New europium coordination polymers with efficient energy transfer from conjugated tetracarboxylate ligands to Eu3+ ion: Syntheses, structures, luminescence and magnetic properties

Article abstract of DOI:10.1039/c1dt10693g

Two novel lanthanide coordination polymers, [Eu₂(EBTC)(DMF) ₅(NO₃)₂]·DMF (1) and [Eu ₂(BBTC)_1.5(CH₃OH)₂(H ₂O)₂]·7DMF·HNO₃ (2) (EBTC

Full text of DOI:10.1039/c1dt10693g

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PAPER
New europium coordination polymers with efficient energy transfer from
conjugated tetracarboxylate ligands to Eu3+ ion: syntheses, structures,
luminescence and magnetic properties†
Li-Feng Wang,a,b Ling-Chen Kang,a Wen-Wei Zhang,*a Fang-Ming Wang,a Xiao-Ming Ren*b and
Qing-Jin Menga
Received 18th April 2011, Accepted 29th June 2011
DOI: 10.1039/c1dt10693g
Two novel lanthanide coordination polymers, [Eu2(EBTC)(DMF)5(NO3)2]·DMF (1) and
[Eu2(BBTC)1.5(CH3OH)2(H2O)2]·7DMF·HNO3 (2) (EBTC4- = 1,1¢-ethynebenzene-
3,3¢,5,5¢-tetracarboxylate; BBTC4- = 1,1¢-butadiynebenzene-3,3¢,5,5¢-tetracarboxylate), were
successfully synthesized from conjugated ligands of EBTC4- and BBTC4-. Although the two
tetracarboxylate ligands have similar structures, their different rigidity/flexibility results in quite
different networks upon complexation. Complex 1 has a two-dimensional (2-D) layered structure with
two crystallographically independent Eu3+ ions, one in a distorted monocapped square-antiprism and
the other in a distorted square-antiprism coordination geometry. Complex 2 exhibits a
three-dimensional (3-D) porous framework, with one type of Eu3+ in a distorted square-antiprism and
the other in a trigondodecahedron environment. Both 1 and 2 emit the intensely red characteristic
luminescence of Eu3+ ion at room temperature, with a long lifetime of up to 1.3 and 0.7 ms, respectively,
during which the ligand emission of EBTC4-/BBTC4- was quenched by the Eu3+ ion, indicating the
existence of efficient energy transfer between the conjugated ligand of EBTC4-/BBTC4- and the Eu3+
ion. Thus, both EBTC4- and BBTC4- are ideal ligands with an “antenna” effect for the Eu3+ ion. The
two complexes show the single-ion magnetic behaviors of Eu3+ with strong spin–orbit coupling
˚
˚
interactions even if there are shorter distances (5.714 A for 1 versus 4.275 and 5.360 A for 2) between
the neighboring Eu3+ ions connected by oxygen atoms of the tetracarboxylates.
ions can be achieved via ligating an organic compound as an
Introduction
“antenna” for energy transfer to lanthanide ions.4 Alternatively,
the magnetic properties of lanthanide ions are also fascinating,
and the strongly unquenched f orbital angular momentum makes
the lanthanide coordination compounds exhibit a pronounced
magnetic anisotropy.2e,5
Lanthanide coordination compounds have appealing lumine-
scence1 and/or magnetic properties,2 and these technologically
important properties make them have potential applications in
sensors, optical storage, and lighting devices.3 Lanthanoid ions,
such as Eu3+ and Tb3+, show very narrow spectral bands, thus
providing the high chromatic purity that is very important for
some practical technical applications. However, the absorption
and fluorescence of these ions are very low since the electronic
transitions are forbidden by parity (Laporte) selection rules.
Fortunately, the intensely characteristic emission of lanthanide
Nowadays, a particular class of materials, known as metal–
organic frameworks (MOFs) or coordination polymers contain-
ing Ln3+ ions (LnMOFs), constitute a robust platform for the
creation of various types of multifunctional materials. LnMOFs,
combining light emission with microporosity, magnetism, catal-
ysis, chirality, ion exchange and molecular separation proper-
ties, present a unique chance of observing synergistic effects,
which assists in defining targets and directing future research.6
An ideal “antenna” ligand for lanthanide ions should have
intense UV absorption and the energy level of its T1 should be
matched, namely close to, but just higher than, the resonance
level of lanthanide ions, and usually the conjugated organic
molecules are good candidates for “antenna” ligands of lanthanide
ions. In addition, lanthanide ions favor binding to atoms that
can act as hard Lewis bases (such as oxygen atoms). Taking
into account the facts above, we designed and synthesized
aState Key Laboratory of Coordination Chemistry, School of Chem-
istry and Chemical Engineering, Nanjing University, Nanjing 210093.
Email: wwzhang@nju.edu.cn
bDepartment of Applied Chemistry, College of Science, Nanjing University
of Technology, Nanjing 210009. E-mail: xmren@njut.edu.cn
† Electronic supplementary information (ESI) available: schematic dia-
gram of the topological network, room-temperature excitation spectra,
PXRD patterns, TGA curves, and selected bond distances and angles
tables of complex 1 and 2. CCDC reference numbers 792723 and 792724.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c1dt10693g
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two conjugated multicarboxylate ligands, 1,1¢-ethynebenzene-
3,3¢,5,5¢-tetracarboxylate (EBTC4-) and 1,1¢-butadiynebenzene-
3,3¢,5,5¢-tetracarboxylate (BBTC4-) (Scheme 1), as the “antenna”
ligands, and further solvothermal synthesized two EuMOFs
with a formula of [Eu2(EBTC)(DMF)5(NO3)2]·DMF (1) and
[Eu2(BBTC)1.5(CH3OH)2(H2O)2]·7DMF·HNO3 (2) and studied
their characteristic fluorescence from the Eu3+ ion.
Preparations
Tetramethyl 1,1¢-butadiynebenzene-3,3¢,5,5¢-tetracarboxylate.
Anhydrous Et3N (20 ml) and tetrahydrofuran (20 ml) were added
to a flask containing a mixture of dimethyl 5-ethynylisophthalate7
(4.2 g, 19 mmol), Pd(PPh3)2Cl2 (0.2710 g, 0.3861 mmol) and
cuprous chloride (0.0764 g, 0.772 mmol). The resulting mixture
◦
was heated at 50 C for 24 h during which oxygen was bubbled
into the solution. The solvent was removed in vacuo and the
residue was extracted with dichloromethane and saturated
ammonium chloride solution, and dried over anhydrous MgSO4.
After filtration and evaporation, flash column chromatography
on silica gel using dichloromethane as eluant gave the product.
Yield 2.9 g (34.7%). Anal. Calcd. For C24H18O8: C, 66.36; H, 4.18.
Found: C, 66.34; H, 4.04. IR (KBr disc, cm-1): 3423 (br), 2950
1
(w), 1728 (s), 1330 (m), 1245 (s), 1001 (m), 752 (m). H NMR
(300 MHz, CDCl3): d 3.97 (s, 12H, CH3), 8.37 (s, 4H, ArH), 8.67
(s, 2H, ArH). 13C NMR (500 MHz, CDCl3): d 52.6 (CH3), 137.3,
130.0, 128.8, 122.5 (Ar), 77.4, 75.1 (C C), 165.2 (C O). MS:
m/z 433.90 (calcd 434.10).
Scheme 1 Illustration for the molecular structures of H4EBTC and
H4BBTC.
1,1¢-Butadiynebenzene-3,3¢,5,5¢-tetracarboxylic acid (H4BBTC).
To a stirred solution of tetramethyl 1,1¢-butadiynebenzene-
3,3¢,5,5¢-tetracarboxylate (0.2 g) in methanol/water (V : V = 9 : 1,
6.0 ml), potassium hydroxide (0.42 g) was added and the mixture
was heated at 80 ◦C for 24 h, then acidified with 6 M hydrochloric
acid. The precipitate was removed by filtration, washed with water
for several times, and dried in vacuo. Yield 0.15 g (85%). Anal.
Calcd. for C20H10O8: C, 63.50; H, 2.66. Found: C, 63.84; H, 2.93.
IR (KBr disc, cm-1): 3415 (br), 3083 (w), 2360 (w), 1707 (s), 1446
(m), 1277 (m), 918 (w), 759 (w). MS: m/z 377.50 [M-1]- (calcd
377.30).
Herein we present the solvothermal syntheses, crystal structures,
photophysical and magnetic properties for the two novel Eu3+
coordination polymers.
Experimental
General
All commercially available chemicals were of analytical grade and
used as received without further purification. 1,1¢-Ethynebenzene-
3,3¢,5,5¢-tetracarboxylic acid (H4EBTC) is prepared according
to the method published before,7 and 1,1¢-butadiynebenzene-
3,3¢,5,5¢-tetracarboxylic acid (H4BBTC) is synthesized by a mod-
ified method according to the literature.8 Elemental analyses (C,
H and N) were carried out on a Perkin–Elmer 240 analyzer. The
FTIR spectra were obtained on a VECTOR TM 22 spectrometer
with KBr pellets in the 400–4000 cm-1 region. The absorption
spectra were obtained from a Shimadzu UV-3100 spectrometer.
The MS spectra were measured on a Bruker Daltonics flexAnalysis
autoflexTOF/TOF spectrometer using cinnamic acid as a matrix,
or on a Finnigan LCQ electron spray mass spectrometer. 1H
and 13C NMR spectra were recorded on a Bruker DRX-500
spectrometer at ambient temperature with tetramethylsilane as
an internal reference. TGA-DTA diagrams were recorded by a
CA Instruments DTA-TGA 2960 type simultaneous analyzer
heating from 293 to 1073 K in nitrogen atmosphere at a rate
of 20 K min-1. Powder X-ray diffraction (PXRD) data were
recorded on a Shimadzu XRD-6000 diffractometer with Cu-Ka
[Eu2(EBTC)(DMF)5(NO3)2]·DMF (1). The mixture of
1,1¢-butadiynebenzene-3,3¢,5,5¢-tetracarboxylic acid (H4EBTC,
5.0 mg), Eu(NO3)3·nH2O (14 mg), DMF (0.4 ml), methanol (two
drops) and HNO3 (0.06 ml, 1 M in DMF) was heated at 85 ◦C for
12 h. After being slowly cooled to room temperature, colorless
block-shaped crystals were achieved (0.30 g, ca. 56% based on
H4EBTC). Anal. Calcd for C36H48Eu2N8O20: C: 35.44; H: 3.98; N:
9.16; Found: C: 35.50; H: 3.94; N: 9.20. IR (KBr disc, cm-1): 3390
(br), 3062 (w), 2931 (w), 1656 (s), 1433 (s), 1381 (s), 1299 (s), 1107
(m), 786 (m), 717 (m), 673 (m).
[Eu2(BBTC)1.5(CH3OH)2(H2O)2]·7DMF·HNO3
(2). Sandy
beige polyhedral crystals were grown from a solution of
1,1¢-butadiynebenzene-3,3¢,5,5¢-tetracarboxylic acid (H4BBTC,
5.0 mg), Eu(NO3)3·nH2O (13 mg), DMF (0.4 ml), methanol (two
drops) and HNO3 (0.03 ml, 1 M in DMF) at 85 ◦C for 17 h (0.23 g,
86% based on H4BBTC). Anal. Calcd for C53H71Eu2N8O26: C:
40.33; H: 4.65; N: 7.28; Found: C: 40.78; H: 4.24; N: 7.00. IR
(KBr disc, cm-1): 3395 (br), 2928 (w), 1657 (s), 1433 (s), 1380 (s),
1103 (m), 780 (m), 717 (m), 672 (m).
˚
(l = 1.54056 A) radiation at room temperature with a scan speed
of 5◦ min-1 and a step size of 0.02◦ in 2q. Photoluminescence
spectra in the solid state were recorded with a Hitachi 850
fluorescence spectrophotometer. The photoluminescence lifetime
was measured with an Edinburgh Instruments FLS920P fluores-
cence spectrometer. Temperature-dependent magnetic susceptibil-
ity measurements were performed on a Quantum Design MPMS-
XL7 SQUID magnetometer under an applied field of 100 Oe over
a temperature range of 1.8–300 K.
Crystallographic analyses
Single crystal X-ray diffraction data were collected on a Bruker
Smart Apex II CCD diffractometer at 291 K using graphite
˚
monochromated Mo/Ka radiation (l = 0.71073 A). The coordi-
nates of the non-hydrogen atoms were refined anisotropically, and
all hydrogen atoms were put in calculated positions and refined
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Table 1 Crystallographic data and structural refinements for complex 1
and 2
role for the structural construction. It has been reported that
3-D cupric microporous paddle-wheel MOFs were generated not
only for H4BBTC,10 but also for H4EBTC.7,11 Since lanthanides
have larger coordination spheres and more flexible coordination
geometries compared with the first-row transition metals,12 they
need more coordination donors with larger space. Thus, besides
ligands of EBTC4- or BBTC4-, not only water, but also some other
bulky molecules or anions, such as DMF, methanol and NO3 ,
coordinate to Eu3+, leading to completely different structures
detailed below.
Complex
1
2b
Empirical formula
Formula weight
T /K
C36H48Eu2N8O20
1216.76
291(2)
C32H21Eu2O16
965.43
291(2)
˚
Wavelength (A)
0.71073
0.71073
-
Crystal size /mm
Crystal system
Space group
0.19 ¥ 0.17 ¥ 0.14
Monoclinic
P21/c
13.989(3)
16.567(3)
21.933(3)
110.731(10)
4754.0(15)
4
0.20 ¥ 0.18 ¥ 0.15
Monoclinic
C2/c
30.208(5)
21.295(5)
18.632(3)
94.434(5)
11950(4)
8
˚
a /A
˚
b /A
Structure description
˚
c /A
b /◦
Structure of [Eu2(EBTC)(DMF)5(NO3)2]·DMF (1). Crystal of
1 belongs tothe monoclinic spacegroup P21/c, with its asymmetric
unit consisting of two different Eu3+ ions, one deprotonated
EBTC4- ligand, five coordinated DMF molecules, two coordinated
NO3- anions and one guest DMF molecule. As shown in Fig.1, the
two crystallographically independent Eu3+ ions exhibit different
coordination geometries. The Eu1 ion is nine-coordinated to form
a distorted monocapped square-antiprism coordination geometry,
in which four oxygen atoms (O2, O5c, O7b and O8b) come from
one EBTC4- ligand, four oxygen atoms (O9, O10, O12 and O13)
3
˚
V /A
Z
F(000)
qmin,max
GOF
2424
1.96–26.00
1.004
3736
1.91–26.00
0.871
◦
/
R1, wR2 [I > 2s(I)]a
0.0577, 0.1250
0.0668, 0.1681
a R = RꢀFo| - |Fcꢀ/R|Fo|; wR2 = {R [w(Fo - Fc2)2]/R (w(Fo2)2]}
.
b The
2
1/2
contribution of solvent electron density was removed by SQUEEZE in its
crystallographic data.
-
from two NO3 anions, and one (O15) from a DMF molecule
isotropically, with the isotropic vibration parameters related to
the non-hydrogen atom to which they are bonded. In complex 2,
solvent molecules in the structure were highly disordered and were
impossible to refine using conventional discrete-atom models. To
resolve this issue, the contribution of solvent electron density was
removed by SQUEEZE routine in PLATON.9 The main data of
collection and refinement details of complexes 1 and 2 are given
in Table 1. Selected bond lengths and angles are listed in Table S1
and Table S2 (ESI†).
to result in a [Eu1O9] unit. The Eu2 ion is eight-coordinated
with a [Eu2O8] unit, which adopts a distorted square-antiprism
coordination geometry bonding to four oxygen donors (O1a, O6b,
O3 and O4) from one EBTC4- ligand and another four (O16, O17,
O18 and O19) from four DMF molecules. The Eu–O bond lengths
˚
vary from 2.303 to 2.534 A, which are in the ranges compatible
with the values of the previously published lanthanide complexes
with carboxylic acids as bridging ligands.13
As illustrated in Fig. 2, four carboxylates of the EBTC4- ligand
coordinate to the Eu3+ ions via two types of coordination modes:
two carboxylates in the trans-direction adopt a bidentate bridging
mode to connect two Eu1 and two Eu2 centers with an Eu1 ◊ ◊ ◊ Eu2
Result and discussion
Syntheses
˚
distance of 5.714 A, and the other two, also in the trans-direction,
Ligand 1,1¢-ethynebenzene-3,3¢,5,5¢-tetracarboxylic acid (H4E-
adopt a chelating mode to coordinate one Eu1 and one Eu2 center,
respectively. Therefore, each EBTC4- ligand links three Eu1 and
three Eu2 ions to generate a binuclear Eu1Eu2(CO2)4 subunit. As
a result, each binuclear subunit bridges four EBTC4- ligands and
each EBTC4- ligand connects four binuclear subunits to form a
ladder-like architecture plane along the crystallographic b-axis
direction. Weak intramolecular hydrogen-bonding interactions
(C25A—H25A ◊ ◊ ◊ O4, with ◦a hydrogen bond distance of 3.00(2)
BTC) was prepared according to the published procedure in the
literature.7
1,1¢-Butadiynebenzene-3,3¢,5,5¢-tetracarboxylic
acid (H4BBTC) was hydrolyzed from tetramethyl 1,1¢-
butadiynebenzene-3,3¢,5,5¢-tetracarboxylate, which is obtained
from oxidative alkyne homocoupling of dimethyl 5-
ethynylisophthalate catalyzed by a combination of copper
and palladium salts.8c Complexes 1 and 2 obtained as crystals
were grown under similar solvothermal conditions from a solution
of the corresponding tetracarboxylate ligand and Eu(NO3)3 in
a mixed solvent of DMF, methanol and HNO3 (1 M in DMF)
at the same temperature of 85 ◦C. It is interesting that the two
complexes exhibit quite different crystal structures. 1 has a 2-D
layered motif, while 2 displays a 3-D microporous network. This
structural distinction may be mainly due to the different rigidity
of the two ligands. Compared with EBTC4-, BBTC4- is more
flexible since the butadiyne group can make the two aromatic
rings rotate more freely than those linked by the ethyne group.
As a result, the butadiyne group in BBTC4- is bent to a different
degree, with one-half of the aromatic rings of the ligands rotated
round the butadiyne group to be almost perpendicularly arranged
in order to satisfy the coordination geometry of Eu3+. Moreover,
the larger center of lanthanide Eu3+ also plays an important
˚
A and an angle of 122.00 ) exist between coordinated DMF
and EBTC4- ligands. Solvent DMF molecules, as well as the
-
coordinated DMF molecules and NO3 anions, exist within the
adjacent ladder-like 2-D layers, leading to 3-D supramolecular
structure formation due to the Van der Waals force interactions.
Furthermore, it is worth noting that EBTC4- almost keeps its
coplanarity and the ethyne group retains its linearity upon
coordination. The dihedral angle between the two aromatic rings
in 1 is 5.875◦, and the bond angles in the CAr—C C—CAr moiety
are 175.76◦ (C8-C9-C10) and 177.36◦ (C9-C10-C11), respectively.
From a topological perspective, taking the center of the aryl
rings as 3-connected nodes and the center of the binuclear as
four connected nodes, a trinodal (3, 4)-connected network was
schematically represented in Fig. S1 (ESI†) with the overall Schla¨fli
symbol of (4.6)2(42.62.82).
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Fig. 1 The local coordination environment of Eu1 and Eu2 ions in 1 with the H atoms omitted for clarity. Color scheme: Eu: teal; O: red; N: blue; C,
gray. Symmetry codes: a = x, 1.5 - y, 0.5 + z; b = 1 + x, 1.5 - y, 0.5 + z; c = 1 + x, y, z.
Fig. 2 Perspective view of the 2-D layers in the ab plane. Light blue polyhedra represent Eu1 ions, and lime polyhedra represent Eu2 ions. Hydrogens,
-
coordinated DMF molecules and NO3 anions are omitted for clarity.
Structure of [Eu2(BBTC)1.5(CH3OH)2(H2O)2]·7DMF·HNO3
(2). Crystal 2 crystallizes in the monoclinic system with a space
group of C2/c. Its asymmetric unit cell contains two crystallo-
graphically unique Eu3+ ions, one and a half BBTC4-, two coordi-
nated water molecules and two coordinated methanol molecules.
As shown in Fig. 3a, two crystallographically independent Eu3+
ions exhibit different coordination geometries in 2 although both
of them are coordinated by eight oxygen atoms. Eu1 displays
distorted square-antiprism coordination geometry with its eight
coordinated oxygen atoms coming from five BBTC4- ligands
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Fig. 3 (a) The asymmetric unit of 2 with the H atoms omitted for clarity. Color scheme: Eu, teal; O, red; C, gray. Symmetry codes: e = x, 1 + y, z; f =
x, 2 - y, 0.5 + z; g = 1.5 - x, 1.5 - y, 1 - z; h = 2 - x, y, 1.5 - z. (b) The local coordination environment of the Eu1 and Eu2 ions in 2. A and B represent
Eu2(CO2)4 and Eu2(CO2)2 subunits, respectively; a and b represent BBTC4- ligands with two different steric forms.
(O3, O4, O5, O6g, O9 and O10g) and two methanol molecules
(O13, O14). Among the five coordinated BBTC4- ligands, one
carboxylate from one BBTC4- coordinates to Eu1 as a chelating
mode, while the other four carboxylates from four different
BBTC4- ligands coordinate to two Eu1 centers in a bidentate
bridging coordination mode to result in an irregular paddle-wheel
binuclear Eu2(CO2)4 (A) unit, with Eu1 ◊ ◊ ◊ Eu1 distance of 4.275
two carboxylates in the cis-positions adopt bidentate mode to
bridge two Eu1 centers, and the other two adopt chelating mode
to bind two Eu2 atoms. In the b-BBTC4- ligand, two opposite
carboxylates in two phenyl rings adopt a bidentate bridging mode
to link two Eu1 and two Eu2 centers, and the other two also in
opposite positions adopt a chelating mode to coordinate Eu1 and
Eu2 center, respectively. Overall, each BBTC4- ligand connects
two Eu2(CO2)4 (A) units and two Eu2(CO2)2 (B) blocks. The bond
˚
A. As for Eu2, it adopts a trigondodecahedron coordination mode,
˚
which bonds to six oxygen donors (O1e, O2g, O7f, O8f, O11 and
O12) from four BBTC4- ligands, and the two remnant sites (O15,
O16) are occupied by two water molecules. Similarly, two Eu2
centers are linked by two carboxylates in a bidentate bridging
coordination mode from two different BBTC4- ligands to form
a binuclear Eu2(CO2)2 (B) subunit, with Eu2 ◊ ◊ ◊ Eu2 distance of
lengths of Eu–O vary from 2.300 to 2.714 A, which are within
the range of those usually encountered for lanthanide oxygen
coordination.
Since the organic BBTC4- building units and the inorganic
Eu2(CO2)4 (A) and Eu2(CO2)2 (B) building blocks in 2 are
alternately linked to each other, a 3-D porous metal–organic
framework is eventually constructed. As shown in Fig. 4, there
exist three types of one-dimensional channels along the c axis,
˚
5.360 A. Furthermore, two more carboxylates from another two
BBTC4- ligands coordinate to Eu2 as chelating mode. Eventually,
two kinds of BBTC4- ligand (a and b) with different steric
configurations generate upon complexation as shown in Fig. 3b.
The torsion angle of the butadiyne group (C29-C30-C30h-C29h)
is 150.973◦ and the dihedral angle between the two aromatic rings
is 8.708◦ in a form, while those in b form are 14.073◦ (C9-C10-
C20-C19) and 87.340◦, respectively. That is to say, the butadiyne
group would bend more in order to keep the coplanarity of the two
aromatic rings in BBTC4- ligand, or the two aromatic rings would
rotate to nearly perpendicular direction. In the a-BBTC4- ligand,
˚
which possesses the open window of 4.3 ¥ 1.8 A (D), 4.8 ¥ 2.5
A (E) and 5.9 ¥ 2.3 A (F), respectively. Moreover, open channel
systems also exist along the a and b axis, with an open window size
of 5.6 ¥ 5.6 A and 5.8 ¥ 2.5 A, respectively. The accessible pore
volume from the single crystal structure is 53.4%, as calculated
using the Platon program.14 When the coordinated methanol and
water molecules are removed, the volume increases up to 61.8%.
A better insight into this framework can be achieved by topology
analysis. With regard to the connectivity describing above, the
˚
˚
˚
˚
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Photophysical properties
The emission spectra upon excitation at 397 nm for 1 and
340 nm for 2 are displayed in Fig. 6, from which the characteristic
intense red luminescence originating from f–f transitions of the
Eu3+ ion are observed. The emission bands centered at 581, 595,
620, 652, 703 nm for 1 versus 580, 593, 615, 652, 695 nm for
7
2 correspond to 5D0→ Fn (n = 0→4) transition of Eu3+ ions,
respectively. Among them, the symmetric forbidden emission
7
5D0→ F0 around 580 nm is almost invisible, the medium-strong
emission around 590 nm is attributed to the magnetic-dipolar
7
5D0 → F1 transition, and the most intense emission around 620 nm
7
is ascribed to the electric-dipolar 5D0→ F2 transition.16 Moreover,
7
the weak emission peaks round 650 nm (5D0→ F3) and 700 nm
7
(5D0→ F4) also correspond to the magnetic dipole transitions.17
7
Since the electric-dipolar 5D0→ F2 transition is hypersensitive
to the coordination environment of the Eu3+ ions, while the
5
7
magnetic-dipolar D0 → F1 transition is fairly insensitive to it,
Fig. 4 Polyhedral representation of 2 seen from a, b, and c directions (the
green and turquoise polyhedra correspond to [Eu1O8] and [Eu2O8] units,
respectively).
5
7
the much stronger intensity of the D0→ F2 transition in these
7
7
complexes (the intensity ratio of I (5D0→ F2) : I (5D0 → F1) is
5.50 and 4.33 for 1 and 2, respectively) indicates that the Eu3+ ion
adopts a noncentrosymmetric coordination environment without
an inversion center,18 which is in good agreement with the result
of single-crystal X-ray analysis.
center of the Eu2(CO2)4 (A) and Eu2(CO2)2 (B) units can be
considered as six-connected nodes, and the aryl rings can be
viewed as three-connected nodes. As a result, a (3,6)-connected
net with a Schla¨fli point symbol of (4.62)2(42.66.88.10) is obtained
for compound 2 (Fig. S2, ESI†).
UV-vis spectra analyses
The absorption spectra of complexes 1 and 2, together with the
ligands of H4EBTC and H4BBTC in the solid state, are shown
in Fig. 5 and Fig. S6†, respectively. The ligands H4EBTC and
H4BBTC exhibit absorptions with intense and sharp bands at ca.
300 nm and 298, 318, 346 nm, respectively, which can be assigned
to the p–p* transitions. Similar absorption bands are observed
in complex 1 (289 and 309 nm) and complex 2 (297, 316 and
339 nm). These bands are also probably due to the p–p* transitions
of the ligands. It is obvious that the p–p* transitions of complex
2 and ligand H4BBTC are bathochromically shifted compared
with those of complex 1 and ligand H4EBTC, respectively, which
is due to a larger p-conjugative effect in compound 2 and
H4BBTC.
Fig. 6 Emission spectra of 1 (a) and 2 (b) in the solid state at ambient
temperature.
As shown in Fig. 6, the p-conjugated ligands in the compounds
H4EBTC and H4BBTC display broad fluorescent emission bands
centered at 402 and 453 nm upon excitation at 342 and 396 nm
in the solid state at room temperature, respectively, and these
Fig. 5 UV-vis spectra of complex 1 and 2.
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emission bands arise from the intramolecular p–p* transition
of the corresponding ligands. In comparison to the fluorescence
spectra of 1, 2 and H4EBTC, H4BBTC, the disappearance of
intramolecular p–p* transition of ligands in the corresponding
Eu3+ coordination polymers indicates that an efficient energy
transfer process occurs from the conjugated tetracarboxylate
ligands to Eu3+ ions in 1 and 2. The energy transfer process
between the ligand and the Eu3+ ions can be described as below:
the EBTC4-/BBTC4- ligand absorbs energy from external source
into S1 from its S0, then proceeds on the internal conversion to
T1 from the lowest vibration level of S1 and further proceeds to
an intramolecular energy transfer from T1 of the ligand to the
5
localized D0 energy level of the central Eu3+ ion, which emits
multiple characteristic emissions of the Eu3+ ion from 5D0→ FJ.15
7
The fluorescence lifetime, t, of the two complexes are investi-
gated in the solid state at room temperature, and the curves of the
fluorescence decay of 1 and 2 are illustrated in Fig. 7. The process
of fluorescence decay for both 1 and 2 follows a single exponential
decay law, as a result, the equation It = A0 + A1 ¥ exp(-t/t) was
utilized for fitting the fluorescence decay curves of 1 and 2, which
gave the best parameters as A0 = 0.02534, A1 = 3.35459 and t =
Fig. 8 Temperature dependence of cM and cMT for 1 (red) and 2 (black).
similar.20 Thus, the magnetic susceptibilities of 1 and 2 can be
fitted with a single-ion Eu3+ model based on eqn (1),21 which only
considers the spin-orbital coupling of Eu3+ ions.
N b2
3kTx B
A
cM
=
+TIP
2
1.308 ms with an overall c of 0.99847, for 1 versus A0 = 0.03528,
⎛
⎜
⎝
⎞
⎟
⎠
⎛
⎜
⎜
⎞
⎟
⎠
⎛
⎜
⎝
⎞
−6x
2
27x
3
135x
5
2
7
2
−x
−3x
A1 = 3.33493 and t = 0.702 ms with an overall c of 0.9983 for 2.
⎟
⎟
⎟
e
⎟
⎜
⎜
A =24+
−
e
+
−
e
+ 189x −
⎟
⎟
⎜
⎜
⎟
⎟
⎟
⎟
⎜
⎝
⎠
2
2
2
(1)
The f–f electronic transitions of lanthanide are forbidden, leading
to long excited state decay time. It was found that the fluorescence
lifetimes of 1 and 2 are of millisecond order, falling in the range of
lanthanide ion fluorescence decay times of 10–2000 ms.
⎛
⎜
⎝
⎞
⎟
⎟
⎠
⎛
⎜
⎜
⎞
⎟
⎟
⎠
⎛
⎜
⎜
⎞
⎟
⎟
⎠
9
2
1458x 11
2457x 13
−10x
−15x
−21x
⎟
⎟
⎟
⎟
⎟
⎟
⎜
+ 405x −
e
+
−
e
+
+
e
⎜
⎜
⎜
⎝
⎝
2
2
2
2
B=1+3e−x +5e−3x +7e−6x +9e−10x +11e−15x +13e−21x
where x = l/kT, with l being the spin-orbital coupling parameter,
k being the Boltzmann constant and TIP being the temperature
independent magnetism. Through fitting the cMT vs. T plots
of the whole temperature range, the best fitting results are
l = 265 cm-1 and TIP = 3.31 ¥ 10-3 with R = R[(cMT)calc.-
(cMT)obsd.]2/R[(cMT)obsd.]2 = 1.7 ¥ 10-4 for 1, and l = 251 cm-1 and
TIP = 4.95 ¥ 10-3 with an agreement factor R of 6.9 ¥ 10-4 for 2. The
l parameters are in good agreement with the literature values.22
The analyses for the magnetic behaviors of 1 and 2 demonstrate
that the Eu3+ ions are well isolated from each other in the magnetic
molecule field, even if the distances between the Eu3+ ions are
rather short.
Fig. 7 Typical luminescence decay profile observed for 1 (black) and 2
(red) in the solid state at room temperature, and the green and blue lines
are the corresponding monoexponential fits of 1 and 2, respectively.
Conclusion
Two Eu3+ coordination polymers, based on the conjugated
tetracarboxylate ligands of EBTC4- and BBTC4-, were successfully
synthesized under similar hydrothermal conditions. Coordination
compound 1 has a 2-D layered structure with a binuclear
Eu1Eu2(CO2)4 subunit linked by EBTC4-, while 2 exhibits a 3-
D microporous network containing two different subunits of
Eu2(CO2)4 and Eu2(CO2)2 connected by BBTC4-. The structural
differences are probably caused by the flexibility/rigidity of the
ligand and the diverse metal center matched by different coordina-
tion geometry. 1 and 2 display single-ion Eu3+ magnetic behaviors.
Moreover, they intensely emit the characteristic red luminescence
of the Eu3+ ion with a long lifetime (1.3 ms for 1 and 0.7 ms for
2) under ultravisible excitation at room temperature. Meanwhile,
the emission bands originating from the intramolecular p*→p
transition of the ligand disappeared, which indicate the existence
of efficient energy transfer from the conjugated tetracarboxylate
Magnetic properties
The variable-temperature magnetic susceptibilities were measured
at an applied magnetic field of 100 Oe in the temperature range of
1.8–300 K for 1 and 2 (Fig. 8). The cMT values are 2.5 cm3 K mol-1
for 1 and 3.0 cm3 K mol-1 for 2, respectively, accounting for the
two Eu3+ ions. In the whole temperature range, as the temperature
decreases, the cMT values gradually decrease and reach nearly zero
at 1.8 K, indicating that the f electrons of the Eu3+ ions depopulate
from their magnetic excited states and populate the diamagnetic
ground state of 7F0 during the cooling process.19
Since the 4f electrons of rare earth ions are shielded by the
outer sphere s and p electrons, the superexchange interactions
between the rare earth ions are usually very weak. Moreover,
by comparing the curves of cM and cMT with those of Eu3+
mononuclear complexes, it is interesting to find that they are very
9496 | Dalton Trans., 2011, 40, 9490–9497
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ligands to the Eu3+ ions. Therefore, the EBTC4-/BBTC4- ligands
are effective “antenna” ligands for the luminescent Eu3+ ion. This
work provides helpful information for the design and construction
of highly luminescent Ln-based coordination polymers, which
have potential applications in optical and electrooptical devices
as well as time-resolved fluorescence assays.
7 Y. X. Hu, S. C. Xiang,W. W. Zhang, Z. X. Zhang, L. Wang, J. F. Bai
and B. L. Chen, Chem. Commun., 2009, 7551.
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Acknowledgements
11 D. F. Sun, S. Q. Ma, J. M. Simmons, J. R. Li, D. Q. Yuan and H. C.
Zhou, Chem. Commun., 2010, 46, 1329.
12 D. J. Tranchemontagne, J. L. Mendoza-Cortes, M. O’Keeffe and O. M.
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This work was supported by the Major State Basic Research
Development Program (Grant 2007CB925103) and the Nature
Science Foundation of China (Grant 20871068).
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©