Toward Supramolecular Soft Materials: Surfactant-Encapsulated Polyoxometalate Complexes

The future design of functional molecular and electronic devices will rely onthe principles of supramolecular chemistry. Combining solid-state inorganicchemistry and molecular membrane chemistry, one can manipulate, orient, andposition inorganic materials based on electrostatic interactions, indicative of thephysical and chemical properties of the inorganic materials. Polyoxometalates(POMs) are the polyoxoanion of the early transition elements, especiallyvanadium, molybdenum, and tungsten, which possess proton conductivity,nonlinear optics, medical, magnetic, electrochemistry, photophysical, and catalyticproperties. Considering the remarkable potential of POMs in above areas and theirdisadvantages such as high solubility in water, low pH stability, and high latticeenergies from material science point of view, it is necessary for us tosystematically investigate the structures and functionalities of surfactant-encapsulated POMs clusters (SEC) as well as the relationship between theirstructures and functionalities. Firstly, we have synthesized and characterized a surfactant-encapsulated 1132004 吉林大学博士学位论文 卜伟锋polyoxomolybdate cluster, and establish methods of preparing and characterizingthe organic/inorganic hybrids in our group. Secondly, we discuss the synthesis andcharacterizations of a surfactant-encapsulated antiferromagnetic Cu4O16-basedcluster. This hetero-architecture material can form stable Langmuir monolayers,but it is significantly distorted when spread onto air/water interface although theprimary core-shell structure seems remained. Its LB and solvent-casting filmshave been prepared by LB technique and solvent-casting method, which areindicative of lamellar structures. But in the latter, DODA alkyl chains are partiallyinterdigitated. Based on the difference of the thin-film structures, they show thedifferent thermal properties. In this magnetic hybrid, DODA alkyl chainsdemonstrate a well-ordered conformation, although they are not a full-extendedconformation. Of difference is that DODA alkyl chains are in a liquid-expandedstate in literatures. Therefore, the conformations of DODA alkyl chains aredirectly related to the surface charge density of POMs. The differentconformations of DODA adsorbed on the surface of POMs lead to the differentpacking structures of SECs. A design principle for the packing structure of SECscould be provided here, which is significant for the design of POM-basedmaterials. At last, our investigation is focused on the europium-substitutedheteropolyoxotungstates (Eu-POMs). A series of surfactant-encapsulatedeuropium-substituted heteropolyoxotungstates have been synthesized, and alsocharacterized in detail. These materials exhibit the characteristic D0→7Fj (j = 0, 1, 52, 3, 4) Eu3+ emission, and their photophysical properties such as excited,emissive spectra, lifetime, and energy transfer from the O→W LMCT state toEu3+ ion are significantly different from those of their corresponding Eu-POMs.Herein, the cationic surfactant as counter-cations not only provides the solubility 1142004 吉林大学博士学位论文 卜伟锋in organic media and mesoscopic supramolecular assemblies for POMs, but alsoexerts a remarkable influence on their electronic structures. Our work rationallyexplains the variety of photophysical properties between POMs/surfactant hybridsand the naked Eu-POMs, and is also significant for their catalysis, electrochemical,and medical applications. Eu-POMs are believed to be oriented in theirself-organizing film, resulting in Eu3+ polarized emission. Of significance is thatthe photophysical properties of SEC-9 in its thin films can be modified by carefulchange of the lamellar distance in diffe...