Synthesis, crystal structure and luminescence properties of lanthanide coordination polymers with a new semirigid bridging thenylsalicylamide ligand

Article abstract of DOI:10.1016/j.jssc.2013.07.014

Two new lanthanide coordination polymers based on a semirigid bridging thenylsalicylamide ligand {[Ln₂L₃(NO₃) ₆]] (C₄H₈O₂)₂}∞ were obtained and characterized by elemen

Full text of DOI:10.1016/j.jssc.2013.07.014

Journal of Solid State Chemistry 205 (2013) 183–189
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Synthesis, crystal structure and luminescence properties of lanthanide
coordination polymers with a new semirigid bridging
thenylsalicylamide ligand
n
Xue-Qin Song , Li Wang, Meng-Meng Zhao, Xiao-Run Wang,
Yun-Qiao Peng, Guo-Quan Cheng
School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new lanthanide coordination polymers based on a semirigid bridging thenylsalicylamide ligand
{[Ln2L3(NO3)6] ꢀ (C4H8O2)2}1 were obtained and characterized by elemental analysis, X-ray diffraction, IR
and TGA measurements. The two compounds are isostructure and possess one dimensional trapezoid
ladder-like chain built up from the connection of isolated LnO3(NO3)3 polyhedra (distorted monocapped
antisquare prism) through the ligand. The photoluminescence analysis suggest that there is an efficient
ligand-to-Ln(III) energy transfer in Tb(III) complex and the ligand is an efficient “antenna” for Tb(III).
From a more general perspective, the results demonstrated herein provide the possibility of controlling
the formation of the desired lanthanide coordination structure to enrich the crystal engineering strategy
and enlarge the arsenal for developing excellent luminescent lanthanide coordination polymers.
& 2013 Elsevier Inc. All rights reserved.
Received 25 May 2013
Received in revised form
12 July 2013
Accepted 14 July 2013
Available online 26 July 2013
Keywords:
Salicylamide ligand
Lanthanide coordination polymer
Crystal structure
Luminescence properties
1. Introduction
We are interested in the synthesis and characterization of lantha-
nide complexes with flexible ligands incorporating salicylamide deri-
Research into utilizing rare earth compounds in industrial,
technological, and medical applications has been continuously
growing over the last few decades. The interest is stimulated not
only by their unique spectroscopic and magnetic properties, but
also by their numerous applications in catalysts, magnets and
luminescent materials [1–4]. Lanthanide coordination polymers in
particular are promising materials because of the intrinsic physical
properties of the trivalent lanthanide ions, such as color-pure
luminescence, large paramagnetism, and the fact that the electro-
static nature of their coordination chemistry allows a large variety
of symmetries and structural patterns to be obtained. One of the
focal points for the design and exploitation of CPs is the rational
choice and assembly of metal ions and versatile bridging organic
ligands. In recent years, various organic ligands used for con-
structing lanthanide coordination polymers have been synthesized
[5–10]. Among these numerous ligands, salicylamide ligands
provide a fascinating prospect in preparing lanthanide-based CPs
as a result of interesting chemical ability to form a wide variety of
frameworks with different dimensionalities and sensitize lantha-
nide luminescence successfully [11].
vatives, and especially in how different types of flexible backbone as
well as different terminal coordination sites can impact the structures
and the luminescence properties [12]. As a continuation of our work
with the lanthanide complex with salicylamide derivatives, we have
successfully obtained a series of lanthanide coordination polymers of
a semirigid bridging ligand with 1,4-dimethoxylbenzene backbone
and terminated with picolysalicylamide recently [11k]. With continued
exploration, we reported herein two novel lanthanide coordina-
tion polymers of similar ligand with thenylsalicylamide terminal
group, namely, 1,4-bis{[(2′-thenylaminoformyl)phenoxyl]methyl}-2,5-
bismethoxybenzene(L) (Scheme 1). As a result, two novel lanthanide
coordination polymers, namely, {[Gd2L3(NO3)6]ꢀ 2C4H8O2}1(1), {[Tb2L3
(NO3)6]ꢀ 2C4H8O2}1(2) were obtained and characterized by elemental
analysis, X-ray diffraction analysis, IR spectroscopy and thermogravi-
metric (TGA) analysis. The two compounds are isostructural and
crystallize in triclinic space group P-1. They possess one dimensional
trapezoid ladder-like chain built up from the connection of isolated
LnO3(NO3)3 polyhedra (distorted monocapped antisquare prism)
through the ligand, which further connected by C–H…O hydrogen
bond to 3D supramolecular architectures. The lowest triplet state
energy levels of the ligand was calculated from the phosphorescence
spectra of the Gd(III) complex at 77 K. The results presented herein
indicated that the new semirigid bridging ligand exhibited a good
antennae effect to the Tb(III) ion due to efficient ligand to metal energy
transfer.
n
Corresponding author. Fax: +86 931 4938755.
E-mail addresses: songxq@mail.lzjtu.cn, songxueqin001@163.com (X.-Q. Song).
0022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jssc.2013.07.014

184
X.-Q. Song et al. / Journal of Solid State Chemistry 205 (2013) 183–189
2. Experimental
and heated to reflux for 12 h. After cooling down, inorganic salts
were separated by filtration and the solvent removed from the
filtrate under reduced pressure. The crude product was recrystal-
lized with ethyl acetate to give a white solid. 2.64 g, Yield 84%. m. p.
158–159 1C. Analytical data, Calc. for C34H32N2O6S2: C, 64.95; H,
5.13; N, 4.46; S, 10.20; Found: C, 64.65; H, 5.14; N, 4.48; S, 10.23; IR
(KBr, υ, cmꢁ1): 3375 (s), 2912(m), 2886(w), 1648 (s,C¼O), 1597 (m),
1536 (s), 1480 (m), 1293 (s), 1226 (s, Ar–O), 752 (s). 1H NMR (CDCl3,
300 MHz): δ: 4.75 (d, 4H, NHCH2, J¼5.2 Hz), 5.26 (s, 4H, OCH2), 6.93
(s, 2H, thiophene), 7.09 (m, 6H), 7.22 (m, 2H), 7.43 (dd, 2H), 7.54 (m,
2H), 8.24 (m, 2H), 8.35 (d, 2H, Ar, J¼2.4 Hz)), 8.90 (t, 2H, NH).
2.1. Materials and instrumentation
Thenylamine and 1,4-bismethoxy-benzene was obtained from
Alfa Aesar Co. Other commercially available chemicals were of
analytical grade and were used without further purification. The
lanthanide nitrates [13] were prepared according to the literature
method.
Carbon, nitrogen, sulfur and hydrogen analyses were per-
formed using an EL elemental analyzer. Melting points were
determined on
a Kofler apparatus. Infrared spectra (4000–
400 cmꢁ1) were obtained with KBr discs on a Therrno Mattson
FTIR spectrometer. Powder X-ray diffraction patterns(PXRD) were
determined with Rigaku-D/Max-II X-ray diffractometer with
graphite-monochromatized Cu-Kα radiation. 1HNMR spectra were
recorded on a Bruker DRX 300 spectrometer in CDCl3 solution
with TMS as internal standard. Thermogravimetric analyses were
carried out on a SDT Q600 thermogravimetric analyzer from room
temperature to 600 1C. A platinum pan was used for heating the
sample with a heating rate of 10 1C/min. Fluorescence measure-
ments of the well grinded thick solid samples were made on
FLS920 of Edinburgh Instrumen. Samples were placed between
two quartz cover slips and the excitation and emission slit of
0.2 nm were used. The 77 K solution-state phosphorescence spec-
tra of the Gd(III) complex was recorded with solution samples (a 1:
1 Ethyl acetate–MeOH (v/v) mixture) loaded in a quartz tube
inside a quartz-walled optical Dewar flask filled with liquid
nitrogen in the phosphorescence mode on a Hitachi F-4500
spectrophotometer and equipped with a xenon lamp as the
excitation source (front-face mode) [14]. Quantum yields were
determined by an absolute method [15] using an integrating
sphere on FLS920 of Edinburgh Instrument. The luminescence
decays were recorded using a pumped dye laser (Lambda Physics
model FL2002) as the excitation source. The nominal pulse width
and the line width of the dye-laser output were 10 ns and
0.18 cmꢁ1, respectively. The emission of the sample was collected
by two lenses in a monochromator (WDG30), detected by a
photomultiplier and processed by a Boxcar Average (EGG model
162) in line with a microcomputer. Reported quantum yields and
luminescence lifetimes are averages of at least three independent
determinations. The estimated errors for quantum yields and
luminescence lifetimes are 10%.
2.3. Synthesis of the complexes
{[Gd2L3(NO3)6] ꢀ 2C4H8O2}1 (1). 62.8 mg (0.1 mmol)
L and
39.9 mg (0.1 mmol) Gd(NO3)3·6H2O were dissolved in a hot ethyl
acetate solution to make a concentrated solution. Then the flask
was cooled, and the mixture was filtered into a sealed 10–20 mL
glass vial for crystallization at room temperature. After about two
weeks, colorless single crystals of 1 suitable for crystal analysis
were obtained. (yield: 72.0 mg, 56% based on Gd(NO3)3·6H2O).
Analytical data (%), Calcd: C, 48.06; H, 4.11; N, 6.11; S, 7.00; Found:
C, 48.18; H, 4.12; N, 6.13; S, 7.01; IR (KBr, ν cmꢁ1): 3428(m), 2924
(m), 2852 (w), 1612 (s), 1564 (m), 1478 (s), 1300 (s), 1235 (m), 1030
(m), 988(m), 815 (w), 756 (m).
{[Tb2L3(NO3)6] ꢀ 2C4H8O2}1 (2). The procedure was the same as
that for Gd(III) complex using Tb(NO3)3·6H2O. Colorless single
crystals were formed after three weeks. (yield: 57.8 mg, 45% based
on Nd(NO3)3·6H2O. Analytical data (%), Calcd: C, 48.00; H, 4.10; N,
6.11, S, 6.99; Found: C, 48.09; H, 4.10; N, 6.12, S, 7.02. IR (KBr, ν):
3436 (m), 2925 (m), 1611 (s), 1561 (m), 1480(s), 1299 (s), 1223 (m),
1032 (m), 988 (m), 815 (w), 755 (m).
2.4. X-ray single-crystal diffraction analysis
Structure diffraction intensities of Gd(III) and Tb(III) complex
were carried out on a Bruker SMART Apex CCD area detector
diffractometer (Mo Kα, λ¼0.71073 Å) at 296 K. Data processing
was accomplished with the SAINT processing program. Multiscan
absorption corrections were applied by using the program SADABS
[18]. The structures were solved with direct methods and refined
with full-matrix least squares on F2 using the SHELXL-97 program
package [19]. All non-hydrogen atoms were subjected to aniso-
tropic refinement, and all hydrogen atoms were added in idealized
2.2. Synthesis of the ligand
positions and refined isotropically. The R1 values are defined
as R1¼Σ║Fo|ꢁ|Fc║/Σ|Fo| and wR2¼{Σ[w(Fo ꢁFc2)2]/Σ[w(Fo2)2]}1/2
.
2
The synthetic route for the ligand (L) is shown in Scheme 1.
2-thenylsalicylamide [16] and 1,4 -bis(bromomethyl)-2,5-dimethoxy-
benzene [17] was prepared according to the literature procedure.
To a solution of 2-thenylsalicylamide (2.45 g, 10.5 mmol) in dry
acetone was added 1.52 g (11 mmol) dried K2CO3, and the mixture
was stirred for 30 min at room temperature, 1.61 g (5 mmol) 1,4 -bis
(bromomethyl)-2,5-dimethoxy-benzene in 20 ml of dry acetone
was added dropwise in 30 min and the resulting solution stirred
During the final steps of refinement high residual electron density
was found near the thiophene, so the atoms of thiophene groups
were considered to be disordered into two different positions.
The disorder of terminal thiophene ring were treated by introducing
a rigid bond restriction that forces both atoms to possess the same
ADP parameters and their occupancies to be complementary
modulated. Crystal parameters, data collection, and details
concerning the structure refinement are summarized in Table 1,
Scheme 1. The synthetic route of the ligand L.

X.-Q. Song et al. / Journal of Solid State Chemistry 205 (2013) 183–189
185
Table 1
Crystal data and structure refinement parameters for Tb(III) and Gd(III) complexes.
Emprical formula
C110H112Gd2N12O40S6
C110H112N12O40S6Tb2
Crystal system, space group
Unit cell dimensions
Triclinic, P-1
Triclinic, P-1
a¼12.5463(3) Å, α¼98.475(5)1
b¼21.801(5) Å, β¼92.173(5)1
c¼23.015(5) Å, γ¼103.235(5)1
6044(2)
2, 1.511
1.279
a¼12.5297(16) Å, α¼98.434(2)1
b¼21.790(3) Å, β¼92.105(2)1
c¼22.969(3) Å, γ¼103.192(3)1
6023.5(13)
2, 1.518
1.357
V/Å3
Z, Dcalcd/kg mꢁ3
μ/mmꢁ1
F(0 0 0)
2800
2804
Theta range for data collection/deg
Completeness to theta¼25.05
Limiting indices
1.79 to 25.02
98.1%
ꢁ14≤h≤14,–25≤k≤25,–27≤l≤23
33510/20925 [R(int)¼0.0379]
20925/14/1655
1.73 to 25.02
98.2%
ꢁ13≤h≤14,–25≤k≤25,–27≤l≤25
33422/20887 [R(int)¼0.0474]
20887/14/1655
Reflections collected/unique
Data/restraints/params
Goodness-of-fit on F2
Final R indices [I42sigma(I)]
R indices (all data)
1.055
1.018
R1¼0.0594, wR2¼0.1513
R1¼0.0896, wR2¼0.1771
2.306 and ꢁ1.603
R1¼0.0739, wR2¼0.1976
R1¼0.1089, wR2¼0.2342
1.934 and ꢁ1.239
Largest diff. peak and hole(e/Å3)
representative bond lengths (Å) and angles (1) are presented in
Table S1. CCDC reference numbers 940194 and 940195. Graphics
were drawn with DIAMOND (Version 3.2) [20].
3. Results and discussion
3.1. Synthesis
The semirigid ligand L was prepared by the ether base coupling
of 1,4-bis(bromomethyl)-benzene and the thenylsalicylamide in a
1:2.25 ratio in dry acetone in the presence of an excess of
anhydrate K2CO3. One of the important issues in determining the
framework structures is the geometry of the organic ligands. In
our study, the terminal coordinating donors are separated by the
benzene backbone at the para position, and two salicylamide arms
are rotationally free and are thus capable of adjusting to match the
metal coordination preference well. The new ligand gave satisfac-
tory 1HNMR, IR spectra, and elemental analyses. The syntheses of
the complexes are straightforward. They were obtained by slow
evaporation of ethyl acetate solution containing ligand L and
corresponding lanthanide(III) ions. Within two weeks or longer
time well-shaped colorless crystals suitable for X-ray analysis are
separated in high yields. The complexes are soluble in DMF, DMSO,
methanol and ethanol, slightly soluble in ethyl acetate, acetonitrile
and acetone, but insoluble in CHCl3 and ether. The characteristic
band of the carbonyl group of free ligand L in IR spectra is shown
at 1648 cmꢁ1. The absence of the band round 1648 cmꢁ1, which is
instead of a new band at ca. 1611 cmꢁ1 of the complexes compared
to free ligands, indicates the complete coordination of the ligand.
Weak absorptions observed in the range of 2900–2950 cmꢁ1 can
be attributed to the ν(CH2) of the ligand. The absorption bands
assigned to the coordinated nitrates were observed as two group
bands at about 1480 cmꢁ1 (ν1) and 1300 cmꢁ1 (ν4) for the
complexes. The differences between the strongest absorption band
ν1 and ν4 of nitrate group lie in ca 180 cmꢁ1, indicating that
coordinated nitrate groups in the complexes are bidentate ligands
[21], the ν3 of free nitrate group disappears in the spectra of the
complexes, implying that three nitrate groups are all in coordina-
tion sphere which are further confirmed by X-ray crystallography
as follows.
Fig. 1. Powder X-ray diffraction patterns of complexes.
(III) complex are almost identical to a pattern simulation from the
single crystal data, which indicates the crystal structures are
representative of the bulk material.
3.2. Crystal structure descriptions
Block crystals of Gd(III) and Tb(III) complex were obtained by
reaction of Ln(NO3)3 ·6H2O and L in ethyl acetate solution. X-ray
crystal structure analyses reveal that Gd(III) and Tb(III) complex
are isostructural and crystallize in the centrosymmetric triclinic
space group P-1. Thus, only Tb(III) is selected for investigation here
in detail as the representative example. The asymmetric unit of Tb
(III) complex is composed of two crystallographically distinct Tb
(III) metal centers, three ligands, six coordinating nitrate anions,
and two ethyl-acetate solvent molecules. A first TbIII ion is
nonacoordinated with three oxygen atoms from the carbonyl
groups of three different ligands (TbꢁO distances are 2.303(6),
2.313(6) and 2.331(6) Å), and three bidentately coordinating
nitrate anion (TbꢁO distances ranging from 2.428(7) to 2.547(8)
Å) (Fig. 2a). The second Tb(III) ion is nonacoordinated as well, with
three oxygen atoms from the carbonyl groups of another three
different ligands (TbꢁO distances are 2.301(6), 2.314(6) and 2.327
(6) Å), and three bidentately coordinating nitrate anion (TbꢁO
The room temperature PXRD patterns of the Gd(III) and Tb(III)
complex of the ligand L as well as that of simulation for Tb(III)
complex single crystal are unfolded from 51 to 501 as shown in
Fig. 1. The powder XRD patterns of as-synthesized Gd(III) and Tb

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X.-Q. Song et al. / Journal of Solid State Chemistry 205 (2013) 183–189
Fig. 2. The coordination environment of Tb1 (a) and Tb2 (b) with thermal ellipsoids at 30% probability(hydrogen atoms are omitted for clarity). Symmetry transformations:
i¼x, y, z; ii¼ ꢁx, ꢁy, ꢁz. Color scheme: Tb atoms, green; C atoms, gray; O atoms, red; N atoms, blue; S atoms, yellow. (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
Fig. 3. The coordination polyhedron of Tb1 (a) and Tb2(b).
Fig. 4. Polyhedron presentation of 1D trapezoid ladder-like chain linked by ligands of Tb1(a) and Tb2(b). (Hydrogen atoms are omitted for clarity.) (c) Schematic
representation of the 1D trapezoid ladder-like chains of Tb(III) complex(Gray nodes represent the anchors of the ligands).
distances ranging from 2.410(7) to 2.539(8) Å) (Fig. 2b). For both
Tb(III) ions, the coordination environment can be considered as a
distorted square antiprism (Fig. 3). All of the bond lengths of Tb–O
fall into the normal ranges. Unexpectedly, in the Tb(III) complex of L
the ligand coordinate in a bidentate bridging mode with two ligands
in a cis conformation and one ligand in a trans conformation which
is unprecedented. The Tb…Tb distances bridged by ligand L with
deformed cis and trans conformation are ca. 12.529(7) and 18.752
(7) Å, respectively, indicating that the trans conformation of L is
more extended than the cis conformation. This arrangement of three
ligands around the terbium ions leads to two trapezoid ladder-like
chains with a intersection angle of 68.861 as shown in Fig. 4. The
“T-shape” unit at Tb(III) center is slightly deviated from an ideal value
(O3–Tb1–O5¼80.86(0)1, O5–Tb1–O8¼154.01(2)1, O3–Tb1–O8¼86.77

X.-Q. Song et al. / Journal of Solid State Chemistry 205 (2013) 183–189
187
(2)1, the Tb1–Tb1–Tb1 angles are 39.03(6)1, 140.96(4)1, 180.00(0)1 and
O21–Tb2–O23¼83.23(5)1, O21–Tb2–O26¼84.37(4)1, O23–Tb2–O26¼
154.66(0)1, the Tb2–Tb2–Tb2 angles are 42.59(3)1, 137.407 (3)1, 180.00
(0)1, respectively). It is worth to note that quadruple intermolecular
hydrogen bonding interactions exist between the adjacent chains of
the Tb1 and Tb2 as shown in Fig. 5. The oxygen atoms of two nitrate
groups (O11, O14, O31 and O33) acted as hydrogen bond acceptor to
interact with the hydrogen atom of the CH3, thiophene and phenyl
group (H9B, H9C, H22 and H78) to form an interesting three-
dimensional supramolecular structures (Fig. 6). The distance between
C7, C12, C24 and O1, O10, O6 is 3.529(8) Å, 3.460(3) Å, 3.416(4) Å and
the corresponding C–H…O angle is 153.34(2)1, 151.09(6)1 and 166.90
(4)1). It is important to note that the etheric oxygen atoms of the two
ligands are not coordinated to the Ln(III) center, but act as the
hydrogen bonding acceptors to form strong intramolecular N–H…O
hydrogen bonds which are stabilized by a stable 6-membered ring.
The binding of lanthanide ions to the carbonyl oxygen atoms of ligands
rather than to etheric oxygen groups appears largely as a consequence
of the cooperative effect of strong intramolecular hydrogen bonds and
steric hindrance inhibiting the formation of a reasonable coordination
geometry that involves the participation of all two ether oxygen
donors of each ligand which is characteristic of lanthanide compound
of salicylamide ligands [11,12].
Compared with the structure of complex of similar ligand
terminated with pyridine group [11k], terminal group-depending
structural variations are observed which can be understood taking
into account the plastic coordinative behavior of this kind of
ligands. By introduction of less electronegative thiophene group
at the salicylamide arms in solvent system with weak coordination
ability, the coordination behavior of the ligand was dramatically
changed because of the presence of sulfur atoms which could
make it less favorable for incorporating hydrogen bonds and
affecting the coordination mode of the resulting bridging ligand.
Consequently, each L ligand allowing the coordination of Ln(III)
simultaneously by the two salicylamide arms of the ligands and
thus the assembly of trapezoid ladder-like chains.
3.3. Luminescence properties
Taking into account the excellent luminescent properties of Tb(III)
ions, the luminescence properties of Tb(III) coordination polymers
and ligand L were investigated. As shown in Fig. 7, the free ligand L
shows an emission band at 452 nm (λex ¼320 nm) which is attributed
to the π-πn transitions. However, upon complexation with Tb(III)
ions this emission is replaced by the typical luminescent color arising
from Tb(III) cation. The excitation spectrum recorded from 250 to
530 nm (Fig. S1) shows a broad excitation band from 280 to 380 nm,
originating from the organic ligand, as well as several weak sharp
signals which can be assigned to direct Tb(III) excitation. While
excitation was done at 325 nm, within the excitation band of the
organic ligand, the emission spectrum only shows Tb(III) emission
and no detectable emission of the organic ligand, thus confirming an
efficient antenna effect for the luminescence of Tb(III) complex.
A depiction of the green luminescence of the Tb(III) compound can
be found in Fig. 8, the emission spectra of the Tb(III) compound shows
the typical f-f based 5D4-7FJ transitions (J¼6ꢁ1) at 478 and 482,
540 and 543, 576 and 582, 620 and 623, 644 and 648, and 672 nm as
5
sharp signals, with the D4-7F5 emission at 543 nm as the emission
Fig. 5. CꢁH…O hydrogen bonding between Tb1 and Tb2 trapezoid ladder-like
chains which are indicated with dashed yellow lines. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of
this article.)
maximum. The 5D4-7FJ transitions with J¼2ꢁ1 around 648 and 672
are present but very weak in intensity.
In addition to steady-state measurements, the luminescence
lifetimes of the Tb (5D4) excited states and quantum yield determi-
nations were determined. The lifetime of luminescence for Tb(III)
complex was measured from the decay profile by fitting with
Fig. 6. The 3D supramolecular structure of Tb(III) complex constructed by CꢁH…O
Fig. 7. Room-temperature emission spectra of ligand L (λex¼320 nm, excitation
and emission passes¼0.2 nm) in the solid state.
hydrogen bonds viewed from the crystallographic a direction.

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X.-Q. Song et al. / Journal of Solid State Chemistry 205 (2013) 183–189
Fig. 10. TGA curves of Tb(III) complex.
crossing to their triplet states and energy transfer from the triplet
state to the 5DJ manifold of the Tb(III) ions, followed by internal
conversion to the emitting 5D4 state. Finally, the Tb(III) ion emits
when the 4f electrons undergo a transition from the excited state of
5D4 to the 7FJ ground states (J¼6–1). Latva′s empirical rule [22] states
that an optimal ligand–to–metal energy transfer process for Tb(III)
Fig. 8. Room-temperature emission spectra for terbium complex of ligand L (λex
325 nm, excitation and emission passes¼0.2 nm) in the solid state.
¼
5
needs ΔE¼3ππnꢁ DJ (2500–4500 cmꢁ1). The triplet energy level of
the ligand (24691 cmꢁ1) is higher than the 5D4 level of Tb(III)
(20400 cmꢁ1) and the energy gap ΔE is relatively large (about
4291 cmꢁ1), which points to an effective intersystem crossing
process. This therefore supports the observation of stronger sensiti-
zation of the terbium complexes because of the larger overlap
between the ligand triplet and terbium ion excited states.
Compared with the Tb(III) complex with the similar ligand
terminated with picolysalicylamide group[11k], the luminescence
lifetimes and quantum yields of Tb(III) complex are relatively
longer and larger suggesting that one or more nonradiative path-
ways are assisting in the deactivation of the excited state and
shortening the observed lifetimes, since the triplet energy level of
the antenna only changed slightly, which excludes the possibility
that the quenching of luminescence is due to a change in the
nature of the antenna triplet state. The most probable explanation
may be the former contain methanol molecules in the inner
coordination sphere, which may quench Tb(III) emitting states.
Fig. 9. Phosphorescence spectra of Gd(III) complex of ligand L in methanol-ethyl
acetate solution at 77 K.
monoexponential decay curve with τ¼1.34 ms. The quantum yields
of luminescence is 73%. The relatively longer luminescence lifetimes
and larger quantum yield values are an indication that the bridging
ligand provides a significant level of protection from nonradiative
deactivation of lanthanide cations by the solvent molecules, as
further supported by the energy transfer discussed below.
3.4. Thermal properties
TGA have been carried out for two complexes which show
similar behavior. The data of Tb(III) complex is taken as an
example to be described in detail. As shown in Fig. 10, the thermal
decomposition of Tb(III) complex occurs in a multi-step process
from ambient temperature to 600 1C to yield Tb4O7 as the final
residue which is confirmed by the similar characteristic absorption
of the residue in IR spectra compared with the standard sample
spectrum of Tb4O7 (total weight loss: 86.26%; calculated: 86.41%).
The first stage takes place from 25 to 118 1C, with a mass loss of
6.32% (calc. 6.39%), which corresponds to the loss of two ethyla-
cetate molecules. Between 216 and 542 1C a total weight loss of
79.94% corresponds to the thermal decomposition of the organic
components and nitrate anions (calculated 80.01%). For Gd(III)
complex the total weight loss was 86.49% with a calculated value
of 86.76% between 25 and 523 1C.
To demonstrate the energy transfer process, the phosphorescence
spectrum of Gd(III) complex of L was measured for the triplet energy-
level data. From the phosphorescence spectra (Fig. 9), the triplet
energy level (3ππn) of Gd(III) complex, which corresponds to its lower
wavelength emission edge, is 24691 cmꢁ1(405 nm). Because the
6
lowest excited state, P7/2 (E(6P7/2)¼32000 cmꢁ1) of Gd(III) is too
high to accept energy from the ligand, the data obtained from the
phosphorescence spectra actually reveal the triplet energy level of
ligand in lanthanide complexes. In general, the sensitization pathway
in luminescent terbium complexes consists of excitation of the
ligands into their excited singlet states, subsequent intersystem

X.-Q. Song et al. / Journal of Solid State Chemistry 205 (2013) 183–189
189
4. Conclusions
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system are all parameters of significant importance in coordination
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Supplementary material
Crystallographic data for the structural analysis have been
deposited with the Cambridge Crystallographic Data Center, CCDC
nos. 940194 and 940195. Copies of this information may be
obtained free of charge from the director, CCDC, 12 Union Road,
Cambridge CB2 IEZ, UK (e-mail: deposit@ccdc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk).
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Acknowledgments
(b) Y. Tang, J. Zhang, W.-S. Liu, M.-Y. Tan, K.-B. Yu, Polyhedron 24 (2005)
1160–1166;
(c) X.-Q. Song, W.-S. Liu, W. Dou, Y.-W. Wang, J.-R. Zheng, Z.-P. Zang, Eur. J.
Inorg. Chem. 11 (2008) 1901–1912;
(d) X.-Q. Song, W. Dou, W.-S. Liu, Y.-L. Guo, X.-L. Tang, Inorg. Chem. Comm. 10
(2007) 1058–1061;
(e) Y.-W. Wang, Y.-L. Zhang, W. Dou, A.-J. Zhang, W.-W. Qin, W.-S. Liu, Dalton
Trans. 39 (2010) 9013–9021;
This work was supported by the National Natural Science
Foundation of China (Grants 21161011) and Gansu Natural Science
Foundation of China(Grants 1212RJZA038).
(f) Q. Wang, X. Yan, H. Zhang, W. Liu, Y. Tang, M. Tan, J. Solid State Chem. 184
(2011) 164–170;
Appendix A. Supporting information
(g) X.-Q. Song, W.K. Dong, Y.J. Zhang, W.-S. Liu, Luminescence. 25 (2010)
328–335;
(h) X.-Q. Song, L. Wang, Q.-F. Zheng, W.-S. Liu, Inorg. Chim. Acta 391 (2012)
171–178;
Supplementary data associated with this article can be found in
the online version at http://dx.doi.org/10.1016/j.jssc.2013.07.014.
(i) X.-Q. Song, L. Wang, M.-M. Zhao, G.-Q. Cheng, X.-R. Wang, Y.-Q. Peng,
Inorg. Chim. Acta 402 (2013) 156–164;
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Y.-Q. Peng, Inorg. Chim. Acta. 404 (2013) 113–122.
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