Investigation Of On-Site Reaction Mechanism Of Symmetrically Functionalized Molecules On Typical Metal Surfaces

On-surface synthesis reaction is a newly and quickly developed topic in surface chemistry,which aims at making use of well-defined solid surfaces as confinement templates to initiate chemical reactions,with the primary goal of synthesizing covalently coupled nanostructures directly on solid surfaces.In practice,on-surface reaction represents the extension of heterogeneous catalysis whereby the initial precursors,the intermediate states,and the reaction products all remain in the adsorbed state,usually in the sub-monolayer regime.On-surface reaction comprises various chemical reactions that would not be easily accessible in standard solution chemistry,such as Glaser-type reaction,Ullman reaction,Bergman reaction,aryl-aryl dehydrogenation,Schiff-base reaction and so on.Since the first report of surface Ullmann coupling realized on Au(111)in 2007,numerous efforts have been invested onto the Ullmann coupling with UHV STM and nc-AFM.Of its advantages,one appealing point is that on-surface Ullman coupling reaction can effectively steer reaction paths and structures of final products.However,the comprehensive understanding of reaction mechanism of surface Ullmann coupling has not been fully established yet,while the contributions from the formation of intermediates states,surface atoms,detached halogen atoms are still under explored to achieve mesoscopic scale covalently bound architectures.In this work,the symmetric molecule,2,7-dibromopyrene(Br₂Py,C₁₆H₈Br₂)was selected as the precursor to learn on-surface reaction.Due to the mild catalytic activity,Br₂Py can form a variety of self-assemble structures on Ag(111).Intact Br₂Py molecules can form hexagonal pattern and ladder-shape structure on Ag(111)held at 280 K.Parts of Br₂Py molecules trigger dissymmetric dehalogenation due to the increase of the amount of Ag adatom and the intensification of the movement of Br₂Py.Organometallic dimer is formed via aryl-Ag coupling after dissymmetric dehalogenation,which consists of Kagome structure,and intact Br₂Py molecules form AAB structure.Both of Kagome and AAB structures are stabilized via hydrogen bond and coordination metal on Ag(111)held at room temperature.Only one structure,brick-wall domain structured by organometallic dimer,was formed after deposition of Br₂Py molecules onto the Ag(111)held at 305 K,which means all Br₂Py molecule were debrominated asymmetrically to produce organometallic dimer.Equally,evaporation rate can also control self-assemble structures of Br₂Py molecules on Ag(111)held at room temperature.Each Br₂Py has more chance to collide with Ag adatom,leading to dissymmetric dehalogenation of all molecules to form organometallic dimer under low evaporation rate(1.0 ML/h).With the increase of evaporation rate(12 ML/h),the diffusion of Ag adatom is inhibited,which leads to captured by Br atom at the end of Br₂Py molecules.Br₂Py together Ag adatom form double-hexagon pattern.Although temperature and evaporation rate can regulate Br₂Py molecules to form a variety of self-assemble structures,building block-intact Br₂Py molecules or organometallic dimers-are gathered together through non-covalent interactions such as halogen bond,hydrogen bond or metal coordination bond.Due to the fact that noncovalent interaction in self-assembly cannot determine the reaction path when encountering strong covalent bonding force in coupling:The organometallic dimer is formed firstly,and then the organometallic long chain,and finally the C-C coupling chain.Although the catalytic activity of Ag(111)substrate is moderate,the interaction between Ag(111)and nanostructures is strong,which is not conducive to the transfer of nanostructures.Therefore,Br₂Py molecules deposited onto Bi-Ag(111)interface to research on-surface reaction.The introduction of Bi dramatically decreases catalytic activity of substrate,resulting in the incapability to break the C-Br bond on the Bi-Ag(111)interface held at room temperature.All intact molecules form single self-assemble structure.Subsequent annealing to 380 K results in disappearances of self-assemble phase and C-C coupled nanostructure formed via on-surface Ullmann reaction,simultaneously.Moreover,a small number of Br₂Py together with Bi adatoms form Bi-containing organometallic dimer and tetramer which are converted to C-C coupled chain as temperature rising up 410 K.Further annealing to 530 K,the length of the C-C coupled nanochain increases to 50 nm.In addition to studying on-surface reaction on metal,semimetal-metal,this paper investigates on-surface on oxide,cerium oxide,also.The adsorption and decomposition of functional molecule N,N?-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxdiimide(EP-PTCDI)on cerium oxide substrate coexisting with trivalent cerium(Ce(III))and tetravalent cerium(Ce(IV))were studied by X-ray photoelectron spectroscopy(XPS).EP-PTCDI molecule forms self-assemble structures on cerium oxide held at room temperature,and starts to undertake on-surface reaction after thermal annealing.As the high temperature(673 K)providing energy enough,hydrogenation reaction of the ketone group was initiated with the assistance of Ce(III).Further annealing to higher temperature,Ce(III)was involved in the decomposition of the EP-PTCDI molecule,and the decomposition sequence of functional groups is as follows:firstly,the ethyl propyl group,followed by the hydrogenated ketone group.The focus of this paper is to investigate the mechanism of on-surface reaction of organic molecules on metal,semimetal-metal,and oxide surfaces,and to identify on-surface reaction pathways by a combination scanning tunneling microscopy(STM),XPS,and density functional theory(DFT).It is hoped that this work will help to understand the mechanism of on-surface reaction and provide a reference for the preparation of nanostructures.