Two series of reactant's ratio-dependent lanthanide organic frameworks derived from nicotinic acid N-oxide and oxalate: Synthesis, crystal structures and luminescence properties

Article abstract of DOI:10.1039/c4dt03668a

Two series of lanthanide(iii)-organic frameworks with the molecular formula [Ln₂(NNO)₂(OX)₂(H₂O)₄]_n (Ln = Eu 1, Tb 2, Sm 3, Dy 4, Gd 5) and [Ln₂(NNO)₄(OX)(H₂O)₂]_n (Ln = Eu 6, Tb 7, Sm 8, Dy 9, Gd 10) were synthesized successfully under the same hydrothermal conditions with nicotinic N-oxide (HNNO) and oxalic acid (H₂OX) as the mixed ligands merely through varying the molar ratio of the reactants. The compounds were characterized by IR, elemental analysis, UV, TG-DTA and powder X-ray diffraction (XRD). X-ray single-crystal diffraction analyses of compounds 1 and 7 selected as representatives and powder XRD analysis of the compounds revealed that both the series of compounds feature three-dimensional (3-D) open frameworks, and crystallize in the triclinic P1 space group while with different unit cell parameters. In compound 1, pairs of Eu³⁺ ions and pairs of NNO^- ligands connect with each other alternately to form a 1-D infinite Eu-NNO double chain, the adjacent 1-D double-chains are then joined together through OX^2- ligands leading to a 2D layer, the 2-D layers are further 'pillared' by OX^2- ligands resulting in a 3-D framework. In compound 7, the 1-D Tb-NNO infinite chain and its 2-D layer are formed in an almost similar fashion to that in compound 1. The difference between the structures of the two compounds 1 and 7 is that the adjacent 2-D layers in compound 7 are further connected by NNO^- ligands resulting in a 3-D framework. The photoluminescence properties and energy transfer mechanism of the compounds were studied systematically. The energy level of the lowest triplet states of the HNNO ligand (23148 cm^-1) was determined based on the phosphorescence spectrum of compound 5 at 77 K. The ⁵D₀ (Eu³⁺) and ⁵D₄ (Tb³⁺) emission lifetimes are 0.46 ms, 0.83 ms, 0.69 ms and 0.89 ms and overall quantum yields are 1.03%, 3.29%, 2.58% and 3.78% for the compounds 1, 2, 6 and 7, respectively. This journal is

Full text of DOI:10.1039/c4dt03668a

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Two series of reactant’s ratio-dependent
lanthanide organic frameworks derived from
nicotinic acid N-oxide and oxalate: synthesis,
crystal structures and luminescence properties†
Cite this: Dalton Trans., 2015, 44,
4601
Yanyan Yu, Lijuan Zhang,* Yunshan Zhou* and Zareen Zuhra
Two series of lanthanide(III)–organic frameworks with the molecular formula [Ln2(NNO)2(OX)2(H2O)4]n (Ln =
Eu 1, Tb 2, Sm 3, Dy 4, Gd 5) and [Ln2(NNO)4(OX)(H2O)2]n (Ln = Eu 6, Tb 7, Sm 8, Dy 9, Gd 10) were syn-
thesized successfully under the same hydrothermal conditions with nicotinic N-oxide (HNNO) and oxalic
acid (H2OX) as the mixed ligands merely through varying the molar ratio of the reactants. The compounds
were characterized by IR, elemental analysis, UV, TG-DTA and powder X-ray diffraction (XRD). X-ray
single-crystal diffraction analyses of compounds 1 and 7 selected as representatives and powder XRD
analysis of the compounds revealed that both the series of compounds feature three-dimensional (3-D)
open frameworks, and crystallize in the triclinic P¯1 space group while with different unit cell parameters.
In compound 1, pairs of Eu3+ ions and pairs of NNO− ligands connect with each other alternately to form
a 1-D infinite Eu-NNO double chain, the adjacent 1-D double-chains are then joined together through
OX2− ligands leading to a 2D layer, the 2-D layers are further ‘pillared’ by OX2− ligands resulting in a 3-D
framework. In compound 7, the 1-D Tb-NNO infinite chain and its 2-D layer are formed in an almost
similar fashion to that in compound 1. The difference between the structures of the two compounds 1
and 7 is that the adjacent 2-D layers in compound 7 are further connected by NNO− ligands resulting in a
3-D framework. The photoluminescence properties and energy transfer mechanism of the compounds
were studied systematically. The energy level of the lowest triplet states of the HNNO ligand (23 148 cm−1
)
Received 30th November 2014,
Accepted 19th January 2015
was determined based on the phosphorescence spectrum of compound 5 at 77 K. The 5D0 (Eu3+) and
5D4 (Tb3+) emission lifetimes are 0.46 ms, 0.83 ms, 0.69 ms and 0.89 ms and overall quantum yields are
1.03%, 3.29%, 2.58% and 3.78% for the compounds 1, 2, 6 and 7, respectively.
DOI: 10.1039/c4dt03668a
www.rsc.org/dalton
5
region due to the shielding of the 4f orbitals by the filled S2
Introduction
5
and P6 subshells.2 However, the lanthanide ions suffer from
In recent years, the rational design and assembly of photo- weak light absorption due to the forbidden f–f transitions,
luminescent lanthanide(III) metal–organic frameworks (Ln- making the direct excitation of the metals very inefficient and
MOFs) has been of considerable interest due to their intri- difficult.3 Such a weak absorption problem can be overcome
guing topology, architectures and potential applications in through introducing conjugate and rigid organic ligands as
fluorescent probes, light-emitting diodes, sensors and fluo- antennas to sensitize the luminescence of lanthanide ions by
rescence labelling. Therefore the synthesis and design of novel energy transfer from the ligand to the lanthanide ion; such a
Ln-MOFs has always been a hot spot in the field of lumines- process is known as the “antenna effect”.4
cent materials science for decades. Trivalent lanthanide ions
Thus, the key to design and synthesis of Ln-MOFs is to
such as Eu3+, Tb3+, Sm3+ and Dy3+ exhibit intense, long-lived, choose appropriate organic ligands with bridging functions
line-like and high monochromatic emission in the visible and conjugate structures as the antenna ligands. As the lantha-
nide ion is oxygenphilic, organic carboxylic acid is a good
choice for the construction of an abundant variety of multidi-
mensional structures of Ln-MOFs.3–5 Among organic car-
boxylic acids, nicotinic acid with an N-heterocyclic rigid ring
State Key Laboratory of Chemical Resource Engineering, Institute of Science, Beijing
University of Chemical Technology, Beijing 100029, P. R. China.
E-mail: zhouys@mail.buct.edu.cn; Tel: +86-10-64414640
†Electronic supplementary information (ESI) available: Chart S1, Tables S1 and
containing N- and O-donors and with conjugate structures are
of particular use in building novel Ln-MOFs with excellent
S2, Fig. S1–S4. CCDC 1020656 and 1023399. For ESI and crystallographic data in
luminescence properties.5 Compared with the parent ligand
CIF or other electronic format see DOI: 10.1039/c4dt03668a
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nicotinic acid, its derivative nicotinic acid N-oxide (HNNO) emission spectra for the complexes in the solid state were
having an oxygen atom in place of a nitrogen donor site can recorded on a Hitachi F-7000FL fluorescence spectropho-
make it easier to obtain Ln-MOFs due to its increasing flexi- tometer with both excitation and emission slits at 5 nm at
bility and coordination ability. Moreover, various organic room temperature. The scan speed was 1200 nm min−1, and
dicarboxylic acids1a,6 are often used to construct Ln-MOFs the PMT voltage was 400 V. The phosphorescence spectra of
composed of mixed ligands with various structures and compound 5 in the solid state at 77 K were recorded using a
desired luminescence properties.7 Among them, oxalic acid Hitachi F-7000 FL fluorescence spectrophotometer with an
with bridging function and a moderate length8 acting as a emission slit of 5 nm. The scan speed was 240 nm min−1 and
linker in the design of Ln-MOFs not only can coordinate to the PMT voltage was 600 V using a xenon arc lamp (150 W) as
lanthanide ions but also can replace solvent molecules to the light source. The luminescence decay curves and photo-
reduce the quenching effect of lanthanide ions.9 Besides, luminescence quantum yields of the samples were obtained
oxalic acid can also participate in the process of intramolecu- using a FLS 980 Combined Fluorescence Lifetime and a Steady
lar energy transfer, so it is a common ligand to construct State Spectrometer.
Ln-MOFs.1a,6,10
Preparation of nicotinic acid N-oxide
Importantly, it is well known that many synthetic factors,
such as the ratio of the reactants, solvents, reaction tempera-
ture, pH value, and types of anions, often affect the results in
constructing the Ln-MOFs, so it is still a great challenge to
control structures with the desired luminescence properties
due to intricate coordination geometry of the central lantha-
nide ions and coordination modes of organic ligands.11 In
order to investigate the influence of different synthetic factors
on the structures of the Ln-MOFs and luminescence pro-
perties, in this work both HNNO and H2OX were selected as
mixed ligands (Chart S1†) to react with lanthanide ions Eu3+,
Tb3+, Sm3+, Dy3+ and Gd3+ under the same hydrothermal con-
ditions leading to two series of Ln-MOFs containing the mixed
ligands merely through adjusting the molar ratio of the reac-
tants: [Ln2(NNO)2(OX)2(H2O)4]n (Ln = Eu 1, Tb 2, Sm 3, Dy 4,
Gd 5) and [Ln2(NNO)4(OX)(H2O)2]n (Ln = Eu 6, Tb 7, Sm 8, Dy
9, Gd 10). Compounds 1 and 7 selected as representatives were
structurally characterized. The luminescence properties and
energy transfer mechanism of the compounds were studied
systematically as well.
Nicotinic acid N-oxide (HNNO) was prepared from nicotinic
acid in the presence of acetic acid and hydrogen peroxide
according to the literature,12 and was checked by IR spectro-
scopy, elemental analysis and 13C NMR spectroscopy. Anal.
calcd for C6H5NO3 (HNNO): C, 51.80; H, 3.62; N, 10.07. Found:
C, 51.83; H, 3.53; N, 10.14%. IR (KBr, ν/cm−1) 3430(m), 3074,
1716(m), 1570, 1483(m), 760(s), 1204(s), 1440(s), 933(s), 822
(m). 13C NMR (ppm): 164.2, 142.1, 138.9, 130.5, 126.7, 125.4
(Fig. S1†).
Synthesis of [Eu2(NNO)2(OX)2(H2O)4]n 1
A mixture of HNNO (0.66 mmol, 0.0916 g), H2OX (0.5 mmol,
0.0630 g), Eu2O3 (0.25 mmol, 0.088 g), and H2O (10 mL) was
placed in a 25 mL Teflon-lined stainless steel vessel, kept
under autogenous pressure at 170 °C for 3 days and slowly
cooled to room temperature at the rate of 10 °C per hour. Col-
orless block shaped single crystals were mechanically collected
under a microscope after drying under vacuum at 20 °C, with a
yield of 0.147 g, 71.2% based on Eu. Anal. calcd (%) for
C16H16Eu2N2O18: C, 23.20; H, 1.95; N, 3.38; Eu, 36.40. Found:
C, 23.10; H, 1.99; N, 3.43; Eu, 36.45. IR (KBr, ν/cm−1) 3610(m),
1653(s), 1593(s), 1411(s), 1317(m), 1229(s), 1177(m), 1134(m),
1026(m), 951(s), 790(s), 675(m), 579(m).
Experimental section
Materials and methods
All materials including solvents were obtained from commer-
cial sources, were of reagent grade and were used without
further purification. IR spectra were recorded on a Nicolet
FTIR-170SX spectrophotometer using KBr pellets in the range
Synthesis of [Tb2(NNO)2(OX)2(H2O)4]n 2
The compound was synthesized in a similar process to that for
compound 1, except that Tb4O7 (0.125 mmol, 0.093 g) was
used instead of Eu2O3. Colorless block shaped crystals were
obtained. Yield: 0.145 g, 68.9% based on Tb. Anal. calcd (%)
for C16H16Tb2N2O18: C, 23.81; H, 1.92; N, 3.33; Tb, 37.74.
4000–400 cm−1 13C NMR spectra were recorded on a Bruker
.
AV 400 spectrometer. Elemental analyses for C, H and N were
performed on a Perkin-Elmer 240C analytical instrument,
while Eu, Tb, Sm, Dy and Gd in all samples, which were dis-
solved in dilute hydrochloric acid, were determined using an
ICPS-7500 model inductively coupled plasma emission
spectrometer (ICP-ES). Thermogravimetric analyses were
carried out on a NETZSCH STA 449C unit at a heating rate of
Found: C, 22.79; H, 1.95; N, 3.30; Tb, 37.60. IR (KBr, ν/cm−1
3610(m), 1651(s), 1592(s), 1410(s), 1317(m), 1229(s), 1177(m),
1134(m), 1026(m), 951(s), 790(s), 675(m), 579(m).
)
Synthesis of [Sm2(NNO)2(OX)2(H2O)4]n 3
5 °C min−1 under air. Powder X-ray diffraction (XRD) measure- The compound was synthesized in a similar process to that for
ments were carried out on a Rigaku D/max 2500 X-ray diffracto- compound 1, except that Sm2O3 (0.25 mmol, 0.087 g) was used
meter with a graphite-monochromatized Cu Kα line (λ = instead of Eu2O3. Yield: 0.153 g, 74.0% based on Sm. Anal.
0.15405 nm) as the incident beam. The scanning rate was set calcd (%) for C16H16Sm2N2O18: C, 23.29; H, 1.95; N, 3.39; Sm,
to 10° min−1 in the 2θ range from 3° to 55°. The excitation and 36.46. Found: C, 23.25; H, 1.98; N, 3.42; Sm, 36.45. IR (KBr,
4602 | Dalton Trans., 2015, 44, 4601–4612
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ν/cm−1) 3610(m), 1654(s), 1594(s), 1411(s), 1317(m), 1230(s), Synthesis of [Dy2(NNO)4(OX)(H2O)2]n 9
1177(m), 1134(m), 1026(m), 951(s), 790(s), 675(m), 579(m).
The compound was synthesized in a similar process to that for
compound 6, except that Dy2O3 (0.25 mmol, 0.093 g) was used
instead of Eu2O3. Yield: 0.163 g, 65.0% based on Dy. Anal.
calcd (%) for C26H20Dy2N4O18: C, 31.18; H, 2.01; N, 5.60; Dy,
32.45. Found: C, 31.13; H, 2.05; N, 5.64; Dy, 32.37. IR (KBr,
ν/cm−1) 3406(m), 1671(s), 1621(s), 1593(s), 1569(s), 1484(w),
1420(s), 1393(m), 1315(m), 1234(s), 1135(m), 1120(m), 1023
(m), 925(s), 792(s), 768(m), 670(m), 448(s).
Synthesis of [Dy2(NNO)2(OX)2(H2O)4]n 4
The compound was synthesized in a similar process to that for
compound 1, except that Dy2O3 (0.25 mmol, 0.093 g) was used
instead of Eu2O3. Yield: 0.139 g, 65.6% based on Dy. Anal.
calcd (%) for C16H16Dy2N2O18: C, 22.63; H, 1.90; N, 3.30; Dy:
38.27. Found: C, 22.63; H, 1.95; N, 3.36; Dy, 38.33. IR (KBr,
ν/cm−1) 3610(m), 1653(s), 1595(s), 1410(s), 1317(m), 1230(s),
1177(m), 1134(m), 1026(m), 951(s), 790(s), 675(m), 579(m).
Synthesis of [Gd2(NNO)4(OX)(H2O)2]n 10
The compound was synthesized in a similar process to that for
compound 6, except that Gd2O3 (0.25 mmol, 0.091 g) was used
instead of Eu2O3. Yield: 0.186 g, 75.0% based on Gd. Anal.
calcd (%) for C26H20Gd2N4O18: C, 31.51; H, 2.03; N, 5.65; Gd,
31.70. Found: C, 31.36; H, 1.96; N, 5.61; Gd, 31.75. IR (KBr,
ν/cm−1) 3406(m), 1671(s), 1621(s), 1593(s), 1569(s), 1484(w),
1420(s), 1393(m), 1315(m), 1234(s), 1135(m), 1120(m), 1023
(m), 925(s), 792(s), 768(m), 670(m), 448(s).
Synthesis of [Gd2(NNO)2(OX)2(H2O)4]n 5
The compound was synthesized in a similar process to that for
compound 1, except that Gd2O3 (0.25 mmol, 0.091 g) was used
instead of Eu2O3. Yield: 0.155 g, 74.0% based on Gd. Anal.
calcd (%) for C16H16Gd2N2O18: C, 22.91; H, 1.92; N, 3.34; Gd,
37.50. Found: C, 22.36; H, 1.96; N, 3.41; Gd, 37.44. IR (KBr,
ν/cm−1) 3610(m), 1654(s), 1596(s), 1410(s), 1317(m), 1233(s),
1177(m), 1134(m), 1026(m), 951(s), 790(s), 675(m), 579(m).
X-ray crystallography
Synthesis of [Eu2(NNO)4(OX)(H2O)2]n 6
Crystallographic data for complexes 1 and 7 were collected at
100 K using an Oxford Diffraction Xcalibur Eos Gemini diffr-
actometer with Mo Kα radiation (λ = 0.71073 Å) in the ω scan
mode. The structure was solved by direct methods and refined
on F2 by full-matrix least-squares using the SHELXTL-97
program package. All the non-H atoms were refined with ani-
sotropic displacement coefficients.13 H atoms were added to a
model in their geometrically ideal positions.14 The crystal
system of compounds 1 and 7 is triclinic and the space group
A mixture of HNNO (1.0 mmol, 0.138 g), H2OX (0.5 mmol,
0.0630 g), Eu2O3 (0.25 mmol, 0.088 g), and H2O (10 mL) was
placed in a 25 mL Teflon-lined stainless steel vessel under stir-
ring, kept under autogenous pressure at 170 °C for 3 days and
slowly cooled to room temperature at the rate of 10 °C per
hour. 0.174 g colorless block shaped single crystals were col-
lected under a microscope after drying under vacuum at 20 °C,
with a yield of 71% based on Eu. Anal. calcd (%) for
C26H20Eu2N4O18: C, 31.85; H, 2.06; N, 5.72; Eu, 31.01. Found:
C, 31.82; H, 2.10; N, 5.68; Eu, 31.05. IR (KBr, ν/cm−1) 3406(m),
1671(s), 1621(s), 1593(s), 1569(s), 1484(w), 1420(s), 1393(m),
1315(m), 1234(s), 1135(m), 1120(m), 1023(m), 925(s), 792(s),
768(m), 670(m), 448(s).
ˉ
is P1. Crystallographic data and experimental details for struc-
tural analyses are summarized in Table 1 and the selected
bond lengths and bond angles of compounds 1 and 7 are
listed in Tables S1 and S2.†
Synthesis of [Tb2(NNO)4(OX)(H2O)2]n 7
Results and discussion
Synthesis
The compound was synthesized in a similar process to that for
compound 6, except that Tb4O7 (0.125 mmol, 0.093 g) was
used instead of Eu2O3. Colorless block shaped crystals were The effect of the molar ratio of the reactants HNNO : H2OX :
obtained. Yield: 0.151 g, ca. 66% based on Tb. Anal. calcd (%) lanthanide oxide on the resulting structures of the synthesized
for C26H20Tb2N4O18: C, 31.41%; H, 2.03%; N, 5.64%; Tb, 31.89. compounds was investigated under the same hydrothermal
Found: C, 31.46; H, 2.07; N, 5.64%; Tb, 31.96. IR (KBr, ν/cm−1
)
reaction conditions. The results indicated that two series of
3406(m), 1671(s), 1621(s), 1593(s), 1569(s), 1484(w), 1420(s), Ln-MOFs were obtained at different molar ratios of the reac-
1393(m), 1315(m), 1234(s), 1135(m), 1120(m), 1023(m), 925(s), tants: [Ln2(NNO)2(OX)2(H2O)4]n (Ln = Eu 1, Tb 2, Sm 3, Dy 4,
792(s), 768(m), 670(m), 448(s).
Gd 5) and [Ln2(NNO)4(OX)(H2O)2]n (Ln = Eu 6, Tb 7, Sm 8, Dy
9, Gd 10). When the molar ratio of the reactants lanthanide
oxide : H2OX : HNNO is 0.25 : 0.5 : 0.66, compounds 1–5 can be
Synthesis of [Sm2(NNO)4(OX)(H2O)2]n 8
The compound was synthesized in a similar process to that for harvested, while compounds 6–10 can be obtained by chan-
compound 6, except that Sm2O3 (0.25 mmol, 0.087 g) was used ging the molar ratio of the reactants lanthanide oxide : H2OX :
instead of Eu2O3. Yield: 0.166 g, 68.0% based on Sm. Anal. HNNO to 0.25 : 0.5 : 1.0. When the dosage of HNNO is in
calcd (%) for C26H20Sm2N4O18: C, 31.95; H, 2.06; N, 5.73; Sm, the range of 0.66–1 mmol, the mixture of the two series of
30.78. Found: C, 31.90; H, 2.07; N, 5.73; Sm, 30.75. IR (KBr, compounds can be acquired (Scheme 1). It should be noted
ν/cm−1) 3406(m), 1671(s), 1621(s), 1593(s), 1569(s), 1484(w), that compound 6 was synthesized successfully, structurally
1420(s), 1393(m), 1315(m), 1234(s), 1135(m), 1120(m), 1023 characterized and reported in our previous work.10a In this
(m), 925(s), 792(s), 768(m), 670(m), 448(s).
work, compounds 6–10 were synthesized using the same
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IR spectra of the free ligand and compounds 1–10
Table 1 Crystal data and structure refinement for compounds 1 and 7
The IR spectra of compounds 1–10 and the free ligand HNNO
are shown in Fig. 1. In the IR spectrum of HNNO, a broad
band in the range of 2200–3000 cm−1 centered at 2500 cm−1
can be assigned to O–H stretching vibrations of the carboxylic
groups.15 The strong band at 3074 cm−1 can be attributed to
the C–H stretching vibrations of the pyridine rings.16 The
strong band centered at 1716 cm−1 arises from the stretching
vibrations of the CvO bond from carboxylic groups of the
ligand.17 Two bands at 1570 and 1483 cm−1 can be assigned to
the stretching vibrations of the CvN bond of the pyridine ring
and one strong band at 1440 cm−1 is due to the characteristic
vibration frequency of the pyridine ring.18 Furthermore, the
band at 1204 cm−1 is from the stretching vibrations of the N–O
group.19
The IR spectra of compounds 2, 3, 4 and 5 are analogous to
that of compound 1, implying their isomorphous nature, so
compound 1 was selected as the representative to discuss here.
In the IR spectrum of compound 1, the N–O stretching
vibration appearing at 1228 cm−1 indicates that the O atom
of the N–O bond coordinates to Eu3+ ions.20 Two bands at
1446 cm−1 and 1653 cm−1 can be assigned to symmetric and
asymmetric stretching of the carboxylate group, respectively,
implying that the carboxylic group of HNNO ligands is depro-
tonated and coordinates to Eu3+ ions,21 which is consistent
with the following X-ray single-crystal structure analysis of
compound 1.
Identification code
1
7
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
C
16H16Eu2N2O18
C26H20Tb2N4O18
994.30
102.5 K
0.71073 Å
Triclinic
P1
a = 8.2954(4) Å
b = 9.7473(5) Å
c = 9.8872(6) Å
α = 101.508(5)°
β = 108.666(5)°
γ = 101.022(4)°
713.62(7) Å3
1
828.23
100(2) K
0.71073 Å
Triclinic
ˉ
ˉ
P1
Unit cell dimensions
a = 6.6723(9) Å
b = 8.3982(13) Å
c = 10.312(2) Å
α = 75.470(16)°
β = 71.695(15)°
γ = 88.038(13)°
530.44(15) Å3
1
Volume
Z
Density(calculate)/Mg
2.593
2.314
m−3
Absorption coefficient/ 5.961
5.013
mm−1
F(000)
Crystal size
396
478
0.34 × 0.35 ×
0.30 mm3
3.22 to 26°
0.20 × 0.15 ×
0.10 mm3
2.98 to 26°
2Θ range for data
collection
Index ranges
−8 ≤ h ≤ 8
−10 ≤ h ≤ 10
−12 ≤ k ≤ 8
−12 ≤ l ≤ 11
5412
−10 ≤ k ≤ 10
−12 ≤ l ≤ 12
3924
Reflections collection
Independent
reflections
2084[R(int) = 0.0345]
2800[R(int) = 0.0424]
Goodness-of-fit on F2
Final R indexes
[I ≥ 2σ(I)]
1.070
1.095
R1 = 0.0245,
wR2 = 0.0572
R1 = 0.0262,
wR2 = 0.0579
0.779/−1.478
R1 = 0.0273,
wR2 = 0.0637
R1 = 0.0292,
wR2 = 0.0648
0.886/−1.579
Final R indexes
[all data]
The IR spectra of compounds 6–10 are almost the same,
hence compound 6 was chosen as the representative for dis-
cussion here. Four peaks at 1420(s), 1393(s), 1593(s), and 1569
(s) cm−1 can be assigned to symmetric and asymmetric stretch-
ing of carboxylate groups of the ligands, and the peak at
1224 cm−1 assigned to the N–O stretching vibration indicated
that carboxylic groups are deprotonated and coordinate
to Eu3+ ions and the O atom of the N–O bond coordinates to
Eu3+ ions.20,21
Largest diff. peak/
hole/e Å−3
CCDC no.
1020656
1023399
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = ∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]1/2
.
Scheme 1 The schematic diagram of synthesis compounds 1–10 (Ln =
Eu, Sm, Dy and Gd, x = 2, y = 3; Ln = Tb, x = 4, y = 7).
published method in order to compare the structural differ-
ences of compounds 1–5 and investigate the relationship
between the structures and luminescence properties of the
compounds.
Fig. 1 IR spectra of nicotinic acid N-oxide and compounds 1–10.
4604 | Dalton Trans., 2015, 44, 4601–4612
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The IR spectra of compounds 6–10 are similar to that of
compounds 1–5, but some differences can be found, which are
caused by the different numbers and different coordination
modes of ligands in the two series of compounds.
Powder XRD patterns
Powder X-ray diffraction experiments were carried out for com-
pounds 1–10. The experimental PXRD patterns of compounds
1–5 with the simulated pattern of compound 1 and com-
pounds 6–10 with the simulated pattern of compound 7 are
shown in Fig. 2 and 3, respectively. The powder XRD patterns
for the as-obtained bulk materials of compounds 1–5 and 6–10
closely match the simulated ones indicating that the phases of
the final resulting compounds 1–5 and 6–10 are pure solid-
state phases, no further by-product in the collected products
was found, and compounds 1–5 and 6–10 are isomorphous,
respectively.
Fig. 3 The powder X-ray diffraction patterns of compounds 6–10 and
the simulated pattern for compound 7.
Description of the structures of compounds 1–10
According to the results of powder X-ray diffraction analysis,
compounds 1–5, namely [Ln2(C6H4NO3)2(C2O4)2(H2O)4]n (Ln =
Eu 1, Tb 2, Sm 3, Dy 4, Gd 5), are isostructural, which are crys-
ˉ
tallized in the triclinic crystal system with the P1 space group.
So, as a representative, the structure of compound 1 was deter-
mined by single crystal X-ray diffraction analysis.
As shown in Fig. 4, the asymmetric unit of compound 1
contains one crystallographically independent Eu3+ ion, one
NNO− ligand, one OX2− ligand and two coordinated water
molecules. Each Eu3+ ion is nine-coordinate with nine oxygen
atoms from three individual NNO− ligands (O3, O2#2, O1#1),
two OX2− (O6, O7#4, O4, O5#3), and two coordinated water
molecules (O8, O9), forming a distorted tricapped trigonal
prism (Fig. S2a†). The Eu–O bond lengths are in the range of
2.3549(33)–2.5299(38) Å, and the O–Eu–O bond angles are in
the range of 65.614(108)–141.744(108)°, which are similar to
those in the reported Eu complexes.1a,c
Fig. 4 Local coordination environment of Eu3+ ions in 1 (atoms com-
posing the asymmetric unit of each material are connected by yellow-
filled bonds, H atoms are omitted for clarity). Symmetry code: (#1) +x,
+y, 1 + z; (#2) 1 − x, 1 − y, 1 − z; (#3) 1 − x, 2 − y, 1 − z; (#4) −x, 1 − y,
2 − z.
Fig. 5 Coordination modes for NNO− and OX2− adopted in compounds
1 and 7.
In compound 1, as illustrated in Fig. 5(a), the NNO− ligand
adopts only one coordination mode; the carboxylate groups
adopt μ2–η1–η1 coordination mode linking two Eu3+ ions and
its N-oxide moiety linking one Eu3+ ion, while each oxalate
links two Eu3+ ions (Fig. 5(c)).
Fig. 2 The powder X-ray diffraction patterns of compounds 1–5 and
the simulated pattern for compound 1.
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Fig. 7 Local coordination environment of Tb3+ ions in 7 (atoms com-
posing the asymmetric unit of each material are connected by yellow-
filled bonds, H atoms are omitted for clarity). Symmetry code: (#1) +x,
1 + y, z; (#2)1 − x, 1 − y, −z; (#3) 1 − x, 1 − y, 1 − z; (#4) 2 − x, 2 − y,
1 − z; (#5) x, y − 1, z.
Fig. 6 (a) The 1-D Eu-NNO double chain along the c direction in com-
pound 1; (b) the 2-D layer composed of the 1-D double chains in the ac
plane in compound 1; (c) the 3-D open framework structure composed
of 2-D layers in compound 1 (colour code: C in pyridine, black; C in
oxalate, black and red; Eu, tea; N, blue; O, red); (d) topology along the a
direction in compound 1 (red node: Eu3+ ion; green node: NNO−; purple
node: OX2−). All H atoms are omitted for clarity.
O2#3, O3, O4, O5#2), one coordinated OX2− (O7#1, O8), and
one water molecule (O9), forming a distorted bicapped trigonal
prism (Fig. S2b†). The Tb–O bond lengths are in the range of
2.3479(6) Å–2.4536(7) Å, and the O–Tb–O bond angles are in
the range of 66.803(13)°–147.492(18)°, which are similar to
those in the reported Tb complexes.
The 3-D framework of compound 7 can be described as
follows: two Tb3+ ions are bridged by a pair of NNO− ligands
(mode Fig. 5(a)) through their carboxylic groups forming a 1-D
double chain along the b direction, in which the neighbouring
Tb⋯Tb distance is ca. 9.75 Å (Fig. 8(a)). Then, the neighbour-
ing 1-D double chains are linked by OX2− ligands (mode Fig.
5(c)) forming a 2D wavelike layer in the ab plane with the
shortest Tb⋯Tb distance being ca. 4.91 Å (Fig. 8(b)). The
adjacent 2-D layers are further linked through NNO− ligands
with its one oxygen atom of the carboxylate linking one Tb3+
ion and its N-oxide moiety linking one Tb3+ ion (mode
The 3-D framework of compound 1 can be described as
follows: two Eu3+ ions are bridged by a pair of NNO− ligands
(mode Fig. 5(a)) by their carboxylic groups, forming a 1D
double infinite chain along the c direction, in which the neigh-
bouring Eu⋯Eu distance is ca. 10.3 Å (Fig. 6(a)). Then, the
adjacent 1-D double chains are linked by OX2− ligands (mode
Fig. 5(c)) forming a 2D wavelike layer in the ac plane with the
shortest interchain Eu⋯Eu distance being ca. 6.3 Å (Fig. 6(b))
which is approximately equal to the bridging OX2− length. The
adjacent 2-D layers are further ‘pillared’ by OX2− ligands
(mode Fig. 5(c)) resulting in a 3-D open framework with
pseudo-1-D channels (Fig. 6(c)), in which the shortest inter-
layer Eu⋯Eu distance is ca. 6.3 Å. Although compound 1 exhi-
bits an interesting 3-D framework structure, no solvent
accessible void can be calculated by the CALC SOLV command
in PLATON.22 The van der Waals radii of the constituent atoms
are responsible for the compact framework structure.
In order to better understand the 3-D open framework of
compound 1, topological analysis of compound 1 was carried
out. Each Eu3+ ion links five ligands (two OX2− ligands and
three NNO− ligands) and therefore is regarded as a pentacor-
ner-shareholder and each NNO− ligand links three Eu3+ ions
and is regarded as a tricorner-shareholder; each OX2− ligand
links two Eu3+ ions and is regarded as a bicorner-shareholder.
As shown in Fig. 6(d), the connectivity of these holders in
compound 1 results in a 3-D framework featuring a binodal
(3,5)-connected net with the point (Schläfli) symbol of
(42·65·83)(42·6).
Single-crystal X-ray diffraction analysis revealed that com-
Fig. 8 (a) The 1-D Tb-NNO double chain along the b direction in com-
pound 7; (b) the 2-D layer composed of the 1-D Tb-NNO double chains
in the ab plane in compound 7; (c) the 3-D open framework structure
composed of 2-D layers in compound 7 (colour code: C in pyridine,
black and yellow; C in oxalate, black; Tb, tea; N, blue; O, red); (d) top-
ology along the a direction in compound 7 (green node: Tb3+ ion; red
node: NNO−; blue node: OX2−). All H atoms are omitted for clarity.
ˉ
pound 7 crystallizes in the triclinic P1 space group. The asym-
metric unit contains one crystallographically independent
Tb3+ ion, two NNO− ligands, half OX2− and one coordinated
water molecule (Fig. 7), each Tb3+ ion is eight-coordinate with
eight oxygen atoms from five individual NNO− ligands (O1#4,
4606 | Dalton Trans., 2015, 44, 4601–4612
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Fig. 5(b)), resulting in a 3-D open framework (Fig. 8(c)), in for the compounds 6 and 8 selected as representatives of the
which the shortest interlayer Tb⋯Tb distance is ca. 7.91 Å. compounds with the formula [Ln2(NNO)4(OX)(H2O)2]n. Good
Although compound 7 exhibits an interesting 3-D framework thermal stability from room temperature to 150 °C for com-
structure, no solvent accessible void has been calculated from pound 1 and from room temperature to 140 °C for compound
the CALC SOLV command in PLATON.21
3 is observed. The first weight loss of 8.62% (calcd 8.70%) for
Topological analysis of the 3-D structure of compound 7 compound 1 in the range of 150 to 175 °C and 8.34% (calcd
was carried out. Each Tb3+ ion links six ligands (one OX2− 8.73%) for compound 3 in range of 145 °C to 173 °C is an
ligand and five NNO− ligands) and therefore is regarded as a endothermic process, corresponding to the loss of coordinated
hexacorner-share holder and the two kinds of NNO− ligands water molecules. Then remarkable thermally stability is
link two or three Tb3+ ions and are regarded as bicorner-share observed again in the range of 170 to 355 °C for compound 1
holders or tricorner-share holders, respectively. Each OX2− and 173 °C to 360 °C for 3. Compounds 1 and 3 from 355 °C
ligand links two Tb3+ ions and is regarded as a bicorner-share- to 650 °C and 360 °C to 740 °C, respectively, decompose and
holder. As shown in Fig. 8(d), the connectivity of these holders lose organic parts by an exothermic process. The weight of the
in compound 7 results in a 3-D framework featuring a binodal remaining residue (calc. 42.50%, found 42.99% for compound
(3,5)-connected net with the point (Schläfli) symbol of 1; calc. 42.27%, found 42.36% for compound 2) matches well
(42·65·83)(42·6).
the composition of Eu2O3 and Sm2O3, respectively.
Comparison of the structures of compound 1 with com-
Compounds 6 and 8 are selected as representatives of the
pound 7 reveals that both consist of Ln-NNO-OX 2-D layers compounds with the formula [Ln2(NNO)4(OX)(H2O)2]n to inves-
composed of Ln-NNO double chains. The difference in their tigate their thermal stability (Fig. 9(c) and (d)). Compounds 6
3D construction lies in the different linkers connecting the and 8 show remarkable thermal stability from room tempera-
adjacent 2-D layers. The adjacent 2-D layers in compound 1 are ture to 300 °C. The weight loss of 64.26% (calc. 64.09%) for
joined together via OX2− ligands as linkers, while the adjacent compound 6 from 300 to 674 °C and 64.25% (calc. 64.31%) for
2-D layers in compound 7 are joined together via NNO− compound 8 from 300 to 670 °C is an exothermic process,
ligands as linkers. Since all the compounds were synthesized corresponding to decomposition of the compounds and loss
under the same hydrothermal conditions, except for the molar of coordinated water molecules and organic moieties. The
ratio of the reactants HNNO : H2OX : Ln2O3, the observed weight of the remaining residue (calc. 35.91%, found 35.74%
difference in the structures can only be attributed to the molar for compound 6; calc. 35.69%, found 35.70% for compound 8)
ratio of the reactants, in other words, simple variation of the matches well the composition of Eu2O3 and Sm2O3,
molar ratio of the reactants can be adopted to produce respectively.
different compounds of different properties.
UV-vis absorption spectrum of HNNO
Thermal stability analysis
As shown in Fig. S3,† the UV-Vis absorption spectrum of
In order to investigate the thermal stability of compounds
HNNO in DMF (c = 1 × 10−4 mol L−1) shows one UV-Vis absorp-
1–10, TG-DTA experiments were performed, under flowing air
tion band at 309 nm, which is assigned to the π→π* transition
in the temperature range of 30 to 850 °C, for compounds 1
of the ligand.23 The molar extinction coefficient value of the
and 3 selected as representatives of the compounds with
ligand at 309 nm is 6.88 × 103 L mol−1 cm−1 implying that the
the formula [Ln2(NNO)2(OX)2(H2O)4]n (Fig. 9(a) and (b)) and
ligand is a good light-harvesting chromophore to sensitize the
lanthanide luminescence.24 The singlet-state energy level can
be calculated to be about 28 653 cm−1 from the UV-Vis absorp-
tion spectrum absorbance edge of HNNO.
Diffuse reflectance spectroscopy
As shown in Fig. 10, the free ligands H2OX and HNNO show
absorption bands at 289 and 309 nm, which are assigned to
the π→π* transition of the ligands.23 The UV-vis spectra of
compounds 1–5 and compounds 6–10 (Fig. 10) are similar to
those of the ligands indicating that the Ln3+ ions do not affect
remarkably the singlet excited state of the free ligands.25
Comparing with the free ligands HNNO and H2OX, there are
small blue shifts or red shifts in the UV-vis spectra of the com-
pounds, which indicates perturbation induced by the metal
coordination or an effective interaction between the
Fig. 9 TG-DTA curves of compounds 1 (a), 3 (b), 6 (c) and 8 (d).
lanthanide cations and the organic ligands.26
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Fig. 11 Schematic energy level diagram and the energy transfer
process in compounds 1–4 and 6–9.
Fig. 10 Solid-state diffuse reflectance spectra of the free ligands and
compounds 1–10.
energy level of the Tb3+ ion (5D4, 20 500 cm−1) is 2648 cm−1
,
Phosphorescence spectrum of compound 5
which is within the optimal energy gap implying that the tran-
sitions from the triplet energy level of HNNO to the Tb3+ ion is
effective. However energy differences between the lowest triplet
level of HNNO and the resonant energy levels of Eu3+ ions
(5D0, 17 500 cm−1), Sm3+ ions (4G5/2, 17 900 cm−1) and Dy3+
ions (F9/2, 20 875 cm−1) are 5648 cm−1, 5248 cm−1 and
2272 cm−1, respectively, which are all outside the optimal
energy gap, indicating that the transitions from the triplet
energy level of HNNO to the Eu3+ ion, Sm3+ ion and Dy3+ ion
are not effective. The lowest triplet level of oxalic acid is
6
Since the lowest excited state P7/2 of the Gd3+ ion is too high
to accept energy from a ligand, the triplet energy level of the
corresponding ligand can be obtained from the phosphor-
escence spectrum of its Gd3+ complex at 77 K.27 As shown in
Fig. S4,† in the phosphorescence spectrum of compound 5,
two phosphorescence bands, peaking around 401 and 432 nm,
can be observed. As the triplet at 24 570 cm−1 (401 nm) is
attributed to the H2OX ligand,28 the lowest triplet energy level
of HNNO can be calculated to be about 23 148 cm−1 (432 nm).
24 570 cm−1 28
the energy differences between the lowest
,
Energy transfer mechanism
triplet of H2OX and the Eu3+ ion (5D0, 17 500 cm−1), the Tb3+
ion (5D4, 20 500 cm−1), the Sm3+ ion (4G5/2, 17 900 cm−1) and
the Dy3+ ion (F9/2, 20 875 cm−1) are 6423, 3423, 6670 and
3695 cm−1, respectively, which indicates that H2OX is a suit-
able sensitizer for the luminescence of Tb3+ rather than the
Eu3+ ion, Sm3+ ion and Dy3+ ion. Thus, among the compounds
1–10, it is predicted that compounds 2 and 7 can emit strong
green characteristic luminescence of Tb3+ ions. The schematic
energy level diagram and the energy transfer process in com-
pounds 1–4 and compounds 6–9 are shown in Fig. 11.
According to Reinhoudt theory,24a the intersystem crossing
process becomes effective when ΔE(1ππ*–3ππ*) of a ligand is at
least 5000 cm−1. The calculated energy gap between the 1ππ*
(28 653 cm−1) and 3ππ* (23 148 cm−1) levels being 5505 cm−1
for HNNO indicates an effective intersystem crossing process
in compounds 1–10 (Table 2).
According to the intramolecular energy transfer mechanism
reported by Latva,29 the energy difference between the lowest
triplet level of a ligand and the resonant energy level of the
Ln3+ ion plays a decisive role in the energy transfer process in
lanthanide complexes. An optimal value of the energy differ-
ence is assumed to exist around 3000 500 cm−1 for the Eu3+
Photoluminescence properties
ion and 2500–4500 cm−1 for the Tb3+ ion,30 the larger or The solid-state photoluminescence spectra recorded at room
smaller energy difference may result in a decrease of photo- temperature for compounds 1 and 6 are depicted in Fig. 12.
luminescence intensities of the lanthanide complexes.5,31
The excitation spectra (Fig. 12(a)) were monitored by the
As listed in Table 2, the energy difference between the intense characteristic emission 5D0→7F1 (617 nm for com-
lowest triplet levels of the HNNO ligand and the resonant pound 1 and 619 nm for compound 6) of the Eu3+ ion.
Table 2 The singlet and triplet energy levels of the HNNO ligand and energy level differences between Ln3+ ions and the HNNO ligand in the com-
plexes 1–4 and 6–9
Ligand 1ππ*/cm−1 3ππ*/cm−1 ΔE(1ππ*–3ππ*)/cm−1 ΔE(3ππ*–5D0)/cm−1 ΔE(3ππ*–5D4)/cm−1 ΔE(3ππ*–4G5/2)/cm−1 ΔE(3ππ*–4F9/2)/cm−1
HNNO 28653
H2OX 38461
23148
23923
5505
14538
5648
6423
2648
3423
5248
6670
2272
3695
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Fig. 13 The solid-state excitation (a) and emission spectra (b) of com-
pound 2 (black) (λex = 288 nm, λem = 545 nm) and 7 (red) (λex = 320 nm,
λem = 545 nm).
Fig. 12 The solid-state excitation spectra (a) and emission spectra (b) of
compound 1 (black) (λem = 617 nm, λex = 284 nm) and compound 6 (red)
(λem = 619 nm, λex = 315 nm).
the direct intra 4f8 excitation.26 The emission spectra (Fig. 12
(b)) of compounds 2 and 7 obtained upon excitation at the
maximum excitation wavelength (288 nm for compound 2 and
320 nm for compound 7) show the characteristic transitions
5D4→7Fj ( j = 3–6) of Tb3+ ions, viz. 5D4→7F6 (490 nm), 5D4→7F5
(544 nm), 5D4→7F4 (584 nm) and 5D4→7F3 (620 nm).
The 5D4→7F5 transition at 545 nm is the strongest emission,
implying intense green luminescence.
The luminescence of compounds 3, 4, 8 and 9 is too weak
to monitor which is mainly due to energy differences between
the lowest triplet levels of the ligands and the resonant energy
levels of Sm3+ and Dy3+ ions are outside the optimum value
and thus the ligands cannot effectively sensitize Sm3+ and Dy3+
luminescence.
Interestingly, it is found that the emission intensities of
compounds 6 and 7 are generally stronger than that of com-
pounds 1 and 2 (Fig. 12 and 13), which can be explained by
the following two reasons: first, there are two OX2− and three
NNO− ligands coordinated to Ln3+ ions in compounds 1 and 2,
while in compounds 6 and 7, there are one OX2− and five
NNO− ligands coordinated to Ln3+ ions, i.e. there are more
NNO− ligands coordinating to Ln3+ ions and they can transfer
energy to Ln3+ ions more effectively to sensitize Ln3+ lumine-
scence in compounds 6 and 7 than that in compounds 1 and 2
(Table 3); second, in compounds 6 and 7, the number of
A strong broad band with peaks at 284 nm and 315 nm can be
attributed to the electronic transitions of the ligands. The
weaker intensities of 4f–4f transitions of Eu3+ ions at 361 nm
(7F0→5D4), 380 nm (7F0→5G2) and 393 nm (7F0→5L6) can be
observed in the excitation spectra of compounds 1 and 6 indi-
cating a more effective luminescence sensitization via the
ligands.27
The emission spectra (Fig. 12(b)) of compounds 1 and 6 at
284 nm and 315 nm, respectively, exhibit the Eu3+ ion charac-
5
teristic sharp bands assigned to D0→7Fj ( j = 0–4) transitions
of the Eu3+ ion: 5D0→7F0 (579 nm), 5D0→7F1 (593 nm),
5D0→7F2 (615 nm), 5D0→7F3 (653 nm) and 5D0→7F4
(697 nm).31 The absence of bands from the higher excited
states such as 5D1 in the emission spectra implies that the
nonradiative relaxation to the 5D0 level is efficient.32 The inten-
sity of the electric dipole transition (5D0→7F2) is the strongest
implying intense red luminescence.33 The intensity ratio
I(5D0→7F2)/I(5D0→7F1) of compounds 1 and 6 are 1.75 and
1.88, respectively, indicates that the coordination environment
of Eu3+ ions in compounds 1 and 6 is lack of inversion
center.34 In addition, the 5D0→7F0 transition of Eu3+ ions
induced by crystal field J mixing is observed in the emission
spectra, which is only allowed for the 10 symmetries, Cs, C1,
C2, C3, C4, C6, C2v, C3v, C4v and C6v according to the ED selec-
tion rule.35 In fact, compounds 1 and 6 are triclinic, and Eu3+
ions occupy a low symmetry crystal field belonging to C1.35
The solid-state photoluminescence spectra of compounds 2
and 7 recorded at room temperature are depicted in Fig. 13.
The excitation spectra of compounds 2 and 7 (Fig. 13(a))
exhibit the intense characteristic emission 5D4→7F5 (544 nm
for compound 2 and 545 nm for compound 7) of Tb3+ ions,
and display a notable broad band with peaks at 287 nm and
320 nm in the range from 200 to 350 nm attributed to the elec-
tronic transitions of the ligands. The quite weak intensities of
4f–4f transitions of Tb3+ ions appearing in the excitation
spectra of compounds 2 and 7 indicate a more effective lumine-
scence sensitization via the ligand excited states than that of
Table 3 Radiative lifetimes (τR), intrinsic quantum yields of the lantha-
nide luminescence step (ΦLn), sensitization efficiencies (ηsens), the experi-
mentally determined luminescence lifetimes (τobs) and overall quantum
yields (Φtot) of compounds 1, 2, 6 and 7
Compound
τR/ms
6.90
τobs/ms
ΦLn/%
6.60
ηsens/%
15.61
Φtot/%
1
2
6
7
0.46
0.83
0.69
0.89
1.03
3.29
2.58
3.78
6.25
11.04
23.37
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The overall quantum yield (Φtot) is calculated from sensitiz-
ation efficiencies (ηsens) and the intrinsic quantum yield (ΦLn
)
of the lanthanide luminescence according to the reported
formula37 Φtot = ηsensΦLn. ΦLn can be calculated from τobs and
natural lifetime (τR)38 using the formula ΦLn = τobs/τR and 1/τR
= AMD,0n3(Itot/IMD), where AMD,0 = 14.65 s−1 which stands for
5
the odds of the spontaneous radiative transition of D0→7F1, n
is the refractive index of medium, Itot is the sum of Eu3+ emis-
sion spectrum peak area integrals, while IMD is the area inte-
gral of the magnetic dipole transition peak area (5D0→7F1).39
Radiative lifetimes (τR), intrinsic quantum yields of the lantha-
nide luminescence (ΦLn), sensitization efficiencies (ηsens), the
experimentally determined luminescence lifetimes (τobs) and
overall quantum yields (Φtot) of compounds 1, 2, 6 and 7 are
listed in Table 3. It can be found that the overall quantum
yields (Φtot) of compounds 1, 2, 6 and 7 are low which may be
caused by the coordination of the water molecules with Ln3+
ions. Both luminescence lifetimes (τobs) and overall quantum
yields (Φtot) of compounds 6 and 7 are larger than that of com-
pounds 1 and 2, which is mainly due to more NNO− ligands
and less water molecules coordinated to Ln3+ ions existing in
compounds 6 and 7 than that in compounds 1 and 2. On the
other hand, the experimentally determined luminescence life-
times (τobs) and overall quantum yields (Φtot) and sensitization
efficiencies (ηsens) of compounds 2 and 7 are larger than that
of compounds 1 and 6, which are mainly caused by the more
effective energy transfer from the ligands to Tb3+ ions in com-
pounds 2 and 7 rather than Eu3+ in compounds 1 and 6,
which is consistent with the luminescence spectral analyses
above.
Fig. 14 Room temperature emission decay curves of compound 1
excited at 284 nm and monitored at 617 nm and of compound 2 excited
at 288 nm and monitored at 545 nm, respectively.
Conclusions
In summary, formation of the two series of 3-D Ln-MOFs with
the formulae [Ln2(NNO)2(OX)2(H2O)4]n and [Ln2(NNO)4(OX)-
(H2O)2]n based on HNNO and H2OX under the same hydro-
thermal conditions unequivocally demonstrates the influence
of synthetic factors (herein by merely changing the molar ratio
of reactants) on the structures of the Ln-MOFs. The energy
transfer mechanism indicated that the Tb3+ luminescence is
well sensitized by HNNO and H2OX ligands, demonstrating
efficient energy transfer from the ligands to Tb3+ ions, while
luminescence of Eu3+, Sm3+ and Dy3+ ions is difficult to be sen-
sitized by the ligands effectively due to unmatched energy
gaps between the triplet state levels of HNNO and H2OX
ligands and 5D0 (Eu3+), 4G5/2 (Sm3+) and F9/2 (Dy3+). The higher
luminescence intensity of compounds 6–7 than that of com-
pounds 1–2 indicated that the synthetic factors not only influ-
ence the structures of the resulting compounds but also affect
their luminescence properties. The thermal stability analysis
results illustrated that compounds 1–10 show remarkable
structure stability between 25 °C and 300 °C. Compounds 2
and 7 have the advantages of good luminescence and good
thermal stability; they are expected to find applications in the
luminescent material field.
Fig. 15 Room temperature emission decay curves of compounds 6 and
7 excited at 315 nm and monitored at 619 nm and excited at 320 nm
and monitored at 545 nm, respectively.
coordination water molecules is less than that in compounds 1
and 2, so the quenching effects of Ln3+ ions caused by the
coordination interaction between coordination water mole-
cules and Ln3+ ions were reduced.33a,36
The room-temperature 5D0(Eu3+) and 5D4(Tb3+) lumine-
scence decay curves were recorded by exciting at the maximum
emission wavelengths of 5D0→7F2 (compound 1, 617 nm; com-
5
pound 6, 615 nm) and D4→7F5 (compounds 2 and 7, 545 nm)
transitions for compounds 1, 6 and 2, 7, respectively. As shown
in Fig. 14 and 15, the luminescence decay curves of com-
pounds 1, 2, 6 and 7 fit well into single-exponential functions.
The lifetime values (τobs) of compounds 1, 2, 6 and 7 were
determined as 0.46 ms, 0.83 ms, 0.69 ms and 0.89 ms,
respectively.
4610 | Dalton Trans., 2015, 44, 4601–4612
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12 V. Derdau, S. Laschat, E. Hupe, W. A. König, I. Dix and
Acknowledgements
P. G. Jones, Eur. J. Inorg. Chem., 1999, 6, 1001–1007.
The financial support from the NSFC, PCSIRT (No. IRT1205), 13 A. Messerschmidt, M. Schneider and R. Huber, J. Appl.
Beijing Engineering Center for Hierarchical Catalysts and the Crystallogr., 1990, 23, 436–439.
Fundamental Research Funds for the Central Universities 14 G. M. Sheldrick, SHELXL-97, Program for the Refinement of
(YS1406) is greatly acknowledged. Prof. Xue Duan from the Crystal Structure, University of Göttingen, Germany, 1997.
Beijing University of Chemical Technology is greatly acknowl- 15 S. Q. Su, W. Chen, C. Qin, Z. Y. Song, G. H. Li, X. Z. Song,
edged for his kind support.
M. Zhu, S. Wang, Z. M. Hao and H. J. Zhang, Cryst. Growth
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