Playing LEGO with macromolecules: Design,synthesis, and self‐organization with metal complexes

Several synthetic strategies for the incorporation of supramolecular binding units into polymers are described. Specifically, terpyridine ligands have been introduced into polymers in such a way that they are distributed either randomly throughout the polymer backbone or at the chain end(s). Two terpyridine ligands form octahedral complexes with a variety of transition‐metal ions, each having different properties. Some general statements regarding metal complex stability are presented as well as a special case representing the selective construction of heteroleptic terpyridine complexes. This leads to a kind of LEGO system for connecting and disconnecting the polymer blocks via metal complexes. Metallo‐supramolecular block copolymers, graft copolymers, and chain‐extended polymers can be designed and prepared with the principles described. Once the design parameters have been derived, thorough control over the final material and its properties can be gained. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1413–1427, 2003     

氢键识别超分子聚合物的新进展

近年来,由于氢键作用对聚合物的热力学性质、微观自组装、结晶及液晶行为的重要影响,氢键识别在超分子聚合物的分子设计与结构控制方面的应用受到广泛关注。本文系统介绍了氢键识别体系的类型与性质,以及分子结构、分子内氢键对氢键识别强度的影响,讨论了羧酸与吡啶间氢键识别体系、与核苷相关的氢键识别体系以及四重氢键识别体系在超分子聚合物中的最新应用,主要介绍了氢键识别超分子聚合物的合成、结构、性质及功能。     

Monosaccharides as Versatile Units for Water‐Soluble Supramolecular Polymers

We introduce monosaccharides as versatile water‐soluble units to compatibilise supramolecular polymers based on the benzene‐1,3,5‐tricarboxamide (BTA) moiety with water. A library of monosaccharide‐based BTAs is evaluated, varying the length of the alkyl chain (hexyl, octyl, decyl and dodecyl) separating the BTA and saccharide units, as well as the saccharide units (α‐glucose, β‐glucose, α‐mannose and α‐galactose). In all cases, the monosaccharides impart excellent water compatibility. The length of the alkyl chain is the determining factor to obtain either long, one‐dimensional supramolecular polymers (dodecyl spacer), small aggregates (decyl spacer) or molecularly dissolved (octyl and hexyl) BTAs in water. For the BTAs comprising a dodecyl spacer, our results suggest that a cooperative self‐assembly process is operative and that the introduction of different monosaccharides does not significantly change the self‐ assembly behaviour. Finally, we investigate the potential of post‐assembly functionalisation of the formed supramolecular polymers by taking advantage of dynamic covalent bond formation between the monosaccharides and benzoxaboroles. We observe that the supramolecular polymers readily react with a fluorescent benzoxaborole derivative permitting imaging of these dynamic complexes by confocal fluorescence microscopy.     

Tuning Aqueous Supramolecular Polymerization by an Acid-Responsive Conformational Switch

Besides their widespread use in coordination chemistry, 2,2’-bipyridines are known for their ability to undergo cis–trans conformational changes in response to metal ions and acids, which has been primarily investigated at the molecular level. However, the exploitation of such conformational switching in self-assembly has remained unexplored. In this work, the use of 2,2’-bipyridines as acid-responsive conformational switches to tune supramolecular polymerization processes has been demonstrated. To achieve this goal, we have designed a bipyridine-based linear bolaamphiphile, 1 , that forms ordered supramolecular polymers in aqueous media through cooperative aromatic and hydrophobic interactions. Interestingly, addition of acid (TFA) induces the monoprotonation of the 2,2’-bipyridine moiety, leading to a switch in the molecular conformation from a linear (trans) to a V-shaped (cis) state. This increase in molecular distortion along with electrostatic repulsions of the positively charged bipyridine-H⁺ units attenuate the aggregation tendency and induce a transformation from long fibers to shorter thinner fibers. Our findings may contribute to opening up new directions in molecular switches and stimuli-responsive supramolecular materials.     

Ferroelectric Coordination Polymers Self‐Assembled from Mesogenic Zinc(II) Porphyrin and Dipolar Bridging Ligands

A new class of ferroelectric coordination‐based polymers has been developed by the self‐assembly of lipophilic zinc porphyrin ( ZnP ) and ditopic bridging ligands. The ligands contain dipolar benzothiadiazole or fluorobenzene units, which are axially coordinated to ZnP with the dipole moments oriented perpendicular to the coordination axes. The coordination‐based polymers show ferroelectric characteristics in the liquid crystalline state, as revealed by distinctive hysteresis in the polarization–electric field (P–E) loops and inversion current peaks in current–voltage (I–V) loops. The observed ferroelectric properties are explainable by flip–flop rotation of the dipolar axle ligands induced by the applied electric field, as demonstrated by the positive‐up–negative‐down (PUND) measurements. The present system provides a new operating principle in supramolecular ferroelectrics.     

Supramolecular polymers based on cyclodextrins

Two types of supramolecular polymers based on cyclodextrins were prepared. One was a host–guest type, and the other was a polyrotaxane type. When a guest part was covalently attached to cyclodextrin, they formed supramolecular dimers, a cyclic daisy chain, supramolecular oligomers, and polymers. t-Boc-cinnamamide-α-cyclodextrin was found to form chiral supramolecular polymers in aqueous solutions. Supramolecular poly[2]rotaxane polymers and supramolecular α,β-cyclodextrin copolymers were obtained. Polyrotaxanes containing β-cyclodextrin or γ-cyclodextrin were prepared. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5113–5119, 2006     

Two-Dimensional Covalent and Supramolecular Polymers: From Monolayer to Bilayer and the Thicker

Selective preparation of two-dimensional polymers (2DPs) and supramolecular polymers (2DSPs) with defined thickness is crucially important for controlling and maximizing their functions, yet it has remained as a synthetic challenge. In the past decade, several approaches have been developed to allow selective preparation of discrete monolayer 2DPs and 2DSPs. Recently, crystal exfoliation and self-assembly strategies have been employed to successfully prepare bilayer 2DP and 2DSP, which represent the first step towards the controlled “growth” of 2D polymers from the thinnest monolayers to thicker few-layers along the third dimension. This Concept review discusses the concept of accurate synthesis of 2D polymers with defined layers. Advances in this research area will pave the way to rational synthetic strategies for 2D polymers with controlled thickness.     

Supramolecular polymers fabricated by orthogonal self-assembly based on multiple hydrogen bonding and macrocyclic host–guest interactions

Supramolecular polymers constructed by orthogonal self-assembly based on multiple hydrogen bonding and macrocyclic host-guest interactions have received increasing attention due to their elegant structures,outstanding properties,and potential applications.Hydrogen bonding endows these supramolecular polymers with good adaptability and reversibility,while macrocyclic host-guest interactions give them good selectivity and versatile stimuli-responsiveness.Therefore,functional supramolecular polymers fabricated by these two highly specific,noninterfering interactions in an orthogonal way have shown wide applications in the fields of molecular machines,electronics,soft materials,etc.In this review,we discuss the recent advances of functional supramolecular polymers fabricated by orthogonal self-assembly based on multiple hydroge n bonding and host-guest interactions.In particular,we focus on crown ether-and pillar[n]arene-based supramolecular polymers due to their compatibility with multiple hydrogen bonds in organic solution.The fabrication strategies,interesting properties,and potential applications of these advanced supramolecular materials are mainly concerned.     

Cover Picture

The cover picture shows a swirling mixture of transition metal fragments and simple organic ligands that assemble into supramolecular coordination compounds, depicted on the perphery of the vortex. The synthesis of such large structures requires strategies that take into account the entropic costs of not forming higher ordered oligomers and polymers. The use of transition metal ions as building blocks for supramolecular compounds has become a widely practiced strategy for making such structures. Within this quickly growing field there have emerged three general synthetic approaches for the metal‐directed formation of supramolecular coordination compounds. These are described by B. J. Holliday and C. A. Mirkin on p. 2022 ff.     

Binding sites on the outside of metallo-supramolecular architectures; engineering coordination polymers from discrete architectures

Aggregation of metallo-supramolecular architectures through additional coordination is explored by introducing metal-binding units onto the outside of the supramolecular architectures. This is achieved within the framework of our imine-based approach to supramolecular architecture, by replacing the pyridylimine units with pyrazylketimine units. An advantage of the design is that it retains the ease-of-synthesis which characterises our imine-based approach. Silver(I) complexes of three pyrazylketimine ligand systems are described. The complexes demonstrate that introducing pyrazine donor units does indeed allow higher-order assembly of the distinct supramolecular architectures into engineered coordination polymers. Two distinct types of aggregation are observed. In the first, the donors on the outside of one architecture bind to the metals of another to link the units into a polymeric array. In the second type, the donors on the outside of the architectures bind to separate metal centres which are themselves not part of the architectures, and these separate metal centres link the units to form the macromolecular array. The weaker donor nature of the pyrazine nitrogens (compared to pyridine) also introduces an additional element into the design; higher coordination numbers are favoured and this can lead to arrays with higher connectivity than those observed in the discrete pyridylimine architectures.     

Urea Derivatives as Functional Molecules: Supramolecular Capsules,Supramolecular Polymers,Supramolecular Gels,Artificial Hosts,and Catalysts

Urea, which has both hydrogen bond acceptor and donor moieties, is an ideal structure for a supramolecular synthon. Various supramolecules having ureido group(s) have been widely developed. This article summarizes recent developments of urea derivatives that exhibit various functions: i) supramolecular capsules that form discrete urea–urea intermolecular hydrogen bonds, ii) supramolecular polymers that form continuous urea–urea intermolecular hydrogen bonds, iii) supramolecular gels that form continuous urea–urea intermolecular hydrogen bonds, iv) artificial host molecules based on the molecular recognition ability of the ureido group, and v) catalytic reactions developed by utilizing the molecular recognition ability of the ureido group.     

Metallogels from Coordination Complexes,Organometallic, and Coordination Polymers

A supramolecular gel results from the immobilization of solvent molecules on a 3D network of gelator molecules stabilized by various supramolecular interactions that include hydrogen bonding, π–π stacking, van der Waals interactions, and halogen bonding. In a metallogel, a metal is a part of the gel network as a coordinated metal ion (in a discrete coordination complex), as a cross‐linking metal node with a multitopic ligand (in coordination polymer), and as metal nanoparticles adhered to the gel network. Although the field is relatively new, research into metallogels has experienced a considerable upsurge owing to its fundamental importance in supramolecular chemistry and various potential applications. This focus review aims to provide an insight into the development of designing metallogelators. Because of the limited scope, discussions are confined to examples pertaining to metallogelators derived from discrete coordination complexes, organometallic gelators, and coordination polymers. This review is expected to enlighten readers on the current development of designing metallogelators of the abovementioned class of molecules.     

Deep Eutectic Supramolecular Polymers: Bulk Supramolecular Materials

Application of new strategies for supramolecular self‐assembly can significantly impact the properties and/or functions of supramolecular polymers. To realize a facial strategy for the development of solvent‐free supramolecular polymers in bulk, “deep eutectic solvents” were employed. Cyclodextrins and natural acids were used to prepare deep eutectic supramolecular polymers ( DESP s). Deep eutectic solvents have special characteristics that endow DESP s with unique macroscopic properties and excellent processability. DESP s exhibit supramolecular adhesion and temperature‐dependent behavior originating from the combined effects of deep eutectic solvents and supramolecular polymerization. Because DESP s are solvent‐free and display interesting macroscopic properties, they have potential as new adaptive materials.     

Supramolecular Interfacial Polymerization: A Controllable Method of Fabricating Supramolecular Polymeric Materials

A new method of supramolecular polymerization at the water–oil interface is developed. As a demonstration, an oil‐soluble supramonomer containing two thiol end groups linked by two ureidopyrimidinone units and a water‐soluble monomer bearing two maleimide end groups are employed. Supramolecular interfacial polymerization can be implemented by a thiol–maleimide click reaction at the water–chloroform interface to obtain supramolecular polymeric films. The glass transition temperature of such supramolecular polymers can be well‐tuned by simply changing the polymerization time and temperature. It is highly anticipated that this work will provide a facile and general approach to realize control over supramolecular polymerization by transferring the preparation of supramolecular polymers from solutions to water–oil interfaces and construct supramolecular materials with well‐defined properties.     

Supramolecular liquid crystallinity as a mechanism of supramolecular polymerization

Two types of supramolecular self-assembling polymers should be distinguished: the open type characterized by units with binding sites exposed to the surface and thus capable of extended growth (e.g. some synthetic and natural supramolecular polymers), and the closed type with binding sites internally compensated yielding well defined oligomeric complexes (e.g. DNA, collagen,haemoglobin). The growth of the open assemblies may be defined as a supramolecular polymerization. Both types form corresponding supramolecular liquid crystalline (SLC) phases, but the behaviour of the closed-type SLC is generally indistinguishable from that of liquid crystals formed by molecularly dispersed units. Quite different is the case of the open-type SLC when the anisotropic forces stabilizing the nematic phase interact with the contact forces that promote the growth of theassembly. In this case a cooperative enhancement of growth occurs simultaneously with the formation of the ordered phase. The theoretical basis for the growth coupled to orientation mechanism, laid down to explain the formation of linear micelle aggregates, may apply to most types of supramolecular polymerization. Outstanding resulting expectations are a large degree of supramolecular polymerization, orientation, superimposition of other growth mechanisms, dampened polydispersity and termination steps.     

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.     

Routes to Hydrogen Bonding Chain‐End Functionalized Polymers

The contribution of supramolecular chemistry to polymer science opens new perspectives for the design of polymer materials exhibiting valuable properties and easier processability due to the dynamic nature of non‐covalent interactions. Hydrogen bonding polymers can be used as supramolecular units for yielding larger assemblies that possess attractive features, arising from the combination of polymer properties and the responsiveness of hydrogen bonds. The post‐polymerization modification of reactive end‐groups is the most common procedure for generating such polymers. Examples of polymerizations mediated by hydrogen bonding‐functionalized precursors have also recently been reported. This contribution reviews the current synthetic routes toward hydrogen bonding sticker chain‐end functionalized polymers.     

Formation of Supramolecular Nanotubes by Self‐assembly of a Phosphate‐linked Dimeric Anthracene in Water

The assembly of supramolecular polymers from a phosphodiester‐linked dimeric anthracene is described. AFM and TEM imaging reveals that the supramolecular polymers self‐assemble into nanotubes in water. Subsequent photodimerization experiments indicate that the supramolecular polymerization occurs via end‐to‐end stacking rather than an interdigitation arrangement of the building blocks.     

Diketopyrrolopyrrole-based functional supramolecular polymers: next-generation materials for optoelectronic applications

The very concept of dye and pigment chemistry that was long known to the industrial world underwent a radical revision after the discovery and commercialization of dyes such as mauveine, indigo, and so on. Apart from their conventional role as coloring agents, organic dyes, and pigments have been identified as indispensable sources for high-end technological applications including optical and electronic devices. Simultaneous with the advancement in the supramolecular chemistry of π-conjugated systems and the divergent evolution of organic semiconductor materials, several dyes, and pigments have emerged as potential candidates for contemporary optoelectronic devices. Of all the major pigments, diketopyrrolopyrrole (DPP) better known as the ‘Ferrari Pigment’ and its derivatives have emerged as a major class of organic functional dyes that find varied applications in fields such as industrial pigments, organic solar cells, organic field–effect transistors, and in bioimaging. Since its discovery in 1974 by Farnum and Mehta, DPP-derived dyes gained rapid attention because of its attractive color, synthetic feasibility, ease of functionalization, and tunable optical and electronic properties. The advancement in supramolecular polymerization of DPP-based small molecules and oligomers with directed morphological and electronic features have led to the development of high performing optoelectronic devices. In this review, we highlight the recent developments in the optoelectronic applications of DPP derivatives specifically engineered to form supramolecular polymers.     

Supramolecular polymerization from polypeptide-grafted comb polymers

The helical and tubular structures self-assembled from proteins have inspired scientists to design synthetic building blocks that can be "polymerized" into supramolecular polymers through coordinated noncovalent interactions. However, cooperative supramolecular polymerization from large, synthetic macromolecules remains a challenge because of the difficulty of controlling the structure and interactions of macromolecular monomers. Herein we report the synthesis of polypeptide-grafted comb polymers and the use of their tunable secondary interactions in solution to achieve controlled supramolecular polymerization. The resulting tubular supramolecular structures, with external diameters of hundreds of nanometers and lengths of tens of micrometers, are stable and resemble to some extent biological superstructures assembled from proteins. This study shows that highly specific intermolecular interactions between macromolecular monomers can enable the cooperative growth of supramolecular polymers. The general applicability of this strategy was demonstrated by carrying out supramolecular polymerization from gold nanoparticles grafted with the same polypeptides on the surface.