Fluorescence-switching 2-D sheet structure formed by self-assembly of cruciform aromatic amphiphiles

We report that aromatic amphiphiles based on cruciform aromatic segments self-assemble into 2-D sheet structures in aqueous environments. Notably, the aromatic amphiphile based on a pyrene unit generates fluorescence-switching 2-D sheet structures. In a pH-neutral condition, the sheets show strong excimer emission. However, upon lowering the pH the excimer emission is quenched due to loosely-packed pyrene units. Subsequent return of pH to a neutral condition leads to full recovery of the excimer emission, indicative of fully-reversible fluorescence emission switching.

Design and Synthesis of New Type of Macrocyclic Architectures Used for Optoelectronic Materials and Supramolecular Chemistry

Supramolecular chemistry has received much attention for decades. Macrocyclic architectures as representative receptors play a vital role in supramolecular chemistry and are applied in many fields such as supramolecular assembly and host-guest recognition. However, the classical macrocycles generally lack functional groups in the scaffolds, which limit their further applications, especially in optoelectronic materials. Therefore, developing a new design principle is not only essential to better understand macrocyclic chemistry and the supramolecular behaviors, but also further expand their applications in many research fields. In recent years, the doping compounds with main-group heteroatoms (B, N, S, O, P) into the carbon-based π-conjugated macrocycles offered a new strategy to build macrocyclic architectures with unique optoelectronic properties. In particular, the energy gaps and redox behavior can be effectively tuned by incorporating heteroatoms into the macrocyclic scaffolds. In this Minireview, we briefly summarize the design and synthesis of new macrocycles, and further discuss the related applications in optoelectronic materials and supramolecular chemistry.

High-Resolution Cryo-Electron Microscopy Reveals the Unique Striated Hollow Structure of Photocatalytic Macrocyclic Polydiacetylene Nanotubes

High-resolution structures are crucial for understanding the functional properties of nanomaterials. We applied single-particle cryo-electron microscopy (cryo-EM), a method traditionally used for structure determination of biological macromolecules, to obtain high-resolution structures of synthetic non-biological filaments formed by photopolymerization of macrocyclic diacetylene (MDA) amphiphilic monomers. Tomographic analysis showed that the MDA monomers self-assemble into hollow nanotubes upon dispersion in water. Single-particle analysis revealed tubes consisting of six pairs of covalently bonded filaments held together by hydrophobic interactions, where each filament is composed of macrocyclic rings stacked in parallel "chair" conformations. The hollow MDA nanotube structures we found may account for the efficient scavenging of amphiphilic pollutants in water and subsequent photodegradation of the guest species.

Stimuli-Responsive Transformable Supramolecular Nanotubes

Supramolecular nanotubes produced by self-assembly of organic molecules can have unique structural features such as a one-dimensional morphology with no branching, distinguishable inner and outer surfaces and membrane walls, or a structure that is hollow and has a high aspect ratio. Incorporation of functional groups that respond to external chemical or physical stimuli into the constituent organic molecules of supramolecular nanotubes allows us to drastically change the structure of the nanotubes by applying such stimuli. This ability affords an array of controllable approaches for the encapsulation, storage, and release of guest compounds, which is expected to be useful in the fields of physics, chemistry, biology, and medicine. In this article, I review the supramolecular nanotubes developed by our group that exhibit morphological transformations in response to pH, chemical reaction, light, temperature, or moisture.

Selective Anticancer Materials by Self-Assembly of Synthetic Amphiphiles Based on N-Acetylneuraminic Acid

N-Acetylneuraminic acid (Neu5Ac), one of the abundant types of sialic acid, is an emerging anticancer agent owing to its ability to target selectins in the plasma membrane of cancer cells. Considering the functionality of Neu5Ac, obtaining novel Neu5Ac-conjugated materials with a selective and an enhanced antitumor activity has remained a challenge. Herein, we report the supramolecular materials of three novel amphiphiles composed of Neu5Ac as a hydrophilic segment and pyrene or adamantane as a hydrophobic segment. The synthetic amphiphiles 1, 2, and 3 self-assembled into ribbons, vesicles, and irregular aggregates in an aqueous solution, respectively. Among the materials, vesicles of amphiphile 2 showed the most substantial selectivity toward cancer cells, followed by cell death due to the production of reactive oxygen species by the pyrene group. The dual advantage of Neu5Ac-selectivity and the pyrene-cytotoxicity of vesicles of amphiphile 2 can provide a strategy for effective anticancer materials.

Switchable Aromatic Nanopore Structures: Functions and Applications

Nanopore structures in nature play a crucial role in performing many sophisticated functions such as signal transduction, mass transport, ion channel, and enzyme reaction. Inspired by pore-forming proteins, considerable effort has been made to design self-assembling molecules that are able to form nanostructures with internal pores in aqueous media. These nanostructures offer ample opportunity for applications because their internal pores are able to perform a number of unique functions required for a confined nanospace. However, unlike nanopore assembly in nature, the synthetic nanopore structures are mostly based on a fixed pore that impedes performing adaptable regulation of properties to environmental change. This limitation can be overcome by integration of hydrophilic oligo(ethylene oxide) dendrons into aromatic building blocks for nanopore self-assembly, because the dendritic chains undergo large conformational changes triggered by environmental change. The transition of the oligoether chains triggers the aromatic nanopore assembly to undergo reversible pore deformation through closing, squeezing, and shape change without structural collapse. These switching properties allow the aromatic nanopore structures to perform adaptable, complex functions which are difficult to achieve using a fixed pore assembly.In this Account, we summarize our recent progress in the development of switchable nanopore structures by self-assembly of rigid aromatic amphiphiles grafted by hydrophilic oligo(ethylene oxide) dendrons in aqueous media. We show that combining oligoether chains into aromatic segments generates switchable aromatic nanopore structures in aqueous media such as hollow tubules, toroidal structures, and 2D porous sheets depending on the shape of the aromatic building block. Next, we discuss the chemical principle behind the switching motion of the aromatic nanopore structures triggered by external stimuli. We show that the internal pores of the aromatic nanostructures are able to undergo reversible switching between open-closed or expanded-contracted states triggered by external stimuli such as temperature, pH, and salts. In the case of toroidal structures, closed ring-like aromatic frameworks can be spirally open triggered by heat treatment, which spontaneously initiate helical polymerization. Additionally, we discuss switchable functions carried out by the aromatic nanopores such as driving helicity inversion of DNA, consecutive enzymatic action, reversible actuation of lipid vesicles, and pumping of captured guests out of internal pores. By understanding the underlying chemical principle required for dynamic mechanical motion, aromatic assembly can be exploited more broadly to create emergent nanopore structures with functions as complex as those of biological systems.

The Role of Peripheral Amide Groups as Hydrogen-Bonding Directors in the Tubular Self-Assembly of Dinucleobase Monomers

Nanotubes are a fascinating kind of self-assembled structure which have a wide interest and potential in supramolecular chemistry. We demonstrated that nanotubes of defined dimensions can be produced from dinucleobase monomers through two decoupled hierarchical cooperative processes: cyclotetramerization and supramolecular polymerization. Here we analyze the role of peripheral amide groups, which can form an array of hydrogen bonds along the tube axis, on this self-assembly process. A combination of 1 H NMR and CD spectroscopy techniques allowed us to analyze quantitatively the thermodynamics of each of these two processes separately. We found out that the presence of these amide directors is essential to guide the polymerization event and that their nature and number have a strong influence, not only on the stabilization of the stacks of macrocycles, but also on the supramolecular polymerization mechanism.

Stimuli-Responsive Supramolecular Chirality Switching and Nanoassembly Constructed by n-Shaped Amphiphilic Molecules in Aqueous Solution

Self-assembled nanomaterials composed of amphiphilic oligomers with functional groups have been applied in the fields of biomimetic chemistry and on-demand delivery systems. Herein, we report the assembly behavior and unique properties of an emergent n-shaped rod-coil molecule containing an azobenzene (AZO) group upon application of an external stimulus (thermal, UV light). The n-shaped amphiphilic molecules comprising an aromatic segment based on anthracene, phenyl linked with azobenzene groups, and hydrophilic oligoether (chiral) segments self-assemble into large strip-like sheets and perforated-nanocage fragments in an aqueous environment, depending on the flexible oligoether chains. Interestingly, the nano-objects formed in aqueous solution undergo a morphological transition from sheets and nanocages to small one-dimensional nanofibers. These molecules exhibit reversible photo- and thermal-responsiveness, accompanied by a change in the supramolecular chirality caused by the conformational transitions of the rod backbone. The architecture of n-shaped amphiphilic molecules with a photosensitive group makes them ideal candidates for intelligent materials for applications in advanced materials science.

Self-division of 2-D sheets in aromatic macrocycle assembly

2-D Sheets from macrocycle assembly undergoes reversible lengthwise division in response to temperature change.

Supramolecular Two-Dimensional Systems and Their Biological Applications

Various biological systems rely on the supramolecular assembly of biomolecules through noncovalent bonds for performing sophisticated functions. In particular, cell membranes, which are 2D structures in biological systems, have various characteristics such as a large surface, flexibility, and molecule-recognition ability. Supramolecular 2D materials based on biological systems provide a novel perspective for the development of functional 2D materials. The physical and chemical properties of 2D structures, attributed to their large surface area, can enhance the sensitivity of the detection of target molecules, molecular loading, and bioconjugation efficiency, suggesting the potential utility of functional 2D materials as candidates for biological systems. Although several types of studies on supramolecular 2D materials have been reported, supramolecular biofunctional 2D materials have not been reviewed previously. In this regard, the current advances in 2D material development using molecular assembly are discussed with respect to the rational design of self-assembling aromatic amphiphiles, the formation of 2D structures, and the biological applications of functional 2D materials.

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.

Self-Assembly of Tetraphenylethylene-Based Amphiphiles in Aqueous Methanol Solution into Two-Dimensional Chiral Sheets for Enantioselective Sorption

Most synthetic building blocks self-assemble into one- or three-dimensional architectures. However, fewer examples have been reported on the aggregation of amphiphiles to form optically-active two-dimensional (2D) structures. Herein, we report the self-assembly of tetraphenylethylene (TPE)-containing hydrophilic dendrons into 2D sheet structures in aqueous methanol solution. TEM and AFM investigations showed that the self-assembly of disubstituted TPE generates helical nanofibers as an intermediate structure which, in turn, laterally associate into a chiral sheet structure with a thickness of 4.6 nm, whereas tetrasubstituted TPE self-assembles into a nonchiral sheet structure with a thickness of 3.8 nm. In great contrast to the nonchiral sheets, the chiral sheets are able to preferentially absorb the d-enantiomer in a racemic phenylalanine derivative solution accompanied by fluorescence enhancement, thus indicating that the single-layered chiral sheets act as an enantioselective membrane that can be used for fluorescence sensing.

Switching between Stacked Toroids and Helical Supramolecular Polymers in Aqueous Nanotubules

Although significant advances have been made in supramolecular tubules, reversible polymerization in the tubular walls while maintaining their intact structure remains a great challenge. Here, reversible helical supramolecular polymerization of stacked toroids is reported, while maintaining tubular structures in aqueous solution. At room temperature, the tubules consist of discrete toroid stackings with hydrophobic interior. Upon heating, the tubules based on toroid stackings undergo a reversible helical supramolecular polymerization to transform into helical tubules by interconnecting between spirally open toroids. The helical polymerization arises from a tilting transition of the closed toroids that transform into spirally open toroids driven by the thermal dehydration of a hydrophilic oligoether dendron surrounding the toroid frameworks.

Time-controllable roll-up onset of polythiophene sheets into nanotubes that exhibit circularly polarized luminescence

Self-assembly of a polythiophene-conjugated glycolipid exclusively produced square sheets a few micrometers on each side. Seventeen hours after the sheets were dispersed in ethanol at 25 °C, they suddenly started to roll up, and eventually they were completely transformed into nanotubes. The onset timing of the roll-up was temperature-dependent. The roll-up involved rearrangement of the molecular packing within the bilayer membranes, which was accompanied by strengthening of the intermolecular hydrogen bonds, alteration of the polythiophene aggregation mode and enhancement of supramolecular chirality due to chiral packing. The nanotubes exhibited not only strong fluorescence derived from J-type aggregation of the polythiophene aromatic moiety but also circularly polarized luminescence (CPL) originating from the left-handed helicity of the polythiophene main chain backbone. Because the CPL onset was concurrent with the sheet roll-up, the CPL onset was also able to be controlled by varying the temperature. Such delayed CPL onset has never been reported in chiral supramolecular structures, in which CPL onset and helicity inversion usually begin immediately upon application of a stimulus and then progress either quickly or gradually. Our findings can be expected to facilitate the development of new stimulus-responsive supramolecular structures that can be used for delayed-action capsules or optical switching devices.

Soft-Matter Nanotubes: A Platform for Diverse Functions and Applications

Self-assembled organic nanotubes made of single or multiple molecular components can be classified into soft-matter nanotubes (SMNTs) by contrast with hard-matter nanotubes, such as carbon and other inorganic nanotubes. To date, diverse self-assembly processes and elaborate template procedures using rationally designed organic molecules have produced suitable tubular architectures with definite dimensions, structural complexity, and hierarchy for expected functions and applications. Herein, we comprehensively discuss every functions and possible applications of a wide range of SMNTs as bulk materials or single components. This Review highlights valuable contributions mainly in the past decade. Fifteen different families of SMNTs are discussed from the viewpoints of chemical, physical, biological, and medical applications, as well as action fields (e.g., interior, wall, exterior, whole structure, and ensemble of nanotubes). Chemical applications of the SMNTs are associated with encapsulating materials and sensors. SMNTs also behave, while sometimes undergoing morphological transformation, as a catalyst, template, liquid crystal, hydro-/organogel, superhydrophobic surface, and micron size engine. Physical functions pertain to ferro-/piezoelectricity and energy migration/storage, leading to the applications to electrodes or supercapacitors, and mechanical reinforcement. Biological functions involve artificial chaperone, transmembrane transport, nanochannels, and channel reactors. Finally, medical functions range over drug delivery, nonviral gene transfer vector, and virus trap.

Supramolecular nanostructures constructed by rod-coil molecular isomers: effect of rod sequences on molecular assembly

Coil-rod-coil molecules, composed of flexible oligoether chains and conjugated rod blocks, have a well-known ability to produce various nanostructures in bulk and in aqueous solution. Herein we report the synthesis and self-assembly of coil-rod-coil molecules based on the sequence of the rod building block and the type of oligoether coil chain. These molecules consist of conjugated rod segments, which are composed of biphenyl, terphenyl, and acetylenic bonds, with chiral oligoether chains as flexible coil segments. The experimental results imply that the sequence of the rod segments markedly influences the self-assembled nanostructures of coil-rod-coil molecules in the bulk state, and that the type of coil chain strongly affects the morphology of the supramolecular nanoassemblies of these molecules in aqueous solution. In the bulk state, molecules 1a and 1b, which contain biphenyl units connected to the end of the coil segments self-organize into a hexagonal perforated lamellar phase, and oblique columnar and body-centred tetragonal structures, respectively. However, molecules 2a and 2b bearing terphenyl units linked to the end of the coil segments self-assemble into lamellar, hexagonal perforated lamellar and hexagonal columnar structures. In aqueous solution, rod-coil molecular isomers with linear chiral oligoether chains self-assemble into helical nanofibres of various lengths. Meanwhile, isomers with chiral oligoether dendron chains self-organize into sheet-like nanoribbons of different sizes.

Morphological Control of Coil-Rod-Coil Molecules Containing m-Terphenyl Group: Construction of Helical Fibers and Helical Nanorings in Aqueous Solution

Rod-coil molecules, composed of rigid segments and flexible coil chains, have a strong intrinsic ability to self-assemble into diverse supramolecular nanostructures. Herein, we report the synthesis and the morphological control of a new series of amphiphilic coil-rod-coil molecular isomers 1-2 containing flexible oligoether chains. These molecules are comprised of m-terphenyl and biphenyl groups, along with triple bonds, and possess lateral methyl or butyl groups at the coil or rod segments. The results of this study suggest that the morphology of supramolecular aggregates is significantly influenced by the lateral alkyl groups and by the sequence of the rigid fragments in the bulk and in aqueous solution. The molecules with different coils self-assemble into lamellar or oblique columnar structures in the bulk state. In aqueous solution, molecule 1a, with a lack of lateral groups, self-assembled into large strips of sheets, whereas exquisite nanostructures of helical fibers were obtained from molecule 1b, which incorporated lateral methyl groups between the rod and coil segments. Interestingly, molecule 1c with lateral butyl and methyl groups exhibited a strong self-organizing capacity to form helical nanorings. Nanoribbons, helical fibers, and small nanorings were simultaneously formed from the 2a-2c, which are structural isomers of 1a, 1b, and 1c. Accurate control of these supramolecular nanostructures can be achieved by tuning the synergistic interactions of the noncovalent driving force with hydrophilic-hydrophobic interactions in aqueous solution.

Supramolecular helical nanostructures from self-assembly of coil-rod-coil amphiphilic molecules incorporating the dianthranide unit

Coil-rod-coil amphipathic oligomers composed of a rigid dianthranide unit and a hydrophilic branched oligoether as the coil segment were synthesized. These amphiphilic molecules self-assemble into clew-like aggregates composed of fibres or helical nanofibers in aqueous solution. Subsequently, supramolecular polymers were produced from the above objects through charge-transfer interactions by adding 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (4F-TCNQ). Interestingly, temperature-sensitive supramolecular chirality was induced by lateral methyl units located at the interface of the rigid and flexible segments. However, upon addition of the electron-acceptor molecule, 4F-TCNQ, strong donor-acceptor interactions restrict any change in supramolecular chirality with temperature.

Static and Dynamic Nanosheets from Selective Assembly of Geometric Macrocycle Isomers

In contrast to the significant advances that have been made in the construction of two-dimensional (2D) nanostructures, the rational modification from static to dynamic 2D sheets remains a great challenge. Static and dynamic sheets formed from selective self-assembly of geometric macrocycle isomers based on anthracene units are presented. The self-assembly of the cis isomer generates static planar sheets, whereas the trans isomer forms dynamic rolled sheets which are reversibly unrolled upon stimulation by a thermal signal. Furthermore, the mixed solution of the two isomers exhibits self-sorting behavior, generating the coexistence of the two independent self-assembled structures, the planar sheets and the folded scrolls. The self-sorted supramolecular objects with considerable shape and size differences are able to be readily separated, one isomer from the other.

Morphology transition in helical tubules of a supramolecular gel driven by metal ions

Our aim to access a particular chemical functionality on helical tubules has been achieved by the rational molecular design and synthesis of glucono-appended cardanol derivatives. For the first time, we report a chiral molecular packing with α-helical tubules, and chiral symmetry-breaking upon exposure to Cu²⁺ that generated the final ordered structure via an in situ morphological transition without undergoing any phase change.

Assembly–disassembly switching of self-sorted nanotubules forming dynamic 2-D porous heterostructure

Self-sorted tubules and sheets are reversibly merged into 2-D porous heterostructure in response to a pH change.

Donor-acceptor interaction-driven self-assembly of amphiphilic rod-coil molecules into supramolecular nanoassemblies

Rigid-flexible amphiphilic molecules consisting of an aromatic segment based on pyrene and biphenyl units and hydrophilic polyethylene oxide chains self-assemble into lamellar, hexagonal columnar, and two-dimensional columnar nanostructures in the bulk state. In aqueous solution, these molecules self-assemble into nanofibers, spherical micelles, and multilayer nanotubes, depending on the chain or rod length of the molecules. Notably, ordered nanostructures of supramolecular polymers, such as single-layer curving fragments, nanofibers, and nanosheets, were constructed through charge-transfer interactions between the nanoobjects and an electron-acceptor molecule, 2,4,5,7-tetranitrofluorenone. These experimental results reveal that diverse supramolecular morphologies can be controlled by tuning rod-coil molecular interactions or charge-transfer interactions between the donor and acceptor molecules.

Self-Assembled Spheres, Flowers, and Fibers from the Same Backbone and Similar Side Chains

Rylene imides (RIs) self-assemble into various nanostructures. Often, the synthesis of unsymmetrical RIs (URIs) is required to achieve nanostructures. However, the synthesis of URIs is nontrivial. Thus, a structurally similar alternative is desirable. iso-Indigo (i-indigo) has a π core and lactam rings that are structurally similar to the RIs. Unsymmetrical iso-indigo (i-indigo) can be easily synthesized by condensing oxindole and isatin. We have synthesized a series of unsymmetrical i-indigo molecules. In these molecules, the π-π interaction, hydrogen bonding, and van der Waals interactions are in operation. Because of these, the molecules self-assemble into spheres, fibers, and dahlia flower morphologies. If the hydrogen bonding interaction is disrupted, then all of them form fibers. Control experiments indicate that the complete absence of hydrogen bonding is deleterious to self-assembly. We also show that the lower analogs of i-indigo are not sufficient to form self-assembled nanostructures.

Control of supramolecular nanoassemblies by tuning the interactions of bent-shaped rod-coil molecules

Rod-coil molecules 1a, 1b and 2a, 2b, consisting of biphenyl and phenyl units connected by an acetylene bond as the rod segment and oligo(ethylene glycol) (OEG) as the coil segment, were synthesized and characterized. Molecules 1a and 1b incorporate a butoxy group at the apex of their bent-shaped rigid building blocks, while both 1b and 2b contain a lateral methyl group between the rod and coil segments. The self-assembling behavior of these molecules was investigated using DSC, SAXS, CD, AFM, and TEM in bulk and aqueous solutions. In the bulk state, 1a self-assembles into oblique columnar structures, whereas 1b, incorporating butoxy and lateral methyl groups, self-assembles into three-dimensional body-centered tetragonal structures. Molecules 2a and 2b with no butoxy groups, and 2b incorporating a lateral methyl group, self-assemble into hexagonal perforated lamellar and oblique columnar structures, respectively. In dilute aqueous solutions, 1a assembles into tubular nanoassemblies, while 1b self-organizes into micelles and nanoparticles. On the other hand, 2a and 2b spontaneously aggregate into nanoribbons and nanofibers. Furthermore, CD experiments together with AFM investigations of 2b indicate the creation of self-organized helical fibers, implying that the lateral methyl group induces the helical stacking of the rod building block. These results reveal that the butoxy and lateral methyl groups between the rod and coil segments dramatically influence the creation of supramolecular nanostructures and morphologies.