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Ionic Complexes of Metal Oxide Clusters for Versatile SelfAssemblies Bao Li, Wen Li, Haolong Li, and Lixin Wu* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry and Institute of Theoretical Chemistry, Jilin University, Changchun 130012, P. R. China CONSPECTUS: The combination of rational design of building components and suitable utilization of driving force affords spontaneous molecular assemblies with well-defined nanostructure and morphology over multiple length scales. The serious challenges in constructing assemblies with structural advantages for the realization of functions programmed into the building components usually lie ahead since the process that occurs does not always follow the expected roadmap in the absence of external intervention. Thus, prefabricated intermediates that help in governing the target self-assemblies are developed into a type of unique building blocks. Metal oxide cluster polyanions are considered as a type of molecular nanoclusters with size scale and structural morphology similar to those of many known inorganic particles and clusters but possess distinctive characteristics. Following the understanding of these clusters in self-assembly and the rationalization of their most efficient design strategy and approach, the obtained fundamental principles can also be applied in common nanoparticle- and cluster-based systems. On the other hand, the deliberate synergy offered by organic countercations that support the self-assembly of these clusters greatly expands the opportunity for the functionalization of complex building units via control of multiple interactions. The ionic combination of the inorganic clusters with hydrophilicity and the cationic organic component with hydrophobicity leads to discrete properties of the complexes. Significantly, the core−shell structure with rigid−flexible features and amphiphilicity will pave the way for hierarchical self-assemblies of the obtained complexes, while the intrinsic characteristics of the metal oxide clusters can be modulated through external physicochemical stimuli. Within this context, over the past decade we have extensively explored the ionic combination of inorganic polyanionic clusters with cationic organic amphiphiles and devoted our efforts to establishing the general rules and structure−property relationships of the formed complexes for constructing self-assemblies at the interface, in solution, and in solid matrixes. Specific interest has been focused on the functional synergy deriving from the incompatible components in highly organized self-assemblies. In this Account, we describe the recent progress on the ionic complexation of polyoxometalate clusters with cationic amphiphiles and the construction of diverse selfassembled nanostructures. First, the fundamental structural characteristics and molecular geometries of the prepared complexes are analyzed. The construction principle and diversity of the self-assembly based on the complexes and the smart stimuli response are then discussed, subject to the adjustment of various non-covalent interactions occurring in the assemblies. Subsequently, we enumerate the functional applications of the ionic complexes assembling into organic, inorganic, and even biological matrixes. The inspiration from the construction of ionic complexation and self-assembly in this Account provides vivid profiles for the design of hybrid materials involving nanoclusters and/or nanoparticles with rich potentials in addition to polyoxometalate chemistry.

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INTRODUCTION Polyoxometalates (POMs) represent a class of polyanionic clusters mainly comprising early-transition-metal oxides.1 Because of their rich chemical composition and nanosized architecture, these inorganic clusters exhibit physicochemical properties of large diversity2 and have been used in the fields of nanoscience, photo-, electro-, and magnetic materials, medicine, and biological systems.3 From the supramolecular point of view, advances in self-assembly chemistry are always accompanied by the new development of molecular blocks,4 and the POMs with uniform and tunable architecture as a unique family possess all of the essential ingredients for directing the assembly of various nanostructures. In addition, the robust self-assembly of POMs offers advantages for themselves to cross the processing barrier. Liu’s group confirmed the self-assembly of bare POMs alone,5 © 2017 American Chemical Society

but the assistance from counterions or grafting groups has also been demonstrated to be effective in both aqueous and organic phases by increasing lateral interactions. Looking back, organic quaternary ammonium salts were used as the counterions to accelerate the crystallization during the POMs’ preparation. Those alkylammonium ions were first used by Venurello et al.6 as phase-transfer agents to drive POMs into the organic phase for homogeneous catalysis. Though the existing state of the formed ionic complexes was not investigated in detail, this pioneering work opened a stage using hydrophobic organic cations. Faulkner’s group recognized the significant role of the POM’s charge in systems assembled through layer-by-layer Received: January 27, 2017 Published: May 16, 2017 1391

DOI: 10.1021/acs.accounts.7b00055 Acc. Chem. Res. 2017, 50, 1391−1399

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Accounts of Chemical Research

Figure 1. Schematic self-assembly nanostructure of Langmuir, LB, and cast films and the reverse vesicle comprising (DODA)4SiW12O40 complexes. The top and middle right show the corresponding SEM and TEM images. Adapted with permission from ref 21. Copyright 2007 Wiley-VCH.

(LBL) deposition of POMs and organic polyelectrolytes onto electrode surfaces.7 In another early result by Ichinose et al.,8 the clusters were allowed to permeate into the ionic area of surfactant bilayers via counterion exchange, achieving in situ formation of electrostatic POM complexes. Coronado and coworkers used ionic interactions at the air−water interface by introducing POMs into the aqueous subphase while spreading cationic surfactants at the air−water interface.9 The POMs as the counterions of the cationic amphiphiles were deposited on the Langmuir−Blodgett (LB) film. Instead of employing POMs for phase transfer catalysis, Kurth and Volkmer first used the phase-transferred POM complexes as discrete building blocks to prepare LB films, taking the assembly of POM clusters to a new stage.10 These achievements and many parallel results pointed out the possible utilization of electrostatic interactions,11 but a substrate or a support for the assembly of POM complexes was still needed until a toroidal POM that can yield the additional interactions was adopted by Polarz et al.12 However, a general discrete POM building block system that by virtue of its ionic interactions with organic cations is suitable for self-assembly under all conditions, including solid surfaces, various interfaces, solutions, and even solid matrixes, is highly desirable. By re-examining these ionic complexes, we have found their inspiring characteristics for self-assembly: (1) they can be considered as polyanion clusters tied together with a collection of organic cations when the POM charges are low but can be regarded as single-molecule reverse micelles when the POM core combines with enough cationic surfactants; (2) the complexes possess satisfactory stability in organic media; (3) the complexes can change their geometrical shape during assembly; (4) the hydrophobicity of the organic cations and the hydrophilicity of the POMs drive a phase separation from each other in solution or at a surface. With these features, we have developed the POM complexes into a new building block system. The fundamental understanding of the ionic complexes has brought unprecedented opportunities for incorporation of POMs into diverse nanostructures toward the fabrication of soft materials. As POMs can be made up of unlimited types of association, our research was mainly focused on the principle of self-assemblies and materials application of these clusters in organized states. In this Account, we unfold ionic complexation and characteristics of POM complexes, their self-assembly structures and formation mechanism, their smart modulation and controlled hybridization, and the combination of POMs with biomolecules and their bioapplications. It could be envisioned that the present POM cluster system can be

extended to other ionic cluster and/or nanoparticle systems so long as the synergistic factors in the complexes are carefully considered.

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IONIC COMBINATION AND SELF-ADAPTIVE ASSEMBLY The POM complexes can be prepared by several methods through an ionic exchange process.13 In a typical procedure, after the cationic amphiphiles in a weakly polar solvent are mixed with bare POMs in water, the clusters transfer into the organic phase spontaneously, forming water-insoluble complexes as a result of the covering by the hydrophobic cations. Significantly, we found that the alkyl chains in the LB film of the prepared complexes bearing clusters of lower charge, such as [SiW12O40]4−, covered with dioctadecyldimethylammonium cation (DODA) exhibit a preferential orientation (Figure 1),14 which is completely different from the spherical packing of the complexes with giant POM cores.15 Moreover, the formed reverse bilayer with oriented alkyl chain packing similar to that of LB films is also observed in the cast film of the same complex in chloroform solution.16,17 These results imply that the complex in organic solvents is structure-changeable and that the incompatible organic periphery tends to stay away from the POM core. Crystal structure analysis of DODA-covered [Mo 6 O 19 ] 2− and cetyltrimethylammonium-enwrapped [SiW12O40]2− ionic complexes demonstrates the identical packing structure.18,19 We further evaluated the state of existence of this type of complex in weakly polar solutions, and the dynamic light scattering (DLS) results indicated their self-assembed state in solution.20 Upon addition of methanol to increase the lateral interactions between alkyl chains, spherical assemblies of DODA4[SiW12O40] were seen in the scanning electronic microscopy (SEM) image (Figure 1). Transmission electron microscopy (TEM) analysis indicated an onionlike vesicular substructure with a layer spacing of ca. 3 nm. The powder X-ray diffraction (XRD) pattern also confirmed the layered structure with identical spacings, corresponding to the length of the reverse bilayer with tilting of the alkyl chain in the interdigitated state. Contact angle measurements on the spincoated film gave a value of 108°, in good accordance with the hydrophobic alkyl chain surface. The ideal cross-sectional area for one surfactant molecule estimated from the π−A isotherm curve matches well with the area when DODA spreads evenly on both sides of the POM used.21 These findings strongly reveal that phase separation driven by the interfacial energy occurs in organic solution, and as a result, the amphiphilic feature in the phase-separated state drives the complex to form 1392


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Figure 2. Schematic mechanism of reverse bilayer, columnar, and rigid spherical packing structures of POM complexes according to the phase separation model with surface occupation on the POM of (a) less than or equal to 1/3, (b) greater than 1/3 but less than or equal to 2/3, and (c) greater than 2/3.

Figure 3. Schematic nanostructure of the [PW12O40]3− complex in various morphologies and the reversible transformations. Adapted from ref 23. Copyright 2009 American Chemical Society.

reverse vesicular assemblies. Thus, it is rational to find that flat reverse bilayers in cast films are formed from the collapsed vesicles during solvent evaporation because of the weak lateral interactions in chloroform in the absence of a polar solvent. To figure out the self-assembly mechanism of this type of complex, we designed a dendritic surfactant, N-(3′,5′-bis[3,5bis(heptyloxy)benzyloxy])benzyl-N,N,N-trimethylammonium bromide (D), to cover a series of POMs with a similar diameter of ca. 1.0 nm but with different charges (4, 5, 8, and 12). Thus, the number of D’s in the complexes increases proportionally to the POMs’ surface charge. The D 4 [SiW 12 O 40 ] and D5[BW12O40] complexes with looser covering form vesicular self-assemblies with a regular reverse bilayer substructure, whereas D7(Na/K)[SiW11O39] and D10Na2[P2W15O56] with tighter covering afford hexagonal columnar assemblies. Apparently, the number of organic cations has a potent effect on the assembled structures of the series of complexes.22 As the surface charges are delocalized in POMs, the separation fashions of common organic components can be ascribed to the occupation of the organic component on the POM core (Figure 2). In an ideal ball model, all POMs can be divided into six equal faces. When the surface occupation is less than or equal to 1/3, the phase separation leads to a linear-shaped complex in which the organic component is located on one or both sides (opposite faces occupied) of the POM ball. With this complex geometry, the reverse bilayer becomes the preferred nanostructure in organic solvent. When the surface occupation is greater than 1/3 but less than or equal to 2/3 (four equatorial faces covered), disklike phase separation occurs, and columnar

packing becomes the favorable packing state. For the case of surface occupation greater than 2/3, no favorable phase separation morphology can occur, and the tight packing of rigid balls turns out to be the main state. The formation mechanism can well explain the self-assembly difference of complexes with giant and small POM cores, which have been reported by our group and other researchers.13−15,20 As an extension, the curved and twisted packing structures can also be well-explained as being derived from the mismatched phase separation of POM complexes. Of course, the occupied (or residual) surface area is closely related to the cluster’s size and charge, the cross-section of the organic molecules, and the solvent polarity. The mechanism can rationalize the general self-assembled structures of most POM complexes. Independent studies confirmed that many complexes having residual surface on POMs self-assemble into reverse bilayer nanostructures with no relevance to the observed morphologies.23 Interestingly, though disklike, conical, and tubular self-assemblies were obtained via control of the solvent polarity and transformations between them were realized, the nanostructures in all of those assemblies had a reverse bilayer structure in which the POMs were located in the middle (Figure 3). Later investigations supported the rule regardless of the use of linear or branched, longer- or shorter-chain surfactants as well as POMs with or without covalent organic modification.21,22 This assembly regularity also discloses the states of existence of POM complexes in homogeneous catalysis, which had not been elaborated in the past years.24 The phase separation of 1393


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Figure 4. Photomodulated reversible change in self-assembly morphology and middle POM section in the reverse bilayer structure comprising azobenzene-grafted disklike clusters covered with DODA. Adapted with permission from ref 29. Copyright 2010 Wiley-VCH.

Figure 5. Schematic illustration of the reversible assembly and disassembly changes of (a) [NaP5W30O110]14− for control of the oxidation of methyl phenyl sulfide in PhMe/THF and (b) [WZn3(H2O)2(ZnW9O34)2]12− for phase transfer catalysis of phenothiazine oxidation in toluene. In both cases, the clusters are covered with azobenzene-terminated surfactant and modulated by UV/visible-light irradiation. The number of surfactant molecules has been reduced for a clear view. Adapted with permission from ref 33, copyright 2012 Wiley-VCH, and ref 34, copyright 2013 American Chemical Society.

hydrophilic and hydrophobic components is useful, and with this feature the POM complexes can be used as supramolecular surfactants to transfer reduced graphene from water to the organic phase.25 POMs with covalent grafting of hydrophobic alkyl chains can be regarded as an example of extreme phase separation in which the organic units are located at a fixed surface area and thus selfassemble into micellar, vesicular, and other organized nanostructures.26 As an example, a lacunary POM with two grafted alkyl chains acts as a normal amphiphile forming wormlike regular micelles.27 Moreover, the POM grafted with one long polymer chain forms a reverse bilayer structure.28

Apparently, the hydrophobic and hydrophilic separation drives the self-assemblies in both polar and nonpolar environments.

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SMART MODULATION OF SELF-ASSEMBLED STRUCTURE AND MORPHOLOGY In principle, all of the factors affecting the nanostructures and morphologies of POM complexes could be used as remote stimuli, and the responses from assembly imply a modulation of the physicochemical property. The electrostatic interaction dominating the connection of the organic cation and POM is sensitive to the extended chemical environment, such as solvent polarity, pH value, salts, and additives. Control of the size 1394


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Figure 6. Synthetic route for hybrid polymers. Gray and dark balls are bromine and Mo atoms in the [Mo6Br8Br6]2− cluster. The hybrid polymers are shown in (a) a photograph and (b) a luminescent picture. Adapted with permission from ref 42. Copyright 2014 The Royal Society of Chemistry.

slowing the reaction, which are derived from the change in dispersion state. When the catalytic cluster [WZn3(H2O)2(ZnW9O34)2]12− is used as the catalytic center instead, the complex performs a phase-transfer catalysis. After performing catalytic oxidation of dye substrates in a lowerpolarity mixed solvent, the complex with a weakly polar transazobenzene periphery separates from the reaction system and transfers spontaneously into the strongly polar aqueous phase upon 365 nm light irradiation (Figure 5, right). After removal of the product solution and addition of fresh substrate, the photosensitive catalyst gets back to the reaction system again upon visible-light (450 nm) irradiation to the aqueous solution for another cycle of catalysis.34 The concept of modulating the self-assembly of POM complexes is greatly expanded by covalent incorporation of functional groups on both the organic component and the inorganic cluster surface. A series of liquid-crystal phase transitions of specially designed POM complexes was realized with and without inserting mesomorphic groups into the organic components.35 Grafting thermosensitive groups such as tris(ethylene oxide) monomethyl ether to the periphery of the POM complexes leads to the formation of a lower critical solution temperature (LCST), while the complexes change from small aggregates/monodisperse to giant aggregates.36 Upon an increase in lateral interaction, the POM complexes behave as supramolecular gels in selected solvents.37 While the assembled structures change with the physical stimuli, such as the solvent as well as the block ratio between the copolymer and POM,38 the luminescence property of the POM clusters was controlled following the control of the assemblies.39 Importantly, the strategy is also effective when other clusters, such as octahedral metal atom clusters bridged by halides, are used for the smart self-assemblies and materials.40

matching of organic and inorganic components is another decisive factor for the assembly morphology and nanostructure. The free surface area adjustment and charge repulsion between POMs affect the self-assembled structure as well. In addition, the interaction between hydrophobic groups contributes to both the self-assembly and structural stability of the complexes. The disklike [Mn(OH)6Mo6O18]3− cluster can be modified covalently. Upon encapsulation of this cluster that grafts photosensitive azobenzene groups symmetrically with DODA, the complex forms ribbonlike self-assemblies.29 Upon irradiation with 365 nm light, the ribbons become shortened and roll into spherical assemblies spontaneously (Figure 4). When the irradiation wavelength is changed to 450 nm, the formed assembled spheres dissociate and return to the ribbon structure accordingly. Such a light-triggered reversible change can pass through several cycles. The reverse bilayer nanostructure is maintained in both states, and detailed structural characterization demonstrated that the hydrogen bond between transazobenzene and the POM cluster rigidifies the middle POM layer so that a planar assembly becomes the dominating morphology. Upon short-wavelength irradiation, the formed cisazobenzene in its bent state breaks the hydrogen bonds, thus making the reverse bilayer flexible for vesicular assemblies, similar to those fabricated from common complexes. Longwavelength light irradiation triggers the backward transformation due to the recovery of the hydrogen bonds, accompanying the re-formation of trans-azobenzene. With the same idea, redox chemistry and host−guest recognition were also used for dynamic modulation of self-assemblies constructed from ferrocene- and azobenzene-grafted [Mn(OH)6Mo6O18]3− complexes.30,31 Furthermore, we realized reversible evolution of a helical ribbon assembly to a spherical reverse vesicular assembly of the (DODA)2Mo6O19 complex via redox stimulation of the POM core.32 The reversible control of assembly and disassembly provides an external modulation for functionalization of POM complexes, such as in responsive catalysis and luminescence. The accompanying configuration change during trans to cis isomerization of azobenzene groups yields large polarity increases that are applicable for controlling the POM complex self-assemblies conveniently. Typically, full encapsulation of a [NaP5W30O110]14− cluster with a surfactant bearing transazobenzene terminals makes the complex dissolve in toluene with monodispersion (Figure 5, left).33 However, the same complex having a cis-azobenzene surface leads to tight packing of rigid balls due to increased surface polarity. The opposite process occurs in a polar solvent such as methanol. Thus, a simple light irradiation controls the assembly and disassembly of the POM complex. This property is favorable for the dynamic control of catalytic oxidation by accelerating and

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POM-COMPLEX-BASED HYBRIDIZATION AND VERSATILE APPLICATION Organic encapsulation offers the POM complexes a great advantage through the covalent combination with inorganic and polymer matrices by grafting of reactive groups at the periphery. With this method, all of the complexes can be connected to the matrixes covalently in a well-dispersed state. As an example, a complex of luminescent [EuW10O36]9− cluster encapsulated with a double-chain cationic surfactant with methyl acrylate tails is introduced into the polymer matrix through copolymerization with methyl methacrylate monomer. The complex with rich reactive groups acting as an additive cross-linker improves the mechanical properties of the prepared polymer while the POM’s luminescence and the transparency of the obtained polymer are well-retained.41 The strategy is 1395


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Figure 7. Preparation of POM complexes with hydroxyl group periphery and their incorporation into silica films (right) for in situ reduction of metal ions and silica bulk (left) for red-luminescent and catalytic materials. Adapted from ref 13, copyright 2008 American Chemical Society, and with permission from ref 46 and 47, copyright 2007 and 2010 Wiley-VCH.

Figure 8. Structural illustration of the magnetic [Gd(β2-SiW11O39)2]13− complex with dual micellar structure loaded with rhodamine B molecules for (a, b) fluorescence imaging of (a) cell and (b) mouse and (c) MRI of the liver. The number of dendrons has been decreased for a clear view. Adapted with permission from ref 52. Copyright 2017 Wiley-VCH.

realized reversibly by photoreduction of POMs in a silica film, which can be further used for in situ preparation of patterned metal nanoparticles by insertion of the silica in a solution of metal ions (Figure 7).46 On the other hand, the covalently anchored complex can act as a microreactor for heterogeneous catalysis, and highly efficient catalytic oxidation of thioethers, olefins, and alcohols has been observed with performances even better than those from the corresponding homogeneous catalysis.47 Evidently, the flexible hydrophobic environment surrounding the POM cluster, like a pseudo-liquid-phase catalyst, improves the oxidation reaction efficiently. Beyond covalent grafting to the matrixes at the periphery, interior functionalization of the encapsulated POM complexes is also applicable. Generally, asymmetric catalysis using a POM catalyst is hard to conduct because of the lack of a chiral center. By the simple use of a covering cationic molecule with a chiral head, the induced-chirality POM complex can be employed for this purpose.48,49 The sustainable chirality is transferred to the POM core through electrostatic interactions, allowing kinetic resolution of aromatic secondary alcohols (benzoin and 1-phenylethanol) to be carried out through selective oxidation with a fairly high enantiomeric excess value.50

powerful and can also be applied to other inorganic cluster systems. As shown in Figure 6, a complex containing an anionic molybdenum halide cluster exhibiting strong deep-red luminescence was grafted into a polymer matrix in a welldispersed state, and the obtained hybrid exhibited characteristics needed for optical materials and devices.42 Following the terminal activation, linear growth of the polymer chain along POM complexes can be realized from activated positions, directing the reaction to form starlike polymers with a POM core.43 The prepared star polymers in monodispersed state with a size of ca. 6 to 12 nm act as unique additives for modulating the self-assembly of linear block copolymers.44 In a further application, graphene shielded by the starlike POM complexes as individual nanosheets becomes more compatible with the polymer matrix and serves as a reinforcing nanofiller for polystyrene coatings.45 In contrast to bare POM heterogeneous catalysts incorporated into silica, the present route presents precise control over the chemical environment surrounding the POMs in the complexes even at a high loading content. Similar to the hybridization of polymers, the encapsulation of [EuP5W30O110]12− in a hydroxyl-terminated surfactant allows the complex to be covalently embedded into a silica matrix via a simple sol−gel reaction. As in an LBL film,11 photochromism is 1396


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Figure 9. Schematic drawing of the synthetic peptide and TEM characterization and packing structure of its ionic self-assembly with H4SiW12O40. Adapted with permission from ref 54. Copyright 2016 Wiley-VCH.

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COASSEMBLY OF POMS WITH BIOMOLECULES AND THEIR BIOAPPLICATIONS The paramagnetic POM clusters are applicable as contrast agents for magnetic resonance imaging (MRI). However, the bare clusters are sensitive to physiological conditions, and electrostatic encapsulation provides an effective approach for structural protection. A cationic surfactant with an oligo(ethylene oxide) monomethyl ether terminal affords a hydrophilic and biocompatible modification for paramagnetic K13[Gd(β2-SiW11O39)2]. The formed complex dissolves in aqueous solution, and the inorganic core shows wider pH and electrolyte stability.51 A stronger contrast effect than with the commercial gadolinium complex is observed, although the capability to accelerate the relaxation of protons encounters an inflection accompanying the formation of self-assembly. By improvement of the organic covering to dendritic cations with oligo(ethylene oxide) monomethyl ether tails,36 the [Gd(β2SiW11O39)2]13− complex encapsulated with a third-generation dendritic cation shows monodispersion and high relaxivity under physiological conditions.52 Noticeably, the complex possesses a regular micellar exterior and a reverse micellar interior, which makes it a robust carrier to adsorb guest cationic molecules stably as a result of the additional electrostatic interaction (Figure 8). Thus, the complex loading a luminescent dye enters the cells without leaching and exhibits bimodal imaging. Both the strongest fluorescent and MRI intensities appear at a circulation time of ca. 60 min simultaneously in an in vivo imaging measurement. The combination of POMs with peptides displays both rich assembly and bioactivity. The complexes of a designed artificial peptide consisting of alternating sequence of hydrophilic and hydrophobic residues (Figure 9) covering POMs such as [SiW12O40]4− display a high binding constant (ca. 6.12 × 107 m−1). The synthetic peptide itself adopts a random-coil conformation but in a mixture of a 2:1 molar ratio with [SiW12O40]4−, the electrostatic complex forms and gradually self-assembles similar to a random coil to a β-sheet state.53 The cryo-TEM image shows the POM incorporated into the core of the peptide bilayers while some amino residues are distributed on the external surface. Unexpectedly, the complex assembly has a very high antimicrobial efficacy for Escherichia coli, while

neither isolated component shows any activity under the same conditions.54

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CONCLUSION AND PERSPECTIVES This Account has provided an overview of the self-assembly principles of structure-deformable POM ionic complexes. Although not all aspects have been included, the versatile potentials of these complexes in nanostructure fabrication, organic and inorganic hybridization, selective catalysis, absorption and luminescence with smart response, bioimaging, and bioactivity have been shown. The complexes comprising multiple components display interesting hierarchical selfassemblies in a controlled mode following a phase-separation regularity. The synergy of the organic component greatly enhances the important role of inorganic clusters as the synthon in the construction of self-assembled nanostructures with incorporated functional information. Under the support of ionic combination with specially designed ionic organic components, the POM family with various architectures and properties can build assemblies without limitation at interfaces and solid surfaces and in solution and solid matrices of inorganic and polymer media. Meanwhile, the strategy used in this Account points to an alternative path for creating new nanostructures.55 Following the present principle, routes to charged nanosized clusters and/or objects are also applicable, as validated by recent publications. Following the self-assembly strategy, the phase separation in the self-assembly of POM complexes can be used for the intermediate layer in solar cell devices by connecting both the metal electrode and the organic active layer. The combination of organic sensitizers with POMs may promote complex assembly for enhanced energy transformation in solutions. From the precise modulation of self-assembled nanostructures, one can see that the electrostatic interaction is also applicable for improving the flexibility of known porous assemblies such as ionic organic frameworks. Apparently, like metal ions, POMs can play the role of nodes to bind to cationic linkers. Through organic encapsulation, the protected POMs with reduced metal ions show biocompatibility and low toxicity and can hopefully be used as both photothermal therapeutic agents and drug carriers. It is envisioned that with more precise consideration of the optimized synergy of organic and inorganic components, 1397


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the POM complexes will be able to draw even more interesting perspectives in supramolecular chemistry and nanomaterials.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bao Li: 0000-0003-0727-9764 Lixin Wu: 0000-0002-4735-8558 Notes

The authors declare no competing financial interest. Biographies Bao Li graduated from Jilin University, China, with a B.S. in 2003 and a Ph.D in chemistry in 2008 under the supervision of Prof. Lixin Wu. He then joined the faculty at the university and became associate professor. His research focuses on the synthesis and functionalization of organically grafted polyoxometalates. Wen Li got a B.S. from Inner Mongolia Normal University in 2001 and a Ph.D. in chemistry in 2006 from Jilin University under the supervision of Prof. Lixin Wu. He then joined the faculty at the university as a lecturer and became an associate professor and then a full professor. His research interest includes tackling the self-assembly and mechanism of polyoxometalate-synthetic peptide complexes. Haolong Li received a B.S. in 2003 and a Ph.D. in chemistry at Jilin University in 2008 from Prof. Lixin Wu’s group. After postdoctoral research as a Humboldt Scholar at the Max Planck Institute for Polymer Research, he became an associate professor at Jilin University in Prof. Wu’s group and won the Excellent Youth Scholars from the NNSFC in 2016. His current interest involves the synergistic selfassembly and functionalization of block copolymers and nanoparticles. Lixin Wu received a B.S. from Heilongjiang University in 1982 and a Ph.D. in Chemistry from Jilin University in 1993. He then joined the faculty and became a full professor in 2000. He has also been a senior postdoctoral fellow (JSPS) at Hokkaido University and a research associate at the University of Hong Kong. He has received a number of honors, including Cross-Century Talent of MOE, Government Grant of China, and Changbaishan Distinguished Professor. His research interests focus on self-assembly and supramolecular chemistry of polyoxometalate-based complexes, the structural basis of cluster− biomolecule complexation, and functionalization of supramolecular polymers.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the National 973 Program (2013CB834503), the NNSFC (21574057, 91227110, 21221063), and the Changbaishan Distinguished Professor Fund of Jilin Province, China.

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