Gelation mechanism and microstructure of organogels formed with various types of gelators

The small‐angle neutron scattering (SANS) and dynamic light scattering (DLS) investigation were carried out for organogels in toluene, formed by organogelators, to elucidate the relationship between the chemical structure and the gelation mechanism as well as the physical properties of the gels. Three different organogelators, that is cyclo(L ‐β‐3,7‐dimethyloctylasparaginyl‐L ‐phenylalanyl) (CPA), trans‐(1R,2R)‐bis(undecylcarbonylamino)cyclohexane (TCH), and N^ε‐lauroyl‐N^α‐stearylaminocarbonyl‐L ‐lysine ethyl ester (LEE), were chosen for comparison. The SANS intensity functions of toluene solutions of these gelators could be reduced with the concentration and were described with a scattering function for thin rods. This indicates that the gels consist of noncorrelated, rod‐like elements aggregated to each other. The characteristic features of the gelation properties, such as the critical gelation concentration, C_gel, the gelation temperature, T_gel, the gel structure, and the gelation mechanism, were different from each other. CPA had the lowest C_gel and became a gel gradually as the temperature decreased, while TCH and LEE had higher C_gels and underwent a sharp sol–gel transition. We conclude that the gelation mechanisms between the CPA and TCH solutions are different. The “CPA type” gelators form a gel by a linear extension of hydrogen‐bonded plane, while the “TCH type” gelators form a twisted wire, because of its strong helicity and crystallizability. In addition, in the latter type, a next generation of fibrils easily stacks on top of the previous ones to form larger fibrils. These models well explain the DLS results and the mechanical properties. That is, the fibrillar stems in CPA gels are rather mobile and fragile, while those in TCH and LEE are frozen and brittle. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3567–3574, 2005     

Chiral Hexa‐ and Nonamethylene‐Bridged Bis(L‐Leu‐oxalamide) Gelators: The First Oxalamide Gels Containing Aggregates with a Chiral Morphology

Chiral amino acid‐ and amino alcohol‐oxalamides are well‐known as versatile and efficient gelators of various lipophilic and polar organic solvents and water. To further explore the capacity of the amino acid/oxalamide structural fragment as a gelation‐generating motif, the dioxalamide dimethyl esters 1₆Me and 1₉Me , and dicarboxylic acid 2₆OH / 2₉OH derivatives containing flexible methylene bridges with odd ( 9 ; n=7) and even ( 6 ; n=4) numbers of methylene groups were prepared. Their self‐assembly motifs and gelation properties were studied by using a number of methods (FTIR, ¹H NMR spectroscopy, CD, TEM, DSC, XRPD, molecular modeling, MMFF94, and DFT). In contrast to the previously studied chiral bis(amino acid or amino alcohol) oxalamide gelators, in which no chiral morphology was ever observed in the gels, the conformationally more flexible 1₆Me , 1₉Me , 2₆OH , and 2₉OH provide gelators that are capable of forming diverse aggregates of achiral and chiral morphologies, such as helical fibers, twisted tapes, nanotubules, straight fibers, and tapes, in some cases coexisting in the same gel sample. It is shown that the differential scanning calorimetry (DSC)‐determined gelation enthalpies could not be correlated with gelator and solvent clogP values. Spectroscopic results show that intermolecular hydrogen‐bonding between the oxalamide units provides the major and self‐assembly directing intermolecular interaction in the aggregates. Molecular modeling studies reveal that molecular flexibility of gelators due to the presence of the polymethylene bridges gives three conformations ( zz , p1 , and p2 ) close in energy, which could form oxalamide hydrogen‐bonded layers. The aggregates of the p1 and p2 conformations tend to twist due to steric repulsion between neighboring iBu groups at chiral centers. The X‐ray powder diffraction (XRPD) results of 1₆Me and 1₉Me xerogels prove the formation of p1 and p2 gel aggregates, respectively. The latter results explain the formation of gel aggregates with chiral morphology and also the simultaneous presence of aggregates of diverse morphology in the same gel system.     

Remarkable Shape‐Sustaining,Load‐Bearing,and Self‐Healing Properties Displayed by a Supramolecular Gel Derived from a Bis‐pyridyl‐bis‐amide of L‐Phenyl Alanine

A series of bis‐amides decorated with pyridyl and phenyl moieties derived from L ‐amino acids having an innocent side chain (L ‐alanine and L ‐phenyl alanine) were synthesized as potential low‐molecular‐weight gelators (LMWGs). Both protic and aprotic solvents were found to be gelled by most of the bis‐amides with moderate to excellent gelation efficiency (minimum gelator concentration=0.32–4.0 wt. % and gel–sol dissociation temperature T_gel=52–110 °C). The gels were characterized by rheology, DSC, SEM, TEM, and temperature‐variable ¹H NMR measurements. pH‐dependent gelation studies revealed that the pyridyl moieties took part in gelation. Structure–property correlation was attempted using single‐crystal X‐ray and powder X‐ray diffraction data. Remarkably, one of the bis‐pyridyl bis‐amide gelators, namely 3,3‐Phe (3‐pyridyl bis‐amide of L ‐phenylalanine) displayed outstanding shape‐sustaining, load‐bearing, and self‐healing properties.     

Mechanical Reinforcement of Molecular Hydrogel by Co‐assembly of Short Peptide‐based Gelators with Different Aromatic Capping Groups

We demonstrated in this study that mixing two short peptide‐based gelators with different aromatic capping groups would result in molecular hydrogels with enhanced mechanical property. We selected gelators of PTZ‐GFFY and Nap‐GFFY for the study. Both PTZ‐GFFY and Nap‐GFFY could independently form molecular hydrogel by a heating‐cooling process. However, the mechanical property of gels of PTZ‐GFFY and Nap‐GFFY was relatively weak with storage moduli (G′) of about 500 and 150 Pa, respectively. A two‐component gel of PTZ‐GFFY and Nap‐GFFY could also form by a heating‐cooling process. Surprisingly, the G′ value of the two‐component gel was about 5000 Pa , which was at least ten times bigger than that of each single‐component gel. This is a novel and simple strategy to improve the mechanical property of molecular hydrogels.     

Multi‐Stimuli‐Responsive Organometallic Gels Based on Ferrocene‐Linked Poly(Aryl Ether) Dendrons: Reversible Redox Switching and Pb2+‐Ion Sensing

We describe the design, synthesis, and “stimuli‐responsive” study of ferrocene‐linked Fréchet‐type [poly(aryl ether)]‐dendron‐based organometallic gels, in which the ferrocene moiety is attached to the dendron framework through an acyl hydrazone linkage. The low‐molecular‐weight gelators (LMWGs) form robust gels in both polar and non‐polar solvent/solvent mixtures. The organometallic gels undergo stimuli‐responsive behavior through 1) thermal, 2) chemical, and 3) electrochemical methods. Among them, conditions 1 and 3 lead to seamlessly reversible with repeated cycles of identical efficiency. Results indicate that the flexible nature of the poly(aryl ether) dendron framework plays a key role in retaining the reversible electrochemical behavior of ferrocene moiety in the LMWGs. Further, the organometallic gelators have exhibited unique selectivity towards Pb²⁺ ions (detection limit ≈10^?8 M ). The metal ion‐sensing results in a gel–sol phase transition associated with a color change visible to the naked eye. Most importantly, decomplexing the metal ion from the system leads to the regeneration of the initial gel morphology, indicating the restoring ability of the organometallic gel. The metal–ligand binding nature has been analyzed by using ¹H NMR spectroscopy, mass spectrometry, and DFT calculations.     

The Effects of Microenvironment Polarity and Dendritic Branching of Aliphatic Hydrocarbon Dendrons on the Self‐Assembly of 2‐Ureido‐4‐pyrimidinones

Two series of aliphatic hydrocarbon‐based G1–G3 dendritic 2‐ureido‐4‐pyrimidinones (UPy) ( S‐Gn )₂ and ( L‐Gn )₂, differing from one another by the distance between the branching juncture to the urea end, were prepared and characterized. These hydrocarbon dendrons were also appended to a p‐aminonitrobenzene solvatochromic chromophore in order to probe their microenvironment polarity. While positive solvatochromism was observed which indicated the chromophore was solvent accessible, there was no significant difference between the microenvironment polarities on going from the G1 to the G3 dendrons. The self‐assembling behavior and tautomeric preference of the dendritic UPy derviatives were examined by ¹H NMR spectroscopy. The dimerization constants (K_dim*) of the DDAA tautomers were unchanged at 10⁷ M ^?1 in CDCl₃ at both 25 and 50 °C, which were comparable to those of UPy compounds bearing other nonpolar substitutents. Furthermore, the lower limits on the K′_dim* of the DADA tautomeric forms of the ( S‐Gn )₂ and ( L‐Gn )₂ series were determined to be 10⁶ and 10⁵ M ^?1 in CDCl₃, respectively. It was found that a closer proximity of the dendron branching juncture to the UPy unit could lead to a destabilization effect on the dimeric states. Hence, the ( L‐Gn )₂ dimers are more stable than those of ( S‐Gn )₂ in the DDAA form, but the latter are more stable than the former in the tautomeric DADA state. This study showed that both the highly nonpolar microenvironment and the proximity of the dendritic branching juncture to the UPy motif could alter the strength and profile of the hydrogen bond‐mediated self‐assembling process.     

A Library of Multipurpose Supramolecular Supergelators: Fabrication of Structured Silica,Porous Plastics,and Fluorescent Gels

Supramolecular gels find applications in various fields. Usually, a specific gelator is useful only for a specific application. This one‐gelator‐one‐application format is one factor that limits the usefulness of supramolecular gels. We report the synthesis of a library of gelators from a common core by using a click‐chemistry approach. Thus, the click reaction of β‐azido‐4,6‐O‐benzylidene–galactopyranoside ( 1 ) with various alkynes gave 11 different gelators having varying gelation abilities. Whereas gelators having alkyl‐chain substituents congealed alkanes and tetraethylorthosilicate (TEOS), the gelators having aromatic substituents congealed aromatic solvents. We exploited this difference in gelling behavior in the templated synthesis of silica rods and porous plastics. The styrene gel of gelator 2 j was polymerized, and the gelator was removed by washing to obtain porous polystyrene. The TEOS gel of gelator 2 b was polymerized to silica, and the gelator template was removed by calcination to give microstructured silica rods. We also developed fluorescent gelator 2 f by this method, which might find applications by virtue of its fluorescence in the assembled state.     

Building Block approach to SiO2–ZrO2 porous materials†

(2‐Hydroxyethyl)trimethylammonium silicate, Si₈O₂₀[N(CH₃)₃(C₂H₄OH)]₈·nH₂O, was allowed to react with zirconium tetrakis(2,4‐pentanedionate) in methanol, resulting in gel formation. The gels were heat‐treated at 650–1000 °C in air. The product at 650 °C showed a specific surface area of 500 m² g⁻¹, and the average pore diameter was ca 4.3 nm, indicating the formation of a thermally stable mesoporous body. Gels with the same composition were also prepared by sol–gel processing using tetraethoxysilane as a silica source. The specific surface area of the product yielded by heating the gels at 650 °C was 425 m² g⁻¹ and the average pore diameter was ca 2.8 nm, which were lower than those of the product from the gels prepared with (2‐hydroxyethyl)trimethylammonium silicate. These differences have been attributed to the difference in nanostructure of the gels, caused by the structure of the silica sources and their polymerization behaviour. Copyright © 1999 John Wiley & Sons, Ltd.     

Multiple‐Stimulus‐Responsive Supramolecular Gels and Regulation of Chiral Twists: The Effect of Spacer Length

A new class of homologous gelators, LG₁₂‐(CH₂)_n‐BSA, composed of bipyridinyl groups, L ‐glutamic moieties having double dodecyl chains, and linked alkyl spacers with different lengths were synthesized. It was found that these gelators could immobilize medium‐polarity solvents readily and the behaviors of these gels showed a dependence on the spacer length. Of all the gels, the LG₁₂‐(CH₂)₁₁‐BSA gels exhibited self‐healing property and multiple‐stimulus responsibility, such as heating, shaking, and sonication. The investigation of CD spectra indicated that the supramolecular chirality, which was attributed to the chiral transfer from the chiral center to the assemblies, was also closely related to the length of methylene spacers. The longer the alkyl spacers, the weaker the transmitted supramolecular chirality. Only LG₁₂‐(CH₂)₁‐BSA gelators, which had the shortest spacers, formed right‐handed nanoscale chiral twists owing to crowded hydrogen bonding interactions. Moreover, the high‐polarity solvent DMF was found to be able to regulate the chiral twist as well as its pitch length readily.     

Hydrophobic End‐Modulated Amino‐Acid‐Based Neutral Hydrogelators: Structure‐Specific Inclusion of Carbon Nanomaterials

Hydrophobic end‐modulated l ‐phenylalanine‐containing triethylene glycol monomethyl ether tagged neutral hydrogelators ( 1 – 4 ) are developed. Investigations determine the gelators’ structure‐dependent inclusion of carbon nanomaterials (CNMs) in the self‐assembled fibrillar network (SAFIN). The gelators ( 1 , 3 , and 4 ) can immobilize water and aqueous buffer (pH 3–7) with a minimum gelator concentration of 10–15 mg mL^?1. The hydrophobic parts of the gelators are varied from a long chain (C‐16) to an extended aromatic pyrenyl moiety, and their abilities to integrate 1 D and 2 D allotropes of carbon (i.e., single‐walled carbon nanotubes (SWNTs) and graphene oxide (GO), respectively) within the gel are investigated. Gelator 1 , containing a long alkyl chain (C‐16), can include SWNTs, whereas the pyrene‐containing 4 can include both SWNTs and GO. Gelator 3 fails to incorporate SWNTs or GO owing to its slow rate of gelation and possibly a mismatch between the aggregated structure and CNMs. The involvement of various forces in self‐aggregated gelation and physicochemical changes occurring through CNM inclusion are examined by spectroscopic and microscopic techniques. The distinctive pattern of self‐assembly of gelators 1 and 4 through J‐ and H‐type aggregation might facilitate the structure‐specific CNM inclusion. Inclusion of SWNTs/GO within the hydrogel matrix results in a reinforcement in mechanical stiffness of the composites compared with that of the native hydrogels.     

Synthesis of transition‐metal‐ion‐selective poly(N‐isopropylacrylamide) hydrogels by the incorporation of an Aza crown ether

A functionalized cyclam was synthesized by the attachment of a polymerizable acryloyl group to one of the four nitrogens on the cyclam molecule. The polymerization of the functionalized cyclam was performed with N‐isopropylacrylamide and N,N′‐methylene bisacrylamide, and the gels obtained were studied in the presence of different transition‐metal‐ion solutions. There was a drastic difference in the phase‐transition temperature (T_c) of the poly(N‐isopropylacrylamide) (PNIPAAm)/cyclam gel in comparison with the pure PNIPAAm gel. For the described system, a T_c shift of 15 °C was obtained. The presence of functionalized cyclam increased the hydrophilicity and T_c of the aforementioned polymer gels in deionized water (at pH 6) because of the presence of protonated amino moieties. The PNIPAAm/cyclam gels showed a dependence of the swelling behavior on pH. T_c of the pure PNIPAAm gel was weakly influenced by the presence of any transition‐metal ions, such as Cu²⁺, Ni²⁺, Zn²⁺, and Mn²⁺. The addition of Cu²⁺ or Ni²⁺ to the PNIPAAm/cyclam gel reduced T_c of the polymer gel, and a shift of approximately 12 °C was observed. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1594–1602, 2003     

Cholesteryl derivatives as phase-selective gelators at room temperature

One cholesterol-based ALS and two dimeric cholesterol-based A(LS)₂ low-molecular-mass organic gelators (LMOGs) containing phthaloyl, isophthaloyl, and terephthaloyl moieties, respectively, were prepared. Gelation test revealed that 2 and 3 are more efficient gelators than 1. Interestingly, 2 and 3 can gel several solvents spontaneously at room temperature and these gels posses thixotropic properties as revealed by rheological studies. More interestingly, 2 and 3 show selective gelation of the solvents from their mixtures with water. The network structures of some gels were investigated by SEM measurements, and the molecular packing mode of the LMOGs in the gel was studied by XRD analysis. Temperature- and concentration-dependent ¹H NMR measurements revealed that hydrogen bonding between the gelator molecules is an important driving force for the gel formation.     

Synthesis of Fluorescent Gelators and Direct Observation of Gelation with a Fluorescence Microscope

Fluorescein‐, benzothiazole‐, quinoline‐, stilbene‐, and carbazole‐containing fluorescent gelators have been synthesized by connecting gelation‐driving segments, including l ‐isoleucine, l ‐valine, l ‐phenylalanine, l ‐leucine residue, cyclo(l ‐asparaginyl‐l ‐phenylalanyl), and trans‐(1R,2R)‐diaminocyclohexane. The emission behaviors of the gelators were investigated, and their gelation abilities studied against 15 solvents. The minimum gel concentration, variable‐temperature spectroscopy, transmission electron microscopy, scanning electron microscopy, fluorescence microscopy (FM), and confocal laser scanning microscopy (CLSM) were used to characterize gelation. The intermolecular hydrogen bonding between the N?H and C=O of amide, van der Waals interactions and π–π stacking play important roles in gelation. The colors of emission are related to the fluorescence structures of gelators. Fibrous aggregates characterized by the color of their emission were observed by FM. 3D images are produced by the superposition of images captured by CLSM every 0.1 μm to a settled depth. The 3D images show that the large micrometer‐sized aggregates spread out three dimensionally. FM observations of mixed gelators are studied. In the case of gelation, two structurally related gelators with the same gelation‐driving segment lead to the gelators build up of the same aggregates through similar hydrogen‐bonding patterns. When two gelators with structurally different gelation‐driving segments induce gelation, the gelators build up each aggregate through individual hydrogen‐bonding patterns. A fluorescent reagent that was incorporated into the aggregates of gels through van der Waals interactions was developed. The addition of this fluorescent reagent enables the successful observation of nonfluorescent gelators’ aggregates by FM.     

Thermoreversible gelation of helical polypeptide/single‐walled carbon nanotubes and their solid‐state structures

Single‐walled carbon nanotubes (SWCNTs) have been functionalized with poly(γ‐benzyl‐L ‐glutamate)s (PBLGs) having well‐defined polymer molecular weight (M_n = 7.5–21.1 kg·mol^?1) and molecular weight distribution (PDI = 1.05–1.20) by a graft‐to method. Toluene solutions containing 5 wt % free PBLG and variable amounts of PBLG‐functionalized SWCNTs (PBLG‐SWCNTs) form gels at room temperature. Differential scanning calorimetry (DSC) analysis reveals that the gelation occurs thermoreversibly, in accord with previous studies on the pristine PBLG/toluene gels. The heat of gel melting (ΔH_m) is slightly elevated for the composite gels compared with the pristine gel, which suggests enhanced interactions between PBLGs in the former. But the gelation temperatures of the composites are unaffected by the presence of PBLG‐SWCNTs. Small‐angle X‐ray scattering (SAXS) analysis of the composite and pristine gels at different temperatures by the Guinier method suggests that PBLG‐SWCNTs promote interactions between PBLG rods, as indicated by the larger PBLG bundle size with increasing PBLG‐SWCNT content in the gel and the melt state. W/SAXS analysis of the dry gels reveals that PBLG‐SWCNTs induce significant changes in the PBLG packing order, resulting in a nematic phase, in contrast to a weakly ordered smectic C phase containing tilted PBLG rods that is observed in the pristine gel. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Imine Gels Based on Ferrocene and Porphyrin and Their Electrocatalytic Property

A series of porphyrin‐based imine gels have been synthesized via dynamic covalent gelation between 5,10,15,20‐tetra(4‐aminophenyl)‐21H,23H‐porphyrin (H₂TAPP) derivatives and various aldehyde compounds. The porphyrin‐ferrocene imine gels based on MTAPP (M=H₂, Ni²⁺, Co²⁺, Pd²⁺ and Zn²⁺) and ferrocene‐1,1′‐dicarbaldehyde (NA) display efficient HER, OER and ORR activities in alkaline media. Among the gels, CoTAPP‐NA shows an HER current density of 10 mA cm^?2 at low overpotential of 470 mV and small Tafel slope of 110 mV decade^?1 in alkaline media. CoTAPP‐NA also exhibits OER catalytic activity with low overpotential (416 mV for 10 mA cm^?2). CoTAPP‐NA shows ability in overall water splitting in alkaline media. In addition, CoTAPP‐NA exhibits onset potential (E_p) of 0.95 V and half‐wave potential (E_1/2) of 0.84 V in 1.0 mol L^?1 KOH solution for oxygen reduction. Moreover, the gel catalyst shows good stability.     

The Synthesis of Amide Dendritic Gelators and its Self-assembly Behavior in MMA

Three kinds of amide dendritic gelators, G₁-C₁₂-G₁, G₂-C₁₂-G₂ and gelator-1, were synthesized, and their self-assemble behavior in methyl methacrylate (MMA) was firstly investigated. The structures of the amide dendritic gelators were confirmed by ¹H-NMR and Mass spectra (MS). The gelation ability of the amide dendritic gelators was researched through tube inversion experiment, the results of which showed that different structures led to quite different gelation ability, including gel-sol temperature and critical concentration to form a stable gel. SEM experiments showed that three kinds of gelator formed different gel morphologies in MMA, all of which were nanoscale gel. All the three amide dendritic gelators could not only form stable gel network at certain temperature with different concentrations in MMA, but also in each case, an optically transparent gel was formed, which indicated dendrimers in the MMA had a good compatibility.     

Triazolyl‐Based Molecular Gels as Ligands for Autocatalytic ‘Click’ Reactions

The catalytic performance of triazolyl‐based molecular gels was investigated in the Huisgen 1,3‐dipolar cycloaddition of alkynes and azides. Low‐molecular‐weight gelators derived from l ‐valine were synthesized and functionalized with a triazole fragment. The resultant compounds formed gels either with or without copper, in a variety of solvents of different polarity. The gelators coordinated Cu^I and exhibited a high catalytic activity in the gel phase for the model reaction between phenylacetylene and benzylazide. Additionally, the gels were able to participate in autocatalytic synthesis and the influence of small structural changes on their performance was observed.     

A Highly Efficient Self‐Assembly of Responsive C2‐Cyclohexane‐Derived Gelators

The rational design and synthesis of a family of effective low‐molecular‐weight gelators (LMWGs) with a modular architecture based on a C₂‐1,4‐diamide cyclohexane core are reported. Due to the high symmetry, the gelators are initially well distributed in solution and no trapped aggregates are formed before the assembly is triggered. The subsequent self‐assembly process, which results in the formation of versatile gels, is highly efficient and can be triggered and tuned due to its remarkable dependence on the pH of solution. The assembly of different gelators is found to be closely related to the pK_a of the corresponding functional substituents on the LMWGs. Primary cell culture experiments reveal that the hydrogels made under physiological conditions are promising as potential tailor‐made microenvironments.     

Metal–Organic Gels Derived from Iron(III) and Pyridine Ligands: Morphology,Self‐Healing and Catalysis for Ethylene Selective Dimerization

Metal‐organic gels showing potential application in catalysis have received much concern. In this work, we designed and synthesized two metal‐organic gels based on coordination between Fe^III and pyridine ligands at room temperature. The gels were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to reveal their assembly structures and morphologies, and it was found the metal‐organic gel derived from di‐topic ligand was composed of three‐dimensional network of nanofibers, while the gel derived from tri‐topic ligand was constituted of sponge‐like structure with amorphous phase. Rheological analysis showed the gel consisting of nanofiber networks displayed self‐healing property. The gels were used as catalysts for selective ethylene dimerization, and the optimum catalysis results of the gel with nanofibers reached the maximal catalytic activity of 1.48×10⁵ g/(mol Fe?h) with C₄ yield more than 90 %, whereas the sponge‐like gel only gave 38 % C₄ products at the same condition. The higher dimerization selectivity of the former Fe^III gel was attributed to its regular assembly structure and lower steric hindrance of the surface metal sites. Due to its catalytic activity, high selectivity and preparation simplicity, the Fe^III gel might be potentially applicable for the preparation of C₄ α‐olefins.