Application of chelate phosphine oxide ligand in eu111 complex with mezzo triplet energy level, highly efficient photoluminescent, and electroluminescent performances

Article abstract of DOI:10.1021/jp055355p

The chelate phosphine oxide ligand bis(2-(diphenylphosphino)phenyl) ether oxide (DPEPO) was used as a unit neutral ligand to prepare the complex Eu(TTA)3(DPEPO) 1 (TTA = 2-thenoyltrifluoroacetonate). Compound 1 has a photoluminescence (PL) quantum yield of 55.3%, which is more than the twice of the PL quantum yield of Eu(TTA)₃(TPPO)₂ (TPPO = triphenylphosphine oxide). Investigation indicated that DPEPO in 1 has the mezzo first triplet excited energy level (T₁) between the first singlet excited energy level (S₁) and T₁ of TTA, which may support one more additional energy transfer routines from the T₁ energy level of DPEPO to that of TTA, and consequently results in the improvement of energy transfer in the E_uIII complex. DPEPO forms a complex with a more rigid and compact structure that can improve energy transfer between ligands and the center EuIII ion, support the higher saturation level by the coordinating ability of the oxygen atom in the ether moiety, and consequently enhance the PL intensity and efficiency of the corresponding Eu^III complex. The multilayered electroluminescent (EL) device of 1 used as the red dopant exhibited an impressive brightness of 632 cd m^-2 at 25 V. The device had the excellent voltage-independent spectral stability with an emission peak at 615 nm. To the best of our knowledge, this luminescence is the brightest emission among Eu complexes with phosphine oxide ligands. The maximum external quantum yield (η_ext) of 2.89% and the maximum current and power efficiency of 4.58 cd A^-1 and 2.05 Im W^-1 were achieved at a low turn-on voltage of 7 V and current density of 0.021 mA cm ^-2. These properties demonstrate that the chelate phosphine oxides ligand DPEPO can not only be favorable to form the rigid and compact complex structure and increase the efficiency of devices, but also reduce the ability of the formation of exciplex. DPEPO shows much better performance compared with the ordinary phosphine oxide ligand triphenylphosphine oxide.

Full text of DOI:10.1021/jp055355p

J. Phys. Chem. B 2006, 110, 3023-3029
3023
Application of Chelate Phosphine Oxide Ligand in EuIII Complex with Mezzo Triplet
Energy Level, Highly Efficient Photoluminescent, and Electroluminescent Performances
Hui Xu, Lian-Hui Wang, Xu-Hui Zhu, Kun Yin, Gao-Yu Zhong, Xiao-Yuan Hou, and
Wei Huang*
Institute of AdVanced Materials (IAM), Fudan UniVersity, Shanghai 200433, P. R. China
ReceiVed: September 21, 2005; In Final Form: December 12, 2005
The chelate phosphine oxide ligand bis(2-(diphenylphosphino)phenyl) ether oxide (DPEPO) was used as a
unit neutral ligand to prepare the complex Eu(TTA)3(DPEPO) 1 (TTA ) 2-thenoyltrifluoroacetonate).
Compound 1 has a photoluminescence (PL) quantum yield of 55.3%, which is more than the twice of the PL
quantum yield of Eu(TTA)3(TPPO)2 (TPPO ) triphenylphosphine oxide). Investigation indicated that DPEPO
in 1 has the mezzo first triplet excited energy level (T1) between the first singlet excited energy level (S1) and
T1 of TTA, which may support one more additional energy transfer routines from the T1 energy level of
DPEPO to that of TTA, and consequently results in the improvement of energy transfer in the EuIII complex.
DPEPO forms a complex with a more rigid and compact structure that can improve energy transfer between
ligands and the center EuIII ion, support the higher saturation level by the coordinating ability of the oxygen
atom in the ether moiety, and consequently enhance the PL intensity and efficiency of the corresponding
EuIII complex. The multilayered electroluminescent (EL) device of 1 used as the red dopant exhibited an
impressive brightness of 632 cd m-2 at 25 V. The device had the excellent voltage-independent spectral
stability with an emission peak at 615 nm. To the best of our knowledge, this luminescence is the brightest
emission among Eu complexes with phosphine oxide ligands. The maximum external quantum yield (ηext) of
2.89% and the maximum current and power efficiency of 4.58 cd A-1 and 2.05 lm W-1 were achieved at a
low turn-on voltage of 7 V and current density of 0.021 mA cm-2. These properties demonstrate that the
chelate phosphine oxides ligand DPEPO can not only be favorable to form the rigid and compact complex
structure and increase the efficiency of devices, but also reduce the ability of the formation of exciplex.
DPEPO shows much better performance compared with the ordinary phosphine oxide ligand triphenylphosphine
oxide.
Introduction
by the introduction of new substituents into the phenanthroline
ring due to the high delocalization of their n electrons. This
As an important family of luminescent materials, lanthanide
complexes have been studied as emitters for OLEDs because
of their nearly monochromic characteristic emission, chemical
environmental stability, and internal device quantum efficiency
that approaches theoretically 100%.1-5 It has been shown that
the introduction of charge injection or transporting moieties into
the ligands can improve the electroluminescent performance of
organic lanthanide metal ion-based materials.6 A group of EuIII
complexes with phenanthroline derivatives as the neutral ligands
have been studied extensively.7-14 The devices of these
complexes showed a line emission from the Eu complex with
maximum brightness values greater than 100 cd m-2,5 and their
device performance was much better than that of the complexes
with another neutral ligand triphenylphosphine oxide (TPPO),
of which devices have turn-on voltages more than 10 V and
weak electroluminescence (EL).13 The reason may be that the
flat structure of the phenanthroline derivatives makes the charge
injection and transport easier. As chelate ligands, phenanthroline
derivatives could form more rigid and stable complexes than
the single-coordinated phosphine oxide ligand TPPO. The
stability of complexes in excited state will influence electrolu-
minescent properties. However, the coordinating ability and the
charge injection and transport ability may be greatly affected
effect limits the modification of phenanthroline derivatives for
further improving the device performances. In the phosphine
oxide ligands, however, the coordinating ability of O atoms can
be less influenced by the groups bonding to phosphorus atoms.
The modification of TPPO would be much easier and more
flexible than phenanthroline derivatives. With the aim of
improving the coordinating ability and the carrier injection and
transport ability of TPPO, in this work we tried to select out
chelate phosphine oxide compounds as the unit neutral ligand.
The application of the chelate phosphine oxide ligands in the
luminescent rare earth (RE) complexes instead of single-
coordinated TPPO can not only enhance the stability of the
complexes but also improve the photoluminescent and elec-
troluminescent properties of the lanthanide complexes. We chose
bis(2-(diphenylphosphino)phenyl) ether oxides (DPEPO) as the
neutral ligand to prepare the tertiary complex Eu(TTA)3-
(DPEPO) 1. The choice of bis(2-(diphenylphosphino)phenyl)
ether oxides is based on its strong ability to coordinate lanthanide
ions, its structure rigidity, and its good electron transporting
ability. The chelate phosphine oxide ligand has the rigid
structure, which not only can enforce the rigidity and form the
more compact complexes so as to improve the triplet energy
transporting from ligands to the center RE ion, but also can
improve energy transfer between the two diphenyl phosphine
oxide moieties through the bridge moieties to eliminate the
* Corresponding author. Tel: +86 (21) 5566 4188; Fax: +86 (21) 6565
5123; E-mail: wei-huang@fudan.edu.cn.
10.1021/jp055355p CCC: $33.50 © 2006 American Chemical Society
Published on Web 02/02/2006

3024 J. Phys. Chem. B, Vol. 110, No. 7, 2006
Xu et al.
SCHEME 1: Molecular Structure and Synthetic
Procedure of Eu(TTA)3(DPEPO) 1
1H NMR (CDC13, 400 MHz): δ (ppm) 7.350-7.155 (m, 22
H, Ph-H), 6.950 (tr, 2 H, J ) 7.2), 6.800 (br, 2 H), 6.688 (q,
JH-H ) 8, JP-H ) 4, Ph-H). LDI-TOP mass (m/z): 539.2 (M+).
Bis(2-(diphenylphosphino)phenyl) ether oxides (DPEPO). A
2.69 g portion of bis(2-(diphenylphosphino)phenyl) ether (5
mmol) was dissolved in 10 mL of 1,4-dioxane at room
temperature. 1 mL of 30% H2O2 (10.5 mmol) was added
dropwise with vigorous stirring. The mixture was then stirred
for another 2 h. Then 10 mL of water was added to stop the
reaction. The mixture was extracted with 3 × 30 mL of
dichloromethane. The oil phase was washed with 2 × 30 mL
of water to remove 1,4-dioxane. The organic layer was dried
with MgSO4. The solvent was removed in vacuo. A white
powder was obtained with yield 2.82 g (99%).
formation of the exciplex.15 Furthermore, in DPEPO the O atom
of the ether bridge is also capable of coordinating to the center
metal.15 This effect can improve the luminescent properties of
the complex. The efficient and stable pure red emission from
the multilayer organic light emitting diodes (OLEDs) based on
1 is demonstrated.
1H NMR (CDC13, 400 MHz): δ (ppm) 7.751-7.558 (m, 9H,
Phen-H); 7.544-7.194 (m, 13H, Phen-H); 7.153 (tr, 2H, J )
8.4, Phen-H); 7.079 (tr, 2H, J ) 7.6, Phen-H); 6.040 (q, 2H,
JH-H ) 7.8, JP-H ) 5.6). LDI-TOP mass (m/z): 571.5 (M+).
Eu(TTA)3(DPEPO) 1.17 A 3.0 mmol sample of 2-thenoyltri-
fluoroacetone (TTA) was dissolved in ethanol, 3.0 mmol of
NaOH in water solution was added to remove the H+ in the
TTA molecule, then 1.0 mmol of EuCl3 6H2O in water solution
was added dropwise and stirred at 60 °C for 30 min. Finally,
1.0 mmol of bis(2-(diphenylphosphino)phenyl) ether oxides
(DPEPO) in ethanol was added in drops and stirred at 60-70
°C for 4 h. A white with pink hue precipitate was formed. The
powder was washed with water and ethanol in reflux and yields
1.005 g of the complex with yield of 72.5%. Its melt point is
Experimental Section
Materials and Instruments. All the reagents and solvents
used for the synthesis of 1 were purchased from Aldrich and
Acros companies. Alq3, BCP, CBP, and NPB used for EL device
fabrication were purchased from Lumthec Corporation. All the
reagents were used without further purification.
1H NMR spectra were recorded using a Varian Mercury plus
400NB spectrometer relative to tetramethylsilane (TMS) as
internal standard. Molecular masses were determined by a
Shimadzu laser desorption/ionization time-of-flight mass spec-
trometer (LDI-TOF-MASS). Elemental analyses were performed
on a Vario ELIII elemental analyzer. Thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC)
were performed on Shimadzu DSC-60A and DTG-60A thermal
analyzers under nitrogen atmosphere at a heating rate of 10 °C/
min. Absorption and photoluminescence (PL) emission spectra
of the target compound were measured in dichloromethane using
a Shimadzu UV-3150 spectrophotometer and a Shimadzu RF-
5301PC spectrophotometer, respectively. Phosphorescence spec-
tra were measured in dichloromethane using an Edinburgh FLS
920 fluorescence spectrophotometer at 77 K cooling by liquid
nitrogen.
1
253 °C. H NMR (CDC13, 400 MHz): δ (ppm) 9.117-8.781
(m, 9H, Phen-H); 7.752-7.432 (m, 17H, Phen-H and Th-H);
7.248-7.106 (m, 5H, Th-H); 6.929 (d, 2H, J ) 5.2, Phen-H);
6.855 (d, 2H, J ) 2.8, Phen-H); 6.653 (tr, 2H, J ) 4.2, Phen-
H); 4.059 (s, 3H, COCHCO). Anal. Calcd. for C60H40-
EuF9O9P2S3: C, 51.99; H, 2.91; Eu, 10.96; O, 10.39; S, 6.94;
Found: C, 51.86; H, 2.95; Eu, 11.03; O, 10.52; S, 7.01. ESI/
MS (m/z): 1386 (M+, 56%), 757 (M2+ + 2 CH3OH, 32%),
1166 (11%), 813 (11%), 571 (DPEPO+, 100%).
Synthesis of Gadolinium Complexes. For triplet energy level
measurement gadolinium complexes comprising neutral ligand
DPEPO without TTAs and comprising TTAs without neutral
ligand were also synthesized.
Preparation of Eu(TTA)3(DPEPO). Bis(2-(diphenylphos-
phino)phenyl) ether. Bis(2-(diphenylphosphino)phenyl) ether
was prepared according to the process described in the literature
(cf. ref 16) with some modifications (Scheme 1). A solution of
1.00 g of diphenyl ether (5.88 mmol) in 10 mL of THF, which
was previously treated with sodium, was added dropwise to a
stirred mixture of 8.1 mL of n-butyllithium (1.6 M in hexane,
12.96 mmol) and 2.08 mL of TMEDA (12.96 mmol) at room
temperature. The reaction mixture was stirred overnight. Then
a solution of 2.4 mL of chlorodiphenylphosphine (13.34 mmol)
in 10 mL of hexanes was added dropwise to the reaction mixture
during 30 min at 0 °C with an ice-water bath. The mixture
reacted at 0 °C for another 1 h. A white precipitate, which was
confirmed as LiCl, was formed. The mixture was then stirred
at room-temperature overnight. Then 10 mL of water was added
into the flask to stop the reaction. The mixture was extracted
with 3 × 30 mL of dichloromethane. The organic layer was
dried with MgSO4. The solvent was removed in vacuo. The
residue was purified by flash column chromatography using
dichloromethane as eluent to yield (4-diphenylphosphanyl-
phenyl) diphenylamine that is a white or sometimes straw yellow
sticky solid in 92% yield. The sticky solid was washed with
acetone and then dried in vacuo. Yield: 2.7 g of white powder
(85%).
Gd(DPEPO)(NO3)3. A 1.0 mmol portion of DPEPO was
dissolved in 50 mL of ethanol. A 1.0 mmol portion of Gd-
(NO3)3(H2O)6 in 0.1 mL of water was added in solution
dropwise under stirring and the solution was heated to refluxing
for 2 h. White precipitates were formed. The resulting solution
was filtered and a white powder was obtained.
Gd(TTA)3(H2O)2. A 3.0 mmol portion of HTTA was dissolved
in 50 mL of ethanol. 3.0 mL of 1.0 mol L-1 NaOH was then
added dropwise, and the mixture was stirred for 15 min. Then
1 mmol of Gd(NO3)3(H2O)6 in 0.1 mL of water was added in
solution dropwise under stirring and the solution was heated to
refluxing for 2 h. A yellow-with-pink powder was obtained by
precipitating from a ethanol and water solution.
Fabrication and Testing of OLEDs. Four-layer OLEDs were
fabricated by vacuum deposition with a configuration of ITO/
NPB (30 nm)/1: CBP (30 nm, 10%)/BCP (40 nm)/Alq3 (30
nm)/LiF (0.5 nm)/Al (100 nm), wherein NPB is N,N-bis(R-
naphthylphenyl)-4,4′-biphenyldiamine as the hole-transporting
layer, CBP is 4,4′-bis(carbazole-9-yl)biphenyl as host material,
BCP is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline as the
electron transporting/hole blocking layer, Alq3 was used as the
electron transporting layer, and ITO and LiF/Al were used as
the anode and cathode, respectively.18 Before loading into a

Novel Luminescent EuIII Complex
J. Phys. Chem. B, Vol. 110, No. 7, 2006 3025
Figure 2. Thermal analysis curves: DSC curve and TGA curve (inset
figure).
Figure 1. 1H NMR (400 MHz) of (a) DPEPO and (b) 1 in CDCl3.
The different kinds of the protons bonding to the diphenyl ether group
were marked with numbers and the corresponding peaks were marked
with the same numbers.
ppm, and one quadruplicate peak at 6.040. The mixed peaks
are attributable to the protons in the four phenyl groups bonding
to the phosphorus atoms and two protons at the 1 position of
the diphenyl ether group. The triplicate peaks at 7.153 and 7.079
ppm are attributable to the protons at the 2 and 3 position of
the diphenyl ether group with J ) 8.4 and 7.6 Hz. The peak of
the protons at 6.040 ppm at the 4 position is quadruplicate rather
than duplicate because of the interaction between the phosphorus
atom and the hydrogen atom. According to the spectrum JP-H
is 5.6 Hz and JH-H is 7.8 Hz. The lanthanide-based complexes
are well-known as shift reagents due to the paramagnetism of
the lanthanide ion. Therefore, the peaks of DPEPO in the
spectrum of 1 are much different with free DPEPO. One of the
mixed peaks of the protons in the four phenyl groups shifts
from about 7.60 ppm to nearly 9 ppm. The other mixed peak
shifts from 7.544 to 7.750 with addition of peaks attributable
to thiophyl groups from TTAs. The other mixed peaks attribut-
able to thiophyl groups are found at the range of 7.248-7.106
ppm. The single peak at 4.059 ppm is attributable to the protons
at the R position (COCHCO) of the three TTAs. The peak
attributable to the protons at the 2 position of the diphenyl ether
group shifts from 7.153 to 6.929 ppm. The peak attributable to
the protons at the 3 position shifts from 7.079 to 6.855 ppm,
and the peak of the protons at the 4 position is found at 6.653
ppm rather than 6.040 ppm former. The split states of the last
three peaks are also changed. Both two former triplicate peaks
change into duplicate peaks and the former quadruplicate peak
changes into a triplicate peak, because Eu(TTA)3 groups can
affect the coupling constants of these peaks from DPEPO.
Thermal Properties. The thermal properties were analyzed
by TGA and DSC (Figure 2). The decomposition temperature
(Td) of 1 is about 317 °C and the melt point (Tm) is 253 °C
with the temperature of glass transition (Tg) found at 93 °C. It
is shown that the thermal properties of 1 were better than that
of Eu(TTA)3(TPPO)2 (Td 300 °C, Tm 167 °C, and no Tg
observed),13 which originates from the more rigid structures of
DPEPO. Furthermore, stronger intermolecular force is proved
by higher Tm of 1, which is about 86 °C more than that of Eu-
(TTA)3(TPPO)2. Higher Td of 1 makes devices fabricated by
vacuum evaporation more feasible. Simultaneously the structure
of O bridge linkage, which limits the movement of diphe-
nylphosphine oxide moieties, increases the rigidity of DPEPO
and further of the whole complex. Compared with Eu(TTA)3-
(TPPO)2, in which TPPO has only one coordinating site so that
deposition chamber, the ITO substrate was cleaned with
detergents and deionized water, dried in an oven at 120 °C for
4 h, and treated with UV-ozone for 25 min. Devices were
fabricated by evaporating organic layers at a rate of 0.1-0.3
nm/s onto the ITO substrate sequentially at a pressure below 1
× 10-6 mbar. Onto the Alq3 layer, a layer of LiF with 0.5 nm
thickness was deposited at a rate of 0.1 nm/s to improve electron
injection. Finally, a 100-nm-thick layer of Al was deposited at
a rate of 0.6 nm s-1 as the cathode. The emission area of the
devices was 0.12 cm2 as determined by the overlap area of the
anode and the cathode. The EL spectra and current-voltage
luminescence characteristics were measured with a Spectrascan
PR 650 photometer and a computer controlled DC power supply
under ambient conditions.
Results and Discussions
Design, Synthesis, and Structure Analysis. With the purpose
of designing chelate phosphine oxides with the unit structure
using in EuIII complexes, the phosphine oxide compounds should
have not only the high coordinating ability but also should have
the good carrier transporting ability. And the structure of the
chelate ligands should improve the energy transfer between
ligands and the center metal ion. Thanks to the pivotal work of
McMillin, the comparison between bis(2-(diphenylphosphino)-
phenyl) ether (DPEphos) and triphenylphosphine in their CuI
complexes was discussed in detail.15,20 DPEphos can reduce the
distance between ligands and the center metal ions, and the O
atom of the ether bridge has the coordinating ability, which could
be inherited in its oxides. We thought the oxides of this ether
should be suitable as the chelate ligand for luminescent EuIII
complexes. Simultaneously, the conjugated moiety in DPEPO
could not only increase the rigidity of the complex, which will
be very important to avoid the structure relaxation in the excited
state, but also improve the thermal stability of the complex.
The 1H NMR spectra of DPEPO and the complex 1 obtained
at 400 MHz in d-chloroform solutions are shown in Figure 1a
and b. The 1H NMR spectrum of DPEPO consists of two mixed
peaks with the chemical shifts in the ranges of 7.751-7.558
and 7.544-7.194 ppm, two triplicate peaks at 7.153 and 7.079

3026 J. Phys. Chem. B, Vol. 110, No. 7, 2006
Xu et al.
Figure 3. Absorption and emission spectra in CH2Cl2 solution (dash
line and solid line) and emission spectrum in solid state (dot line).
Figure 4. Absorption and photoluminescence spectra of DPEPO (solid
lines) and TTA (dot lines) in CH2Cl2 solution.
it can move easier with little activating energy, 1 with the more
rigid structure needs higher temperature to overcome the
potential barriers when making its segments move, which should
be the reason 1 has a relatively high Tg. High Tg is supposed to
be good for increasing the device lifetime.
Optical Properties. Dilute CH2Cl2 solution of 1 (1 × 10-5
mol L-1) absorbs at 338 nm (Figure 3). When 1 was excited
with the appropriate radiations, the sharp characteristic emission
at 612 nm of the EuIII ion from 5D0
accompanied by the weak emissions corresponding to D0 -
7Fj (j ∼ 0, 1, 3). The solid-state emission of 1 was at 615 nm
with 3 nm shifting red.
-
7F2 was observed
5
The PL quantum yield of 1 in CH2Cl2 solution (1 × 10-5
mol L-1) was measured to be 55.3% using Ru(bpy)3Cl2 as the
standard. This PL quantum yield, which is more than the twice
of that of Eu(TTA)3(TPPO)2 (26.77%),19 indicates that DPEPO
is the better second ligand to improve the PL properties of the
Eu complex than TPPO. Compared with the single coordinated
ligand TPPO, the chelate phosphine oxide ligand has different
effects on the PL properties of its complex. First, with the
linkage of the two diphenylphosphine oxide groups by the ether
bridge, the volume of single DPEPO is smaller than two TPPO
and the O-Eu-O angle in 1 decreases compared with Eu-
(TTA)3(TPPO)2 because the bridge linkage restricts the move-
ment of the two diphenylphosphine oxide groups. This compact
structure makes the solvent molecule harder to get into the
structure of 1 so as to reduce the solvent-induced quenching.15
Simultaneously, the compact structure of 1 makes the three
TTAs get closer to the center metal so as to improve the triplet
energy transfer from TTAs to EuIII ion. This rigid structure
reduces the structure relaxation in the excited state as well.20
Finally, the oxygen atom in the ether bridge also has the
capability of coordinating to the center metal.20 Higher coor-
dinating saturation level is better to get a highly efficient
luminescent EuIII complex by increasing the interaction between
DPEPO and EuIII.
Figure 5. Phosphorescence spectra of complexes Gd(DPEPO)(NO3)3
(solid line) and Gd(TTA)3(H2O)2 (dot line).
TABLE 1: Singlet and Triplet State Energy Levels of the
Ligands
singlet energy
level (eV)
triplet energy
level (eV)
ligand
DPEPO
TPPO21
Phen21
TTA
3.94
4.51
3.87
3.12
2.99
2.35
2.09
2.35
possibility of the highly efficient energy transfer from DPEPO
to TTA.
To demonstrate the energy transfer process, the phosphores-
cence spectra of complexes Gd(DPEPO)(NO3)3 and Gd(TTA)3-
(H2O)2 were measured for the triplet energy level data of the
ligands (Figure 5). From the phosphorescence spectra, the triplet
energy levels of Gd(DPEPO)(NO3)3 and Gd(TTA)3(H2O)2,
which correspond to their peak emission wavelengths, are 2.99
eV (415 nm) and 2.35 eV (527 nm), respectively. Because the
6
lowest excited state P7/2 of GdIII is too high to accept energy
Energy Transfer between Ligands and EuIII. It is known
that in the lanthanide complexes neutral ligands often play a
role of absorbing and transporting energy to the other ligands
or the center metal ions. According to absorption and photo-
luminescence spectra of DPEPO and TTA (Figure 4), it is found
that there is a large area overlap between the emission peak of
DPEPO and the absorption peak of TTA from 285 to 398 nm,
which is almost equal to the range of the emission peak of
DPEPO. It means that nearly all the radiations from DPEPO
can be absorbed by TTA. This wide overlap shows the
from ligands, the data obtained from the phosphorescence
spectra actually reveal the triplet energy levels of DPEPO and
TTA in lanthanide complexes. The singlet state energy levels
of DPEPO and TTA are estimated by referencing their absor-
bance edges, which are 3.94 eV (315 nm) and 3.12 eV (398
nm), respectively. For comparison, the singlet and triplet state
energy levels are summarized in Table 1 and illustrated in Figure
6.
In Figure 6, the S1 energy levels of all the ligands match that
of TTA and the 5D0 energy level of EuIII. So the energy absorbed

Novel Luminescent EuIII Complex
J. Phys. Chem. B, Vol. 110, No. 7, 2006 3027
Figure 7. Current density-luminance-voltage characteristics.
Figure 8. Current and power efficiency curves.
Figure 6. Schematic energy level diagram and the energy transfer
process: S1, first excited singlet state; T1, first excited triplet state.
by the ligands could be transferred to the TTA or EuIII ion
directly. But it is known that the gap between two energy levels
should be intermediate. Too big or small would decrease the
efficiency of energy transfer. The energy gap between the S1
energy levels of DPEPO and TTA is 0.82 eV, which is very
close to the 0.75 eV between the S1 energy levels of phen and
TTA but much less than 1.29 eV between the S1 energy levels
of TPPO and TTA. Compared with TPPO, the energy gap
between DPEPO and TTA is more appropriate to improve
energy transfer from S1 of DPEPO to that of TTA. At the same
time the T1 energy level of phen is much lower than that of
TTA. So reverse energy transfer occurs.21 The reverse energy
transfer makes part of energy in T1 state of TTA lose rather
than transfer to EuIII. As the T1 state of phen is in the same
5
energy level as D0 of EuIII, it is difficult to make the positive
energy transfer occur. This effect results in the decrease of PL
intensity of Eu(TTA)3(phen). Similarly, the positive energy
transfer from T1 of TPPO to that of TTA is also very difficult
due to their same T1 energy levels. But the T1 energy level of
DPEPO is 0.64 eV higher than that of TTA, therefore the energy
from the T1 excited state of DPEPO can efficiently transfer to
the T1 excited state of TTA. Furthermore, part of S1 excited
states energy of DPEPO may transfer to its own T1 states. So
this part of energy can transfer to the T1 excited state of TTA
and further to EuIII efficiently. The process can be summarized
as five steps. Energy transfers from the S1 excited state of
DPEPO to that of TTA, then transfers from S1 to T1 excited
state of TTA; or energy transfers from S1 to T1 excited state of
DPEPO and then transfers from T1 excited state of DPEPO to
that of TTA. Finally energy transfers from T1 excited state of
TTA to 5D0 of EuIII. Obviously, different from phen and TPPO,
DPEPO may have one more additonal energy transfer routine
between T1 excited states of DPEPO and TTA, which can
improve the energy transfer from the ligands to the center ions.
EL Performances of Devices. Compound 1 was used as a
dopant emitter to fabricate electroluminescent devices with the
structure ITO/NPB (30 nm)/1:CBP (30 nm, 10%)/BCP (40 nm)/
Alq3 (30 nm)/LiF (0.5 nm)/Al (100 nm).
chelate phosphine oxide ligands with the more conjugated
structure in 1. Notably, the device has the maximum brightness
reaching to 632 cd m-2 at 25 V. To the best of our knowledge,
that is the highest brightness among the Eu complexes with
phosphine oxide ligand to date. However, the devices with Eu-
(TTA)3(TPPO)2 serving as a dopant had much weaker EL
emission intensity.13
The current and power efficiency curves of the device are
shown in Figure 8. The maximum external quantum yield (ηext)
of 2.89% and the maximum current and power efficiencies of
4.58 cd A-1 and 2.05 lm W-1 were achieved at a much lower
current density of 0.021 mA cm-2 at 7 V. At the practical
brightness of 100 cd m-2, the current and power efficiency
remained 1.98 cd A-1 and 0.445 lm W-1 (corresponding to ηext
of 1.25%) at a voltage of 14 V and a current density of 4.82
mA cm-2. Compound 1 is among the highest efficient europium-
based electroluminescent materials. The EL efficiency is much
better than that of devices using Eu(TTA)3(TPPO)2 as a dopant,
which originates from the introduction of the chelate phosphine
oxide ligand. The structure of the O bridge linking the two
diphenylphosphine oxide moieties increases the conjugated area
of the neutral ligand, which can improve carrier injection and
transport in 1. In another aspect, the introduction of chelate
phosphine oxide ligand also makes the complex more rigid,
which reduces the structure relaxation in the excited state.
Furthermore, the coordinating ability of the ether oxygen atom
in DPEPO as mentioned above can not only saturate the
coordination degree of the center RE ion to improve the
electroluminescent properties of the complex,11 but also
makes the energy transfer from DPEPO to EuIII ion easier and
The current density-luminance-voltage (I-L-V) charac-
teristics of devices are shown in Figure 7. The device gave off
the pure characteristic emission of EuIII ion at 616 nm and had
a considerably low onset voltage of 7.0 V at a luminance of 1
cd m-2, compared with 12 V of the device containing Eu(TTA)3-
(phen) and 14 V of the device containing Eu(TTA)3(TPPO)2.9,13
The lower turn-on voltage of the device may originate from
the improvement of the carrier’s injecting ability by introducing

3028 J. Phys. Chem. B, Vol. 110, No. 7, 2006
Xu et al.
Summary
In conclusion, a novel luminescent EuIII complex with chelate
phosphine oxide ligand, Eu(TTA)3(DPEPO), was designed and
synthesized. It is demonstrated that the chelate phosphine oxide
ligand DPEPO can not only be favorable to form the rigid
structure of the complex that is important to avoid the structure
relaxation in excited state, but also can improve energy transfer
between two diphenyl phosphine oxide moieties so as to reduce
the formation of exciplex. Furthermore, the compact structure
of DPEPO and the interaction between the O atom of ether
moiety and EuIII make the energy transfer from TTAs to EuIII
more efficient. In particular, DPEPO has the T1 energy level
between the S1 and T1 energy levels of TTA. This might enable
energy transfer from the T1 excited state of DPEPO to that of
TTA, which cannot occur in the complexes containing TPPO
or phen. These effects make the complex have the prominent
PL efficiency of 55.3% and the EL maximum current efficiency
of 4.58 cd A-1 (corresponding to ηext of 2.89%). Eu(TTA)3-
(DPEPO) seems attractive for practical applications due to the
excellent spectral stability, high brightness, and high current
and power efficiency at the brightness of 100 cd m-2. It is shown
that DPEPO could serve as an effective potential ligand to afford
high performance organolanthanide-based electroluminescent
materials. Most importantly, this compound shows the huge
potential of the chelate phosphine oxide derivatives serving as
ligands of the luminescent lanthanide complexes due to the easy
modification and synthesis.
Figure 9. EL spectra of device at applied voltages of 7, 14, 18, and
25 V.
TABLE 2: EL Performances of the Europium Complexes
max.
max.
max.
turn-on
current
energy
brightness voltage efficiency efficiency
dopant
(cd m-2
)
(V)
(cd A-1
)
(lm W-1
)
9Eu(TTA)3phen
505
< 200
634
12
14
7
13Eu(TTA)3(TPPO)2
Eu(TTA)3(DPEPO)
Acknowledgment. This work was financially supported by
the National Natural Science Foundation of China under Grants
60325412, 90406021, and 50428303 as well as the Shanghai
Commission of Science and Technology under Grants
03DZ11016, and 04XD14002 and the Shanghai Commission
of Education under Grants 2003SG03 and 2004SG06.
4.58
2.05
more efficient. The existence of this oxygen atom supports
another efficient channel of energy transfer between DPEPO
and EuIII.
The performances of OLEDs based on Eu(TTA)3phen or Eu-
(TTA)3(TPPO)2 doped into CBP as the emitter in the literature
are listed in Table 2 for comparison.9,13 It is shown that the EL
properties of Eu(TTA)3(DPEPO) exceed these popular europium
complexes in the brightness, turn-on voltage, and efficiency.
The electroluminescent spectra of the device at different
voltages are also shown in Figure 9. All of the figures show
the monochromatic light emission at 616 nm, which demon-
strates that charge trapping is the dominating luminescence
process.22 The EL spectra remain very stable when increasing
the voltage in the devices. It is not observed of any exciplex
emission and the disturbance of the emission from CBP and
Alq3. This is distinct from those devices where the exciplex
emission from the host materials doped with Eu(TTA)3(TPPO)2
could be observed clearly when the voltage increased.23 Thus
the present material and devices exhibited improved color
stability, which should be also the consequence of using the
chelate phosphine oxide ligand DPEPO instead of TPPO. The
O bridge moiety in DPEPO can improve the energy transfer
between two diphenylphosphine oxide groups to decrease the
formation of exciplex. Almost the same electroluminescence
spectra detected in a very wide voltage range from 7.0 to 25.0
V show that at high current density and voltage carriers were
still caught in the emitting layer by CBP and 1 rather than
entering into the NPB layer without recombination, and the
absence of the emission from CBP shows us the efficient energy
transfer between CBP and the complex at high voltages. It means
that DPEPO can improve carrier injection and nearly all of the
energy absorbed by DPEPO can be transferred efficiently to
EuIII ion.
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