Multifunctional amino-decorated metal-organic frameworks: Nonlinear-optic, ferroelectric, fluorescence sensing and photocatalytic properties

Article abstract of DOI:10.1039/c2jm34349e

Solvothermal reactions of 2-amino-1,4-benzene dicarboxylic acid (NH ₂bdcH₂) and N-auxiliary ligands in the presence of manganese(ii), zinc(ii) and cobalt(ii) salts have given rise to four new metal-organic frameworks, namely, [Mn₅(NH₂bdc) ₅(bimb)₅·(H₂O)_0.5] _n (bimb = 4,4′-bis(1-imidazolyl)biphenyl) (1), [Zn(NH ₂bdc)(bix)·(DMF)₂]_n (bix = 1,4-bis(imidazol-1-ylmethyl)benzene) (2), [Co(NH₂bdc)(bix) ·(DMF)·(CH₃CH₂OH)]_n (3) and [Co(NH₂bdc)(bpp)]_n (4) (bpp = 1,3-bis(4-pyridyl)propane). Single-crystal X-ray diffraction analyses revealed that MOF 1 displays 5-fold interpenetrating 4-connected dia 3D net; MOFs 2 and 3 are isomorphic, which possess 3-fold interpenetrating dia 3D nets; MOF 4 exhibits 4-connected sql 2D net. Noncentrosymmetric structures and multifunctionality in 1-3 are established by varying ligands and metal centers. In the solid state, polar MOFs 1-3 exhibit nonlinear-optic (NLO) and MOF 1 demonstrates typical ferroelectric behaviour with a remnant electric polarization (P_r) of 1.2 μC cm^-2 and an electric coercive field (E_c) of 0.35 kV cm^-1. In addition, MOF 2 could be a potential luminescent probe for detecting nitrobenzene or 2-nitrotoluene via fluorescence enhancement and has been evaluated as a promising visible-light-driven photocatalyst for degradation of organic pollutants.

Full text of DOI:10.1039/c2jm34349e

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PAPER
Multifunctional amino-decorated metal–organic frameworks: nonlinear-optic,
ferroelectric, fluorescence sensing and photocatalytic properties†
a
Lili Wen, Li Zhou, Bingguang Zhang, Xianggao Meng, Hua Qu and Dongfeng Li
a
b
a
a
a
*
*
Received 4th July 2012, Accepted 10th September 2012
DOI: 10.1039/c2jm34349e
Solvothermal reactions of 2-amino-1,4-benzene dicarboxylic acid (NH2bdcH2) and N-auxiliary ligands
in the presence of manganese(II), zinc(II) and cobalt(II) salts have given rise to four new metal–organic
frameworks, namely, [Mn5(NH2bdc)5(bimb)5$(H2O)0.5]n (bimb ¼ 4,40-bis(1-imidazolyl)biphenyl) (1),
[Zn(NH2bdc)(bix)$(DMF)2]n (bix ¼ 1,4-bis(imidazol-1-ylmethyl)benzene) (2),
[Co(NH2bdc)(bix)$(DMF)$(CH3CH2OH)]n (3) and [Co(NH2bdc)(bpp)]n (4) (bpp ¼ 1,3-bis(4-pyridyl)
propane). Single-crystal X-ray diffraction analyses revealed that MOF 1 displays 5-fold
interpenetrating 4-connected dia 3D net; MOFs 2 and 3 are isomorphic, which possess 3-fold
interpenetrating dia 3D nets; MOF 4 exhibits 4-connected sql 2D net. Noncentrosymmetric structures
and multifunctionality in 1–3 are established by varying ligands and metal centers. In the solid state,
polar MOFs 1–3 exhibit nonlinear-optic (NLO) and MOF 1 demonstrates typical ferroelectric
behaviour with a remnant electric polarization (Pr) of 1.2 mC cmꢀ2 and an electric coercive field (Ec) of
0.35 kV cmꢀ1. In addition, MOF 2 could be a potential luminescent probe for detecting nitrobenzene or
2-nitrotoluene via fluorescence enhancement and has been evaluated as a promising visible-light-driven
photocatalyst for degradation of organic pollutants.
accessible components with multifunctionality is seldom
reported.8
Introduction
Over the past few years, metal–organic frameworks (MOFs)
have attracted tremendous attention owing to their intriguing
structural topologies and wide potential applications including
gas storage and separation, catalysis, enantioselective separation,
luminescent sensing, and magnetic materials.1–4 Even more
remarkably, these applications can be combined and integrated
into individual frameworks to form multifunctional MOFs.5
Despite impressive progress in the area of MOFs, currently, it
remains a significant challenge to introduce, in a rational and
systematic way, more complex functionality within the lattice.
The strategy to incorporate ‘‘functional’’ metal centers or func-
tional groups on the organic bridging ligands into specific MOFs
has been proved to be very effective to address this challenge.6 In
particular, employing some amine-derivatized MOFs to realize
high storage and high selectivity toward CO2 has recently been
attempted.7 However, examples of this family from readily
For this purpose, we have chosen 2-amino-1,4-benzene
dicarboxylic acid (NH2bdcH2) as a ligand, which is a particularly
effective building block because (1) it has many potential coor-
dination modes, allowing for the construction of a topologically
diverse family of materials;9 (2) the polar NH2bdcH2 would be
more effective in constructing the noncentrosymmetric frame-
work, which is an essential requirement for nonlinear-optical
(NLO) and ferroelectric behavior; (3) the amino substituent
acting as an auxochromic and bathochromic group in the
aromatic ring, may shift the absorption wavelength and promote
charge transfer interactions of the resulting architecture, which is
promising for the development of luminescent probes and
visible-light photocatalysts.
Solvothermal reactions of NH2bdcH2 and N-auxiliary ligands
with transition-metal centers (Mn2+/Zn2+/Co2+) have yielded
[Mn5(NH2bdc)5(bimb)5$(H2O)0.5]n
zolyl)biphenyl) (1), [Zn(NH2bdc)(bix)$(DMF)2]n (bix ¼ 1,4-
(bimb ¼ 4,40-bis(1-imida-
aKey Laboratory of Pesticide
& Chemical Biology of Ministry of
bis(imidazol-1-ylmethyl)benzene)
(2),
[Co(NH2bdc)(bix)$
Education, College of Chemistry, Central China Normal University,
Wuhan, 430079, P. R. China. E-mail: wenlili@mail.ccnu.edu.cn; dfli@
mail.ccnu.edu.cn; Fax: +86 27 67867232; Tel: +86 27 67862900
bKey Laboratory of Catalysis and Materials Science of the State Ethnic
Affairs Commission & Ministry of Education, South-Central University
for Nationalities, Wuhan 430074, P. R. China
(DMF)$(CH3CH2OH)]n (3) and [Co(NH2bdc)(bpp)]n (4) (bpp ¼
1,3-bis(4-pyridyl)propane) based on single crystal X-ray struc-
tural, thermogravimetric (TG), and elemental analyses. MOF 1
displays a 5-fold interpenetrating 4-connected dia 3D net; MOFs
2 and 3 are isomorphic, and possess 3-fold interpenetrating dia
3D nets; MOF 4 exhibits a 4-connected sql 2D net. Non-
centrosymmetric structures and multifunctionality in 1–3 are
established by varying ligands and metal centers. In the solid
† Electronic supplementary information (ESI) available: Synthesis
conditions, full preparation details and characterization data. CCDC
866677 and 866678. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2jm34349e
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J. Mater. Chem., 2012, 22, 22603–22609 | 22603

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state, polar MOFs 1–3 exhibit nonlinear-optic (NLO) properties
and MOF 1 demonstrates typical ferroelectric behaviour with a
remnant electric polarization (Pr) of 1.2 mC cmꢀ2 and an electric
coercive field (Ec) of 0.35 kV cmꢀ1. More exciting, MOF 2 could
be a potential luminescent probe for detecting nitrobenzene or 2-
nitrotoluene via fluorescence enhancement and has been evalu-
ated as a promising visible-light-driven photocatalyst for the
degradation of organic pollutants. To the best of our knowledge,
this is the first example of MOFs exhibiting two interesting
combined characteristics.
Photocatalytic experiments
The visible light source was a 500 W halogen lamp with a UV
cutoff filter (providing visible light with l > 400 nm) and the UV
light source was a 350 W xenon lamp. A suspension of powdered
catalyst (50 mg) in a fresh aqueous solution of X3B (50 mL, 6 ꢂ
10ꢀ6 mol Lꢀ1) at pH 6 was first sonicated for 5 min, and shaken
at a constant rate in the dark overnight (to establish an
adsorption–desorption equilibrium of X3B on the sample
surface). At given irradiating intervals, a series of suspensions of
a certain volume were collected and filtered through a membrane
filter (pore size, 0.45 mm) to remove suspended catalyst particles,
and the filtrate was analyzed on the UV/vis spectrometer. The
organic dye concentration was estimated by the absorbance at
510 nm, which directly relates to the structure change of its
chromophore. The UV/vis spectra for solution samples were
obtained on a Shimadzu UV 2450 spectrometer.
Experimental
Materials and physical measurements
The reagents and solvents employed were commercially
available and used as received. Ligand bimb was prepared by
literature methods.10 C, H and N microanalyses were carried
out with a Perkin-Elmer 240 elemental analyzer. IR spectra
were recorded on KBr pellets on a Bruker Vector 22 spec-
trophotometer in the 4000–400 cmꢀ1 region. Thermogravi-
metric analyses were performed on a simultaneous SDT 2960
thermal analyzer under flowing N2 with a heating rate of
10 ꢁC minꢀ1 from ambient temperature to 700 ꢁC. The
powder XRD data were collected on a Siemens D5005
Synthesis of [Mn5(NH2bdc)5(bimb)5$(H2O)0.5]n (1)
A mixture of MnCl2$4H2O (19.7 mg, 0.10 mmol), NH2bdcH2
(18.1 mg, 0.10 mmol), bimb (28.6 mg, 0.10 mmol) in DMA
(1 mL) and H2O (1 mL) was placed in a parr Teflon-lined
stainless steel vessel (25 cm3), and then the vessel was sealed and
heated at 140 ꢁC for 2 days. After the mixture was slowly cooled
to room temperature, yellow crystals of 1 were collected (yield:
75% based on Mn). Anal. Calcd. For C26H19.20MnN5O4.10: C,
59.80; H, 3.71; N, 13.41%; found: C, 59.82; H, 3.75; N, 13.46%.
IR spectrum (cmꢀ1): 3446m, 3337m, 3134m, 2361w, 1621s,
1542s, 1517s, 1430s, 1383s, 1335m, 1309m, 1254m, 1119w,
1064m, 1005w, 962w, 897w, 841m, 819w, 779w, 739m, 655m,
624w, 575w, 526m.
ꢀ
diffractometer with Cu Ka radiation (l ¼ 1.5418 A) over the
2q range 5–50ꢁ at room temperature. A pulsed Q-switched
Nd:YAG laser at a wavelength of 1064 nm was used to
generate an SHG signal from samples. The backward scat-
tered SHG light was collected using a spherical concave
mirror and passed through a filter which transmits only
532 nm radiation. The solid-state diffuse-reflectance UV/Vis
spectra for powder samples were recorded on a Perkin-Elmer
Lambda 35 UV/Vis spectrometer equipped with an integrating
sphere by using BaSO4 as a white standard.
Synthesis of [Zn(NH2bdc)(bix)$(DMF)2]n (2)
A mixture of Zn(NO3)2$6H2O (29.7 mg, 0.10 mmol), NH2bdcH2
(18.1 mg, 0.10 mmol), bix (23.8 mg, 0.10 mmol), DMF(3 mL)
was placed in a 5 mL glass vial, which was sealed and heated at
110 ꢁC for 2 days. After the mixture was slowly cooled to room
temperature, light yellow crystals of 2 were collected (yield: 55%
based on Zn). Anal. Calcd. for C28H33N7O6Zn: C, 53.57; H, 5.30;
N, 15.63; found: C, 53.63; H, 5.25; N, 15.66%. IR spectrum
(cmꢀ1): 3428m, 3325m, 3130m, 2929w, 1670s, 1611s, 1576s,
1524m, 1438m, 1365s, 1251s, 1093s, 1030w, 952m, 831m, 772m,
720w, 656m, 624w, 579w.
P-E hysteresis loop and I–V curves
The single crystal with approximate size of 1.0 ꢂ 0.13 ꢂ 0.13
mm3 for 1 was selected to measure the dielectric hysteresis loop
on a ferroelectric tester (Radient Technology), with electrodes
made of Cu wire with about 150 mm diameter covered by Ag-
conducting glue on one relatively larger crystallographic face.
When the applied electric field varied from positive to negative
voltage, an electric hysteresis loop was observed.
Synthesis of [Co(NH2bdc)(bix)$(DMF)$(CH3CH2OH)]n (3)
Compound 3 was prepared in a similar way to 2, using Co(NO3)$
6H2O (0.10 mmol) instead of Zn(NO3)2$6H2O. Purple crystals of
3 were collected (yield: 70% based on Co). Anal. Calcd. for
C26H30CoN6O6: C, 53.71; H, 5.20; N, 14.45%; found: C, 53.75;
H, 5.26; N, 14.48%. IR spectrum (cmꢀ1): 3427m, 3326m, 3128m,
1661s, 1617s, 1556s, 1522s, 1425s, 1369w, 1283s, 1254m, 1236m,
1148w, 1106m, 1090m, 1030w, 944w, 828w, 773m, 749w, 725w,
660m,621s, 577s, 513s.
Fluorescence measurements
Compound 2 was immersed in fresh distilled methanol for 4 days
to remove the nonvolatile solvates (DMF), and the extract was
decanted every day and fresh methanol was replaced. Then it was
activated by drying under a dynamic vacuum at 80 ꢁC overnight.
The activated sample (30 mg) of 2 was immersed in methanol
solvent (20 mL) with a different organic solvent (0.2 mL) to form
solvent included compouꢁnd 2, which was collected by filtration
and dried for 24 h at 20 C. The sample was pressed on a glass
slide for the photoluminescence studies. Fluorescence measure-
Synthesis of [Co(NH2bdc)(bpp)]n (4)
ments were recorded with
spectrophotometer.
a
Hitachi 850 fluorescence
A mixture of Co(NO3)$6H2O (29.1 mg, 0.10 mmol), NH2bdcH2
(18.1 mg, 0.10 mmol) and bpp (19.8 mg, 0.10 mmol), DMF
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(1 mL) and ethanol (1 mL) was placed in a 5 mL glass vial, which
was sealed and heated at 110 ꢁC for 2 days. After slowly cooled to
room temperature, purple crystals of 4 were collected (yield: 29%
based on Co). Anal. Calcd. for C21H19CoN3O4: C, 57.79; H,
4.39; N, 9.63%; found: C, 57.82; H, 4.44; N, 9.64%. IR spectrum
(cmꢀ1): 3448m, 3345w, 2929w, 1619s, 1558s, 1497m, 1428s,
1385s, 1330w, 1257m, 1141s, 1069w, 1023w, 959w, 892w, 843m,
812m, 773m, 705w, 614w, 577w, 519w.
Results and discussion
Description of the crystal structures
Interestingly, crystallographic analysis showed that 1 crystal-
lizes in the acentric space group Cc. The asymmetric unit of 1
consists of five independent Mn(II) atoms, five individual
NH2bdc2ꢀ anions and five distinct bix bridges, as well as two
disordered free water molecules with occupancy of 0.2 and 0.3,
respectively (Fig. S1a†). All Mn(II) centers adopt distorted
octahedral coordination geometry defined by four oxygen
atoms from two NH2bdc2ꢀ moieties and two nitrogen atoms
from separate bimb bridges (Fig. 1a). The NH2bdc2ꢀ moieties
connect Mn(II) atoms in bidentate-chelating mode to form one-
dimensional (1D) chains, which were pillared by the rigid bimb
ligands to afford a three-dimensional (3D) porous structure
with large channels of size ca. 20.5 ꢂ 19.4, 17.9 ꢂ 10.9 and 34.6
X-ray crystallography
Suitable single crystals of 1–4 were selected and mounted in air
onto thin glass fibers. 1, 3, 4 were measured at room temperature
on a Bruker SMART APEX CCD-based diffractometer with
ꢀ
graphite-monochromatic Mo Ka radiation (l ¼ 0.71073 A).
Data reductions and absorption corrections were performed with
the SAINT and SADABS software packages, respectively.11 X-
ray intensity data for 2 were collected on a Rigaku Saturn 724 +
CCD X-ray diffractometer with graphite monochromatic Mo
2
ꢀ
ꢂ 10.9 A (based on Mn/Mn separation) along the a-, b- and
c-directions, respectively (Fig. S1b–d†). The channels nearly
disappear with a free volume of only 5.6% upon interpenetra-
tion.14 Further examinations reveal that 1 adopts a five inter-
ꢀ
Ka radiation (l ¼ 0.71073 A) at 173 K. CrystalClear software
6
€
penetrating 4-connected dia net with a Schlafli symbol of 6
(Rigaku Inc., 2007) was used for data reduction and empirical
absorption correction. The structures of all the crystals were
solved by direct method using SHELXS-97 (ref. 12) and devel-
oped by conventionally alternating cycles of least-squares
refinement on F2 (SHELXL-97)13 and different Fourier
synthesis. All the non-hydrogen atoms were refined anisotropi-
cally. The hydrogen atom positions were generated theoretically,
allowed to ride on their respective parent atoms and included in
the structure factor calculations with assigned isotropic thermal
parameters. To assist the refinement, several restraints were
applied: for 3, the disordered C13/C14/C15/C16/C17/C18 atoms
of benzene rings were restrained in order to obtain reasonable
thermal parameters; the N1 atom of NH2bdcH2 ligand is disor-
dered to three positions (N1/N10/N100) in compound 4. Crystal-
lographic data and other pertinent information for 1–4 are
summarized in Table 1, while the selected bond lengths and
angles are listed in Table S1.† More details on the crystallo-
graphic studies as well as atom displacement parameters are
presented in the CIF.
(Fig. 1b).15
To investigate the influence of the N-auxiliary ligands and
metal centers on the structure of the complexes, the reaction of
flexible bix and NH2bdcH2 with Zn(NO3)2$6H2O was carried
out. A new complex 2 with acentric space group Cc was obtained.
The Zn(II) centers adopt a distorted tetrahedral coordination
geometry, composed of two oxygen atoms from two NH2bdc2ꢀ
anions and two separate bix nitrogen atoms (Fig. 1c). For the bix
ligand, two terminal imidazole groups assume a trans confor-
mation, additionally, their planes are steeply titled 99.6 and
102.9ꢁ with respect to the average plane of the phenyl, respec-
tively. In 2, each NH2bdc2ꢀ anion coordinates to two Zn atoms
with monodentate coordination modes, generating a 1D chain
along the c axis, which is further linked by bix to achieve a 3D
porous structure. The individual 3D network is ‘‘open’’ with
2
ꢀ
larger hexagonal channels of size ca. 23.9 ꢂ 20.7 A , smaller ‘‘p’’-
2
ꢀ
shaped channels of size ca. 20.2 ꢂ 14.2 A and rectangular
2
ꢀ
channels of size ca. 13.9 ꢂ 11.0 A (based on Zn–Zn distances)
Table 1 Crystal data and structure refinement for 1–4
1
2
3
4
Formula
C26H19.2MnN5O4.1
522.20
Monoclinic
Cc
34.569(3)
17.656(2)
20.504(2)
90
108.326(3)
90
11 880(2)
20
1.460
0.600
48 525
22 713
0.0570
0.1342
C28H33N7O6Zn
628.98
Monoclinic
Cc
20.661(4)
11.567(2)
14.227(3)
90
116.01(3)
90
3055.7(13)
4
1.367
C26H30CoN6O6
581.49
Monoclinic
Cc
20.960(4)
11.530(2)
14.211(3)
90
116.24(3)
90
3080.4(13)
4
1.254
C21H19CoN3O4
436.32
Orthorhombic
Pbca
11.7678(14)
17.166(2)
19.269(2)
90
90
90
3892.5(8)
8
1.489
Formula weight
Crystal system
Space group
ꢀ
a/A
ꢀ
b/A
ꢀ
c/A
a/ꢁ
b/ꢁ
g/ꢁ
3
ꢀ
V/A
Z
rcalcd
g
ꢀ1 cmꢀ3
m/mmꢀ1
0.855
8464
4519
0.0587
0.1331
0.603
12 113
5741
0.0526
0.1058
0.915
26 667
3584
0.0395
0.0942
Collected reflections
Unique reflections
R1 [I > 2s(I)]
wR2 (all data)
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by powder X-ray diffraction (PXRD). The PXRD patterns of all
as-synthesized products closely match the simulated ones from
the single-crystal data, indicating that products are in a pure
phase (Fig. S5–S8†).
NLO and ferroelectric properties
1–3 crystallize in a noncentrosymmetric space group Cc that
belongs to a polar point group (Cs), which is associated with a
second harmonic generation (SHG) response and ferroelectric
behavior. Our preliminary measurements on powdered samples
of 1–3 suggest that, at ambient temperature, 1 and 2 are
approximately 0.5 times as SHG active as urea, 3 is 0.4 times as
active. Additionally, the ferroelectric properties were examined.
Because the crystal sizes of 2 and 3 are relatively small, only the
single crystal of 1 was selected to measure the dielectric hysteresis
loop on a ferroelectric tester. An electric hysteresis loop is clearly
observed for the single crystal of 1 at room temperature (Fig. 2);
further, the extremely low leakage currents demonstrate that the
observed hysteresis loops are clearly due to ferroelectricity
(Fig. S9†). Experimental results indicate the presence of electric
hysteresis loops with a remnant electric polarization (Pr) of
1.2 mC cmꢀ2 and an electric coercive field (Ec) of 0.35 kV cmꢀ1
;
the spontaneous saturation polarization (Ps) of complex 1 is ca.
2.556 mC cmꢀ2, which is obviously significantly higher than those
found in
a typical ferroelectric compound (e.g., NaK-
C4H4O6$4H2O (Rochelle salt), Ps z 0.25 mC cmꢀ2), a MOF with
tetrazole Cd(TBP)(Cl) (TBP ¼ N-(40-tetrazoyl-benzyl)proline)
(Ps z 0.50 mC cmꢀ2), and polythreading MOFs containing
opposite charged motifs (Ps z 0.57–0.81 mC cmꢀ2) as well as
inorganic–organic hybrids (TAMS)Bi2Cl8 (TAMS2+ ¼ trime-
thylamino-N-methyl stilbazolium, Ps z 0.08 mC cmꢀ2), but
smaller than that found in a typical ferroelectric BaTiO3
synthesized by the peptide-assisted synthesis method
(Ps z 6.5 mC cmꢀ2).17 More importantly, a low applied electric
field can polarize this sample to be ferroelectric, meaning
practical utilization, compared to BaTiO3 where its Ec is about
7–8 kV cmꢀ1, much larger than that of compound 1. Such
properties probably originate from the permanent dipoles of
nonsymmetric NH2bdcH2 (1.9880 Debye) and the ionic
displacement polarizations.18 Notably, compound 1 is a 5-fold
interpenetrating architecture, the charge centers for a single net
do not easily coincide with each other in the crystal state, which
would be more effective in increasing the polarity than common
neutral complexes and therefore exhibit NLO and ferroelectric
properties.
Fig. 1 Coordination environments of central metal atoms and ligands in
1 (a), 2 (c), and 4 (e); schematic view of the 5-fold interpenetrating for 1
(b) and the 3-fold interpenetrating for 2 (d) as well as the 2D framework
of 4 (f).
along the c-, a- and b-directions (Fig. S2†). Its structure can be
simplified to a three interpenetrating 4-connected dia net
(Fig. 1d). Despite the framework interpenetration, the structure
retains a 39.8% solvent accessible void volume (1215.0 out of the
3
ꢀ
3055.7 A unit cell volume), where DMF molecules reside as
labile ligands.
When Zn(NO3)2$6H2O was replaced by Co(NO3)$6H2O,
complex 3 was obtained. Because the structure is very similar to
that of compound 2, we do not describe it any further.
Changing the N-auxiliary ligand from bix to more flexible bpp
leads to
a change in the final architecture from non-
centrosymmetric for 3 to symmetric for 4. Compound 4 crys-
tallizes in the centric space group Pbca. All Co atoms situated in
a tetragonal pyramid arrangement, are coordinated by three
oxygen atoms from two NH2bdc2ꢀ fragments and two bpp
nitrogen atoms (Fig. 1e). For the bpp ligand, two terminal
pyridine groups assume a cis conformation and are steeply titled
at 103.4ꢁ. Each NH2bdc2ꢀ anion links two Co atoms by
adopting bidentate-chelating and monodentate coordination
modes. The extension of the structure into a 2D sheet is
accomplished by NH2bdc2ꢀ and bpp linking the metal centers
(Fig. S4†). Topologically, the 2D network can be seen as a 4-
4
2
16
€
connected sql net with Schlafli symbol (4 6 ) (Fig. 1f).
Obviously, from the crystal structures described above, the
metal centers and dissimilar bridging backbones (rigid or flex-
ible) have great impacts on the structures of the complexes. The
phase purity of the bulk material was independently confirmed
Fig.
2 Dielectric hysteresis loops for single crystal 1 at room
temperature.
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Fluorescence sensing
The emission spectra for 2 exhibit a broad peak at 426 nm when
excited at 380 nm, which is more likely to be due to ligand-to-
metal charge transfer (LMCT) than to intraligand transitions,
since a broad emission (lmax ¼ 551 nm) is observed for
NH2bdcH2 (Fig. S10†). Motivated by pioneering works to make
use of microporous MOFs for detecting aromatic compounds,19
the potential of complex 2 for sensing aromatic compounds was
explored. The addition of a small amount of different aromatic
compounds to methanol differentially affects the luminescence
intensities (Fig. 3). Of particular importance is the significant
enhancing effect of nitrobenzene and 2-nitrotoluene on the
luminescence emission of complex 2. Upon irradiation at 380 nm,
complex 2 gives two new emission bands at 443 and 480 nm in the
presence of either nitrobenzene or 2-nitrotoluene addition to the
original peak at 426 nm. This is very different from the behaviors
of luminescent MOFs reported earlier, for which nitro-contain-
ing aromatics gave similar quenching profiles. The work repre-
sents the very first example of the MOF family that exhibits such
phenomena.
Fig. 4 UV/Vis diffuse-reflectance spectra of compounds 1–4 with BaSO4
as the background.
carried out after the dark adsorption–desorption equilibrium
was achieved. The distinctly shortened degradation time
compared with the control experiments underlines that catalyst
2 is active for the decomposition of X3B (Scheme 1) under
visible-light irradiation (Fig. 5). The kinetic data for the
degradation of X3B can be well fitted by the apparent first-order
rate equation, the rate constant under visible-light irradiation
was found to be 0.33 hꢀ1 for that of 2. In contrast, compounds
3 and 4 are unable to degrade X3B efficiently under the same
conditions (Fig. S12†). By careful comparison, it could be noted
that the LMCT of 2 could occur in the visible region (550 nm),
whereas that of other samples lie in the UV region, which may
be the reason for the higher visible-light responsive catalytic
activities for compound 2. After photocatalysis, compound 2
was recycled by filtration and powder XRD demonstrates that
complex 2 is rather stable during photocatalysis. In addition,
the presence of tert-butyl alcohol (TBA, a widely used _OH
scavenger) greatly depressed the photodegradation rate of X3B
on catalyst 2, that is, the relevant rate constant for 2 sharply
decreased to 0.027 hꢀ1 under visible light. The _OH quenching
experiment result suggests that the photodegradation of X3B on
catalyst 2 is predominately through attack of _OH radicals. A
simplified model of the possible photocatalytic reaction mech-
anism is proposed, see Scheme S1.† To exclude the possibility
that the photocatalytic properties of 2 result from dissolved
molecular or oligomeric fragments of solid catalysts in the
photocatalytic process, other control experiments were con-
ducted. The reaction suspensions after 10 h of irradiation were
filtered to remove the solid catalyst particles, and fresh X3B was
added into the respective filtrates for catalysis testing. Without
solid catalyst in the reaction system, the fresh X3B was not
degraded during another 10 h of irradiation under halogen
lamp, which indicates that the solution contains no photo-
catalytically active fragments. Clearly the photocatalytic activ-
ities arise solely from solid 2.
Photocatalytic activities
To elucidate the photoresponse wavelength region, the solid
state diffuse-reflectance UV/vis spectra for the as-synthesized
compounds were recorded (Fig. 4). Compounds 2–4 consist of
absorption components both in the UV and visible region
whereas compound 1 only exhibits UV absorptions. The
absorption bands are around 380, 390, 380 and 390 for 1–4,
respectively, which could be attributed to ligand-to-metal
charge transfer (LMCT). It should be noted that the absorption
band occurring in the visible region at 550 nm for 2 could also
be assigned to LMCT. In the case of 3 and 4, additional clear
peaks in the visible region were observed at 548(586) and
534(583) nm, which probably respectively originate from the d–
d spin-allowed transition of the Co2+ (d7) ions. The absorption
of 1 in the visible region is not as distinct as that of other
samples, which may result from the spin-forbidden d–d of the
Mn2+ (d5) ion.
The presence of visible region transitions motivated us to
explore applications of 2–4 in heterogeneous photocatalysis.20
The photodegradation experiment under visible irradiation was
Fig. 3 Comparison of the solid state luminescence intensity of 2 intro-
duced into different analytes (0.2 mL) in CH3OH solutions (20 mL).
Scheme 1 Molecular structure of X3B.
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and W. B. Lin, J. Am. Chem. Soc., 2005, 127, 8940; (c) U. Mueller,
M. Schubert, F. Teich, H. Puetter, K. Schierle-Arndt and J. Pastre,
J. Mater. Chem., 2006, 16, 626; (d) J. Y. Lee, O. K. Farha,
J. Roberts, K. A. Scheidt, S. B. T. Nguyen and J. T. Hupp, Chem.
Soc. Rev., 2009, 38, 1450; (e) A. C. McKinlay, R. E. Morris,
P. Horcajada, G. Ferey, R. Gref, P. Couvreur and C. Serre, Angew.
Chem., Int. Ed., 2010, 49, 6260; (f) Z. B. Ma and B. Moulton,
Coord. Chem. Rev., 2011, 255, 1623; (g) M. Dinca and J. R. Long,
J. Am. Chem. Soc., 2005, 127, 9376; (h) D. Dubbeldam,
C. J. Galvin, K. S. Walton, D. E. Ellis and R. Q. Snurr, J. Am.
Chem. Soc., 2008, 130, 10884; (i) J. R. Li, R. J. Kuppler and
H. C. Zhou, Chem. Soc. Rev., 2009, 38, 1477; (j) S. K. Henninger,
H. A. Habib and C. Janiak, J. Am. Chem. Soc., 2009, 131, 2776; (k)
M. H. Zeng, Q. X. Wang, Y. X. Tan, S. Hu, H. X. Zhao,
L. S. Long and M. Kurmoo, J. Am. Chem. Soc., 2010, 132, 2561; (l)
X. Bao, P. H. Guo, J. L. Liu, J. D. Leng and M. L. Tong, Chem.–
Eur. J., 2011, 17, 2335; (m) M. Eddaoudi, J. Kim, N. Rosi,
D. Vodak, J. Wachter, M. O’Keeffe and O. M. Yaghi, Science,
2002, 295, 469; (n) T. Ahnfeldt, N. Guillou, D. Gunzelmann,
Fig. 5 Control experiments on the photodegradation of X3B. (I) X3B/
visible light (without catalyst); (II) X3B/compound 2/TBA/visible light;
(III) X3B/compound 2/dark; (IV) X3B/compound 2/visible light.
Conclusion
ꢁ
I. Margiolaki, T. Loiseau, G. Ferey, J. Senker and N. Stock,
Angew. Chem., Int. Ed., 2009, 48, 5163; (o) M. D. Allendorf,
C. A. Bauer, R. K. Bhakta and R. J. T. Houk, Chem. Soc. Rev.,
2009, 38, 1330; (p) J. Zhang, S. M. Chen, H. Valle, M. Wong,
C. Austria, M. Cruz and X. H. Bu, J. Am. Chem. Soc., 2007, 129,
14168; (q) J. J. Perry, J. A. Perman and M. J. Zaworotko, Chem.
Soc. Rev., 2009, 38, 1400; (r) S. Horike, Y. Inubushi, T. Hori,
T. Fukushima and S. Kitagawa, Chem. Sci., 2012, 3, 116.
2 (a) S. Kaskel and R. Fischer, J. Mater. Chem., 2012, 22, 10093; (b)
D. K. Cao, M. J. Liu, J. Huang, S. S. Bao and L. M. Zheng, Inorg.
Chem., 2011, 50, 2278; (c) X. M. Zhang, T. Jiang, H. S. Wu and
M. H. Zeng, Inorg. Chem., 2009, 48, 4536; (d) D. Liu, Z. G. Ren,
H. X. Li, J. P. Lang, N. Y. Li and B. F. Abrahams, Angew. Chem.,
Int. Ed., 2010, 49, 4767; (e) X. Q. Zhao, B. Zhao, S. Wei and
P. Cheng, Inorg. Chem., 2009, 48, 11048; (f) Z. M. Zhang, S. Yao,
Y. G. Li, R. Clerac, Y. Lu, Z. M. Su and E. B. Wang, J. Am.
Chem. Soc., 2009, 131, 14600; (g) S. M. Zhang, Z. Chang, T. L. Hu
and X. H. Bu, Inorg. Chem., 2010, 49, 11581; (h) P. Yang, X. He,
M. X. Li, Q. Ye, J. Z. Ge, Z. X. Wang, S. R. Zhu, M. Shao and
H. L. Cai, J. Mater. Chem., 2012, 22, 2398; (i) D. B. Dang,
P. Y. Wu, C. He, Z. Xie and C. Y. Duan, J. Am. Chem. Soc., 2010,
132, 14321.
3 (a) M. K. Sharma and P. K. Bharadwaj, Inorg. Chem., 2011, 50, 1889;
(b) M. C. Das and P. K. Bharadwaj, J. Am. Chem. Soc., 2009, 131,
10942; (c) P. Mahata and S. Natarajan, Inorg. Chem., 2007, 46,
1250; (d) J. A. Zhao, L. W. Mi, J. Y. Hu, H. W. Hou and
Y. T. Fan, J. Am. Chem. Soc., 2008, 130, 15222; (e) Y. S. Xue,
Y. B. He, S. B. Ren, Y. F. Yue, L. Zhou, Y. Z. Li, H. B. Du,
X. Z. You and B. L. Chen, J. Mater. Chem., 2012, 22, 10195; (f)
Y. G. Huang, F. L. Jiang and M. C. Hong, Coord. Chem. Rev.,
2009, 253, 2814; (g) F. Wang, Z. S. Liu, H. Yang, Y. X. Tan and
J. Zhang, Angew. Chem., Int. Ed., 2011, 50, 450; (h) X. L. Zheng,
In conclusion, a series of diverse 4-connected amino-decorated
MOFs based on mixed-ligand systems are prepared, which reveal
interesting multiple physical properties. Single-crystal X-ray
diffraction analyses revealed that MOF 1 displays a 5-fold
interpenetrating 4-connected dia 3D net; MOFs 2 and 3 are
isomorphic, and possess 3-fold interpenetrating dia 3D nets;
MOF 4 exhibits a 4-connected sql 2D net. Noncentrosymmetric
structures and multifunctionality in 1–3 are established by
varying ligands and metal centers. In the solid state, polar MOFs
1–3 exhibit nonlinear-optic (NLO) properties and MOF 1
demonstrates typical ferroelectric behavior with a remnant
electric polarization (Pr) of 1.2 mC cmꢀ2 and an electric coercive
field (Ec) of 0.35 kV cmꢀ1, which features MOF 1 as a potential
material in microelectronics (e.g. tunable capacitors), computing
(non-volatile memory devices) and transducers. In addition,
MOF 2 could be a potential luminescent probe for detecting
nitrobenzene or 2-nitrotoluene via fluorescence enhancement and
has been evaluated as a promising visible-light-driven photo-
catalyst for degradation of organic pollutants. To the best of our
knowledge, this is the first example that exhibits the combination
of two interesting characters in MOFs, indicative of the very
promise of such MOF material for homeland security, environ-
mental and humanitarian implications. A comprehensive study
of complex 2 is currently underway to fully understand the
mechanism of the fluorescent enhancing effect on nitrobenzene
or 2-nitrotoluene.
€
Y. Liu, M. Pan, X. Q. Lu, J. Y. Zhang, C. Y. Zhao, Y. X. Tong
and C. Y. Su, Angew. Chem., Int. Ed., 2007, 46, 7399; (i) Y. Gong,
Y. C. Zhou, T. F. Liu, J. Lu, D. M. Proserpio and R. Cao, Chem.
Commun., 2011, 47, 5982.
4 (a) S. S. Chen, M. Chen, S. Takamizawa, M. S. Chen, Z. Su and
W. Y. Sun, Chem. Commun., 2011, 47, 752; (b) H. D. Guo,
X. M. Guo, S. R. Batten, J. F. Song, S. Y. Song, S. Dang,
G. L. Zheng, J. K. Tang and H. J. Zhang, Cryst. Growth Des.,
2009, 9, 1394; (c) H. Wu, H. Y. Liu, Y. Y. Liu, J. Yang, B. Liu and
J. F. Ma, Chem. Commun., 2011, 47, 1818; (d) J. Q. Liu, B. Liu,
Y. Y. Wang, P. Liu, G. P. Yang, R. T. Liu, Q. Z. Shi and
S. R. Batten, Inorg. Chem., 2010, 49, 10422; (e) Q. Yue, L. Yan,
J. Y. Zhang and E. Q. Gao, Inorg. Chem., 2010, 49, 8647; (f)
H. Y. He, D. Q. Yuan, H. Q. Ma, D. F. Sun, G. Q. Zhang and
H. C. Zhou, Inorg. Chem., 2010, 49, 7605.
5 (a) S. J. Garibay, Z. Q. Wang, K. K. Tanabe and S. M. Cohen, Inorg.
Chem., 2009, 48, 7341; (b) K. H. Li, D. H. Olson, J. Y. Lee, W. H. Bi,
K. Wu, T. Yuen, Q. Xu and J. Li, Adv. Funct. Mater., 2008, 18, 2205;
(c) S. L. Qiu and G. S. Zhu, Coord. Chem. Rev., 2009, 253, 2891; (d)
J. S. Hu, L. F. Huang, X. Q. Yao, L. Qin, Y. Z. Li, Z. J. Guo,
H. G. Zheng and Z. L. Xue, Inorg. Chem., 2011, 50, 2404.
Acknowledgements
We greatly appreciate financially support from NNSF of China
(no. 201171062 and 20801021), Program for New Century
Excellent Talents in University of China (NCET-10-040),
Program for Distinguished Young Scientist of Hubei Province
(2010CDA087), Self-determined research funds of CCNU from
the colleges’ basic research and operation of MOE
(CCNU10A01008, CCNU11C01002), and Program for Chang-
jiang Scholars and Innovative Research Team in University
(PCSIRT, No. IRT0953).
Notes and references
6 (a) J. W. Yoon, S. H. Jhung, J.-S. Chang and A. K. Cheetham, J. Am.
Chem. Soc., 2006, 128, 16846; (b) J. An, S. J. Geib and N. L. Rosi, J.
Am. Chem. Soc., 2010, 132, 38; (c) J. B. Lin, J. P. Zhang and
1 (a) J. S. Seo, D. Whang, H. Lee, S. I. Jun, J. Oh, Y. J. Jeon and
K. Kim, Nature, 2000, 404, 982; (b) C. D. Wu, A. G. Hu, L. Zhang
22608 | J. Mater. Chem., 2012, 22, 22603–22609
This journal is ª The Royal Society of Chemistry 2012

View Online
X. M. Chen, J. Am. Chem. Soc., 2010, 132, 6654; (d) F. Debatin,
A. Thomas, A. N. Kelling, B. Z. Hedin, I. Senkovska, S. Kaskel,
V. A. Blatov, L. Carlucci, G. Ciani and D. M. Proserpio,
CrystEngComm, 2004, 6, 377.
€
€
M. Junginger, H. Muller, U. Schilde, C. Jager, A. Friedrich and
H.-J. Holdt, Angew. Chem., Int. Ed., 2010, 49, 1258.
16 (a) J. F. Eubank, H. Mouttaki, A. J. Cairns, Y. Belmabkhout,
L. Wojtas, R. Luebke, M. Alkordi and M. Eddaoudi, J. Am. Chem.
Soc., 2011, 133, 14204; (b) C. Ren, Y. N. Zhang, W. J. Shi, B. Liu,
Y. Y. Wang and Q. Z. Shi, CrystEngComm, 2011, 13, 5179.
17 (a) H. Zhao, Z. R. Qu, Q. Ye, B. F. Abrahams, Y. P. Wang, Z. G. Liu,
Z. L. Xue, R. G. Xiong and X. Z. You, Chem. Mater., 2003, 15, 4166;
(b) Q. Ye, Y. M. Song, G. X. Wang, D. W. Fu, K. Chen,
P. W. H. Chan, J. S. Zhu, D. S. Huang and R. G. Xiong, J. Am.
Chem. Soc., 2006, 128, 6554; (c) X. Y. Duan, Q. J. Meng, Y. Su,
Y. Z. Li, C. Y. Duan, X. M. Ren and C. S. Lu, Chem.–Eur. J.,
2011, 17, 9936; (d) G. Xu, Y. Li, W. W. Zhou, G. J. Wang,
X. F. Long, L. Z. Cai, M. S. Wang, G. C. Guo, J. S. Huang,
G. Bator and R. Jakubas, J. Mater. Chem., 2009, 19, 2179; (e)
G. Ahmad, M. B. Dickerson, Y. Cai, S. E. Jones, E. M. Ernst,
J. P. Vernon, M. S. Haluska, Y. Fang, J. Wang, G. Subramanyam,
R. R. Naik and K. H. Sandhage, J. Am. Chem. Soc., 2008, 130, 4.
18 H. R. Zhao, D. P. Li, X. M. Ren, Y. Song and W. Q. Jin, J. Am.
Chem. Soc., 2010, 132, 18.
7 (a) R. Vaidhyanathan, S. S. Iremonger, G. K. H. Shimizu, P. G. Boyd,
S. Alavi and T. K. Woo, Science, 2010, 330, 650; (b)
R. Vaidhyanathan, S. S. Iremonger, K. W. Dawson and
G. K. H. Shimizu, Chem. Commun., 2009, 5230; (c) B. S. Zheng,
J. F. Bai, J. G. Duan, L. Wojtas and M. J. Zaworotko, J. Am.
Chem. Soc., 2011, 133, 748; (d) S. Couck, J. F. M. Denayer,
ꢁ
G. V. Baron, T. Remy, J. Gascon and F. Kapteijn, J. Am. Chem.
Soc., 2009, 131, 6326; (e) Y. Zou, S. Hong, M. Park, H. Chun and
M. S. Lah, Chem. Commun., 2007, 5182.
8 H. L. Jiang, Y. Tatsu, Z. H. Lu and Q. Xu, J. Am. Chem. Soc., 2010,
132, 5586.
9 (a) C. A. Black, J. S. Costa, W. T. Fu, C. Massera, O. Roubeau,
ꢁ
S. J. Teat, G. Aromı, P. Gamez and J. Reedijk, Inorg. Chem., 2009,
48, 1062; (b) C. P. Li, J. Chen, Q. Yu and M. Du, Cryst. Growth
Des., 2010, 10, 1623; (c) X. Y. Chen, B. Zhao, W. Shi, J. Xia,
P. Cheng, D. Z. Liao, S. P. Yan and Z. H. Jiang, Chem. Mater.,
2005, 17, 2866; (d) Z. Y. Fu, J. L. Yi, Y. Chen, S. J. Liao, N. Guo,
G. D. Yang, Y. X. Lian and X. T. Wu, Eur. J. Inorg. Chem., 2008, 628.
10 J. Fan and B. E. Hanson, Chem. Commun., 2005, 2327.
11 Bruker Analytical X-Ray, Madison, WI, 2008.
12 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008,
64, 112.
13 G. M. Sheldrick, SHELXL97, Program for Crystal Structure
19 (a) S. Pramanik, C. Zheng, X. Zhang, T. J. Emge and J. Li, J. Am.
Chem. Soc., 2011, 133, 4153; (b) D. Tanaka, S. Horike,
S. Kitagawa, M. Ohba, M. Hasegawa, Y. Ozawac and K. Toriumi,
Chem. Commun., 2007, 3142; (c) H. Xu, F. Liu, Y. J. Cui,
B. L. Chen and G. D. Qian, Chem. Commun., 2011, 47, 3153; (d)
Y. J. Cui, Y. F. Yue, G. D. Qian and B. L. Chen, Chem. Rev.,
2012, 112, 1126.
€
€
ꢁ
Refinement, University of Gottingen, Gottingen, Germany, 1997.
14 A. L. Spek, J. Appl. Crystallogr., 2003, 36, 7.
20 (a) C. G. Silva, I. Luz, F. X. L. Xamena, A. Corma and H. Garcıa,
Chem.–Eur. J., 2010, 16, 11133; (b) M. C. Das, H. Xu, Z. Y. Wang,
G. Srinivas, W. Zhou, Y. F. Yue, V. N. Nesterov, G. D. Qian and
B. L. Chen, Chem. Commun., 2011, 47, 11715; (c) Y. H. Fu,
D. R. Sun, Y. J. Chen, R. K. Huang, Z. X. Ding, X. Z. Fu and
Z. H. Li, Angew. Chem., Int. Ed., 2012, 51, 3364.
15 (a) X. L. Wang, C. Qin, E. B. Wang and Z. M. Su, Chem. Commun.,
2007, 4245; (b) A. J. Blake, N. R. Champness, P. Hubberstey,
€
W.-S. Li, M. A. Withersby and M. Schroder, Coord. Chem. Rev.,
1999, 183, 117; (c) S. R. Batten, CrystEngComm, 2001, 18, 1; (d)
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 22603–22609 | 22609