Eu-MOFs with 2-(4-carboxyphenyl)imidazo[4,5-f]-1,10-phenanthrolineand ditopic carboxylates as coligands: Synthesis, structure, high thermostability, and luminescence properties

Article abstract of DOI:10.1021/ic501253c

(Graph Presented) Hydrothermal reactions of europium(III) salt with 2-(4-carboxyphenyl)imidazo[4,5- f ]-1,10-phenanthroline and dicarboxylic acid as coligands - benzene-1,4-dicarboxylic acid, 4,4′-biphenyldicarboxylic acid, 2,5-dibromoterephthalic acid, and naphthalene-1,4-dicarboxylic acid - lead to four europium fluorescent materials (1-4). Structural analyses reveal that 1 - 4 have binuclear 3D metal - organic frameworks with different channels, void volumes, and conjugated structures tuned by ditopic carboxylates. There are no latticed and coordinated water molecules occurring in 1 - 3, while the free water molecules fill in 1D channels of 4. 4′ was readily obtained via water removal of 4. Thermal analyses of all compounds show the high thermal stability of the main framework up to 450 °C. Optical studies indicate that 1 - 4 and 4′ show the characteristic red luminescence emission of the EuIII ion in the visible regions at room temperature. On the basis of emission spectra, their luminescence lifetimes were determined. In particular, compound 4??? shows a longer lifetime (τ = 0.942 ms) and significantly enhanced quantum yield (39%) compared with those of 1 (11%, 0.770 ms), 2 (4%, 0.414 ms), 3 (18%, 0.807 ms), and 4 (26%, 0.858 ms).

Full text of DOI:10.1021/ic501253c

Article
pubs.acs.org/IC
Eu-MOFs with 2‑(4-Carboxyphenyl)imidazo[4,5‑f ]‑1,10-
phenanthroline and Ditopic Carboxylates as Coligands: Synthesis,
Structure, High Thermostability, and Luminescence Properties
Sheng Zhang,†,‡ Yang Yang, Zheng-Qiang Xia, Xiang-Yu Liu, Qi Yang, Qing Wei, Gang Xie,
†,‡
†
†,§
†
†
†
,†
†
San-Ping Chen,* and Sheng-Li Gao
†Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry and
Materials Science, Northwest University, Xi’an, Shaanxi 710069, P. R. China
§
School of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, China
*
S Supporting Information
ABSTRACT: Hydrothermal reactions of europium(III) salt with 2-(4-
carboxyphenyl)imidazo[4,5-f ]-1,10-phenanthroline and dicarboxylic
acid as coligandsbenzene-1,4-dicarboxylic acid, 4,4′-biphenyldicar-
boxylic acid, 2,5-dibromoterephthalic acid, and naphthalene-1,4-dicar-
boxylic acidlead to four europium fluorescent materials (1−4).
Structural analyses reveal that 1−4 have binuclear 3D metal−organic
frameworks with different channels, void volumes, and conjugated
structures tuned by ditopic carboxylates. There are no latticed and
coordinated water molecules occurring in 1−3, while the free water
molecules fill in 1D channels of 4. 4′ was readily obtained via water
removal of 4. Thermal analyses of all compounds show the high ther-
mal stability of the main framework up to 450 °C. Optical studies
indicate that 1−4 and 4′ show the characteristic red luminescence
III
emission of the Eu ion in the visible regions at room temperature. On
the basis of emission spectra, their luminescence lifetimes were determined. In particular, compound 4′ shows a longer lifetime
(
(
τ = 0.942 ms) and significantly enhanced quantum yield (39%) compared with those of 1 (11%, 0.770 ms), 2 (4%, 0.414 ms), 3
18%, 0.807 ms), and 4 (26%, 0.858 ms).
8
INTRODUCTION
states from the surrounding ligand field. So, numerous chro-
mophoric antenna ligands, in particular, π-conjugated organic
■
In recent years, lanthanide metal−organic frameworks (Ln-
MOFs) have been of intense interest owing to their fantastic
9
chromophores, have received the most attention. Herein, long-
conjugated multidentate 2-(4-carboxyphenyl)imidazo[4,5-f ]-
,10-phenanthroline (HNCP) is selected as the main ligand.
1
−3
topological structures and intriguing photoluminescence. The
former benefits from the varied coordination geometries and
abundant coordination numbers of LnIII ions, while the latter
profits from the sharp and long-lifetime characteristic 4f−4f
6
1
On the one hand, it has three potential coordination domains to
coordinate with the Ln ions, including the bidentate phenanthro-
line moiety, the amino group of the imidazolyl ring, and the
III
2
emission bands of Ln ions because of the shielding of the 5s 5p
filled subshells. Especially, EuIII ions have been regarded as
10
terminal carboxylate group (Scheme 1). When HNCP is used
attractive for employment as red emitters in multicolor displays
and organic light-emitting diodes. Eu-MOFs may thus become
outstanding candidates for exploiting optical devices. As is
known, it is inevitable to face two main problems for the con-
struction of Ln-MOFs.
as the bridging ligand, the phenanthroline N and carboxylate O
atoms in either end can multiply connect lanthanide metal cen-
tral ions. On the other hand, it is born with conjunctive aromatic
rings, which not only increases the rigidity of the structures but
also provides a powerful absorbing sensitizer. Additionally, a
certain degree of rigidity and robustness of the phenanthroline
moiety are also helpful for keeping the coordination sphere from
reducing nonradiative deactivation. These merits lay a solid foun-
dation for the design of compounds with excellent luminescence
properties.
4
3d
First, in general, the spin/parity-forbidden nature of the f−f
transitions results in abominably low mole extinction coef-
5
ficients. The characteristic emissions of europium compounds
primarily stem from electric-dipole (ED) transitions forbidden
by Laporte’s rule, associated with the change of parity.
Nevertheless, when an organic ligand functioning as the
sensitizer is introduced to conquer the weak absorption induced
by those forbidden rules, transitions become partly permitted
because of J mixing and the admixture of vibrational states or the
3
d,6,7
Received: May 30, 2014
Published: October 6, 2014
©
2014 American Chemical Society
10952
dx.doi.org/10.1021/ic501253c | Inorg. Chem. 2014, 53, 10952−10963

Inorganic Chemistry
Article
a
Scheme 1. Diverse Coordination Domains of HNCP
Scheme 2. Ligands Used in the Paper
a
Color code: red, terminal carboxylate group; blue, amino group of the
imidazolyl ring; green, bidentate phen moiety.
Second, the luminescence properties of Ln-MOFs could be
noticeably influenced by the species of guest molecules and
11
the coordination environments of metal centers. According to
12
the theory of hard and soft acids and bases, Ln ions as hard acids
are likely to be hard donor atoms, such as O atom. Consequently,
the first coordination sphere of Ln ions is easy to connect with
III
empty Al O crucible was used as the reference. Powder X-ray diffrac-
2 3
tion (PXRD) was carried out on a Bruker D8-ADVANCE diffracto-
meter equipped with Cu Kα at a scan speed of 1°/min. Other chemicals
and reagents were obtained by commercial purchase and used with-
some solvents, especially water molecules. Water molecules are
highly effective nonradiative relaxers to quench luminescence
III
because of the loss of excited-state energy from the Ln ions
1
out further purification. The H NMR spectrum was recorded on an
through the vibration energy of the close-proximity O−H oscil-
INOVA-400 Varian 400 MHz instrument in a dimethyl sulfoxide
1
3
lator. Meanwhile, these solvents limit the extension of the
(DMSO)-d solution at room temperature. Electrospray ionization mass
6
dimension of Ln-MOFs and also influence the thermal stabi-
spectrometry (ESI-MS) spectra were recorded using an HCT mass
spectrometer (Bruker Daltonics) under the conditions at which the
needle and capillary exit voltages were set to 4 kV and 200 V, respec-
tively; the desolvation chamber temperature was set to 573 K. Elemental
analyses (C, H, and N) were performed on an Elementar Vario EL III
analyzer. IR spectra were recorded on a Tensor 27 spectrometer (Bruker
1
1d,13d,14
lity.
As is reported many references, MOFs of structure,
stability, and function can be tuned via the linker design, such as
13a,15
ditopic carboxylate linkers.
The merits are as follows: (1)
They can be used as the second ligands to take the solvent’s place,
force the spatial dimension of the structure to be higher, and
further improve the stability of the framework. (2) The 3D Ln-
MOF systems can be functionalized with different organic groups
of the ligands, such as Br, NH , OC H , OC H , C H , and
Optics, Ettlingen, Germany) with pressed KBr pellets in the range of
−1
4
000−500 cm . Optical diffuse reflectance was carried out with a
Shimadzu UV-2450 spectrophotometer. The solid-state photolumines-
cence analyses and lifetime measurements were performed on an
Edinburgh FLSP920 fluorescence spectrometer. The overall quantum
2
3
7
5
11
2
4
C H . Additionally, their pore sizes could be stretched through
4
4
1
6
yields (Φ
(
) were measured by both relative and absolute methods
long molecular struts, such as biphenyl, tetrahydropyrene,
pyrene, and terphenyl. (3) Aromatic carboxylic acids frequently
acted as the sensitizers. (4) The carboxyl groups display various
coordination modes, affecting intramolecular energy transfer in
compounds. Extending the idea above, if ditopic carboxylate
linkers are selected as coligands, do the substituents in the
aromatic ring and long molecular struts affect the structures and
thus the luminescence properties of the final Eu-MOFs?
overall
17,18
see the Supporting Information, SI).
Synthesis of HNCP. A phenanthren-9-ol oxidation reaction with
bis(trifluoroacetoxy)iodobenzene [(CF CO ) IC H , BTI] was carried
3
2
2
6
5
out in aqueous N,N-dimethylformamide (DMF) at 0 °C, obtaining
1
2
0-phenanthroline-5,6-dione. 10-Phenanthroline-5,6-dione (0.1 mmol,
10 mg) was dissolved, together with 4-carboxybenzaldehyde (0.1 mmol,
150 mg), in 10 mL of acetic acid. The mixture system temperature was
increased to 100 °C and the mixture stirred for 30 min, to which 1.46 g of
dry ammonium acetate (19.0 mmol, 20 equiv) was added with stirring
at 120 °C for 2 h. Then the reaction system was reduced to room
temperature, filtered, and washed with a lot of water, acetone, and ether to
yield the desired product. Yield: 286 mg (88%). Mp: >300 °C. After
several days, a few dark-orange HNCP crystals were obtained by slow
evaporation of the above filtrate, which was determined by single-crystal
In this paper, we prepared a series of Eu-MOFs based on the
HNCP ligand by introducing various aromatic dicarboxylic acids
as coligands (1,4-H bdc = benzene-1,4-dicarboxylic acid, 4,4′-
2
H bpdc = 4,4′-biphenyldicarboxylic acid, 2,5-H Br bdc = 2,5-
2
2
2
dibromoterephthalic acid, and H ndc = naphthalene-1,4-
2
dicarboxylic acid; Scheme 2), aiming to shed some light on
new Eu-MOF molecular materials for use in a new generation of
miniaturized devices. Fortunately, four new binuclear europium-
−1
X-ray diffraction (Table 1). IR (KBr pellet, cm ): 3375(b), 3082(b),
2
1
648(b), 2532(b), 1940(w), 1695(s), 1616(m), 1556(s), 1481(m),
441(m), 1362(m), 1306(s), 1283(m), 1097(s), 959(m), 862(w),
(
III) compounds [Eu(NCP)(L)] [L = 1,4-H bdc (1), 2,5-
808(s), 716(s), 696(w), 619(m), 525(w), 467(w) (see Figure S1 in the
n
2
1
H Br bdc (2), 4,4′-H bpdc (3)] and [Eu(NCP)(ndc)(H O) ]
SI). H NMR (DMSO-d
.47 (d, 2H), 7.02 (d, 2H), 6.80 (d, 2H) (see Figure S2 in the SI). Anal.
Calcd for C H N O (340.34): C, 70.57; H, 3.56; N, 16.47. Found: C,
): δ 12.93 (s, 1H), 7.89 (d, 2H), 7.75 (d, 2H),
6
2
2
2
2
2 n
7
(
4) based on the HNCP ligand and various second ligands were
hydrothermally synthesized and structurally characterized. On
the basis of thermal analysis of compound 4, compound 4′ was
readily prepared by guest dehydration of 4. The luminescence
properties of all compounds were discussed.
20 12
4
2
+
7
0.85; H, 3.44; N, 16.91. ESI-MS: m/z 341.11 (100%; [M + H] ).
Synthesis of [Eu(NCP)(1,4-bdc)]n (1). EuCl ·6H O (0.0366 g,
3
2
0
.1 mmol), HNCP (0.0170 g, 0.05 mmol), 1,4-H bdc (0.0083 g,
2
0
.05 mmol), acetone (5 mL), and H O (1 mL) were mixed and stirred
2
for 20 min. The reaction system above was sealed in a 10 mL Teflon-
lined stainless steel autoclave and maintained at 190 °C for 3 days. Upon
reduction of the temperature to room temperature at a rate of 5 °C/h,
yellow-brown block crystals of compound 1 were gained in 48% yield.
Anal. Calcd for C H EuN O (655.40): C, 51.22; H, 2.30; N, 8.54.
Found: C, 51.36; H, 2.29; N, 8.60. IR (KBr, cm ): 3423(b), 2366(w),
1709(m), 1613(s), 1553(s), 1398(b), 1221(w), 1176(w), 1147(w),
1129(w), 1111(w), 1096(w), 1073(m), 1040(w), 1011(w), 977(w),
EXPERIMENTAL SECTION
Materials and Instrumentation. The 2-(4-carboxyphenyl)-
imidazo[4,5-f ]-1,10-phenanthroline (HNCP) ligand was prepared
using phenanthren-9-ol as the raw material via a two-step reaction
■
28
15
4
6
−1
(
4
Scheme 3). The thermogravimetric (TG) data for compounds 1−4 and
′ were collected on a NETZSCH STA 449F3 instrument under a N
2
atmosphere at a heating rate of 10 °C/min from 30 to 1000 °C; an
1
0953
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Inorganic Chemistry
Article
Scheme 3. Synthetic Procedures for the HNCP Ligand
Table 1. Crystal Data and Structure Refinement Summary for the HNCP·3H O Ligand and Compounds 1−4
2
HNCP·3H O
1
2
3
4
2
empirical formula
fw
C H N O
C H EuN O
C H Br EuN O
C H EuN O
C H EuN O
32 21 4 8
2
0
18
4
5
28 15
4
6
28 13
2
4
6
34 19
4
6
394.38
655.40
813.20
731.49
741.49
cryst syst
space group
a (Å)
monoclinic
monoclinic
monoclinic
triclinic
orthorhombic
Pbca
P2 /c
1
c2/c
P2 /n
1
P1
̅
3.7161(8)
24.686(5)
21.803(4)
90
24.1650(15)
11.2586(7)
20.1253(12)
90
10.0700(7)
23.4076(16)
11.5729(8)
90
9.5948(19)
11.227(2)
14.752(3)
82.390(4)
83.808(4)
83.401(4)
1557.7(5)
2
16.6588(15)
17.6541(17)
19.2806(18)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
96.552(5)
90
95.3670(10)
90
104.6890(10)
90
90
90
1987.0(7)
4
5451.4(6)
8
2638.7(3)
4
5670.3(9)
8
Z
3
D (g/cm )
1.318
1.597
2.047
1.560
1.737
c
T (K)
μ (mm−1)
296(2)
0.097
296(2)
2.349
296(2)
5.458
296(2)
296(2)
2.274
2.064
F(000)
824
2576
1560
724
2944
reflns collected/unique
R(int)
10699/3903
0.1113
14428/5367
0.0362
14099/5122
0.0452
8303/5984
0.0684
30226/5848
0.0985
data/restraints/param
GOF on F2
3903/12/263
1.083
5367/0/354
1.066
5122/0/370
1.017
5984/18/382
1.056
5848/0/406
1.000
a
R1 [I > 2σ(I)]
0.0903
0.0390
0.0412
0.1014
0.0435
b
wR2 (all data)
0.3155
0.1577
0.1433
0.2849
0.0928
a
R1 = ∑(F − F )/∑F . wR2 = [∑w(F − F ) /∑w(F ) ]1/2.
b
2
2
2
2 2
o
c
o
o
c
o
9
6
44(w), 882(w), 863(w), 841(m), 807(w), 789(w), 734(m), 694(w),
34(w), 568(w), 524(w).
3 days. After slow cooling to room temperature at a rate of 5 °C/h,
yellow block crystals of compound 4 were gained in 38% yield. Anal.
Calcd for C H EuN O (741.50): C, 51.83; H, 2.85; N, 7.56. Found:
C, 51.63; H, 2.29; N, 7.60. IR (KBr, cm ): 3421(b), 1617(s), 1550(m),
1468(m), 1428(s), 1370(m), 1274(s), 1214(w), 1189(w), 1156(w),
1104(w), 1079(w), 1041(w), 961(w), 864(w), 835(w), 811(w),
789(w), 771(w), 736(w), 668(w), 640(w), 568(w), 532(w).
Synthesis of [Eu(NCP)(2,5-Br bdc)] (2). EuCl ·6H O (0.0366 g,
2
n
3
2
32 21
4
8
−1
0
0
.1 mmol), HNCP (0.0170 g, 0.05 mmol), H Br bdc (0.0156 g,
2
2
.05 mmol), acetone (3 mL), and H O (3 mL) were mixed and stirred
2
for 20 min. The system above was sealed in a 10 mL Teflon-lined
stainless steel autoclave and maintained at 190 °C for 3 days. After the
heat was tardily reduced to room temperature at a rate of 5 °C/h, yellow-
brown block crystals of compound 2 were gained in 37% yield. Anal.
Calcd for C H Br EuN O (813.2): C, 41.39; H, 1.61; N, 6.90. Found:
Synthesis of 4′. The crystals of compound 4 were put in a drying
tubular oven and kept heating at 200 °C for about 0.5 h, and yellow
powder of 4′ was gained. Anal. Calcd for C32
H17EuN O (705.50): C,
4 6
54.48; H, 2.43; N, 7.94. Found: C, 54.36; H, 2.33; N, 7.78. IR (KBr,
28
13
2
4
6
−1
C, 41.36; H, 1.29; N, 6.60. IR (KBr, cm ): 3415(b), 2360(w), 1681(w),
−
1
1
1
1
7
601(s), 1575(w), 1555(w), 1535(w), 1479(w), 1452(w), 1392(b),
326(w), 1280(w), 1243(w), 1194(w), 1137(w), 1097(w), 1071(m),
044(w), 1018(w), 975(w), 948(w), 882(w), 839(w), 805(w), 759(w),
36(w), 719(m), 643(w), 633(w), 557(w), 530(w).
cm ): 1622(s), 1545(m), 1477(m), 1410(s), 1380(m), 1298(s),
1223(w), 1111(w), 1180(w), 1114(w), 1059(w), 1067(w), 955(w),
837(w), 822(w), 866 (w), 777(w), 770(w), 722(w), 651(w), 633(w),
587(w), 520(w).
Synthesis of [Eu(NCP)(4,4′-bpdc)]n (3). EuCl ·6H O (0.0366 g,
X-ray Crystallography. Single-crystal X-ray diffraction data for
3
2
0
0
2
.1 mmol), HNCP (0.0170 g, 0.05 mmol), 4,4′-H bpdc (0.0121 g,
HNCP·3H O and compounds 1−4 were collected on a Bruker SMART
2
2
.05 mmol), DMF (2 mL), and H O (4 mL) were mixed and stirred for
0 min. The reaction system was sealed in a 10 mL Teflon-lined stainless
APEXII CCD diffractometer, equipped with graphite-monochromat-
ized Mo Kα radiation (λ = 0.71073 Å) using ω and φ scan modes. All of
the structures were solved by direct methods and refined with full-matrix
2
steel autoclave and maintained at 170 °C for 3 days. After the heat
was tardily reduced to room temperature at a rate of 5 °C/h, orange
block crystals of compound 3 were gained in 38% yield. Anal. Calcd for
C H EuN O (731.51): C, 55.83; H, 2.62; N, 7.66. Found: C, 55.36; H,
2
least-squares refinements based on F using SHELXS-97 and SHELXL-
1
9
97. H atoms were placed in geometrically calculated positions. All non-
H atoms were refined anisotropically. Detailed crystallographic data and
34
19
4
6
−1
2.92; N, 7.60. IR (KBr, cm ): 3444(b), 1598(m), 1577(m), 1511(m),
1405(s), 1322(w), 1178(w), 1093(w), 1075(w), 1043(w), 852(w), 814(w),
791(w), 773(w), 718(w), 735(w), 686(w), 641(w), 567(w), 530(w).
Synthesis of [Eu(NCP)(ndc)] ·2nH O (4). EuCl ·6H O (0.0366 g,
structure refinement parameters of HNCP·3H O and compounds 1−4
2
are given in Table 1. Selected bond distances, angles, and hydrogen
bonds are listed in Tables S1 and S2 in the SI.
n
2
3
2
0
.1 mmol), HNCP (0.0170 g, 0.05 mmol), H ndc (0.0216 g, 0.1 mmol),
2
RESULTS AND DISCUSSION
■
acetone (1 mL), H O (5 mL), and dropping triethylamine (70 μL) were
mixed and stirred for 20 min. The system above was sealed in a 10 mL
Teflon-lined stainless steel autoclave and maintained at 170 °C for
2
Description of the Structures. The ligand HNCP crys-
tallizes in the monoclinic system with space group P2 /c, and the
1
1
0954
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Inorganic Chemistry
Article
Figure 1. (a) Perspective view of HNCP·3H O. (b) Intramolecular hydrogen-bonding interactions in HNCP·3H O. (c) Hydrogen-bonding
2
2
interactions in the 1D chain of HNCP·3H O. Yellow sticks point partly to HNCP linkers.
2
asymmetric unit has three water molecules (Figure 1a). The
mean plane of the [4,5-f ]-1,10-phenanthroline moiety and the
benzene ring (C14−C19) form a dihedral angle of 23.48(15)°.
As shown in Figure 1b, the O atom (O4) from the free water
molecule is located in the plane of the HNCP organic molecule,
which links the N atom (N4) from the imidazo group, the O
atom (O1) from the carboxylic group, and the O atom (O5)
from the other free water molecule by O4−H4A···N4, O1−H1···
O4#2, and O4−H4B···O5 bonds to form the intramolecular
hydrogen-bond system. The O atom (O3) from another free
water molecule and the H atom from the imidazophenantholine
ring connect with each other by O3−H3B···N3#3 and N3−
H3N···O3#1 hydrogen bonds to generate the other intramole-
cular hydrogen-bond system. Notably, O3 and N4 are employed
as vital atoms of the spacer, which extend the HNCP molecules
to shape a boundless 1D zigzag chain via hydrogen-bond systems
(Figure 1c). Then, the chains are connected with each other by
intermolecular O1−H1···O4#2 hydrogen bonds to yield a 2D
network. Furthermore, the hydrogen-bonding and π−π-stacking
interactions between the parallel phenanthroline and phenyl
rings extend the 2D network to a 3D supermolecular structure.
The average distance of π−π stacking is 3.4473 Å (Figure 2).
Single-crystal X-ray diffraction analysis shows that compounds
Figure 2. 3D supramolecular network of HNCP·3H O.
2
bond angles are in the ranges of 95.18(14)−103.10(12)° and
71.74(13)−143.16(11)°, respectively. The bond lengths and
20
angles of 1 are in the normal ranges.
1
−4 have binuclear 3D structures. Compound 1 crystallizes in
These dimers [Eu (COO) ] are linked to each other by 2-fold
2 4
−
the monoclinic system with space group C2/c. As shown in
antiparallel NCP anions to obtain a 1D double chain (see Figure
3d), in which the face-to-face π−π interactions between the
benzene rings are shown with a distance of 3.5345 Å. Along the
ac plane, the completely deprotonated 1,4-bdc2 anions con-
nect the 1D chains to form an infinite 2D network structure, and
each distorted grid has a size of 20.9997(8) Å × 11.4028(6) Å
(Figure 3e). Along the crystallographic b axis, the dimers are
III
−
Figure 3a, it contains one Eu ion, one NCP anion, and one 1,4-
bdc2 anion in the asymmetric unit. The nine-coordinated Eu
−
III
−
ions are occupied by seven O atoms and two N atoms to generate
a distorted three-capped triangular prism (Figure 3b). Two N
−
atoms are from a chelating NCP anion [Eu1−N1 = 2.660(4) Å
and Eu1−N2 = 2.611(4) Å] and seven O atoms from the carbo-
−
2−
−
2−
xylic groups of three μ -COO -bridging 1,4-bdc anions [Eu1−
connected by μ -COO -bridging 1,4-bdc anions serving as
2
2
O1 = 2.521(3) Å, Eu1−O2 = 2.436(3) Å, Eu1−O5 = 2.348(3) Å,
pillars to enlarge the 2D network structure into a 3D framework,
which presents 1D open rhombic channels along every crys-
tallographic orientation of the a, b, and c axes, and no guest
molecules encased in them (Figure 3f−h). The rhombic channels
own the same size of 11.4028(6) Å × 11.2586(8) Å along crys-
tallographic the a and c axes, which is slightly smaller than that
along the b axis. Moreover, the estimated solvent-accessible void
volume is 18.3% of the total crystal volume using the PLATON/
−
−
and Eu1−O6 = 2.335(3) Å ] and three μ -COO -bridging NCP
3
anions [mode I in Scheme 4; Eu1−O4#1 = 2.363(3) Å, Eu1−
O4 = 2.612(4) Å, and Eu1−O3 = 2.513(4) Å]. The two neigh-
III
boring Eu ions are quadruply bridged by the carboxyl groups
2−
−
from two 1,4-bdc and two NCP anions to yield a binuclear
europium(III) dimer [Eu (COO) ; Figure 3c]. In the dimer, the
2
4
Eu···Eu distance is 3.9265(3) Å. In the whole compound 1, the
N−Eu−N bond angle is 62.29(12)°. The O−Eu−O and O−Eu−N
21
VOID routine.
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III
1
Figure 3. Structure of compound 1. (a) Coordination environment of the Eu ion (symmetry code: #1, −x + 1, y, −z + / ). (b) Coordination
2
III
III
2−
−
polyhedral geometry of the Eu ion. (c) Dimer of Eu ions bridged by the carboxyl groups from two 1,4-bdc and two NCP anions. (d) 1D infinite
−
double chain linked by these dimers [Eu (COO) ] and 2-fold antiparallel NCP anions along the b axis. (e) These 1D chains connected by the
2
4
completely deprotonated 1,4-bdc2 anions to form an infinite 2D network structure in the ac plane. (f) 2D layer in the ab plane. (g) 3D framework
−
assembled via 1,4-H bdc ligands along the a axis. (h) Space-filling diagram of the packing down the b axis, showing the rhombic channels [the orange
2
polyhedron stands for the europium(III) dimer]. H atoms are omitted for clarity.
−
2−
Compound 2 belongs to the monoclinic system with space
O atoms from two NCP and three 2,5-Br bcd anions to form
2
III
group P2 /n. As shown in Figure 4a, it contains one Eu ion, one
the coordination sphere (Figure 4a). Two N atoms belong to a
chelating NCP anion [Eu−N1 = 2.529(6) Å and Eu−N2 =
2.587(6) Å], two O atoms belong to the μ -COO -bridging
1
−
2−
−
NCP anion, and one Br bcd anion in the asymmetric unit.
The central Eu ion is eight-coordinated through two N and six
2
III
−
2
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Scheme 4. Two Coordination Modes of HNCP in
Compounds 1−4 (M = Metal)
(Figure 5e), the dimer can be regarded as a 4-connected node and
2−
−
the 4,4′-bpdc and NCP anions act as linkers, which can be
considered to simplify the square-grid-like topology with these
grids ofdimensionsof 14.9253(23)Å × 15.5914(24) Å (calculated
III
from the distance of the neighboring Eu atoms). Furthermore,
2−
the 4,4′-bpdc anions expand these 2D sheets to a pillar-layered
D framework (Figure 5f). The 3D topology of 3 is shown in
Figure 5g. A broad unoccupied void space [14.9253(23) Å ×
5.5914(24) Å × 15.5363(22) Å] exists in 3 (Figure 5h) that is
3
1
smaller than that of compound 1 (Figure 5g). Because of the
torsion of the single bond (C−C) and the various coordination
modes of the carboxyl in biphenyldicarboxylic acid, the layers
easily tend to intercross in the 3D structure. The theoretical total
accessible volume of compound 3 obtained is 16.7% using the
PLATON/VOID routine.
Compound 4 crystallizes in the orthorhombic system with
space group Pbca. As shown in Figure 6a, the asymmetric unit con-
−
NCP anions [Eu−O5 = 2.348(3) Å and Eu−O6 = 2.335(3) Å],
−
2−
and the others belong to three μ -COO -bridging 2,5-Br bdc
anions [Eu−O3 = 2.523(4) Å, Eu−O4 = 2.612(4) Å, Eu−O1 =
21(3) Å, and Eu−O2 = 2.436(3) Å]. Its coordination geometry
is a distorted two-capped triangular prism (Figure 4b). In 2, the
N−Eu−N bond angle is 65.01(19)°, which is larger than that of
compound 1. The O−Eu−O and O−Eu−N bond angles are in
the ranges of 53.22(17)−146.67(19)° and 69.3(19)−157.48(19)°,
respectively. The carboxyl groups of 2,5-H Br bdc link with
two neighboring Eu ions to produce a dimer, and the distance
of Eu···Eu is 5.360(6) Å.
Different from the above compound 1, the adjacent Eu ions
are linked by the μ -COO -bridging NCP anions (mode II) to
shape a series of left- and right-handed helical zigzag chains along
the a axis (Figure 4c). These helical zigzag chains are connected
to each other by the μ -COO -bridging 2,5-Br bdc anions to
yield a 1D double chain. The double chains are further connected
by the μ -COO -bridging 2,5-Br bdc anions to generate a 2D-
layered structure in the ab plane (Figure 4c,d). The 2D structure
can be taken as a four-connected 2D (4,4) network using the
dimers as nodes (Figure 4d), in which each rhombic grid
structure possesses a size of 18.9174(10) Å × 19.7843(71) Å.
Along the crystallographic b axis, the layers are stacked together
through the interlaminar μ -COO -bridging 2,5-Br bdc anions
to obtain a 3D framework.
Compound 3 pertains to the triclinic system with space group
2
2
5
21
III
−
tains one Eu ion, two free water molecules, one NCP anion,
and one ndc2 anion. The geometrical configuration is defined as
a distorted two-capped triangular prism by six O atoms from the
−
2
2
III
−
2−
carboxyl groups of the NCP and ndc anions [Eu1−O1 =
.526(4) Å, Eu1−O2 = 2.411(4) Å, Eu1−O3 = 2.433(4) Å,
Eu1−O4 = 2.916(4) Å, Eu1−O5 = 2.365(4) Å, and Eu1−O6 =
III
2
−
−
2
−
2
.372(4) Å] and two N atoms from one chelating NCP anion
[
Eu1−N1 = 2.575(4) Å and Eu1−N2 = 2.628(4) Å; Figure 6b].
III
−
2−
The two Eu ions are quadruply bridged by the carboxylate
groups of the NCP and ndc anions to form a binuclear dimer
2
2
−
2−
−
2−
Eu (COO) , and the distance of Eu···Eu is 4.107(4) Å (Figure 6c).
2
4
2
2
2−
It is worth noting that the ndc anions not only take part in the
bridging of the adjacent Eu ion but also hold the remaining four
III
sites (Figure 6d). In order to describe the framework of com-
pound 4, the free water molecules are omitted in the follow-
ing descriptions. In 4, the carboxyl groups of HNCP are also
completely deprotonated by applying coordination mode II
(Scheme 4) and link dimers to shape an infinitely 1D double
chain along the crystallographic a axis (Figure 6c). These 1D
chains are further linked to each other by carboxyl O atoms of
−
2−
2
2
III
−
̅
P1. The asymmetric unit contains one Eu ion, one NCP anion,
2−
III
2−
and one 4,4′-bpdc anion (Figure 5a). Each Eu ion is eight-
ndc anions in the ab plane to generate a 2D square-grid-like
−
coordinated by the two N atoms from a chelating NCP anion
coordination network. The approximate dimensions of these
−
(
N1 and N2) and the six O atoms from two μ -COO -bridging
grids are 16.6588(9) Å × 11.6138(9) Å (calculated from the
2
−
−
III
NCP anions (O1 and O2) and two μ -COO -bridging 4,4′-
distance of the neighboring Eu atoms). To simplify the
2
bpdc2 anions (O3, O4, O5, and O6) to shape a distorted two-
−
−
2−
structure of 4, the two parallel NCP and ndc anions are
considered as one linker, respectively. The Eu (COO) cluster
can be reduced to a 4-connected node, which connects the
surrounding Eu (COO) clusters together by those linkers to
capped triangular-prismatic EuO N polyhedron (Figure 5b).
O5 and N1 inhabit the position of the two caps, and N2, O1, O2,
O3, O4, and O6 lie in the vertex position of the triangular prism.
2
4
6
2
2
4
III
As shown in Figure 5c, two eight-coordinated Eu− ions are
generate a 2D network, as shown in Figure 6e. In the vertical
engaged by two carboxylate groups from two NCP anions to
direction of the ab plane, the remaining four sites in the dimer
III
2−
2−
shape the binuclear dimer [Eu (COO) ]. In the dimer, the Eu
polyhedron are held by two ndc anions (Figure 6f). The ndc
2
2
2−
ions are coordinated by completely deprotonated 4,4′-bpdc
anions join up with these 2D layers to shape a 3D structure
(Figure 6f,g). The space-filling diagram of the packing shows the
rhombic channels along the a axis (Figure 6h). According to
−
and NCP anions in a bridging bidentate fashion (mode I in
Scheme 4). The average distances of Eu−O and Eu···Eu are
.294(11) and 5.4271(15) Å, respectively. The axial positions of
the paddlewheel are occupied by the N atoms, and the Eu−N
bond lengths of the dimer range from 2.504(14) to 2.560(12) Å,
21
2
PLATON calculations, compound 4 shows a smaller void
volume (8.0%) than 1 and 3.
It is worth noting that these binuclear europium(III) 3D
architectures based on the HNCP ligand can be achieved via
employment of a different second ligand (dicarboxylic acid). The
structural analyses are concluded: (1) HNCP has more potential
domains, strong electoral donor atoms (N and O), and V-shaped
III
20
which corresponds well to the known Eu −N bond distances.
The N−Eu−N bond angle is 63.4(4)°. The O−Eu−O and O−
Eu−N bond angles are in the ranges of 51.9(4)−153.8(5)°and
7
5.8(4)−156.9(4)°, respectively.
III
As depicted in Figure 5c, these [Eu (COO) ] dimers are linked
dicarboxylate, which is beneficial for linking Eu ions together.
2
2
−
by two antiparallel NCP anions to give rise to a 1D limitless
(2) The introduction of a second dicarboxylic acid is helpful to
extend the structure to higher dimensionality. The sizes, shapes,
and substituent groups of the second ligands with various
conjugated electronic structures are found to play important
2−
double chain. The double chains are connected by 4,4′-bpdc
anions to form the 2D honeycomb lamellar rhombic-shaped
network (Figure 5d). To better understand the structure of 3
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III
III
Figure 4. Structure of compound 2. (a) Coordination environment of the Eu ion. (b) Coordination polyhedron geometry of the Eu ion. (c) Adjacent
III
−
−
Eu ions connected by the μ -COO -bridging NCP anions (mode II) to shape a series of left- and right-handed helical zigzag chains and further yield a
2
−
2−
−
1
D double chain connected by the μ -COO -bridging 2,5-Br bdc anions along the a axis. (d) These double chains connected by the μ -COO -
2
2
2
2
−
−
bridging 2,5-Br bdc anions to generate a 2D-layered structure in the ab plane. (e) 2D-layered structure linked by μ -COO bridging to generate a 3D
2
2
framework along the a axis [the purple polyhedron stands for the europium(III) dimer]. H atoms are omitted for clarity.
roles in tuning the channels, total accessible volumes, and con-
jugated structures of the compounds, further effecting the
luminescence properties of the final Eu-MOFs.
Thermal Analysis and PXRD Results. PXRD experiments
were measured for compounds 1−4. The patterns for these
compounds are essentially in accordance with the simulated ones
from the single-crystal structure analysis, indicating the pure
samples of compounds 1−4 (Figure S3 in the SI).
To estimate the stabilities of these MOFs, TG analyses of 1−4
were carried out in the temperature range of 30−1000 °C at a
heating rate of 10 °C/min under a N atmosphere, as shown in
2
Figure 7. The TG curves of 1−3 are very similar. 1 is regarded as a
representative to analyze the TG performances. The thermal
behavior of compound 1 does not show weight loss before
490 °C (470 °C, 2; 458 °C, 3; 451 °C, 4) and exhibits a rather
high thermal stability because of no coordinated water molecules
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Figure 6. Structure of compound 4. (a) Coordination environment of
III
III
the Eu ion. (b) Coordination polyhedron geometry of the Eu ion. (c)
Figure 5. Structure of compound 3. (a) Coordination environment of
III
−
2−
Eu ions bridged by the carboxylate groups of NCP and ndc anions
III
III
the Eu ion. (b) Coordination polyhedron geometry of the Eu ion. (c)
to form a binuclear dimer Eu (COO) . HNCP ligands are omitted for
clarity. (c) These dimers linked by the carboxyl groups of HNCP to
2
4
These [Eu (COO) ] dimers linked by two antiparallel HNCP ligands to
give rise to a 1D limitless double chain. (d) Double chains connected by
,4′-bpdc anions to form the 2D honeycomb lamellar rhombic-shaped
network [the blue polyhedron stands for the europium(III) dimer]. (e)
Schematic illustrating the 2D topology with the dimer as a 4-connected
2
2
shape an infinitely 1D double chain along the crystallographic a axis. (e)
2−
4
−
Schematic illustrating the 2D topology with two parallel NCP and
2−
ndc anions as linkers and the Eu (COO) cluster as a 4-connected
2
4
2−
node. (f) 3D framework assembled via ndc anions (water molecules
are omitted for clarity). (g) Schematic illustrating the 3D topology. (h)
Space-filling diagram of the packing down the a axis, showing the
rhombic channels. H atoms are omitted for clarity.
2
−
−
node and the 4,4′-bpdc and NCP anions as linkers. (f) 3D framework
2
−
assembled via 4,4′-bpdc anions. (g) Schematic illustrating the 3D
2
−
topology (green sticks point to the 4,4′-bpdc anion linkers). (h)
Space-filling diagram of the packing down the b axis, showing the
rhombic channels. H atoms are omitted for clarity.
Compound 4 displays two steps of weight loss. The first step of
weight loss begins at 84 °C and ends at 112 °C. The weight loss
of 4.2% corresponds to the removal of two lattice water mole-
cules (calcd 4.8%). Furthermore, the dehydrated compound is
stable up to decomposition of the organic components. The
decomposition process begins at 450 °C and ends at 585 °C.
The final residue (23.92%) could correspond to Eu O (calcd
or solvent molecules in the coordination architectures. Upon an
increase in the temperature, the compound experiences a one-
step weight loss of 72.51% between 490 and 650 °C, which is
ascribed to decomposition of the organic components. The final
mass of the residue is 27.49% (21.64%, 2; 24.92%, 3) of the
original weight, which matches well with the values expected for
the formation of Eu O (calcd: 26.85%, 1; 20.65%, 2; 24.06%, 3).
2
3
23.73%).
2
3
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compounds and the free ligand indicate that coordination of the
III
Eu ion does not have an obvious effect on the singlet excited
state of the free ligand. However, a slight blue shift is discernible
in the absorption maximum of the five compounds (λmax = 293
nm), compared with the absorption maxima of the HNCP ligand
(
λmax = 307 nm), which could be attributed to the lack of
aggregation in the MOFs, whereas the free ligand exhibits a
22
certain degree of π-stacking interactions (Figure 2). The molar
4
absorption coefficients of 1−4 and 4′ at 307 nm of 4.909 × 10 ,
4
4
4
4
−1
4
.792 × 10 , 4.949 × 10 , 4.819 × 10 , and 4.822 × 10 mol
−1
cm , respectively, are similar to that of the H TDC ligand
4.697 × 10 mol cm ), which is consistent with crystallo-
2
4
−1
−1
(
graphic analysis. The large molar absorption coefficient of the
H TDC ligand shows a powerful ability to absorb light.
2
As shown in Figure 8, the excitation spectra of the HNCP
ligand and compounds 1−4 and 4′ were obtained by surveying
Figure 7. TG analysis curves of compounds 1−4 and 4′.
The DSC curves of compounds 1−4 are shown in Figure S4 in
the SI. Compounds 1−3 show broad upward exothermic peaks,
indicating the thermal decomposition processes of one-step
weight loss. For compound 4, the DSC curves present a small
downward endothermic peak and a broad upward exothermic
peak, which are ascribed to the loss of free water molecules and
the decomposition process of the organic components.
In addition, temperature-dependent PXRD (see Figure S5 in
the SI) of compounds 1−4 after calcination at different elevated
temperatures was also studied. The patterns for these com-
pounds closely match the uncalcined compounds before 450 °C.
With an increase of the temperature to 500 °C, the characteristic
peaks of these compounds are significantly reduced. On the basis
of temperature-dependent PXRD and TG analyses of compound
Figure 8. Room temperature excitation spectra for 1−4 and 4′.
4
, the crystals of 4 are calcined in an oven at 200 °C for about
III
the emission wavelength of the Eu ions at 612 nm in the solid
0
.5 h, losing the latticed water molecules and obtaining com-
state. They all present a broad excitation band between 300 and
425 nm, which can be assigned to the π−π* electron transition of
the HNCP ligand. Compared with the HNCP ligand, a slight
blue shift in the range of 300−425 nm is discernible in their
excitation profile, which is ascribed to energy transfer between
the HNCP ligand and the Eu center. The excitation spectra of
all compounds display another series of sharp excitation bands
with low intensity. These weak signals are characteristic of direct
excitation of the Eu ion, resulting from transitions between the
F and L states and transitions between the F and D
states. These transitions are overlapped by a broad excitation
band, and their intensities are weaker than that of absorption of
the organic ligands. It demonstrates that luminescence sensi-
tization by excitation of the ligand is much more efficient than
pound 4′. IR spectroscopy analysis reveals the absence of the
−1
peak at around 3421(b) cm in 4′, indicating no lattice water
molecules. The TG curve of 4′ is very similar to the second step
of the TG curve of dehydration of 4 (Figure 7). The PXRD
pattern of dehydrated 4 agrees well with that simulated from
single-crystalX-raydiffractiondata of4, whichindicates a structural
maintenance upon guest water removal (Figure S3 in the SI).
In parallel with TG analysis and temperature-dependent
PXRD, IR (see Figure S6 in the SI) and fluorescence (see Figure
S7 in the SI) spectra of 1−3 and 4′ after calcination at different
elevated temperatures were also studied in order to further prove
the thermal stabilities of these europium(III) compounds. In the
IR spectrum of these calcined samples, the characteristic bands
III
III
7
5
7
5
0
6
0,1
2,1
23
−1
of the carboxylate groups at ∼1690/∼1400 cm for (CO)
III
24
−1
direct excitation of the Eu ion absorption level.
stretching or flexural vibrations and amino groups at ∼1300 cm
As shown in Figure 9, the solid-state luminescent properties of
compounds 1−4 and 4′ were measured in the range of 500−750
nm at room temperature. The emission spectra of 1−4 and 4′
for (C−N) stretching vibrations still exist and are identical with
those of compounds without calcination. The fluorescence spec-
trum of these calcined compounds at raised temperature also
presents negligible differences. Many phenomena above indicate
that the structures of synthetic compounds 1−3 and 4′ are fairly
show the typical europium red emissions under ligand excitation
III
(
396 nm). In addition, a series of faint f−f transitions of the Eu
ion at 580, 591, 619, 651, and 699 nm are also observed,
corresponding to the D → F , D → F , D → F , D →
F , and D → F transitions, respectively. No broad and
strong emission band bridging through the ligand suggests
that the absorbed energy is effectively transferred to the emitting
III
stable up to 450 °C. As a consequence, the Eu -MOFs of 1−3
5
7
5
7
5
7
5
and 4′ are excellent candidates for luminescence materials when
0
0
0
1
0
2
0
7
5
7
25
good thermal stability is demanded.
3
0
4
Photophysical Property. As shown in Figure S8 in the SI,
the UV−vis absorption spectra of the free HNCP ligand and
compounds 1−4 and 4′ were measured in a DMSO solution (c =
III
5
7
level of the Eu center by the ligand. The weak D
→ F
transitions are regarded as both strictly forbidden
3
and
0
0
−5
5
7
2
× 10 M). The absorption maxima for the HNCP ligand is
D
0
→ F
26
5
7
assigned to the singlet−singlet n → π* or π → π* electronic
transitions.
in magnetic-dipole (MD) and ED schemes. The D → F
transition is relatively stronger, which is a MD-allowed transition.
0
1
3
d,20
The similar trends in absorption spectra of all
1
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stabilization of the HNCP ligand, offering an effective pathway of
energy transfer. Additionally, no solvent molecules or aqua ligands
exist in 1−3 and 4′, which can dramatically reduce the energy loss
1
3
from high-energy vibrations, such as O−H vibrations.
In order to obtain a better acquaintance of the luminescence
efficiencies, the quantum yield is another significant parameter
III
to portray the emission process of the Eu ion besides the
luminescence lifetime, which not only demonstrates the ability
III
of the HNCP ligand sensitizing the emission of the Eu center
but also draws conclusions between the structures and photo-
3d
luminescence properties. The overall quantum yield (Φoverall)
for an europium(III) compound regards the system as a “black
box”, in which the inner process is not tangibly measured. Being
deemed as the compound absorbing a photon (i.e., the antenna is
30
excited), Φoverall can be calculated as
Φ
= ΦoverallΦLn
(1)
overall
Figure 9. Room temperature emission spectra for 1−4 and 4′.
where Φ
and Φ correspond to the efficiency of energy
III
transfer
Ln
5
7
shifting from the ligand to the Eu ion and the intrinsic quantum
It is clear that the hypersensitive D → F transition (ED)
0
2
yield of the Ln ion, respectively. ΦLn can be defined as
displays the highest intensity at 619 nm, indicating the existence
of a highly polarizable chemical environment around the Eu
III
ARAD
ARAD + ANR
τ
obs
5
7
ΦLn
=
=
ion. It is documented that the intensity of the D → F
0
2
τRAD
(2)
III
transition increases as the site symmetry of Eu decreases,
5
7
while the D → F transition is MD in origin and insensitive to
site symmetry. Obviously, the luminescent intensity ratio of
D → F )/( D → F ) is 11.50. The results indicate no
inversion center in the site of the Eu ion and a brilliant-red
emission of these compounds, matching the single-crystal
X-ray analysis of the compounds.
In eq 2, ARAD, A , τ , and τ represent the radiative decay
RAD
rate, nonradiative decay rate, D lifetimes, and radiative life-
times, respectively.
The intrinsic quantum yields of the Eu ion could not be
decided experimentally upon direct f−f excitation in view of the
very low absorption intensity. Consequently, the radiative life-
times of Eu ( D ) are calculated from eq 3,
0
1
NR obs
2
7
5
0
5
7
5
7
(
0
2
0
1
III
28
III
29
5
31
The luminescent decay lifetime measurements of compounds
0
1
−4 and 4′ were performed by a pulsed tungsten lamp at room
⎛
⎞
1
τRAD
3
ITOT
temperature (298 K). Meanwhile, the lifetime values (τ) of the
ARAD
=
= AMD,0n ⎜
⎟
⎠
5
D level were obtained by fitting with a monoexponential curve.
⎝ I
0
MD
(3)
The luminescent decay profiles are shown in Figures S9−S12
in the SI, and the lifetime values are given in Table 2. The
Here, AMD,0 is the deactivation rate related to the spontaneous
5
7
emission probability for the D → F transition in vacuum,
0
1
−
1
equal to 14.65 s ; n is the refractive index of the medium, which
equals 1.5 in the solid sample used in the calculation for coor-
Table 2. Radiative (ARAD) and Nonradiative (A ) Decay
Rates, D Lifetimes (τ ), Radiative Lifetimes (τRAD),
NR
5
0
obs
dination compounds; I /I signifies the ratio of the total area
tot MD
Intrinsic Quantum Yields (Φ ), Energy-Transfer Efficiencies
Ln
5
7
of the integrated D → F (J = 0−4) emissions of the corrected
0
J
(
1
Φ ), and Overall Quantum Yields (Φ
) for Compounds
overall
sen
europium(III) emission spectrum to the area of the intensity of
−4 and 4′
5
7
32
the MD D → F transition. For compounds 1−4 and 4′, the
0
1
A
A
τRAD
(μs)
Φ
Φ
Φ
calculated values of A , A , and τ
via eq 3 are given in
RAD
RAD
)
NR
)
Ln
(%)
transfer
(%)
overall
(%)
RAD
NR
−
1
−1
compound (s
(s
τobs (μs)
5
Table 2, combined with the lifetime values (τ) of the D level,
and finally the Φ calculated values are 64%, 29%, 71%, 82%, and
0
a
1
830
712
875
954
470 769 ± 2
692 413 ± 2
330 807 ± 2
85 858 ± 2
1204
1404
1143
1048
987
64
17
11
Ln
b
1
9
12
95%, respectively.
a
2
3
4
29
71
82
95
14
4
Compound 1 exhibits slightly higher luminescence efficiency
compared with 2. On the basis of their structures, the poor
luminescence efficiency noted in compound 2 may be attributed
to the electron-accepting group (Br) of ditopic carboxylate
linkers involving in the coordination sphere of 2, which easily
causes fluorescence quenching to reduce the fluorescence quan-
b
1
4
4
a
25
18
b
2
7
19
a
32
26
b
3
4
28
a
III
4′
1013
53 942 ± 2
41
39
tum yield of the Eu ion. Moreover, the richer π-conjugated 1,4-
b
4
4
42
H bdc second ligand than 2,5-H Br bdc influences Φtransfer. 3, 4,
2
2
2
a
b
Relative quantum yield. Absolute quantum yield.
and 4′ exhibit relatively high quantum yields and radiative decay
rates because of the distorted eight-coordinated dodecahedral
III
33
experimental curves fit well with single-exponential decays, which
points to a single chemical environment around the Eu ion. The
environment of the Eu ion. In this coordinate environment,
III
5
7
the hypersensitive D → F ED transition is allowed by
0 2
luminescence lifetimes of compounds 1−4 and 4′, unlike the
symmetry-related selection rules and occurs quite easily. So, the
inner coordination sphere may result in a following increase in
the overall quantum yield. The nonradiative decay rates of 3, 4,
and 4′ decrease by 29%, 82%, and 89% approximately, compared
to that of 1, because of the extended conjugate system through
similarity of their photoluminescence spectra, are quite different
(
0.770 ms for 1; 0.414 ms for 2; 0.807 ms for 3; 0.858 ms for 4;
.942 ms for 4′). These long lifetimes may be attributed to the
strong chelating coordination ability and steric architectonic
0
1
0961
dx.doi.org/10.1021/ic501253c | Inorg. Chem. 2014, 53, 10952−10963

Inorganic Chemistry
Article
the introduction of one phenyl group in the 4 or 2 positions and
presents an ingenious design to adjust MOF-based fluorescent
materials.
4
and 2 positions of the 1,4-H bdc ligand. However, it is obvious
2
that the quantum yields of 4 and 4′ are superior to that of 3,
which is probably due to H ndc containing a better delocalized
ASSOCIATED CONTENT
Supporting Information
X-ray crystallographic files (CIF), selected bond lengths and
2
■
conjugated system in contrast to 4,4′-H bpdc, leading to a sign-
2
S
*
ificant enhancement in the overall quantum yield and sensi-
tization and thus resulting in a more efficient ligand-to-metal and
angles, hydrogen-bonding intereactions, photoluminescence
5
less nonradiative D relaxation process. Noticeably, the photo-
1
0
properties, IR, H NMR, solid-state emission, and UV−vis
luminescent properties of 4′ (the dehydrated product of 4) are
obviously modified, along with enhancement of the intensity of
the emission bands and an increase in the lifetime and quantum
yield to 0.942 ms and 39%. This phenomenon is attributed to the
quenching effect of the luminescent state through high-frequency-
absorption spectra, PXRD patterns, DCS plots, and lumines-
cence decay profiles. This material is available free of charge via
the Internet at http://pubs.acs.org. Crystallographic data for the
structures reported in this paper are also deposited in the
Cambridge Crystallographic Data Center with CCDC reference
numbers 949867, 948244, 948245, 948246, and 948430 for the
HNCP ligand and compounds 1−4, respectively.
13
vibrating water molecules. Compound 4 has a higher quantum
yield and longer lifetime than those for the other compounds
besides 4′, despite the presence of solvent molecules in compound
4
. The better delocalized conjugated system possibly compensates
AUTHOR INFORMATION
Corresponding Author
E-mail: sanpingchen@126.com or sanpingchen312@gmail.
com. Tel.: +86(29)81535025. Fax: +86(29)81535025.
■
*
for the quenching effect of the uncoordinated water molecules in
the channels.
In a word, the quantum yields and lifetimes of compounds
1
−4 and 4′ follow a sequence of 4′ > 4 > 3 > 1 > 2, which
emphasizes the significant impact of the different sizes, shapes,
and substituent groups of ditopic carboxylate linkers and the
latticed water molecules on luminescent properties. Additionally,
the energy gap between the ligand triplet state and the lowest-
Author Contributions
These authors have equal contribution to this work.
‡
Notes
The authors declare no competing financial interest.
III
lying excited state of the Eu ion, the coordinated solvent mole-
cules, and the conjugation effect of chromophores and/or coli-
gands also generate cooperative effects in the luminescence
properties of Eu-MOFs, as evidenced by reported Eu-MOFs (see
Table S3 in the SI).
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the National
Natural Science Foundation of China (Grants 21127004,
1203149, 21373162, 21173168, and 21463020) and the Nature
2
Science Foundation of Shanxi Province (Grants FF10091,
SJ08B09, and 11JS110).
CONCLUSION
A series of 3D binuclear Eu -MOFs have been successfully
■
III
designed and synthesized based on the HNCP ligand with
different aromatic dicarboxylic acids as coligands under hydro-
thermal conditions, which show different channels, void volumes,
and conjugated structures tuned by these coligands. The rigidity
of the HNCP ligand and the introduction of bicarboxylic acid
ligands favor the formation of coordination polymers with high
thermal stability. Thermal analyses state that compounds 1−4
and the dehydrated product 4′ of 4 have remarkable thermal
stabilities of the main framework until 450 °C. The fluorescence
spectra of eurpopium(III) compounds 1−4 and 4′ exhibit the
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