Modular Molecular Self‐Assembly for Diversified Chiroptical Systems

Bottom‐up multicomponent molecular self‐assembly is an efficient approach to fabricate and manipulate chiral nanostructures and their chiroptical activities such as the Cotton effect and circular polarized luminescence (CPL). However, the integrated coassembly suffers from spontaneous and inherent systematic pathway complexity with low yield and poor fidelity. Consequently, a rational design of chiral self‐assembled systems with more than two components remains a significant challenge. Herein, a modularized, ternary molecular self‐assembly strategy that generates chiroptically active materials at diverse hierarchical levels is reported. N‐terminated aromatic amino acids appended with binding sites for charge transfer and multiple hydrogen bonds undergo the evolution of supramolecular chirality with unique handedness and luminescent color, generating abundant CPL emission with high luminescence dissymmetry factor values in precisely controlled modalities. Ternary coassembly facilitates high‐water‐content hydrogel formation constituted by super‐helical nanostructures, demonstrating a helix to toroid topological transition. This discovery would shed light on developing complicated multicomponent systems in mimicking biological coassembly events.     

Assembled Plasmonic Asymmetric Heterodimers with Tailorable Chiroptical Response

Directed nanocrystal (NC) heteroassemblies could potentially achieve tailorable multiplex circular dichroism (CD) bands. Here, for the first time, we developed assembly of nanoparticle (NP)‐nanorod (NR) chiral heterodimers with chiral molecules to explore their chiroptical activities. The experimental results revealed that plasmonic CD responses were in the region from 520 to 750 nm, which was in agreement with the theoretical simulation. Importantly, the CD band could be regulated by controlling the gaps between adjacent NCs and altering the building blocks of the assemblies. These results show that the plasmonic chiroptical response of NP‐NR heterodimers could come from the finger‐crossed chiral construction of adjacent NC in the heterodimers and the formation of plasmonic hot‐spots in the assemblies could further enhance the plasmonic CD. This work provides a new opportunity to create heterogeneous nanoscale plasmonic objects with tailorable chiroptical response for application in biosensors, in vivo chiral medical carriers and negative refractive index materials.     

Active Self‐Assembly of Train‐Shaped DNA Nanostructures via Catalytic Hairpin Assembly Reactions

Biomolecular self‐assembly is a powerful approach for fabricating supramolecular architectures. Over the past decade, a myriad of biomolecular assemblies, such as self‐assembly proteins, lipids, and DNA nanostructures, have been used in a wide range of applications, from nano‐optics to nanoelectronics and drug delivery. The method of controlling when and where the self‐assembly starts is essential for assembly dynamics and functionalization. Here, train‐shaped DNA nanostructures are actively self‐assembled using DNA tiles as artificial “carriages,” hairpin structures as “couplers,” and initiators of catalytic hairpin assembly (CHA) reactions as “wrenches.” The initiator wrench can selectively open the hairpin couplers to couple the DNA tile carriages with high product yield. As such, DNA nanotrains are actively prepared with two, three, four, or more carriages. Furthermore, by flexibly modifying the carriages with “biotin seats” (biotin‐modified DNA tiles), streptavidin “passengers” are precisely arranged in corresponding seats. The applications of the CHA‐triggered self‐assembly mechanism are also extended for assembling the large DNA origami dimer. With the creation of 1D architectures established, it is thought that this CHA‐triggered self‐assembly mechanism may provide a new element of control for complex autonomous assemblies from a variety of starting materials with specific sites and times.     

Unconventional Nanofabrication for Supramolecular Electronics

The scientific effort toward achieving a full control over the correlation between structure and function in organic and polymer electronics has prompted the use of supramolecular interactions to drive the formation of highly ordered functional assemblies, which have been integrated into real devices. In the resulting field of supramolecular electronics, self‐assembly of organic semiconducting materials constitutes a powerful tool to generate low‐dimensional and crystalline functional architectures. These include 1D nanostructures (nanoribbons, nanotubes, and nanowires) and 2D molecular crystals with tuneable and unique optical, electronic, and mechanical properties. Optimizing the (opto)electronic properties of organic semiconducting materials is imperative to harness such supramolecular structures as active components for supramolecular electronics. However, their integration in real devices currently represents a significant challenge to the advancement of (opto)electronics. Here, an overview of the unconventional nanofabrication techniques and device configurations to enable supramolecular electronics to become a real technology is provided. A particular focus is put on how single and multiple supramolecular fibers and gels as well as supramolecularly engineered 2D materials can be integrated into novel vertical or horizontal junctions to realize flexible and high‐density multifunctional transistors, photodetectors, and memristors, exhibiting a set of new properties and excelling in their performances.     

Reconfigurable Chiral Self‐Assembly of Peptides through Control of Terminal Charges

Self‐assembly of chiral nanostructures is of considerable interest, since the ability to control the chirality of these structures has direct ramifications in biology and materials science. A new approach to design chiral nanostructures from self‐assembly of N‐(9‐fluorenylmethoxycarbonyl)‐protected phenylalanine‐tryptophan‐lysine tripeptides is reported. The terminal charges can induce helical twisting of the assembled β‐sheets, enabling the formation of well‐defined chiral nanostructures. The degree and direction of twisting in the β‐sheets can be precisely tailored through in situ pH and temperature modulations. This enables the assembly of reconfigurable chiral nanomaterials with easily adjustable size and handedness. These results offer new insight into the mechanism of helical twist formation, which may enable the precise assembly of highly dynamical materials with potential applications in biomedicine, chiroptics, and chiral sensing.     

The Role of Surface Groups in Dictating the Chiral-Solvent-Induced Assembly of Carbon Dots into Structures Exhibiting Circularly Polarized Luminescence

Here, the use of achiral nanoparticles and solvent-induced chirality transfer is combined for the making of large structures exhibiting chiroptical properties in the form of circularly polarized luminescence (CPL). The nanoparticles that the authors use are carbon dots (C-Dots) that are known for their bright luminescence and the ability to tune their surface moieties by using different precursors in their synthesis. Here, the result of adding the chiral solvent limonene into an aqueous solution of various C-Dots is explored, differentiated by their surface group. It is shown that only nitrogen-containing C-Dots with amine functional groups see the emergence of a CPL signal and the formation of a large fibrillar assembled structure. The various forces happening in the interface between the C-Dots and the limonene phase and the role of the amine groups in both the chirality transfer interactions and the interactions between C-Dots in the assembly process are discussed, whereas these two processes intertwine with each other. The ability to form fluorescent chiral structures exhibiting CPL from achiral nanoparticles and the understanding of the various interactions in this process are both important to the rationale design of any supramolecular chiral assemblies.     

Tuning the Amphiphilicity of Building Blocks: Controlled Self‐Assembly and Disassembly for Functional Supramolecular Materials

Amphiphilicity is one of the molecular bases for self‐assembly. By tuning the amphiphilicity of building blocks, controllable self‐assembly can be realized. This article reviews different routes for tuning amphiphilicity and discusses different possibilities for self‐assembly and disassembly in a controlled manner. In general, this includes irreversible and reversible routes. The irreversible routes concern irreversible reactions taking place on the building blocks and changing their molecular amphiphilicity. The building blocks are then able to self‐assemble to form different supramolecular structures, but cannot remain stable upon loss of amphiphilicity. Compared to the irreversible routes, the reversible routes are more attractive due to the good control over the assembly and disassembly of the supramolecular structure formed via tuning of the amphiphilicity. These routes involve reversible chemical reactions and supramolecular approaches, and different external stimuli can be used to trigger reversible changes of amphiphilicity, including light, redox, pH, and enzymes. It is anticipated that this line of research can lead to the fabrication of new functional supramolecular assemblies and materials.     

Supramolecular Helices: Chirality Transfer from Conjugated Molecules to Structures

Different scales of chirality endow a material with many excellent properties and potential applications. In this review, using π‐conjugated molecules as functional building blocks, recent progress on supramolecular helices inspired by biological helicity is summarized. First, induced chirality on conjugated polymers and small molecules is introduced. Molecular chirality can be amplified to nanostructures, superstructures, and even macroscopic structures by a self‐assembly process. Then, the principles for tuning the helicity of supramolecular chirality, as well as formation of helical heterojunctions, are summarized. Finally, the potential applications of chiral structures in chiral sensing and organic electronic devices are critically reviewed. Due to recent progress in chiral structures, an interdisciplinary area called “chiral electronics” is expected to gain wide popularity in the near future.     

Ultra-stable silica-coated chiral Au-nanorod assemblies: Core–shell nanostructures with enhanced chiroptical properties

Chiral nano-assemblies with amplified optical activity have attracted particular interest for their potential application in photonics, sensing and catalysis. Yet it still remains a great challenge to realize their real applications because of the instability of these assembled nanostructures. Herein, we demonstrate a facile and efficient method to fabricate ultra-stable chiral nanostructures with strong chiroptical properties. In these novel chiral nanostructures, side-by-side assembly of chiral cysteine-modified gold nanorods serves as the core while mesoporous silica acts as the shell. The chiral core–shell nanostructures exhibit an evident plasmonic circular dichroism (CD) response originating from the chiral core. Impressively, such plasmonic CD signals can be easily manipulated by changing the number as well as the aspect ratio of Au nanorods in the assemblies located at the core. In addition, because of the stabilization effect of silica shells, the chiroptical performance of these core–shell nanostructures is significantly improved in different chemical environments.

Open image in new window

     

Supramolecular Chiral 2D Materials and Emerging Functions

Chiral materials are widely applied in various fields such as enantiomeric separation, asymmetric catalysis, and chiroptical effects, providing stereospecific conditions and environments. Supramolecular concepts to create the chiral materials can provide an insight for emerging chiro-optical properties due to their well-defined scaffolds and the precise functionalization of the surfaces or skeletons. Among the various supramolecular chiral structures, 2D chiral sheet structures are particularly interesting materials because of their extremely high surface area coupled with many unique chemical and physical properties, thereby offering potential for the next generation of functional materials for optically active systems and optoelectronic devices. Nevertheless, relatively limited examples for 2D chiral materials exhibiting specific functionality have been reported because incorporation of molecular chirality into 2D architectures is difficult at the present stage. Here, a brief overview of the recent advances is provided on the construction of chiral supramolecular 2D materials and their functions. The design principles toward 2D chirality and their potential applications are also discussed.     

Achieving chiral resolution in self-assembled supramolecular structures through kinetic pathways

Chiral phase transitions were studied in a self-assembled 2,6-dibromoanthraquinones supramolecular system prepared on Au(111) using scanning tunneling microscopy. As the molecules were deposited at about 150?K, they formed heterochiral chevron structures (a racemate) consisting of two alternating prochiral molecular rows. When the as-deposited sample was warmed to 300?K followed by cooling to 80?K, phase-separated homochiral structures (a conglomerate), as well as the chevron structures, were observed. We propose molecular models for the structures that are in good agreement with ab?initio studies and can be explained by hydrogen bonds and halogen bonds. We found that heterochiral chevron structures were more stable than homochiral structures due to two additional [Formula: see text] halogen bonds per molecule. We considered kinetic pathways for the phase transitions that were made possible via a disordered liquid phase entropically stabilized at 300?K. We show how chiral resolution can be achieved by exploiting kinetic paths allowed in supramolecular systems.     

Chirality and Chiroptical Effects in Plasmonic Nanostructures: Fundamentals,Recent Progress,and Outlook

Strong chiroptical effects recently reported result from the interaction of light with chiral plasmonic nanostructures. Such nanostructures can be used to enhance the chiroptical response of chiral molecules and could also significantly increase the enantiomeric excess of direct asymmetric synthesis and catalysis. Moreover, in optical metamaterials, chirality leads to negative refractive index and all the promising applications thereof. In this Progress Report, we highlight four different strategies which have been used to achieve giant chiroptical effects in chiral nanostructures. These strategies consecutively highlight the importance of chirality in the nanostructures (for linear and nonlinear chiroptical effects), in the experimental setup and in the light itself. Because, in the future, manipulating chirality will play an important role, we present two examples of chiral switches. Whereas in the first one, switching the chirality of incoming light causes a reversal of the handedness in the nanostructures, in the second one, switching the handedness of the nanostructures causes a reversal in the chirality of outgoing light.     

Biocatalytically Triggered Co‐Assembly of Two‐Component Core/Shell Nanofibers

For the development of applications and novel uses for peptide nanostructures, robust routes for their surface functionalization, that ideally do not interfere with their self‐assembly properties, are required. Many existing methods rely on covalent functionalization, where building blocks are appended with functional groups, either pre‐ or post‐assembly. A facile supramolecular approach is demonstrated for the formation of functionalized nanofibers by combining the advantages of biocatalytic self‐assembly and surfactant/gelator co‐assembly. This is achieved by enzymatically triggered reconfiguration of free flowing micellar aggregates of pre‐gelators and functional surfactants to form nanofibers that incorporate and display the surfactants’ functionality at the surface. Furthermore, by varying enzyme concentration, the gel stiffness and supramolecular organization of building blocks can be varied.     

Combined Tumor Environment Triggered Self‐Assembling Peptide Nanofibers and Inducible Multivalent Ligand Display for Cancer Cell Targeting with Enhanced Sensitivity and Specificity

Many new technologies, such as cancer microenvironment‐induced nanoparticle targeting and multivalent ligand approach for cell surface receptors, are developed for active targeting in cancer therapy. While the principle of each technology is well illustrated, most systems suffer from low targeting specificity and sensitivity. To fill the gap, this work demonstrates a successful attempt to combine both technologies to simultaneously improve cancer cell targeting sensitivity and specificity. Specifically, the main component is a targeting ligand conjugated self‐assembling monomer precursor (SAM‐P), which, at the tumor site, undergoes tumor‐triggered cleavage to release the active form of self‐assembling monomer capable of forming supramolecular nanostructures. Biophysical characterization confirms the chemical and physical transformation of SAM‐P from unimers or oligomers with low ligand valency to supramolecular assemblies with high ligand valency under a tumor‐mimicking reductive microenvironment. The in vitro fluorescence assay shows the importance of supramolecular morphology in mediating ligand–receptor interactions and targeting sensitivity. Enhanced targeting specificity and sensitivity can be achieved via tumor‐triggered supramolecular assembly and induces multivalent ligand presentation toward cell surface receptors, respectively. The results support this combined tumor microenvironment‐induced cell targeting and multivalent ligand display approach, and have great potential for use as cell‐specific molecular imaging and therapeutic agents with high sensitivity and specificity.     

Multi‐Component Organic Layers on Metal Substrates

Increasingly high hopes are being placed on organic semiconductors for a variety of applications. Progress along these lines, however, requires the design and growth of increasingly complex systems with well‐defined structural and electronic properties. These issues have been studied and reviewed extensively in single‐component layers, but the focus is gradually shifting towards more complex and functional multi‐component assemblies such as donor–acceptor networks. These blends show different properties from those of the corresponding single‐component layers, and the understanding on how these properties depend on the different supramolecular environment of multi‐component assemblies is crucial for the advancement of organic devices. Here, our understanding of two‐dimensional multi‐component layers on solid substrates is reviewed. Regarding the structure, the driving forces behind the self‐assembly of these systems are described. Regarding the electronic properties, recent insights into how these are affected as the molecule's supramolecular environment changes are explained. Key information for the design and controlled growth of complex, functional multicomponent structures by self‐assembly is summarized.     

Endowing Perovskite Nanocrystals with Circularly Polarized Luminescence

Perovskite nanocrystals are attracting great interest due to their excellent photonic properties. Here, through a supramolecular self‐assembly approach, the perovskite nanocrystals (NCs) with a novel circularly polarized luminescence (CPL) are successfully endowed. It is found that the achiral perovskite NCs can coassemble with chiral gelator in nonpolar solvents, in which the gelator molecules modify the surface of the perovskite NCs. Through such cogelation, the molecular chirality can transfer to the NCs resulting in CPL signals with a dissymmetric factor (g_lum) up to 10^?3. Furthermore, depending on the molecular chirality of the gelator, the CPL sense can be selected and the mirror‐imaged CPL is obtained. Such gels can be further embedded into the polymer film to facilitate flexible CPL devices. It is envisaged that this approach will afford a new insight into the designing of the functional chiroptical materials.     

Supramolecular Chiroptical Switches Based on Achiral Molecules

Chirality plays an important role in biological and material sciences. By introducing chiral elements into functional materials, new properties are created and an increase in information density can be achieved. Chiral properties of functional materials do not only rely on molecular structure, but also on supramolecular interaction between the building blocks. In contrast to the generally accepted opinion that chiral systems should include chiral molecules, this Research News introduces the role of achiral molecules in realizing chiral properties in films and gel‐like materials. Even a system that is entirely composed of achiral molecules can exhibit interesting chiroptical properties in supramolecular ultrathin films. This article demonstrates how achiral molecules can be assembled into supramolecular chiral films and organogels. It further shows how the incorporated achiral molecules can be used to switch the chiral properties of these supramolecular films and organogels.     

Peptide‐Based Nanotubes and Their Applications in Bionanotechnology

In nature, biological nanomaterials are synthesized under ambient conditions in a natural microscopic‐sized laboratory, such as a cell. Biological molecules, such as peptides and proteins, undergo self‐assembly processes in vivo and in vitro, and these monomers are assembled into various nanometer‐scale structures at room temperature and atmospheric pressure. The self‐assembled peptide nanostructures can be further organized to form nanowires, nanotubes, and nanoparticles via their molecular‐recognition functions. The application of molecular self‐assemblies of synthetic peptides as nanometer‐scale building blocks in devices is robust, practical, and affordable due to their advantages of reproducibility, large‐scale production ability, monodispersity, and simpler experimental methods. It is also beneficial that smart functionalities can be added at desired positions in peptide nanotubes through well‐established chemical and peptide syntheses. These features of peptide‐based nanotubes are the driving force for investigating and developing peptide nanotube assemblies for biological and non‐biological applications.     

Unraveling the Solution‐State Supramolecular Structures of Donor–Acceptor Polymers and their Influence on Solid‐State Morphology and Charge‐Transport Properties

Polymer self‐assembly in solution prior to film fabrication makes solution‐state structures critical for their solid‐state packing and optoelectronic properties. However, unraveling the solution‐state supramolecular structures is challenging, not to mention establishing a clear relationship between the solution‐state structure and the charge‐transport properties in field‐effect transistors. Here, for the first time, it is revealed that the thin‐film morphology of a conjugated polymer inherits the features of its solution‐state supramolecular structures. A “solution‐state supramolecular structure control” strategy is proposed to increase the electron mobility of a benzodifurandione‐based oligo(p‐ phenylene vinylene) (BDOPV)‐based polymer. It is shown that the solution‐state structures of the BDOPV‐based conjugated polymer can be tuned such that it forms a 1D rod‐like structure in good solvent and a 2D lamellar structure in poor solvent. By tuning the solution‐state structure, films with high crystallinity and good interdomain connectivity are obtained. The electron mobility significantly increases from the original value of 1.8 to 3.2 cm² V^?1 s^?1. This work demonstrates that “solution‐state supramolecular structure” control is critical for understanding and optimization of the thin‐film morphology and charge‐transport properties of conjugated polymers.     

Spiral Patterning of Au Nanoparticles on Au Nanorod Surface to Form Chiral AuNR@AuNP Helical Superstructures Templated by DNA Origami

Plasmonic motifs with precise surface recognition sites are crucial for assembling defined nanostructures with novel functionalities and properties. In this work, a unique and effective strategy is successfully developed to pattern DNA recognition sites in a helical arrangement around a gold nanorod (AuNR), and a new set of heterogeneous AuNR@AuNP plasmonic helices is fabricated by attaching complementary‐DNA‐modified gold nanoparticles (AuNPs) to the predesigned sites on the AuNR surface. AuNR is first assembled to one side of a bifacial rectangular DNA origami, where eight groups of capture strands are selectively patterned on the other side. The subsequently added link strands make the rectangular DNA origami roll up around the AuNR into a tubular shape, therefore giving birth to a chiral patterning of DNA recognition sites on the surface of AuNR. Following the hybridization with the AuNPs capped with the complementary strands to the capture strands on the DNA origami, left‐handed and right‐handed AuNR@AuNP helical superstructures are precisely formed by tuning the pattern of the recognition sites on the AuNR surface. Our strategy of nanoparticle surface patterning innovatively realizes hierarchical self‐assembly of plasmonic superstructures with tunable chiroptical responses, and will certainly broaden the horizon of bottom‐up construction of other functional nanoarchitectures with growing complexity.