Preparation And Solution Self-assembly Behavior Of Polyoxometalate Clusters Modified By Organic Cations

Polyoxometalates?POM?are inorganic metal oxygen cluster compounds formed through oxygen coordination bridges between the high valent pre-transition metals?V,Nb,Ta,Mo,W,etc.?and oxygen atoms.It has good electron and proton transport and storage capacity,special redox potential and strong proton acidity.Because POM has abundant chemical composition and excellent physical and chemical properties,it has shown great application prospects in many aspects.Functional organic-inorganic hybrid molecules are obtained by chemical or physical modification of functional organic groups into inorganic polyoxometalates,which have potential applications in bio-recognition,drug transportation and new intelligent materials.At the same time,these molecules are amphiphilic,so they can self-assembled in solution to form aggregates with various morphologies and rich functions.By applying external stimuli to the system or changing the assembly environment?temperature,solvent composition,etc.?,the aggregation behavior of complex molecules in solution can be regulated,which provides a way to discover new intelligent materials and expand the application range of self-assembly.Based on this,in this paper,the complexes composited of two kinds of organic cations cladding polyoxometalate clusters were synthesized by electrostatic interaction.With the non-covalent interaction as the driving force,the aggregation morphologys and structures of the complexes changed with different solvent polarity.The main research contents of this study included the following three parts mainly:Firstly,the single vacancy Wells-Dawson POM(K₁₀[?-P₂W₁₇O₆₁]·20H₂O,abbreviated as P₂W₁₇)which reduced symmetry relative to that of Keggin POM was synthesized.?3-2,3-epoxypropyl?trimethoxysilane?KH-560?was used to modify P₂W₁₇.Then,the POM modified by organic cations complex NBu₄-P₂W₁₇-EPO was synthesized by electrostatic binding P₂W₁₇-EPO with tetrabutylammonium bromide?NBu₄Br?.The structure and morphology of the complex were characterized by FT-IR and XRD.Dynamic light scattering?DLS?was used to investigate the effects of solvent polarity,solvent ratio and the concentration of the complex on the particle size of its self-assembly.The results showed that NBu₄-P₂W₁₇-EPO could be self-assembled into sphere particles with a certain size in the mixed solvent of acetone and H₂O,and as the time went on,the discrete spherical particles in the solution were further assembled and evolved into a cubic shape with a regular structure.And the particle size of the assembly decreased gradually with the decrease of acetone/H₂O ratio.The mixture of acetone and H₂O?v:v=1:1?was used as solvent,the size of the assembly was not concentration-dependent in the range of 0.01-0.3 mg/mL.Next,the double-chain cationic surfactant?DDAB-11-ol?was firstly synthesized by the reaction of 11-bromo-1-undecanol with N,N-dimethyldodecylamine.Then DDAB-11-ol was reacted with zinc phthalocyanine?ZnTNPc?to form a phthalocyanine-modified double-chain cationic surfactant ZnTNPc-O-DDAB.Finally,the complex ZnTNPc-O-DDAB/PW₁₂2 was obtained by the electrostatic bonding of ZnTNPc-O-DDAB with Keggin phosphotungstic acid(H₃PW₁₂O₄₀,abbreviated as PW₁₂).The structure and composition of ZnTNPc-O-DDAB/PW₁₂2 were characterized by FT-IR,¹H-NMR and elemental analysis.The binding ratio of ZnTNPc-O-DDAB and PW₁₂2 was determined by TG to be 2.8:1,and the composite had good thermal stability.The complex ZnTNPc-O-DDAB/PW₁₂2 showed good visible light absorption and fluorescence emission properties.That was to say,the light absorption range for PW₁₂could be broadened into the visible region.At the same time,the Q-band fluorescence emission of the visible light-induced ZnTNPc-O-DDAB showed the concentration quenching effect;while the complex ZnTNPc-O-DDAB/PW₁₂2 was formed after adding PW₁₂2 into the ZnTNPc-O-DDAB solution,making the Q-band fluorescence emission enhanced.In addition,the electron transfer between ZnTNPc-O-DDAB and PW₁₂2 was confirmed by UV-vis,FL,electrochemical test,which laid a certain foundation for its application in the field of photocatalysis.Last,the self-assembly behavior of ZnTNPc-O-DDAB/PW₁₂2 in DMSO/H₂O solvent was studied.ZnTNPc-O-DDAB/PW₁₂2 in DMSO/H₂O?v:v=4:1?mixed solvents could aggregate spontaneously to form spherical micelles with a size of about 300 nm.After the spherical micelles separated by centrifugation were re-dispersed in water for freeze-drying,the hollow nanospheres were obtained again,and the size of the spheres decreased,and the formation of the hollow structure followed the Ostwald mechanism.In addition,the secondary self-assembly of ZnTNPc-O-DDAB/PW₁₂2 assembly in DMSO/H₂O?v:v=4:1?solution separated by centrifugation was investigated in water.It was found that the secondary self-assembly of ZnTNPc-O-DDAB/PW₁₂2 spherical assembly in water is mainly affected by the amount of water added and the time of self-assembly.When the water content was 10 mL,the original spherical structure of the composite was reassembled along a certain shape trajectory with the prolongation of the secondary assembly time,and the polygonal morphology was formed.At 2 h,the proto-spherical particles had been gradually gathered and accumulated in order to form a layered butterfly-like,six-petal flower-like morphology.In addition,when the secondary assembly time was fixed at 2 h,the more water was used,the denser the formed butterfly and petal-shaped assemblies.It is assumed that the driving force of the assembly process may be the hydrophobic interaction between the zinc phthalocyanine molecules and the hydrophobic segments in the complex molecule.It was assumed that the driving force of the assembly process may be the?-?accumulation between ZnTNPc parts and the hydrophobic interaction of hydrophobic segments in the complex molecules.Finally,the more stable hollow nanospheres were obtained by solvothermal reaction of spherical micelles at 180°C for 10 h,and the spheres still retained the basic structure of phthalocyanine.