Crystal structure and highly luminescent properties studies of bis-β-diketonate lanthanide complexes

Article abstract of DOI:10.1021/ic302726z

A new bis(β-diketonate), 1,3-bis(4,4,4-trifluoro-1,3-dioxobutyl)phenyl (BTP), which contains a trifluorinated alkyl group, has been prepared for the synthesis of two series of dinuclear lanthanide complexes with the general formula Ln₂(BTP)₃L₂ [Ln³⁺ = Eu ³⁺, L = DME(1), bpy(2), and phen(3); Ln³⁺ = Sm ³⁺, L = DME(4), bpy(5), and phen(6); DME = ethylene glycol dimethyl ether, bpy = 2,2′-bipyridine, phen = 1,10-phenanthroline]. The crystal structure of the free ligand has been determined and shows a twisted arrangement of the two binding sites around the 1,3-phenylene spacer. X-ray crystallographic analysis reveals that complexes 1, 2, 4, and 5 are triple-stranded dinuclear structures formed by three bis-bidentate ligands with two lanthanide ions. The room-temperature photoluminescence (PL) spectra of complexes 1-6 show that this bis-β-diketonate can effectively sensitize rare earths (Sm³⁺ and Eu³⁺) and produce characteristic emissions of the corresponding Eu³⁺ and Sm³⁺ ions. In addition, two bidentate nitrogen ancillary ligands, 2,2′-bipyridine (bpy) and 1,10-phenanthroline (phen), have been employed to enhance the luminescence quantum yields and lifetimes of both series of Eu³⁺ and Sm ³⁺ complexes.

Full text of DOI:10.1021/ic302726z

Article
pubs.acs.org/IC
Crystal Structure and Highly Luminescent Properties Studies of Bis-β-
diketonate Lanthanide Complexes
Jing Shi, Yanjun Hou,* Wenyi Chu, Xiaohong Shi, Huiquan Gu, Bilin Wang, and Zhizhong Sun*
Key Laboratory of Chemical Engineering Process and Technology for High-Efficiency Conversion, School of Chemistry and Materials
Science, Heilongjiang University, No.74, Xuefu Road, Nangang District, Harbin 150080, People’s Republic of China
S
* Supporting Information
ABSTRACT: A new bis(β-diketonate), 1,3-bis(4,4,4-trifluoro-
1,3-dioxobutyl)phenyl (BTP), which contains a trifluorinated
alkyl group, has been prepared for the synthesis of two series
of dinuclear lanthanide complexes with the general formula
Ln2(BTP)3L2 [Ln3+ = Eu3+, L = DME(1), bpy(2), and
phen(3); Ln3+ = Sm3+, L = DME(4), bpy(5), and phen(6);
DME = ethylene glycol dimethyl ether, bpy = 2,2′-bipyridine,
phen = 1,10-phenanthroline]. The crystal structure of the free
ligand has been determined and shows a twisted arrangement
of the two binding sites around the 1,3-phenylene spacer. X-
ray crystallographic analysis reveals that complexes 1, 2, 4, and 5 are triple-stranded dinuclear structures formed by three bis-
bidentate ligands with two lanthanide ions. The room-temperature photoluminescence (PL) spectra of complexes 1−6 show that
this bis-β-diketonate can effectively sensitize rare earths (Sm3+ and Eu3+) and produce characteristic emissions of the
corresponding Eu3+ and Sm3+ ions. In addition, two bidentate nitrogen ancillary ligands, 2,2′-bipyridine (bpy) and 1,10-
phenanthroline (phen), have been employed to enhance the luminescence quantum yields and lifetimes of both series of Eu3+
and Sm3+ complexes.
1.33 times more intense luminescence signal than the
corresponding mononuclear analogue.14 But we found that
only triple-stranded dinuclear lanthanide complexes are proved
by single crystal X-ray diffraction. In contrast, we report not
only four single crystal structures of triple-stranded dinuclear
lanthanide complexes but also that the luminescence signal of
the europium BTP complex is 1.67 times more than that of
corresponding mononuclear analogue. Additionally, we display
that the quantum yields of complexes 2 and 3 are higher than
that of the corresponding analogue, which are illustrated in
Table 3.
We all know that the energy of C−H oscillators required is
more than that of C−F oscillators, resulting in lower
luminescence intensities and shorter excited-state lifetimes.
Thus, the replacement of the C−H bonds with lower-energy
C−F oscillators plays a critical role in the design of high
performance luminescent molecular devices.15,16 Additionally,
the β-diketonate ligand is one of the important “antennas”
which effectively transfer the energy of the ligand to metal; the
lanthanide-centered luminescent properties are enhanced.
On the basis of the above-mentioned consideration, we
sought a new bis-β-diketonate, 1,3-bis(4,4,4-trifluoro-1,3-
dioxobutyl)phenyl (BTP), ligand for the formation of dinuclear
lanthanide complexes, which is composed of the highly
electron-withdrawing −CF3 groups, which minimize the energy
INTRODUCTION
■
Extensive interest has been focused on the investigation of
photophysical properties of lanthanide ions owing to the
widespread applications in display devices, luminescent sensors,
probes for clinical use, tunable lasers, optical amplifiers, and
efficient light conversion molecular devices, to name a few.1−6
The current increasing interest in lanthanide β-diketonate
complexes has been received due to high molar absorption
coefficients of the β-diketonate ligands, long luminescent
lifetimes, and high luminescence quantum efficiency.7,8 On
one hand, bis-β-diketonate ligands have more negatively
charged binding sites, which can form stable triple-stranded
dinuclear structures. On the other hand, bis-β-diketonate
ligands can effectively transfer the intramolecular energy to
the central ion, contributing to bringing characteristic
luminescence properties.9 Previously, various β-diketonate
ligands have been documented,10 and thenoyltrifluoroacetone
(TTA) and acylpyrazole derivates are the most famous β-
diketonate ligands for sensitizing Eu3+ luminescence.11 But
there exist few studies of bis-β-diketonate lanthanide complexes
with triple-stranded dinuclear helicate structure.12 Bassett et al.
have prepared two bis-β-diketonate ligands for examination of
dinuclear lanthanide complex formation and investigation of
their properties as sensitizers for lanthanide luminescence.13
However, it is a pity that no crystallographic data are available
for the proposed triple-stranded dinuclear structure; only
molecular simulation structure is shown. Recently, Li et al. have
shown that a europium bis-β-diketonate complex exhibited a
Received: December 12, 2012
Published: April 19, 2013
© 2013 American Chemical Society
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Table 1. Crystal Data and Structure Refinement for Complexes 1, 2, 4, and 5
parameters
formula
1
2
4
5
C50H38Eu2F18O16
1540.72
buff
C64H38Eu2F18N4O12Cl4
1842.70
C50H38Sm2F18O16
1537.50
buff
C62H34Sm2F18N4O12Cl4
1839.48
Mr
color
buff
buff
cryst syst
space group
a (Å)
monoclinic
C2/c
monoclinic
monoclinic
C2/c
monoclinic
P1
̅
P1
̅
15.632(3)
12.913(2)
29.373(5)
90
12.3302(4)
15.1832(5)
19.6047(7)
96.92
15.6073(8)
12.9464(6)
29.3024(14)
90
12.3311(4)
15.1841(5)
19.6059(7)
96.92
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
92.006(2)
90
103.56
91.55
92.0160(10)
90
103.56
91.55
5925.6(18)
4
3536.0(2)
2
5917.1(5)
4
3536.7(2)
2
ρ (g cm3)
1.727
1.731
1.726
1.727
μ (mm−1
)
2.217
2.017
2.085
1.904
F (000)
3016
1804
3008
1800
R1 [I > 2σ(I)]
wR2 [I > 2σ(I]
R1 (all data)
wR2 (alldat)
GOF on F2
0.0425
0.1125
0.0532
0.1204
1.035
0.0329
0.0876
0.0381
0.0919
1.038
0.0297
0.0356
0.0965
0.0408
0.1013
1.034
0.0656
0.0332
0.0672
1.159
room temperature and cited relative to a reference solution of
losses compared with C−H bonds, that could better promote
the solubility of bis-β-diketonate lanthanide complexes in
different organic solvents. Further investigation of photo-
luminescence studies reveals that bis-β-diketonate complex
[Eu2(BTP)3(H2O)4] exhibited a 1.67 times more intense
luminescence signal than the corresponding mononuclear
analogue [Eu(BTFA)3(H2O)2].
Ru(bpy)3Cl2 (Φ = 55.3%),17 and through the following expression:
n2Aref
I
φ
=
φ
ref
overall
nr2efAIref
(1)
In eq 1, n, I, and A denote the refractive index of solvent, the area of
the emission spectrum, and the absorbance at the excitation
wavelength, respectively, and φref represents the quantum yield of
the standard Ru(bpy)3Cl2 solution. The subscript ref denotes the
reference, and the absence of a subscript implies an unknown sample.
Synthesis of 1,3-bis(4,4,4-trifluoro-1,3-dioxobutyl)phenyl
(BTP). A mixture of sodium ethoxide (1.4 g, 20 mmol) and ethyl
trifluoroacetate (2.9 g, 20 mmol) in 40 mL of dry THF
(tetrahydrofuran) was stirred for 10 min, followed by the addition
of 1,3-bis-acetophenone (2.97 g, 8.4 mmol). Then, it was further
stirred at room temperature for 24 h (Scheme 1). The resulting
EXPERIMENTAL SECTION
■
Materials and Instrumentation. The commercially available
chemicals were analytical reagent grade and used without further
purification. LnCl3·6H2O was prepared according to the literature by
dissolving lanthanide oxide in a slight excess of hydrochloric acid. The
solution was evaporated, and the precipitate was collected from
water.13 Elemental analyses were performed on an Elementar Vario EL
cube analyzer. Fourier transform infrared (FT-IR) spectra were
obtained on a Perkin-Elmer Spectrum One spectrophotometer by
using KBr disks in the range 4000−370 cm−1. Ultraviolet (UV) spectra
were recorded on a Perkin-Elmer Lambda 25 spectrometer. Thermal
analyses were conducted on a Perkin-Elmer STA 6000 with a heating
rate of 10 °C·min−1 in a temperature range from 30 to 800 °C under a
Scheme 1. Synthesis of the BTP
1
N2 atmosphere. The H NMR spectra were recorded on a Bruker
Avance III 400 MHz spectrometer in CDCl3 solution. Excitation and
emission spectra were measured by an Edinburgh FLS 920
fluorescence spectrophotometer. Luminescence lifetimes were re-
corded on a single photon counting spectrometer from Edinburgh
Instrument (FLS 920) with a microsecond pulse lamp as the
excitation. The data were analyzed by software supplied by Edinburgh
Instruments. Suitable single crystals of 1, 2, 4, and 5 were selected for
single crystal diffraction analysis (CCDC Nos. 886032, 899276,
898750, and 899343 contain supplementary crystallographic data for
complexes 1, 2, 4, and 5). Diffraction intensity data were collected on a
Bruker SMART APEX II X-ray diffractometer with graphite-
monochromated Mo Kα radiation (l = 0.71073 Å). All data were
collected at a temperature of 20 2 °C. The structures were solved
using direct methods and refined on F2 by full-matrix least-squares
using the SHELXTL-97 program. The Ln3+ ions were easily located,
and then non-hydrogen atoms (C, N, O, and F) were placed from the
subsequent Fourier-difference maps. A summary for data collection
and refinements is given in Table 1. The overall quantum yields of
both europium and samarium complexes were measured in CH3CN at
mixture was poured into 100 mL of ice water and acidified to pH 2−3
using hydrochloric acid (2 M), and the resulting white precipitate was
filtered and dried in a vacuum. Recrystallization from isopropanol gave
white flake crystals (2.7 g, 82%). Elemental analysis (%) calcd for
C14H8F6O4 (354.03): C, 47.47; H, 2.28. Found: C, 47.45; H, 2.29. IR
(KBr, cm−1): 3435 (w), 3125 (w), 1591 (s), 1269 (s), 1207 (s), 1158
(s), 1080 (s), 93 (m), 787 (m), 723 (w), 709 (w), 678 (w), 627 (m),
577 (m). 1HNMR (CDCl3, 400 MHz): 14.83 (s, 2H), 8.49 (s, 1H),
8.16 (d, J = 1.0 Hz, 2H), 7.68 (t, J = 2.0 Hz, 1H), 6.64 ppm (s, 2H)
(Figure S1, Supporting Information). ESI-TOF m/z = 354 (M−)
(Figure S2, Supporting Information).
Synthesis of Gd2(BTP)3(H2O)4. An aqueous solution of
GdCl3·6H2O (1.0 mmol) was added to a solution of BTP (1.5
mmol) in methanol in the presence of NaOH (1.5 mmol).
Precipitation took place immediately, and the reaction mixture was
stirred for 10 h at room temperature. The product was filtered; washed
with methanol, water, and then methanol; dried; and stored in a
desiccator. The complex was then purified by recrystallization from a
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Scheme 2. Synthesis of Complexes 1−6
dichloromethane−methanol mixture. Yield: 82%. Elemental analysis
(%) calcd for C42H32Gd2F18O16 (1449.99): C, 34.81; H, 2.23. Found:
C, 34.83; H, 2.24. IR (KBr)νmax: 3419 cm−1 (s, νO−H), 1619 cm−1 (s,
νCO), 1538 cm−1 (s), 1294 cm−1 (s), 1137 cm−1 (s, νC−F), 783 cm−1
(m, νCF3).
Synthesis of Gd(bpy)2(NO3)3. To a 50 mL methanol solution
containing 2,2′-bipyridine (2.0 mmol), Gd(NO3)3(H2O)6 (1.0 mmol)
was added dropwise under constant stirring, and then, the solution was
refluxed for 1 h at 65 °C. The resulting solution was filtered to obtain a
white powder. Yield: 85%. Elemental analysis (%) calcd for
C20H20GdN6O6 (597.70): C, 40.19; H, 3.37; N, 14.06. Found: C,
40.18; H, 3.38; N, 14.06. IR (KBr) νmax: 1578 cm−1 (s, νN−O), 1458
cm−1 (s), 1415 cm−1 (s), 1312 cm−1 (s), 1039 cm−1 (s, νC−N), 757
cm−1 (m, νN−O).
Synthesis of Gd(phen)2(NO3)3. To a 50 mL methanol solution
containing 1,10-phenanthroline (2.0 mmol), Gd(NO3)3(H2O)6 (1.0
mmol) was added dropwise under constant stirring, and then, the
solution was refluxed for 1 h at 65 °C. The resulting solution was
filtered to obtain a white powder. Yield: 89%. Elemental analysis (%)
calcd for C24H20GdN6O6 (645.70): C, 44.64; H, 3.12; N, 14.87.
Found: C, 44.66; H, 3.11; N, 14.86. IR (KBr) νmax: 1559 cm−1 (s,
νN−O), 1422 cm−1 (s), 1295 cm−1 (s), 1026 cm−1 (s, νC−N), 739 cm−1
(m, νN−O).
Synthesis of Eu2(BTP)3(H2O)4 and Sm2(BTP)3(H2O)4. To a
methanol solution of BTP (1g, 2.8 mmol), NaOH (0.2 g, 5.6 mmol)
was added, and the mixture was allowed to stir for 5 min. To this
methanol solution, LnCl3·6H2O (1.86 mmol) was added dropwise,
and the mixture was allowed to stir for 24 h at ambient temperature.
Water was then added to this mixture, and the precipitate thus formed
was filtered, washed with water, and dried in the air. Single crystals
were obtained in about two weeks by recrystallization from DME/
hexane.
Eu2(BTP)3(DME)2 (1). Elemental analysis (%) calcd for
C50H38Eu2F18O16 (1540.72): C, 38.98; H, 2.49. Found: C, 38.97; H,
2.51. IR (KBr) νmax: 1619 cm−1 (s, νCO), 1537 cm−1 (s), 1293
cm−1(s), 1137 cm−1 (s, νC−F), 782 cm−1 (m, νCF3).
Sm2(BTP)3(DME)2 (4). Elemental analysis (%) calcd for
C50H38Sm2F18O16 (1537.50): C, 39.06; H, 2.49. Found: C, 39.04; H,
246. IR (KBr) νmax: 1621 cm−1 (s, νCO), 1538 cm−1 (s), 1294 cm−1
(s), 1137 cm−1 (s, νC−F), 781 cm−1 (m, νCF3).
Synthesis of 2, 3, 5, and 6. Complexes 2, 3, 5, and 6 were
prepared by stirring solutions of Ln2(BTP)3(H2O)4 and the two molar
nitrogen donors in CH3OH for 10 h at 65 °C. The products were
isolated and purified following the aforementioned method. Single
crystals of complexes 2 and 5 were harvested in about two weeks by
recrystallization from chloroform and acetone/hexane.
Eu2(BTP)3(bpy)2(CH2Cl2)2 (2). Yield: 91%. Elemental analysis (%)
calcd for C64H38Eu2F18N4O12Cl4 (1842.70): C, 41.71; H, 2.08; N, 3.04.
Found: C, 41.73; H, 2.07; N, 3.06. IR (KBr) νmax: 1618 cm−1 (s,
νCO), 1327 cm−1 (s), 1281 cm−1 (s), 1128 cm−1 (s, νC−F), 755 cm−1
(m, νCF3).
Eu2(BTP)3(phen)2 (3). Yield: 89%. Elemental analysis (%) calcd for
C66H34Eu2F18N4O12 (1722.03): C, 46.06; H, 1.99; N, 3.26. Found: C,
46.04; H, 1.98; N, 328. IR (KBr) νmax: 1622 cm−1 (s,νCO), 1294 cm−1
(s), 1190 cm−1 (s), 1139 cm−1 (s, νC−F), 780 cm−1(m, νCF3).
Sm2(BTP)3(bpy)2(CH2Cl2)2 (5). Yield: 93%. Elemental analysis (%)
calcd for C64H38Sm2F18N4O12Cl4 (1839.48): C, 41.79; H, 2.08; N,
3.05. Found: C, 41.76; H, 2.07; N, 3.03. IR (KBr) νmax: 1616 cm−1 (s,
νCO), 1294 cm−1 (s), 1190 cm−1 (s), 1139 cm−1 (s, νC−F), 780 cm−1
(m, νCF3).
Sm2(BTP)3(phen)2 (6). Yield: 88%. Elemental analysis (%) calcd
for C66H34Sm2F18N4O12 (1720.03): C, 46.15; H, 2.00; N, 3.26. Found:
C, 46.17; H, 2.02; N, 3.28. IR (KBr) νmax: 1622 cm−1 (s, νCO), 1294
cm−1 (s), 1187 cm−1 (s), 1137 cm−1 (s, νC−F), 778 cm−1(m, νCF3).
Computational Details. The DFT-B3LYP (density functional
method)18 and CIS (configuration interaction with single excitations)
approaches19 using the 6-31G(d) basis set have been employed to
optimize the geometry structures in the ground and excited states,
respectively. On the basis of the optimized geometry structures in the
ground and excited states, the absorption and emission properties in
the acetonitrile media can be calculated by time-dependent density
functional theory (TDDFT)20 using 6-31G(d, p) basis set associated
with the polarized continuum model (PCM).21 All of the calculations
Eu2(BTP)3(H2O)4. Yield: 87%. Elemental analysis (%) calcd for
C42H32Eu2F18O16 (1438.59): C, 35.07; H, 2.24. Found: C, 35.08; H,
2.24. IR (KBr)νmax: 3420 cm−1 (s, νO−H), 1619 cm−1 (s, νCO), 1539
cm−1 (s), 1294 cm−1 (s), 1136 cm−1 (s, νC−F), 783 cm−1 (m, νCF3).
Sm2(BTP)3(H2O)4. Yield: 87%. Elemental analysis (%) calcd for
C42H32Sm2F18O16 (1435.39): C, 35.14; H, 2.25. Found: C, 35.15; H,
2.24. IR (KBr)νmax: 3419 cm−1 (s, νO−H), 1620 cm−1 (s, νCO), 1536
cm−1 (s), 1293 cm−1 (s), 1137 cm−1 (s, νC−F), 781 cm−1 (m, νCF3).
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were accomplished by using the Gaussian 03 software package.22 An
analytical frequency analysis provides evidence that the calculated
species represents a true minimum without imaginary frequencies on
the respective potential energy surface. In the calculation of the optical
absorption spectrum, the lowest spin-allowed singlet−singlet tran-
sitions, up to energy of ∼5 eV, were taken into account.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of the Ligand and
Complexes. The synthetic method of the complexes 1−6 is
shown in Scheme 2. The IR spectra of both complexes 1 and 4
display the typical broad absorption in the region 3000−3500
cm−1, corresponding to the presence of solvent molecules in
the complexes 1 and 4. In addition, the absence of the broad
band in the region 3000−3500 cm−1 for complexes 2, 3, 5, and
6 reveals that solvent molecules have been replaced by the
bidentate neutral donors.23 The carbonyl stretching frequency
of BTP (1591 cm−1) is red-shifted in complexes 1−6 (1619
cm−1 in 1; 1618 cm−1 in 2; 1623 cm−1 in 3; 1621 cm−1 in 4;
1616 cm−1 in 5; 1622 cm−1 in 6), which suggests the
involvement of carbonyl oxygen in the complex coordinated
with the Ln3+ ion. It is clear from the thermogravimetric
analysis data that complex Eu2(BTP)3(H2O)4 undergoes a mass
loss of about 5% (calcd 5.1%) in the first step (140−200 °C),
which corresponds to the loss of the coordinated water
molecules, and then, it undergoes a single step decomposition
(Figure S3, Supporting Information). On the contrary,
complexes 2 and 3 are more stable than complex
Eu2(BTP)3(H2O)4, and they undergo single-step decomposi-
tion at 330 °C, illustrating that there are no solvents in
complexes 2 and 3 (Figures S4 and S5, Supporting
Information). Thermogravimetry differential scanning calorim-
etry (TG−DSC) curves of complexes Sm2(BTP)3(H2O)4 and 5
and 6 are similar to those of complexes Eu2(BTP)3(H2O)4 and
2 and 3 (Figures S6, S7, and S8, Supporting Information).
X-Ray Structural Characterization. X-ray crystallographic
analysis reveals that complexes 1 and 4 and complexes 2 and 5
are isomorphic, respectively. Crystal data and data collection
parameters for complexes 1, 2, 4, and 5 are described in Table
1. The selected bond lengths and angles for complexes 1,2, 4,
and 5 are described in Table S1 in the Supporting Information.
The structural analysis reveals that complex 1, crystallizing in
the monoclinic space group C2/c, is a triple-stranded dinuclear
helicate featured by the coordination of three bis-β-diketonate
ligands to two crystallographically equivalent Eu3+ ions, as
shown in Figure 1. The crystallographically distinct Eu3+ ion is
ligated to six oxygen atoms from the three bis-diketonate and
two oxygen atoms from DME, resulting in the distorted square
antiprism geometry. The Eu−O distances are in the range of
2.339(4)−2.531(4) Å, which are in accordance with reported
values in europium monodiketone complexes24 and longer than
reported values in europium bis-diketonate complexes.14
Figure 1. Molecular structure of complex 1.
Figure 2. Molecular structure of complex 2.
H···F and C−H···O play a critical role in the stabilization of the
structures. Nonclassic hydrogen bond lengths (Å) and angles
(deg) for complexes 1, 2, 4, and 5 are described in Table S2 in
the Supporting Information.
UV−Vis Spectra. The UV−vis absorption spectra of the
free ligand BTP and complexes 1−3 were measured in CH3CN
solution (c = 1 × 10−5 M; Figure 3). The maximum absorption
In the structure of complex 2 (Figure 2), two DME
molecules are substituted by the ancillary ligand of bpy so that
each central Eu3+ ion is coordinated with six oxygen atoms from
three BTP ligands and two nitrogen atoms from bpy. The
coordination geometry of the Eu3+ ion in complex 2 can be
described as a distorted square antiprism. In complex 2, the
average bond distances of Eu−N and Eu−O are 2.587 Å and
2.341 Å, respectively; the asymmetric unit and coordination
geometry of complexes 4 and 5 are given in Figures S9 and S10
in the Supporting Information. There is no classic hydrogen
bond in the four complexes 1, 2, 4, and 5; interactions of C−
Figure 3. UV−vis absorption spectra of BTP, complexes 1−3 in
CH3CN (c = 1 × 10−5 M).
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Figure 4. Spatial plots of the selected frontier molecular orbits of the ground (left) and excited (right) states of BTP.
band at 325 nm for BTP, is attributed to the singlet−singlet
π−π* electronic transition of the aromatic rings in the bis-β-
diketonate. Compared with the ligand, the maximum
absorption peaks are blue-shifted or red-shifted for complexes
1−3, which is attributed to the perturbation induced by the
coordination of the Eu3+ ion. The spectral patterns of the Eu3+
complexes in CH3CN are similar to that of the free ligand,
indicating that the coordination of the europium ions does not
significantly influence the energy of the singlet states of the bis-
β-diketonate. The molar absorption coefficient values of
complexes 1−3 are calculated as 8.00 × 104 (317 nm), 7.30
× 104 (325 nm), and 7.26 × 104 (327 nm) mol−1·dm3·cm−1,
respectively, which are about three times that of the ligand
(2.68 × 104 L·mol−1·cm−1 at 325 nm), suggesting the presence
of three ligands in the corresponding complexes.
Computational Studies. As shown in Figure 4, it is noted
that the electronic cloud distribution of the HOMO in the
ground and singlet excited state localizes at the COCHC-
(OH)PhC(OH)CHCO group, while the one of the LUMO
localizes at the whole molecule. Herein, the HOMO and
LUMO energies were calculated by DFT. The HOMO and
LUMO levels for the ground and singlet excited state were
found to be −6.69 and −2.59 eV and −6.42 and −2.61 eV,
respectively. By comparison of the electronic cloud distribution
of the HOMO in the ground, the electronic cloud distribution
of the HOMO in the singlet excited state is asymmetrical, and
one of two −CF3 group have rotated to some extent. In
addition to these above, the electronic cloud distribution of the
−CF3 group in the LUMO of the singlet excited state is poor,
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Table 2. Absorptions of BTP Calculated with the TDDFT Methods
Sn
confign
H→L
CI codff
E/nm (eV)
oscillator
assignt
S1
S5
S6
0.6409
0.5325
0.6238
328 (3.78)
313 (3.97)
289 (4.28)
0.0613
1.1847
0.0374
JICOCHC(OH)CC→JI*BTP
JICOCHC(OH)PhC(OH)CHCO→JI*BTP
JICOCHC(OH)PhC(OH)CHCO→JI*BTP
H−1→L
H−1→L+1
Figure 5. Excitation and emission spectra of complexes (a) 1−3 in CH3CN (c = 1 × 10−5 M) and (b) 4−6 in CH3CN (c = 1 × 10−5 M).
Table 3. Radiative (ARAD) and Nonradiative (ANR) Decay Rates, Observed Luminescence Lifetime (τobs), Intrinsic Quantum
Yield (Φln), Sensitization Efficiency (Φsens), and Overall Quantum Yield (Φoverall) for Complexes 1−3 and the Reported
Analogous Lanthanide Complexes at 298 K
complexes
I7F2/I7F1
ARAD (s−1
)
ANR (s−1
)
τobs (us)
Φln (%)
Φsens (%)
Φoverall (%)
Eu2(BTP)3(DME)2(1)
8.81
9.08
1032
1207
1456
1128
857
6601
2816
601
131
366
486
506
906
548
14
30
70
57
78
47
93
87
79
82
83
83
13
26
55
47
65
39
14
Eu2(BTB)3(C2H5OH)2(H2O)2
Eu2(BTP)3(bpy)2(CH2Cl2)2)(2)
12.43
18.30
7.32
14
Eu2(BTB)3(bpy)2
851
Eu2(BTP)3(phen)2(3)
246
14
Eu2(BTB)3(phen)2
15.56
863
973
5
which matches well with the goal using the highly electron-
withdrawing property of −CF3 groups.
because the corresponding transition D0→7F1 is a magnetic
transition. The intensity ratio of I7F2/I7F1 is 8.81 for complex 1,
while it increased to 12.43 for 2 and 7.32 for 3, suggesting that
the Eu3+ ion is coordinated in a local site without any inversion
center. Furthermore, the emission spectra of complexes 1−3
show only one peak for the 5D0→7F0 transition and three stark
components for the 5D0→7F1 transition, indicating the presence
of a single chemical environment around the Eu3+ ion. The
emission bands around 582 and 650 nm are very weak, and
owing to their corresponding transitions 5D0→7F0,3 are
forbidden both in magnetic and electric dipole schemes. The
solid PL spectra of complexes 1−3 recorded at 298 K are
shown in Figures S11−S13 in the Supporting Information. PL
spectra of complexes 4−6 in CH3CN are shown in Figure 4b.
Upon excitation at 334 nm, which is the maximum of the
As shown in Table 2, the lowest excitation energy calculated
by TD-DFT is 3.78 eV. It is obvious that S5 and S6 high-energy
absorption transitions are mainly due to the HOMO − 1 →
LUMO and HOMO − 1 → LUMO + 1 frontier molecular
orbital transitions. The calculated singlet−singlet transitions of
BTP are in reasonable agreement with the experimental data.
Photoluminescence (PL) Properties of Complexes 1−
6. PL spectra of complexes 1−3 in CH3CN are shown in Figure
5a.
In comparison with the absorption spectra of complexes 1−
6, the excitation spectra show narrow bands around 324 nm for
1−3 and 334 nm for 4−6. Thus, the excitation and absorbance
wavelength of complexes 1−6 overlap very well within the
range, indicating characteristic lanthanide emission is sensitized
through the ligand. Upon excitation at 324 nm, which is the
maximum of the excitation spectrum, complexes 1−3 showed
the characteristic emission bands of the Eu3+ ion corresponding
excitation spectrum, complexes 4−6 show characteristic narrow
band emissions of the Sm3+ ion corresponding to the G5/2
→
4
6HJ (J = 5/2, 7/2, 9/2, 11/2) transitions. The three expected
4
6
peaks for the G5/2 → H5/2−9/2 transitions are well resolved.
The most intense peak is the hypersensitive transition 4G5/2
6H9/2 at 650 nm.
→
5
7
5
to the D0 → FJ (J = 0−4) transitions. Among them, the D0
→ 7F2 transition at λ = 612 nm is the strongest emission as an
induced electric dipole transition, and its corresponding
intensity is very sensitive to the coordination environment.
This very intense 5D0→7F2 peak, pointing to a highly
polarizable chemical environment around the Eu3+ ion, is
responsible for the brilliant red emission of complexes 1−3.25,26
The intensity of the emission band at 592 nm is relatively
weak and independent of the coordination environment
The 5D0 lifetimes (τobs) were determined from the
luminescent decay profiles for complexes 1−3 at room
temperature by fitting with monoexponential curves, proposing
the presence of a single chemical environment around the
emitting Eu3+ ion, and the values are compiled in Table 3.
Typical decay profiles of complexes 1−3 are shown in Figure
S14 in the Supporting Information. The relatively shorter
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lifetime observed for complex 1 (τobs = 131 μs) can be caused
by dominant nonradiative decay channels associated with
vibronic coupling for the presence of water molecules, as well
documented in many of the hydrated europium β-diketonate
complexes. On the other hand, the relative longer lifetimes have
among their analogues of complexes Eu2(BTB)3L2 (BTB =
3,3′-bis(4,4,4-trifluoro-1,3-dioxobutyl)biphenyl, L = H2O/
C2H5OH, bpy, and phen). Perceptibly, the sensitization
efficiency (Φsens) of BTP to the Eu3+ ion in complexes 1−3
is found to be promising.
To examine the effect of BTP on sensitizing
Eu3+luminescence, the luminescent signal intensities of
complexes Eu2(BTP)3(H2O)4 and Eu(BTFA)3(H2O)2 are
compared in Figure 6. Although both ligands possess similar
been observed for complexes 2 (τobs = 486 μs) and 3 (τobs
=
906 μs) because of their less important nonradiative
deactivation pathways. The lifetimes observed for complexes
4 (τobs = 15 μs), 5 (τobs = 58 μs), and 6 (τobs = 100 μs) are
relatively shorter than those of Eu3+ complexes. The decay
profiles of complexes 4−6 are shown in Figure S15 in the
Supporting Information.
The luminescence quantum yield Φ is an important
parameter for evaluation of the efficiency of the emission
process in luminescent materials. However, the overall
quantum yield (Φoverall) of the lanthanide complex treats the
system as a “black box” where the internal process is not
explicitly considered. The overall luminescence quantum yield
(Φoverall) for a lanthanide complex can be determined
experimentally under excitation of the ligand. Given that the
complex absorbs a photon, the sensitization efficiency can be
defined as eq 2:27
ϕ
ϕln
overall
ϕ
=
sens
(2)
Figure 6. Emission spectra of isoabsorptive solutions (A = 0.1) of (a)
Eu2(BTP)3(H2O)4 and (b) Eu(BTFA)3(H2O)2 in CH3CN, λexc = 350
nm.
The intrinsic quantum yields of europium could not be
determined experimentally upon direct f−f excitation because
of very low absorption intensity. However, they can be
estimated using eq 3, after calculating the radiative lifetime
(τrad; eq 4):28
triplet states, interestingly, the signal intensity of complex
Eu2(BTP)3(H2O)4 is 1.67 times higher than that of the
mononuclear analogue Eu(BTFA)3(H2O)2. It proposes that
BTP could be a better candidate on sensitizing Eu3+ ion
luminescence than BTFA, which might be attributed to the
rigidity of the triple-stranded helicate.32 The rigid structure
restricts the thermal vibration of the ligand and reduces the
energy loss by radiationless decay.
ARAD
ARAD + ANR
τ
τrad
obs
ϕln =
=
(3)
⎛
⎞
Itot
I
MD
1
τrad
ARAD
=
= AMD,0n3
⎜
⎝
⎟
⎠
(4)
Intramolecular Energy Transfer between Ligand and
Eu3+ Ion. In general, the widely accepted energy transfer
mechanism in lanthanide complexes is that proposed by Crosby
et al.31 In order to make energy transfer effective, the triplet
where AMD,0 = 14.65 s−1 is the spontaneous emission
probability of the magnetic dipole D0 → F1 transition, n is
5
7
the refractive index of the medium, Itot is the total integrated
5
5
emission of the D0→7FJ transitions, and IMD is the integrated
states energy level of the ligand cannot be less than the D0
emission of the 5D0
→
7F1 transition. ARAD and ANR are
level (17 500 cm−1) of the Eu3+ ion. Moreover, the triplet states
5
radiative and nonradiative decay rates, respectively.
energy level of the ligand is close to the D1 level (19 100
The parameters characterizing the photophysical properties
of solid-state samples of the complexes are summarized in
cm−1), and the energy difference is about 1200 cm−1, the
strongest fluorescence emission that will occur. That is usually
considered energy level matching.33 To elucidate the energy
transfer processes in the europium complex, the energy levels of
the relevant electronic states should be estimated. The singlet
and triplet energy levels of BTP and bidentate nitrogen donors
are estimated by referring to their wavelengths of UV−vis
absorbance edges and the lower wavelength emission peaks of
the corresponding phosphorescence spectra. On account of the
difficulty in observing the phosphorescence spectra of the
ligand, the emission spectra of the complex Gd2(BTP)3(H2O)4
at 77 K can be used to estimate the triplet state energy level.
Because the lowest lying excited level (6P7/2→8S7/2) of Gd(III)
is located at 32 150 cm−1, the triplet state energy level of the
ligand is not significantly affected by the Gd3+ ion.34 As shown
in Figure 5, the triplet energy level of Gd2(BTP)3(H2O)4,
which corresponds to the lower emission peak wavelength, is
20 080 cm−1 (498 nm). The single state energy (1ππ*) level of
BTP is estimated by referencing its absorbance edge, which is
22 936 cm−1 (436 nm). The singlet levels of the ancillary
Table 3. The overall luminescence quantum yield (Φoverall
)
observed for complex Eu2(BTP)3(H2O)4 is the lowest among
the three complexes. It is understandable that the presence of
the O−H oscillators in the close proximity of the Eu3+ center
could effectively quench the luminescence via vibrational
relaxations.29 In addition, the substitution of four water
molecules by two DME molecules in complex 1 results in an
increase in emission intensity. On the other hand, the
substitution of the solvent molecules by the bidentate nitrogen
donors in complexes 2 and 3 lead to an increase in the observed
quantum yields. Thus, complexes 1−3 show increasing
luminescence quantum yields in a sequence of 1 < 2 < 3,
which is in agreement with the literature.30,31 As such, a similar
behavior is observed in determining the luminescence quantum
yields of complexes 4−6 in acetonitrile solution (2% in 4, 6% in
5, 13% in 6, respectively). In comparison with the luminescence
quantum yields reported in the literature (Table 3), the overall
quantum yields of complexes 2 and 3 separately rank the first
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Figure 7. (a) (1) UV−vis absorption spectrum of BTP. (2) Room temperature emission spectrum of BTP. (3) Phosphorescence spectrum of
Gd2(BTP)3(H2O)4 at 77 K. (4) Room temperature emission spectrum of bpy. (5) Phosphorescence spectrum of Gd(bpy)2(NO3)3 at 77 K. (b) (1)
UV−vis absorption spectrum of BTP. (2) Room temperature emission spectrum of BTP. (3) Phosphorescence spectrum of Gd2(BTP)3(H2O)4 at 77
K. (4) Room temperature emission spectrum of phen. (5) Phosphorescence spectrum of Gd(phen)2(NO3)3 at 77 K.
ligands bpy (29 900 cm−1) and phen (31 000 cm−1) were taken
from the literature.35 The triplet (T1) energy levels of bpy and
phen are calculated by referring to the lower wavelength
emission edges of the corresponding phosphorescence spectra
of Gd3+ complexes, which are 21 367 cm−1 (468 nm) and 20
790 cm−1 (481 nm), respectively (Figure 5a,b).
Eu3+ ion directly or through the triplet state of BTP (overlap of
the phosphorescence spectra of Gd2(BTP)3(H2O)4 and
Gd(bpy)2(NO3)3). Similar processes can be observed for
complex 3 (Figure 7b).
According to the above experimental results, the schematic
energy level diagram and the energy transfer process that
possibly takes place in complex 2 are shown in Figure 8. The
Generally, the sensitization pathway in luminescent euro-
pium complexes consists of excitation of the ligands into their
excited singlet states, subsequent intersystem crossing of the
ligands to their triplet states, and energy transfer from the
5
triplet state to the DJ manifold of the Eu3+ ion, followed by
5
internal conversion to the emitting D0 state; finally, the Eu3+
ion emits when transition to the ground state occurs. Moreover,
the electron transition from the higher excited states, such as
5D3 (24 800 cm−1), 5D2 (21 500 cm−1), and 5D1 (19 100 cm−1),
5
to D0 (17 500 cm−1) becomes feasible by internal conversion,
and most of the photophysical processes take place in this
orbital. Consequently, most europium complexes give rise to
typical Eu(III) emission bands at ∼581, 593, 614, 654, and 702
5
nm corresponding to the deactivation of the excited state D0
to the ground states 7FJ (J = 0−4). Therefore, the energy level’s
match of the triplet state of the ligands to 5D0 of Eu3+ is one of
the key factors which affect the luminescent properties of the
properties of the europium complexes.36
Figure 8. Schematic energy level diagram and energy transfer process
for complex 2. S1, first excited singlet state; T1, first excited triplet
state.
To better investigate the energy transfer process between the
ligand BTP and the neutral ligands, the overlap of the
absorption and photoluminescence spectra of BTP and
bidentate nitrogen donors should be discussed in detail. In a
typical example of complex 2, there is an overlap between the
room temperature emission spectrum of bpy and the
absorption spectra of the BTP in the region 307−372 nm
(Figure 7a). It indicates that the radiative energy that comes
from the singlet state of bpy can in part be absorbed by BTP.
The singlet state of bpy can also transfer energy to the triplet
level of BTP or to its own triplet level, which can be proved
from the overlap between the room temperature emission of
bpy and the phosphorescence spectra of Gd2(BTP)3(H2O)4
and Gd(bpy)2(NO3)3. The singlet level of the BTP can transfer
energy to the triplet state of bpy or to its own triplet level
(overlap between the room temperature emission of BTP and
the phosphorescence spectra of bpy or BTP). The triplet level
of the neutral ligand, bpy, can also transfer energy to the central
triplet levels of the ligand BTP (20 080 cm−1), bpy (21 367
cm−1), and phen (20 709 cm−1) are obviously higher than the
5D0 level (17 500 cm−1) of Eu3+ ion, and their energy gaps
ΔE(3ππ*−5D0) are 2580, 3867, and 3209 cm−1, respectively,
which are too high to allow an effective back energy transfer.
According to Latva’s empirical rule,33 an optimal ligand-to-
metal energy transfer process for Eu3+ needs the energy gap
ΔE(3ππ* − 5D0) > 2500 cm−1; thus the ligand-to-metal transfer
process could occur effectively for complexes 1−3. At the same
4
time, the G5/2 level of the Sm3+ ion is 17 924 cm−1, and the
energy gaps ΔE(3ππ*−4G5/2) are 2156, 3443, and 2785 cm−1,
respectively, which are too high to allow an effective back
energy transfer. Moreover, the triplet levels of the ligand BTP
5
(20 080 cm−1) is also higher than the D1 level (19 100 cm−1)
of the Eu3+ ion, thus energy transfer from the triplet state of
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5
BTP to the D1 level, followed by radiationless deactivation to
and Technology Innovation Talents Special Funds Projects
(No. RC2012XK001005).
the 5D0 level. The energy gap between the 1ππ* and 3ππ* states
of BTP, bpy, and phen are 2856, 8533, and 10 210 cm−1,
respectively. According to Reinhoudt’s empirical rule, the
intersystem crossing process becomes effective when ΔE
(1ππ*−3ππ*) is at least 5000 cm−1.37 One can conclude that
the effective intersystem crossing and ligand to metal energy
transfer processes can be found in all complexes 1−3,
confirming that the ligands turn out to be quite efficient in
sensitizing the Eu3+ ion luminescence.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic data for complexes 1, 2, 4, and 5, NMR
spectrum for BTP, ESI-TOF spectrum for BTP, TG-DSC
curves for complexes Eu2 (BTP)3 (H2 O)4 , 2, 3,
Sm2(BTP)3(H2O)4, 5, and 6, PL spectra of complexes 1−3
in the solid state, luminescence decay profiles of complexes 1−
6, and additional figures. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: houyj@hlju.edu.cn; sunzz@hlju.edu.cn.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We are grateful for financial support by National Natural
Science Foundation of China (No. 50975058), Heilongjiang
Province (Nos. B201207 and B201208), the Education
Department of Heilongjiang Province of China
(Nos.12511383 and 12521413), and Harbin Municipal Science
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