Controlling On-surface Synthesis Of Covalent Nanostructure Via Metal-organic Coordination Template

Covalent organic nanoarchitectures synthesized by on-surface coupling reactions in a"bottom-up" manner have high thermal stability and novel electronic/spintronic properties,and have great application prospects in molecular electronics and molecular nanosensors.Numerous nanoarchitectures have been achieved through invoking the intra/inter molecular covalent connections of the organic precursors with predesigned reactive sites.It is essential to realize the controllable synthesis of molecular nanostructure,through inspecting insights into the mechanism of surface covalent coupling reaction and various factors affecting the reaction.However,due to the non-selectivity and irreversibility of the covalent coupling,it is still a great challenge to precisely control surface couplings to obtain specific covalent nanostructures with a single-atom precision.In this thesis,a series of covalent nanostructures are successfully synthesized by on-surface covalent coupling reaction of the brominated organic precursor molecules with functional groups.The on-surface synthesis processes are revealed by using ultra-high vacuum scanning tunneling microscope(UHV-STM)at a single-molecule level.The metal-organic coordination motifs are used to guide surface covalent coupling reaction pathways.The main results include:(1)Using 2,9-dibromo-1,10-phenanthroline(DBP)molecule,we synthesized a covalent macrocyclic nanostructure on a Cu(111)surface.Through the replacement of organic ligands and coordination metal atoms,it was proved that the high yield of macrocyclic structures resulted from the Cu-N metal-organic coordination template.The Cu-N template was formed between the DBP molecule and the Cu adatom released from the copper surface.Such a template effect enhances the cyclization but suppresses the polymerization of the phenanthroline-based precursors,achieving a high yield of the macrocyclic structures.(2)On Cu(111)surfaces,we controled the reaction paths of 6,6’-dibromo-2,2’-bipyridine(DBB)molecules(with a flexible conformation)by introducing Fe-N metal organic coordination motifs,and synthesized a covalent hexagonal structure(namely,SPy).After inspecting the reaction processes of DBB molecules in the presence of Fe atoms at various annealing temperatures,we found a "butterfly" structure was an intermediate species necessary for the formation of the final hexagonal structure cyclo-Sexipyridine(SPy).Finally,we proposed the formation mechanism of covalent hexagonal.In addition,we introduced the Fe-N coordination to the reaction of 2,9-dibromo-1,10-phenanthroline(DBP)(whose conformaiton is inflexible)molecules to investigate the reaction of DBP molecules under different annealing conditions.Finally,by comparing the experimental results for the reactions of DBB and DBP molecules in the presence of Fe-N coordination template,we conclud that the Fe-N coordination template effect enhances the cyclization of DBB molecules with flexible conformaiton structure,but suppresses the cyclization of DBP molecules with inflexible conformaiton.(3)On Au(111)surfaces,we invesigated a selective coupling reaction between 6,6’-dibromo-2,2’-bipyridine(DBB)and 1,4-bis(6,6"-dibromo-[2,2’:6’,2"-terpyridin]-4’-yl)benzene(p-DBTB)molecules,and successfully synthesized,via the Fe-N metal-organic coordination template,a covalent pentagonal structure.