| Supramolecular self-assembly is a powerful technique for manufacturing functional nanomaterials.The properties and functions of supramolecular selfassemblies are closely related to the packing structure of the components,so it is very important to develop methods to regulate the packing structure of assemblies.At present,the main methods of structural regulation include control of assembly pathway,multi-component co-assembly,post-treatment,and so on.Since the difficulty of these methods increases as the dimension of the assemblies increases,successful examples of structural regulation are still mainly limited to zerodimensional(0D)and one-dimensional(1D)self-assembly systems,which have relatively simple interactions.Similar to two-dimensional(2D)covalent materials,2D supramolecular self-assembled materials have significant differences in optical,electrical and mechanical properties from materials with other dimensions,but examples that can achieve controlled regulation of complicated 2D packing structures are limited.Au(Ⅰ)-thiolate 2D assemblies are stable layered networks formed through strong aurophilic interactions between linear Au(Ⅰ)-thoilate coordination polymer chains,which are usually obtained from the reaction of chlorauric acid and thiols.Au(Ⅰ)-thiolate 2D assemblies have a wide range of applications in synthesis of gold nanoparticles/clusters,fabrication of bio-mimic-structures,sensing and detection.Although Au(Ⅰ)-thiolate 2D assemblies have shown excellent application potential in many fields,their molecular-level assembly processes are still unclear,and there is a lack of study on the regulation of their 2D packing structure.To solve these problems,in this thesis,with Au(Ⅰ)-cysteine as an object of research,its molecularlevel assembly processes are studied in detail;methods,such as pathway control and multi-ligand co-assembly,are established for control synthesis of its 2D polymorphs and fine-tune the 2D packing structure;the dependence of chiroptical properties on packing structure is demonstrated.The research work includes the following parts:In the first part of the work,molecular-level assembly processes of Au(Ⅰ)-cysteine are studied in detail;2D supramolecular polymorphism in the system is found,and control synthesis of two 2D supramolecular polymorphs(Nanosheets I and Nanosheets II)is achieved by regulating the assembly pathways.Results show that the formation of Au(Ⅰ)-cysteine 2D assemblies undergoes complex structural transformation process from the coordination polymer chain to 1D helical belts and to 2D lamellar assemblies finally.Due to the complex intermolecular interactions within the assemblies,1D helical belts transform into two types of 2D assemblies,that is,there are two competing assembly pathways.With either chlorauric acid/cysteine or 1D helical belts/cysteine as raw materials,the effects of experimental parameters such as cysteine equiv.to gold,p H value,and concentration,on the assembly pathways are found to be complicated,no matter the synthesis is done at room temperature or boiling temperature.Low equivalent Cys,high p H value and low temperature are beneficial to the synthesis of Nanosheets I;moderate p H and high temperature are beneficial to the synthesis of Nanosheets II.Different from 0D and1 D systems,in which pathway control can usually be achieved with the change of only one experimental parameters,it requires the variation of multiple experimental parameters of cysteine equiv.,p H value and temperature,for pathway control in this 2D system,and assembly pathway control has been extended from a relatively simple 0D and1 D system widely reported to a more complex 2D system.In the second part of the work,a combination of multiple characterization methods is used to reveal the difference of packing structure between Nanosheets I and Nanosheets II;irregular intermolecular interactions are confirmed in these two structures;combining with the structural characteristics of the assembly,the assembly process,the relationship between experimental parameters and pathway control,etc,the principle of realizing pathway control is expounded,and the differences in chiroptical activity of assemblies with different packing structures are shown.Structural characterizations show that the carboxyl groups in ligands of Nanosheets I exist in protonation form,and the inter-ligand interaction is mainly intramolecular hydrogen bond,while the carboxyl groups in ligands of Nanosheets II exist both in protonation form and deprotonation form,and the inter-ligand interactions are both intramolecular hydrogen bonds and intermolecular electrostatic interactions/hydrogen bonds.Because of the above differences,Nanosheets I and II have different packing parameters.Importantly,although Nanosheets II are shown to be more regular than Nanosheets I,the interactions between ligands in both 2D assemblies are irregular,which is significantly different from the regular(or uniform)structural model for other reported supramolecular polymorphs.The ligand conformations cannot be completely regularized during the assembly process is related to the fact that the assemblies are built up with coordination polymers with multiple interactions and the space-confinement effect of the 2D system.Due to the space-confinement,the two 2D structures cannot inter-convert to each other or transform to a more regular structure through heat treatment or other treatments after their formation.Different experimental parameters mainly are found to affect different stages of complex structural transformation processes to affect the assembly pathway.The two 2D polymorphs of Nanosheet I and II demonstrate significantly enhanced chiroptical activity(dissymmetric factor)compared with the 1D helical belts.This kind of assembly with clear structure,the same composition but different packing structures developed by the system show different chiroptical activity activities,which provides the possibility for in-depth study of the important issue of factors affecting the chiroptical activity of assemblies,but there is no clear conclusion yet.In the last part of the work,the co-assembly of three ligands of cysteine,3-mercaptopropionic acid and 1-thioglycerol is used to further fine-tune the packing structure of Au(Ⅰ)-thiolate 2D assembly.Results show that although the conditions for single-ligand assembly are quite different for the three ligands,they can assemble together by copolymerization first.The optimal conditions(assembly pathways)of the three-ligand co-assemblies are not in the middle of the optimal conditions(assembly pathways)of the single-ligands assembly but are similar to the optimal assembly conditions(assembly pathway)of 3-mercaptopropionic acid,which provides repulsion force in the assembly process.The incorporation ratio of different ligands can be controlled by their feeding ratio.Compared with two-ligand coassembly,three-ligand can effectively avoid phase separation by generation of random copolymer without forming single-ligand domains in the assemblies when their feeding ratios are not significantly different,in this way,the packing structure of the assembly can be adjusted in a wide range of ligand composition.In summary,in this work,molecular-level assembly processes of Au(Ⅰ)-cysteine are disclosed;controlled synthesis of two 2D polymorphs of Au(Ⅰ)-cysteine nanosheets(Nanosheet I and II)is achieved by pathway control;the structural differences between the two polymorphs are revealed with multiple characterization methods,and supramolecular polymorphs with irregular intermolecular interactions are confirmed.The significant influences of the packing structures on chiroptical activity are demonstrated.Further,a three-ligand co-assembly method is developed to fine-tune the 2D packing structure.This study provides an example for successful pathway control and structural regulation of 2D assemblies,and it may provide methods for the property optimization and function regulation of Au(Ⅰ)-thiolate 2D assemblies for their applications in a wide field. |
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