From hydrogen bonding to metal coordination and back: Porphyrin-based networks on Ag(111)

Regulation of a Porphyrin Derivative Containing Two Symmetric Benzoic Acids by Different Pyridines

A porphyrin derivative called 5,15-di(4-carboxyphenyl)porphyrin (H₂DCPp) with carboxyl groups successfully self-assembled on a highly oriented pyrolytic graphite (HOPG) surface and its co-assembly structures with three kinds of pyridine molecules were investigated by scanning tunneling microscopy (STM) with atomic resolution. H₂DCPp arranged in a long-range ordered structure, and both 1,4-bis (pyridin-4-ylethynyl) benzene (BisPy), 4,4'-bipyridine (BP) and 1,3,5-tris(pyridin-4-ylethynyl) benzene (TPYB) molecules successfully regulated the host molecules as guest molecules. The well-organized model optimized by density functional theory (DFT) calculations reveals the detailed behavior of the assembly characteristics and regulation of porphyrin derivatives, which is helpful for the research and development of solar cells and nanodevices.

Structural Transformation of Surface-Confined Porphyrin Networks by Addition of Co Atoms

The self-assembly of a nickel-porphyrin derivative (Ni-DPPyP) containing two pyridyl coordinating sites and two pentyl chains at trans meso positions was studied with scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) on Au(111). Deposition of Ni-DPPyP onto Au(111) gave rise to a close-packed network for coverages smaller or equal to one monolayer as revealed by STM and LEED. The molecular arrangement of this two-dimensional network is stabilized via hydrogen bonds formed between the pyridyl's nitrogen and hydrogen atoms from the pyrrole groups of neighboring molecules. Subsequent deposition of cobalt atoms onto the close-packed network and post-deposition annealing at 423 K led to the formation of a Co-coordinated hexagonal porous network. As confirmed by XPS measurements, the porous network is stabilized by metal-ligand interactions between one cobalt atom and three pyridyl ligands, each pyridyl ligand coming from a different Ni-DPPyP molecule.

Surface-mediated ordering of pristine Salen molecules on coinage metals

The conformational isomers of Salen molecules and their self-assembled structures on coinage metal surfaces.

One-Dimensional Double Wires and Two-Dimensional Mobile Grids: Cobalt/Bipyridine Coordination Networks at the Solid/Liquid Interface

Various architectures have been generated and observed by STM at a solid/liquid interface resulting from an in situ chemical reaction between the bipyridine terminal groups of a ditopic ligand and Co(II) ions. Large monodomains of one-dimensional (1D) double wires are formed by Co(II)/ligand coordination, with polymer lengths as long as 150 nm. The polymers are organized as parallel wires 8 nm apart, and the voids between wires are occupied by solvent molecules. Two-dimensional (2D) grids, showing high surface mobility, coexist with the wires. The wires are formed from linear chain motifs where each cobalt center is bonded to two bipyridines. 2D grids are generated from a bifurcation node where one cobalt bonds to three bipyridines. Surface reconstruction of the grids and of the 1D wires was observed under the STM tip. As an exciting result, analysis of these movements strongly indicates surface reactions at the solid/liquid interface.

Self-Assembly and External Modulation of a Flexible Porphyrin Derivative on Highly Oriented Pyrolytic Graphite

With the aid of scanning tunneling microscopy, we have examined the two-dimensional hydrogen-bonded networks of carboxyl-functionalized porphyrin derivative H₂TCPp molecules at the heptanoic acid/HOPG interface. Moreover, we have successfully modulated the self-assembly structure of H₂TCPp by introducing 1,2-di(4-pyridyl)ethylene molecules into the assembled system. By performing density functional theory calculations, we also revealed the formation mechanisms of the different assemblies and the modulation process. Comparing the self-assembly structures at the liquid/solid interface with those in bulk crystals, we have obtained deep insight into the differences in H₂TCPp assemblies between 2D and 3D networks. Furthermore, this research is expected to deepen our understanding of on-surface phenomena and to provide a feasible process toward 2D assembly regulation.

Metal dependent motif transition in a self-assembled monolayer of bipyridine derivatives via coordination: An STM study

Low-dimensional molecular motifs with diversity developed via the on-surface chemistry are attracting growing interest for their potential in advanced nanofabrication. In this work, scanning tunneling microscopy was employed to investigate the in situ and ex situ metal coordinations between 4,4'-ditetradecyl-2,2'-bipyridine (bpy) and Zn(ii) or Cu(ii) ions at a highly oriented pyrolytic graphite (HOPG)/1-phenyloctane interface under ambient conditions. The results demonstrate that the bpy adopts a flat-lying orientation with its substituted alkyl chains in a tail-to-tail arrangement in a bpy monolayer. For the in situ coordination, the bpy/Zn(ii) and bpy/Cu(ii) complexes are aligned in edge-on fashions, wherein the bpy stands vertically on the HOPG surface and interdigitates at the alkyl chains. In the two-dimensional arrays of ex situ coordinated complexes, metal dependent motifs have been observed with Zn(ii) and Cu(ii), wherein the bipyridine moieties are parallel to the graphite surface. These results suggest that the desired on-surface coordination architectures may be achieved by the intentional selection of the metal centers.

Confinement properties of 2D porous molecular networks on metal surfaces

Quantum effects that arise from confinement of electronic states have been extensively studied for the surface states of noble metals. Utilizing small artificial structures for confinement allows tailoring of the surface properties and offers unique opportunities for applications. So far, examples of surface state confinement include thin films, artificial nanoscale structures, vacancy and adatom islands, self-assembled 1D chains, vicinal surfaces, quantum dots and quantum corrals. In this review we summarize recent achievements in changing the electronic structure of surfaces by adsorption of nanoporous networks whose design principles are based on the concepts of supramolecular chemistry. Already in 1993, it was shown that quantum corrals made from Fe atoms on a Cu(1 1 1) surface using single atom manipulation with a scanning tunnelling microscope confine the Shockley surface state. However, since the atom manipulation technique for the construction of corral structures is a relatively time consuming process, the fabrication of periodic two-dimensional (2D) corral structures is practically impossible. On the other side, by using molecular self-assembly extended 2D porous structures can be achieved in a parallel process, i.e. all pores are formed at the same time. The molecular building blocks are usually held together by non-covalent interactions like hydrogen bonding, metal coordination or dipolar coupling. Due to the reversibility of the bond formation defect-free and long-range ordered networks can be achieved. However, recently also examples of porous networks formed by covalent coupling on the surface have been reported. By the choice of the molecular building blocks, the dimensions of the network (pore size and pore to pore distance) can be controlled. In this way, the confinement properties of the individual pores can be tuned. In addition, the effect of the confined state on the hosting properties of the pores will be discussed in this review article.

Formation of Ordered Coronene Clusters in Template Utilizing the Structural Transformation of Hexaphenylbenzene Derivative Networks on Graphite Surface

In the present work, we report the fabrication of regular coronene (COR) clusters on surfaces in ambient conditions in the two-dimensional network formed by hexaphenylbenzene derivatives (HPB) via structural transformation. HPB could form a stable snowflake network structure on the highly oriented pyrolytic graphite surface at the air-solid interface. When COR molecules were introduced into the system, the HPB snowflake network could transform to honeycomb structures, and the COR heptamers were subsequently aggregated and entrapped into the cavity. Scanning tunneling microscopic was employed to monitor the assembly behavior of both HPB and HPB/COR at a submolecule scale level, and density functional theory calculations were utilized to reveal that the structural transformation and the entrapment are the energetically favorable. The pores formed from HPB might also give a clue to immobilizing some functional molecule clusters, like COR, to fabricate their ordered monolayer in ambient conditions, so as to obtain complex supramolecular surface structures.

Observation and modeling of conformational molecular structures driving the self-assembly of tri-adamantyl benzene on Ag(111)

A STM image of the hexagonal network of tri-adamantyl benzene molecules on Ag(111).