Helical Superstructures Research Articles

Transfer, Amplification, Storage, and Complete Self-Recovery of Supramolecular Chirality in an Achiral Polymer System.

Supramolecular chirality and its complete self-recovery ability are highly mystical in nature and biological systems, which remains a major challenge today. Herein, we demonstrate that partially cross-linked azobenzene (Azo) units can be employed as the potential chiral trigger to fully heal the destroyed helical superstructure in achiral nematic polymer system. Combining the self-assembly of Azo units and terminal hydroxyl groups in polymer side chains allows the vapor-induced chiral nematic phase and covalent fixation of the superstructure via acetal reaction. The induced helical structure of Azo units can be stored by inter-chain cross-linking, even after removal of the chiral source. Most interestingly, the stored chiral information can trigger perfect chiral self-recovery (CSR) behavior after being destroyed by UV light, heat, and solvents. The results pave a new way for producing novel chiroptical materials with reversible chirality from achiral sources.

Spontaneous chiral self-assembly of CdSe@CdS nanorods

Spontaneous chiral self-assembly of CdSe@CdS nanorods

Self-assembled helical superstructures of polystyrene-b-poly(2-vinyl pyridine) with inversed helicity from induced chirality

This work aimed to examine the self-assembling behaviors of polystyrene-b-poly(2-vinyl pyridine) (PS-P2VP) complexed with chiral dopants, R-/S-mandelic acid (R-/S-MA). Induced vibrational circular dichroism (VCD) due to intra-chain chiral interaction suggests the formation of helical P2VP with preferred handedness by complexation with R-/S-MA, demonstrating the induced circular dichroism (ICD) behaviors for the C-C single bond without high-degree tacticity. Owing to the microphase separation, helical superstructures with controlled helicity could be formed from the self-assembly of PS-P2VP with ICD. Most interestingly, the self-assembled helical ribbon exhibits reversed helical sense to the helical P2VP, as evidenced by induced VCD through inter-chain chiral interaction and rationalized using cholesteric hard-core model due to a large value of P/d ratio.

Molecular Triangles: A New Class of Macrocycles

ConspectusMacrocycles have had a profound influence on the establishment of supramolecular chemistry because of their abundant molecular recognition and self-assembly characteristics. The design of new macrocyclic receptors that can be tailored by synthesis to display new and exotic properties is an important research objective for chemists and materials scientists. Rigid macrocycles with π-conjugated aromatic units, in contrast with flexible ones, tend to possess large interior and exterior π-surfaces in addition to persistent shapes. These features not only endow these macrocycles with a wide range of host-guest properties but also render them ideal building blocks for constructing a diverse variety of supramolecular architectures. The incorporation of π-conjugated units into macrocycles also imbues them with a wealth of optical, electronic, and magnetic properties, resulting in their broad application in materials science and molecular nanotechnology.Recently, we have designed and synthesized a new class of macrocycles, namely, molecular triangles, which have rigid structures with triangular geometries. They consist of three chiral trans-1,2-cyclohexano apexes and three aromatic tetracarboxylic diimide linkers, such as pyromellitic diimide, naphthalene diimide, and perylene diimide. Benefiting from the availability of facile synthetic protocols, the geometries and properties of these rigid molecular triangles can be altered at will. By combining these tetracarboxylic diimide linkers, we have been able to synthesize both molecular equilateral and isosceles triangles. During the past few years, we have conducted research in a systematic manner on the structural features and self-assembly characteristics of these molecular triangles. The following points are worthy of note regarding these molecular triangles: (i) They possess shape-persistent inner cavities of a highly electron-deficient nature. These features endow them with the ability to complex with anions and electron-rich molecules, forming supramolecular nanotubes and two-dimensional tilings. (ii) Those with intrinsic chirality are able to self-assemble into solid-state nonhelical or single-handed helical superstructures, inducing selective chirality transfer from the macrocycles to their crystalline supramolecular assemblies. (iii) The triangular arrangement of aromatic tetracarboxylic diimide linkers contributes to through-space electron delocalization encompassing the entire macrocycle, conferring exotic electronic and spin properties. To date, the family of molecular triangles has exhibited a range of physicochemical properties, such as anion recognition, chiral assembly, supramolecular gelation, energy storage, solid-state luminescence, and nonlinear optical response.In this Account, we summarize our recent progress in research into these molecular triangles. We present an overview of their design and synthesis, as well as a general summary of their structural features. Thereafter, we discuss state-of-the-art developments in relation to their molecular recognition properties and their assembly characteristics. In addition, we highlight the potential applications of these molecular triangles and their complexes with a range of solvents and electron-rich molecules. Finally, we speculate on further structural modifications and application-oriented explorations based on this class of molecular triangles.

Single-molecule mechanics of synthetic aromatic amide helices: Ultrafast and robust non-dissipative winding

Summary Because of proteins’ many degrees of conformational freedom, programming protein folding dynamics, overall elasticity, and motor functions remains an elusive objective. Instead, smaller and simpler objects, such as synthetic foldamers, may be amenable to design. However, little is known about their mechanical performance. Here, we show that reducing molecular size may not compromise mechanical properties. We report that helical aromatic oligoamides as small as 1 nm possess outstanding elasticity and outperform most natural helices. Using single-molecule force spectroscopy, we characterize their folding trajectories and intermediate states. We show that they cooperatively and reversibly unwind at high forces. They extend up to 3.8 times their original length and rewind against considerable forces on a timescale of 10 μs. Pulling and relaxing cycles follow the same trace up to a very high loading rate, indicating that the mechanical energy accumulated during the stretching does not dissipate and is immediately reusable.

Geminate labels programmed by two-tone microdroplets combining structural and fluorescent color

Creating a security label that carries entirely distinct information in reflective and fluorescent states would enhance anti-counterfeiting levels to deter counterfeits ranging from currencies to pharmaceuticals, but has proven extremely challenging. Efforts to tune the reflection color of luminescent materials by modifying inherent chemical structures remain outweighed by substantial trade-offs in fluorescence properties, and vice versa, which destroys the information integrity of labels in either reflection or fluorescent color. Here, a strategy is reported to design geminate labels by programming fluorescent cholesteric liquid crystal microdroplets (two-tone inks), where the luminescent material is ‘coated’ with the structural color from helical superstructures. These structurally defined microdroplets fabricated by a capillary microfluidic technique contribute to different but intact messages of both reflective and fluorescent patterns in the geminate labels. Such two-tone inks have enormous potential to provide a platform for encryption and protection of valuable authentic information in anti-counterfeiting technology.

Local Crystallinity in Twisted Cellulose Nanofibers.

Cellulose is crystallized by plants and other organisms into fibrous nanocrystals. The mechanical properties of these nanofibers and the formation of helical superstructures with energy dissipating and adaptive optical properties depend on the ordering of polysaccharide chains within these nanocrystals, which is typically measured in bulk average. Direct measurement of the local polysaccharide chain arrangement has been elusive. In this study, we use the emerging technique of scanning electron diffraction to probe the packing of polysaccharide chains across cellulose nanofibers and to reveal local ordering of the chains in twisting sections of the nanofibers. We then use atomic force microscopy to shed light on the size dependence of the inherent driving force for cellulose nanofiber twisting. The direct measurement of crystalline twisted regions in cellulose nanofibers has important implications for understanding single-cellulose-fibril properties that influence the interactions between cellulose nanocrystals in dense assemblies. This understanding may enable cellulose extraction and separation processes to be tailored and optimized.

Dielectric Thermal Smart Glass Based on Tunable Helical Polymer-Based Superstructure for Biosensor with Antibacterial Property.

A dielectric thermal smart glass (DTSG) based on the dielectric heating optical (DHO) effect in tunable helical polymer-based superstructures—cholesteric liquid crystals (CLCs)—was exhibited in this study. Field-induced dielectric heating can strongly affect the orientation of liquid crystals and change its optical properties. The purpose of this research focuses on dual-frequency CLC materials characterized by their specific properties on dielectric relaxation and demonstrates their potential for antibacterial biosensor applications. The developed DTSG is driven by voltages with modulated frequencies. The principal of DTSG in transparent states are a planar (P) state and a heated planar (HP) state reflecting infrared light, operated with the voltage at low and high frequencies, respectively. The scattering states are a focal conic (FC) state and a heated FC (HFC) state, with an applied frequency near the crossover frequency. The biomolecule detection of the antibacterial property was also demonstrated. The detection limitation of the DTSG biosensor was found to be about 0.5 µg/mL. The DTSG material has many potential industrial applications, such as in buildings, photonic devices, and biosensor applications.

Optical Techniques in the Determination of Pitch Lengths in the Cholesteric and Chiral Smectic C Phases

There is no denying that the future of liquid crystals is in the display industry. The use of the twisted nematic (TN) and the superwisted nematic (STN) is widespread. The twisting in these devices is achieved either by use of optical components such as polarizers or the use of chiral dopants in a guest-host system. The later has gained much more attention. This is because not only are the accompanying optical components not needed, but any desired colour can be achieved by varying the pitch length of the helix formed by the chiral nematic (cholesteric) compound. In such application, the ability to determine the pitches of the resulting helical superstructure accurately and quickly is essential. Currently there are many methods for measuring the pitch of a helical super structure. These methods mostly employ optical or spectroscopic techniques. In this paper, four such methods namely, Fingerprint, Diffraction, Selective Reflection and Cano-wedge, are discussed in terms of their theoretical foundation, suitability to given situations and the validity of the results obtained. The suitability of the method depends among other things on the type of mesophase and the length of the pitch in relation to the wavelengths of the visible light. While the Fingerprint and Selective reflection may be used for both cholesteric and chiral smectic C (SmC*), Diffraction and Cano techniques are most suitable for the cholesterics.

Photo-tunable epsilon-near-zero behavior in a self-assembled liquid crystal - nanoparticle hybrid material.

Dynamic tuning of electromagnetic response is an important parameter to realize exotic applications of optical metamaterials. Self-assembly achieved via the incorporation of soft materials is an attractive approach to achieve tunable optical properties. Among the soft materials, liquid crystals are highly sought after due to the inherent soft-stimuli responsiveness. This article reports experimental evidence of tunable epsilon-near-zero (ENZ) behavior brought about by an optical field in a self-assembled liquid crystal – nanoparticle system. The material consists of Au nanoparticles capped with a photo-active chiral liquid crystal ligand. In the liquid crystalline state, the system self-assembles into a helical lamellar superstructure, confirmed by polarizing optical microscopy, HRTEM, XRD, and circular dichroism studies. Upon irradiation with UV light, the localized surface plasmon resonance peak of Au red-shifts by ∼10 nm and gets restored with white light illumination. The effective permittivity of the system obtained from ellipsometry indicates ENZ behavior in the visible spectrum with a bandwidth of ∼45 nm which gets enhanced by a factor of 1.6 on UV illumination. Theoretical calculations, carried out using the effective medium approach, support the experimental findings, making the system an efficient ENZ metamaterial in the optical regime.

Chiral plasmonic liquid crystal gold nanoparticles: self-assembly into a circular dichroism responsive helical lamellar superstructure.

Owing to their proven and promising potential in various technological endeavors ranging from catalysis and sensing to invisibility cloaks made from metamaterials, chiral plasmonic superstructures resulting from the directed self-assembly of optically active metal nanoparticles (MNPs) have been pursued intensively in recent years. Several strategic efforts have emerged especially to accomplish advanced nanomaterials assembling into liquid crystalline (LC) helical structures, where MNPs are regularly packed in fluid/frozen arrays/layers or wires (columns). While the helical fluid columnar arrays (molecular wires) showing circular dichroism (CD) have been realized, the discovery of fluid chiral lamellar ordering, where the dielectric and conducting regimes are arranged alternatively, has hitherto remained highly elusive. Herein we report the first examples of monodisperse LC-gold NPs (LC-GNPs) self-assembling into a fluid/frozen lamellar structure exhibiting CD activity. Notably, these new, exceptional LC-GNPs have been realized by simple, hassle-free protocols that involve the room temperature addition of LC dimer-like arylamines to Au(iii), where the amines not only reduce Au(iii) to Au(0) but also bind strongly to the central GNP scaffold. Their molecular structure, mesomorphism, and ability to interact with circularly polarized light have been evidenced unambiguously and could play an important role in realizing metamaterials in the visible region.

Induction, fixation and recovery of self-organized helical superstructures in achiral liquid crystalline polymer

A flexible chiral storage based on an achiral polymer system can be successfully achieved by chiral doping and covalent cross-linking.

Multi-Modal Chiral Superstructures in Self-Assembled Anthracene-Terminal Amino Acids with Predictable and Adjustable Chiroptical Activities and Color Evolution.

Deep understanding of structure-property relationship between packing of chiral building units and their chiroptical behaviors would significantly facilitate the rational design and fabrication of the emerging chiroptical materials such as circularly polarized luminescence (CPL) emissive materials. In this paper, we unveil the universal existence of supramolecular tilt helical superstructures in self-assembled π-conjugated amino acid derivatives. A series of coded amino acid methyl esters were conjugated to anthracene segments at N-terminus, which afforded 21 and 31 symmetry supramolecular tilt chirality in solid-states. Helical assemblies enabled diversified Cotton effects and CPL performances, which were in accordance with the tilted chirality between anthracene segments. Such correlation shows fine universality, whereby the chiroptical prediction could be realized. Furthermore, on top of charge-transfer complexation, manipulation of CPL emission colors and handedness were realized.

Multi‐Modal Chiral Superstructures in Self‐Assembled Anthracene‐Terminal Amino Acids with Predictable and Adjustable Chiroptical Activities and Color Evolution

AbstractDeep understanding of structure–property relationship between packing of chiral building units and their chiroptical behaviors would significantly facilitate the rational design and fabrication of the emerging chiroptical materials such as circularly polarized luminescence (CPL) emissive materials. In this paper, we unveil the universal existence of supramolecular tilt helical superstructures in self‐assembled π‐conjugated amino acid derivatives. A series of coded amino acid methyl esters were conjugated to anthracene segments at N‐terminus, which afforded 21 and 31 symmetry supramolecular tilt chirality in solid‐states. Helical assemblies enabled diversified Cotton effects and CPL performances, which were in accordance with the tilted chirality between anthracene segments. Such correlation shows fine universality, whereby the chiroptical prediction could be realized. Furthermore, on top of charge‐transfer complexation, manipulation of CPL emission colors and handedness were realized.

π-Extended Diaza[7]helicenes by Hybridization of Naphthalene Diimides and Hexa-peri-hexabenzocoronenes.

The synthesis of an unprecedented, π‐extended hexabenzocorene (HBC)‐based diaza[7]helicene is presented. The target compound was synthesized by an ortho‐fusion of two naphthalene diimide (NDI) units to a HBC‐skeleton. A combination of Diels–Alder and Scholl‐type oxidation reactions involving a symmetric di‐NDI‐tolane precursor were crucial for the very selective formation of the helical superstructure via a hexaphenyl‐benzene (HPB) derivative. The formation of the diaza[7]helicene moiety in the final Scholl oxidation is favoured, affording the symmetric π‐extended helicene as the major product as a pair of enantiomers. The separation of the enantiomers was successfully accomplished by HPLC involving a chiral stationary phase. The absolute configuration of the enantiomers was assigned by comparison of circular dichroism spectra with quantum mechanical calculations.

Electro- and Photo-Driven Orthogonal Switching of a Helical Superstructure Enabled by an Axially Chiral Molecular Switch

Self-organized functional soft materials, enabled by specific chemical architectures, are currently attracting tremendous attention because of their stimuli-responsive attributes and applications in advanced technological devices. A novel axially chiral molecular switch containing two azo linkages and six terminal alkyl chains on two elongated rod-shaped wings, that exhibits superior solubility, high helical twisting power, and reversible photoisomerization in an achiral liquid crystal host, is synthesized and utilized in the development of a photoresponsive, self-organized helical superstructure, that is, cholesteric liquid crystal (CLC). The planar CLC adopts a standing helix (SH) configuration because of surface alignment layers on the substrates. This SH can be transitioned to a lying helix configuration, enabling tunable diffraction gratings under the application of electric field. Adjustment of the initial pitch of the planar CLC by photoirradiation yields the diffraction gratings with stripes either parallel or perpendicular to the rubbing direction upon the application of an appropriate electric field. Tunable beam steering along orthogonal directions has been demonstrated. Such tunable stimuli-responsive soft materials fabricated with artificial chiral switches show great potential in optics, photonics, and beyond.

Glycomacrocycle-Based Azobenzene Derivatives as Chiral Dopants for Photoresponsive Cholesteric Liquid Crystals.

We demonstrate photoresponsive cholesteric liquid crystals (CLCs) doped with glycomacrocyclic azobenzene derivatives, which exhibit large conformational changes, providing dynamic control of helical superstructures in response to a light stimulus. An unprecedented shortening of the helical pitch length and the empowerment of helical twisting power up to 500% are observed upon trans (E) to cis (Z) photoisomerization. Light-driven dynamic helix twisting and untwisting behavior affords the first example of glycomacrocyclic azobenzene-based CLCs, which can drive the mechanical movement of micro-objects. Two modes of rotation-two-directional or one-directional rotational motion (crankshaft mode)-are realized. In particular, the latter mode based on the reversible cholesteric texture transition between homogeneous stripes and focal conics leads to the accumulation of the rotation angles achieving the amplified mechanical movements.

Sunlight‐Driven Self‐Organized Helical Superstructure Chromotropic Device

AbstractSunlight‐driven chromotropic devices have attracted much attention and been intensively studied in recent years because of their applications in the fields of soft robotics, camouflage, and anti‐counterfeiting. Herein, a sunlight‐driven helical superstructure chromotropic device is reported via doping a photo‐responsive molecular motor molecule into the nematic liquid crystal host to form cholesteric liquid crystals (CLCs). In designed CLC chromotropic devices, the color changing range is capable of covering the visible region upon exposure to sunlight of different intensities. The photo‐responsive behaviors of the CLC chromotropic devices, including initial state, photo‐stable state, and transforming mode can be accurately controlled by manipulating their composition. Moreover, it is feasible to mass produce CLC chromotropic devices in various forms from rigid to soft and from film to fiber. They are demonstrated as smart “skins” capable of imitating the colors of surrounding environments. Their rewritable feature under sunlight offers a promising platform for designing and fabricating advanced multifunctional devices for real‐world applications.

Spontaneous Resolution of Chiral Multi-Thiolate-Protected Ag30 Nanoclusters.

Despite significant progress achieved in the preparation of chiral nanoparticles, the enantioseparation of racemates still presents a big challenge in nanomaterial research. Herein, we report the synthesis and structural characterization of racemic anisotropic nanocluster Ag30(C2B10H9S3)8Dppm6 (Ag30-rac), which is protected by mixed carboranetrithiolate and phosphine ligands. Spontaneous self-resolution of the racemates was realized through conglomerate crystallization in dimethylacetamide (DMAc). The homochiral nanoclusters in the racemic conglomerates adopt enantiomeric helical self-assemblies (R/L-Ag30). Diverse noncovalent interactions as the driving force in directing superstructure packing were elucidated in detail. Furthermore, the nanoclusters show red luminescence in both solid and solution states, and the racemic conglomerates display a mirror-image CPL response. This work provides atom-precise helical nanoparticle superstructures that facilitate an in-depth understanding of the helical-assembly mechanism.

Liquid Thermo-Responsive Smart Window Derived from Hydrogel

Summary Buildings account for 40% of global energy consumption, while windows are the least energy-efficient part of buildings. Conventional smart windows only regulate solar transmission. For the first time, we developed high thermal energy storage thermo-responsive smart window (HTEST smart window) by trapping the hydrogel-derived liquid within glasses. The excellent thermo-responsive optical property (90% of luminous transmittance and 68.1% solar modulation) together with outstanding specific heat capacity of liquid gives the HTEST smart window excellent energy conservation performance. Simulations suggested that HTEST window can cut off 44.6% heating, ventilation, and air-conditioning (HVAC) energy consumption compared with the normal glass in Singapore. In outdoor demonstrations, the HTEST smart window showed promising energy-saving performance in summer daytime. Compared with conventional energy-saving glasses, which need expensive equipment, the thermo-responsive liquid-trapped structure offers a disruptive strategy of easy fabrication, good uniformity, and scalability, together with soundproof functionality that opens an avenue for energy-saving buildings and greenhouses.