Kinetically Controlled Self-assembly Pathways And Mechanism Of Au(?)-thiolate Nanosheets

Life system represents the perfect embodiment and zenith of self-assembly.So far,artificial supramolecules are far behind natural supramolecules in terms of structure and functionality.The gap stems from the fact that natural supramolecular systems are complex in compositions,so they often have multiple self-assembly pathways and multiple metastable structures,and their structures and functions are closely related to the self-assembly pathways.In contrast,most artificial supramolecular systems are simple in compositions,and their structures and functions are mainly determined by thermodynamic stability.Thus,it is of great significance to establish complex artificial supramolecular systems and control the self-assembly pathways for the improvement of structural complexity and functionality of the assemblies.Recently,some pioneering studies have successfully realized the kinetic control of some complex self-assembly processes and achieved metastable nanostructures and functions that are otherwise thermodynamically inaccessible.However,multiple parallel and competing pathways often coexist in a complex system,therefore,it is still a great challenge to precisely control the self-assembly pathways and lead to different metastable structures selectively.Among the vast self-assembled materials,Au???-thiolate coordination polymer?ATCP?is a special supramolecular building block.The uniqueness of ATCP materials is that the precursors are formed by complex chemical reactions,and there are multiple-type and multiple-strength weak interactions,such as Au-S coordination,aurophilic interactions between Au???atoms,and inter-ligand interactions including H-bonding and electrostatic interactions depending on the ligands.Therefore,the assembly processes of ATCP are extremely complicated,and its controllable self-assembly is a great challenge.However,the controllable variables such as the relative rate of different reactions,the type and strength of weak interactions and the reaction environment are closely related to the self-assembly processes,which provides many possible methods for controlling the assembly pathways,so ATCP is an ideal system to study complex kinetic processes.The gold?I?-3-mercaptopropionic acid?Au???-MPA?lamellar assemblies have been chosen as the object of study,which are prepared by the reaction of HAuCl₄,MPA-Na and NaOH.Our previous work showed that two competing self-assembly pathways coexisted in Au???-MPA lamellar assemblies,which respectively formed thermodynamically favoured stable multilayer spindle-shaped nanosheets and kinetically favoured metastable single layer triangular nanosheets.Here we explored the methods to control the self-assembly pathways of Au???-MPA and its mechanism.First,we investigated the effects of the degree of hydrolysis of chlorauric acid on the self-assembly pathways.By changing the feeding order of the reactants from mixing NaOH and MPA-Na first to mixing NaOH and HAuCl₄ first,kinetically favored product of single layer assemblies can be obtained.Compared with other methods,such as changing temperature,solvent,concentration and so on,the unique advantage of this method is that it does not change the thermodynamic parameters,which provides convenience for exploring the influence of kinetic effects on the self-assembly,and reflects kinetics play an important role in controlling the morphology of assembly.It lays a foundation for developing more kinetic control tools.Then,we studied how the hydrolysis of HAuCl₄ affect the selection of assembly pathways.Results show the reactivity of Au???is dependent on the degree of its hydrolysis,low degree of hydrolysis of Au???will increase its reactivity with thiol,and lead to aggregated string-shaped preassemblies.In the next step,while high degree of hydrolysis of Au???will decrease its reactivity with thiol,and lead to dispersed string-shaped preassemblies.The dispersed string-shaped preassemblies stack in order and assembled to triangular nanosheets,while the agglomerated strings melt and transform to spindle-shaped nanosheets in situ.Based on the above conclusion,we also effectively controlled the pathway of Au???-MPA to form single layer triangular nanosheets by lowering the temperature or concentration of reaction.Meanwhile,we controlled the string-shaped preassemblies to seriously aggregate with each other by reducing pH value,realizing controlled self-assembly of thermodynamically stable Au???-MPA nanosheets.In short,in view of the fact that most artificial supramolecular assemblies are thermodynamically stable structures,lacking the metastable structure and function,it has become a frontier research in the field of self-assembly that kinetic control over the self-assembly pathways of artificial supramolecules and selectively prepared metastable assemblies.In this thesis,gold?I?-thiolate lamellar assemblies was studied.We controlled the dispersion state of string-shaped preassemblies by changing the feeding order and amount of the reactants,and then selectively prepared the kinetic and thermodynamic products.This work revealed the relationship between the feeding order and amount of the reactants on the dispersion state of string-shaped preassemblies and assembly pathway,and achieved the controllable self-assembly of metastable single-layer assemblies and stable multi-layer assemblies.The study of this work provides an effective tool for controlling the self-assembly pathways and preparing kinetic products,which lays the foundation for further research on the function of metastable assemblies.