Organic molecules are envisioned as the building blocks for design and fabrication of functional devices in future, owing to their versatility, low cost and flexibility. Although some devices such as organic light-emitting diode (OLED) have been already applied in our daily lives, the field is still in its infancy and numerous challenges still remain. In particular, fundamental understanding of the process of organic material fabrication at a molecular level is highly desirable. This thesis focuses on the design and fabrication of supramolecular and macromolecular nanostructures on a Au(111) surface through self-assembly, polymerization and a combination of two. We used scanning tunneling microscopy (STM) as an experimental tool and Monte Carlo (MC) and kinetic Monte Carlo (KMC) simulat...[ Read more ]

Organic molecules are envisioned as the building blocks for design and fabrication of functional devices in future, owing to their versatility, low cost and flexibility. Although some devices such as organic light-emitting diode (OLED) have been already applied in our daily lives, the field is still in its infancy and numerous challenges still remain. In particular, fundamental understanding of the process of organic material fabrication at a molecular level is highly desirable. This thesis focuses on the design and fabrication of supramolecular and macromolecular nanostructures on a Au(111) surface through self-assembly, polymerization and a combination of two. We used scanning tunneling microscopy (STM) as an experimental tool and Monte Carlo (MC) and kinetic Monte Carlo (KMC) simulations as theoretical tools to characterize the structures of these systems and to investigate the mechanisms of the self-assembly and polymerization processes at a single-molecular level.

The results of this thesis consist of four parts as below:

Part I addresses the mechanisms of two-dimensional multicomponent supramolecular self-assembly via pyridyl-Fe-terpyridyl coordination. Firstly, we studied four types of self-assembled metal-organic systems exhibiting different dimensionalities using specifically-designed molecular building blocks. We found that the two-dimensional system is under thermodynamic controls while the systems of lower dimension are under kinetic controls. Secondly, we studied the self-assembly of a series of cyclic supramolecular polygons. Our results indicate that the yield of on-surface cyclic polygon structures is very low independent of temperature and concentration and this phenomenon can be attributed to a subtle competition between kinetic and thermodynamic controls. These results shed light on thermodynamic and kinetic controls in on-surface coordination self-assembly.

Part II addresses the two-dimensional supramolecular self-assembly of porphyrin derivatives. Firstly, we investigated the coordination self-assembly of a series of peripheral bromo-phenyl and pyridyl substituted porphyrins with Fe. 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, but contained nanopores that are functionalized by bromine substitutes. Secondly, we studied a two-dimensional coordination networks formed by 5,10,15,20-tetra(4-pyridyl)porphyrin and Fe. We discovered a novel coordination motif in which a pair of vertically aligned Fe atoms is ligated by four equatorial pyridyl groups. Lateral manipulation, vertical manipulation and tunneling spectroscopy were employed to characterize the networks. These novel coordination networks decorated with Br or vertically aligned Fe atoms may provide potential functions as nano-receptor, molecular magnetism or catalyst.

Part III addresses the mechanism of on-surface Ullmann coupling reaction. We studied Pd- and Cu-catalyzed Ullmann coupling reactions between phenyl bromide functionalized porphyrin derivatives. We discovered that the reactions catalyzed by Pd or Cu can be described as a two-phase process that involves an initial activation followed by C-C bond formation. Analysis of rate constants of the Pd-catalyzed reactions allowed us to determine its activation energy as (0.41 ± 0.03) eV. These results provide a quantitative understanding of on-surface Ullmann coupling reaction.

Part IV addresses the on-surface self-assembly driven by a combination of coordination bonds and covalent bonds. Firstly, we utilized metal-directed template to control the on-surface polymerization process. 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. 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. Secondly, we demonstrated a two-step approach for assembling metal-organic coordination network exhibiting very large pores. The first step involves obtaining one kind of building blocks via on-surface Ullmann coupling and the second step is coordination self-assembly. Moreover, the modulation of the surface-state electrons in the network was studied. These results provide new approaches to design and fabricate on-surface nanostructures.

In summary, we resolved the structures and studied the on-surface assembly and reaction mechanisms of supramolecular and macromolecular nanostructures at a sub-molecular level. These fundamental studies may shed lights on design and fabrication of low-dimensional organic materials.