Responsive nanostructures from aqueous assembly of rigid-flexible block molecules

Controlling aqueous self-assembly mechanisms by hydrophobic interactions

We report an innovative template-assisted synthetic protocol for the selective functionalization of terminal triple bonds in oligophenyleneethynylenes (OPE) by pre-organization in aqueous solution. By this approach, three new OPE-based bolaamphiphiles substituted with hydrophilic poly(2-ethyl-2-oxazoline) (PEtOx) chains of different length have been synthesized. The chain length was observed to strongly influence the aqueous supramolecular polymerization: bolaamphiphiles with longer hydrophilic chains aggregate into spherical nanoparticles in a stepwise fashion, whereas 2D anisotropic platelets are formed cooperatively if shorter PEtOx chains are used. Our results demonstrate that hydrophobic interactions can be strong enough to trigger cooperative effects in aqueous self-assembly processes.

Inversion of supramolecular chirality in bichromophoric perylene bisimides: influence of temperature and ultrasound

The supramolecular helicity in the self-assembled nanostructures of two perylene bisimide bichromophoric systems could be controlled by varying the preparatory methods. The self-assembly of the compounds under different conditions was investigated in detail by using absorption, fluorescence, CD, FTIR, XRD, TEM, and SEM techniques. These studies reveal that the heating-cooling method results in aggregates with ordered molecular packing and enhanced optical chirality. Ultrasonication leads to molecular aggregates with less ordered packing wherein the supramolecular chirality was reversed relative to the sample prepared via a heating-cooling method. This heating-cooling method proved to be superior in terms of nanofiber synthesis, yielding fibers with extended length and a prominent helical twist. At higher concentration, both compounds exhibited a gelation property in benzonitrile. The tunable chiroptical properties in these supramolecular systems make them potential candidates for applications in the field of optical and electronic device fabrication based on organic nanostructures.

Self-assembled nanostructures of amphiphilic zinc(II) salophen complexes: role of the solvent on their structure and morphology

This contribution explores the effect of several solvent properties, such as volatility, polarity, and Lewis basicity on the formation of molecular self-assembled nanostructures in the solid state, obtained either by casting of related solutions or by complete solvent evaporation, using seven solvents representative of common classes of coordinating organic solvents, of an amphiphilic Zn(II) Schiff-base complex. In all cases, the existence of well-defined X-ray diffraction patterns, for both the cast and powder samples, indicates a strong tendency towards the molecular self-assembly of such complexes. While nanostructures formed in acetone, THF, pyridine, and DMF have a lamellar organization, those formed in ACN, ethanol, and DMSO exhibit a 2D columnar square structure. Field emission scanning electron microscopy analysis indicates that nanostructures formed in volatile acetone, THF, ACN, and ethanol solvents show a fibrous morphology, while those formed in less volatile pyridine, DMF, and DMSO have a ribbon appearance. Overall, the results indicate that while the formation of such nanostructures is independent of the Lewis basicity of the solvent, the solvent polarity affects their structure - more polar solvents favour higher symmetry structures - and the solvent volatility influences their morphology and ordering in the cast films - lower volatility of the solvent parallels the formation of much more ordered structures. Therefore, the appropriate choice of solvent allows control of the structure, morphology, and ordering of these molecular assemblies.

Tuning the self-assembly of rectangular amphiphilic cruciforms

The self-assembly of a series of nonionic amphiphilic cruciforms based on the 1,2,4,5-tetrakis(phenylethynyl)benzene (TPEB) skeleton, in which the peripheral substituents have been modified to modulate the morphology of the supramolecular structures, is reported. The presence of linear paraffinic and hydrophilic chains in TPEBs 1 and 2 gives rise to two-dimensional structures of high aspect ratio. In contrast, the incorporation of dendronized hydrophilic chains results in the formation of twisted ribbons in amphiphile 3 and impedes the organized self-assembly of TPEB 4. Theoretical calculations show that the self-assembly of these amphiphiles might be initiated with the formation of π-stacked dimeric units. Compound 2, which self-assembles into different morphologies depending on the solvent, interacts by π-stacking and also by the interdigitation of the peripheral decyl tails to generate bidimensional supramolecular structures. The steric demand exerted by the dendronized polar wedges in 3 and 4 strongly conditions their supramolecular organization. This steric demand together with the interdigitation of the decyl chains results in the self-assembly of cruciform 3 into helical aggregates. However, the lack of the paraffinic chains in 4 impedes this helical organization, and the formation of amorphous material is visualized. The joint experimental and theoretical study presented herein provides relevant guidelines for the modulated self-assembly of nonionic amphiphilic molecules.

Hierarchical molecular self-assemblies: construction and advantages

Hierarchical molecular self-assembly offers many exotic and complicated nanostructures which are of interest in nanotechnology and material science. In the past decade, various strategies leading to hierarchical molecular self-assemblies have been developed. In this review we summarize the recent advances in the creation and application of solution-based self-assembled nanostructures that involve more than one level of arrangement of building blocks. The strategies for construction hierarchical self-assembled structures and the advantages brought up by these assemblies are focused on. The following contents are included: (1) general approaches to fabricate hierarchical self-assembly, including self-assemblies based on supramolecules and specially designed block copolymers; (2) the advantages brought about by the hierarchical self-assembly, including the fabrication of special self-assembled structures, rich responsiveness to external stimuli, and the materials' performance.

A supramolecular microgel glutathione peroxidase mimic with temperature responsive activity

Glutathione peroxidase (GPx) protects cells from oxidative damage by scavenging surplus reactive oxygen species (ROS). Commonly, an appropriate amount of ROS acts as a signal molecule in the metabolism. A smart artificial GPx exhibits adjustable catalytic activity, which can potentially reduce the amount of ROS to an appropriate degree and maintain its important physiological functions in metabolism. To construct an optimum and excellent smart artificial GPx, a novel supramolecular microgel artificial GPx (SM-Te) was prepared based on the supramolecular host-guest interaction employing the tellurium-containing guest molecule (ADA-Te-ADA) and the cyclodextrin-containing host block copolymer (poly(N-isopropylacrylamide)-b-[polyacrylamides-co-poly(6-o-(triethylene glycol monoacrylate ether)-β-cyclodextrin)], PPAM-CD) as building blocks. Subsequently, based on these building blocks, SM-Te was constructed and the formation of its self-assembled structure was confirmed by dynamic light scattering, NMR, SEM, TEM, etc. Typically, benefitting from the temperature responsive properties of the PNIPAM scaffold, SM-Te also exhibited similar temperature responsive behaviour. Importantly, the GPx catalytic rates of SM-Te displayed a noticeable temperature responsive characteristic. Moreover, SM-Te exhibited the typical saturation kinetics behaviour of a real enzyme catalyst. It was proved that the changes of the hydrophobic microenvironment and the pore size in the supramolecular microgel network of SM-Te played significant roles in altering the temperature responsive catalytic behaviour. The successful construction of SM-Te not only overcomes the insurmountable disadvantages existing in previous covalent bond crosslinked microgel artificial GPx but also bodes well for the development of novel intelligent antioxidant drugs.

Two-dimensional self-assembly of amphiphilic porphyrins on a dynamically shrinking droplet surface

Developing a new field of molecular self-assembly in the sub-micrometer regime-with precision as high as that used to make discrete nano-sized molecular architectures through molecular design-is a major challenge for supramolecular chemistry. At present, however, there is no effective strategy for controlling the assembling molecules when their quantity is greater than one thousand. Herein, we propose a potential solution by exploiting a novel supramolecular system in conjunction with dynamically shrinking oil droplets, enabling more than a thousand component molecules to organize simultaneously into the form of sub-micrometer-scale ring structures. In our developed system, amphiphilic porphyrins, having potential two-dimensional assembling ability, were compartmentalized into droplets with narrow distributions and molecular numbers. These droplets were subsequently transformed into discrete ring-like structures during the process of solvent removal from the inner organic layer, i.e., shrinking the droplets. Unique self-assembled structures, which are not accessible through conventional supramolecular strategies, can be selectively created depending on the initial stage of the droplet.

Triphenylalanine peptides self-assemble into nanospheres and nanorods that are different from the nanovesicles and nanotubes formed by diphenylalanine peptides

Understanding the nature of the self-assembly of peptide nanostructures at the molecular level is critical for rational design of functional bio-nanomaterials. Recent experimental studies have shown that triphenylalanine(FFF)-based peptides can self-assemble into solid plate-like nanostructures and nanospheres, which are different from the hollow nanovesicles and nanotubes formed by diphenylalanine(FF)-based peptides. In spite of extensive studies, the assembly mechanism and the molecular basis for the structural differences between FFF and FF nanostructures remain poorly understood. In this work, we first investigate the assembly process and the structural features of FFF nanostructures using coarse-grained molecular dynamics simulations, and then compare them with FF nanostructures. We find that FFF peptides spontaneously assemble into solid nanometer-sized nanospheres and nanorods with substantial β-sheet contents, consistent with the structural properties of hundred-nanometer-sized FFF nano-plates characterized by FT-IR spectroscopy. Distinct from the formation mechanism of water-filled FF nanovesicles and nanotubes reported in our previous study, intermediate bilayers are not observed during the self-assembly process of FFF nanospheres and nanorods. The peptides in FFF nanostructures are predominantly anti-parallel-aligned, which can form larger sizes of β-sheet-like structures than the FF counterparts. In contrast, FF peptides exhibit lipid-like assembly behavior and assemble into bilayered nanostructures. Furthermore, although the self-assembly of FF and FFF peptides is mostly driven by side chain-side chain (SC-SC) aromatic stacking interactions, the main chain-main chain (MC-MC) interactions also play an important role in the formation of fine structures of the assemblies. The delicate interplay between MC-MC and SC-SC interactions results in the different nanostructures formed by the two peptides. These findings provide new insights into the structure and self-assembly pathway of di-/tri-phenylalanine peptide assemblies, which might be helpful for the design of bioinspired nanostructures.

Environment-sensitive fluorescent supramolecular nanofibers for imaging applications

The combination of an environment-sensitive fluorophore, 4-nitro-2,1,3-benzoxadiazole (NBD), and peptides have yielded supramolecular nanofibers with enhanced cellular uptake, brighter fluorescence, and significant fluorescence responses to external stimuli. We had designed and synthesized NBD-FFYEEGGH that can form supramolecular nanofibers and emit brighter than its counterpart of NBD-EEGGH without the self-assembling property. The nanofibers of NBD-FFYEEGGH could specifically bind to Cu(2+), leading to the formation of fluorescence quenched elongated nanofibers. This fluorescence quenching property was enhanced in self-assembling nanofibers and could be applied for detection of Cu(2+) in vitro and within cells. In a further step, an enzyme-cleavable DEVD peptide was placed between NBD-FFY and the copper binding tripeptide GGH. The resulting self-assembling peptide NBD-FFFDEVDGGH also showed strong fluorescence quenching to Cu(2+). Upon the enzymatic cleavage to remove the Cu(2+)-binding GGH tripeptide from the peptide, the fluorescence was restored. The cellular uptake of nanofibers was better than that of free molecules because of endocytosis. The supramolecular nanofibers with fluorescence turn-on property could therefore be applied for detection of caspase-3 activity in vitro and within cells. We believe that the combination of environment-sensitive fluorescence and fast responses of supramolecular nanostructures would lead to a useful platform to detect many important analytes.

Direct exfoliation of carbon allotropes with structural analogues of self-assembled nanostructures and their photovoltaic applications

The self-assembled nanostructures of amphiphilic molecules enabled a direct exfoliation of carbon allotropes, which were successfully introduced into the HTL layer.

Dual responsive supramolecular amphiphiles: guest molecules dictate the architecture of pyridinium-tailored anthracene assemblies

By introducing an electron-deficient guest molecule and a counter anion, the assembly morphology of 1-[11-(2-anthracenylmethoxy)-11-oxoundecyl]pyridinium bromide (2-AP) was transformed to microsheets and nanofibers from microtubes, respectively.

Oligo(phenylenevinylene) hybrids and self-assemblies: versatile materials for excitation energy transfer

The engineering of the nanostructure of OPV based self-assemblies allows control of photoinduced energy transfer processes, leading to materials exhibiting tunable luminescence colours, including white.

Bulk and solution properties of a thermo-responsive rod–coil block polymer based on poly(N-isopropylacrylamide)

Molecular weight dependence on thermoresponsive behaviors of rod–coil diblock copolymers (x indicates the DP of rod PHIPPVTA blocks).

Hierarchical growth of fluorescent dye aggregates in water by fusion of segmented nanostructures

Dye aggregates are becoming increasingly attractive for diverse applications, in particular as organic electronic and sensor materials. However, the growth processes of such aggregates from molecular to small assemblies up to nanostructures is still not properly understood, limiting the design of materials' functional properties. Here we elucidate the supramolecular growth process for an outstanding class of functional dyes, perylene bisimides (PBIs), by transmission electron microscopy (TEM), cryogenic scanning electron microscopy (cryo-SEM), and atomic force microscopy (AFM). Our studies reveal a sequential growth of amphiphilic PBI dyes from nanorods into nanoribbons in water by fusion and fission processes. More intriguingly, the fluorescence observed for higher hierarchical order nanoribbons was enhanced relative to that of nanorods. Our results provide insight into the relationship between molecular, morphological, and functional properties of self-assembled organic materials.

Development of toroidal nanostructures by self-assembly: rational designs and applications

Toroidal nanostructures are symmetrical ring-shaped structures with a central internal pore. Interestingly, in nature, many transmembrane proteins such as β-barrels and α-helical bundles have toroidal shapes. Because of this similarity, toroidal nanostructures can provide a template for the development of transmembrane channels. However, because of the lack of guiding principles for the construction of toroids, researchers have not widely studied the self-assembly of toroidal nanostructures as compared with the work on other supramolecular architectures. In this Account, we describe our recent efforts to construct toroidal nanostructures through the self-assembly of rationally designed building blocks. In one strategy for building these structures, we induce interfacial curvatures within the building blocks. When we laterally graft a bulky hydrophilic segment onto a p-oligophenyl rod or β-sheet peptides, the backbones of the self-assembled structures can bend in response to the steric effect of these large side groups, driving the p-oligophenyl rod or β-sheet peptides to form nanosized toriods. In another strategy, we can build toroids from bent-shaped building blocks by stacking the macrocycles. Aromatic segments with an internal angle of 120° can associate with each other in aqueous solution to form a hexameric macrocycle. Then these macrocycles can stack on top of each other via hydrophobic and π-π interactions and form highly uniform toroidal nanostructures. We provide many examples that illustrate these guiding principles for constructing toroidal nanostructures in aqueous solution. Efforts to create toroidal nanostructures through the self-assembly of elaborately designed molecular modules provide a fundamental approach toward the development of artificial transmembrane channels. Among the various toroids that we developed, a few nanostructures can insert into lipid membranes and allow limited transport in vesicles.

Cooperatively enhanced receptors for biomimetic molecular recognition

The concept of preorganization suggests that organizing a receptor around its guest during binding is detrimental, because the cost of conformational change is assumed to be paid out of the binding energy. Although this concept has historically guided the synthesis of a great many synthetic hosts, in recent years, chemists have begun to synthesize receptors that resemble proteins in their cooperative conformational changes. Such changes could enhance the host-guest interactions, in particular if the binding of the guest triggers previously unengaged noncovalent interactions within the host. These hosts, referred to as cooperatively enhanced receptors, corroborate with their biological counterparts to support the approach of creating high-affinity receptors through the combined strategies of cooperativity and preorganization. Solvents, often the invisible participants of any solution-based supramolecular process, should be properly considered in the design of synthetic receptors, whether preorganized or cooperatively enhanced.

Development of structural complexity by liquid-crystal self-assembly

Since the discovery of the liquid-crystalline state of matter 125 years ago, this field has developed into a scientific area with many facets. This Review presents recent developments in the molecular design and self-assembly of liquid crystals. The focus is on new exciting soft-matter structures distinct from the usually observed nematic, smectic, and columnar phases. These new structures have enhanced complexity, including multicompartment and cellular structures, periodic and quasiperiodic arrays of spheres, and new emergent properties, such as ferroelctricity and spontaneous achiral symmetry-breaking. Comparisons are made with developments in related fields, such as self-assembled monolayers, multiblock copolymers, and nanoparticle arrays. Measures of structural complexity used herein are the size of the lattice, the number of distinct compartments, the dimensionality, and the logic depth of the resulting supramolecular structures.

Hierarchical self-assembly: well-defined supramolecular nanostructures and metallohydrogels via amphiphilic discrete organoplatinum(II) metallacycles

Metallacyclic cores provide a scaffold upon which pendant functionalities can be organized to direct the formation of dimensionally controllable nanostructures. Because of the modularity of coordination-driven self-assembly, the properties of a given supramolecular core can be readily tuned, which has a significant effect on the resulting nanostructured material. Herein we report the efficient preparation of two amphiphilic rhomboids that can subsequently order into 0D micelles, 1D nanofibers, or 2D nanoribbons. This structural diversity is enforced by three parameters: the nature of the hydrophilic moieties decorating the parent rhomboids, the concentration of precursors during self-assembly, and the reaction duration. These nanoscopic constructs further interact to generate metallohydrogels at high concentrations, driven by intermolecular hydrophobic and π-π interactions, demonstrating the utility of coordination-driven self-assembly as a first-order structural element for the hierarchical design of functional soft materials.

Poly(oxyethylene sugaramide)s: unprecedented multihydroxyl building blocks for tumor-homing nanoassembly

Hydrogen bonding is a major intermolecular interaction for self-assembly occurring in nature. Here we report novel polymeric carbohydrates, i.e., poly(oxyethylene galactaramide)s (PEGAs), as biomimetic building blocks to construct hydrogen bond-mediated self-assembled nanoparticles that are useful for biomedical in vivo applications. PEGAs were conceptually designed as a biocompatible hybrid between polysaccharide and poly(ethylene glycol) (PEG) to attain multivalent hydrogen bonding as well as fully hydrophilic, non-ionic and antifouling characteristics. It was revealed that PEGAs are capable of homospecies hydrogen bonding in water and constructing multi-chain assembled nanoparticles whose structural integrity is highly stable with varying concentration, temperature and pH. Using near-infrared fluorescence imaging we demonstrate facile blood circulation and efficient tumor accumulation of the self-assembled PEGA nanoparticles that were intravenously injected into mice. These in vivo behaviors elucidate the combined merits of our design strategy, i.e., biocompatible chemical constitution capable of multivalent hydrogen bonding, antifouling properties, minimal cell interaction and mesoscopic colloidal self-assembly, as well as size-motivated tumor targeting.

Stimulus-responsive azobenzene supramolecules: fibers, gels, and hollow spheres

Novel, stimulus-responsive supramolecular structures in the form of fibers, gels, and spheres, derived from an azobenzene-containing benzenetricarboxamide derivative, are described. Self-assembly of tris(4-((E)-phenyldiazenyl)phenyl)benzene-1,3,5-tricarboxamide (Azo-1) in aqueous organic solvent systems results in solvent dependent generation of microfibers (aq DMSO), gels (aq DMF), and hollow spheres (aq THF). The results of a single crystal X-ray diffraction analysis of Azo-1 (crystallized from a mixture of DMSO and H2O) reveal that it possesses supramolecular columnar packing along the b axis. Data obtained from FTIR analysis and density functional theory (DFT) calculation suggest that multiple hydrogen bonding modes exist in the Azo-1 fibers. UV irradiation of the microfibers, formed in aq DMSO, causes complete melting while regeneration of new fibers occurs upon visible light irradiation. In addition to this photoinduced and reversible phase transition, the Azo-1 supramolecules display a reversible, fiber-to-sphere morphological transition upon exposure to pure DMSO or aq THF. The role played by amide hydrogen bonds in the morphological changes occurring in Azo-1 is demonstrated by the behavior of the analogous, ester-containing tris(4-((E)-phenyldiazenyl)phenyl)benzene-1,3,5-tricarboxylate (Azo-2) and by the hydrogen abstraction in the presence of fluoride anions.

Synthesis of polyaniline with low polydispersity by using a supramolecular ionic assembly as the reaction medium

A supramolecular ionic assembly comprised of an anionic oligo(phenylene ethynylene) and anilinium cations provides a unique reaction medium in which anilinum cations are concentrated and aligned. The oxidative polymerization (see figure) of aniline using the supramolecular ionic assembly (gray) yielded polyaniline (green/blue) with a number-average molar mass of 20,500 and polydispersity of 1.3.

Smart wormlike micelles

A major scientific challenge of the past decade pertaining to the field of soft matter has been to craft 'adaptable' materials, inspired by nature, which can dynamically alter their structure and functionality on demand, in response to triggers produced by environmental changes. Amongst these, 'smart' surfactant wormlike micelles, responsive to external stimuli, are a particularly recent area of development, yet highly promising, given the versatility of the materials but simplicity of the design-relying on small amphiphilic molecules and their spontaneous self-assembly. The switching 'on' and 'off' of the micellar assembly structures has been reported using electrical, optical, thermal or pH triggers and is now envisaged for multiple stimuli. The structural changes, in turn, can induce major variations in the macroscopic characteristics, affecting properties such as viscosity and elasticity and sometimes even leading to a spontaneous and effective 'sol-gel' transition. These original smart materials based on wormlike micelles have been successfully used in the oil industry, and offer a significant potential in a wide range of other technological applications, including biomedicine, cleaning processes, drag reduction, template synthesis, to name but a few. This review will report results in this field published over the last few years, describe the potential and practical applications of stimuli-responsive wormlike micelles and point out future challenges.