Constructions And Properties Of Stimuli-Responsive Coordination-driven Self-Assembly Systems

Supramolecular chemistry is an emerging realm that embraces the recognition and assembly of multiple chemical components through non-covalent interactions.In common non-covalent interactions,coordination bonds have high directionality and strong bond energy,and are therefore widely used as a driving force for the constructions of supramolecular metal-organic assemblies,such as metallacycles,metallacages,and helicates.However,due to the intrinsic dynamic nature,coordination bonds can sometimes result in systems composing of multiple separate assemblies,and it is difficult to apply separation methods in traditional molecular chemistry to supramolecular metal-organic assemblies.This has led to an increasing interest in convenient and controllable stimuli-responsive methods where external physical and chemical stimuli,such as p H,light,temperature,concentration,and guest templates,are utilized to achieve structural transformations between different assemblies in mixed systems.Additionally,modifying some functional units in coordination assemblies can serve as good output signals.These studies provide a foundation for constructing supramolecular functional materials with "switchable" functions.Starting from the coordination-driven self-assembly strategy for constructing metal-organic assemblies,this thesis investigates the structural transformations that occur within different assembly systems under multiple stimuli.The main content is divided into three parts:Chapter 1 is an introduction,illustrating the basic concepts of supramolecular chemistry and the important role of coordination-driven self-assembly.Then,several types of supramolecular metal-organic assemblies constructed by coordination-driven self-assembly strategies are described in detail,including metallacycles,metallacages,and helicates.Next,a type of mild and efficient way to induce structural transformations between metal-organic assemblies,that is,stimuli response,is introduced,and common physical and chemical conditions used for stimuli response,such as light,concentration,temperature,solvents,and guests,are summarized.Several output signals that can intuitively reflect the structural transformation of supramolecular assemblies are also introduced,including color,phase transition,and conductivity.Finally,based on the results of literature research,the problems and challenges existing in this field at present are identified,including the lack of diversity in the stimulus response regulation methods of structural transformations,the lack of quantitative analysis of thermodynamics and kinetics,and the limited types of output signals reflecting structural transformations.This helps to determine the research goal of this paper: to design and synthesize metal-organic assembly systems with multi-stimulus response capabilities,and to systematically reveal the mechanisms behind the structural transformations of these assemblies through qualitative and quantitative analysis.Additionally,new functional units are introduced as output signals to intuitively reflect the transformations between assemblies.In Chapter 2,we constructed two metallacycles containing free radicals and studied the changes in the free radical signals during the structural transformation of the assemblies.First,we synthesized ligands containing bipyridine moieties and achieved assembly using a directed bonding strategy.Then,we confirmed the accuracy of the structures using single-crystal X-ray diffraction.Next,we utilized the reversibility of Pt-N coordination bonds and the dynamic reversibility of imine bonds to achieve the transformation of the free radical macrocycle structures using p H control methods,respectively.However,in the subsequent EPR testing,there was no significant change in the spectra before and after the transformation,which was attributed to the weak spin-spin interaction in the system and could be explained by the long distance between the free radicals within and between the molecules in the single crystal structure.In Chapter 3,we used the subcomponent self-assembly strategy to construct metalorganic assemblies with geometrical configurations of tetrahedral cages and triple helicates,and achieved multiple stimulus response regulation of assembly structure transformations.First,we synthesized linear ligands containing pyridine aldehyde functional groups,and then determined the suitable building blocks required for the stimulus response system by changing the type of metal ions,amines,and ligand connecting lengths.We found that the structure transformation from the tetrahedral cage to the triple helicate could be promoted under conditions of solvent diffusion,dilution,and heating,and we revealed the mechanism behind this transformation through quantitative analysis of thermodynamics and kinetics: the transformation is an enthalpy-disfavored but entropy-favored process,and the energy difference between the two assemblies is small,allowing for the transformation to occur under mild conditions.Additionally,we introduced a new functional unit as an output signal to reflect the transformation between the two assemblies.In summary,this paper designed and synthesized a supramolecular metal-organic assembly system that can be controlled by stimuli response for structural transformation based on directional bonding and subcomponent self-assembly strategies,and systematically studied its structure and properties.In the tetrahedral cage-triple helicate system,we analyzed the thermodynamics and kinetics of a series of transformation conditions and revealed that conditions favoring entropy are conducive to the transformation to triple helix,while those with significant energy advantages can offset the advantage of entropy.In the free radical macrocycle system,the structure transformation can be achieved using p H control methods,but the EPR spectra did not show significant changes with the structural transformation,which could be explained by the long distance between the free radicals within and between the molecules in the single crystal structure.Therefore,this thesis provides inspiration for the constructions of stimuli-responsive metal-organic assembly systems.