Low Molecular Mass Organogelator Research Articles

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.

Crystal or Low Molecular Mass Organogel Based on Sugar-Derived Chiral Pyrano[2,3-b]naphtho[1,2-e]pyrans

[structure: see text] 2-C-Acetoxymethyl glycal derivatives reacted with beta-naphthol in the presence of InCl(3) (30 mol %) to give chiral pyrano[2,3-b]naphtho[1,2-e]pyrans in good to excellent yields via stereoselective Ferrier rearrangement-tandem cyclization. The major glucose-derived product exhibited crystal packing with C-H...O and C-H...pi (arene) interactions and the major galactose-derived product, a low molecular mass organogelator (LMOG), formed a rare type of micro tubular gel assembly in n-hexane.

Synthesis and self-assembly of dichalcone substituted carbazole-based low-molecular mass organogel

We report the synthesis and self-assembly of a new pi-conjugated dichalcone substituted carbazole-based low molecular mass organogelator. It could form stable gels in most halogen-aromatic solvents. The transmission electron microscopy (TEM) images revealed that the gel formed fibrous structures with diameter of 50-100 nm, which consisted of several thinner fibers. The FT-IR, UV-vis and XRD results suggested that the H-bonds and pi-pi interactions were the main driving forces for the formation of the self-assembled gel, in which the U-shaped molecules were stacked into lamellar structures. The fluorescent spectra showed that the emission of the xerogel red-shifted markedly compared with the sol state, which resulted from the aggregation of the molecules.

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.

Kinetics of 5α-Cholestan-3β-ylN-(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.

Self-assembled helical spherical-nanotubes from an l-glutamic acid based bolaamphiphilic low molecular mass organogelator

An L-glutamic acid based bolaamphiphile was observed to self-assemble into a novel "helical spherical-nanotube" with a molecular monolayered structure within a tubular wall.

Flow-Induced Alignment of Fiberlike Supramolecular Self-Assemblies during Organogel Formation with Various Low Molecular Mass Organogelator−Solvent Systems

Organogelators are small organic molecules that self-assemble at low temperature in long entangled and connected fibers to give birth to a random gel. In this paper, we describe a systematic procedure to prepare easily orientated samples by using a weak flow. We demonstrate that applying a mechanical shear stress during the cooling of a hot isotropic solution of a low molecular mass gelator can lead not only to the alignment of the growing fibers in a parallel fashion within the bulk material but also to centimeter-sized, cylinder-shaped objects displaying strong anisotropic features and well-defined organization at the nanometer level. This work opens exciting ways toward organic materials exhibiting enhanced mechanical, photonic, magnetic, or electronic properties along a given direction.