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Cite this: DOI: 10.1039/c4sm01684j

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Metallo/clusto hybridized supramolecular polymers Haolong Li and Lixin Wu* The introduction of metal centers to a supramolecular polymer system is an important approach to fabricate hybrid supramolecular polymers with synergistic properties between their inorganic and organic components, which is mainly realized through two strategies: one is the embedment of metal ions through metal–ligand coordination to form metallo-supramolecular polymers (MSPs); the other is using metal-containing clusters as hybrid building blocks to prepare clusto-supramolecular polymers (CSPs). The available paradigms of MSPs and CSPs not only exhibit the unique functions of metal centers but also hold the good processing ability and the stimuli-responsibility of dynamically bonded polymeric

Received 30th July 2014 Accepted 15th September 2014

structures, thus representing a new class of hybrid soft materials. In this review, the development and

DOI: 10.1039/c4sm01684j

recent progress of MSPs and CSPs are discussed in detail, including their structure design, synthetic procedures and related properties. Finally, challenges and potential areas in metal-containing

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supramolecular polymers are outlooked.

1. Introduction Supramolecular polymers,1 refer to the polymeric assemblies formed through the noncovalent interactions of monomers, such as hydrogen bonding,2,3 host–guest recognition,4–6 metal– ligand coordination,7 p–p stacking,8 electrostatic interaction,9 and charge transfer interaction.10 Due to the dynamic binding feature, the assembly and disassembly processes of

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, China. E-mail: [email protected]

Haolong Li was born in 1981 in Jilin, China. He received his B.S. (2003) and Ph.D. (2008) from Jilin University under the supervision of Professor Lixin Wu. Aer a postdoctoral training from 2009 to 2011 as an Alexander von Humboldt Scholar at the Max Planck Institute for Polymer Research, working with Prof. Christoph Bubeck and Prof. Klaus M¨ ullen, he joined Jilin University as an associate Professor in 2011. His research interests focus on organic–inorganic hybrid supramolecular polymers and cluster-functionalized polymer nanocomposites.

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supramolecular polymers can occur reversibly under certain external stimuli, which enable supramolecular polymers as typical so matter. In the past two decades, supramolecular polymers have attracted increasing attention, because they not only bring an innovative concept to design new supramolecular materials, but also exhibit potential applications in self-healing, microelectronic as well as biomedical areas.11,12 However, most of the available supramolecular polymers are formed of pure organic monomers, the organic–inorganic hybrid supramolecular polymers are less studied, which may be due to the

Lixin Wu received his Ph.D. degree in 1993 from the Institute of Theoretical Chemistry at Jilin University in Changchun, China. Then, he took the experience of postdoctoral research at Changchun Institute of Physics (CAS), China, and the Research Institute for Electronic Sciences at Hokkaido University in Japan. Dr Wu became full professor at the year of 2000 and worked at the State Key Lab of Supramolecular Structure and Materials at Jilin University. His research interest concentrates on the self-assemblies and structures of polyoxometalate-based supramolecular complexes, hybrid liquid crystal, surface patterning related colloid and interface phenomena.

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difficulty in synthesizing hybrid building blocks with inorganic motifs. In conventional polymer chemistry, the incorporation of metal centers is an important approach to create hybrid macromolecular structures with a synergetic property of organic and inorganic parts, which is normally realized through the covalent embedment of metal atoms or metal-containing clusters in polymer backbones.13–15 Recently, this concept has been extended to the supramolecular polymer system. Since metal ions possess diverse properties in electronics, optics, magnetics and catalysis, their incorporation brings a variety of features and leads to functional metallo-supramolecular polymers (MSPs).16–22 Furthermore, precisely tailoring the structures of ligands and choosing different metal ions afford a large topological diversity of the MSPs. On the other hand, structurally well-dened metal-containing clusters, as the assemblies of multicomponent metal ions, normally exhibited more abundant properties than a single metal ion. The incorporation of metal-containing clusters to covalent polymers to prepare polymer nanocomposites has been widely explored, for example, to obtain cluster-reinforced polymer materials or to improve the optical performance of polymer matrices.14,15 However, the research combining cluster chemistry with supramolecular polymers is still in it is infancy and only a few studies have been reported. Similar to the denition of MSPs, here we use clusto-supramolecular polymers (CSPs) to describe the supramolecular polymers containing clusters as hybrid centers. The localization of metallo/clusto centers in the different positions of the backbone of supramolecular polymers leads to different chain structures, which can be mainly classied into linear and branched types, as shown in Scheme 1. These structures are similar to those of covalent polymers, but their dynamic binding feature and hybridized compositions provide many unique properties beyond covalent polymers.

Review

In this review, we will introduce the available metallo/clusto approaches that have been successfully used to fabricate metalcontaining hybrid supramolecular polymers. The concept of the structure design of these polymers and their structurally related properties will be discussed in detail. Finally, the potential applications and future challenges in this area will be outlooked.

2. Metallo-supramolecular polymers (MSPs) The incorporation of metal centers into polymeric structures is an important approach to obtain hybrid polymer materials, which can readily combine the good processing ability of polymers and the versatile functions of metal components. According to the binding stability of metal centers, the metalcontaining polymers are mainly divided into metallo-polymers and metallo-supramolecular polymers (MSPs). The former are based on the strong covalent bonds that result in irreversible binding; while the MSPs are formed by noncovalent coordination interactions, leading to reversible binding structures. Generally, there are three strategies to fabricate MSPs: (i) the connection of preformed metal–ligand complexes through an organic reaction; (ii) the formation of polymer chains through the coordination interaction between metal ions and organic ligands; (iii) the coordination of metal ions with a preformed polymer containing chelating ligands. In the rst two strategies, metal ions act as the joints in the main chains of MSPs; as for the third one, metal ions are modied on the side chains of MSPs like pendants. The exibility in varying the structure of ligands and the position of metal ions offers a large structural diversity of MSPs. Given that several excellent reviews about MSPs are available,13,16–22 herein we focus on the MSPs containing metal–ligand joints in their main chains and discuss them according to their topological structures. 2.1

Scheme 1 Schematic illustration of the metallo/clusto hybridized supramolecular polymers with linear and branched structures.

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Linear structures

Most of the reported linear MSPs possess a chain-extended structure consisting of ditopic ligands and metal ions. The early attempts started from the use of monodentate pyridine–metal interaction which has been widely applied in the construction of solid state coordination polymers.23 However, the interactions between pyridine and metal ions are not strong enough to form MSPs with typical polymer properties. In this context, other ligands with high binding constants were explored in succession. Serrano and coworkers synthesized a series of metal-containing liquid-crystal MSPs by the condensation of a Schiff-base Cu(II) complex and acid dichlorides. The MSPs were soluble in chlorinated solvents.24 The Mn obtained from vapor pressure osmometry (VPO) corresponded to a relatively low degree of polymerization (DP) of 5 to 6. Shortly aer that, Eisenbach and Schubert reported the fabrication of a MSP by the coordination of Cu(I) ions with a bipyridine (abbreviated as bpy)-ended polyether. The MSP exhibited an increased viscosity in comparison with the pristine ligand. Gel permeation chromatography (GPC) results showed a 3-fold increase of the molar

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Review

mass aer the complex formation.25 Although these early MSPs are in the oligomeric region, their design concept initiated the further improvement of MSPs with a denite polymeric structure. The rst well-dened MSP was pioneered by Rehahn group, which was based on a ditopic ligand bearing two end-capped phenanthroline groups, as shown in Fig. 1.26 The ligand was able to coordinate with Cu(I) or Ag(I) ions in a nonpolar solvent like tetrachloroethane, forming extended MSPs with a highmolecular-mass (DP $ 30). However, the MSPs decomposed to low-molecular-mass aggregates in the presence of small amounts of coordinating solvents such as acetonitrile due to ligand exchange. To fabricate more stable MSPs, Rehahn group incorporated Ru(II) complexes with high binding constants as the joints in the main chain of MSPs.27,28 Their strategy combined a bitopic ligand tetrapyridophenazine and a 2,20 -bpy based complex Ru(bpy)Cl3 as “metal monomer” which contains a bidentate ligand. The formed MSPs have a rigid main chain and high overall charge, yet are readily soluble in a variety of organic solvents and also in pure water. When the substituted group R at the bpy unit was phenyl group, the Mw reached 47 000. On the other hand, Ru(II)–polypyridine complexes can be directly used as monomers for condensation polymerization due to their high chemical stabilities.29–31 MacDonnell developed a stereospecic synthesis strategy by using two enantiomerically pure Ru(II)–tris(bpy) complexes as monomers.32 The bridging of these complexes resulted in the formation of soluble chiral MSP with a DP of 13 to 15 and a Mn of 12 000 to 14 000. 2,20 :60 ,200 -Terpyridine (abbreviated as tpy),33 is the most frequently used ligand for the construction of MSPs, in particular for linear MSPs. The three nitrogen atoms on tpy enable it to act as a tridentate ligand to form stable complexes by binding a broad variety of transition metal ions.34,35 Furthermore, the substitution chemistry of the 40 -position of the tpy unit is very mature, which offers the possibility to modify it with extended chains, bridging spacers and even cross-linked cores to form different ligands for MSPs. The basic concept of using tpy-based polytopic ligands as building blocks to fabricate MSPs was introduced by Constable and Cargill Thompson.36 They presented that tpy units linked by suitable spacers provide a powerful route to designed co-ordination polymers. Kurth and coworkers developed the hierarchical selfassembly chemistry of simple tpy-based MSPs by using them as

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metallo-supramolecular polyelectrolytes.37 Due to the high charged density, these MSPs are well soluble in aqueous solution and can be alternatively assembled with negatively charged polyelectrolyte by layer-by-layer technique, generating multilayered thin lms with a smooth and parallel interface.38 Moreover, upon replacing the initial counterions of the MSPs by anionic amphiphilic molecules like dihexadecylphosphate (DHP), polyelectrolyte–amphiphile complexes (PACs) were prepared. The resulting PACs have a stratied architecture where the MSP chains are embedded in between the amphiphile layers,39 as shown in Fig. 2a. Due to the hydrophobic surface, the PACs are soluble in common organic solvents, forming Langmuir monolayers at the air–water interface which are readily transferred onto solid substrates to attain highly ordered LB lms. Additionally, straight and epitaxially oriented chains of PACs containing rigid ditopic ligands were obtained on long alkane modied graphite substrates, offering an entry

Schematic representation of (a) the formation route of PAC and (b) the structure induced reversible spin transition of PAC under the variation of temperature. Reproduced with permission from ref. 42. Copyright ACS Publishing, 2005.

Fig. 2

Fig. 1 Schematic representation of the formation of MSPs based on the coordination of a ditopic phenanthroline ligand and Cu(I) or Ag(I) ion. Reproduced with permission from ref. 26. Copyright RSC Publishing, 1996.

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to fabricate hybrid devices in the nanoscale.40 The coexistence of rigid-rod polymeric backbones and the exible side chains endows PACs thermotropic liquid crystal properties.41 Importantly, the phase transition could be used to adjust the magnetic properties of metal ions like Fe(II) and Ni(II), giving rise to a responsive spin-crossover behavior, as shown in Fig. 2b.42,43 Rehahn and coworkers synthesized a soluble Ru(II)–tpybased MSP following two different routes,44 as shown in Fig. 3: (i) the coordination of a telechelic tpy-capped ligand with an activated Ru(II) species; (ii) the covalent connection of preformed bifunctional Ru(tpy)2 complexes and bridging units by a Pd-catalyzed coupling condensation. Viscosity and 1H NMR measurements indicated that the DP of the MSP obtained by the former route reached 30, while the latter only produced oligomeric compounds. Anyway, the substitution of two hexyl chains on the rigid backbone of this MSP offered an important contribution on its good solubility. In comparison with the rigid rod-like spacers mentioned above, the incorporation of exible oligomeric or polymeric chains to bridge the tpy groups is more reasonable to realize MSPs with typical polymer properties. Schubert and coworkers carried out systematic studies in this area. They synthesized a series of tpy-capped ditopic ligands containing different lengths of poly(ethylene glycol) (PEG) spacers. The coordination of these ligands with metal ions like Fe(II) produces water soluble MSPs, as shown in Fig. 4.45,46 Besides, they also present a delicate

Review

strategy to fabricate block MSPs by using two tpy-ended monotopic ligands containing different polymer chains to coordinate with one metal ion.47 When the two polymer chains had distinct hydrophobicity, the block MSPs exhibited an amphiphilic character and could self-assemble into micelles like covalent block copolymers.48,49 Moreover, phase separation easily occurred in the block MSP lms, which has been successfully used to construct nanoporous structures, as shown in Fig. 5.50,51 The incorporation of functional groups into the spacer part of tpy-based ditopic ligands can offer functions to the nal MSPs. W¨ urthner and coworkers reported the synthesis of a uorophore perylene bisimide bridging ligand and used it to coordinate with Zn(II) to form a MSP with high luminescence.52 Due to the high binding constant of the Zn(II)–tpy center, the MSP showed an average molecular mass of 20 000, corresponding to a DP value of 15.53 The introduction of electroactive oligo-p-phenylenevinylene (OPV) functional units was achieved by Meijer group, and the formation of a MSP composed of Fe(II) ions and an OPV-derivative ditopic ligand was demonstrated by UV-vis spectroscopy.54 Except for tpy-based derivatives, other ditopic ligands were also explored to fabricate linear MSPs. Stuart and coworkers developed a class of water soluble ditopic ligands based on pyridine-2,6-dicarboxylic acid groups connected at the 4-position of the pyridine ring by tetra- or hexaethylene oxide spacers (C4 and C6), as shown in Fig. 6a.55 The ligands could form soluble reversible MSPs with a series of metal ions such as Zn(II), Nd(II), Fe(II) and Fe(III).55,56 The MSPs behaved as negatively charged polyelectrolytes, which allow them to be easily deposited onto electrodes to fabricate electrochemical devices through an electrostatic layer-by-layer assembly technique.57,58 When mixing the aqueous solutions of MSPs with a block copolymer which contains a polycationic block and a neutral block, a hierarchical self-assembly occurred between these components driven by electrostatic interaction, leading to the

Schematic representation of a linear main chain MSP based on the terpyridine Ru(II) complex in 1-butanol–DMA mixing solution. Reproduced with permission from ref. 44. Copyright ACS Publishing, 1999. Fig. 3

Building blocks with oligomeric and polymeric ethylene glycol spacers used by Schubert for the construction of water soluble MSPs, mentioned in ref. 45 and 46.

Fig. 4

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Fig. 5 Schematic representation of the chemical structure of the block MSP PS375-[Ru]-PEO225 and their phase separation structure used for fabricating nanopores. Reproduced with permission from ref. 50. Copyright Wiley-VCH Publishing, 2007.

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Fig. 6 (a) Molecular structures of ditopic ligands C4 and C6; (b) schematic illustration of the formation of C3Ms micelles in aqueous solution. Reproduced with permission from ref. 59. Copyright ACS Publishing, 2012.

formation of complex coacervate core micelles (C3Ms), as shown in Fig. 6b.56 Recently, Yan and coworkers reported that the Fe(III)-containing C3Ms micelles exhibited an interesting redox-induced uptake function for charged species.59 Under a redox stimulus, the micelles had excess charges, which led to the uptake of oppositely charged species like polyelectrolytes into the micelles to restore the charge balance. Furthermore, it was found that such an uptake process induced the morphological transformation of the micelles, for example from spheres to bers. The redox-responsive C3Ms micelles represent a paradigm of “adaptive self-assembly”, which concept is

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promising to be extended to novel smart systems with redoxtriggered properties. Another representative ditopic ligand was reported by Rowan and coworkers, in which they modied 2,6-bis(10 -methylbenzimidazolyl)pyridine (abbreviated as Mebip) groups onto the both ends of different polymeric spacers and obtained a series of telechelic macromolecular ligands, as shown in Fig. 7a–c.7,60–62 The coordination of these ligands with divalent metal ions such as Fe(II), Co(II), Zn(II) and Cd(II) ions leads to linear MSPs with typical polymer properties. Mechanically stable lms can be easily prepared by casting the MSPs from solution. In the case of employing a long poly(tetrahydrofuran) chain (Mn ¼ 2900) as the spacer part, the formed MSP lms showed a thermoplastic elastomeric behavior in which the metal–ligand complexation blocks and the so polymer segments are phase separated.60 Furthermore, when the Mebip groups were modied at the ends of conjugated spacers, like poly(p-phenylene ethynylene), light emitting MSP materials could be obtained.61,62 Recently, Rowan, Weder and coworkers presented an optically healable MSP formed of a Mebip-end rubbery amorphous poly(ethylene-co-butylene) ligand and metal ions, as shown in Fig. 7c.7 Upon the irradiation of UV light, the metal–ligand complexes were excited and the adsorbed energy was converted into heat, which resulted in temporary dissociation of the complexes and a decrease of the MSP viscosity, thus causing a quick defect healing. This innovative concept of light–heat conversion induced preparation is promising to be extended to a wide range of MSP materials.

(a)–(c) are the Mebip-based ditopic macromolecular ligands reported by Rowan and coworkers; (d) represents the photo-healing mechanism of the MSP based on (c) and Zn(II) ions. Reproduced with permission from ref. 7. Copyright Nature Publishing, 2011.

Fig. 7

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Among the wide range of metal–ligand motifs, phosphinecapped Pt(II) nodes with rigid organic donors are intriguing due to their advantage in directional bonding, which is important for the construction of specic coordination structures. Huang and coworkers successfully combined the organo-Pt(II) chemistry with supramolecular polymerization. They synthesized a dipyridyl donor containing a central Z-congured stiff-stilbene unit. In the presence of two 180 di-Pt(II) acceptors, the dipyridyl donors were linked to form size-controllable discrete organoPt(II) metallacycles.63 These metallacycles further transformed into extended MSP chains upon photoirradiation because of the conformational switching of the stiff-stilbene ligand. This transformation is accompanied by interesting morphological changes from spherical nanoparticles to long nanobers. These results present a novel self-assembly pathway to advanced supramolecular assemblies that can respond to external stimuli. Recently, the orthogonal combination of metal–ligand coordination with other noncovalent interactions has also been employed to fabricate MSPs.64 Zhang and coworkers reported the metal-coordination promoted supramolecular polymerization of cucurbit[8]uril (CB[8])-based linear SP.65 As shown in Fig. 8, they synthesized a bifunctional monomer, 100 -(naphthalen-2-ylmethyl)-60 -(pyridin-2-yl)-[2,20 : 40,400-terpyridine]-100 -ium bromide (NTPY), which bears both a naphthalene moiety and a terpyridine moiety. The terpyridine moiety of NTPY can bind with metal ions like Fe(II); meanwhile, the naphthalene moiety can interact with CB[8] through host–guest recognition. The two noncovalent interactions link the NTPY together to form SP chains in aqueous solution. Furthermore, it was found that the rigid and bulky terpyridine–Fe linkers can not only effectively enhance the monomer's solubility but also avoid its cyclization, thus remarkably promoting supramolecular polymerization. These results provide a new route to fabricate water-soluble SPs.

Review

2.2

Branched structures

Through appropriately adjusting the ligand structures and choosing metal ions with different coordination numbers, MSPs with a branched or cross-linked structure can be constructed. Currently, the most frequently studied branched MSPs are the star-like ones in which the metal–ligand complexes act as the cores and the polymer chains are graed on the ligands as arms. In this area, bpy-based ligands are widely used because they not only can form a tris-complex with abundant metal ions but also is readily functionalized at the 4- and/or 40 -position, allowing a precise adjustment of the arm numbers. Fraser and coworkers presented a pioneering route to synthesize metalcentered star-like MSPs by using metal–tris-bpy complexes as metallo-initiators in combination with living polymerization, as shown in Fig. 9.66 In this work, the 4- and 40 -positions of bpy were pre-functionalized by halomethyl groups which could initiate the ring-opening polymerization of oxazoline derivatives. Well-dened star-like MSPs with a narrow molecular weight distribution were obtained by this way. In the case of polymerization with a labile hexafunctional Fe(II)-centered initiator, DP > 200 is attainable. When using di-, tetra- and hexafunctional Ru(II)-centered initiators, star-like MSPs with varied arm numbers were readily obtained.67 Moreover, star-like MSPs with block copolymer arms were prepared by the sequential addition of two different monomers to the metalloinitiators.68 Through the functionalization of suitable initiator groups on the bpy ring, other living polymerization methods such as atom transfer radical polymerization (ATRP) and reversible addition–fragmentation chain transfer polymerization (RAFT),69–72 have also been realized in the metallo-initiator system, which generated star-like MSPs with diverse arm compositions like PS or PMMA chains. On the other hand, the coordination core of star-like MSPs, that is the initiator part, was also improved by adopting different meta-ligand binding motifs. Schubert and coworkers performed systematic work in this area. They used a bis(bromomethyl)bipyridine Cu(I)

Fig. 8 Schematic representation of supramolecular polymerization of the CB[8]-based monomer promoted by the coordination with Fe(II) ions. Reproduced with permission from ref. 65. Copyright RSC Publishing, 2013.

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complex as the initiator to prepare a MSP with four polyoxazoline arms.73,74 Besides, they incorporated grid-forming ligands into the star-like MSPs system,75,76 as shown in Fig. 10, which extended the metallo-initiator approach to larger and higher organized architectures. Except for stars, MSP networks have also been fabricated by using the similar route for covalent polymer chemistry where linear chains are connected by a cross-linker. For metal-coordination networks, the cross-linkers could be polytopic ligands or lanthanide ions because they can provide more than two coordination sites. Kurth and coworkers reported that the introduction of a tris-tpy ligand into metal ions induced the formation of cross-linked MSP networks.77 When the crosslinked degree is equal or below 9.5%, soluble homogenous MSP networks with positive charges were obtained, which could be used as cationic polyelectrolytes to fabricate smooth hybrid multilayered lms through the layer-by-layer self-assembly technique. As aforementioned, Mebip-ended ditopic ligands can act as the linear binding units when coordinating with divalent transition metal ions. Rowan and coworkers demonstrated that these ligands can also bind with large lanthanide ions such as La(III) and Eu(III) in a ratio of 3 : 1, forming crosslinked networks.78,79 They used a low-molecular-weight poly(pxylene) (PPX) derivative containing two end-capped Mebip groups. The coordination of this macromonomer with the mixture of Fe(II) and La(III) ions produced MSP materials exhibiting both an excellent mechanical property and a solution-processing ability, as shown in Fig. 11. This cannot be attained with traditional PPX materials.79 The introduced La(III) ions could bind three Mebip groups and thus acted as crosslinkers, which offered an important contribution in achieving the good mechanical properties.

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Huang and coworkers developed a series of MSP networks based on the combination of host–guest recognition and metal– ligand coordination. The ditopic monomers in their work have similar structures possessing a 1,2,3-triazole group between two terminal functional groups which are the complementary host and guest, respectively, as monomer 1 and 2 shown in Fig. 12. In general, crown ether and cryptand were selected as the host groups, while paraquat and dialkylammonium were used as the guest groups.80,81 The ditopic monomers can self-assemble to form linear polymers in acetonitrile at high concentrations. The addition of PdCl2(PhCN)2] that can bind 1,2,3-triazole motif led to the transition to cross-linked supramolecular polymers. This process was reversed by the addition of competing ligand PPh3.80 Moreover, the conversion from the linear to cross-linked structure can result in a reversible sol–gel transition, when crown ether and dialkylammonium were chosen to construct the recognition system and Pd(II) ions were used to provide cross-linking sites.81 The supramolecular gel can be molded into shape-persistent, free-standing objects, and also

Fig. 11 Schematic representation of the solution-casting fabrication of cross-linked films through the coordination of a Mebip-ended PPX building block with the mixed metal ions of both Fe(II) and La(III). Reproduced with permission from ref. 79. Copyright ACS Publishing, 2006.

Fig. 9 Schematic representation of the synthetic route of a meta-centered star-like MSP based on the metal–bpy complex initiated polymerization of 2-ethyl-2-oxazoline. Reproduced with permission from ref. 66. Copyright ACS Publishing, 1997.

Fig. 10 Schematic representation of formation of a grid-cored star-like MSP. Reproduced with permission from ref. 75. Copyright ACS

Publishing, 2003.

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Fig. 12 Schematic representation of the metal coordination mediated reversible conversion between linear and cross-linked supramolecular polymers. Reproduced with permission from ref. 80. Copyright Wiley-VCH Publishing, 2010.

responded to the multiple stimuli of temperature, pH value and potassium ions. These studies demonstrated that the rational combination of metal–ligand coordination with other noncovalent interactions in an orthogonal way can pave a new route to smart MSP materials with controllable topology and responsibilities. Recently, Huang and coworkers incorporated metallacyclic cores into the SP system through delicate molecular design, which greatly enriched the topological structures and functionalities of available MSPs.82–84 Initially, they synthesized two amphiphilic rhomboids by using dicarboxylate ligands containing PEG chains to coordinate with organo-Pt(II) acceptors. The rhomboids can self-assemble into diverse and well-dened nanostructures such as 0D micelles, 1D nanobers, or 2D nanoribbons in solution. Moreover, these nanostructures can further form metallohydrogels at high concentrations, driven by intermolecular hydrophobic and p–p interactions, which represented a paradigm of the hierarchical design of functional so materials.82 Huang and coworkers further extended the concept regarding metallacyclic coordination-driven selfassembly to the orthogonal and complex SP system. On one hand, they decorated the donor part of the metallacycles with the well-known quadruple hydrogen bonding motif, 2-ureido-4pyrimidinone (UPy), and selected the dendronized organo-Pt(II) as the acceptors. The formed rhomboids subsequently polymerize into dendronized organo-Pt(II) metallacyclic polymers

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through H-bonding UPy groups, which possess both the structural features of conventional dendronized polymers and the dynamic reversibility of SPs.83 On the other hand, they developed a strategy to unify the spontaneous formation of metallacyclic cores with the host–guest chemistry to deliver a 3D MSP network.84 As shown in Fig. 13, they synthesized a benzo-21crown-7 (B21C7)-functionalized dipyridyl ligand 3, which can self-assemble into a hexagonal metallacycle 1 on the coordination with an organo-Pt(II) acceptor 2. These hexagons subsequently form a MSP network upon cross-linking the B21C7 moieties with a bisammonium salt 4. The resulting MSP network exhibited gelation behavior at high concentrations and sol–gel transitions based on temperature or the addition of competing cations for the crown-ether. Based on these results, it can be envisioned that hierarchical orthogonal self-assembly will become a powerful strategy to access novel SP materials with fascinating properties.

3. Clusto-supramolecular polymers (CSPs) Metal-containing clusters are structurally well-dened assemblies of pure metal (e.g. metal clusters) or metal and hetero elements (e.g. metal-oxide clusters), and thus they generally exhibit more diverse functionalities in comparison with simple metal ions. Polyoxometalates (POMs) represent a large class of

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(a) Self-assembly of B21C7-functionalized discrete metallacyclic hexagon 1 and (b) schematic representation of the formation of a crosslinked 3D MSP network from self-assembly of hexagon 1 and bisammonium salt 4. Reproduced with permission from ref. 84. Copyright ACS Publishing, 2014.

Fig. 13

discrete metal-oxide clusters with abundant compositions and structures as well as versatile functions in catalysis, optics, electronic and magnetics.85–87 Moreover, the post-functionalization chemistry of POMs has been developed maturely, allowing the modication of organic groups on the frameworks of POMs through covalently graing or on the surfaces of POMs through electrostatically adsorbing.88 Aer rational modications, POMs have been successfully incorporated into the supramolecular polymer system to construct CSPs. However, to the best of our knowledge, the CSPs containing metal clusters in their backbones have not been reported. Actually, all the available metal-hybridized CSPs are formed by POM clusters, which are discussed in this section. 3.1

Linear structures

Upon symmetrically modifying with two supramolecular binding groups, the obtained POM derivatives can act as hybrid monomers to form a linear supramolecular polymer chain

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through the noncovalent connection of these groups. In this context, the disc-like Anderson-type POMs like [MnMo6O24]3 (MnMo6) have attracted particular attention because organic groups are readily graed to the both sides of these clusters by a widely used Tris-ligand esterication method.89 Wu group carried out a series of studies on the MnMo6-containing mainchain CSPs driven by hydrogen bonding interaction.90–93 Initially, they synthesized an azobenzene (Azo) graed MnMo6 cluster (Azo-MnMo6) with dimethyldioctadecylammonium (DODA) as the counterion.90 It was found that C–H/O hydrogen bonds existed between the terminal oxygen atoms of MnMo6 and hydrogen atoms of the phenyl ring of Azo groups in the trans-form, which drove Azo-MnMo6 to self-assemble to MSP bers. Interestingly, the reversibility of the trans–cis isomerization of Azo groups could induce the association and dissociation of the hydrogen bonds. As a result, a photoresponsive morphological transformation of Azo-MnMo6 from bers to spheres was achieved, which is instructive for the design of

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POMs-based smart materials. To introduce more typical hydrogen bonding interactions into this system, Wu group prepared a pyridine-graed MnMo6 cluster (Py–MnMo6) with tetrabutylammonium (TBA) counterions and used it as building blocks to construct CSPs through hydrogen bonding with dicarboxylic acid like maleic acid.91 The diacid molecules played the role of bridge for the CSPs formation. The CSP chains could further bundle to bers and even intertwine to networks due to their lateral hydrogen bonding interaction, which caused a gelation behavior, as shown in Fig. 14. Because hydrogen bonds are sensitive to pH values, the gels show a quick and reversible response to the addition of base like 4-dimethylaminopyridine (DMAP) and diacid. The distance between Py–MnMo6 units secluded by diacid was proven to be a critical factor for the gelation, which is related to the TBA density along the CSP chains and the solvent–ber interfacial energy. It readily controls the gelation behavior of the hybrid CSPs through adjusting the length of diacid. This work paved a new route to incorporate hard inorganic clusters into so materials. The concept of using hydrogen bonds to drive the formation of POM-containing CSPs was further extended by Wu group to stronger donor and accepter systems. They graed two adeninyl groups onto a MnMo6 cluster and encapsulated the cluster by cationic surfactants with different lengths of alkyl chains. The hybrid cluster could form linear CSP chains through the double hydrogen bonds between the adeninyl groups.92 Interestingly, the CSPs showed a dynamic self-assembly behavior which tightly relied on the ambient temperature and the chain length of surfactants, forming diverse hybrid nanostructures such as bers, rods, tubes and spheres. Moreover, the transformation between different morphologies was reversible during the heating or cooling processes. The mechanism of this transformation was demonstrated to be a synergy effect of the

Review

variation of the hydrogen bonds between adjacent clusters and the surfactant rearrangement surrounding the clusters. These results illuminated that hydrogen bonds can be rationally employed as a driving force to construct CSPs with smart responsive properties. To further increase the stability and improve the processing ability of MnMo6-based CSPs, Wu group employed complementary hydrogen bonds as the driving force. They synthesized a pair of hybrid MnMo6 clusters bearing symmetrically graed adenine and thymine groups onto the both sides, respectively, as shown in Fig. 15a.93 Meanwhile, tetradecyl ammonium (TDA), as a surfactant possessing high density of alkyl chains, was encapsulated on the clusters as counterions to enhance the hydrophobicity. The hybrid clusters were well soluble in chloroform, in which the low polar environment favors the hydrogen bonds between the complementary base pairs, leading to a stable CSP with a typical rheological behavior of non-Newtonian uid. Importantly, the high viscosity of the CSP enables it to be readily processed by mold casting and electrospinning methods like conventional polymers, as shown in Fig. 15b and c. 3.2

Branched structures

The branched CSPs are a very new area and only a few studies have been reported which all concern the star-like CSPs. In these studies, POMs play the role of the inorganic core on which polymer arms are modied. Bu and coworkers developed an “arm-rst” approach to obtain star-like CSPs through the electrostatic combination of a Keplerate-type POM cluster [Mo132O372(CH3COO)30(H2O)72]42 (Mo132) with ammonium terminated polystyrene (PS),94 as shown in Fig. 16. They

(a) Schematic illustration of the hybrid CSP chain formed by the MnMo6 clusters modified with a complementary base pair; (b) photos of different shaped monoliths of the CSP prepared by mold casting; (c) SEM image of CSP fibers prepared by electrospinning. Reproduced with permission from ref. 93. Copyright RSC Publishing, 2013. Fig. 15

Fig. 14 Schematic illustration of the gelation mechanism of CSPs based on Py–MnMo6 clusters [MnMo6O18{(OCH2)3CNHCOC5H4N}2]3 and dicarboxylic acid linking by hydrogen bonds. Reproduced with permission from ref. 91. Copyright RSC Publishing, 2012.

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synthesized a series of four cationic PS chains with gradually increased molecular mass (DP ¼ 124, 169, 218 and 290) by the ATRP method and then used them to replace the counterions of Mo132. The resulting star-like CSPs have been proven to have a core-shell structure in which the PS chains adopted a close packing state on the cluster surface. Interestingly, the CSPs were monodispersed in chloroform but transformed into vesicular aggregates in chloroform–methanol mixed solvents. This polarity-induced self-assembly behavior is demonstrated to be driven by the delicate balance between van der Waals attractive forces and the repulsive entropic effect between adjacent CSPs.95 Recently, Bu and coworkers reported that the vesicular aggregates of CSPs can act as a host capsule for the inclusion of a spherical polymer micelle, realizing hierarchical supramolecular recognition.96 These results indicate that CSPs provide a suitable model platform to study the intra/inter interactions of star polymers. As aforementioned, when the counterions on POMs are organic molecules with enough hydrophobicity, the hybrid clusters are soluble in non-aqueous solvents even in those of low-polarity such as chloroform and toluene.97 Furthermore, the electrostatic interactions between organic cations and POM anions are very stable in non-aqueous solvents, allowing the self-assembly of hybrid clusters to have a variety of morphologies like onion-like spheres,98,99 helical bers,100 and cones,101 without the dissociation of the complex structure. Wu group reported that a hybrid POM with the terminal group of methyl methacrylate can be in situ copolymerized into the polymethylmethacrylate matrix while retaining the electrostatic complexation.102 Such a structural stability enables hybrid POMs to act as “supra-core” for further graing polymer chains. Based on this, Wu group presented a “core-rst” approach to prepare POM-cored star-like CSPs,103 as shown in Fig. 17. They chose a highly luminescent [EuW10O36]9 (EuW10) cluster as the functional core. Firstly, the EuW10 cluster was encapsulated by cationic molecules with trithioester terminal groups through electrostatic interaction, and then PS chains were graed by a

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reversible addition–fragmentation chain-transfer polymerization (RAFT) method. The core–shell structure of the star-like CSPs was conrmed by TEM measurement, and the DP values of the PS arms were well controlled with a PDI less than 1.1. Through gradually increasing the length PS arms, the CSPs showed a structurally dependent self-assembly behavior in solution, at the air–water interface and in the solid state. Importantly, the distance between the EuW10 cores of adjacent CSPs in lms can be precisely adjusted by PS arms, which cause a tunable color purity of EuW10 luminescence. This tight structure–property relationship is promising to be extended to

Fig. 17 Schematic representation of the “core-first” fabrication route of the EuW10-cored star-like CSP. Reproduced with permission from ref. 103. Copyright RSC Publishing, 2014.

Fig. 16 Schematic representation of the “arm first” fabrication route of Mo132-cored star-like CSP. Reproduced with permission from ref. 94. Copyright RSC Publishing, 2012.

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other star-like CSPs containing POMs with other functions like electronic and magnetic properties. Through the rational design of the supramolecular interaction groups and the number of interaction sites on POMs, CSP networks can also be fabricated. Wu group developed a selfcrosslinking POM system based on the aforementioned AzoMnMo6 clusters.104 Two b-cyclodextrin (CD)-graed pyridine cations were modied onto the clusters through electrostatic interaction, as shown in Fig. 18. By this way, two host b-CD units and two guest Azo units were integrated in one POM cluster. The strong inclusion of trans-Azo groups in b-CD resulted in self-crosslinking of Azo-MnMo6 clusters, forming micrometerscale right-handed twisted MSP assemblies with clear lamellar structures. It was proven that the inner chirality of CD cavity transferred to the Azo groups through the host–guest recognition and amplied into the whole assembly, which contributed to the right-handed twisted structure. Upon UV-irradiation, the trans–cis isomerization of Azo groups occurred, destructing the host–guest interaction. As a consequence, the twisted assemblies transformed to irregular nanobers. Such a photo-triggered morphological transformation was well reversible. These results provide a new approach to bring supramolecular chirality in CSP systems.

Review

4. Metallo/clusto multiple hybrid supramolecular polymers One fascinating feature of POMs is their structural diversity, not only in the cluster frameworks but also in their derivatives. The post-functionalization of POMs allows covalently graing organic ligands like pyridine-based groups onto the framework of POMs. When a POM cluster has more than two graed sites, it can act as a hybrid ligand and further coordinate with metal ions to form linear SP chains or SP networks containing multiple hybrid components. Hasenknopf and coworkers synthesized an Anderson-type MnMo6 cluster with symmetric pyridine pendants, as shown in Fig. 19a. The addition of PdCl2(C6H5CN)2 to the acetonitrile solution of the clusters can cause the coordination of pyridine to Pd(II) ions and the formation of a cyclic or linear SP structure, as the geometric constraints of the MnMo6 cluster prevent the binding of both pyridine rings on one cluster to the same Pd(II) ion.105 Interestingly, the SP formed a transparent gel with obvious birefringence in acetonitrile–methanol mixed solvents, as shown in Fig. 19b and c. This phenomenon indicates that the SP forms polymeric strands which are further arranged into an

Fig. 18 (a) Schematic representation of the MnMo6 clusters modified with both Azo and b-CD groups and (b) the photo-triggered reversible selfcrosslinking process of MnMo6 clusters. Reproduced with permission from ref. 104. Copyright RSC Publishing, 2013.

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anisotropic structure by supramolecular interactions with neighboring strands or solvents. Among the various organically functionalized POMs, Lindqvist-type organoimido derivatives are particularly interesting due to the triple bonding feature of imido metal nitrogen linkage which can make the delocalization of organic p electrons to the POMs cluster.106 Peng and coworkers reported the rst hybrid in which a hexamolybdate cluster and transitionmetal complexes were linked by an extended p-conjugated bridge.107 In their synthetic approach, two terpyridyl ligands were graed to the cluster rst and then the coordination with Fe(II) ions was carried out. The hybrid SP is slightly soluble in DMSO and thus its optical property can be studied by UV-vis spectroscopy. Compared with the model Fe(II)–bis(terpyridine) complex, the metal-to-ligand charge transfer (MLCT) bands of the hybrid SP is red-shied by nearly 40 nm, which is due to the electron-withdrawing nature of the Mo^N bond and the extended p conjugation of ligands in the polymeric structure.

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This unique electron delocalization property is promising to be used in the fabrication of electronic and photoelectronic devices. Besides the covalent graing, the organic ligand can also be modied onto POMs through a noncovalent fashion like electrostatic interaction. Wu group presented an approach to prepare the hybrid POMs with multiple coordinating groups on its periphery.108 They synthesized a dual functional surfactant PyC10C12N which contains an ammonium group and a 4-(4pyridylvinylene)phenyl group on its two terminal points. The encapsulation of the [Eu(BW11O39)2]15 cluster by the equal molar ratio of PyC10C12N and didodecyldimethylammonium bromide resulted in a hybrid complex with six PyC10C12N cations. In the presence of Zn(II) ions, the complexes were crosslinked and assembled into nanoscale SP particles. Through adjusting the ratio of Zn(II) ions to pyridyl groups, the size SP particles could be precisely controlled. This work provides a fully supramolecular approach to fabricate metallo/clusto multiple hybrid SPs. Except for the modication of POMs with organic groups, different POMs have been used as subunits to fabricate clusto macromolecules which can subsequently self-assemble into large aggregated structures in solution.109,110 Wei and coworkers synthesized a paddle-wheel macrocluster by using carboxylic acid functionalized hexamolybdate derivatives as building blocks and copper ions as linkers, as shown in Fig. 20. The macroclusters spontaneously assembled into large and vesicular-like aggregates in suitable solvents, exhibiting a hierarchical organization process from small POM clusters to nanoscale complexes and then to supramolecular structures. The study demonstrated a facile and controllable strategy to fabricate POM-based macrostructures with interesting properties.

5. Fig. 19 (a) Structure illustration of the pyridine-grafted MnMo6 cluster

whose molecular formula is [MnMo6O18{(OCH2)3CNHCOC5H4N}2]3. The cluster core is represented by coordination polyhedra (blue Mo, purple Mn), the organic ligands as ball-and-stick models (black C, red O, light blue N, pink H); (b) and (c) photos of the transparent SP gel taken under ambient light and polarized light, respectively. Reproduced with permission from ref. 105. Copyright RSC Publishing, 2003.

Conclusions and outlook

The introduction of metal centers into the SPs system through metallo/clusto approaches not only creates organic–inorganic hybrid structures but also leads to abundant new properties. The hybrid SPs possess the advantage that combines the functionality of the metal components such as luminescence and magnetics with the processing ability of the polymeric structures. Furthermore, due to the reversible noncovalent bonding

Fig. 20 Coordination assembly of carboxylic acid functionalized hexamolybdate with Cu(II) ions. Reproduced with permission from ref. 110. Copyright ACS Publishing, 2013.

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feature, the hybrid SPs are typical so materials with stimuliresponsive property which is difficult to be realized in metalcontaining inorganic materials. Although fruitful results have been achieved in metallo/clusto hybridized SPs, many promising areas still remain unexplored. From the aspect of structure design, the synthetic chemistry of MSPs is mature and many routes are available to obtain MSPs with different topological structures. Future efforts may be attracted to the orthogonal combination of metal–ligand interaction with other noncovalent interactions as driving forces to create multi-responsive structures. In comparison with MSPs, the chemistry of CSPs is still in its infancy. More methodologies are needed to be developed to produce CSPs with precisely controllable structures. The selection of inorganic motifs can be extended to other cluster systems like pure metal clusters to bring new properties. From the aspect of function design, exploring the synergistic properties of different components in metal-hybrid SPs is an important direction, for example, combination of the light–heat conversion ability of the metal–ligand coordination moiety and the phase separation behavior of the rubbery polymer moiety in a MSP has been used to realize an optically healing function. On the other hand, the CSPs particularly star-like CSPs, are suitable additives to be incorporated to conventional polymer matrices to fabricate polymer nanocomposites, due to the small size and uniform distribution of the clusters. The self-assembly of CSPs is promising to create diverse hybrid microphases inside the nanocomposites, which may lead to improved mechanical and optical properties of the polymer matrices. As a whole, the hybridization concept based on the incorporation of metal centers strongly impels the expansion of the SPs system. It is an interdisciplinary paradigm using supramolecular chemistry to bridge polymer chemistry and inorganic chemistry. It can be envisioned that the metallo/clusto approach is promising to generate a new class of hybrid so materials, thus providing an excellent complement to conventional hybrid polymers.

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