Helical ribbon aggregate composed of a crown-appended cholesterol derivative which acts as an amphiphilic gelator of organic solvents and as a template for chiral silica transcription

Growth actuated bending and twisting of single crystals

Crystals of a variety of substances including elements, minerals, simple salts, organic molecular crystals, and high polymers forgo long-range translational order by twisting and bending as they grow. These deviations have been observed in crystals ranging in size from nanometers to centimeters. How and why so many materials choose dramatic non-crystallographic distortions is analyzed, with an emphasis on crystal chemistries that give rise to stresses operating either on surfaces of crystallites or within the bulk.

A highly selective, Hg2+ triggered hydrogelation: modulation of morphology by chemical stimuli

The morphology of a supramolecular hydrogel based on 4′-[4-(4-aminophenyl)phenyl]-2,2′:6′,2′′-terpyridine, exclusively with Hg²⁺, can be tuned by external chemical stimuli.

Experimental modelling of single-particle dynamic processes in crystallization by controlled colloidal assembly

A comprehensive review of the experimental modeling of single particle dynamics in crystallization is presented.

Self-Assembly and Nanostructures in Organogels Based on a Bolaform Cholesteryl Imide Compound with Conjugated Aromatic Spacer

The self-assembly of small functional molecules into supramolecular structures is a powerful approach toward the development of new nanoscale materials and devices. As a class of self-assembled materials, low weight molecular organic gelators, organized in special nanoarchitectures through specific non-covalent interactions, has become one of the hot topics in soft matter research due to their scientific values and many potential applications. Here, a bolaform cholesteryl imide compound with conjugated aromatic spacer was designed and synthesized. The gelation behaviors in 23 solvents were investigated as efficient low-molecular-mass organic gelator. The experimental results indicated that the morphologies and assembly modes of as-formed organogels can be regulated by changing the kinds of organic solvents. Scanning electron microscopy and atomic force microscopy observations revealed that the gelator molecule self-assemble into different aggregates, from wrinkle and belt to fiber with the change of solvents. Spectral studies indicated that there existed different H-bond formations between imide groups and assembly modes. Finally, some rational assembly modes in organogels were proposed and discussed. The present work may give some insight to the design and character of new organogelators and soft materials with special structures.

Spacer effect on nanostructures and self-assembly in organogels via some bolaform cholesteryl imide derivatives with different spacers

In this paper, new bolaform cholesteryl imide derivatives with different spacers were designed and synthesized. Their gelation behaviors in 23 solvents were investigated, and some of them were found to be low molecular mass organic gelators. The experimental results indicated that these as-formed organogels can be regulated by changing the flexible/rigid segments in spacers and organic solvents. Suitable combination of flexible/rigid segments in molecular spacers in the present cholesteryl gelators is favorable for the gelation of organic solvents. Scanning electron microscopy and atomic force microscopy observations revealed that the gelator molecules self-assemble into different aggregates, from wrinkle and belt to fiber with the change of spacers and solvents. Spectral studies indicated that there existed different H-bond formations between imide groups and assembly modes, depending on the substituent spacers in molecular skeletons. The present work may give some insight into the design and character of new organogelators and soft materials with special molecular structures.

Emerging chirality in nanoscience

Chirality in nanoscience may offer new opportunities for applications beyond the traditional fields of chirality, such as the asymmetric catalysts in the molecular world and the chiral propellers in the macroscopic world. In the last two decades, there has been an amazing array of chiral nanostructures reported in the literature. This review aims to explore and categorize the common mechanisms underlying these systems. We start by analyzing the origin of chirality in simple systems such as the helical spring and hair vortex. Then, the chiral nanostructures in the literature were categorized according to their material composition and underlying mechanism. Special attention is paid to highlight systems with original discoveries, exceptional structural characteristics, or unique mechanisms.

pH sensitive tubules of a bile acid derivative: a tubule opening by release of wall leaves

Tubules formed by self-assembly of organic molecules have vast potential for nanotechnology applications, and the introduction of sensitivity to stimuli into self-assembly tubules represents a particularly attractive feature. Here we report the preparation and characterization of a molecule obtained by chemical modification of a natural bile acid, a biological surfactant, that self-assembles in pH sensitive tubules in aqueous solutions. The tubules, which are rigid, single-walled and with a diameter of 60 nm, form at pH 8-9 and open up when the pH is increased. The transition is reversible, it occurs in the pH range of 9-10 with an opening mechanism that is remarkably different from those so far proposed in the literature. It involves a release of wall layers similar to leaves, and is determined by a drastic pH-triggered change in the molecular arrangement, which in turn induces a radical modification of the wall curvature. The description of the morphological transformation is provided by means of cryogenic transmission electron microscopy and represents, to our knowledge, the first detailed visualization of pH stimulated tubule opening. UV and circular dichroism spectroscopies are used to investigate the evolution at the molecular level.

Synthesis of chiral TiO₂ nanofibre with electron transition-based optical activity

The optical chirality induced at the absorption bands due to electronic exciton coupling of the transition dipole moments between chromophores in close proximity is ubiquitous in helical organic materials. However, inorganic materials with optical activity resulting from electronic transitions have not been explored. Here we report the synthesis of chiral TiO₂ fibres via transcription of the helical structure of amino acid-derived amphiphile fibres through coordination bonding interactions between the organics and the TiO₂ source. Upon calcination, the as-prepared amorphous TiO₂ double-helical fibres with a pitch length of ~100 nm were converted to double-helical crystalline fibres with stacks of anatase nanocrystals in an epitaxial helical relationship. Both the amorphous and anatase crystalline helical TiO₂ fibres exhibited optical response to circularly polarized light at the absorption edge around ~350 nm. This was attributed to the semiconductor TiO₂-based electronic transitions from the valence band to the conduction band under an asymmetric electric field.

Optical activity resulting from electronic transitions in chiral inorganic materials is rare. Liu et al. report the synthesis of amino acid-derived amphiphile templated chiral TiO₂ fibres, which exhibit an optical response to polarized light resulting from valence to conduction band electronic transitions.

Drug-loaded nanoparticles and supramolecular nanotubes formed from a volatile microemulsion with bile salt derivatives

The main objective of this study was to form nanoparticles of a model hydrophobic drug, celecoxib, from a volatile microemulsion stabilized by a bile salt derivative. Nanoparticles were obtained by conversion of the microemulsion nanodroplets with the dissolved drug into solid nanometric particles. The use of bile salt derivatives as the surfactants for the formation of a microemulsion enabled significantly higher loading of the drug in both the microemulsion and nanoparticles, compared with the native bile salt. In addition, superior stability of the particles was achieved with the bile salt derivatives, and drug crystallization was inhibited. Interestingly, differences in particle stability and crystallization inhibition were observed between two bile salt derivatives differing only by one hydroxyl group on the bile salt backbone, indicating the delicate balance of interactions in the system. For one of the derivatives, upon dispersion of the nanoparticles in water, they spontaneously arranged into well-defined elongated nanometric tubules as detected and attested by cryo-TEM. It was found that the drug present in nanoparticles induces formation of the nanotubes.

Chirality of the 1,4-phenylene-silica nanoribbons at the nano and angstrom levels

We reported the preparation of chiral 1,4-phenylene-silicas, using a sol-gel transcription approach, by self-assembly using low-molecular-weight gelators as templates. The silicas exhibited chirality at both the nano and angstrom levels. However, the relation between the chirality at the nano level and that at the angstrom levels has not been well studied. In this study, chiral 1,4-phenylene-silica nanoribbons were prepared by the self-assemblies of three chiral cationic gelators derived from amino acids as templates. These samples were characterized using field-emission scanning electron microscopy, transmission electron microscopy, x-ray diffraction, and circular dichroism. The results indicated that the handedness of the nanoribbons and the stacking of the aromatic rings were controllable. Although the nanoribbons exhibited left-handedness at the nano level, the stacking of the aromatic rings could exhibit left- or right-handedness. The handedness of the nanoribbons at the nano level was controlled by the organic self-assembly of the gelator. However, the stacking of the aromatic rings seemed to be controlled by the gelator itself.

Controlled synthesis and tunable properties of ultrathin silica nanotubes through spontaneous polycondensation on polyamine fibrils

This paper describes a facile approach to a biomimetic rapid fabrication of ultrathin silica nanotubes with a highly uniform diameter of 10 nm and inner hollow of around 3 nm. The synthesis is carried out through a spontaneous polycondensation of alkoxysilane on polyamine crystalline fibrils that were conveniently produced from the neutralization of a solution of protonated linear polyethyleneimine (LPEI-H(+)) by alkali compounds. A simple mixing the fibrils with alkoxysilane in aqueous solution allowed for the rapid formation of silica to produce LPEI@silica hybrid nanotubes. These 10-nm nanotubes were hierarchically organized in a mat-like morphology with a typical size of 1-2 micrometers. The subsequent removal of organic LPEI via calcination resulted in silica nanotubes that keep this morphology. The morphology, the structure, the pore properties and the formation mechanism of the silica nanotubes were carefully investigated with scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller measurements (BET), and X-ray diffraction (XRD). Detailed studies demonstrated that the formation of the nanotubes depends on the molar ratio of [OH]/[CH2CH2NH] during the neutralization as well as on the basicity of the alkali compound and on the concentration of the silica source. The synthesis of silica nanotubes established here could be easily applied to a fabrication on the kilogram scale. Silica nanotubes that were obtained from the calcination of hybrid nanotubes of LPEI@silica in an N2 atmosphere showed a distinct photoluminescence centered at 540 nm with a maximum excitation wavelength of 320 nm. Furthermore, LPEI@silica hybrid nanotubes were applied to create silica-carbon composite nanotubes by alternative adsorption of ionic polymers and subsequent carbonization.

Synthesis and self-assembly of perylenetetracarboxylic diimide derivatives with helical oligo(L-lactic acid)n segments

Three perylenetetracarboxylic diimide (PDI) derivatives consisting of a short oligo(L-lactic acid)(n) (O-LLA) segment at one imide nitrogen were synthesized. The polymers were characterized by (1)H NMR and gel permeation chromatography (GPC). Their properties were investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD) experiments, scanning electron microscopy (SEM), electronic absorption, and circular dichroism (CD) spectroscopy. The self-assembly behavior of these PDIs in molten state as well as in solvent was examined. It was found that the structure and the morphology of the self-assembly of these polymers depend on the relative length of the O-LLA segment. The PDIs with longer O-LLA chains present liquid crystal properties with an obvious phase transition from disordered phase to an ordered (α) phase, which cannot be found for the PDIs with short O-LLA segments. The long O-LLA segments also caused a left-handed helicity for the aggregates of the PDIs from solution. This research demonstrated that one can control the order, aggregation mode, and morphology of the molecular aggregates by changing the length of the O-LLA chains. This information can be useful in the design of new organic materials that exhibit molecular aggregation.

A crown ether appended super gelator with multiple stimulus responsiveness

A crown ether appended super gelator is designed and synthesized. It can gel a variety of organic solvents and shows excellent gelation properties with both low critical gelation concentration and short gelation time. Due to the introduction of the crown ether moiety and a secondary ammonium unit, the supramolecular gels show reversible gel-sol transitions. The supramolecular gels can also be molded into shape-persistent and free-standing objects.

A multiresponsive, shape-persistent, and elastic supramolecular polymer network gel constructed by orthogonal self-assembly

A cross-linked supramolecular polymer network gel is designed and prepared, which shows reversible gel-sol transitions induced by changes in pH, temperature, cation concentration, and metal co-ordination. The gel pore size is controlled by the amount of cross-linker added to the system, and the material can be molded into shape-persistent, free-standing objects with elastic behavior. These features are all due to the dynamically reversible host-guest complexation and good mechanical properties of the cross-linked polymer network. No single organogel has previously been reported to possess all of these features, making this supramolecular gel an unprecedentedly intelligent soft material.

Tuning the supramolecular chirality of one- and two-dimensional aggregates with the number of stereogenic centers in the component porphyrins

A synthetic strategy was developed for the preparation of porphyrins containing between one and four stereogenic centers, such that their molecular weights vary only as a result of methyl groups which give the chiral forms. The low-dimensional nanoscale aggregates of these compounds reveal the profound effects of this varying molecular chirality on their supramolecular structure and optical activity. The number of stereogenic centers influences significantly the self-assembly and chiral structure of the aggregates of porphyrin molecules described here. A scanning tunneling microscopy study of monolayers on graphite shows that the degree of structural chirality with respect to the surface increases almost linearly with the number of stereogenic centers, and only one handedness is formed in the monolayers, whereas the achiral compound forms a mixture of mirror-image domains at the surface. In solution, four hydrogen bonds induce the formation of an H-aggregate, and circular dichroism measurements and theoretical studies indicate that the compounds self-assemble into helical structures. Both the chirality and stability of the aggregates depend critically on the number of stereocenters. The chiral porphyrin derivatives gelate methylcyclohexane at concentrations dependent on the number and position of chiral groups at the periphery of the aromatic core, reflecting the different aggregation forces of the molecules in solution. Increasing the number of stereogenic centers requires more material to immobilize the solvent, in all likelihood because of the greater solubility of the porphyrins. The vibrational circular dichroism spectra of the gels show that all compounds have a chiral environment around the amide bonds, confirming the helical model proposed by calculations. The morphologies of the xerogels (studied by scanning electron microscopy and scanning force microscopy) are similar, although more fibrous features are present in the molecules with fewer stereogenic centers. Importantly, the presence of only one stereogenic center, bearing a methyl group as the desymmetrizing ligand, in a molecule of considerable molecular weight is enough to induce single-handed chirality in both the one- and two-dimensional supramolecular self-assembled structures.

Pyridinium based amphiphilic hydrogelators as potential antibacterial agents

The numerous applications of hydrogelators have led to rapid expansion of this field. In the present work we report the facile synthesis of amphiphilic hydrogelators having a quaternary pyridinium unit coupled to a hydrophobic long alkyl chain through an amide bond. Different amphiphiles with various hydrophobic chain length and polar head groups were rationally designed and synthesized to develop a structure-property relation. A judicious combination of hydrophilic and hydrophobic segments led to the development of pyridinium based amphiphilic hydrogelators having a minimum gelation concentration of 1.7%, w/v. Field emission scanning electronic microscopy (FESEM), atomic force microscopy (AFM), photoluminescence, FTIR studies, X-ray diffraction (XRD) and 2D NOESY experiments were carried out to elucidate the different non-covalent interactions responsible for the self-assembled gelation. The formation of three-dimensional supramolecular aggregates originates from the interdigitated bilayer packing of the amphiphile leading to the development of an efficient hydrogel. Interestingly, the presence of the pyridinium scaffold along with the long alkyl chain render these amphiphiles inherently antibacterial. The amphiphilic hydrogelators exhibited high antibacterial activity against both Gram-positive and Gram-negative bacteria with minimum inhibitory concentration (MIC) values as low as 0.4 μg/mL. Cytotoxicity tests using MTT assay showed 50% NIH3T3 cell viability with hydrogelating amphiphile 2 up to 100 μg/mL.

What kind of "soft materials" can we design from molecular gels?

Since their discovery, over the years, molecular gels have been constantly drawing the attention of chemists from various scientific fields. Their structural softness together with the orderliness at the molecular level provides such molecules immense potential for the amplification of their properties. Using this chemistry, one can easily realize a macroscopic outcome from a molecular level modulation. This phenomenon is governed by the principle of supramolecular interactions that introduce a unique "reversibility" to the system. The new generation of gel chemistry is now tending more towards the development of new attractive functions to create smart materials for achieving outstanding response or unprecedented selectivity over a process. However, for the successful implementation of this mission, it is really essential to correlate gel functions with their structures. This focus review is an attempt to find such a correlation, which can motivate and stimulate this existing field towards precisely designing molecular gels for the desired functions.