Functional π-Gelators and Their Applications

Dendron-mediated self-assembly, disassembly, and self-organization of complex systems have been investigated. The most ideal building blocks would need to display shape persistence in solution and in the solid state, since only this feature provides access to the use of complementary methods of structural analysis. Most supramolecular dendrimers are chiral even when they are constructed from nonchiral building blocks and are equipped with mechanisms that amplify chirality. This poses additional challenges associated with the understanding of the structural origin of chirality in different supramolecular structures through combinations of structural analysis methods. While many supramolecular structures assembled from dendrimers and dendrons resemble some of the related morphologies generated from block-copolymers, they are much more complex and are not determined by the volume ratio between the dissimilar parts of the molecule.

Dendron-Mediated Self-Assembly, Disassembly, and Self-Organization of Complex Systems

Supramolecular gels are a fascinating class of soft materials. Their gelators can self-assemble into nano- or micro-scale superstructures, such as fibers, ribbons, sheets and spheres in an appropriate solvent, thereby resulting in the formation of 3D networks. The dynamic and reversible nature of the non-covalent interactions that contribute to the formation of these network structures together gives these supramolecular gels the inherent ability to respond to external stimuli. However, the dynamic nature of supramolecular gels, which endows them with unique properties, makes their characterization diversified at the same time. Therefore, we present here a review summarizing various methods for characterizing supramolecular gels, including nuclear magnetic resonance spectroscopy, computational techniques, X-ray techniques, microscopy techniques, dynamic light scattering, thermal analysis, and rheology. Based on the gelation mechanisms and influencing factors of supramolecular gels, suitable and sufficient characterization methods should be carefully employed to make full use of their respective advantages to better investigate these materials.

Characterization of supramolecular gels

Visible light sensitive photoinitiating systems: Recent progress in cationic and radical photopolymerization reactions under soft conditions

Large, hydrophilic polyoxoanions with high solubility in water and/or other polar solvents demonstrate unique solution behavior by self-assembling into single layer, hollow, spherical “blackberry” structures, which is obviously different from small, simple ions. These macroions cannot be treated as insoluble colloidal suspensions because they form stable “real solutions”. Counterion-mediated attraction is considered as the main driving force for the self-assembly behavior. The size disparity between the macroions and their counterions results in macroion–counterion pairing which leads to the inter-macroanionic attraction. The blackberries, with robust membranes semi-permeable to cations, can adjust their size accurately and reversibly in response to the change of solvent polarity and charge density of individual macroions. The inorganic macroions with well-defined size, shape, mass, charge density, but no intramolecular interactions, are ideal model systems to study the intermolecular interactions in polyelectrolyte and bio-macromolecular solutions. The blackberry structures show certain similarities to spherical viral capsids, from the overall structure to the formation kinetics. More amazingly, these inorganic macroions demonstrate some features usually believed to belong only to complex biological molecules, such as the self-recognition in dilute solutions. Meanwhile, polyoxometalates-based organic–inorganic hybrid materials demonstrate amphiphilic properties by self-assembling into vesicles and reverse vesicles in polar and non-polar solvents, respectively, and form monolayer at the water/air interface. Different from conventional amphiphiles, these hybrids show pH-dependent and counterion-dependent self-assembly behaviors with controllable functionality, e.g. fluorescence and catalytic activity, due to the high and tunable charges and the functionalities of POM polar head groups.

Solution behaviors and self-assembly of polyoxometalates as models of macroions and amphiphilic polyoxometalate–organic hybrids as novel surfactants

An organic−inorganic hybrid polymer composed of a Dawson trivanadium-substituted heteropolytungstate ((Bu4N)5[H4P2W15V3O62]) cluster and PS chain are originally designed and first synthesized via in situ ATRP Further characterization includes NMR spectroscopy, FT-IR spectroscopy, and GPC which proved the purity of the material By means of a cation exchange process, a hybrid polymer (Bu4N)+-POM-PS was tuned into a novel giant amphiphile H+-POM-PS Immediately, individual molecules self-assembled into kinetically favored hybrid vesicles in DMF Our work provides a controllable and rational way to fabricate stable and well-defined POM−polymer hybrid polymers that can be optimized for potential applications

Synthesis of Polyoxometalate−Polymer Hybrid Polymers and Their Hybrid Vesicular Assembly

The originally synthesized hydrophobic hybrid polymer Bu 4 N + -POM-PS is tuned into a giant amphiphile composed of a hydrophilic H + -POM head and a hydrophobic PS tail through protonation. Immediately, the hybrid amphiphiles self-assemble into vesicles. A further annealing treatment induces an evolution from vesicles to tubular aggregates containing H + -POM nanotubes wrapped with PS coronas. After this transformation, the tubular aggregates grow further and then arrange in a parallel manner to form domains. This interesting morphology evolution provides us an opportunity to understand the intriguing aggregation behavior of the hybrid amphiphiles and, meanwhile, to generate POM nanotubes which might be utilized to create nanomaterials for potential applications.

An Intriguing Morphology Evolution of Polyoxometalate‐Polystyrene Hybrid Amphiphiles from Vesicles to Tubular Aggregates

Newly designed and synthesized polyoxometalate cluster-contained hybrid molecules with two dendritic poly(urethane amide) wings are interesting as gelators that form hybrid organogels of self-assembled hybrid nano-ribbons.

Polyoxometalate cluster-contained hybrid gelator and hybrid organogel: a new concept of softenization of polyoxometalate clusters

Amphiphilic diblock codendrimers consisting of dendrons of hydroxyl-containing poly(methallyl dichloride) (PMDC) and long alkyl-containing poly(urethane amide) (PUA) were synthesized in different generations. These codendrimers were found to self-assemble into ribbonlike aggregates in organic solvent and further formed three-dimensional networks and behaved macroscopically as gels. The width of the self-assembled ribbons decreases with the generation of both dendritic blocks. Multiple intermolecular hydrogen bonds between amide and hydroxyl groups were found to be the main driving force to form these self-assembled gels.

Self-assembled structures in organogels of amphiphilic diblock codendrimers.

A poly(urethane amide) (PUA) dendron with long alkyl chains on its periphery was synthesized and then attached to the backbone of a polyelectrolyte, in which each unit contained a positive charge, by ionizing the carboxyl groups on the apexes of the dendrons to form a dendronized polymer. We found that both the PUA dendron and the dendronized polymer could form organogels in toluene. Interestingly, both the minimum gelation concentration and the gelation time of the dendronized polymer gelator were greatly reduced compared with the dendron alone. Our investigations showed that in the gel phase the intermolecular hydrogen bonding between adjacent dendrons creates similar supramolecular structures in both the dendron and the dendronized polymer gelator, which immobilize solvent molecules by means of interactions between dendrons and solvent molecules. Further studies on the gelation kinetics indicated that the polyelectrolyte backbone plays an important role in prearranging the attached dendritic gelators orderly and quickly into the supramolecular structures through a nucleation-elongation mechanism. Therefore, the gel-forming ability of the dendritic PUA gelator is enhanced by being complexed with the polyelectrolyte. In this work, this positive macromolecular effect is discussed in detail.

Zijian Zhang

Papers

Synthesis of Polyoxometalate−Polymer Hybrid Polymers and Their Hybrid Vesicular Assembly

An Intriguing Morphology Evolution of Polyoxometalate‐Polystyrene Hybrid Amphiphiles from Vesicles to Tubular Aggregates

Polyoxometalate cluster-contained hybrid gelator and hybrid organogel: a new concept of softenization of polyoxometalate clusters

Self-assembled structures in organogels of amphiphilic diblock codendrimers.

Enhancing gelation ability of a dendritic gelator through complexation with a polyelectrolyte.