Charge-transfer phenomena in novel,dual-component,sugar-based organogels

The synthesis of a new saccharide-based gelator (2) containing a donor moiety has been described. Gelation experiments of a dual-component gel consisting of a saccharide-based gelator bearing an acceptor group (1) and of 2 have been performed in a variety of organic solvents and water. Moreover, gelation tests at different molar ratios of 1 and 2 have been performed in water, octanol, and diphenyl ether. In these last two solvents a gel color change was observed, from colorless to yellow, upon cooling of the sample to room temperature. This phenomenon was further investigated by UV-visible spectroscopy, which revealed the presence of charge-transfer interactions in the gel, in octanol. Temperature-dependence UV spectroscopy confirmed that such interactions occur in the gel but not in the corresponding solution sample. Furthermore, T(gel) measurements show that dual-component gels of 1 and 2 present increased thermal stability at a 50:50 ratio of the two gelators, in dependence of the solvent. Transmission electron microscopy (TEM) images of the single-component gels in diphenyl ether revealed that they consist of a fibrous network, while the dual-component gel presents a novel, helical, fibrous-bundle structure.     

Synthesis of Trideca-O-methyl-alpha-pedunculagin. Diastereo-Favoritism Studies on Intramolecular Ester-Cyclization of Axially Chiral Diphenic Acids with Carbohydrate Core

Total synthesis of trideca-O-methyl-alpha-pedunculagin was achieved by a simple sequence. The key step is the synthesis of methyl 4,6-O-benzylidene-2,3-O-[(S)-4,4',5,5',6,6'-hexamethoxydiphenoyl]-alpha-D-glucopyranoside through intramolecular ester-cyclization of racemic hexamethoxydiphenoyl chloride with methyl 4,6-O-benzylidene-alpha-D-glucopyranoside at the 2,3-position. The diastereoselectivity results obtained in the intramolecular cyclization of hexamethoxydiphenic acid to the carbohydrate core raises a very interesting point in considering the pathway of (R)-diphenic acid biosynthesis.     

Inclusion of DNA into organic gelator fibers made of amphipathic molecules and its controlled release

When methyl 4,6-O-(p-nitrobenzylidene)-alpha-D-glucopyranoside (p-NO(2)Glu) was dissolved in water, p-NO(2)Glu molecules self-assembled to form a fiber (elemental fiber), and as a result, the solution became a partially transparent gel. When an equal (or more) amount of DNA was added to the gel, a white and crystalline gel was obtained. Energy-dispersive X-ray spectroscopy coupled with TEM and confocal microscopy suggested that DNA was included in the gel fibers made of p-NO(2)Glu molecules. The results imply that p-NO(2)Glu molecules are self-assembled to form an elemental fiber and these elemental fibers and DNA are twisted together to form higher hierarchic fibers. When the complexed gel made of plasmid DNA (pDNA) and p-NO(2)Glu was added to E. coli T7 S30 extract solution, the pDNA had less expression ability compared with naked one. When we added methyl-beta-cyclodextrin (MbetaCyD), the expression rate was recovered with increasing added amount of MbetaCyD. The present paper shows inclusion and controlled release of DNA from a novel supporting material of DNA and that technology could play an important role in the development of localized approaches to gene therapy.     

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.     

荧光探针法研究可聚合有机分子(BMDM)/二苯醚的凝胶化过程

用荧光素作荧光探针 ,研究了可聚合凝胶因子 4,4′ 二 (α 甲基丙烯酰氧基 1 ,3 亚乙氧基羰基丙酰氨基 )二苯甲烷 (BMDM)在二苯醚的凝胶化过程中的聚集状态 .研究结果发现 ,与荧光素在二苯醚溶液中的荧光光谱相比 ,在凝胶中出现了 1 0～2 5nm的红移 ;而且发现分子凝胶的凝胶化时间随着BMDM浓度的增加而减小 .偏振荧光法研究分子凝胶的取向发现 ,该分子凝胶的形成过程是从各向同性的溶液(P0 =0 )向各向异性的溶胶 (P>0 )转变 ,再向各向同性的凝胶 (Pgel≈ 0 )转变的过程     

Solvent/gelator interactions and supramolecular structure of gel fibers in cyclic bis-urea/primary alcohol organogels

An organogel system consisting of trans-(1S,2S)-bis(ureidododecyl)cyclohexane (SS-BUC) and a series of primary alcohols was explored with optical polarizing microscopy (OPM), electron microscopy, circular dichroism (CD), wide-angle X-ray scattering (WAXS), and synchrotron small-angle X-ray scattering (SAXS). OPM, SAXS, and especially WAXS showed that the gel fiber of SS-BUC/methanol gels essentially consists of SS-BUC crystal itself. SAXS showed that the SS-BUC crystal in the gel takes a lamella with a domain spacing of 5.2 nm. When we left the gel at room temperature, the spacing decreased to 3.1 nm after several months. This distance change may correspond to the structural transition from a double-layer structure to an intercalated-layer structure, which was proposed by Feringa et al. (Chem.-Eur. J. 1999, 5, 937-950) as a possible arrangement of the molecular packing. When the gels in ethanol, propanol, butanol, or octanol were examined, they never showed crystalline peaks in WAXS and SAXS, indicating the amorphous nature of the gels. With increasing the alkyl chain length from ethanol to octanol, dramatic changes were observed in the CD spectrum in the 200-500-nm range. Because these CD changes are correlated to the absorbance of urea, those can be considered as the evidence that the solvents strongly relate to the spatial arrangement between the adjacent urea groups. For the amorphous gels, the cross-sectional correlation function [gammaCu] was directly obtained by the inverse Hankel transform of the SAXS data. The value of gammaCu for the gels is decreased with increasing u (distance between the two scattering bodies, see eq 5). Furthermore, it more rapidly decreases than that of the rigid cylinder model. This feature can be explained by the speculation that many solvent molecules permeate into the SS-BUC fiber. There was a clear difference between ethanol and the other gels, indicating that the solvents with a longer alkyl chain give the more permeated and diffused fiber. This permeated fiber (i.e., wet fiber) can rationalize the dramatic CD change, by presuming that the permeated solvent molecules alter the molecular stacking form.     

Structural characteristics of crosslinking domain in gellan gum gel

Gellan gum samples having different counterions of TMA⁺ (tetramethyl-ammonium ion), Na⁺, K⁺ and Cs⁺ were prepared, and the small-angle X-ray scattering was observed from the aqueous solutions of those samples. A sharp peak appears in the SAXS profile at low temperature, indicating the electrostatic interaction between the domains composed of multiple gellan gum chains. The SAXS profile revealed a rigid rod-like characteristics of gellan gum chain. The cross-sectional radius of gyration was evaluated from the scattering profiles, and the gelation mechanism was discussed from the change of the cross-sectional radius of gyration upon gel-sol transition. Taking the difference of the scattering power of counter ions into account, the ion condensation due to gelation was evaluated from the excess scattering of Cs⁺ gellan gum with respect to K⁺ gellan gum in respective aqueous solutions.     

水分子凝胶聚集态的DSC和AFM研究

合成了一种新型凝胶因子，能在很低的浓度下使水发生凝胶化，形成水分子凝 胶。通过原子力显微镜(AFM)以及透射电子显微镜(TEM)对水分子凝胶的微观形态进 行了表征，表明凝胶因子可以在水中聚集、自组装成延伸的螺旋缠绕细纤维结构， 并且得出了纤维束的平均直径在100nm左右，平均孔径在100nm左右。利用示差扫描 量热(DSC)的数据，计算了水分子凝胶体系的平均孔径大小在50～100nm左右，与 AFM和TEM观测的结果较吻合，从而验证了DSC理论推导计算的正确性。同时还得到 了不同浓度的水分子凝胶的凝胶—溶胶相转变温度Tcs在55—72℃之间，而且随着 凝胶因子浓度的增加，水分子凝胶体系的平均孔径呈减小的趋势，凝胶—溶胶相转 变温度呈上升的趋势。     

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.     

Phase transitions in poly(heptane-1,7-diyl biphenyl-4,4′-dicarboxylate). SAXS,WAXS and DSC study

The main-chain thermotropic liquid-crystalline poly(heptane-1,7-diyl biphenyl-4,4′-dicarboxylate) (P7MB) was investigated by time-resolved small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), and differential scanning calorimerty (DSC). Nonisothermal crystallisation with different rates of cooling and heating was used. On cooling, two phase transitions are observed, isotropic melt - smectic (I-Sm) and Sm- three-dimensional crystalline structure (Sm-Cr), whereas on heating only one transition is observed, Cr-I transition. The transition enthalpies were calculated. Temperature dependences of d-spacings of all crystalline peaks and of the peak observed at high values of scattering vector in the SAXS region were derived. The temperature dependence of the degree of crystallinity was established, based on the integrated intensities of the crystalline peaks and amorphous halo in WAXS.     

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.     

Micro‐phase‐separation behavior of amphiphilic polyurethanes involving poly(ethylene oxide) and poly(tetramethylene oxide)

The temperature dependence of thermal, morphological, and rheological properties of amphiphilic polyurethanes was examined with differential scanning calorimetry (DSC), wide‐angle X‐ray scattering (WAXS), small‐angle X‐ray scattering (SAXS), rheological measurements, and Fourier transform infrared spectroscopy. Multiblock (MPU) and triblock (TPU) polyurethanes were synthesized with two crystallizable segments—poly(ethylene oxide) (PEO) as a hydrophilic block and poly(tetramethylene oxide) (PTMO) as a hydrophobic block. DSC and WAXS measurements demonstrated that the microphase of MPUs in the solid state is dominantly affected by the PEO crystalline phase. However, high‐order peaks were not observed in the SAXS measurements because the crystallization of the PEO segments in MPUs was retarded by poor sequence regularity. The microphase in the melt state was induced by the hydrogen bonding between the N? H group of hexamethylene diisocyanate linkers and the ether oxygen of PEO or PTMO blocks. As the temperature increased, the smaller micro‐phase‐separated domains were merged into the larger domains, and the liquidlike ordering was eventually disrupted because of the weakening hydrogen bonding. However, the fully homogeneous state of an MPU with a molar ratio of 5/5 PEO/PTMO (MPU55) was not confirmed even at much higher temperatures with both SAXS and rheological measurements. However, the SAXS patterns of TPU showed weak but broad second‐order peaks below the melting temperature of the PEO block. Compared with MPU55, the ordering of the TPU crystalline lamellar stacks was enhanced because of the high sequence regularity and the low hydrogen‐bonding density. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2365–2374, 2003     

Low-molecular-weight gelators: elucidating the principles of gelation based on gelator solubility and a cooperative self-assembly model

This paper highlights the key role played by solubility in influencing gelation and demonstrates that many facets of the gelation process depend on this vital parameter. In particular, we relate thermal stability ( T gel) and minimum gelation concentration (MGC) values of small-molecule gelation in terms of the solubility and cooperative self-assembly of gelator building blocks. By employing a van't Hoff analysis of solubility data, determined from simple NMR measurements, we are able to generate T calc values that reflect the calculated temperature for complete solubilization of the networked gelator. The concentration dependence of T calc allows the previously difficult to rationalize "plateau-region" thermal stability values to be elucidated in terms of gelator molecular design. This is demonstrated for a family of four gelators with lysine units attached to each end of an aliphatic diamine, with different peripheral groups (Z or Boc) in different locations on the periphery of the molecule. By tuning the peripheral protecting groups of the gelators, the solubility of the system is modified, which in turn controls the saturation point of the system and hence controls the concentration at which network formation takes place. We report that the critical concentration ( C crit) of gelator incorporated into the solid-phase sample-spanning network within the gel is invariant of gelator structural design. However, because some systems have higher solubilities, they are less effective gelators and require the application of higher total concentrations to achieve gelation, hence shedding light on the role of the MGC parameter in gelation. Furthermore, gelator structural design also modulates the level of cooperative self-assembly through solubility effects, as determined by applying a cooperative binding model to NMR data. Finally, the effect of gelator chemical design on the spatial organization of the networked gelator was probed by small-angle neutron and X-ray scattering (SANS/SAXS) on the native gel, and a tentative self-assembly model was proposed.     

Structure characterization of cellulose nanofiber hydrogel as functions of concentration and ionic strength

Carboxylated cellulose nanofibers (CNFs), having an average width of 7 nm and thickness of 1.5 nm, were produced by TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation method. The fiber cross-sectional dimensions were determined using small-angle X-ray scattering (SAXS), transmission electron microscopy and atomic force microscopy techniques, where the rheological properties under different concentration and ionic strength were also investigated. The formation of hydrogel was evidenced by increasing the CNF concentration or ionic strength of the solvent (water), while the gel structure in ion-induced CNF hydrogels was found to be relatively inhomogeneous. The gelation behavior was closely related to the segmental aggregation of charged CNF, which could be quantitatively characterized by the correlation length (ξ) from the low-angle scattering profile and the scattering invariant (Q) in SAXS.     

Characterization of pore structure in metal-organic framework by small-angle X-ray scattering

MOF-5-like crystals were studied by small-angle X-ray scattering (SAXS) to reveal, both quantitatively and qualitatively, their real structural details, including pore surface characteristics, pore shape, size distribution, specific surface area (SSA), spatial distribution, and pore-network structure. A combined SAXS and wide-angle X-ray scattering (WAXS) experiment was conducted to investigate the variation of the pore structure with the MOF-5 crystalline phase produced at different cooling rates. The SSA of the MOF-5 crystals synthesized herein spanned a broad range from approximately 3100 to 800 m2/g. The real pore structures were divided into two regimes. In regime I the material consisted mainly of micropores of radius approximately 8 A as well as mesopores of radius 120 approximately 80 A. The structure in regime II was a fractal network of aggregated mesopores with radius >or=32 A as the monomer, reducing SSA and hydrogen uptake capacity at room temperature. The two regimes can be manipulated by controlling the synthesis parameters. The concurrent evolution of pore structure and crystalline phase during heating for solvent removal was also revealed by the in-situ SAXS/WAXS measurement. The understanding of the impact of the real pore structure on the properties is important to establish a favorable synthetic approach for markedly improving the hydrogen storage capacity of MOF-5.     

Equilibrium short‐range order in the isotropic phase of a poly(ester‐imide) with a C22H44 alkane spacer

Small‐angle X‐ray scattering (SAXS) was performed on a sample of poly(4,4′‐ phthaloimidobenzoyldoeicosamethyleneoxycarbonyl) (PEIM‐22) as a function of temperature. Wide‐angle X‐ray diffraction and differential scanning calorimetry were used to follow the isotropization of the crystalline PEIM‐22. The crystals of PEIM‐22 consist of biphasic layers up to the isotropization temperature. A series of SAXS peaks are observed for the crystals between θ = 0.3 and 3.5°. The width of these peaks indicates the formation of a smectic‐like, crystalline layer structure of a coherently scattering domain size of only 3–4 repeating units. In the isotropic phase, a single, broader peak remained at a spacing of ≈2.6 nm, suggesting even at high temperature the existence of equilibrium, short‐range, local order. The SAXS profiles were calculated based on a model of alternating layers of a linear, paracrystalline lattice. The results were discussed together with similar data on model compounds in the literature, and it is suggested that the short‐range order in the isotropic phase is due to a nanometer‐scale separation of the polar, aromatic phthaloimidobenzoyl from the flexible doeicosamethyleneoxycarbonyl. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 611–621, 2000     

Thermoplastic elastomers: Structural and morphological aspects of ether–esteramide copolymers

Polyesteramide 6NT6–polytetramethylene ether glycol (PTMEG) copolymers, precipitated from dilute solution, have been studied by small-angle x-ray scattering (SAXS) and wide-angle x-ray scattering (WAXS) with variation of the PTMEG content. A comparison of the structural characteristics of these materials has been made with those of the 6NT6 homopolymer. Chain folding is assumed as the crystallization mechanism for the low-PTMEG-content copolymers, with possible inclusion of polyether segments within the 6NT6 crystal lattice. WAXS data support the view of a weakening of the 6NT6 crystalline packing in the equatorial planes as the reason for broadening of the diffraction peaks, for changes in their relative intensities, and for the increase in the equatorial interplanar distances in the high-PTMEG-content samples. The annealing behavior has also been investigated.     

In situ studies of monosaccharide gelation using the small-angle X-ray scattering time-resolved method

The process of gelation of one of the monosaccharides, alpha-galactose-based gelator, with benzene as a solvent, has been studied. Small-angle X-ray scattering (SAXS) synchrotron time-resolved measurements were performed throughout the gelation process. The obtained SAXS measurements were elaborated using such methods as the fractal analysis, Fourier transform to get distance distribution functions, and simulation of a cluster model. We obtained the picture of the mesostructure development from the sol state to the gel state. Our results indicate that the fractal-type aggregates exist in the sol and the process of gelation is accompanied by the structural transition. This transition causes the aggregates to become smaller and denser, and their shapes differ from those of the sol. The complex method of SAXS data treatment we established seems to be a useful tool also for further studies of monosaccharide gelation.     

Nanostructure of calcium alginate aerogels obtained from multistep solvent exchange route

Ca-alginate materials were studied by small-angle X-ray scattering (SAXS) at different steps of conversion from gel to aerogel in order to determine the relation between the polymer organization at the nanoscale in the gels and the final dry aerogel. In all cases, i.e. before and after the different exchanges of solvents and after the formation of the aerogel, the SAXS patterns exhibit an asymptotic behavior at low q values (in the experimental q range 7x10(-3) up to 2.10(-2) A(-1)) close to I(q) approximately q(-1), indicative of randomly oriented rod-like scattering objects. The evolution of the diameter of such rod-like objects was thus deduced from the maxima observed on Kratky plots, i.e. I(q) q2 vs q. The results are in perfect agreement qualitatively (rod-like anisometry type of the scattering objects) and quantitatively (diameter of the rods) with direct SEM observations of the morphology of aerogels and with the results of N2 adsorption on the aerogel. This is evidence that in the chosen experimental processing conditions, the morphology of the aerogel depends on the morphology of pre-existing objects within the gel, i.e. that the structure of the aerogel provides a correct image of the structure of the parent gel.