Fine-tuning of two-dimensional metal-organic nanostructures via alkali-pyridyl coordination

Revealing the Orientation Selectivity of Tetrapyridyl-Substituted Porphyrins Constrained in Molecular "Klotski Puzzles"

Understanding and controlling molecular orientations in self-assembled organic nanostructures are crucial to the development of advanced functional nanodevices. Scanning tunneling microscopy (STM) provides a powerful toolbox to recognize molecular orientations and to induce orientation changes on surfaces at the single-molecule level. Enormous effort has been devoted to directly controlling the molecular orientations of isolated single molecules in free space. However, revealing and further controlling molecular orientation selectivity in constrained environments remain elusive. In this study, by a combination of STM imaging/manipulations and density functional theory calculations, we report the orientation selectivity of tetrapyridyl-substituted porphyrins in response to various local molecular environments in artificially constructed molecular "Klotski puzzles" on Au(111). With the assistance of STM lateral manipulations, "sliding-block" molecules were able to enter predefined positions, and specific molecular orientations were adopted to fit the local molecular environments, in which the intermolecular interaction was revealed to be the key to achieving the eventual molecular orientation selectivity. Our results demonstrate the essential role of local molecular environments in directing single-molecule orientations, which would shed light on the design of molecular structures to control preferred orientations for further applications in molecular nanodevices.

Engineering Two-Dimensional Multilevel Supramolecular Assemblies from a Bifunctional Ligand on Au(111)

Herein, we demonstrate the supramolecular assemblies from a bifunctional ligand on Au(111), towards engineering two-dimensional (metal-) organic multilevel nanostructures. The bifunctional ligand employed, including two Br atoms and one carboxylic terminal, offers multiple bonding motifs with different configurations and binding energies. These bonding motifs are highly self-selective and self-recognizable, and thus afford the formation of subunits that contribute to engineering multilevel self-assemblies. Our scanning tunneling microscopy experiments, in combination with the density functional theory calculations, revealed various hydrogen, halogen and alkali-carboxylate bonding motifs dictating the different levels of the assemblies. The multilevel assembly protocol based on a judicious choice of multiple bonding motifs guarantees a deliberate control of surface-confined (metal-) organic nanostructures. Our findings may present new opportunities for the fabrication of complex two-dimensional (metal-) organic nanostructures with potential in applications of functionally diverse nanomaterials.

Construction and Structural Transformation of Metal-Organic Nanostructures Induced by Alkali Metals and Alkali Metal Salts

Metal-organic nanostructures are attractive in a variety of scientific fields, such as biomedicine, energy harvesting, and catalysis. Alkali-based metal-organic nanostructures have been extensively fabricated on surfaces based on pure alkali metals and alkali metal salts. However, their differences in the construction of alkali-based metal-organic nanostructures have been less discussed, and the influence on structural diversity remains elusive. In this work, from the interplay of scanning tunneling microscopy imaging and density functional theory calculations, we constructed Na-based metal-organic nanostructures by applying Na and NaCl as sources of alkali metals and visualized the structural transformations in real space. Moreover, a reverse structural transformation was achieved by dosing iodine into the Na-based metal-organic nanostructures, revealing the connections and differences between NaCl and Na in the structural evolutions, which provided fundamental insights into the evolution of electrostatic ionic interactions and the precise fabrication of alkali-based metal-organic nanostructures.

Exploiting Cooperative Catalysis for the On-Surface Synthesis of Linear Heteroaromatic Polymers via Selective C-H Activation

Regiospecific C−H activation is a promising approach to achieve extended polymers with tailored structures. While a recent on‐surface synthetic approach has enabled regioselective homocoupling of heteroaromatic molecules, only small oligomers have been achieved. Herein, selective C−H activation for dehydrogenative C−C couplings of hexaazatriphenylene by Scholl reaction is reported for the first time. By combining low‐temperature scanning tunneling microscopy (STM) and atomic force microscopy (AFM), we revealed the formation of one‐dimensional polymers with a double‐chain structure. The details of the growth process are rationalized by density functional theory (DFT) calculations, pointing out a cooperative catalytic action of Na and Ag adatoms in steering the C−H selectivity for the polymerization.

Simple lattice model of self-assembling metal-organic layers of pyridyl-substituted porphyrins and copper on Au(111) surface

A simple lattice model of metal-organic adsorption layers self-assembling on a Au(111) surface and based on pyridyl-substituted porphyrins differing in the number of functional groups and their position has been proposed. The model has been parameterized using DFT methods. The ground state analysis of the considered model demonstrates the variety of surface-confined metal-organic networks (SMONs) containing square, linear, and discrete elements appearing in the adsorption layer depending on the partial pressure of the components. The SMONs comprising more symmetrical molecules with a greater number of pyridyl substituents in the porphyrin core exhibit more diverse phase behavior. Structures of the phase diagrams were verified at nonzero temperatures using Grand Canonical Monte Carlo simulations. It was found that the continuous SMONs have higher thermal stability at relatively low partial pressures of the organic component, while the linear and discrete SMONs are more thermally stable at high pressure. Depending on the partial pressure of the organic component, thermal destruction of continuous SMONs occur either through the formation of defects/islands having structures of the linear SMONs, or through the sublimation of individual structural elements. Melting of linear SMONs reveals the appearance of 2D pores or islands of a purely organic phase. The latter fact is confirmed by the experimentally observed coexistence of these phases.