Nian Lin

Charge-transfer-induced structural rearrangements at both sides of organic/metal interfaces

Organic/metal interfaces control the performance of many optoelectronic organic devices, including organic light-emitting diodes or field-effect transistors. Using scanning tunnelling microscopy, low-energy electron diffraction, X-ray photoemission spectroscopy, near-edge X-ray absorption fine structure spectroscopy and density functional theory calculations, we show that electron transfer at the interface between a metal surface and the organic electron acceptor tetracyano-p-quinodimethane leads to substantial structural rearrangements on both the organic and metallic sides of the interface. These structural modifications mediate new intermolecular interactions through the creation of stress fields that could not have been predicted on the basis of gas-phase neutral tetracyano-p-quinodimethane conformation.

Single-Molecule Resolution of an Organometallic Intermediate in a Surface-Supported Ullmann Coupling Reaction

We have studied the organometallic intermediate of a surface-supported Ullmann coupling reaction from 4, 4″-dibromo-p-terphenyl to poly(para-phenylene) by scanning tunneling microscopy/spectroscopy and density functional theory calculations. Our study reveals at a single-molecular level that the intermediate consists of biradical terphenyl (ph)(3) units that are connected by single Cu atoms through C-Cu-C bridges. Upon further increasing the temperature, the neighboring biradical (ph)(3) units are coupled by C-C bonds forming poly(para-phenylene) oligomers while the Cu atoms are released.

Self-recognition and self-selection in multicomponent supramolecular coordination networks on surfaces

Self-recognition, self-selection, and dynamic self-organization are of fundamental importance for the assembly of all supramolecular systems, but molecular-level information is not generally accessible. We present direct examples of these critical steps by using scanning tunneling microscopy to study mixtures of complementary organic ligands on a copper substrate. The ligands coordinate cooperatively with iron atoms to form well ordered arrays of rectangular multicomponent compartments whose size and shape can be deliberately tuned by selecting ligands of desired length from complementary ligand families. We demonstrate explicitly that highly ordered supramolecular arrays can be produced from redundant ligand mixtures by molecular self-recognition and -selection, enabled by efficient error correction and cooperativity, and show an example of failed self-selection due to error tolerance in the ligand mixture, leading to a disordered structure.

Coordination and Metalation Bifunctionality of Cu with 5,10,15,20-Tetra(4-pyridyl)porphyrin: Toward a Mixed-Valence Two-Dimensional Coordination Network

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.

Surface-confined supramolecular coordination chemistry.

The non-covalent synthesis of coordination compounds and networks provides promising avenues towardsmetal-containing supermolecules and nanostructured materials with ultimate feature definition. An importantfactor for their further development, and their integration and exploitation in nanoscale functional systems,is the capability to prepare or organize them at well-defined substrates or templated environments. Supramolecularengineering on atomistically controlled surfaces has been propelled by the direct insight into low-dimensionalcoordination systems provided by scanning tunneling microscopy observations. Here we discuss the principlesof surface-confined supramolecular coordination chemistry, emphasizingself-assembly protocols conducted on surface atomic lattices employing metal centers to direct the organizationof molecular ligands and the template-induced organization of prefabricated metallosupramolecular species.The presented exemplary molecular-level studies elucidate the arrangement of organic adsorbates and transitionmetal adatoms on low-index metal and graphite surfaces. They reveal the interplay between molecule-adatom,intermolecular, and adsorbate-substrate interactions, which need to be balanced for the fabricationof low-dimensional nanostructures. The control and understanding of both the nature of metal-ligandinteractions and the resulting supramolecular organization on solid surfaces is decisive for the designof advanced architectures with concomitant functions. The realized metallosupramolecular compounds andarrays combine the properties of their constituent metal ions and organic ligands, and feature versatilestructural characteristics as well as attractive functional aspects: their redox, magnetic, spin-state,and electronic transitions.

Porphyrin-Based Two-Dimensional Coordination Kagome Lattice Self-Assembled on a Au(111) Surface

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.

Modular assembly of low-dimensional coordination architectures on metal surfaces

The engineering of highly organized molecular architectures has attracted strong interest because of its potential for novel materials and functional nanoscopic devices. An important factor in the development, integration, and exploitation of such systems is the capability to prepare them on surfaces or in nanostructured environments. Recent advances in supramolecular design on metal substrates provide atomistic insight into the underlying self-assembly processes, mainly by scanning tunneling microscopy observations. This review summarizes progress in noncovalent synthesis strategies under ultra-high vacuum conditions employing metal ions as coordination centers directing the molecular organization. The realized metallosupramolecular compounds and arrays combine the properties of their constituent metal ions and organic ligands, and present several attractive features: their redox, magnetic and spin-state transitions. The presented exemplary molecular level studies elucidate the arrangement of organic adsorbates on metal surfaces, demonstrating the interplay between intermolecular and molecule–substrate interactions that needs to be controlled for the fabrication of low-dimensional structures. The understanding of metallosupramolecular organization and metal–ligand interactions on solid surfaces is important for the control of structure and concomitant function.

Steering On-Surface Polymerization with Metal-Directed Template

On-surface polymerization represents a novel bottom-up approach for producing macromolecular structures. To date, however, most of the structures formed using this method exhibit a broad size distribution and are disorderly adsorbed on the surface. Here we demonstrate a strategy of using metal-directed template to control the on-surface polymerization process. We chose a bifunctional compound which contains pyridyl and bromine end groups as the precursor. Linear template afforded by pyridyl-Cu-pyridyl coordination effectively promoted Ullmann coupling of the monomers on a Au(111) surface. Taking advantage of efficient topochemical enhancement owing to the conformation flexibility of the Cu-pyridyl bonds, macromolecular porphyrin structures that exhibit a narrow size distribution were synthesized. We used scanning tunneling microscopy and kinetic Monte Carlo simulation to gain insights into the metal-directed polymerization at the single molecule level. The results reveal that the polymerization process profited from the rich chemistry of Cu which catalyzed the C-C bond formation, controlled the size of the macromolecular products, and organized the macromolecules in a highly ordered manner on the surface.

Metal-organic coordination interactions in Fe-Terephthalic acid networks on Cu(100)

Metal-organic coordination interactions are prime candidates for the formation of self-assembled, nanometer-scale periodic networks with room-temperature structural stability. We present X-ray photoelectron spectroscopy measurements of such networks at the Cu(100) surface which provide clear evidence for genuine metal-organic coordination. This is evident as binding energy shifts in the O 1s and Fe 3p photoelectron peaks, corresponding to O and Fe atoms involved in the coordination. Our results provide the first clear evidence for charge-transfer coordination in metal-organic networks at surfaces and demonstrate a well-defined oxidation state for the coordinated Fe ions.

Thermodynamics and Selectivity of Two-Dimensional Metallo-supramolecular Self-Assembly Resolved at Molecular Scale

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.