Self-assembly Of Multi-pyridyl Molecules On Metal Surfaces

Two-dimensional adsorbed system has attracted extensive attentions because of its potential applications in molecular electronics, molecular recognition and catalysis, etc. As a hot research field, there still exist a lot of open questions, especially, the influence of the intermolecular interaction and the molecule-substrate interaction. Molecules containing pyridyl group is important in construction of diverse supramolecular structures with hydrogen bond, which are kinds of important building blocks in self-assemly of molecular patterns. This thesis focuses on the self-assembly of the molecules with multi-pyridyl groups on metal surfaces.In chapter I, we briefly introduce the powerful instrument used in surface science:scanning tunneling microscopy (STM). In the following part, we give the overview of the self-assembly systems on solid surfaces, including the basic principles, significance and present progresses. In the end, we briefly describe the ultra-high vacuum low temperature STM system used in this work.In chapter II, we present the study on the self-assembly of 2,4,6-tris(2-pyridyl)-s-triazine (TPTZ) on Au (111) surface using STM. By systematically changing the coverage of the TPTZ molecules, the self-assemblies of TPTZ form homochiral suprastructures with tunable period undergoes spontaneous chiral resolution. The TPTZ molecules form enantiomorphous domains composed of rhombic supercells with various periods depending on the coverage of molecules, that is,'1×1'and'2 x 2'structures at low coverages, and'6×6','7×7',and'8×8'structures at higher coverages. In a unit cell of a certain enantiomer, the two triangular half-unit cells, consisting of adsorbed TPTZ molecules, are centrosymmetric to each other. The molecules inside each half-unit cell are bound to each other through a single-CH···N-hydrogen bond, while the molecules at the boundaries between half-unit cells are bound through double-CH···N-hydrogen bonds. The STM images and the DFT calculations reveal that the molecules in an enantiomorphous domain adopt the same adsorption orientation of either R-TPTZ or L-TPTZ, which indicates that the adsorbed TPTZ molecules on Au(111) undergo spontaneous chiral resolution. The subtle balance between the intermolecular interaction and the molecule-substrate interaction tunes the period of the superstructure. The total interaction energy densities obtained from the DFT calculations explain the experimental observations quantitatively.In chapterⅢ, following the study of TPTZ, we present the further investigation of functional properties of TPTZ assemblies. By applying proper bias voltage, we found the electric-field causes one of the three pyridyl groups of the TPTZ molecules becoming bright at the domain boundary of the supramolecular pattern. We suggest that in this process the pyridyl group undergoes a conformation change. The theoretical simulations indicate such conformation change is because of the rotation of pyridyl group with N tilting to the Au substrate. The molecules at the boundary may have some free space for the rotation of the pyridyl group, and the intermolecular interaction between the molecules at different domains may help to stabilize such conformation. However, the intermolecular interaction of inner molecules of the same domain has much larger steric hindrance, preventing the rotation of the molecules.In chapterⅣ, we investigated the self-assembly of 3,6-Di-2-yridyl-1,2,4,5-tetrazine (DPTZ) molecules on Ag (111) and Au (111) surfaces. On both of the surfaces, one dimensional chain is formed at low coverage and the structure of the SAM become much complex with the coverage increasing. The STM experiments combined with DFT calculations reveals that the directional hydrogen bond and charge transfer between molecule and substrate induced dipole-dipole repulsion is responsible for the formation of one dimensional chain. The honeycomb network formed on Ag (111) surface kept on 250 K indicates that the low temperature of substrate will trap the molecules in the kinetic limited state. Taking account of the influence of substrate, the chains on Ag (111) surface behave quite different from those on Au (111) surface...