CO-functionalized tip. We captured a series of γ-ortho cyclodehydrogenation products, wherein the maximum extent to which the reaction can progress is associated with 7-fold C-C formation to afford nearly planar γ-ortho fused porphyrins with 66 conjugated π-electrons. In addition, a small number of molecules undergo C-C bond dissociation of meso-phenyl at elevated temperature, producing fully planar γ-ortho fused products lacking one or two phenyl moieties. Scanning tunneling spectroscopy measurements and DFT calculations suggest the electronic gap of the γ-ortho fused porphyrin decreases compared to that of the precursor. The HOMO and LUMO of the planar γ-ortho fused products are localized on the partially fused benzo moieties and the meso-position, respectively.

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.

Manifold dynamic non-covalent interactions for steering molecular assembly and cyclization.

Deciphering rich non-covalent interactions that govern many chemical and biological processes is crucial for the design of drugs and controlling molecular assemblies and their chemical transformations. However, real-space characterization of these weak interactions in complex molecular architectures at the single bond level has been a longstanding challenge. Here, we employed bond-resolved scanning probe microscopy combined with an exhaustive structural search algorithm and quantum chemistry calculations to elucidate multiple non-covalent interactions that control the cohesive molecular clustering of well-designed precursor molecules and their chemical reactions. The presence of two flexible bromo-triphenyl moieties in the precursor leads to the assembly of distinct non-planar dimer and trimer clusters by manifold non-covalent interactions, including hydrogen bonding, halogen bonding, C-H⋯π and lone pair⋯π interactions. The dynamic nature of weak interactions allows for transforming dimers into energetically more favourable trimers as molecular density increases. The formation of trimers also facilitates thermally-triggered intermolecular Ullmann coupling reactions, while the disassembly of dimers favours intramolecular cyclization, as evidenced by bond-resolved imaging of metalorganic intermediates and final products. The richness of manifold non-covalent interactions offers unprecedented opportunities for controlling the assembly of complex molecular architectures and steering on-surface synthesis of quantum nanostructures.

Tailoring long-range superlattice chirality in molecular self-assemblies via weak fluorine-mediated interactions.

Controllable fabrication of enantiospecific molecular superlattices is a matter of imminent scientific and technological interest. Herein, we demonstrate that long-range superlattice chirality in molecular self-assemblies can be tailored by tuning the interplay of weak intermolecular non-covalent interactions between hexaphenylbenzene-based enantiomers. By means of high-resolution scanning tunneling microscopy measurements, we demonstrate that the functionalization of a hexaphenylbenzene-based molecule with fluorine (F) atoms leads to the formation of molecular self-assemblies with distinct long-range chiral recognition patterns. We employed density functional theory calculations to quantify F-mediated lone pair F⋯π, C-H⋯F, and F⋯F interactions attributed to the distinct enantiospecific molecular self-organizations. Our findings underpin a viable route to fabricate long-range chiral recognition patterns in supramolecular assemblies by engineering the weak non-covalent intermolecular interactions.

Ultrahigh-yield on-surface synthesis and assembly of circumcoronene into a chiral electronic Kagome-honeycomb lattice.

On-surface synthesis has revealed remarkable potential in the fabrication of atomically precise nanographenes. However, surface-assisted synthesis often involves multiple-step cascade reactions with competing pathways, leading to a limited yield of target nanographene products. Here, we devise a strategy for the ultrahigh-yield synthesis of circumcoronene molecules on Cu(111) via surface-assisted intramolecular dehydrogenation of the rationally designed precursor, followed by methyl radical-radical coupling and aromatization. An elegant electrostatic interaction between circumcoronenes and metallic surface drives their self-organization into an extended superlattice, as revealed by bond-resolved scanning probe microscopy measurements. Density functional theory and tight-binding calculations reveal that unique hexagonal zigzag topology of circumcoronenes, along with their periodic electrostatic landscape, confines two-dimensional electron gas in Cu(111) into a chiral electronic Kagome-honeycomb lattice with two emergent electronic flat bands. Our findings open up a new route for the high-yield fabrication of elusive nanographenes with zigzag topologies and their superlattices with possible nontrivial electronic properties.

On-Surface Synthesis and Characterization of [7]Triangulene Quantum Ring.

The ability to engineer geometrically well-defined antidots in large triangulene homologues allows for creating an entire family of triangulene quantum rings (TQRs) with tunable high-spin ground state, crucial for next-generation molecular spintronic devices. Herein, we report the synthesis of an open-shell [7]triangulene quantum ring ([7]TQR) molecule on Au(111) through the surface-assisted cyclodehydrogenation of a rationally designed kekulene derivative. Bond-resolved scanning tunneling microscopy (BR-STM) unambiguously imaged the molecular backbone of a single [7]TQR with a triangular zigzag edge topology, which can be viewed as [7]triangulene decorated with a coronene-like antidot in the center. Additionally, dI/dV mapping reveals that both inner and outer zigzag edges contribute to the edge-localized and spin-polarized electronic states of [7]TQR. Both experimental results and spin-polarized density functional theory calculations indicate that [7]TQR retains its open-shell septuple ground state (S = 3) on Au(111). This work demonstrates a new route for the design of high-spin graphene quantum rings for future quantum devices.

Atomically-precise dopant-controlled single cluster catalysis for electrochemical nitrogen reduction.

The ability to precisely engineer the doping of sub-nanometer bimetallic clusters offers exciting opportunities for tailoring their catalytic performance with atomic accuracy. However, the fabrication of singly dispersed bimetallic cluster catalysts with atomic-level control of dopants has been a long-standing challenge. Herein, we report a strategy for the controllable synthesis of a precisely doped single cluster catalyst consisting of partially ligand-enveloped Au4Pt2 clusters supported on defective graphene. This creates a bimetal single cluster catalyst (Au4Pt2/G) with exceptional activity for electrochemical nitrogen (N2) reduction. Our mechanistic study reveals that each N2 molecule is activated in the confined region between cluster and graphene. The heteroatom dopant plays an indispensable role in the activation of N2 via an enhanced back donation of electrons to the N2 LUMO. Moreover, besides the heteroatom Pt, the catalytic performance of single cluster catalyst can be further tuned by using Pd in place of Pt as the dopant.

Real-Space Imaging of a Single-Molecule Monoradical Reaction.

Organic radicals consisting of light elements exhibit a low spin-orbit coupling and weak hyperfine interactions with a long spin coherence length, which are crucial for future applications in molecular spintronics. However, the synthesis and characterization of these organic radicals have been a formidable challenge due to their chemical instability arising from unpaired electrons. Here, we report a direct imaging of the surface chemical transformation of an organic monoradical synthesized via the monodehydrogenation of a chemically designed precursor. Bond-resolved scanning tunneling microscopy unambiguously resolves various products formed through a complex structural dissociation and rearrangement of organic monoradicals. Density functional theory calculations reveal detailed reaction pathways from the monoradical to different cyclized products. Our study provides unprecedented insights into complex surface reaction mechanisms of organic radical reactions at the single molecule level, which may guide the design of stable organic radicals for future quantum technology applications.

Imprinting Ferromagnetism and Superconductivity in Single Atomic Layers of Molecular Superlattices.

Ferromagnetism and superconductivity are two antagonistic phenomena since ferromagnetic exchange fields tend to destroy singlet Cooper pairs. Reconciliation of these two competing phases has been achieved in vertically stacked heterostructures where these two orders are confined in different layers. However, controllable integration of these two phases in one atomic layer is a longstanding challenge. Here, an interlayer-space-confined chemical design (ICCD) is reported for the synthesis of dilute single-atom-doped TaS2 molecular superlattice, whereby ferromagnetism is observed in the superconducting TaS2 layers. The intercalation of 2H-TaS2 crystal with bulky organic ammonium molecule expands its van der Waals gap for single-atom doping via co-intercalated cobalt ions, resulting in the formation of quasi-monolayer Co-doped TaS2 superlattices. Isolated Co atoms are decorated in the basal plane of the TaS2 via substituting the Ta atom or anchoring at a hollow site, wherein the orbital-selected p-d hybridization between Co and neighboring Ta and S atoms induces local magnetic moments with strong ferromagnetic coupling. This ICCD approach can be applied to various metal ions, enabling the synthesis of a series of crystal-size TaS2 molecular superlattices.