Synthesis, crystal structure, and luminescent properties of novel Eu 3+ heterocyclic β-diketonate complexes with bidentate nitrogen donors

Article abstract of DOI:10.1021/ic061425a

New tris(heterocyclic β-diketonato)europium(III) complexes of the general formula Eu(PBI)₃·L [where HPBI = 3-phenyl-4-benzoyl-5- isoxazolone and L = H₂O, 2,2′-bipyridine (bpy), 4,4′-dimethoxy-2,2′-bipyridine (dmbpy), 1,10-phenanthroline (phen), or 4,7-diphenyl-1,10-phenanthroline (bath)] were synthesized and characterized by elemental analysis, Fourier transform infrared spectroscopy (FT-IR), ¹H NMR, high-resolution mass spectrometry, thermogravimetric analysis, and photoluminescence (PL) spectroscopy. Single-crystal X-ray structures have been determined for the complexes Eu(PBI)₃· H₂O·EtOH and Eu(PBI)₃·phen. The complex Eu(PBI)₃·H₂O·EtOH is mononuclear, and the central Eu³⁺ ion is coordinated by eight oxygen atoms to form a bicapped trigonal prism coordination polyhedron. Six oxygens are from the three bidentate HPBI ligands, one is from a water molecule, and another is from an ethanol molecule. On the other hand, the crystal structure of Eu(PBI) ₃·phen reveals a distorted square antiprismatic geometry around the europium atom. The room-temperature PL spectra of the europium(III) complexes are composed of the typical Eu³⁺ red emission, assigned to transitions between the first excited state (⁵D₀) and the multiplet (⁷F_0-4). The results demonstrate that the substitution of solvent molecules by bidentate nitrogen ligands in Eu-(PBI) ₃·H₂O·EtOH richly enhances the quantum yield and lifetime values. To elucidate the energy transfer process of the europium complexes, the energy levels of the relevant electronic states have been estimated. Judd-Ofelt intensity parameters (Ω₂ and Ω₄) were determined from the emission spectra for Eu ³⁺ ion based on the ⁵D₀ - → ⁷F₂ and ⁵D₀ → ⁷F₄ electronic transitions, respectively, and the ⁵D₀ → ⁷F₁ magnetic dipole allowed transition was taken as the reference. The high values obtained for the 4f-4f intensity parameter Ω₂ for europium complexes suggest that the dynamic coupling mechanism is quite operative in these compounds.

Full text of DOI:10.1021/ic061425a

Inorg. Chem. 2006, 45, 10651−10660
+
Synthesis, Crystal Structure, and Luminescent Properties of Novel Eu3
Heterocyclic
â-Diketonate Complexes with Bidentate Nitrogen Donors
S. Biju,† D. B. Ambili Raj,† M. L. P. Reddy,*,† and B. M. Kariuki‡
Chemical Science and Technology DiVision, Regional Research Laboratory, CSIR,
ThiruVananthapuram 695 019, India, and School of Chemical Sciences, UniVersity of Birmingham,
Birmingham, U.K. B15 2TT
Received July 29, 2006
New tris(heterocyclic
â-diketonato)europium(III) complexes of the general formula Eu(PBI)3‚L [where HPBI ) 3-phenyl-
4-benzoyl-5-isoxazolone and
L
)
H2O, 2,2 -bipyridine (bpy), 4,4 -dimethoxy-2,2 -bipyridine (dmbpy), 1,10-
′
′
′
phenanthroline (phen), or 4,7-diphenyl-1,10-phenanthroline (bath)] were synthesized and characterized by elemental
analysis, Fourier transform infrared spectroscopy (FT-IR), 1H NMR, high-resolution mass spectrometry, thermo-
gravimetric analysis, and photoluminescence (PL) spectroscopy. Single-crystal X-ray structures have been determined
for the complexes Eu(PBI)3‚H2O‚EtOH and Eu(PBI)3‚phen. The complex Eu(PBI)3‚H2O‚EtOH is mononuclear, and
the central Eu3+ ion is coordinated by eight oxygen atoms to form a bicapped trigonal prism coordination polyhedron.
Six oxygens are from the three bidentate HPBI ligands, one is from a water molecule, and another is from an
ethanol molecule. On the other hand, the crystal structure of Eu(PBI)3‚phen reveals a distorted square antiprismatic
geometry around the europium atom. The room-temperature PL spectra of the europium(III) complexes are composed
+
of the typical Eu3 red emission, assigned to transitions between the first excited state (5D0) and the multiplet
(7F0 4). The results demonstrate that the substitution of solvent molecules by bidentate nitrogen ligands in Eu-
-
(PBI)3‚H2O‚EtOH richly enhances the quantum yield and lifetime values. To elucidate the energy transfer process
of the europium complexes, the energy levels of the relevant electronic states have been estimated. Judd−Ofelt
+
5
intensity parameters (Ω2 and
Ω
4) were determined from the emission spectra for Eu3 ion based on the D0 f 7F2
and D0 f 7F4 electronic transitions, respectively, and the D0 f 7F1 magnetic dipole allowed transition was taken
5
5
as the reference. The high values obtained for the 4f
−4f intensity parameter Ω2 for europium complexes suggest
that the dynamic coupling mechanism is quite operative in these compounds.
Introduction
of a central lanthanide ion and chelating organic ligand as a
photosensitizer. The antenna chromophore moiety efficiently
absorbs and transfers light to the central lanthanide ion by
the energy transfer process. This sensitization process is much
more effective than the direct excitation of the Ln3+ ions,
since the absorption coefficients of organic chromophores
are many orders of magnitude larger than the intrinsically
low molar absorption coefficients (typically 1-10 M-1 cm-1)
of lanthanide ions.6
Lanthanide complexes with organic ligands are of great
interest for a wide range of photonic applications such as
tunable lasers, amplifiers for optical communications, com-
ponents of the emitting materials in multilayer organic light-
emitting diodes. and efficient light conversion molecular
devices.1-5 In most cases, the luminescent complexes consist
* To whom correspondence should be addressed. Telephone: 91-471-
2515360. Fax: 91-471-2491712. E-mail: mlpreddy@yahoo.co.uk.
† CSIR.
The â-diketone ligand is one of the important “antennas”,
from which the energy can be effectively transferred to Ln3+
ions for high harvest emissions and which has the following
‡ University of Birmingham.
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10.1021/ic061425a CCC: $33.50
Published on Web 11/15/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 26, 2006 10651

Biju et al.
advantages.5 The â-diketone ligand has strong absorption
within a large wavelength range for its π-π* transition and
consequently has been targeted for its ability to sensitize the
luminescence of the Ln3+ ions. Further, it has the ability to
form stable and strong adducts with Ln3+ ions, which can
have practical usage.7,8 A large number of highly coordinated
complexes of lanthanide tris(â-diketonates) containing sev-
eral nitrogen ligands such as 1,10-phenanthroline,9 4,7-
disubstituted-1,10-phenanthrolines,10 2,2′-bipyridine,10 4,4′-
disubstituted-2,2′-bipyridines,10 1,4-diaza-1,3-butadienes,11
and 2,2′:6′,6′′-terpyridine12,13 have been reported as efficient
light conversion molecular devices. Highly efficient photo-
luminescent and electroluminescent performances have also
been observed in the europium(III)-(tris-2-thenoyltrifluo-
roacetonato)phosphine oxide complexes.14 Molecular lan-
thanide chelates containing 4-acyl-5-pyrazolonate ligands
have also been successfully used in the production of
emission layers in organic electroluminescent devices.15,16
Recently, the authors have developed promising light con-
version molecular devices based on 3-phenyl-4-aroyl-5-
isoxazolonate complexes of Eu3+ with phosphine oxides,
which provides stable eight- and nine-coordinated lanthanide
complexes with high quantum efficiency.17,18 In the present
paper we report the synthesis, crystal structures, and pho-
tophysical properties of new europium(III) complexes of
3-phenyl-4-benzoyl-5-isoxazolone (HPBI) with various bi-
dentate nitrogen ligands having electron-donating and electron-
withdrawing groups.
Elemental analyses were performed with a Perkin-Elmer Series
2 Elemental Analyser 2400. A Nicolet FT-IR 560 Magna Spec-
trometer using KBr (neat) was used to obtain IR spectral data, and
a Bruker 300 MHz NMR spectrometer was used to obtain 1H NMR
spectra of the compounds in CDCl3 or acetone-d6 media. Thermo-
gravimetric analyses were carried out using a TGA-50H (Shimadzu,
Japan). Mass spectra were recorded using a JEOL JSM 600 fast
atom bombardment (FAB) high-resolution mass spectrometer
(HRMS). Diffuse reflectance spectra of the europium complexes
and the standard phosphor were recorded on a Shimadzu UV-2450
UV-vis spectrophotometer using BaSO4 as a reference. Absor-
bances of the ligands and corresponding europium complexes in
CH3CN solution were measured with a UV-vis spectrophotometer
(Shimadzu, UV-2450). Photoluminescence (PL) spectra were
recorded using a Spex-Fluorolog DM3000F spectrofluorometer with
a double grating 0.22 m Spex 1680 monochromator and a 450 W
Xe lamp as the excitation source using the front face mode. The
lifetime measurements were carried out at room temperature using
a Spex 1934 D phosphorimeter.
The overall quantum yields (Φoverall) were measured at room
temperature using the technique for powdered samples described
by Bril et al.,19 through the following expression:
1 - rst Ax
Φoverall
)
Φst
(1)
(
1 - r )(A )
x
st
where rst and rx are the diffuse reflectance (with respect to affixed
wavelength) of the complexes and of the standard phosphor,
respectively, and Φst is the quantum yield of the standard phosphor.
The terms Ax and Ast represent the areas under the complex and
standard emission spectra, respectively. To have absolute intensity
values, BaSO4 was used as a reflecting standard. The standard
phosphor used was sodium salicylate (Merck), whose emission
spectra are formed by a large broad band peaking around 425 nm,
with a constant Φ value (60%) for excitation wavelengths between
220 and 380 nm. Three measurements were carried out for each
sample, so that the presented Φoverall value corresponds to the
arithmetic mean value. The errors in the quantum yield values
associated with this technique were estimated within 10%.19
X-ray single-crystal data were recorded at room temperature on
a Bruker Smart 6000 diffractometer equipped with a CCD detector
and a copper tube source. Data were processed using SAINTPLUS
(SAINTPLUS, program suite for data processing, Bruker AXS, Inc.,
Madison, WI). Structures were solved and refined using SHELXL-
97.20 The uncoordinating ethanol molecule in Eu(PBI)3‚H2O‚EtOH
is disordered with an occupancy of one-half. The water protons
were not located, and hydroxyl protons were placed in positions
calculated for optimum hydrogen bonding. Non-hydrogen atoms
were refined anisotropically, and a riding model was used for C-H
hydrogen atoms. Table 1 shows crystal data, structure refinement
parameters, atomic coordinates, and isotropic displacement param-
eters. The dichloromethane site has 70% occupancy, and hydrogen
atom positions were constrained geometrically during refinement
in the complex Eu(PBI)3‚phen.
Experimental Section
Materials and Instrumentation. The following commercially
available chemicals were used without further purification: eu-
ropium(III) nitrate hexahydrate, 99.9% (Acros Organics); gadolin-
ium(III) nitrate hexahydrate, 99.9% (Acros Organics); 1,10-
phenanthroline monohydrate (Merck); 2,2′-dipyridyl, 99+% (Aldrich);
4,7-diphenyl-1,10-phenanthroline, 97% (Aldrich); 4-4′-dimethoxy-
2-2′-bipyridine, 97% (Aldrich). The ligand HPBI was synthesized
in our laboratory as mentioned in our previous publication.18 All
other chemicals used were of analytical reagent grade.
(7) Yang, L.; Gong, Z.; Nie, D.; Lou, B.; Bian, Z.; Guan, M.; Huang, C.;
Lee, H. J.; Baik, W. P. New J. Chem. 2006, 30, 791-796.
(8) Binnemans, K. Handbook on the Physics and Chemistry of Rare
Earths; Elsevier: Amsterdam, 2005; Vol. 35, Chapter 225, pp 107-
272.
(9) Watson, W. H.; Williams, R. J.; Stemple, N. R. J. Inorg. Chem. 1972,
34, 501-508.
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Pucci, D. Inorg. Chem. 2005, 44, 1818-1825.
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Goncalves, I. S.; Ribeiro-Claro, P. J. A. Eur. J. Inorg. Chem. 2004,
3913-3919.
(12) Fukuda, Y.; Nakao, A.; Hiyashi, K. J. Chem. Soc., Dalton Trans. 2002,
527-533.
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Acta 2003, 344, 37-42.
Synthesis of Eu(PBI)3‚C2H5OH‚H2O (1). An ethanolic solution
of Eu(NO3)3‚6H2O (0.5 mmol) was added to a solution of HPBI
(1.5 mmol) in ethanol in the presence of NaOH (1.5 mmol).
(14) Hasegawa, Y.; Yamamuro, M.; Wada, Y.; Kanehisa, N.; Kai, Y.;
Yanagida, S. J. Phys. Chem. A 2003, 107, 1697-1702.
(15) Xin, H.; Shi, M.; Gao, X. C.; Huang, Y. Y.; Gong, Z. L.; Nie, D. B.;
Cao, H.; Bian, Z. Q.; Li, F. Y.; Huang, C. H. J. Phys. Chem. B 2004,
108, 10796-10800.
(19) (a) Bril, A.; De Jager-Veenis, A. W. J. Electrochem. Soc. 1976, 123,
396-398. (b) Mello Donega, C. D.; Junior, S. A.; de Sa, G. F. Chem.
Commun. 1996, 11, 1199-1200. (c) Carlos, L. D.; Mello Donega, C.
D.; Albuquerque, R. Q.; Junior, S. A.; Menezes, J. F. S.; Malta, O. L.
Mol. Phys. 2003, 101, 1037-1045
(16) Shi, M.; Li, F.; Yi, T.; Zhang, D.; Hu, H.; Huang, C. Inorg. Chem.
2005, 44, 8929-8936.
(17) Pavithran, R.; Reddy, M. L. P.; Alves, S., Jr.; Freire, R. O.; Rocha,
G. B.; Lima, P. P. Eur. J. Inorg. Chem. 2005, 20, 4129-4137.
(18) Pavithran, R.; Saleesh Kumar, N. S.; Biju, S.; Reddy, M. L. P.; Alves,
S., Jr.; Freire, R. O. Inorg. Chem. 2006, 45, 2184-2192.
(20) Sheldrick, G. M. SHELXL97. Program package for crystal structure
determination; University of Go¨ttingen: Go¨ttingen, Germany, 1998.
10652 Inorganic Chemistry, Vol. 45, No. 26, 2006

New Eu(PBI)3‚L Complexes
Table 1. Crystal Data, Collection, and Structure Refinement
Parameters for Complexes 1 and 4
H, 3.69; N, 76. IR (KBr) νmax: 1637, 1607, 1601, 1483, 1437, 1182,
1021, 906 cm-1. m/z ) 1300.47 (M+) + Na.
parameters
1
4
Results and Discussion
empirical formula
fw
C51H41EuN3O11.50
1031.83
monoclinic
P21/a
C60.70H39.40Cl1.40EuN5O9
1184.36
triclinic
Structural Characterization of Europium(III) Com-
plexes. The synthesis procedure of europium complexes 1-5
is shown in Schemes 1 and 2. The microanalyses and HRMS
studies of complexes 1-5 show that Eu3+ ion has reacted
with HPBI in a metal-to-ligand mole ratio of 1:3 and one
molecule of bidentate nitrogen ligand is involved in com-
plexes 2-5. The IR spectrum of complex 1 shows a broad
absorption in the region 3000-3500 cm-1, indicating the
presence of solvent molecules in the complex. The existence
of solvent molecules in lanthanide complexes with hetero-
cyclic â-diketones such as 1-phenyl-3-methyl-4-acylpyra-
zolones is well documented.21,22 On the other hand, the
absence of the broad band in the region 3000-3500 cm-1
for complexes 2-5 suggests that water and solvent molecules
have been displaced by the bidentate nitrogen donors. The
carbonyl stretching frequency of HPBI (1699 cm-1) has been
shifted to lower wavenumbers in complexes 1-5 (1641 cm-1
in 1; 1638 cm-1 in 2; 1640 cm-1 in 3; 1640 cm-1 in 4; 1637
cm-1 in 5), indicating the involvement of carbonyl oxygen
in the complex formation with Eu3+ ion. Further, the red
shifts observed in the CdN stretching frequencies of nitrogen
donors (1615 cm-1) in complexes 2-5 (to 1605 cm-1 in 2;
1607 cm-1 in 3; 1605 cm-1 in 4; 1607 cm-1 in 5) show the
involvement of nitrogen in the complex formation with Eu3+
ion.
crystal system
space group
cryst size (mm3)
temperature (K)
a (Å)
P1h
0.20 × 0.20 × 0.10
0.20 × 0.20 × 0.10
296(2)
296(2)
15.6795(4)
21.3364(7)
16.1558(6)
90
118.076(2)
90
4768.8(3)
4
1.437
9.950
2092
0.0348
0.0895
10.5185(4)
16.7762(8)
16.8211(7)
80.343(3)
75.786(2)
72.842(3)
2734.7(2)
2
1.438
9.348
1195
0.0473
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
F
calcd (g cm-3
)
µ (mm-1
F(000)
)
R1 [I > 2σ(I)]
wR2 [I > 2σ(I)]
R1 (all data)
wR2 (all data)
GOF
0.1049
0.0734
0.1165
1.009
0.0473
0.0965
1.019
Precipitation took place immediately, and the reaction mixture was
stirred for 10 h at room temperature (Scheme 1). The product was
filtered, washed with ethanol, with water, and then with ethanol,
and dried and stored in a desiccator. The complex was then purified
by recrystallization from dichloromethane-ethanol mixture. Ele-
mental analysis (%): Calcd for C50H38EuN3O11 (1007.965): C,
59.58; H, 3.8; N, 4.17. Found: C, 59.75; H, 4.01; N, 4.21. IR (KBr)
ν
max: 3300, 1641, 1614, 1483, 1389, 1184, 910, 760 cm-1. m/z )
967 (M+ - H2O, C2H5OH) + Na.
Synthesis of Gd(PBI)3‚2H2O. An aqueous solution of Gd(NO3)3‚
6H2O (0.5 mmol) was added to a solution of HPBI (1.5 mmol) in
ethanol 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 ethanol,
with water, and then with ethanol, and dried and stored in a
desiccator. The complex was then purified by recrystallization from
dichloromethane- ethanol mixture. Elemental analysis (%): Calcd
for C48H34GdN3O11 (986.06): C, 58.46; H, 3.45; N, 4.26. Found:
C, 58.40; H, 3.61; N, 4.24. IR (KBr) νmax: 3300, 1640, 1612, 1483,
1389, 1184, 910 cm-1. m/z ) 950 (M+ - 2H2O).
Synthesis of Complexes 2-5. Synthesis routes of the complexes
2-5 are shown in Scheme 2. All these complexes were prepared
by stirring equimolar solutions of Eu(PBI)3‚C2H5OH‚H2O and
nitrogen donors in CHCl3 for 24 h at room temperature. The
products were obtained after solvent evaporation and are purified
by recrystallization from dichloromethane-ethanol mixture.
Eu(PBI)3‚bpy (2). Elemental analysis (%): Calcd for C58H38-
EuN5O9 (1100.19): C, 63.32; H, 3.48; N, 6.37. Found: C, 62.99;
H, 3.54; N, 6.08. IR (KBr) νmax: 1638, 1605, 1602, 1483, 1389,
910, 760 cm-1. m/z ) 1123.3 (M+) + Na.
Eu(PBI)3‚dmbpy (3). Elemental analysis (%): Calcd for C60H42-
EuN5O11 (1160.19): C, 62.12; H, 3.65; N, 6.04. Found: C, 61.98;
H, 3.61; N, 6.47. IR (KBr) νmax: 1640, 1607, 1603, 1483, 1434,
1388, 1182, 1024 cm-1. m/z ) 1183.7 (M+) + Na.
It is clear from the thermogravimetric analysis data that
complex 1 (Figure S1 in Supporting Information) undergoes
a mass loss of 7% (calculated: 6.3%) up to 150 °C, which
corresponds to the removal of coordinated solvent molecules.
On the other hand, in complexes 2-5 (Figures S2-S4 in
Supporting Information) there is no mass loss up to 200 °C,
indicating the absence of solvent molecule in the coordination
sphere. Further, decomposition takes place in the region
200-800 °C for all the complexes.
The structure of Eu(PBI)3‚H2O‚EtOH (1) and Eu(PBI)3‚
phen (4) were characterized by single-crystal X-ray crystal-
lography. Details of crystal data and data collection para-
meters for complexes 1 and 4 are given in Table.1. The
selected bond lengths and bond angles for complexes are
listed in Table 2. The asymmetric unit of Eu(PBI)3‚H2O‚
EtOH is shown in Figure 1, and crystal packing is shown in
Figure 2. The central Eu3+ ion is coordinated with eight
oxygen atoms: six from the three bidentate HPBI ligands,
one from a water molecule, and another from an ethanol
molecule. The coordination geometry of the metal center is
best described as a bicapped trigonal prism. The average
bond length between the europium ion and the isoxazolone
oxygen atoms is 2.395 Å, which is slightly shorter than that
of europium and water oxygen atom (2.399 Å) and also that
Eu(PBI)3‚phen (4). Elemental analysis (%): Calcd for C60H38-
EuN5O9 (1124.96): C, 64.06; H, 3.4; N, 6.23. Found: C, 63.91;
H, 3.39; N, 6.53. IR (KBr) νmax: 1638, 1605, 1600, 1482, 1434,
1388, 1182, 1024 cm-1. m/z ) 1148.5 (M+) + Na.
Eu(PBI)3‚bath (5). Elemental analysis (%): Calcd for C60H38-
EuN5O9 (1277.15): C, 67.71; H, 3.63; N, 5.48. Found: C, 67.68;
(21) Pettinari, C.; Marchetti, F.; Cingolann, A.; Drozdov, A.; Timokhin,
I.; Troyanov, S. I.; Tsaryuk, V.; Zolin, V. Inorg. Chim. Acta 2004,
357, 4181-4190.
(22) Shen, L.; Shi, M.; Li, F.; Zhang, D.; Li, X.; Shi, E.; Yi, T.; Du, Y.;
Huang, C. Inorg. Chem. 2006, 45, 6188-6197.
Inorganic Chemistry, Vol. 45, No. 26, 2006 10653

Biju et al.
Scheme 1. Synthesis Route of the Complex Eu(PBI)3‚C2H5OH‚H2O
Scheme 2. Synthesis Route for Complexes 2-5
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Complexes
1 and 4
europium ion due to electrostatic effects. Similar behavior
has been reported elsewhere in the X-ray single-crystal
structure of Tb(PMPP)3‚2H2O (where PMPP ) 1-phenyl-
3-methyl-4-propionyl-5-pyrazolone).23 The ethanol molecule
coordinating to the cation donates a hydrogen bond to a
nitrogen atom of a ligand molecule of a neighboring cluster
[N(3)‚‚‚O(10) ) 2.913(2)], and pairs of complex molecules
are linked by two of these bonds as shown in Figure 2a.
The disordered ethanol molecule accepts one hydrogen bond
1
4
Eu-O(1)
2.457(2)
2.373(2)
2.388(2)
2.382(3)
2.330(2)
2.438(2)
2.469(2)
2.399(3)
Eu-O(1)
Eu-O(2)
Eu-O(4)
Eu-O(5)
Eu-O(7)
Eu-O(8)
Eu-N(4)
Eu-N(5)
2.378(4)
2.361(4)
2.385(4)
2.376(4)
2.441(3)
2.340(4)
2.550(4)
2.614(4)
Eu-O(2)
Eu-O(4)
Eu-O(5)
Eu-O(7)
Eu-O(8)
Eu-O(10)
Eu-O(11)
from the water molecule coordinating the cation [O(11)water
‚
O(1)-Eu-O(2)
O(4)-Eu-O(5)
O(7)-Eu-O(8)
O(10)-Eu-O(11)
73.45(8)
71.12(8)
72.28(8)
144.17(3)
O(1)-Eu-O(2)
O(4)-Eu-O(5)
O(7)-Eu-O(8)
N(4)-Eu-N(5)
71.47(13)
72.32(13)
71.65(12)
63.67(14)
‚‚O(12)ethanol ) 2.702(2)] and donates a bond to a ligand
nitrogen atom of a neighboring cluster [O(12)ethanol‚‚‚N(2)
) 2.913(2)] to form bridges between pairs of clusters (Figure
2b). The water molecule also hydrogen bonds with a nitrogen
of europium and ethanol oxygen atom (2.469 Å). This may
be due to the result of the negative charge of the isoxazolone
oxygen, which could be more strongly coordinated to the
(23) Zhou, D.; Li, Q.; Huang, C.; Yao, G.; Umetani, S.; Matsui, M.; Ying,
L.; Yu, A.; Zhao, X. Polyhedron 1997, 16, 1381-1389.
10654 Inorganic Chemistry, Vol. 45, No. 26, 2006

New Eu(PBI)3‚L Complexes
Figure 1. (a) Asymmetric unit and (b) crystal packing viewed down the axis of complex 1. Hydrogen atoms have been omitted for clarity.
atom of a neighboring cluster [N(1)‚‚‚O(11)water ) 2.809-
(2)] to form chains parallel to the a-axis.
throline (Eu-O bonds 2.31-2.41 Å in DBM; Eu-N, 2.66
Å in phen).24
In complex 4, the central Eu3+ ion is coordinated by six
oxygen atoms from three HPBI ligands and two nitrogen
atoms from a bidentate phenanthroline ligand, resulting in a
coordination number of eight. The coordination geometry
can be described as a distorted square antiprism with six
oxygen atoms and two nitrogen atoms (Figure 3). A similar
geometric structure was reported elsewhere for the tris-
(acetylacetonate)europium(III) phenanthroline and tris(diben-
zolmethanido)europium(III) phenanthroline complexes by
single-crystal X-ray diffraction studies.24 The average Eu-N
bond distance (2.58 Å) is longer than the Eu-O bonds of
HPBI ligands (2.34-2.44 Å), as observed in the X-ray single-
crystal data of the complex tris(4,4,5,5,6,6,6-heptafluoro-1-
(2-naphthyl)hexane-1,3-dione)europium(III) with phenan-
throline (Eu-O bonds 2.34-2.40 Å in HFNH; Eu-N, 2.60
Å in phen)25 and tris(dibenzolmethanido)europium(III) phenan-
UV-Vis Spectra. UV-vis absorption spectra of the
ligands (HPBI, bpy, dmbpy, phen, bath) and their Eu(III)
complexes measured in CH3CN (c ) 2 × 10-5 M) are shown
in Figures 4 and 5, respectively. The maximum absorption
band at 316 nm for HPBI, 311 nm for complex 1, and the
hump observed around 311 nm in complexes 2-5 are
attributed to singlet-singlet π-π* enol absorption of
heterocyclic â-diketonate. Compared with the ligand HPBI
(λmax) 316 nm), the absorption maxima are blue shifted to
311 nm in complexes 1-5. The absorption maxima at 290,
298, 289, and 289 nm in complexes 2, 3, 4, and 5,
respectively, are due to the 1π-π* absorption of the aromatic
rings of bidentate nitrogen donors. These values also shows
a blue shift of 1, 1, 9, and 12 nm, respectively, in complexes
compared to free nitrogen donors (291, 299, 298, and 301
nm). The spectral shapes of the complexes in CH3CN are
similar to that of the free ligands, suggesting that the
coordination of Eu3+ ion does not have a significant influence
(24) Ahmed, M. O.; Liao, J. L.; Chen, X.; Chen, S. A.; Kaldis, J. H. Acta
Crystallogr. 2003, E59, m29-m32.
(25) Yu, J.; Zhou, L.; Zhang, H.; Zheng, Y.; Li, H.; Deng, R.; Peng, Z.;
Li, Z. Inorg. Chem. 2005, 44, 1611-1618.
1
on the π-π* state energy. However, a small blue shift
Inorganic Chemistry, Vol. 45, No. 26, 2006 10655

Biju et al.
Figure 2. (a) Linkage of clusters by ethanol molecules and (b) hydrogen bonding involving water molecules. Some hydrogen atoms have been omitted for
clarity.
Figure 3. Asymmetric unit of complex 4.
observed in the absorption maximum of all the complexes
Figure 4. UV-visible absorption spectra of ligands in acetonitrile (c )
2 × 10-5 M).
is due to the perturbation induced by the metal coordination.
The determined molar absorption coefficient values of the
complexes 1, 2, 3, 4, and 5 at 311 nm, 2.82 × 104, 2.94 ×
104, 2.90 × 104, 2.92 × 104, and 2.96 × 104 L mol-1 cm-1,
respectively, are about 3 times higher than that of the HPBI
(9.4 × 103 at 316 nm), indicating the presence of three
â-diketonate ligands in the corresponding complexes. Further,
the higher molar absorption coefficient of HPBI reveals that
the â-diketonate ligand has a strong ability to absorb light.
PL Properties of Complexes 1-5. The normalized
excitation spectra of the europium complexes 1-5 (in solid
state) at room temperature, monitored around the peak of
5
7
the intense D0 f F2 transition of the Eu3+ ion, are shown
in Figure 6. The excitation spectra of all the complexes
exhibit a broad band between 250 and 450 nm and a series
of sharp lines characteristic of the Eu3+ energy level structure,
10656 Inorganic Chemistry, Vol. 45, No. 26, 2006

New Eu(PBI)3‚L Complexes
Figure 5. UV-visible absorption spectra of HPBI and complexes 1-5
in acetonitrile (c ) 2 × 10-5 M).
Figure 7. Room-temperature PL spectrum of complexes 1-5 excited at
their maximum emission wavelengths (367, 369, 371, 350, and 360 nm,
respectively, for complexes 1, 2, 3, 4, and 5).
chemical environment around the Eu3+ ion that is responsible
for the brilliant red emission of these complexes. A relevant
feature that may be noted for complexes 1-5 is the very
high intensity of the 5D0 f 7F2 transition, relative to the 5D0
7
f F1 lines, indicating that the Eu3+ ion coordinated in a
local site without an inversion center. Further, the emission
spectra of the complexes show only one peak for 5D0 f 7F0
5
7
transition and three stark components for the D0 f F1
transition, indicating the presence of a single chemical
environment around the Eu3+ ion.
The lifetime values (τobs) of the 5D0 level were determined
from the luminescence decay profiles for complexes 1-5 at
room temperature by fitting with a monoexponential curve,
and they are depicted in Table 3. Typical decay profiles of
complexes 2-5 are shown in Figure S6 (Supporting Infor-
mation). The relatively shorter lifetime obtained for complex
1 may be due to dominant nonradiative decay channels
associated with vibronic coupling due to the presence of
solvent molecules, as is well documented for many of the
hydrated europium â-diketonate complexes.5 Longer lifetime
values have been observed for complexes 2-5 compared to
complex 1, due to the absence of nonradiative pathways.
Judd-Ofelt theory is a useful tool for analyzing f-f
electronic transitions.27 Interaction parameters of ligand fields
are given by the Judd-Ofelt parameters Ωλ (where λ ) 2,
4, and 6). In particular, Ω2 is more sensitive to the symmetry
and sequence of ligand fields. To produce faster Eu3+
radiation rates, antisymmetrical Eu3+ complexes with larger
Ω2 parameters need to be designed. The experimental Ω2
and Ω4 intensity parameters were determined from the
Figure 6. Excitation spectrum of 5D0 emission (λmax ) 613 nm) of Eu3+
complexes 1-5 at 303 K.
7
5
5
assigned to transitions between the F0,1 and the L6, D3,2,1
levels. This transition is weaker than the absorption of the
organic ligands and is overlapped by a broad excitation band,
which proves that luminescence sensitization via excitation
of the ligand is much more efficient than the direct excitation
of the Eu3+ ion absorption level. The broad band may be
related to the excited states of ligands or to the ligand-to-
metal charge transfer (LMCT) transitions resulting from the
interaction between the ion and the ligand’s first coordination
shell.5,26
The room-temperature normalized emission spectra of
europium complexes 1-5 (in solid state) under the excitation
wavelengths that maximize the Eu3+ emission intensity are
shown in Figure 7. The emission spectra of the europium
complexes display characteristic sharp peaks in the 575-
5
7
emission spectra given in Figure 7 by using the D0 f F2
5
7
5
7
725 nm region associated with the D0 f FJ transitions of
and D0 f F4 electronic transitions, respectively, and by
expressing the emission intensity IJ ) pωJ0ARAD(J) N(5D0)
in terms of the area under the emission curve. Here pωJ0 is
the transition energy and N is the population of the 5D0 level.
5
7
the Eu3+ ion. The five expected peaks for the D0 f F0-4
5
transitions are well resolved and the hypersensitive D0 f
7F2 transition is very intense, pointing to a highly polarizable
(26) Braga, S. S.; Ferreira, R. S.; Goncalves, I. S.; Pillinger, M.; Rocha,
J.; Teixeira-Dias, J. C.; Carlos, L. D. J. Phys. Chem. B 2004, 106,
11430-11437.
(27) (a) Judd, B. R. Phys. ReV. 1962, 127, 750-761. (b) Ofelt, G. S. J.
Chem. Phys. 1962, 37, 511-519. (c) Werts, M. H. V.; Jukes, R. T.
F.; Verhoeven, W. J. Phys. Chem. Chem. Phys. 2002, 4, 1542-1548.
Inorganic Chemistry, Vol. 45, No. 26, 2006 10657

Biju et al.
5
Table 3. Experimental Intensity Parameters (Ω2,4), R02, Radiative (ARAD) and Nonradiative (ANR) Decay Rates, D0 Lifetime (τobs), Intrinsic Quantum
Yields (ΦLn, %; Φtransfer, %), and Overall Quantum Yield (Φoverall, %) for Complexes 1-5 at 303 K
complex
10-20Ω2 (cm2)
10-20Ω4 (cm2)
R02
A
RAD (s-1
)
ANR (s-1
)
τ
obs (µs)
Φ
Ln (%)
Φ
transfer (%)
Φ
overall (%)
1
2
3
4
5
26.47
19.25
17.25
15.66
20.19
14.29
3.22
1.91
1.53
2.06
0.0097
0.0063
0.0082
0.0072
0.0054
1059
6911
6094
5543
7023
2941
3314
2373
4213
2878
250
978
1181
1025
1010
26
68
72
57
71
8.0
22
25
20
22
2.2
15
18
11
15
The radiative emission rates, ARAD(J), are given by28,29
ARAD
ΦLn
)
)
(4)
ARAD + ANR
4e2ω3
3pc3
1
5
2
ARAD
)
ø
Ω
7FJ||U(λ)|| D0
(2)
∑
λ
1
2J + 1
λ
τobs
(5)
ARAD + ANR
where e is the electronic charge, ω is the angular frequency
of the transition, p is Planck’s constant over 2π, c is the
velocity of light, ø is the Lorentz local field correction term
given by n(n2 + 2)2/9, n being the refraction index, and
where ARAD and ANR are the radiative and nonradiative decay
rates, respectively. ARAD can be calculated using eq 2, and
the nonradiative rates ANR can be obtained from the calculated
ARAD and the observed lifetime (τobs) using eq 5.
5
2
7FJ||U(λ)|| D0 are the squared reduced matrix elements
whose values are 0.0032 and 0.0023 for J ) 2 and 4,
Table 3 gives the overall quantum yield (Φoverall), ARAD
and ANR, intrinsic quantum yield (ΦLn), and Φtransfer values
for complexes 1-5. The R02 intensity parameter, defined as
the ratio between the intensities of the 5D0 f 7F0 and 5D0 f
7F2 transitions, gives information on the J-mixing effect
respectively.30 The magnetic dipole allowed 5D0
f
7F1
transition was taken as reference,28,31 in vacuo ARAD(5D0f7F1)
) 14.65 s-1.26 An average index of refraction equal to 1.5
was considered, leading to ARAD(5D0f7F1) ≈ 50 s-1.32 The
5
7
5
7
Ω6 parameter was not determined since the D0 f F5,6
transitions could not be experimentally detected. Table 3 lists
the Ω2 and Ω4 intensity parameters estimated for complexes
1-5. A point to be noted in these results is the relatively
high values of the Ω2 parameter for complexes 1-5. This
might be interpreted as being a consequence of the hyper-
sensitive behavior of the 5D0 f 7F2 transition.33 The dynamic
coupling mechanism is, therefore, dominant, indicating that
the Eu3+ ion is in a highly polarizable chemical environment.
The overall quantum yield (Φoverall) of the europium
complex treats the complex as a “black box” where the
internal process is not explicitly considered: given that the
complex absorbs a photon (i.e., the antenna is excited), the
overall quantum yield can be defined as34
associated with D0 f F0 transition and is also given in
Table 3. According to energy gap theory, radiationless
transitions are prompted by ligands and solvents with high-
frequency vibrational modes. Creation of Eu3+ complexes
with higher quantum yields is directly linked to suppression
of radiationless transitions caused by vibrational excitations
in surrounding media.14,35-37 It is clear from Table 3 that
complex 1, having solvent molecules in the coordination
sphere, exhibits a lower quantum yield. This is due to the
presence of O-H oscillators in this system, which effectively
quench the luminescence of the Eu3+ ion. On the other hand,
complexes 2-5 exhibit high quantum yield and lifetime
values due to the displacement of solvent molecules from
the coordination sphere by the bidentate nitrogen donors.
Among complexes 2 and 3, the latter shows a better quantum
yield due to the enhanced basicity of the coordinating
nitrogen atoms upon the substitution of two electron-donating
groups (OCH3) in the para,para′-position in the bipyridine
molecule. On the other hand, among complexes 4 and 5,
again the latter exhibits a high quantum yield due to the
extended conjugation obtained by the introduction of two
phenyl groups in the 4,7-positions of the phenanthroline
ligand. The intrinsic quantum yield and 5D0 lifetime obtained
in the present study for the Eu3+ complexes 2-5 were found
to be promising when compared to that observed for the
various Eu3+ â-diketonate-bidentate nitrogen donor com-
plexes reported so far in the literature (Table 4).
Φoverall ) ΦtransferΦLn
(3)
Here Φtransfer is the efficiency of energy transfer from the
ligand to Eu3+ and ΦLn is the intrinsic quantum yield of the
lanthanide ion. The latter can be calculated as
(28) Malta, O. L.; Brito, H. F.; Menezes, J. F. S.; Goncalves e Silva, F.
R.; Alves, S., Jr.; de Andrade, A. M. V. J. Lumin. 1997, 75, 255-
268.
(29) Malta, O. L.; dos Santos, M. A. C.; Thompson, L. C.; Ito, N. K. J.
Lumin. 1996, 69, 77-84.
(30) Carnall, W. T.; Crosswhite, H. M. Energy leVels, Structure and
Transition Probabilities of the TriValent Lanthanides in LaF3; Argonne
National Laboratory: IL, 1977.
(31) Carlos, L. D.; Messaddeq, Y.; Brito, H. M.; Sa´ Ferreira, R. A.; V. de
Zea Bermudez, V.; Ribeiro, S. J. L. AdV. Mater. 2000, 12, 594-598.
(32) Molina, C.; Moreira, P. J.; Gonc¸alves, R. R.; Sa´ Ferreira, R. A.;
Messaddeq, Y.; Ribeiro, S. J. L.; Soppera, O.; Leite, A. P.; Marques,
P. V. S.; de Zea Bermudez, V.; Carlos, L. D. J. Mater. Chem. 2005,
15, 3937-3945.
(33) Peacock, R. D. Struct. Bonding (Berlin) 1975, 22, 83-121.
(34) (a) Xiao, M.; Selvin, P. R. J. Am. Chem. Soc. 2001, 123, 7067-7073.
(b) Comby, S. Imbert, D. Anne-Sophie, C, Bunzli, J. C. G.;
Charvonniere, L. J.; Ziessel, R. F. Inorg. Chem. 2004, 43, 7369-
7379. (c) Quici, S.; Cavazzini, M.; Marzanni, G.; Accorsi, G.;
Armaroli, N.; Ventura, B.; Barigelletti, F. Inorg. Chem. 2005, 44, 529-
537.
Energy Transfer between Ligands and Eu3+. To dem-
onstrate the energy transfer process, the phosphorescence
(35) Peng, C.; Zhang, H.; Yu, J.; Meng, Q.; Fu, L.; Li, H.; Sun, L.; Guo,
X. J. Phys. Chem. B 2005, 109, 15278-15287.
(36) Wada, Y.; Ohkubo, T.; Ryo, M.; Nakarawa, T.; Hasegawa, Y.;
Yanagida, S. J. Am. Chem. Soc. 2000, 122, 8583-8584.
(37) Hasegawa, Y.; Ohkubo, T.; Sogabe, K.; Kawamura, Y.; Wada, Y.;
Nakashima, N.; Yanagida, S. Angew. Chem., Int. Ed. 2000, 39, 357-
360.
10658 Inorganic Chemistry, Vol. 45, No. 26, 2006

New Eu(PBI)3‚L Complexes
Table 4. Solid State Photophysical Data for D0 Luminescence of Some Selected Eu3+ Complexes at Room Temperature
5
complexa
A
RAD (s-1
)
ANR (s-1
)
τ
obs (µs)
Φ
Ln (%)
Eu(PBI)3‚phen
Eu(PBI)3‚bpy
5543
6911
436
580
600
4213
3314
993
569
900
1025
978
700
210
662
620
57
68
31
50
40
51
Eu(tta)3‚phen35
Eu(btfa)3‚phen5
Eu(NTA)3‚phen38
Eu(NTA)3‚bpy39
816
797
a tta ) 2-thenoyltrifluoroacetonate; btfa ) 4,4,4-trifluoro-1-phenyl-1,3-butanedionate; NTA ) 1-(2-naphthoyl)-3,3,3-trifluroacetonate.
spectra of the complex Gd(PBI)3‚2H2O was measured for
the triplet energy level data of the ligand HPBI. From the
phosphorescence spectra (Figure S7 in the Supporting
Information), the triplet energy level (3ππ*) of Gd(PBI)3‚
2H2O, which corresponds to its peak emission wavelength,
is 20 366 cm-1 (491 nm). Because the lowest excited state
6P7/2 of Gd3+ is too high to accept energy from a ligand, the
data obtained from the phosphorescence spectra actually
reveal the triplet energy level of HPBI in europium com-
plexes. The singlet state energy (1ππ*) level of HPBI is
estimated by referencing its absorbance edge, which is 27 397
cm-1 (365 nm). The singlet and triplet energy levels of bpy
(29 900 and 22 900 cm-1) and phen (31 000 and 22 100
cm-1) were taken from the literature.40
Figure 8. Schematic energy level diagram and the energy transfer
process: S1, first excited singlet state; T1, first excited triplet state.
In general, the sensitization pathway in luminescent
europium complexes consists of excitation of the ligands into
their excited singlet states, subsequent intersystem crossing
of the ligands to their triplet states, and the energy transfer
needs ∆E(3ππ*-5D0) > 2500 cm-1 and hence the energy
transfer process is effective for complexes 1-5. It is also
noted that the energy gaps between the 1ππ* and 3ππ* levels
are 7031, 7000, and 8900 cm-1 for HPBI, bpy, and phen,
respectively. According to Reinhoudt’s empirical rule, the
intersystem crossing process becomes effective when
∆E(1ππ*-3ππ*) is at least 5000 cm-1;42 thus HPBI, bpy,
and phen are effective sensitizers for Eu3+ and the intersys-
tem crossing processes in complexes 1-5 are effective.
5
from the triplet state to the DJ manifold of the Eu3+ ions,
5
followed by internal conversion to the emitting D0 state.
Finally, the Eu3+ ion emits when transition to the ground
state occurs.16 Moreover, the electron transition from the
higher excited states, such as 5D3 (24 800 cm-1), 5D2 (21 200
cm-1), and 5D1 (19 000 cm-1), to 5D0 (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 emission bands at
∼581, 593, 614, 654, and 702 nm corresponding to the
deactivation of the excited state 5D0 to the ground states 7FJ
(J ) 0-4). Thus, matching the energy levels of the triplet
Conclusions
The photophysical properties of new heterocyclic â-dike-
tonate europium complexes Eu(PBI)3‚H2O‚EtOH (1), Eu-
(PBI)3‚bpy (2), Eu(PBI)3‚dmbpy (3), Eu(PBI)3‚phen (4), and
Eu(PBI)3‚bath (5) were investigated. For the first time the
single-crystal X-ray structures of novel europium-3-phenyl-
4-benzoyl-5-isoxazolonate complexes were established. The
sensitization mechanism for luminescent europium com-
plexes involves a usual triplet pathway, in which the transfer
of energy absorbed by the ligand to the Eu3+ ion takes place
from the ligand-centered triplet excited state. The charac-
teristic emission spectra of Eu3+ complexes show a very high
intensity for the hypersensitive 5D0 f 7F2 transition, pointing
to a highly polarizable chemical environment around the Eu3+
ion. The results show that the substitution of solvent
molecules by bidentate nitrogen ligands in Eu(PBI)3‚H2O‚
EtOH complex greatly enhances the metal-centered lumi-
nescence quantum yields and lifetime values. The intrinsic
luminescent quantum yields of Eu3+ ion in complexes 2-4
are in the range 57-72%, and these values are promising,
5
state of the ligands to D0 of Eu3+ is one of the key factors
that affect the luminescent properties of the europium
complexes. Based on the above experimental results, the
energy level diagram and the possible energy transfer
pathways are shown in Figure 8. The triplet energy levels
of HPBI (20 366 cm-1), bpy (22 900 cm-1), and phen (22 100
5
cm-1) are higher than the D0 level (17 500 cm-1) of Eu3+,
and their energy gaps ∆E(3ππ*-5D0) between ligand and
metal-centered levels are too high to allow an effective back
energy transfer. According to Latva’s empirical rule,41 an
optimal ligand-to-metal energy transfer process for Eu3+
(38) Fernandes, J. A.; Ferreira, R.A. S.; Pillnger, M.; Carlos, L. D.; Jepsen,
J.; Hazell, A.; Ribeiro-Claro, P.; Goncalves, I. S. J. Lumin. 2005, 113,
50-63.
(39) Fu, L.; Sa´ Ferreira, R. A.; Silva, N. J. O.; Fernandes, J. A.; Ribeiro-
Claro, P.; Gonc¸alves, I. S.; de Zea Bermudez, V.; Carlos, L. D. J.
Mater. Chem. 2005, 15, 3117-3125.
(40) Yu, X.; Su, Q. J. Photochem. Photobiol., A: Chem. 2003, 155, 73-
78.
(41) Latva, M.; Takalo, H.; Mukkala, V. M.; Matachescu, C.; Rodriguez-
Ubis, J. C.; Kanakare, J. J. Lumin. 1997, 75, 149-169.
(42) Steemers, F. J.; Verboom, W.; Reinhoudt, D. N.; Vander Tol, E. B.;
Verhoeven, J. W. J. Am. Chem. Soc. 1995, 117, 9408-9414.
Inorganic Chemistry, Vol. 45, No. 26, 2006 10659

Biju et al.
compared to other Eu3+-â-diketonate complexes involving
various nitrogen donors reported so far in the literature. Thus
our results demonstrate that 3-phenyl-4-benzoyl-5-isox-
azolonate complexes of Eu3+ involving bidentate nitrogen
may find potential application as emitting materials in organic
light-emitting diodes.
India. The authors also thank Prof. T. K. Chandrashekar,
Director Regional Research Laboratory, Trivandrum, India,
and Prof. C. K. Jayasankar, S. V. University, Tirupathi, India,
for their constant encouragement and valuable discussions.
Supporting Information Available: Crystallographic data, TG
data, luminescence decay profiles, and phosphorescence spectra.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. The authors acknowledge financial
support from the Defence Research and Development
Organization and University Grants Commission, New Delhi,
IC061425A
10660 Inorganic Chemistry, Vol. 45, No. 26, 2006