Review pubs.acs.org/CR
Supramolecular Polymers: Historical Development, Preparation, Characterization, and Functions Liulin Yang, Xinxin Tan, Zhiqiang Wang, and Xi Zhang* Key Laboratory of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China 5.2. Viscometry 5.3. NMR Spectroscopy 5.4. AFM-Based Single-Molecule Force Spectroscopy 5.5. Light Scattering 5.6. Small-Angle Neutron Scattering 5.7. Asymmetric Flow Field-Flow Fractionation 5.8. Other Methods 6. Application of Supramolecular Polymers 6.1. Optoelectronic Materials 6.2. Supramolecular Self-Healing Materials 6.2.1. Self-Healing Materials Based on Multiple Hydrogen Bonding 6.2.2. Self-Healing Materials Based on MetalCoordination Bonding 6.2.3. Self-Healing Materials Based on π−π Interactions 6.3. Biomedical Applications 7. Conclusions and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments References
CONTENTS 1. Introduction 2. Driving Forces for Supramolecular Polymers 2.1. Multiple Hydrogen Bonds 2.1.1. Hydrogen-Bonding Arrays 2.1.2. Hydrogen-Bonded Supramolecular Polymers 2.2. Metal-Coordination Bonds 2.2.1. Ligands and Metal Ions 2.2.2. Counterions 2.2.3. Stoichiometric Ratio 2.3. Host−Guest Interactions 2.3.1. Cyclodextrin-Based Supramolecular Polymers 2.3.2. Cucurbit[8]uril-Based Supramolecular Polymers 2.3.3. Crown Ether-Based Supramolecular Polymers 2.3.4. Calixarene-Based Supramolecular Polymers 2.3.5. Pillararene-Based Supramolecular Polymers 2.4. Aromatic Donor−Acceptor Interactions 2.5. Multiple Driving Forces 3. Supramolecular Polymers with Branch-like and Hyperbranch-like Topology 3.1. Supramolecular Dendrimers 3.2. Supramolecular Hyperbranched Polymers 4. New Strategies for Controllable Supramolecular Polymerization 4.1. Polymerization of a Supramonomer 4.2. Self-Sorting Controlled Supramolecular Polymerization 4.3. Stimulus-Controlled Supramolecular Polymerization 4.4. Living Supramolecular Polymerization 5. Characterization of Supramolecular Polymers 5.1. Size Exclusion Chromatography
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1. INTRODUCTION Supramolecular polymers, originating from the integration of polymer science and supramolecular chemistry, have been a rapidly developing interdisciplinary research area. In the broad sense, the term “supramolecular polymers” can be defined as any type of assembly formed from one or more molecular components via reversible bonds. 1 However, to avoid confusion, a limited definition regarding supramolecular polymers as “polymeric arrays of monomeric units that are held together by highly directional and reversible noncovalent interactions, resulting in polymeric properties in solution and bulk” 2,3 is more explicit and recommended. A variety of noncovalent interactions (e.g., multiple hydrogen bonding, metal coordination, host−guest interactions, and aromatic stacking) have been employed as driving forces to construct supramolecular polymers. Monomers for building linear supramolecular polymers can be classified into five major types: AA type, AB type, AA/BB
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Special Issue: 2015 Supramolecular Chemistry Received: November 5, 2014 Published: March 13, 2015 7196
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Table 1. Typical Examples of Multiple-Hydrogen-Bonding Arrays and Their Binding Constants
Three major mechanisms for supramolecular polymerization, including isodesmic, ring−chain equilibrium, and cooperative growth, have been thoroughly discussed in a previous review3 by Meijer et al., and thus, we just make a brief introduction as follows. The isodesmic mechanism is similar to the step polymerization that obeys Flory’s “principle of equal reactivity”. This implies that the reactivity of the end groups does not change during the supramolecular polymerization process. The ring−chain equilibrium mechanism depicts an equilibrium between the linear chain and cyclic counterpart during the supramolecular polymerization process. For the cooperative mechanism, the process of supramolecular polymers involves at least two stages, nucleation and elongation. The first step is a linear isodesmic polymerization with a binding constant Kn for the addition of each monomer, and a nucleus of degree of polymerization is formed. The second step involves another linear isodesmic elongation process, and due to the cooperative effect, the supramolecular polymerization proceeds by a binding constant Ke higher than Kn. Although much insight into the process of supramolecular polymerization has been gained, it still remains a great challenge for designing controlled supramolecular polymer-
type, ABBA type, and aromatic-stacking type. An AA-type monomer is a kind of self-complementary dimer, which is common in a multiple-hydrogen-bonding system. The complementary heterodimer AB-type monomer that connects each moiety in a head-to-tail manner is useful in building supramolecular homopolymers or alternating block copolymers. The homoditopic AA and BB monomers can polymerize via heterodimerization in an alternating manner. Both AB-type and AA/BB-type monomers are the most commonly used monomers for supramolecular polymers. The unique ABBAtype monomers are powerful in building linear supramolecular polymers especially in dilute solution, which can effectively inhibit dimerization and cyclization. Monomers based on an aromatic donor−acceptor interaction alone might be too weak and less directional to drive supramolecular polymerization. Therefore, directing groups based on solvophobic, hydrogenbonding, dipole−dipole, and donor−acceptor interactions on the periphery of an aromatic core should be introduced to promote the formation of supramolecular polymers. For supramolecular branched or hyperbranched polymers, two kinds of monomers, ABn (n ≥ 2) and Am/Bn (m, n ≥ 2), are most commonly used. 7197
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hydrogen bonds, the binding constant of DAA−AAD arrays 2 is significantly higher than that of ADA−DAD arrays 1 (Ka ≈ 104 vs 102 M−1), on account of the additional attractive secondary electrostatic interactions in DAA−AAD arrays.10 Later on, Zimmerman et al. confirmed that the AAA−DDD arrays 3 indeed have a significantly higher binding constant (Ka > 105 M−1 in CDCl3), due to the solely attractive secondary interactions.12 According to this secondary interaction rule, multiplehydrogen-bonding arrays were designed, with binding constants even higher than those of triple-hydrogen-bonding arrays. In 1997, Meijer et al. reported a self-complementary quadruplehydrogen-bonding unit, ureidopyrimidinone (UPy) 4, with a dimerization constant Kdim of up to 6 × 107 M−1 in CHCl3 (Table 1).16,17 The high dimerization constant of UPy derives from its AADD array of donor and acceptor groups. Analogous to UPy, Zimmerman et al. reported a DeAP unit, 5, that contained a self-complementary AADD array with a dimerization constant Kdim = 9 × 107 M−1 in CHCl3.18 Although these AADD arrays exhibit a high dimerization constant, different tautomeric forms are present which may significantly decrease their binding constants.19 For UPy, when the tautomeric equilibrium is shifted to the pyrimidin-4-ol tautomer, as a DADA hydrogen-bonding array, the dimerization binding constant is lowered to 9 × 105 M−1 in CHCl3, due to more repulsive secondary interactions.20 The strongest quadruplehydrogen-bonding arrays should be AAAA−DDDD arrays. Leigh et al. have reported a cationic AAAA−DDDD complex which exhibits a binding constant as high as 1012 M−1 in CH2Cl2.21 Sextuple-hydrogen-bonding arrays have also been developed, and one of the representatives is between diaminopyridinesubstituted isophthalamide and barbiturates 6. The binding constant for this heterocomplementary recognition pair is up to 1.37 × 106 M−1 in CDCl3.22 Another example of a sextuplehydrogen-bonding array was reported by Corbin and Zimmerman. 2 3 They designed heterocomplementary bis(ureidonaphthyridine)s and bis(naphthyridinourea)s; however, the individual units formed an intramolecular complex, which reduced the duplex stability. Gong et al. reported a very stable sextuple-hydrogen-bonding array, 7, with a Ka ≈ 109 M−1 in CDCl3, in which intramolecular hydrogen bonding played a key role in providing a preorganized, planar arrangement of the donor and acceptor groups.24,25 2.1.2. Hydrogen-Bonded Supramolecular Polymers. Multiple hydrogen bonds were the first kind of noncovalent bond that was adopted to fabricate supramolecular polymers. In 1990, Lehn et al. first reported this type of supramolecular polymer. Bifunctional diamidopyridines and uracil derivatives were mixed to form linear polymeric chains 8 via triplehydrogen-bonding interactions (Figure 1).26 In contrast to the behavior of each monomer, the supramolecular polymers displayed liquid crystallinity in the solid phase. To achieve supramolecular polymers with a high degree of polymerization (DP), multiple-hydrogen-bonding arrays with a higher binding constant, such as quadruple-hydrogen-bonding arrays, were employed. In 1997, Meijer et al. reported that bifunctional AAtype monomers containing two UPy end groups were able to form linear supramolecular polymers 9 (Figure 1).16 The molecular mass of supramolecular polymers can be tuned by varying the solvent and concentration. Addition of monofunctional units caused decreases of the solution viscosity as well as the DP. The calculated DP for the pure monomer was about
ization systems, affording supramolecular polymers with a controlled structure such as molecular mass, polydispersity, and the corresponding properties. Given the mechanism of controlled polymerization for conventional polymers, supramolecular polymerization following the cooperative nucleation−elongation process, which is analogue to chain-growth polymerization, should be one of the right candidates to achieve this aim. Besides this direction, considering the unique characteristics of supramolecular polymerization, other effective approaches are expected to be developed to control this process. In this review, we will summarize some of the latest progress in this regard. Characterization of supramolecular polymers, especially the parameters of average molar mass and polydispersity, is essential and obligatory for supramolecular polymers. However, the dynamic nature of supramolecular polymers makes their characterization difficult. The change of concentration, temperature, or stimuli from the environment may significantly influence the original molecular structure of supramolecular polymers. Many well-established characterization methods fail to work as well as when applied to conventional polymers. Only partial information can be obtained from individual methods. Therefore, a combination of different characterization methods is required. We have already presented a review4covering this topic, so here we will summarize the latest progress on the characterization of supramolecular polymers. The development of supramolecular polymers has brought new vigor into material science. Compared with conventional polymers, the polymeric structure and reversible noncovalent interactions endow supramolecular polymers with not only traditional polymeric characteristics, but also dynamic properties to achieve functions such as high response to stimuli, environmental adaptation, and self-repairing capacity. Considering that there are several wonderful reviews about functional supramolecular polymers,5−8 in this review, we mainly highlight some of the recent advances in specific application areas, such as optoelectronic materials, self-healing materials, and biomedical materials.
2. DRIVING FORCES FOR SUPRAMOLECULAR POLYMERS 2.1. Multiple Hydrogen Bonds
The hydrogen bond is a type of dipole−dipole attraction between an electronegative atom and a hydrogen atom bonded to another electronegative atom, such as nitrogen, oxygen, or fluorine. It was first suggested by Moore and Winmill in 1912.9 Nowadays, the hydrogen bond is one of the most widely applied noncovalent interactions in the field of supramolecular chemistry. 2.1.1. Hydrogen-Bonding Arrays. Hydrogen bonds are weaker than covalent and ionic bonds, with an energy typically between 5 and 30 kJ/mol. A single hydrogen bond is not strong enough to fabricate supramolecular polymers. However, both the strength and the directionality can be increased when multiple hydrogen bonds are arrayed to create hydrogenbonding arrays. Typical examples of multiple-hydrogenbonding arrays are summarized in Table 1. The binding strength between arrays is also dependent on the order of the donor and acceptor in the arrays (Table 1), which was first pointed out by Jorgensen et al.10,11 and further confirmed by Zimmerman et al.12−15 It was found that although the DAA− AAD and ADA−DAD arrays have an equal number of 7198
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was very favored over homodimerization by a factor of >20:1 at high concentration (>0.1 M). To prepare a strict alternating supramolecular copolymer, multiple-hydrogen-bonding arrays with highly stable heterodimerization but minimal homodimerization need to be designed. This goal was achieved by Zimmerman et al., who reported a non-self-complementary unit, ureidoguanosine. This quadruple-hydrogen-bonding unit could strongly combine with an Napy unit (Ka ≈ 107 M−1 in CDCl3), while exhibiting negligible self-association (Kdim = 230 M−1 in CDCl3).34,35 Mixing the two bifunctional macromonomers 13 and 14 gave rise to a supramolecular copolymer with a strictly alternating sequence of the two monomers,36 while both of the monomers alone did not form polymers (Figure 3).
Figure 1. Supramolecular polymers self-assembled from typical triple-, quadruple-, and sextuple-hydrogen-bonding arrays.
700 at 40 mM in CHCl3. Since then, this unit has been widely used for constructing supramolecular polymers.17,27−29 Lehn et al. also reported a series of supramolecular polymers based on barbiturate/cyanurate−isophthaloyldiamidopyridine sextuple arrays 10 (Figure 1).30,31 Rigid fibers were observed in toluene, while small-angle neutron scattering in decane demonstrated the formation of gel-like assemblies. Heterodimerizing motifs are attractive for constructing supramolecular block copolymers. Meijer et al. prepared AA/ BB-type supramolecular block copolymers using Napy and UPy motifs,32 in view of the strong and selective complexation of these two units.33 They designed a poly(tetrahydrofuran) macromonomer, 11, with UPy units as the chain ends and a ditopic Napy monomer, 12 (Figure 2). The composition of the supramolecular polymers could be tuned from a pure homopolymer to an alternating heteropolymer by controlling the stoichiometry of UPy and Napy groups. This behavior was shown to be concentration dependent, and heterodimerization
Figure 3. Structures of bifunctional macromonomers that can form a strict alternating supramolecular copolymer. Reprinted from ref 36. Copyright 2006 American Chemical Society.
The molar ratio of AA/BB-type monomers is one of the major factors that limits the DP of supramolecular polymers. Meijer et al. addressed this limitation by using AB-type monomers to prepare supramolecular polymers.37 Polymerization of a bifunctional UPy−Napy monomer was expected to obey the ring−chain equilibrium mechanism. Thus, at low concentrations ( 1014 M−2 for the 3-fold chelating terpyridine unit. For fully reversible coordination polymers, the relationship between DP and K is given by the expression DP ∝ (K[M])1/2.51 According to this relationship, the DP of MSPs constructed from 1 mM solution can be estimated. In the case of a binding constant of K = 105 M−1, only oligomers with DP ≈ 10 can be obtained, whereas, in the case of a binding constant of K = 107 M−1, polymers with DP ≈ 100 may be achieved. It should be pointed out that the rational design of the molecular structure of ligands and the directionality of coordination bonding also play key roles in the construction of linear MSPs. A rigid ligand with a nonlinear coordination configuration often leads to the formation of macrocyclic entities, such as polygons, polyhedrons, and MOFs.52 Therefore, a ditopic ligand with a flexible linker or coordination systems with linear configurations are recommended for the construction of linear MSPs with a high DP. At the early stage of this line of research, MSPs were usually found to be either insoluble or decomposable in solution, exchanging some of their original ligands for solvent molecules; thus, a profound characterization was precluded.8,46 Rehahn et al. reported the first example of MSP 17 with a high molecular mass that is soluble and inert in noncoordinating solvent (Figure 6).60 A ligand monomer, 4,4″-bis[(9-aryl)-2-o-phenanthroline]-2′,5′-di-n-hexyl-p-terphenyl, was mixed with equimolar metal monomer [Cu(MeCN)4]PF6, broad 1H NMR peaks for the polymeric repeating units were observed, and the lack of end-group absorptions implied the DP was higher than 30. The polymer was found to be stable in noncoordinating solvent, such as 1,1,2,2-tetrachloroethane. However, when a trace of coordinating solvent (acetonitrile) or a specific free ligand was added to the solution, the polymer became more dynamic with rapid exchange of ligands. On the basis of this research, Rehahn et al. also studied a series of MSPs with different metal ions, including copper(I) and silver(I), and different structures of ophenanthroline-based bisbidentate ligands.61,62 These systems could be considered supramolecular polymers in coordinating
Figure 4. Poly(ethylene/butylene) with OH end groups (a) and poly(ethylene/butylene) functionalized with multiple-hydrogenbonded units (b). Reprinted with permission from ref 42. Copyright 2000 John Wiley & Sons, Inc.
rates, leading to thermally responsive materials with a high sensitivity in the solid state. Hailes et al. developed a bifunctional monomer based on ureidocytosine motif 15, which did not undergo tautomeric changes in C6D6, C6D5CD3, and the solid state (Figure 5).43 Diffusion coefficient measure-
Figure 5. Bifunctional macromonomers based on ureidocytosine 15 and cytosine- and adenine-end-capped motifs 16.
ment indicated that it formed oligomers in dilute CHCl3 solutions (6.5 mM) but polymers in concentrated solutions (37 mM). Rowan et al. investigated the nucleobase 16 (e.g., cytosine and adenine) as self-assembly units to construct supramolecular polymers in the solid state aided by phase segregation (Figure 5).44,45 They found that nucleobaseterminated poly(THF) macromonomers could be meltprocessed into self-supporting films and fibers while the corresponding poly(THF) (Mn = ca. 1.4 kDa) was a soft waxy material. The rheological behavior of the supramolecular polymers was strongly influenced by the nucleobase groups. In the case of cytosine-end-capped material, a network structure was observed at room temperature. For the adenine-endcapped material, incomplete association was observed at lower temperatures with a transition to a network at higher temperatures prior to a complete loss of polymer-like properties. The development of such materials, which exhibit very low melt viscosities, potentially allows access to thermally rehealable plastics as well as easy-to-process materials. 2.2. Metal-Coordination Bonds
The incorporation of metal-coordination bonds into metalcontaining polymers has been extensively studied. Metalcontaining polymers have attracted great interest due to their combination of the properties of organic polymers and the magnetic, electronic, optical, and catalytic potential of metals.46 7200
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Table 2. Typical Examples of Building Motifs for Metallosupramolecular Polymers (MSPs)
was highly dependent on the hydrophilicity of the monomer64 and the counterions.65 The 2,2′,6′2″-terpyridine (or TPY for short) ligand is one of the most important ligands for fabricating MSPs. TPY contains three nitrogen atoms, so it can act as a tridentate ligand and form stable complexes by chelating various transition-metal ions.66 Therefore, it has been widely used to fabricate MSPs.67−70 In 1995, Constable71 et al. proposed that MSPs could be obtained by addition of metal ions to TPYfunctionalized monomers. Kurth and co-workers prepared this MSP through the complexation between 1,4-bis(2,2′,6′,2″-terpyrid-4′-yl)benzene and iron(II) ions.72 The molecular mass was determined by analytical ultracentrifuga-
solvent, although they do not show supramolecular behavior in noncoordinating solvent. Compared with the 2,9-substituted phenanthroline ligand, 2,9-unsubstituted phenanthroline ligands show a higher coordination ability with metal ions, owing to the lack of steric hindrance. Higuchi et al. employed this unsubstituted phenanthroline with a dioctylfluorene spacer as a monomer ligand to prepare a copper-containing supramolecular polymer, 18 (Figure 6). A high molecular mass (Mw = 7.6 × 103 kDa) of the supramolecular polymer was obtained, which could act as a green electrochromic material.63 Ionic conductive materials were also developed based on the phenanthroline ligand chelated with a Ni(II) ion; the conductivity of these materials 7201
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Figure 6. MSP 17, which is soluble and dynamic in coordinating solvent, and MSP 18 based on a phenanthroline ligand with a fluorene spacer act as green electrochromic materials.
potential or thermal treatment. On the basis of this strategy, a further detailed study of MSPs based on a TPY−iron(II) complex system was significantly extended.75,76 The reversible formation of the metal−ligand bond was studied by addition of the strong chelating ligand HEDTA (N-(2-hydroxyethyl)ethylenediamine-N,N′,N′-triacetic acid) in aqueous solution. The monomer ligand was cut off, precipitated, and then isolated with addition of HEDTA. Upon addition of FeCl2, the supramolecular polymer formed almost immediately. The fabrication and the study of the reversibility of supramolecular polymers based on the TPY−iron(II) complex system were also demonstrated by other research works.78−81 In addition to the TPY−iron(II) complex system, complexes of TPY with other transition-metal ions have also been incorporated into fabricating MSPs. Schubert and co-workers compared the thermodynamic stability of TPY-based supramolecular polymers with different metal ions through viscometry experiments.75 The sequence of maximum values of the viscosities for polymers with different metal ions is in the order iron(II) > nickel(II) > cobalt(II) > copper(II) > cadmium(II). When excess metal ions were added, the relative viscosity decreased significantly in the cases of cadmium(II), copper(II), and cobalt(II); however, the decrease was much less pronounced for iron(II) and nickel(II). All of these results are in agreement with literature data for the stability constants of the formed biscomplexes.57,82 Würthner et al. prepared TPY−Zn(II)-based supramolecular polymers bearing a perylene bisimide group in the main chain.83 The average molecular mass of the supramolecular polymer was about 20 kDa, corresponding to approximately 15 repeating units in a 1 mM solution of CHCl3/CH3OH (60 vol %/40 vol %).49 Various TPY-based supramolecular polymers containing a Zn2+ ion with high photoluminescence quantum yields and electroluminescence performance have been reported,84,85 which will be further discussed in section 6. The dynamic nature of the metal−ligand bond is one of the key features of MSPs. Different metal ions exhibit different binding kinetics and thermodynamics, which makes these
tion. From the Swedberg formula, the lower limit of the molecular mass was roughly estimated to be 14.9 kDa, corresponding to 25 repeating units. A crystal structure of this MSP was obtained through electron diffraction in combination with molecular modeling, which evidenced the polymeric chain structure.73 Early research of MSPs based on the TPY ligand suffered from restricted solubility; in other cases, the solubilizing groups had to be introduced by rather complicated procedures. To improve the solubility of MSPs, Schubert et al. introduced a simple synthetic strategy to construct water-soluble MSPs (Figure 7).74−77 Bifunctional monomers which consisted of
Figure 7. Schematic representation of water-soluble terpyridine ligands.
poly(oxytetramethylene) 19 or poly(ethylene oxide) 20 flanked with two TPY groups were synthesized through the Williamson reaction. This simple and high-yield strategy facilitates the preparation of TPY-terminated oligomers and polymers. The addition of octahedral coordinating transition-metal ions (colbat(II) or zinc(II)) to these TPY ligands leads to the formation of MSPs. These supramolecular polymers can be reversed by changing the pH or applying an electrochemical 7202
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systems even more versatile. Würthner et al. compared the kinetic stability of the complexation of several first-row transition-metal ions with the TPY ligand.49 The results are consistent with kinetic data82 for the reaction of various M(TPY)2 complexes with excess TPY in water. For example, t1/2 values for Fe2+, Ni2+, and Co2+ are 8400, 610, and 60 min, respectively, whereas t1/2 is less than 0.1 min for the Cu2+ and Zn2+ complexes. Schubert and co-workers investigated the characterization and the stability of MSPs based on TPY−metal ion complexes utilizing gel permeation chromatography (GPC).86 GPC studies were carried out on a series of monoend-capped TPY−poly(ethylene oxide)−metal (Fe2+, Co2+, and Ni2+) complexes with different sample injection volumes. In the Fe2+ and Co2+ systems (Figure 8), lowering the volume of the
Figure 9. Formation of MSPs using BIP-based ditopic ligand monomers with metal salts in acetonitrile. Reprinted from ref 100. Copyright 2006 American Chemical Society.
Figure 8. GPC elution curves for the bis-TPY ligand and Co2+ complex system (different injection volumes) and their corresponding starting material. Reprinted with permission from ref 86. Copyright 2007 John Wiley & Sons, Inc.
An interesting aspect of these studies is that the BIP ligand can form 2:1 metal ion complexes with transition-metal ions and 3:1 complexes with lanthanide ions; therefore, supramolecular gels94,100 or films103−105 can be created by mixing the telechelic chelating ligand with a combination of lanthanoid and transition-metal ions (Figure 9). These metallosupramolecular gels or films exhibit thermo-, chemo-, optico-, and mechanoresponses, as well as light-emitting and shape-memory104 properties. A unique multidentate ligand comes from porphyrin derivatives.54,111 Porphyrin-based model compounds have been developed as artificial light-harvesting systems and molecular photonic devices to mimic natural light-harvesting complexes. However, construction of these model compounds often involves tedious organic synthesis. To overcome this problem, supramolecular porphyrin polymers have attracted great attention. Hunter et al. reported a metalloporphyrin supramolecular polymer, 22, through the coordination of cobalt porphyrin and two pyridine ligands, one on each face of the porphyrin (Figure 10).54 The binding constant K = (1.0 ± 0.5) × 106 M−1 is high enough to build a supramolecular polymer. The DP increased with the concentration of the monomer characterized by GPC, indicating that the polymers in solution were constitutionally dynamic. The DP reached approximately 100 repeating units with a mean molecular mass of 136 kDa, which was significantly larger than those of the porphyrin systems obtained from covalent reactions (DP = 10−15). Almost simultaneously, Kobuke et al. reported another example of metalloporphyrin supramolecular polymer 23 through the coordination of zinc porphyrin and imidazolyl groups (Figure 10).111 Considering the pentacoordination of Zn2+, an ABBAt y p e m o no m e r w h i c h co n ta i ne d t w o co nn e c t e d porphyrinatozinc(II) groups flanked with two imidazolyl groups was designed, and supramolecular polymers were obtained by complementary coordination at each imidazolyl-
injected sample resulted in a detectable shift to the macromonomer in GPC spectra, indicating a degradation of polymer chains due to the shearing and dilution effects of GPC. To develop novel MSPs as promising electrochromic materials, a series of TPYs with pyridine-ring-functionalized ligands coupled with rigid, linear spacers, in contrast to the unsubstituted TPY ligand, were studied.87−93 Supramolecular polymers based on electron-donating-group-substituted TPY ligands show rapid switching rates, good reversibility, high stability, and optical memory, which are attractive candidates for smart responsive electrochromic materials. A tridentate ligand, 2,6-bis(1′-methylbenzimidazolyl)pyridine (BIP for short), which is analogous to the TPY ligand, was also developed for the construction of MSPs (Figure 9).58,94−110 By titrating a metal ion (Zn2+) solution into a bis-BIP monomer, 21, solution, the formation of MSPs was monitored by viscometry.96 A steady increase of the reduced viscosity up to a Zn2+/ligand ratio of 1 indicated the formation of MSPs; this was followed by a decrease of the reduced viscosity beyond this point, confirming the depolymerization of the MSPs. In addition to the Zn2+ ion, other transition-metal ions such as Cd2+, Co2+, and Fe2+ were also adopted to fabricate BIP-based supramolecular polymers.97 Different metal ions impart different properties onto the MSPs, which allows fine-tuning of the properties of these materials. Thermal data suggest that supramolecular polymers with weaker binding, more labile metal ions (e.g., Cd2+, Zn2+) show less thermal stability than those with stronger binding, less labile metal ions (e.g., Co2+, Fe2+). BIP-based MSPs consisting of copper ions (Cu+ and Cu2+) show an interesting redox-responsive property, where the DP and the viscosity of their solutions depend on the oxidation state of the copper ions.108 7203
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adjusted from 1:2 to 1:6, according to the ligand structure. Such nonstoichiometric binding is possible through the electrostatic interactions between amphiphiles and supramolecular polymers as depicted in Figure 11. Perfectly rodlike chains of these complexes formed from rigid ditopic ligand can be observed under atomic force microscopy (AFM).124 PACs are soluble in common organic solvents, form Langmuir monolayers at the air−water interface, and are readily transferred onto solid substrates. Moreover, PACs display thermotropic liquid crystalline phases.125−127 The change of counterions can also endow the materials with new properties. The introduction of amphiphiles as counterions gives rise to a structure-induced spin-crossover, shown in Langmuir−Blodgett (LB) multilayers of iron-based PACs.127 The phase transition upon heating induces the deformation of the coordination geometry around the central metal ions, which is strong enough to reduce the strength of the ligand field, giving rise to spin transition from a low-spin state to a high-spin state. The counterions also affect the ionic conductivity of the MSP films.65 MSPs with “hard” counteranions show high conductivity: the order of the hardness in the bases is chloride > nitrate > acetate > acetylacetonate, which is consistent with the order of the ionic conductivity in the supramolecular polymers. 2.2.3. Stoichiometric Ratio. The preparation of MSPs normally involves a simple mix of ligands and metal ions in solution, and the polymerization follows a step-by-step mechanism. Therefore, to obtain MSPs with a high molecular mass, it is of paramount importance to control the ligand to metal ion ratio very accurately. At least two methods were introduced to monitor the stoichiometric ratio precisely: one is conductometry, and the other is viscosimetry. Conductivity measurement was introduced by Kurth and coworkers128 to monitor the stoichiometric ratio during selfassembly of TPY-based MSPs. Due to the charge of the MSPs, conductometry is well suited to follow the self-assembly, and in addition it is of excellent accuracy. Figure 12 shows conductivity measurements as a function of y = [M]/[L] for the titration of iron(II), cobalt(II), and nickel(II) acetate to bisTPY ligand. The conductivity decreases as the metal ion is titrated to the solution until a minimum is reached at y = 1. As y exceeds 1, further addition of metal salt results in an increase of the conductivity. The advantages of conductivity are the ease of use, even for air-sensitive compounds, high accuracy, and in situ detection.
Figure 10. (Top) Metalloporphyrin supramolecular polymers driven by the coordination of cobalt porphyrin and pyridine groups. Reprinted with permission from ref 52. Copyright 2000 John Wiley & Sons, Inc. (Bottom) Metalloporphyrin supramolecular polymers driven by the coordination of zinc porphyrin and imidazolyl groups. Reprinted with permission from ref 111. Copyright 2000 John Wiley & Sons, Inc.
porphyrin unit. The average DP was about 80 repeat units analyzed by GPC. Besides the ligand−metal ion complex systems mentioned above, a lot of other ligand−metal ion systems, such as bis(pyrazolyl)pyridine−Fe 2+ , 112 bis(pyrazolyl)pyridine− Zn2+,113 pyridine-2,6-dicarboxylate ligand coordinated with Zn2+, Nd3+, and La3+,114,115 pyridine−Pt2+ and pyridine−Pd2+ systems, 116,117 4,4′-biphenyldicarboxylic acid−terbium (Tb3+),118 tetra-2-pyridyl-1,4-pyrazine−Fe2+,119 acyl hydrazone ligands with transition-metal ions,120 azo aromatic ligand− Co3+,121,122 and azo aromatic ligand−Fe3+,122 have been well documented for the construction of functional MSPs. 2.2.2. Counterions. The solubility of MSPs can be adjusted by the choice of counterions. In the case of noncoordinating counterions, MSPs are generally soluble in aqueous media through colloidal stabilization. With counterions such as PF6−, Cl−, or amphiphiles with a negatively charged headgroup, MSPs are soluble in organic solvents. The replacement of acetate counterions with amphiphiles such as dihexadecyl phosphate (DHP) leads to formation of MSP−amphiphile complexes (PACs).123 The ratio of metal ion to amphiphile can be readily
Figure 11. Self-assembly of MSPs and dihexadecyl phosphate (DHP) affords PACs with a monolayer or double-layer structure depending on the metal ion to amphiphile ratio.125,126 Right: Melting of the alkyl chains in the amphiphilic mesophase induces a spin transition from a diamagnetic low-spin state to a paramagnetic high-spin state. Reprinted from ref 127. Copyright 2005 American Chemical Society. 7204
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mer can inhibit the formation of a cyclic oligomer and promote the linear growth of polymer chains, resulting in a high DP. This is especially important for some kinetically inert MSPs, because once the cyclic chains are formed, they can hardly be broken and re-formed even with concentration of the solution after polymerization. Finally, variation of the temperature not only brings important modification of the aimed MSPs, but also influences the balance between the thermodynamic stability and kinetic lability of these polymers. 2.3. Host−Guest Interactions
Host−guest interactions are driving forces130 for host−guest complexes which involve a cooperative effect of various noncovalent interactions, such as hydrophobic interactions, multiple hydrogen bonds, ionic bonds, van der Waals forces, and electrostatic interactions. Although a large number of host−guest systems have been reported, common host molecules involved in fabricating supramolecular polymers include cyclodextrins (CDs), crown ethers, cucurbit[8]urils (CB[8]’s), calixarenes, and pillar[n]arenes.130,131 Guest molecules are general organic compounds that can be incorporated inside the cavity of hosts. Several typical host molecules and their corresponding guest molecules are listed in Table 3. The chemical basis of the host−guest complexation is based on complementary shape and size fit between the host and guest molecules. Various types of monomers and their corresponding supramolecular polymers based on host−guest interactions are summarized in Table 4. Usually two types of ditopic monomers that employ hosts (A) and guests (B) are designed to fabricate supramolecular polymers: AB-type heteroditopic monomers and AA/BB-type homoditopic monomers. However, both of them share a common problem, which is a competition between linear polymerization and oligomerization (intramolecular complexation, dimerization, or oligomeric cyclization). A strategy to inhibit the formation of oligomers is to increase the concentration of monomers, since oligomers are predominant in dilute solution. For CB[8]-based systems, considering their poor solubility in water solution, a unique ABBA-type monomer was designed to promote linear supramolecular polymerization in dilute solution. 2.3.1. Cyclodextrin-Based Supramolecular Polymers. Cyclodextrins are cyclic oligomers built up from six, seven, or eight glucopyranose units, linked by α-(1−4)-glycosidic linkages, named α-, β-, and γ-cyclodextrins, respectively.132 They form inclusion complexes in water with a variety of organic molecules. The host−guest interaction between cyclodextrins and guest molecules has been adopted as one of the earliest driving forces to prepare polyrotaxanes and supramolecular polymers.131 In an early stage of this line of research, numerous monosubstituted CDs, as AB-type monomers, have been prepared and characterized. However, most of them form intramolecular complexes or dimers.133 As one of the pioneering works, Harada et al. found that 6-cinnamoyl-α-CD (6-cin-α-CD) formed intermolecular complexes.134 Vapor pressure osmometry measurements demonstrated that the 6cin-α-CD showed a concentration dependency, while α-CD did not. The molecular mass of this complex was estimated to be 3 kDa, corresponding to a trimer. The failure in forming linear polymers was due to the phenyl ring being included from the narrower side of CD (C6-primary OH group side), which forced it to be more inclined to form a dimer or cyclic oligomer.
Figure 12. MSP synthesis under conductometric control. Conductometric titrations of the ditopic ligand L1 and iron(II), cobalt(II), nickel(II), and zinc(II) acetate in acetic acid solution (□, ○, Δ, and ◇). Reprinted with permission from ref 128. Copyright 2010 Royal Society of Chemistry.
Viscosimetry is another method to monitor the supramolecular polymerization process.75,129 The advantage of this method is that both charged and uncharged species can be monitored. Schubert and co-wokers employed viscometry experiments to monitor the self-assembly process in preparing TPY-based supramolecular polymers with different metal ions (Figure 13).75 A stepwise addition of different metal salt
Figure 13. Viscosimetry experiments. Stepwise addition of various metal salts to a solution (c = 20 mg/mL methanol) of bis-TPY monomer led to different maximum values of the relative viscosity for an equivalent addition of the metal salt. Reprinted from ref 75. Copyright 2003 American Chemical Society.
solutions (cadmium(II) acetate, copper(II) acetate, cobalt(II) acetate, nickel(II) acetate, iron(II) sulfate) to a solution of bisTPY monomer resulted for all salts in an increase in the relative viscosity up to a maximum value at a metal ion:telechelic ratio of 1:1, followed by a decrease for higher ratios. The sequence of maximum values of the viscosity as well as the extent of relative viscosity decrease demonstrated the different binding strengths of these metal ions. Other factors such as solvent, concentration, and temperature also play important roles in the preparation of MSPs. As we mentioned before,60−62 the solvent can compete with the ligand in chelating the metal ions, thus influencing the stability of the MSP structures, in particular in the case of weak metallocomplexes. Since the polymerization of MSPs follows a polycondensation mechanism, a high concentration of mono7205
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Table 3. Several Typical Host Molecules and Their Corresponding Guest Molecules
was obtained, which was even larger than the supramolecular polymers with a rigid and hydrophobic linker.138 A possible interpretation of these results was the solvation effect of the linker, which avoided the formation of intrachain coiled moieties.139 It was suggested that supramolecular polymer chains with a coiled conformation could trap the terminal group and then stop the polymer growth, on the basis of the smallangle X-ray scattering (SAXS) and molecular dynamic study. Therefore, large supramolecular polymers could be prepared by improving the solvent quality or tuning the hydrophilicity of the linkers. In addition to the dimer with the same type of CD, different types of CDs (α- and β-CDs) can be attached together to form a heterodimer. When the heterodimer was mixed with a guest dimer containing an adamantane group (a guest for βCD) and a phenyl group (a guest for α-CD), they also gave supramolecular polymers.140 To combine photochemistry with supramolecular chemistry, an interesting work on a photoswitchable CD-based supramolecular polymer employing γ-CD and a dimer, 24, with coumarin as an end group was reported by Tian and coworkers.141 γ-CD accommodated two coumarin units due to its large hydrophobic cavity (7.5−8.3 Å).132 Supramolecular polymers were fabricated by mixing γ-CD with the dimer
An exception was 6-adamantylamido-β-CD, which was reported to form a linear supramolecular polymer characterized by X-ray single-crystal diffraction.135 The success of this example may be due to the steric effect of the bulky adamantly group, which impedes the formation of dimers and cyclic oligomers. To inhibit the dimerization or cyclization, the guest substituent group was then attached to one of the secondary hydroxyl groups of CD (the wider side). In this case, for example, the phenyl ring of 3-cin-α-CD was included from the wider side of CD (secondary OH group side), leading to the formation of linear supramolecular polymers. The molecular mass was about 20 kDa, which corresponded to a repeating unit of 20.136 A mixture of AA/BB homoditopic monomers such as a CD dimer and a guest dimer also forms supramolecular polymers. When a β-CD dimer and an adamantane dimer were mixed in aqueous solutions, supramolecular polymers or oligomers formed depending on the rigidity or length of the linkers in the monomers.137,138 It was found that the longer the hydrophobic linker (alkyl segment), the lower the molecular mass of supramolecular polymers.137 However, when the linker was a long hydrophilic PEG chain (Mn = 600 Da), a supramolecular polymer with molecular mass Mn = 100 kDa 7206
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Table 4. Representative Types of Host−Guest Monomers and Their Corresponding Supramolecular Polymers, Taking CD and CB[8] as Examples
offer opportunities for the development of new supramolecular polymers. 144 However, due to the difficulties in the modification of CBs, supramolecular polymers based on CB[5], CB[6], and CB[7] are still rare. CB[8] is unique because of its ability to bind two guests in its relatively large cavity. Therefore, CB[8] is the most potential host in the CB family for fabricating supramolecular polymers.145 However, limited by the poor solubility ( 150 kDa) were obtained owing to the relatively high binding constant of log Ka = 5.1 ± 0.5 in CHCl3 at room temperature. Electron-deficient planar aromatic guests, such as trinitrofluorenone, trinitrobenzene, and tetracyanobenzene, can be “clamped” by a bisporphyrin tweezer via charge-transfer interaction, leading to a π donor−acceptor-type host−guest complex. This unique interaction was incorporated into a heteroditopic monomer, 53, to construct supramolecular polymers (Figure 31).197 The head-to-tail complexation
Figure 29. Schematic illustration of the construction of supramolecular polymers from a pillar[5]arene dimer and ditopic and tritopic guest molecules. Reprinted with permission from ref 182. Copyright 2014 Royal Society of Chemistry.
for the linear supramolecular polymer was estimated to be 89, and the DP for the hyperbranched one was estimated to be 113. 2.4. Aromatic Donor−Acceptor Interactions
Monomers based on aromatic donor−acceptor interactions alone might be too weak and less directional to drive supramolecular polymerization. However, the introduction of directing groups on the periphery of an aromatic core can promote the formation of supramolecular polymers. A lots of works focusing on the formation of nanofiber assemblies through columnar-like stacking of monomers based on π−π interactions have been reported and reviewed.189−192 In this review, we are focusing more on linear main-chain supramolecular polymers fabricated through strong aromatic donor− acceptor interactions. Molecular tweezers,193 as noncyclic host molecules with open cavities capable of binding guest molecules through aromatic donor−acceptor interactions, are one of the representatives. Supramolecular polymers with incorporated fullerene have attracted interest in construction of functionalized supramolecular polymers, due to their unique physical and chemical properties.194 It was found that a π-extended analogue of tetrathiafulvalene (exTTF) could act as a receptor for C60.
Figure 31. Aromatic donor−acceptor interaction-induced supramolecular polymerization. Reprinted with permission from ref 197. Copyright 2012 John Wiley & Sons, Inc.
between the bisporphyrin tweezer and the 4,5,7-trinitrofluorenone-2-carboxylate (TNF) moiety resulted in large upfield shifts of the TNF protons, indicating that the TNF moiety was located within the porphyrin tweezer, thereafter forming supramolecular polymers. A molecular tweezer possessing two pincer-like alkynylplatinum(II) terpyridine units connected by a rigid spacer is able to recognize a variety of electron-rich arenes through charge-transfer interactions.198,199 Wang and coworkers reported supramolecular polymers by incorporating this tweezer−guest molecular recognition (Figure 32).200 They designed heteroditopic AB-type monomers 54, which consisted of a tweezer group and a pyrene group on each side.
Figure 30. Schematic representation of AB-type monomers consisting of, for 51, C60 with a pincerlile exTTF and, for 52, C60 with a macrocyclic molecule containing two exTTF motifs. 7213
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Figure 32. Schematic representation of the supramolecular polymers derived from the pincer-like monomer and pyrene. Reprinted with permission from ref 200. Copyright 2014 John Wiley & Sons, Inc.
interactions to promote the formation of supramolecular polymers. To combine multiple ion pairing and hydrogen bonding, Schmuck et al. developed a class of receptor molecules, 55, 2(guanidiniocarbonyl)-1H-pyrroles, for the recognition of carboxylates in polar solvents (Figure 33a).209,210 A typical
Supramolecular polymers formed through the tweezer−pyrene complexation (K ≈ 104 M−1) on the basis of a ring−chain equilibrium mechanism. Concentration and temperature dependence effects were observed for these supramolecular polymers. Until now, not enough effort has been devoted to developing these tweezer−guest motifs as building blocks for supramolecular polymers. Compared to the rigid macrocyclic hosts, these noncyclic tweezers, on account of their flexible configurations, can sandwich a variety of guest moieties and overcome the limitations encountered by the macrocyclic hosts, such as a tedious synthesis, a low yield, poor solubility, and a limited number of guest species. Therefore, these tweezer− guest recognition motifs may represent a more versatile strategy for the fabrication of supramolecular polymers. Although this strategy is promising, in most cases, the binding strength is not strong enough. Because of the flexible configurations of molecular tweezers, they can adapt to various guest moieties and suffer the relatively low binding constant compared to macrocylic hosts. Therefore, to achieve a high DP, it is highly desirable to develop motifs with a high binding constant, which can really advance this driving force as a versatile strategy to construct functional supramolecular polymers. 2.5. Multiple Driving Forces
Figure 33. (a) (Guanidiniocarbonyl)pyrroles efficiently bind carboxylates in polar solvents due to a combination of ion pair formation and additional hydrogen bonds. (b) Supramolecular polymers constructed from self-assembly of a heteroditopic monomer based on two orthogonal binding interactions, metal−ligand interaction and ion pair interaction. Reprinted from ref 211. Copyright 2005 American Chemical Society.
As mentioned above, supramolecular polymers are usually constructed by employing one type of noncovalent interaction, while, in nature, the combination of multiple noncovalent interactions in an orthogonal manner in living creatures is ubiquitous. Inspired by biological systems, chemists have been trying to fabricate supramolecular polymers driven by multiple noncovalent interactions.201−208 The combination of different and orthogonal supramolecular building blocks not only enriches the library of supramolecular polymers, but also allows control of the supramolecular polymerization, achieving supramolecular polymers with well-defined structure and tailormade functionality. As already reviewed by Wang,202 the integration of multiple noncovalent interactions mainly involves metal−ligand coordination, multiple hydrogen bonding, host− guest interactions, and π−π interactions. Therefore, we just highlight several works which employ orthogonal multiple
representative is a self-complementary zwitterion that forms extremely stable dimers in DMSO with a binding constant of up to 1010 M−1. Such a high binding constant is ascribed to the strong ion pairing combined with multiple hydrogen bonds.211 On the basis of these studies, linear supramolecular polymers were constructed through the orthogonal combination of three binding interactions, ion pairing, hydrogen bonding, and metal−ligand interaction (Figure 33b).212,213 7214
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Review
polymer. A similar effect was achieved while using CB[8] to enhance the rigidity of the spacer, which was illustrated in Figure 26. An example of a pillararene-based alternating supramolecular polymer promoted by self-sorting was reported by Jia and Li et al.216 They designed a double-threaded dimer by means of an AB2-type heterotritopic pillar[5]arene derivative, containing a pillararene unit and two 4-bromobutyl guest units, which afforded the first example of [c2]daisy chains from heterotritopic monomers 58 (Figure 36). This [c2]daisy chain
cowas and (p-
Figure 34. Schematic diagram of supramolecular polymers constructed by 6-substituted β-CD and α-CD. Reprinted from ref 214. Copyright 2005 American Chemical Society.
aminohydrocinnamoyl)-β-CD formed an intramolecular complex in an aqueous solution. When adamantanecarboxylic acid (AdCax) was added to the solution, AdCax was included in the β-CD cavity in place of the aminocinnamoyl group. The aminocinnamoyl group was released from the β-CD cavity to be exposed in water. Then α-CD was added, which bound the cinnamoyl group to give a pseudo-[2]rotaxane. Then another stopper group, trinitrobenzenesulfonic acid sodium salt (TNB), was attached to the aminocinnamoyl group to obtain a [2]rotaxane. When AdCx was removed by extraction with organic solvents, TNB was incorporated inside the cavity of the adjacent β-CD molecule, thus forming supramolecular polymers. In this system, the α-CD acts as a rigid bulky group to promote the linear supramolecular polymerization. Supramolecular polymerization of a calixarene dimer and guest molecules can be boosted by the integration of α-CD on the polymer main chain (Figure 35).215 Compared with binary supramolecular polymer 57@56, the threading of α-CD on guest molecule 57 effectively enhanced the rigidity of the spacer, leading to a larger polymeric size and better size distribution of the ternary 57@α-CD-56 supramolecular
Figure 36. Chemical structures of the heterotritopic pillar[5]arene derivative and homoditopic pillar[5]arene derivative and the corresponding supramolecular polymer promoted by self-sorting of these monomers. Reprinted with permission from ref 216. Copyright 2013 Royal Society of Chemistry.
monomer has two additional free guest moieties at both ends and can be regarded as a homoditopic BB-type monomer, 59. While complexed with a homoditopic AA-type pillar[5]arene monomer, alternating supramolecular polymers containing [c2]daisy chains formed. These results have provided a facile route for fabricating novel [c2]daisy chain supramolecular copolymers. We have reported recently that supramolecular polymerization based on a CB host and guest could be promoted and controlled by the orthogonal interactions of CB[8], CB[7], and guest molecules.217 To this end, a monomer, 60, that contained one p-phenylene moiety in the middle flanked by two naphthalene moieties as end groups was designed and synthesized (Figure 37). The p-phenylene moiety selectively bound to cucurbit[7]uril (CB[7]), thus allowing free access to the naphthalene end groups for 2:1 complexation with CB[8]. An equimolar mixture of these three components resulted in the formation of a strong host−guest complex between CB[7] and the p-phenylene moiety to give a rigid and bulky linker between the two naphthalene units, thus serving to prohibit monomer cyclization and dimerization and facilitate the linear supramolecular polymerization.
Figure 35. Structural illustration of the building blocks and schematic representation of the comparison of binary and ternary supramolecular polymers. Reprinted with permission from ref 215. Copyright 2013 Royal Society of Chemistry. 7215
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Figure 37. Chemical structures of monomer 60, CB[7], and CB[8]. Schematic diagram of cucurbituril-based supramolecular polymerization promoted by self-sorting upon equimolar mixing of CB[7], CB[8], and 60 and controlled depolymerization of the supramolecular polymers in excess of CB[7]. Reprinted with permission from ref 217. Copyright 2014 John Wiley & Sons, Inc.
Figure 38. First example of supramolecular dendrimers formed by a homotritopic core unit, a heterotritopic AB2 branch unit, and an end cap.
3. SUPRAMOLECULAR POLYMERS WITH BRANCH-LIKE AND HYPERBRANCH-LIKE TOPOLOGY
heterodimer UC·64, while the dimer of tU remains unchanged. On the basis of these results, a uniform supramolecular dendrimer was obtained through the assembly of building blocks 64 and 65.220 According to the same strategy, uniform supramolecular dendrimers were also constructed by the orthogonal self-assembly of multiple hydrogen bonding and metal-coordination bonding221 or an isocyanuric acid−Hamilton receptor pair and a quadruple-hydrogen-bonding dimer.222 In summary, the advantages of supramolecular dendrimers are mainly due to their dynamics and reversibility. However, because of the dynamics of noncovalent interactions, it is not easy to keep supramolecular dendrimers with a uniform structure in solution. Although the strategy to utilize different interactions for each generation has resolved this conflict to some extent, it still involves tedious organic synthesis. Therefore, supramolecular polymers with a hyperbranched structure seem to be more versatile than supramolecular dendrimers.
3.1. Supramolecular Dendrimers
A broad definition of supramolecular dendrimers contains dendrimers that are constructed from the recognition of dendritic building blocks.218 However, in this review we focus on a narrow definition of a supramolecular dendrimer, where the repetition motif is no longer based on dendritic subunits. A supramolecular dendrimer constructed from a heterotritopic AB2 unit, which contains two Hamilton receptors as well as a complementary cyanuric acid substrate, was first reported by Hirsch et al.219 The formation of different generations of supramolecular dendrimers can be obtained by tuning the stoichiometric mixing of core unit 61, branching unit 62, and end cap 63 in a ratio of 1:(3 × 2n − 3):(3 × 2n) (Figure 38). This investigation, for the first time, shows the possibility of a programmed self-assembly of discrete supramolecular dendrimers from nondendritic units. However, in such a system, due to the identical recognition motifs, “wrong” combinations, such as a core unit combined with an end-cap unit, are also possible; thus, the final product may consist of structurally different assemblies. This problem can be solved when different and orthogonal recognitions are utilized for each generation.220−222 As shown in Figure 39, triurea (tU) derivatives and urea-containing calix[4]arenes (UCs) combine themselves to form dimers. With addition of tetratosylurea-substituted calix[4]arenes 64, the homodimer of UC is replaced by
3.2. Supramolecular Hyperbranched Polymers
In recent decades, a large number of supramolecular hyperbranched polymers have been reported, not only due to their easy preparation, but also because of their potential applications in many fields such as biochemistry, medicine, and materials science. As an early work, Meijer et al. introduced the classic multiple-hydrogen-bonding array, the Upy unit, to construct supramolecular hyperbranched polymers.223 A trifunctional copolymer of propylene oxide and ethylene oxide macromonomer terminated with Upy units was prepared, which then 7216
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Figure 39. Chemical structure and schematic illustration of building blocks for supramolecular dendrimers with a uniform structure in solution. Reprinted from ref 220. Copyright 2005 American Chemical Society.
preparation process and easier combination of multiple functions. Taking advantage of the easy modification of macrocyclic host molecules, e.g., crown ether229−231 and pillararene,232 AB2type monomers which simultaneously contain host and guest moieties are most commonly used to construct supramolecular hyperbranched polymers. Gibbson et al. reported the construction of supramolecular hyperbranched polymers from self-assembly of an AB2 monomer containing complementary recognition sites, a bis(m-phenylene)-32-crown-10 and two paraquat moieties.229 On account of the relatively low binding constant of this host−guest system, a high monomer concentration (>100 mM) was required for a high degree of supramolecular polymerization. Supramolecular hyperbranched polymers fabricated through the recognition between dibenzo[24]crown-8 (DB24C8) moieties and diakylammonium ion centers showed a pH response.230,231 A polymer structure formed under acidic condition, while it degraded in basic solution. Upon successive addition of acid and base to the polymer solution, reversible changes in the fluorescence intensity could be observed over at least 10 cycles.231 While incorporating metal complexes, a reversible luminescence supramolecular hyperbranched polymer was constructed, taking advantage of its pH response.230 Supramolecular hyperbranched polymers can be constructed by the combination of different noncovalent interaction modes. An interesting example comes from Wang et al., who constructed supramolecular hyperbranched polymers via a “tweezering-directed self-assembly” strategy, combining the inclusion mode from a crown ether−guest system and the tweezering mode from a tweezer-like molecule (Figure 40).233 It was shown that a
formed supramolecular hyperbranched polymers. The rheological behavior of this polymeric network, which shows a welldefined viscoelastic transition, differs from the rheological behavior of the phase-separated polymer networks, such as sugar-based natural materials. Another unique property of this supramolecular network is its reversibility, which was demonstrated by the addition of a monofunctional Upy unit acting as a chain stopper. The result is a degradation of the polymer network, resulting in a dramatic decrease of the viscosity. However, polymer networks based on weak secondary interactions do not show this response. The advantage of this supramolecular network based on multiple hydrogen bonding is the high “virtual” molecular mass in combination with an excellent processability, while the disadvantages could be the low creep resistance and the poor water resistance. In addition to multiple-hydrogen-bonding systems, a series of supramolecular hyperbranched polymers based on host−guest interactions have been reported. A typical system based on βCD and a variety of guest molecules have been widely studied, and functional groups are incorporated to endow supramolecular hyperbranched polymers with various functions, such as enzymatic catalysis and thermo- and photoresponsiveness.219,224−228 As one of the pioneering works, Tato et al. synthesized supramolecular hyperbranched polymers through the recognition of β-CD trimer and sodium deoxycholate224 or adamantyl dimer.225 Dendrimer-like self-assembly structures were observed under microscopy. Porphyrin- and telluriumcontaining supramolecular hyperbranched polymers based on a cycledextrin−adamantyl system appear to be artificial enzymes with both SOD and GPx activity.226 Compared with other methods, this artificial enzyme shows advantages of a simple 7217
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Figure 40. Schematic representation of the construction of a supramolecular hyperbranched polymer from host−guest and tweezering binding modes. Reprinted with permission from ref 233. Copyright 2014 Royal Society of Chemistry.
an azastilbene unit as a guest moiety (Figure 42).235 The transformation from supramolecular hyperbranched polymers
bis[alkynylplatium(II) terpyridine] tweezer, 66, could clamp an alkynylgold(III) diphenylpyridine guest, 67, leading to the formation of a supramolecular A2 monomer terminated by a crown ether. After the addition of a B3 secondary ammonium salt monomer, 68, a supramolecular hyperbranched polymer containing two kinds of binding modes was successfully constructed. We have studied the construction of hyperbranched supramolecular polymers driven by CB[8] host-enhanced π−π interaction. Considering that CB[8] can bind two guest molecules into the cavity, we just need to prepare a three-arm molecule, 69, which possesses one guest moiety (e.g., a naphthalene group) in each arm, to fabricate a hyperbranched supramolecular polymer in aqueous solution (Figure 41).234 CB[8]-based supramolecular hyperbranched polymers can be switched into covalent hyperbranched polymers by introducing
Figure 42. Illustration of the transformation from supramolecular hyperbranched polymers into covalent hyperbranched polymers by UV irradiation. Reprinted with permission from ref 235. Copyright 2014 Royal Society of Chemistry.
to covalently hyperbranched polymers can be achieved by the [2 + 2] cycloaddition of azastilbene groups in the cavity of CB[8] after UV irradiation. The switching can induce a change in fluorescence emission color from light orange to light blue. The method of combining supramolecular polymerization with photochemistry may be helpful in constructing hyperbranched polymers with advanced tunable optical properties. A unique water-soluble 2-dimensional supramolecular hyperbranched polymer was constructed from the recognition of a 4,4-bipyridin-1-ium (BP)-containing tritopic molecule and CB[8] (Figure 43).236 The rigid and steric BP molecule forces the formation of a single-layer 2D periodic hexagonal pore array in solution, which was characterized by X-ray scattering and various microscopies. The obtained 2D porous architecture represents a new type of supramolecular polymer, which exhibits periodic structural ordering similar to that of MOFs
Figure 41. Illustration of the formation of supramolecular hyperbranched polymers through the recognition of CB[8] and a naphthalene-containing three-arm monomer, 69. Reprinted with permission from ref 234. Copyright 2013 Royal Society of Chemistry. 7218
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molecular polymers. The ternary complex supramonomer was chosen because of the high binding constant between CB[8] and the N-terminal tripeptide group Phe-Gly-Gly and therefore to ensure the structural stability of the supramonomer throughout the polymerization process. On the basis of orthogonal processes of host−guest interaction and click polymerization, we demonstrated that a supramolecular polymer could be formed by mixing all of the moieties simultaneously in an aqueous solution; therefore, a “one-pot” strategy to simplify the fabrication of supramolecular polymers is further developed. Supramolecular polymers can also be constructed by noncovalent polymerization of supramonomers.238 Supramonomers were obtained by host−guest interaction between tripeptide derivative FGG-Azo and CB[8]. Then supramolecular polymers were formed spontaneously by mixing the supramonomers with bis(β-cyclodextrin)s in a molar ratio of 1:1 through the complexation of azobenzene groups and βcyclodextrin. It should be noted that the solubility of FGG-Azo and CB[8] is less than 0.1 mM at room temperature. However, once supramonomers are formed, the solubility is significantly enhanced and the concentration of the solution can be prepared as high as 4 mM, thus promoting the formation of a supramolecular polymer. In most cases, supramolecular polymerization is similar to step polymerization, meaning that the distribution of molecular mass is usually broad, which is hard to control. In the case of polymerization of a supramonomer, the uncontrollable noncovalent polymerization can be switched into controllable covalent polymerization; thus, supramolecular polymers being formed in this way may have a controllable polydispersity. It is highly anticipated that the concept of a supramonomer not only enriches the preparation methods for supramolecular polymers, but also gives higher flexibility in designing functional supramolecular polymers.
Figure 43. Schematic representation of the 2D honeycomb SOF (supramolecular organic framework). Reprinted from ref 236. Copyright 2013 American Chemical Society.
and covalent organic frameworks (COFs) and may offer unprecedented accessibility and processability of soft layered nanostructures. In addition to preparation of MOFs and COFs on the basis of coordination bonds and covalent bonds, this study opens a new horizon of supramolecular approach for fabrication of two-dimensional materials.
4. NEW STRATEGIES FOR CONTROLLABLE SUPRAMOLECULAR POLYMERIZATION 4.1. Polymerization of a Supramonomer
Supramonomer refers to a monomer that is fabricated by noncovalent interactions, but is able to polymerize using covalent or noncovalent polymerization methods.237,238 This is similar to the macromonomer concept in traditional polymer science. In contrast to the commonly used method, the concept of a supramonomer provides a new perspective for the fabrication of supramolecular polymers with controlled structures. Lehn et al. reported the synthesis of a supramolecular polymer by formation of covalent C−C bonds between the ditopic supramolecular building blocks undergoing C−C connection through aerial oxidation.239 We have demonstrated that supramolecular polymers can be constructed through click polymerization of a supramonomer (Figure 44).237 For this purpose, we prepared a complex of tripeptide derivative 71 and CB[8] as a supramonomer and then polymerized the supramonomer through click polymerization to obtain supra-
4.2. Self-Sorting Controlled Supramolecular Polymerization
Self-sorting has been used successfully to fabricate alternating supramolecular polymers. We wondered if it could be employed to promote and control the process of supramolecular polymerization. As shown in Figure 37, a ternary system including a guest molecule that could be recognized by CB[8] and CB[7] simultaneously and orthogonally was designed. The molar mass of supramolecular polymers could be controlled by tuning the ratio of CB[7] to monomer, not only during the polymerization process but also during the
Figure 44. Supramolecular polymer synthesized by click polymerization from a supramonomer. Reprinted with permission from ref 237. Copyright 2014 Royal Society of Chemistry. 7219
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depolymerization (Figure 45). When CB[7] was gradually added to the solution of Naph−Phen−Naph−CB[8], the
Figure 46. Schematic representation of the photocontrollable supramolecular polymerizations of monomers with different configurations of the stilbene moiety. Reprinted with permission from ref 244. Copyright 2013 John Wiley & Sons, Inc.
then to neutral form, resulting in the degradation of the supramolecular polymers.179 Tian et al. reported pH and redox dual-responsive supramolecular polymers based on the same host molecule, but a guest containing viologen and dimethylamino groups, as shown in Figure 47.246 These supramolecular polymers could reversibly respond to both electrochemical redox and pH variance, achieving switching between unimers and polymers. Although a vast number of works have focused on the construction of stimulus-responsive supramolecular materials, not enough attention has been paid to the controlled supramolecular polymerization by employing external stimuli. Most of the reported works mainly focus on the switch between polymerization and depolymerization, which are still in its infancy in controlled supramolecular polymerization. However, with deeper research, including further study of the kinetics and mechanisms of supramolecular polymerization under external stimuli, it is highly anticipated that we can push these methods toward stimulus-controlled supramolecular polymerization.
Figure 45. Ratio dependence of the molar mass of supramolecular polymers in the CB[7] titration experiments. Reprinted with permission from ref 217. Copyright 2014 John Wiley & Sons, Inc.
molecular mass of the supramolecular polymers increased first when the ratio was 1. A maximum of molecular mass (Mw = 9.7 × 104 g· mol−1, PDI = 1.5) was reached while all the components were equimolar. It is expected that different self-sorting systems could be utilized to achieve supramolecular polymerization in a controlled manner, therefore enriching the field of supramolecular polymer chemistry with important advances toward the realization of molecular mass and structural control. 4.3. Stimulus-Controlled Supramolecular Polymerization
One of the specific characteristics of supramolecular polymers is their response to external stimuli, such as pH, temperature, photoirradiation, and chemical or electrochemical redox.6,240 Under a specific stimulus, supramolecular polymers can achieve controllable conformations or polymerization/depolymerization switching and function adjustment. Here we mainly highlight some examples in controlling the construction of supramolecular polymers under external stimuli. Light stimulus is attractive as it allows remote activation and control without adding or generating extra chemicals.241,242 To study the photocontrollable self-assembly, supramolecular polymers comprised of stilbene as a photoresponsive moiety were reported by Yang et al.243,244 The self-assembly behavior and physical properties of the supramolecular polymers can be regulated by photoisomerization of the stilbene. For example, Upy-based supramolecular polymers with Z or E configurations of stilbene followed ring−chain equilibrium and isodesmic growth mechanisms, respectively. As shown in Figure 46, (Z)monomer formed cyclic dimers at low concentrations and linear supramolecular polymers at high concentrations, while (E)-monomer formed linear supramolecular polymers first and further supramolecular networks driven by π−π interactions between stilbenes as the concentration increased. Redox reactions are basic and complementary chemical reactions that have been widely used in modulating the selfassembly behavior of supramolecular polymers.6,240 Redoxcontrolled constructions of supramolecular polymers consisting of a viologen guest as a redox-responsive moiety were studied by several research groups.245,246 Liu et al. constructed psulfonatocalix[4]arene- and viologen-based supramolecular polymers in aqueous solution. Upon electrochemical reduction, the viologen converted from a cation to a radical cation and
4.4. Living Supramolecular Polymerization
Despite the good understanding of the supramolecular polymerization mechanisms,3 living supramolecular polymerization still remains an important challenge. Inspired by conventional living polymerization mechanisms, kinetic control over the initiation and propagation steps should be involved in living supramolecular polymerization. To this end, the nucleation−elongation processes in cooperative supramolecular polymerization may serve as the right candidate for living supramolecular polymerization. Although it is difficult to achieve, there have been successful demonstrations in recent years. To control the shape, size, and stability of supramolecular polymers in an aqueous environment, Meijer et al. proposed a strategy by tuning the balance between attractive forces (e.g., dipole interactions, π−π stacking, and solvophobic effects) and repulsive electrostatic interactions of a discotic amphiphile monomer (Figure 48).247 It was demonstrated that the polymerization mechanism could be switched from a cooperative nucleation−elongation mechanism to an isodesmic mechanism by increasing the peripheral Coulombic repulsions of the metal complexes, resulting in significantly different morphologies of the assemblies: from elongated rodlike columnar assemblies 73 to discrete spherical objects 74. More interestingly, cooperative assembly of 74 can be restored by increasing the ionic strength of the buffered environment, e.g., 0.5 M NaCl solution, leading to the formation of high aspect ratio rodlike supramolecular polymers. This approach, by 7220
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Figure 47. Reversible polymerization/depolymerization of supramolecular polymers induced by the stimuli of pH and electrochemical redox. Reprinted from ref 240. Copyright 2014 American Chemical Society.
Figure 48. Schematic representation of the fluorinated discotic amphiphiles 73 [M(III) = Gd(III)] and 74 [M(III) =Y(III)] and the self-assembly of discotic amphiphiles. The hydrophobic core (solid circle) directs the self-assembly into a helical architecture. The hydrophobic amino acid substituents (dashed line) in the second layer determine the stability of the helix. The peripheral hydrophilic metal complex groups induce a repulsive Coulombic force to the helix. Reprinted with permission from ref 247. Copyright 2010 National Academy of Sciences.
subtly tuning the balance between positive and repulsive interactions between building blocks, paves the way for controlled supramolecular polymerization. An elegantly designed system for living supramolecular polymerization based on the cooperative mechanism was reported by Sugiyasu and Takeuchi and co-workers.248 They have designed a porphyrin molecule bearing hydrogen-bonding moieties and long alkyl chains, which self-assembled through π−π stacking of the porphyrin planes and hydrogen bonding of
the amide groups (Figure 49). Two different types of aggregates for this porphyrin molecule were observed: one was kinetically trapped J-aggregates assembled through an isodesmic pathway, and the other was thermodynamicaly stable H-aggregates assembled through a nucleation−elongation pathway. The presence of a second off-pathway (J-aggregation into small clusters) aggregate led to the accumulation of a large amount of “dormant” J-aggregates. However, while a tiny amount of short H-aggregate seed was added to the solution of 7221
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qualitative information. Considering that we have reviewed the characterization methods for supramolecular polymers recently,4 we will make a brief introduction of the methods that we have discussed before, but focus on some new methods that were not covered well by our last review. 5.1. Size Exclusion Chromatography
Size exclusion chromatography (SEC), especially gel permeation chromatography (GPC), is a very mature and widely used characterization method for conventional polymers. However, a serious degradation effect induced by shearing forces in the column packing, accompanied by the nonspecific adsorption and dilution effect, makes SEC unsuitable for most of the supramolecular polymers. However, in some cases, especially those involving metal-containing or multiple-hydrogen-bonding arrays with a high binding constant, the application of SEC has achieved modest success.86,249 As was mentioned in section 2.2.1, Schubert et al. investigated the stability of MSPs with different metal ions when characterized by SEC.86 They found no relationship between the concentration and the chromatographic results for Ni2+−TPY complexes (log K = 21.8), but a degradation of polymer chains for Fe2+−TPY and Co2+−TPY systems, owing to their relatively low binding constants (log K(Fe) = 20.9, log K(Co) = 18.3). This study clearly demonstrated the degradation effect of SEC for supramolecular polymers. 5.2. Viscometry Figure 49. Schematic representation of porphyrin-containing molecule 75 and energetic landscape representing the pathway complexity in supramolecular polymerization of 75, illustrated on the basis of the thermodynamic parameters determined by van’t Hoff plots. Reprinted with permission from ref 248. Copyright 2014 Nature Publishing Group.
Viscometry is a classical method to characterize conventional polymers. The intrinsic viscosity increases with increasing molecular weight, and their relationship can be expressed by the empirical Mark−Houwink equation: [η] = KMα. In this equation, for a given polymer, both K and α are empirical constants which can be obtained from calibration experiments with a series of standard polymer samples with various molecular weights.250 However, it is difficult to find a suitable covalent model to obtain these parameters for the dynamic systems. Therefore, in most cases, this method is only used to study supramolecular polymerization in a qualitative way, for example, to deduce the critical polymerization concentration.37,162,166,251
J-aggregates as an initiator, these dormant J-aggregates transformed into fibrous H-aggregates with a narrow polydispersity (PDI = 1.1). AFM images demonstrated the living character of fiber growth with each addition batch to afford fibers of homogeneous length and width. This “nucleated polymerization with a competing off-pathway aggregate” is a potent methodology for realizing living supramolecular polymerization.
5.3. NMR Spectroscopy
NMR can be used to determine the Mn of a supramolecular polymer by end-group analysis based on properly simplified assumptions. For example, assume that complete complexation will cause a chemical shift, Δδ, and ΔδC at a certain monomer concentration. Defining p = Δδ/ΔδC, the DP can be easily calculated to be 1/(1 − p).165 DOSY is becoming a popular method for characterization of supramolecular polymers.174,197,252 The diffusion coefficient is inversely proportional to the size of supramolecular polymers. Furthermore, the size of supramolecular polymers can be qualitatively estimated according to the Stokes−Einstein equation. This relationship between the diffusion coefficient and the size of supramolecular polymers holds true for many systems. The shortcoming for this method is that the average molar mass of supramolecular polymers can only be roughly estimated, since the model for the Stokes−Einstein equation is not always true for different supramolecular polymer systems.
5. CHARACTERIZATION OF SUPRAMOLECULAR POLYMERS Supramolecular polymerization is essentially a self-assembly process with a certain thermodynamic equilibrium constant. Therefore, it is possible to estimate the average molar mass of supramolecular polymers through theoretical models. The DP can be simply estimated as DP ≈ (KaC)1/2, where Ka is the equilibrium constant and C is the total monomer concentration, based on the isodesmic model.2 However, for supramolecular polymerization systems that do not follow the isodesmic mechanism, in other words, the equilibrium constant Ka varies with the chain growth of a supramolecular polymer, it is difficult to express them by general fitting equations. From another point of view, it still remains a challenge to directly characterize the average molar mass of supramolecular polymers. Due to the dynamic nature of supramolecular polymers, many well-established characterization methods for conventional polymers may not work well. In most cases, only partial information can be obtained from a certain method. Therefore, it is necessary to combine various characterization methods with a proper theoretical estimation to obtain
5.4. AFM-Based Single-Molecule Force Spectroscopy
AFM-based SMFS, which can be employed to measure minute forces and to record exceptionally small distances simultaneously, has been a powerful tool in the study of single polymer chains.150,152,253−257 In a general SMFS experiment, a polymer 7222
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Figure 50. Supramolecular polymers formed through the host-enhanced charge-transfer interaction and their characterization by SMFS. Reprinted with permission from ref 150. Copyright 2010 John Wiley & Sons, Inc.
polymer chain can show force signals and the point at which the polymer chain picked up is out of control, it is still difficult to obtain precise information about the length of supramolecular polymers.
chain could be adsorbed onto the AFM tip as well as onto the substrate, thus forming the so-called polymer bridge between the AFM tip and the substrate. When the AFM tip is separated from the substrate, the polymer chain will be stretched and deflect the cantilever. The deflection of the cantilever and the displacement of the piezo will be recorded and converted to force−extension curves. In our previous work, we have developed a series of CB[8]containing supramolecular polymers.145 SMFS is an important tool to characterize these supramolecular polymers. For example, we designed a DADV monomer in which anthracene and viologen moieties were the electron donor and acceptor, respectively, as shown in Figure 50. With the electron donor and the electron acceptor encapsulated in CB[8], the monomers can be driven by the host-enhanced charge-transfer interaction to join together in a head-to-tail fashion to form the supramolecular polymers. Supramolecular polymers were physically adsorbed onto the substrate and then picked up by the AFM tip through a physical absorption force. In this system, two types of force−extension curves were obtained. In one type of force−extension curve, the force rose monotonically with increasing extension and then dropped to zero rapidly upon the rupture of the polymer bridge. Fitting the force−extension curves by the modified freely jointed chain (M-FJC) model gave a Kuhn length of about 2.2 nm, indicating that the supramolecular polymers are quite rigid. In the other type of force−extension curve, a long plateau was observed, corresponding to the detachment of the supramolecular polymer chain that adsorbs onto the substrate with a trainlike conformation. The lengths of the plateaus in all the force− extension curves were statistically analyzed, and the Gaussian fitting gives the most probable length as 60 nm. As shown in the examples above, SMFS can give direct evidence of the formation of a chainlike supramolecular polymeric structure. Moreover, the elasticity of supramolecular polymer chains can also be obtained from the force−extension curves. However, since only a sufficiently long supramolecular
5.5. Light Scattering
Light scattering, including static light scattering (SLS) and dynamic light scattering (DLS), is a well-established method in determining the molecular size and morphology of analytes. For SLS, information on the weight-average molar mass (Mw), radius of gyration (Rg), and second virial coefficient (A2) can be gained. In most cases, DLS is used to obtain the size distribution information from aggregates formed by supramolecular polymers. As the intensity of scattered light scales with the radius of the particle, r6, smaller analytes are mostly discriminated in batch light scattering, if larger aggregates are present in solution. This often results in a misleading size distribution and prohibits quantification of analytes. 5.6. Small-Angle Neutron Scattering
Small-angle neutron scattering (SANS) is a powerful tool and has been widely used in the study of materials with nanoscale, including polymers, colloids, and biological systems. In the regime where wave numbers are small compared with wideangle Bragg diffraction but relatively large on the scale of SANS, correlations between opposite interface segments are random and are averaged out. For smooth interfaces, the scattering intensity can be predicted by Porod’s law: I=
CP + BGD Q4
In this equation, I is the normalized scattering intensity, BGD is the background intensity of incoherent scattering, CP is the Porod constant, directly proportional to the surface area, and Q is the magnitude of the scattering vector [Q = (4π/λ) sin(θ/2), where λ is the neutron wavelength and θ is the angle of scatter]. Analysis of the SANS data can give information about the size 7223
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and shape of the polymers. For example, the value of Rg as well as the Porod exponent can be obtained. In the case of polymers with a good solvent, the Porod exponent is about 1.6. For a Θ solvent, the value is 2. For a poor solvent, the value is between 2 and 3, indicating a chain collapse of polymers. Besides conventional polymers, the method of SANS can be used in the characterization of supramolecular polymers.151,258 For example, in cooperation with Qiu, we used this technique to characterize the CB[8]-containing supramolecular polymers. The result confirmed the formation of supramolecular polymers with long linear chains (Table 5). Since water is not a good Table 5. SANS Data for a CB[8]-Containing Supramolecular Polymer concn/mM
Porod exponent
Rg/nm
0.1 0.2 0.3
2.5 2.6 2.8
163.0 167.2 112.4
solvent for these supramolecular polymers, the Porod exponent of the supramolecular polymers is around 2.5 in aqueous solution, indicating that the supramolecular polymers are collapsed coils in water.
Figure 51. (a) Comparison of the molecular mass/size detection range for different techniques including AF4. (b) Schematic representation of the principles of AF4.
We first used AF4 to characterize supramolecular polymers in a supramolecular polymerization system promoted and controlled through self-sorting.217 As shown in Figure 52, the
5.7. Asymmetric Flow Field-Flow Fractionation
Asymmetric flow field-flow fractionation (AF4), as one of the representatives of the field-flow fractionation technique, was introduced by Giddings and Wahlund in 1987.259 AF4 connecting in-line to ultraviolet, differential refraction, and multiangle light scattering (MALS) detectors provides information on the molar mass, polydispersity, size, and shape/conformation of the samples. Due to the lack of a stationary phase, it represents a gentle separation and characterization method, where nonspecific interactions and shear forces are reduced to a minimum, allowing for a broad separation range from nano- up to micrometers (Figure 51a). These advantages make AF4 a powerful and versatile technique in characterization of macromolecules and supramolecular assemblies.260−262 The separation in AF4 is achieved solely by a flow in an empty channel where a perpendicular flow force is applied (Figure 51b). The channel consists of two plates joined together, which are separated by a spacer. The bottom plate is porous, covered by a semipermeable membrane, which acts as an accumulation wall. The membrane with a typical cutoff of 5 or 10 kDa is permeable for the eluent, but impermeable for the polymer molecules and colloidal particles, and therefore keeps the sample in the channel so that it is directed by flow to the channel outlet. The eluent flow creates a parabolic flow profile within the channel, and the eluent moves more slowly closer to the channel walls compared to the channel center. During the transport of the sample by the eluent, a cross-flow is applied to drag the molecules/particles to the surface of the membrane. Meanwhile, the diffusivity acts as a counteracting force, until a steady-state distribution of the sample in the channel is reached. Smaller molecules/particles with higher diffusion coefficients move closer to the channel center, where the eluent flow is faster. The velocity gradient inside the channel separates the analytes according to their size in such a way that smaller molecules elute before the larger ones. This means that AF4 separation is the opposite of SEC separation, in which the large molecules elute first.
Figure 52. Ratio-dependent AsF-FFF elution curves of (a) Naph− Phen−Naph−0.5CB[7]−CB[8], (b) Naph−Phen−Naph−0.6CB[7]− CB[8], (c) Naph−Phen−Naph−0.8CB[7]−CB[8], and (d) Naph− Phen−Naph−1.0CB[7]−CB[8] obtained by the MALS detector. Reprinted with permission from ref 217. Copyright 2014 John Wiley & Sons, Inc.
intensity of the MALS signal increased and the peak shifted to the right when the molar ratio of CB[7] and Naph−Phen− Naph−CB[8] increased from 0.5 to 1.0, indicating higher molecular mass supramolecular polymers. The highest molecular mass of the supramolecular polymers was calculated to be 97 kDa with a polydispersity of 1.5. According to these results, AF4 coupled to different detectors can provide information on the molecular mass and polydispersity for supramolecular polymers. In contrast to individual light scattering, the prior fractionation by AF4 allows the investigation of complex heterogeneous and polydisperse mixtures. Although the universality for different supramolecular polymer systems needs to be explored further, this technique 7224
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Figure 53. Absorbance changes of an MSP 76 film coated on an ITO electrode in an argon-saturated acetonitrile solution of n-Bu4NClO4 (0.1 M) by stepping the potential (vs Ag/AgCl) from 0.0 to +1.6 V and back to 0.0 V. Reprinted with permission from ref 87. Copyright 2007 John Wiley & Sons, Inc.
6.1. Optoelectronic Materials
shows great potential in resolving the characterization of supramolecular polymers.
Electrochromic materials change their optical properties by application of an external electric potential. Electrochromic materials have a wide range of applications such as electronic papers and smart windows.267 Metal oxides such as WO3 and polymers such as Prussian blue, poly[3,4-(ethylenedioxy)thiophene] (PEDOT), or polyaniline (PANI) are among the known materials for electrochromic applications. However, the general drawbacks of these materials are their prohibitive costs, small optical contrast, and low switching speeds. Metallosupramolecular polymers may be a promising alternative electrochromic material which exhibits outstanding properties in performance, variability, and fabrication efficiency.63,268 Kurth et al. prepared a series of TPY-based MSPs with a functional group at the pyridine periphery close to the metal ion and studied the structure−property relationships of MSPs as electrochromic materials.87,269 The electrochromic nature of the MSPs is revealed by monitoring the absorbance as a function of the applied potential. As a representative example, Figure 53 depicts the spectral changes of MSP 76.87 Upon increasing the potential from 0.0 to +1.6 V, the absorbance of the polymer at ca. 585 nm decreases, and finally, the ITO slide is completely colorless at +1.6 V. Subsequently stepping back the potential leads to the re-emergence and, finally, full recovery of the absorbance. The substitution groups can influence the electrochromic properties of MSPs through steric or electronic effects. Electron-donating groups give rise to faster switching rates, while electron-withdrawing ones lower the switching rates and decrease the stability. Subsequently, they fabricated a TPY−Fe2+-based MSP film by a dip-coating process, which shows outstanding electrochromic properties.268 The MSP film can be reversibly switched from Fe2+ (blue) to Fe3+ (colorless) by applying a potential of 4.1 V vs Li/Li+. A very high optical contrast ΔT of 71% at a wavelength of 590 nm and a coloration efficiency of around 525 cm2 C−1 can be realized. The devices show a long-term stability of about 10 000 cycles, indicating that the TPY−Fe2+-based MSP is a very promising electrochromic material for future applications of smart windows. To use electrochromic materials in display devices, it is necessary to tune the color of the materials. The colors of MSPs can be easily adjusted by changing the metal species and/ or by modifying the organic ligands. For example, TPY−Fe2+based MSPs with pyridine groups bearing electron-releasing methoxy groups are blue powders, while those with pyridine groups bearing electron-withdrawing bromo groups are green.87 Normally, the color of Fe-based MSPs can be purple, blue, or
5.8. Other Methods
Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) is an important method to study the accurate molecular mass of polymers. However, it is difficult to study the molecular mass of supramolecular polymers because of their poor stability in the experimental condition. Since only short species or oligomers can be detected in most cases, MS is not a common method to characterize supramolecular polymers so far. Vapor pressure osmometry (VPO) is an old method to determine the molecular mass of polymers. The Mn , concentration, and vapor pressure can be related through Raoult’s law. Although it has been used in supramolecular polymer systems, VPO is not an appropriate method for polymers with molar mass higher than 30 kDa, and is hardly suited for characterizing charged metallopolymers. Analytical ultracentrifugation (AUC) is a useful technique commonly used for the characterization of proteins and protein complexes, as well as synthetic polymers, including polyelectrolytes, metallosupramolecular polymers, and so on.263−265 This method allows the direct determination of the average molar mass and polydispersity of polymers. For example, the AUC method was used to characterize block copolymers connected by metal coordination interactions, such as PS− [Ru]2+−PEO.266 Compared with other methods, samples are characterized in their native state under solution conditions. Therefore, no interactions with matrixes or surfaces and no dilution effect exists during analysis, which is particularly beneficial for supramolecular systems. Although it is still little used for supramolecular polymers, this technique is highly anticipated to be one of the options.
6. APPLICATION OF SUPRAMOLECULAR POLYMERS Polymers based on covalent bonds have been, are, and will be major materials. As a supplement to covalently attached polymers, the nature of noncovalent interactions gives the supramolecular polymers the ability to be dynamic and reversible. Therefore, these kinds of supramolecular polymers can be used as recyclable, degradable, stimulus-responsive, and self-healing materials. In addition, various moieties can be assembled during the process of supramolecular polymerization, leading to engineering or integration of functional materials. 7225
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green, that of Co-based MSPs can be yellow or orange, and that of Ru-based polymers is red (Figure 54).269 In addition, more
Figure 55. Ligands with diverse rigid linear π-conjugated spacers.
the substitution effect was the Fe2+ MSPs with bpy ligands substituted by triethylene glycol (TEG) chains. The Fe2+ ion is known to quench fluorescence, but the TEG-substituted Fe2+(TPY)2 MSP shows a retention of quantum yield that is nearly 3-fold higher than that of the unsubstituted analogue.91 The electro-optical properties of a series of Zn2+(TPY)2-based MSPs with diverse rigid linear π-conjugated moieties were also investigated (Figure 56).84,271 Tuning of the photophysical properties was enabled; thus, bright purple to green photoluminescent emission (quantum yields of 0.12−0.81) was observed, with the emission color strongly depending on the nature of the conjugated groups.84 Besides MSPs, other kinds of supramolecular polymers (e.g., host−guest systems) bearing chromophore230,272 or amine-rich273 moieties are also found to be potential photoluminescent materials. White organic light-emitting materials are of great interest both in the scientific and in the industrial communities. Usually, the white light results from a simultaneous emission of red, green, and blue chromophores or of two complementary colors (e.g., orange and blue). When polymeric materials are applied, two approaches are usually adopted: one is mixing of different polymers, and the other is synthesis of a single polymer bearing different chromophores. However, the first approach often suffers a phase separation, and the second one requires additional synthetic work for every single mixing ratio of the different chromophores in the materials. The use of a dynamic supramolecular polymer could combine the advantages of both approaches and diminish the disadvantages, because mixing of supramolecular homopolymers in a solvent will result in statistical copolymers. In an elegant study from Schubert,274 a series of Zn2+(bpy)2 MSPs with diverse central chromophores (Figure 57), which emitted blue, green, and red light, were deeply studied (Figure 58). The dynamic nature of the MSPs enables the systematic assembly of a library of statistical copolymers by simply mixing the respective homopolymer solutions. Depending on the ratios used and the consequent energy-transfer processes, the resulting emission colors can be tailored, for example, white light. These MSPs can be further utilized to assemble thin films of statistical copolymers in a simple and material-saving manner by inkjet printing.
Figure 54. Colors of MSPs with different TPY derivatives and metal ions. Reprinted from ref 269. Copyright 2008 American Chemical Society.
than two metal ions can be introduced into the polymers, such as Fe2+ and Ru2+, and the color of the MSPs can be tuned from purple to orange to light green at different potentials. Such a multicolor electrochromic change is the result of different redox potentials of iron and ruthenium ions.270 Photoluminescent polymers have received significant interest especially in the fields of organic light-emitting diodes (OLEDs) and plastic solar cells. Introduction of metal ions into conjugated polymers has been investigated with regard to higher electroluminescence quantum yields. However, the purification and processing of such polymers is a difficult task. Supramolecular polymers containing photoactive building blocks may be an option to overcome these difficulties. Würthner et al. synthesized perylene bisimide dyes bearing TPY ligands and assembled MSPs with Zn2+ and Fe2+ ions.83 It was found that the fluorescence quantum yield of the Zn2+based MSP was 0.61 in DMF, while that of the Fe2+-based MSP was quenched to 0.04 in CHCl3. This result shows that the luminescent properties of MSPs can be tuned by changing the metal ion species. The luminescent properties of the bis-TPY are profoundly affected by the nature of the functional groups at the peripheral pyridine and the spacers (Figure 55).84,88 In general, attachment of an electron-donating OMe group to the pyridine ring or extending appropriately the conjugation length of the spacers gives rise to an increased quantum yield, while electron-withdrawing groups, such as Br and CN, generally lead to a decreased emission efficiency.88 The λem can be tuned to span the region from ca. 370 nm to more than 500 nm, leading to different emission colors such as purple, bright blue, and bright green. Thus, these building blocks are promising candidates for the construction of metallosupramolecular light-emitting polymers. An interesting example of elucidating
6.2. Supramolecular Self-Healing Materials
The design of self-healing polymers also demands multiple healing cycles, thus requiring other strategies rather than just relying on covalent bondings. Self-healing materials based on supramolecular interactions stand a good chance of meeting such a requirement, therefore putting them on the frontline of 7226
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Figure 56. Schematic representation of the MSPs with diverse conjugated spacers. Reprinted with permission from ref 84. Copyright 2009 John Wiley & Sons, Inc.
Figure 57. Schematic representation of the synthesis of the Zn2+(bpy)2 MSPs with diverse chromophores. Reprinted with permission from ref 274. Copyright 2013 Royal Society of Chemistry.
at this temperature. Cooling to room temperature resulted in a film with the original properties.276 It is not necessary to use very strong hydrogen-bonding motifs, and even relatively weak hydrogen interactions are reasonably applicable in forming supramolecular networks. Leibler et al. prepared a supramolecular network by mixing ditopic and multitopic molecules, bearing (amidoethyl)imidazolidone 95, bis(amidoethyl)urea 96, and diamidotetraethyltriurea 97 side moieties which acted as multiple-hydrogenbonding cross-linkers (Figure 59).277 This rubber-like system shows recoverable extensibility up to several hundred percent and little creep under load. When broken or cut, it can be simply repaired by bringing together fractured surfaces to selfheal at room temperature. The process can be repeated many times with the extensibility well recuperated. Another example of a self-healing supramolecular network formed through weak multiple hydrogen bonding was reported by Binder.278 Bivalent poly(isobutylene)s (PIBs) 98 with
applications. The concept of supramolecular self-healing materials relies on the use of noncovalent bonds to generate reversibility and dynamic networks, which are able to heal the damaged sites.275 We therefore discuss supramolecular network formation based on hydrogen bonding, metal-coordination bonding, and π−π stacking in relation to a self-healing behavior. 6.2.1. Self-Healing Materials Based on Multiple Hydrogen Bonding. One of the most impressive examples of self-healing materials is commercially available under the brand name SupraB (by Suprapolix), based on Meijer’s quadruple hydrogen bond. Due to the thermoreversible hydrogen bonding of UPy groups, the material combines strong elastic behavior at room temperature (hydrogen bonds closed) and low viscous melt behavior at elevated temperatures (hydrogen bonds open).42 When the polymeric film was damaged by scratching, it spontaneously self-healed when heated to 140 °C, benefitting from the rather low melt viscosity 7227
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the battery. To overcome this challenge, the authors fabricated a self-healing electrode by coating silicon microparticles with a thin layer of a hydrogen-bond-directed self-healing supramolecular polymer network (Figure 61a,b). This hydrogenbonding network was modified from the classical synthetic route demonstrated by Leibler et al.277 The designed amorphous structure and low glass-transition temperature (Tg) allow the polymer chains at the fractured interfaces to rearrange, approach, and intermix, driven by the dynamic reassociation of hydrogen bonds at room temperature. These self-healing anodes have an excellent cycle life, which is 10 times longer than that of state-of-the-art anodes made from silicon microparticles (SiMPs) and still retain a high capacity (up to ∼3000 mA·h·g−1, Figure 61c). The similar supramolecular polymer networks composited with nickel microparticles have been used to fabricate selfhealing electronic skins, which show remarkable mechanical and electrical self-healing properties at ambient conditions (Figure 62).280 On rupture, the initial conductivity is repeatedly restored with ∼90% efficiency after a 15 s healing time, and the mechanical properties are completely restored after ∼10 min. The composite resistance varies inversely with the applied flexion and tactile forces and therefore acts as an excellent electronic sensor. These results demonstrate that the repeatable self-healing capability of natural skin can be mimicked in conductive and piezoresistive materials, thus potentially expanding the scope of applications of current electronic skin systems. 6.2.2. Self-Healing Materials Based on Metal-Coordination Bonding. Metallosupramolecular polymers 99 comprising a rubbery, amorphous poly(ethylene-co-butylene) core with 2,6-bis(19-methylbenzimidazolyl)pyridine (Mebip) ligands at the termini coordinated with Zn(II) or La(III) metal ions can be mended through exposure to light (Figure 63).107 On exposure to ultraviolet light, the metal−ligand motifs are electronically excited and the absorbed energy is converted into heat. This causes temporary disengagement of the metal−ligand motifs and a concomitant reversible decrease in the polymers’ molecular mass and viscosity, thereby allowing quick and efficient defect healing. The concept of photothermally induced healing of supramolecular materials seems to
Figure 58. (a) Mixing triangle of MSPs P2, P3, and P11. (b) Picture of the corresponding solutions in a quartz microtiter plate (λex = 365 nm). (c) Position of the photoluminescence (PL) obtained from wells in the Commission Internationale de l’Elcairage (CIE) color space. Reprinted with permission from ref 274. Copyright 2013 Royal Society of Chemistry.
different molecular masses were functionalized with barbituric acid. Rheology experiments in the melt state (20 °C, Figure 60a) showed a rubbery plateau at high frequencies and a terminal flow region in the low-frequency range, indicating the formation of dynamic supramolecular tie points between barbituric acid groups (Figure 60b). Small disks prepared from PIBs show self-healing within several hours at ambient temperatures after the two parts are brought into contact again (Figure 60c). The self-healing feature is particularly desirable for energy storage because the lifetimes of many rechargeable batteries are limited by the mechanical fractures over the cycling process. An outstanding application of self-healing supramolecular polymers in improving the cycle life of rechargeable lithium batteries was reported by Bao, Cui, and co-workers.279 A silicon anode, which has a theoretical specific capacity 10 times higher than that of conventional graphite anodes, suffers from extreme volume expansion by up to three charge−discharge processes. These volumetric changes can cause cracking and pulverization in the electrode, which irreversibly degrades the performance of
Figure 59. A mixture of fatty acid and triacid was condensed first with diethylenetriamine and then reacted with urea, giving a mixture of oligomers equipped with complementary hydrogen-bonding groups. The hydrogen bond acceptors are shown in red, donors in green. Reprinted with permission from ref 277. Copyright 2008 Nature Publishing Group. 7228
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Figure 60. (a) Frequency sweep measurement of a PIB bearing two barbituric acid groups. (b) Formation of a dynamic, reversible supramolecular network of a barbituric acid-functionalized PIB. (c) Self-healing experiment of a barbituric acid-functionalized PIB with Mn = 28 kDa. Reprinted with permission from ref 275. Copyright 2013 John Wiley & Sons, Inc.
Figure 62. Self-healing conductor based on a composite of self-healing polymer and metal particles. (a) Illustration of the self-healing composite. (b) Healed film being flexed to show its mechanical strength and flexibility after only 5 min of healing at room temperature. Reprinted with permission from ref 280. Copyright 2012 Nature Publishing Group.
be applicable to various supramolecular polymers with a binding motif that is sufficiently dynamic. Coordination between iron and catechol ligands has been correlated to the hardness and high extensibility of the cuticle of mussel byssal threads and proposed to endow self-healing properties. Inspired by this biological material, a simple method to control catechol−Fe3+ interpolymer cross-linking via pH was developed for preparing self-healing polymer networks with near-covalent elastic moduli (Figure 64).281,282 The uniqueness of this system is the ability of mono-, bis-, or triscatechol−Fe formation at different pHs, thus providing the ability of crosslinking control without Fe3+ precipitation. The ultrahigh binding constant of tris- and biscatechol−Fe3+ complexes (K ≈ 1037−1040) endows the strength of metal−ligand bonds as high as covalent bonds; moreover, metal−ligand bonds can
Figure 61. (a) Schematic illustration of the design and behavior of selfhealing electrodes. (b) Chemical structure of the self-healing supramolecular polymer. (c) Capacity retention of SiMP electrodes with different polymer additives, including the SHP (self-healing polymer)/CB composite and conventional polymer binders. The SiMP electrode with SHP showed a much longer cycling lifetime than that of the conventional polymer. Reprinted with permission from ref 279. Copyright 2013 Nature Publishing Group.
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Figure 63. Mechanism and synthesis of photohealable metallosupramolecular polymers. Reprinted with permission from ref 107. Copyright 2011 Nature Publishing Group.
Figure 64. pH-dependent stoichiometry of Fe3+−catechol complexes. Reprinted with permission from ref 281. Copyright 2011 National Academy of Sciences.
Figure 65. (a) Polyimide copolymer containing electron-deficient naphthalene diimide units and pyrenyl-end-capped polyamide bearing electronrich pyrene groups. (b) Folding of the two polymers due to π−π stacking. Reprinted with permission from ref 284. Copyright 2009 Royal Society of Chemistry.
The same strategy was extended by combining π−π interactions between a polyimide copolymer bearing naphthalene diimide units and a pyrenyl-end-capped polyamide, with hydrogen bonding between urea groups near the pyrene groups. Upon mixing these two polymers, the resulting material again reveals triggered self-healing at elevated temperature with high healing efficiencies.
spontaneously re-form after breaking, thus healing the damage of the material. 6.2.3. Self-Healing Materials Based on π−π Interactions. Self-healing supramolecular polymers based on π−π interactions were first reported by Burattini et al.283−286 As shown in Figure 65, a supramolecular network was achieved by employing chain-folding polyimide (electron acceptor 100) and pyrenyl (electron donor 101) end-capped chains with polyamide as a spacer.284 The Tg of the network can be tuned to achieve self-healing at a relatively wide temperature range (∼50−100 °C). A film with deep red color was obtained by solution casting, which could be rehealed by simply pressing the broken parts together and heating to 80 °C. Upon heating, the stacking interactions were interrupted, enabling destruction of the supramolecular network structure and thus healing of the breakage. A 100% of recovery of the tensile modulus was achieved, and the healing cycle could be repeated several times.
6.3. Biomedical Applications
Supramolecular polymers can be endowed with unique properties, such as biodegradation, responsiveness to various biological stimuli, and easy incorporation of bioactive components, thereby showing great potential for application in the biomedical field. Although a vast number of supramolecular polymeric assemblies have been studied for biomedical applications,287 we just select several examples of typical supramolecular polymers and illustrate their potential 7230
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Figure 66. (a) Modular approach for preparing bioactive supramolecular materials. (b) Fibroblast cell adhesion and spreading in vitro on different drop-cast films consisting of mixtures of PCLdiUPy with both UPy-GRGDS and UPy-PHSRN (left panel) or PCLdiUPy alone (right panel) after 48 h for cell culturing without fetal bovine serum (FBS). (c) In vivo behavior of bioactive supramolecular polymeric materials. Solution-cast supramolecular PCLdiUPy films with UPy-GRGDS and UPy-PHSRN peptides (left panel) and without peptides (right panel) were subcutaneously implanted in mice. The magnification of the enlargement is 400×. Reprinted with permission from ref 288. Copyright 2005 Nature Publishing Group.
Figure 67. Structure of multiple-hydrogen-bonded supramolecular polymers and the corresponding hydrogel designed for protein delivery. Reprinted with permission from ref 291. Copyright 2012 John Wiley & Sons, Inc.
fibroblase cells to the supramolecular polymeric materials. An even more striking effect is seen in vivo where the formation of single giant cells at the interface between bioactive material and tissue is triggered. Protein delivery has been regarded as a safe and efficient method for curing diseases. Supramolecular hydrogels are regarded as emerging candidates for encapsulation and release of proteins. Meijer and co-workers reported a series of transient supramolecular networks based on multiple-hydrogen-bonded supramolecular polymers 102 (Figure 67), which display a
applications in the fields of tissue engineering, protein delivery, drug delivery, gene transfection, and bioimaging. Hydrogen-bonded supramolecular polymers have shown their advantages and potential in tissue engineering.288−290 These supramolecular polymers are suitable for bioactive materials owing to their processability, biodegradation, and biocompatibility. Meijer and co-workers fabricated UPy-based supramolecular polymers by simply mixing UPy-functionalized polymers with UPy-modified bioactive molecules (Figure 66).288 The in vitro results showed strong, specific binding of 7231
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Figure 68. (a) Chain-extended UPy-modified hydrogelators 103 and 104 and end-functionalized UPy-hydrogelator 105. (b) Macrophage infiltration in the cortex and capsule under the implantation site. Tissue slices were stained for the macrophage marker ED1 15 days after implantation. Reprinted with permission from ref 292. Copyright 2012 Elsevier Inc.
example of such a kind of supramolecular polycations as a nonviral vector for gene delivery.293 This class of cationic supramolecular polymers was constructed by the host−guest interaction between a ferrocene dimer and a β-CD dimer and had the ability to effectively condense DNA and rapidly release DNA triggered by H2O2 (Figure 69). A moderate gene transfection efficacy (∼106 RLU mg−1 of protein) in COS-7 cells was achieved. However, a much lower transfection efficacy
nonlinear behavior in their formation as well as in their selfhealing and erosion properties, demonstrating an ideal material for protein delivery in regenerative medical applications.291 As proof-of-concept, a bioactive, bone morphogenetic protein was incorporated as cargo into these transient supramolecular networks and then was implanted under the kidney capsule of mice. The supramolecular polymer could not be macroscopically detected by eye in the explanted kidneys after 7 days, indicating that they have eroded and that the proteins are delivered. It is expected that this study provides a modular approach in the field of regenerative medicine. Besides protein delivery, a similar supramolecular polymer system has also been utilized for in vivo intrarenal drug delivery.292 Dankers et al. synthesized two different classes of supramolecular hydrogelators through linking the UPy units to PEG chains by two methods: chain-extended hydrogelators containing UPy moieties in the backbone (103 and 104) and bifunctional hydrogelators end-functionalized with UPy units (105) (Figure 68). Molecules 103 and 104 formed strong, shape-persistent, and slow-eroding hydrogels, which were suitable for long-term intrarenal delivery of organic drugs. In comparison, molecule 105 formed a weaker, elastic, and fasteroding hydrogel, which was suitable for short-term, fast delivery of drugs to the kidney cortex. The favorable biological behavior of these supramolecular hydrogels makes them exquisite candidates for subcapsular drug delivery and opens up a new way for intrarenal therapy. The biomedical applications of linear supramolecular polymers constructed from small-molecule building blocks have rarely been studied, owing to the fact that they are highly susceptible to depolymerization under exposure to external stimuli. As a preliminary attempt, Zhu et al. reported an
Figure 69. Schematic representation of a cationic supramolecular polymer constructed via host−guest interactions between a ferrocene dimer and a β-CD dimer as well as their H2O2-induced pDNA release behavior. Reprinted with permission from ref 293. Copyright 2013 Royal Society Chemistry. 7232
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(∼103 RLU mg−1 of protein) in cancer cells was obtained, because the excess of H2O2 in cancer cells triggered the faster and earlier DNA release. Therefore, cationic supramolecular polymers with a higher binding constant (Ka ≥ 106 M−1), higher molecular mass (DP ≥ 40, Mn ≥ 200 kDa), and higher surface charge density are suggested to improve the transfection efficiency. Supramolecular polymers also have shown their potential application in bioimaging and diagnosis. Liu et al. reported a supramolecular polymer based on host−guest interactions between a β-CD dimer, 106, and a Mn3+−porphyrin derivative, 107, which features noncytotoxic and efficient magnetic resonance imaging (MRI) enhancement properties (Figure 70).294 In in vitro MR imaging characterization, a considerably
7. CONCLUSIONS AND OUTLOOK In this review, we have summarized various driving forces for supramolecular polymerization, outlined the preparation of supramolecular polymers with different topologies, discussed the latest advances for controlled supramolecular polymerization, gave a brief overview of the characterization methods of supramolecular polymers, and highlighted the potential applications of supramolecular polymers. With great advances in the past two decades, it has been witnessed that supramolecular polymers have grown into an important and interdisciplinary field that has drawn increasing interest from academic and industrial communities. Although various noncovalent interactions have been incorporated into fabricating supramolecular polymers, it is still necessary to develop new driving forces with a high binding constant to expand the library of supramolecular polymers. Compared with the vast number of linear main chain supramolecular polymers, other topologies of supramolecular polymers such as side chain, branched, and hyperbranched copolymers295 etc. should be paid more attention. To construct supramolecular polymers with a high degree of polymerization, several principle rules should be followed. A high binding constant is the primary request for achieving supramolecular polymers with a high degree of polymerization. For systems with relatively low binding constants, a high concentration should be a good strategy to inhibit oligomerization and cyclization, thus increasing the degree of polymerization. In addition to these strategies, a careful and rational design of monomers, especially the linkers of ditopic monomers, is another key point. A fundamental principle is to design linear monomers with rigid or bulky linkers, which can effectively hinder the cyclization of oligomers. For AA/BB-type monomers, a mismatch of length between two linkers for AA and BB monomers also works well in preventing dimerization. For host molecules with poor solubility, e.g., cucurbituril, ABBA-type guest monomers combined with electrostatic repulsive force have been demonstrated to be outstanding candidates with optimized architecture. As one of the challenges, the characterization of supramolecular polymers has made significant progress in recent years. New methods, such as SANS and AF4, have shown their power and potential in the characterization of supramolecular polymers. It is worth mentioning that the AF4 system, taking advantage of convenience, rapidness, and effectiveness with abundant information, is anticipated to be a universal characterization tool for supramolecular polymers as well as GPC for conventional polymers. Supramolecular polymers with controlled structures are of great importance for the study of the structure−function relationship, which is the cornerstone of supramolecular polymer materials with highly customized functions. Although it is a great challenge, we are on the way. Several exciting breakthroughs have been made under the endeavor of scientists. With the deep study of the mechanism and kinetics of supramolecular polymerization,296,297 we have reason to believe that more and more approaches can be developed in achieving controlled supramolecular polymerization. Undoubtedly, the application of supramolecular polymers is the impetus for the advance of this research area. So far, multiple-hydrogen-bond-based supramolecular polymers and metallosupramolecular polymers have had a strong impact on material science, while the application study of supramolecular polymers based on other driving forces still needs to
Figure 70. (a) Schematic representation of supramolecular polymers based on host−guest interactions between a β-CD dimer and a Mn3+− porphyrin derivative. (b) Representative 2D coronal T1-weighted MR images of the mice at preinjection and 2, 5, 10, 20, and 25 min after intravenous injection of supramolecular polymer agents at 0.03 mmol of Mn/kg. Reprinted from ref 294. Copyright 2013 American Chemical Society.
enhanced T1 relaxation was observed by the supramolecular polymer compared to the commercially available Gd contrast agent. Further in vivo MR imaging investigation in mice revealed prominently positive contrast enhancement of the supramolecular polymer within the blood, kidney, and urinary bladder of the mice. 7233
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be enhanced. In addition, we need to further extend more application areas, such as biomedical materials and smart responsive materials, to inject new vigor into this emerging research field.
AUTHOR INFORMATION Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest. Biographies
Zhiqiang Wang is a professor in the Department of Chemistry of Tsinghua University, Beijing, China. He received his B.Sc. in Organic Chemistry, M.Sc. in Physical Chemistry, and Ph.D. in Polymer Chemistry and Physics at Jilin University. He has been working on interfacial self-assembly and supramolecular polymers.
Liulin Yang received his Ph.D. in Polymer Physics and Chemistry at Xiamen University under the supervision of Prof. Yanming Dong in 2011. His graduate work concerned the study of dendronized carbohydrate liquid crystals. In 2012, he joined Prof. Xi Zhang’s
Xi Zhang is a professor in the Department of Chemistry of Tsinghua University, Beijing, China. He received his B.Sc. in Analytical Chemistry and M.Sc. and Ph.D. in Polymer Chemistry and Physics at Jilin University under the supervision of Prof. Jiacong Shen. During his Ph.D. study, he spent more than one year at the Institute of Organic Chemistry, University of Mainz, Germany, as a joint-training Ph.D. student under the supervision of Prof. Helmut Ringsdorf. He joined the Department of Chemistry at Jilin University as a lecturer in 1992 and was then promoted to be a professor in 1994. He moved to Tsinghua University in late 2003. He has been recognized by various honors and awards, such as the National Natural Science Award (2004), Member of the Chinese Academy of Sciences (2007), and AkzoNobel Chemical Science Award (2010). His main scientific interests are in the areas of the cutting edge of supramolecular chemistry and polymer chemistry, including supra-amphiphiles, supramolecular polymers, Se-containing polymers, organized molecular films, and single-molecule force spectroscopy.
group as a postdoctoral fellow at Tsinghua University. His research interests mainly focus on the hierarchical and controllable selfassembly of supramolecules, including supra-amphiphiles and supramolecular polymers.
ACKNOWLEDGMENTS We gratefully acknowledge the comments and advice of Dr. Oren A. Scherman, Zehuan Huang, Yunhao Bai, E. I. M. Hicham, and Qiao Song. This work is financial supported by the Key Projects of the National Natural Science Foundation of China (NSFC) (Grant 21434004), the National Basic Research Program of China (Grant 2013CB834502), and the Foundation for Innovative Research Groups of the NSFC (Grant 21121004).
Xinxin Tan received his B.Sc. degree in 2010 from the Department of Chemistry, Tsinghua University, and his undergraduate work was performed in the group of Prof. Xi Zhang. In the same year, he started his Ph.D. research in the same group. His research interests mainly focus on supramolecular polymers and AFM-based single-molecule force spectroscopy. 7234
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Supramolecular Polymers: Historical Development, Preparation, Characterization, and Functions Liulin Yang, Xinxin Tan, Zhiqiang Wang, and Xi Zhang* Key Laboratory of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China 5.2. Viscometry 5.3. NMR Spectroscopy 5.4. AFM-Based Single-Molecule Force Spectroscopy 5.5. Light Scattering 5.6. Small-Angle Neutron Scattering 5.7. Asymmetric Flow Field-Flow Fractionation 5.8. Other Methods 6. Application of Supramolecular Polymers 6.1. Optoelectronic Materials 6.2. Supramolecular Self-Healing Materials 6.2.1. Self-Healing Materials Based on Multiple Hydrogen Bonding 6.2.2. Self-Healing Materials Based on MetalCoordination Bonding 6.2.3. Self-Healing Materials Based on π−π Interactions 6.3. Biomedical Applications 7. Conclusions and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments References
CONTENTS 1. Introduction 2. Driving Forces for Supramolecular Polymers 2.1. Multiple Hydrogen Bonds 2.1.1. Hydrogen-Bonding Arrays 2.1.2. Hydrogen-Bonded Supramolecular Polymers 2.2. Metal-Coordination Bonds 2.2.1. Ligands and Metal Ions 2.2.2. Counterions 2.2.3. Stoichiometric Ratio 2.3. Host−Guest Interactions 2.3.1. Cyclodextrin-Based Supramolecular Polymers 2.3.2. Cucurbit[8]uril-Based Supramolecular Polymers 2.3.3. Crown Ether-Based Supramolecular Polymers 2.3.4. Calixarene-Based Supramolecular Polymers 2.3.5. Pillararene-Based Supramolecular Polymers 2.4. Aromatic Donor−Acceptor Interactions 2.5. Multiple Driving Forces 3. Supramolecular Polymers with Branch-like and Hyperbranch-like Topology 3.1. Supramolecular Dendrimers 3.2. Supramolecular Hyperbranched Polymers 4. New Strategies for Controllable Supramolecular Polymerization 4.1. Polymerization of a Supramonomer 4.2. Self-Sorting Controlled Supramolecular Polymerization 4.3. Stimulus-Controlled Supramolecular Polymerization 4.4. Living Supramolecular Polymerization 5. Characterization of Supramolecular Polymers 5.1. Size Exclusion Chromatography
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1. INTRODUCTION Supramolecular polymers, originating from the integration of polymer science and supramolecular chemistry, have been a rapidly developing interdisciplinary research area. In the broad sense, the term “supramolecular polymers” can be defined as any type of assembly formed from one or more molecular components via reversible bonds. 1 However, to avoid confusion, a limited definition regarding supramolecular polymers as “polymeric arrays of monomeric units that are held together by highly directional and reversible noncovalent interactions, resulting in polymeric properties in solution and bulk” 2,3 is more explicit and recommended. A variety of noncovalent interactions (e.g., multiple hydrogen bonding, metal coordination, host−guest interactions, and aromatic stacking) have been employed as driving forces to construct supramolecular polymers. Monomers for building linear supramolecular polymers can be classified into five major types: AA type, AB type, AA/BB
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Table 1. Typical Examples of Multiple-Hydrogen-Bonding Arrays and Their Binding Constants
Three major mechanisms for supramolecular polymerization, including isodesmic, ring−chain equilibrium, and cooperative growth, have been thoroughly discussed in a previous review3 by Meijer et al., and thus, we just make a brief introduction as follows. The isodesmic mechanism is similar to the step polymerization that obeys Flory’s “principle of equal reactivity”. This implies that the reactivity of the end groups does not change during the supramolecular polymerization process. The ring−chain equilibrium mechanism depicts an equilibrium between the linear chain and cyclic counterpart during the supramolecular polymerization process. For the cooperative mechanism, the process of supramolecular polymers involves at least two stages, nucleation and elongation. The first step is a linear isodesmic polymerization with a binding constant Kn for the addition of each monomer, and a nucleus of degree of polymerization is formed. The second step involves another linear isodesmic elongation process, and due to the cooperative effect, the supramolecular polymerization proceeds by a binding constant Ke higher than Kn. Although much insight into the process of supramolecular polymerization has been gained, it still remains a great challenge for designing controlled supramolecular polymer-
type, ABBA type, and aromatic-stacking type. An AA-type monomer is a kind of self-complementary dimer, which is common in a multiple-hydrogen-bonding system. The complementary heterodimer AB-type monomer that connects each moiety in a head-to-tail manner is useful in building supramolecular homopolymers or alternating block copolymers. The homoditopic AA and BB monomers can polymerize via heterodimerization in an alternating manner. Both AB-type and AA/BB-type monomers are the most commonly used monomers for supramolecular polymers. The unique ABBAtype monomers are powerful in building linear supramolecular polymers especially in dilute solution, which can effectively inhibit dimerization and cyclization. Monomers based on an aromatic donor−acceptor interaction alone might be too weak and less directional to drive supramolecular polymerization. Therefore, directing groups based on solvophobic, hydrogenbonding, dipole−dipole, and donor−acceptor interactions on the periphery of an aromatic core should be introduced to promote the formation of supramolecular polymers. For supramolecular branched or hyperbranched polymers, two kinds of monomers, ABn (n ≥ 2) and Am/Bn (m, n ≥ 2), are most commonly used. 7197
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hydrogen bonds, the binding constant of DAA−AAD arrays 2 is significantly higher than that of ADA−DAD arrays 1 (Ka ≈ 104 vs 102 M−1), on account of the additional attractive secondary electrostatic interactions in DAA−AAD arrays.10 Later on, Zimmerman et al. confirmed that the AAA−DDD arrays 3 indeed have a significantly higher binding constant (Ka > 105 M−1 in CDCl3), due to the solely attractive secondary interactions.12 According to this secondary interaction rule, multiplehydrogen-bonding arrays were designed, with binding constants even higher than those of triple-hydrogen-bonding arrays. In 1997, Meijer et al. reported a self-complementary quadruplehydrogen-bonding unit, ureidopyrimidinone (UPy) 4, with a dimerization constant Kdim of up to 6 × 107 M−1 in CHCl3 (Table 1).16,17 The high dimerization constant of UPy derives from its AADD array of donor and acceptor groups. Analogous to UPy, Zimmerman et al. reported a DeAP unit, 5, that contained a self-complementary AADD array with a dimerization constant Kdim = 9 × 107 M−1 in CHCl3.18 Although these AADD arrays exhibit a high dimerization constant, different tautomeric forms are present which may significantly decrease their binding constants.19 For UPy, when the tautomeric equilibrium is shifted to the pyrimidin-4-ol tautomer, as a DADA hydrogen-bonding array, the dimerization binding constant is lowered to 9 × 105 M−1 in CHCl3, due to more repulsive secondary interactions.20 The strongest quadruplehydrogen-bonding arrays should be AAAA−DDDD arrays. Leigh et al. have reported a cationic AAAA−DDDD complex which exhibits a binding constant as high as 1012 M−1 in CH2Cl2.21 Sextuple-hydrogen-bonding arrays have also been developed, and one of the representatives is between diaminopyridinesubstituted isophthalamide and barbiturates 6. The binding constant for this heterocomplementary recognition pair is up to 1.37 × 106 M−1 in CDCl3.22 Another example of a sextuplehydrogen-bonding array was reported by Corbin and Zimmerman. 2 3 They designed heterocomplementary bis(ureidonaphthyridine)s and bis(naphthyridinourea)s; however, the individual units formed an intramolecular complex, which reduced the duplex stability. Gong et al. reported a very stable sextuple-hydrogen-bonding array, 7, with a Ka ≈ 109 M−1 in CDCl3, in which intramolecular hydrogen bonding played a key role in providing a preorganized, planar arrangement of the donor and acceptor groups.24,25 2.1.2. Hydrogen-Bonded Supramolecular Polymers. Multiple hydrogen bonds were the first kind of noncovalent bond that was adopted to fabricate supramolecular polymers. In 1990, Lehn et al. first reported this type of supramolecular polymer. Bifunctional diamidopyridines and uracil derivatives were mixed to form linear polymeric chains 8 via triplehydrogen-bonding interactions (Figure 1).26 In contrast to the behavior of each monomer, the supramolecular polymers displayed liquid crystallinity in the solid phase. To achieve supramolecular polymers with a high degree of polymerization (DP), multiple-hydrogen-bonding arrays with a higher binding constant, such as quadruple-hydrogen-bonding arrays, were employed. In 1997, Meijer et al. reported that bifunctional AAtype monomers containing two UPy end groups were able to form linear supramolecular polymers 9 (Figure 1).16 The molecular mass of supramolecular polymers can be tuned by varying the solvent and concentration. Addition of monofunctional units caused decreases of the solution viscosity as well as the DP. The calculated DP for the pure monomer was about
ization systems, affording supramolecular polymers with a controlled structure such as molecular mass, polydispersity, and the corresponding properties. Given the mechanism of controlled polymerization for conventional polymers, supramolecular polymerization following the cooperative nucleation−elongation process, which is analogue to chain-growth polymerization, should be one of the right candidates to achieve this aim. Besides this direction, considering the unique characteristics of supramolecular polymerization, other effective approaches are expected to be developed to control this process. In this review, we will summarize some of the latest progress in this regard. Characterization of supramolecular polymers, especially the parameters of average molar mass and polydispersity, is essential and obligatory for supramolecular polymers. However, the dynamic nature of supramolecular polymers makes their characterization difficult. The change of concentration, temperature, or stimuli from the environment may significantly influence the original molecular structure of supramolecular polymers. Many well-established characterization methods fail to work as well as when applied to conventional polymers. Only partial information can be obtained from individual methods. Therefore, a combination of different characterization methods is required. We have already presented a review4covering this topic, so here we will summarize the latest progress on the characterization of supramolecular polymers. The development of supramolecular polymers has brought new vigor into material science. Compared with conventional polymers, the polymeric structure and reversible noncovalent interactions endow supramolecular polymers with not only traditional polymeric characteristics, but also dynamic properties to achieve functions such as high response to stimuli, environmental adaptation, and self-repairing capacity. Considering that there are several wonderful reviews about functional supramolecular polymers,5−8 in this review, we mainly highlight some of the recent advances in specific application areas, such as optoelectronic materials, self-healing materials, and biomedical materials.
2. DRIVING FORCES FOR SUPRAMOLECULAR POLYMERS 2.1. Multiple Hydrogen Bonds
The hydrogen bond is a type of dipole−dipole attraction between an electronegative atom and a hydrogen atom bonded to another electronegative atom, such as nitrogen, oxygen, or fluorine. It was first suggested by Moore and Winmill in 1912.9 Nowadays, the hydrogen bond is one of the most widely applied noncovalent interactions in the field of supramolecular chemistry. 2.1.1. Hydrogen-Bonding Arrays. Hydrogen bonds are weaker than covalent and ionic bonds, with an energy typically between 5 and 30 kJ/mol. A single hydrogen bond is not strong enough to fabricate supramolecular polymers. However, both the strength and the directionality can be increased when multiple hydrogen bonds are arrayed to create hydrogenbonding arrays. Typical examples of multiple-hydrogenbonding arrays are summarized in Table 1. The binding strength between arrays is also dependent on the order of the donor and acceptor in the arrays (Table 1), which was first pointed out by Jorgensen et al.10,11 and further confirmed by Zimmerman et al.12−15 It was found that although the DAA− AAD and ADA−DAD arrays have an equal number of 7198
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was very favored over homodimerization by a factor of >20:1 at high concentration (>0.1 M). To prepare a strict alternating supramolecular copolymer, multiple-hydrogen-bonding arrays with highly stable heterodimerization but minimal homodimerization need to be designed. This goal was achieved by Zimmerman et al., who reported a non-self-complementary unit, ureidoguanosine. This quadruple-hydrogen-bonding unit could strongly combine with an Napy unit (Ka ≈ 107 M−1 in CDCl3), while exhibiting negligible self-association (Kdim = 230 M−1 in CDCl3).34,35 Mixing the two bifunctional macromonomers 13 and 14 gave rise to a supramolecular copolymer with a strictly alternating sequence of the two monomers,36 while both of the monomers alone did not form polymers (Figure 3).
Figure 1. Supramolecular polymers self-assembled from typical triple-, quadruple-, and sextuple-hydrogen-bonding arrays.
700 at 40 mM in CHCl3. Since then, this unit has been widely used for constructing supramolecular polymers.17,27−29 Lehn et al. also reported a series of supramolecular polymers based on barbiturate/cyanurate−isophthaloyldiamidopyridine sextuple arrays 10 (Figure 1).30,31 Rigid fibers were observed in toluene, while small-angle neutron scattering in decane demonstrated the formation of gel-like assemblies. Heterodimerizing motifs are attractive for constructing supramolecular block copolymers. Meijer et al. prepared AA/ BB-type supramolecular block copolymers using Napy and UPy motifs,32 in view of the strong and selective complexation of these two units.33 They designed a poly(tetrahydrofuran) macromonomer, 11, with UPy units as the chain ends and a ditopic Napy monomer, 12 (Figure 2). The composition of the supramolecular polymers could be tuned from a pure homopolymer to an alternating heteropolymer by controlling the stoichiometry of UPy and Napy groups. This behavior was shown to be concentration dependent, and heterodimerization
Figure 3. Structures of bifunctional macromonomers that can form a strict alternating supramolecular copolymer. Reprinted from ref 36. Copyright 2006 American Chemical Society.
The molar ratio of AA/BB-type monomers is one of the major factors that limits the DP of supramolecular polymers. Meijer et al. addressed this limitation by using AB-type monomers to prepare supramolecular polymers.37 Polymerization of a bifunctional UPy−Napy monomer was expected to obey the ring−chain equilibrium mechanism. Thus, at low concentrations ( 1014 M−2 for the 3-fold chelating terpyridine unit. For fully reversible coordination polymers, the relationship between DP and K is given by the expression DP ∝ (K[M])1/2.51 According to this relationship, the DP of MSPs constructed from 1 mM solution can be estimated. In the case of a binding constant of K = 105 M−1, only oligomers with DP ≈ 10 can be obtained, whereas, in the case of a binding constant of K = 107 M−1, polymers with DP ≈ 100 may be achieved. It should be pointed out that the rational design of the molecular structure of ligands and the directionality of coordination bonding also play key roles in the construction of linear MSPs. A rigid ligand with a nonlinear coordination configuration often leads to the formation of macrocyclic entities, such as polygons, polyhedrons, and MOFs.52 Therefore, a ditopic ligand with a flexible linker or coordination systems with linear configurations are recommended for the construction of linear MSPs with a high DP. At the early stage of this line of research, MSPs were usually found to be either insoluble or decomposable in solution, exchanging some of their original ligands for solvent molecules; thus, a profound characterization was precluded.8,46 Rehahn et al. reported the first example of MSP 17 with a high molecular mass that is soluble and inert in noncoordinating solvent (Figure 6).60 A ligand monomer, 4,4″-bis[(9-aryl)-2-o-phenanthroline]-2′,5′-di-n-hexyl-p-terphenyl, was mixed with equimolar metal monomer [Cu(MeCN)4]PF6, broad 1H NMR peaks for the polymeric repeating units were observed, and the lack of end-group absorptions implied the DP was higher than 30. The polymer was found to be stable in noncoordinating solvent, such as 1,1,2,2-tetrachloroethane. However, when a trace of coordinating solvent (acetonitrile) or a specific free ligand was added to the solution, the polymer became more dynamic with rapid exchange of ligands. On the basis of this research, Rehahn et al. also studied a series of MSPs with different metal ions, including copper(I) and silver(I), and different structures of ophenanthroline-based bisbidentate ligands.61,62 These systems could be considered supramolecular polymers in coordinating
Figure 4. Poly(ethylene/butylene) with OH end groups (a) and poly(ethylene/butylene) functionalized with multiple-hydrogenbonded units (b). Reprinted with permission from ref 42. Copyright 2000 John Wiley & Sons, Inc.
rates, leading to thermally responsive materials with a high sensitivity in the solid state. Hailes et al. developed a bifunctional monomer based on ureidocytosine motif 15, which did not undergo tautomeric changes in C6D6, C6D5CD3, and the solid state (Figure 5).43 Diffusion coefficient measure-
Figure 5. Bifunctional macromonomers based on ureidocytosine 15 and cytosine- and adenine-end-capped motifs 16.
ment indicated that it formed oligomers in dilute CHCl3 solutions (6.5 mM) but polymers in concentrated solutions (37 mM). Rowan et al. investigated the nucleobase 16 (e.g., cytosine and adenine) as self-assembly units to construct supramolecular polymers in the solid state aided by phase segregation (Figure 5).44,45 They found that nucleobaseterminated poly(THF) macromonomers could be meltprocessed into self-supporting films and fibers while the corresponding poly(THF) (Mn = ca. 1.4 kDa) was a soft waxy material. The rheological behavior of the supramolecular polymers was strongly influenced by the nucleobase groups. In the case of cytosine-end-capped material, a network structure was observed at room temperature. For the adenine-endcapped material, incomplete association was observed at lower temperatures with a transition to a network at higher temperatures prior to a complete loss of polymer-like properties. The development of such materials, which exhibit very low melt viscosities, potentially allows access to thermally rehealable plastics as well as easy-to-process materials. 2.2. Metal-Coordination Bonds
The incorporation of metal-coordination bonds into metalcontaining polymers has been extensively studied. Metalcontaining polymers have attracted great interest due to their combination of the properties of organic polymers and the magnetic, electronic, optical, and catalytic potential of metals.46 7200
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Table 2. Typical Examples of Building Motifs for Metallosupramolecular Polymers (MSPs)
was highly dependent on the hydrophilicity of the monomer64 and the counterions.65 The 2,2′,6′2″-terpyridine (or TPY for short) ligand is one of the most important ligands for fabricating MSPs. TPY contains three nitrogen atoms, so it can act as a tridentate ligand and form stable complexes by chelating various transition-metal ions.66 Therefore, it has been widely used to fabricate MSPs.67−70 In 1995, Constable71 et al. proposed that MSPs could be obtained by addition of metal ions to TPYfunctionalized monomers. Kurth and co-workers prepared this MSP through the complexation between 1,4-bis(2,2′,6′,2″-terpyrid-4′-yl)benzene and iron(II) ions.72 The molecular mass was determined by analytical ultracentrifuga-
solvent, although they do not show supramolecular behavior in noncoordinating solvent. Compared with the 2,9-substituted phenanthroline ligand, 2,9-unsubstituted phenanthroline ligands show a higher coordination ability with metal ions, owing to the lack of steric hindrance. Higuchi et al. employed this unsubstituted phenanthroline with a dioctylfluorene spacer as a monomer ligand to prepare a copper-containing supramolecular polymer, 18 (Figure 6). A high molecular mass (Mw = 7.6 × 103 kDa) of the supramolecular polymer was obtained, which could act as a green electrochromic material.63 Ionic conductive materials were also developed based on the phenanthroline ligand chelated with a Ni(II) ion; the conductivity of these materials 7201
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Figure 6. MSP 17, which is soluble and dynamic in coordinating solvent, and MSP 18 based on a phenanthroline ligand with a fluorene spacer act as green electrochromic materials.
potential or thermal treatment. On the basis of this strategy, a further detailed study of MSPs based on a TPY−iron(II) complex system was significantly extended.75,76 The reversible formation of the metal−ligand bond was studied by addition of the strong chelating ligand HEDTA (N-(2-hydroxyethyl)ethylenediamine-N,N′,N′-triacetic acid) in aqueous solution. The monomer ligand was cut off, precipitated, and then isolated with addition of HEDTA. Upon addition of FeCl2, the supramolecular polymer formed almost immediately. The fabrication and the study of the reversibility of supramolecular polymers based on the TPY−iron(II) complex system were also demonstrated by other research works.78−81 In addition to the TPY−iron(II) complex system, complexes of TPY with other transition-metal ions have also been incorporated into fabricating MSPs. Schubert and co-workers compared the thermodynamic stability of TPY-based supramolecular polymers with different metal ions through viscometry experiments.75 The sequence of maximum values of the viscosities for polymers with different metal ions is in the order iron(II) > nickel(II) > cobalt(II) > copper(II) > cadmium(II). When excess metal ions were added, the relative viscosity decreased significantly in the cases of cadmium(II), copper(II), and cobalt(II); however, the decrease was much less pronounced for iron(II) and nickel(II). All of these results are in agreement with literature data for the stability constants of the formed biscomplexes.57,82 Würthner et al. prepared TPY−Zn(II)-based supramolecular polymers bearing a perylene bisimide group in the main chain.83 The average molecular mass of the supramolecular polymer was about 20 kDa, corresponding to approximately 15 repeating units in a 1 mM solution of CHCl3/CH3OH (60 vol %/40 vol %).49 Various TPY-based supramolecular polymers containing a Zn2+ ion with high photoluminescence quantum yields and electroluminescence performance have been reported,84,85 which will be further discussed in section 6. The dynamic nature of the metal−ligand bond is one of the key features of MSPs. Different metal ions exhibit different binding kinetics and thermodynamics, which makes these
tion. From the Swedberg formula, the lower limit of the molecular mass was roughly estimated to be 14.9 kDa, corresponding to 25 repeating units. A crystal structure of this MSP was obtained through electron diffraction in combination with molecular modeling, which evidenced the polymeric chain structure.73 Early research of MSPs based on the TPY ligand suffered from restricted solubility; in other cases, the solubilizing groups had to be introduced by rather complicated procedures. To improve the solubility of MSPs, Schubert et al. introduced a simple synthetic strategy to construct water-soluble MSPs (Figure 7).74−77 Bifunctional monomers which consisted of
Figure 7. Schematic representation of water-soluble terpyridine ligands.
poly(oxytetramethylene) 19 or poly(ethylene oxide) 20 flanked with two TPY groups were synthesized through the Williamson reaction. This simple and high-yield strategy facilitates the preparation of TPY-terminated oligomers and polymers. The addition of octahedral coordinating transition-metal ions (colbat(II) or zinc(II)) to these TPY ligands leads to the formation of MSPs. These supramolecular polymers can be reversed by changing the pH or applying an electrochemical 7202
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systems even more versatile. Würthner et al. compared the kinetic stability of the complexation of several first-row transition-metal ions with the TPY ligand.49 The results are consistent with kinetic data82 for the reaction of various M(TPY)2 complexes with excess TPY in water. For example, t1/2 values for Fe2+, Ni2+, and Co2+ are 8400, 610, and 60 min, respectively, whereas t1/2 is less than 0.1 min for the Cu2+ and Zn2+ complexes. Schubert and co-workers investigated the characterization and the stability of MSPs based on TPY−metal ion complexes utilizing gel permeation chromatography (GPC).86 GPC studies were carried out on a series of monoend-capped TPY−poly(ethylene oxide)−metal (Fe2+, Co2+, and Ni2+) complexes with different sample injection volumes. In the Fe2+ and Co2+ systems (Figure 8), lowering the volume of the
Figure 9. Formation of MSPs using BIP-based ditopic ligand monomers with metal salts in acetonitrile. Reprinted from ref 100. Copyright 2006 American Chemical Society.
Figure 8. GPC elution curves for the bis-TPY ligand and Co2+ complex system (different injection volumes) and their corresponding starting material. Reprinted with permission from ref 86. Copyright 2007 John Wiley & Sons, Inc.
An interesting aspect of these studies is that the BIP ligand can form 2:1 metal ion complexes with transition-metal ions and 3:1 complexes with lanthanide ions; therefore, supramolecular gels94,100 or films103−105 can be created by mixing the telechelic chelating ligand with a combination of lanthanoid and transition-metal ions (Figure 9). These metallosupramolecular gels or films exhibit thermo-, chemo-, optico-, and mechanoresponses, as well as light-emitting and shape-memory104 properties. A unique multidentate ligand comes from porphyrin derivatives.54,111 Porphyrin-based model compounds have been developed as artificial light-harvesting systems and molecular photonic devices to mimic natural light-harvesting complexes. However, construction of these model compounds often involves tedious organic synthesis. To overcome this problem, supramolecular porphyrin polymers have attracted great attention. Hunter et al. reported a metalloporphyrin supramolecular polymer, 22, through the coordination of cobalt porphyrin and two pyridine ligands, one on each face of the porphyrin (Figure 10).54 The binding constant K = (1.0 ± 0.5) × 106 M−1 is high enough to build a supramolecular polymer. The DP increased with the concentration of the monomer characterized by GPC, indicating that the polymers in solution were constitutionally dynamic. The DP reached approximately 100 repeating units with a mean molecular mass of 136 kDa, which was significantly larger than those of the porphyrin systems obtained from covalent reactions (DP = 10−15). Almost simultaneously, Kobuke et al. reported another example of metalloporphyrin supramolecular polymer 23 through the coordination of zinc porphyrin and imidazolyl groups (Figure 10).111 Considering the pentacoordination of Zn2+, an ABBAt y p e m o no m e r w h i c h co n ta i ne d t w o co nn e c t e d porphyrinatozinc(II) groups flanked with two imidazolyl groups was designed, and supramolecular polymers were obtained by complementary coordination at each imidazolyl-
injected sample resulted in a detectable shift to the macromonomer in GPC spectra, indicating a degradation of polymer chains due to the shearing and dilution effects of GPC. To develop novel MSPs as promising electrochromic materials, a series of TPYs with pyridine-ring-functionalized ligands coupled with rigid, linear spacers, in contrast to the unsubstituted TPY ligand, were studied.87−93 Supramolecular polymers based on electron-donating-group-substituted TPY ligands show rapid switching rates, good reversibility, high stability, and optical memory, which are attractive candidates for smart responsive electrochromic materials. A tridentate ligand, 2,6-bis(1′-methylbenzimidazolyl)pyridine (BIP for short), which is analogous to the TPY ligand, was also developed for the construction of MSPs (Figure 9).58,94−110 By titrating a metal ion (Zn2+) solution into a bis-BIP monomer, 21, solution, the formation of MSPs was monitored by viscometry.96 A steady increase of the reduced viscosity up to a Zn2+/ligand ratio of 1 indicated the formation of MSPs; this was followed by a decrease of the reduced viscosity beyond this point, confirming the depolymerization of the MSPs. In addition to the Zn2+ ion, other transition-metal ions such as Cd2+, Co2+, and Fe2+ were also adopted to fabricate BIP-based supramolecular polymers.97 Different metal ions impart different properties onto the MSPs, which allows fine-tuning of the properties of these materials. Thermal data suggest that supramolecular polymers with weaker binding, more labile metal ions (e.g., Cd2+, Zn2+) show less thermal stability than those with stronger binding, less labile metal ions (e.g., Co2+, Fe2+). BIP-based MSPs consisting of copper ions (Cu+ and Cu2+) show an interesting redox-responsive property, where the DP and the viscosity of their solutions depend on the oxidation state of the copper ions.108 7203
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adjusted from 1:2 to 1:6, according to the ligand structure. Such nonstoichiometric binding is possible through the electrostatic interactions between amphiphiles and supramolecular polymers as depicted in Figure 11. Perfectly rodlike chains of these complexes formed from rigid ditopic ligand can be observed under atomic force microscopy (AFM).124 PACs are soluble in common organic solvents, form Langmuir monolayers at the air−water interface, and are readily transferred onto solid substrates. Moreover, PACs display thermotropic liquid crystalline phases.125−127 The change of counterions can also endow the materials with new properties. The introduction of amphiphiles as counterions gives rise to a structure-induced spin-crossover, shown in Langmuir−Blodgett (LB) multilayers of iron-based PACs.127 The phase transition upon heating induces the deformation of the coordination geometry around the central metal ions, which is strong enough to reduce the strength of the ligand field, giving rise to spin transition from a low-spin state to a high-spin state. The counterions also affect the ionic conductivity of the MSP films.65 MSPs with “hard” counteranions show high conductivity: the order of the hardness in the bases is chloride > nitrate > acetate > acetylacetonate, which is consistent with the order of the ionic conductivity in the supramolecular polymers. 2.2.3. Stoichiometric Ratio. The preparation of MSPs normally involves a simple mix of ligands and metal ions in solution, and the polymerization follows a step-by-step mechanism. Therefore, to obtain MSPs with a high molecular mass, it is of paramount importance to control the ligand to metal ion ratio very accurately. At least two methods were introduced to monitor the stoichiometric ratio precisely: one is conductometry, and the other is viscosimetry. Conductivity measurement was introduced by Kurth and coworkers128 to monitor the stoichiometric ratio during selfassembly of TPY-based MSPs. Due to the charge of the MSPs, conductometry is well suited to follow the self-assembly, and in addition it is of excellent accuracy. Figure 12 shows conductivity measurements as a function of y = [M]/[L] for the titration of iron(II), cobalt(II), and nickel(II) acetate to bisTPY ligand. The conductivity decreases as the metal ion is titrated to the solution until a minimum is reached at y = 1. As y exceeds 1, further addition of metal salt results in an increase of the conductivity. The advantages of conductivity are the ease of use, even for air-sensitive compounds, high accuracy, and in situ detection.
Figure 10. (Top) Metalloporphyrin supramolecular polymers driven by the coordination of cobalt porphyrin and pyridine groups. Reprinted with permission from ref 52. Copyright 2000 John Wiley & Sons, Inc. (Bottom) Metalloporphyrin supramolecular polymers driven by the coordination of zinc porphyrin and imidazolyl groups. Reprinted with permission from ref 111. Copyright 2000 John Wiley & Sons, Inc.
porphyrin unit. The average DP was about 80 repeat units analyzed by GPC. Besides the ligand−metal ion complex systems mentioned above, a lot of other ligand−metal ion systems, such as bis(pyrazolyl)pyridine−Fe 2+ , 112 bis(pyrazolyl)pyridine− Zn2+,113 pyridine-2,6-dicarboxylate ligand coordinated with Zn2+, Nd3+, and La3+,114,115 pyridine−Pt2+ and pyridine−Pd2+ systems, 116,117 4,4′-biphenyldicarboxylic acid−terbium (Tb3+),118 tetra-2-pyridyl-1,4-pyrazine−Fe2+,119 acyl hydrazone ligands with transition-metal ions,120 azo aromatic ligand− Co3+,121,122 and azo aromatic ligand−Fe3+,122 have been well documented for the construction of functional MSPs. 2.2.2. Counterions. The solubility of MSPs can be adjusted by the choice of counterions. In the case of noncoordinating counterions, MSPs are generally soluble in aqueous media through colloidal stabilization. With counterions such as PF6−, Cl−, or amphiphiles with a negatively charged headgroup, MSPs are soluble in organic solvents. The replacement of acetate counterions with amphiphiles such as dihexadecyl phosphate (DHP) leads to formation of MSP−amphiphile complexes (PACs).123 The ratio of metal ion to amphiphile can be readily
Figure 11. Self-assembly of MSPs and dihexadecyl phosphate (DHP) affords PACs with a monolayer or double-layer structure depending on the metal ion to amphiphile ratio.125,126 Right: Melting of the alkyl chains in the amphiphilic mesophase induces a spin transition from a diamagnetic low-spin state to a paramagnetic high-spin state. Reprinted from ref 127. Copyright 2005 American Chemical Society. 7204
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mer can inhibit the formation of a cyclic oligomer and promote the linear growth of polymer chains, resulting in a high DP. This is especially important for some kinetically inert MSPs, because once the cyclic chains are formed, they can hardly be broken and re-formed even with concentration of the solution after polymerization. Finally, variation of the temperature not only brings important modification of the aimed MSPs, but also influences the balance between the thermodynamic stability and kinetic lability of these polymers. 2.3. Host−Guest Interactions
Host−guest interactions are driving forces130 for host−guest complexes which involve a cooperative effect of various noncovalent interactions, such as hydrophobic interactions, multiple hydrogen bonds, ionic bonds, van der Waals forces, and electrostatic interactions. Although a large number of host−guest systems have been reported, common host molecules involved in fabricating supramolecular polymers include cyclodextrins (CDs), crown ethers, cucurbit[8]urils (CB[8]’s), calixarenes, and pillar[n]arenes.130,131 Guest molecules are general organic compounds that can be incorporated inside the cavity of hosts. Several typical host molecules and their corresponding guest molecules are listed in Table 3. The chemical basis of the host−guest complexation is based on complementary shape and size fit between the host and guest molecules. Various types of monomers and their corresponding supramolecular polymers based on host−guest interactions are summarized in Table 4. Usually two types of ditopic monomers that employ hosts (A) and guests (B) are designed to fabricate supramolecular polymers: AB-type heteroditopic monomers and AA/BB-type homoditopic monomers. However, both of them share a common problem, which is a competition between linear polymerization and oligomerization (intramolecular complexation, dimerization, or oligomeric cyclization). A strategy to inhibit the formation of oligomers is to increase the concentration of monomers, since oligomers are predominant in dilute solution. For CB[8]-based systems, considering their poor solubility in water solution, a unique ABBA-type monomer was designed to promote linear supramolecular polymerization in dilute solution. 2.3.1. Cyclodextrin-Based Supramolecular Polymers. Cyclodextrins are cyclic oligomers built up from six, seven, or eight glucopyranose units, linked by α-(1−4)-glycosidic linkages, named α-, β-, and γ-cyclodextrins, respectively.132 They form inclusion complexes in water with a variety of organic molecules. The host−guest interaction between cyclodextrins and guest molecules has been adopted as one of the earliest driving forces to prepare polyrotaxanes and supramolecular polymers.131 In an early stage of this line of research, numerous monosubstituted CDs, as AB-type monomers, have been prepared and characterized. However, most of them form intramolecular complexes or dimers.133 As one of the pioneering works, Harada et al. found that 6-cinnamoyl-α-CD (6-cin-α-CD) formed intermolecular complexes.134 Vapor pressure osmometry measurements demonstrated that the 6cin-α-CD showed a concentration dependency, while α-CD did not. The molecular mass of this complex was estimated to be 3 kDa, corresponding to a trimer. The failure in forming linear polymers was due to the phenyl ring being included from the narrower side of CD (C6-primary OH group side), which forced it to be more inclined to form a dimer or cyclic oligomer.
Figure 12. MSP synthesis under conductometric control. Conductometric titrations of the ditopic ligand L1 and iron(II), cobalt(II), nickel(II), and zinc(II) acetate in acetic acid solution (□, ○, Δ, and ◇). Reprinted with permission from ref 128. Copyright 2010 Royal Society of Chemistry.
Viscosimetry is another method to monitor the supramolecular polymerization process.75,129 The advantage of this method is that both charged and uncharged species can be monitored. Schubert and co-wokers employed viscometry experiments to monitor the self-assembly process in preparing TPY-based supramolecular polymers with different metal ions (Figure 13).75 A stepwise addition of different metal salt
Figure 13. Viscosimetry experiments. Stepwise addition of various metal salts to a solution (c = 20 mg/mL methanol) of bis-TPY monomer led to different maximum values of the relative viscosity for an equivalent addition of the metal salt. Reprinted from ref 75. Copyright 2003 American Chemical Society.
solutions (cadmium(II) acetate, copper(II) acetate, cobalt(II) acetate, nickel(II) acetate, iron(II) sulfate) to a solution of bisTPY monomer resulted for all salts in an increase in the relative viscosity up to a maximum value at a metal ion:telechelic ratio of 1:1, followed by a decrease for higher ratios. The sequence of maximum values of the viscosity as well as the extent of relative viscosity decrease demonstrated the different binding strengths of these metal ions. Other factors such as solvent, concentration, and temperature also play important roles in the preparation of MSPs. As we mentioned before,60−62 the solvent can compete with the ligand in chelating the metal ions, thus influencing the stability of the MSP structures, in particular in the case of weak metallocomplexes. Since the polymerization of MSPs follows a polycondensation mechanism, a high concentration of mono7205
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Table 3. Several Typical Host Molecules and Their Corresponding Guest Molecules
was obtained, which was even larger than the supramolecular polymers with a rigid and hydrophobic linker.138 A possible interpretation of these results was the solvation effect of the linker, which avoided the formation of intrachain coiled moieties.139 It was suggested that supramolecular polymer chains with a coiled conformation could trap the terminal group and then stop the polymer growth, on the basis of the smallangle X-ray scattering (SAXS) and molecular dynamic study. Therefore, large supramolecular polymers could be prepared by improving the solvent quality or tuning the hydrophilicity of the linkers. In addition to the dimer with the same type of CD, different types of CDs (α- and β-CDs) can be attached together to form a heterodimer. When the heterodimer was mixed with a guest dimer containing an adamantane group (a guest for βCD) and a phenyl group (a guest for α-CD), they also gave supramolecular polymers.140 To combine photochemistry with supramolecular chemistry, an interesting work on a photoswitchable CD-based supramolecular polymer employing γ-CD and a dimer, 24, with coumarin as an end group was reported by Tian and coworkers.141 γ-CD accommodated two coumarin units due to its large hydrophobic cavity (7.5−8.3 Å).132 Supramolecular polymers were fabricated by mixing γ-CD with the dimer
An exception was 6-adamantylamido-β-CD, which was reported to form a linear supramolecular polymer characterized by X-ray single-crystal diffraction.135 The success of this example may be due to the steric effect of the bulky adamantly group, which impedes the formation of dimers and cyclic oligomers. To inhibit the dimerization or cyclization, the guest substituent group was then attached to one of the secondary hydroxyl groups of CD (the wider side). In this case, for example, the phenyl ring of 3-cin-α-CD was included from the wider side of CD (secondary OH group side), leading to the formation of linear supramolecular polymers. The molecular mass was about 20 kDa, which corresponded to a repeating unit of 20.136 A mixture of AA/BB homoditopic monomers such as a CD dimer and a guest dimer also forms supramolecular polymers. When a β-CD dimer and an adamantane dimer were mixed in aqueous solutions, supramolecular polymers or oligomers formed depending on the rigidity or length of the linkers in the monomers.137,138 It was found that the longer the hydrophobic linker (alkyl segment), the lower the molecular mass of supramolecular polymers.137 However, when the linker was a long hydrophilic PEG chain (Mn = 600 Da), a supramolecular polymer with molecular mass Mn = 100 kDa 7206
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Table 4. Representative Types of Host−Guest Monomers and Their Corresponding Supramolecular Polymers, Taking CD and CB[8] as Examples
offer opportunities for the development of new supramolecular polymers. 144 However, due to the difficulties in the modification of CBs, supramolecular polymers based on CB[5], CB[6], and CB[7] are still rare. CB[8] is unique because of its ability to bind two guests in its relatively large cavity. Therefore, CB[8] is the most potential host in the CB family for fabricating supramolecular polymers.145 However, limited by the poor solubility ( 150 kDa) were obtained owing to the relatively high binding constant of log Ka = 5.1 ± 0.5 in CHCl3 at room temperature. Electron-deficient planar aromatic guests, such as trinitrofluorenone, trinitrobenzene, and tetracyanobenzene, can be “clamped” by a bisporphyrin tweezer via charge-transfer interaction, leading to a π donor−acceptor-type host−guest complex. This unique interaction was incorporated into a heteroditopic monomer, 53, to construct supramolecular polymers (Figure 31).197 The head-to-tail complexation
Figure 29. Schematic illustration of the construction of supramolecular polymers from a pillar[5]arene dimer and ditopic and tritopic guest molecules. Reprinted with permission from ref 182. Copyright 2014 Royal Society of Chemistry.
for the linear supramolecular polymer was estimated to be 89, and the DP for the hyperbranched one was estimated to be 113. 2.4. Aromatic Donor−Acceptor Interactions
Monomers based on aromatic donor−acceptor interactions alone might be too weak and less directional to drive supramolecular polymerization. However, the introduction of directing groups on the periphery of an aromatic core can promote the formation of supramolecular polymers. A lots of works focusing on the formation of nanofiber assemblies through columnar-like stacking of monomers based on π−π interactions have been reported and reviewed.189−192 In this review, we are focusing more on linear main-chain supramolecular polymers fabricated through strong aromatic donor− acceptor interactions. Molecular tweezers,193 as noncyclic host molecules with open cavities capable of binding guest molecules through aromatic donor−acceptor interactions, are one of the representatives. Supramolecular polymers with incorporated fullerene have attracted interest in construction of functionalized supramolecular polymers, due to their unique physical and chemical properties.194 It was found that a π-extended analogue of tetrathiafulvalene (exTTF) could act as a receptor for C60.
Figure 31. Aromatic donor−acceptor interaction-induced supramolecular polymerization. Reprinted with permission from ref 197. Copyright 2012 John Wiley & Sons, Inc.
between the bisporphyrin tweezer and the 4,5,7-trinitrofluorenone-2-carboxylate (TNF) moiety resulted in large upfield shifts of the TNF protons, indicating that the TNF moiety was located within the porphyrin tweezer, thereafter forming supramolecular polymers. A molecular tweezer possessing two pincer-like alkynylplatinum(II) terpyridine units connected by a rigid spacer is able to recognize a variety of electron-rich arenes through charge-transfer interactions.198,199 Wang and coworkers reported supramolecular polymers by incorporating this tweezer−guest molecular recognition (Figure 32).200 They designed heteroditopic AB-type monomers 54, which consisted of a tweezer group and a pyrene group on each side.
Figure 30. Schematic representation of AB-type monomers consisting of, for 51, C60 with a pincerlile exTTF and, for 52, C60 with a macrocyclic molecule containing two exTTF motifs. 7213
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Figure 32. Schematic representation of the supramolecular polymers derived from the pincer-like monomer and pyrene. Reprinted with permission from ref 200. Copyright 2014 John Wiley & Sons, Inc.
interactions to promote the formation of supramolecular polymers. To combine multiple ion pairing and hydrogen bonding, Schmuck et al. developed a class of receptor molecules, 55, 2(guanidiniocarbonyl)-1H-pyrroles, for the recognition of carboxylates in polar solvents (Figure 33a).209,210 A typical
Supramolecular polymers formed through the tweezer−pyrene complexation (K ≈ 104 M−1) on the basis of a ring−chain equilibrium mechanism. Concentration and temperature dependence effects were observed for these supramolecular polymers. Until now, not enough effort has been devoted to developing these tweezer−guest motifs as building blocks for supramolecular polymers. Compared to the rigid macrocyclic hosts, these noncyclic tweezers, on account of their flexible configurations, can sandwich a variety of guest moieties and overcome the limitations encountered by the macrocyclic hosts, such as a tedious synthesis, a low yield, poor solubility, and a limited number of guest species. Therefore, these tweezer− guest recognition motifs may represent a more versatile strategy for the fabrication of supramolecular polymers. Although this strategy is promising, in most cases, the binding strength is not strong enough. Because of the flexible configurations of molecular tweezers, they can adapt to various guest moieties and suffer the relatively low binding constant compared to macrocylic hosts. Therefore, to achieve a high DP, it is highly desirable to develop motifs with a high binding constant, which can really advance this driving force as a versatile strategy to construct functional supramolecular polymers. 2.5. Multiple Driving Forces
Figure 33. (a) (Guanidiniocarbonyl)pyrroles efficiently bind carboxylates in polar solvents due to a combination of ion pair formation and additional hydrogen bonds. (b) Supramolecular polymers constructed from self-assembly of a heteroditopic monomer based on two orthogonal binding interactions, metal−ligand interaction and ion pair interaction. Reprinted from ref 211. Copyright 2005 American Chemical Society.
As mentioned above, supramolecular polymers are usually constructed by employing one type of noncovalent interaction, while, in nature, the combination of multiple noncovalent interactions in an orthogonal manner in living creatures is ubiquitous. Inspired by biological systems, chemists have been trying to fabricate supramolecular polymers driven by multiple noncovalent interactions.201−208 The combination of different and orthogonal supramolecular building blocks not only enriches the library of supramolecular polymers, but also allows control of the supramolecular polymerization, achieving supramolecular polymers with well-defined structure and tailormade functionality. As already reviewed by Wang,202 the integration of multiple noncovalent interactions mainly involves metal−ligand coordination, multiple hydrogen bonding, host− guest interactions, and π−π interactions. Therefore, we just highlight several works which employ orthogonal multiple
representative is a self-complementary zwitterion that forms extremely stable dimers in DMSO with a binding constant of up to 1010 M−1. Such a high binding constant is ascribed to the strong ion pairing combined with multiple hydrogen bonds.211 On the basis of these studies, linear supramolecular polymers were constructed through the orthogonal combination of three binding interactions, ion pairing, hydrogen bonding, and metal−ligand interaction (Figure 33b).212,213 7214
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Chemical Reviews An elegant system was designed by Harada and workers,214 in which the supramolecular polymerization promoted by the orthogonal interactions of α- and β-CDs their guest molecules (Figure 34). It was found that
Review
polymer. A similar effect was achieved while using CB[8] to enhance the rigidity of the spacer, which was illustrated in Figure 26. An example of a pillararene-based alternating supramolecular polymer promoted by self-sorting was reported by Jia and Li et al.216 They designed a double-threaded dimer by means of an AB2-type heterotritopic pillar[5]arene derivative, containing a pillararene unit and two 4-bromobutyl guest units, which afforded the first example of [c2]daisy chains from heterotritopic monomers 58 (Figure 36). This [c2]daisy chain
cowas and (p-
Figure 34. Schematic diagram of supramolecular polymers constructed by 6-substituted β-CD and α-CD. Reprinted from ref 214. Copyright 2005 American Chemical Society.
aminohydrocinnamoyl)-β-CD formed an intramolecular complex in an aqueous solution. When adamantanecarboxylic acid (AdCax) was added to the solution, AdCax was included in the β-CD cavity in place of the aminocinnamoyl group. The aminocinnamoyl group was released from the β-CD cavity to be exposed in water. Then α-CD was added, which bound the cinnamoyl group to give a pseudo-[2]rotaxane. Then another stopper group, trinitrobenzenesulfonic acid sodium salt (TNB), was attached to the aminocinnamoyl group to obtain a [2]rotaxane. When AdCx was removed by extraction with organic solvents, TNB was incorporated inside the cavity of the adjacent β-CD molecule, thus forming supramolecular polymers. In this system, the α-CD acts as a rigid bulky group to promote the linear supramolecular polymerization. Supramolecular polymerization of a calixarene dimer and guest molecules can be boosted by the integration of α-CD on the polymer main chain (Figure 35).215 Compared with binary supramolecular polymer 57@56, the threading of α-CD on guest molecule 57 effectively enhanced the rigidity of the spacer, leading to a larger polymeric size and better size distribution of the ternary 57@α-CD-56 supramolecular
Figure 36. Chemical structures of the heterotritopic pillar[5]arene derivative and homoditopic pillar[5]arene derivative and the corresponding supramolecular polymer promoted by self-sorting of these monomers. Reprinted with permission from ref 216. Copyright 2013 Royal Society of Chemistry.
monomer has two additional free guest moieties at both ends and can be regarded as a homoditopic BB-type monomer, 59. While complexed with a homoditopic AA-type pillar[5]arene monomer, alternating supramolecular polymers containing [c2]daisy chains formed. These results have provided a facile route for fabricating novel [c2]daisy chain supramolecular copolymers. We have reported recently that supramolecular polymerization based on a CB host and guest could be promoted and controlled by the orthogonal interactions of CB[8], CB[7], and guest molecules.217 To this end, a monomer, 60, that contained one p-phenylene moiety in the middle flanked by two naphthalene moieties as end groups was designed and synthesized (Figure 37). The p-phenylene moiety selectively bound to cucurbit[7]uril (CB[7]), thus allowing free access to the naphthalene end groups for 2:1 complexation with CB[8]. An equimolar mixture of these three components resulted in the formation of a strong host−guest complex between CB[7] and the p-phenylene moiety to give a rigid and bulky linker between the two naphthalene units, thus serving to prohibit monomer cyclization and dimerization and facilitate the linear supramolecular polymerization.
Figure 35. Structural illustration of the building blocks and schematic representation of the comparison of binary and ternary supramolecular polymers. Reprinted with permission from ref 215. Copyright 2013 Royal Society of Chemistry. 7215
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Figure 37. Chemical structures of monomer 60, CB[7], and CB[8]. Schematic diagram of cucurbituril-based supramolecular polymerization promoted by self-sorting upon equimolar mixing of CB[7], CB[8], and 60 and controlled depolymerization of the supramolecular polymers in excess of CB[7]. Reprinted with permission from ref 217. Copyright 2014 John Wiley & Sons, Inc.
Figure 38. First example of supramolecular dendrimers formed by a homotritopic core unit, a heterotritopic AB2 branch unit, and an end cap.
3. SUPRAMOLECULAR POLYMERS WITH BRANCH-LIKE AND HYPERBRANCH-LIKE TOPOLOGY
heterodimer UC·64, while the dimer of tU remains unchanged. On the basis of these results, a uniform supramolecular dendrimer was obtained through the assembly of building blocks 64 and 65.220 According to the same strategy, uniform supramolecular dendrimers were also constructed by the orthogonal self-assembly of multiple hydrogen bonding and metal-coordination bonding221 or an isocyanuric acid−Hamilton receptor pair and a quadruple-hydrogen-bonding dimer.222 In summary, the advantages of supramolecular dendrimers are mainly due to their dynamics and reversibility. However, because of the dynamics of noncovalent interactions, it is not easy to keep supramolecular dendrimers with a uniform structure in solution. Although the strategy to utilize different interactions for each generation has resolved this conflict to some extent, it still involves tedious organic synthesis. Therefore, supramolecular polymers with a hyperbranched structure seem to be more versatile than supramolecular dendrimers.
3.1. Supramolecular Dendrimers
A broad definition of supramolecular dendrimers contains dendrimers that are constructed from the recognition of dendritic building blocks.218 However, in this review we focus on a narrow definition of a supramolecular dendrimer, where the repetition motif is no longer based on dendritic subunits. A supramolecular dendrimer constructed from a heterotritopic AB2 unit, which contains two Hamilton receptors as well as a complementary cyanuric acid substrate, was first reported by Hirsch et al.219 The formation of different generations of supramolecular dendrimers can be obtained by tuning the stoichiometric mixing of core unit 61, branching unit 62, and end cap 63 in a ratio of 1:(3 × 2n − 3):(3 × 2n) (Figure 38). This investigation, for the first time, shows the possibility of a programmed self-assembly of discrete supramolecular dendrimers from nondendritic units. However, in such a system, due to the identical recognition motifs, “wrong” combinations, such as a core unit combined with an end-cap unit, are also possible; thus, the final product may consist of structurally different assemblies. This problem can be solved when different and orthogonal recognitions are utilized for each generation.220−222 As shown in Figure 39, triurea (tU) derivatives and urea-containing calix[4]arenes (UCs) combine themselves to form dimers. With addition of tetratosylurea-substituted calix[4]arenes 64, the homodimer of UC is replaced by
3.2. Supramolecular Hyperbranched Polymers
In recent decades, a large number of supramolecular hyperbranched polymers have been reported, not only due to their easy preparation, but also because of their potential applications in many fields such as biochemistry, medicine, and materials science. As an early work, Meijer et al. introduced the classic multiple-hydrogen-bonding array, the Upy unit, to construct supramolecular hyperbranched polymers.223 A trifunctional copolymer of propylene oxide and ethylene oxide macromonomer terminated with Upy units was prepared, which then 7216
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Figure 39. Chemical structure and schematic illustration of building blocks for supramolecular dendrimers with a uniform structure in solution. Reprinted from ref 220. Copyright 2005 American Chemical Society.
preparation process and easier combination of multiple functions. Taking advantage of the easy modification of macrocyclic host molecules, e.g., crown ether229−231 and pillararene,232 AB2type monomers which simultaneously contain host and guest moieties are most commonly used to construct supramolecular hyperbranched polymers. Gibbson et al. reported the construction of supramolecular hyperbranched polymers from self-assembly of an AB2 monomer containing complementary recognition sites, a bis(m-phenylene)-32-crown-10 and two paraquat moieties.229 On account of the relatively low binding constant of this host−guest system, a high monomer concentration (>100 mM) was required for a high degree of supramolecular polymerization. Supramolecular hyperbranched polymers fabricated through the recognition between dibenzo[24]crown-8 (DB24C8) moieties and diakylammonium ion centers showed a pH response.230,231 A polymer structure formed under acidic condition, while it degraded in basic solution. Upon successive addition of acid and base to the polymer solution, reversible changes in the fluorescence intensity could be observed over at least 10 cycles.231 While incorporating metal complexes, a reversible luminescence supramolecular hyperbranched polymer was constructed, taking advantage of its pH response.230 Supramolecular hyperbranched polymers can be constructed by the combination of different noncovalent interaction modes. An interesting example comes from Wang et al., who constructed supramolecular hyperbranched polymers via a “tweezering-directed self-assembly” strategy, combining the inclusion mode from a crown ether−guest system and the tweezering mode from a tweezer-like molecule (Figure 40).233 It was shown that a
formed supramolecular hyperbranched polymers. The rheological behavior of this polymeric network, which shows a welldefined viscoelastic transition, differs from the rheological behavior of the phase-separated polymer networks, such as sugar-based natural materials. Another unique property of this supramolecular network is its reversibility, which was demonstrated by the addition of a monofunctional Upy unit acting as a chain stopper. The result is a degradation of the polymer network, resulting in a dramatic decrease of the viscosity. However, polymer networks based on weak secondary interactions do not show this response. The advantage of this supramolecular network based on multiple hydrogen bonding is the high “virtual” molecular mass in combination with an excellent processability, while the disadvantages could be the low creep resistance and the poor water resistance. In addition to multiple-hydrogen-bonding systems, a series of supramolecular hyperbranched polymers based on host−guest interactions have been reported. A typical system based on βCD and a variety of guest molecules have been widely studied, and functional groups are incorporated to endow supramolecular hyperbranched polymers with various functions, such as enzymatic catalysis and thermo- and photoresponsiveness.219,224−228 As one of the pioneering works, Tato et al. synthesized supramolecular hyperbranched polymers through the recognition of β-CD trimer and sodium deoxycholate224 or adamantyl dimer.225 Dendrimer-like self-assembly structures were observed under microscopy. Porphyrin- and telluriumcontaining supramolecular hyperbranched polymers based on a cycledextrin−adamantyl system appear to be artificial enzymes with both SOD and GPx activity.226 Compared with other methods, this artificial enzyme shows advantages of a simple 7217
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Figure 40. Schematic representation of the construction of a supramolecular hyperbranched polymer from host−guest and tweezering binding modes. Reprinted with permission from ref 233. Copyright 2014 Royal Society of Chemistry.
an azastilbene unit as a guest moiety (Figure 42).235 The transformation from supramolecular hyperbranched polymers
bis[alkynylplatium(II) terpyridine] tweezer, 66, could clamp an alkynylgold(III) diphenylpyridine guest, 67, leading to the formation of a supramolecular A2 monomer terminated by a crown ether. After the addition of a B3 secondary ammonium salt monomer, 68, a supramolecular hyperbranched polymer containing two kinds of binding modes was successfully constructed. We have studied the construction of hyperbranched supramolecular polymers driven by CB[8] host-enhanced π−π interaction. Considering that CB[8] can bind two guest molecules into the cavity, we just need to prepare a three-arm molecule, 69, which possesses one guest moiety (e.g., a naphthalene group) in each arm, to fabricate a hyperbranched supramolecular polymer in aqueous solution (Figure 41).234 CB[8]-based supramolecular hyperbranched polymers can be switched into covalent hyperbranched polymers by introducing
Figure 42. Illustration of the transformation from supramolecular hyperbranched polymers into covalent hyperbranched polymers by UV irradiation. Reprinted with permission from ref 235. Copyright 2014 Royal Society of Chemistry.
to covalently hyperbranched polymers can be achieved by the [2 + 2] cycloaddition of azastilbene groups in the cavity of CB[8] after UV irradiation. The switching can induce a change in fluorescence emission color from light orange to light blue. The method of combining supramolecular polymerization with photochemistry may be helpful in constructing hyperbranched polymers with advanced tunable optical properties. A unique water-soluble 2-dimensional supramolecular hyperbranched polymer was constructed from the recognition of a 4,4-bipyridin-1-ium (BP)-containing tritopic molecule and CB[8] (Figure 43).236 The rigid and steric BP molecule forces the formation of a single-layer 2D periodic hexagonal pore array in solution, which was characterized by X-ray scattering and various microscopies. The obtained 2D porous architecture represents a new type of supramolecular polymer, which exhibits periodic structural ordering similar to that of MOFs
Figure 41. Illustration of the formation of supramolecular hyperbranched polymers through the recognition of CB[8] and a naphthalene-containing three-arm monomer, 69. Reprinted with permission from ref 234. Copyright 2013 Royal Society of Chemistry. 7218
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molecular polymers. The ternary complex supramonomer was chosen because of the high binding constant between CB[8] and the N-terminal tripeptide group Phe-Gly-Gly and therefore to ensure the structural stability of the supramonomer throughout the polymerization process. On the basis of orthogonal processes of host−guest interaction and click polymerization, we demonstrated that a supramolecular polymer could be formed by mixing all of the moieties simultaneously in an aqueous solution; therefore, a “one-pot” strategy to simplify the fabrication of supramolecular polymers is further developed. Supramolecular polymers can also be constructed by noncovalent polymerization of supramonomers.238 Supramonomers were obtained by host−guest interaction between tripeptide derivative FGG-Azo and CB[8]. Then supramolecular polymers were formed spontaneously by mixing the supramonomers with bis(β-cyclodextrin)s in a molar ratio of 1:1 through the complexation of azobenzene groups and βcyclodextrin. It should be noted that the solubility of FGG-Azo and CB[8] is less than 0.1 mM at room temperature. However, once supramonomers are formed, the solubility is significantly enhanced and the concentration of the solution can be prepared as high as 4 mM, thus promoting the formation of a supramolecular polymer. In most cases, supramolecular polymerization is similar to step polymerization, meaning that the distribution of molecular mass is usually broad, which is hard to control. In the case of polymerization of a supramonomer, the uncontrollable noncovalent polymerization can be switched into controllable covalent polymerization; thus, supramolecular polymers being formed in this way may have a controllable polydispersity. It is highly anticipated that the concept of a supramonomer not only enriches the preparation methods for supramolecular polymers, but also gives higher flexibility in designing functional supramolecular polymers.
Figure 43. Schematic representation of the 2D honeycomb SOF (supramolecular organic framework). Reprinted from ref 236. Copyright 2013 American Chemical Society.
and covalent organic frameworks (COFs) and may offer unprecedented accessibility and processability of soft layered nanostructures. In addition to preparation of MOFs and COFs on the basis of coordination bonds and covalent bonds, this study opens a new horizon of supramolecular approach for fabrication of two-dimensional materials.
4. NEW STRATEGIES FOR CONTROLLABLE SUPRAMOLECULAR POLYMERIZATION 4.1. Polymerization of a Supramonomer
Supramonomer refers to a monomer that is fabricated by noncovalent interactions, but is able to polymerize using covalent or noncovalent polymerization methods.237,238 This is similar to the macromonomer concept in traditional polymer science. In contrast to the commonly used method, the concept of a supramonomer provides a new perspective for the fabrication of supramolecular polymers with controlled structures. Lehn et al. reported the synthesis of a supramolecular polymer by formation of covalent C−C bonds between the ditopic supramolecular building blocks undergoing C−C connection through aerial oxidation.239 We have demonstrated that supramolecular polymers can be constructed through click polymerization of a supramonomer (Figure 44).237 For this purpose, we prepared a complex of tripeptide derivative 71 and CB[8] as a supramonomer and then polymerized the supramonomer through click polymerization to obtain supra-
4.2. Self-Sorting Controlled Supramolecular Polymerization
Self-sorting has been used successfully to fabricate alternating supramolecular polymers. We wondered if it could be employed to promote and control the process of supramolecular polymerization. As shown in Figure 37, a ternary system including a guest molecule that could be recognized by CB[8] and CB[7] simultaneously and orthogonally was designed. The molar mass of supramolecular polymers could be controlled by tuning the ratio of CB[7] to monomer, not only during the polymerization process but also during the
Figure 44. Supramolecular polymer synthesized by click polymerization from a supramonomer. Reprinted with permission from ref 237. Copyright 2014 Royal Society of Chemistry. 7219
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depolymerization (Figure 45). When CB[7] was gradually added to the solution of Naph−Phen−Naph−CB[8], the
Figure 46. Schematic representation of the photocontrollable supramolecular polymerizations of monomers with different configurations of the stilbene moiety. Reprinted with permission from ref 244. Copyright 2013 John Wiley & Sons, Inc.
then to neutral form, resulting in the degradation of the supramolecular polymers.179 Tian et al. reported pH and redox dual-responsive supramolecular polymers based on the same host molecule, but a guest containing viologen and dimethylamino groups, as shown in Figure 47.246 These supramolecular polymers could reversibly respond to both electrochemical redox and pH variance, achieving switching between unimers and polymers. Although a vast number of works have focused on the construction of stimulus-responsive supramolecular materials, not enough attention has been paid to the controlled supramolecular polymerization by employing external stimuli. Most of the reported works mainly focus on the switch between polymerization and depolymerization, which are still in its infancy in controlled supramolecular polymerization. However, with deeper research, including further study of the kinetics and mechanisms of supramolecular polymerization under external stimuli, it is highly anticipated that we can push these methods toward stimulus-controlled supramolecular polymerization.
Figure 45. Ratio dependence of the molar mass of supramolecular polymers in the CB[7] titration experiments. Reprinted with permission from ref 217. Copyright 2014 John Wiley & Sons, Inc.
molecular mass of the supramolecular polymers increased first when the ratio was 1. A maximum of molecular mass (Mw = 9.7 × 104 g· mol−1, PDI = 1.5) was reached while all the components were equimolar. It is expected that different self-sorting systems could be utilized to achieve supramolecular polymerization in a controlled manner, therefore enriching the field of supramolecular polymer chemistry with important advances toward the realization of molecular mass and structural control. 4.3. Stimulus-Controlled Supramolecular Polymerization
One of the specific characteristics of supramolecular polymers is their response to external stimuli, such as pH, temperature, photoirradiation, and chemical or electrochemical redox.6,240 Under a specific stimulus, supramolecular polymers can achieve controllable conformations or polymerization/depolymerization switching and function adjustment. Here we mainly highlight some examples in controlling the construction of supramolecular polymers under external stimuli. Light stimulus is attractive as it allows remote activation and control without adding or generating extra chemicals.241,242 To study the photocontrollable self-assembly, supramolecular polymers comprised of stilbene as a photoresponsive moiety were reported by Yang et al.243,244 The self-assembly behavior and physical properties of the supramolecular polymers can be regulated by photoisomerization of the stilbene. For example, Upy-based supramolecular polymers with Z or E configurations of stilbene followed ring−chain equilibrium and isodesmic growth mechanisms, respectively. As shown in Figure 46, (Z)monomer formed cyclic dimers at low concentrations and linear supramolecular polymers at high concentrations, while (E)-monomer formed linear supramolecular polymers first and further supramolecular networks driven by π−π interactions between stilbenes as the concentration increased. Redox reactions are basic and complementary chemical reactions that have been widely used in modulating the selfassembly behavior of supramolecular polymers.6,240 Redoxcontrolled constructions of supramolecular polymers consisting of a viologen guest as a redox-responsive moiety were studied by several research groups.245,246 Liu et al. constructed psulfonatocalix[4]arene- and viologen-based supramolecular polymers in aqueous solution. Upon electrochemical reduction, the viologen converted from a cation to a radical cation and
4.4. Living Supramolecular Polymerization
Despite the good understanding of the supramolecular polymerization mechanisms,3 living supramolecular polymerization still remains an important challenge. Inspired by conventional living polymerization mechanisms, kinetic control over the initiation and propagation steps should be involved in living supramolecular polymerization. To this end, the nucleation−elongation processes in cooperative supramolecular polymerization may serve as the right candidate for living supramolecular polymerization. Although it is difficult to achieve, there have been successful demonstrations in recent years. To control the shape, size, and stability of supramolecular polymers in an aqueous environment, Meijer et al. proposed a strategy by tuning the balance between attractive forces (e.g., dipole interactions, π−π stacking, and solvophobic effects) and repulsive electrostatic interactions of a discotic amphiphile monomer (Figure 48).247 It was demonstrated that the polymerization mechanism could be switched from a cooperative nucleation−elongation mechanism to an isodesmic mechanism by increasing the peripheral Coulombic repulsions of the metal complexes, resulting in significantly different morphologies of the assemblies: from elongated rodlike columnar assemblies 73 to discrete spherical objects 74. More interestingly, cooperative assembly of 74 can be restored by increasing the ionic strength of the buffered environment, e.g., 0.5 M NaCl solution, leading to the formation of high aspect ratio rodlike supramolecular polymers. This approach, by 7220
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Figure 47. Reversible polymerization/depolymerization of supramolecular polymers induced by the stimuli of pH and electrochemical redox. Reprinted from ref 240. Copyright 2014 American Chemical Society.
Figure 48. Schematic representation of the fluorinated discotic amphiphiles 73 [M(III) = Gd(III)] and 74 [M(III) =Y(III)] and the self-assembly of discotic amphiphiles. The hydrophobic core (solid circle) directs the self-assembly into a helical architecture. The hydrophobic amino acid substituents (dashed line) in the second layer determine the stability of the helix. The peripheral hydrophilic metal complex groups induce a repulsive Coulombic force to the helix. Reprinted with permission from ref 247. Copyright 2010 National Academy of Sciences.
subtly tuning the balance between positive and repulsive interactions between building blocks, paves the way for controlled supramolecular polymerization. An elegantly designed system for living supramolecular polymerization based on the cooperative mechanism was reported by Sugiyasu and Takeuchi and co-workers.248 They have designed a porphyrin molecule bearing hydrogen-bonding moieties and long alkyl chains, which self-assembled through π−π stacking of the porphyrin planes and hydrogen bonding of
the amide groups (Figure 49). Two different types of aggregates for this porphyrin molecule were observed: one was kinetically trapped J-aggregates assembled through an isodesmic pathway, and the other was thermodynamicaly stable H-aggregates assembled through a nucleation−elongation pathway. The presence of a second off-pathway (J-aggregation into small clusters) aggregate led to the accumulation of a large amount of “dormant” J-aggregates. However, while a tiny amount of short H-aggregate seed was added to the solution of 7221
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qualitative information. Considering that we have reviewed the characterization methods for supramolecular polymers recently,4 we will make a brief introduction of the methods that we have discussed before, but focus on some new methods that were not covered well by our last review. 5.1. Size Exclusion Chromatography
Size exclusion chromatography (SEC), especially gel permeation chromatography (GPC), is a very mature and widely used characterization method for conventional polymers. However, a serious degradation effect induced by shearing forces in the column packing, accompanied by the nonspecific adsorption and dilution effect, makes SEC unsuitable for most of the supramolecular polymers. However, in some cases, especially those involving metal-containing or multiple-hydrogen-bonding arrays with a high binding constant, the application of SEC has achieved modest success.86,249 As was mentioned in section 2.2.1, Schubert et al. investigated the stability of MSPs with different metal ions when characterized by SEC.86 They found no relationship between the concentration and the chromatographic results for Ni2+−TPY complexes (log K = 21.8), but a degradation of polymer chains for Fe2+−TPY and Co2+−TPY systems, owing to their relatively low binding constants (log K(Fe) = 20.9, log K(Co) = 18.3). This study clearly demonstrated the degradation effect of SEC for supramolecular polymers. 5.2. Viscometry Figure 49. Schematic representation of porphyrin-containing molecule 75 and energetic landscape representing the pathway complexity in supramolecular polymerization of 75, illustrated on the basis of the thermodynamic parameters determined by van’t Hoff plots. Reprinted with permission from ref 248. Copyright 2014 Nature Publishing Group.
Viscometry is a classical method to characterize conventional polymers. The intrinsic viscosity increases with increasing molecular weight, and their relationship can be expressed by the empirical Mark−Houwink equation: [η] = KMα. In this equation, for a given polymer, both K and α are empirical constants which can be obtained from calibration experiments with a series of standard polymer samples with various molecular weights.250 However, it is difficult to find a suitable covalent model to obtain these parameters for the dynamic systems. Therefore, in most cases, this method is only used to study supramolecular polymerization in a qualitative way, for example, to deduce the critical polymerization concentration.37,162,166,251
J-aggregates as an initiator, these dormant J-aggregates transformed into fibrous H-aggregates with a narrow polydispersity (PDI = 1.1). AFM images demonstrated the living character of fiber growth with each addition batch to afford fibers of homogeneous length and width. This “nucleated polymerization with a competing off-pathway aggregate” is a potent methodology for realizing living supramolecular polymerization.
5.3. NMR Spectroscopy
NMR can be used to determine the Mn of a supramolecular polymer by end-group analysis based on properly simplified assumptions. For example, assume that complete complexation will cause a chemical shift, Δδ, and ΔδC at a certain monomer concentration. Defining p = Δδ/ΔδC, the DP can be easily calculated to be 1/(1 − p).165 DOSY is becoming a popular method for characterization of supramolecular polymers.174,197,252 The diffusion coefficient is inversely proportional to the size of supramolecular polymers. Furthermore, the size of supramolecular polymers can be qualitatively estimated according to the Stokes−Einstein equation. This relationship between the diffusion coefficient and the size of supramolecular polymers holds true for many systems. The shortcoming for this method is that the average molar mass of supramolecular polymers can only be roughly estimated, since the model for the Stokes−Einstein equation is not always true for different supramolecular polymer systems.
5. CHARACTERIZATION OF SUPRAMOLECULAR POLYMERS Supramolecular polymerization is essentially a self-assembly process with a certain thermodynamic equilibrium constant. Therefore, it is possible to estimate the average molar mass of supramolecular polymers through theoretical models. The DP can be simply estimated as DP ≈ (KaC)1/2, where Ka is the equilibrium constant and C is the total monomer concentration, based on the isodesmic model.2 However, for supramolecular polymerization systems that do not follow the isodesmic mechanism, in other words, the equilibrium constant Ka varies with the chain growth of a supramolecular polymer, it is difficult to express them by general fitting equations. From another point of view, it still remains a challenge to directly characterize the average molar mass of supramolecular polymers. Due to the dynamic nature of supramolecular polymers, many well-established characterization methods for conventional polymers may not work well. In most cases, only partial information can be obtained from a certain method. Therefore, it is necessary to combine various characterization methods with a proper theoretical estimation to obtain
5.4. AFM-Based Single-Molecule Force Spectroscopy
AFM-based SMFS, which can be employed to measure minute forces and to record exceptionally small distances simultaneously, has been a powerful tool in the study of single polymer chains.150,152,253−257 In a general SMFS experiment, a polymer 7222
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Figure 50. Supramolecular polymers formed through the host-enhanced charge-transfer interaction and their characterization by SMFS. Reprinted with permission from ref 150. Copyright 2010 John Wiley & Sons, Inc.
polymer chain can show force signals and the point at which the polymer chain picked up is out of control, it is still difficult to obtain precise information about the length of supramolecular polymers.
chain could be adsorbed onto the AFM tip as well as onto the substrate, thus forming the so-called polymer bridge between the AFM tip and the substrate. When the AFM tip is separated from the substrate, the polymer chain will be stretched and deflect the cantilever. The deflection of the cantilever and the displacement of the piezo will be recorded and converted to force−extension curves. In our previous work, we have developed a series of CB[8]containing supramolecular polymers.145 SMFS is an important tool to characterize these supramolecular polymers. For example, we designed a DADV monomer in which anthracene and viologen moieties were the electron donor and acceptor, respectively, as shown in Figure 50. With the electron donor and the electron acceptor encapsulated in CB[8], the monomers can be driven by the host-enhanced charge-transfer interaction to join together in a head-to-tail fashion to form the supramolecular polymers. Supramolecular polymers were physically adsorbed onto the substrate and then picked up by the AFM tip through a physical absorption force. In this system, two types of force−extension curves were obtained. In one type of force−extension curve, the force rose monotonically with increasing extension and then dropped to zero rapidly upon the rupture of the polymer bridge. Fitting the force−extension curves by the modified freely jointed chain (M-FJC) model gave a Kuhn length of about 2.2 nm, indicating that the supramolecular polymers are quite rigid. In the other type of force−extension curve, a long plateau was observed, corresponding to the detachment of the supramolecular polymer chain that adsorbs onto the substrate with a trainlike conformation. The lengths of the plateaus in all the force− extension curves were statistically analyzed, and the Gaussian fitting gives the most probable length as 60 nm. As shown in the examples above, SMFS can give direct evidence of the formation of a chainlike supramolecular polymeric structure. Moreover, the elasticity of supramolecular polymer chains can also be obtained from the force−extension curves. However, since only a sufficiently long supramolecular
5.5. Light Scattering
Light scattering, including static light scattering (SLS) and dynamic light scattering (DLS), is a well-established method in determining the molecular size and morphology of analytes. For SLS, information on the weight-average molar mass (Mw), radius of gyration (Rg), and second virial coefficient (A2) can be gained. In most cases, DLS is used to obtain the size distribution information from aggregates formed by supramolecular polymers. As the intensity of scattered light scales with the radius of the particle, r6, smaller analytes are mostly discriminated in batch light scattering, if larger aggregates are present in solution. This often results in a misleading size distribution and prohibits quantification of analytes. 5.6. Small-Angle Neutron Scattering
Small-angle neutron scattering (SANS) is a powerful tool and has been widely used in the study of materials with nanoscale, including polymers, colloids, and biological systems. In the regime where wave numbers are small compared with wideangle Bragg diffraction but relatively large on the scale of SANS, correlations between opposite interface segments are random and are averaged out. For smooth interfaces, the scattering intensity can be predicted by Porod’s law: I=
CP + BGD Q4
In this equation, I is the normalized scattering intensity, BGD is the background intensity of incoherent scattering, CP is the Porod constant, directly proportional to the surface area, and Q is the magnitude of the scattering vector [Q = (4π/λ) sin(θ/2), where λ is the neutron wavelength and θ is the angle of scatter]. Analysis of the SANS data can give information about the size 7223
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and shape of the polymers. For example, the value of Rg as well as the Porod exponent can be obtained. In the case of polymers with a good solvent, the Porod exponent is about 1.6. For a Θ solvent, the value is 2. For a poor solvent, the value is between 2 and 3, indicating a chain collapse of polymers. Besides conventional polymers, the method of SANS can be used in the characterization of supramolecular polymers.151,258 For example, in cooperation with Qiu, we used this technique to characterize the CB[8]-containing supramolecular polymers. The result confirmed the formation of supramolecular polymers with long linear chains (Table 5). Since water is not a good Table 5. SANS Data for a CB[8]-Containing Supramolecular Polymer concn/mM
Porod exponent
Rg/nm
0.1 0.2 0.3
2.5 2.6 2.8
163.0 167.2 112.4
solvent for these supramolecular polymers, the Porod exponent of the supramolecular polymers is around 2.5 in aqueous solution, indicating that the supramolecular polymers are collapsed coils in water.
Figure 51. (a) Comparison of the molecular mass/size detection range for different techniques including AF4. (b) Schematic representation of the principles of AF4.
We first used AF4 to characterize supramolecular polymers in a supramolecular polymerization system promoted and controlled through self-sorting.217 As shown in Figure 52, the
5.7. Asymmetric Flow Field-Flow Fractionation
Asymmetric flow field-flow fractionation (AF4), as one of the representatives of the field-flow fractionation technique, was introduced by Giddings and Wahlund in 1987.259 AF4 connecting in-line to ultraviolet, differential refraction, and multiangle light scattering (MALS) detectors provides information on the molar mass, polydispersity, size, and shape/conformation of the samples. Due to the lack of a stationary phase, it represents a gentle separation and characterization method, where nonspecific interactions and shear forces are reduced to a minimum, allowing for a broad separation range from nano- up to micrometers (Figure 51a). These advantages make AF4 a powerful and versatile technique in characterization of macromolecules and supramolecular assemblies.260−262 The separation in AF4 is achieved solely by a flow in an empty channel where a perpendicular flow force is applied (Figure 51b). The channel consists of two plates joined together, which are separated by a spacer. The bottom plate is porous, covered by a semipermeable membrane, which acts as an accumulation wall. The membrane with a typical cutoff of 5 or 10 kDa is permeable for the eluent, but impermeable for the polymer molecules and colloidal particles, and therefore keeps the sample in the channel so that it is directed by flow to the channel outlet. The eluent flow creates a parabolic flow profile within the channel, and the eluent moves more slowly closer to the channel walls compared to the channel center. During the transport of the sample by the eluent, a cross-flow is applied to drag the molecules/particles to the surface of the membrane. Meanwhile, the diffusivity acts as a counteracting force, until a steady-state distribution of the sample in the channel is reached. Smaller molecules/particles with higher diffusion coefficients move closer to the channel center, where the eluent flow is faster. The velocity gradient inside the channel separates the analytes according to their size in such a way that smaller molecules elute before the larger ones. This means that AF4 separation is the opposite of SEC separation, in which the large molecules elute first.
Figure 52. Ratio-dependent AsF-FFF elution curves of (a) Naph− Phen−Naph−0.5CB[7]−CB[8], (b) Naph−Phen−Naph−0.6CB[7]− CB[8], (c) Naph−Phen−Naph−0.8CB[7]−CB[8], and (d) Naph− Phen−Naph−1.0CB[7]−CB[8] obtained by the MALS detector. Reprinted with permission from ref 217. Copyright 2014 John Wiley & Sons, Inc.
intensity of the MALS signal increased and the peak shifted to the right when the molar ratio of CB[7] and Naph−Phen− Naph−CB[8] increased from 0.5 to 1.0, indicating higher molecular mass supramolecular polymers. The highest molecular mass of the supramolecular polymers was calculated to be 97 kDa with a polydispersity of 1.5. According to these results, AF4 coupled to different detectors can provide information on the molecular mass and polydispersity for supramolecular polymers. In contrast to individual light scattering, the prior fractionation by AF4 allows the investigation of complex heterogeneous and polydisperse mixtures. Although the universality for different supramolecular polymer systems needs to be explored further, this technique 7224
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Figure 53. Absorbance changes of an MSP 76 film coated on an ITO electrode in an argon-saturated acetonitrile solution of n-Bu4NClO4 (0.1 M) by stepping the potential (vs Ag/AgCl) from 0.0 to +1.6 V and back to 0.0 V. Reprinted with permission from ref 87. Copyright 2007 John Wiley & Sons, Inc.
6.1. Optoelectronic Materials
shows great potential in resolving the characterization of supramolecular polymers.
Electrochromic materials change their optical properties by application of an external electric potential. Electrochromic materials have a wide range of applications such as electronic papers and smart windows.267 Metal oxides such as WO3 and polymers such as Prussian blue, poly[3,4-(ethylenedioxy)thiophene] (PEDOT), or polyaniline (PANI) are among the known materials for electrochromic applications. However, the general drawbacks of these materials are their prohibitive costs, small optical contrast, and low switching speeds. Metallosupramolecular polymers may be a promising alternative electrochromic material which exhibits outstanding properties in performance, variability, and fabrication efficiency.63,268 Kurth et al. prepared a series of TPY-based MSPs with a functional group at the pyridine periphery close to the metal ion and studied the structure−property relationships of MSPs as electrochromic materials.87,269 The electrochromic nature of the MSPs is revealed by monitoring the absorbance as a function of the applied potential. As a representative example, Figure 53 depicts the spectral changes of MSP 76.87 Upon increasing the potential from 0.0 to +1.6 V, the absorbance of the polymer at ca. 585 nm decreases, and finally, the ITO slide is completely colorless at +1.6 V. Subsequently stepping back the potential leads to the re-emergence and, finally, full recovery of the absorbance. The substitution groups can influence the electrochromic properties of MSPs through steric or electronic effects. Electron-donating groups give rise to faster switching rates, while electron-withdrawing ones lower the switching rates and decrease the stability. Subsequently, they fabricated a TPY−Fe2+-based MSP film by a dip-coating process, which shows outstanding electrochromic properties.268 The MSP film can be reversibly switched from Fe2+ (blue) to Fe3+ (colorless) by applying a potential of 4.1 V vs Li/Li+. A very high optical contrast ΔT of 71% at a wavelength of 590 nm and a coloration efficiency of around 525 cm2 C−1 can be realized. The devices show a long-term stability of about 10 000 cycles, indicating that the TPY−Fe2+-based MSP is a very promising electrochromic material for future applications of smart windows. To use electrochromic materials in display devices, it is necessary to tune the color of the materials. The colors of MSPs can be easily adjusted by changing the metal species and/ or by modifying the organic ligands. For example, TPY−Fe2+based MSPs with pyridine groups bearing electron-releasing methoxy groups are blue powders, while those with pyridine groups bearing electron-withdrawing bromo groups are green.87 Normally, the color of Fe-based MSPs can be purple, blue, or
5.8. Other Methods
Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) is an important method to study the accurate molecular mass of polymers. However, it is difficult to study the molecular mass of supramolecular polymers because of their poor stability in the experimental condition. Since only short species or oligomers can be detected in most cases, MS is not a common method to characterize supramolecular polymers so far. Vapor pressure osmometry (VPO) is an old method to determine the molecular mass of polymers. The Mn , concentration, and vapor pressure can be related through Raoult’s law. Although it has been used in supramolecular polymer systems, VPO is not an appropriate method for polymers with molar mass higher than 30 kDa, and is hardly suited for characterizing charged metallopolymers. Analytical ultracentrifugation (AUC) is a useful technique commonly used for the characterization of proteins and protein complexes, as well as synthetic polymers, including polyelectrolytes, metallosupramolecular polymers, and so on.263−265 This method allows the direct determination of the average molar mass and polydispersity of polymers. For example, the AUC method was used to characterize block copolymers connected by metal coordination interactions, such as PS− [Ru]2+−PEO.266 Compared with other methods, samples are characterized in their native state under solution conditions. Therefore, no interactions with matrixes or surfaces and no dilution effect exists during analysis, which is particularly beneficial for supramolecular systems. Although it is still little used for supramolecular polymers, this technique is highly anticipated to be one of the options.
6. APPLICATION OF SUPRAMOLECULAR POLYMERS Polymers based on covalent bonds have been, are, and will be major materials. As a supplement to covalently attached polymers, the nature of noncovalent interactions gives the supramolecular polymers the ability to be dynamic and reversible. Therefore, these kinds of supramolecular polymers can be used as recyclable, degradable, stimulus-responsive, and self-healing materials. In addition, various moieties can be assembled during the process of supramolecular polymerization, leading to engineering or integration of functional materials. 7225
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green, that of Co-based MSPs can be yellow or orange, and that of Ru-based polymers is red (Figure 54).269 In addition, more
Figure 55. Ligands with diverse rigid linear π-conjugated spacers.
the substitution effect was the Fe2+ MSPs with bpy ligands substituted by triethylene glycol (TEG) chains. The Fe2+ ion is known to quench fluorescence, but the TEG-substituted Fe2+(TPY)2 MSP shows a retention of quantum yield that is nearly 3-fold higher than that of the unsubstituted analogue.91 The electro-optical properties of a series of Zn2+(TPY)2-based MSPs with diverse rigid linear π-conjugated moieties were also investigated (Figure 56).84,271 Tuning of the photophysical properties was enabled; thus, bright purple to green photoluminescent emission (quantum yields of 0.12−0.81) was observed, with the emission color strongly depending on the nature of the conjugated groups.84 Besides MSPs, other kinds of supramolecular polymers (e.g., host−guest systems) bearing chromophore230,272 or amine-rich273 moieties are also found to be potential photoluminescent materials. White organic light-emitting materials are of great interest both in the scientific and in the industrial communities. Usually, the white light results from a simultaneous emission of red, green, and blue chromophores or of two complementary colors (e.g., orange and blue). When polymeric materials are applied, two approaches are usually adopted: one is mixing of different polymers, and the other is synthesis of a single polymer bearing different chromophores. However, the first approach often suffers a phase separation, and the second one requires additional synthetic work for every single mixing ratio of the different chromophores in the materials. The use of a dynamic supramolecular polymer could combine the advantages of both approaches and diminish the disadvantages, because mixing of supramolecular homopolymers in a solvent will result in statistical copolymers. In an elegant study from Schubert,274 a series of Zn2+(bpy)2 MSPs with diverse central chromophores (Figure 57), which emitted blue, green, and red light, were deeply studied (Figure 58). The dynamic nature of the MSPs enables the systematic assembly of a library of statistical copolymers by simply mixing the respective homopolymer solutions. Depending on the ratios used and the consequent energy-transfer processes, the resulting emission colors can be tailored, for example, white light. These MSPs can be further utilized to assemble thin films of statistical copolymers in a simple and material-saving manner by inkjet printing.
Figure 54. Colors of MSPs with different TPY derivatives and metal ions. Reprinted from ref 269. Copyright 2008 American Chemical Society.
than two metal ions can be introduced into the polymers, such as Fe2+ and Ru2+, and the color of the MSPs can be tuned from purple to orange to light green at different potentials. Such a multicolor electrochromic change is the result of different redox potentials of iron and ruthenium ions.270 Photoluminescent polymers have received significant interest especially in the fields of organic light-emitting diodes (OLEDs) and plastic solar cells. Introduction of metal ions into conjugated polymers has been investigated with regard to higher electroluminescence quantum yields. However, the purification and processing of such polymers is a difficult task. Supramolecular polymers containing photoactive building blocks may be an option to overcome these difficulties. Würthner et al. synthesized perylene bisimide dyes bearing TPY ligands and assembled MSPs with Zn2+ and Fe2+ ions.83 It was found that the fluorescence quantum yield of the Zn2+based MSP was 0.61 in DMF, while that of the Fe2+-based MSP was quenched to 0.04 in CHCl3. This result shows that the luminescent properties of MSPs can be tuned by changing the metal ion species. The luminescent properties of the bis-TPY are profoundly affected by the nature of the functional groups at the peripheral pyridine and the spacers (Figure 55).84,88 In general, attachment of an electron-donating OMe group to the pyridine ring or extending appropriately the conjugation length of the spacers gives rise to an increased quantum yield, while electron-withdrawing groups, such as Br and CN, generally lead to a decreased emission efficiency.88 The λem can be tuned to span the region from ca. 370 nm to more than 500 nm, leading to different emission colors such as purple, bright blue, and bright green. Thus, these building blocks are promising candidates for the construction of metallosupramolecular light-emitting polymers. An interesting example of elucidating
6.2. Supramolecular Self-Healing Materials
The design of self-healing polymers also demands multiple healing cycles, thus requiring other strategies rather than just relying on covalent bondings. Self-healing materials based on supramolecular interactions stand a good chance of meeting such a requirement, therefore putting them on the frontline of 7226
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Figure 56. Schematic representation of the MSPs with diverse conjugated spacers. Reprinted with permission from ref 84. Copyright 2009 John Wiley & Sons, Inc.
Figure 57. Schematic representation of the synthesis of the Zn2+(bpy)2 MSPs with diverse chromophores. Reprinted with permission from ref 274. Copyright 2013 Royal Society of Chemistry.
at this temperature. Cooling to room temperature resulted in a film with the original properties.276 It is not necessary to use very strong hydrogen-bonding motifs, and even relatively weak hydrogen interactions are reasonably applicable in forming supramolecular networks. Leibler et al. prepared a supramolecular network by mixing ditopic and multitopic molecules, bearing (amidoethyl)imidazolidone 95, bis(amidoethyl)urea 96, and diamidotetraethyltriurea 97 side moieties which acted as multiple-hydrogenbonding cross-linkers (Figure 59).277 This rubber-like system shows recoverable extensibility up to several hundred percent and little creep under load. When broken or cut, it can be simply repaired by bringing together fractured surfaces to selfheal at room temperature. The process can be repeated many times with the extensibility well recuperated. Another example of a self-healing supramolecular network formed through weak multiple hydrogen bonding was reported by Binder.278 Bivalent poly(isobutylene)s (PIBs) 98 with
applications. The concept of supramolecular self-healing materials relies on the use of noncovalent bonds to generate reversibility and dynamic networks, which are able to heal the damaged sites.275 We therefore discuss supramolecular network formation based on hydrogen bonding, metal-coordination bonding, and π−π stacking in relation to a self-healing behavior. 6.2.1. Self-Healing Materials Based on Multiple Hydrogen Bonding. One of the most impressive examples of self-healing materials is commercially available under the brand name SupraB (by Suprapolix), based on Meijer’s quadruple hydrogen bond. Due to the thermoreversible hydrogen bonding of UPy groups, the material combines strong elastic behavior at room temperature (hydrogen bonds closed) and low viscous melt behavior at elevated temperatures (hydrogen bonds open).42 When the polymeric film was damaged by scratching, it spontaneously self-healed when heated to 140 °C, benefitting from the rather low melt viscosity 7227
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the battery. To overcome this challenge, the authors fabricated a self-healing electrode by coating silicon microparticles with a thin layer of a hydrogen-bond-directed self-healing supramolecular polymer network (Figure 61a,b). This hydrogenbonding network was modified from the classical synthetic route demonstrated by Leibler et al.277 The designed amorphous structure and low glass-transition temperature (Tg) allow the polymer chains at the fractured interfaces to rearrange, approach, and intermix, driven by the dynamic reassociation of hydrogen bonds at room temperature. These self-healing anodes have an excellent cycle life, which is 10 times longer than that of state-of-the-art anodes made from silicon microparticles (SiMPs) and still retain a high capacity (up to ∼3000 mA·h·g−1, Figure 61c). The similar supramolecular polymer networks composited with nickel microparticles have been used to fabricate selfhealing electronic skins, which show remarkable mechanical and electrical self-healing properties at ambient conditions (Figure 62).280 On rupture, the initial conductivity is repeatedly restored with ∼90% efficiency after a 15 s healing time, and the mechanical properties are completely restored after ∼10 min. The composite resistance varies inversely with the applied flexion and tactile forces and therefore acts as an excellent electronic sensor. These results demonstrate that the repeatable self-healing capability of natural skin can be mimicked in conductive and piezoresistive materials, thus potentially expanding the scope of applications of current electronic skin systems. 6.2.2. Self-Healing Materials Based on Metal-Coordination Bonding. Metallosupramolecular polymers 99 comprising a rubbery, amorphous poly(ethylene-co-butylene) core with 2,6-bis(19-methylbenzimidazolyl)pyridine (Mebip) ligands at the termini coordinated with Zn(II) or La(III) metal ions can be mended through exposure to light (Figure 63).107 On exposure to ultraviolet light, the metal−ligand motifs are electronically excited and the absorbed energy is converted into heat. This causes temporary disengagement of the metal−ligand motifs and a concomitant reversible decrease in the polymers’ molecular mass and viscosity, thereby allowing quick and efficient defect healing. The concept of photothermally induced healing of supramolecular materials seems to
Figure 58. (a) Mixing triangle of MSPs P2, P3, and P11. (b) Picture of the corresponding solutions in a quartz microtiter plate (λex = 365 nm). (c) Position of the photoluminescence (PL) obtained from wells in the Commission Internationale de l’Elcairage (CIE) color space. Reprinted with permission from ref 274. Copyright 2013 Royal Society of Chemistry.
different molecular masses were functionalized with barbituric acid. Rheology experiments in the melt state (20 °C, Figure 60a) showed a rubbery plateau at high frequencies and a terminal flow region in the low-frequency range, indicating the formation of dynamic supramolecular tie points between barbituric acid groups (Figure 60b). Small disks prepared from PIBs show self-healing within several hours at ambient temperatures after the two parts are brought into contact again (Figure 60c). The self-healing feature is particularly desirable for energy storage because the lifetimes of many rechargeable batteries are limited by the mechanical fractures over the cycling process. An outstanding application of self-healing supramolecular polymers in improving the cycle life of rechargeable lithium batteries was reported by Bao, Cui, and co-workers.279 A silicon anode, which has a theoretical specific capacity 10 times higher than that of conventional graphite anodes, suffers from extreme volume expansion by up to three charge−discharge processes. These volumetric changes can cause cracking and pulverization in the electrode, which irreversibly degrades the performance of
Figure 59. A mixture of fatty acid and triacid was condensed first with diethylenetriamine and then reacted with urea, giving a mixture of oligomers equipped with complementary hydrogen-bonding groups. The hydrogen bond acceptors are shown in red, donors in green. Reprinted with permission from ref 277. Copyright 2008 Nature Publishing Group. 7228
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Figure 60. (a) Frequency sweep measurement of a PIB bearing two barbituric acid groups. (b) Formation of a dynamic, reversible supramolecular network of a barbituric acid-functionalized PIB. (c) Self-healing experiment of a barbituric acid-functionalized PIB with Mn = 28 kDa. Reprinted with permission from ref 275. Copyright 2013 John Wiley & Sons, Inc.
Figure 62. Self-healing conductor based on a composite of self-healing polymer and metal particles. (a) Illustration of the self-healing composite. (b) Healed film being flexed to show its mechanical strength and flexibility after only 5 min of healing at room temperature. Reprinted with permission from ref 280. Copyright 2012 Nature Publishing Group.
be applicable to various supramolecular polymers with a binding motif that is sufficiently dynamic. Coordination between iron and catechol ligands has been correlated to the hardness and high extensibility of the cuticle of mussel byssal threads and proposed to endow self-healing properties. Inspired by this biological material, a simple method to control catechol−Fe3+ interpolymer cross-linking via pH was developed for preparing self-healing polymer networks with near-covalent elastic moduli (Figure 64).281,282 The uniqueness of this system is the ability of mono-, bis-, or triscatechol−Fe formation at different pHs, thus providing the ability of crosslinking control without Fe3+ precipitation. The ultrahigh binding constant of tris- and biscatechol−Fe3+ complexes (K ≈ 1037−1040) endows the strength of metal−ligand bonds as high as covalent bonds; moreover, metal−ligand bonds can
Figure 61. (a) Schematic illustration of the design and behavior of selfhealing electrodes. (b) Chemical structure of the self-healing supramolecular polymer. (c) Capacity retention of SiMP electrodes with different polymer additives, including the SHP (self-healing polymer)/CB composite and conventional polymer binders. The SiMP electrode with SHP showed a much longer cycling lifetime than that of the conventional polymer. Reprinted with permission from ref 279. Copyright 2013 Nature Publishing Group.
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Figure 63. Mechanism and synthesis of photohealable metallosupramolecular polymers. Reprinted with permission from ref 107. Copyright 2011 Nature Publishing Group.
Figure 64. pH-dependent stoichiometry of Fe3+−catechol complexes. Reprinted with permission from ref 281. Copyright 2011 National Academy of Sciences.
Figure 65. (a) Polyimide copolymer containing electron-deficient naphthalene diimide units and pyrenyl-end-capped polyamide bearing electronrich pyrene groups. (b) Folding of the two polymers due to π−π stacking. Reprinted with permission from ref 284. Copyright 2009 Royal Society of Chemistry.
The same strategy was extended by combining π−π interactions between a polyimide copolymer bearing naphthalene diimide units and a pyrenyl-end-capped polyamide, with hydrogen bonding between urea groups near the pyrene groups. Upon mixing these two polymers, the resulting material again reveals triggered self-healing at elevated temperature with high healing efficiencies.
spontaneously re-form after breaking, thus healing the damage of the material. 6.2.3. Self-Healing Materials Based on π−π Interactions. Self-healing supramolecular polymers based on π−π interactions were first reported by Burattini et al.283−286 As shown in Figure 65, a supramolecular network was achieved by employing chain-folding polyimide (electron acceptor 100) and pyrenyl (electron donor 101) end-capped chains with polyamide as a spacer.284 The Tg of the network can be tuned to achieve self-healing at a relatively wide temperature range (∼50−100 °C). A film with deep red color was obtained by solution casting, which could be rehealed by simply pressing the broken parts together and heating to 80 °C. Upon heating, the stacking interactions were interrupted, enabling destruction of the supramolecular network structure and thus healing of the breakage. A 100% of recovery of the tensile modulus was achieved, and the healing cycle could be repeated several times.
6.3. Biomedical Applications
Supramolecular polymers can be endowed with unique properties, such as biodegradation, responsiveness to various biological stimuli, and easy incorporation of bioactive components, thereby showing great potential for application in the biomedical field. Although a vast number of supramolecular polymeric assemblies have been studied for biomedical applications,287 we just select several examples of typical supramolecular polymers and illustrate their potential 7230
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Figure 66. (a) Modular approach for preparing bioactive supramolecular materials. (b) Fibroblast cell adhesion and spreading in vitro on different drop-cast films consisting of mixtures of PCLdiUPy with both UPy-GRGDS and UPy-PHSRN (left panel) or PCLdiUPy alone (right panel) after 48 h for cell culturing without fetal bovine serum (FBS). (c) In vivo behavior of bioactive supramolecular polymeric materials. Solution-cast supramolecular PCLdiUPy films with UPy-GRGDS and UPy-PHSRN peptides (left panel) and without peptides (right panel) were subcutaneously implanted in mice. The magnification of the enlargement is 400×. Reprinted with permission from ref 288. Copyright 2005 Nature Publishing Group.
Figure 67. Structure of multiple-hydrogen-bonded supramolecular polymers and the corresponding hydrogel designed for protein delivery. Reprinted with permission from ref 291. Copyright 2012 John Wiley & Sons, Inc.
fibroblase cells to the supramolecular polymeric materials. An even more striking effect is seen in vivo where the formation of single giant cells at the interface between bioactive material and tissue is triggered. Protein delivery has been regarded as a safe and efficient method for curing diseases. Supramolecular hydrogels are regarded as emerging candidates for encapsulation and release of proteins. Meijer and co-workers reported a series of transient supramolecular networks based on multiple-hydrogen-bonded supramolecular polymers 102 (Figure 67), which display a
applications in the fields of tissue engineering, protein delivery, drug delivery, gene transfection, and bioimaging. Hydrogen-bonded supramolecular polymers have shown their advantages and potential in tissue engineering.288−290 These supramolecular polymers are suitable for bioactive materials owing to their processability, biodegradation, and biocompatibility. Meijer and co-workers fabricated UPy-based supramolecular polymers by simply mixing UPy-functionalized polymers with UPy-modified bioactive molecules (Figure 66).288 The in vitro results showed strong, specific binding of 7231
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Figure 68. (a) Chain-extended UPy-modified hydrogelators 103 and 104 and end-functionalized UPy-hydrogelator 105. (b) Macrophage infiltration in the cortex and capsule under the implantation site. Tissue slices were stained for the macrophage marker ED1 15 days after implantation. Reprinted with permission from ref 292. Copyright 2012 Elsevier Inc.
example of such a kind of supramolecular polycations as a nonviral vector for gene delivery.293 This class of cationic supramolecular polymers was constructed by the host−guest interaction between a ferrocene dimer and a β-CD dimer and had the ability to effectively condense DNA and rapidly release DNA triggered by H2O2 (Figure 69). A moderate gene transfection efficacy (∼106 RLU mg−1 of protein) in COS-7 cells was achieved. However, a much lower transfection efficacy
nonlinear behavior in their formation as well as in their selfhealing and erosion properties, demonstrating an ideal material for protein delivery in regenerative medical applications.291 As proof-of-concept, a bioactive, bone morphogenetic protein was incorporated as cargo into these transient supramolecular networks and then was implanted under the kidney capsule of mice. The supramolecular polymer could not be macroscopically detected by eye in the explanted kidneys after 7 days, indicating that they have eroded and that the proteins are delivered. It is expected that this study provides a modular approach in the field of regenerative medicine. Besides protein delivery, a similar supramolecular polymer system has also been utilized for in vivo intrarenal drug delivery.292 Dankers et al. synthesized two different classes of supramolecular hydrogelators through linking the UPy units to PEG chains by two methods: chain-extended hydrogelators containing UPy moieties in the backbone (103 and 104) and bifunctional hydrogelators end-functionalized with UPy units (105) (Figure 68). Molecules 103 and 104 formed strong, shape-persistent, and slow-eroding hydrogels, which were suitable for long-term intrarenal delivery of organic drugs. In comparison, molecule 105 formed a weaker, elastic, and fasteroding hydrogel, which was suitable for short-term, fast delivery of drugs to the kidney cortex. The favorable biological behavior of these supramolecular hydrogels makes them exquisite candidates for subcapsular drug delivery and opens up a new way for intrarenal therapy. The biomedical applications of linear supramolecular polymers constructed from small-molecule building blocks have rarely been studied, owing to the fact that they are highly susceptible to depolymerization under exposure to external stimuli. As a preliminary attempt, Zhu et al. reported an
Figure 69. Schematic representation of a cationic supramolecular polymer constructed via host−guest interactions between a ferrocene dimer and a β-CD dimer as well as their H2O2-induced pDNA release behavior. Reprinted with permission from ref 293. Copyright 2013 Royal Society Chemistry. 7232
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(∼103 RLU mg−1 of protein) in cancer cells was obtained, because the excess of H2O2 in cancer cells triggered the faster and earlier DNA release. Therefore, cationic supramolecular polymers with a higher binding constant (Ka ≥ 106 M−1), higher molecular mass (DP ≥ 40, Mn ≥ 200 kDa), and higher surface charge density are suggested to improve the transfection efficiency. Supramolecular polymers also have shown their potential application in bioimaging and diagnosis. Liu et al. reported a supramolecular polymer based on host−guest interactions between a β-CD dimer, 106, and a Mn3+−porphyrin derivative, 107, which features noncytotoxic and efficient magnetic resonance imaging (MRI) enhancement properties (Figure 70).294 In in vitro MR imaging characterization, a considerably
7. CONCLUSIONS AND OUTLOOK In this review, we have summarized various driving forces for supramolecular polymerization, outlined the preparation of supramolecular polymers with different topologies, discussed the latest advances for controlled supramolecular polymerization, gave a brief overview of the characterization methods of supramolecular polymers, and highlighted the potential applications of supramolecular polymers. With great advances in the past two decades, it has been witnessed that supramolecular polymers have grown into an important and interdisciplinary field that has drawn increasing interest from academic and industrial communities. Although various noncovalent interactions have been incorporated into fabricating supramolecular polymers, it is still necessary to develop new driving forces with a high binding constant to expand the library of supramolecular polymers. Compared with the vast number of linear main chain supramolecular polymers, other topologies of supramolecular polymers such as side chain, branched, and hyperbranched copolymers295 etc. should be paid more attention. To construct supramolecular polymers with a high degree of polymerization, several principle rules should be followed. A high binding constant is the primary request for achieving supramolecular polymers with a high degree of polymerization. For systems with relatively low binding constants, a high concentration should be a good strategy to inhibit oligomerization and cyclization, thus increasing the degree of polymerization. In addition to these strategies, a careful and rational design of monomers, especially the linkers of ditopic monomers, is another key point. A fundamental principle is to design linear monomers with rigid or bulky linkers, which can effectively hinder the cyclization of oligomers. For AA/BB-type monomers, a mismatch of length between two linkers for AA and BB monomers also works well in preventing dimerization. For host molecules with poor solubility, e.g., cucurbituril, ABBA-type guest monomers combined with electrostatic repulsive force have been demonstrated to be outstanding candidates with optimized architecture. As one of the challenges, the characterization of supramolecular polymers has made significant progress in recent years. New methods, such as SANS and AF4, have shown their power and potential in the characterization of supramolecular polymers. It is worth mentioning that the AF4 system, taking advantage of convenience, rapidness, and effectiveness with abundant information, is anticipated to be a universal characterization tool for supramolecular polymers as well as GPC for conventional polymers. Supramolecular polymers with controlled structures are of great importance for the study of the structure−function relationship, which is the cornerstone of supramolecular polymer materials with highly customized functions. Although it is a great challenge, we are on the way. Several exciting breakthroughs have been made under the endeavor of scientists. With the deep study of the mechanism and kinetics of supramolecular polymerization,296,297 we have reason to believe that more and more approaches can be developed in achieving controlled supramolecular polymerization. Undoubtedly, the application of supramolecular polymers is the impetus for the advance of this research area. So far, multiple-hydrogen-bond-based supramolecular polymers and metallosupramolecular polymers have had a strong impact on material science, while the application study of supramolecular polymers based on other driving forces still needs to
Figure 70. (a) Schematic representation of supramolecular polymers based on host−guest interactions between a β-CD dimer and a Mn3+− porphyrin derivative. (b) Representative 2D coronal T1-weighted MR images of the mice at preinjection and 2, 5, 10, 20, and 25 min after intravenous injection of supramolecular polymer agents at 0.03 mmol of Mn/kg. Reprinted from ref 294. Copyright 2013 American Chemical Society.
enhanced T1 relaxation was observed by the supramolecular polymer compared to the commercially available Gd contrast agent. Further in vivo MR imaging investigation in mice revealed prominently positive contrast enhancement of the supramolecular polymer within the blood, kidney, and urinary bladder of the mice. 7233
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be enhanced. In addition, we need to further extend more application areas, such as biomedical materials and smart responsive materials, to inject new vigor into this emerging research field.
AUTHOR INFORMATION Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest. Biographies
Zhiqiang Wang is a professor in the Department of Chemistry of Tsinghua University, Beijing, China. He received his B.Sc. in Organic Chemistry, M.Sc. in Physical Chemistry, and Ph.D. in Polymer Chemistry and Physics at Jilin University. He has been working on interfacial self-assembly and supramolecular polymers.
Liulin Yang received his Ph.D. in Polymer Physics and Chemistry at Xiamen University under the supervision of Prof. Yanming Dong in 2011. His graduate work concerned the study of dendronized carbohydrate liquid crystals. In 2012, he joined Prof. Xi Zhang’s
Xi Zhang is a professor in the Department of Chemistry of Tsinghua University, Beijing, China. He received his B.Sc. in Analytical Chemistry and M.Sc. and Ph.D. in Polymer Chemistry and Physics at Jilin University under the supervision of Prof. Jiacong Shen. During his Ph.D. study, he spent more than one year at the Institute of Organic Chemistry, University of Mainz, Germany, as a joint-training Ph.D. student under the supervision of Prof. Helmut Ringsdorf. He joined the Department of Chemistry at Jilin University as a lecturer in 1992 and was then promoted to be a professor in 1994. He moved to Tsinghua University in late 2003. He has been recognized by various honors and awards, such as the National Natural Science Award (2004), Member of the Chinese Academy of Sciences (2007), and AkzoNobel Chemical Science Award (2010). His main scientific interests are in the areas of the cutting edge of supramolecular chemistry and polymer chemistry, including supra-amphiphiles, supramolecular polymers, Se-containing polymers, organized molecular films, and single-molecule force spectroscopy.
group as a postdoctoral fellow at Tsinghua University. His research interests mainly focus on the hierarchical and controllable selfassembly of supramolecules, including supra-amphiphiles and supramolecular polymers.
ACKNOWLEDGMENTS We gratefully acknowledge the comments and advice of Dr. Oren A. Scherman, Zehuan Huang, Yunhao Bai, E. I. M. Hicham, and Qiao Song. This work is financial supported by the Key Projects of the National Natural Science Foundation of China (NSFC) (Grant 21434004), the National Basic Research Program of China (Grant 2013CB834502), and the Foundation for Innovative Research Groups of the NSFC (Grant 21121004).
Xinxin Tan received his B.Sc. degree in 2010 from the Department of Chemistry, Tsinghua University, and his undergraduate work was performed in the group of Prof. Xi Zhang. In the same year, he started his Ph.D. research in the same group. His research interests mainly focus on supramolecular polymers and AFM-based single-molecule force spectroscopy. 7234
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