Supramolecular Polymers in Aqueous Media

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.

Tubular supramolecular alternating copolymers fabricated by cyclic peptide-polymer conjugates

Supramolecular copolymers are an emerging class of materials, which bring together different properties and functionalities of multiple components via noncovalent interactions. While it is widely acknowledged that the repeating unit sequence plays an essential role on the performance of these materials, mastering and tuning the supramolecular copolymer sequence is still an open challenge. To date, only statistical supramolecular copolymers have been reported using cyclic peptide-polymer conjugates as building blocks. To enrich the diversity of tubular supramolecular copolymers, we report here a strategy of controlling their sequences by introducing an extra complementary noncovalent interaction. Hence, two conjugates bearing one electron donor and one electron acceptor, respectively, are designed. The two conjugates can individually assemble into tubular supramolecular homopolymers driven by the multiple hydrogen bonding interactions between cyclic peptides. However, the complementary charge transfer interaction between the electron donor and acceptor makes each conjugate more favorable for complexing with its counterpart, resulting in an alternating sequence of the supramolecular copolymer. Following the same principle, more functional supramolecular alternating copolymers are expected to be designed and constructed via other complementary noncovalent interactions (electrostatic interactions, metal coordination interactions, and host-guest interactions, etc.).

Layered assembly of cationic and anionic supramolecular polymers

The chemical synthesis and the supramolecular assembly of an aromatic oligoamine are described. The self-assembly of the cationic oligomers in aqueous solution leads to the formation of vesicular objects. The assembly process of the oligomers is monitored by absorption and fluorescence spectroscopy and the formed vesicles are characterized by atomic force and transmission electron microscopy. The electrostatic complementarity of anionic supramolecular polymers sheets and the cationic vesicles is used for a layered assembly process.

Cucurbituril Based Luminescent Materials in Aqueous Media and Solid State

Cucurbit[n]urils, the pumpkin shaped macrocyclic host molecules possessing a hydrophobic cavity and two identical carbonyl portals, have drawn a lot of attention in recent years due to their high-affinity yet dynamic molecular recognition properties in water. The reversible and stimuli-responsive nature of their host-guest complexes imparts "smart" features leading to materials with intriguing optical, mechanical and morphological properties. In this review, we focus on the design of cucurbituril based luminescent materials in aqueous media as well in solid or film state. The design principles of fluorescent complexes, small assemblies as well as supramolecular polymers along with their stimuli-responsive properties and applications in diverse areas such as optoelectronic devices, light harvesting, anti-counterfeiting and information technology, cell imaging, etc are highlighted with selected examples from recent literature. We also discuss examples of room temperature phosphorescent materials derived from purely organic luminogens in the presence of cucurbiturils.

Unveiling Polymerization Mechanism in pH-regulated Supramolecular Fibers in Aqueous Media

An amine functionalized C₃‐symmetric benzotrithiophene (BTT) monomer has been designed and synthetized in order to form pH responsive one‐dimensional supramolecular polymers in aqueous media. While most of the reported studies looked at the effect of pH on the size of the aggregates, herein, a detailed mechanistic study is reported, carried out upon modifying the pH to trigger the formation of positively charged ammonium groups. A dramatic and reversible change in the polymerization mechanism and size of the supramolecular fibers is observed and ascribed to the combination of Coulombic repulsive forces and higher monomer solubility. Furthermore, the induced frustrated growth of the fibers is further employed to finely control the one‐dimensional supramolecular polymerisation and copolymerization processes.

Self-assembly of neutral platinum complexes possessing chiral hydrophilic TEG chains

Neutral platinum complexes that possess chiral triethylene glycol (TEG) moieties were synthesized. The platinum complexes formed helically twisted stacked assemblies in chloroform and toluene, which were studied by ¹H NMR, UV/vis spectroscopy, and emission spectroscopy. On the other hand, emissive micellar aggregates were observed in a THF/water mixed solvent. Dynamic light scattering (DLS) experiments revealed that micellar aggregates with a diameter (d) of ≈100 nm emitted strong light, whereas the monomeric form and large aggregates (d > 500 nm) did not show luminescence efficiently. Furthermore, the micellar aggregates were twisted chirally, where the twisted direction was determined by the chirality of the TEG moieties. The assemblies were observed to be solvent responsive, which allows for the modulation of the nanostructure by changing the solvent polarity.

A comprehensive survey upon diverse and prolific applications of chitosan-based catalytic systems in one-pot multi-component synthesis of heterocyclic rings

Heterocyclic compounds are among the most prestigious and valuable chemical molecules with diverse and magnificent applications in various sciences. Due to the remarkable and numerous properties of the heterocyclic frameworks, the development of efficient and convenient synthetic methods for the preparation of such outstanding compounds is of great importance. Undoubtedly, catalysis has a conspicuous role in modern chemical synthesis and green chemistry. Therefore, when designing a chemical reaction, choosing and or preparing powerful and environmentally benign simple catalysts or complicated catalytic systems for an acceleration of the chemical reaction is a pivotal part of work for synthetic chemists. Chitosan, as a biocompatible and biodegradable pseudo-natural polysaccharide is one of the excellent choices for the preparation of suitable catalytic systems due to its unique properties. In this review paper, every effort has been made to cover all research articles in the field of one-pot synthesis of heterocyclic frameworks in the presence of chitosan-based catalytic systems, which were published roughly by the first quarter of 2020. It is hoped that this review paper can be a little help to synthetic scientists, methodologists, and catalyst designers, both on the laboratory and industrial scales.

Water-Soluble Supramolecular Polymers of Paired and Stacked Heterocycles: Assembly, Structure, Properties, and a Possible Path to Pre-RNA

The hypothesis that RNA and DNA are products of chemical and biological evolution has motivated our search for alternative nucleic acids that may have come earlier in the emergence of life-polymers that possess a proclivity for covalent and non-covalent self-assembly not exhibited by RNA. Our investigations have revealed a small set of candidate ancestral nucleobases that self-assemble into hexameric rosettes that stack in water to form long, twisted, rigid supramolecular polymers. These structures exhibit properties that provide robust solutions to long-standing problems that have stymied the search for a prebiotic synthesis of nucleic acids. Moreover, their examination by experimental and computational methods provides insight into the chemical and physical principles that govern a particular class of water-soluble one-dimensional supramolecular polymers. In addition to efficient self-assembly, their lengths and polydispersity are modulated by a wide variety of positively charged, planar compounds; their assembly and disassembly are controlled over an exceedingly narrow pH range; they exhibit spontaneous breaking of symmetry; and homochirality emerges through non-covalent cross-linking during hydrogel formation. Some of these candidate ancestral nucleobases spontaneously form glycosidic bonds with ribose and other sugars, and, most significantly, functionalized forms of these heterocycles form supramolecular structures and covalent polymers under plausibly prebiotic conditions. This Perspective recounts a journey of discovery that continues to reveal attractive answers to questions concerning the origins of life and to uncover the principles that control the structure and properties of water-soluble supramolecular polymers.

Elucidating dynamic behavior of synthetic supramolecular polymers in water by hydrogen/deuterium exchange mass spectrometry

A comprehensive understanding of the structure, self-assembly mechanism, and dynamics of one-dimensional supramolecular polymers in water is essential for their application as biomaterials. Although a plethora of techniques are available to study the first two properties, there is a paucity in possibilities to study dynamic exchange of monomers between supramolecular polymers in solution. We recently introduced hydrogen/deuterium exchange mass spectrometry (HDX-MS) to characterize the dynamic nature of synthetic supramolecular polymers with only a minimal perturbation of the chemical structure. To further expand the application of this powerful technique some essential experimental aspects have been reaffirmed and the technique has been applied to a diverse library of assemblies. HDX-MS is widely applicable if there are exchangeable hydrogen atoms protected from direct contact with the solvent and if the monomer concentration is sufficiently high to ensure the presence of supramolecular polymers during dilution. In addition, we demonstrate that the kinetic behavior as probed by HDX-MS is influenced by the internal order within the supramolecular polymers and by the self-assembly mechanism.

Directional Supramolecular Polymerization in a Dynamic Microsolution: A Linearly Moving Polymer's End Striking Monomers

Although directional chain reactions are common in nature's self-assembly processes and in covalent polymerizations, it has been challenging to perform such processes in artificial one-dimensional self-assembling systems. In this paper, we describe a system, employing perylene bisimide (PBI) derivatives as monomers, for selectively activating one end of a supramolecular polymer during its growth and, thereby, realizing directional supramolecular polymerization. Upon introduction of a solution containing only a single PBI monomer into the microflow channel, nucleation was induced spontaneously. The dependency of the aggregation efficiency on the flow rate suggested that the shear force facilitated collisions among the monomers to overcome the activation energy required for nucleation. Next, by introducing a solution containing both monomer and polymer, we investigated how the shear force influenced the monomer-polymer interactions. In situ fluorescence spectra and linear dichroism revealed that growth of the polymers was accelerated only when they were oriented under the influence of shear stress. Upon linear motion of the oriented polymer, polymer growth at that single end became predominant relative to the nucleation of freely diffusing monomers. When applying this strategy to a two-monomer system, the second (less active) monomer reacted selectively at the forward-facing terminus of the first polymer, leading to the creation of a diblock copolymer through formation of a molecular heterojunction. This strategy-friction-induced activation of a single end of a polymer-should be applicable more generally to directional supramolecular block copolymerizations of various functional molecules, allowing molecular heterojunctions to be made at desired positions in a polymer.

Molecular Recognition Driven Bioinspired Directional Supramolecular Assembly of Amphiphilic (Macro)molecules and Proteins

Bioinspired self-assembly has been explored with diverse synthetic scaffolds, among which amphiphiles are perhaps the most extensively studied systems. Classical surfactants or amphiphilic block copolymers, depending on the hydrophobic-hydrophilic balance, produce distinct nanostructures, which hold promise for applications ranging from biology to materials sciences. Nevertheless, their immiscibility-driven aggregation does not provide the opportunity to precisely regulate the internal order, morphology, or functional group display, which is highly desirable, especially in the context of biological applications.A new class of amphiphiles have emerged in the recent past in which the hydrophilic segment(s) is appended with a hydrophobic supramolecular-structure-directing-unit (SSDU), consisting of a π-conjugated chromophore and a H-bonding group. Self-recognition of the SSDU by attractive directional interactions governs the supramolecular assembly, which is fundamentally different than the repulsive solvent-immiscibility driven aggregation of traditional amphiphiles. Such SSDU-appended hydrophilic polymers exhibit entropy-driven highly stable self-assembly producing distinct nanostructures depending on the H-bonding functional group. For example, polymers with the hydrazide-functionalized SSDU attached form a polymersome, while in a sharp contrast, the same polymers when connected to an amide containing SSDU produce a cylindrical micelle via a spherical-micelle intermediate. This relationship holds true for a series of SSDU-attached hydrophilic polymers irrespective of the hydrophobic/hydrophilic balance or chemical structure, indicating that the supramolecular-assembly is primarily controlled by the specific molecular-recognition motif of the SSDU, instead of the packing parameter-based norms. Beyond synthetic polymers, SSDU-attached proteins also exhibit similar molecular-recognition driven self-assembly as well as coassembly with SSDU-attached polymers or hydrophilic wedges, producing multi-stimuli-responsive nanostructures in which the protein gains remarkable protection from thermal denaturation or enzymatic hydrolysis and exhibits redox-responsive enzymatic activity.Furthermore, SSDU-derived bola-shape π-amphiphiles have been recognized as a useful scaffold for the synthesis of unsymmetric polymersomes, rarely reported in the literature. The building block consists of a hydrophobic naphthalene-diimide (NDI) π-system attached to a hydrophilic functional group (ionic or nonionic) and a nonionic wedge on its two opposite arms. Extended H-bonding among the hydrazide groups, placed only on one side of the central chromophore by design, ensures stacking of the NDIs with parallel orientation and induces a preferred direction of curvature so that the H-bonded chain and consequently the functional groups attached to the same side remain at the inner-wall of the supramolecular polymersome. Automatically, the functional groups, located on the other side, are displayed at the outer surface. This design works for different amphiphiles, which by virtue of efficient and predictable functional group display, strongly influences the multivalent binding with different biological targets resulting in efficient enzyme inhibition, glycocluster effect, or antibacterial activity, depending on the nature of the functional group. By taking advantage of the electron accepting nature of the NDI, electron rich pyrene-containing amphiphiles can be costacked in alternating sequence, producing temperature and redox-responsive supramolecular polymers with NDI/pyrene stoichiometry-dependent morphology, lower critical solution temperature (LCST), functional group display, and antibacterial activity.

Hierarchical self-assemblies of carnosine asymmetrically functioned perylene diimide with high optoelectronic response

HYPOTHESIS

Taking advantage of photoinduced electron transfer, one dimensional organic nanomaterials with tunable donor-acceptor (D-A) interface provide a promising avenue to get high optoelectric properties. However, strong charge transfer interaction between D and A segments impedes the formation of long-range ordered structure, which limits the charge transport through efficient π electronic delocalization. Incorporation of chiral peptide offering various hydrogen bonding (H-bonding) along with asymmetric molecular structure enables substantially controllable D-A interface and tunable organization of the π-conjugates.

EXPERIMENTS

A new amphiphilic perylene diimide (CUPDI) with PDI as an acceptor is designed and synthesized. A polar chiral dipeptide composed of β-alanine and l-histidine with the imidazole ring as the donor i.e., l-carnosine, is incorporated at one of imides. Transition of various supramolecular assemblies of CUPDI is realized by changing CUPDI concentration and solvents. The photoelectronic properties of the assemblies are investigated as well as their association with the microstructure of the nanomaterials.

FINDINGS

Delicately tuned hydrogen bonds between the peptides and π-π interaction between PDI cores in different solvents enable the formation of assemblies with multifarious microstructures such as small spherical aggregates, nanowires with uniform diameter, nanobelts, and irregular aggregates. The maximum amount of photocurrent enhancement is up to 1.08 µA observed for the nanobelt, four times higher than that of irregular aggregates. However, the nanowires show the best performance of 7.1-fold in response to ammonia. Thus, the photoelectric performances are strongly dependent on the the molecular arrangement within the nanomaterials.

Dually cross-linked single networks: structures and applications

Cross-linked polymers have attracted an immense attention over the years, however, there are many flaws of these systems, e.g. softness and brittleness; such materials possess non-adjustable properties and cannot recover from damage and thus are limited in their practical applications. Supramolecular chemistry offers a variety of dynamic interactions that when integrated into polymeric gels endow the systems with reversibility and responsiveness to external stimuli. A combination of different cross-links in a single gel could be the key to tackle these drawbacks, since covalent or chemical cross-linking serve to maintain the permanent shape of the material and to improve overall mechanical performance, whereas non-covalent cross-links impart dynamicity, reversibility, stimuli-responsiveness and often toughness to the material. In the present review we sought to give a comprehensive overview of the progress in design strategies of different types of dually cross-linked single gels made by researchers over the past decade as well as the successful implementations of these advances in many demanding fields where versatile multifunctional materials are required, such as tissue engineering, drug delivery, self-healing and adhesive systems, sensors as well as shape memory materials and actuators.

Globular Aggregates Stemming from the Self-Assembly of an Amphiphilic N-Annulated Perylene Bisimide in Aqueous Media

Herein, we describe the synthesis of highly emissive amphiphilic N-annulated PBI 1 decorated with oligo ethylene glycol (OEG) side chains. These polar side chains allow the straightforward solubility of 1 in solvents of different polarity such as water, iPrOH, dioxane, or chloroform. Compound 1 self-assembles in aqueous media by π-stacking of the aromatic units and van der Waals interactions, favored by the hydrophobic effect. The hypo- and hypsochromic effect observed in the UV-Vis spectra of 1 in water in comparison to chloroform is diagnostic of H-type aggregation. Solvent denaturation experiments allow deriving the free Gibbs energy for the self-assembly process in aqueous media and the factor m that is indicative of the influence exerted by a good solvent in the stability of the final aggregates. The ability of compound 1 to self-assemble in water yields globular aggregates that have been visualized by TEM imaging.

Self-Assembly of Photoresponsive Molecular Amphiphiles in Aqueous Media

Amphiphilic molecules, comprising hydrophobic and hydrophilic moieties and the intrinsic propensity to self-assemble in aqueous environment, sustain a fascinating spectrum of structures and functions ranging from biological membranes to ordinary soap. Facing the challenge to design responsive, adaptive, and out-of-equilibrium systems in water, the incorporation of photoresponsive motifs in amphiphilic molecular structures offers ample opportunity to design supramolecular systems that enables functional responses in water in a non-invasive way using light. Here, we discuss the design of photoresponsive molecular amphiphiles, their self-assembled structures in aqueous media and at air-water interfaces, and various approaches to arrive at adaptive and dynamic functions in isotropic and anisotropic systems, including motion at the air-water interface, foam formation, reversible nanoscale assembly, and artificial muscle function. Controlling the delicate interplay of structural design, self-assembling conditions and external stimuli, these responsive amphiphiles open several avenues towards application such as soft adaptive materials, controlled delivery or soft actuators, bridging a gap between artificial and natural dynamic systems.

Enzyme-Induced Supramolecular Order in Pyrene Dipeptide Hydrogels for the Development of an Efficient Energy-Transfer Template

Peptide self-assembly is gathering much attention due to the precise control it provides for the arrangement of functional moieties for the fabrication of advanced functional materials. It is desirable to use a physical, chemical, or biological trigger that can control the self-assembly process. In the current article, we have applied an enzyme to induce the peptide self-assembly of an aromatic peptide amphiphile, which modulates the supramolecular order in the final gel phase material. We accessed diverse peptide hydrogels from identical gelator concentrations by simply changing the enzyme concentration, which controlled the reaction kinetics and influenced the dynamics of self-assembly. Depending upon the concentration of the enzyme, a bell-shaped relationship was observed in terms of intermolecular interactions, morphology, and properties of the final gel phase material. The access of non-equilibrium structures was further demonstrated by fluorescence emission spectroscopy, circular dichroism spectroscopy, atomic force microscopy, transmission electron microscopy, and rheology. This strategy is applied to construct a charge-transfer hydrogel by doping the donor hydrogel with an acceptor moiety, which exhibits efficient energy transfer. Interestingly, such structural control at the nanoscopic level can further tune the energy-transfer efficiency by simply modulating the enzyme concentration.

Solvent Effects in Supramolecular Chemistry: Linear Free Energy Relationships for Common Intermolecular Interactions

The proper choice of solvent is of major importance for all studies in supramolecular chemistry, including molecular recognition in host-guest systems, intramolecular folding, self-assembly, and supramolecular polymerization. In this Perspective, the usefulness of linear free energy relationships (LFERs) is highlighted to unravel the effect of solvents on coordinate bonding (e.g., cation-crown ether), hydrogen bonding, halogen bonding, dipolar aggregation, and π-π-stacking. For all of these intermolecular interactions widely applied in supramolecular systems, LFER relationships between the Gibbs binding energies and common solvent polarity scales including E_T(30), π*, α or β based on solvatochromic dyes, scales derived from binding processes such as Gutmann donor and acceptor numbers or hydrogen bond donor and acceptor scales, or physical functions like the Kirkwood-Onsager or the Liptay-Onsager functions could be demonstrated. These relationships can now be applied toward a better understanding of the prevailing intermolecular forces for supramolecular interactions. They further enable a rational selection of the most suitable solvent for the preparation of self-assembled materials and the estimation of binding constants without the need for time-consuming comprehensive investigations of solvents.

Post-Functionalization of Supramolecular Polymers on Surface and the Chiral Assembly-Induced Enantioselective Reaction

Although post-functionalization is extensively used to introduce diverse functional groups into supramolecular polymers (SPs) in solution, post-functionalization of SPs on surfaces still remains unexplored. Here we achieved the on-surface post-functionalization of two SPs derived from 5,10,15-tri-(4-pyridyl)-20-bromophenyl porphyrin (Br-TPyP) via cross-coupling reactions on Au(111). The ladder-shaped, Cu-coordinated SPs preformed from Br-TPyP were functionalized through Heck reaction with 4-vinyl-1,1'-biphenyl. In the absence of Cu, Br-TPyP formed chiral SPs as two enantiomers via self-assembly, which were functionalized via divergent cross-coupling reaction with 4-isocyano-1,1'-biphenyl (ICBP). Surprisingly, this reaction was discovered as an enantioselective on-surface reaction induced by the chirality of SPs. Mechanistic analysis and DFT calculations indicated that after debromination of Br-TPyP and the first addition of ICBP, only one attack direction of ICBP to the chiral SP intermediate is permissive in the second addition step due to the steric hindrance, which guaranteed the high enantioselectivity of the reaction.

From Photoinduced Supramolecular Polymerization to Responsive Organogels

Controlling supramolecular polymerization by external stimuli holds great potential toward the development of responsive soft materials and manipulating self-assembly at the nanoscale. Photochemical switching offers the prospect of regulating the structure and properties of systems in a noninvasive and reversible manner with spatial and temporal control. In addition, this approach will enhance our understanding of supramolecular polymerization mechanisms; however, the control of molecular assembly by light remains challenging. Here we present photoresponsive stiff-stilbene-based bis-urea monomers whose trans isomers readily form supramolecular polymers in a wide range of organic solvents, enabling fast light-triggered depolymerization-polymerization and reversible gel formation. Due to the stability of the cis isomers and the high photostationary states (PSS) of the cis-trans isomerization, precise control over supramolecular polymerization and in situ gelation could be achieved with short response times. A detailed study on the temperature-dependent and photoinduced supramolecular polymerization in organic solvents revealed a kinetically controlled nucleation-elongation mechanism. By application of a Volta phase plate to enhance the phase-contrast method in cryo-EM, unprecedented for nonaqueous solutions, uniform nanofibers were observed in organic solvents.

Enthalpically and Entropically Favorable Self-Assembly: Synthesis of C4h -Symmetric Tetraazatetrathia[8]circulenes by Regioselective Introduction of Pyridine Rings

Self-assembly of π-conjugated molecules in solution generally occurs owing to either an enthalpic or an entropic gain; however, designing π-conjugated systems that simultaneously exhibit enthalpically and entropically favorable self-assembly behavior is challenging. Herein, the self-assembly behavior of tetraazatetrathia[8]circulenes is disclosed, which is driven by both enthalpy and entropy. Single-crystal X-ray diffraction analysis demonstrated that molecules of these tetraazatetrathia[8]circulenes form face-to-face stacked dimers with a 1D columnar structure owing to the circularly arranged dipole moments. Importantly, concentration- and temperature-dependent ¹ H NMR spectra revealed that the formation of self-assemblies of tetraazatetrathia[8]circulenes in chloroform and methanol is favored by both enthalpic and entropic factors. The unique association behavior is due to the presence of sp² -hybridized nitrogen atoms, which weakly coordinate to the hydrogen atoms of these solvents and reduce the π-electron density of the circulene cores.

Supramolecular Depolymerization in the Mixture of Two Poor Solvents: Mechanistic Insights and Modulation of Supramolecular Polymerization of Ionic π-Systems

Solvents are fundamentally essential for the synthesis and processing of soft materials. Supramolecular polymers (SPs), an emerging class of soft materials, are usually stable in single and mixtures of poor solvents. In contrast to these preconceived notions, here we report the depolymerization of SPs in the mixture of two poor solvents. This surprising behavior was observed for well-known cationic perylene diimides (cPDIs) in the mixtures of water and amphiphilic organic solvents such as isopropanol (IPA). cPDIs form stable SPs in water and IPA but readily depolymerize into monomers in 50-70 vol% IPA containing water. This is due to the selective solvation of the π-surface of cPDIs by alkyl chains of IPA and ionic side chains by water, as evidenced by molecular dynamic simulations. Moreover, by systematically changing the ratio between water and amphiphilic organic solvent, we could achieve an unprecedented supramolecular polymerization both by increasing and decreasing the solvent polarity.

Peptide-Protein Interactions: From Drug Design to Supramolecular Biomaterials

The self-recognition and self-assembly of biomolecules are spontaneous processes that occur in Nature and allow the formation of ordered structures, at the nanoscale or even at the macroscale, under thermodynamic and kinetic equilibrium as a consequence of specific and local interactions. In particular, peptides and peptidomimetics play an elected role, as they may allow a rational approach to elucidate biological mechanisms to develop new drugs, biomaterials, catalysts, or semiconductors. The forces that rule self-recognition and self-assembly processes are weak interactions, such as hydrogen bonding, electrostatic attractions, and van der Waals forces, and they underlie the formation of the secondary structure (e.g., α-helix, β-sheet, polyproline II helix), which plays a key role in all biological processes. Here, we present recent and significant examples whereby design was successfully applied to attain the desired structural motifs toward function. These studies are important to understand the main interactions ruling the biological processes and the onset of many pathologies. The types of secondary structure adopted by peptides during self-assembly have a fundamental importance not only on the type of nano- or macro-structure formed but also on the properties of biomaterials, such as the types of interaction, encapsulation, non-covalent interaction, or covalent interaction, which are ultimately useful for applications in drug delivery.

Nanoscale Assembly of Functional Peptides with Divergent Programming Elements

Self-assembling peptides are being applied both in the biomedical area and as building blocks in nanotechnology. Their applications are closely linked to their modes of self-assembly, which determine the functional nanostructures that they form. This work brings together two structural elements that direct nanoscale self-association in divergent directions: proline as a β-breaker and the β-structure-associated diphenylalanine motif, into a single tripeptide sequence. Amino acid chirality was found to resolve the tension inherent to these conflicting self-assembly instructions. Stereoconfiguration determined the ability of each of the eight possible Pro-Phe-Phe stereoisomers to self-associate into diverse nanostructures, including nanoparticles, nanotapes, or fibrils, which yielded hydrogels with gel-to-sol transition at a physiologically relevant temperature. Three single-crystal structures and all-atom molecular dynamics simulations elucidated the ability of each peptide to establish key interactions to form long-range assemblies (i,e., stacks leading to gelling fibrils), medium-range assemblies (i.e., stacks yielding nanotapes), or short-range assemblies (i.e., dimers or trimers that further associated into nanoparticles). Importantly, diphenylalanine is known to serve as a binding site for pathological amyloids, potentially allowing these heterochiral systems to influence the fibrillization of other biologically relevant peptides. To probe this hypothesis, all eight Pro-Phe-Phe stereoisomers were tested in vitro on the Alzheimer's disease-associated Aβ(1-42) peptide. Indeed, one nonfibril-forming stereoisomer effectively inhibited Aβ fibrillization through multivalent binding between diphenylalanine motifs. This work thus defined heterochirality as a useful feature to strategically develop future therapeutics to interfere with pathological processes, with the additional value of resistance to protease-mediated degradation and biocompatibility.

Self-Assembly of Bolaamphiphiles into 2D Nanosheets via Synergistic and Meticulous Tailoring of Multiple Noncovalent Interactions

A bolaamphiphile possessing a hydrophobic skeleton and two hydrophilic groups at both ends represents an important class of building blocks toward a rich variety of self-assembled materials for use in ion transport, optoelectronic devices, and drug and gene delivery. Herein, we report a one-step synthesis of an array of rationally designed anionic bolaamphiphiles and unravel the correlation between molecular structure of anionic bolaamphiphiles and their disparate self-assemblies via synergistic and meticulous tailoring of a set of interactions. Intriguingly, by delicately regulating the interactions among these supramolecular interactions, two-dimensional (2D) nanosheets are crafted via self-assembly of anionic bolaamphiphiles. Particularly, single-layered 2D nanosheets are formed through the synergy of aromatic π-π stacking, hydrophobic, hydrogen-bonding, and electrostatic repulsion interactions. In contrast, the selective converting of anionic headgroups of bolaamphiphiles into nonpolar alkyl chain screens the electrostatic repulsion between neighboring bolaamphiphiles while keeping the other segments of bolaamphiphiles intact, thereby allowing them to self-assemble into multilayered 2D nanosheets. Interestingly, the intrinsically charged 2D nanosheets could anchor oppositely charged metal nanoparticles via electrostatic attraction. Conceptually, anionic bolaamphiphile-derived 2D nanosheets may function as a substrate to position a diversity of nanocrystals and conjugated polymers for a broad range of applications in catalysis, optical devices, and photothermal therapy.

Mimicking Biological Recognition: Lessons in Binding Hydrophilic Guests in Water

Selective molecular recognition of hydrophilic guests in water plays a fundamental role in a vast number of biological processes, but synthetic mimicry of biomolecular recognition in water still proves challenging both in terms of achieving comparable affinities and selectivities. This Review highlights strategies that have been developed in the field of supramolecular chemistry to selectively and non-covalently bind three classes of biologically relevant molecules: nucleotides, carbohydrates, and amino acids. As several groups have systematically modified receptors for a specific guest, an evolutionary perspective is also provided in some cases. Trends in the most effective binding forces for each class are described, providing insight into selectivity and potential directions for future work.

Pathway and Length Control of Supramolecular Polymers in Aqueous Media via a Hydrogen Bonding Lock

Programming the organization of π-conjugated systems into nanostructures of defined dimensions is a requirement for the preparation of functional materials. Herein, we have achieved high-precision control over the self-assembly pathways and fiber length of an amphiphilic BODIPY dye in aqueous media by exploiting a programmable hydrogen bonding lock. The presence of a (2-hydroxyethyl)amide group in the target BODIPY enables different types of intra- vs. intermolecular hydrogen bonding, leading to a competition between kinetically controlled discoidal H-type aggregates and thermodynamically controlled 1D J-type fibers in water. The high stability of the kinetic state, which is dominated by the hydrophobic effect, is reflected in the slow transformation to the thermodynamic product (several weeks at room temperature). However, this lag time can be suppressed by the addition of seeds from the thermodynamic species, enabling us to obtain supramolecular polymers of tuneable length in water for multiple cycles.

100th Anniversary of Macromolecular Science Viewpoint: User's Guide to Supramolecular Peptide-Polymer Conjugates

This Viewpoint highlights the design principles and development of peptide-based supramolecular polymers. Here we delve deep into the practicalities of synthesizing and characterizing these macromolecular structures and provide a thorough overview of the benefits and challenges that come with these systems. This Viewpoint emphasizes to beginners and experts alike the importance of understanding the fundamental behavior and self-assembly processes when designing these complex and dynamic functional materials.

Introducing the Dihydro-1,3-azaboroles: Convenient Entry by a Three-Component Reaction, Synthetic and Photophysical Application

The (Fmes)BH₂·SMe₂ reagent (7) reacts sequentially with an acetylene and, e.g., xylylisonitrile in a convenient three-component reaction to give a series of unprecedented dihydro-1,3-azaborole derivatives 16. The tolane-derived example 16a was deprotonated and used as a ligand in organometallic chemistry. Compounds 16 served as the starting materials for the straightforward synthesis of various dihydro-1,3-azaborinine derivatives by treatment with an isonitrile. Several diaryldihydro-1,3-azaboroles showed interesting photophysical properties such as aggregation-induced emission and high fluorescence quantum yields.

Tuning the Molecular Packing of Self-Assembled Amphiphilic PtII Complexes by Varying the Hydrophilic Side-Chain Length

Understanding the relationship between molecular design and packing modes constitutes one of the major challenges in self‐assembly and is essential for the preparation of functional materials. Herein, we have achieved high precision control over the supramolecular packing of amphiphilic Pt^II complexes by systematic variation of the hydrophilic side‐chain length. A novel approach of general applicability based on complementary X‐ray diffraction and solid‐state NMR spectroscopy has allowed us to establish a clear correlation between molecular features and supramolecular ordering. Systematically increasing the side‐chain length gradually increases the steric demand and reduces the extent of aromatic interactions, thereby inducing a gradual shift in the molecular packing from parallel to a long‐slipped organization. Notably, our findings highlight the necessity of advanced solid‐state NMR techniques to gain structural information for supramolecular systems where single‐crystal growth is not possible. Our work further demonstrates a new molecular design strategy to modulate aromatic interaction strengths and packing arrangements that could be useful for the engineering of functional materials based on Pt^II and aromatic molecules.

Supramolecular polymerization of sulfated dendritic peptide amphiphiles into multivalent L-selectin binders

The synthesis of a sulfate-modified dendritic peptide amphiphile and its self-assembly into one-dimensional rod-like architectures in aqueous medium is reported. The influence of the ionic strength on the supramolecular polymerization was probed via circular dichroism spectroscopy and cryogenic transmission electron microscopy. Physiological salt concentrations efficiently screen the charges of the dendritic building block equipped with eight sulfate groups and trigger the formation of rigid supramolecular polymers. Since multivalent sulfated supramolecular structures mimic naturally occurring L-selectin ligands, the corresponding affinity was evaluated using a competitive SPR binding assay and benchmarked to an ethylene glycol-decorated supramolecular polymer.

Regulating the Supramolecular Polymerization of Fibrous Crystalline Structures in Aqueous Solution

Inspired by protein polymerizations, much progress has been made in making "polymer-like" supramolecular structures from small synthetic subunits through non-covalent bonds. A few regulation mechanisms have also been explored in synthetic platforms to create supramolecular polymers and materials with dynamic properties. Herein, a type of reactive regulator that facilitates the dimerization of the monomer precursors through dynamic bonds to trigger the supramolecular assembly from small molecules in an aqueous solution is described. The supramolecular structures are crystalline in nature and the reaction coupled assembly strategy can be extended to a supramolecular assembly of aromatic amide derivatives formed in-situ. The method may be instructive for the development of supramolecular nanocrystalline materials with desired physical properties.

Influence of the Z/E Isomerism on the Pathway Complexity of a Squaramide-Based Macrocycle

The rising interest on pathway complexity in supramolecular polymerization has prompted the finding of novel monomer designs able to stabilize kinetically trapped species and generate supramolecular polymorphs. In the present work, the exploitation of the Z/E (geometrical) isomerism of squaramide (SQ) units to produce various self-assembled isoforms and complex supramolecular polymerization pathways in methylcyclohexane/CHCl₃ mixtures is reported for the first time. This is achieved by using a new bissquaramidic macrocycle (MSq) that self-assembles into two markedly different thermodynamic aggregates, AggA (discrete cyclic structures) and AggB (fibrillar structures), depending on the solvent composition and concentration. Remarkably, UV-vis, ¹ H NMR, and FT-IR experiments together with quantum-chemical calculations indicate that these two distinct aggregates are formed via two different hydrogen bonding patterns (side-to-side in AggA and head-to-tail in AggB) due to different conformations in the SQ units (Z,E in AggA and Z,Z in AggB). The ability of MSq to supramolecularly polymerize into two distinct aggregates is utilized to induce the kinetic-to-thermodynamic transformation from AggA to AggB, which occurs via an on-pathway mechanism. It is believed that this system provides new insights for the design of potential supramolecular polymorphic materials by using squaramide units.

Supramolecular cancer nanotheranostics

Among the many challenges in medicine, the treatment and cure of cancer remains an outstanding goal given the complexity and diversity of the disease. Nanotheranostics, the integration of therapy and diagnosis in nanoformulations, is the next generation of personalized medicine to meet the challenges in precise cancer diagnosis, rational management and effective therapy, aiming to significantly increase the survival rate and improve the life quality of cancer patients. Different from most conventional platforms with unsatisfactory theranostic capabilities, supramolecular cancer nanotheranostics have unparalleled advantages in early-stage diagnosis and personal therapy, showing promising potential in clinical translations and applications. In this review, we summarize the progress of supramolecular cancer nanotheranostics and provide guidance for designing new targeted supramolecular theranostic agents. Based on extensive state-of-the-art research, our review will provide the existing and new researchers a foundation from which to advance supramolecular cancer nanotheranostics and promote translationally clinical applications.

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.

Multiswitchable photoacid-hydroxyflavylium-polyelectrolyte nano-assemblies

Light- and pH-responsive nano-assemblies with switchable size and structure are formed by the association of a photoacid, anthocyanidin, and a linear polyelectrolyte in aqueous solution. Specifically, anionic disulfonated naphthol derivatives, neutral hydroxyflavylium, and cationic poly(allylamine) are used as building blocks for the ternary electrostatic self-assembly, forming well-defined supramolecular assemblies with tunable sizes of 50 to 500 nm. Due to the network of possible chemical reactions for the anthocyanidin and the excited-state dissociation of the photoacid upon irradiation, different ways to alter the ternary system through external triggering are accessible. The structure and trigger effects can be controlled through the component ratios of the samples. Dynamic and static light scattering (DLS, SLS) and ζ-potential measurements were applied to study the size and the stability of the particles, and information on the molecular structure was gained by UV-vis spectroscopy. Isothermal titration calorimetry (ITC) provided information on the thermodynamics and interaction forces in the supramolecular assembly formation.

The effect of amide bond orientation and symmetry on the self-assembly and gelation of discotic tripeptides

A series of discotic tripeptides containing a rigid aromatic core and l-phenylalanine have been developed. The orientation of the amide bonds yielded variations of the structure and self-assembly properties of the compounds. The aggregation behavior of the discotic tripeptides was studied by various spectroscopic techniques. The morphology of the resulting aggregates was studied by field emission electron microscopy and atomic force microscopy. These studies showed that the orientation of the amide bonds has a strong influence on the intermolecular interactions, resulting in huge differences in the aggregation properties, and morphology of the discotic tripeptides. Only the C₃-symmetric discotic tripeptides formed organogels. The supramolecular aggregation mechanism of N-centered and C[double bond, length as m-dash]O-centered discotic tripeptides for forming 3-fold intermolecular H-bonded helical column were the same, there was only a smaller enthalpy change due to the occurrence of longer distances for the N-HO[double bond, length as m-dash]C bonds of the N-centered discotic tripeptide. Whereas, the C₂-symmetric discotic tripeptides 2 and 3 adopted a 6-fold intermolecular H-bonded dimer structure. Thus, this report presents a valuable approach for the fine-tuning of the discotic tripeptide based functional material.

Self-assembly of supramolecular chemoenzymatic poly-l-phenylalanine

The self-assembly behavior of chemoenzymatic high molecular weight (ca. 30 000 Da) poly-l-phenylalanine (ePLP) and the nano-morphologies thereof are investigated.

Unexpected thermo-responsiveness of bisurea-functionalized hydrophilic polymers in water

HYPOTHESIS

Thermoresponsive polymers are important materials for various applications. However, the number of polymers that exhibit this property in the temperature range of interest remains limited. The development of novel rational design strategies through the understanding of the thermal transition's origin is therefore of utmost importance.

EXPERIMENTS

Bisurea-functionalized water-soluble polymers were synthesized by RAFT polymerization. After direct dissolution in water, the supramolecular assemblies were analyzed by cryo-TEM and SANS. Their temperature-dependent water-solubility was characterized by various techniques, namely DLS, SANS, DSC, IR, to understand the origin of the temperature sensitivity.

FINDINGS

The supramolecular assemblies exhibit an unexpected temperature-dependent solubility. For instance, a cloud point of only 39 °C was measured for poly(N,N-dimethylacrylamide) assemblies. This property is not restricted to one specific polymer but is rather a general feature of bisurea-functionalized polymers that form supramacromolecular bottlebrushes in water. The results highlight the existence of two distinct transitions; the first one is a visually perceptible cloud point due to the aggregation of individual micelles, presumably driven by the hydrophobic effect. The second transition is related to the dissociation of intermolecular bisurea hydrogen bonds. Finally, we show that it is possible to widely tune the cloud point temperature through the formation of co-assemblies.

Supramolecular brush polymers prepared from 1,3,4-oxadiazole and cyanobutoxy functionalised pillar[5]arene for detecting Cu2+

The self-assembly of an A1/A2 disubstituted pillar[5]arene was used to construct a supramolecular brush polymer.