A novel low-molecular-mass gelator with a redox active ferrocenyl group: tuning gel formation by oxidation

A novel low-molecular-mass gelator containing a redox-active ferrocenyl group, cholesteryl glycinate ferrocenoylamide (CGF), was intentionally designed and prepared. It was demonstrated that the gelator gels 13 out of the 45 solvents tested. Scanning electron microscopy (SEM) measurements revealed that the gelator self-assembled into different supramolecular network structures in different gels. Chemical oxidation of the ferrocenyl residue resulted in phase transition of the gel from gel state to solution state. FTIR and (1)H NMR spectroscopy studies revealed that hydrogen bonding between the gelator molecules in the gel was one of the main driving forces for the formation of the gels.     

一种基于胆固醇的荧光小分子胶凝剂的合成及其胶凝行为

合成并表征了一种含7-硝基苯并-2-氧杂-1,3-二唑基(7-Nitrobenzo-2-oxa-1,3-diazol-4-yl)的胆固醇衍生物(NBD-C), 考察了其在30种溶剂中的胶凝行为. 实验结果表明, NBD-C对乙腈具有很强的胶凝作用, 且该凝胶体系具有显著的剪切触变性. 对干凝胶的显微分析发现, 在不同溶剂中, NBD-C具有不同的聚集结构. 红外光谱(FTIR)、核磁共振光谱(¹H NMR)和荧光光谱研究结果表明, 除了胆固醇的范德华堆积作用之外, 分子间氢键作用也是该化合物聚集的重要驱动力.     

A new urea gelator: incorporation of intra- and intermolecular hydrogen bonding for stable 1D self-assembly

A new bisurea gelator derived from 2,6-diaminopyridine has been developed. It efficiently gelates common organic and liquid crystalline (LC) solvents by forming elongated self-assembled fibres in solvents. X-Ray crystallography and 1H NMR measurements reveal that two urea groups in pyridine-based bisurea compounds form different hydrogen bonding patterns. One of two urea units is involved in intramolecular hydrogen bonding with the pyridyl nitrogen, while the other urea unit forms bifurcated intermolecular hydrogen bonding. This hydrogen-bonded structure is key for the fibrous self-assembly along with the efficient gelation. In addition, LC gels based on the pyridine-based gelator exhibit good electrooptic properties. These results indicate that the pyridine-based bisurea compound is a good gelator not only effective in gelation but also useful as a component of functional soft materials.     

一种含芘葡萄糖衍生物的合成及其胶凝行为

合成并表征了一种荧光活性小分子胶凝剂——芘磺酰基-丙二胺-葡萄糖(PSDAPG), 考察了其在36种常见溶剂中的胶凝行为. 结果发现, PSDAPG可使其中16种溶剂胶凝. 对癸醇, PSDAPG表现出罕见的超级胶凝能力, 室温下最低胶凝浓度(MGC)达7.0×10-4 g·mL-1. 此外, PSDAPG还是一种既可胶凝水又可胶凝有机溶剂的双性胶凝剂. 扫描电镜(SEM)、傅立叶变换红外光谱(FTIR)、核磁共振(1HNMR)和荧光光谱研究表明,在不同溶剂中, PSDAPG具有不同的聚集结构, 除了芘基之间的疏水π-π堆积作用外, 氢键作用是PSDAPG自发形成三维网络结构的重要驱动力. 实验研究还表明, 溶液态和凝胶态的PSDAPG荧光光谱均同时呈现芘的单体荧光和激基缔合物荧光光谱特征, 但两者的光谱形貌差异显著. 随凝胶的形成, 体系单体荧光发射增强, 激基缔合物荧光发射减弱,表明形成的三维网络结构阻碍了PSDAPG中芘单元的运动性, 使得以Birks途径形成激基缔合物的效率降低.     

Optically transparent hydrogels from an auxin-amino-acid conjugate super hydrogelator and its interactions with an entrapped dye

Low-molecular-weight organic hydrogelators (LMHGs) that can rigidify water into soft materials are desirable in various applications. Herein, we report the excellent hydrogelating properties of a simple synthetic auxin-amino-acid conjugate, naphthalene-1-acetamide of L-phenylalanine (1-NapF, M(w)=333.38?Da), which gelated water even at 0.025?wt?%, thereby making it the most-efficient LMHG known. Optically transparent gels that exhibited negligible scattering in the range 350-900?nm were obtained. A large shift from the theoretical pK(a) value of the gelator was observed. The dependence of the minimum gelator concentration (MGC) and the gel-melting temperatures on the pH value indicated the importance of H-bonding between the carboxylate groups on adjacent phenylalanine molecules in the gelator assembly. FTIR spectroscopy of the xerogels showed a β-sheet-like assembly of the gelator. Variable-temperature (1)H?NMR spectroscopy demonstrated that π stacking of the aromatic residues was also partly involved in the gelator assembly. TEM of the xerogel showed the presence of a dense network of thin, high-aspect-ratio fibrillar assemblies with diameters of about 5?nm and lengths that exceeded a few microns. Rheology studies showed the formation of stable gels. The entrapment of water-soluble dyes afforded extremely fluorescent gels that involved the formation of J-aggregates by the dye within gel. A strong induced-CD band established that the RhoB molecules were interacting closely with the chiral gelator aggregates. H-bonding and electrostatic interactions, rather than intercalation, seemed to be involved in RhoB binding. The addition of chaotropic reagents, as well as increasing the pH value, disassembled the gel and promoted the release of the entrapped dye with zero-order kinetics.     

A synthetic tripeptide as a novel organo-gelator: a structural investigation

A self-associating synthetic tripeptide [Boc-Ala(1)-Aib(2)-β-Ala(3)-OMe (Aib: α-amino-isobutyric acid, β-Ala: β-alanine)] forms thermoreversible transparent gels in various organic solvents and this offers the first example of a peptide gelator whose molecular self-assembly afforded for gelation has been characterised by single-crystal X-ray diffraction and FT-IR and NMR spectroscopic studies. The crystal structure of an analogous synthetic non-gelator tripeptide [Boc-Ala(1)-Gly(2)-β-Ala(3)-OMe] is also discussed in light of the self-assembly of the gelator tripeptide.     

Chiral bis(amino alcohol)oxalamide gelators-gelation properties and supramolecular organization: racemate versus pure enantiomer gelation

Four new chiral bis(amino alcohol)oxalamides (1-4: amino alcohol=leucinol, valinol, phenylglycinol, and phenylalaninol, respectively) have been prepared as low-molecular-weight organic gelators. Their gelation properties towards various organic solvents and mixtures were determined and these were then compared to related bis(amino acid) oxalamide gelators. Spectroscopic (FTIR, (1)H NMR) and X-ray diffraction studies revealed that the primary organization motif of (S,S)-1 and racemate 1 (rac-1) in lipophilic solvents involved the formation of inverse bilayers. The X-ray crystal structure of (S,S)-1 also shows this type of bilayer organization. The crystal structure of rac-2 reveals meso bilayers of hydrogen-bonded aggregates. Within the bilayers formed, the gelator molecules are connected by cooperative hydrogen bonding between oxalamide units and OH groups, while the interbilayer interactions are realized through lipophilic interactions between the iBu groups of leucinol. Oxalamide meso-1 lacks any gelation ability and crystallizes in monolayers. In dichloromethane rac-1 forms an unstable gel; this is prone to crystallization as a result of the formation of symmetrical meso bilayers. In contrast, in aromatic solvents rac-1 forms stable gels; this indicates that enantiomeric bilayers are formed. Oxalamide rac-1 is capable of gelling a volume of toluene three times larger than (S,S)-1. A tranmission electron microscopy investigation of rac-1 and (S,S)-1 toluene gels reveals the presence of thinner fibers in the former gel, and, hence, a more compact network that is capable of immobilizing a larger volume of the solvent. The self-assembly of these types of gelator molecules into bilayers and subsequent formation of fibrous aggregates can be explained by considering the strength and direction of aggregate forces (supramolecular vectors) in three-dimensional space.     

Optically Transparent Hydrogels from an Auxin–Amino‐Acid Conjugate Super Hydrogelator and its Interactions with an Entrapped Dye

Low‐molecular‐weight organic hydrogelators (LMHGs) that can rigidify water into soft materials are desirable in various applications. Herein, we report the excellent hydrogelating properties of a simple synthetic auxin–amino‐acid conjugate, naphthalene‐1‐acetamide of L ‐phenylalanine ( 1‐NapF , M_w=333.38 Da), which gelated water even at 0.025 wt %, thereby making it the most‐efficient LMHG known. Optically transparent gels that exhibited negligible scattering in the range 350–900 nm were obtained. A large shift from the theoretical pK_a value of the gelator was observed. The dependence of the minimum gelator concentration (MGC) and the gel‐melting temperatures on the pH value indicated the importance of H‐bonding between the carboxylate groups on adjacent phenylalanine molecules in the gelator assembly. FTIR spectroscopy of the xerogels showed a β‐sheet‐like assembly of the gelator. Variable‐temperature ¹H NMR spectroscopy demonstrated that π stacking of the aromatic residues was also partly involved in the gelator assembly. TEM of the xerogel showed the presence of a dense network of thin, high‐aspect‐ratio fibrillar assemblies with diameters of about 5 nm and lengths that exceeded a few microns. Rheology studies showed the formation of stable gels. The entrapment of water‐soluble dyes afforded extremely fluorescent gels that involved the formation of J‐aggregates by the dye within gel. A strong induced‐CD band established that the RhoB molecules were interacting closely with the chiral gelator aggregates. H‐bonding and electrostatic interactions, rather than intercalation, seemed to be involved in RhoB binding. The addition of chaotropic reagents, as well as increasing the pH value, disassembled the gel and promoted the release of the entrapped dye with zero‐order kinetics.     

Bile acid alkylamide derivatives as low molecular weight organogelators: systematic gelation studies and qualitative structural analysis of the systems

A series of amino- and hydroxyalkyl amides of bile acids have been synthesized and characterized by Fourier transform infrared spectroscopy (FTIR), (1)H and (13)C nuclear magnetic resonance spectroscopy (NMR), as well as electrospray ionization mass spectrometry (ESI-MS) measurements. The ability of the synthesized molecules to promote gel formation was systematically investigated. Out of 396 combinations formed by 11 compounds and 36 different solvents, 22 gel-containing systems were obtained with 1% (w/v) gelator concentration. Apart from one exception, the gelator compounds were lithocholic acid derivatives. This challenges the general trend of bile acid-based physical gelators, according to which the gelation ability of lithocholic acid derivatives is poor. A correlation between the values of Kamlet-Taft parameters and solvent preferences for gelators was observed. The morphologies of the solid and gel structures studied with scanning electron microscopy (SEM) showed variability from fibers to spherical microscale aggregates, the latter of which are unique among bile acid-based organogels. The gels exhibited more complex behavior than was previously established with bile acid derivatives, judging by the microscale diversity present in gelating and non-gelating systems and the tendency for polymorphism. This study underlines the importance of both the molecular and colloidal scale aspects of the gelation phenomenon.     

Synthesis,optical properties,and aggregation behavior of a triad system based on perylene and oligo(p-phenylene vinylene) units

A donor-acceptor-donor triad molecule with a perylene bisimide derivative as electron acceptor, and an oligo(p-phenylene vinylene) (OPV) derivative as electron donor was synthesized (OPV-PERY-OPV). The structure of the triad was characterized by (1)H and (13)C NMR spectroscopy, size-exclusion chromatography (SEC), and MALDI-TOF spectrometry. Absorbance spectra and CD spectroscopic measurements of the triad molecule indicated the formation of aggregates in solvents such as toluene, chloroform, and tetrachloroethane, whereas it was present in the molecularly dissolved state in THF. The (1)H NMR spectra of the molecule in chloroform had, unexpectedly, four doublet peaks for the perylene protons, instead of the two doublets that is generally seen in N,N'-substituted perylene molecules. To understand the aggregation behavior and the splitting of the signals in the (1)H NMR spectra, a simple model compound was synthesized, in which the OPV units were replaced by phenyl groups (Ph-PERY-Ph). (1)H NMR spectra in CDCl(3) and tetrachloroethane again had four doublet peaks for the perylene protons, whereas in THF the perylene protons gave only a single peak. NOE and COSY spectroscopy were used to assign the peaks to their corresponding perylene protons. UV/Vis and CD spectroscopic measurements indicated that, similar to the OPV-PERY-OPV triad molecule, the model compound Ph-PERY-Ph was also present in the aggregated form in solvents such as toluene, chloroform, and tetrachloroethane, and in the molecularly dissolved state in THF. IR measurements of the model molecule in the first set of solvents indicated carbamate bond (bond;OCObond;NHbond;)-induced intermolecular hydrogen bonding, whereas in THF, the molecule was mostly present in the free form. CPK models with a dimeric structure, in which two perylene molecules are held together by intermolecular hydrogen bonding with the perylene core shifted slightly with respect to one another, could account for the optical properties and the observation of the four different peaks in the (1)H NMR spectra in polar solvent. Temperature-dependent (1)H NMR spectroscopic, UV/Vis, and CD measurements indicated that the transition from the aggregated to the molecularly dissolved state took place at higher temperatures. The electrochemical studies indicated that OPV-PERY-OPV was both p- and n-dopable, whereas Ph-PERY-Ph was only n-dopable. Cyclic voltammetry measurements of Ph-PERY-Ph in THF had two reduction peaks corresponding to the reduction of the perylene core to the monoanion and dianion, respectively. In dichloromethane, however, an additional reduction peak at lower potential was observed. This new reduction peak might arise from the hydrogen-bonded species.     

Synthesis and structure-property relationships of amphiphilic organogelators

A series of low-molecular-weight amphiphilic molecules was synthesized and investigated for their ability to gel organic solvents. These amphiphilic molecules are composed of a head-group moiety capable of forming intermolecular associations through hydrogen bonds and n-alkyl chains of various lengths. This paper describes the continuation of recently published work, in which this class of gelators was presented for the first time. Here, the focus addresses systematically three different structural variations of the head-group moiety: 1) the rigidity, 2) the type and strength of the hydrogen-bond-forming units, and 3) the reduction of the intermolecular interaction by incorporating a lateral substituent. The gelation behavior was investigated in p-xylene and in several polar solvents. The aim was to establish structure-property relationships and to provide organogelators capable of forming gels at low concentrations with adjustable sol-gel transition temperatures and good optical quality. For some individual compounds of the series this property profile was achieved. For the investigated solvents, with particular compounds, sol-gel transition temperatures above 100 degrees C with a reasonable concentration of gelator are obtainable.     

Water-induced physical gelation of organic solvents by N-(n-alkylcarbamoyl)-L-alanine amphiphiles

A series of amino acid-based gelators N-(n-alkylcarbamoyl)-L-alanine were synthesized, and their gelation abilities in a series of organic solvents were tested. No gelation was observed in pure solvents employed. All the amphiphilic molecules were found to form stable organogels in the solvents in the presence of a small amount of water, methanol, or urea. The volume of solvent gelled by a given amount of the gelator was observed to depend upon the volume of added water. The gelation behavior of the amphiphiles in a given solvent containing a known volume of water was compared. The effects of chirality and substitution on the acid group on the gelation ability were examined. Although the corresponding N-(n-tetradecylcarbamoyl)-DL-alanine was found to form only weak organogel in pure solvents, the achiral amphiphilic compound N-(n-tetradecylcarbamoyl)-β-alanine, however, did not form gel in the absence of water. The methyl ester of N-(n-tetradecylcarbamoyl)-L-alanine was also observed to form gels in the same solvents, but only in the presence of water. The organogels were characterized by several techniques, including (1)H NMR, Fourier transform IR, X-ray diffraction, and field emission scanning electron microscopy. The thermal and rheological properties of the organogels were studied. The mechanical strength of the organogel formed by N-(n-tetradecylcarbamoyl)-DL-alanine was observed to increase upon the addition of water. It was concluded that water-mediated intermolecular hydrogen-bonding interaction between amphiphiles caused formation of supramolecular self-assemblies.     

Thermal properties of the gel made by low molecular weight gelator 1,2-O-(1-ethylpropylidene)-alpha-D-glucofuranose with toluene and molecular dynamics of solvent

The studies of the gel-to-sol phase transition by the Raman, FT-IR, and 1H NMR methods of the gel made by low molecular weight organogelator 1,2-O-(1-ethylpropylidene)-alpha-D-glucofuranose with toluene as the solvent are reported. The FT-IR spectra revealed the existence of a hydrogen bond network formed by gelator molecules in the crystalline and gel phase. In both phases, the network formation is dominated by the gelator self-interaction. Upon gelation, only one stretching band of infrared absorption modes nualpha, assigned to the O(6)H hydroxyl protons of gelator, is shifted by Deltaupsilonalpha = 25 cm-1, which indicates the involvement of this proton in the interaction with the solvent molecules. The phase transition measurements performed as a function of gelator concentration allowed the calculation of the energy correlated with the transition from gel to solution phase. The obtained value of 72 kJ/mol is the largest one reported up until now for monosaccharide-based gels. The analysis of the temperature measurements of the toluene 1H NMR spectra provides evidence for a different chemical environment of toluene molecules in the gel. The toluene spin-lattice relaxation in bulk and gel indicate that the viscosity is most likely the main factor that influences the dynamics of toluene.     

Synthesis, self-assembling properties, and atom transfer radical polymerization of an alkylated L-phenylalanine-derived monomeric organogel from silica: a new approach to prepare packing materials for high-performance liquid chromatography

The monomer N'-octadecyl-N(alpha)-(4-vinyl)-benzoyl-L-phenylalanineamide (4) based on L-phenylalanine has been simply but effectively synthesized, and its self-assembling properties have been investigated. FTIR and a variable-temperature (1)H NMR spectroscopic investigation demonstrated that the aggregation of compound 4 in various organic solvents is due to the formation of intermolecular hydrogen bonds among the amide moieties. UV/Vis measurements indicated that the multiple pi-pi interactions of the phenyl groups also contribute to the self-assembly. As was observed by (13)C cross-polarization magic-angle spinning (CP/MAS) NMR and variable-temperature (1)H NMR measurements, the ordered alkyl chains also played an important role in the molecular aggregation by van der Waals interactions. Compound 4 was polymerized by surface-initiated atom transfer radical polymerization from porous silica gel to prepare a packing material for HPLC. The results of thermogravimetric analysis showed that a relatively large amount of polymer was grafted onto the silica surface. The organic phase on silica was in a noncrystalline solid form in which the long alkyl chain exists in a less-ordered gauche conformation. Analysis of chromatographic performance for polyaromatic hydrocarbon samples showed higher selectivity than conventional reversed-phase HPLC packing materials.     

L-苯丙氨酸衍生物与脂肪胺构筑双组分超分子凝胶

设计合成了系列单链L-苯丙氨酸衍生物,该系列衍生物单组分没有胶凝性能. 选择脂肪胺作为配对物,与L-苯丙氨酸衍生物组成双组分体系后能够胶凝许多有机溶剂形成凝胶. 流变学测试显示该凝胶体系弹性模量（G’）比粘性模量（G"）约高一个数量级,有着很好的机械性能,并且呈现出典型的类固体的流变学行为. 傅里叶变换红外（FT-IR）光谱、核磁共振（NMR）谱、小角X射线衍射（SAXS）和扫描电镜（SEM）结果表明,凝胶中胶凝剂分子形成纤维状或片层状的聚集体,羧基（―COOH）和氨基（―NH₂）的酸碱作用、酰胺基团间（―CONH―）的氢键作用以及分子间范德华作用力是形成该凝胶的主要驱动力. 凝胶中胶凝剂分子自组装形成具有周期性的层状有序结构,层状结构进一步组装形成纤维状聚集体,最终形成三维网状结构阻碍溶剂流动形成凝胶.     

Multidrug‐Containing,Salt‐Based,Injectable Supramolecular Gels for Self‐Delivery,Cell Imaging and Other Materials Applications

Both molecular and crystal‐engineering approaches were exploited to synthesize a new class of multidrug‐containing supramolecular gelators. A well‐known nonsteroidal anti‐inflammatory drug, namely, indomethacin, was conjugated with six different l ‐amino acids to generate the corresponding peptides having free carboxylic acid functionality, which reacted further with an antiviral drug, namely, amantadine, a primary amine, in 1:1 ratio to yield six primary ammonium monocarboxylate salts. Half of the synthesized salts showed gelation ability that included hydrogelation, organogelation and ambidextrous gelation. The gels were characterized by table‐top and dynamic rheology and different microscopic techniques. Further insights into the gelation mechanism were obtained by temperature‐dependent ¹H NMR spectroscopy, FTIR spectroscopy, photoluminescence and dynamic light scattering. Single‐crystal X‐ray diffraction studies on two gelator salts revealed the presence of 2D hydrogen‐bonded networks. One such ambidextrous gelator (capable of gelling both pure water and methyl salicylate, which are important solvents for biological applications) was promising in both mechanical (rheoreversible and injectable) and biological (self‐delivery) applications for future multidrug‐containing injectable delivery vehicles.     

Improved Efficiency of Molecular‐Gel Formation by Adjusting Preorganization of Amino‐Acid‐Derived Flexible Molecules: A NMR and Thermodynamic study

The efficiency of the formation of molecular gels of simple derivatives of l ‐valine and l ‐isoleucine is greatly improved in different organic solvents when a hexyl fragment is replaced by a bulkier cyclohexyl one. A study using NMR and IR spectroscopy provides information on the preferred conformations of the molecules, indicating that the cyclohexyl moiety precludes intramolecular H bonding and preorganises the system for intermolecular interactions, which are responsible for fiber formation. NMR data of the gels provides thermodynamic data on fibrillization, revealing that the origin of this effect is mainly entropic. Electron microscopy (SEM and TEM) images show fibrillar and tape‐like objects, which are observed commonly in molecular gels. Rheological measurements reveal significant differences between cyclohexyl and hexyl appended gelators. These findings could contribute to the rational design of small, flexible, building blocks for self‐assembly.     

Low Molecular Weight Gelators for Organic Fluids: Gelation Using a Family of Cyclo(dipeptide)s

Simple cyclo(dipeptide)s consisting of diverse amino acids are able to cause physical gelation in a wide variety of organic fluids, including edible oils, glyceryl esters, alcohols, and aromatic molecules. Minimum gel concentrations, FTIR spectroscopy, NMR spectroscopy, and electron micrograph are used to characterize gel phenomenon. The intermolecular hydrogen bonding between N-H and C=O in cyclo(dipeptide)s plays an important role in gelation. FTIR and X-ray diffraction data suggest that the aggregate responsible for gel is an assembly of hydrogen-bonded molecular ladders, which are initially formed from numerous molecules through intermolecular hydrogen bonding. The ladder-like aggregates are intertwined and interlocked, and finally immobilize organic fluids. The gelation ability is discussed in connection with the three-component solubility parameters of solvents. Copyright 2000 Academic Press.     

Proton-sensitive fluorescent organogels

A 1,10-phenanthroline-appended cholesterol-based gelator (1) and its nongelling reference compound (2) were synthesized. Among 19 solvents tested herein, gelator 1 could gelate 11 solvents including alcohols, dipolar aprotic solvents, organic acids and a base (triethylamine), indicating that 1 acts as a versatile gelator. The TEM observation gave a visual image showing that fibrillar aggregates are entangled in the three-dimensional network structure. In the fluorescence measurements, most gels afforded an emission maximum at 394 nm (purple emission), whereas only the acetic acid gel afforded an emission maximum at 522 nm (yellow emission). Thus, the influence of protonation of the 1,10-phenanthroline nitrogens (by trifluoroacetic acid) on the fluorescence properties in the gel phase was investigated in detail. The results have established that the fluorescence intensity of 1 x H+ becomes particularly strong in the gel phase, presumably because of the energy transfer from neutral 1* to protonated 1 x H+ and the restriction of the 1 x H+ molecular motion. The finding suggests the possibility that the gel system would be useful not only as a new proton-sensitive fluorescence system but also as a new medium for designing efficient energy transfer systems.