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We demonstrate an enhancement of cyclization against polymerization for the on-surface debrominative coupling reaction, by using the metal-organic Cu-N coordination template to direct the reaction pathways. Experiments performed by using ultrahigh-vacuum scanning tunneling microscopy (UHV-STM), with the substitution of metal-coordination centers, metallic substrates and functional organic ligands, corroborate the template effect of the Cu-N coordination.

We have achieved an on-surface synthesis of giant conjugated macrocycles of diameter ~7 nm consisting of up to 30 subunits. The synthesis was initiated from a debrominative coupling of the molecular precursors occurring on a hot Ag(111) surface, leading to the formation of arched oligomeric chains and macrocycles. These products, revealed by scanning tunneling microscopy in combination with density functional theory, were covalent oligomers assisted by C-Ag organometallic bonds between parallel molecular subunits, due to a site-selective debromination and the asymmetric molecular conformation. A consecutive cyclodehydrogenation activated by thermal treatments at higher temperatures steered the final conjugation of the macrocycles. Our finding provides a novel design strategy toward π-conjugated macrocycles, and opens up new opportunities for precise synthesis of organic nanostructures.

Using scanning tunnelling microscopy (STM), we demonstrate that Au-pyridyl coordination can be used to assemble two-dimensional coordination network structures on metal surfaces. The polymorphism of the coordination network structures can be manipulated at both the micro- and nanoscale. Using the same organic ligand, we assembled two distinct polymorphic network structures, which were assisted by threefold Au-pyridyl coordination on Ag(111) with predeposited Au atoms (α-network), and by twofold Au-pyridyl coordination on Au(111) (β-network), respectively. Specifically on the Au(111) surface, single-oriented β-network domains as large as ≈400 nm were selected by thermal annealing. We ascribe this global control strategy to distinct Au bonding modes tuned by molecule-substrate interactions. Using an STM tip, we succeeded in creating α-network domains (≈10 nm) locally within the homogeneous β-network domain areas on Au(111) in a controlled manner.

On-surface Ullmann coupling reaction of aryl chlorides has been achieved on Cu(111), Ag(111), and Au(111), and the mechanism has been investigated on the single molecule level using scanning tunneling microscopy and density functional theory. The different reactivity of the aryl halides was utilized to design a stepwise on-surface synthesis, which affords a zigzag template and then converts to 2D porous networks.

A self-assembled Fe-porphyrin coordination chain structure on a Au(111) surface is investigated by scanning tunneling microscopy (STM), revealing structural reconstruction resulting from an alternative change of molecular orientations and spontaneous formation of uniformly sized Fe polynuclears. The alternation of the molecular orientations is ascribed to the cooperation of the attractive coordination and the intermolecular steric repulsion as elucidated by high-resolution STM observations. Furthermore, chemical control experiments are carried out to determine the number of atoms in an Fe polynuclear, suggesting a tentative Fe dinuclear-module that serves not only as a coordination center to link porphyrin units together but also as a "dangling" site for further functionalization by a guest terpyridine ligand. The chain structure and the Fe polynuclears are stable up to 320 K as revealed by real-time STM scanning. Annealing at higher temperatures converts the chain structure into a two-dimensional coordination structure.

Hierarchical control of chemical reactions is being considered as one of the most ambitious and challenging topics in modern organic chemistry. In this study, we have realized the one-by-one scission of the X–H bonds (X = N and C) of aromatic amines in a controlled fashion on the Cu(111) surface. Each dehydrogenation reaction leads to certain metal–organic supramolecular structures, which were monitored in single-bond resolution via scanning tunneling microscopy and noncontact atomic force microscopy. Moreover, the reaction pathways were elucidated from X-ray photoelectron spectroscopy measurements and density functional theory calculations. Our insights pave the way for connecting molecules into complex structures in a more reliable and predictable manner, utilizing carefully tuned stepwise on-surface synthesis protocols.

Single molecules of 1,1,2,3,4,5-hexaphenylsilole adsorbed on Cu(111) have been investigated using low-temperature scanning tunneling microscopy, scanning tunneling spectroscopy, and quantum chemistry calculations. Two adsorption states have been identified, showing distinctive tunneling conductance. The molecules can switch their states under tip influence. Theoretical calculations indicate that the two states are associated with molecules adsorbed at two 90°-rotated orientations, and the tunneling conductance is attributed to molecular orbitals that spatially bridge tip-to-substrate gap. Our findings demonstrate a decisive dependence of single-molecule conductance on the molecular orientation with respect to electrodes.

Using scanning tunneling microscopy, the coordination self-assembly of a series of peripheral bromo-phenyl and pyridyl substituted porphyrins with Fe was studied on an Au(111) surface. The porphyrins functionalized with two trans-pyridyl groups afford extended hexagonal frameworks and the porphyrins functionalized with three pyridyl groups generate discrete rosette and extended chiral kagome framework structures. The self-assembly of the porphyrin derivatives in which phenyl groups are substituted by bromo-phenyl results in coordination networks exhibiting identical structures to that of the parent compounds. These structures contain nanocavities decorated with Br, which provide potential for covalent functionalization.

We investigated the coordination self-assembly and metalation reaction of Cu with 5,10,15,20-tetra(4-pyridyl)porphyrin (2HTPyP) on a Au(111) surface by means of scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. 2HTPyP was found to interact with Cu through both the peripheral pyridyl groups and the porphyrin core. Pairs of pyridyl groups from neighboring molecules coordinate Cu(0) atoms, which leads to the formation of a supramolecular metal-organic coordination network. The network formation occurs at room temperature; annealing at 450 K enhances the process. The interaction of Cu with the porphyrin core is more complex. At room temperature, formation of an initial complex Cu(0)-2HTPyP is observed. Annealing at 450 K activates an intramolecular redox reaction, by which the coordinated Cu(0) is oxidized to Cu(II) and the complex Cu(II)TPyP is formed. The coordination network consists then of Cu(II) complexes linked by Cu(0) atoms; that is, it represents a mixed-valence two-dimensional coordination network consisting of an ordered array of Cu(II) and Cu(0) centers. Above 520 K, the network degrades and the Cu atoms in the linking positions diffuse into the substrate, while the Cu(II)TPyP complexes form a close-packed structure that is stabilized by weak intermolecular interactions. Density functional theory investigations show that the reaction with Cu(0) proceeds via formation of an initial complex between metal atom and porphyrin followed by formation of Cu(II) porphyrin within the course of the reaction. The activation barrier of the rate limiting step was found to be 24-37 kcal mol(-1) depending on the method used. In addition, linear coordination of a Cu atom by two CuTPyP molecules is favorable according to gas-phase calculations.

The coordination assembly of 1,3,5-trispyridylbenzene with Cu on a Au(111) surface has been investigated by scanning tunneling microscopy under ultrahigh vacuum conditions. An open two-dimensional (2D) metal-organic network of honeycomb structure is formed as the 2D network covers partial surface. Upon the 2D network coverage of the entire surface, further increment of molecular density on the surface results in a multistep nonreversible structural transformation in the self-assembly. The new phases consist of metal-organic networks of pentagonal, rhombic, zigzag, and eventually triangular structures. In addition to the structural change, the coordination configuration also undergoes a change from the two-fold Cu-pyridyl binding in the honeycomb, pentagonal, rhombic and zigzag structures to the three-fold Cu-pyridyl coordination in the triangular structure. As the increment of molecular packing density on the surface builds up intrinsic in-plane compression pressure in the 2D space, the transformation of the structure, as well as the coordination binding mode, is attributed to the in-plane compression pressure. The quantitative structural analysis of the various phases upon molecular density increment allows us to construct a phase diagram of network structures as a function of the in-plane compression.

We investigated the thermodynamic processes of two-dimensional (2D) metallo-supramolecular self-assembly at molecular resolution using scanning tunneling microscopy and variable-temperature low-energy electron diffraction. On a Au(111) substrate, tripyridyl ligands coordinated with Cu in a twofold Cu−pyridyl binding mode or with Fe in a threefold Fe−pyridyl binding mode, forming a 2D open network structure in each case. The network structures exhibited remarkable thermal stability (600 K for the Cu-coordinated network and 680 K for the Fe-coordinated network). The Fe−pyridyl binding was selected thermodynamically as well as kinetically in self-assembly involving both modes. The selectivity can be effectively suppressed in a specifically designed self-assembly route.

Surface-supported supramolecular self-assembly has been used to generate multicomponent two-dimensional metal−organic coordination networks on a Au(111) surface. The networks consist of linker ligands of 4′,4′′′′-(1,4-phenylene)bis(2,2′:6′,2′′-terpyridine) and nodal ligands of 5,10,15,20-tetra(4-pyridyl)porphyrin that are connected by pyridine−Fe−terpyridine motifs. Scanning tunneling microscopy revealed the coexistence of two polymorphic types of network structures (rhombus and Kagome). Through control of the dosage of the constituent ligands, homogeneous structural phases were obtained selectively. In particular, the rhombus structure could be converted into the more complex and more open Kagome structure by inclusion of guest molecules. Finally, coordination networks providing structural and chemical homogeneity were realized by judiciously choosing the dosages of the constituent ligands and the chemical substitution of the porphyrin ligands.

Self-assemblies of porphyrin-based molecules (tetra-pyridyl-porphyrin (TPyP) and ZnTPyP) on different single-crystal metallic substrates have been investigated using scanning tunneling microscopy (STM) in the ultra high vacuum (UHV) environment.

Four types of metal-organic structures exhibiting specific dimensionality were studied using scanning tunneling microscopy and Monte Carlo simulations. The four structures were self-assembled out of specifically designed molecular building blocks via the same coordination motif on an Au(111) surface. We found that the four structures behaved differently in response to thermal annealing treatments: The two-dimensional structure was under thermodynamic control while the structures of lower dimension were under kinetic control. Monte Carlo simulations revealed that the self-assembly pathways of the four structures are associated with the characteristic features of their specific heat. These findings provide insights into how the dimensionality of supramolecular coordination structures affects their thermodynamic properties.

A two-dimensional network of metal-coordinated Kagome lattice has been self-assembled by free-base tetrapyridyl porphyrin molecules on a Au(111) surface. Au ad-atoms offered by the surface coordinate the pyridyl functions of the neighboring porphyrins in a 2-fold linear coordination. Comparison of the self-assemblies of the same molecules on various surfaces (Au, Ag, and Cu) underlines the fact that both structural and chemical characteristics of surfaces play important roles in determining the supramolecular adlayer structures.

On a Au(111) surface, a mixture of two types of molecular ligands assembles with Cu generating two homogeneous and one heterogeneous metallo-supramolecular structures owing to same pyridyl–Cu–pyridyl coordination. The proportion of the three phases in the output structures is regulated by adjusting the ratio of the two components in the input molecular mixture. As the ratio approaches the stoichiometric value of the heterogeneous structure, one of the ligands is 100% incorporated in the heterogeneous phase, manifesting a unique amplification behaviour in this multiple output self-assembly.