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Supramolecular architectures assembled from amphiphilic hybridpolyoxometalates

Dong Li, Panchao Yin and Tianbo Liu*

Received 5th October 2011, Accepted 22nd November 2011DOI: 10.1039/c2dt11882c

Polyoxometalate (POM)-based inorganic–organic molecular hybrid clusters have been recentlyrecognized as good candidates to design novel multi-functional materials. Tremendous efforts have beeninvested in synthesizing many interesting hybrid structures with exceptional chemical and physicalproperties. Grafting organic ligands to the POM clusters render these functional clusters amphiphilicproperties. Here we summarize the current progresses and provide some perspectives, from colloidalchemists’ point of view, on the self-assembly of the amphiphilic POM–organic hybrids in solution and atinterfaces, as well as the related consequent novel features such as enhanced fluorescent properties.

1. Introduction

For the past twenty years, we have witnessed significant progressin the development of inorganic–organic hybrid materials, fromsolid state dye lasers,1 molecular electronics2 and photovoltaiccells3 to light emitting diodes.4 These achievements not onlyreshaped some fundamental knowledge of material sciences, butalso led to new products that have changed our daily life. Theirinteresting macroscopic properties originated from the synergisticcombination of the two microscopic components. Among differ-ent inorganic clusters, polyoxometalates (POMs)5,6 are nowsome of the most important candidates for the synthesis ofhybrid complexes because of their diversified and well-definedmolecular structures, exceptional catalytic properties and poten-tial applications in various fields.7–13

Poly-oxo-metalates represent a large group of transition metal-oxide clusters that are linked through bridging oxygen atomswith central metal ions in their highest oxidation states. Owingto the tuneable valence and coordination geometry of the centralmetal ions, various POMs with different sizes, shapes andcharges have been synthesized.10 The field of polyoxometalateshas been rapidly expanding from isopolyoxometalates to hetero-polyoxometalates, from early transition metal POMs (Mo, V, Cr,Fe, W, Mn, etc.) to late transition metal POMs (U, Nb, Au, Pd,etc.), and from pure inorganic molecular clusters to hybrid clus-ters. Fig. 1 gives several examples of well-characterized POMmolecular clusters.

Chemically grafting organic ligands to POMs is challengingbut also highly rewarding. Such inorganic–organic molecularhybrids are expected to render amphiphilic properties to thePOMs and consequently improve their applications by expandingtheir compatibility in organic media. Furthermore, these organicligands can also be applied to adjust some important features ofPOMs, including electronic and luminescent properties.14

Dong Li

Dong Li was born in Jinan,China, in 1983. He received hisBSc from Shandong University,China in 2006 and MSc fromLehigh University in 2009. Heis currently pursuing a PhD inPhysical Chemistry under thesupervision of Prof. Tianbo Liu.His research focuses on the sol-ution behaviors of macro-cations in solutions and theself-assembly of viral capsids.He is a Constance N. Busch fel-lowship recipient. Panchao Yin

Panchao Yin is a PhD studentin the research group of Prof.Tianbo Liu, at the ChemistryDepartment of Lehigh Univer-sity, studying the self-assemblyof macro-ions in solution. Heobtained his BSc degree inPolymer Science and Engineer-ing from Department of Chemi-cal Engineering in TsinghuaUniversity, China.

Department of Chemistry, Lehigh University, 6 E Packer Avenue,Bethlehem, PA 18015, USA. E-mail: liu@lehigh.edu; Fax: 1-610-758-2935http://www.lehigh.edu/~inliu

This journal is © The Royal Society of Chemistry 2012 Dalton Trans., 2012, 41, 2853–2861 | 2853

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Exploring the amphiphilic nature of such hybrids and under-standing their self-assembly behaviour in solution and at inter-faces would be an important initial steps for scientists.

Amphiphilic molecules which are ubiquitous in nature nor-mally combine hydrophilic and hydrophobic componentstogether into one structure. Such an arrangement gives them theability to interact with two different phases and self-organizeinto highly ordered structures.15 We are interested in, from thecolloidal chemists’ point of view, exploring the amphiphilicnature of the hybrid POMs and their nano-scaled assemblies.

2. Synthesis of amphiphilic hybrid POMs

The majority of inorganic–organic hybrid POMs can beclassified into two groups, the hybrids with weak interactions(e.g. electrostatic interactions, hydrogen bonding, or van derWaals interactions etc.) and the hybrids with strong interactions(e.g. covalent bonds) between the inorganic and organic com-ponents.16 This article will mostly focus on the second scenario.

2.1. Amphiphilic hybrid POMs with non-covalent bonds

For the first group, hydrophilic POM macroions interact withorganic cations or cationic surfactants mainly through electro-static interactions to construct inorganic–organic amphiphilichybrids. One example is the surfactant encapsulated POM clus-ters (SECs).17 These clusters normally consist of a core–shellstructure having hydrophilic POMs in the centre surrounded byhydrophobic functional groups.18–23 The surface properties ofPOMs may still be retained, according to a recent study ofthe Li+ uptake and release process from SECs.24 A similar syn-thetic approach can be extended to fabricate POM/polymerhybrids.25–27 Mizuno’s group has reported a group of organicmacrocations/POM ionic crystals.28–31 Owing to the hydrophilicand hydrophobic channels inside these ionic crystals, theydemonstrated exceptional adsorption and catalytic properties.32

Cronin et al. showed that some protonated bulky organic amines

can not only serve as counter-cations but also influence the finalPOM structure by limiting the reorganization rate of differentPOM isomers in solution.33–36 More details regarding thesehybrids formed by non-chemical bonds can be found in the cor-responding early reviews.37,38

2.2. Amphiphilic POMs with covalent bonds

The covalently modified amphiphilic hybrid POMs are attractivebecause: 1. the terminal and bridging oxygen atoms are relativelyreactive and can be replaced by other atoms or form directM–O–R bonds; 2. some POM clusters possess multiple sitesavailable for functionalization, which can be done by linkingone or more hydrophobic organic functional groups to onePOM; 3. the amphiphilic nature of these hybrids extends thefunctionality of POM clusters in organic media; 4. amphiphilichybrid POMs can probably be used as multifunctional oxidationor acidification catalysts with good selective recognition of sub-strates. Although there are many different synthetic pathways tocovalently link organic functional groups with POMs, we willonly focus on several facile preparation methods (some com-monly used synthetic strategies are summarized in Fig. 2). Moredetails regarding the synthesis of hybrid POMs can be found inanother well-written review.16

2.2.1 Organoimido derivatives of POMs. Since Zubieta’sgroup reported the first example,39 the organoimido derivativesof POM have been extensively investigated and a number oforganoimido derivatives of the Lindqvist hexamolybdate ion,[Mo6O19]

2−, have been reported.40,41 The hexamolybdate ion[Mo6O19]

2− is a chemically robust cluster with good thermalstability. The terminal oxygen atoms are reactive enough to bereplaced directly by various nitrogenous species, for instance,

Tianbo Liu

Tianbo Liu received his BSdegree in Chemistry fromPeking University in 1994. Hereceived his PhD in Chemistryfrom SUNY at Stony Brook in1999, with Professor BenjaminChu. After two years as a post-doctoral associate, he startedindependent research at thePhysics Department of Broo-khaven National Laboratory. InJanuary 2005 he moved toDepartment of Chemistry,Lehigh University, where he is

an associate professor of Chemistry. His laboratory focuses onthe fundamental behavior of complex solutions, especially hydro-philic macroions, inorganic–organic hybrid surfactants, andother colloidal and biological systems.

Fig. 1 Several well-characterized polyoxometalate clusters: (a) Lindq-vist; (b) Anderson; (c) Keggin; (d) Dawson; and (e) {Mo154}. Copyright2003 Nature Publishing Group.

2854 | Dalton Trans., 2012, 41, 2853–2861 This journal is © The Royal Society of Chemistry 2012

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diazenido, diazoalkyl, and imido groups.40,42,43 The six terminaloxo groups and some bridging oxo groups in the hexamolybdatecluster can be partially or completely substituted with organo-imido ligands,44 as shown in Fig. 3. Recently, a large number ofmonosubstituted, disubstituted and polysubstituted organoimidoderivatives of hexamolybdate have been synthesized and structu-rally characterized.40,45,46 Also, the synthesis of such clusterscan be dramatically improved in the presence of dicyclohexyl-carbodiimide (DCC).47 Since the π electrons in the organic com-ponent may extend their conjugation to the inorganic frameworkand dramatically modify the electronic structure and redox prop-erties of the corresponding POMs, exciting synergistic effectsdue to the close interaction of delocalized organic p–π orbitswith the POM cluster’s d–π orbits are expected for the POMorganoimido derivatives with aromatic functional groups.48 The

multi-stage redox properties of these POMs and the possibilityof generating mixed-valence electronic structures make themattractive building blocks for the development and design of newelectrical and magnetic nano-scale materials.

2.2.2 Tris-Anderson hybrid POMs. Another strategy tocovalently modify POMs is through the use of a “tris” (tris(hydroxymethyl)aminomethane) linker with three pendanthydroxyl groups. It is an one-pot reaction of [α-Mo8O26]

4− pre-cursor, M(acac)3 (M = MnIII, FeIII) or M(OAc)2 (M = ZnII, NiII)and tris derivatives49,50 in acetonitrile under refluxing conditions.The trisalkoxo ligand with a secondary functional group can befurther modified through an imination or amidation reaction. Asthe result, a variety of tripods that allow further functionalizationthrough imine and peptide bonds are generated, as shown inFig. 4.51–53 Only recently, the unsymmetrical tris-Andersonhybrid POMs with two different functional groups attached tothe same central POM was achieved by Cronin’s group.54,55 Notonly limited to the Anderson POMs, amphiphilic hybridLindqvist and Dawson POMs capped with tris functional groupshave also been synthesized. Zubieta et al. synthesized a series ofhybrid polyoxovanadate [V6O13Hx{(OCH2)3CR}2]

n− (x, n = 0,2;2,0; 4,2; 6,2; R = NO2, CH2OH, CH3) with trisalkoxo μ-bridgingtripodal ligands.56 Hill and co-workers developed a way to func-tionalize Dawson type POMs with the tris ligand.57 These state-of-the-art synthetic tools could provide numerous hybrid POMswith great potential as multi-functional materials.

2.2.3 POM-modified polymers. In those hybrids, the POMclusters could serve as functional groups on side-chains ordirectly get involved in the main polymer chains. The first cova-lently bonded POM–polymer hybrid was reported by Judeinsteinin which a lacunary Keggin cluster was linked to a polystyreneor polymethacrylate backbone through the formation of Si–Obonds.58 Later, Maatta et al. reported a polymer–POM hybridsynthesized via free radical-copolymerization.59 Peng and co-workers have recently incorporated hexamolybdate clusters intopoly(phenylene ethynylene) as side-chain pendants throughthe Pd-catalyzed coupling reactions.60 Fluorescence studiesdemonstrated that polymers with conjugated POMs exhibited aconsiderably higher fluorescence quenching effect than those

Fig. 2 Several commonly applied synthetic strategies to covalently linkorganic groups to the POM units. Copyright 2010 American ChemicalSociety.

Fig. 3 Covalently modified Lindqvist type POMs through the for-mation of organoimido bond at terminal and/or bridging oxygen atoms.(a) Mono-substituted, (b) di-substituted and (c) hexa-substituted Lindq-vist POMs through terminal oxygen. (d) Covalent modification of Lindq-vist POMs through bridging oxygen. Reprinted with permission fromref. 40, 44 and 45. Copyright 1992, 2000 American Chemical Society.2008 John Wiley and sons.

Fig. 4 The formation of tris-Anderson hybrid POMs with differentorganic functional groups. Reprinted with permission from ref. 51.Copyright 2010 the Royal Society of Chemistry.

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without conjugated POMs, indicating that the photo-inducedelectron transfer is more effective through conjugated bridges.Using the same approach, main-chain-hexamolybdate-containinghybrid polymers were also achieved by this group.61

3. Amphiphilic POMs based supramolecularassemblies

3.1. One dimensional (1D) assemblies

1D nanostructures with low dimensionality and high aspect ratiopossess unique optical and photoelectronic properties.62 Thesematerials can be incorporated in future electronic and photonicdevices such as photodetectors, light emitting diodes (LEDs),and field effect transistors (FET).63,64

One strategy to construct 1D supramolecular assembly takesadvantage of self-assembly through weak interactions (hydrogenbonding, van der Waals interactions, hydrophobic interactions,and π–π stacking interactions). A typical example is the 1Dnanofibrils self-organized at the solvent/air interface, which isreported by Cronin’s group (Fig. 5).55 Three different AndersonPOM based hybrids can self-assemble into single-layered, longnanofibrils with the length of several microns and they arestacked together through multiple weak interactions.

3.2. Thin films formed by amphiphilic hybrid POMs

Thin films are important for photoluminescent sensors, electro-chromic devices and catalysis. However, POMs alone lack theability to form stable films; therefore a special film-formingmatrix such as surfactants or polymers is needed. Chambers andco-workers reported the synthesis of the first bis(alkyl) substi-tuted, amphiphilic, asymmetrical POM species, a bis(dodecyl)derivative {[CH3(CH2)3]4N}4{[CH3(CH2)11Si]2OSiW11O39}. Itreversibly forms stable Langmuir–Blodgett (LB) monolayer atthe air–water interface.65 The stability of the LB film dependedlargely on the organosilyl groups rather than the bulky counter-cations of tetrabutylammonium (TBA). Our recent studies on a

group of novel POM-(organic linker)-POM dumbbell typeamphiphilic hybrids also show the formation of LB films at theair–water interface with TBA as counter-cations.66 These nano-dumbbells are hydrophilic on both ends, and the middle linkerpart is hydrophobic, as shown in Fig. 6. The air/water interfacialbehaviors, obtained from the π–A isotherms, for hybrids withlinear alkyl chain linkers are relatively similar. However, hybridswith bipyridine and ether linkers present a different air/water be-havior. The liquid expanded and liquid condensed phases areclearly located and connected through a plateau. We believe thatthe hydrophobicity and composition of the organic linkers playdominant roles.

Self-assembled monolayers (SAMs) represent an attractiveapproach to anchor hybrid POMs to the surface, which exhibits ahigh degree of structural order, and can be patterned easily.Therefore, it allows a better control of the assembled structures.Cronin’s group reported an interesting self-assembled monolayerof hybrid POMs on gold surface which shows cell adhesionproperties.67 As shown in Fig. 7, a monolayer of 16-mercapto-hexadecanoic acid (MHA) moieties was stamped on goldsurface, which was further covalently coupled with Mn-Ander-son POMs via N3-(dimethylaminopropyl)-N′-ethylcarbodiimidehydrochloride (EDC)/N-hydroxysuccinimide (NHS). Finally,different functional groups, such as pyrene, were grafted onto thePOMs. Human fibroblasts have high affinity to the pyrene plat-form, and the central POMs are essential to the cell adhesion

Fig. 5 Self-assembled monolayer of amphiphilic hybrid AndersonPOMs on Si–OH. (a) and (b) SFM images; (c) a cartoon showing theproposed hierarchical arrangement of hybrid POMs in the nanofibrilsthrough multiply weak interactions. Reprinted with permission from ref.55. Copyright 2010 American Chemical Society.

Fig. 6 Monolayer formation for the dumbbell-shaped hybrid surfac-tants at the water/vapor interface: (a) liquid expansion (LE)/G phase, (b)LE phase, and (c) liquid condensed (LC) phase. TBA+ counter-cationsare not shown. Reprinted with permission from ref. 66. Copyright 2011American Chemical Society.

Fig. 7 The structures of self-assembled monolayers of Mn-AndersonPOM/pyrene complexes on gold surface with selective cell adhesionproperties. Self-assembled monolayers contain only POMs or pyrene hasno cell adhesion properties. Reprinted with permission from ref. 67.Copyright 2009 American Chemical Society.

2856 | Dalton Trans., 2012, 41, 2853–2861 This journal is © The Royal Society of Chemistry 2012

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performance. Similar strategies can be found in Errington’s andTour’s papers.68,69

3.3. Supramolecular assemblies formed in solution

Micelles and vesicles. In solution the POM–organic hybridscan be treated as ionic surfactants with large polar head groups(the POMs). Amphiphilic surfactants can interact with twoimmiscible solvent phases and lower the interfacial tension, andself-organize into supramolecular architectures when their con-centration is above the critical association concentration (CAC).Sulfate, sulfonate, phosphate, carboxylate and ammonium arecommon head groups for regular ionic surfactants, which aremuch smaller than the POMs. Consequently, POM-based surfac-tants could greatly change the surfactant packing parameter Ns,which is widely applied to explain and predict the self-assemblybehavior of a surfactant through the relationship Ns = Vc/(LcA0)≈ Ac/A0, in which Vc is the volume of the hydrocarbon tail, Lc isthe length of the hydrocarbon tail, and A0 is the area per headgroup.70 When Ns is small (<0.37), spherical micelles areexpected to form in solution; when Ns is close to 0.5, cylindermicelles become the favourable assemblies; when Ns approaches1, lamella phase or vesicles should be observed. If Ns is largerthan 1, reverse vesicles or micelles are two possible structures(as illustrated in Fig. 8). Therefore, it is important fundamentallyto study how the giant POM affects the surfactant packing par-ameter. Moreover, by tethering different organic functionalgroups to the hydrocarbon tails, the POM-based hybrid surfac-tants could show better bio-compatibility, better catalytic per-formance and stronger sorption ability in different systems.

As shown in Table 1, different interesting supramolecularassemblies have been reported for various amphiphilic hybridPOMs, and the morphologies of the final assembled structuresare largely influenced by several important factors, such as mol-ecular morphology, hydrophobic chain length, solute con-centration, solvent polarity, the type of counterions, pH andother external stimuli. Typical examples of diblock POM hybridsare a group of [PW11O39(SiCn)2]

3− surfactants (indicated as H1in Table 1), reported by Polarz and co-workers.72 They were con-structed by covalently linking two organo-alkoxysilane CnSi-(OCH2CH3)3 (n = 8, 12, 18) chains with a [PW11O39]

3− keggincluster under acidic conditions. Liquid crystalline phases were

observed at high hybrid concentrations. The amphiphilic proper-ties of this hybrid are significantly enhanced after replacinginitial counterions of TBA+ with H+, Na+ or K+. Small-angle-X-ray-scattering (SAXS) studies confirmed a hexagonal cylinderpacking (space group P6/mm) with a core–shell structure formedin solution. The hybrid [PW11O39(SiC16)2]Na3 can form micellesin diluted aqueous solutions. The size of the micelles alsodepends on the length of the surfactant tails.

Triblock POM-based hybrids are arranged in either anorganic–inorganic–organic (O–I–O) or an inorganic–organic–inorganic (I–O–I) fashion. Cronin and co-workers synthesizedan O–I–O type hybrid [n-Bu4N]3[MnMo6O18{(OCH2)3-CNHCO-(CH2)14CH3}2] (H4 in Table 1) recently.73 A similarhybrid surfactant of Mn-Anderson-C6 with shorter chains (H3 inTable 1) was also synthesized. H4 is insoluble in water butquite soluble in acetonitrile (MeCN) or MeCN–water mixtures toform stable and homogeneous solutions, probably due to thebulky TBA+ counterions. The formation of vesicular structure isobserved for both H3 and H4 in mixed solvents of MeCN–water, as conformed by dynamic and static light scattering (DLSand SLS) techniques.74 It is reasonable to consider the POM partfaces outside while the two alkyl chains bend and stay inside toform the solvent-phobic layer in the vesicles. However, com-pared with H4, fewer vesicles are formed for H3 in the sametype of solvent, probably due to their less amphiphilic nature andthe shorter alkyl chains which makes the bending of the chainsmore difficult. However, quite different from common ionic sur-factants (for example, sodium dodecyl sulfate or phosphorlipids) which tend to form vesicles and reach equilibrium rela-tively fast, it took weeks and months for the amphiphilic hybridPOMs to reach equilibrium. The size of the vesicles continues togrow during the following two months, until it stops at a hydro-dynamic radius (Rh) of 115 nm. Generally, a lipid surfactantsuch as dipalmitoylphophatidylcholine (DPPC), which possessesthe same double alkyl chains as H4, tends to form smaller vesi-cles (∼10 nm) in aqueous solution. Therefore, the much largervesicles formed by the H4 surfactant indicate that the giant polarhead groups will significantly affect the packing of individualhybrid molecules on the vesicle surface. Moreover, since theAnderson cluster has a stiff plate structure, a high bendingenergy is required to bend two alkyl chains in order to form avesicular structure. In a separate study, a reversed vesicular phasewas observed for these two hybrid surfactants in toluene–MeCNmixed solvent. The formation of reverse vesicles further estab-lishes the amphiphilic nature of the H3 and H4 hybrids.

The solution behaviour of the hybrids will change when thealkyl chain length becomes very short. The hybrid H2 was con-structed by covalently grafting four organoruthenium(II) unitsonto the rim of the central cavity in the wheel-shaped POM K16-

Li11[{K(H2O)}3{Ru(p-cymene)-(H2O)}4P8W49O186(H2O)2],making the hydrophobic p-cymene groups point out on bothsides of the “wheel”.75 The hydrophilic and the hydrophobicmotifs of H2 are tightly attached to each other, making thewhole molecule rigid. H2 has high solubility in water and water–acetone mixtures up to 80 vol% acetone content. A slow self-association process was observed in samples containing 55–80vol% acetone, and spherical, hollow structures were determinedby DLS and SLS.76 It is found that the size of the assembliesalways decreases with increasing the less polar solvent content,

Fig. 8 Illustration of how different geometries of surfactant moleculeswill lead to various supramolecular assemblies.71

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which is contradictory to other hydrophilic POM macroions.77 Itis possible that the four organic ligands on the hybrids’ surfaceprovide additional hydrophobic interactions in regulating thesupramolecular assemblies’ size. Such assemblies are black-berry-type structures instead of amphiphilic vesicular structures.

A typical example of the I–O–I type POM based hybrids wassynthesized by linking two V3-capped Wells–Dawson-typecluster TBA5H4[P2V3W15O62] with a linear bis-(tris) ligands.78

One of these hybrids is shown in Table 1 as H8, which isapproximately 3.4 nm long, representing the first example of a“dumbbell-shaped” hybrid. These dumbbells form an LB film atwater/air interfaces, and vesicular structures in water–acetonemixed solutions. A linear relationship is found between the Rh ofthe vesicles and the inverse of the solvent dielectric constant.This is a typical feature for a charge-regulated self-assemblyprocess.79 Moreover, at a fixed solvent content (50 : 50 v%water–acetone), the vesicle size shows a concentration depen-dence in the range of 0.05–0.25 mg mL−1. DLS studies indicatethat the average size of the vesicular structures remains constant(∼60 nm) when the H8 concentration is less than 0.15 mg mL−1.At higher concentrations, the Rh value increases up to 120 nm.

The formation of vesicular structures by the current dumbbellhybrids is unique due to their shapes and charges. Consideringthe geometry of the current hybrids, the packing parameter Ns

must be close to 1, suggesting the formation of either the lamellastructure or the bilayer vesicle structure. No micelle formationwas observed. Meanwhile, the high surface charge density onthe two POM heads hinders the close contact of individualdumbbells. In order to overcome the electrostatic repulsionbetween the two POM heads and trigger the vesicle formation,the TBA countercations must play an important role. We specu-late that TBAs interact with the hybrid POMs in two possibleways: one is to decrease the electrostatic repulsion between the

highly charged dumbbell head groups, and the other is toincrease the hydrophobic interactions by inserting the fourn-C4H9 chains of the TBA into the organic linkers of the hybriddumbbells. From zeta potential studies we confirmed theformer78 and from NMR studies we confirmed the latter.80 Amodel is presented in Fig. 9 to demonstrate the possible vesiclestructure. The vesicles are still negatively charged overall, but

Table 1 The structure of some amphiphilic hybrid POMs with covalently grafted organic functional groups and their self-assembled supramoleculararchitectures in selected solvents

Hybrid POMs formula Abbreviation Structure SolventSupramolecularassemblies

[n-Bu4N]3[PW11O39(SiCn)2] (n = 8, 12, 18) H1 Water Micelle/liquid crystal

K16Li11[{K(H2O)}3{Ru(p-cymene)-(H2O)}4P8W49O186(H2O)2]

H2 Water–CH3CN “Blackberry” typestructure

[n-Bu4]3[MnMo6O18{(OCH2)3-CNHCO-(CH2)2CH3}2] H3 Toluene–CH3CN

Vesicles/reversedvesicles

[n-Bu4]3[MnMo6O18{(OCH2)3-CNHCO-(CH2)14CH3}2] H4 Toluene–CH3CN

Vesicles/reversedvesicles

[n-Bu4N]2[V6O13{(OCH2)3C(NH(CO)(CH2)3C16H9)}2] H5 Water–DMSO Vesicles

[n-Bu4N]2[V6O13{(OCH2)3C(NH(CO)CH2CH2CH2C16H9)}2]

H6 Water–DMSO Vesicles

[n-Bu4N]2[V6O13{(OCH2)3CCH2OOC(CH12)16CH3}2] H7 Water–acetone Vesicles

[n-Bu4N]10H2[{P2V3W15O59(OCH2)3CCH2}2O] H8 Water–acetone Vesicles

Fig. 9 Top: Vesicular structures formed by H8 hybrid dumbbells inwater/acetone mixed solvents. The TBA counterions are closely associ-ated with the POM polar heads and also contributing their hydrophobictails in the solvent-phobic layer. Bottom: TEM images of the vesicularstructures formed by H8 hybrid dumbbells in water–acetone mixed sol-vents. Reprinted with permission from ref. 78. Copyright 2009 JohnWiley and Sons.

2858 | Dalton Trans., 2012, 41, 2853–2861 This journal is © The Royal Society of Chemistry 2012

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most of the remaining counterions are closely associated to theexternal and internal surfaces of the vesicles.

Five dumbbell shaped hybrids with different central linkergroups were systematically studied for comparison.66 The vesicleformation is found to be an entropy-driven process, and theentropy term TΔS is much larger compared with conventionalsurfactants, such as the ionic Gemini surfactants. In addition, thelength of the organic linker may show a direct relation to theTΔS value, as for a longer linker the entropy gain tends to bemore negative because more hydrogen bonds need to be broken.Moreover, the vesicle size can be easily and accurately tuned bysimply changing the solvent polarity, as shown in Fig. 10.

These results indicate that the vesicle formation is a quitegeneral behavior for the new types of surfactants with large inor-ganic polar head groups. In addition, the unprecedented for-mation of such structures from hybrids with large redox-activePOM head groups could pave the way for many future materialsapplications.

Controllable supramolecular assemblies with fluorescenceproperties. Supramolecular assemblies which can respond tosingle or multiple external stimuli, such as temperature, ionicstrength, pH, redox, light, ultrasound and magnetic field, are ofgreat interest.81–84 In 2007, Cronin and co-workers reported thesynthesis of pyrene modified hybrid POMs.53 Recently, Hillet al. developed two amphiphilic hybrid POMs with one or twopyrene groups attached to the central polyoxovanadate, indicatedas H5 and H6 in Table 1.80 Pyrene is a well-characterized fluoro-phore to probe the polarity of the local micro-environment,either in a hydrophobic or hydrophilic media, from the changeof specific emission peaks. It was found that these two amphi-philic POMs could form fluorescent-active vesicular structures inwater–DMSO mixed solvents by folding the pyrene groups intothe middle hydrophobic layer, as shown in Fig. 11. By replacingthe original bulky TBA (or TMA, TEA) counterions withprotons, obvious emission peak shifts were noticed in the

fluorescence spectra, which were attributed to the formation ofpyrene excimers. Since it is well documented that the formationof pyrene excimers largely depends on spatial distance of twopyrene monomers (<0.5 nm), an estimation of the distance ofadjacent H6 molecules could be made through monitoring thechange of emission pattern. Indeed, the emission spectra ofpyrene gradually changed with the solution pH values, and sodid the vesicle size and zeta potential value of the vesicles.Moreover, from the 2D NOESY NMR study, for the first time, itwas observed that the amphiphilic TBA counterions interact withpyrene groups in the hydrophobic layer of vesicles. This infor-mation is important to understand how the hybrid surfactantsarrange themselves to form closely packed regions in the supra-molecular structures and could be used to explain how thecounter-ions perturbs the solvent-phobic layer formation.

Luminescence can also be directly emitted from the POMs. Itis generally believed that POMs are a group of good lumines-cence quenchers because of their ability to accept one or mul-tiple electrons to stabilize fragile structures.85 There are verylimited examples that show isopolyoxometalates can emit lumi-nescence.86 However, recently it was noticed that one amphiphi-lic hybrid polyoxovanadate (TBA)2V6O13{(OCH2)3CCH2

OOC(CH2)16CH3}2 (H7), with two long hydrocarbon chainscould emit strong blue luminescence with proton or sodiumbeing the counterions.87 The luminescence intensity was furtherenhanced when the hybrids self-assembled into vesicles in sol-ution. It is possible that the origin of the emission involves anemissive state derived from a ligand-to-metal charge transfer(LMCT) process.

By combining both the surfactant encapsulation and thecovalent grafting strategies together, Wu’s group recentlyreported a new amphiphilic hybrid POM system, which couldself-assemble in solution and demonstrated interesting smartmorphological changes between fibrous and spherical aggregatesunder photo-irradiation, as shown in Fig. 12.88 This hybrid POMis synthesized by first covalently grafting the azobenzene group

Fig. 10 The size of hybrid vesicles shows a linear relationship withsolvent polarity. Hybrid 1 TBA10H2[(P2V3W15O59(OCH2)3CNHCO)2];hybrid 2 TBA10H2[(P2V3W15O59(OCH2)3CNHCO)2CH2]; hybrid 3TBA10H2 [(P2V3W15O59(OCH2)3CNHCOCH2)2]; hybrid 4 TBA10H2

[(P2V3W15O59(OCH2)3CNHCOC5H3N)2]; hybrid 5 TBA10H2[(P2V3

W15O59(OCH2)3CCH2)2O]. Reprinted with permission from ref. 66.Copyright 2011 American Chemical Society.

Fig. 11 Top: Fluorescent vesicles self-assembled from H6 hybridPOMs in water–DMSO mixed solvents. Bottom: (a) Fluorescencespectra of H5 and H6 with different counterions. (b) Plot of the pyrenemonomer fluorescence peak I(375 nm)/I(395 nm) versus the counterionsize for hybrid clusters H5 and H6 with different counterions. Reprintedwith permission from ref. 80. Copyright 2011 American ChemicalSociety.

This journal is © The Royal Society of Chemistry 2012 Dalton Trans., 2012, 41, 2853–2861 | 2859

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onto the Mn-Anderson type cluster and then encapsulating thewhole cluster into a shell of dimethyldioctadecylammonium(DODA) surfactants through electrostatic interactions. Upon UVirradiation at 365 nm for 30 min, a gradual morphology changewas observed for these hybrid POMs from cross-linked fibrous/rodlike assemblies to spheres with smooth surface in the mixedsolvent of CHCl3 and CH3OH. The spherical assemblies couldturn back to rodlike structures when visible-light irradiation wasapplied. This photo-induced reversible structural transition wasbelieved to derive from the trans-to-cis isomerization of azogroups.

4. Conclusions and future work

The exploration of the amphiphilic hybrid POMs and theirassemblies is still in its infancy. Only a few amphiphilic hybridPOMs have been studied and their assemblies show variousinteresting properties. However, systematic studies of theirchemical and physical properties are needed for a comprehensiveunderstanding of their fascinating features. The current achieve-ments and remaining problems have inspired chemists to designrational synthetic strategies and synthesize novel amphiphilichybrid POMs. There are several directions in this field deservingmore attention and they may rapidly expand during the nextdecade. For example, amphiphilic hybrid POMs with chiralligands attached may demonstrate interesting self-assembly beha-viours and they could find important applications for enanotiose-lective catalysis and separation. The incorporation of aminoacids or peptides with POMs is expected to create novel hybridmaterials that combine the biological functionality of biomole-cules with good chemical stability of POMs. Moreover, surfacemodification of POMs with dendrimers, cyclodextrins, or func-tional polymers could be synthesized in the near future. Smartsupramolecular assemblies that can response to different environ-mental stimuli is another important research direction for design-ing and synthesizing amphiphilic hybrid POMs, which may beemployed as drug delivery vehicles and magnetic resonanceimaging (MRI) agents.

Acknowledgements

T. L. thanks NSF (CHE1026505) and Alfred P. Sloan Foundationfor support. We also thank Dr Zicheng Xiao and Fadi Haso forhelpful discussion.

Notes and references

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