Regulation Of The Structure Activity Relationship Of Molecular Monolayers By Iron/Copper Cations And Water Molecules

Molecular monolayers have many excellent properties due to their ultra-thin thickness and flexible controllability,so they are widely used in many fields.In this paper,the modulation effects of Cu²⁺/Fe³⁺ions on the monolayer structure-activity relationship of 2,4,6-tris（4-carboxyphenyl）-1,3,5-triazine（TATB）molecules on Highly Oriented Pyrolytic Graphite（HOPG）surface have been investigated under atmospheric conditions by scanning tunneling microscopy（STM）.And the modulation effects of water molecules on the monolayer structure-activity relationship of 4,4’’-dibromo-p-terphenyl（DBTP）、5,15-（di-4-ethynylphenyl）porphyrin（DEPP）、4-Azidobenzoic Acid（ABA）molecules on metal surfaces in the ultra-high vacuum environment.The specific results are as follows:1.Cu²⁺/Fe³⁺ions coordinate with TATB molecules through O-Cu-O or O-Fe-O,thereby driving the monolayer to transform from the honeycomb hydrogen bond network structure of pure TATB to a metal-organic coordination structure.Interestingly,after the concentration of Cu²⁺ions in solution is doubled,one Cu²⁺ion maintains coordination with two COOH groups but the number of COOH groups involved in the coordination also doubles,which results in a change in the coordination structure.Increasing the Fe³⁺ion concentration causes each Fe³⁺ion to switch from coordinating with three COOH groups to coordinating with two COOH groups,which lead to the change of coordination structure.It suggests that adjusting the concentration of metal ions in the mixed solution can effectively regulate the metal-organic coordination structure of TATB on the HOPG surface,which is rarely reported.It is further demonstrated by density functional theory（DFT）calculations that metal-organic coordination bonds are the main driving force for the surface monolayer structure after the incorporation of Cu²⁺/Fe³⁺ions.The results indicate that the introduction of iron/copper cations can effectively tune the structure-activity relationship of TATB monolayers.2.Water molecules can induce the metal-organic coordination structure formed by ABA,DBTP and DEPP molecules on the metal single crystal before the introduction of water into a disordered structure or an ordered hydrogen bond structure.Deposition of water molecules can induce the ordered N-Cu-N coordination dimer chain structure formed by ABA molecules on the Cu（110）surface into a disordered structure,and can also induce the ordered C-Cu-C coordination chain structure formed by DBTP molecules on the Cu（110）surface into a disordered structure.With the increase of the deposition amount of water molecules,the pinwheel-like Kagomé-I metal coordination structure formed by ABA molecules on the Cu（111）surface will first transform into a hexagonal petal-like hydrogen bond structure,and then into another Kagomé-II structure.The further deposition of water molecules cannot induce the change of Kagomé-II structure.A sufficient amount of water molecules can induce both the pinwheel-like Kagomémetal coordination structure and the grid-like hydrogen bond structure formed by ABA molecules on the Ag（111）surface into a chain hydrogen bond structure of water incorporation.It is worth mentioning that the reversible cyclic transformation of the monolayer structure of ABA molecules on the Cu（100）surface and DEPP molecules on the Ag（110）surface can be achieved by water molecule induction and heat treatment,respectively.ABA molecules deposited on the surface of Cu（100）will first form a N-Cu-N coordination dimer chain structure,which will transform into a star-shaped N-Cu-N coordination tetramer structure after heating.The deposition of water molecules can induce the star-shaped N-Cu-N coordination tetramer structure to first transform into the N-Cu-N coordination dimer chain structure.The coordination dimer chain structure first transform into a grid-like hydrogen bond structure under the induction of water molecules,and then into a linear hydrogen bond structure of water incorporation.The linear hydrogen bond structure will transform back to the grid-like hydrogen bond structure after heating.DEPP molecules self-assembly into an ordered close-packed structure on the Ag（110）surface,which is transformed into a C-Ag-C metal coordination chain structure after heating,and the deposited water molecules induce the coordination chain structure to turn back the ordered close-packed structure.These results indicate that water molecules can regulate the structure-activity relationship of monolayers,and even achieve reversible cyclic transformation of monolayer structures on some surface through water molecule induction and heat treatment.