Synthesis, Structural Characterization, and Thermoresponsivity of Hybrid Supramolecular Dendrimers Bearing a Polyoxometalate Core

Abstract

A series of cationic dendrons bearing triethylene glycol monomethyl ether terminal groups of different generations have been synthesized and used to encapsulate an inorganic polyanionic cluster [K(12.5)Na(1.5)(NaP5W30O110)] through electrostatic interactions. The resulting dendritic cation-encapsulated polyoxometalate (POM) complexes, cluster-dendrimers, are soluble in water and exhibit lower critical solution temperatures (LCST). The thermoresponsivities of these complexes in aqueous solutions were studied by turbidimetry and variable-temperature (1)H NMR spectroscopy. The observed cloud points show a remarkable dependence on the generation of the dendrons. Complexes composed of first-generation dendrons exhibit no obvious thermoresponsive properties, but for complexes bearing second-generation dendrons, the LCST decreases as the number of dendritic cations around the POM cluster increases. Complexes composed of third-generation cations underwent reversible aggregation and disaggregation upon heating and cooling, respectively. This thermally induced self-aggregation was characterized by DLS and TEM. In addition, the effects of salt and solvent on the LCST were investigated. This research demonstrates a new type of thermoresponsive dendritic organic-inorganic hybrid complex and provides a general route to the endowment of POMs with temperature-sensitive properties through electrostatic interactions.

Synthesis of Well‐Defined Dawson‐Type Poly(N,N‐diethylacrylamide) Organopolyoxometalates

“…1) was still stable and its color was maintained ( Figure SI-8). [20,42,43] Conclusion Polyoxometalate-poly(N,N-diethylacrylamide) (POM-PDEAAm) hybrids have been synthesized in a controlled way for the first time by polymerizing DEAAm from a suitable organo-POM trithiocarbonate RAFT agent. In contrast, the solution obtained with POM-COOH as a photocatalyst (expt.…”

Self-assembly of Oligo(ethylene oxide)-linked Diammonium Ions with Polyoxometalates into Ordered Polyhedron Nanocrystals in Aqueous Media

“…12,13 Likewise, the introduction of cationic dendrons with triethylene glycol monomethyl ether as counter ions caused the POMs to show aggregation above the lower critical solution temperature (LCST) of the dendrons. 14 We also reported the introduction of chiral ammonium ions and consequent chiral nanoassembly of POMs. 15,16 The electrostatic complexation of POMs with monocationic lipids provides lipophilic complexes, in which the long alkyl chains of lipid molecules inevitably serve as insulators, which are unfavorable from the perspective of maximizing electronic interactions among the accumulated POMs.…”

Photoresponsive Nanosheets of Polyoxometalates Formed by Controlled Self‐Assembly Pathways

“…[2,[14][15][16] They spontaneously form vesicles in aqueous-organic solvent mixtures, [14,16] or micelles [15] in aqueous media. However, the site-selective modification approach requires considerable synthetic effort and it inevitably loses the intrinsic electronic properties of POMs.A dditionally,s ymmetric introduction of hydrophobic units [14,16] is not as uitable molecular design for aqueous selfassembly.Meanwhile,the third approach utilizes electrostatic interactions between anionic POMs and cationic amphiphiles, [17][18][19][20][21][22][23][24][25][26] similar to that developed for 1D metal complexes. [27,28] These lipophilic amphiphile-POM complexes are dispersible in nonpolar organic solvents,g iving aggregates with globular,corn-shaped, fibrous,orhelical superstructures, depending on the constituents.…”

“…[27,28] These lipophilic amphiphile-POM complexes are dispersible in nonpolar organic solvents,g iving aggregates with globular,corn-shaped, fibrous,orhelical superstructures, depending on the constituents. [17][18][19][20][21][22][23][24][25][26] All these approaches, however, provide unpredictable 3D curved aggregate morphologies that are difficult to control and not suitable for technological applications.O ne of the most valuable nanoarchitectures is an extended 2D nanosheet, which can be accumulated into multilayered films.A lthough this type of nanostructure is greatly anticipated, no general method for self-assembly of spherical POMs into extended nanosheets exists to date.T oachieve this goal, the energy landscapes and pathways in self-assembly need to be controlled at the molecular level.…”

“…To self-assemble anionic POMs in regular 2D arrays without covalent modification, we came up with an idea to introduce soft counterions with am olecular cross-sectional area comparable to those of POMs.T hese counterions are required to show structural flexibility so that they accommodate densely aligned spherical POM molecules.S uch an adaptive association feature would be realized by introducing flexible oligo(ethylene oxide) groups in the counterions. Although POMs with bulky cationic dendrons containing triethylene glycol chains have been reported, [26] only polydispersed globular structures were formed. Thus,n og eneral guidelines have been available for the design of POM-cation pairs that hierarchically self-assemble into regular 2D structures.…”