Photoluminescence and electroluminescence properties of three ternary lutetium complexes

Article abstract of DOI:10.1039/b304451c

Three lutetium complexes, Lu(PMIP)₃(TPPO)₂ (A), Lu(PMIP)₃Bipy (B) and Lu(PMIP)₃Phen (C) (where PMIP, TPPO, Bipy and Phen stand for 1-phenyl-3-methyl-4-isobutyryl-5-pyrazolone, triphenylphosphinoxide, 2,2′-bipyridine and 1,10-phenanthroline, respectively), were synthesized and complex A was characterized by single-crystal X-ray diffraction analysis. Complex A was found to crystallize in the P2₁/n(14) space group. The photoluminescence spectra of the complexes reveal that all the complexes have a similar spectrum peaking near 440 nm; however, the intensity of the spectrum of complex A is about 100 times higher than that of the other two complexes. When these complexes were utilized to prepare light-emitting devices, that with complex A gave a blue light, originating from this complex, with the highest brightness of 119 cd m ^-2 at an applied voltage of 19 V, while the same configuration devices using the other two complexes exhibited green light peaking at 512 nm, which arises from exciplexes formed at the interface of TPD and the corresponding lutetium complex. A maximum luminance of 1010 cd m^-2 at 16 V from the exciplex was obtained with the highest power efficiency of 0. 13 lm W^-1 at 9 V and a turn-on voltage as low as 3 V for the device with a (ITO)/TPD(10 nm)/B(50 nm)/BCP(20 nm)/AIQ(40 nm)/Mg_0.9Ag _0.1(100 nm)/Ag (100 nm) configuration.

Full text of DOI:10.1039/b304451c

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Photoluminescence and electroluminescence properties of three
ternary lutetium complexes
Hao Xin,a Mei Shi,b Fu You Li,ab Min Guan,a De Qing Gao,a Chun Hui Huang,*ab
Kurash Ibrahimc and Feng Qin Liuc
a
State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University,
Beijing 100871, P. R. China. E-mail: hch@chem.pku.edu.cn; Fax: +86-10-62751708;
Tel: +86-10-62757156
Institute of Advanced Materials and Technology, Fudan University, Shanghai 200433,
b
P. R. China
Photoemission Spectroscopy Station, Institute of High Energy Physics, Chinese Academy of
Sciences, Beijing 100039, P. R. China
c
Received (in Montpellier, France) 23rd April 2003, Accepted 16th June 2003
First published as an Advance Article on the web 28th August 2003
Three lutetium complexes, Lu(PMIP)3(TPPO)2 (A), Lu(PMIP)3Bipy (B) and Lu(PMIP)3Phen (C) (where
PMIP, TPPO, Bipy and Phen stand for 1-phenyl-3-methyl-4-isobutyryl-5-pyrazolone, triphenylphosphinoxide,
2,20-bipyridine and 1,10-phenanthroline, respectively), were synthesized and complex A was characterized by
single-crystal X-ray diffraction analysis. Complex A was found to crystallize in the P21/n (14) space group. The
photoluminescence spectra of the complexes reveal that all the complexes have a similar spectrum peaking
near 440 nm; however, the intensity of the spectrum of complex A is about 100 times higher than that of the
other two complexes. When these complexes were utilized to prepare light-emitting devices, that with complex
A gave a blue light, originating from this complex, with the highest brightness of 119 cd mꢀ2 at an applied
voltage of 19 V, while the same configuration devices using the other two complexes exhibited green light
peaking at 512 nm, which arises from exciplexes formed at the interface of TPD and the corresponding lutetium
complex. A maximum luminance of 1010 cd mꢀ2 at 16 V from the exciplex was obtained with the highest power
efficiency of 0.13 lm Wꢀ1 at 9 V and a turn-on voltage as low as 3 V for the device with a (ITO)/TPD(10 nm)/
B(50 nm)/BCP(20 nm)/AlQ(40 nm)/Mg0.9Ag0.1(100 nm)/Ag (100 nm) configuration.
Introduction
Experimental
Organic light-emitting devices (OLEDs) are of great interest
because of their efficient emission in the visible region and their
possible application in the fabrication of flat panel full color
displays.1–6 Rare earth complexes, as organic light-emitting
materials, can be classified as either central-ion-emission or
ligand-emission complexes. The central-ion-emission com-
plexes, especially europium and terbium b-diketonates, have
aroused considerable attention due to their sharp emission
band and theoretical 100% quantum efficiency.7–9 Some rare
earth complexes, such as those of Y, La, Gd and Lu with
the 4f electronic configuration of their central ions being
empty, half-filled or filled, have no emission in the visible light
region and therefore belong to the group of ligand-emission
complexes, just like the complex AlQ. As electroluminescence
materials, these kinds of rare earth complexes have been
studied relatively rarely. In our group, devices based on
Y(PMIP)3Bipy,10 Gd(PMIP)3Bipy11 and Gd(PMIP)3Phen12
have been fabricated and reported.
In this paper, the synthesis of three ternary lutetium com-
plexes, tris(1-phenyl-3-methyl-4-isobutyryl-5-pyrazolone)bis-
(triphenylphosphinoxide) lutetium, Lu(PMIP)3(TPPO)2 (A),
tris(1-phenyl-3-methyl-4-isobutyryl-5-pyrazolone)-2,20-bipyr-
idine lutetium, Lu(PMIP)3Bipy (B), and tris(1-phenyl-3-
methyl-4-isobutyryl-5-pyrazolone)-1,10-phenanthroline lute-
tium, Lu(PMIP)3Phen (C), and a study of their photolumines-
cence (PL) and electroluminescence (EL) properties is
reported.
Materials
1-Phenyl-3-methyl-4-isobutyryl-5-pyrazolone (PMIP) was
synthesized in our laboratory.8 The hole transport material
N,N0-diphenyl-N,N0-bis(3-methylphenyl)-1,10-biphenyl-4,40-
diamine (TPD, 99%) was purchased from Aldrich and the hole
blocking material 2,9-dimethyl-4,7-diphenyl-1,10-phenylan-
throline (BCP, 98%) was obtained from Acros; they were used
as supplied. Tris(8-hydroxyquinolinato) alumimium (AlQ) was
synthesized in our lab and sublimed two times before use.
Indium tin oxide (ITO) glass substrate with a sheet resistance
of 15 O cmꢀ2 was kindly sent by China Southern Glass Hold-
ing Co. Ltd. as a gift.
Synthesis
The complexes (molecular structures shown in Fig. 1) were
synthesized by the method reported previously.8 For example:
for Lu(PMIP)3(TPPO)2 , to a 50 ml ethanol (95%) solution
containing 1 mmol LuCl3 , 3 mmol PMIP and 2 mmol TPPO,
3 ml of 1.0 mol Lꢀ1 NaOH was added dropwise under stirring
and the solution was refluxed for 24 hours. The resulting solu-
tion was filtered and a colorless crystal was obtained after 2
days. It was purified by recrystallization from ethanol solution
with a yield 85%. M.p.: 158–159 ꢁC. Anal. found (%): C 64.23;
H 5.21; N 5.75; calcd (%) for C78H75N6O8P2Lu: C 63.90; H
5.16; N 5.75. Complexes B and C were prepared and recrystal-
lized by the same procedure from ethanol–chloroform (2 : 1
DOI: 10.1039/b304451c
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face profile measuring system and an IL-1000 quartz crystal
monitor. Synchrotron radiation photon-electron spectroscopy
(SRPES) was detected by a photon-electron spectrometer (vsw
Scientific Instruments, Ltd.) through synchrotron radiation
under ultrahigh vacuum (3–4 ꢂ 10ꢀ8 Pa). The photon-electron
spectrum was recorded under 32.7 eV synchrotron radiation
light.
Results and discussion
Crystallography
The crystallographic data for complex A are listed in Table 1.
An ORTEP diagram for the common repeating unit is shown
in Fig. 2. The lutetium metal ion is octa-coordinated and the
geometry is a distorted bicapped trigonal prism. The PMIP
oxygen atoms [O(2), O(5), O(4), O(1)] and two TPPO oxygen
atoms [O(7), O(8)] form the trigonal prism. Another two oxy-
gen atoms [O(3) and O(6)] cap the two quadrilateral faces
O(1)–O(4)–O(8)–O(2) and O(7)–O(4)–O(8)–O(5). The average
Fig. 1 Chemical structures of the complexes.
˚
˚
V/V) solution with yields higher than 80%. Lu(PMIP)3Bipy
(m.p.: 254–255 ꢁC): Anal. found (%): C 58.31; H 5.05; N
10.06; calcd (%) for C52H53N8O6Lu: C 58.80; H 4.99; N
10.55. Lu(PMIP)3Phen (m.p.: 324–326 ꢁC): Anal. found (%):
C 59.14; H 4.63; N 9.85; calcd (%) for C54H53N8O6Lu: C
59.71; H 4.92; N 10.32.
Lu–O distance is 2.304 A [2.211(2)–2.453(3) A], which is
little smaller than the sum of radii of Lu3+ (0.977 A, eight
˚
13
coordinated) and O2ꢀ (1.42 A).
˚
Film formation properties
All three complexes can be evaporated to form homogeneous
ultrathin films. As an example, FT-IR spectra of complex A
as an 80 nm thick film evaporated on Si substrate and in the
solid state are shown in Fig. 3. It is clear that the two spectra
are identical with each other, indicating the complex is not
decomposed during evaporation. Complexes B and C show
the same behavior.
Crystallography
A crystal of complex A was mounted on glass fiber and trans-
ferred to a RIGAKU RAXIS RAPID imaging plate diffrac-
tometer with graphite-monochromated Mo Ka radiation
˚
(l ¼ 0.71073 A). The structure was solved by Patterson meth-
ods (SHELXS-97) and refined by full-matrix least-squares
methods using the program SHELXL-97. All non-hydrogen
atoms were refined anisotropically. The hydrogen atom posi-
tions were calculated in ideal geometries. The final cycle of
full-matrix least-squares refinement was based on 10 676
observed reflections [I > 2.0s(I)] and 857 variable parameters
and converged with R1 ¼ 0.0698, wR2 ¼ 0.0778. The maxi-
mum and minimum peaks on the final differential Fourier
UV-Vis spectra
UV-Vis spectra of the three complexes, the neutral ligands and
NaPMIP are shown in Fig. 4(a). The absorbance of TPPO is
mainly located in the higher energy range (less than 240 nm),
while the absorbances of Bipy and Phen are in a much lower
energy range, from 300 to 215 nm. However, in the ground
state, the UV-Vis spectra of all the three complexes are just
the sum of those of PMIPꢀ and the corresponding neutral
ligand, no obvious change is observed.
ꢀ3
˚
map were 0.863 and ꢀ0.630 e A
.
CCDC reference number 198777. See http://www.rsc.org/
suppdata/nj/b3/b304451c/ for crystallographic data files in
CIF or other electronic format.
PL properties
Normalized PL spectra (excitation and emission) of the com-
plexes are shown in Fig. 4(b). All three complexes show similar
emission spectra peaking near 440 nm; however, their intensity
Preparation of EL devices
Organic layers were sequentially deposited in one run by high
vacuum (9 ꢂ 10ꢀ4 Pa) thermal evaporation onto a pre-cleaned
ITO glass substrate. A shadow mask with 5 mm diameter
openings was used to define the cathode of a 200 nm thick
layer of Mg0.9Ag0.1 alloy, with a 100 nm thick Ag cap. The
thickness of the deposited layer and the evaporation speed of
the individual materials were monitored in vacuum with quartz
crystal monitors. The deposition rates were maintained to be
0.1–0.3 nm sꢀ1 for organic materials and 1.0–1.3 nm sꢀ1 for
the Mg0.9 :Ag0.1 alloy.
Table 1 Crystallographic data for Lu(PMIP)3(TPPO)2
Empirical formula
FW
C78H75N6O8P2Lu
1461.35
Monoclinic
P21/n (14)
13.2633(6)
23.0476(13)
22.9725(6)
91.390(14)
7020.3(5)
4
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
Angle/deg
3
˚
U/A
Apparatus
Z
calcd/gꢃcmꢀ3
1.383
293(2)
FT-IR spectra were taken on a Nicolet MAGNA-IR spec-
trometer. The photoluminescence and electroluminescence
spectra were measured on a Hitachi F-4500 fluorescence
spectrophotometer. UV-visible absorption spectra were
recorded using a Shimadzu 3100 UV-VIS-NIR spectropho-
tometer. The brightness was determined by a ST-86LA spot
photometer and a close-up lens with a focal spot of 5 mm in
diameter. The layer thickness was measured by a Dektak sur-
D
T/K
˚
l/A
0.71073
1.512
0.0698
m/mmꢀ1
a
R1 [I > 2s(I)]
wR2 a [I > 2s(I)]
0.0778
P
P
P
P
a
2
R1
¼
||Fo| ꢀ |Fc| |/ |Fo| wR2 ¼ { [w(Fo ꢀ Fc2)]/ [w(Fo2)2]}1/2
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Fig. 2 ORTEP diagram of Lu(PMIP)3(TPPO)2 with the thermal
ellipsoids drawn at the 30% probability level and the H atoms removed
for clarity.
is quite different: that of complex A is about 100 times higher
than that of complexes B and 2 : 1. Lutetium complexes are
ligand-emission materials, whose fluorescence originates from
the anionic ligand (in the three complexes, it is PMIPꢀ1). From
the UV-vis spectra in Fig. 4(a), it is clear that the absorption of
TPPO is located in the higher energy range of less than 240
nm while that of PMIPꢀ1, Bipy and Phen are in the same range
from 300 to 215 nm,as has been mentioned above; this indi-
cates that the singlet energy level of TPPO is higher than those
of the other three. In other words, the energy absorbed by
TPPO can be transferred to PMIPꢀ1 and strength its emission,
while the energy absorbed by Bipy and Phen cannot transfer to
PMIPꢀ1 efficiently, which leads to different PL intensities for
the three complexes.
Fig. 4 (a) UV-vis absorbance spectra of the complexes measured
from their CHCl3 solutions. (b) Normalized photoluminescence spec-
tra (excitation and emission) of the complexes A (—), B (---) and
C (ꢃ ꢃ ꢃ).
HOMO and LUMO energy levels
The ultraviolet photoelectron spectrum (UPS) was measured
from an evaporated ultrathin film (on ITO substrate) of each
complex under ultrahigh vacuum conditions using synchrotron
radiation as the energy source (hn, in this work it was 32.7 eV),
from which the first ionization energy (Ip) could be obtained
according to the equation reported by Tang et al.:14 Ip ¼
hn ꢀ f, where f stands for the width of the UPS spectrum.
The energy of the highest occupied molecular orbital (HOMO)
of each complex is equal to its Ip value. The f values obtained
were 26.15, 26.43 and 26.10 eV for complexes A, B and C,
respectively, so the HOMO level values for complexes A, B
and C are 6.55, 6.27 and 6.60 eV, respectively. The energy of
the lowest unoccupied molecular orbital (LUMO) of the com-
plex is estimated from its HOMO level and the energy gap
between the HOMO and LUMO, which can be obtained by
referring to the absorption spectrum of each complex; these
values are 3.35, 3.31 and 3.35 eV for complexes A, B and C,
respectively. Thus, the LUMO values are 3.20, 2.96 and 3.25 eV
for complexes A, B and C, respectively.
EL of complex A
The electroluminescence spectra of the devices based on com-
plex A are shown in Fig. 5. For a double layer device, A1, with
Fig. 5 EL spectra of devices using complex A as the emitting mate-
rial. Device A1: ITO/TPD(40 nm)/A(40 nm)/Mg0.9Ag0.1 ; device A2:
ITO/TPD(40 nm)/A(40 nm)/AlQ(40 nm)/Mg0.9Ag0.1 ; device A3:
Fig. 3 FT-IR spectra of complex A and its 80 nm-evaporated film on
a Si substrate.
ITO/TPD(20 nm)/A(40 nm)/BCP(20 nm)/AlQ(40 nm)/Mg0.9Ag0.1 .
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the configuration ITO/TPD(40 nm)/A(40 nm)/Mg0.9Ag0.1
,
the blue EL emission peaking at 403 nm was from TPD; no
emission from complex A was detected, which reveals that
complex A has good electron-transport characteristics. When
AlQ was used as the electron injection and transport material
to form device A2 with the configuration ITO/TPD(40 nm)/
A(40 nm)/AlQ(40 nm)/Mg0.9Ag0.1 , the emission (Fig. 5) peak-
ing at 513 nm was from AlQ, indicating that this complex has
good hole-transport properties as well. When a hole-blocking
layer (BCP) was introduced between AlQ and complex A
in device A3, ITO/TPD(20 nm)/A(40 nm)/BCP(20 nm)/
AlQ(40 nm)/Mg0.9Ag0.1 , a blue light (Fig. 5) was observed
and the maximum luminance of this device was 119 cd mꢀ2
at 19 V. The emission peaking at 465 nm was mainly from
complex A, although it is a little red-shifted compared to its
PL, with a shoulder from TPD. Fig. 6 depicts the energy level
diagram complete with the Fermi level positions for the indium
tin oxide (ITO) anode and Mg0.9Ag0.1 cathode. Other energy
levels shown in Fig. 6 are gathered from the literature.15–17
The EL spectrum can be explained as follows: the electron
injection barrier from complex A to TPD (0.7 eV) and the hole
injection barrier from TPD to complex A (0.7 eV) are equal; at
the same time, as complex A has very good electron-transport
characteristics, for device A1 the recombination occurs in the
TPD layer and blue light from TPD is observed. For device
A2, although the hole injection barrier (0.7 eV) from TPD into
the complex layer is much higher than electron injection from
AlQ to the complex layer (ꢀ0.2 eV), the holes and electrons
recombine in the AlQ layer, caused by the high hole-transport
rate in TPD and the low electron-transport rate in AlQ.18
When a BCP layer was inserted between complex A and
AlQ, the holes are prevented from entering the ALQ layer,
so device A3 emits light mainly from complex A.
Fig. 7 The normalized EL spectra of devices using complex B as
emitter. Device B1: ITO/TPD(40 nm)/B(40 nm)/Mg0.9Ag0.1 ; device
B2: ITO/TPD(20 nm)/B(60 nm)/Mg0.9Ag0.1 device B3: ITO/
TPD(10 nm)/B(50 nm)/BCP(20 nm)/AlQ(40 nm)/Mg0.9Ag0.1 . Insert:
PL spectra of TPD, complex B and their 1:1 mixture by coating them
on quartz slides from their corresponding CHCl3 solutions, the excita-
tion wavelengths are 380, 360 and 380 nm, respectively.
;
the UV-vis spectrum of this mixture is just the sum of its com-
ponents, indicating that the new species is not formed in the
ground state. Our previous work19 has proven that the exciplex
is formed between the excited stated of TPD and the ground
state of the complex in the interface. When a positive voltage
was applied to the ITO electrode, a maximum luminance of
160 cd mꢀ2 at 19 V from device B1 was observed. When the
thickness of TPD was decreased while the device kept a fixed
total thickness, the relative emission intensity from TPD
decreased and, at the same time, the light emitted changed
from blue green to pure green as the thickness of TPD changed
from 40 to 5 nm (Fig. 7, device B2).
EL of complex B
In order to gain higher performance from the exciplex,
device B3 with the configuration ITO/TPD(10 nm)/B(50
nm)/BCP(20 nm)/ALQ(40 nm)/Mg0.9Ag0.1 was fabricated.
As complex B has good hole-transport features and the
HOMO and LOMO energy levels of complex B (6.3 and
3.0 eV, respectively) are similar to those of complex A, it is
necessary to introduce BCP as the hole-blocking layer in
order to obtain emission from the exciplex. A green light
(Fig. 7) with a maximum brightness as high as 1010 cd
mꢀ2 at 16 V and a maximum luminous efficiency of 0.13
lm Wꢀ1 at 9 V was achieved with a turn-on voltage of 3 V
from this device. Such an improvement of the luminance
and the low turn-on voltage is the contribution of the
hole-transport properties of the complex and employment
of the electron injection layer of AlQ. It is well-known that
AlQ can carry a higher current density,11 which balances
carrier injection in the interface of TPD and the complex.
The EL spectra of the devices using complex B as emitter are
shown in Fig. 7. Device B1 is a double layer device with the
same configuration (ITO/TPD(40 nm)/B(40 nm)/Mg0.9Ag0.1
)
as device A1, however, its EL spectrum is quite different; in
spite of the minor emission from TPD peaking at 403 nm, a
new emission peaking at 512 nm appeared and no emission
from complex B was detected. This new emission is from the
exciplex formed between TPD and complex B. It is known that
the formation of an exciplex is a result of the charge transfer
between a donor and an accepter at their interface in the
excited state, which often occurs between planar molecules.
Neutral ligand Bipy becomes a rigid plane after it is coordi-
nated to Lu3+ and allows complex B to easily form an exciplex,
which is proven by the PL of their 1 : 1 mixture (Fig. 7, inset),
in which a new emission having the same feature as the EL
emission peaking at 512 nm is observed. On the other hand,
Fig. 8 The current density–voltage and luminance–voltage character-
istics of device B2: (ITO)/TPD(10 nm)/B(50 nm)/BCP(20 nm)/
AlQ(40 nm)/Mg0.9Ag0.1 .
Fig. 6 The energy diagram of the materials used in device A3.
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The current density–voltage and luminance–voltage charac-
teristics of device B3 are shown in Fig. 8.
of formation of the exciplex. A device with the configuration
ITO/TPD(10 nm)/B(50 nm)/BCP(20 nm)/AlQ(40 nm)/
Mg0.9Ag0.1 presented a maximum luminance of 1010 cd mꢀ2
at 16 V with the highest power efficiency, 0.13 lm Wꢀ1 at 9
V, and a turn-on voltage as low as 3 V; this is the highest
brightness obtained from an exciplex so far. Therefore, the
three lutetium complexes all have potential applications as
blue or green light-emitting materials. Our results indicate
not only that complexes with a high PL intensity can be used
as emitters in OLEDs, but also that those showing weak or
no PL have potential applications if they can form exciplexes
with a high EL efficiency.
EL of complex C
Complex C shows the same properties as complex B, that is, it
easily forms an exciplex with TPD emission peaking at 512 nm.
The EL spectra shown in Fig. 9 indicates this clearly. A double
layer device C1 with the configuration ITO/TPD(15 nm)/C(95
nm)/Mg0.9Ag0.1 achieved a highest brightness of 475 cd m2ꢀ1
at 17 V, which is greatly enhanced compared to the double
layer device B1, ITO/TPD(40 nm)/B(40 nm)/Mg0.9Ag0.1,
indicating that carrier injection is well-balanced in this device.
In order to eliminate the emission from TPD, in device C2 with
the configuration ITO/TPD(5 nm)/C(40 nm)/BCP (20 nm)/
AlQ(40 nm)/Mg0.9Ag0.1 the thickness of TPD was reduced
to 5 nm; as expected, no emission at 403 nm from TPD was
detected (Fig. 9, solid line), however, the performance of
device C2 decreased to 776 cd mꢀ2 at 17 V, caused by the
thinned TPD layer, which reduced hole injection.
Acknowledgements
The authors thank the State Key Program of Funda-
mental Research on Rare Earth Functional Materials (G
1998061308), NHTRDP 863 Program (2002AA243080) and
NNSFC (20221101) for financial support.
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Fig. 9 The normalized EL spectra of devices using complex C
as emitter. Device C1: ITO/TPD(15 nm)/C(95 nm)/Mg0.9Ag0.1 ; dev-
ice C2: ITO/TPD(5 nm)/C(40 nm)/BCP(20 nm)/AlQ(40 nm)/
Mg0.9Ag0.1
.
New J. Chem., 2003, 27, 1485–1489
1489