Ferroelectric switchable behavior through fast reversible de/adsorption of water spirals in a chiral 3D metal-organic framework

Article abstract of DOI:10.1021/ja403449k

A polar homochiral 3D MOF [{Co₂(L)(bpe)(H₂O)} ·5H₂O]_n constructed with cobalt(II) and a new ligand N-(1,3-dicarboxy-5-benzyl)-carboxymethylglycine (H₄L) accommodates ordered helical water streams in its helical grooves. It provides the first example of switchable ferroelectric and optical behavior through two-step reversible single-crystal to single-crystal transformation (SCSC) upon desorption/adsorption of water spirals and coordinated water molecules, respectively.

Full text of DOI:10.1021/ja403449k

Communication
pubs.acs.org/JACS
Ferroelectric Switchable Behavior through Fast Reversible De/
adsorption of Water Spirals in a Chiral 3D Metal−Organic Framework
Xi-Yan Dong,† Bo Li,† Bin-Bin Ma,‡ Shi-Jun Li,† Ming-Ming Dong,† Yan-Yan Zhu,†
Shuang-Quan Zang,*,† You Song,*,‡ Hong-Wei Hou,† and Thomas. C. W. Mak*,†,§
†College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China
‡State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing National Laboratory of
Microstructures, Nanjing University, Nanjing 210093, China
§Department of Chemistry and Center of Novel Functional Molecules, The Chinese University of Hong Kong, Shatin, New
Territories, Hong Kong SAR, China
S
* Supporting Information
stability. However, two-step explicit fast reversible single-crystal
ABSTRACT: A polar homochiral 3D MOF [{Co2(L)-
(bpe)(H2O)}·5H2O]n constructed with cobalt(II) and a
new ligand N-(1,3-dicarboxy-5-benzyl)-carboxymethylgly-
cine (H4L) accommodates ordered helical water streams in
its helical grooves. It provides the first example of
switchable ferroelectric and optical behavior through
two-step reversible single-crystal to single-crystal trans-
formation (SCSC) upon desorption/adsorption of water
spirals and coordinated water molecules, respectively.
to single-crystal (SCSC) transformation involving switchable FE
and optical properties induced by desorption/adsorption of free
and coordinated waters, respectively, has not been reported
hitherto.
Herein, we present an unprecedented homochiral 3D MOF
[{Co2(L)(bpe)(H2O)}·5H2O]n (1) {H4L = N-(1,3-dicarboxy-5-
benzyl)carboxymethylglycine, bpe = 1,3-bis(4-pyridyl)ethene},
prepared by hydrothermal reaction of Co(NO3)2·6H2O, H4L,
and NaOH in H2O-MeOH. MOF 1 contains helical water
columns and undergoes two-step reversible SCSC trans-
formation concomitant with FE and optical switchable behavior
upon desorption/adsorption of helical water columns and aqua
ligands, respectively.
etal−organic frameworks (MOFs) have potential appli-
Mcations in gas sorption, separation and storage, catalysis,
magnetism, and optics.1 Noncentrosymmetric and chiral MOF-
based ferroelectric (FE)2 or special dielectric3 materials have
attracted much recent interest, whereas some progress has been
made with other inorganic, organic−inorganic, and organic
ferroelectrics.4 Small polar guest molecules confined in MOFs,
such as water, methanol, ethanol, and organic cations often
display exceptional conductivity,5 antiferroelectric6 or ferro-
electric7 behavior, and interesting dielectric behavior.3 Ordered
wires of hydrogen-bonded water molecules in both carbon
nanotubes and nanopore membranes can exhibit spontaneous
electric polarization based on molecular dynamics simulation.8
Kobayashi’s group revealed that the chain-like water clusters
confined in channels of porous coordination polymer crystals
showed antiferroelectric behavior at high temperature.6a Long et
al. demonstrated that a (H2O)12n wire confined within a 3D
porous MOF, even in a centrosymmetric space group, can elicit a
significant FE response.9 To date, reports on ordered helical
water chains confined in homochiral MOFs mainly focus on
structural aspects,10 but their electric polarity has not been
extensively investigated.
MOF 1 crystallizes in hexagonal space group P61.13 The
asymmetric unit consists of two independent Co2+ ions, one L,
one bpe, one aqua ligand, and six free water molecules with a total
site occupation factor of 5 (Supporting Information, Figure S1).
Independent Co1 and Co2 ions possessing distorted octahedral
geometry are bridged by two carboxylate units (O7−C13−O8
and O5−C12−O6 of carboxyl groups in bidentate mode) and
one μ-O (O1) atom of another carboxylate group, resulting in a
binuclear [Co2C2O5] motif with a Co···Co distance of 3.52 Å.
Each binuclear [Co2C2O5] unit links four L, one aqua ligand
(O1W), and two bpe to form a 6-connected node as a secondary
unit. Each L serves as a μ7-bridge to coordinate seven Co ions. A
trans-bpe is coordinated to different Co ions through its terminal
N atoms (Co1−N2 2.106 Å, Co2−N3 2.127 Å) in two binuclear
[Co2C2O5] units (see Figures S1 and S4). The binuclear
[Co2C2O5] motif and ligand (L) assemble two outer coaxial
unequal double-helical chains designated as A and B. (Figure 1
and Figure S4). In helix A, the Co(II) dimers are interlinked
through m-phenylene spacers, while they are connected by trans-
carboxyl groups in helix B (Figure 1a and Figure S2a,b). Helices
A and B possess the same long pitch and are interconnected by
C7−N1 bonding (Figure S2c). A view down the c-axis easily
identifies a larger hydrophilic helical channel (diameter of 16.22
Å) surrounded by host double-helical chains, with O7 (phenyl
carboxylate) and O1W of helix A and uncoordinated O2
In addition, single-crystal to single-crystal (SCSC) trans-
formation accompanying drastically switchable physical property
change has potential applications in molecular devices such as
switches and sensors in luminescence, magnetism, and
electricity.11,12 Robust MOFs can retain their integrity when
the guest molecules reversibly adsorb/desorb, thus providing a
platform to directly and accurately investigate the properties of
confined guest molecules or their influence on host-framework
Received: April 10, 2013
Published: July 5, 2013
© 2013 American Chemical Society
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Figure 1. (a) Side view of double helix composed of unequal helix A and
B, which are shown in sky blue and rose color, respectively. (b) View of
helical assembly of bpe molecules. (c) View of the chiral host backbone
of MOF 1. Inner bpe (gray) molecules resemble supporting trestles that
consolidate the double helix by Co−N bonds (two-colored line). (d)
Hydrogen bonding interactions between water molecules and host
chains; (e) View of helical water column; (f) Hydrogen-bonded helical
water column in the groove of host chiral backbone of 1 (O7W′ is
omitted for clarity).
Figure 2. (a) In situ variable-temperature powder XRD of 1 upon both
heating and cooling (25−250 °C). (b) Time-dependent IR spectra upon
cooling. Color code: magenta line, as-synthesized complex 1; black line,
dehydrated phase 2; cyan line, rehydrated form 1′.
(iminodiacetate) of helix B pointing to the inner wall of the
channel (Figure S2d). Moreover, binuclear [Co2C2O5] units
extend outward to participate in construction of the adjacent
helical chains. As a result, each double helix is fused to the
adjacent six ones to form a 3D homochiral host framework
(Figure S3). Trans-bpe ligands presenting the same chiral
distribution apparently sustain the helical channel by Co−N
bonding with helices A and B and intermolecular π···π
interactions between adjacent bpe molecules further reinforce
the structure (Figure 1c and Figure S4). Hence double-helical
chains and bpe molecules cooperatively construct the chiral host
rigid backbone to generate a helical groove, resulting in a helical
column of water clusters intertwining the helical bpe (Figure S5).
Four lattice water molecules (O3W, O5W, O6W, O7W)
assemble into a tetramer (O7W−H7WA···O3W O7W···O3W
2.72(3) Å; O7W−H7WB···O5W O7W···O5W 2.73(3) Å;
O6W−H6WA···O3W O6W···O3W 2.79(0) Å; O6W−
H6WB···O5W, O6W···O5W 3.25(3) Å) and the other two
(O2W, O4W) suspend from O5W, which are strung together by
a H-bond between O5W and O6W (O5W−H5WB···O6W,
O5W···O6W 3.04(3) Å) to produce a water column (Figure
1d,e). This helical water column is further supported and
stabilized by maintaining hydrogen bond networks to O1W
(aqua ligand), O7 and O2 in the wall of the hydrophilic channel
(O1W···O2W 2.89(9) Å, O5W···O7 2.98(0) Å and O3W···O2A
2.90(0) Å, O3W···O2B 3.27(0) Å) (Figure 1d,e and Supporting
Information, Table S6). In this way, the host helical groove
transfers its chirality to the helical guest water column and hence
determines its orientation.
endothermic peaks (at 108 and 170 °C) in DSC results can be
attributed to the loss of free and coordinated water molecules,
respectively (Figure S6).
In situ variable-temperature powder XRD (Figure 2a) showed
that the powder pattern of 1 remains unchanged until 150 °C,
indicating that its framework is stable after free-water removal.
Some diffraction peaks disappear above 200 °C on heating, but
then return at 150 °C upon cooling. No new peaks emerge and
numerous major peaks maintain all along, indicating that the
compound can preserve its rigid and stable framework at high
temperature, and crystal-to-crystal transformation is fast and
reversible. It is noted that the dehydrated phase of 1 cannot be
isolated using the usual technique of high-temperature
evacuation to expel solvent because its rehydration is completed
within several minutes in air. Time-dependent IR spectra of an
as-prepared sample of 1 show IR bands of water in the region
3600−3100 cm−1 (Figure S7), which nearly disappear in the
dehydrated phase, but upon cooling in air they are restored to
their original intensity within several minutes (Figure 2b). Other
main IR bands remain unchanged, further suggesting that the
host backbone is preserved before and after dehydration.
Inspired by the above results and similar reports,14 we
performed in situ variable-temperature SCXRD on 1 to explore
the possibility of SCSC transformation. When single crystals of 1
were used for VT-SCXRD measurement under hot N2 flow at
323 K, all free water molecules easily escape to give the
dehydrated form [Co2(L)(bpe)(H2O)]n (2) (Figure 3b and
Table S1). X-ray analysis of 213 revealed that its crystal structure
is virtually identical to that of 1 (a, b, c, V = 15.929, 15.929, 21.366
Å, 4694.9 Å3 in 1; and 15.883, 15.883, 21.379 Å, 4670.8 Å3 in
2).13
TG−DSC measurement was performed with freshly prepared
crystals of 1 to determine the number of guest water molecules
(n) present in the complex and to examine its thermal stability.
Thermogravimetric analysis (TGA) in air revealed that the
weight loss of 12.53% (calcd 12.59%) up to 150 °C and 2.47%
(calcd 2.51%) between 150 and 200 °C correspond to the loss of
five free and one coordinated H2O per formula unit, respectively.
The framework remains thermally stable up to 350 °C in air. Two
[Co2(L)(bpe)]n (3) can be produced in situ at 400 K under
hot N2 flow. Crystal structure analysis showed that removal of the
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via fast dehydration/rehydration processes may be attributed to
robustness and hydrophilicity of the host backbone. Such two-
step dehydration involving free and coordinated water and fast
rehydration in SCSC is rare,15 which may find some use in
switching and sensor applications.
The most striking result in this study is ascribed to dramatic
water-dependent FE switchable feature. Since a water helix is
confined in the host channel of 1 (polar point group C6) at room
temperature, its FE properties were examined. Considering the
crystal symmetry, the spontaneous polarization vector is parallel
to the c axis. Single crystals of suitable size were selected to
measure the dielectric hysteresis loop with electrodes made of Au
wires covered by Ag-conducting glue on the approximate
crystallographic (001) plane. Electric hysteresis loops were
observed at room temperature under variable electric field
(Figure S9). The leakage current density in the order of 10−8−
10−7 A·cm−2 at fields of 20 kV/cm (Figure S10) demonstrates
that the observed hysteresis loops are clearly due to
ferroelectricity.2b From the structural viewpoint, the existence
of polar infinite H-bonded water columns and the spatial
asymmetry may be the key factors for spontaneous polarization.
However, it remains challenging to clarify the distinct origin of
observed ferroelectricity in 1 owing to its very complex crystal
structure. It is fortunate that 1 can undergo reversible SCSC via
cyclic dehydration/rehydration and retain its polar robust
network, which facilitates assessment of the contribution of the
water spirals to polarization. Hence the temperature dependence
of the polarization (P) was measured on a single crystal under an
appropriate electric field (E) of 3.75 kV cm−1 parallel to the
crystallographic c-axis (Figure 4). The hysteresis loop of as-
Figure 3. Schematic representation of the two-step reversible SCSC
transformation between 1, 2, and 3 and concomitant change in
structures and properties; H2O molecules are shown as large red
spheres; hydrogen bonds are indicated by green dash lines. The first step
(1−2−1′) between structures a and b involves desorption/adsorption
of free water molecules and ferroelectricity disappearing/appearing. The
coordination environment of Co(II) and color of single crystal remain
unchanged. The second step [2−3−(2′)1′] between structures b and c
involves desorption/adsorption of aqua ligands, transformation of
coordination environment of Co1 from octahedral to trigonal-
bipyramidal, and color change of the single crystal from red to dark
brown.
coordinated H2O molecules causes Co1 to adopt trigonal-
bipyramid coordination (Figure 3), in contrast to its octahedral
environment in 1 and 2 (Figure S8 and Table S4).
Accompanying this change in the Co1 coordination environment
is a color change from red to dark brown (Figure 3c). However, it
is noteworthy that the conformation of bpe changes dramatically
to stabilize the backbone of 3 (2: C21−C20−C19, 127.35°;
C16−C19−C20, 125.74°; N2−N3, 9.350 Å. 3: C21−C20−C19,
115.82°; C16−C19−C20, 116.92°; N2−N3, 9.365 Å) (Figure 3
and Figure S8 and Tables S3,S4), resulting in a smaller channel
along the c axis with a hexagonal-star cross-section. From 2 to 3,
the unit-cell volume just slightly decreases (from 4670.83 to
4632.50 Å3), but the void increases appreciably (from 26.4 to
27.9%) due to the loss of aqua ligand (O1W), being consistent
with the robustness of the host framework. Exposure of single
crystals of 2 and 3 to moisture for a few minutes results in their
reverse transformation to give the rehydrated phase 1′13 which is
identical to 1 (Figure 3 and Table S5) and concomitant with
color reversal to red (Figure 3). The reversible and repeatable
optical properties of the crystal upon loss/recapture of water can
be compared to thermal-induced optical switching. Both free and
coordinated water molecules revert to their role in 1, such that
the length of Co1−O1W remains unchanged (1: Co1−O1W,
2.134(4) Å. 1′: Co1−O1W, 2.134(5) Å) and H-bonded water
spirals take on a homomorphous shape except for tiny regulation
of the hydrogen bonds between guest water molecules (Figure 3,
Table S6 and S7). Noteworthy is the fact that the conformation
of bpe is restored to its original one (1′: C21−C20−C19,
128.19°; C16−C19−C20, 128.31°; N2−N3, 9.365 Å. 1: C21−
C20−C19, 126.47°; C16−C19−C20, 126.70°; N2−N3, 9.370
Å) (Figure S8). Unfortunately, the intermediate 2′ [Co2(L)-
(bpe)(H2O)]n with only coordinated O1W retrapped in the
framework could not be arrested, such that guest and
coordinated waters nearly revert instantly and indistinguishably
into 1′ in air as soon as the hot N2 flow is shut off (SI). The fast
reversible SCSC transformations [1−2−1′ or 1−2−3−(2′)1′]
Figure 4. Hysteresis loops for the crystalline phase of 1 (as prepared at
RT), 2 (dehydrated at 323K), and 1′ (rehydrated at RT) at field (E//c)
of 3.75 kV/cm.
prepared 1 at room temperature is marked as a red line (On
state). With temperature increasing to 323 K, the P−E loop
shrinks to a dark loop (Off state), corresponding to the
dehydrated form of 2, which displays very small polarization
induced by the chiral host backbone in the polar space group.2,4c
The P−E loop (green line) nearly returns to the initial state
(On state, Figure 4) after exposure of 2 in moisture for a few
minutes to rehydrate it to 1′, indicating that FE polarization is
almost completely restored. This reversible On/Off polarization
behavior joins hands with a fast and reversible dehydration/
rehydration of free water molecules (Figure 3), which can be
termed “water-dependent FE switching”. These experimental
results demonstrated that the FE activity (On state) mainly
originates from 1D H-bonded guest water columns confined in
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the channel, which was further confirmed by QM/MM
calculation (Supporting Information, section S9). The computed
results indicate that the water molecules in the groove contribute
more than 90% dipole moment of the c-axis direction of the
whole model system (36 H2O molecules included in one pitch of
the helical groove). Clearly, the polarization of the water spiral is
of critical importance to the inherent ferroelectricity of 1, which
is consistent with ferroelectric measurements.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, crystal structure, TGA and DSC data,
crystallographic data and data collection, structure solution and
refinement procedures, P−E hysteresis loop, I−V curves, and
dielectric spectra, calculations for polarization, CIF data. This
material is available free of charge via the Internet at http://pubs.
acs.org.
AUTHOR INFORMATION
Corresponding Author
zangsqzg@zzu.edu.cn; yousong@nju.edu.cn; tcwmak@cuhk.
edu.hk
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (No. 20901070, No. 91022031 and No.
21021062) and Zhengzhou University.
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