Organogelation by polymer organogelators with a L-lysine derivative: formation of a three-dimensional network consisting of supramolecular and conventional polymers

Polymer compounds consisting of a L-lysine derivative and conventional polymers, such as poly(ethylene glycol), polycarbonate, polyesters, and poly(alkylene), have been synthesized and their organogelation properties examined in various solvents. These polymer compounds function as good organogelators that form organogels in many organic solvents and oils. The organogelation ability is almost independent of the polymer backbone. Observation by field-emission scanning electron microscopy (FE-SEM) demonstrates that the polymer organogelators form a supramolecular polymer with a diameter of several tens of nanometers and create a three-dimensional network in organogels. FT-IR spectroscopic analysis shows that the supramolecular polymer is mainly formed by the self-assembly of L-lysine segments through hydrogen-bonding and van der Waals interactions. Furthermore, the organogels formed by the polymer organogelators have a lower gel-sol temperature and higher gel strength than those of a low-molecular-weight model organogelator.     

Urea and thiourea derivatives as low molecular-mass organogelators

The properties of a series of organogels consisting of a urea or thiourea derivative with one or two n-alkyl substitutuents at the nitrogen atoms (a low molecular-mass organogelator (LMOG)) and an organic liquid are described. They include N,N'-dimethylurea, the LMOG of lowest molecular mass (M(W) 88) we are aware of. The efficiencies of the LMOGs, based the diversity of liquids gelated, the minimum amount required for gelation of a liquid at room temperature, and the temporal and thermal stabilities of the gels formed, have been investigated as a function of the number, length, and substitution pattern of their n-alkyl chains. The gels are thermally reversible and require generally very low concentrations (<2 wt %) of an LMOG. Some of the LMOGs with shorter chains are more efficient than their longer chained analogues. The structural and thermodynamic properties of the gels have been examined by IR, DSC, and X-ray diffraction techniques. Polarizing optical microscopic analyses of the gels show that the nature of gelator aggregates depends mainly on the alkyl chain length. Changes in the aggregation ability have been examined systematically by perturbing the molecular structure.     

Novel organogelators based on pyrazine-2,5-dicarboxylic acid derivatives and their mesomorphic behaviors

A series of new low molecular organogelators (LMOGs) with thermotropic mesophase were synthesized via the reaction of 3,6-dimethyl-pyrazine-2,5-dicarboxylic acid with p-alkoxyl anilines. These compounds readily formed stable gels in a variety of organic solvents and their self-assembly behavior, structure–property relationship were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), ¹H nuclear magnetic resonance (¹H NMR), Fourier transform infrared spectroscopy (FTIR) and ultra-violet–visible spectroscopy (UV). The results showed a combination of intra-hydrogen bonding, π–π stacking and van der Waals interaction resulted in the aggregation of the organogelators to form three-dimension fibrous networks. The gels formed were multi-responsive to environmental stimuli, such as temperature, fluorinion, and shear stress. More importantly, all the organogelators exhibited thermotropic hexagonal column mesophase as revealed by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and variable temperature XRD studies. A control compound was synthesized and its gelling ability was also checked.     

Molecular organogels of the sodium salt of (R)-12-hydroxystearic acid and their templated syntheses of inorganic oxides

(R)-12-Hydroxystearic acid (HSA), a natural product from castor oil, is a well-known low-molecular mass organogelator (LMOG). Here, we demonstrate that the sodium salt of HSA, HSA-S, is an extremely versatile and efficient LMOG. Furthermore, its self-assembled fibrillar networks (SAFINs) in gels with ethanol, benzene, tetrahydrofuran, and dimethyl sulfoxide, as well as the gel of HSA with benzene, are shown to act as templates during the sol-gel polymerization of tetraethyl orthosilicate (TEOS) in the absence or presence of an external catalyst. The templated, fiber-like objects obtained after calcinations have been characterized. The shape of the templated silica is strongly influenced by the catalyst applied. In addition, it has been possible to effect the formation of assemblies of nanoscale objects of Fe₂O₃ and CuO by polymerization of appropriate precursors in HSA-S based gels and in suspensions, respectively, followed by drying and calcination. The procedures employed are efficient and inexpensive protocols to make porous nanomaterials using organogels. Typically, templated syntheses of such materials in organogels have employed less accessible and more structurally complex LMOGs than HSA-S or HSA. Electrostatic interactions via Na⁺ bridges or H-bonding between silicate intermediates and gelator strands are proposed to be a primary driving force for templating.     

Gelation‐Induced Enhanced Fluorescence Emission from Organogels of Salicylanilide‐Containing Compounds Exhibiting Excited‐State Intramolecular Proton Transfer: Synthesis and Self‐Assembly

Self‐assembly structure, stability, hydrogen‐bonding interaction, and optical properties of a new class of low molecular weight organogelators (LMOGs) formed by salicylanilides 3 and 4 have been investigated by field‐emission scanning electron microscopy (FESEM), X‐ray diffraction (XRD), UV/Vis absorption and photoluminescence, as well as theoretical studies by DFT and semiempirical calculations with CI (AM1/PECI=8) methods. It was found that salicylanilides form gels in nonpolar solvents due to π‐stacking interaction complemented by the presence of both inter‐ and intramolecular hydrogen bonding. The supramolecular arrangement in these organogels predicted by XRD shows lamellar and hexagonal columnar structures for gelators 3 and 4 , respectively. Of particular interest is the observation of significant fluorescence enhancement accompanying gelation, which was ascribed to the formation of J‐aggregates and inhibition of intramolecular rotation in the gel state.     

N-alkyl perfluoroalkanamides as low molecular-mass organogelators

A new class of low molecular-mass organogelators (LMOGs), N-alkyl perfluoroalkanamides, F(CF(2))(n)CONH(CH(2))(m)H, is described. The molecules are designed to exploit the incompatibilities of their three molecular parts, and the results demonstrate that this strategy can be used to tune molecular aggregation and gel stability. The gelating properties of these LMOGs have been examined in a wide variety of organic liquids (including alkanes, alcohols, toluene, n-perfluorooctane, CCl(4), and DMSO) as a function of the N-alkyl and perfluoroalkyl chain lengths by X-ray diffraction, polarizing optical microscopy, infrared spectroscopy, differential scanning calorimetry, and small-angle neutron scattering (SANS). The gels are thermally reversible and require generally very low concentrations (<2 wt %) of LMOG. Several of the gels are stable for very long periods at room temperature. The incompatibility of the fluorocarbon and hydrocarbon segments causes the LMOGs to aggregate, probably into lamellae within the fibrils that constitute the basic unit of the gel networks. The SANS studies show that the cross-sections of fibers in the gel networks of LMOGs with shorter perfluoroalkyl chains are much larger than those with longer ones. Comparisons with the gelating properties of some analogous esters (F(CF(2))(n)CO(2)(CH(2))(m)H) and diblock perfluoroalkylalkanes (F(CF(2))(n)(CH(2))(m)H) indicate that additional ordering within the aggregate units is enforced by the intermolecular H bonding among amide groups that is evidenced by IR spectroscopy. Analyses of these results and structure/solvent correlations are provided.     

Organogels formed in various organic solvents by different alkyl L-phenylalanine dihydrazide derivatives

A new group of organogelators, L-phenylalanine dihydrazide derivatives were synthesized, which can self-assemble in various organic solvents and turned them into thermally reversible physical supramolecular organogels at extremely low concentrations (<2 wt %). Scanning electron microscopy measurements revealed that the gelator self-assembled into different supramolecular network structures in different solvents. FT-IR spectroscopy studies revealed that intermolecular hydrogen bonding between N-H and C=O of amide group and hydrophobic interaction of the alkyl groups were the driving forces for the formation of the gels. Based on the data of XRD and molecular modeling, one possible packing mode for the formation of organogelator aggregates was proposed.     

Super organogelator based on tetrathiafulvalene with four amide derivatives and its F4TCNQ charge-transfer complex

A new series of tetrathiafulvalene-based organogelators endowed with four hydrophobic chains incorporating amide groups was synthesised and characterised. The resulting transparent organogels were obtained with organic solvents such as cyclohexane, carbon tetrachloride and chlorobenzene. Additionally, the length of the alkyl chain influenced the gelation ability of organogels. Considering the results, we concluded that compounds were ‘super gelators’. Interestingly, the gelators reacted with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane to form charge-transfer (CT) complexes and binary organogels. ¹HNMR and FT-IR revealed that cooperation of hydrogen bonding, π–π and CT interactions was the main driving force for formation of the native and CT gels. The scanning electron microscopy images of native xerogels revealed characteristic gelation morphologies of three-dimensional cross-linking networks, whereas the morphologies of CT complex xerogels showed amorphous rod-like aggregates. X-ray powder diffraction studies suggested that both gelator and CT complex maintained lamellar molecular packing mode in organogel phase.     

Gelation of perfluorinated liquids by N-alkyl perfluoroalkanamides

Gels comprised of low-molecular-mass organic gelators (LMOGs), N-alkyl perfluoroalkanamides [F(CF2)(m)CONH(CH2)(n)H; FmNHn], and several perfluorinated liquids are described. The gelation ability of the amides has been compared to that of two analogous alkyl perfluoroalkanoates. The properties of these gels have been correlated with the N-alkyl and (to a lesser extent) perfluoroalkyl chain lengths in the FmNHn by X-ray diffraction, polarizing optical microscopy, infrared spectroscopy, and small-angle neutron scattering. The gels are thermally reversible and require generally very low concentrations (<2 wt %) of LMOG. Several of the gels have been stable at room temperature for >1 year, thus far. The incompatibility of the fluorocarbon and hydrocarbon segments causes the LMOGs to aggregate into lamellae within the fibrils that constitute the basic unit of the gel networks. IR spectroscopic studies of these gels indicate that additional ordering within the aggregate units is enforced by intermolecular H-bonding among amide groups.     

Functional organogel based on a hydroxyl naphthanilide derivative and aggregation induced enhanced fluorescence emission

A new class of low molecular weight organogelator (LMOG) of hydroxyl naphthanilide moiety was suitably designed and synthesized and it forms gels through noncovalent interactions in hydrocarbon solvents. Self-assembly structure, hydrogen bonding interaction, and photophysical properties of organogelator 3-hydroxy-naphthalene-2-carboxylic acid (2-heptylcarbamoyl-phenyl)-amide (2) have been investigated by field emission scanning electron microscope (FE-SEM), FT-IR, UV–vis absorption and photoluminescence combined with theoretical studies by hybrid density-functional theory (DFT) B3LYP and semi-empirical calculations AM1 with CI methods. It was found that gelation is completely thermoreversible, and it occurs due to the aggregation of the organogelator resulting in the formation of a fibrous network due to the π–π stacking interaction complemented by the presence of both inter- and intra-molecular hydrogen bonding. The self-assembled fibrillar networks in the gels were distinctly evidenced by SEM observations. FT-IR studies confirm that the common driving force for aggregation in the organogels and microsegregation in the mesophase is the occurrence of a tight intermolecular hydrogen bonded network that does not persist in diluted solution. Gelator 2 is very weakly fluorescent in solution, but its intensity is increased by almost 30–32 times in their respective gelled state depending on the nature of the gelling solvents. The aggregation induced emission enhancement is ascribed to the formation of J-aggregation and inhibition of intramolecular rotation in the gel state.     

Immobilization of Styrene‐Substituted 1,3,4‐Oxadiazoles into Thermoreversible Luminescent Organogels and Their Unexpected Photocatalyzed Rearrangement

A series of styrene‐substituted 1,3,4‐oxadiazoles has been designed and investigated as new low‐molecular‐weight organogelators. The photophysical properties of the resulting thermoreversible organogels have been characterized by UV/Vis absorption and luminescence spectroscopies. Surprisingly, the gelation ability of the oxadiazoles depended on the presence of the styrene moiety as gelation of the investigated oxadiazoles did not take place in its absence. Gel formation was accompanied by a modification of the fluorescence of the organogelators in the supramolecular state. UV irradiation of the gels caused a rearrangement of the immobilized 1,3,4‐oxadiazoles bearing a styrene moiety by a tandem [4+2] and [3+2] cascade reaction. Structure modification and color change of the gels were also evident upon irradiation.     

Kinetics of 5alpha-cholestan-3beta-yl N-(2-naphthyl)carbamate/n-alkane organogel formation and its influence on the fibrillar networks

The kinetics and mode of nucleation and growth of fibers by 5alpha-cholestan-3beta-yl N-(2-naphthyl)carbamate (CNC), a low-molecular-mass organogelator (LMOG), in n-octane and n-dodecane have been investigated as their sols were transformed isothermally to organogels. The kinetics has been followed in detail by circular dichroism, fluorescence, small-angle neutron scattering, and rheological methods. When treated according to Avrami theory, kinetic data from the four methods are self-consistent and describe a gelation process involving one-dimensional growth and "instantaneous nucleation". As expected from this growth model, polarized optical micrographs of the self-assembled fibrillar networks (SAFINs) show fibrous aggregates. However, their size and appearance change abruptly from spherulitic to rodlike as temperature is increased. This morphological change is attended by corresponding excursions in static and kinetic CD, fluorescence and rheological data. Furthermore, the rheological measurements reveal an unusual linear increase in viscoelastic moduli in the initial stages of self-assembly. Each of the methods employed becomes sensitive to changes of the system at different stages of the transformation from single molecules of the LMOG to their eventual SAFINs. This study also provides a methodology for investigating aggregation phenomena of some other self-assembling systems, including those of biological and physiological importance.     

Dipeptide-based low-molecular-weight efficient organogelators and their application in water purification

The development of new low-molecular-weight gelators for organic solvents is motivated by several potential applications of gels as advanced functional materials. In the present study, we developed simple dipeptide-based organogelators with a minimum gelation concentration (MGC) of 6-0.15 %, w/v in aromatic solvents. The organogelators were synthesized using different L-amino acids with nonpolar aliphatic/aromatic residues and by varying alkyl-chain length (C-12 to C-16). The self-aggregation behavior of these thermoreversible organogels was investigated through several spectroscopic and microscopic techniques. A balanced participation of the hydrogen bonding and van der Waals interactions is crucial for efficient organogelation, which can be largely modulated by the structural modification at the hydrogen-bonding unit as well as by varying the alkyl-chain length in both sides of the hydrophilic residue. Interestingly, these organogelators could selectively gelate aromatic solvents from their mixtures with water. Furthermore, the xerogels prepared from the organogels showed a striking property of adsorbing dyes such as crystal violet, rhodamine 6G from water. This dye-adsorption ability of gelators can be utilized in water purification by removing toxic dyes from wastewater.     

Distinct kinetic pathways generate organogel networks with contrasting fractality and thixotropic properties

The kinetics of the isothermal transformation of sols, comprised of a low molecular-mass organogelator (LMOG) and an organic liquid, to their organogel phases have been followed by circular dichroism (CD), fluorescence, small angle neutron scattering (SANS), and rheological methods. The thixotropic properties (in the sense that severe shearing followed by rest lead to reestablishment of viscoelasticity) of the gels have been examined as well by rheological measurements. The compositions of the samples were either 5alpha-cholestan-3beta-yl N-(2-naphthyl) carbamate (CNC) in an n-alkane (n-octane or n-dodecane) or 3beta-cholesteryl N-(2-naphthyl) carbamate (CeNC) in ethyl acetate. Values of Df, the mass fractal dimension of the microcrystalline self-assembled fibrillar networks (SAFINs) in the gels, have been extracted from the kinetic data using a model developed by Dickinson (J. Chem. Soc., Faraday Trans. 1997, 93, 111). The Df values, 1.1-1.3 for the CeNC gels and 1.3-1.4 or 1.6-1.8 (depending on the temperature of incubation of the sol phase) for CNC gels, are consistent with the gel network structures observed by optical microscopy. In addition, comparison of the temperature dependence of both n (the Avrami component) and K (the Avrami "rate constant") for CeNC/ethyl acetate gelation with those reported previously for gelation of CNC/n-alkane sols demonstrate that the very small change of a single bond in CNC to a double bond in CeNC causes significant differences in their gelation abilities and gel properties. The rheological measurements on CNC/n-alkane gels with spherulitic SAFIN units, formed by incubation of their sols at < or =28 degrees C, indicate that they are thixotropic. Gels with the same chemical composition but formed by incubation of their sols at > or =30 degrees C, leading to fiberlike SAFIN units, remain liquidlike after shearing regardless of the periods they are at rest. The time-dependent viscoelastic properties of the gel networks are treated according to a stretched exponential model. The observations from these studies provide detailed insights into the mechanisms of formation of molecular organogel phases and demonstrate the extreme sensitivity of the SAFINs and viscoelastic properties of such organogels to slight modifications in LMOG structure or sample history.     

Poly(aryl ether) Dendrons with Monopyrrolotetrathiafulvalene Unit‐Based Organogels exhibiting Gel‐Induced Enhanced Emission (GIEE)

A series of poly(aryl ether) dendrons with a monopyrrolo‐tetrathiafulvalene unit linked through an acyl hydrazone linkage were designed and synthesized as low molecular mass organogelators (LMOGs). Two of the dendrons could gelate the aromatic solvents and some solvent mixtures, but the others could not gel all solvents tested except for n‐pentanol. A subtle change on the molecular structure produces a great influence on the gelation behavior. Note that the dendrons could form the stable gel in the DMSO/water mixture without thermal treatment and could also form the binary gel with fullerene (C₆₀) in toluene. The formed gels undergo a reversible gel–sol phase transition upon exposure to external stimuli, such as temperature and chemical oxidation/reduction. A number of experiments (SEM, FTIR spectroscopy, ¹H NMR spectroscopy, and UV/Vis absorption spectroscopy, and XRD) revealed that these dendritic molecules self‐assembled into elastically interpenetrating one‐dimensional fibrillar aggregates and maintain rectangular molecular‐packing mode in organogels. The hydrogen bonding, π–π, and donor–acceptor interactions were found to be the main driving forces for formation of the gels. Moreover, the gel system exhibited gel‐induced enhanced emission (GIEE) property in the visible region in spite of the absence of a conventional fluorophore unit and the fluorescence was effectively quenched by introduction of C₆₀.     

Effect of the structure of gelators on electro-optical properties of liquid crystal physical gels

Low molecular mass organic gelator (LMOG) as an important component of liquid crystal physical gel has a great influence on the electro-optical properties. In this paper, three analogues of amide gelator were synthesized and employed as LMOGs in nematic liquid crystal 5CB. Both hydrogen-bonding and pi-pi-stacking interactions in the gel phase were found to stabilize the self-assembled structure. It was observed that the morphology was highly dependent on the crystallinity of gelators, which was affected by the intensity of hydrogen bonding. The thicker fibril was obtained with higher crystallinity of LMOG, while the thinner fibril was obtained with lower crystallinity. Moreover, the electro-optical properties of liquid crystal physical gels were proposed to be related to the interaction between the fibrils and the liquid crystal molecules.     

环境响应的智能小分子有机凝胶材料

小分子有机凝胶(low molecule organogel,LMOG)是近年来逐渐发展起来的一类新型自组装材料,随着研究的深入,LMOG的功能化特别是对环境有智能响应的凝胶体系引起人们极大的研究兴趣。本文综述了4类智能响应的凝胶体系的研究进展,即: 光响应小分子凝胶体系,主要是凝胶因子内含有偶氮苯、二芳乙烯等光致变色基团;电化学响应小分子凝胶体系,主要是凝胶因子内含有四硫富瓦烯等电化学响应基团;离子(分子)响应的小分子凝胶体系,通过凝胶和客体离子(分子)间通过电荷转移或结构形变等形式实现响应;超声波响应小分子凝胶体系,在超声波外力的存在下,使分子结构以有利于形成分子间氢键的形式存在,从而形成稳定凝胶。     

Boron Effect on Sugar-Based Organogelators

The reaction of several alkylglucosides with phenyl boronic acid permitted easy access to a series of alkylglucoside phenyl boronate derivatives. This type of compound has structures similar to those of known benzylidene glucoside organogelators except for the presence of a boronate function in place of the acetal one. Low to very low concentrations of these amphiphilic molecules produced gelation of several organic solvents. The rheological properties of the corresponding soft materials characterized them as elastic solids. They were further characterized by SEM to obtain more information on their morphologies and by SAXS to determine the type of self-assembly involved within the gels. The sensitivity of the boronate function towards hydrolysis was also investigated. We demonstrated that a small amount of water (5 % v/v) was sufficient to disrupt the organogels leading to the original alkylglucoside and phenyl boronic acid; an important difference with the stable benzylidene-based organogelators. Such water-sensitive boronated organogelators could be suitable substances for the preparation of smart soft material for topical drug delivery.     

Thixotropic twin-dendritic organogelators

Twin-dendritic organogelators have been prepared through selective functionalization of N-(3-aminopropyl)-1,3-propanediamine (APPDA) with self-assembling dendrons by using 1,1'-carbonyldiimidazole (CDI). Subsequent modification of the APPDA linker provided an additional degree of structural diversity by which to tailor the gelator self-assembly in bulk or in the gel state. These compounds are able to gel cyclohexane, toluene, n-butyl acetate, ethyl acetate, dichloromethane, and tetrahydrofuran. 3,4-Disubstituted apical branching units provided the most efficient organogelators and show a propensity to form thixotropic gels, wherein the gel recovers its elasticity after being subjected to shear. Structural and retrostructural analysis of the twin-dendritic organogelators reveals the bulk structural characteristics to be indicative of the subsequent gel properties. Diverse self-organized arrays were identified in bulk and all are able to form gels, thus indicating the role of quasiequivalence in mediating self-assembly in the gel state. Furthermore, we have found that porous columnar mesophases provide a strategy by which to prepare thixotropic gels. We demonstrate the importance of weak lateral hydrogen bonding within a column stratum versus hydrogen bonding along the length of the column for forming porous columnar mesophases and, by extension, thixotropic gels.