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.     

Insight on the NMR study of supramolecular gels and its application to monitor molecular recognition on self-assembled fibers

The 1H NMR study of supramolecular gels formed by two organogelators derived from valine is described. The analysis of the variation of chemical shift values and relaxation times in the gel samples reveals that in these systems only discrete species are observed by 1H NMR. The reduced T2 values and negative NOEs that are measured upon gel formation can be ascribed to an exchange between discrete organogelator species and the gel network. This process is found to be fast in the time scale of 1H NMR relaxation and slow in the NMR observation frequency time scale. It is shown here that other molecules, aside from the gelator itself, can interact with the gel network and this process can be monitored easily by measurement of relaxation times. As a proof of principle, the selective interaction of 2,2'-bis(hydroxymethyl)biphenyl over diphenylmethane with the self-assembled fibers formed by one of the gelators in benzene is described.     

Bis(amino acid) oxalyl amides as ambidextrous gelators of water and organic solvents: supramolecular gels with temperature dependent assembly/dissolution equilibrium

Bis(LeuOH) (1a), bis-(ValOH) (2a) and bis(PhgOH) (5a) (Phg denotes (R)-phenylglycine) oxalyl amides are efficient low molecular weight organic gelators of various organic solvents and their mixtures as well as water, water/DMSO, and water/DMF mixtures. The organisational motifs in aqueous gels are dominated primarily by lipophilic interactions while those in organic solvents are formed by intermolecular hydrogen bonding. Most of the gels are thermoreversible and stable for many months. However, 2a forms unstable gels with organic solvents which upon ageing transform into variety of crystalline shapes. For some 1a/alcohol gels, a linear correlation between alcohol dielectric constants (epsilon) and gel melting temperatures (Tg) was found. The 1H NMR and FTIR spectroscopic investigations of selected gels reveal the existence of temperature dependent network assembly/dissolution equilibrium. In the 1H NMR spectra of gels only the molecules dissolved in entrapped solvent could be observed. By using an internal standard, the concentration of dissolved gelator molecules could be determined. In FTIR spectra, the bands corresponding to network assembled and dissolved gelator molecules are simultaneously present. This enabled determination of the Kgel values by using both methods. From the plots of InKgel versus 1/T, the deltaHgel values of selected gels have been determined (-deltaHgel in 10-36 kJ mol(-1) range) and found to be strongly solvent dependent. The deltaHgel values determined by 1H NMR and FTIR spectroscopy are in excellent agreement. Crystal structures of 2a and rac-5a show the presence of organisational motifs and intermolecular interactions in agreement with those in gel fibres elucidated by spectroscopic methods.     

New application of proton nuclear spin relaxation unraveling the intermolecular structural features of low-molecular-weight organogel fibers

Proton nuclear spin relaxation has been for the first time extensively used for a structural and dynamical study of low-molecular-weight organogels. The gelator in the present study is a modified phenylalanine amino acid bearing a naphthalimide moiety. From T(1) (spin-lattice relaxation time in the laboratory frame) and T(1ρ) (spin-lattice relaxation time in the rotating frame) measurements, it is shown that the visible gelator NMR spectrum below the liquid-gel transition temperature corresponds to a so-called isotropic compartment, where gelator molecules behave as in a liquid phase but exchange rapidly with the molecules constituting the gel structure. This feature allows one to derive, from accessible parameters, information about the gel itself. Nuclear Overhauser effect spectroscopy (NOESY) experiments have been exploited in view of determining not only cross-relaxation rates but also specific longitudinal rates. The whole set of relaxation parameters (at 25 °C) leads to a correlation time of 5 ns for gelator molecules within the gel structure and 150 ps for gelator molecules in the isotropic phase. This confirms, on one hand, the flexibility of the organogel fibers and, on the other hand, the likely presence of clusters in the isotropic phase. Concerning cross-relaxation rates, a thorough theoretical investigation in multispin systems of direct and relayed correlations in a NOESY spectrum allows one to make conclusions about contacts (around 2-3 ?) not only between naphtalimide moieties of different gelator molecules but also between the phenyl ring and the naphtalimide moiety again of different gelator molecules. As a result, not only is the head-to-tail structure of amino acid columns confirmed but also the entangling of nearby columns by the naphthalimide moieties is demonstrated.     

Fluoride-responsive gelator and colorimetric sensor based on simple and easy-to-prepare cyano-substituted amide

A new and easy-to-prepare gelator based on cyano-substituted amide (BPNIA) was designed and synthesized. BPNIA could form thermoreversible gel in DMSO-H(2)O (v/v, 9?:?1) and ultrasound-stimulated gel in DMSO. FT-IR, UV-vis and XRD spectra indicated that the gelator molecules self-assemble into a fibrous network resulting from the cooperation of intermolecular hydrogen bonding, π-π stacking and cyano interactions. BPNIA can act as a highly selective colorimetric sensor for fluoride in DMSO, overcoming the interference of H(2)PO(4)(-), AcO(-) and other halide anions. The deprotonation of the NH groups is responsible for the dramatic color change from colorless to yellow. Interestingly, the organogel of BPNIA could allow a two channel fluoride response by proton controlled reversible sol-gel transition and color changes.     

Supramolecular gels formed from multi-component low molecular weight species

Low molecular weight supramolecular gels consist of small molecules (gelators) that in an appropriate solvent self-assemble into nano- or micro-scale network structures resulting in the formation of a gel. Most supramolecular gels consist of two parts, namely the solvent and the gelator. However, the concept of multi-component supramolecular gels, in which more than one compound is added to the solvent, offers a facile way (e.g. by changing the ratio of the different components) to tailor the properties of the gel. The simplest multi-component gels consist of two components added to the solvent and are the most widely studied to date. There are three general classes of such multi-component gels that have been investigated. The first class requires all the added components to access the gel; that is, no component forms a gel on its own. A second class uses two (or more) gelators which can either co-assemble or self-sort into distinct assemblies and the final class consists of one (or more) gelator and one (or more) non-gelling additive which can impact the assembly process of the gelator and therefore the gel's properties.     

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.     

Studies on organogelation of self assembling bis urea type low molecular weight molecules

A novel class of toluene based bis urea compounds carrying linear fatty acid units and semicarbazide linkages has been synthesised. The compounds were exhibiting thermoreversible gelation at concentrations below 10 mg/mL in common organic solvents, both aliphatic and aromatic. The effect of the chain length variation of fatty acid units on gelation properties like gelation concentration, gelation time and gel melting temperatures were studied. Choosing a particular gelator of fixed chain length and a specific solvent, the effect of the concentration on the gelation properties were studied. The thermal studies using DSC revealed the presence of phase transitions corresponding to the premelting and melting of the gels during the heating cycle. The morphology of the xerogels studied using SEM revealed a three dimensional network structure while the WAXS studies showed no crystallinity in the xerogels. IR spectra of the gels (solvent subtracted) and solutions in the corresponding solvent showed that a high degree of inter-molecular H bonding exists and absorptions corresponding to NH stretching shifted to lower wave numbers. Thus simple bisurea type of compounds exhibiting gelation ability in a wide range of solvents can be used for making functional gels for various applications.     

On the relation between the solvent parameters and the physical properties of methyl-4,6-O-benzylidene-α-d-glucopyranoside organogels

This paper reports the mechanisms of gel formation, the thermal properties and the microstructures of the networks of the gels composed of methyl-4,6-O-benzylidene-α-d-glucopyranoside and selected organic solvents: p-xylene, benzene, toluene, diphenyl ether and tetraethoxysilane. The Fourier transform infrared measurements together with simulation spectra, the air bath method and Polarized Optical Microscopy were employed in our studies. The experimental data show that the solvent has an influence on the microstructure of the gel network but there is no predictable influence of the solvent polarity on the shape of the formed gelator aggregates and correspondingly on the fibrous assemblies as revealed by the different microstructure of the gel network. Independently of the solvent polarity, the studied gelator, like other methyl-4,6-O-benzylidene derivatives of monosaccharides, formed gels through the formation of a hydrogen-bond network. The solvent parameters, such as the dielectric constant, Hildebrand solubility parameter, the polarity scale E_T and the Kamlet–Taft parameters were considered to quantify solvent effects on the gelation. The conclusions about the correlations are of interest but only to this particular sugar based gels.     

Stimuli-Responsive Polymers in Solution Investigated by NMR and Infrared Spectroscopy

Summary: Temperature-induced and solvent composition-induced phase separation in solutions of poly(N-isopropylmethacrylamide) (PIPMAm) and other thermoresponsive polymers as studied by NMR and infrared (IR) spectroscopy is discussed. The fraction p of phase-separated units (units with significantly reduced mobility) and subsequently, e.g., thermodynamic parameters characterizing the coil-globule phase transition induced by temperature, were determined from reduced integrated intensities in high-resolution ¹H NMR spectra. This approach can be especially useful in investigations of phase separation in solutions of binary polymer systems. Information on behaviour of water during temperature-induced phase transition was obtained from measurements of ¹H NMR relaxation times of HDO molecules. NMR and IR spectroscopy were used to investigate PIPMAm solutions in water/ethanol (D₂O/EtOH) mixtures where the phase separation can be induced by solvent composition (cononsolvency). Some differences in globular-like structures induced by temperature and solvent composition were revealed by these methods.     

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.     

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.     

基于均苯三甲酸与对羟基吡啶的超分子水凝胶

以均苯三甲酸和对羟基吡啶为原料, 采用简便方法合成了一种新的凝胶因子, 并采用1H NMR、IR和元素分析确认其结构. 红外光谱中2849和1894 cm－1处出峰证明羧基与吡啶基间形成了氢键. 在凝胶化过程中, 凝胶因子可自组装形成纤维状网络结构. 随着凝胶因子浓度的增加, 纤维搭接逐渐致密, 凝胶网络密度逐渐增大, 可冻结水含量逐渐增加. 因此, 通过改变凝胶因子浓度可有效控制凝胶的结构及性能. 该凝胶因子在较低浓度下形成的超分子水凝胶在100 ℃下也能够稳定存在.     

Self‐Assembly of a Chiral Lipid Gelator Controlled by Solvent and Speed of Gelation

Glutamine derivative 1 with two‐photon absorbing units has been synthesized and was found to show gelation ability in some solvents. Its self‐assembly in the gel phase could be controlled by the solvent and speed of gelation. For example, in DMSO the organogelator self‐assembled into H‐aggregates with weak exciton coupling between the aromatic moieties. On the other hand, in DMSO/diphenyl ether (1:9, v/v) the molecules formed 1D aggregates, but with strong exciton coupling due to the small distance between the chromophores. Moreover, the formation of these two kinds of aggregates could be adjusted by the ratio of DMSO to diphenyl ether. In DMSO/toluene, DMSO/butanol, DMSO/butyl acetate, and DMSO/acetic acid systems similar results were observed. Therefore, conversion of the packing model occurs irrespective of the nature of the solvent. Notably, a unique sign inversion in the CD spectra could be realized by controlling the speed of gelation in the DMSO/diphenyl ether (1:9, v/v) system. It was found that a low speed of gelation induces the gelator to adopt a packing model with strong π–π interactions between the aromatic units. Moreover, the gels, when excited at 800 nm, emit strong green fluorescence and the quantum chemical calculations suggest that intramolecular charge transfer leads to two‐photon absorption of the gelator molecule.     

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.     

Influence of solvent on the thermal stability and organization of self-assembling fibrillar networks in methyl-4,6-O-(p-nitrobenzylidene)-α-d-glucopyranoside gels

Sugar based low-molecular-mass organogelator (LMOG) methyl-4,6-O-(p-nitrobenzylidene)-α-d-glucopyranoside, is a unique gelator because its small and weakly-interacting molecules can form large supramolecular structures in nonpolar, but also in polar, solvents and cause their gelation. The self-assembling properties of the gelator were studied in selected nonpolar and polar solvents. It was shown that the driving forces for both types of solvent are the intermolecular hydrogen bond interaction. The effect of the nature of the solvent on the thermal stability of the gels and on the three-dimensional network organization was determined. Different solvent parameters, such as dielectric constant, one-component solubility parameter, the polarity parameter and the Kamlet-Taft parameters were considered to quantify solvent effects on the gelation. Some correlation between these parameters and the gel stability, microstructure and the enthalpy of the phase transition were established. The effort to correlate the Kamlet-Taft parameters to the thermal stability and gelation ability is also possible but applies only to the studied gelator.     

NMR Investigations of Temperature-Induced Phase Transition in Aqueous Polymer Solutions

The different dynamics of polymer segments forming phase-separated globular structures in aqueous (D₂O) solutions affects both the shape of NMR spectra and NMR relaxation times of polymer and solvent. Two types of the approach are discussed. The first one is based on the reduction of integrated intensities of polymer NMR lines in high-resolution NMR spectra in the system undergoing the coil-globule phase transition. The fraction p of phase-separated units (units with significantly reduced mobility) and subsequently, e.g., thermodynamic parameters ΔH and ΔS characterizing the coil-globule phase transition can be determined. The second approach is based on measurements of ¹H NMR relaxation times of water (HDO) which provide information on behaviour of water during phase transition. The power of both approaches is demonstrated on results obtained with solutions of several thermoresponsive homopolymers and copolymers.     

In Situ Gel‐to‐Crystal Transition and Synthesis of Metal Nanoparticles Obtained by Fluorination of a Cyclic β‐Aminoalcohol Gelator

A new fluorinated version of a cyclic β‐aminoalcohol gelator derived from 1,2,3,4‐tetrahydroisoquinoline is presented. The gelator is able to gel various nonprotic solvents through OH???N hydrogen bonds and additional CH???F interactions due to the introduction of fluorine. A bimolecular lamellar structure is formed in the gel phase, which partly preserves the pattern of molecular organization in the single crystal. The racemate of the chiral gelator shows lower gelation ability than its enantiomer because of a higher tendency to form microcrystals, as shown by X‐ray diffraction analysis. The influence of fluorination on the self‐assembly of the gelator and the properties of the gel was investigated in comparison to the original fluorine‐free gel system. The introduction of fluorine brings two new features. The first is good recognition of o‐xylene by the gelator, which induces an in situ transition from gels of o‐xylene and of an o‐xylene/toluene mixture to identical single crystals with unique tubular architecture. The second is the enhanced stability of the toluene gel towards ions, including quaternary ammonium salts, which enables the preparation of a stable toluene gel in the presence of chloroaurate or chloroplatinate. The gel system can be used as a template for the synthesis of spherical gold nanoparticles with a diameter of 5 to 9 nm and wormlike platinum nanostructures with a diameter of 2 to 3 nm and a length of 5 to 12 nm. This is the first example of a synthesis of platinum nanoparticles in an organogel medium. Therefore, the appropriate introduction of a fluorine atom and corresponding nonbonding interactions into a known gelator to tune the properties and functions of a gel is a simple and effective tactic for design of a gel system with specific targets.     

Photoinduced gelation by stilbene oxalyl amide compounds

Oxalyl amide derivatives bearing 4-dodecyloxy-stilbene as a cis-trans photoisomerizing unit were synthesized. The trans derivative acted as a versatile gelator of various organic solvents, whereas the corresponding cis derivative showed a poor gelation ability or none at all. In diluted solution (c = 2.0 x10(-5) mol dm(-3), ethanol), the cis isomer was photochemically converted into the trans isomer within 4 min. Depending on the radiation wavelength, the trans isomer was stable or liable to photodecomposition. When exposed to irradiation, a concentrated solution of the cis isomer (c = 2.0 x 10(-2) mol dm(-3), ethanol) turned into a gel. The FT-Raman, FT-IR, and 1H NMR spectra demonstrated that the gelation process occurred because of a rapid cis --> trans photoisomerization followed by a self-assembly of the trans molecules. Apart from the formation of hydrogen bonding between the oxalyl amide parts of the molecules, confirmed by FT-IR spectroscopy, it was assumed that the pi-pi stacking between the trans-stilbene units of the molecule and a lipophilic interaction between long alkyl chains were the interactions responsible for gelation.