Design and self-assembly of L-glutamate-based aromatic dendrons as ambidextrous gelators of water and organic solvents

Enantioseparation via Chiral Supramolecular Gels Comprising Ambidextrous Gelators Based on β-Peptide-type Primary Amines

While β-peptides have been paid attention due to their diverse secondary structures, their application to the design of low-molecular-weight gelators (LMWGs) is less explored. In this work, chiral cyclic β-amino acid-based β-peptides were developed as ambidextrous LMWGs, wherein multiple hydrogen bonds between the amide moieties led to high gelation ability. Their molecular assembly was elucidated using spectroscopies, microscopy, and X-ray analysis. Further, the supramolecular gel was used as a platform for the enantioselective extraction of (S)-naproxen from its racemate under optimized conditions. These findings have expanded the utility of β-peptides and shown the potential of supramolecular gels as a distinct dynamic medium for enantiomer separation.

Living Hybrid Exciton Materials: Enhanced Fluorescence and Chiroptical Properties in Living Supramolecular Polymers with Strong Frenkel/Charge-Transfer Exciton Coupling

A family of chiral perylene diimides (PDIs) was newly developed as excellent circularly polarized luminescence (CPL) materials. They are asymmetrically derivatized with a double-alkyl-chained L- or D-glutamate unit and a linear or branched alkyl chain. When water is added to the tetrahydrofuran (THF) solution of glutamate-PDI-linear-alkyl chain compounds, kinetically formed H-aggregates are formed in globular nanoparticles (NPs). These NPs undergo spontaneous transformation into thermodynamically stable nanotubes via helical nanostructures, which showed structured broad spectra originating from the strong coupling of delocalized Frenkel excitations (FE) and charge transfer excitations (CTE). Significant enhancement of circular dichroism (CD), fluorescence quantum yield, and circularly polarized luminescence (CPL) with luminescence dissymmetry factor (glum) are observed during the transformation of NPs to the FE/CTE-coupled helical and tubular structures. This transformation process is significantly accelerated by applying physical stimuli, i.e., ultrasonication or adding helical aggregates as seed crystals, a feature unique to living supramolecular polymerization. Meanwhile, the branched chain-containing PDIs only form H-aggregates and did not show FE/CTE hybrid exciton states with living supramolecular polymerization properties. This study unveils that suitably designed chiral PDI derivatives show FE/CTE coupling accompanied by high fluorescence quantum yields, enhanced chiroptical properties, and supramolecular living polymerization characteristics.

Interfacial self-assembly of a chiral pyrene exciplex into a superhelix with enhanced circularly polarized luminescence

We found that the interfacially confined self-assembly of pyrene and phenanthrene glutamides can form strong exciplexes and amorphous superhelices, which show intensity-enhanced and sign-inverted CPL activity with improved quantum yield compared to a pyrene excimer. This work unveils the predominant role of supramolecular nanostructures over molecular configurations on CPL performance.

Chiroptical Synaptic Heterojunction Phototransistors Based on Self-Assembled Nanohelix of π-Conjugated Molecules for Direct Noise-Reduced Detection of Circularly Polarized Light

High-performance chiroptical synaptic phototransistors are successfully demonstrated using heterojunctions composed of a self-assembled nanohelix of a π-conjugated molecule and a metal oxide semiconductor. To impart strong chiroptical activity to the device, a diketopyrrolopyrrole-based π-conjugated molecule decorated with chiral glutamic acid is newly synthesized; this molecule is capable of supramolecular self-assembly through noncovalent intermolecular interactions. In particular, nanohelix formed by intertwinded fibers with strong and stable chiroptical activity in a solid-film state are obtained through hydrogen-bonding-driven, gelation-assisted self-assembly. Phototransistors based on interfacial charge transfer at the heterojunction from the chiroptical nanohelix to the metal oxide semiconductor show excellent chiroptical detection with a high photocurrent dissymmetry factor of 1.97 and a high photoresponsivity of 218 A W-1 . The chiroptical phototransistor demonstrates photonic synapse-like, time-dependent photocurrent generation, along with persistent photoconductivity, which is attributed to the interfacial charge trapping. Through the advantage of synaptic functionality, a trained convolutional neural network successfully recognizes noise-reduced circularly polarized images of handwritten alphabetic characters with better than 89.7% accuracy.

Chiral Inversion and Recovery of Supramolecular Luminescent Copper Nanocluster Hydrogels Triggered by Polyethyleneimine and Polyoxometalates

Construction of controllable chiroptical supramolecular luminescence systems is of great significance for developing intelligent chiral luminescence materials with precise and effective regulation and understanding chirality-switching phenomena in biological systems, which has attracted extensive attention. Because chiral metal nanoclusters (NCs) can provide facilities for the study of nanoscale chiral effects, in this study, we select chiral glutathione-stabilized copper NCs (G-SH-Cu NCs) to construct a supramolecular luminescent hydrogel with achiral branched polyethyleneimine (PEI) and polyoxometalates [Na₉(EuW₁₀O₃₆)·32H₂O, denoted as EuW₁₀]. Thus, a chiral property precise controlled system was constructed by self-assembly. Interestingly, the addition of PEI to G-SH-Cu NC solution induced the formation of luminescent hydrogels with chiral inversion, while further addition of EuW₁₀ not only enhanced the luminescence of the hydrogel but also recovered the chiroptical properties. The chiral inversion behavior is possibly ascribed to the hydrogen bond interaction/electrostatic interaction between G-SH-Cu NCs and PEI in the chiral inversion process, while the competition of hydrogen bonding interaction (between G-SH-Cu NCs and PEI) and electrostatic interaction (between PEI and EuW₁₀) was accountable for the chiral recovery process. Manipulation of chirality inversion in the metal NC-containing coassemblies is rare, while this work establishes a feasible strategy to modulate the chiral inversion behavior of Cu NCs, which not only produces new physicochemical properties of metal NCs through synergistic behavior but also offers a feasible way to realize the potential application of chiroptical materials.

Quinoline-cored Poly(Aryl Ether) Dendritic Organogels with Multiple Stimuli-Responsive and Adsorptive Properties

Functional supramolecular gel materials have potential applications in sensors, optical switches, artificial antennae, drug delivery and so on. In this paper, quinoline-cored poly(aryl ether) dendritic organogelators were designed, synthesized and fully characterized. The gelation behaviour of the dendritic organogelator was tested in organic solvents, mixed solvents and ionic liquids. The dendron Q-G1 was found to be an efficient and versatile organogelator toward various apolar and polar organic solvents with the critical gelation concentrations (CGCs) approaching 1.2×10^-2  mol/L, indicating one dendritic organogelator could immobilize 1.2×10³ solvent molecules in the organogel network. Interestingly, these dendrons exhibited excellent gel formation in ionic liquids. Notably, these dendritic organogels were found to display multiple stimuli-responsive properties toward external stimuli including heat, ultrasound and shear stress, with a reversible sol-gel phase transition. In addition, the dendritic organogel could effectively adsorb heavy metals and organic dyes. The removal rate of Pb²⁺ was up to 20% and the adsorption rate for Rhodamine B was as high as 89%.

Designer Peptide and Protein Dendrimers: A Cross-Sectional Analysis

Dendrimers have attracted immense interest in science and technology due to their unique chemical structure that offers a myriad of opportunities for researchers. Dendritic design allows us to present peptides in a branched three-dimensional fashion that eventually leads to a globular shape, thus mimicking globular proteins. Peptide dendrimers, unlike other classes of dendrimers, have immense applications in biomedical research due to their biological origin. The diversity of potential building blocks and innumerable possibilities for design, along with the fact that the area is relatively underexplored, make peptide dendrimers sought-after candidates for various applications. This review summarizes the stepwise evolution of peptidic dendrimers along with their multifaceted applications in various fields. Further, the introduction of biomacromolecules such as proteins to a dendritic scaffold, resulting in complex macromolecules with discrete molecular weights, is an altogether new addition to the area of organic chemistry. The synthesis of highly complex and fully folded biomacromolecules on a dendritic scaffold requires expertise in synthetic organic chemistry and biology. Presently, there are only a handful of examples of protein dendrimers; we believe that these limited examples will fuel further research in this area.

Circularly Polarized Luminescence from a Pyrene-Cyclodextrin Supra-Dendron

Soft nanomaterials with circularly polarized luminescence (CPL) have been currently attracting great interest. Here, we report a pyrene-containing π-peptide dendron hydrogel, which shows 1D and 2D nanostructures with varied CPL activities. It was found that the individual dendrons formed hydrogels in a wide pH range (3-12) and self-assembled into helices with pH-tuned pitches. Through chirality transfer, the pyrene unit could show CPL originated from both the monomer and excimer bands. When cyclodextrin was introduced, different supra-dendrons were obtained with β-cyclodextrin (PGAc@β-CD) and γ-cyclodextrin (PGAc@γ-CD) through host-guest interactions, respectively. Interestingly, the PGAc@β-CD and PGAc@γ-CD supra-dendrons self-assembled into 2D nanosheet and entangled nanofibers, respectively, showing cyclodextrin induced circularly polarized emission from both the monomer and excimer bands of the pyrene moiety. Thus, through a simple host-guest interaction, both the nanostructures and the chiroptical activities could be modulated.

A supramolecular peptide polymer from hydrogen-bond and coordination-driven self-assembly

A terpyridine- and guanine-functionalized peptide was developed that could form different morphologies by self-assembly or coordination with Fe²⁺ in dimethyl sulfoxide.

Proton triggered circularly polarized luminescence in orthogonal- and co-assemblies of chiral gelators with achiral perylene bisimide

Acid–base exposure switched circularly polarized luminescence was achieved in a coassembled gel which is composed of a chiral gelator and achiral perylene bisimide.

Peptide-based ambidextrous bifunctional gelator: applications in oil spill recovery and removal of toxic organic dyes for waste water management

A low molecular weight peptide-based ambidextrous gelator molecule has been discovered for efficient control of water pollution. The gelator molecules can gel various organic solvents with diverse polarity, e.g. n-hexane, n-octane, petroleum ether, petrol, diesel, aromatic solvents like chlorobenzene, toluene, benzene, o-xylene and even aqueous phosphate buffer of pH 7.5. These gels have been thoroughly characterized using various techniques including field emission scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray powder diffraction analysis, small angle X-ray scattering and rheological experiments. Interestingly, hydrogel obtained from the gelator molecule has been found to absorb toxic organic dyes (both cationic and anionic dyes) from dye-contaminated water. The gelator molecule can be reused for several cycles, indicating its possible future use in waste water management. Moreover, this gelator can selectively gel petrol, diesel, pump oil from an oil-water mixture in the presence of a carrier solvent, ethyl acetate, suggesting its efficient application for oil spill recovery. These results indicate that the peptide-based ambidextrous gelator produces soft materials (gels) with dual function: (i) removal of toxic organic dyes in waste water treatment and (ii) oil spill recovery.

Amphiphilic oligoamides as versatile, acid-responsive gelators

Among six oligoamides, G1–G6, G2 stands out as a versatile gelator to form stable hydrogels as well as several organogels. The hydrogel of G2 is able to encapsulate and release medicinally important substances with acid-responsiveness.

Chiral Nanoarchitectonics: Towards the Design, Self-Assembly, and Function of Nanoscale Chiral Twists and Helices

Helical structures such as double helical DNA and the α-helical proteins found in biological systems are among the most beautiful natural structures. Chiral nanoarchitectonics, which is used here to describe the hierarchical formation and fabrication of chiral nanoarchitectures that can be observed by atomic force microscopy (AFM), scanning tunneling microscopy (STM), scanning electron microscopy (SEM), or transmission electron microscopy (TEM), is one of the most effective ways to mimic those natural chiral nanostructures. This article focuses on the formation, structure, and function of the most common chiral nanoarchitectures: nanoscale chiral twists and helices. The types of molecules that can be designed and how they can form hierarchical chiral nanoarchitectures are explored. In addition, new and unique functions such as amplified chiral sensing, chiral separation, biological effects, and circularly polarized luminescence associated with the chiral nanoarchitectures are discussed.

Glutamic acid derivatives as gelators for electrolyte of lithium ion batteries

Organic gelators were synthesized to prepare gel electrolyte with wider electrochemical stable window.

An additional fluorenylmethoxycarbonyl (Fmoc) moiety in di-Fmoc-functionalized L-lysine induces pH-controlled ambidextrous gelation with significant advantages

In recent years, several fluorenylmethoxycarbonyl (Fmoc)-functionalized amino acids and peptides have been used to construct hydrogels, which find a wide range of applications. Although several hydrogels have been prepared from mono Fmoc-functionalized amino acids, herein, we demonstrate the importance of an additional Fmoc-moiety in the hydrogelation of double Fmoc-functionalized L-lysine [Fmoc(Nα)-L-lysine(NεFmoc)-OH, (Fmoc-K(Fmoc))] as a low molecular weight gelator (LMWG). Unlike other Fmoc-functionalized amino acid gelators, Fmoc-K(Fmoc) exhibits pH-controlled ambidextrous gelation (hydrogelation at different pH values as well as organogelation), which is significant among the gelators. Distinct fibrous morphologies were observed for Fmoc-K(Fmoc) hydrogels formed at different pH values, which are different from organogels in which Fmoc-K(Fmoc) showed bundles of long fibers. In both hydrogels and organogels, the self-assembly of Fmoc-K(Fmoc) was driven by aromatic π-π stacking and hydrogen bonding interactions, as evidenced from spectroscopic analyses. Characterization of Fmoc-K(Fmoc) gels using several biophysical methods indicates that Fmoc-K(Fmoc) has several advantages and significant importance as a LMWG. The advantages of Fmoc-K(Fmoc) include pH-controlled ambidextrous gelation, pH stimulus response, high thermal stability (∼100 °C) even at low minimum hydrogelation concentration (0.1 wt%), thixotropic property, high kinetic and mechanical stability, dye removal properties, cell viability to the selected cell type, and as a drug carrier. While single Fmoc-functionalized L-lysine amino acids failed to exhibit gelation under similar experimental conditions, the pH-controlled ambidextrous gelation of Fmoc-K(Fmoc) demonstrates the benefit of a second Fmoc moiety in inducing gelation in a LMWG. We thus strongly believe that the current findings provide a lead to construct or design various new synthetic Fmoc-based LMW organic gelators for several potential applications.

How important are dispersion interactions to the strength of aromatic stacking interactions in solution?

The similarity of aromatic stacking energies in solution for varying sized surfaces suggests that dispersion interactions are a minor contributor.

In this study, the contributions of London dispersion forces to the strength of aromatic stacking interactions in solution were experimentally assessed using a small molecule model system. A series of molecular torsion balances were designed to measure an intramolecular stacking interaction via a conformational equilibrium. To probe the importance of the dispersion term, the size and polarizability of one of the aromatic surfaces were systematically increased (benzene, naphthalene, phenanthrene, biphenyl, diphenylethene, and diphenylacetylene). After correcting for solvophobic, linker, and electrostatic substituent effects, the variations due to polarizability were found to be an order of magnitude smaller in solution than in comparison to analogous computational studies in vacuo. These results suggest that in solution the dispersion term is a small component of the aromatic stacking interaction in contrast to their dominant role in vacuo.

Synthesis and spectroscopic characterization of chiral biphenyl-cholesterol gels

The synthesis of 4-(3-cholesteroxycarbonylpropyloxy)biphenyl (BO4-chol), 4-(7-cholesteroxycarbonylheptyloxy)biphenyl (BO8-chol), and 4,4'-bis(7-cholesteroxycarbonyl heptyloxy)biphenyl (BBO8-chol) is reported. These gelators form 1% and 2% (w/w) stable gels in n-octanol. The gels formed from single cholesterol gelators (BO4-chol and BO8-chol) exhibit lower phase transition temperatures (Tg) (62-65, 68-69 °C) than the gel obtained from the bischolesterol gelator BBO8-chol (96-98 °C). All three gelators form chiral gels in n-octanol as observed by induced circular dichroism (ICD) spectroscopy. The effect of two cholesterol moieties versus one cholesterol unit linked to a biphenyl molecule by a flexible chain, and the effect of the chain length on the gelation ability of these three novel gelators was investigated by circular dichroism (CD), absorption, and fluorescence spectroscopies. The gels obtained from BO4-chol and BO8-chol exhibit biphasic circular dichroism spectra with opposite chirality. The ICD spectra of both BO8-chol and BBO8-chol gels show a positive ICD band followed by a negative band at room temperature. However, while BO8-chol gel ICD absorptions decrease equally as temperature increases, BBO8-chol gel shows an inversion of the Cotton effect bands between 50 and 60 °C until completely disappearing above the phase transition temperature. SEM was used for the investigation of the morphology of the xerogels. On the basis of XRD data and molecular modeling, we propose packing modes for the formation of the organogelator aggregates.

Fluorescent hydrogels with tunable nanostructure and viscoelasticity for formaldehyde removal

Hydrogels with ultrahigh water content, ∼99 wt %, and highly excellent mechanical strength were prepared by 4'-para-phenylcarboxyl-2,2':6',2″-terpyridine (PPCT) in KOH aqueous solution. The self-assembled structure, rheological properties, and the gel-sol transformation temperature (Tgel-sol) of PPCT/KOH hydrogels that depend on PPCT and KOH concentrations were studied, indicating easily controllable conditions for producing hydrogels in PPCT and KOH mixtures. An important finding was that the hydration radius (Rh) of cations (M(+) = Li(+), Na(+), K(+), Cs(+), NH4(+), (CH3)4N(+), (CH3CH2)4N(+), (CH3CH2CH2)4N(+), (CH3CH2CH2CH2)4N(+)) plays a vital role in gelation of PPCT/MOH systems. To produce hydrogels in PPCT/MOH systems, the Rh of M(+) must be in a suitable region of 3.29 to 3.58 Å, e.g., K(+), Na(+), Cs(+), and the capability of M(+) for inducing PPCT to form hydrogels is K(+) > Na(+) > Li(+), which is followed by the Hofmeister series. The hydrogels of PPCT and KOH mixtures are responsive to external stimuli including temperature and shearing force, and present gelation-induced enhanced fluorescence emission property. The states of being sensitive to the stimuli can readily recover to the original hydrogels, which are envisaged to be an attracting candidate to produce self-healing materials. A typical function of the hydrogels of PPCT and KOH mixtures is that formaldehyde (HCHO) can speedily be adsorbed via electrostatic interaction and converted into nontoxic salts (HCOOK and CH3OK), making it a promising candidate material for HCHO removal in home furnishings to reduce indoor environmental pollutants.

Control of supramolecular chirality of nanofibers and its effect on protein adhesion

Chiral nanostructure, such as the double helix of DNA and α-helix of protein, plays an important role in biochemistry and material sciences. In the organism system, the biological entities always exhibit homochirality and show preference toward one specific enantiomer. How the opposite enantiomers will affect the chirality of the supramolecular nanostructures and their interactions with the biological molecules remains an important issue. In this study, two gelators bearing amphiphilic l-glutamide and d- or l-pantolactone (abbreviated as DPLG and LPLG) were designed, and their self-assembly behavior and interactions with proteins were investigated. It was found that both of the gelators could form gels in the mixed solvent of ethanol and water, and the corresponding gels were characterized with UV-vis spectroscopy, circular dichroism, Fourier transform infrared spectroscopy, X-ray diffraction, and atomic force microscopy. Although both gels formed nanofiber structures and showed many similar properties, their supramolecular chiralities were opposite, which was determined by the chirality of the terminal group. The chirality of the nanofibrous structure is found to influence the protein adhesion significantly. Quartz crystal microbalance technique was used to investigate the adsorption of human serum albumin on the nanofibrous structures. It was revealed that supramolecular nanostructure of DPLG exhibited stronger adhesive ability than that of LPLG, while there is no clear difference at a molecular level. This suggested that slightly different interactions between d and l substances with the biological molecules could be amplified when they formed chiral nanostructures. Molecular dynamic simulations were performed to verify the interaction between the two gelators and protein molecules. A possible model was proposed to explain the interaction between the nanofibers and the proteins.

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.

Effect of solvent hydrophobicity on gelation kinetics and phase diagram of gelatin ionogels

We present a systematic investigation of the effect of solvent hydrophobicity (alkyl chain length) on the gelation kinetics and the phase states of the polypeptide gelatin in imidazolium based ionic liquid (IL) solutions. We have observed that IL concentration and hydrophobicity had dramatic influences on the thermal and viscoelastic properties of gelatin ionogels. Gelation concentration cg was observed to increase from 1.75 to 2.75% (w/v) while the gelation temperature Tg was found to decrease from 32 to 26 °C with increase in 1-octyl-3-methyl imidazolium chloride [C8mim][Cl] (most hydrophobic) concentration as compared to the case of the least hydrophobic IL 1-ethyl-3-methyl imidazolium chloride [C2mim][Cl], where the corresponding changes were marginal. Gradual softening of the gel with increase in hydrophobicity and concentration of IL was clearly noticed. The viscosity of the gelling sol diverged as ηr ∼ ε(1)(-k) and storage modulus of gel grew as G0 ∼ ε(1)(t) where ε1 = |1 - c/cg| with the exponents having values k = 1.2-1.8 ± 0.08 and t = 1.2-1.6 ± 0.08, close to but not exactly the same as predicted by the percolation model: k = 0.7-1.3 and t = 1.9. Thus, the gelation kinetics involved in the growth of interconnected networks could be conceived to follow an anomalous percolation model. The temporal growth of self-assembled structures followed a power law dependence given by: ηr ∼ ε(2)(-α) and Rh ∼ ε(2)(-β) where ε(2) = t > tg (α = 1-2.9 ± 0.08 and β = 1-2.7 ± 0.08). The low frequency storage modulus G0, gelation temperature Tg, gelation concentration cg and gelation time tg adequately defined the sol-gel phase diagram. Results clearly revealed that by adjusting the hydrophobic chain length and concentration of IL it was possible to customize both thermal and mechanical properties of these ionogels to match specific application requirements.