Self-assembly And Reaction Of Functionalized Biphenyl Molecules On Ag(111)

The "bottom-up" surface synthesis establishes an unprecedented avenue toward the fabrication of low-dimensional carbon-based functional nanomaterials.Compared to in-solution organic chemistry,chemical reactions of precursors in surface synthesis primarily take place on solid surfaces.Thus,the surface reactions are likely catalyzed and confined by two-dimensional surfaces,which is a major characteristic for surface synthesis.The surface synthesis can be classified into two perspectives,namely,surface self-assembly and surface reaction.The former mainly refers to the construction of various supramolecular nanostructures via noncovalent interactions including hydrogen bonds,coordination bonds,van der Waals force,and electrostatic interaction.The latter addresses the synthesis of covalently bonded products through C-C coupling reactions.Researchers have already prepared a plethora of nanomaterials bearing unique structures and properties such as polycyclic benzenes,graphene nanoribbons,and two-dimensional supramolecules.However,surface synthesis still faces huge challenges,for example,how to effectively control surface reaction pathways.Precursor design is one of numerous methods to modulate surface synthesis.Typically precursors can be categorized into multiphenyl,acene,heteroatom carbocycle,alkanes and olefins.Molecules featuring multiple phenyls,particularly,biphenyl-type molecules,have receive the most research interests.The investigation into surface self-assembly and reaction of biphenyl-type molecules could provide paramount guidance to researches of the complicated surface synthesis systems.In this thesis,we studied the self-assembly and reaction of biphenyl molecules on surfaces via combined molecular beam epitaxy(MBE),scanning tunneling microscopy(STM),and x-ray photoemission spectroscopy(XPS).Supramolecular tessellations and polycyclic aromatic hydrocarbon were successfully fabricated on metal surfaces.The driving forces for supramolecular tessellations formation and mechanisms underlying aryl-aryl coupling were also revealed.The major results are presented as follows:1)Deposition of 4,4’-dihydroxybiphenyl(DHBP)on the Ag(111)surface at 100 K followed by the annealing at different temperatures results in the stepwise dehydrogenation of hydroxyls.This leads to symmetry transitions of the adsorbed molecules in the sequence of symmetry-asymmetry—symmetry.An evolution of supramolecular tessellations(and close-packed rhombic structure)is observed during the annealing process,including regular(square and hexagonal),semi-regular(tetragonal),demi-regular(Kagome-like and rhombi-hexa-octagonal),and dual(penta-hexa-octagonal)tessellations.In these spontaneously assembled tessellations,an evolution of desymmetrization,then later reestablishment of high-symmetric vertices,is observed with increasing surface temperature.This is determined by the combined influence of the symmetry of adsorbed molecules themselves,symmetry of the substrate lattice,the symmetry match between molecule and lattice,and molecular thermodynamics on the surface.We demonstrate that this desymmetrization and resymmetrization strategy of supramolecular tessellations is efficient and offers rational engineering of various tessellations on surfaces.2)The catalytic effect of Ce atoms on C-Br bond breakage was proved by the comparison of two groups of samples:4-bromoacetylene-biphenyl(BEBP)/Ag(111)&Ce+BEBP/Ag(111).Deposetion of the 4-bromoethynyl-biphenyl(BEBP)molecules onto Ag(111)at 120 K results in the formation of a rare zipper-like and ordered rhombic-like supramolecular structures via Br…H bonds.Meanwhile,some C-Br bonds are broken,but no metal-organic intermediates are formed.In addition,the highly selective synthesis of alkyne dimers is achieved by steering the on-surface Glaser reaction via kinetics and thermodynamics.On the contrary,Ce atoms and BEBP molecules are co-depositied on Ag(111)at 120 K,leading to the complete brekage of the C-Br bond.The metal-organic dimers are observed,which proves that Ce has catalytic effect on the breaking of C-Br bond.On the contrary,Ce and BEBP molecules were co-deposited on Ag(111)at 120 K,leading to the breakage of C-Br bond completely.This work shows that Ce atoms can effectively reduce the activation barrier of C-Br bond and proves that the high chemical activity of Ce atoms in catalytic of C-X bond on the surface.3)The detailed reaction pathway of 2-2 ’-dibromobiphenyl(DBBP)on the Ag(111)surface and the synthesis mechanism of acene have been studied.We demonstrate that reaction pathway of intermolecular C-C coupling of 2,2’-Dibromo-Biphenyl on a Ag(111)surface is different from that in solution.The planar dibenzo[e,l]pyrene is obtained with high selectivity on the Ag(111)surface via aryl-aryl coupling rather than nonplanar saddle-shaped tetraphenylene formed via double Ullmann coupling.The highly regioselective aryl-aryl coupling can be attributed to the 2D confinement of the surface which brings steric hindrance between monomers.Our study demonstrates that PAHs can be prepared differently from those in solution via on-surface synthesis,especially planar molecules.