Exploring the programmable assembly of a polyoxometalate-organic hybrid via metal ion coordination

Article abstract of DOI:10.1021/ja404777g

The conformational flexibility and programmed assembly of a dumbbell-shaped polyoxometalate-organic hybrid molecule comprising two Dawson-type polyoxometalates linked by a 2,2′-bipyridine unit, which can be coordinate to metal ions, in this case of Zn²⁺, are described. SAXS, UV/vis, and NMR spectroscopic techniques confirm that the hybrid molecules exist as the trans dumbbell in metal-ion-free solutions and can be reversibly transformed into the cis dumbbell through coordination upon the addition of ZnCl₂ into a DMSO solution containing the hybrid. Subsequent addition of EDTA reverses the switching process by extracting the Zn²⁺ cations from the hybrid. During the interchange process between trans and cis dumbbells, a further reorganization of the hybrid molecules occurs through bond rotation to minimize steric clashes between the polyoxometalate subunits, in order to stabilize the corresponding dumbbell conformation. The Zn²⁺- controlled conformational transformation of the hybrid can be further utilized to manipulate the hybrid's solvophobic interaction-driven self-assembly behavior in the metal-ion driven reversible formation of 140 nm sized vesicles, studied by laser light scattering techniques.

Full text of DOI:10.1021/ja404777g

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Article
Exploring the Programmable Assembly of a
Polyoxometalate-Organic Hybrid via Metal Ion Coordination
Panchao Yin, Tao Li, Ross S. Forgan, Claire Lydon, Xiaobing Zuo, Zhaoxiong
Norm Zheng, Byeongdu Lee, De-Liang Long, Leroy Cronin, and Tianbo Liu
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja404777g • Publication Date (Web): 09 Aug 2013
Downloaded from http://pubs.acs.org on August 13, 2013
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Exploring
the
Programmable
Assembly
via Metal
of
a
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Polyoxometalate-Organic
Coordination
Hybrid
Ion
†
‡
¶
¶
‡
Panchao Yin, Tao Li, Ross S. Forgan, Claire Lydon, Xiaobing Zuo, Zhaoxiong Norm
,
¶
,†,±
†
‡
¶
Zheng, Byeongdu Lee, Deliang Long, Leroy Cronin,* and Tianbo Liu*
†Department of Chemistry, Lehigh University, Bethlehem, PA 18015, USA
‡X-ray Science Division, Advanced Photo Source, Argonne National Laboratory, Argonne, IL 60439, USA
¶WestCHEM, School of Chemistry, University of Glasgow, Glasgow, G12 8QQ, UK
±Department of Polymer Science, The University of Akron, Akron, OH 44325, USA
*The correspondence should be addressed.
ABSTRACT: The conformational flexibility and programmed assembly of a dumbbellꢀshaped
polyoxometalateꢀorganic hybrid molecule comprising two Dawsonꢀtype polyoxometalates linked
2
+
by a 2,2’ꢀbipyridine unit which can be coordinate to metal ions, in this case of Zn , is described.
SAXS, UV/Vis, and NMR spectroscopic techniques confirm that the hybrid molecules exist as
the trans dumbbell in metalꢀionꢀfree solutions, and can be reversibly transformed into the cis
dumbbell through coordination upon the addition of ZnCl into a DMSO solution containing the
2
2+
hybrid. Subsequent addition of EDTA reverses the switching process by extracting the Zn
cations from the hybrid. During the interchange process between trans and cis dumbbells, a
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further reorganization of the hybrid molecules occurs through bond rotation to minimize steric
clashes between the polyoxometalate subunits, in order to stabilize the corresponding dumbbell
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+
conformation. The Zn ꢀcontrolled conformational transformation of the hybrid can be further
utilized to manipulate the hybrid’s solvophobic interactionꢀdriven selfꢀassembly behavior in the
metalꢀion driven reversible formation of 140 nm sized vesicles, studied by laser light scattering
techniques.
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INTRODUCTION
Molecular switches are molecules that can be reversibly shifted between two or more stable
states in response to environmental stimuli, e.g. pH, light, temperature, redox potential, electric
1
ꢀ14
field, and the presence of ligands or metal ions.
In particular, the control of supramolecular
assembly via metal ion coordination is a very useful approach to preparing artificial molecular
13,15,16
switches,
and is vital in many biological behaviors, e.g. metal ionꢀdirected protein folding
2
+
+
and selfꢀassembly, Ca induced contraction or relaxation of the human heart, Na stimulated
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+
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nerve impulses, and the Ca ꢀgated K ion channel found inside cell membranes.
It remains
challenging, however, to build artificial metalꢀionꢀdriven molecular switches based on relatively
simple molecules that operate on the tens of kDa scale of proteins and demonstrate significant
modification of selfꢀassembly behavior. Molecular switches that respond to metal ion stimuli can
suffer from the fact that complexation often only affects significantly the local area around the
metal binding site, limiting the functionality of the switches. Moreover, molecular switches that
display metalꢀionꢀresponsive folding and selfꢀassembly behavior could be thought of as simple
models to help us understand the corresponding biological process involving proteins and RNAs.
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Polyoxometalates (POMs) are a large group of structurally wellꢀdefined molecular metalꢀoxide
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clusters (ca. 1 ~ 6 nm) with diverse physical properties and applications.
Chemically grafting
organic ligands/chains onto the POMs results in POMꢀorganic hybrids, which can maintain,
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modify, or enhance the optical,
electrical,
thermal, and fluorescence
properties of
the POMs. The functionality (e.g. selfꢀassembly behavior and catalytic activity) of POMꢀbased
hybrids relies significantly on their molecular conformation and, therefore, POMs which exhibit
conformational changes in response to external stimuli could be used as ‘switches’ to enhance or
remove these functions. Herein, we report, to the best of our knowledge, the first example of a
POMꢀbased metalꢀionꢀdriven molecular switch, which demonstrates a reversible switching
2
+
process upon the coordination of Zn cations. Moreover, during the interchange between the
trans and cis dumbbells, a further reorganization of the hybrid molecules occurs through bond
rotation to minimize steric clashes between the polyoxometalate subunits, in order to stabilize the
corresponding dumbbell conformation. This controllable metal ion translocation is used to
reversibly tune the packing parameter of the POMꢀbased hybrid (which can be treated as an
amphiphile) and its consequent selfꢀassembly behavior in the metalꢀion driven reversible
formation of 140 nm sized vesicles, as studied by laser light scattering (LLS) and TEM
1
techniques. The working cycles of the molecular switch were also monitored by UVꢀvis and Hꢀ
NMR spectroscopy, and the switching process was elucidated both by Synchrotron SmallꢀAngle
XꢀRay Scattering (SAXS) and 2D Nuclear Overhauser Effect Spectroscopy (NOESY).
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RESULTS AND DISCUSSION
Molecular Structure of the Hybrid Molecular Switch. Hybrid 1, TBA H [{P V W O
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(OCH ) CNHCO} (C H N) ] (TBA, tetrabutylammonium), has been reported and fully
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characterized in a previous publication. The hybrid has a dumbbellꢀlike shape with the two
Dawsonꢀtype POMs at opposite ends linked by a 2,2’ꢀbipyridine unit, as evidenced by the crystal
structure of a partially cationꢀexchanged sample (see experimental section for details) displayed
in Figure 1. The covalent but nonꢀconjugated linkage between the organic and inorganic building
blocks of 1 ensures the molecule is quite stable whilst the functionalities of different components
3
are maintained. The size of the molecule is ca. 1 × 1 × 4 nm and it has a molecular weight of ca.
10.6 kDa. The 2,2’ꢀbipyridine unit has been explored with respect to the incorporation into the
design of switchable molecules because of the metalꢀionꢀinduced conformational rotation around
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the central CꢀC single bond which occurs upon coordination of the metal ion.
The purpose of
designing hybrid 1 is to transfer the metalꢀionꢀresponsive behavior from the bipyridine unit to the
hybrid POMs, study the interaction between the highly charged and bulky POM units in transꢀ
and cisꢀ dumbbell, and control the hydrophobicityꢀdriven selfꢀassembly behavior through the
introduction of metal ions. The POM units are connected to the 4, 4’ positions of the
heteroaromatic rings, which not only can extend the bipyridine unit’s conformation change to the
entire hybrid molecule, but also attenuate somewhat the possible steric hindrance between the
two POM units during the rotation process (Figure 2). Based on the above assumption, the
hybrid can exist in two conformations: the trans dumbbell in a metalꢀionꢀfree environment, as
illustrated in the solidꢀstate structure, and the cis dumbbell driven by the coordination of the two
nitrogen atoms of the central 2,2’ꢀbipy unit to a metal ion.
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Figure 1. Representation of the molecular structure of the anionic component of hybrid 1.
Counterions and solvent molecules have been removed for clarity. Color code: Wꢀblue, Vꢀgrey,
Pꢀorange spheres; Cꢀgrey, Oꢀred, Nꢀblue, Hꢀwhite sticks.
Figure 2. The reversible conformational transformation that occurs when 1 (denoted the trans
dumbbell herein) is complexed by a divalent metal chloride to form the complexed (cis)
dumbbell.
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UV/vis Monitoring the Trans to Cis Dumbbell Transformation Process. Titration of ZnCl2
into a DMSO solution of the pure bipyridine ligand without POM attachments results in a
classical bathochromic shift of the bipyridineꢀbased absorption from λmax of ca. 300 to 330 nm,
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which is attributed to the complexation between Zn ions and the ligand (Figures S1 and S2 in
supporting information). A resolved absorption peak for the bipyridine moiety is not observed in
the UV/vis spectrum of hybrid 1, because the strong absorption peak of the Dawsonꢀtype POM
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unit obscures it. Despite this, a similar bathochromic shift can be observed when Zn is added to
a DMSO solution of 1 (Figure 3). For a control titration experiment, ZnCl is titrated against the
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39
dumbbellꢀshaped hybrid 2 which has an oxalateꢀbased linker rather than a bipyridine one
(Figure S3 and S4 in supporting information). It induces no obvious change in the UV/vis
spectrum, ruling out the possibility that the interaction between the anionic POM subunit and
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+
2+
Zn ions contributes to the bathochromic shift. As a result, the addition of Zn is expected to
trigger the formation of a Znꢀbipyridine complex and consequently change the conformation of
the dumbbell structure from trans to cis.
Figure 3. UVꢀVis monitoring of the titration of ZnCl into a DMSO solution of hybrid 1.
2
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H-NMR Monitoring of the Reversible Transformation Process. HꢀNMR spectroscopy was
used to obtain detailed information regarding the reversibility of the metalꢀionꢀdriven switching
1
process. HꢀNMR spectroscopic studies in DMSOꢀd on the bipyridine ligand without POM
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attachments shows downfield shifts and broadening of the resonances associated with the
heteroaromatic rings and amino groups in the spectrum of the ligand upon the addition of ZnCl2.
2
+
The signal changes reach a maximum after the Zn ions reach a molar ratio of 3.5:1 to
bipyridine (i.e., 3.5 equivalents, see Figure S5). This peak shifting and broadening effect is
characteristic of complexation. A similar phenomenon can be observed in the corresponding
experiments (Figure 4) with hybrid 1. After titrating larger amounts of ZnCl into the hybrid
2
solution (up to 40 equivalents), the signals corresponding to Ha and Hc at 7.85 and 8.75 ppm,
respectively, gradually decrease, coinciding with the appearance and growth of two new peaks at
2+
8
.15 and 9.05 ppm, indicating the Zn ꢀcomplexation controlled trans to cis transformation.
Meanwhile, the peaks assigned to Hb and Hd, at 8.45 and 8.85 ppm, respectively, continuously
move downfield to 8.55 and 8.90 ppm, respectively. As high as ca. 40 equivalents of ZnCl are
2
needed to fully convert the trans dumbbell of hybrid 1 to the cis dumbbell, probably due to the
repulsive interaction between the giant Dawson clusters during the rotation process.
Interestingly, the downfield shifted peaks return to their original upfield positions after the
addition
of
ca.
20
equivalents
of
(TBA) ꢁEDTA
(tetraꢀnꢀbutylammonium
4
ethylenediaminetetraacetate, see experimental section), suggesting that the EDTA chelates the
2+
Zn ions by extracting them from the cis dumbbell, which subsequently converts by rotation
2
+
around the C–C bond to the trans dumbbell configuration. The addition of Zn ions and their
later removal with EDTA in the DMSO solution of hybrid 1 can be considered as a running cycle
1
2+
of a molecular switch (Figure 2). Further HꢀNMR studies confirm that the Zn ꢀcontrolled
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switching process occurs seamlessly for at least 5 cycles without decomposition of the hybrids or
dysfunctionality (Figure S7 in support information).
Additionally, the trans to cis dumbbell transformation leads to an observable change in the
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diffusion speed of the hybrid molecule, as measured by 2D diffusion ordered H NMR
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spectroscopy (DOSY). The DOSY results divulge diffusion coefficients (D) of 7.9 x 10 m s
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2 ꢀ1
for the trans dumbbell (before adding ZnCl ) and 1.3 x 10 m s for the cis dumbbell. Based on
2
the StokesꢀEinstein diffusion equation (Equation provided in experimental section), the
hydrodynamic radii (R ) of the trans dumbbell and the cis dumbbell can be calculated as 1.4 nm
h
and 0.8 nm, respectively. This observation could be explained by the shorter distance between
two Dawsonꢀtype POMs in the cis dumbbell than in the trans dumbbell.
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Figure 4. Stacked partial HꢀNMR spectra (500 MHz, DMSOꢀd , 298 K) covering the addition
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of 0ꢀ40 equivalents of ZnCl to 25 mg of the hybrid 1 dissolved in DMSOꢀd , showing signal
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broadening and shifting upon complexation of the Zn to the bipyridine unit. The spectrum of
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2+
the sample after subsequent addition of (TBA) ꢁEDTA (top) which chelates the Zn cations
4
shows signals commensurate with the metalꢀfree hybrid. The assignment of the resonances in 1
1
1
was confirmed by a Hꢀ H COSY spectrum (see supporting information).
Observation of Coordination Driven Switch by SAXS and NOESY. The molecular
framework of hybrid 1 is essentially rigid and only five single bonds are allowed to rotate (the
pair of C4ꢀN1, the pair of C3ꢀC4, and C1ꢀC1´ in Figure 1 and 2); these bonds control the
molecular conformation. However, due to the symmetrical structure of the Dawsonꢀtype POM,
rotation of the pair of C5ꢀN1 single bonds has no effect on changing the hybrid’s conformation.
The rotation of C1ꢀC1´ bond, and thus the cisꢀtrans switch, highly relies on the complexation of
the bipyridine chelating unit with metal ions. In addition, the C3ꢀC4 bond is critical for the study
of the coꢀconformation of both the trans and the cis dumbbells. Our experimental results and the
consideration of minimized energy of the dumbbell structure suggest that the amide groups and
the neighboring heteroaromatic rings are expected to adopt a coꢀplanar orientation because of the
conjugative effect, which will limit the possible coꢀconformations for both the cis dumbbell and
the trans dumbbell to only 3 each (Figure 1, Figure 5, and discussions in supporting information).
SAXS, a powerful technique for characterizing nanostructures in solution, was used to determine
40ꢀ44
the coꢀconformation of hybrid 1 in DMSO solution.
The SAXS profiles of hybrid 1 before
(
open square) and after (open circle) adding ZnCl (Figure 6) are remarkably different, exhibiting
2
ꢀ1
peak maxima at q = 0.27, and 0.33 Å , respectively (Figure 6a). Furthermore, the pair distance
45
distribution functions (PDDF) in real space, p(r), were obtained using the program GNOM as
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shown in Figure 6b. All PDDFs exhibit a dumbbellꢀtype bimodal feature, where the first peak
describes the intraꢀsubunit (Dawsonꢀtype POM) distances and is the same for both dumbbells,
while the second peak describes the interꢀsubunit pair distances. The position of the second peak
roughly measures the center to center distance of the two POM subunits of the dumbbellꢀshaped
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hybrid. The second peak maximum is shifted from 28.5 Å to 21.2 Å after adding ZnCl , as a
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result of the metalꢀbinding induced trans to cis switch; the POM subunit separation is
considerably shorter in the cis dumbbell. In order to further probe the solution coꢀconformations,
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the theoretical PDDF for each coꢀconformation was calculated using the program SolX
and
superimposed upon the experimental PDDFs in Figure 6c. All the three trans coꢀconformations
fit well with the experimental curve due to the similar subunit separations in each. For the cis
dumbbell, however, the calculated PDDF for cisꢀII displays the best fit to the experimental data
of the cis dumbbell solution, indicating that it is the most populated coꢀconformation, with the
probable slight shoulder at ca 35 Å suggesting a small contribution from the other coꢀ
conformations is possible, i.e., the dumbbells exhibit some dynamic behavior in DMSO.
To further examine the coꢀconformations, 2D NOESY NMR spectroscopic measurements were
employed. 2D NOESY NMR spectroscopy can identify spins undergoing crossꢀrelaxation and
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measure their crossꢀrelaxation rates. The intensity of NOESY crossꢀpeaks are dependent on the
distance between protons and normally a signal is observed only if the corresponding protonꢀ
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proton distance is < 5 Å. As such, NOESY experiments were expected to complement the
SAXS studies by determining the extent of protonꢀproton interactions, and the proximity of the
organic fragments. The NOESY spectrum recorded at room temperature on a sample of 1
dissolved in DMSOꢀd , i.e., the trans dumbbell, exhibited a much stronger correlation between
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the amido proton (Hd) and Hc compared to that between Hd and Ha, implying that transꢀIII is
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the major coꢀconformation of the Znꢀfree hybrid in solution (Figures 5 and 7a), as opposed to the
transꢀI coꢀconformation observed in our particular solidꢀstate sample (Figure 1). In contrast, the
NOESY spectrum of a DMSO solution of 1 with 40 equivalents of ZnCl added to induce full
2
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complexation (i.e., to form the cis dumbbell) exhibits significant correlations between Hd and
both Ha and Hc. This observation appears to correlate well with the SAXS result, which
suggests that cisꢀII is the major coꢀconformation of the cis dumbbell solution (Figure 5 and 7b).
1
It should be noted that cisꢀII has reduced symmetry and, as a result, we would expect the Hꢀ
NMR spectrum to exhibit six resonances for the aromatic protons since all are heterotopic, but
only three broad resonances are observed. We attribute this observation to the dynamic exchange
between coꢀconformations, through rotation of the C3ꢀC4 bond, which is occurring fast on the
NMR timescale and giving a spectrum which is the average of all coꢀconformations of the cis
dumbbells, major or minor, sampled in solution.
In theory, addition of ZnCl to 1 to stimulate the rotation of the C1ꢀC1´ bond would change the
2
expected major coꢀconformation, transꢀIII, into cisꢀIII if the C3ꢀC4 bonds do not rotate.
However, the interꢀPOM distance (center to center) in cisꢀIII is quite short (1.2 nm) and so steric
hindrance/static charge repulsion is expected to be the driving force for one of the C3ꢀC4 bonds
to rotate by 180° and relieve this unfavorable strain by increasing the distance between the
Dawson POM subꢀunits to 2.0 nm (Figure 5, 7c and video MV1 in supporting information). This
2
+
further rotation of the C3ꢀC4 bond could explain why a large excess of Zn is required for the
transformation from the trans to the cis dumbbell.
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Figure 5. Calculated structures of the three possible coꢀconformations which can be generated
by rotation around the C3ꢀC4 bonds for both the trans dumbbell (Znꢀfree) and cis dumbbell (Zn
complex), respectively.
SAXS was also used to confirm that the cis dumbbell could be converted back to the trans
2+
dumbbell when the Zn ions are removed from solution by the addition of a competing chelating
agent. After addition of (TBA) ꢁEDTA, (shown in Figure 6a open triangle), the d spacing
4
increases to around ca. 28 Å, based on the p(r) curve in Figure 6b (open square) indicating the
hybrids have reversibly changed back again to the trans dumbbell, (Figure 7c).
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Figure 6. a) SAXS curves of DMSO solutions of the metalꢀfree trans dumbbell of 1 (open
squares), the cis dumbbell generated after adding an excess of ZnCl (open circles), and the trans
2
dumbbell which is regenerated after adding TBA ꢁEDTA to the solution containing the cis
4
dumbbell (open triangles). The SAXS profiles are vertically offset for clarity. b) Pair distance
distribution functions (PDDFs), of the three samples obtained using the program GNOM. c) The
overlay of calculated PDDFs of the three possible trans coꢀconformations and the experimentally
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derived PDDF of the trans dumbbell. The calculated PDDFs were computed directly from the
molecular coordinates using the program SolX. d) The overlay of calculated PDDFs of the three
possible cis coꢀconformations and the experimental derived PDDF of the cis dumbbell.
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Figure 7. Labelled partial Hꢀ H NOESY NMR spectra (500 MHz, DMSOꢀd , 298 K) of (a)
6
2
+
hybrid 1 without Zn and (b) hybrid 1 with 40 equivalents of ZnCl added. The difference in
2
relative strengths of the HaꢀHd and HcꢀHd correlation cross peaks indicate that 1 predominantly
2+
adopts the transꢀIII coꢀconformation and, in the presence of Zn , the cisꢀII coꢀconformation,
although dynamic exchange is possible. Structural diagrams for both dumbbells are given for
guidance only, to show both possible orientations of the amide groups with respect to rotation
around the C3ꢀC4 bond, and the subsequent close contacts between protons.
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Metal-Ion-Driven Self-Assembly/Disassembly behavior of Hybrid in Solution. It has been
demonstrated previously that dumbbellꢀshaped hybrid POMs are amphiphilic molecules capable
3
9,50,51
of forming vesicular nanostructures.
The selfꢀassembly behavior of amphiphilic surfactants
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is controlled by various factors especially their packing parameter (P), which is defined
as v /a l , where v is the surfactant tail volume, l is the tail length, and a is the equilibrium area
o
e o
o
o
e
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per molecule at the aggregate surface.
In the current case, the metalꢀionꢀdriven
conformational change of 1 was expected to alter its packing parameter (P), which might be
utilized to manipulate its solvophobicꢀdriven selfꢀassembly behaviour. The trans dumbbells,
when dissolved in DMSO/methanol (1:1 v/v) solution, exhibit a stable and low scattered intensity
(ca. 40 kcps; c.f. scattered intensity for benzene is ca. 120 kcps) in timeꢀresolved static light
scattering (SLS) studies, suggesting that they exist as solvated single molecules. In contrast, the
scattered intensity of this solution of 1 increased to ca. 9,000 kcps ꢀ without any precipitation ꢀ
after the addition of ca. 18 equivalents of ZnCl , suggesting the formation of large structures. A
2
typical CONTIN analysis from a dynamic light scattering (DLS) study on these solutions
indicated the existence of assemblies with an average R of 138±7 nm, and with a narrow size
h
2
+
distribution (Figure 8a), only when Zn is present. The average radius of gyration (R ) of these
g
assemblies that is obtained from the SLS measurement, 144±7 nm, correlates closely with Rh,
suggesting a hollow spherical vesicular structure for the assemblies. This observation is further
confirmed by TEM studies (Figure 8b). By removing ZnCl from the cis dumbbell through the
2
addition of (TBA) ꢁEDTA, the conformation of 1 should change back to trans and the vesicles
4
disassemble, as indicated by the drastic drop of the scattered intensity back to 47 kcps. This
controllable selfꢀassembly and disassembly processes can be repeated with the same sample by
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adding and removing ZnCl over several cycles (TOC graph and experimental details in support
2
information).
A control experiment using the nonꢀfunctional hybrid 2 shows that no large assemblies are
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2+
observed, even after 54 equivalents of Zn ions are added (see experimental details in support
information), which rules out the possibility that any POM subunitꢀZnCl interaction is the major
2
driving force for the formation of these vesicular structures. The trans dumbbell of 1 is of
approximately cylindrical shape with a diameter of ca. 1 nm; therefore the closest approach of
the heteroaromatic rings in two adjacent hybrid molecules can be no less than 1 nm based on a
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parallelꢀpacking model (see figure S9 in supporting information).
Since a distance of ca. 0.34
nm between the aromatic groups is necessary to form faceꢀtoꢀface πꢀπ stacking contacts, it is
difficult for the hydrophobic organic linker of the trans dumbbell to directly interact with one
another due to the steric hindrance of the larger POM subunits. In a previous study, the Znꢀfree
(i.e. trans dumbbell) of 1 was observed to form vesicles in highly polar solvents (water/acetone
mixtures) since the alkyl tails on the TBAs are expected to strongly interact with the organic
39,50
linkers in these highly polar solvents and fill the solvophobic region
. We expect that the
interaction between the TBA cations and the organic linkers becomes weaker in comparatively
less polar solvents, e.g. the DMSO/methanol mixtures utilized in this study, and, therefore, no
assemblies of the trans dumbbell of 1 were observed. The cis dumbbell is a Vꢀshaped molecule,
with a torsion angle of 108° and a P value of 0.55, which is much large than that of the trans
dumbbell, 0.40. In the proposed packing model in figure 8c, two cis dumbbell can strongly
interact with each other through hydrophobic interactions and the πꢀπ stacking of adjacent
heteroaromatic rings, yielding interꢀPOM distances of > 2 nm, enabling two possible packing
modes of cis dumbbell to form vesicles. Firstly, a monolayer structure, where two polar head
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groups of 1 are located on the outer and inner surfaces, respectively, with the linkers forming the
solvophobic layer of the vesicle; and secondly, a bilayer structure, where the two polar heads are
on the same surface with the linkers residing inside the vesicle shell. The ‘bent’ feature of cisꢀ
dumbbell prefers the bilayer model of vesicles because of the easiness of packing; however, we
cannot rule out the possibility of the monolayer vesicle model. The wellꢀestablished theory
suggests that a surfactant with packing parameter 0.5 to 1.0 is able to assemble into bilayer
structures. The cisꢀdumbbell has a packing parameter of 0.55, therefore more likely to form
bilayer vesicles.
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Figure 8. a) DLS results of the assemblies of Hybrid 1 in Methanol/DMSO mixed solvents at
45° scattering angle. b) TEM image of the large assembly. c) The bilayer model for the vesicle
structure.
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CONCLUSIONS
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In summary, we have shown that the bipyridineꢀbased dumbbellꢀshaped POM hybrid 1, the first
POMꢀbased metalꢀionꢀdriven molecular switch, can be switched via the addition and removal of
2
+
Zn which in turn can dramatically change the supramolecular aggregation of the clusters. The
hybrid exists as the trans dumbbell in metal ionꢀfree solutions and can be converted to the cis
dumbbell upon the coordination of zinc(II) and this is fully reversible, switching the
conformation of the hybrid cluster over multiple cycles with controlled addition and removal of
2
+
Zn ions in a DMSO solution of the hybrid. A multiꢀresponsive switching behavior is observed
during the transformation process and the transformation can be further utilized to manipulate its
selfꢀassembly behavior, which is believed to mimic the behavior of metal ionꢀdirected folding
and assembly process of proteins. The current study could be helpful in not only understanding
the metal ionꢀcontrolled folding and assembly behavior of biomacromolecules and designing
proteinꢀmimic smart molecules with controlled folding and selfꢀassembly behavior, but also
developing POMꢀbased materials with tunable/modified properties (e.g. catalytic properties and
encapsulation/delivery).
EXPERIMENTAL SECTION
Single Crystal X-Ray Diffraction. A sample of 1 was dissolved in dimethylformamide and
+
+
subjected to partial cation exchange (TBA to H ) using Amberlite 15 cation exchange resin.
Vapor diffusion of diethyl ether into the solution yielded single crystals, one of which was
selected and mounted onto a rubber loop using Fomblin oil. XꢀRay diffraction intensity data
were collected at 150(2) K on a Bruker Apex II Quasar CCD diffractometer (λMoꢀKα = 0.71073 Å)
equipped with a graphite monochromator. Structure solution and refinement were carried out
5
4
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with SHELXSꢀ97 and SHELXLꢀ97 via WinGX interface. The main dumbbell clusters were
well defined during structure refinement but the tetrabutylammonium cations were only partially
57
modelled with parts missing in the deep solvent area. Platon SQUEEZE procedure has been
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applied to the structure data to improve the structure quality and to determine the solvent void
volume of crystal lattice.
Crystal
data
3
and
2 3
structure
refinements
for
=
(
C H N) H [{P V W O (OCH ) CNHCO} C H N ]: C H N O P V W , Fw
16 36
10
2
2
15 59
2
10
6
2
180 382 14 126
4
6
30
ꢀ1
3
10704.02 g mol ; orange block crystal: 0.12 × 0.06 × 0.03 mm . Monoclinic, space group P2 /n,
a = 15.4135(5), b = 28.3495(10), c = 34.4392(12) Å, β = 93.178(2) , V = 15025.6(9) Å , Z = 2, ρ
1
o
3
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–3
=
2.366 g cm , λ (MoꢀKα) = 0.71073, 84776 reflections measured, 23950 unique (R = 0.1054)
int
which were used in all calculations. Final R1 = 0.0822 and wR2 = 0.2384 (all data). CCDC
reference number CCDC 936592.
SAXS experimental section. The SAXS experiments were performed at 12ꢀIDꢀB station with
Xꢀray energy of 12 KeV at the Advanced Photon Source of the Argonne National Laboratory.
The sample to detector distance was about 2 m. A 2D CCD detector was used to acquire images
with typical exposure times in the range of 1.0 s.
TEM. The TEM images were taken on a JEOL JEMꢀ2000 electron microscope operated at 200
kV. Samples for the TEM analysis were prepared by dropping a small volume of the solution
sample onto a holey carbon film on copper grid. EDS experiments were carried out under the
same condition with EDS attachments (Oxford) in JEMꢀ2000.
Static light scattering. A commercial Brookhaven Instrument LLS spectrometer equipped with
a solidꢀstate laser operating at 532 nm was used for measurement of both SLS and DLS. SLS
experiments were performed at scattering angles (θ) between 20 and 100º, at 2º intervals.
However, due to the large fluctuations in scattered intensities at low scattering angles, we
removed the data from 20ꢀ40º in the final analysis. Derived from RayleighꢀGansꢀDebye
5
8
equation , partial Zimm plot was used to analyze the SLS data to obtain the radius of gyration
2
g
2
(R ). The partial Zimm plot stems from the following approximate formula: 1/I = C(1+R *q /3).
g
2
Here R is determined from the slope and the intercept of a plot of 1/I vs. q .
g
Dynamic light scattering. DLS measures the intensity–intensity time correlation function by
(1)
means of a BIꢀ9000AT multiꢀchannel digital correlator. The field correlation function |g (τ)|
5
9
was analyzed by the constrained regularized CONTIN method to yield information on the
distribution of the characteristic line width Γ. The normalized distribution function of the
characteristic line width, G(Γ), so obtained, can be used to determine an average apparent
2
translational diffusion coefficient, Dapp =Γ/q . The hydrodynamic radius R is related to D via the
h
Stokes–Einstein equation: R = kT/(6πηD) where k is the Boltzmann constant and η the viscosity
h
of the solvent at temperature T. From DLS measurements, we can obtain the particleꢀsize
distribution in solution from a plot of Γ*G(Γ) versus R . The R of the particles is obtained by
h
h
extrapolating Rh,app to zero scattering angle. The normalized distribution function of the
characteristic line width, G(Γ), so obtained, can be used to determine an average apparent
2
translational diffusion coefficient, Dapp =Γ/q . The hydrodynamic radius R is related to D via the
h
Stokes–Einstein equation: R = kT/(6πηD) where k is the Boltzmann constant and η the viscosity
h
of the solvent at temperature T. From DLS measurements, we can obtain the particleꢀsize
distribution in solution from a plot of Γ*G(Γ) versus R . The R of the particles is obtained by
h
h
extrapolating Rh,app to zero scattering angle.
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1
D NMR and 2D COSY, DOSY, and NOESY NMR Experiments. All the NMR spectra were
recorded on Bruker Avance 500 MHz spectrometer equipped with a BBO probe at 298 K. 2D
COSY spectra were collected under standard conditions. 2D NOESY were performed with
mixing times ranging from 50 to 500 ms. The appropriate mixing time for trans dumbbell and cis
dumbbell solution were determined to be 70 ms and 300 ms.
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DOSY was performed on a Bruker Avance 500 MHz spectrometer with the magnetic field
gradient (g) varying from 0 to 32 G/cm in 16 ~ 32 steps. The length of the gradient (d ) was from
6
0
000 ms to 8000 ms, and the time interval between two pulsedgradients (D) was from 0.1 s to
.15 s. All spectra were taken at room temperature. After the data collection, FIDs were
processed and analyzed with the NMR software TopSpin 2.0 provided by Bruker. Both T1/T2
relaxation and CONTIN methods were used to fit the raw data. The observed proton signal I in a
standard DOSY spectrum is expressed through equation 1:
Equation (1)
where I is the reference intensity, γ is the gyromagnetic ratio of the proton. If only one
0
2
diffusive component exists in the solution, a straight line will occur in a plot of ln(I/I ) versus g ,
and the apparent diffusion coefficient (D) can be calculated from the slope using linear
regression analysis.
0
ASSOCIATED CONTENT
Supporting Information. All experimental details, Figure S1ꢀS11, single crystal data, and video.
This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*Eꢀmail: tliu@uakron.edu (T. Liu), lee.cronin@glasgow.ac.uk (L. Cronin)
ACKNOWLEDGEMENTS
T. Liu acknowledges support from the NSF (CHE1026505), the A. P. Sloan Foundation, and
Lehigh University. L. Cronin acknowledges support from EPSRC and WestCHEM. T. Li and X.
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Zuo are thankful for the use of the Advanced Photon Source, an Office of Science User Facility
operated for the U. S. Department of Energy (DOE) Office of Science by Argonne National
Laboratory, was supported by the U.S. DOE under Contract No. DEꢀAC02ꢀ06CH11357.
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