Controllable Self-assembly And Functionalization Of Poiyoxometalate-based Supramolecular Complexes

Polyoxometalates （POMs） are discrete, molecularly defined, nanoscale （0.5–6nm）inorganic metal-oxide clusters with structural variety and interesting functionalproperties leading to rich applications in optics, magnetics, medicine and especiallycatalysis, which are often used as intriguing inorganic building blocks. However, POMsusually exist as inorganic crystals or powder, which are difficult to be processed tofabricate soft materials, as a result, limiting their applications.The organic modification of POMs can efficiently improve their machinability.Among the strategies, a facile and simple approach currently used is to replace thecounterions of POMs by organic cations, resulting in organic cation-encapsulated POMs（OCEPs）, which can lead to increased stability of inner POMs, improved solubility andmachinability. The OCEP complex possesses a discrete supramolecular architecture inwhich the organic molecules are not fixed at certain sites and able to rearrange on POMsurface. Therefore, the OCEPs are usually used as supramolecular building blocks forthe fabrication of assemblies with novel structures and functions.Because the OCEPs are a type of classic amphiphiles bearing a hydrophobic shell anda hydrophilic POM core in a well-defined composition, they often assemble in solution. The structures of organic and inorganic parts, concentration, temperature, solvent,assembling time and so forth are the key factors for the morphology and structure of theassemblies, which can be enriched by varying the factors. As we known, the structuredetermines the property, so the functions might differ a lot with the different assembledstructures. Therefore, the functions of OCEPs would benefit from the variety of theirassemblies.Catalysis is one of the most important properties of POMs, and widely studied. Whenthe counterions are protons, the POMs can be dissociated completely in water as strongacids, and behaved as acid catalysts. The POMs can reversibly gain and loss electronswithout structural change, so they exhibit strong redox ability as excellent redoxcatalysis. Nowadays, how to control the catalytic reactions and recycle the homogenouscatalysts are two key problems people always care in the catalytic field based on POMs.One can make the functions of POMs smart and rich through integrating the functionsand synergy effects in OCEPs. In this dissertation, we focus on the design of organicand inorganic parts, manipulation of the assembled structure of OCEPs, the mechanismof structure evolution, the controllable catalysis based on different existing state ofOCEPs, and the recycle of the homogeneous POM catalysts.Firstly, to construct the novel assembled structures and study the mechanism of theformation and evolution of assembled structures, the dendritic molecule bearing fourheptyl chains, denoted as D, was synthesized and characterized. We chose four types ofanionic POMs with nearly the same size and morphology but different numbers ofcharges, to be encapsulated through two phase encapsulation method, resulting in fourcomplexes OCEP-D₄, OCEP-D₅, OCEP-D₇, and OCEP-D₁₀with different numbers of D.¹H NMR spectra and Fourier transform infrared （IR） spectra confirm the presence of thestrong electrostatic interaction between cationic dendrons and POMs, the well-keptstructures of dendrons and POMs. The elemental analysis and thermogravimetric （TG）results suggest the chemical formulae of OCEP-D₄, OCEP-D₅, OCEP-D₇, and OCEP-D₁₀to be D₄SiW₁₂O₄₀, D₅BW₁₂O₄₀, D₇Na(SiW₁₁O₃₉) or D₇K（SiW11O39）, andD₁₀Na₂P₂W₁₅O₅₆. We initially investigated the self-assembling behavior of the dendriticmolecule in organic solution by transmission electron microscopic （TEM）, X–raydiffraction （XRD） and contact angle measurements. The measurement results imply thatthe dendritic molecule assembles into vesicular assemblies with a reverse bilayerstructure in solution. Then, we study the self-assembling behavior of the complexesthrough dynamic light scattering （DLS）, XRD, contact angle, SEM and TEMcharacterizations, confirming that OCEP-D4and OCEP-D5assemble into thevesicle-like structure with reverse bilayers, while OCEP-D₇and OCEP-D10with morecationic dendrons can make themselves keep away from each other in solution, resultingin the mono-dispersed state. We examined the solid structures of the complexes viaXRD and TEM characterizations, which infer OCEP-D4and OCEP-D₅possess lamellarstructure, in contrast, OCEP-D₇and OCEP-D₁0prefer to form hexagonal columnarassemblies rather than lamellar assemblies in solid. We propose the mechanism of thestructure evolvement through the interpretation of the simulation. The structuralevolvement is due to the orientation change of the cationic dendrons on the periphery ofOCEPs as a result of their varied number in one complex. The present researchdemonstrates a new kind of dendritic complexes, provides a facile and feasible route forcontrolling their assembled structures in solution and solid by simply alternating thenumber of cationic dendrons in the complexes, and provides a general pathway toconstruct other types of self-assemblies based on POMs.Secondly, to construct a smart supramolecular self-assembly system based on POMs,we herein synthesized an azobenzene-terminated cationic surfactant （AzoC₆）₂N, whichwas then used to encapsulate a typical POM P₅W₃₀through two phase encapsulationmethod, giving a photo-responsive complex, Azo-SEP.1H NMR and IR spectra indicatethat the frame structure of POM and cationic surfactant are well kept in the complex,and the strong electrostatic interaction exists between the surfactant and the POM. Based on the elemental analysis and TG results, twelve （AzoC₆）₂N cations are inferredto be combined in one complex with the chemical formula [（AzoC₆）₂N]₁₂K₂(P₅W₃₀) or[（AzoC₆）₂N]₁₂KNa(P₅W₃₀). The UV-Vis and1H NMR spectral changes confirm thereversible photoisomerization behavior of the azobenzene groups in Azo-SEP insolutions. The digital photographs, DLS, SEM, TEM and XRD results strongly verifythe photo-responsive assembly and disassembly of Azo-SEP in two solutions withopposite behavior. According to the contact angle measurement, the reversible assemblyand disassembly behavior is tuned by remarkable polarity changes of Azo-SEP inducedby the trans-cis isomerization of azobenzene groups. Furthermore, the aggregated anddispersed states of Azo-SEP can be translated into pronounced activity differences inPOM-catalyzed oxidation reaction for photo-modulating catalysis. The controllableassembly of POM complex enables the controllable functions of it.Thirdly, to extend the dynamic reversible assembly of POM complex from singlephase to two phase system, we use the cationic surfactant （AzoC₆）₂N to encapsulate asandwich type POM Zn₅W₁₉through two phase encapsulation method, giving anotherphoto-responsive complex, Azo-SEP-1.1H NMR and IR spectra indicate that thestructure of Zn₅W₁₉and cationic surfactant are well kept in the complex, and the strongelectrostatic interaction exists between the surfactant and the POM. According to theelemental analysis and TG results, nine （AzoC₆）₂N cations are inferred to be combinedin one complex with the chemical formula [（AzoC₆）₂N]₉Na₃[WZn₃（H₂O）₂(ZnW₉O₃₄)₂].The UV-Vis and1H NMR spectral changes confirm the reversible photoisomerizationbehavior of the azobenzene groups in Azo-SEP-1in solutions, and present thepercentage of isomerization ratio. The digital photographs, DLS, SEM, TEM and XRDresults confirm the photo-responsive assembly and disassembly of Azo-SEP-1alsoaccur in toluene （PhMe） and DMF/H2O （1:1in v/v） mixed solutions with oppositebehavior. Combining the two solutions together, digital photographs and UV-Vismeasurement results show the successful photo-induced reversible phase transfer of Azo-SEP-1between PhMe and DMF/H₂O mixed solvents. Upon visible light irradiation,most of the azobenzene in the complex are in the trans-state, and the complex ismonodisperse in PhMe phase with week polarity. In contrast, under UV light irradiation,most of the azobenzene in the complex are in the cis-state, and the complex is inaggregated state forming small assemblies in DMF/H₂O phase with strong polarity.Therefore, the POM complex can undergo a reversible phase transfer upon the UV andvisble light irradiations with the transfer percentage larger than96%. According to IRspectra, the structure of the complex is well kept during the transfer process. Thereversible phase transfer behavior is also triggered by the remarkable polarity changesof Azo-SEP induced by the trans-cis isomerization of azobenzene groups. Thelight-controlled phase tag can be further used to separate the homogeneous catalystsbased on POM.In conclusion, we focus on the controllable assembly of OCEP supramolecularcomplex, and aim to enrich, improve and manipulate the functions by controllableassembly of OCEPs. We have achieved the controllable assemblies through altering thenumber of organic cations or triggering by light in single phase, discussed themechanism of controllable assembly, and realized the reversible phase transfer betweentwo phase termini, as well as clarified the cause for phase transfer. Furthermore, wehave regulated the functions of POM complex by the controllable assembly property.We believe that the present investigation may provide a general pathway to constructnovel self-assemblies based on POMs and similar systems, and promising buildingblocks as well as strategy for controllable functional materials.