Construction Of A Series Of Photo-responsive Ligands And Supramolecular Nanocages Based On Feringa’s Second-generation Motor And Their Functional Studies

Discrete functional metal-organic self-assembled nanocages are molecular factories that integrate functional organic molecules into the self-assembled cage’s backbone and execute precise tasks within the confined space.In recent two decades,the cage has raised the extensive interest of researchers so that plenty of applications have been investigated in the fields of catalytic regulation,allosteric receptors,stimuli-responsive materials,guest transportation,etc.Molecular motors are classical photo/heating responsive motifs.Due to the accessible modifiability and uniform rotating cycle,the backbone of the second-generation motor has been widely introduced into distinct molecular systems to undertake photo/heating responsive tasks,like cargo transportation,gas adsorption,regime-switching,and other application scenes.By introducing the second-generation molecular motor into the self-assembled nanocage to construct novel stimuli-responsive supramolecular systems,the molecular motor can perform useful tasks within the confined space.These motorized self-assembled cages are intelligent systems worth in-depth research.This thesis aims to rationally design and systematically synthesize a series of motor ligands with different lengths and obtain self-assembled nanocages of different sizes by generating coordination bonds with metal ions.The rotational dynamics behavior of the molecular motors anchored in the cage’s backbone,the traditional host-guest recognition function of the cage,and other physical and chemical properties brought by the assembly behavior will be discussed in detail in this thesis.The thesis mainly includes the following three chapters:The first chapter is about the frontier research status of this field.It introduces the functional application of the second-generation molecular motors in the confined nanospace in different microscales,the wide application of self-assembled nanocages in various fields,and the research progress of photo-responsive metal-organic self-assembled cages.In the second chapter,we designed and synthesized a novel pyridine-modified banana-shaped motor ligand L¹,which can self-assemble with the square-planar-coordinated Pd²⁺to obtain[Pd₂L₄]-type nanocage.Firstly,the traditional photoisomerization and thermal helical inversion of L¹ were characterized by 1D NMR and UV-Vis spectroscopy.As a result,the motor unit maintained the unidirectional 360°rotation cycle.The high-resolution ESI-MS and 1D/2D NMR spectra confirmed that L¹ and Pd²⁺have self-assembled into the uniform[Pd₂L₄]-type cage.The combination of molecular dynamics simulation and small-angle X-ray scattering（SAXS）revealed the three-dimensional structure of the coordination cage.The rotation function of the motor block remains even if it is embedded in the coordination cage,and its rotation behavior can lead to the overall size of the cage being larger than before.Furthermore,the self-assembled nanocage can encapsulate guest molecule C₆₀ by a structure-adaptive process:after encapsulation,the rotor unit turns to the outside of the cage,and the cage backbone is flattened to suit guest molecules.Combining 1D NMR spectra,cyclic voltammetry,SAXS,and molecular dynamics simulation,a novel strategy has been developed to analyze the structures of nanocage-based systems that cannot crystallize.In chapter 3,based on the research results in chapter 2,another two motor ligands L² and L³,with longer lengths,are designed and synthesized.L² can still maintain the rotational function like traditional molecular motors.However,the photoisomerization of L³ is no longer a unimolecular reaction,and polymerization occurs simultaneously during light irradiation,which greatly compromises its subsequent functional application.For the self-assembly of L²and Pd²⁺,the catenated cage[Pd₄L₈]was generated.To suppress the effects of steric hindrance,the metal precursor[Pd Cl₂（CH₃CN）₂]is used.Finally,a mixture of[Pd₂L₂]-type coordinating metallocycle and free ligand L² has been obtained.Additionally,ligand L² showed an aggregation-induced emission（AIE）phenomenon,and the fluorescence intensity of L² in PSS after illumination is very close to stable L²,but the quenching effect is stronger when the aggregation degree increases.However,after coordinating with Pd²⁺,the AIE property no longer exists.The functional application of this kind of motorized ligand with the AIE property in the complex backbone provides a broad application prospect.