Constitutional Dynamics of Metal–Organic Motifs on a Au(111) Surface

Constitutional dynamic chemistry (CDC), including both dynamic covalent chemistry and dynamic noncovalent chemistry, relies on reversible formation and breakage of bonds to achieve continuous changes in constitution by reorganization of components. In this regard, CDC is considered to be an efficient and appealing strategy for selective fabrication of surface nanostructures by virtue of dynamic diversity. Although constitutional dynamics of monolayered structures has been recently demonstrated at liquid/solid interfaces, most of molecular reorganization/reaction processes were thought to be irreversible under ultrahigh vacuum (UHV) conditions where CDC is therefore a challenge to be achieved. Here, we have successfully constructed a system that presents constitutional dynamics on a solid surface based on dynamic coordination chemistry, in which selective formation of metal–organic motifs is achieved under UHV conditions. The key to making this reversible switching successful is the molecule–substrate interaction as revealed by DFT calculations.     

Stabilizing and Organizing Bi3Cu4 and Bi7Cu12 Nanoclusters in Two‐Dimensional Metal–Organic Networks

Multinuclear heterometallic nanoclusters with controllable stoichiometry and structure are anticipated to possess promising catalytic, magnetic, and optical properties. Heterometallic nanoclusters with precise stoichiometry of Bi₃Cu₄ and Bi₇Cu₁₂ can be stabilized in the scaffold of two‐dimensional metal–organic networks on a Cu(111) surface through on‐surface metallosupramolecular self‐assembly processes. The atomic structures of the nanoclusters were resolved using scanning tunneling microscopy and density functional theory calculations. The nanoclusters feature highly symmetric planar hexagonal shapes and core–shell charge modulation. The clusters are arranged as triangular lattices with a periodicity that can be tuned by choosing molecules of different size. This work shows that on‐surface metallosupramolecular self‐assembly creates unique possibilities for the design and synthesis of multinuclear heterometallic nanoclusters.     

Tailoring Large Pores of Porphyrin Networks on Ag(111) by Metal–Organic Coordination

The engineering of nanoarchitectures to achieve tailored properties relevant for macroscopic devices is a key motivation of organometallic surface science. To this end, understanding the role of molecular functionalities in structure formation and adatom coordination is of great importance. In this study, the differences in formation of Cu‐mediated metal–organic coordination networks based on two pyridyl‐ and cyano‐bearing free‐base porphyrins on Ag(111) are elucidated by use of low‐temperature scanning tunneling microscopy (STM). Distinct coordination networks evolve via different pathways upon codeposition of Cu adatoms. The cyano‐terminated module directly forms 2D porous networks featuring fourfold‐coordinated Cu nodes. By contrast, the pyridyl species engage in twofold coordination with Cu and a fully reticulated 2D network featuring a pore size exceeding 3 nm² only evolves via an intermediate structure based on 1D coordination chains. The STM data and complementary Monte Carlo simulations reveal that these distinct network architectures originate from spatial constraints at the coordination centers. Cu adatoms are also shown to form two‐ and fourfold monoatomic coordination nodes with monotopic nitrogen‐terminated linkers on the very same metal substrate—a versatility that is not achieved by other 3d transition metal centers but consistent with 3D coordination chemistry. This study discloses how specific molecular functionalities can be applied to tailor coordination architectures and highlights the potential of Cu as coordination center in such low‐dimensional structures on surfaces.     

Step‐Mediated Anisotropic Adsorption and Condensation of tert‐Butylamine on Cu(111)

Scanning tunneling microscopy (STM) combined with density functional theory (DFT) calculations were applied in studying the anisotropic adsorption and condensation of tert‐butylamine (t‐BA) molecules in the vicinity of the steps on the Cu(111) surface. The preferential adsorption at the upper step edges and uneven distribution of t‐BA in the vicinity of the steps illustrate the asymmetric electronic structure of the surface steps. Our observation demonstrates that the adsorption and diffusion of a polar molecule would be significantly mediated by steps on metal surfaces due to the molecule–step interaction and the intermolecular interactions.     

Controllable Scission and Seamless Stitching of Metal–Organic Clusters by STM Manipulation

Scanning tunneling microscopy (STM) manipulation techniques have proven to be a powerful method for advanced nanofabrication of artificial molecular architectures on surfaces. With increasing complexity of the studied systems, STM manipulations are then extended to more complicated structural motifs. Previously, the dissociation and construction of various motifs have been achieved, but only in a single direction. In this report, the controllable scission and seamless stitching of metal–organic clusters have been successfully achieved through STM manipulations. The system presented here includes two sorts of hierarchical interactions where coordination bonds hold the metal–organic elementary motifs while hydrogen bonds among elementary motifs are directly involved in bond breakage and re‐formation. The key to making this reversible switching successful is the hydrogen bonding, which is comparatively facile to be broken for controllable scission, and, on the other hand, the directional characteristic of hydrogen bonding makes precise stitching feasible.     

Competitive Metal Coordination of Hexaaminotriphenylene on Cu(111) by Intrinsic Copper Versus Extrinsic Nickel Adatoms

The interplay between the self-assembly and surface chemistry of 2,3,6,7,10,11-hexaaminotriphenylene (HATP) on Cu(111) was complementarily studied by high-resolution scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) under ultra-high vacuum conditions. To shed light on the competitive metal coordination, comparative experiments were carried out on pristine and nickel-covered Cu(111). Directly after room-temperature deposition of HATP onto pristine Cu(111), self-assembled aggregates were observed by STM, and XPS results indicated still protonated amino groups. Annealing up to 200 °C activated the progressive single deprotonation of all amino groups as indicated by chemical shifts of both the N 1s and C 1s core levels in the XP spectra. This enabled the formation of topologically diverse π–d conjugated coordination networks with intrinsic copper adatoms. The basic motif of these networks was a metal–organic trimer, in which three HATP molecules were coordinated by Cu₃ clusters, as corroborated by the accompanying density functional theory (DFT) simulations. Additional deposition of more reactive nickel atoms resulted in both chemical and structural changes with deprotonation and formation of bis(diimino)–Ni bonded networks already at room temperature. Even though fused hexagonal metal-coordinated pores were observed, extended honeycomb networks remained elusive, as tentatively explained by the restricted reversibility of these metal–organic bonds.     

Two‐Dimensional Self‐Assembly of a Porphyrin–Polypyridyl Ruthenium(II) Hybrid on HOPG Surface through Metal–Ligand Interactions

The synthesis and self‐assembly behavior of porphyrin–polypyridyl ruthenium(II) hybrid, which consists of a flexible alkyl chain attached with two conjugated moieties is described. The electronic absorption spectrum and emission spectra show that the [C₈‐TPP‐(ip)Ru(phen)₂](ClO₄)₂, abbreviated as (C₈ip)TPPC has optical properties. Scanning tunneling microscopy (STM) studies found that the π–π interaction and metal–ligand interaction allow (C₈ip)TPPC to form self‐assembled structure and have an edge‐on orientation on the highly oriented pyrolytic graphite (HOPG) surface. The multidentate structure in (C₈ip)TPPC molecules act as linkers between the molecules and form metal–ligand coordination, which forces the assembly process in the direction of stable columnar arrays. In addition, although the sample was stored for two months in ambient conditions, STM experiments showed that the order of (C₈ip)TPPC self‐assembly only slightly decreased which indicates that the self‐assembled monolayer is stable. This work demonstrates that introducing a metal‐ligand in the porphyrin‐polypyridyl compound is a useful strategy to obtain novel surface assemblies.     

The direct observation of 2H‐DPP metalation on Pd(111) and Cu/Pd(111) surface

The metalation behaviors of 5,15‐diphenylporphyrin (2H‐DPP) on Pd(111) and Cu/Pd(111) have been investigated using scanning tunneling microscopy and density functional calculations. We show that 2H‐DPP molecules deposited on Pd(111) surface form Pd‐DPP with a proportion of about 75% already at room temperature (RT). This is in contrast to non‐metalation adsorption of 2H‐DPP on Cu–Pd alloy at RT. Annealing to 323 K facilitates the metalation of 2H‐DPP on Cu–Pd alloy island. The comparison of the results indicates that the metalation of 2H‐DPP calls for both enough surface free energy of approaching N? H bond and enough reactivity of breaking N? H bond. Copyright © 2016 John Wiley & Sons, Ltd.     

Molecular and Supramolecular Networks on Surfaces: From Two‐Dimensional Crystal Engineering to Reactivity

The invention of the scanning tunneling microscope has led to the visualization of molecules in real space on atomically flat conductive substrates. This has boosted research into supramolecular chemistry on surfaces. In this Review, we highlight recent developments in the design and functionality of supramolecular surface patterns, with special attention paid to those networks which are chiral or contain a high degree of porosity as well as to the reactivity, which is one of the most important recent developments in supramolecular surface chemistry.     

Separation of Arylenevinylene Macrocycles with a Surface‐Confined Two‐Dimensional Covalent Organic Framework

A two‐dimensional surface covalent organic framework, prepared by a surface‐confined synthesis using 4,4′‐azodianiline and benzene‐1,3,5‐tricarbaldehyde as the precursors, was used as a host network to effectively immobilize arylenevinylene macrocycles (AVMs). Thus AVMs could be separated from their linear polymer analogues, which are the common side‐products in the cyclooligomerization process. Scanning tunneling microscopy investigations revealed efficient removal of linear polymers by a simple surface binding and solvent washing process.     

Controlling the Self‐Metalation Rate of Tetraphenylporphyrins on Cu(111) via Cyano Functionalization

The reaction rate of the self‐metalation of free‐base tetraphenylporphyrins (TPPs) on Cu(111) increases with the number of cyano groups (n=0, 1, 2, 4) attached at the para positions of the phenyl rings. The findings are based on isothermal scanning tunneling microscopy (STM) measurements. At room temperature, all investigated free‐base TPP derivatives adsorb as individual molecules and are aligned with respect to densely packed Cu substrate rows. Annealing at 400 K leads to the formation of linear dimers and/or multimers via CN‐Cu‐CN bonds, accompanied by self‐metalation of the free‐base porphyrins following a first‐order rate equation. When comparing the non‐cyano‐functionalized and the tetracyano‐functionalized molecules, we find a decrease of the reaction rate by a factor of more than 20, corresponding to an increase of the activation energy from 1.48 to 1.59 eV. Density functional theory (DFT) calculations give insights into the influence of the peripheral electron‐withdrawing cyano groups and explain the experimentally observed effects.     

Comparing the Self-Assembly of Sexiphenyl-Dicarbonitrile on Graphite and Graphene on Cu(111)

A comparative study on the self-assembly of sexiphenyl-dicarbonitrile on highly oriented pyrolytic graphite and single-layer graphene on Cu(111) is presented. Despite an overall low molecule–substrate interaction, the close-packed structures exhibit a peculiar shift repeating every four to five molecules. This shift has hitherto not been reported for similar systems and is hence a unique feature induced by the graphitic substrates.     

Diradical Organic One‐Dimensional Polymers Synthesized on a Metallic Surface

We report on the synthesis and characterization of atomically precise one‐dimensional diradical peripentacene polymers on a Au(111) surface. By means of high‐resolution scanning probe microscopy complemented by theoretical simulations, we provide evidence of their magnetic properties, which arise from the presence of two unpaired spins at their termini. Additionally, we probe a transition of their magnetic properties related to the length of the polymer. Peripentacene dimers exhibit an antiferromagnetic (S=0) singlet ground state. They are characterized by singlet–triplet spin‐flip inelastic excitations with an effective exchange coupling (J_eff) of 2.5 meV, whereas trimers and longer peripentacene polymers reveal a paramagnetic nature and feature Kondo fingerprints at each terminus due to the unpaired spin. Our work provides access to the precise fabrication of polymers featuring diradical character which are potentially useful in carbon‐based optoelectronics and spintronics.     

Intermixed Adatom and Surface‐Bound Adsorbates in Regular Self‐Assembled Monolayers of Racemic 2‐Butanethiol on Au(111)

In situ scanning tunneling microscopy combined with density functional theory molecular dynamics simulations reveal a complex structure for the self‐assembled monolayer (SAM) of racemic 2‐butanethiol on Au(111) in aqueous solution. Six adsorbate molecules occupy a (10×√3)R30° cell organized as two RSAuSR adatom‐bound motifs plus two RS species bound directly to face‐centered‐cubic and hexagonally close‐packed sites. This is the first time that these competing head‐group arrangements have been observed in the same ordered SAM. Such unusual packing is favored as it facilitates SAMs with anomalously high coverage (30 %), much larger than that for enantiomerically resolved 2‐butanethiol or secondary‐branched butanethiol (25 %) and near that for linear‐chain 1‐butanethiol (33 %).