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1. Metal Ion-Induced Formation of Metallogels by Visible-Light-Responsive Phenylalanine-Functionalized Arylazopyrazole Ligands

   https://doi.org/10.3390/photochem3040026  

   Browning, Mikayla; Jefferson, Alexandra; Geter, Jazz; Ghebreyessus, Kesete (December 2023, Photochem)

   A visible-light-responsive arylazopyrazole-functionalized phenylalanine (4-MeS-AAP-NF) derived ligand was designed and synthesized, and it was found to form metallogels with reversible photo-responsive properties in mixed methanol/water (MeOH/H2O) solvents. The gelation behavior of the 4-MeS-AAP-NF ligand in the presence of different divalent metal ions in mixed methanol/water (MeOH/H2O) solvents at pH~11.60 was studied. It was found that the 4-MeS-AAP-NF ligand alone could not self-assemble to form any gels. However, in the presence of divalent metal ions, it readily formed the assembled metallogels in an alkaline aqueous/methanol solution with various morphologies. The results suggest that the gelation process was triggered by divalent metal ions. The presence of the AAP moiety in the gel matrix rendered the metallogel assemblies photo-responsive, and the reversible gel-to-sol phase transition was studied by UV-vis spectroscopy. The gels showed a slow, reversible visible-light-induced gel-to-sol phase transition under blue (λ = 405 nm) and then sol-to-gel transition by green light (λ = 530 nm) irradiation, resulting in the re-formation of the original gel state. The morphology and viscoelastic properties of the yellow–orange opaque metallogels were characterized by scanning electron microscopy (SEM) and rheological measurement, respectively. 

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2. 4,6-O-Phenylethylidene Acetal Protected D-Glucosamine Carbamate-Based Gelators and Their Applications for Multi-Component Gels

   https://doi.org/10.3390/gels8030191  

   Sharma, Pooja; Wang, Guijun (March 2022, Gels)

   The self-assembly of carbohydrate-based low molecular weight gelators has led to useful advanced soft materials. The interactions of the gelators with various cations and anions are important in creating novel molecular architectures and expanding the scope of the small molecular gelators. In this study, a series of thirteen new C-2 carbamates of the 4,6-O-phenylethylidene acetal-protected D-glucosamine derivatives has been synthesized and characterized. These compounds are rationally designed from a common sugar template. All carbamates synthesized were found to be efficient gelators and three compounds are also hydrogelators. The resulting gels were characterized using optical microscopy, atomic force microscopy, and rheology. The gelation mechanisms were further elucidated using 1H NMR spectroscopy at different temperatures. The isopropyl carbamate hydrogelator 7 formed hydrogels at 0.2 wt% and also formed gels with several tetra alkyl ammonium salts, and showed effectiveness in the creation of gel electrolytes. The formation of metallogels using earth-abundant metal ions such as copper, nickel, iron, zinc, as well as silver and lead salts was evaluated for a few gelators. Using chemiluminescence spectroscopy, the metal–organic xerogels showed enzyme-like properties and enhanced luminescence for luminol. In addition, we also studied the applications of several gels for drug immobilizations and the gels showed sustained release of naproxen from the gel matrices. This robust sugar carbamate-derived gelator system can be used as the scaffold for the design of other functional materials with various types of applications. 

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3. Supramolecular Gelation of Triphenylamine Bis-Urea Macrocycles in Toluene

   https://doi.org/10.1021/acs.macromol.3c02402  

   Prakash, Rahul; Esmaeili, Mohsen; Gbadamosi, Fahidat A.; Pellechia, Perry J.; Sadati, Monirosadat; Shimizu, Linda S. (February 2024, Macromolecules)

   Herein, we investigate supramolecular gelation behavior of a dendronized triphenylamine bis-urea macrocycle (1) in toluene in the presence and absence of sulfoxide chain stoppers. Macrocycle 1 assembles in the sol phase through intermolecular hy-drogen bonding interactions, spontaneously transitioning into a gel state when left undisturbed at room temperature. In tolu-ene, 1 displays a critical gelation concentration of 0.066 wt%, classifying it as a super-gelator. Furthermore, it exhibits a thermoreversible gel-sol phase transition as well as thixotropic behavior. Temperature-dependent 1H NMR spectroscopy is employed to probe the sol phase assembly of 1 with the size variations at different temperatures assessed by 2D DOSY. Rheological experiments at 10 °C were used to measure gelation response to mechanical stimuli. An amplitude sweep test highlights a linear viscoelastic region. Additionally, the self-healing behavior of gel 1 was verified through a series of strain cycles, where it showed complete recovery. Addition of chain stoppers 10% versus 1 of dimethyl sulfoxide (DMSO) and diphenyl sulfoxide (DPS) lead to weaker gels with smaller differences between the storage and the loss moduli. Rheological analysis revealed slower/partial recovery for the gel containing chain stoppers. Gels assembled from macrocyclic building blocks may retain homogeneous binding cavity and channels offering novel functional properties. 

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4. Less is more when forming gels by dilution

   https://doi.org/10.1126/science.abo7656  

   Webber, Matthew J. (July 2022, Science)

   Soft materials that change form or function in response to environmental or user-applied stimuli have a wide range of biomedical applications ( 1 ). Gels can form in water from weakly interacting molecules but can return to the state of a flowing liquid suspension, known as a sol, upon changes in the concentration of the molecules or the applied temperature. This behavior is known as a reentrant phase transition. A gel-to-sol phase transition typically arises from a reduction in concentration, meaning that a gel becomes a sol upon dilution and a sol becomes a gel with increased concentration. On page 213 of this issue, Su et al. ( 2 ) demonstrate a system that exhibits a sol-to-gel transition when diluted, inverting the common behavior of gels. Their observations offer insight into systems that undergo reentrant phase transitions in biology. 

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5. Synthesis of a Series of Trimeric Branched Glycoconjugates and Their Applications for Supramolecular Gels and Catalysis

   https://doi.org/10.3390/molecules28166056  

   Bietsch, Jonathan; Chen, Anji; Wang, Dan; Wang, Guijun (August 2023, Molecules)

   Carbohydrate-derived molecular gelators have found many practical applications as soft materials. To better understand the structure and molecular gelation relationship and further explore the applications of sugar-based gelators, we designed and synthesized eight trimeric branched sugar triazole derivatives and studied their self-assembling properties. These included glucose, glucosamine, galactose, and maltose derivatives. Interestingly, the gelation properties of these compounds exhibited correlations with the peripheral sugar structures. The maltose derivative did not form gels in the tested solvents, but all other compounds exhibited gelation properties in at least one of the solvents. Glucose derivatives showed superior performance, followed by glucosamine derivatives. They typically formed gels in toluene and alcohols; some formed gels in ethanol-water mixtures or DMSO water mixtures. The glycoclusters 9 and 10 demonstrated rate acceleration for the copper-catalyzed azide-alkyne cycloaddition (CuAAC) reactions. These were further studied for their metallogels formation properties, and the copper metallogels from compound 9 were successfully utilized to catalyze click reactions. These metallogels were able to form a gel column, which was effective in converting the reactants into the triazole products in multiple cycles. Moreover, the same gel column was used to transform a second click reaction using different reactants. The synthesis and characterization of these compounds and their applications for catalytic reactions were discussed. 

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