Self-Assembly of n-Shaped Rod–Coil Molecules into Thermoresponsive Nanoassemblies: Construction of Reversible Helical Nanofibers in Aqueous Environment

Donor-Acceptor Assembly of Amphiphilic Molecules Based on 9,10-Distyrylanthracene Derivatives with Terminal Naphthalene Groups

Amphiphilic rod-coil compounds have excellent photophysical properties and can be assembled into supramolecular nanostructures of different sizes in water or polar solvents. Herein, we synthesized the amphiphilic compounds 2N-DSA, 4N-DSA, and 6N-DSA with 9,10-distyrylanthracene (DSA) as the core and a naphthalene unit as the terminal group that connected DSA through a tetraethylene glycol chain. These compounds have excellent aggregation-induced emission (AIE) properties in aqueous solution and are assembled into worm-like fragments or different sizes of spherical assemblies, defending the volume ratio of the rod to coil segments. Notably, the donor-acceptor interaction between DSA and electron- deficient compounds 2,4,6-trinitrophenol (TNP), 2,4,5,7-tetranitrofluorenone (TNF), and tetraethylene glycol dinitrobenzoate (TGDNB) forms a charge transfer complex, which can be used as a nanoreactor to improve the yield of the Suzuki coupling reaction about 8-10 times. The experimental results reveal that the synergy effect of the donor-acceptor, intermolecular π-π stacking, and hydrophobic-hydrophilic interactions significantly influences the morphology of aggregates and the efficiency of the nanoreactor.

Chirality-induced supramolecular nanodishes: enantioselectivity and energy transfer

Self-assembly is one of the most important issues of fabricating materials with precise chiral nanostructures. Herein, we constructed a chiral assembly system from amphiphiles containing hydrophobic/hydrophilic chiral coils bonded to hexabiphenyl, exhibiting controllable enantioselectivity over various aggregation behaviors. The chiral coils aroused various steric hindrances affecting intrinsic stacking tendency and compactness, leading to different aggregating behaviors, as concluded from the self-assembly investigation. The strong π-π stacking interaction between the long hexabiphenyl groups gave rise to a relatively compact arrangement in the aqueous solution, whereas the methyl side groups on the coil segments raised steric hindrance at the rigid-flexible interface, resulting in loose stacking and formation of nanostructures with a larger curvature. Compared with the achiral molecule 1 that formed micron-sized large sheets, molecules 2-4 containing chiral coils aggregated into nanodishes, which looked exactly like mosquito-repellent incense, to overcome surface tension. The helical structures effectively amplified chirality and exhibited strong circular dichroism (CD) signals, which indicate enantioselectivity. In addition, the relatively loose packing behavior permitted their co-assembly with a dye and aided efficient energy transfer, providing a foundation for the chiral application of supramolecules. Thus, by introducing a simple methyl side group in amphiphilic molecules, asymmetric synthesis and energy transfer efficiency can be realized.

Chirality Amplification Over the Morphology Control of the Rod-Coil Molecules with Lateral Methyl Groups

In the context of sustainable development, research regarding chirality has aroused enormous attention. Concurrently, chiral self-assembly is one of the most important subjects in supramolecular research, which can broaden the applications of chiral materials. This study focuses on the morphology control of amphiphilic rod-coil molecules composed of the rigid hexaphenyl unit and flexible oligoethylene and butoxy groups containing lateral methyl groups, carried out using an enantioseparation application. The methyl side chain being located on different blocks influences the driving force through steric hindrance, which determines the direction and degree of tilted packing during the π-π stacking of the self-assembly process. Interestingly, the amphiphilic rod-coil molecules aggregated into long helical nano-fibers, which further hierarchically aggregated into nano-sheets or nano-tubes upon increasing the concentration of the THF/H₂O solution. In particular, the hierarchical-chiral assembly effectively amplified the chirality and was validated by the strong Cotton signals; playing a vital role in the enantioselective nucleophilic substitution reaction. These results provide new insights into the applications of chiral self-assemblies and soft chiral materials.

Morphology-dependent photoresponsive behaviors of a self-assembled system based on a single cyanostilbene derivative

In recent years, photoresponsive supramolecular self-assemblies have shown great potential in various fields. However, it is still a great challenge to integrate and control multiple photoresponsive behaviors in a self-assembled system. Herein, we design a novel cyanostilbene-based molecule VOE. In the aggregated state, it has different photoresponsive behaviors under different morphologies. When VOE molecules are dispersed in a 70% H₂O/THF mixture, two different assembly morphologies are obtained as the aging time changes. One is weakly emissive nanoparticles with amorphous packing arrangements, and the other is highly emissive microbelts with well-ordered stacking modes. When they are irradiated with blue light (420 nm), the disordered assembly structure of nanoparticles leads to a [2+2] cycloaddition reaction, while a Z/E isomerization reaction occurs in ordered packed microbelts. Therefore, we can use a self-assembled system to generate two different morphologies, enabling completely different emissions and photoresponsive behaviors.

Thermoresponsive dendritic oligoethylene glycols

Monodispersed molecules of low molar masses showing thermoresponsiveness are appealing both for mechanism investigation of the thermally-modulated dehydration and aggregation on molecular levels and for designing functional intelligent materials. In the present report, thermoresponsive properties of a homologous series of monodispersed dendritic macromolecules carrying three-, four- or six-fold dendritic oligoethylene glycol (OEG) segments were investigated. These dendritic macromolecules carry either methoxyl or ethoxyl terminals, and have different cores (alcohol, methyl ester or methacryloyl) to exhibit different overall hydrophilicity. They show characteristic thermoresponsive properties with sharp phase transitions when suitable structural units are combined. Three structural factors determine their phase transition temperatures, including the cores, branching density and peripheral terminals. Thermally-induced collapse and aggregation are monitored with temperature-varied NMR spectroscopy at the microscale level and optical microscopy at the macroscale level. At elevated temperature, these dendritic macromolecules undergo fast exchange between the dehydrated and the hydrated states. These dendritic macromolecules afford structure-dependent confinement to guest dyes through their multi-valent interactions.

Thermoresponsive Supramolecular Assemblies from Dendronized Amphiphiles To Form Fluorescent Spheres with Tunable Chirality

Balance between self-association of structural units and self-repulsion from crowding-induced steric hindrance accounts for the supramolecular assembly of the amphiphilic entities to form ordered structures, and solvation provides a toolbox to conveniently modulate the assemblies through differential interactions to various structural units. Here we report solvation-modulated supramolecular chiral assembly in aqueous solutions of amphiphilic dendronized tetraphenylethylenes (TPEs) with three-folded dendritic oligoethylene glycols (OEGs) through dipeptide Ala-Gly linkage. These dendronized amphiphiles can form supramolecular spheres with enhanced supramolecular chirality, which is tunable and dependent on solvation. These nanosized spherical aggregates exhibit thermoresponsive behavior, and their cloud point temperatures are dependent on mixed solvent of water and THF. The phase transition temperatures increase with water fractions due to water-driven shifting of OEG moieties from interiors of the aggregates to their peripheries. Furthermore, the thermally induced dehydration and collapse of OEG moieties mediate the reversible aggregation and deaggregation between the spheres, imparting tunable aggregation-induced fluorescent emission (AIE) and supramolecular chirality. Both experimental results and molecular dynamic simulations have highlighted that reversible chirality transformations of the amphiphilic dendronized assemblies mediated by solvation through change solvent quality or thermally dehydration are dependent on the balance between interactions of OEG dendrons with TPE moieties and with the solvent molecules.

Dendronized polydiacetylenes via photo-polymerization of supramolecular assemblies showing thermally tunable chirality

Transformation of supramolecular chiral assemblies into covalent polymers integrates characteristics of supramolecular chemistry together with covalent entities, leading to fabrication of covalent chiral materials through versatile supramolecular chiral assemblies. Here, we report supramolecular assembly of an amphiphilic dendronized 10,12-pentacosadiynoic amide (PCDA) in aqueous solutions to form twisted ribbons, which were transferred into covalent dendronized polydiacetylenes (PDAs) via photopolymerization. These supramolecular dendronized PCDA and the corresponding covalent dendronized PDAs showed unprecedent thermoresponsive properties. The thermally-induced dehydration and aggregations tuned reversibly their chiralities, which can be visually inspected through colour changes.

Self-assembly of propeller-shaped amphiphilic molecules: control over the supramolecular morphology and photoproperties of their aggregates

The aggregation-induced emission (AIE) effect is an important feature for luminescence studies, which can offer a broader range of applications for fluorescent materials. Herein, we report the morphological control and photoproperties of amphipathic propeller-shaped rod-coil molecules based on a benzene-1,3,5-tricarboxamide (BTA) unit, which restricts the intramolecular rotation and leads to the AIE effect during the self-assembly process. Investigations on the assembly of these molecules have revealed that tetragonal perforated lamella, hexagonal columnar, body-centered tetragonal micellar, and hexagonal close-packed nanostructures were spontaneously formed in the solid-state. In the solution-state, these molecules assemble into nanosheet-like aggregates, bowl-like objects, and spherical nanoparticles, respectively. The morphology of the molecular aggregates can be controlled by modifying the molecular chain length or introducing lateral methyl groups in the coil chain. Notably, these molecular assemblies exhibit strong AIE phenomena in a mixed THF/H2O solution and can be used as smart soft materials due to the restriction of their intramolecular motion.

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.

The relationship between molecular structure and supramolecular morphology in the self-assembly of rod-coil molecules with oligoether chains

Controlling the morphology of rod-coil molecular aggregates is crucial for studying and obtaining functional materials with exceptional properties. In this paper, we report the construction of rod-coil molecular nanoaggregates with well-defined structures. The rod-coil molecules, labeled 1a-1d, consist of a rod section, composed of phenyl and biphenyl groups, and oligoether chains with 7 and 12 repeating units. The final assembled structures showed either oblique or hexagonal columnar structures, depending on the length of the coils in the bulk state. Interestingly, in water, molecules 1a and 1c self-assemble into scrolled nanofibers and cylindrical micelles. Instead, molecules 1b and 1d, which have methyl groups decorated at the interface of the rod and coil sections, self-organize into helical nanofibers and nanorings, respectively. Thus, controlling the length of the coil chains and inserting lateral methyl groups is an effective strategy to construct precise rod-coil molecular assemblies in the bulk and in aqueous solution.

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.

Construction of Various Supramolecular Assemblies from Rod-Coil Molecules Containing Biphenyl and Anthracene Groups Driven by Donor-Acceptor Interactions

Rod-coil amphiphilic functional molecules, comprising a rigid aromatic building block and hydrophilic oligoether dendrons as the coil segments, were synthesized. These compounds exhibit a powerful self-organizing ability to form supramolecular nanoparticles and long nanofibers in tetrahydrofuran/water solution, by controlling the intermolecular interaction of the rigid blocks. These molecules are able to form supramolecular polymers and, subsequently, to form sheetlike nanoaggregates, through charge-transfer interactions by the addition of a guest molecule, tetracyanoquinodimethane. Notably, upon addition of water-soluble 2,4,6-trinitrophenol, the self-assembly of these molecules exhibits the antagonistic effect owing to donor-acceptor and hydrophobic-hydrophilic interactions among the molecules. The experimental results reveal that various morphologies of rod-coil molecular assemblies can be obtained by tuning the molecular interaction and the hydrophilicity of guest electron-acceptor molecules. Interestingly, the cross-coupling reaction between phenylboronic acid and chlorobenzene occurs within the charge complexes of these molecular aggregates. This occurs in the nanoenvironment that affords an extremely concentrated reaction zone and reduces the activation energy barrier required for the cross-coupling reaction.

Construction of Supramolecular Nanostructures from V-Shaped Amphiphilic Rod-Coil Molecules Incorporating Phenazine Units

A series of bent-shaped molecules, consisting of dibenzo[a,c]phenazine and phenyl groups connected together as a rod segment, and poly(ethylene oxide) (PEO) with a degree of polymerization (DP) of 6 as the coil segment, were synthesized. The self-assembling behavior of these molecules by differential scanning calorimetry (DSC), thermal optical polarized microscopy (POM), small-angle X-ray scattering spectroscopy (SAXS), atomic force microscopy (AFM), and transmission electron microscopy (TEM), revealed that carboxyl or butoxy carbonyl groups at the 11 position of dibenzo[a,c]phenazine noticeably influence self-organization of molecules into supramolecular aggregates in bulk and aqueous solutions. Molecules 1 and 2 with chiral or non-chiral PEO coil chains and the carboxyl group at the rod segments self-organize into a hexagonal perforated lamellar structure and a hexagonal columnar structure in the solid state. In aqueous solution, molecules 1 and 2 self-assemble into diverse lengths of nanofibers, whereas molecules 3 and 4 with butoxy carbonyl groups exhibit a self-organizing capacity to form diverse sizes of spherical aggregates.

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.

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.