Construction of Stimuli-Responsive Functional Materials via

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Construction of Stimuli-Responsive Functional Materials via Hierarchical Self-Assembly Involving Coordination Interactions Published as part of the Accounts of Chemical Research special issue “Supramolecular Chemistry in Confined Space and Organized Assemblies”. Li-Jun Chen and Hai-Bo Yang*

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Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Chang-Kung Chuang Institute, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062, People’s Republic of China CONSPECTUS: Supramolecular self-assembly, which creates the ordered structures as a result of spontaneous organization of building blocks driven by noncovalent interactions (NCIs), is ubiquitous in nature. Recently, it has become increasingly clear that nature often builds up complex structures by employing a hierarchical self-assembly (HSA) strategy, in which the components are brought together in a stepwise process via multiple NCIs. Inspired by the dedicated biological structures in nature, HSA has been widely explored to construct well-defined assemblies with increasing complexity. The employment of direct metal−ligand bonds to drive the formation of discrete metallosupramolecular architectures has proven to be a highly efficient strategy to prepare structurally diverse architectures like two-dimensional (2-D) polygons and three-dimensional (3-D) polyhedra with well-defined shapes, sizes, and geometries. Such well-defined organometallic assemblies provide an ideal platform for designing novel artificial supramolecular systems with the increasing complexity though HSA. The presence of a well-defined organometallic scaffold brings an additional dimension to the final nanoscale structures. Moreover, the multilevel dynamic nature of hierarchical self-assemblies brings more structural and functional possibilities of resultant supramolecular systems. This Account will focus on our recent advance on construction of stimuli-responsive functional materials through HSA involving coordination interactions. In our study, a series of functionalized metallacycles were first constructed through coordination-driven self-assembly (CDSA). Then, the secondary noncovalent interaction sites were integrated within the functionalized metallacycle system via either preassembly or postassembly approach. Different segments, such as alkyl chains, dendrimers, cholesteryl moiety, covalent macrocycles, and even polymeric fragments, which could provide hydrophobic and hydrophilic interactions, van der Waals forces, hydrogen bonding, CH−π and π−π interactions, and host−guest interactions, have been utilized to provide the secondary NCIs. Further self-assembly of functionalized metallacycles gives rise to the formation of complex higher-order structures driven by other NCIs by taking advantages of orthogonal property of coordination bonds with other NCIs. By changing the type of additional NCIs embodied in building blocks, different supramolecular architectures, such as the ordered nanostructures, supramolecular polymers and gels, fluorescent materials and sensors, have been successfully prepared with the tailored chemical and physical properties. In particular, the dynamic nature of coordination bonds as well as other NCIs endows final assemblies with stimuli-responsive functions. Collectively, our studies suggest that combining coordination and other NCIs in a well-defined and precise manner is a highly efficient strategy to achieve the complex architectures and functional materials. Therefore, it is very promising to develop the desired functional materials with high precision and fidelity by employing HSA involving coordination interactions.

1. INTRODUCTION One of the primary motivations for exploring supramolecular chemistry is to design and construct novel, robust, and functional materials.1 Many examples from nature have © XXXX American Chemical Society

Received: June 30, 2018

A

DOI: 10.1021/acs.accounts.8b00317 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Figure 1. Schematic representation of two synthetic strategies to incorporate the secondary NCIs into metallosupramolecular polygons.

progress in the construction of functional materials, especially those with stimuli-responsive properties, through HSA involving coordination interactions.

demonstrated the power of hierarchical self-assembly (HSA) in constructing structurally complex and functional architectures such as DNA and proteins, in which multiple components are brought together through a stepwise process driven by multiple noncovalent interactions (NCIs).2 These bring to light that HSA is a prerequisite to obtain highly ordered complex functional assemblies. Aiming to obtain a better understanding of such biological processes in nature, a great deal of effort has been devoted toward investigating artificial functional supramolecular systems by employing HSA.3 In particular, the power and versatility of HSA allows for the construction of nanoscale materials with well-defined structures and intriguing functions.4 Over the past few decades, coordination-driven self-assembly (CDSA) has received considerable attention because of the large number of diverse structural motifs that are accessible via coordination chemistry.5 Lehn and Sauvage have demonstrated the feasibility and efficiency of CDSA for constructing infinite helicates, grids, muscles and racks.6 In addition, Fujita,7 Stang,8 Raymond,9 Mirkin,10 Newkome,11 Nitschke,12 Shionoya,13 and others have successfully developed a variety of coordinationbased paradigms for the self-assembly of discrete 2-D polygons and 3-D polyhedra with well-defined shapes and sizes. These metallosupramolecular structures have become very popular not only for their aesthetic attributes but also due to their broad applications in fields such as catalysis, sensing, and drug delivery.14−17 Note that the formation of metal−ligand bonds can occur in parallel with other NCIs.18 The orthogonal combination of coordination bonds with other different NCIs should provide a highly efficient strategy for constructing supramolecular complexes with the higher-order structures and desired functionalities. Moreover, due to the reversible and dynamic nature of coordination bonds as well as other NCIs, an additional element of stimuli-responsive ability could be included into such HSA systems, thus leading to new functional materials with stimuli-dependent properties and even selfhealing capabilities. Our group has had a long-standing interest in development of novel functional materials through HSA. In this brief Account, we focus on our recent discoveries and

2. STRATEGY FOR CONSTRUCTING FUNCTIONAL MATERIALS BASED ON DISCRETE METALLACYCLES THROUGH HIERARCHICAL SELF-ASSEMBLY Previous studies have demonstrated that the incorporation of functional groups into or onto ligands does not hinder the CDSA, thus resulting in the formation of functionalized supramolecular architectures.19 For example, some interesting moieties, such as, crown ethers, cavitands, dendrimers, fullerenes, and ferrocenyl units, have been introduced into or onto the metallosupramolecular scaffolds to generate functionalized supramolecular architectures by employing either a pre- or postself-assembly functionalization strategy.20 Likewise, we developed two synthetic strategies to incorporate the secondary NCIs into metallosupramolecular polygons that can combine metal− ligand interactions and other secondary NCIs in a single process while keeping the metallosupramolecular scaffold intact (Figure 1): (I) a pre-assembly functionalization approach employs the building blocks containing both coordination sites and the secondary NCI sites; (II) a post-assembly modification approach employs the building blocks with coordination sites and reaction sites, wherein the secondary NCIs could be introduced through post-assembly modification reaction. In both cases, the metal−ligand interaction is the primary driving force to produce discrete metallacycles with precise shape and size, and then other NCIs introduced by either pre-assembly or post-assembly approach work as the secondary driving force to produce the higher-order complex assemblies. It is worth mentioning that in some cases the metallacycle skeleton itself can act as a secondary interaction site to assembly with other external molecules producing the complex supramolecular architectures. The motivation for applying discrete coordination-driven selfassembled metallacycles to obtain functional materials via HSA arises from a number of inherent characteristics of metallacycles. B


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Accounts of Chemical Research Table 1. Summary of Motifs That We Have Selected To Drive HSA

have adopted a series of functional segments to realize HSA, some of which offer one type of noncovalent interaction, while others contain different types of NCI sites. The introduction of these functional segments should be orthogonal with metallacycle skeleton that is crucial to keep the metallosupramolecular scaffold intact. Some of the motifs that we have selected to drive HSA of metallacycles, and the type of NCIs that they can provide are summarized in Table 1. For example, the introduction of alkyl chain segments into or onto the precursors can result in the formation of amphiphilic metallacycles, which can subsequently hierarchically self-assemble into highly ordered nanostructures driven by hydrophobic/hydrophilic effects. Likewise, Fréchet dendrimers can provide π−π and CH−π interactions and nontypical hydrogen bonds, cholesteryl moieties feature van der Waals forces, pillar[n]arenes or crown ethers can act as host to bind guest, and alkynylplatinum(II) bzimpy (bzimpy = 2,6-bis(benzimidazol-2′-yl)pyridine) units may display typical Pt···Pt, π−π, and CH−π interactions. These functional segments were introduced into the metallacycles via the pre-assembly functionalization approach. Meanwhile, some polymeric moieties such as poly(N-isopropylacrylamide)

First, the dimension of a metallacycle can be readily tuned without the significant changes in the synthesis protocol. Second, the formation of metal−ligand bonds can occur in parallel with other NCIs during first-level assembly because the energies of metal−ligand bonds are generally higher than those of other weak interactions. Third, self-assembled polygons provide a well-defined, rigid scaffold for precise control over the position and number of noncovalent interaction moieties that can eventually tune the dimension of resultant materials. Moreover, multiple functional moieties can be combined in one system effectively through rational choice of building blocks. Finally, the internal cavities of some supramolecular metallacycles as well as the dynamic nature of coordination bonds manifest their host−guest and stimuli-responsive capabilities, which are promising for broad applications of resultant materials.

3. MOTIFS USED TO INDUCE HIERARCHICAL SELF-ASSEMBLY Various NCIs such as hydrophobic or hydrophilic effects, hydrogen bonding, and host−guest interaction can be used as the secondary driving forces to induce HSA of discrete metallacycles. Thus, we C


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Figure 2. Molecular structures of some nonfunctional di-Pt(II) acceptors.

Figure 3. Self-assembly of metallacycles 5−7 (a) and their hierarchical self-assembled morphologies (b, e for 5; c, f for 6; and d, g for 7). Adapted with permission from ref 22. Copyright 2014 Wiley-VCH.

Other than the size and shape of metallacycles, small modification of the functional segments also influences the dimension of resultant nanostructures. For example, as for triangular metallodendrimers that were self-assembled from a 60° poly(benzyl ether) dendritic dipyridyl donors and 180° diplatinum(II) acceptor 3a, the first-generation triangular metallodendrimer self-assembled into spherical nanostructures, while nanofibers were observed for the second-generation metallodendrimer under the same conditions.23 Another example is presented by the self-assembly behaviors of two similar rhomboidal metallacycles 8 and 9 decorated with cholesteryl moieties.24 It was found that nanostructures generated from those two rhomboidal metallacycles displayed totally different features. For instance, tadpole-shaped nanostructures were observed in CH2Cl2/CH3OH mixed solvents (v/v = 40/60) for metallacycle 9, while metallacycle 8 aggregated into flower-like nanostructures. An interesting finding is that the solvent polarity is also of pivotal importance of the aggregation morphologies of these complexes. As shown in Figure 4b,c, metallacycle 8 displayed a morphology transformation from the regular fluff sphere-like structures to fluff sticks to nanoflowers and further evolved into the helical bundles, while the vesicle−tadpole− vesicle transformation was found for metallacycle 9 with the increase of solvent polarity. Likewise, Stang, Huang, and co-workers also realized the construction of 0-D micelles, 1-D nanofibers, and 2-D nanoribbons by using a similar HSA strategy through the introduction of hydrophilic poly(ethylene glycol) units onto the metallacycles.25

(PNIPAAM) or poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) units, which possess multiple noncovalent interaction sites can be introduced into the metallacyclic scaffolds by using a post-assembly modification approach though post-assembly reversible addition−fragmentation chain transfer (RAFT) polymerization. It is worth mentioning that most of these functional units were incorporated as part of dipyridine donors. As for the acceptor, we can choose either nonfunctional di-Pt(II) acceptors with different binding angles (Figure 2) or modified di-Pt(II) acceptors substituted with the functional groups.

4. HIERARCHICAL COMPLEX STRUCTURES AND STIMULI-RESPONSIVE ASSEMBLIES FROM FUNCTIONAL MATALLACYCLES 4.1. Hierarchical Complex Nanostructures

It is a significant issue to control morphology at different scales in the fields of nanoscience and nanotechnology.21 Regarding the formation of coordination metallacycles as a primary selfassembly, obtained metallacycles may further work as building blocks for highly ordered nanoarchitectures. Examples are the hierarchical self-assemblies from supramolecular metallacycles with different-numbered hydrophobic alkyl chains. In 2014, we designed and synthesized a series of metallacycles decorated with alkyl chains, which were found to be capable of yielding various ordered nanostructures.22 Stirring mixtures of 120° dipyridyl donor 4 with long aliphatic chains and 60°, 120°, or 180° di-Pt(II) acceptors 1, 2a, or 3a, respectively, resulted in the formation of rhomboid 5 and hexagons 6 and 7 (Figure 3a). It was found that rhomboidal complex 5 self-assembled into nanofibers, while hexagonal complexes 6 and 7 displayed nanospherical morphologies under the same conditions (Figure 3b−g), indicating that the geometry and alkyl chain number of the obtained metallacycles influenced their self-organization patterns.

4.2. Stimuli-Responsive Nanostructures from Functional Metallacycles

Using the strategy of HSA, we have realized a straightforward fabrication of stimuli-responsive nanostructures. For example, we designed and prepared a new family of dimethyl isophthalate (DMIP)-functionalized poly(benzyl ether) dendrimer ([G-0]− [G-2]) containing hexagonal metallodendrimers from ligand D


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Figure 4. Chemical structures (a) and morphology transformations of cholesteryl rhomboidal metallacycles 8 (b) and 9 (c) in various solvents. Adapted with permission from ref 24. Copyright 2016 The Royal Society of Chemistry.

Figure 5. (a) The illustration of HSA from dendrite-functionalized ligands 10a−c and halide-responsive release of fluorescent molecules, (b) chemical structures of dendritic segments, and the emission (c) and intensity (d) change of the release process. Adapted with permission from ref 26. Copyright 2014 American Chemical Society.

construction of smart nanocontainers can be realized by using these stimuli-responsive nanostructures for incorporation of guest molecules such as fluorescent molecules (Figure 5a). More importantly, upon the treatment with bromide, the spherical aggregates were disassembled and released the loaded guest

10a−c that were able to hierarchically self-assemble into the nanoscale, monodisperse vesicle-like structures.26 Upon the addition of bromide, the disassembly of such vesicle-like structures into micelle structures was observed due to the disassembly of hexagonal metallodendrimers. Thus, the idea of E


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Figure 6. (a) Self-assembly of pillar[5]arene-decorated metallacycles 12 and 13 from ligand 11, (b) 2-D DOSY plot of 12⊃(14)3 at different concentrations, (c) TEM images of supramolecular polymers 12⊃(14)3, (d) photograph of gel formation from 12⊃(14)3 and SEM image of the xerogel, and (e) reversible gel−sol transitions of supramolecular polymer gels triggered by a variety of stimuli. Adapted with permission from ref 31. Copyright 2014 American Chemical Society.

Figure 7. Chemical structures of building blocks 15−17 and schematic presentation of the formation of cross-linked supramolecular polymers. Adapted with permission from ref 34. Copyright 2015 The Royal Society of Chemistry.

molecules (Figure 5c,d). This example represents one of the very few successful examples of stimuli-responsive systems triggered by anions and expected to find applications in drug delivery and cancer therapy in the future. It is worth mentioning that Sessler and co-workers have made a number of anion responsive systems that change their nature and morphology as a result of anion binding.27,28

Cross-linked supramolecular polymers were obtained through host−guest chemistry between such metallacycles and various neutral ditopic guests, such as 14, as detected by 2-D DOSY (Figure 6b) and TEM (Figure 6c). Continuous increase of concentration to a relatively high level (ca. 23 mM), supramolecular polymer gel (Figure 6d) was obtained. Moreover, those gels displayed reversible gel−sol transitions stimulated by various external stimuli, such as temperature, competitive guest, and halide (Figure 6e) due to the dynamic nature of HSA. The host−guest interaction always displays high selectivity, which allows for the formation of complex self-sorting systems.32 When a pillar[5]arene host unit and a neutral guest moiety were separately decorated on dipyridyl donors, a metallacycle possessing three pillar[5]arene units and a metallacycle decorated three neutral guest units were successfully obtained in one pot CDSA though size-control self-sorting strategy.33 When two dipyridyl building blocks 15 containing benzo-21-crown-7 and 16 with dibenzo-24-crown-8 moieties were employed to interact with acceptor 2a and a ditopic guest 17 that contained both benzylammonium and (9-anthracylmethyl)benzylammonium moieties, the steric effect controlled self-sorting resulted in the formation of stimuli-responsive cross-linked supramolecular polymers and supramolecular polymer metallogels (Figure 7).34

5. SUPRAMOLECULAR POLYMERS AND GELS FROM FUNCTIONAL METALLACYCLES 5.1. Stimuli-Responsive Supramolecular Polymers and Organometallic Gels

Scientific researchers have been attempting to construct artificial supramolecular polymers and gels because they can combine the traditional polymeric properties with unique functions like selfhealing properties.29 We were wondering whether the HSA strategy could be also applied to prepare supramolecular polymers and gels. To answer this question, we introduced a pillar[n]arene moiety into the building blocks since pillar[n]arenes have proven to be excellent host molecules.30 As depicted in Figure 6, a pillar[n]arene species 11 was adopted to coordinate with acceptors 3a,b, resulting the formation of two different sized hexakis-pillar[5]arene metallacycles 12 and 13.31 F


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Figure 8. (a) Graphical representation of synthesis of metallacycles 19 and 20 and star supramolecular polymers 21 and 22 and the mechanism (b) and photographs (c) of self-healing of the supramolecular polymeric hydrogel. Adapted with permission from ref 41. Copyright 2016 American Chemical Society.

resultant star polymers in water was observed. Moreover, supramolecular hydrogels were obtained from star polymers 21,22 without a heating−cooling process. Interestingly, as shown in Figure 8b,c, a self-healing phenomenon was observed for the obtained polymeric hydrogels because of the dynamic nature of Pt−N bonds in the hexagonal metallacycles. To further explore the potential of these hydrogels as biomaterials, we linked another block segment PDMAEMA with CO2-responsive properties and low cytotoxity42 into the supramolecular metallacycle by stepwise post-assembly polymerization.43 From trithioester substituted metallacycle 19, the star-shaped supramolecular block copolymer 23 was prepared by stepwise polymerizing DMAEMA and NIPAAM (Figure 9a). As expected, copolymer 23 featured CO2-induced morphology transformation and CO2-triggered thermoresponsive behavior. Furthermore, after an aqueous polymer solution was treated with CO2, supramolecular polymeric hydrogels were obtained upon heating to 34 °C. Furthermore, a reverse gel-to-sol transition was observed by bubbling N2 into the resultant hydrogels. The resultant polymeric hydrogel also displayed injectable property. When copolymer 23 was dissolved in water at ambient temperature accompanied by sulforhodamine B (a fluorescent dye) after ventilation with CO2 for 1 min, supramolecular hydrogel formed immediately upon injection into the aqueous media at 34 °C and swelled with the increase of time (Figure 9b). The in vitro cytotoxicity investigation of block copolymers 23 indicated that the resultant supramolecular hydrogel displayed good cytocompatibility (Figure 9c). Since CO2 is an important metabolite in cells with good biocompatibility and biomembrane permeability, we envisioned that the obtained injectable and biocompatible supramolecular hydrogels might be very promising as biological materials.

Aside from host−guest interaction as the secondary driving force, other segments can also induce the formation of supramolecular organometallic gels. For example, two discrete hexagonal ring-in-ring architectures with the rigid backbones and soft alkyl chains could undergo HSA into three-dimensional networks and generate translucent supramolecular metallogels in DMSO or DMF driven by π−π and hydrophobic interactions.35 The second-generation rhomboidal metallodendrimer generated from 120° dendritic donor ligand 10c with acceptor 1 could form supramolecular gels in several solvents and exhibited a relatively stronger gelation ability compared to that of the dendritic precursor 10c under the certain conditions.36 In addition, driven by the hydrogen bonding of the amide groups along with van der Waals forces between hydrophobic alkyl long chains, the ordered nanofibers and supramolecular organometallic gels were also obtained.37 The dynamic nature of metal−ligand bonds imposed all these supramolecular polymers and gels reversible stimuliresponsive gel−sol phase transition properties. 5.2. Self-Healing and Injectable Supramolecular Polymeric Hydrogels

During the past decades, hydrogels have been widely used as biological materials because of their good biocompatibility and flexibility.38 In order to broaden their application in biological science, the design and construction of supramolecular polymeric hydrogels from metallacycles are indeed desired. By combining CDSA and the RAFT polymerization,39 we obtained a new series of star-shaped supramolecular polymers containing PNIPAAM moieties40 and well-defined metallacyclic cores via post-assembly polymerization strategy.41 A new 120° dipyridyl donor 18 decorated with chain transfer agents (CTAs) was adopted to coordinate with 120° di-Pt(II) acceptors 2a,b, forming the discrete hexagonal metallacycles 19,20 decorated with three CTAs moieties. Subsequent polymerization with 2′, 2-azobis(isobutyronitrile) (AIBN) as initiator afforded the starshaped supramolecular polymers 21,22 (Figure 8). A typical lower critical solution temperature (LCST) behavior of such

6. FLUORESCENT MATERIALS AND SENSORS 6.1. Fluorescent Hierarchical Self-Assemblies

A typical advantage of CDSA is that various functional units can be orthogonally incorporated into one metallacycle system.44 G


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Figure 9. (a) Graphical representation of the construction of star supramolecular block copolymers 23, (b) photographs of the injectable hydrogel over time at 34 °C, and (c) cell viability of MC-3T3 treated with different concentrations of copolymers 23. Adapted with permission from ref 43. Copyright 2017 American Chemical Society.

Figure 10. (a) Graphical representation of TPE containing star supramolecular polymer 24, (b) confocal microscopy image of the actin filament in bone marrow stromal cell (BMSC) cytoskeleton (stained red with 568 phalloidin), (c) fluorescent nanoparticles (green), (d) the nuclei of BMSCs (stained blue by Hoechst), and (e) the merged view of cell endocytosis (scale bar = 50 mm). Adapted with permission from ref 47. Copyright 2017 The Royal Society of Chemistry.

PNIPAAM arms, from which ordered, fluorescent nanoparticles spontaneously formed due to their amphiphilic properties.47 The obtained metallacycle displayed typical AIE characteristics benefiting from the introduction of the TPE moiety. Moreover, those TPE-based supra-amphiphilic metallacycles exhibited good biocompatibility, thus can be applied in cell imaging (Figure 10).

Thus, the combination of functional units containing NCI sites with other functional groups in metallacycles can integrate the properties of different functional units, thus leading to new hierarchical structures with promising functionality. For example, the integration of fluorogens with aggregation-induced emission characteristics (AIEgens)45 endows the assemblies with AIE properties while maintaining their stimuli-responsive properties. We recently reported the construction of a new family of cross-linked AIE supramolecular polymer gel from two well-defined metallacycles decorated with both tetra-phenylethylene (TPE) and pillar[5]arene moieties. Along with the formation of supramolecular gel, the typical AIE phenomena were observed.46 Moreover, such soft material displayed “on− off” of fluorescence accompanied by multiple stimuli-responsive gel−sol transitions. Combining exo-functionalization strategy and post-assembly polymerization, we recently prepared a new discrete TPE-based supra-amphiphilic metallacycle 24 decorated with three

6.2. Fluorescent Sensor though Hierarchical Self-Assembly

It should be noted that many self-assembled metallacycles are positively charged because of the formation of coordination bonds between electric donor ligand and oxidized metal ions, which might be employed to interact with anions through electrostatic interactions.48 In 2015, we presented an example of polyanion-induced HSA of discrete metallacycles.49 Heparin, a sulfated glycosaminoglycan polymer with multiple negative charges that has been widely used as an anticoagulant drug,50 was selected to induce HSA. The introduction of TPE endowed H


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Figure 11. (a) Schematic representation of the HSA between tris-TPE metallacycle 25 and heparin, emission spectral changes (b) and photographs (c) of 25 with the addition of heparin, and (d) (I − I0)/I0 values as a function of the concentrations of heparin, ChS, and HA. Adapted with permission from ref 49. Copyright 2015 American Chemical Society.

We found that the introduction of the alkynylplatinum(II) bzimpy moiety53,54 into a discrete metallacycle could endow new functionalized metallacycle 28 (Figure 13a) with interesting photophysical properties.55 Triggered by CH2Cl2 vapor or mechanical grinding, metallacycle 28 displayed a reversible color change between yellow and red (Figure 13b). The detailed mechanism studies by PXRD, SEM, and TEM investigations revealed that, with the existence of CH2Cl2, the metallacyclic scaffold displayed chair conformation and favored a close molecular stacking through intermolecular Pt···Pt and π−π interactions, induced by C−H···π interactions between CH2Cl2 molecules and the alkynyl-Pt(II) bzimpy units, while loose stacking of alkynylPt(II) bzimpy moieties was found with the absence of CH2Cl2. Besides the high selectivity toward CH2Cl2 vapor molecules, metallacycle 28 displayed ultrastability compared to many known vapochromic materials, retaining the red color in air for several months at room temperature or even under vacuum for more than 1 week. It is worth mentioning that 195Pt solid-state NMR experiments were adopted to investigate the local environment of the Pt atoms in this highly stable vapochromic system, providing a new way to study Pt···Pt interaction. Another family of alkynylplatinum(II) bzimpy-functionalized metallacycles56 were found to display switchable emission in different cyclohexane/CH2Cl2 compositions and spontaneously self-assemble into metallogel that was selectively responsive to the aromatic guest of coronene mainly driven by Pt···Pt and π−π interactions at room temperature. Very recently, we also found that a new discrete alkynylplatinum(II) bzimpy functionalized supramolecular metallacage57 displayed a solvent triggered emission switch and enhancement due to the change of intermolecular Pt···Pt and π−π interactions.

the resultant metallacycle 25 with interesting sensing functionality. Entangled pearl-necklace networks were obtained through complexation between 25 and heparin driven by multiple electrostatic interactions (Figure 11a). Obvious emission intensity changes (Figure 11b,c) accompanied by the HSA process with a linear relationship demonstrated metallacycle 25 could work as a turn-on sensing probe for heparin. It was found that metallacycle 25 displayed high selectivity to heparin even in comparison with structurally similar analogues such as chondroitin 4-sulfate (ChS) and hyaluronic acid (HA) (Figure 11d). 6.3. White-Light Emission Materials and Vapochromic Materials

If we look back to the building blocks and functional components for the construction of metallacycles, some building blocks can display both self-assembly ability and interesting optical properties. Thus, optical materials can be obtained by using these components as building blocks. For example, TPE moieties have been incorporated into supramolecular metalloarchitectures to build luminescent materials based on their AIE properties.51 We recently reported the emissive materials from the self-assembly of full conjugation of TPE with 2,2′:6′, 2″-terpyridine (TPY).52 Rosette-like metallo-architectures ranging from generation G1−G3 (G2 (26) and G3 (27) are shown in Figure 12) were obtained from the self-assembly of TPE−TPY ligands with Cd(II). Along with the increasing structural complexity, the multiple bulky TPY groups and multivalent interactions enhanced the restriction of intramolecular rotation to immobilize TPE fluorophores into metallosupramolecular architectures. Both 26 and 27 formed the tubular structures through the stacking of individual supramolecular rosettes (TEM image and packing cartoon of 27 are shown in Figure 12b). Supramolecular rosette 26 exhibited the highly pure white-light emission property under wide range of good/poor solvents ratios (Figure 12c−e), providing an alternative strategy in the seeking of novel light-emitting materials.

7. CONCLUSION AND PERSPECTIVES The development of new strategy for supramolecular selfassembly has always been one of the most important aspects I


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Figure 12. (a) Schematic representation of discrete hexamer 26 (G2) and heptamer 27 (G3), (b) TEM image of nanotubes assembled from 27 (scale bar, 200 nm) and proposed stacking structure, and (c) CIE 1931 chromaticity diagram, (d) quantum yields, and (e) photographs of 26 in CH3CN/methanol with various methanol fractions. Adapted with permission from ref 52. Copyright 2018 Springer Nature.

Figure 13. (a) Chemical structure of metallacycle 28 and (b) photographs showing the dynamic color changes under ambient light for the reversible vapochromic phenomenon of 28. Adapted with permission from ref 55. Copyright 2016 American Chemical Society.

ORCID

within supramolecular chemistry since it may provide a highly efficient approach for construction of new supramolecular architectures with precise structures and desired functionalities. As indicated in this Account, the well-defined discrete metallacycles provide a very promising platform for constructing novel supramolecular nanostructures and advanced functional materials via HSA. The judicious combination of reversible coordination bonds with other NCIs allows for the fabrication of a variety of self-assembled complex nanostructures and stimuliresponsive functional materials. Although some ordered nanostructures and functional materials have been constructed via HSA involving coordination interactions, there are still many uncharted terrains for chemists to explore. For example, further studies concerning the mechanism of HSA processes and the deeper insight into the kinetics and thermodynamics of these self-assembly systems are still needed. Moreover, in addition to the chemical stimulus presented in this Account, there is anticipation that new triggers such as light, gas, and sonication will soon regulate the properties of hierarchical self-assemblies. Finally, the efforts should be devoted to improving the chemical, physical, and mechanical properties of the resultant supramolecular structures and to establish the structure−property relationships for the final supramolecular assemblies.

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Hai-Bo Yang: 0000-0003-4926-1618 Funding

This work was financially supported by NSFC/China (Nos. 21625202 and 91427304), 973 Program (No. 2015CB856600), STCSM (No. 16XD1401000), and Program for Changjiang Scholars and Innovative Research Team in University. Notes

The authors declare no competing financial interest. Biographies Li-Jun Chen received her Ph.D. degree in chemistry from East China Normal University in 2016. Then, she joined Prof. Hai-Bo Yang’s group at East China Normal University as a Postdoctoral Fellow. Hai-Bo Yang obtained his Ph.D. degree at the Institute of Chemistry, Chinese Academy of Sciences (ICCAS), in Beijing in 2004. Then he joined Professor Peter J. Stang’s group at University of Utah as a Postdoctoral Fellow. Since the end of 2008, he has started his independent research as a PI at East China Normal University in Shanghai. Prof. Yang’s research interests span the areas of supramolecular self-assembly, supramolecular polymers, and rotaxane dendrimers.

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AUTHOR INFORMATION

Corresponding Author

REFERENCES

(1) Steed, J. W.; Atwood, J. L. Supramolecular Chemistry; Wiley: New York, 2000.

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