Scanning Tunneling Microscopic Studies Of Surface Dynamic Covalent Chemistry Based On Schiff Base Reaction

With the wide study of surface reaction,the strategy for the construction of surface nanostructures has gradually developed from molecular self-assembly to covalent reaction.Due to the reversibility of Schiff base reaction,it can be employed to prepare novel nanoarchitectures at the interface based on principles of dynamic covalent chemistry（DCC）.In addition,host-guest chemistry also provides an important approach to construct surface nanoarchitectures.The host-guest approach can be divided mainly into two categories,one of which is the filling of guest molecule in the cavity of the preformed porous host structure,and the other is guest-induced structural transformation of the host networks.Based on the inducement effect,guest molecule can also behave as an amplifier for the promotion of specific component in the surface dynamic covalent library（DCL）.Therefore,the combination of dynamic covalent chemistry and host-guest interaction provides a new approach for the construction of novel surface nanostructures.The main content is as follows:We have investigated the host-guest interaction between a two-dimensional covalent organic framework（2D COF）,which was synthesized on a highly oriented pyrolytic graphite（HOPG）surface with benzene-1,3,5-tricarbaldehyde and p-phenylenediamine as the precursors,and three guest molecules,namely,coronene,copper phthalocyanine（Cu Pc）,and fluorine-substituted copper phthalocyanine(F₁₆Cu Pc).The host-guest interaction and dynamic behavior were investigated by scanning tunneling microscopy（STM）and density functional theory（DFT）calculations.Coronene shows no selectivity in terms of entrapment in the host pores and,at high concentration,fills all the cavities with fuzzy appearance in the STM image,indicating the mobility of the coronene molecules within the pores.With good match of size and symmetry,Cu Pc and F₁₆Cu Pc molecules are more stably entrapped in the hexagonal cavities.And due to the formation of weak hydrogen bonds between the F₁₆Cu Pc molecules and the host pores,F₁₆Cu Pc shows the strongest interaction with the host network and can be selectively entrapped when coexisting with coronene.Surface-promoted product selection from a dynamic covalent library was investigated in detail at the octanoic acid/HOPG interface by virtue of STM,using4-aminobenzoicacid,5-aminoisophthalicacid,and2,5-bis（octyloxy）-terephthalaldehyde as precursors.Furthermore,the presence of a proper template molecule,in the current case coronene,can further promote the amplification of specific product at the liquid/solid interface by forming supramolecular host-guest architectures.All these results pointed out that,at the liquid/solid interface,not only the adsorption energy of individual molecules but more importantly the total energy of the supramolecular assembly plays a determining role in the selection of products from a dynamic covalent library.Thus besides the assembling properties of the product itself,the addition of a proper guest which can form stable two-dimensional host-guest assembly with a specific product can also play an important role in product selection at the interface.Investigation of DCC between imine and aldehyde was carried out at the octanoic acid/HOPG interface using 2,5-bis（octyloxy）-terephthalaldehyde,4,4’-biphenyldicarboxaldehyde,3,3’-dihydroxy-4,4’-biphenyldicarboxaldehyde and 5-aminoisophthalic acid as precursors.It was found that metathesis took place between 2,5-bis（octyloxy）-terephthalaldehyde and the diimine formed by4,4’-biphenyldicarboxaldehyde and 5-aminoisophthalic acid,while the zigzag pattern of the diimine formed by 3,3’-dihydroxy-4,4’-biphenyldicarboxaldehyde and 5-aminoisophthalic acid was not influenced by the addition of2,5-bis（octyloxy）-terephthalaldehyde,due to the existence of hydroxy group.Besides,coronene-induced structural transformation was observed for both binary and tri-component DCLs,contributing to the amplification of specific product in a DCL.Preparation of novel nanoarchitectures via binary DCC at the n-octanoic acid/HOPG interface was achieved,with 1,3,5-benzenetricarboxaldehyde employed as a tritopic ligand,and 5-aminoisophthalic acid and 4-aminobenzoic acid as nucleophiles,through surface-assisted Schiff base reaction.A two-dimensional crystal of racemate was formed by the diimine molecules which was generated by 1,3,5-benzenetricarboxaldehyde and 4-aminobenzoic acid.In addition,both transimination and metathesis between aldehyde and imine were promoted due to the amplification effect of coronene.