Synthesis And Thermoresponsivity Of Dendritic Polyoxometalate Supramolecular Complexes

The thermoresponsive materials have drawn considerable attention in both technologicalapplication and fundamental research over recent years. Due to their ability to undergolarge changes in physical-chemical and colloidal properties triggered by temperature,thermoresponsive materials have been widely applied in the field of self-assembly,biomedical and nanotechnology. Thermoresponsive material is generally defined as onethat exhibits UCST (upper critical solution temperature) or LCST (lower critical solutiontemperature) behavior. Compared to the UCST property, the LCST property is more oftenstudied, at which temperature the molecular undergoes a phase transition from a solublestate to an insoluble state in aqueous solution.To give one molecular a LCST property,conventionally, the thermoresponsive units are necessary and often grafted to matrix bycovalent bonding. In contrast to the covalently bonding, non-covalent synthesis is regardedas a kind of self-assembly method that can even construct topologies that are not availablewith conventional method. However, the non-covalent synthesis of thermoresponsivemolecular is rarely reported. Therefore, developing novel thermoresponsive materialsbased on non-covalent synthesis remains an important challenge.Polyoxometalates (POMs) are discrete, molecularly defined inorganic metal oxideclusters with structural variety and excellent functional properties that provide them richapplication in catalysis, optics, magnetics and medicine. To tailor the compatibility ofPOMs with organic materials and biological tissues, two methods have been made thatconsist of covalent grafting and electrostatically binding. Although many successfulexamples of fabricating POM/organic molecular hybrid materials through covalentbonding have been reported, this method has its own limitation. The POM derivativesused in this way are all originated from the modification of POMs, but the POMs that canbe further modified (typically hexamolybdates or lacunary POMs) are only a minority in the POM family. For most of common POMs, this method is not available. In contrast, amore powerful method is to modify the POMs with cationic surfactants throughelectrostatic interaction because every POM takes intrinsic negative charge. The resultingsurfactant-encapsulated complexes (SECs) are compatible with organic matrixes anddisplay significant property in self-assemblies, catalysis and magnetic resonance imaging,while the basic physical and chemical properties of POMs are retained. Among theSECs, stimuli-responsive SECs are of increasing interest for their excellent ability tomodulate self-assembled structures, morphologies, and functions in response toenvironment stimuli such as light and electrical field. However, until now, to the best ofour knowledge, there is only one report based on thermoresponsive POM hybrid. In thatpaper, they introduced the thermoresponsive polymer (PDEAAm-NH2) to the Dawsontype POM by covalent bonding and the resulting hybrid complex exhibited LCSTphenomenon. Nevertheless, covalent grafting is not good for the POM without modifiedsite. So fabricating the thermoresponsive organo-POM complexes through electrostaticinteraction are still required.To achieve the goals spoken above, we carried out two works as follows.Firstly, we designed and synthesized a series of surfactants with lots of triethyleneglycol (TEG) groups in the periphery that could increase the water-solubility of thesemolecules. Then we used these surfactants to encapsulate the POM with fourteen chargesthrough electrostatic interaction. We got three kinds of two generation complexes by tuningthe beginning ratio of dendritic molecules to POM. The results of NMR and IR confirmedthat the POM was encapsulated by the surfactants via electrostatic interaction successfully.EA and TGA told us the chemical formula of these complexes. All complexes werewater-soluble at room temperature.Secondly, we investigated the thermoresponsive property of these complexes. Beforedoing that, the results of NMR in D2O confirmed that the POM combines with thesurfactant strongly through electrostatic interaction and the complex is stable in aqueoussolution. The aqueous solutions of surfactants and (D-1)13P5W30kept transparent over theexperimental temperature range (20-100oC). However, the solutions of D-2and D-3complexes turned milky when heating up to a specific temperature, indicating that the complexes of D-2and D-3exhibit the LCST phenomenon. Next, we measured the LCSTsof complexes and studied their thermoresponsive behavior by turbidity measurementsusing UV/Vis spectroscopy. The LCST of complexes is found to decrease with increasingthe generation. For the D-2complexes, the LCST decreased with increasing the number ofthe D-2surfactants. DLS and TEM were used to investigate the thermally inducedself-assembly of complex in aqueous solution. Upon heating and cooling, the complexassemblies in water underwent a reversible aggregation and disaggregation process. Byusing the variable temperature1H NMR, we got a deeper insight about the temperaturesensitivity of the complex.Thirdly, we investigated the influence of salts and solvent on the LCST of complex,which was confirmed to be similar to conventional thermoresponsive polymer such asPoly(isopropylacrylamide). Moreover, the salt effect on the thermal behavior of complexeswas studied to give the potential applications of these complexes in biomedical areas.In conclusion, these novel thermoresponsive complexes show excellentthermoreponsive behavior like conventional thermoresponsive polymers. Moreover, themethod of non-covalent electrostatic interaction may provide a facile and universalapproach to fabricate thermoresponsive organo-POM complex. Furthermore, the POMfamily may provide this kind of thermoresponsive materials with the potential to serve asfunctional materials for catalysis or imaging， which may open up a new door to thedevelopment of novel functional smart materials.