Dynamic Self-assembly Of Lindqvist-type Polyoxometalate Supramolecular Complex

The study of dynamic self-assembly systems not only contributes to deepen the understanding of supramolecular self-assembly,but also provides new ideas study of smart materials and devices.Polyoxometalates are a class of inorganic polyanion clusters with defined structure and rich functions,which are a typical type of inorganic functional clusters that show response to external stimuli and are one of the excellent building blocks of dynamic supramolecular self-assembly systems.Polyoxometalates can be modified with cationic organic surfactants,biomolecules and polymers through electrostatic encapsulation and covalent grafting and finally realize synergistically functionalized.The self-assembly of chiral supramolecules with helical structures has attracted much attention.Dynamic chiral self-assembly plays a key role in the design and development of smart materials such as chiral switches and asymmetric catalysis.Based on this knowledge,we devote ourself greatly to realize the helical self-assembly from the perspective of‘spontaneous symmetry destructing',by using a POM complex with a linear structure as the building block.We selected the octadecyltrimethylammonium bromide?ODTA·Br?with a long alkyl chain as the cationic surfactant to replace the external counterions of Lindqvist hexamolybdate cluster complex?ODTA?₂[Mo₆O₁₉].The complex was characterized in detail by ¹H-NMR,IR,TGA and elemental analysis.The results provide that we have successfully obtained an organic/inorganic hybrid polyoxometalate supramolecular complex?ODTA?₂[Mo₆O₁₉]with a symmetric linear structure via simple ion replacement.The assembly structure of the complex was controlled by changing the polarity of the solvent.Scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,and X-ray photoelectron spectroscopy?XRD?confirmed that the helical self-assembly morphology of Lindqvist-type polyoxometalate complex was achieved in a mixed solvent of acetonitrile/isopropanol.Reducted with UV light and then oxidized in the air,the polyoxometalate complex showed obvious reversible photochromic behavior,and the topology of polyoxometalate remains the same even after undergoing the stepwise multi-electron redox.And we also found that the assembly morphology of complex changed at the same time as the photochromism,and gradually changed from a helical strip to a spherical assembly.More specifically,this transformation of morphology caused by UV light irradiation can be restored after air oxidation,and back again to the original spiral assembly.And this redox stimulation-driven reversible morphological transformation can be realized in multiple cycles.The ¹H-NMR and XRD results showed that redox-regulated reversible self-assembly is driven by the changes of the electrostatic attraction between inorganic clusters and organic cations and the electrostatic repulsion between inorganic clusters.It is a viable method to construct a helical self-assembly system by using a complex with a symmetrical structure.The photochromic properties of polyoxometalate are used to realize the dynamic reversible regulation of polyoxometalate chiral self-assembly.The results of this work will help to better understand the mechanism of dynamic self-assembly.And such nanostructures that are responsive to specific conditions will help promote the accurate preparation of advanced functional materials.