Structural Control Of On-Surface Covalent Reactions

The "bottom-up" synthesis of low-dimensional nanostructures by on-surface covalent coupling reactions has promising prospects for the future applications in molecular electronics.Via the covalent coupling between molecular precursors,the structural and chemical properties of the resultant covalent nanostructures can be controlled with the single-atomic precision.Thereby,it is essential to realize the controllable synthesis of nanostructures,through inspecting insights into the mechanism of surface covalent coupling reaction and the thermodynamic and kinetic factors affecting the reaction.However,due to the non-selectivity and irreversibility of the covalent coupling,it is still a great challenge to construct specific covalent nanostructures by on-surface covalent coupling reactions.In this thesis,porous nitrogen-doped nanoribbons are successfully synthesized by on-surface covalent coupling reactions of the organic precursor molecules with functional groups,revealed by using scanning tunneling microscopy(STM),in combination with X-ray photoelectron spectroscopy(XPS)and density functional theory(DFT).The reactions are successfully controlled by introducing metal-organic coordination bonds as template.The main results include:1.Using 6,6,-Dibromo-2,2,:6’,2,-terpyridine(DT),a molecule consisting of three pyridines(py)and two bromine terminals,as the molecular precursors.Nitrogen-doped porous nanoribbons with different widths were synthesized on Au(111)by selective dehydrogenation coupling between nanochains.The same reaction was carried out on Cu(111)and Ag(111)surfaces to compare the activation ability of C-H bonds on different substrates.2.Investigating the template effect of the coordination bonding over the on-surface covalent coupling.Considering that the pyridine(py)in DT molecule can coordinate with Fe,we introduced Fe-py coordination motifs into the reaction process,realizing the cyclization of DT molecules,which was completely different from the direct coupling mode of DT molecules.Furthermore,we designed a molecular precursor 1,4-bis(6,6,-dibromo+-[2,2’:6’,2’-terpyridin]-4’-yl)benzene(p-DBTB)that possess two DT terminals.Due to the conformational flexibility,p-DBTB has various adsorption configurations on Au(111).In the absence of Fe,the direct covalent coupling of the molecules led to a disordered two-dimensional network structure.With the introduction of Fe-py coordination motifs,The molecular configurations were solely selected out,while the covalent bonding modes were limited in;leaving only a cyclic bonding mode.Finally,the pure organic porous nanoribbons were synthesized.The template effect of the Fe-py coordination towards the covalent reactions was further confirmed by DFT calculations and XPS measurements.In addition,by using an isomer precursor 1,3-bis(6,6,-dibromo-[2,2,:6’,2’-terpyridin]-4’-yl)benzene(m-DBPB),we demonstrated the generality of this template effect.3.Utilizing the different activation temperatures of Br-benzene and Br-pyridine,by using 6,6,-dibromo-4,-(4-bromophenyl)-2,2’:6’,2,-terpyridine(Br-DBT)molecules,to realize the selective synthesis of one-dimensional linear polymers and two-dimensional network structures via step-by-step synthesis.