The effect of different neutral ligands on photoluminescence and electroluminescence properties of ternary terbium complexes

Article abstract of DOI:10.1021/jp037816h

Three terbium complexes Tb(tba-PMP)₃(TPPO) (A), Tb(tba-PMP) ₃(H₂O) (B), and Tb(tba-PMP)₃(Phen) (C) (where tba-PMP, TPPO, and Phen stand for ?2-diketone 1-phenyl-3-methyl-4-(tert-butylacetyl)-5-pyrazolone, triphe

Full text of DOI:10.1021/jp037816h

10796
J. Phys. Chem. B 2004, 108, 10796-10800
The Effect of Different Neutral Ligands on Photoluminescence and Electroluminescence
Properties of Ternary Terbium Complexes
†
‡
†
†
†
†
†
Hao Xin, Mei Shi, Xi Cun Gao, Yan Yi Huang, Ze Liang Gong, Dao Bo Nie, Hong Cao,
†
‡
,†,‡
Zu Qiang Bian, Fu You Li, and Chun Hui Huang*
State Key Laboratory of Rare Earth Materials Chemistry and Applications,
Peking UniVersity, Beijing, 100871, P. R. China, and Institute of AdVanced Materials and Technology,
Fudan UniVersity, Shanghai, 200433, P. R. China
ReceiVed: December 11, 2003; In Final Form: April 14, 2004
Three terbium complexes Tb(tba-PMP)
3 3 2 3
(TPPO) (A), Tb(tba-PMP) (H O) (B), and Tb(tba-PMP) (Phen) (C)
(where tba-PMP, TPPO, and Phen stand for â-diketone 1-phenyl-3-methyl-4-(tert-butylacetyl)-5-pyrazolone,
triphenyl phosphine oxide, and 1,10-phenanthroline, respectively) with different neutral ligands were synthesized
and characterized, and the mechanism of how the neutral ligands affect photoluminescence (PL) and
electroluminescence (EL) properties of terbium complexes was studied. Experiments revealed neutral ligand
TPPO and Phen strongly affect the terbium complex PL intensity, TPPO enhanced the PL intensity of complex
A, while Phen reduced the PL intensity of complex C compared to that of complex B. Investigation indicated
this is caused by the different excited energy levels between tba-PMP, TPPO, and Phen, which were obtained
from their phosphorescence spectra measured with their corresponding gadolinium complexes Gd(tba-PMP)
O)(EtOH), Gd(TPPO) (NO , and Gd(Phen) (NO . Compared to complex B, the energy absorbed by
TPPO can be efficiently transferred to tba-PMP and the central ion Tb due to its excited singlet and triplet
3
-
(H
2
2
3
)
3
2
3 3
)
3
+
5
3+
energy levels matching that of tba-PMP and the D
4
energy level of Tb , and consequently enhancing the
PL intensity of A, while on the contrary, negative energy transfer occurred between Phen and tab-PMP or
3+
5
3+
4
Tb since the triplet energy level of Phen is lower than that of tba-PMP and the D energy level of Tb as
well. Experimental results show terbium complex electroluminescence (EL) is greatly dependent on its PL
2
intensity, the performance achieved based on complexes A, B, and C being 9540 cd/m and 7.2 lm/W, 3230
2
2
cd/m and 1.17 lm/W, and 690 cd/m and 0.13 lm/W, respectively, with the power efficiency ratio A:B:C )
5
5:9:1, which was greatly enlarged compared to their PL intensity ratio A:B:C ) 2.1:1.3:1.
Introduction
mechanism of how the neutral ligand affects the lanthanide
complex PL intensity is still not clear, and in addition,
â-diketone eb-PMP is a liquid, which not only leads to difficult
purification (by chromatograph) during the synthesis of ligand,
but also makes the complex melting point relatively low (159.4
Lanthanide complexes, especially terbium and europium
complexes,1
-10
are of great interest in the field of electrolumi-
nescence (EL) because their emission is due to the electronic
transitions of the central ions that give a sharp narrow spectral
°
C), which is a disadvantage for practical application. In this
1
0
characteristic and 100% emission quantum efficiency. Re-
work, a new ligand 1-phenyl-3-methyl-4-(tert-butylacetyl)-5-
pyrazolone (tba-PMP) with a higher melting point (89-90 °C)
was synthesized, which is easily purified through recrystalli-
zation with a yield higher than 80%. By using tba-PMP as an
anionic ligand together with different neutral ligand, three
terbium complexes Tb(tba-PMP)3(TPPO) (A, mp 211.7 °C), Tb-
11
cently, we reported a new terbium complex Tb(eb-PMP)3TPPO)
eb-PMP and TPPO stand for 1-phenyl-3-methyl-4-(2-ethylbu-
(
tyryl)-5-pyrazolone and triphenyl phosphine oxide, respectively),
with experiments indicating that carrier transport was relatively
balanced in this complex, which led to the exciton being easily
confined in the emitting layer, and a performance of 8800 cd/
(
(
tba-PMP)3(H2O) (B, mp 249.1 °C), and Tb(tba-PMP)3(Phen)
C, mp 298.2 °C) were synthesized and their PL and EL
2
m , 9.4 lm/W, and external quantum efficiency 21 cd/A was
achieved in a device with the configuration of ITO/TPD (20
nm)/Tb(eb-PMP)3TPPO (50 nm)/BCP (20 nm)/AlQ (30 nm)/
Mg:Ag, which confirmed that a high-efficiency EL performance
could be obtained from undoped lanthanide complex. However,
our further experimental results12 showed that balanced carrier
injection is not the only factor that affects terbium complex EL
performance, another key aspect being the complex PL intensity.
Complexes with higher PL intensity will achieve higher EL
efficiency when the carrier injection is well-balanced. Our results
also revealed that TPPO could strengthen terbium complex PL
intensity while Phen gave the negative effect.1 However, the
properties are investigated and compared. By measuring the
excited energy level of tba-PMP, TPPO, and Phen from the
phosphorescence spectra of their corresponding gadolinium
complexes Gd(tba-PMP)3(H2O)(EtOH), Gd(TPPO)2(NO3)3, and
Gd(Phen)2(NO3)3, the energy transfer mechanism between
different ligands and the central terbium ion is suggested, which
is the reason for the different PL intensities of the three
complexes.
Experimental Section
2,13
Materials. 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline
BCP, 98%) and tert-butylacetyl chloride (99%) were obtained
from Acros; N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-bi-
phenyl-4,4′-diamine (TPD, 99%) was purchased from Aldrich;
*
To whom correspondence should be addressed. E-mail: hch@
(
qchem.pku.edu.cn. Phone: +86-(10) 62757156. Fax: +86-(10) 62757156
†
‡
Peking University.
Fudan University.
1
0.1021/jp037816h CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/24/2004

Ternary Terbium Complexes
J. Phys. Chem. B, Vol. 108, No. 30, 2004 10797
N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-diphenyl-4,4′-di-
amine (NPB) was kindly supplied by Technical Institute of
Physics and Chemistry, Chinese Academy of Science as a gift.
All were used as supplied. Tris(8-hydroxyquinolinato)alumim-
ium (AlQ) was synthesized in our lab and sublimated two times
before use. Indium tin oxide (ITO) glass substrate with a sheet
resistance of 15 Ω/0 was kindly supplied by China Southern
Glass Holding Co. Ltd.
Apparatus. NMR spectra were recorded on a Bruker
DRX300 instrument. The crystal was mounted on a glass fiber
and transferred to a RIGAKU RAXIS RAPID imaging plate
diffractmeter with graphite-monochromatized Mo KR radiation
(λ ) 0.71073 Å). The structure was solved by the Patterson
methods (SHELXS-97) and refined by the full-matrix least-
squares method, using the program SHELXL-97. TG analysis
was measured on a SDT 2960 simultaneous TG-DTA (Thermal
Analysis, U.S.A) under N2 with a temperature rising rate 10
deg/min. The photoluminescence (PL) and electroluminescence
(
EL) spectra were measured with a Hitachi F-4500 fluorescence
Figure 1. Chemical and crystal structures of complexes A, B, and C.
Ellipsoids are drawn at the 30% probability level and hydrogen atoms
were omitted for clarity.
spectrophotometer. Luminance-voltage (L-V) characteristics
were measured with a computer-controlled Keithley 2400
Sourcemeter unit with a calibrated silicon diode. FT-IR spectra
were taken on a Nicolet MAGNA-IR spectrometer. UV-visible
absorption spectra were recorded with a Shimadzu 3100 UV-
vis-NIR spectrophotometer. In the phosphorescence measure-
ment, the fourth harmonic 266 nm of the pulsed GCR-4 Nd:
YAG Laser (Spectra-Physics, USA) with 30 Hz repetition rate
and 6 ns pulse width was used as the excitation source, which
is close to the peak wavelength of the excitation band of the
terbium complexes. The emission from the samples was
collected by a set of focus lenses, dispersed by a Spectra-pro-
Anal. Calcd for C50H65N6O8Tb: C, 57.91; H, 6.32; N, 8.36.
Found: C, 57.47; H, 5.88; N, 8.36. Tb(tab-PMP)3(Phen) (C):
Anal. Calcd for C61H66N8O6Cl3Tb: C, 57.53; H, 5.19; N, 8.80.
Found: C, 58.01; H, 5.49; N, 8.86.
Synthesis of Gadolinium Complexes. Gadolinium com-
plexes comprising solely neutral ligands without anionic ligands
or comprising only tab-PMP ligand without neutral ligand were
also synthesized for triplet energy level measurement. Gd(tba-
PMP)3(H2O)2: To a 50-mL ethanol solution containing 1 mmol
of Gd(NO3)3(H2O)6 and 3 mmol of 1-phenyl-3-methyl-4-(tert-
butylacetyl)-5-pyrazolone (tba-PMP) was added 3 mL of 1.0
5
00 mm focal length monochromator (ARC) with a 2400
grooves/mm grating and detected by the PD 493 photomultiplier
tube (ARC). To reduce the laser scattering light, a BG-24 filter
-1
mol‚L NaOH dropwise under stirring and the solution was
(
Schott) was placed in front of the monochromator. The output
of the photomultiplier was sent to the SD 2000 boxcar system
Ocean Optics) for averaging and then transferred to the
heated to refluxing for 5 h. The resulting solution was filtered
and the colorless crystal was obtained after 2 days. The crystal
was then purified by recrystallization from ethanol solution. Gd-
(TPPO) (NO ) : To a 50-mL ethanol solution containing 2
(
computer. All the experiments are completed at room temper-
2
3 3
ature.14
mmol of TPPO was added 1 mmol of Gd(NO ) (H O) dropwise
3
3
2
6
Synthesis of tba-PMP. 1-Phenyl-3-methyl-pyrazolone-5
under stirring and the solution was heated to refluxing for 2 h.
The resulting solution was filtered and a white powder was
obtained. Gd(Phen)2(NO3)3: The synthesis of Gd(phen)2(NO3)3
was the same as that for complex Gd(TPPO)2(NO3)3.
(
PMP) (7.5 g, 0.05 mol) was dissolved in 150 mL of dried 1,4-
dioxane and the solution was heated to 70 °C under stirring for
0 min. After calcium hydroxide (9 g, 0.18 mol) and barium
1
hydroxide (1 g, 0.03 mol) were added in small portions, tert-
butylacetyl chloride (8 mL, 0.058 mol) was added dropwise.
The resulting mixture was refluxed for 24 h. The cloudy pinkish
mixture was cooled to room temperature and then poured into
a stirred solution of ice-cold hydrochloric acid (350 mL of a 3
mol L solution) and yellow product precipitated. The product
was purified by recrystallization from ethanol solution and a
yellow needle crystal was obtained with a yield of 80%. Mp
Results and Discussion
Crystallography. Single crystals of complexes A and B were
obtained by recrystallization from their ethanol solution and
complex C was obtained from its CHCl3 solution. Complex B
lost the ethanol molecule in the single crystal. Their chemical
and crystal structures are shown in Figure 1. For complexes A
and B, the central Tb(III) ions are surrounded by seven oxygen
atoms, six of which are from three â-diketonates, with the other
being respectively from triphenyl phosphine oxide and the water
molecule; for complex C, the central Tb(III) ion is surrounded
by six oxygen and two nitrogen atoms, respectively from
â-diketonates and 1,10-phenanthroline, and in addition, there
-1
8
1
9-90 °C. Anal. Calcd for C16H20N2O2: C 70.23; H 7.38; N
1
0.24. Found: C 70.66; H 7.65; N 10.07. H NMR (200 Hz,
CDCl3) δ:7.87 (2H), 7.45 (2H), 7.28 (1H), 2.62 (2H), 2.48 (3H),
1
.86 (4H), 1.12 (9H).
Synthesis of Terbium Complexes. Complexes A, B, and C
8
were synthesized according to the method reported previously.
For complexes A and B, the reaction was carried out in ethanol
solution, while for complex C, the solvent was a mixture of
ethanol and CHCl3 ((v/v) 1:1). The complexes were all purified
by recrystallization from the same solvent as the reaction with
a yield no less than 80%. Tb(tab-PMP)3(TPPO) (A): Anal.
Calcd for C66H72N6O7PTb: C, 63.14; H, 5.74; N, 6.70. Found:
C, 63.32; H, 5.78; N, 6.76. Tb(tab-PMP)3(H2O) (EtOH) (B):
exists a CHCl molecule in the crystal of complex C. The
3
average Tb-O distance is 2.323 (range 2.281(2)-2.399(4) Å),
2.255 (range 2.282(4)-2.404(4) Å), and 2.341 Å (range 2.287-
(4)-2.396(4) Å) for complexes A, B, and C, respectively, which
is reasonable and coincides with the standard bond length for
different coordination numbers in terbium complexes.15
TG Analysis. The TG curves of complexes A, B, and C are
shown in Figure 2. It is clear that complex A is very stable at

10798 J. Phys. Chem. B, Vol. 108, No. 30, 2004
Xin et al.
Figure 2. TG curves of complexes A, B, and C.
Figure 3. UV-vis spectra of the three complexes and neutral ligands.
temperatures below 300 °C while complexes B and C lost their
water, ethanol, and CHCl3 molecules, the lost weight ratio being
7
.4% and 8.2% for complexes B and C, respectively, which
match the weight percent (6.18% and 9.30%) of the correspond-
ing lost molecules within experimental error.
UV-Vis. UV-vis absorbance spectra of the three complexes
and the ligands measured from their CHCl3 solutions are shown
in Figure 3. Complexes A and C had similar UV-Vis
characteristics with one main peak at around 265 nm, originating
from tab-PMP1 (the same as complex B), and the other
absorbance at lower wavelength contributed by the neutral ligand
TPPO and Phen, respectively; the absorbance of Phen became
blue-shifted in complex C due to the coordination of the nitrogen
atoms.
-
Figure 4. Phosphorus spectra of complexes Gd(tab-PMP)
3 2 2
(H O) , Gd-
(TPPO (NO ) , and Gd(Phen) (NO ) .
)2
3
3
2
3 3
Phosphorescence Spectra. The phosphorescence spectra of
complexes Gd(tba-PMP)3(H2O)2, Gd(TPPO)2(NO3)3, and Gd-
(Phen)2(NO3)3 are shown in Figure 4. From the phosphorescence
spectra, the triplet energy levels, which correspond to their peak
emission wavelength, are 2.51 (465 nm), 2.35 (530 nm), and
2
.09 eV (595 nm) respectively for Gd(tba-PMP)3(H2O)2, Gd-
(TPPO)2(NO3)3, and Gd(Phen)2(NO3)3.
Photoluminescence. PL spectra (excitation and emission) of
the three complexes measured from their 80-nm evaporated
films are given in Figure 5. Since during evaporation complexes
B and C lost their water and ethanol or CHCl3 molecules,
respectively, confirmed by TG, the emitting species were
respectively Tb(tba-PMP)3 and Tb(tba-PMP)3(Phen) for com-
plexes B and C (here and after) in the evaporated films. All
three complexes had the same excitation spectra and emitted
the characteristic bands of the central Tb3 ion with the main
Figure 5. Photoluminescence spectra (excitation and emission) of
complex A, B, and C measured from their 80 nm vacuum evaporated
films on quartz substrates.
+
neutral ligands. Compared to complex B without neutral ligand,
TPPO strengthened the PL intensity of complex A, while on
the contrary, Phen made the PL intensity of complex B decrease.
This phenomenon could be explained by the energy transfer
between ligands and the central metal ion. The singlet state
energy levels of tab-PMP, TPPO, and Phen are estimated by
3
+
peak at around 545 nm, assigned to the Tb f-f transition of
5
7
D4 f F5. However, the emission intensity (integral area) is
quite different for each complex with a ratio of A:B:C ) 2.1:
.3:1. Obviously, this difference no doubt originated from the
different coordination environments caused by the different
1

Ternary Terbium Complexes
J. Phys. Chem. B, Vol. 108, No. 30, 2004 10799
Figure 6. Schematic energy level diagram and the energy transfer
Figure 7. Normalized EL spectra of device A1 (ITO/TPD (20 nm)/A
(50 nm)/BCP (20 nm)/AlQ (30 nm)/Mg0.9Ag0.1), device B1(ITO/TPD
process:. S
1
, first excited singlet state; T
1
, first excited triplet state.
(
30 nm)/B (30 nm)/BCP (30 nm)/AlQ (20 nm)/Mg0.9Ag0.1), and
TABLE 1: The Singlet and Triplet State Energy Level of
the Ligands
device C1 (ITO/TPD (20 nm)/C (50 nm)/BCP (20 nm)/AlQ (30 nm)/
Ag0.9Ag0.1) at an applied voltage of 10 V.
singlet energy
level/eV
triplet energy
level/eV
TABLE 2: PL and EL Property Comparison of the Three
Complexes A, B, and C
complexes
tab-PMP
TPPO
Phen
3.81
4.51
3.87
2.51
2.35
2.09
rel PL
intensity
brightness
(cd/m )
power eff
(lm/W)
2
complexes
A
B
C
2.1
1.3
1
9540
3230
690
7.12
1.17
0.13
referencing their absorbance edge; they are 3.81 (325 nm), 4.51
275 nm), and 3.87 eV (320 nm), respectively. For comparison
(
and clarity, the singlet and triplet energy levels of the ligands
are summarized in Table 1 and illustrated in Figure 6. In
complex A, the S1 energy level of TPPO matches that of tba-
2
obtained from device B1 was 3230 cd/m (18 V) and 1.17 lm/
W, similar to that of its analogue complex Tb(PMIP)3 (2800
2
5
3+
cd/m , 2.61 lm/W). The best EL (Figure 7) presentation obtained
from complex C was 690 cd/m and 0.13 lm/W in a device
device C1) having a structure of (ITO/TPD (20 nm)/C (50 nm)/
PMP and the D4 energy level of Tb , so the energy absorbed
2
by TPPO could be transferred to tba-PMP or Tb3+ ion directly,
(
leading to the enhancement of the PL intensity of complex A.
For complex C, as the S1 state of phen is in the same energy
level as tba-PMP, it is difficult for the positive energy transfer
to occur, while at the same time, since the T1 energy level of
BCP (20 nm)/AlQ (30 nm)/Mg0.9Ag0.1), presenting the same
magnitude to its analogue complex Tb(PMIP)3(Phen).16
The PL properties and the best EL performance of the three
complexes obtained from their corresponding multilayer devices
are summarized in Table 2. The PL intensity of complex A was
twice that of complex B, while the EL efficiency achieved from
complex A was more than 6 times higher than that from complex
B. Similarly, the PL intensity of complex C decreased to half
that of complex B and the EL efficiency obtained from complex
C decreased by nearly one magnitude. This result indicated the
terbium complex PL intensity greatly affects its EL performance.
5
3+
Phen is much lower than that of tba-PMP and D4 of Tb , the
reverse energy transfer occurred, resulting in a PL decrease of
complex C compared to complex B. In addition, the main
emission of complexes B and C peaked at 545 nm and split
into two peaks at 542 and 548 nm and the half-width became
wider compared to complex C; this may be caused by the
different coordination environments of the central ions as the
polymerization may occur because of their unsaturated coor-
dination number.13
Electroluminescence. Our previous report has confirmed that
balanced carrier injection and a well-confined recombination
zone in the emitting layer are crucial for lanthanide complex
EL performance enhancement.13 As complex A has a similar
structure to complex Tb(eb-PMP)3(TPPO), it shows the same
EL properties. In device A1 with the configuration of ITO/TPD
Conclusions
In summary, we report the effect of the neutral ligands (TPPO,
Phen) on the PL and EL properties of the terbium complexes.
Our experimental results revealed that neutral ligands strongly
affect the terbium complexes PL quantum yield due to their
different excited singlet and triplet energy levels. Compared to
complex B, if the energy absorbed by the neutral ligand (TPPO)
can be efficiently transferred to negative-charged ligand (tba-
(20 nm)/complex A (50 nm)/BCP (20 nm)/AlQ (30 nm)/Mg0.9-
Ag0.1, the recombination zone was well confined in the complex
layer (EL spectrum shown in Figure 7) and a performance of
2
3+
brightness 9540 cd/m , power efficiency 7.12 lm/W, and peak
PMP) or the central Tb ion, the PL of the complex is
external quantum efficiency 18.1 cd/A was achieved from this
device, and the peak power efficiency occurred at a luminance
enhanced, but if the reverse energy transfer occurs, the complex
PL intensity decreases. Our experiments further confirmed that
high PL yield and well-balanced carrier injection are both
important for terbium complexes EL performance. High PL
intensity complexes can be synthesized by utilizing more
suitable ligands (both negatively charged and neutral ligands);
the second issue can be satisfied by both ligand modification
2
of 447 cd/m and it was still higher than 2.0 lm/W at a brightness
2
11
4
000 cd/m , similar to complex Tb(eb-PMP)3(TPPO).
Although complex B has as good hole-transport properties
as complex Tb(PMIP),1
1,12
after introducing the hole-blocking
layer BCP and altering the thickness of different layers to adjust
the injection ratio of electron and hole, device B1 with the
configuration of ITO/TPD (30 nm)/complex B (30 nm)/BCP
2
and device optimization. The performance of 9640 cd/m , 7.12
lm/W, and external quantum efficiency 18.1 cd/A achieved from
complex Tb(tab-PMP)3(TPPO) (A) was one of the best perfor-
mances of terbium complexes. Our experimental results provide
(30 nm)/AlQ (20 nm)/Mg0.9Ag0.1 emitted pure green light
originating from complex B (Figure 7), and the performance

10800 J. Phys. Chem. B, Vol. 108, No. 30, 2004
Xin et al.
a useful guide for future work in lanthanide complex EL
performance enhancement.
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Acknowledgment. The authors thank the State Key Program
of Fundamental Research on Rare Earth Functional Materials
G 1998061308), NHTRDP 863 Program (2002AA324080),
NNSFC (20221101), and NNSFC(50372002) for financial
support. The authors also thank Prof. Xin Sheng Zhao and Prof.
Guo Qiang Yang for their helpful discussion.
(
Phys. Lett. 1998, 72, 2217.
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(
2
Supporting Information Available: X-ray crystallographic
data for complexes A, B, and C (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
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