Structural and photophysical studies on geometric (Er2Yb 2/Yb2Er2) and configurational (EuTb 3/Eu3Tb) isomers of heterotetranuclear lanthanide(III) complexes

Article abstract of DOI:10.1021/cg3015546

Heterotetranuclear geometrical (Er₂Yb₂/Yb ₂Er₂) and configuational (EuTb₃/Eu ₃Tb) isomeric lanthanide(III) complexes have been synthesized and characterized by spectroscopy as well as X-ray

Full text of DOI:10.1021/cg3015546

Article
pubs.acs.org/crystal
Structural and Photophysical Studies on Geometric (Er2Yb2/Yb2Er2)
and Configurational (EuTb3/Eu3Tb) Isomers of Heterotetranuclear
Lanthanide(III) Complexes
Hai-Bing Xu,*,†,‡ Jian-Guo Deng,‡ Li-Yi Zhang,† and Zhong-Ning Chen*,†
†State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou, Fujian 350002, China
‡New Materials R&D Center, Institute of Chemical Materials, China Academy of Engineering Physics, Chengdu, Sichuan 621900,
China
S
* Supporting Information
ABSTRACT: Heterotetranuclear geometrical (Er2Yb2/Yb2Er2) and configuational
(EuTb3/Eu3Tb) isomeric lanthanide(III) complexes have been synthesized and char-
acterized by spectroscopy as well as X-ray crystallography. The geometric Er2Yb2/
Yb2Er2 isomers exhibit dual emissions from both erbium(III) and ytterbium(III) ions.
For the EuTb3/Eu3Tb configurational isomers, the TbIII subunits transfer energy to the
EuIII centers in the EuTb3 complex, whereas the TbIII ion in the TbEu3 complex serves
mainly as a structural stabilizer.
emissions (including near-infrared and visible emissions) sen-
sitized by another lanthanide subunit have been less explored in
heterometallic lanthanide(III) complexes.8a
Recently, we established a feasible synthetic approach to
access hybrid lanthanide materials using potential bridging
ligands. For instance, 8-hydroxyquinoline (Hq) and its derivatives
display various coordination modes including chelating (I), bridg-
ing with μ-phenol (II), and chelating-bridging with μ- (III) and
μ3-phenoxo modes (IV) (Scheme 1). Reactions of bis(2-methyl-
INTRODUCTION
■
Because of the forbidden Laporte rule for the lanthanide(III)
ions, introduction of a sensitizer with a high extinction coeffi-
cient as light-harvesting antenna is usually necessary for achiev-
ing highly efficient lanthanide luminescence.1−3 So far, antenna
chromophores have included organic ligands,4,5 organometallic
chromophores,5−7 and lanthanide subunits.8−10 The multifunc-
tional luminescent lanthanide materials with the first two classes of
sensitizers have been extensively investigated in the literature and
have found many applications in immunoassays,11 dual-imaging
agents,12 DNA13 and metal-ion14 recognition, singlet oxygen15
and fluoride4,16 luminescence probes, electronic luminescence
materials,17 and single-molecule magnets.18 Heterometallic com-
plexes containing two or more different lanthanide ions are parti-
cularly attractive because of their potential to achieve dual emis-
sions and up-conversion, which are useful in the dual detection of
analytes combined with magnetic resonance imaging (MRI) or
X-ray techniques for luminescence imaging in cells,19−21 multiple
signal detection,22,23 barcoded materials,24 and bioimaging nano-
probes,25 among others. Nevertheless, because of the similar coor-
dination preferences together with a smooth and monotonic
contraction from La to Lu (from 1.216 to 1.032 Å), it is very
challenging to control the synthesis of such heterometallic lantha-
nide complexes.8 Although there are few reports on such f−f′
mixed-metal arrays (excluding statistical crystallization9), the syn-
thetic routes are quite complicated,8 and none of the hetero-
metallic lanthanide(III) arrays have been characterized unambigu-
ously by X-ray crystallography.8 Additionally, studies on lanthanide
Scheme 1. Coordination Modes of q−
8-hydroxyquinolinato)aluminum(III) [ Al(Mq)2] or/and tetra-
meric bis(8-hydroxyquinolinato) zinc(II) [(Znq2)4] with the Ln-
(β-diketonate) subunit afforded Al3Ln24 or Zn2Ln26 complexes.
These p−f or d−f heteronuclear complexes exhibit typical white-
light emission because of the complementary light-emitting colors
from both the residual luminescence of Al(Mq)2 or Znq2 and the
sensitized emission of the Eu(III) subunit.
Received: October 23, 2012
Revised: November 15, 2012
Published: December 6, 2012
© 2012 American Chemical Society
849
dx.doi.org/10.1021/cg3015546 | Cryst. Growth Des. 2013, 13, 849−857

Crystal Growth & Design
Article
ethanol to give 1410 mg (84%) of NH4L as an off-white solid, which
was used without further purification. mp: 270−275 °C. ESI-MS
(DMSO): m/z 320 [M−NH4]−.
Taking advantage of the various coordination modes of
8-hydroxyquinolinato (Scheme 1) and acetate (Scheme S1,
Supporting Information), we have been able to establish a
convenient and feasible synthetic route to access geometric and
configurational f−f′ mixed metal isomers. Herein, we describe the
two f−f′ heterometallic geometric isomers {Erq2(hfac)}2[(μ3-
OH)]2[Ybq(hfac)]2 (Er2Yb2, 1) and {Ybq2(hfac)}2[(μ3-OH)]2-
[Erq(hfac)]2 (Yb2Er2, 2), together with the two configurational
isomers {Eu(CH3COO)3}{Tb(L)(hfac)2}3 (EuTb3, 3) and
Tris(4-nitrophenyl-2,2′-bipyridine-6-carboxylato) Lantha-
nide Complexes (3a and 4a). An aqueous (5 mL) solution of
Ln(OTf)3 (0.1 mmol) was added dropwise to an aqueous (80 mL)
solution of NH4L (101 mg, 0.3 mmol) with stirring. A precipitate
immediately formed with stirring at room temperature for 3 h. The
precipitate was filtered, washed with water and ethanol, and then dried.
3a (EuL3). Anal. Calcd for C51H30EuN9O12·3H2O: C, 52.50; H,
3.11; N, 10.80. Found: C, 52.18; H, 3.03; N, 10.69. IR (KBr, cm−1):
1653 s (CO). Yield: 98%.
{Tb(CH3COO)3}{Eu(L)(hfac)2}3 (TbEu3, 4), where q−
=
8-hydroxyquinolinato, hfac− = hexafluoroacetylacetonate, and
L− = 4-nitrophenyl-2,2′-bipyridine-6-carboxylato. As revealed
by photophysical studies, Er2Yb2/Yb2Er2 geometric isomers
exhibit typical dual emissions from both erbium and ytterbium
ions. For LnLn′3 configurational isomers, the TbIII subunits trans-
fer energy to the EuIII center in the EuTb3 complex, whereas the
TbIII ion in the TbEu3 array acts mainly as a structural stabilizer.
4a (TbL3). Anal. Calcd for C51H30TbN9O12·3H2O: C, 52.19; H,
3.09; N, 10.74. Found: C, 52.65; H, 2.97; N, 10.42. IR (KBr, cm−1):
1653 s (CO). Yield: 95%.
EuTb3 (3) and Eu3Tb (4) Complexes. 3a or 4a and 3 equiv of
Ln(hfac)2(CH3COO)(H2O)2 (Ln = Tb, 3; Eu, 4) were added to
dichloromethane with stirring, giving a clear solution. After being
filtered, the concentrated dichloromethane solutions were layered with
n-hexane to afford the products as pale yellow crystals.
3 (EuTb3). Yield: 78%. Anal. Calcd for C87H45EuF36N9O30Tb3·-
3H2O·0.5C6H14: C, 34.80; H, 1.88; N, 4.06. Found: C, 34.56; H, 1.85;
N, 3.93. ICP analysis showed that the molar ratio of Tb3+ to Eu3+ was
9.06:2.74, corresponding to 22.5% Eu3+ of the total metal composition. ESI-
MS (CH3OH−CH2Cl2, m/z): ESI-MS (CH3OH−CH2Cl2, m/z): 1455
([M − 2CH3COO− + H2O]2+), 1463 ([M − 2CH3COO− + 2H2O]2+),
1473 ([M − 2CH3COO− + 3H2O]2+). IR (KBr, cm−1): 1651 s (CO).
4 (TbEu3). Yield: 75%. Anal. Calcd for C87H45TbF36N9O30Eu3·-
C6H14·CH2Cl2: C, 35.66; H, 1.94; N, 3.98. Found: C, 35.87; H, 1.86;
N, 3.98. ICP analysis showed that the molar ratio of Tb3+ to Eu3+ was
3.07:9.92, corresponding to 75.5% Eu3+ of the total metal composi-
tion. ESI-MS (CH3OH−CH2Cl2, m/z): 2692 ([M − L + H2O]+). IR
(KBr, cm−1): 1651 s (CO).
EXPERIMENTAL PROCEDURES
■
Material and Reagents. All manipulations were performed under
a dry argon atmosphere using Schlenk techniques and a vacuum-line
system. The solvents were dried, distilled, and degassed prior to use,
except those for spectroscopic measurements, which were of spec-
troscopic grade. Lanthanide oxide, 8-hydroxyquinoline (Hq), 4-nitro-
benzaldehyde (alfa), sodium pyruvate, and hexafluoroacetylacetone
(Hhfac) were obtained from commercial sources. Ln(hfac)3(H2O)2,
Ln(hfac)2(CH3COO)(H2O)2 (Ln = Nd, Eu, Tb, Yb), and Lnq3·H2O
were prepared by the literature procedures.16,26,28c
[{Ln(q)2(hfac)}2(μ3-OH)2{Ln′(q)(hfac)}2] (Ln = Er, Yb; Ln′ = Yb,
Er). Lnq3·H2O (Ln = Er, 1; Yb, 2) and equimolar Ln′(hfac)3(H2O)2
(Ln′ = Yb, 1; Er, 2) were added to dichloromethane with stirring for
30 min, giving a clear solution. After filtration, the concentrated dichloro-
methane solutions were layered with n-hexane to afford the products as
yellow crystals.
GdTb3. This compound was prepared by the same procedure as
used for 3 but with GdL3 and 3 equiv of Tb(hfac)2(CH3COO)(H2O)2
for reaction. Yield: 70%. Anal. Calcd for C87H45GdF36N9O30Tb3·-
3H2O: C, 34.06; H, 1.68; N, 4.11. Found: C, 34.03; H, 1.65; N, 4.13. R
(KBr, cm−1): 1653 s (CO).
1 (Er2Yb2). Yield: 78%. Anal. Calcd for C74H42Er2F24N6O16Yb2·H2O:
C, 36.64; H, 1.83; N, 3.46. Found: C, 36.91; H, 1.78; N, 3.18. ICP
analysis showed that the molar ratio of Yb3+ to Er3+ was 7.85:7.29,
corresponding to 47.3% Er3+ of the total metal composition. ESI-MS
(CH3OH−CH2Cl2, m/z): 995 ([M − 2hfac]2+). IR (KBr, cm−1): 2855
m, 2929 m (H−O), 1655 s (CO).
LaEu3. This compound was prepared by the same procedure as used
for 3 but with LaL3 and 3 equiv of Eu(hfac)2(CH3COO)(H2O)2.
Yield: 76%. Anal. Calcd for C87H45LaF36N9O30Eu3·3H2O: C, 34.50; H,
1.70; N, 4.16. Found: C, 34.53; H, 1.75; N, 4.17. IR (KBr, cm−1): 1655 s
(CO).
2 (Yb2Er2). Yield: 75%. Anal. Calcd for C74H42Yb2F24N6O16Er2·H2O:
C, 36.64; H, 1.83; N, 3.46. Found: C, 36.71; H, 1.84; N, 3.38. ICP
analysis showed that the molar ratio of Yb3+ to Er3+ was 7.09: 8.03,
corresponding to 52.3% Er3+ of the total metal composition. ESI-MS
(CH3OH−CH2Cl2, m/z): 995 ([M − 2hfac]2+). IR (KBr, cm−1): 2854 m,
2930 m (H−O), 1653 s (CO).
La3Eu. This compound was prepared by the same procedure as used
for 4 but with EuL3·H2O and 3 equiv of La(hfac)2(CH3COO)(H2O)2.
Yield: 78%. Anal. Calcd for C87H45EuF36N9O30La3·3H2O: C, 34.80; H,
1.71; N, 4.20. Found: C, 34.76; H, 1.77; N, 4.15. IR (KBr, cm−1): 1655
s (CO).
Physical Measurements. Elemental analyses (C, H, N) were
carried out on a Perkin-Elmer model 240C elemental analyzer. ICP
data were measured on a Jobin Yvon Ultima2 inductively coupled
plasma OES spectrometer. Electrospray ion mass spectroscopy (ESI-
MS) was performed on a Finnigan LCQ mass spectrometer using
dichloromethane and methanol mixture as the mobile phase. UV−vis
absorption spectra were measured on a Perkin-Elmer Lambda 25 UV−
vis spectrophotometer. Infrared (IR) spectra were recorded on a
Magna750 FT-IR spectrophotometer with KBr pellets. Emission and
excitation spectra in the UV−vis region were recorded on a Perkin-
Elmer LS 55 luminescence spectrometer with a red-sensitive photo-
multiplier-type R928 detector. Emission lifetimes in the solid state and
in degassed solutions were determined on an Edinburgh Analytical
Instrument (F900 fluorescence spectrometer) using an LED laser at
397-nm excitation, and the resulting emission was detected by a
thermoelectrically cooled Hamamatsu R3809 photomultiplier tube.
The instrument response function at the excitation wavelength was
deconvolved from the luminescence decay. Near-infrared (NIR) emis-
sion spectra were measured on an Edinburgh FLS920 fluorescence
spectrometer equipped with a Hamamatsu R5509-72 supercooled
photomultiplier tube at 193 K and a TM300 emission monochromator
with an NIR grating blazed at 1000 nm. The NIR emission spectra
Gd2Yb2. This compound was prepared by the same procedure as
used for 1 but with Gdq3·H2O and equimolar Yb(hfac)3(H2O)2. Yield:
72%. Anal. Calcd for C74H42Gd2F24N6O16Yb2·H2O: C, 36.95; H, 1.84;
N, 3.49. Found: C, 37.01; H, 1.89; N, 3.58. IR (KBr, cm−1): 2855 m,
2929 m (H−O), 1655 s (CO).
Yb2Gd2. This compound was prepared by the same procedure as
used for 2 but with Ybq3·H2O and equimolar Gd(hfac)3(H2O)2. Yield:
70%. Anal. Calcd for C74H42Yb2F24N6O16Gd2·H2O: C, 36.95; H, 1.84;
N, 3.49. Found: C, 36.99; H, 1.85; N, 3.44. IR (KBr, cm−1): 2855 m,
2929 m (H−O), 1655 s (CO).
Ammonium 4-Nitrophenyl-2,2′-bipyridine-6-carboxylate
(NH4L). To a 500 mL round-bottom flask were added an ethanol
(200 mL) solution of 4-nitrobenzaldehyde (2.42 g, 16 mmol) and an
aqueous (30 mL) solution of sodium pyruvate (2.64 g, 24 mmol). The
mixture was neutralized with HCl with stirring at 298 K for two days,
giving (E)-4-(4-nitrophenyl)-2-oxobut-3-enoic acid as a pale yellow
precipitation. (E)-4-(4-Nitrophenyl)-2-oxobut-3-enoic acid (1.11g,
5 mmol), 2-pyridacylpyridinium iodide (1.63g, 5 mmol), and ammo-
nium acetate (8.1 g, 105 mmol) were then added to 100 mL of
methanol. The solution was heated under reflux for 1 day to produce
an off-white precipitate. After being allowed to cool to room tem-
perature, the precipitate was filtered and washed with water and
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Article
Table 1. Crystallographic Data of 1−4
1·H2O
2·H2O
3·3H2O·6C6H14
4·3C6H14
empirical formula
formula weight
space group
a (Å)
C74H44Er2F24N6O17Yb2
C74H44Yb2F24N6O17Er2
2425.76
P2(1)/n
12.969(4)
24.709(3)
13.227(3)
90
C123H135EuF36N9O33Tb3
3580.12
C105H87Eu3F36N9O30Tb
3253.64
2425.76
P2(1)/n
13.1738(8)
24.5677(11)
13.2235(7)
90
R3
̅
R3
̅
24.8037(6)
24.8037(6)
42.7922(19)
90
24.8541(7)
24.8541(7)
42.446(4)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
100.718(3)
90
101.432(10)
90
90
90
120
120
V (Å3)
4205.1(4)
2
4154.6(16)
2
22799.6(13)
6
22707(2)
6
Z
ρcalc (g/cm−3
)
1.925
1.937
1.564
1.428
μ (mm−1
)
4.299
4.351
1.900
1.793
radiation (λ, Å)
0.71073
293(2)
0.0379
0.1020
1.059
0.71073
293(2)
0.71073
293(2)
0.0598
0.1717
1.089
0.71073
293(2)
0.0515
0.1382
1.061
temp (K)
a
R1(Fo)
0.0562
b
2
wR2(Fo )
0.1514
GOF
1.065
a
b
2
2
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = [∑w(Fo − Fc2)2/∑w(Fo )2]1/2
.
were corrected using a calibration curve supplied with the instru-
ment. The emission quantum yields (Φ) of 3 and 4 in degassed
dichloromethane solutions at room temperature were calculated as
Φs = Φr(Br/Bs)(ns/nr)2(Ds/Dr) using [Ru(bpy)3]Cl2 in acetonitrile as
the standard (Φem = 0.062), where the subscripts r and s denote
reference standard and the sample solution, respectively, and n, D, and
Φ are the refractive index of the solvent, the integrated intensity, and
the luminescence quantum yield, respectively. The quantity B was cal-
culated by B = 1 − 10−AL, where A is the absorbance at the excitation
wavelength and L is the optical path length.10 All solutions used for the
determination of emission lifetimes and quantum yields were prepared
under a vacuum in a 10 cm3 round-bottom flask equipped with a side-
arm 1-cm fluorescence cuvette and sealed from the atmosphere by a
quick-release Teflon stopper. Solutions used for luminescence deter-
mination were prepared after rigorous removal of oxygen by three
successive freeze−pump−thaw cycles.
Crystal Structural Determination. Single crystals of 1·H2O,
2·H2O, 3·3H2O·6C6H14, and 4·3C6H14 suitable for X-ray diffraction
were grown by layering n-hexane onto the dichloromethane solutions.
Crystals coated with epoxy resin or sealed in capillaries with mother
liquors were measured on a Siemens SMART CCD diffractometer by the
ω-scan technique at room temperature using graphite-monochromated
Mo Kα radiation (λ = 0.71073 Å). Lp and absorption corrections were
applied in the reflection reduction process. The structures were solved
by direct methods, and the heavy atoms were located from E-map. The
remaining non-hydrogen atoms were determined from the successive
difference Fourier syntheses. The non-hydrogen atoms were refined
anisotropically except for the F atoms, and the hydrogen atoms were
generated geometrically with isotropic thermal parameters. The struc-
tures were refined on F2 by full-matrix least-squares methods using the
SHELXTL-97 program package. For 1−4, the refinements were
carried out by fixing the C−F distances (1.32 0.01 Å) with the occu-
pancy factor of each Fn and F′n being 0.5. Crystallographic data of 1−4
are summarized in Table 1. The unit cell contains 36 hexane and 18
hydrate molecules for 3 and 18 hexane molecules for 4, which were
treated as a diffuse contribution to the overall scattering without speci-
fic atom positions by SQUEEZE/PLATON.
dichloromethane solutions were layered with n-hexane to afford
geometric isomers of heterotetranuclear lanthanide(III) com-
plexes as yellow crystals. ICP data indicated that the molar
ratios of ErIII to YbIII in 1 and 2 are about 1:1.08 and 1.13:1,
respectively. ESI-MS (CH3OH−CH2Cl2) studies revealed the
presence of positive ion fragments 995 ([M − 2hfac]2+)
(Figures S1 and S2, Supporting Information). On one hand, as
8-hydroxyquinolinato (q−) in the precursor Lnq3·H2O exhibits
chelating coordination mode (I). The unsaturated coordinated
q− has potential bonding ability, inducing a further binding to
Ln′(hfac)3(H2O)2. On the other hand, as Lnq3·H2O was iso-
lated from a basic aqueous solution with pH ≈ 9 whereas
Ln′(hfac)3(H2O)2 was isolated from an acidic solution with
pH ≈ 6, a neutralization reaction might occur upon mixing
Lnq3·H2O with Ln′(hfac)3(H2O)2 in dichloromethane solutions.
These are likely the two reasons that make the reaction occur, so
that μ3-OH bridged Er2Yb2/Yb2Er2 isomers were successfully
isolated.
Precursors LnL3 were synthesized by the reaction of NH4L
with Ln(OTf)3 in aqueous solutions. The EuTb3/Eu3Tb con-
figurational isomers were prepared by the reaction of LnL3 with
3 equiv of Ln′(hfac)2(CH3COO)(H2O)2 in dichloromethane
solutions until the solution became clear. After filtration, crys-
tallization by layering n-hexane onto the concentrated dichloro-
methane solutions afforded the configurational heterotetranu-
clear lanthanide isomers as pale yellow crystals. As the car-
boxylates in the precursor LnL3 exhibit monodentate coordina-
tion, this implies that the COO− group has further coordination
(Scheme S1, Supporting Information) capability to link Ln′-
(hfac)2(CH3COO)(H2O)2 so as to isolate LnLn′3 heterote-
tranuclear complexes. ICP data indicated that the molar ratios
of EuIII to TbIII in 3 and 4 are 1:3.30 and 3.23:1, respectively.
The complexes were all characterized by elemental and ICP
analyses, IR spectroscopy, ESI-MS (Figures S3 and S4, Supporting
Information), and X-ray crystallography. The crystallographic data
of 1·H2O (CCDC 808800), 2·H2O (CCDC 808803),
3·3H2O·6C6H14 (CCDC 808801), and 4·3C6H14 (CCDC
808802) are summarized in Table 1.
RESULTS AND DISCUSSION
■
Syntheses. The synthetic routes of heterometallic lantha-
nide isomers are shown in Scheme 2. The geometric isomers
1 and 2 were prepared by mixing equimolar Lnq3·H2O and
Ln′(hfac)3(H2O)2 in dichloromethane solutions with stirring
until the solutions became clear. After filtration, the concentrated
Crystal Structures. Selected bond lengths and angles of 1−
4 are presented in Table 2. As 1 and 2 are geometric isomers,
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Crystal Growth & Design
Article
Scheme 2. Synthetic Routes to Lanthanide Isomers 1−4
Table 2. Selected Bond Distances (Å) and Angles (deg) for Compounds 1·H2O, 2·H2O, 3·3H2O·6C6H14, and 4·3C6H14
1·H2O
2·H2O
3·3H2O·6C6H14
4·3C6H14
Er1−O1
Er1−O2
Er1−O3
Er1−O4(H)
Er1−N1
2.334(4)
2.293(4)
2.348(4)
2.284(4)
2.504(5)
2.464(6)
2.335(4)
2.367(4)
2.324(4)
2.300(3)
2.484(5)
3.7782(4)
3.4587(4)
95.60(14)
95.83(15)
107.94(16)
111.04(15)
96.76(14)
108.09(14)
Er1−O1
Er1−O2
Er1−O3
Er1−O4(H)
Yb1−N1
Yb1−N2
Yb1−O1
Yb1−O2
Yb1−O3
Yb1−O4(H)
Er1− N3
Er1−Yb1
Er1−Yb1′
Yb1−O1−Er1′
Er1−O2−Yb1′
Er1−O3−Yb1
Yb1−O4(H)−Er1
Yb1−O4(H)−Er1′
Er1−O4(H)−Er1′
2.368(7)
2.326(7)
2.315(7)
2.298(7)
2.461(9)
2.493(10)
2.296(7)
2.353(7)
2.344(8)
2.289(7)
2.483(9)
3.7755(11)
3.4545(7)
95.6(2)
Tb1−O1
2.484(4)
2.426(4)
2.357(4)
2.598(5)
2.529(4)
Eu1−O1
Eu1−O2
Eu1−O7
Eu1−N1
Eu1−N2
2.495(3)
2.435(4)
2.370(4)
2.613(4)
2.549(4)
Tb1−O2′
Tb1−O7
Tb1−N1
Tb1−N2
Er1−N2
Yb1−O1′
Yb1−O2′
Yb1−O3
Yb1−O4(H)
Yb1−N3
Eu1−O1
Eu1−O7
Eu1−O8
2.467(3)
2.448(4)
2.432(4)
Tb1−O1
Tb1−O7
Tb1−O8
2.482(3)
2.459(4)
2.436(4)
Er1−Yb1
Tb1−Eu1
4.0196(3)
Tb1−Eu1
4.038
Er1−Yb1′
Er1−O1−Yb1′
Er1−O2−Yb1′
Er1−O3−Yb1
Er1−O4(H)−Yb1
Er1−O4(H)−Yb1′
Yb1−O4(H)−Yb1′
Eu1−O1−Tb1
Tb1−O7−Eu1
108.56(13)
113.56(15)
Tb1−O1−Eu1
Eu1−O7−Tb1
108.42(12)
113.46(14)
95.2(3)
108.3(3)
110.8(3)
96.6(3)
108.4(3)
only the structure of 1 is described here (Figure 1). In 1, the
Er2Yb2 tetranuclear cluster can be viewed as a combination of
two Er(q)2(hfac) units and two Yb(q)(hfac) components
linked by two μ3-OH−. The Yb···Yb distance is 3.76 Å, and the
Er···Er distance is 6.19 Å. The Er···Yb separations are 3.78 and
3.46 Å. As shown in Figure 1b, it is intriguing that the Er2Yb2
structure affords two incomplete cubanes that share a common
plane built by Yb, O4 (μ3-OH−), Yb′, and O4′ (μ3-OH−).
In 1, two ErIII ions exhibit eight-coordinated environments from
one hfac−, two chelating 8-hydroxyquinolinato, one μ-phenoxo of
8-hydroxyquinolinato, and one μ3-OH−, forming a square antiprism
geometry. The two YbIII ions are also eight-coordinated from
one hfac−, one chelating 8-hydroxyquinolinato, two μ-phenoxos
of 8-hydroxyquinolinato, and two μ3-OH−, also giving a dis-
torted square-antiprism. The Er−O3′ [2.315(7) Å] and Yb−O3
[2.344(8) Å] distances for μ-phenoxo of 8-hydroxyquinolinato
are much longer than the Er−O4 [2.298(7) Å] and Yb−O4
[2.289(7) Å] distances for μ3-OH−. The Er′−O3−Yb bond
angle [108.3(3)°] is larger than that of Er−O4−Yb [96.6(3)°].
The 8-hydroxyquinolinato exhibits a chelating-bridging mode
as shown in Scheme 1 (mode III). The bridging μ3-OH caps
ErYb2 atoms, with Er(Yb)−O distances of 2.284(4), 2.299(4),
and 2.343(4) Å and Er(Yb)−O−Yb angles in the range of
96.7−110.1°.
As 3 and 4 are configurational isomers, only the structure of
3 is described here. In 3 (Figure 2), the EuTb3 tetranuclear
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Figure 1. (a) Perspective view of the asymmetric unit of 1 with atomic
labeling scheme. (b) View of 1 showing two incomplete cubanes that
share a common plane. The F atoms on the trifluoromethyl are
omitted for clarity.
cluster can be viewed as a fan structure composed of three
Tb(hfac)2(L) fragments linked by the central EuIII ion through
μ3-acetate coordination mode. The EuTb3 core is arranged into
a distorted triangular pyramid with three TbIII ions in the basal
plane and the EuIII ion at the apex. The three TbIII atoms
are nine-coordinated with two hfac−, one chelating L−, one
μ3-CH3COO−, and one μ3-COO− from another bridging L−,
affording a distorted capped square antiprism. The apical EuIII atom
is also nine-coordinated to oxygen atoms from three chelating
CH3COO− groups and three μ3-COO− groups from the bridging
L−, to give a distorted capped square antiprism. Although the EuIII
atom in the precursor EuL3 is also nine-coordinated to three
oxygen atoms and six nitrogen atoms from L−, it is likely that the
better coordination capability of oxygen than nitrogen atoms to
the lanthanide(III) ion induces the structural translation to occur.
The μ3-CH3COO− adopts a chelating-bridging bonding mode
so as to link three Tb(hfac)2(L) fragments with central EuIII
ion. The bridging L− is also featured with a μ3-COO− mode to
link EuIII and TbIII ions into a symmetric fashion with Tb···Tb
and Eu···Tb distances across μ3-COO− of 6.522(10) and 4.016(9) Å,
respectively.
Figure 2. (a) Perspective view of the asymmetric unit of 3 with atomic
labeling scheme. (b) View of 3 showing a fan structure composed of
three Tb(hfac)2(L) fragments linked by the central EuIII ion through
μ3-acetate. The F atoms on the trifluoromethyl are omitted for clarity.
well-known that the hexafluoroacetylacetonato (hfac−) ion can
be easily substituted with external donor atoms.29 On the other
hand, the molar ratio of ErIII to YbIII ions established by ICP is
about 1:1. All of these facts indicate that the lanthanide(III)
ions with two chelating hydroxyquinolinato ligands are from
the Lnq3·H2O precursors.
Similarly, as configurational isomers 3 (EuTb3) and 4 (Eu3Tb)
are hexagonal with the R3 space group, the asymmetric unit of the
̅
complex contains only two lanthanide ions with Ln1/3Ln′ for 3 or
LnLn′1/3 for 4. On one hand, the same coordination environment
of the three lanthanide(III) ions in the basal plane is different
from that of the apex lanthanide(III) ion. On the other hand, the
molar ratio of EuIII to TbIII ions established by ICP is about
1:3 for 3 and 3:1 for 4. All of these facts suggest that the three
metal ions in the basal plane are the same and from the
As the geometric isomers (1 and 2) are monoclinic with the
space group of P2(1)/n (Table 1), the asymmetric unit of the
complex contains only two lanthanide ions with Ln′Ln array
(Figure 1a). On one hand, the lanthanide-β-diketonate complex
is less stable than the lanthanide-quinolinolate complex, as it is
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precursor Tb/Eu(hfac)2(CH3COO)(H2O)2, whereas the
apex ion is another lanthanide(III) ion from the EuL3/TbL3
precursor.
Additionally, as shown in Table 1 and Figure 3a, the geo-
metric isomer’s crystal contains some organic solvents. If the
organic solvent has some interactions with Eu3+, the correspond-
ing signals are also shifted to low field. Interestingly, the signals of
the ethanol (quartet) are coincident with those of hfac− in LaEu3
(Figure 3a, bottom), so as to combine the two kinds of signals
1H NMR Spectrometry. To further confirm the structures,
we resorted to 1H NMR spectroscopy to determine the relative
numbers of La2Eu2 and Eu2La2 or Eu3La and La3Eu pairs. For
1
1
the predictability of europium’s H NMR spectra, Eu3+ was
together, inducing the H NMR profiles and the integral areas
used as a shift reagent. Paramagnetic Eu3+ together with dia-
with apparent discrepancy with respect to expectations. In turn,
the mixed signals of solvent and CH3COO− in LaEu3 could also
be separated by the shift reagent Eu3+ in EuLa3 (Figure 3a, top),
resulting the unexpected 1H NMR profiles and the integral areas.
As both hfac− and 8-hydroxyquinolinato are chelated to the
1
magnetic La3+ would help to confirm the structures. The H
NMR spectra of the two pairs of complexes are shown in Figure 3
and Figures S5 and S6 (Supporting Information).
1
La3+ and Eu3+ ions, the difference in the H NMR spectra of
La2Eu2 and Eu2La2 is the integral area of 8-hydroxyquinolinato
chelating to the Eu3+ ions. As shown in Figure 3b, the 1H NMR
spectra of hfac− are split into two bands (3.47 and 3.54 ppm)
with similar integral areas, indicating that the same number
hfac− groups are chelated to the La3+ and Eu3+ ions in La2Eu2
and Eu2La2. Meanwhile, there are two sets of 8-hydroxyquino-
linato signals in both La2Eu2/Eu2La2 complexes. One is observed
at ca. 6.93, 7.02, 7.97, 9.98, and 11.2 ppm, and the other appears
at 7.11, 7.54, 8.72, 10.2, and 11.31 ppm, respectively. The integral
area of the first set is about half that of the second group in
complex La2Eu2, whereas that of the first group is twice that of
the second in complex Eu2La2, implying that the second group
corresponds to the proton signals of 8-hydroxyquinolinato
1
chelated to the Eu3+ ions. Thus, H NMR studies further con-
firm that the structures are consistent with those revealed by
X-ray crystallography.
Photophysical Properties. The UV−vis absorption and
luminescence data of 1−4 are summarized in Table 3. 1 and 2
(Figure S7, Supporting Information) display mainly two sets of
absorption bands, one from the ligand-centered transition of
q− at ca. 245 nm (for 1) or 257 nm (for 2) and the other at ca.
310 nm due to intraligand π−π* transition of hfac−.6,28,30 In
addition, a broad low-energy absorption trailing to 420 nm arises
probably from the [n → π*] (q−) transitions.27,31 In the elec-
tronic spectra of 3 and 4 (Figure S7, Supporting Information),
the absorption band at ca. 285 nm is due to the intraligand transi-
tion of L−, and the one at ca. 305 nm is also from hfac−.6,28,30
Upon excitation at 250 nm < λex < 420 nm, 1 and 2 display
typically dual luminescence at ca. 1000 nm from YbIII ion and
ca. 1535 nm from ErIII ion in both the solid state and dichloro-
methane solution (Figure 4 and Figure S8, Supporting Information)
at ambient temperature. As the precursor Ln(hfac)3(H2O)2 is
nonemissive upon excitation beyond 350 nm,6,28,30 the lantha-
nide emission in 1 and 2 should be sensitized by energy trans-
fer from 8-hydroxyquinolinato with near-visible irradiation at
350 nm < λex < 420 nm. Because YbIII ion can serve as a sen-
sitizer of ErIII, substantial energy transfer is operating from the
YbIII chromophore to the ErIII subunit,1 as the lifetime of resi-
dual YbIII emission is 1.26 μs in 1 and 1.55 μs in 2 at ambient
temperature. The Yb → Er energy-transfer rates kET can thus be
estimated from the equation6,7,28,30
1
Figure 3. Part of the H NMR spectra highlighting the proton signals
of (a) hfac− and acetate for LaEu3/EuLa3 and (b) 8-hydroxyquino-
linato and hfac− for La2Eu2/Eu2La2 in CDCl3 at 400 MHz.
According to the crystal structures of Er2Yb2/Yb2Er2, the
differences between the pair La2Eu2/Eu2La2 are the numbers of
8-hydroxyquinolinato chelating to the Eu3+ ions. As for LaEu3
and EuLa3, the differences in chemical environments are the
chelating ligands around Eu3+. For LaEu3, six hfac− groups are
bound to the Eu3+ ions, whereas three acetates are cheated to
the Eu3+ ion in EuLa3. Taking into account hfac− as the internal
standard substance, we could find the shift tendency with
Eu3+ as the shift reagent. Therefore, we highlight part of the 1H
NMR spectra on hfac− and 8-hydroxyquinolinato in the pair of
La2Eu2/Eu2La2 together with hfac− and acetate in LaEu3/EuLa3
to distinguish the chemical environment of Eu3+.
kET = 1/τ − 1/τ0
(1)
As shown in Figure 3a, the chemical shift of acetate chelating
to La3+ is about 2.07 ppm in complex LaEu3. Once the acetate
is chelated to the shift reagent Eu3+ in the complex EuLa3, the
corresponding signal is shifted to low field at around 2.51 ppm.
In the same way, the signal of hfac− chelating to the La3+ ion at
3.58 ppm in EuLa3 is also low-field-shifted to 3.78 ppm upon
hfac− binding to the shift reagent Eu3+ in LaEu3.
where τ is the lifetime of residual Yb-based emission in 1 and 2
species and τ0 is the lifetime in the reference Gd2Yb2 (1.94
0.03 μs) and Yb2Gd2 (2.64 0.05 μs) complexes, which lack
the corresponding energy transfer. The energy-transfer rates (kET)
can thus be calculated as kET = 1/τEr Yb − 1/τGd Yb = (2.8 0.4) ×
2
2
2
2
105 s−1 for 1 and kET = 1/τYb Er − 1/τYb Gd = (2.7 0.4) × 105 s−1
2
2
2
2
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O−H···F interactions could suppress the vibrational nonradia-
tive processes, so as to improve the lanthanide(III) luminescence
from f−f′ transitions.4,16 As anticipated, titration of 1 and 2
with [Bu4N]F in aerated dichloromethane solutions indeed
induced a marked enhancement of the lanthanide(III) lumines-
cence. As shown in Figure 5, addition of 2 equiv of F− to the
Table 3. Absorption and Luminescence Data of 1−4
λem/nm
a
λabs/nm
(τem
)
at
b
compound medium
(ε/M−1 cm−1
)
298 K
Φem × 10−3
d
1 (Er2Yb2) CH2Cl2 234 (190400),
1000 (1.26
0.63
256 (153500),
304 (55900),
371 (13900)
0.05 μs),
d
1535(1.13
0.07 μs)
0.14
Gd2Yb2
CH2Cl2
1000 (1.94
0.03 μs)
d
2 (Yb2Er2) CH2Cl2 239 (119000),
257 (147700),
1000 (1.55
0.78
0.07 μs),
d
307 (51300),
1535(1.09
0.13 μs)
0.13
371 (15500)
Yb2Gd2
CH2Cl2
1000 (2.64
0.05 μs)
3 (EuTb3) Solid
543 (144
0.5 μs),
615 (0.98
0.03 ms)
b
CH2Cl2 284 (109200),
303 (113600)
543 (19.6
0.2 μs),
13
615 (1.22
0.03 ms)
4 (TbEu3) Solid
543 (0.66
Figure 5. Emission spectra of 1 (λex = 333 nm) with different molar
ratios of [Bu4N]F in dichloromethane solution at 298 K. The molar
ratios between 1 and [Bu4N]F were 1:0 (gray), 1:0.2 (black), 1:1
(blue), 1:2 (red), and 1:4 (cyan).
0.01 μs),
615 (0.87
0.01 ms)
c
b
CH2Cl2 284 (100200),
303 (109500)
543 (− ),
27
615 (1.05
0.05 ms)
dichloromethane solution of 1 caused about a 2-fold enhance-
ment in the YbIII emissive intensity. However, when more than
2 equiv of F− was added, the lanthanide(III) emission gradually
attenuated. This is because the lanthanide(III) ions were gra-
dually ligated by excess fluoride so as to destroy the original
structures.4,16
GdTb3
CH2Cl2
543 (28.2
0.8 μs)
a
Excitation wavelengths in lifetime measurements: 400 nm for 1 and 2,
b
340 nm for 3 and 4. Quantum yields of 3 and 4 in degassed
dichloromethane determined relative to that of Ru(bpy)3(PF6)2
c
(Φ = 0.062) in degassed acetonitrile. Undetectable in our instrument.
d
As the molar absorption coefficient at 980 nm for YbIII is
much larger than that for ErIII ion, it is likely that the adjacent
YbIII ions could significantly promote the up-conversion effi-
ciency of ErIII ion. However, such phenomena were not observed
in our cases upon laser irradiation at 980 nm with 460 mW. The
reason might be that the energy from the excited states of 2H11/2
(525 nm) and 4S3/2 (545 nm) to the 4I9/2 level are slightly higher
than that of the triplet excited state of the ligand (q−) (around
17100 cm−1, 585 nm), thus quenching the up-conversion emis-
sion completely. In fact, it is challenging to attain up-conversion
emissions in a single molecule, as both the direct coordination
of μ3-OH− to the lanthanide(III) ions and the presence of
organics can rapidly quench the up-converted emission.4,16,32
Nonetheless, the work presented here does show the potential
for the fabrication of such complex for up-conversion
applications.
Quantum yields of Yb and Er complexes estimated by the equation
Φ
Ln = τobs/τ0, in which τobs is the observed emission lifetime and τ0 is
the radiative or natural lifetime with τ0 = 2 ms for YbIII and 8.26 ms for
ErIII.7 These values refer to the lanthanide-based emission process only
and take no account for the efficiency of intersystem crossing and
energy-transfer processes.
Upon excitation at 250 nm < λex < 380 nm, 3 also typically
displays dual luminescence at ca. 543 nm from TbIII ion and ca.
615 nm from EuIII ion with microsecond to millisecond range
of lifetimes in both the solid state and dichloromethane solu-
tion at ambient temperature (Table 1 and Figure 8). Titration
experiments were performed to examine whether there is a
communication between Tb3+ and Eu3+ centers through photo-
induced energy transfer. As shown in Figure 6, titration of Tb-
(hfac)2(CH3COO)(H2O)2 with EuL3 in dichloromethane solu-
tion induces significant attenuation of the TbIII emission. Con-
versely, titration of EuL3 with Tb(hfac)2(CH3COO)(H2O)2
(Figure S9, Supporting Information) in dichloromethane solu-
tion results in marked enhancement of the EuIII-based emission.
Both titration experiments suggest that the emissive energy of
Figure 4. Emission spectra of 1 (blue) and 2 (red) (λex = 400 nm) in
the solid state at 298 K.
for 2, which are much lower than the rates of d → f energy
transfer using an organometallic chromophore as a near-infrared-
emitting sensitizer of the lanthanide ions.6,28,30a By comparison of
the rates of Yb → Er energy transfer between 1 and 2, it was
found that the Yb → Er energy transfer in 1 is slightly more
efficient than that in 2.
Because of the direct linkage of μ3-OH− to the lanthanide-
(III) ions in 1 and 2, the nonradiative O−H vibration excita-
tion could severely quench the lanthanide(III) luminescence.33
It has been demonstrated that using fluoride to form strong
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Supporting Information), which is attributed to the low-lying
excited states of L− and its inability for transfer energy to the TbIII
centers, suggest a fast thermally activated back energy-transfer
from the EuIII to the TbIII subunit, because some energy levels of
the TbIII ion are too close to the emissive level of the EuIII ion. As
shown in Figure 8, there is almost unobservable emission ascribed
to TbIII ion in the emission spectrum of 4. Thus, the TbIII ion in 4
behaves mainly as a structural stabilizer.
Figure 6. Titration of Tb(hfac)2(CH3COO)(H2O)2 with EuL3 in
dichloromethane solution, showing significant attenuation of the TbIII-based
emission (λex = 335 nm). The molar ratios between Tb(hfac)2(CH3COO)-
(H2O)2 and EuL3 were 3:0 (cyan), 3:0.6 (red), 3:1.2 (blue).
the TbIII subunit is transferred to the EuIII ion in 3, inducing
sensitized EuIII-centered emission with the TbIII subunit as the
antenna chromophore.
The Tb → Eu energy-transfer rate kET was estimated by eq 1,
where τ is the lifetime of residual Tb-based emission in 3 and
τ0 (28.2 0.8 μs) is the lifetime in the reference GdTb3 com-
plex, which lacks energy transfer from the TbIII triplet states.
Figure 8. Emission spectra of 3 (red) and 4 (blue) (λex = 335 nm) in
dichloromethane solution at 298 K.
The energy-transfer rate (kET) can thus be calculated as kET
=
1/τEuTb − 1/τGdTb = (1.6 0.15) × 104 s−1, which is much lower
3
3
CONCLUSIONS
than the rates of d → f energy transfer using organomentallic
chromophores as the sensitizers of EuIII luminescence.6a,30b
Upon direct excitation of the terbium absorption band at
488 nm, however, the emission signal of europium(III) ion at
615 nm was not detected. This phenomenon does not mean
that Tb → Eu energy transfer is not operating. It is likely that
the EuL3 complex absorbs some of the incident light at the
excitation wavelength, and less light is thus available to excite
the Tb complex. To verify this opinion, titration experiments
using the Gd-based complex were carried out. As shown in
Figure 7, titration of EuL3 with Gd(hfac)2(CH3COO)(H2O)2
■
A convenient and feasible synthetic approach has been established
for the specific preparation of geometric and configurational
heterometallic isomers containing different lanthanides by incor-
porating a lanthanide subunit with another lanthanide precursor.
Because of the incomplete energy transfer, the geometric lanthanide
isomers (1 and 2) exhibit dual emissive states arising from both
lanthanide chromophores. For configurational lanthanide isomers,
the TbIII subunits transfer energy to the EuIII centers in 3 (EuTb3),
whereas the TbIII ion in 4 (Eu3Tb) acts mainly as a structural
stabilizer. It is demonstrated that energy-transfer rates with one
lanthanide subunit as the sensitizer for another typical lanthanide
emission in f−f heteronuclear complexes are lower than those with
d-block organometallic chromophores as antennas for sensitized
lanthanide luminescence in d−f heteronuclear complexes. This
synthetic strategy would open up a significant approach for the
design of heterometallic complexes containing two different
lanthanide(III) ions, thus providing new possibilities for choosing
specific metals with attractive properties in developing higher-
performance dual-color probes and up-conversion materials.
ASSOCIATED CONTENT
* Supporting Information
Additional experimental and spectroscopic data together with
X-ray crystallographic files of compounds 1−4. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
Figure 7. Titration of EuL3 with Gd(hfac)2(CH3COO)(H2O)2 in
dichloromethane solutions, showing marked enhancement of the EuIII-
based emission (λex = 335 nm). The molar ratios of EuL3 to
Gd(hfac)2(CH3COO)(H2O)2 were 1:0 (cyan), 1:1 (red), 1:2
(magenta), and 1: 3 (blue).
AUTHOR INFORMATION
Corresponding Author
*E-mail: hbxu0513@yahoo.com.cn (H.-B.X.), czn@fjirsm.ac.cn
(Z.-N.C.).
■
in dichloromethane solution also showed marked enhancement
of the EuIII-based emission. This suggests that the hfac− ligands
also act as partial sensitizers for Eu-based emission in our case.
In the same way, titration of TbL3 with Eu(hfac)2(CH3COO)-
(H2O)2 in dichloromethane solution induces trivial enhancement
of the TbIII-based emission. The reverse titration procedure,
however, leads to a slight attenuation of the EuIII-based emission
and trivial enhancement of the TbIII-based emission (Figures S10
and S11, Supporting Information). These facts together with
undetectable TbIII emission for the precursor TbL3 (Figure S10,
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge financial support from the NSFC (20901077,
20931006, U0934003, and 91122006), the NSF of Fujian Province
(2011J01065), and CAEP (2012B0302039).
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Article
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Chen, Z. N. Dalton Trans. 2011, 40, 5549.
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