Inversing supramolecular chirality and boosting circularly polarized luminescence of pyrene moieties via a gel matrix

Alkyl-substituted l/d-glutamide derivatives (L/D-SG) were designed as gelators to fabricate host gel matrices. Pyrene-appended l/d-glutamide derivatives (Py-LG/Py-DG) were employed as guest luminophores to investigate chiral packing and emission behavior in gel matrices. It was found that Py-LG and Py-DG are prone to form P- and M-chirally packed assemblies in DMSO gels, respectively. However, the chiral packing was inversed, and CPL was boosted after Py-LG/Py-DG was embedded in the L/D-SG gel matrix. M-chiral packing together with left-handed excimer emission ((-)-CPL) was observed in the Py-LG immobilized L-SG gel matrix, while P-chiral packing together with right-handed excimer emission ((+)-CPL) was found in the Py-DG immobilized L-SG gel matrix. It is more interesting to find that the molecular chirality of the matrix gelator did not affect the supramolecular chirality of pyrene assemblies. Either l or a d-matrix gelator can inverse the supramolecular chirality of the pure gel, but did not follow the chirality of the matrix. It was found that the gel matrix converts intralayer pyrene-pyrene (Py-Py) packing in the pure pyrene gel to interlayer Py-Py packing, thus giving an opposite chirality. The study not only deepened our understanding of the supramolecular chirality transfer but also unveiled the effects of an inert gel matrix in regulating the chiroptical properties.

Stimuli responsive dynamic transformations in supramolecular gels

Supramolecular gels are formed by the self-assembly of small molecules under the influence of various non-covalent interactions. As the interactions are individually weak and reversible, it is possible to perturb the gels easily, which in turn enables fine tuning of their properties. Synthetic supramolecular gels are kinetically trapped and usually do not show time variable changes in material properties after formation. However, such materials potentially become switchable when exposed to external stimuli like temperature, pH, light, enzyme, redox, and chemical analytes resulting in reconfiguration of gel matrix into a different type of network. Such transformations allow gel-to-gel transitions while the changes in the molecular aggregation result in alteration of physical and chemical properties of the gel with time. Here, we discuss various methods that have been used to achieve gel-to-gel transitions by modifying a pre-formed gel material through external perturbation. We also describe methods that allow time-dependent autonomous switching of gels into different networks enabling synthesis of next generation functional materials. Dynamic modification of gels allows construction of an array of supramolecular gels with various properties from a single material which eventually extend the limit of applications of the gels. In some cases, gel-to-gel transitions lead to materials that cannot be accessed directly. Finally, we point out the necessity and possibility of further exploration of the field.

Catalytic Gels for a Prebiotically Relevant Asymmetric Aldol Reaction in Water: From Organocatalyst Design to Hydrogel Discovery and Back Again

This paper reports an investigation into organocatalytic hydrogels as prebiotically relevant systems. Gels are interesting prebiotic reaction media, combining heterogeneous and homogeneous characteristics with a structurally organized active "solid-like" catalyst separated from the surrounding environment, yet in intimate contact with the solution phase and readily accessible via "liquid-like" diffusion. A simple self-assembling glutamine amide derivative 1 was initially found to catalyze a model aldol reaction between cyclohexanone and 4-nitrobenzaldehyde, but it did not maintain its gel structure during reaction. In this study, it was observed that compound 1 could react directly with the benzaldehyde to form a hydrogel in situ based on Schiff base 2 as a low-molecular-weight gelator (LMWG). This new dynamic gel is a rare example of a two-component self-assembled LMWG hydrogel and was fully characterized. It was demonstrated that glutamine amide 1 could select an optimal aldehyde component and preferentially assemble from mixtures. In the hunt for an organocatalyst, reductive conditions were applied to the Schiff base to yield secondary amine 3, which is also a highly effective hydrogelator at very low loadings with a high degree of nanoscale order. Most importantly, the hydrogel based on 3 catalyzed the prebiotically relevant aldol dimerization of glycolaldehyde to give threose and erythrose. In buffered conditions, this reaction gave excellent conversions, good diastereoselectivity, and some enantioselectivity. Catalysis using the hydrogel of 3 was much better than that using non-assembled 3-demonstrating a clear benefit of self-assembly. The results suggest that hydrogels offer a potential strategy by which prebiotic reactions can be promoted using simple, prebiotically plausible LMWGs that can selectively self-organize from complex mixtures. Such processes may have been of prebiotic importance.

Supramolecular chiroptical switches

Recent progress in chiroptical switches including on/off, amplification, and inversion of the chiral signals such as ECD and CPL in supramolecular assemblies is shown.

Photoluminescence spectral broadening, chirality transfer and amplification of chiral perovskite materials (R-X-p-mBZA)2PbBr4 (X = H, F, Cl, Br) regulated by van der Waals and halogen atoms interactions

We introduced halogen-substituted chiral molecules as A-site cations to synthesize a series of novel organic–inorganic hybrid 2D chiral perovskite materials (R-X-p-mBZA)₂PbBr₄ (X = H, F, Cl, Br; p: para-position; mBZA = α-methylbenzylamine).

Chirality transfer based on dynamic covalent chemistry: from small chiral molecules to supramolecules

Chirality transfer from small molecule to supramolecule was successfully achieved via regulation of dynamic covalent chemistry.

Ultra-fast self-healing PVA organogels based on dynamic covalent chemistry for dye selective adsorption

Organogels can repair the damage rapidly, and the relative xerogels can adsorb methylene blue with high efficiency.

Chirality and energy transfer amplified circularly polarized luminescence in composite nanohelix

Transfer of both chirality and energy information plays an important role in biological systems. Here we show a chiral donor π-gelator and assembled it with an achiral π-acceptor to see how chirality and energy can be transferred in a composite donor-acceptor system. It is found that the individual chiral gelator can self-assemble into nanohelix. In the presence of the achiral acceptor, the self-assembly can also proceed and lead to the formation of the composite nanohelix. In the composite nanohelix, an energy transfer is realized. Interestingly, in the composite nanohelix, the achiral acceptor can both capture the supramolecular chirality and collect the circularly polarized energy from the chiral donor, showing both supramolecular chirality and energy transfer amplified circularly polarized luminescence (ETACPL).

Unexpected right-handed helical nanostructures co-assembled from l-phenylalanine derivatives and achiral bipyridines

Achiral bipyridines can co-assemble with l-phenylalanine derivatives into unexpected right-handed helical nanostructures rather than left-handed helix by utilizing intermolecular hydrogen bonding formed between pyridyl and carboxylic groups.

The construction of chiral supramolecular systems with desirable handedness is of great importance in materials science, chemistry, and biology since chiral nanostructures exhibit fascinating photophysical properties and unique biological effects. Herein, we report that achiral bipyridines can co-assemble with l-phenylalanine derivatives into unexpected right-handed helical nanostructures rather than a left-handed helix via intermolecular hydrogen bonding interactions formed between the pyridyl and carboxylic groups. This study opens up a route to develop chiral nanostructures with desirable handedness via the co-assembly of simple molecular building blocks and provides a straightforward insight into the chirality control of nanostructures in supramolecular systems.

A Chiroptical Logic Circuit Based on Self-Assembled Soft Materials Containing Amphiphilic Spiropyran

A chiral logic circuit is proposed based on the multiple chiroptical responsiveness of a supramolecular gel material. The gel is fabricated by mixing a chiral gelator and a spiropyran derivative. Chiral responsiveness including the chiral switch and the logic gate is realized through the combined chirality transfer, photochromism, and acidichromism of the system.

A strategy for tuning achiral main-chain polymers into helical assemblies and chiral memory systems

A general strategy to tune the achiral main chain polymers into helical nanoassemblies was proposed based on the co-gelation approach. As an example, two achiral main chain polymers, PCz8 and PSi8, were selected, and their co-assembly with an amphiphilic l- or d-glutamide gelator was investigated. Although the polymers could not form gels individually, they could form co-gels with the gelator and the resultant gels exhibited macroscopic supramolecular chirality, which could be confirmed by CD spectra and SEM observations. Moreover, the supramolecular chirality can be memorized even after the gelator molecules were removed. Remarkably, either the gelator-containing or gelator-free chiral polymer assemblies showed circularly polarized luminescence (CPL), which is usually inherent to intrinsic chiral polymers. It was suggested that during the co-gelation, the chirality of the gelator was transferred to and memorized by the achiral polymers. The approach seems to be general and we provided the first example to tune the achiral polymers into helical assemblies through the co-gelation.

Chiroptical vesicles and disks that originated from achiral molecules

We report a chiral gel of vesicles and disklike micelles that originated from achiral molecules. The supramolecular chirality was obtained via regulating pH, in which a sol-gel-sol transition in a colloidal system consisting of a gelator, 4,4-di(2,3-dicarboxylphenoxyl)azobenzene (AzoNa4), and a zwitterionic surfactant, tetradecyldimethylamine oxide (C14DMAO), happened. The supramolecular chirality was related to the state of aggregation, i.e., only the condensed gels show chiral sense and sols are chiral-silent. The coexistence of vesicles and disklike micelles was captured for the first time in supramolecular chiral hydrogels by cryo- and freeze-fracture transmission electron microscopy (cryo- and FF-TEM) observations. Ascribed to the photoisomerization of the azobenzene units, upon alternative UV/visible light irradiation, the gel chirality can be switched reversibly with the macroscopic changes between vesicles/disks and wormlike micelles. A pH- and light-dual-responsive chiroptical switch can be constructed, which may require understanding the regulating membrane permeability and reagent release of structural transformation through photoisomerization and also require understanding the origin of gelation-induced supramolecular chirality completely based on achiral molecules.

Self-assembled switching gels with multiresponsivity and chirality

A multiresponsive hydrogel material consisting of a commercial cationic surfactant and an azobenzene derivative functionalized with four carboxylic acid groups was constructed. The achiral azobenzene molecule as a gelator produces chirality at the supramolecular level in the presence of H(+). The acid-induced gelation and morphology change of supramolecular gels were investigated in detail by cryogenic transmission electron microscopy (cryo-TEM), rheological measurements, circular dichroism (CD), and (1)H NMR spectra. Based on the results, a mechanism of the intermolecular H-bond-directed gelation and supramolecular chirality was proposed. Other than the pH sensitivity, the microstructure and the chirality of the hydrogel demonstrate reversible switching behavior in response to photoirradiation, on account of the photoisomerization of the azobenzene derivative. Accordingly, a chiroptical switch comprising four different states in response to pH and light stimuli is strategically constructed. Not only does the present system provide a good opportunity for investigating the gelation-induced supramolecular chirality by symmetry breaking totally based on achiral molecules, but it also proposes a new strategy to build multiresponsive supramolecular switches as particularly attractive for the future development of functional materials.

Gelation induced supramolecular chirality: chirality transfer, amplification and application

Supramolecular chirality defines chirality at the supramolecular level, and is generated from the spatial arrangement of component molecules assembling through non-covalent interactions such as hydrogen bonding, van der Waals interactions, π-π stacking, hydrophobic interactions and so on. During the formation of low molecular weight gels (LMWGs), one kind of fascinating soft material, one frequently encounters the phenomenon of chirality as well as chiral nanostructures, either from chiral gelators or even achiral gelators. A view of gelation-induced supramolecular chirality will be very helpful to understand the self-assembly process of the gelator molecules as well as the chiral structures, the regulation of the chirality in the gels and the development of the "smart" chiral materials such as chiroptical devices, catalysts and chiral sensors. It necessitates fundamental understanding of chirality transfer and amplification in these supramolecular systems. In this review, recent progress in gelation-induced supramolecular chirality is discussed.

Self-assembly of triangular amphiphiles into diverse nano/microstructures and release behavior of the hollow sphere

Amphiphilic triangular derivatives containing imine bond have been designed and used as building blocks for the construction of soft colloidal materials. The acylhydrazone derivative was found to form various nano/microstructures in different solvents. The microspheres, flower-like, and hollow spheres were formed in the polar solvents, while organogels with microporous structures were formed in nonpolar solvents. However, the simple Schiff base amphiphile without amide group did not form any ordered structures in the tested solvents. Therefore, besides the π-π stacking between the aromatic core and the van der Waals interactions between the alkyl chains, the hydrogen bonding between the acylhydrazone moieties played an important role in promoting the formation of various organized structures. More interestingly, the hollow sphere structure which was formed in THF could be used to encapsulate and release the rhodamine B. It was found that rhodamine B was encapsulated within their interiors under neutral conditions and released in an acidic condition due to the breakage of the imine bond.

Limonene induced chiroptical generation and inversion during aggregation of achiral polyfluorene analogs: structure-dependence and mechanism

We demonstrate an element-dependent chiroptical inversion and structural dependence of π-conjugated polymers with the help of limonene chirality in aggregation states.