Diverse lanthanide coordination polymers tuned by the flexibility of ligands and the lanthanide contraction effect: Syntheses, structures and luminescence

Article abstract of DOI:10.1039/c1dt10931f

Two new flexible exo-bidentate ligands were designed and synthesized, incorporating different backbone chain lengths bearing two salicylamide arms, namely 2,2′-(2,2′-oxybis(ethane-2,1-diyl)bis(oxy))bis(N- benzylbenzamide) (L^I) and 2,2′-(2,2′-(ethane-1,2- diylbis(oxy))bis(ethane-2,1-diyl))bis(oxy)bis(N-benzylbenzamide) (L ^II). These two structurally related ligands are used as building blocks for constructing diverse lanthanide polymers with luminescent properties. Among two series of lanthanide nitrate complexes which have been characterized by elemental analysis, TGA analysis, X-ray powder diffraction, and IR spectroscopy, ten new coordination polymers have been determined using X-ray diffraction analysis. All the coordination polymers exhibit the same metal-to-ligand molar ratio of 2:3. L^I, as a bridging ligand, reacts with lanthanide nitrates forming two different types of 2D coordination complexes: herringbone framework {[Ln₂(NO₃) ₆(L^I)₃·mC₄H₈O ₂]_∞ (Ln = La (1), and Pr (2), m = 1, 2)} as type I,; and honeycomb framework {[Ln₂(NO₃)₆(L ^I)₃·nCH₃OH]_∞ (Ln = Nd (3), Eu (4), Tb (5), and Er (6), n = 0 or 3)} as type II, which change according to the decrease in radius of the lanthanide. For L^II, two distinct structure types of 1D ladder-like coordination complexes were formed with decreasing lanthanide radii: [Ln₂(NO₃)₆(L ^II)₃·2C₄H₈O₂] _∞ (Ln = La (7), Pr (8), Nd (9)) as type III, [Ln ₂(NO₃)₆(L^I)₃· mC₄H₈O₂·nCH₃OH] _∞ (Ln = Eu (10), Tb (11), and Er (12), m, n = 2 or 0) as type IV. The progressive structural variation from the 2D supramolecular framework to 1D ladder-like frameworks is attributed to the varying chain length of the backbone group in the flexible ligands. The photophysical properties of trivalent Sm, Eu, Tb, and Dy complexes at room temperature were also investigated in detail.

Full text of DOI:10.1039/c1dt10931f

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PAPER
Diverse lanthanide coordination polymers tuned by the flexibility of ligands
and the lanthanide contraction effect: syntheses, structures and luminescence†
Xiaoyan Zhou,a Yanling Guo,b Zhaohua Shi,a Xueqin Song,c Xiaoliang Tang,a Xiong Hu,a Zhentong Zhu,a
Pengxuan Lia and Weisheng Liu*a
Received 17th May 2011, Accepted 19th October 2011
DOI: 10.1039/c1dt10931f
Two new flexible exo-bidentate ligands were designed and synthesized, incorporating different
backbone chain lengths bearing two salicylamide arms, namely 2,2¢-(2,2¢-oxybis(ethane-2,1-diyl)-
bis(oxy))bis(N-benzylbenzamide) (LI) and 2,2¢-(2,2¢-(ethane-1,2-diylbis(oxy))bis(ethane-2,1-
diyl))bis(oxy)bis(N-benzylbenzamide) (LII). These two structurally related ligands are used as building
blocks for constructing diverse lanthanide polymers with luminescent properties. Among two series of
lanthanide nitrate complexes which have been characterized by elemental analysis, TGA analysis, X-ray
powder diffraction, and IR spectroscopy, ten new coordination polymers have been determined using
X-ray diffraction analysis. All the coordination polymers exhibit the same metal-to-ligand molar ratio
of 2 : 3. LI, as a bridging ligand, reacts with lanthanide nitrates forming two different types of 2D
coordination complexes: herringbone framework {[Ln2(NO3)6(LI)3·mC4H8O2]• (Ln = La (1), and Pr (2),
m = 1, 2)} as type I,; and honeycomb framework {[Ln2(NO3)6(LI)3·nCH3OH]• (Ln = Nd (3), Eu (4), Tb
(5), and Er (6), n = 0 or 3)} as type II, which change according to the decrease in radius of the
lanthanide. For LII, two distinct structure types of 1D ladder-like coordination complexes were formed
with decreasing lanthanide radii: [Ln2(NO3)6(LII)3·2C4H8O2]• (Ln = La (7), Pr (8), Nd (9)) as type III,
[Ln2(NO3)6(LI)3·mC4H8O2·nCH3OH]• (Ln = Eu (10), Tb (11), and Er (12), m, n = 2 or 0) as type IV. The
progressive structural variation from the 2D supramolecular framework to 1D ladder-like frameworks
is attributed to the varying chain length of the backbone group in the flexible ligands. The
photophysical properties of trivalent Sm, Eu, Tb, and Dy complexes at room temperature were also
investigated in detail.
Introduction
The construction of coordination polymers has attracted consid-
erable interest in coordination chemistry and materials science
because of their intriguing structural diversities and potential
aKey Laboratory of Nonferrous Metal Chemistry and Resources Utiliza-
tion of Gansu Province and State Key Laboratory of Applied Organic
applications in functional materials, nanotechnology and bio-
logical recognition.1 The field of lanthanide–organic frameworks
(LnOFs)2 is rapidly growing owing to the magnetic and electronic
properties of 4f ions, which should exhibit interesting properties
and have potential applications in catalysis,3 sensors,4 contrast
agents,5 non-linear optics,6 displays,7 and electroluminescent
devices.8 As compared to the fruitful production of metal–organic
frameworks (MOFs) with d-block transition metal ions, the
design and control of lanthanide-based frameworks is currently
a formidable task due to their high and variable coordination
numbers and flexible coordination environments.9 However, the
flexibility of the lanthanide ions can be profitable in giving a variety
of structures dictated by the different sizes of the lanthanide ions
and can result in fascinating and interesting structures.10
Chemistry, College of Chemistry and Chemical Engineering, Lanzhou
University, Lanzhou, 730000, China. E-mail: liuws@lzu.edu.cn; Fax: +86-
931-8912582; Tel: +86-931-8915151
bSchool of Nuclear Science and Technology, Lanzhou University, Lanzhou,
730000, China
cSchool of Chemical and Biological Engineering, Lanzhou Jiaotong Univer-
sity, Lanzhou, 730070, China
† Electronic supplementary information (ESI) available: Details of crys-
tallographic parameters, data collection and refinements, and selected
bond distances and angles for polymers 1–12 (except for 5, 6), tables of
interatomic distances and angles for 1–12 (except 5, 6), selected torsion
angles of complexes 2, 4, 9, and 11, absorption spectra of ligands LI, LII
and their lanthanide complexes, XRPD patterns of the Eu3+, Tb3+ and
Er3+ complexes with LI (4–6), the conformations of ligand LII in complex
11, TGA diagrams, the room temperature solid-state emission spectra
for LI, LII, Sm3+ and Dy3+ complexes with LI, the room temperature
solid-state phosphorescence lifetimes of the Sm3+, Eu3+, Tb3+, and Dy3+
complexes with LI, the phosphorescence spectra of the Gd3+ complexes
with LI and LII. CCDC reference numbers 817219–817228. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1dt10931f
In recent years, various organic ligands used for constructing
lanthanide coordination polymers have been synthesized.11–16
Among these numerous ligands, the amide type ligands have
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Dalton Trans., 2012, 41, 1765–1775 | 1765
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been attracting more attention for the preparation of rare earth
complexes, which have strong a coordination capability towards
lanthanide ions and possess strong luminescence properties.17 We
are interested in the synthesis and characterization of lanthanide
complexes with flexible ligands incorporating salicylamide deriva-
tives, and especially in how different types of flexible backbone
as well as different terminal coordination sites can impact the
structures and the luminescence properties.18
and the resulting solution was stirred and heated to reflux for
24 h. The mixture was cooled to room temperature and distilled
water (60 mL) was added; the turbid solution was then extracted
with chloroform (3 ¥ 50 mL). The organic phase was washed
with saturated salt water and dried with anhydrous Na2SO4. The
solvent was removed under reduced pressure to give a crude
product. The pure product was obtained using chromatography
on silica gel.
In this study, we designed and synthesized two new flexi-
ble exo-bidentate ligands 2,2¢-(2,2¢-oxybis(ethane-2,1-diyl)-
bis(oxy))bis(N-benzylbenzamide) (LI), and 2,2¢-(2,2¢-(ethane-
1,2-diylbis(oxy))bis(ethane-2,1-diyl))bis(oxy)bis(N-benzylbenza-
mide) (LII) (Scheme 1). As a result, a series of novel coordi-
nation complexes, namely, [La2(NO3)6(LI)3·2C4H8O2]• (1),
[Pr2(NO3)6(LI)3·C4H8O2]• (2), [Nd2(NO3)6(LI)3·3CH3OH]• (3),
[Eu2(NO3)6(LI)3·3CH3OH]• (4), [La2(NO3)6(LII)3·2C4H8O2]• (7),
[Pr2(NO3)6(LII)3·2C4H8O2]• (8), [Nd2(NO3)6(LII)3·2C4H8O2]• (9),
[Eu2(NO3)6(LII)3·2C4H8O2]• (10), [Tb2(NO3)6(LII)3·2C4H8O2·
2CH3OH]• (11), [Er2(NO3)6(LII)3·2C4H8O2]• (12), were
synthesized and characterized via single-crystal X-ray diffraction
analysis. These compounds represent good examples of the effect
of the lanthanide contraction with LI and LII.19 The contraction
of the radius of lanthanide ions imposes evident influences upon
the coordination geometry and the global network structures.
Generally, the Ln series can be divided into three groups according
to their masses: La–Pm (Group 1), Sm–Dy (Group 2), and Ho–Lu
(Group 3).20 Indeed, the title compounds can be grouped into
four types: for LI, the large La and Pr lanthanides form type I
structures; the small ones, Nd, Eu, Tb, and Er, form type II. For
LII, the large La, Pr, and Nd lanthanides form type III structures;
the small ones, Eu, Tb, and Er, form type IV. It is noteworthy that
subtle variation in the chain length of the backbone affects the
molecular structures, which change from a 2D supramolecular
framework to a 1D ladder-like framework. The luminescence
properties of the resulting complexes formed with Ln3+ (Ln = Sm,
Eu, Tb and Dy) were studied in detail.
2,2¢-(2,2¢-oxybis(ethane-2,1-diyl)bis(oxy))bis(N -benzylbenza-
mide) (LI). Chromatography (0–40% ethyl acetate in petroleum
ether gradient). Yield: 67%. Mp: 90.3–91.0 ◦C. Anal. calcd (found)
for C32H32N2O5: C, 73.26 (73.18); H, 6.15 (6.09); N, 4.93 (4.85)%.
IR (KBr, n, cm-1): 3362 (s), 2927 (m), 1638 (vs), 1598 (s), 1536
(vs), 1486 (m), 1449 (m), 1306 (m), 1244 (s), 1054 (m), 960 (w),
759 (s). (br, broad; w, weak; m, medium; s, strong; vs, very strong)
1H NMR (CDCl3, 400 MHz, ppm): d 3.32 (t, 4H, –CH2–), 3.84
(t, 4H, –CH2–), 4.60 (d, 4H, NH–CH2–), 6.78 (d, 2H, Ar–H), 7.10
(t, 2H, Ar–H), 7.19 (t, 2H, Ar–H), 7.20–7.44 (m, 12H, Ar–H),
8.22 (m, 4H, –NH–, Ar–H), 13C NMR (CDCl3, 100 MHz, ppm):
d 43.75, 67.62, 68.87, 76.68, 77.00, 77.32, 112.39, 121.73, 127.22,
127.54, 128.54, 132.43, 132.69, 138.78, 156.43, 165.18. ESI-MS:
m/z 625.4 (M + H+).
2,2¢ - (2,2¢ - (ethane - 1,2 - diylbis(oxy))bis(ethane - 2,1 - diyl))bis-
(oxy)bis(N-benzylbenzamide) (LII). Chromatography (0–40%
ethyl acetate in petroleum ether gradient). Yield: 78%. Mp: 103.4–
104.2 ◦C. Anal. calcd (found) for C34H36N2O6: C, 71.81 (71.76); H,
6.38 (6.36); N, 4.84 (4.88)%. IR (KBr, n, cm-1): 3377 (s), 2902 (br,
m), 1645 (vs), 1598 (m), 1545 (s), 1486 (m), 1451 (m), 1298 (m),
1
1242 (s), 931 (w), 768 (s). H NMR (CDCl3, 400 MHz, ppm): d
3.12 (s, 4H, –CH2–), 3.56 (t, 4H, –CH2–), 4.08 (t, 4H), 4.64 (d, 4H,
NH–CH2–), 6.85, 6.87 (d, 2H, Ar–H), 7.07 (t, 2H, Ar–H), 6.96 (t,
2H, Ar–H), 7.20–7.41 (m, 12H, Ar–H), 8.20–8.22 (t, 2H, Ar–H),
8.41 (s, 2H, –NH–). 13C NMR (CDCl3, 100 MHz, ppm): d 43.69,
67.83, 69.02, 70.31, 76.68, 77.00, 77.32, 112.51, 121.60, 121.81,
127.10, 127.49, 127.59, 128.48, 132.34, 132.65, 138.92, 156.63,
165.26. ESI-MS: m/z 669.4 (M + H+).
Synthesis of lanthanide nitrate complex
To a clear solution of the ligand (0.05 mmol) in ethyl acetate
(10 mL) was added lanthanide nitrate (0.05 mmol) in 2 mL of
methanol. The resulting solution was left stirring for 4 h at room
temperature to afford a pale white solid, which was filtered off,
washed three times with ethyl acetate, and dried in a vacuum
over P2O5 for 24 h to give the analytically pure product in 65–
80% yield. The complex was dissolved in a methanol/ethyl acetate
solution to make a concentrated solution in a round-bottom flask.
Then the mixture was filtered into a sealed 25 mL glass vial
for crystallization at room temperature. After about two weeks,
crystals suitable for analysis were obtained. IR and elemental
analysis data for all complexes are summarized in Table S1, ESI.†
Scheme 1 Schematic illustration of ligands LI and LII.
Experimental section
Synthesis of ligand
General synthetic procedure for LI and LII. 2,2¢-(2,2¢-oxybis-
(ethane-2,1-diyl)bis(oxy))bis(N-benzylbenzamide) (LI), and 2,2¢-
(2,2¢-(ethane-1,2-diylbis(oxy))bis(ethane-2,1-diyl))bis(oxy)bis(N-
benzylbenzamide) (LII) were prepared by the following synthetic
routes. To a solution of salicylamide21 (2.044 g, 9 mmol) in dry
dimethylformamide (DMF) was added K2CO3 (1.656 g, 12 mmol)
and the mixture was stirred and heated for 1 h. 2,2¢-oxybis(ethane-
Materials and methods
The commercially available chemicals were used without further
purification. All of the solvents used were of analytical reagent
grade.
Elemental analyses were conducted using an Elemental Vario
EL. FT-IR spectra were recorded on a Nicolet FT-170SX
2,1-diyl)bis(4-methylbenzenesulfonate)
or
2,2¢-(ethane-1,2-
diylbis(oxy))bis(ethane-2,1diyl)bis-(4-methylbenzenesulfonate)22
(4 mmol) in 30 mL of DMF were added dropwise over 30 min,
1766 | Dalton Trans., 2012, 41, 1765–1775
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Table 1 Crystal data and structure refinement parameters for coordination polymers 1–4, 7
1
2
3
4
7
Empirical formula
Temperature/K
Formula weight
Crystal system
Space group
C96H96La2N12O33
296 (2)
2223.67
Monoclinic
P21
11.9319 (10)
22.7781 (19)
21.1246 (17)
90
104.303 (4)
90
5563.4 (8)
2
1.327
0.85
2460
0.044
29 677/17 682
17 682/391/1300
1.036
R1 = 0.0462
wR2 = 0.0695
R1 = 0.0999
wR2 = 0.0767
C96H96N12O33Pr2
296 (2)
2227.67
Monoclinic
P21
11.961 (2)
22.678 (3)
21.079 (3)
90
104.390 (7)
90
5538.2 (15)
2
1.336
0.95
2276
0.069
30 333/17 856
17 856/535/1234
0.982
R1 = 0.0555
wR2 = 0.0854
R1 = 0.1297
wR2 = 0.0988
C96H96N12Nd2O33
293 (2)
2234.33
C96H96Eu2N12O33
296 (2)
2249.77
C51H54LaN6O18
296 (2)
1177.91
Hexagonal
Hexagonal
Triclinic
¯
¯
¯
R3
R3
P1
˚
a/A
19.755 (3)
19.755 (3)
53.209 (11)
90
19.682 (5)
19.682 (5)
53.28 (3)
90
11.721 (7)
14.110 (9)
19.522 (12)
81.873 (7)
88.382 (8)
66.118 (7)
2921 (3)
2
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
90
120
17983 (5)
6
1.238
0.93
6840
0.043
27 977/5953
5953/291/394
1.068
R1 = 0.0473
wR2 = 0.1411
R1 = 0.0642
wR2 = 0.1507
90
120
17876 (12)
6
1.254
1.12
6876
0.068
38 376/7408
7408/72/418
1.043
R1 = 0.0712
wR2 = 0.2049
R1 = 0.1317
wR2 = 0.2666
3
˚
V/A
Z
D
calcd/kg m-3
1.339
0.80
1206
0.049
14 041/9690
9690/0/685
1.007
R1 = 0.0580
wR2 = 0.1077
R1 = 0.0996
wR2 = 0.1173
m/mm-1
F(000)
Independent reflections (Rint
Reflns collectes/unique
Data/restraints/params
Goodness-of-fit on F2
Final R indices [I > 2s(I)]
)
R indices (all data)
Residual electron density
-1
˚
/e A
-0.378 to 0.422
0.000(8)
-0.352 to 0.443
0.003(11)
-0.421 to 1.264
None
-0.825 to 2.410
None
-0.400 to 0.535
None
Flack parameter and esd
instrument using KBr discs in the 400–4000 cm-1 region. Melting
points were determined on a Kofler apparatus. X-Ray powder
diffraction (XRPD) patterns were obtained on a Rigaku D/Max-
IIX-ray diffractometer with graphite-monochromatized Cu-Ka
radiation. 1H NMR (400 MHz) and 13C NMR (100 MHz)
spectra were measured on a Bruker DRX 400 spectrometer in
d-CHCl3 solution with TMS as internal standard. ESI-TOF mass
spectra were measured on a Mariner MS spectrometer. Absorption
spectra were recorded using a Varian Cary 100 spectrophotometer
and fluorescence measurements were made on a Hitachi F-4500
spectrofluorimeter equipped with quartz cuvettes of 1 cm path
length with a xenon lamp as the excitation source. An excitation
and emission slit of 1.0 or 2.5 nm were used for fluorescence
measurements in the solid state. The 77 K solution-state phos-
phorescence spectra were recorded with solution samples loaded
in a quartz tube inside a quartz-walled optical Dewar flask filled
with liquid nitrogen in the phosphorescence mode. Fluorescent
quantum yields were determined by an absolute method in the
solid state using an integrating sphere (150 mm diameter, BaSO4
coating) on a FLS920 machine from Edinburgh Instruments.
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 cm-1, respectively. The emission of
a sample was collected by two lenses into a monochromator
(WDG30), detected by a photomultiplier and processed by a
Boxcar Average (EGG model 162) in line with a microcomputer.
Thermogravimetric analyses (TGA) were performed on a P.E.
Diamond TG/DTA/SPAECTRUN ONE thermal analyzer up
X-ray diffraction studies. Single-crystal X-ray diffraction mea-
surements were carried out on a Bruker SMART 1000 CCD
diffractometer operating at 50 KV and 30 mA using Mo-Ka
˚
radiation (l = 0.71073 A). Some crystals were mounted inside
a Lindemann glass capillary for data collection using the SMART
and SAINT software.23 An empirical absorption correction was
applied using the SADABS program.24 The structure was solved
by direct methods and refined by full-matrix least-squares on
F2 using the SHELXTL-97 program package.25 In the X-ray
structure refinements, however, the solvent molecules of com-
plexes 1–4 and 7–12 could not be located because of their high
thermal disorder, and the final structure models were refined
without the solvent molecules by using a SQUEEZE routine of
PLATON. Details of crystallographic parameters, data collection
and refinements are listed in Table 1 and 2. Selected bond
distances and angles for polymers 1–12 (except for 5, 6) are
listed in Tables S4–S13, ESI,† respectively. To explore the other
Ln3+ complex structures of LI, X-ray powder diffraction (XRPD)
of complexes 4–6 (Eu3+, Tb3+ and Er3+) were investigated. The
XRPD patterns (Fig. S1, ESI†) of the complexes show the
main reflections are nearly identical to the those of complex 4,
which supports the notion that these three complexes are isostru-
ctural.
Results and discussion
Structure description
Structures of 1, 2 (type I). The single-crystal X-ray analysis
reveals that 1 and 2 are isostructural and crystallize in the space
group P21, showing a slight contraction of the Ln coordination
sphere as expected. Thus, only 2 is selected for investigation
here in detail as the representative example. In complex 2, the
◦
◦
to 700 C at a heating rate of 10 C min-1 in an air atmosphere.
All the measurements were performed at room temperature unless
otherwise stated. Reported luminescence lifetimes are the averages
of at least three independent determinations.
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Table 2 Crystal data and structure refinement parameters for coordination polymers 8–12
8
9
10
11
12
Empirical formula
Temperature (K)
M
C51H54N6O18Pr
296 (2)
1179.91
C51H54N6NdO18
293 (2)
1183.25
C51H54EuN6O18
293 (2)
1190.96
C51H54N6O18Tb
293 (2)
1197.94
C51H54ErN6O18
293 (2)
1206.26
Crystal system
Triclinic
Triclinic
Triclinic
Triclinic
Triclinic
¯
¯
¯
¯
¯
Space group
P1
P1
P1
P1
P1
˚
a/A
11.732 (8)
14.149 (10)
19.594 (14)
81.871 (7)
88.384 (7)
66.055 (7)
2941 (4)
11.7509 (8)
14.0863 (9)
19.5636 (14)
81.903 (3)
88.260 (3)
66.215 (3)
2932.5 (3)
2
11.4625 (3)
15.9942 (4)
17.5529 (4)
78.5740 (10)
86.7910 (10)
77.0770 (10)
3074.14 (13)
2
11.4447 (7)
15.9902 (11)
17.5724 (12)
78.616 (3)
87.019 (3)
76.896 (3)
3070.4 (4)
2
11.3947 (5)
15.8766 (6)
17.5415 (7)
78.8930 (10)
87.493 (2)
76.8540 (10)
3032.4 (2)
2
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Z
2
D
calcd/kg m-3
1.332
0.90
1210
0.073
1.340
0.96
1308
0.021
1.287
1.09
1218
0.025
1.296
1.23
1354
0.019
1.321
1.45
1228
0.019
m/mm-1
F(000)
Independent reflections (Rint
)
Reflections collected/unique
Data/restraints/params
Goodness-of-fit on F2
21 052/10 784
10 784/0/673
0.947
R1 = 0.0644
wR2 = 0.1355
R1 = 0.1058
wR2 = 0.1471
-1.365 to 1.450
None
15 991/10 800
10 800/0/685
1.035
R1 = 0.0315
wR2 = 0.0620
R1 = 0.0392
wR2 = 0.0643
-0.435 to 0.478
None
16 732/11 304
11 304/18/661
1.049
R1 = 0.0417
wR2 = 0.0863
R1 = 0.0630
wR2 = 0.0926
-0.4443 to 0.316
None
16705/11282
11282/0/673
1.011
R1 = 0.0325
wR2 = 0.0703
R1 = 0.0427
wR2 = 0.0731
-0.566 to 0.550
None
15968/11152
11152/0/607
1.047
R1 = 0.0393
wR2 = 0.0877
R1 = 0.0554
wR2 = 0.0928
-0.598 to 0.874
None
Final R indices [I > 2s(I)]
R indices (all data)
-1
˚
Residual electron density/e A
Flack parameter and esd
Fig. 1 (a) An Oak Ridge Thermal Ellipsoid Plot (ORTEP) diagram of the crystal structure of complex 2, showing 30% probability thermal motion
ellipsoids. All hydrogen atoms were omitted for clarity. (b) View of the 2D herringbone of 2 in the view towards plane (h, k, l = 1, 0, 1). (c) Intramolecular
p–p stacking interaction in 2.
asymmetric unit consists of three ligands and two crystallo-
graphically independent Pr ions. As shown in Fig. 1a, the Pr1
center is nine-coordinated and is surrounded by three carbonyl
oxygen atoms coming from three different ligands (O5, O11, O15)
and six nitrate O (O16, O18, O19, O21, O22, O24), and the
coordination geometry around Pr1 may be described as a distorted
monocapped square antiprism. Pr2 also bonds to three carbonyl
oxygen atoms coming from three different ligands (O1, O6, O10)
and six nitrate O (O25, O27, O28, O30, O31, O33), forming a
more distorted monocapped square antiprism. The Pr–O (nitrate)
O6 = 82.0(2)◦, O6–Pr2–O10 = 80.8(3)◦ O1–Pr2–O10 = 153.1(2)◦).
The simplest cyclic motif of the 2D network is a large chair-
conformation hexagonal 96-membered ring consisting of six LI
linkers alternately bridging six Pr(NO3)3 moieties, in which three
nonadjacent praseodymium atoms are coplanar, paralleling the
remaining three nonadjacent praseodymium atoms with a plane-
˚
to-plane distance of 6.679 A, and the metal–metal distances in each
˚
hexagonal ring are slightly different: Pr1 ◊ ◊ ◊ Pr1 = 14.8427(17) A,
˚
˚
Pr1 ◊ ◊ ◊ Pr2 = 16.3167(18) A, Pr2 ◊ ◊ ◊ Pr2 = 15.6332(18) A. In this
framework, each Pr ion can be considered as 3-connecting nodes
to link 2-connecting LI and Pr2(NO3)6(LI)3 units as repeating units,
generating a (6, 3) topological network. It is noteworthy that in
complex 2, the dihedral angle between the phenyl ring (C11) and
the adjacent benzylamine ring (C38) around Pr2 is 7.653◦, together
˚
bond lengths span the range of 2.514(10)–2.577(7) A, however,
the Pr–O (carbonyl) bond lengths vary obviously from 2.347(7)
˚
to 2.430(7) A, both of which fall into normal ranges. As seen
from Fig. 1b, each metal center forms a “T-joint” connecting
unit for the bidentate bridging flexible ligand, and these “T-
shaped” units are arranged to produce a 2D network, which can
be described and classified as being a herringbone grid motif. The
geometry of the salicylamide groups around the praseodymium
atom slightly deviates from ideal values (O5–Pr1–O11 = 84.2(2)◦,
O11–Pr1–O15 = 85.8(2)◦, O5–Pr1–O15 = 153.8(2)◦; O1–Pr2–
˚
with the center-to-center distance of 4.16 A, implying weak p–
p interaction between the two rings (Fig. 1c).26 To the best of
our knowledge, this type of non-interpenetrating neutral (6, 3)
topological framework with a herringbone grid motif constructed
by linking flexible ligands with rare earth ions is unpreceden-
ted.
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Fig. 2 (a) An ORTEP diagram of the crystal structure of complex 4, showing the 30% probability thermal motion ellipsoids. All hydrogen atoms were
omitted for clarity. (b) View of the 2D honeycomb framework of complex 4 along the c axis. (c) Intramolecular p–p stacking interactions in 4.
Fig. 3 (a) An ORTEP diagram of the crystal structure of complex 9, showing the 30% probability thermal motion ellipsoid. All hydrogen atoms were
deleted for clarity. (b) View of the 1D infinite neutral coordination ladder-like structure of the Nd3+ complex with LII (9) along the a axis. The side rails
of the ladder formed the alternate left-handed and right-handed helical chains through the rungs. Some atoms of the ligand and nitrates are omitted for
clarity. (c) Intramolecular p–p stacking interactions in complex 9.
Structures of 3, 4 (type II). Complexes 3 and 4 are isomor-
phous, belonging to the hexagonal system with space group R3.
To confirm the role of different backbone groups in the self-
assembly process, LII was designed with the longer chain length
of triethylene glycol instead of diethylene glycol forming the
backbone to perform the reaction with lanthanide nitrates. Single-
crystal X-ray analysis revealed that the La, Pr, Nd, Eu, Tb, and
Er complexes 7–12 of ligand LII crystallized in the triclinic system,
with space group P1, forming two types of ladder-like frameworks.
Representative bond lengths and angles are listed in Tables S8–S13,
ESI.†
¯
Hence, only the structure of complex 4 is discussed in detail.
The structural analysis reveals that 4 has crystallographic C3
symmetry. So, an asymmetric unit contains 2 Eu3+ each residing
on a 3-fold axis, 1 ligand, and 2 nitrates. As shown in Fig. 2a,
the Eu3+ center lies in a distorted monocapped square antiprism
coordination environment, defined by three O-donors from the
carbonyl groups of three LI ligands (O1 and O5), and six O-donors
from three bidentate coordinated nitrate groups (O6, O8, O9 and
O11). The Eu3+ cation acts as a three-connecting node linking
neighboring Eu3+ cation centers via the ligands, forming a (6, 3)
topological network. As can be seen from Fig. 2b, each metal
center forms a three-dimensional “Y-geometry” connecting unit
with the bidentate bridging flexible ligand rather than a “T-shape”
unit, the 2D network of 4 can be described as a honeycomb-like
topological net instead of a herringbone grid motif (complex 2).
The geometry of the salicylamide groups around the europium
atom is mutually perpendicular (O1–Eu1–O1 = 84.2(2)◦, O5–Eu2–
O5 = 83.4(3)◦), the metal–metal distances in each hexagonal ring
¯
Structures of 7–9 (type III). The complexes 7–9 are isomor-
phous, with minor differences in their comparative geometries
reflecting the normal lanthanide contraction effect. Therefore, the
crystal structure of complex 9 is described in detail to introduce
the structure type III. The structural analysis reveals that 9 has
crystallographic i symmetry. So the asymmetric unit contains 1
Nd3+, 3/2 ligands, and 3 nitrates. As shown in Fig. 3a, each
Nd3+ center adopts a distorted monocapped square antiprism
coordination sphere, which is defined by three bidentate nitrates,
and three carbonyl donors from three different LII ligands. Among
them, six oxygen atoms (O10, O12, O13, O15, O16 and O18)
belong to the bidentate nitrate groups, and the Nd–O bond lengths
are 2.551(2), 2.527(2), 2.5259(19), 2.5654(19), 2.5352(19), and
˚
are exactly the same: Eu1 ◊ ◊ ◊ Eu2 = 14.207(19) A, and the Eu–
Eu–Eu internal angle is 87.690◦, which is distinctly different from
those in 2. Each phenyl ring (C11) around Eu1 inclines parallel
to the benzylamine ring (C6) from the adjacent ligands (center-to-
˚
2.543(2) A, respectively; the other three oxygen atoms (O1, O6
◦
˚
center distance of 4.12 A and dihedral angle of 13.05 ), resulting
in a weak intramolecular p–p stacking interaction. Additionally,
as shown in Fig. 2c, another weak intramolecular p–p stacking
interaction of the phenyl (C19)–benzylamine (C6) ring is found,
with center-to-center distances of 4.403 (dihedral angles 18.57◦).26
and O7) are from carbonyl groups, and the Nd–O bond lengths
˚
are 2.4010(18), 2.3979(18), and 2.3602(17) A, respectively. All of
the bond lengths for Nd–O fall into the normal ranges.
The structure is comprised of ladder-like polymeric units, as
illustrated in Fig. 3b, bearing, as extensions of each rung, two
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additional exo-bidentate dangling LII ligands. The geometry of
the salicylamide groups around the neodymium atom deviates
from an ideal “T-shape”: O7–Nd1–O6 = 86.62(7)◦, O7–Nd1–O1 =
80.80(7)◦, O6–Nd1–O1 = 89.34(6)◦. However, the Nd–Nd–Nd
angles have ideal values, of 98.884◦, 81.116◦, and 180.0◦. The
9. There were no interactions between the adjacent ligands. The
aromatic rings of neighboring ligands were too distant to take part
in p–p interactions, with the shortest distance between centroids
28
˚
being 4.92 A.
Comparison of the structures. In the investigation of the self-
assembly between lanthanide nitrate salts and two structurally-
related flexible exo-bidentate ligands, ten novel crystal products
were isolated, and four types of structures were obtained. In
the four types of coordination polymers: 2D herringbone, 2D
honeycomb and two different types of 1D ladder-like framework,
the Ln3+ binds three ligands and the coordination sphere is
˚
Nd ◊ ◊ ◊ Nd distances are 14.0863(9) A along the side rails and
˚
16.660(1) A within the rungs. The Nd centers allow the rail of
the ladder to be an infinite, neutral coordination, single-stranded
helical chain folded in opposite directions. The left-hand and right-
hand helical rails are alternately connected, via the rungs. The
repeating period in the helical chain contains one Nd–LII unit.
The ladder is composed of large 76-membered rings. As seen from
Fig. 3c, the centroid distance between the benzylamine ring (C40)
2
completed by h -nitrates, and is located in the distorted mono-
capped square antiprism to different degrees. There is no solvent
taking part in coordination. To further understand the effects
of the lanthanide contraction and the different chain lengths
of ligands on the assembly of topological architectures in these
polymers in more detail, the conformation analyses of ligands
are presented. The distortion angles of the flexible backbone of
ligands in these polymers (2, 4, 9, and 11) are summarized in Table
S3, ESI,† and display distinct distortion fashions. Thus, various
frameworks were obtained. In the herringbone framework, as
shown in Fig. 5a, the three crystallographically independent LI
units in complex 2 adopt right-handed helical (A), nonhelical
(B), and left-handed helical (C) conformations. Among them,
two right-helical conformations of LI arrange in a head-to-tail
fashion and are perpendicular to each other, which assemble with
Pr1 ions. A similar situation is also present in the left-handed
helical conformation (C), which binds Pr2 ions. The nonhelical
conformation (B) of LI is coordinated to two different Pr ions (Pr1,
Pr2). Therefore, the three crystallographically independent LI
units with Pr ions arrange in an unusual framework (Fig. 5b). The
dominant conformation found for ligand LI is helical, arranging
with their two salicylamide arms in a gathered-round fashion
and acting as a middle spacer, with a shortest O ◊ ◊ ◊ O distance of
˚
and the benzene ring (C25) from an adjacent ligand is 4.40 A, and
the dihedral angle between the two rings is 11.725◦. These indicate
the existence of weak intramolecular p–p stacking interactions.26
Molecular ladders have been observed in some rigid systems with
transition metals27 and in all previous cases the metal junction
of the ladder has been a pseudo-octahedral or octahedral centre
generating the flat ladder with all linking ligands coplanar. In the
case reported here, we use the flexible ligand with lanthanide ions
to form a non-interpenetrated ladder-like structure, and the two
side rails of the molecular ladder pose the helical conformation,
which remain limited according to the literature.
Structures of 10–12 (type IV). The complexes 10–12 are iso-
morphous with minor differences in their comparative geometries
reflecting the normal lanthanide contraction effect. Therefore, the
crystal structure of complex 11 was described in detail to introduce
the structure type IV. In complex 11, each Tb3+ is in a distorted
monocapped square antiprism O9 coordination sphere formed by
three bidentate nitrates (O10, O12, O13, O15, O16 and O18), and
three carbonyl donors (O1, O6 and O7) from three different LII
ligands (Fig. 4a), which is similar to the coordination environment
of Nd3+ in complex 9. Complexes 9 and 11 are similar with respect
to the coordination geometry of the Ln3+ ions and having a 1D
ladder-like structure, except for the arrangement of LII (Fig. 4b).
In complex 11, the Tb ◊ ◊ ◊ Tb distances along the side rails (17.572
A) and within the rungs (17.411 A) are much longer than those
in 9, indicating that the LII in complex 11 has a more extended
conformation. Therefore, the rails of the ladder framework are just
folded in opposite directions, and do not form a helical rail (Fig.
4c). The “T-shape” unit at Tb3+ center is slightly deviated from an
ideal value (O6–Tb1–O1 = 150.65(3)◦, O6–Tb1–O7 = 79.96(3)◦,
O1–Tb1–O7 = 83.71(3)◦), the Tb–Tb–Tb angles are 50.707(0)◦,
129.293(1)◦, 180.00◦, respectively, which is different from complex
˚
˚
11.0179(104) A, 11.1728(110) A between the two carbonyl oxygen
donor atoms (A, C). Relatively large twisting is observed between
the two phenyl rings with a dihedral angle of 74.954◦, 78.918◦ (A,
C). The portion of the ligand LI in the X-ray structure featuring
nonhelical conformation allowed its salicylamide arms to stretch
out. Moreover, it acts as a longer spacer with a shortest O ◊ ◊ ◊ O
˚
˚
˚
distance of 11.8616(92) A (B) between the two carbonyl oxygen
donor atoms. It is notable that the distortion fashion of the –
CH2CH2– groups is significantly different from that observed in
the LI ligand featuring a helical configuration (Table S3, ESI†), and
a small twisting is observed between the two phenyl rings with a
Fig. 4 (a) An ORTEP diagram of the crystal structure of complex 11, showing the 30% probability thermal motion ellipsoids. All hydrogen atoms were
omitted for clarity. (b) View of the 1D infinite neutral coordination ladder-like structure of the Tb3+ complex of LII (11) along the a axis. Some atoms of
the ligand and nitrates are omitted for clarity. (c) View of the 1D infinite neutral coordination ladder-like structure of 11 along the c axis, which indicates
the side rails form a nonhelical chain. Some atoms of the ligand and nitrates and crystalline ethyl acetate molecules are omitted for clarity.
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Fig. 5 (a) View of the conformations of ligands LI in complex 2. All hydrogen atoms were deleted for clarity. (b) The schematic illustration for the
assembly of the herringbone framework from “T-shaped” units (complex 2).
Fig. 6 (a) View of the conformation of ligand LI in complex 4. All hydrogen atoms were deleted for clarity. (b) The schematic illustration for the assembly
of the honeycomb framework from “Y-geometry” connecting units (complex 4).
Fig. 7 (a) View of the conformations of ligands LII in complex 9 (A, B). All hydrogen atoms were omitted for clarity. (b) The schematic illustration of
the assembly of the ladder-like framework from “T-shaped” units (complex 9).
dihedral angle of 23.225◦. The honeycomb framework, as depicted
in Fig. 6a, contains one crystallographically independent LI, which
adopts a helical rather than a nonhelical conformation, appearing
largely as a consequence of the lanthanide contraction effect. Each
LI ligand in 4 acts as shorter spacer, the shortest O ◊ ◊ ◊ O distance
mation (B). The two crystallographically independent LII units
are mutually perpendicular, and are assembled with Nd3+ ions
in a 1D framework (Fig. 7b). In the helical conformation of LII
(A), it is worth pointing out that the shortest twisting is observed
between the two phenyl rings with a dihedral angle of 66.676◦,
which is slightly smaller than those observed in complex 2. In
this type of LII ligand with a nonhelical conformation, there is
an i symmetrical center which is in the center of C51–C51, so the
two phenyl rings are parallel to each other. The shortest O ◊ ◊ ◊ O
distances between the two carbonyl oxygen donor atoms in LII
˚
of the two carbonyl oxygen donor atoms is 10.6166(109) A, which
is much shorter than those of LI in 2. It indicates that the LI
ligand in 4 has a more gathered-round conformation. The small
twisting between the two phenyl rings is 39.822◦, which is smaller
than those helical conformations of LI in 2. As shown in Fig. 6b,
the Eu3+ cation acts as a three-connecting node (“Y-geometry”)
linking neighboring Eu3+ cation centers via ligands, which adopt
the same helical conformation and arrange in a head-to-head
fashion, forming a 2D framework.
˚
˚
are 11.3848(31) A (A) and 12.7856(29) A (B), respectively. For
complex 11, the two crystallographically independent LII ligands
also adopt the helical (A¢) and nonhelical (B¢) conformations,
which form the side rails and rungs of the ladder, respectively
(Fig. S5, ESI†). The i symmetrical center at the center of the
C51–C51 bond in the nonhelical fashion was also observed in 11.
The dihedral angle of the two phenyl rings in the helical fashion
In the ladder-like framework, complex 9, as shown in Fig. 7a,
contains two crystallographically independent LII units, which
adopt the helical conformation (A) and the nonhelical confor-
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is 69.599◦ (A¢), which is similar to that in complex 9, whereas
the shortest O ◊ ◊ ◊ O distances between the two carbonyl oxygen
3.4%, calculated 3.8%). The second step in the range of 234–
526 ◦C corresponds to the decomposition of the organic ligands,
as indicated by a significant weight loss (83.1%). No attempt was
made to identify the products of decomposition. Complex 7 is
donor atoms in LII are 13.7708(14) A (A¢) and 13.6232(14) A
(B¢), respectively, which are much longer than those observed
in 9, indicating that the LII in complex 11 has a more extended
conformation. A possible explanation for this change might be
that such an arrangement may reduce the steric hindrance and
make the whole framework more stable.
˚
˚
◦
stable before 143 C, after which it shows two weight loss steps.
The first step (143–191 ◦C) corresponds to the removal of two
solvent molecules per formula unit. The observed weight loss of
7.1% is slightly higher than the calculated value (6.5%). The second
step from 242 to 540 ◦C corresponds to the decomposition of the
organic ligands. The total observed weight loss is 84.6% and the
final residuals were not characterized.
These two ligands based on the backbone-dependent structural
variation can be understood taking into account the flexible
coordination behavior, modulated by changing the chain length
of the ligands. Compared with LI, although it has not affected
the coordination mode of the functional terminal groups, the
flexibility of LII is significantly changed due to the longer chain
of the backbone, which induces more extended orientations of the
salicylamide arms to reduce the steric hindrance. Consequently,
the two-dimensional (6, 3) topological networks with LI degrade
to one-dimensional frameworks with LII.
As is known, the effect of the lanthanide contraction may play
an important role in the formation of structures. The two flexible
ligands referred to in this study seem to be more sensitive to
the larger ionic radius of the lanthanides rather than the smaller
ionic radius. The complexes 1–6 were obtained from the same
starting materials and experimental method, however, two distinct
structural types owing to the different Ln radii were obtained:
La, Pr ions form a 2D herringbone framework, whereas Nd, Eu,
Tb, and Er ions form a 2D honeycomb framework. Similarly, for
complexes 7–12 La, Pr, and Nd form a 1D ladder-like framework,
in which the two rails of the ladder are composed of a helical chain,
whereas Eu, Tb, and Er form another ladder-like framework with
nonhelical rails. Additionally, as the ionic radii of the Ln ions
decreases, a general trend for reduction in Ln–O bond lengths and
coordination numbers is observed.
Luminescence properties of ligands and their complexes in the solid
state
The solid-state luminescent properties of free ligands and their
nitrate-coordinated complexes (Sm3+, Eu3+, Tb3+, and Dy3+) were
investigated at room temperature. Upon UV irradiation, the free
ligands emit a strong blue luminescence (apparent lmax at ca.
442 nm for LI and ca. 437 nm for LII) (Fig. S8, S9, ESI†). However,
upon complexation, this emission is replaced by the typical
luminescence color arising from the corresponding lanthanide
cation (Sm3+, Eu3+, Tb3+, and Dy3+). These colors can also be easily
visualized using a standard laboratory UV lamp (lex ª 365 nm).
The room temperature excitation and emission spectra of the
Eu3+ complex of ligand LI in the solid state are shown in Fig. 9.
The excitation spectrum was obtained by monitoring the 617 nm
5
7
line of the D0 → F2 emission. It only exhibits a series of sharp
lines characteristic of the Eu3+ energy-level structure that can be
assigned to transitions between the 7F0,1 and the 5L6, 5D2,1 levels.29
This indicates that Eu3+ luminescence is not efficiently sensitized by
the ligand. There are five characteristic peaks of the Eu3+ complex
shown in the emission spectrum, which is characteristic of the
metal in the 550–700 nm region, with well-resolved transitions
5
that are anticipated from the metal-centered D0 excited state to
Thermal analysis
the 7FJ ground-state multiplet. Maximum intensities at 580, 593,
617, 652, and 694 nm, respectively, were observed for the J =
0, 1, 2, 3, and 4 transitions. The most intense emission in the
Complexes 1–12 were subjected to TGA studies under an air
atmosphere. TGA curves of complexes 1–12 are similar and exhibit
two main weight loss steps (Fig. S6, S7, ESI†), and complexes 2
and 7 were used as two representatives (Fig. 8). The TGA curve
of complex 2 reveals two main weight loss steps. The mass loss in
the first step (an endothermic peak at 141 ◦C) can be attributed to
the liberation of one solvent molecule per formula unit (observed
7
luminescent spectrum is the 5D0 → F2 transition, which is the so-
called hypersensitive transition and is responsible for the brilliant-
red emission of compound 4.30 It is noteworthy that the emission
Fig. 9 Room-temperature excitation and emission spectra for the Eu3+
complex of ligand LI (lex = 396 nm, excitation and emission passes =
2.5 nm) in the solid state with emission monitored at approximately 617
nm.
Fig. 8 The TGA curves of complexes 2 and 7.
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5
7
intensity of the band arising from the D0 → F2 transition is
lifetimes of 0.040 ms for the Sm3+ complex and 0.050 ms for the
Dy3+ complex (Fig. S14, S15, ESI†).
5
7
stronger than that of D0 → F1. Since the former transition is
electric dipole in nature, its intensity is strongly influenced by the
crystal field, while the latter transition is magnetic dipole in origin
and less sensitive to its environment. It indicates the absence of
inversion symmetry at the Eu3+ site, which is in agreement with
the results of the single crystal X-ray analysis. The fluorescence
quantum yield, U, of the Eu3+ complex in the solid state was
found to be 10.62% using an integrating sphere. The luminescence
lifetime value for the 5D0 level of the Eu3+ complex was determined
to be 0.746 ms from the luminescence decay profile at room
temperature by fitting with a monoexponential curve (Fig. S10,
ESI†).
When Tb3+ is introduced into the LI ligand, the excitation
spectrum of LI displayed characteristic emission of Tb3+ ion in the
solid state (Fig. 10). The emission spectrum of the Tb3+ complex
exhibits the characteristic emission bands for Tb3+ centered at
490, 545, 583, and 621 nm, as shown in Fig. 10, which are assigned
Furthermore, the normalized emission spectra for the four
complexes of LII were studied. The relevant photophysical data are
summarized in Table S2, ESI.† The long luminescence lifetimes
and large quantum yield values are an indication that these
two flexible exo-bidentate ligands provide a significant level of
protection from non-radiative deactivation of lanthanide cations
by the solvent molecules, which is consistent with the single
crystal analysis. We note that both the luminescence lifetimes and
quantum yield values were slightly influenced by the backbone
groups. In contrast to the complexes of LI, the intensities of the
four complexes of LII are relatively stronger; the corresponding
quantum yields and lifetimes are also relatively larger and longer.
The triplet energy level of the antenna only changed slightly
with the chain length of ligands (vide infra), so we exclude the
possibility that the increased luminescence is due to the change in
the nature of the antenna triplet state. The possible explanation
for this change might be the structure of the complexes, in
that the 1D ladder-like structure of LII would enhance the
energy transfer sensitization of the lanthanide emission as well
as reduce the deactivation of its excited state via non-radiative
processes.
5
7
5
7
to the transition of D4 to FJ (J = 6, 5, 4, 3). The D4 → F6
is the second largest peak observed. The 5D4
→
7F4 and 5D4
7
→ F3 transitions consist of two peaks. The ligand fluorescence
disappears completely, indicating a very efficient energy transfer
from the ligand to the Tb3+ center. The fluorescence quantum yield
(U) of the terbium nitrate complex in the solid state was found to
be 32.51% using an integrating sphere at room temperature. The
luminescence lifetime value for the 5D4 level of the Tb3+ complex
was determined to be 1.150 ms from the luminescence decay profile
at room temperature by fitting with a monoexponential curve (Fig.
S11, ESI†).
Comparing the emission spectra of the four complexes with
either LI or LII (Table S2, ESI†), we can see that these two
ligands provide preferable sensitization of the Tb3+ lumines-
cence by an efficient antenna effect. This behavior can be
rationalized by the ligand-to-metal energy transfer discussed
below.
Energy transfer between the ligand and Ln3+
In the energy transfer from the ligand to the metal, the triplet-
state energy of the ligand is regarded as an important factor in the
excitation of the lanthanide cation.31 In order to acquire the triplet-
state energy levels (T0) of ligands LI and LII, the low-temperature
(77 K) phosphorescence spectra of the Gd3+ complexes of LI and
LII were measured in a 1 : 1 methanol-ethyl acetate (v/v) glassing
solvent, shown in Fig. S16 and S17, ESI.† From the phosphores-
cence spectra, the triplet energy levels (T0) of the Gd3+ complexes,
corresponding to their lower wavelength emission edges, are 24 390
cm-1 (410 nm) and 24 510 cm-1 (408 nm). Because the lowest excited
state, 6P7/2 (E (6P7/2) = 32 000 cm-1) of Gd3+ is much higher than the
lowest-lying triplet levels of most chromophores, the data obtained
from the phosphorescence spectrum actually reveals the triplet
energy level of the ligand in lanthanide complexes. Latva et al.’s
empirical rule32,33 states that an optimal ligand-to-metal energy
transfer process for Ln3+ needs (DE = E(T0) - E(5DJ )) 2500–4000
cm-1 for Eu3+ and 2500–4500 cm-1 for Tb3+. The experimentally
observed T0 energy levels of ligands LI and LII are ca. 3990 and
Fig. 10 Room-temperature excitation and emission spectra for the Tb3+
complex of ligand LI (lex = 323 nm, excitation and emission passes = 1.0
nm) in the solid state.
The potential for antenna-modified ligand LI to sensitize other
visible emitting lanthanide cations, notably Sm3+ and Dy3+, was
also investigated. For the Sm3+ complex of LI, two characteristic
bands at 563 and 598 nm were observed, which are attributed
5
4110 cm-1 higher than the D4 level of Tb3+ (20 400 cm-1), which
4
6
to G5/2 → HJ (J = 5/2, 7/2) transitions (Fig. S12, ESI†). For
facilitates efficient energy transfer. By contrast, the lowest-energy
Eu3+ acceptor levels are at 17 300 (5D0), which is significantly lower
in energy than the T0 energy levels of the two ligands. Therefore, it
supports the observation of stronger sensitization of the terbium
complexes than the europium complexes because of the larger
energy gap between the ligand triplet and the europium ion excited
states.
Dy3+, two characteristic narrow bands can be seen in the emission
spectrum, which are attributed to transitions of 482 nm (4F9/2
→
6H15/2) and573 nm(4F9/2 → H13/2) (Fig. S13, ESI†). Quantum yield
measurements for the Sm3+ and Dy3+ complexes in the solid state
were found to be 0.84 and 5.05, respectively. The luminescence
decays are best described by a single-exponential process with
6
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M. Iglesias, C. Ruiz-Valero and M. A. Monge, Chem. Commun., 2002,
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Conclusions
We have presented here two novel structurally-related flexible exo-
bidentate ligands, with different backbone chain lengths, which
result in the formation of lanthanide complexes with diverse
structures. X-ray crystallography revealed that the polymers have
novel 2D herringbone frameworks (1–2) and 2D honeycomb
frameworks (3–6) as well as two different types of 1D ladder-
like structures (7–9, 10–12). The structural diversity is mainly
caused by the variation in the flexibility of ligands and the
lanthanide contraction effect. It is obvious that varying the chain
length of the backbone from triethylene glycol to diethylene
glycol affects the flexibility of ligands, which considerably changes
the overall frameworks from two-dimensional topology nets to
one-dimensional ladder-like structures. The resulting polymers
provide the lanthanide ions with a nona-coordination environ-
ment and prevent solvent and water molecules from entering
the coordination sphere, effectively resulting in an enhanced
sensitization of the lanthanide due to a decrease in the vibronic
quenching effect. The photophysical properties of the lanthanide
complexes also demonstrate that the two salicylamide based
ligands exhibit a good antenna effect for Ln3+ cations and
provide excellent sensitization of the Tb3+ luminescence through
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a
particularly efficient ligand-to-lanthanide energy transfer
process.
Acknowledgements
This work was financially supported by the National Natural Sci-
ence Foundation of China (Grants 20771048, 20931003, 21001059
and J0730425) and the Fundamental Research Funds for the
Central Universities (lzujbky-2009-k06).
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