Helical Self-assembly Research Articles

Tetraphenylethylene-based conjugated polymers with valine-containing side chains: Aggregation-induced emission, helical self-assembly, complexation-induced circularly polarized luminescence

Realizing high circularly polarized luminescence (CPL) in polymer system is an important but challenging task. Herein, we provide a strategy for designing conjugated polymers with high CPL performance upon the complexation with mandelic acids. Two tetraphenylethylene (TPE)-based conjugated polymers, l-P1 and d-P1, were synthesized by Sonogashira polycoupling reactions between the TPE-containing diethynyl monomer and dibromo aryl monomers carrying valine methyl ester groups. Both the polymers showed aggregation-induced emission (AIE) properties and the capabilities to self-assemble into regular helical nanostructures on nanoscale and fluorescent microfibers on microscale. They exhibited weak circular dichroism (CD) signals; upon the complexation with R/S-mandelic acids, their CD absorption and fluorescence emission can be efficiently enhanced. The composite films of l/d-P1 and R/S-mandelic acids exhibited excellent complexation-induced CPL properties with the absolute luminescence dissymmetric factor (glum) up to 0.01 and the fluorescence quantum yield up to 26.8%; the glum values showed little dependence on their emission wavelength. Various types of carboxyl acids have been screened and only mandelic acids show significantly chiroptical induction. The specific interaction between mandelic acids and l/d-P1 also makes the latter promising candidates for the enantioselective recognition of chiral organic molecules.

Chiral Supramolecular Organogel Constructed Using Riboflavin and Melamine: Its Application in Photo-Catalyzed Colorimetric Chiral Sensing and Enantioselective Adsorption.

The synthesis of a chiral supramolecular organogel via the hierarchical helical self-assembly of optically active riboflavin and melamine derivatives is described herein. Owing to the photocatalysis of riboflavin and the supramolecular chirality induced in the helically stacked riboflavin/melamine complex, the gel is observed to act as a light-stimulated chiral sensor of optically active alcohols by detecting the change in color from yellow to green. The gel also served as an efficient chiral adsorbent, enabling optical resolution of a racemic compound with high chiral recognition ability.

Helical self-assembly of an unusual pseudopeptide: crystallographic evidence

Abstract Pseudopeptides are a versatile class of organic building blocks having potential applications in a wide range of domains. In the current study, N and C termini protected l-alanine based short pseudopeptide was synthesized, where 5-aminoisophthalic acid (5-AIA), a rigid non-proteogenic γ-amino butyric acid was incorporated as C-terminal residue. The single crystal X-ray analysis revealed that the l-Ala residue of the aforesaid peptide adopts ϕ and ψ values characteristic of polyproline II conformation. Self-assembly of the pseudopeptide seems to represent a supramolecular helical architecture via NH⋯O, CH⋯O hydrogen bonding and π–π interactions.

Regular double-helix self-assembled from two poly(p_phenylene) chains under the inducement of metal nanoclusters

Molecular dynamics simulations demonstrate that metal nanoclusters (NCs) can induce two poly(p_phenylene) (PP) chains to self-assemble into regular double-helix through noncovalent interactions. In the process, the van der Waals (vdW) energy dominates the helically self-assembly of PP chains. The corresponding mechanism is revealed from the evolution of atomic conformations and the variation of total potential energy including its energy components. When adsorption energy of PP and NCs between 1.32 and 12.87 kcal/mol·Å, NCs can induce the helical self-assembly of two PP chains to form regular double-helix. The NC diameter and relative position of PP and NCs have a great influence on the formation of the double helix. Furthermore, the PP chain interacting with different number of Ni NCs are also studied.

Helical self-assembly and Fe3+ detection of V-shaped AIE-active chiral tetraphenylbutadiene-based polyamides.

Chiral aggregation-induced emission (AIE) molecules have drawn attention for their helical self-assembly and special optical properties. The helical self-assembly of AIE-active chiral non-linear main-chain polymers would produced some desired optical features. In this work, a series of V-shaped chiral AIE-active polyamides P1-C3, P1-C6, P1-C12 and linear P2-C3, P2-C6, bearing n-propyl/hexyl/dodecyl side-chains, based on tetraphenyl-butadiene (TPB) were prepared. All of the target main-chain polymers exhibit distinct AIE characteristics. The polymer P1-C6 with moderate length alkyl chains shows better AIE properties. The V-shaped main-chains and the chiral induction of (1R,2R)-(+)-1,2-cyclohexanediamine in each repeatingunit promote the polymer chains display helical conformations, and multiple helical polymer chains induce nano-fibers helicity when the polymer chains aggregate and self-assemble in THF/H2O mixtures. Simultaneously, the helical conformation polymer chains and helical nano-fibers cause P1-C6 produce strong CD signals with positive Cotton effect. Moreover, P1-C6 could also occur fluorescence quenching response to Fe3+ selectively with a low detection limit of 3.48 μmol/L.

Indolyl imine substituted BODIPY systems; synthesis, photophysical, and biological properties

In the study, 4,6-dimethoxy-2,3-diphenylindole-7-carbaldheyde 2, served as a valuable precursor for the reaction with both aniline and BODIPY-linked aniline, generated novel indolyl-imine Schiff base ligands 3 and 7. These indolyl-imine ligand systems were then employed to create the corresponding N–N six-membered boron complexes, 4 (precursor) and 8. The structure of novel compounds were confirmed by 1H and 13C NMR, and high-resolution mass spectrometry. The final evidences of indolyl-imine 7 and its corresponding boron complex 8 were further supported by the single X-ray crystal structures. Single crystal X-ray diffraction analysis revealed the formation of short “C–H⋯F”, “C–H···π”, “C–H⋯O” and “C–H⋯B” interactions, which led to the impressive supramolecular tubular-like (7) and helical (8) self-assemblies. The investigations of the UV–Vis absorption and fluorescence emission properties of the targeted compounds 4, 7 and 8 were carried out using absorption and fluorescence emission (2D and 3D) spectroscopic techniques. Fluorescence properties of the targeted compounds were found to be solvent-dependent. DFT optimized structures and the TD-DFT calculations were in alignment with the experimental studies. Additionally, the cytotoxic properties of the target compounds were examined in human breast cancer cells, MCF-7 in vitro as preliminary biological study.

Architecture-Controllable Single-Crystal Helical Self-assembly of Small-Molecule Disulfides with Dynamic Chirality.

Beyond the common supramolecular helical polymers in solutions, controlling single-crystal helical self-assembly with precisely defined chirality and architectures has been challenging. Here, we report that simply merging static homochiral amino acids with dynamic chiral disulfides can produce a class of building blocks featuring supramolecular helical single-crystal self-assembly with unusual stereodivergency. Analysis of 20 single-crystal structures of 1,2-dithiolanes gives an atom-precision understanding of the chirality transfer from the molecular to supramolecular level, featuring homochiral and heterochiral helical supramolecular self-assembly in the solid state. The underlying structure-assembly relationship reveals that the synergistic interplay of intermolecular H-bonds and the 1,2-dithiolane ring with adaptive chirality plays a key role in determining the assembly pathway, also involving the effects of residue groups, substituents, molecular stacking, and solvents. The confinement effect in the solid state can stabilize the dynamic stereochemistry of disulfide bonds and selectively result in specific conformers that can minimize the energy of global supramolecular systems. We envision that these results represent a starting point to use dynamic chiral disulfide as a functional entity in supramolecular chemistry and may inspire a new class of supramolecular helical polymers with dynamic functions.

Massive, soft, and tunable chiral photonic crystals for optical polarization manipulation and pulse modulation

Photonic crystals enable modulation of light waves in space, time, and frequency domains; in particular, chiral photonic crystals are uniquely suitable for polarization rotation and switching of complex vector fields. Current development of chiral photonic crystals, nevertheless, are still confronted with limitations of one form or the other such as large optical losses, limited or absence of tunability, narrow operation bandwidth, and/or insufficient optical thickness for practical implementation. In this work, we show that cholesteric liquid crystals as 1D tunable chiral photonic crystals are promising alternatives to not only address all these issues and deficiencies but also enable new photonic applications in wider temporal and spectral realms. Our work entails a detailed study of the dynamical evolution of cholesteric helical self-assembly and defect formation in the bulk of thick cholesteric liquid crystals under various applied electric field conditions and a thorough exploration of how applying fields of vastly different frequencies can eliminate and/or prevent the formation of unremovable defects and to control the alignment of cholesteric helices in the entire bulk. We have developed a dual-frequency field assembly technique that enables robust room-temperature fabrication of stable well-aligned cholesteric liquid crystals to unprecedented thickness (containing thousands of grating periods) demanded by many photonic applications. The resulting chiral photonic crystals exhibit useful much-sought-after capabilities impossible with other existing or developing chiral photonic crystals—compactness (single, flat, millimeter-thick optical element), high transmission, dynamic tunability, large polarization rotation, and various switching/modulation possibilities for ultrafast and continuous-wave lasers in the visible, near- and mid-infrared regimes.

Observation of helical self-assembly in cyclic triphosphazene-based columnar liquid crystals bearing chiral mesogenic units

Cyclotriphosphazene-based discotic liquid crystals are reported exhibiting helicoildal columnar arrangement and fluorescence turn-on phenomenon.

Helical Self-Assembly of Poly(para-phenylene) Chains Induced by Fullerenes

Molecular dynamics simulations show that two poly(para-phenylene) (PPP) chains can self-assemble helically to form a regular double-helix structure under the inducement of a fullerene molecule. The cross section of the PPP double helix shows a dumbbell-like shape consisting of two highly strained bulbs on two edges. The contribution of system energy and each energy component to the helical self-assembly is discussed, and the conditions and mechanism are explained. The fullerene diameter, PPP length, temperature, and relative position all have great influence on the helical self-assembly process. The thermal stability of the formed double helix is further tested. Multiple fullerenes, arranged in a string, can easily cause the helical self-assembly of two PPP chains. Three to six PPP chains have a certain probability of forming regular multiple helices under the inducement of fullerenes with an appreciate diameter. This work provides a new idea and theoretical basis for the controllable fabrication of regular helical polymers and related functional nanodevices.

The Effects of Lengths of Flavin Surfactant N-10-Alkyl Side Chains on Promoting Dispersion of a High-Purity and Diameter-Selective Single-Walled Nanotube.

Flavin with defined helical self-assembly helps to understand chemical designs for obtaining high-purity semiconducting (s)-single-walled carbon nanotubes (SWNT) in a diameter (dt)-selective manner for high-end applications. In this study, flavins containing 8, 12, 16, and 20 n-alkyl chains were synthesized, and their single/tandem effects on dt-selective s-SWNT dispersibility were investigated at isomolarity. Flavins with n-dodecyl and longer chain lengths (FC12, FC16, and FC20) act as good surfactants for stable SWNT dispersions whereas n-octyl flavin (FC8) exhibits poor dispersibility owing to the lack of SWNT buoyancy. When used with small-dt SWNT, FC8 displays chirality-selective SWNT dispersion. This behavior, along with various flavin helical motifs, prompts the development of criteria for ‘side chain length (lS)’ required for stable and dt-selective SWNT dispersion, which also explains lS-dependent dt-enrichment behavior. Moreover, SWNT dispersions with flavins with dodecyl and longer lS exhibit increased metallic (m)-SWNT, background absorption-contributing carbonaceous impurities (CIs) and preferential selectivity of s-SWNT with slightly larger dt. The increased CIs that affect the SWNT quantum yield were attributed to a solubility parameter. Furthermore, the effects of flavin lS, sonication bath temperature, centrifugal speed, and surfactant concentration on SWNT purity and s-/m-SWNT ratio were investigated. A tandem FC8/FC12 provides fine-tuning of dt-selective SWNT dispersion, wherein the FC8 ratio governs the tendency towards smaller dt. Kinetic and thermodynamic assemblies of tandem flavins result in different sorting behaviors in which wide dt-tunability was demonstrated using kinetic assembly. This study highlights the importance of appropriate side chain length and other extrinsic parameters to obtain dt-selective or high-purity s-SWNT.

Novel liquid crystals with circularly polarized luminescence induced by tartaric core

This work not only developed series of novel CPL molecules, but also confirmed that the method of designing liquid crystals with chiral core surrounded by peripheral achiral AIEgens was an effective strategy to construct the CPL materials. Three bis-cyanostilbene derivatives bridging by tartaric spacer (TC-5, TC-8 and TC-12) were prepared in yields of 81%-86%. They exhibited the orderly helical molecular stacking in mesophase and emitted the strong fluorescence at aggregated states. They possessed good CD and CPL properties based on the chiral transfer from tartrate core to peripheral cyanostilbene units. The orderly helical self-assembly in mesophase displayed the better CPL emission than the disorderly aggregation in solid films. The alkyl chains made important influence on CPL properties and TC-8 with octyl chain was the optimized structure for producing the best chiral transfer and the strongest CPL signal.

ß-Sheet Induced Helical Self-Assembly Structure Formation by Dityrosine Dipeptide: Crystallographic Evidence and Other Biophysical Studies

Self-assembled structures derived from short peptides are a versatile class of organic building blocks which have shown great potential in a wide range of domains. In the current study, side-chain protected dityrosine based short peptide (TP) was synthesized, and its conformation accompanied by a self-assembly pattern was investigated through several spectroscopic studies and single crystal X-ray analysis. The single crystal X-ray analysis of TP confirmed that it exhibited a ß-sheet pattern which further self-assembled to form ß-sheet-promoted helical architectures by various noncovalent interactions. To the best of our knowledge, this is the first crystallographic report of a side-chain protected dityrosine based short peptide adopting ß-sheet-promoted helical structures. Morphological analysis of TP also revealed ß-sheet as well as helical conformations. NMR study suggested that both amide hydrogens of TP are involved in intermolecular hydrogen bonding. Moreover, CD spectroscopy established the self-assembly phenomenon of TP in the solution state by showing both corresponding ß-sheet and α-helix bands. Hirshfeld surface analysis and DFT study also concluded similar results. These kinds of small peptide units mimicking important protein secondary structures like helical assembly would be of pivotal significance as they may act as small peptidomimetics, mimicking the protein "Hotspot" area.

Helical self-assembly of a mucin segment suggests an evolutionary origin for von Willebrand factor tubules

The glycoprotein von Willebrand factor (VWF) contributes to hemostasis by stanching injuries in blood vessel walls. A distinctive feature of VWF is its assembly into long, helical tubules in endothelial cells prior to secretion. When VWF is released into the bloodstream, these tubules unfurl to release linear polymers that bind subendothelial collagen at wound sites, recruit platelets, and initiate the clotting cascade. VWF evolved from gel-forming mucins, the polymeric glycoproteins that coat and protect exposed epithelia. Despite the divergent function of VWF in blood vessel repair, sequence conservation and shared domain organization imply that VWF retained key aspects of the mucin bioassembly mechanism. Here, we show using cryo-electron microscopy that the ability to form tubules, a property hitherto thought to have arisen as a VWF adaptation to the vasculature, is a feature of the amino-terminal region of mucin. This segment of the human intestinal gel-forming mucin (MUC2) was found to self-assemble into tubules with a striking resemblance to those of VWF itself. To facilitate a comparison, we determined the residue-resolution structure of tubules formed by the homologous segment of VWF. The structures of the MUC2 and VWF tubules revealed the flexible joints and the intermolecular interactions required for tubule formation. Steric constraints in full-length MUC2 suggest that linear filaments, a previously observed supramolecular assembly form, are more likely than tubules to be the physiological mucin storage intermediate. Nevertheless, MUC2 tubules indicate a possible evolutionary origin for VWF tubules and elucidate design principles present in mucins and VWF.

Transition from lamellar to nanostructure mesophases in azobenzene-based hockey-stick polycatenars

Tuning from 1D to 3D mesophases by alkyl chain engineering. Multichain π-conjugated hockey-stick molecules form lamellar SmA and meso-structure Ia3̄d with continuous networks. The effect of the position of the central bent-core unit on helical self-assembly is discussed.

Supramolecular helical self-assembly of small peptides

In this highlight, we describe the construction of supramolecular single/double/triple-helical assemblies from small di/tri/tetrapeptides and their applications.

Transformable Helical Self-Assembly for Cancerous Golgi Apparatus Disruption.

Golgi apparatus is a major subcellular organelle responsible for drug resistance. Golgi apparatus-targeted nanomechanical disruption provides an attractive approach for killing cancer cells by multimodal mechanism and avoiding drug resistance. Inspired by the poisonous twisted fibrils in Alzheimer's brain tissue and enhanced rigidity of helical structure in nature, we designed transformable peptide C6RVRRF4KY that can self-assemble into nontoxic nanoparticles in aqueous medium but transformed into left-handed helical fibrils (L-HFs) after targeting and furin cleavage in the Golgi apparatus of cancer cells. The L-HFs can mechanically disrupt the Golgi apparatus membrane, resulting in inhibition of cytokine secretion, collapse of the cellular structure, and eventually death of cancer cells. Repeated stimulation of the cancers by the precursors causes no acquired drug resistance, showing that mechanical disruption of subcellular organelle is an excellent strategy for cancer therapy without drug resistance. This nanomechanical disruption concept should also be applicable to multidrug-resistant bacteria and viruses.

Distance Matters: Biasing Mechanism, Transfer of Asymmetry, and Stereomutation in N-Annulated Perylene Bisimide Supramolecular Polymers.

The synthesis of two series of N-annulated perylene bisimides (PBIs), compounds 1 and 2, is reported, and their self-assembling features are thoroughly investigated by a complete set of spectroscopic measurements and theoretical calculations. The study corroborates the enormous influence that the distance between the PBI core and the peripheral groups exerts on the chiroptical properties and the supramolecular polymerization mechanism. Compounds 1, with the peripheral groups separated from the central PBI core by two methylenes and an ester group, form J-type supramolecular polymers in a cooperative manner but exhibit negligible chiroptical properties. The lack of clear helicity, due to the staircase arrangement of the self-assembling units in the aggregate, justifies these features. In contrast, attaching the peripheral groups directly to the N-annulated PBI core drastically changes the self-assembling properties of compounds 2, which form H-type aggregates following an isodesmic mechanism. These H-type aggregates show a strong aggregation-caused quenching (ACQ) effect that leads to nonemissive aggregates. Chiral (S)-2 and (R)-2 experience an efficient transfer of asymmetry to afford P- and M-type aggregates, respectively, although no amplification of asymmetry is achieved in majority rules or “sergeants-and-soldiers” experiments. A solvent-controlled stereomutation is observed for chiral (S)-2 and (R)-2, which form helical supramolecular polymers of different handedness depending on the solvent (methylcyclohexane or toluene). The stereomutation is accounted for by considering the two possible conformations of the terminal phenyl groups, eclipsed or staggered, which lead to linear or helical self-assemblies, respectively, with different relative stabilities depending on the solvent.

Axially chiral propeller-shaped spiroborates with aggregation-modulated circularly polarized luminescence

Modulating CPL with large luminescent dissymmetry factors and reversal signals is a great challenge in developing chiroptical small organic molecules (SOMs) materials. We herein report a series of axially chiral propeller-shaped pyridyl-/benzothiazolyl-enolato-naphtholate spiroborates ((R)-/(S)-2a-2d). Of these spiroborates, the CPL signals of (R)-/(S)-2c-2d could be effectively modulated by increasing water fraction (fw) from 50% to 90% in the THF-H2O solution. (R)-/(S)-2c exhibit aggregation-amplified CPL with the dissymmetry factor (glum) up to 2.38 × 10−2 and (R)-/(S)-2d show aggregation-induced CPL. In addition, the CPL signals of (R)-/(S)-2c exhibit reversible inversion with the fw increasing from 50% to 90%. The amplification and inversion of the CPL signals are revealed via the helical self-assemblies observed by scanning electron microscopy (SEM) images and the promotion of the population of emissive intramolecular charge transfer (ICT) state.

Light-Click In Situ Self-Assembly of Superhelical Nanofibers and Their Helicity Hierarchy Control

In situ self-assembly has emerged as a powerful technology in preparation of nanoparticles owing to its solution-processing-free advantage and scalable production. This new technology allows us to facilely obtain a wide range of nanoobjects of architecture and dimension, including spheres, cylinders, vesicles, and tubes; however, it has limitation in the construction of sophisticated superstructures such as helix or superhelix. Here, we describe a new in situ self-assembly method, light-click in situ self-assembly, which can induce the concurrence of polymer coupling and self-assembly through photoclick chemistry. We use this technique to the direct synthesis of helical assemblies. Using two clickable molecules as the reactive precursors, their photoligation can synchronize with their self-assembly process and in situ fabricate ergodic helical nanostructures with chiral bias, evolving from nonhelical filaments to single-stranded helical fibrils to multistranded helical fibrils to complex superhelical fibers. Such helically hierarchical shape evolution is dependent on the photoreaction extent. Quantifying the relationship of reaction conversion and resulting morphology can establish a full phase diagram of helical self-assembly. Moreover, utilizing this strategy, one can on-demand pause and resume the assembly process by switching on and off the light stimulus in order to achieve temporal control of the helicity hierarchy.