Structural and Functional Evolution of a Library of Constitutional Dynamic Polymers Driven by Alkali Metal Ion Recognition

Efficient enumeration-selection computational strategy for adaptive chemistry

Design problems of finding efficient patterns, adaptation of complex molecules to external environments, affinity of molecules to specific targets, dynamic adaptive behavior of chemical systems, reconstruction of 3D structures from diffraction data are examples of difficult to solve optimal design or inverse search problems. Nature inspires evolution strategies to solve design problems that are based on selection of successful adaptations and heritable traits over generations. To exploit this strategy in the creation of new materials, a concept of adaptive chemistry was proposed to provide a route for synthesis of self-adapting molecules that can fit to their environment. We propose a computational method of an efficient exhaustive search exploiting massive parallelization on modern GPUs, which finds a solution for an inverse problem by solving repetitively a direct problem in the mean field approximation. One example is the search for a composition of a copolymer that allows the polymer to translocate through a lipid membrane at a minimal time. Another example is a search of a copolymer sequence that maximizes the polymer load in the micelle defined by the radial core-shell potentials. The length and the composition of the sequence are adjusted to fit into the restricted environment. Hydrogen bonding is another pathway of adaptation to the environment through reversible links. A linear polymer that interacts with water through hydrogen bonds adjusts the position of hydrogen bonds along the chain as a function of the concentration field around monomers. In the last example, branching of the molecules is adjusted to external fields, providing molecules with annealed topology, that can be flexibly changed by changing external conditions. The method can be generalized and applied to a broad spectrum of design problems in chemistry and physics, where adaptive behavior in multi-parameter space in response to environmental conditions lead to non-trivial patterns or molecule architectures and compositions. It can further be combined with machine learning or other optimization techniques to explore more efficiently the parameter space.

Host-guest complexes vs. supramolecular polymers in chalcogen bonding receptors: an experimental and theoretical study

We describe here a comparative study between two tripodal anion receptors based on selenophene as the binding motif. The receptors use benzene or perfluorobenzene as a spacer. The presence of the electron-withdrawing ring activates the selenium atom for anion recognition inducing the formation of self-assembled supramolecular structures in the presence of chloride or bromide anions, which are bonded by the cooperative action of hydrogen and chalcogen bonding interactions. DOSY NMR and DLS experiments provided evidence for the formation of the supramolecular structures only in the presence of a perfluorobenzene based anion receptor while the analogous benzene one shows the classical anion/receptor complex without the participation of the selenium atom. The energetic and geometric features of the complexes of both receptors with the Cl and Br anions have been studied in solution. These results combined with the molecular electrostatic potential (MEP) surface plots allow us to rationalize the quite different behaviors of both receptors observed experimentally.

Energy-Dissipating Polymeric Silicone Surfactants

Materials that are able to withstand impact loadings by dissipating energy are crucial for a broad range of different applications, including personal protective applications. Shear-thickening fluids (STFs) are often used for this purpose, but their preparation is still limited, in part, to high production costs. It is demonstrated that polymeric surfactants comprised of linear telechelic sugar-modified silicones-with neither additives nor particles-generate transient polymer networks (TPNs) that represent a promising alternative to STFs. The reported polymers have distinct viscoelastic properties and can turn from a liquid into a rubbery network when force is applied. Saccharide-modified silicones with short chains (degree of polymerization (DP) ≈ 34, 68) are solids, but become energy-absorbing viscoelastic fluids when diluted in low-viscosity silicone oils; longer silicones (DP ≈ 338, 675) with low saccharide contents are viscoelastic fluids at room temperature. Excellent damping properties are found for the reported silicone surfactants, even those containing only 0.1% saccharides. The degree of energy absorption can be tailored simply by controlling the sugar/silicone ratio.

Supramolecular Self-Assembly Driven Selective Sensing of Phosphates

A new bis-heteroleptic Ru^II complex (1[PF₆]₂) with iodotriazole as the anion binding group along with the attached pyrene moiety is developed to investigate anion sensing properties and the origin of its selectivity toward a particular class of anions. Selective sensing of phosphates over other anions in both the solution and solid states by 1[PF₆]₂ is clearly evident from the perturbation of the absorption band and a large degree of amplification of ³MLCT emission band in the presence of phosphates. Importantly, macroscopic investigation such as Scanning Electron Microscopy (SEM) and Dynamic Light Scattering (DLS) indicated the formation of supramolecular architecture in the presence of dihydrogen phosphate via halogen bonding interaction and π-π stacking of pyrene moieties. Such macroscopic property is further corroborated by solution and solid state spectroscopic studies, e.g., ¹H-DOSY NMR, single crystal X-ray crystallography, and solid state photoluminescence (PL) spectroscopy.

Multiple adaptation of constitutional dynamic networks and information storage in constitutional distributions of acylhydrazones

We report a study of the behavior of four dynamic covalent libraries (DCLs) based on acylhydrazones aAbB and of the corresponding square constitutional dynamic networks (CDNs) NA-ND under the effect of three agents, namely, metal cations, base + metal cations and light irradiation; in particular, the successful switching of the CDN NB between two orthogonal distributions results, respectively, from metallo-selection and photo-selection. The four DCLs undergo triple adaptation when subjected to the three agents with the generation of specific CDN distributions characteristic of each of the four DCLs. The ternary outputs displayed by the DCLs present three states (-1, 0 and 1) related to three different constitutional distributions expressed in response to the triple inputs applied. This latter process amounts to the storage of molecular information in dynamic distributions rather than in selective interactions between complementary entities undergoing molecular recognition.

pH-Responsive Dipeptide-Based Dynamic Covalent Chemistry Systems Whose Products and Self-Assemblies Depend on the Structure of Isomeric Aromatic Dialdehydes

Facile control over preparation of organic building blocks and self-assembled aggregations to construct the desired materials remains challenges. This article reports selective dynamic covalent bonds formation and the corresponding self-assembly behaviors by using a dipeptide, glycylglycine (GlyGly), reacting with isomeric aromatic dialdehydes o-phthalaldehyde (OPA), p-phthalaldehyde (PPA), and m-phthalaldehyde (MPA) to demonstrate diversified aggregation forms caused by structure topology variations. Under alkaline condition, the aldehyde groups of phthalaldehydes can be connected with the amino groups of GlyGly by imine bonds as the dynamic chemical bonds. Owing to the fact that formation and dissociation of the imine bonds were reversibly pH-responsive, the reactions and aggregates assembled by their products were also reversibly controlled by changing pH. Three products, including two-armed product (OPGG, in which two GlyGly molecules were connected with one OPA molecule), single-armed product (PPG, in which only one GlyGly molecule was connected with a PPA molecule), and a mixture product (MPGG and MPG), as well as their different self-assembly behaviors, were obtained from OPA/GlyGly, PPA/GlyGly, and MPA/GlyGly systems, respectively, at the same condition of pH 8.6 in 90% methanol aqueous solution. However, for OPA/GlyGly system, another different type of product with benzopyrrole structure (OPG) was obtained by nucleophilic substitution via mixing OPA and GlyGly in water, which generated organic nanoparticles. Based on the results above, we conjectured the differences in dynamic covalent bond formation and supramolecular assembly clearly were influenced by the structure topologies of phthalaldehydes (OPA, PPA, and MPA). The experimental phenomenon verified the hypothesis as well, which may guide us to realize facile construction of selective reaction products and intelligent reversibly responsive materials with diverse morphologies and functions.

4-Sulfocalix[4]arene/Cucurbit[7]uril-Based Supramolecular Assemblies through the Outer Surface Interactions of Cucurbit[n]uril

Upon mixing of aqueous solutions of the freely soluble building blocks cucurbit[7]uril (Q[7]) and 4-sulfocalix[4]arene (SC[4]A), white microcrystals instantly separate in near-quantitative yield. The driving force for this assembly is suggested to be the outer-surface interaction of the Q[n]. Dynamic light scattering, scanning electron microscopy, and NMR (diffusion-ordered NMR spectroscopy) analyses have confirmed the supramolecular aggregation of Q[7] and SC[4]A. Titration ¹H NMR spectroscopy and isothermal titration calorimetry have shown that the interaction ratio of Q[7] and SC[4]A is close to 3:1. Moreover, the Q[7]/SC[4]A-based supramolecular assembly can accommodate molecules of some volatile compounds or luminescent dyes. Thus, this work offers a simple and highly efficient means of preparing adsorbent or solid fluorescent materials.

Reversible conversion between a pleated oligo-tetrathiafulvalene radical foldamer and folded donor-acceptor [3]pseudorotaxane under redox conditions

Linear tetrathiafulvalene (TTF) oligomers were synthesized, which could not only form a pleated TTF˙⁺ radical cation foldamer under oxidation conditions, but also interlocked with CBPQT⁴⁺ to form folded donor-acceptor [3]pseudorotaxane in the neutral state of TTF. Moreover, switchable transformation between these two folded supramolecular structures was achieved under the alternative regulation of the redox states of TTF units.

A folding-directed catalytic microenvironment in helical dynamic covalent polymers formed by spontaneous configuration control

Helical dynamic covalent polymers with a folding-directed hollow tubular structure were investigated for catalysis.

Dynamic covalent polymers

This Highlight presents an overview of the rapidly growing field of dynamic covalent polymers. This class of polymers combines intrinsic reversibility with the robustness of covalent bonds, thus enabling formation of mechanically stable, polymer-based materials that are responsive to external stimuli. It will be discussed how the inherent dynamic nature of the dynamic covalent bonds on the molecular level can be translated to the macroscopic level of the polymer, giving access to a range of applications, such as stimuli-responsive or self-healing materials. A primary distinction will be made based on the type of dynamic covalent bond employed, while a secondary distinction will be based on the consideration whether the dynamic covalent bond is used in the main chain of the polymer or whether it is used to allow side chain modification of the polymer. Emphasis will be on the chemistry of the dynamic covalent bonds present in the polymer, in particular in relation to how the specific (dynamic) features of the bond impart functionality to the polymer material, and to the conditions under which this dynamic behavior is manifested. © 2016 The Authors. Journal of Polymer Science Part A: Polymer Chemistry Published by Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 3551-3577.

Facile Stimuli-Responsive Transformation of Vesicle to Nanofiber to Supramolecular Gel via ω-Amino Acid-Based Dynamic Covalent Chemistry

This paper reports an interesting type of self-assembly systems based on dynamic covalent bonds. The systems are pH-responsible and reversible, which could be utilized for controlling the morphology transformation of the assemblies. In alkaline conditions, the amine group of 11-aminoundecanoic acid (AUA) can connect with the aldehyde group of benzaldehyde (BA) or 1-naphthaledhyde (NA) by dynamic covalent bond to form a small organic building block accompanied by the morphological transformation from vesicles to fibers. When pH is lowered to a neutral value, the dynamic covalent bonds (imine bonds) can be hydrolyzed, leading to the dissociation of fibers and appearance of spherical aggregates. The transformation was confirmed reversible as fibers appeared again when the pH was changed back to alkaline value. In addition, a reversibly controlled gel was designed based on the nanofiber formation. NaCl, which is capable of greatly enhance the nanofiber density and cross-linking, was used to induce the growth of free-standing gel from free-flowing nanofiber system, and the resultant gel was proven to be pH-reversible.

Emergent Behaviors in Kinetically Controlled Dynamic Self-Assembly of Synthetic Molecular Systems

Living systems are categorically a kinetic state of matter that exhibits complex functions and emergent behaviors. By contrast, synthetic systems are relatively simple and are typically controlled by the thermodynamic parameters. To understand this inherent difference between the biological and synthetic systems, novel approaches are of vital importance. In this regard, we have designed a three-component molecular system (a triad) by conjugating an amino acid with two functional molecules (naphthalenediimide and pyrene), which facilitates kinetically controlled self-assemblies. Herein, we describe three different molecular aggregation states of triads (entitled State I, State II, and State III) and also the dynamic pathway complexities associated with their transformations from one state to another. By meticulously employing the triads of different molecular aggregation states and the stereochemical information of the amino acid, we report emergent behaviors termed "supramolecular speciation" and "supramolecular regulation". Further, we present a hitherto unknown emergent property in a self-assembled state under the majority-rules experiment, which has been termed "super-nonlinearity". This work provides novel insights into complex synthetic systems having unprecedented functions and properties. Such emergent behaviors of synthetic triads that involve an interplay among complex interactions may find relevance in the context of prebiotic chemical evolution.

Dynamic Peptide Library for the Discovery of Charge Transfer Hydrogels

Coupling of peptide self-assembly to dynamic sequence exchange provides a useful approach for the discovery of self-assembling materials. In here, we demonstrate the discovery and optimization of aqueous, gel-phase nanostructures based on dynamically exchanging peptide sequences that self-select to maximize charge transfer of n-type semiconducting naphthalenediimide (NDI)-dipeptide bioconjugates with various π-electron-rich donors (dialkoxy/hydroxy/amino-naphthalene or pyrene derivatives). These gel-phase peptide libraries are characterized by spectroscopy (UV-vis and fluorescence), microscopy (TEM), HPLC, and oscillatory rheology and it is found that, of the various peptide sequences explored (tyrosine Y-NDI with tyrosine Y, phenylalanine F, leucine L, valine V, alanine A or glycine G-NH2), the optimum sequence is tyrosine-phenylalanine in each case; however, both its absolute and relative yield amplification is dictated by the properties of the donor component, indicating cooperativity of peptide sequence and donor/acceptor pairs in assembly. The methodology provides an in situ discovery tool for nanostructures that enable dynamic interfacing of supramolecular electronics with aqueous (biological) systems.

Reversible control of pore size and surface chemistry of mesoporous silica through dynamic covalent chemistry: philicity mediated catalysis

Here, we report the synthesis of adaptive hybrid mesoporous silica having the ability to reconfigure its pore properties such as pore size and philicity in response to the external environment. Decyl chains were reversibly appended to the pore walls of silica through imine motifs as dynamic covalent modules to switch the pore size and philicity in response to pH. This switching of pore properties was used to gate the access of reactants to the gold nanoparticles immobilized inside the nanopores, thus enabling us to turn-on/turn-off the catalytic reaction. The use of such dynamic covalent modules to govern pore properties would enable the realization of intelligent hybrids capable of controlling many such chemical processes in response to stimuli.

Polyisobutylene-Based pH-Responsive Self-Healing Polymeric Gels

This work demonstrates the successful application of dynamic covalent chemistry for the construction of self-healing gels from side-chain primary amine leucine pendant diblock copolymers of polyisobutylene (PIB) ((P(H2N-Leu-HEMA)-b-PIB)) in the presence of PIB based dialdehyde functionalized cross-linker (HOC-PIB-CHO) through imine (-HC═N-) bond formation without aiding any external stimuli. Gels were synthesized in 1,4-dioxane at room temperature at varied wt % of gelator concentration, [H2N]/[CHO] ratios and molecular weight of the block segments. The mechanical property of gels was examined by rheological measurements. We observed higher value of storage modulus (G') than the loss modulus (G″) within the linearity limits of deformation, indicating the rheological behavior in the gel is dominated by an elastic property rather than a viscous property. The G' values significantly depend upon the extent of cross-linking in the gel network. To establish self-healing property of the gels, rheology analysis through step-strain measurements (strain = 0.1 to 200%) at 25 °C was performed. The polymeric gel network shows reversible sol-gel transition for several cycles by adjusting the pH of the medium with the help of hydrochloric acid (HCl) and triethylamine (Et3N) triggers. FT-IR spectroscopy established formation of imine bonds in the gel network and these gels showed poor swelling behavior in various organic solvents because of the small interstitial porosity, confirmed by field emission-scanning electron microscopy (FE-SEM).

Perspectives in chemistry--aspects of adaptive chemistry and materials

Chemistry, pure and applied, is a science and an industry. By its power over the expressions of matter, it also displays the creativity of art. It has expanded from molecular to supramolecular chemistry and then, by way of constitutional dynamic chemistry, towards adaptive chemistry. Constitutional dynamics allow for adaptation, through component exchange and selection in response to physical stimuli (e.g. light, photoselection), to chemical effectors (e.g. metal ions, metalloselection) or to environmental effects (e.g. phase change) in equilibrium or out-of-equilibrium conditions, towards the generation of the best-adapted/fittest constituent(s) in a dynamic set. Such dynamic systems can be represented by two-dimensional or three-dimensional dynamic networks that define the agonistic and antagonistic relationships between the different constituents linked through component exchange. The introduction of constitutional dynamics into materials science opens perspectives towards adaptive materials and technologies, presenting attractive behavioral features (such as self-healing). In particular, dynamic polymers may undergo modification of their properties (mechanical, optical, etc.) through component exchange and recombination in response to physical or chemical agents. Constitutional adaptive materials open towards a systems materials science and offer numerous opportunities for soft-matter technologies.

Bipyridinium radical cation dimerization-driven polymeric pleated foldamers and a homoduplex that undergo ion-tuned interconversion

Radical cation dimerization induces bipyridinium-derived polymers to form pleated secondary structures and a homoduplex which can be tuned by ammonium to interconvert.

Donor–acceptor interaction-driven folding of linear naphthalene–glycol oligomers templated by a rigid bipyridinium rod

The construction of folded and helical supramolecular structures through the self-assembly of a series of flexible linear oligomers induced by a rigid rod-like template has been demonstrated.

Dynamic combinatorial/covalent chemistry: a tool to read, generate and modulate the bioactivity of compounds and compound mixtures

Reversible covalent bond formation under thermodynamic control adds reactivity to self-assembled supramolecular systems, and is therefore an ideal tool to assess complexity of chemical and biological systems. Dynamic combinatorial/covalent chemistry (DCC) has been used to read structural information by selectively assembling receptors with the optimum molecular fit around a given template from a mixture of reversibly reacting building blocks. This technique allows access to efficient sensing devices and the generation of new biomolecules, such as small molecule receptor binders for drug discovery, but also larger biomimetic polymers and macromolecules with particular three-dimensional structural architectures. Adding a kinetic factor to a thermodynamically controlled equilibrium results in dynamic resolution and in self-sorting and self-replicating systems, all of which are of major importance in biological systems. Furthermore, the temporary modification of bioactive compounds by reversible combinatorial/covalent derivatisation allows control of their release and facilitates their transport across amphiphilic self-assembled systems such as artificial membranes or cell walls. The goal of this review is to give a conceptual overview of how the impact of DCC on supramolecular assemblies at different levels can allow us to understand, predict and modulate the complexity of biological systems.

Relative contractile motion of the rings in a switchable palindromic [3]rotaxane in aqueous solution driven by radical-pairing interactions

Artificial muscles are an essential component for the development of next-generation prosthetic devices, minimally invasive surgical tools, and robotics. This communication describes the design, synthesis, and characterisation of a mechanically interlocked molecule (MIM), capable of switchable and reversible linear molecular motion in aqueous solution that mimics muscular contraction and extension. Compatibility with aqueous solution was achieved in the doubly bistable palindromic [3]rotaxane design by using radical-based molecular recognition as the driving force to induce switching.

Dynamic uptake and release from poly(methacryloyl hydrazide) microgel particles through reversible hydrazide-aldehyde chemistry

The uptake and release time-profiles of aldehydes from aqueous formulations can be fine tuned using hydrazide functional microgels.

Experimental and theoretical methods for the analyses of dynamic combinatorial libraries

Progresses in spatial and temporal analytical tools open new avenues for the study and control of increasingly complex chemical systems.

Adaptation of dynamic covalent systems of imine constituents to medium change by component redistribution under reversible phase separation

A dynamic covalent library of interconverting imine constituents, dissolved in an acetonitrile/water mixture, undergoes constitutional reorganization upon phase separation induced by a physical stimulus (heat) or a chemical effector (inorganic salt, carbohydrate, organic solvent). The process has been made reversible, regenerating the initial library upon phase reunification. It represents the behavior of a dynamic covalent library upon reversible phase separation and its adaptation to a phase change, with up-regulation in each phase of the fittest constituents by component selection. Finally, the system exemplifies the splitting of a 2D (square) constitutional dynamic network into a 3D (cube) one.

Molecular recognition: from solution science to nano/materials technology

In the 25 years since its Nobel Prize in chemistry, supramolecular chemistry based on molecular recognition has been paid much attention in scientific and technological fields. Nanotechnology and the related areas seek breakthrough methods of nanofabrication based on rational organization through assembly of constituent molecules. Advanced biochemistry, medical applications, and environmental and energy technologies also depend on the importance of specific interactions between molecules. In those current fields, molecular recognition is now being re-evaluated. In this review, we re-examine current trends in molecular recognition from the viewpoint of the surrounding media, that is (i) the solution phase for development of basic science and molecular design advances; (ii) at nano/materials interfaces for emerging technologies and applications. The first section of this review includes molecular recognition frontiers, receptor design based on combinatorial approaches, organic capsule receptors, metallo-capsule receptors, helical receptors, dendrimer receptors, and the future design of receptor architectures. The following section summarizes topics related to molecular recognition at interfaces including fundamentals of molecular recognition, sensing and detection, structure formation, molecular machines, molecular recognition involving polymers and related materials, and molecular recognition processes in nanostructured materials.

Stimuli-responsive supramolecular polymeric materials

Supramolecular materials, dynamic materials by nature, are defined as materials whose components are bridged via reversible connections and undergo spontaneous and continuous assembly/disassembly processes under specific conditions. On account of the dynamic and reversible nature of noncovalent interactions, supramolecular polymers have the ability to adapt to their environment and possess a wide range of intriguing properties, such as degradability, shape-memory, and self-healing, making them unique candidates for supramolecular materials. In this critical review, we address recent developments in supramolecular polymeric materials, which can respond to appropriate external stimuli at the fundamental level due to the existence of noncovalent interactions of the building blocks.

Highly efficient ring-opening reaction of azlactone-based copolymer platforms for the design of functionalized materials

Azlactone-based homopolymers and copolymers were successfully synthesized using the reversible addition-fragmentation chain transfer (RAFT) process. The functional monomer 2-styryl-4,4-dimethylazlactone (SDA) was first homopolymerized in bulk then copolymerized with styrene, leading to (co)polymers with low polydispersity indices (PDI = 1.10-1.20). The reactive azlactone rings, located along the backbone of the copolymers were subjected to highly efficient ring-opening reactions with functionalized primary amine derivatives incorporating a fluorescent (naphthalene) or an electrochemical (ferrocene) probes, a biological fragment (glutathione), a sugar unit (β-cyclodextrin), or an oligomeric fluorinated moiety, leading to materials with various interesting properties.

Designed post-self-assembly structural and functional modifications of a truncated tetrahedron

Post-self-assembly modifications of a discrete metal-organic supramolecular structure have been developed. Such modifications allow the properties of the self-assembled supramolecular species to be changed in a simple and efficient manner (>90% yield). Initiated by the application of chemical stimuli, the post-self-assembly modifications described herein result in three distinct changes to the supramolecular system: an individual building-block component change, an overall structural modification, and a functional evolution of a [6+4] metal-organic supramolecular structure. The three modifications have been carefully examined by a range of characterization methods, including NMR and UV-vis spectroscopy, electrospray ionization mass spectrometry, pulsed field gradient spin echo NMR measurements, electrochemical analysis, and computational simulations.

Multicomponent assembly of cavitand-based polyacylhydrazone nanocapsules

The thermodynamically controlled syntheses of different di-, tetra-, and hexacavitand polyacylhydrazone nanocapsules are reported. [2+4]-, [4+8]-, and [6+12]-nanocapsules assemble upon reacting a tetraformyl cavitand with two equivalents of isophthalic dihydrazide, or terephthalic dihydrazide in the presence of trifluoroacetic acid, whereby the building blocks are linked together through 8, 16, or 24 newly formed acylhydrazone bonds. Futhermore, the reaction of the tetraformylcavitands with different trigonal planar trihydrazides, simultaneously leads to the formation of [2+4]- and [6+8]-nanocapsules in varying ratios that depend on the cavitand to trihydrazide ratio and the nature of the cavitand and trihydrazide building blocks. The product ratios are rationalized with the different conformational strain of the acylhydrazone linkages in these nanocapsules. Diffusion NMR experiments with the hexacavitand polyacylhydrazone nanocapsules yield solvodynamic radii that range from 1.6 to 2.5 nm, consistent with estimates from force field calculations, and support, that these capsules have solvent filled, spherical interiors, the sizes of which approaches those of smaller proteins.