Self-Assembly of Racemic Alanine Derivatives: Unexpected Chiral Twist and Enhanced Capacity for the Discrimination of Chiral Species

Chiral Solvent-Induced Homochiral Hierarchical Molecular Assemblies at the Liquid/Solid Interface

We herein investigate the formation of homochiral hierarchical self-assembled molecular networks (SAMNs) via chirality induction by the coadsorption of a chiral solvent at the liquid/graphite interface by means of scanning tunneling microscopy (STM). In a mixture of achiral solvents, 1-hexanoic acid, and 1,2,4-trichlorobenzene, an achiral dehydrobenzo[12]annulene (DBA) derivative with three alkoxy and three hydroxy groups in an alternating manner forms chiral hierarchical triangular cluster structures through dynamic self-sorting. Enantiomorphous domains appear in equal probability. On the other hand, in chiral 2-methyl-1-hexanoic acid as a solvent, this molecule produces (i) homochiral small triangular clusters at a low solute concentration, (ii) a chirality-biased hierarchical structure consisting of triangular cluster structures with different cluster sizes at a medium concentration, and (iii) a dense structure with no chirality bias at a high concentration. We attribute the concentration-dependent degree of the chirality transmission to the number of coadsorbed solvent molecules in the SAMNs and to the difference in nucleus structure and size in the initial stage of the SAMN formation.

Water-Regulated Evolution of Inversion, Reinversion, and Amplification of Circularly Polarized Luminescence of Supramolecular Organogels Based on Glutamide-Cyanostilbene Amphiphile

Water incorporated with supramolecular building blocks in organic solvents can play a key role in the circularly polarized luminescence (CPL) inversion and amplification of supramolecular assemblies. Herein, we demonstrate that fine-tuning the water content regulated the assembly structure evolution and made the circular dichroism and CPL sign of the system undergo intriguing inversion, reinversion, and amplification processes based on a unique and interesting glutamide-cyanostilbene system, as supported by morphology, spectroscopic observations, and time-dependent density functional theory calculation.

Photoisomerization and thermal reconstruction induced supramolecular chirality inversion in nanofiber determined by minority isomer

Here, amphiphile GCH based on glutamide-cyanostilbene is designed and synthesized, it is found that it can assembly in acetonitrile, and shows circular dichroism signals. After Z-E isomerizaition by UV irradiation, the CD signal of the assembly can be inverted. Unexpectedly, after another heating and cooling process, the circular dichroism signals can be totally inverted even though the E-isomers are in minority. Finally, the molecular dynamics (MD) simulations deeply elucidate the supramolecuar chirality inversion mechanism. This work brings some new insights into the control of chirality inversion, which may provide a perspective for the smart chiroptical materials construction.

Electromagnetic Response of Multistage-Helical Nano-Micro Conducting Polymer Structures and their Enhanced Attenuation Mechanism of Multiscale-Chiral Synergistic Effect

Nowadays, the rapidly development of advanced antidetection technology raises stringent requirements for microwave absorption materials (MAMs) to focus more attention on wider bandwidth, thinner thickness, and lower density. Adding magnetic medium to realize broadband absorption may usually result in the decline of service performance and accelerating corrosion of MAMs. Chiral MAMs can produce extra magnetic loss without adding magnetic medium due to the unique electromagnetic cross polarization effect. However, more efforts should be taken to furtherly promote efficient bandwidth of chiral MAMs and reveal attenuation mode and modulation method of chiral structure. Herein, a novel superhelical nano-microstructure based on chiral polyaniline and helical polypyrrole is successfully achieved via in situ polymerization strategy. The enhanced multiscale-chiral synergistic effect contributes to broaden effective absorption bandwidth, covering 8.6 GHz at the thickness of 3.6 mm, and the minimum reflection loss can reach -51.3 dB simultaneously. Besides, to further explain response modes and loss mechanism of superhelical nano-microstructures, the electromagnetic simulation and test analysis are applied together to reveal their synergistic enhancement attenuation mechanism. Taken together, this strategy gives a new thought of how to design, prepare, and optimize the hierarchical structure materials to achieving broadband and high-performance microwave absorption.

Amplification of Molecular Asymmetry during the Hierarchical Self-Assembly of Foldable Azobenzene Dyads into Nanotoroids and Nanotubes

The amplification of molecular asymmetry through self-assembly is a phenomenon that not only comprehends the origin of homochirality in nature but also produces chiroptically active functional materials from molecules with minimal enantiomeric purity. Understanding how molecular asymmetry can be transferred and amplified into higher-order structures in a hierarchical self-assembly system is important but still unexplored. Herein, we present an intriguing example of the amplification of molecular asymmetry in hierarchically self-assembled nanotubes that feature discrete and isolatable toroidal intermediates. The hierarchical self-assembly is initiated via asymmetric intramolecular folding of scissor-shaped azobenzene dyads furnished with chiral side chains. When scalemic mixtures of the enantiomers are dissolved in a non-polar solvent and cooled to 20 °C, single-handed nanotoroids are formed, as confirmed using atomic force microscopy and circular dichroism analyses. A strong majority-rules effect at the nanotoroid level is observed and can be explained by a low mismatch penalty and a high helix-reversal penalty. The single-handed nanotoroids stack upon cooling to 0 °C to exclusively afford their respective single-handed nanotubes. Thus, the same degree of amplification of molecular asymmetry is realized at the nanotube level. The internal packing efficiency of molecules within nanotubes prepared from the pure enantiomers or their scalemic mixtures is likely different, as suggested by the absence of higher-order structure (supercoil) formation in the latter. X-ray diffraction analysis of the anisotropically oriented nanotube films revealed looser molecular packing within the scalemic nanotubes, which clearly reflects the lower enantiomeric purity of their internal chiral side chains.

Facile Stratification-Enabled Emergent Hyper-Reflectivity in Cholesteric Liquid Crystals

Cholesteric liquid crystals (CLCs) are chiral photonic materials with selective reflection in terms of wavelength and polarization. Helix engineering is often required in order to produce desired properties for CLC materials to be employed for beam steering, light diffraction, scattering, and adaptive or broadband reflection. Here, we demonstrate a novel photopolymerization-enforced stratification (PES)-based strategy to realize helix engineering in a chiral CLC system with initially one handedness of molecular rotation throughout the layer. PES plays a crucial role in driving the chiral dopant bundle consisting of two chiral dopants of opposite handedness to spontaneously phase separate and create a CLC bilayer structure that reflects left- and right-handed circularly polarized light (CPL). The initially hidden chiral information therefore becomes explicit, and hyper-reflectivity, i.e., reflecting both left- and right-handed CPL, successfully emerges from the designed CLC mixture. The PES mechanism can be applied to structure a wide range of liquid crystal (LC) and polymer materials. Moreover, the engineering strategy enables facile programming of the center wavelength of hyper-reflection, patterning, and incorporating stimuli-responsiveness in the optical device. Hence, the engineered hyper-reflective CLCs offer great promise for future applications, such as digital displays, lasing, optical storage, and smart windows.

Synthetic approaches to metal-coordination-directed macrocyclic complexes

Metal-coordination-directed macrocyclic complexes, in which macrocyclic architectures are formed by metal-ligand coordination interactions, have emerged as attractive supramolecular scaffolds for the creation of materials for applications in biosensing and therapeutics. Despite recent progress, uncontrolled multicyclic cages and linear oligomers/polymers is the most likely outcome from metal-ligands assembly, representing a challenge to current synthetic methods. Herein we outlined the state-of-art synthetic approaches to the metal-coordination-directed macrocyclic complexes by using foldable ligands or through assembly of amphiphilic ligands. This mini-review offers a guideline for the efficient preparation of metal-coordination-directed macrocyclic complexes with predictable and controllable structures, which may find applications in many biology-related areas.

Chiral Recognition with Broad Selective Sensor Arrays

The detection and discrimination of chiral analytes has always been a topical theme in food and pharmaceutical industries and environmental monitoring, especially when dealing with chiral drugs and pesticides, whose enantiomeric nature assessment is of crucial importance. The typical approach matches novel chiral receptors designed ad hoc for the discrimination of a target enantiomer with emerging nanotechnologies. The massive synthetic efforts requested and the difficulty of analyzing complex matrices warrant the ever-growing exploitation of sensor array as an alternative route, using a limited number of chiral or both chiral and achiral sensors for the stereoselective identification and dosing of chiral compounds. This review aims to illustrate a little-explored winning strategy in chiral sensing based on sensor arrays. This strategy mimics the functioning of natural olfactory systems that perceive some couples of enantiomeric compounds as distinctive odors (i.e., using an array of a considerable number of broad selective receptors). Thus, fundamental concepts related to the working principle of sensor arrays and the role of data analysis techniques and models have been briefly presented. After the discussion of existing examples in the literature using arrays for discriminating enantiomers and, in some cases, determining the enantiomeric excess, the remaining challenges and future directions are outlined for researchers interested in chiral sensing applications.

Homo- and heterometallic chiral dynamic architectures from allyl-palladium(II) building blocks

New chiral tetranuclear square-like homo- and heterometallamacrocycles containing allyl-palladium and either {Pd(P-P)*} or {Pt(P-P)*} optically pure moieties (P-P* = (2S,3S)-O-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphanyl)butane ((S,S)-DIOP) and (2S,4S)-2,4-bis(diphenylphosphanyl)pentane ((S,S)-BDPP)) have been obtained by the self-assembly of [Pd(η³-2-Me-C₃H₄)(4-PPh₂py)₂]⁺ and [M(P-P)*(H₂O)₂]²⁺ building blocks in a 1 : 1 molar ratio. The supramolecular assemblies thus prepared [{Pd(η³-2-Me-C₃H₄)}₂(4-PPh₂py)₄{M(P-P)*}₂](CF₃SO₃)₆ (M = Pd, Pt) have been fully characterised by multinuclear NMR spectroscopy and MS spectrometry. The structures display remarkable differences on their dynamic behaviour in solution that depend on the lability and thermodynamic strength of M-py bonds. The structural characteristics of the new metallamacrocyles obtained have also been unambiguously established by XRD analysis. The architectures have been assayed as catalytic precursors in the asymmetric allylic alkylation reaction.

Possible chemical and physical scenarios towards biological homochirality

The single chirality of biological molecules in terrestrial biology raises more questions than certitudes about its origin. The emergence of biological homochirality (BH) and its connection with the appearance of life have elicited a large number of theories related to the generation, amplification and preservation of a chiral bias in molecules of life under prebiotically relevant conditions. However, a global scenario is still lacking. Here, the possibility of inducing a significant chiral bias "from scratch", i.e. in the absence of pre-existing enantiomerically-enriched chemical species, will be considered first. It includes phenomena that are inherent to the nature of matter itself, such as the infinitesimal energy difference between enantiomers as a result of violation of parity in certain fundamental interactions, and physicochemical processes related to interactions between chiral organic molecules and physical fields, polarized particles, polarized spins and chiral surfaces. The spontaneous emergence of chirality in the absence of detectable chiral physical and chemical sources has recently undergone significant advances thanks to the deracemization of conglomerates through Viedma ripening and asymmetric auto-catalysis with the Soai reaction. All these phenomena are commonly discussed as plausible sources of asymmetry under prebiotic conditions and are potentially accountable for the primeval chiral bias in molecules of life. Then, several scenarios will be discussed that are aimed to reflect the different debates about the emergence of BH: extra-terrestrial or terrestrial origin (where?), nature of the mechanisms leading to the propagation and enhancement of the primeval chiral bias (how?) and temporal sequence between chemical homochirality, BH and life emergence (when?). Intense and ongoing theories regarding the emergence of optically pure molecules at different moments of the evolution process towards life, i.e. at the levels of building blocks of Life, of the instructed or functional polymers, or even later at the stage of more elaborated chemical systems, will be critically discussed. The underlying principles and the experimental evidence will be commented for each scenario with particular attention on those leading to the induction and enhancement of enantiomeric excesses in proteinogenic amino acids, natural sugars, and their intermediates or derivatives. The aim of this review is to propose an updated and timely synopsis in order to stimulate new efforts in this interdisciplinary field.

A biocompatible hydrogel as a template for oxidative decomposition reactions: a chemodosimetric analysis and in vitro imaging of hypochlorite

The self-assembly properties of new biocompatible, thermoreversible fluorescent hydrogels, composed of amino acid residues have been reported. A unique gel-to-sol transition is triggered by chemodosimetric interaction in the presence of hypochlorite.

Chiral V-shaped Pyrenes: Hexagonal Packing, Superhelix, and Amplified Chiroptical Performance

Here, we designed symmetric and dissymmetric chiral V-shaped pyrenes by linking achiral pyrenes to trans-1,2-cyclohexane diamine scaffolds with varied spacers to investigate their circular dichroism (CD) and circularly polarized excimer emission (CPEE). In molecular solution, the symmetric V-shaped molecules (P1, P2, P3) displayed spacer-dependent CD and CPEE originating from the intramolecular excimers while the dissymmetric V-shaped B was silent in CD and CPEE. Upon self-assembly, the chiral V-shaped conformation guided a helical hexagonal packing. Notably, P1 self-assembled into delicate superhelices with optimum chiroptical activities and the largest g_CD for pyrene derivatives to date. The dissymmetric B formed two distinct hexagonal aggregates as twists and rectangular nanotubes with greatly amplified CPEE. This work demonstrates unprecedented hexagonal superhelices from chiral V-shaped scaffolds and provides a deep insight into the relationship between molecular conformation, supramolecular architectures, and their chiroptical performance.

Self-Assembly of Peptide Hierarchical Helical Arrays with Sequence-Encoded Circularly Polarized Luminescence

Self-assembled peptide materials with sequence-encoded properties have attracted great interest. Despite their intrinsic chirality, the generation of circularly polarized luminescence (CPL) based on the self-assembly of simple peptides has been rarely reported. Here, we report the self-assembly of peptides into hierarchical helical arrays (HHAs) with controlled supramolecular handedness. The HHAs can emit full-color CPL signals after the incorporation of various achiral fluorescent molecules, and the g_lum value is 40 times higher than that of the CPL signal from the solutions. By simply changing the amino acid sequence of the peptides, CPL signals with opposite handedness can be generated within the HHAs. The peptide HHAs can provide hydrophobic pockets to accommodate the fluorescent molecules with helical arrangement through strong aromatic stacking interactions, which are responsible for the CPL signals. This work provides a pathway to construct highly ordered chiral materials, which have broad applications in the chiroptical field.

Bamboo-like π-Nanotubes with Tunable Helicity and Circularly Polarized Luminescence

We report the fabrication of an exotic bamboo-like π-nanotube via the hierarchical self-assembly of a dipeptide-substituted naphthalenediimide gelator with tunable helicity and circularly polarized luminescence (CPL). It was found that in the presence of trifluoroacetic acid (TFA) the gelator molecules self-assembled into a bamboo-like π-nanotube, which is composed of truncated nanocones and CPL active. When defining the diameter ratio of the lower to upper edge of each nanocone as a parameter to express the helicity of different nanotubes, it was found that both the helicity and CPL of these nanotubes can be adjusted by the amount of TFA. Moreover, the helicity of the nanotube can be conveyed to the achiral quantum dots (QDs) and produce a hybrid nanotube/QDs CPL active materials with adjustable dissymmetry factor. This work finds a new type self-assembled bamboo-like π-nanotube and unveils their helicity and CPL control.

Self-Templated, Enantioselective Assembly of an Amyloid-like Dipeptide into Multifunctional Hierarchical Helical Arrays

Chiral self-assembly of peptides has attracted great interest owing to their promising applications in biomedicine, chemistry, and materials science. However, compared with the rich knowledge about their chiral self-assembly at the molecular or nanoscale, the formation of long-range-ordered hierarchical helical arrays (HHAs) from simple peptides remains a formidable challenge. Herein, we report the self-templated assembly of an amyloid-like dipeptide into long-range-ordered HHAs by their spontaneous fibrillization and hierarchical helical assembly within a confined film. The chiral interactions between the peptide and diamines result in geometry frustration and the phase transition of self-assembling peptide films from achiral spherulite structures into chiral HHAs. By changing the chirality and enantioselective interactions, we can control the phase behavior, handedness, and chiroptics of the self-assembled HHAs precisely. Moreover, the redox activity of the HHAs allows the in situ decoration of nanoparticles with high catalytic activity. These results provide insights into the chiral self-assembly of peptides and the fabrication of highly ordered materials with complex architectures and promising applications in chiroptics and catalysis.

An injectable peptide hydrogel with excellent self-healing ability to continuously release salvianolic acid B for myocardial infarction

Drug-loaded hydrogels can improve blood supply and inhibit extracellular matrix degradation after myocardial infarction. However, due to the continual dynamic motion of cardiac tissue, the hydrogel structure cannot be reconstructed in time, causing accelerated degradation and drug burst release. Here, a novel, superior, self-healing elastin-mimic peptide hydrogel (EMH) was fabricated for the local delivery of salvianolic acid B (SaB). The self-healing ability of EMH is enhanced by SaB-loaded polydopamine nanoparticles (SaB-PDA). In vitro, the pre-hydrogel (SaB-PDA/pre-EMH) is endowed with excellent biocompatibility and a low viscosity, making it suitable for intramyocardial injection. Once injected into the myocardial infarction (MI) region, SaB-PDA/pre-EMH can form SaB-PDA/EMH with great mechanical strength under the action of upregulated transglutaminase (TGase) in heart tissue post-MI. The superior self-healing ability of SaB-PDA/EMH allows for an increase in retention time in the beating ventricular wall. Therefore, with long-term release of SaB, SaB-PDA/EMH can inhibit ventricular remodeling and promote angiogenesis for MI treatment.

Switchable Circularly Polarized Luminescence in Supramolecular Gels through Photomodulated FRET

While the tremendous deal of efforts has been dedicated to the design and fabrication of materials with circularly polarized luminescence (CPL), the development of the chiroptical switch between different CPL signals is one of the important routes toward its application. Here, we prepared a supramolecular gel from the coassemblies containing a chiral gelator (9-fluoren-methoxycarbonyl-functionalized glutamate derivatives, FLG), a fluorescent molecule [(rhodamine B, RhB) or (2',7'-dichlorofluorescein sodium salt, DCF)], and a photochromic molecule [1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)-3,3,4,4,5,5-hexafluoro-1-cyclopentene, DAE], thus constructing photomodulated switchable CPL soft materials. It was found that FLG could form supramolecular gel in ethanol and self-assemble into left-handed twisted nanostructures. During the formation of a co-gel with RhB (or DCF) and DAE, the chirality of FLG could be effectively transferred to both the fluorescent and photochromic components, which induced them with chiroptical properties including CPL and circular dichroism (CD). DAE undergoes a reversible transition between the achromatous open state and the dark purple closed state in the co-gel under alternating irradiation with UV and visible light. During such a process, an intermolecular Förster resonance energy transfer (FRET) behavior from fluorescent RhB to ring-closed DAE caused the emission quenching of RhB, which led to CPL silence of RhB in the co-gel. Subsequent irradiation with visible light caused the restoration of the emission and CPL activity with the restored open state. These changes could be repeated many times upon alternate UV and visible irradiation. Therefore, a reversible CPL switch was fabricated in supramolecular gels through the photomodulated FRET process.

Effect of Stereochemistry on Chirality and Gelation Properties of Supramolecular Self-Assemblies

Although chiral nanostructures have been fabricated at various structural levels, the transfer and amplification of chirality from molecules to supramolecular self-assemblies are still puzzling, especially for heterochiral molecules. Herein, four series of C₂ -symmetrical dipeptide-based derivatives bearing various amino acid sequences and different chiralities are designed and synthesized. The transcription and amplification of molecular chirality to supramolecular assemblies are achieved. The results show that supramolecular chirality is only determined by the amino acid adjacent to the benzene core, irrespective of the absolute configuration of the C-terminal amino acid. In addition, molecular chirality also has a significant influence on the gelation behavior. For the diphenylalanine-based gelators, the homochiral gelators can be gelled through a conventional heating-cooling process, whereas heterochiral gelators form translucent stable gels under sonication. The racemic gels possess higher mechanical properties than those of the pure enantiomers. All of these results contribute to an increasing knowledge over control of the generation of specific chiral supramolecular structures and the development of new optimized strategies to achieve functional supramolecular organogels through heterochiral and racemic systems.

Chiral supramolecular polymerization of dicyanostilbenes with emergent circularly polarized luminescence behavior

A novel type of circularly polarized luminescence (CPL) active systems have been constructed via chiral supramolecular polymerization of dicyanostilbene-based monomers.

Supramolecular Chiral Aggregates Exhibiting Nonlinear CD-ee Dependence

Although a linear relationship between the optical activity (normally the CD signal) and the enantiomeric excess (ee) of chiral auxiliaries has been the most commonly observed dependence in dynamic supramolecular helical aggregates, positive nonlinear CD-ee dependence, known as the "majority-rules effect" (MRE), indicative of chiral amplification, has also been well documented and to some extent understood. In sharp contrast, the negative nonlinear CD-ee dependence has been much less reported and is not well understood. Here, the state of the art of both the positive and negative nonlinear CD-ee dependence in noncovalently bound supramolecular helical aggregates is summarized, with the hope that the vast examples of supramolecular aggregates showing positive nonlinear dependence, in terms of the methods of investigations, variations in the structure of the building block (single species or multiple species), and theoretical modeling using the mismatch penalty energy and helix reversal penalty energy, would help to guide the design of building blocks to form aggregates showing negative nonlinear dependence, and thus to understand the mechanisms. The potential applications of those functional supramolecular aggregates are also discussed.

Visible Enantiomer Discrimination via Diphenylalanine-Based Chiral Supramolecular Self-Assembly on Multiple Platforms

The development of enantioselective recognition is of great significance in medical science and pharmaceutical industry, which associates with the molecular recognition phenomenon widely observed in biological systems. In particular, the facile and straight achievement of visual enantioselective recognition has been drawing increasing consideration, but it is still a challenge. Herein, a heterochiral diphenylalanine-based gelator (LFDF) is synthesized, presenting left-handed nanofibers during self-assembly in ethanol, which accomplishes the phenylalaninol enantiomer recognition on multiple platforms. When adding l- or d-phenylalaninol into LFDF supramolecular solution followed by ultrasonic treatment, precipitate and gel are formed, respectively. Meanwhile, LFDF supramolecular gel completely collapses in a minute after dropping l-phenylalaninol, while the gel almost remains when d-type is employed. Moreover, a fluorescent supramolecular xerogel (ThT-LFDF) is fabricated by combining the LFDF gelator with thioflavine T (ThT), which could detect l-phenylalaninol accompanying with fluorescence quenching while d-type with barely decreasing. And the ThT-LFDF xerogel system shows a good sensitivity (reaches to ppm) for the detection of l-phenylalaninol. It is found that the chirality of the assembled nanofibers, as well as amino and carboxyl of phenylalaninol, plays a critical role on the discrimination process. The multiple and visible enantioselective recognition of phenylalaninol through chiral supramolecular self-assemblies shows potential applications in the fields of medical science and pharmaceutical industry.

Emergence of Homochiral Benzene-1,3,5-tricarboxamide Helical Assemblies and Catalysts upon Addition of an Achiral Monomer

Chirality amplification refers to the ability of a small chiral bias to fully control the main chain helicity of polymers and assemblies. Further implementation of functional chirally amplified helices as switchable asymmetric catalysts, chiral sensors, and circularly polarized light emitters will require a greater control of the energetics governing these chirality amplification effects. In this work, we report on the counterintuitive ability of an achiral molecule to suppress conformational defects in supramolecular helices, thus leading to the emergence of homochirality in a system containing a very small chiral bias. We focus our investigation on supramolecular helices composed of an achiral benzene-1,3,5-tricarboxamide (BTA) ligand, coordinated to copper, and an enantiopure BTA comonomer. Amplification of chirality as probed by varying the amount (sergeants and soldiers effect) or the optical purity (diluted majority-rules effect) of the enantiopure comonomer are modest in this initial system. However, both effects are hugely enhanced upon addition of a second achiral BTA monomer, leading to a perfect control of the helicity either by means of a remarkably low amount of sergeants (0.5%) or a small bias from a racemic mixture of enantiopure comonomers (10% ee). Such an enhancement in the amplification of chirality is only achieved by mixing the three components, i.e. the two achiral and the enantiopure comonomers, highlighting a synergistic effect upon coassembly of the three monomers. Investigation of the role of the achiral additive by multifarious analytical techniques supports its ability to stabilize the helical coassemblies and suppress helix reversals: i.e., conformational defects. Implementation of these helical copper precatalysts in the hydrosilylation of 1-(4-nitrophenyl)ethanone confirms that the effect of the achiral BTA additive is also operative under the conditions of the catalytic experiment. A highly enantioenriched product (90% ee) is produced by a supramolecular catalyst operating with ppm levels of chiral species.

Design of Chiral Supramolecular Polymers Exhibiting a Negative Nonlinear Response

Many synthetic and supramolecular chiral polymeric systems are known to exhibit the "majority rules effect" (MRE), a positive nonlinear response in which a small enantiomeric excess (ee) of the chiral building blocks leads to unproportionally large chiroptical signals near zero ee. In contrast, the opposite "racemate rules effect" (RRE), a negative nonlinear response in which the chiroptical signals are flat near zero ee, while giving large nonlinear chiroptical responses to ee at high values, has only been occasionally observed. The origin of this unusual ee dependence remains elusive largely because few systems have been established that exhibit this effect. Herein, we present a design approach that enables the development of chiral supramolecular polymers with a pronounced negative nonlinear response akin to RRE. This is achieved by in situ generating a bidentate inducer for supramolecular polymerization that exists in both meso- and homochiral forms upon reacting with chiral guests. The presence of the meso-inducer creates an aggregate structure that has a little response in the circular dichroism (CD) spectra as a function of ee at a particular wavelength, but a homochiral inducer gives large changes in response to ee at this wavelength. This allowed for an RRE-like response to be observed when the CD intensity of the supramolecular polymers was plotted against the ee of the chiral guests that generate the meso- and homochiral inducers without the necessity of the racemic guest preferentially being incorporated into the polymer.

Solvent-Induced Supramolecular Assembly of a Peptide-Tetrathiophene-Peptide Conjugate

The assembly of a peptide-tetrathiophene-peptide (PTP) conjugate has been investigated in mixed solvents, which has different polarities by changing the solvent proportions. It was found that PTP can form fibers in THF/hexane solutions with 40–80%v of hexane. The fibers were stable and did not change on time. On the other hand, PTP formed ordered structures in a mixed solution with the water content from 40 to 60%v. For the as-prepared solutions, two nanostructures vesicles and parallelogram sheets were obtained. The parallelogram sheets could transform into vesicles on time. The fibers showed supramolecular chirality, however, there was no Cotton effect for vesicles and parallelogram sheets. UV-vis, FL, XRD, FT-IR, and CD spectra together with SEM, AFM, TEM were used to characterize the nanostructures and properties of the assemblies. Molecular packing mechanism was proposed based on the experimental data.

Polyamine-induced, chiral expression from liquid crystalline peptide nanofilaments to long-range ordered nanohelices

We reported the condensation and transformation of peptide micelles into well-defined nanohelices through the incorporation of natural polyamines. The liquid-crystalline peptide micelles are assembled by a short dipeptide amphiphile driven by strong electrostatic repulsions and aromatic stacking attractions. By incorporating polyamines into the peptide solutions, like-charge attractions were achieved to induce the condensation of the like-charged nanofilaments into giant bundles. Intriguingly, by increasing the temperature or electrostatic screening effects, the nanofilaments within the bundles fuse with each other into well-defined flat ribbons which then spontaneously twisted into macroscopically aligned nanohelices. Moreover, the chiral interactions between the aromatic groups of adjacent peptides are inverted from right-handedness to left-handedness during the formation of nanohelices. The results provide new insights into the chiral evolution during peptide self-assembly and offer opportunities for the design of peptide materials with new properties, such as anisotropic hydrogels and long-range ordered chiral nanostructures.

Organic Chiral Charge Transfer Magnets

In this work, through designing organic helix donor-acceptor complexes, one type of room-temperature chiral magnet was reported. Within these chiral charge transfer magnets, circularly polarized light could induce a larger saturation magnetization compared to linearly polarized light illumination with identical intensity. Moreover, the transmission light polarization from chiral magnets could be tuned via applying the magnetic field. Overall, room-temperature organic chiral magnets with optomagnetic effects will enhance the function of organic magnetochiral materials.

Homochiral nanotubes from heterochiral lipid mixtures: a shorter alkyl chain dominated chiral self-assembly

It is an important topic to achieve homochirality both at a molecular and supramolecular level. While it has long been regarded that "majority rule" guides the homochiral self-assembly from an enantiomer mixture, it still remains a big challenge to manipulate the global homochirality in a complex system containing chiral species that are not enantiomers. Here, we demonstrate a new example wherein homochiral nanotubes self-assembled from a mixture of heterochiral lipids that deviated from the "majority rule". We have found that when two heterochiral lipids with mirror headgroups but a 2-methylene discrepancy in alkyl chain length are mixed, homochiral nanotubes are always formed regardless of their mixing ratio. Remarkably, the helicity of the nanotube is exclusively controlled by the molecular chirality of the lipids with shorter alkyl chains, i.e., the chiral self-assembly was dominated by the lipid with the shorter alkyl chain. MD simulation reveals that the match of both the alkyl chain length and hydrogen-bonding between two kinds of lipids plays an important role in the assembly. This work provides a new insight into the supramolecular chirality of complex systems containing multi chiral species.

Recent developments in the construction of metallacycle/metallacage-cored supramolecular polymers via hierarchical self-assembly

Supramolecular polymers have received considerable attention during the last few decades due to their scientific value in polymer chemistry and profound implications for future developments of advanced materials. Discrete supramolecular coordination complexes (SCCs) with well-defined size, shape, and geometry have been widely employed to construct hierarchical systems by coordination-driven self-assembly with the spontaneous formation of metal-ligand bonds, which results in the formation of well-defined two-dimensional (2D) metallacycles or three-dimensional (3D) metallacages with high functionalities. The incorporation of discrete SCCs into supramolecular polymers by the orthogonal combination of metal-ligand coordination and other noncovalent interactions or covalent bonding could further facilitate the construction of novel supramolecular polymers with hierarchical architectures and multiple functions including controllable uptake and release of guest molecules, providing a flexible platform for the development of smart materials. In this review, the recent progress in metallacycle/metallacage-cored supramolecular polymers that were constructed by the combination of metal-ligand interactions and other orthogonal interactions (including hydrophobic or hydrophilic interactions, hydrogen bonding, van der Waals forces, π-π stacking, electrostatic interactions, host-guest interactions and covalent bonding) has been discussed. In addition, the potential applications of metallacycle/metallacage-cored supramolecular polymers in the areas of light emitting, sensing, bio-imaging, delivery and release, etc., are also presented.

Chiral nanostructures self-assembled from nitrocinnamic amide amphiphiles: substituent and solvent effects

Chiral nanostructures, such as α-helical proteins and double helix DNA, are widely found in biological systems and play a significant role in the biofunction of life. These structures are essentially fabricated through the covalent or noncovalent bonds between small chiral molecules. It is thus an important issue to understand how small chiral molecules can form chiral nanostructures. Here, using a series of isomeric nitrocinnamic amide derivatives, we have investigated the self-assembly behavior and the effect of the substituent position as well as the solvent on the formation of chiral nanostructures. It was found that totally different chiral nanostructures were formed due to the different positions of the nitro group on the cinnamic amide. Moreover, it was found that the chiral sense of the self-assembled nanostructures can be regulated by the solvent whereby helicity inversion was observed. This work provides a simple way to regulate the self-assembly pathway via molecular design and choice of solvent for the controlled creation of chiral nanostructures.

Chirality-Enabled Liquid Crystalline Physical Gels with High Modulus but Low Driving Voltage

Self-supporting liquid crystalline physical gels with facile electro-optic response are highly desirable, but their development is challenging because both the storage modulus and driving voltage increase simultaneously with gelator loading. Herein, we report liquid crystalline physical gels with high modulus but low driving voltage. This behavior is enabled by chirality transfer from the molecular level to three-dimensional fibrous networks during the self-assembly of 1,4-benzenedicarboxamide phenylalanine derivatives. Interestingly, the critical gel concentration is as low as 0.1 wt %. Our findings open doors to understanding and exploiting the role of chirality in organic gels.

A self-assembled photoresponsive gel consisting of chiral nanofibers

A novel compound based on a glutamic acid skeleton, containing azobenzene as a photoresponsive group and ureidopyrimidinone (UPy) as a connection site, was designed and synthesized. The monomer is capable of forming an organogel in nonpolar organic solvents and different types of nanostructures in other solvents. The state of the gel and the chirality of the nanostructures could both be adjusted by subsequent light irradiation at different wavelengths. The helical nanofiber-like morphology was verified in the internal structure of the gel. The performance of this gel was investigated by a series of methods, such as UV–vis absorption spectroscopy, circular dichroism, scanning electron microscopy and rheological techniques. This work provides a new method for facile synthesis of chiro-optical gels.

Alanine-Based Chiral Metallogels via Supramolecular Coordination Complex Platforms: Metallogelation Induced Chirality Transfer

Chiral self-assemblies constantly attract great interest because of their potential to provide insight into biological systems and materials science. Herein we report on the efficient preparation of alanine-based chiral metallacycles, rhomboids 1D and 1L and hexagons 2D and 2L using a Pt(II) ← pyridyl directional bonding approach. The metallacycles are subsequently assembled into nanospheres at low concentration, that generate chiral metallogels at high concentration driven by hydrogen bonding, hydrophobic and π-π interactions. The gels consist of microscopic chiral nanofibers with well-defined helicity, as confirmed by circular dichroism (CD) and scanning (SEM) and transmission electron (TEM) microscopies. Given these results, we expect this technique will not only unlock interesting new approaches to understand homochirality in nature but also allow the design of versatile soft materials containing chiral supramolecular cores.

Chiral magnetic-nanobiofluids for rapid electrochemical screening of enantiomers at a magneto nanocomposite graphene-paste electrode

The development of highly sensitive and selective enantiomeric platforms towards the rapid screening of active pharmaceutical ingredients (APIs) is nowadays a crucial challenge in several fields related to pharmacology, biomedicine, biotechnology and (bio)sensors. Herein, it is presented a novel, facile and generic methodology focused on exploiting the synergistically and electrocatalytic properties of chiral magnetic-nanobiofluids (mNBFs) with electrochemical enantiobiosensing at a magneto nanocomposite graphene paste electrode (mNC-GPE). The feasibility of this approach has been validated by chirally recognizing tryptophan (TRP) enantiomers as a proof-of-concept. For this aim, a specific chiral mNBF based on an aqueous dispersion of cobalt ferrite loaded with gold nanoparticles carrying a thiolated β-cyclodextrin (β-CD-SH/Au/CoFe₂O₄-NPs) has been synthesized and used towards the supramolecular discrimination of TRP enantiomers at an advanced graphene-paste transducer via cyclic voltammetry. This strategy, which is the first demonstration of applicability of chiral mNBFs for electrochemical enantiorecognition, opens up new approaches into enantio(bio)sensing.