Dendritic europium complex as a single dopant for white-light electroluminescent devices

Article abstract of DOI:10.1039/b504738b

Three novel dendritic europium complexes containing grafted carbazole units as their functional peripheries were prepared and characterized. The Foerster energy transfer from the peripheries of carbazole units to β-diketonates as well as from the ligands

Full text of DOI:10.1039/b504738b

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www.rsc.org/materials | Journal of Materials Chemistry
Dendritic europium complex as a single dopant for white-light
electroluminescent devices{
Shangfeng Li,a Gaoyu Zhong,b Weihong Zhu,a Fuyou Li,b Jianfeng Pan,a Wei Huangb and He Tian*a
Received 5th April 2005, Accepted 14th June 2005
First published as an Advance Article on the web 30th June 2005
DOI: 10.1039/b504738b
Three novel dendritic europium complexes containing grafted carbazole units as their functional
peripheries were prepared and characterized. The Fo¨rster energy transfer from the peripheries of
carbazole units to b-diketonates as well as from the ligands to the central Eu(III) ions in these
complexes has been demonstrated. Under the indirect excitation of carbazole units, a
5
7
characteristic red emission peak at 615 nm due to the Eu(III) ion transition of D0 A F2 was
observed for the complexes in solution or solid films. As a result of the light-harvesting and site
isolation of carbazole dendrons, compared with the reference compound Eu(BPPD)3(Phen), PL
efficiencies of Eu(MCPD)3(Phen), Eu(BCPD)3(Phen) and Eu(TCPD)3(Phen) in the film state are
3.3, 7.9 and 4.5 fold increased, respectively. Notably, white light emission with the CIE
coordinates of (0.333, 0.348) has been achieved with a single dopant of Eu(TCPD)3(Phen) in an
EL device of configuration ITO/NPB/CBP : Eu(TCPD)3(Phen)/BCP/Mg : Ag. Its maximum
external quantum efficiency is more than 1.1% and maximum brightness reached 229 cd m22. The
origin of the blue and green emission is attributed to the dendron-grafted carbazole units forming
an exciplex and an electroplex with CBP (4,49-N,N9-dicarbazole biphenyl) and ligand
b-diketonate, respectively. The results suggest that modifying ligands can not only tune the
carrier-transporting properties of complexes, but also provide a useful clue to the use of
electroplexes or exciplexes to realize broad or even white electroluminescence.
intensity and limits the application for OLED.4 In addition,
Introduction
due to their low thermal stability, the lanthanide complexes
are only suitable for applications in an appropriate tem-
perature range.
Although numerous investigations on organic electrolumines-
cent (EL) devices using organic dyes and polymers have been
extensively reported, lanthanide complexes are still of great
interest because they exhibit extremely sharp emission bands
due to their 4f electrons.1 Furthermore, their internal quantum
efficiencies are not a limitation (up to 100%, theoretically)
because both the singlet and triplet states are involved in the
luminescence process. Recently, a remarkable EL break-
through (12 000 cd m22, 11.4 lm W21) obtained from a
terbium complex Tb(eb-PMP)3TPPO has been reported.2
However, lanthanide complexes have not shown satisfying
EL performance.3 The major drawbacks of lanthanides are
their low extinction coefficients and efficient luminescence
quenching by matrix vibrations like hydroxyl groups via
non-radiative pathways, which decreases their luminescence
However, these drawbacks are thought to be avoidable
by using an appropriate dendritic multichromophore as an
antenna system.5 In a dendritic antenna, the peripheral donor
units can collect photons, and transfer excitation energy to a
coordinated lanthanide ion, thus greatly enhancing lumines-
cence efficiency. Moreover, the site-isolation or dilution effect
of dendrons enables the destructive quenching by the chemical
surroundings to be avoided. However, in comparison with
dendritic iridium complexes,5a,b research on europium den-
drimers is very rare except for several cases with simple non-
functional peripheries.6 Also little work has been done about
white light OLEDs made from dendritic europium complexes
with functionalized dendrons. In our previous work,7 we used
the Fre´chet-type poly(aryl ether) to develop a dendritic
europium complex Eu(BPPD)3(Phen) (tris[1-[3,5-bis(phenyl-
methoxy)phenyl]-3-phenylpropane-1,3-dione](1,10-phenanthroline)-
europium(III)), which shows enhanced luminescence by the
dendron isolation effect. Here we report three novel
dendritic europium complexes Eu(MCPD)3(Phen) (tris[1-
[4-[4-(9H-carbazol-9-yl)butoxy]phenyl]-3-phenylpropane-1,3-
dione](1,10-phenanthroline)europium(III)), Eu(BCPD)3(Phen)
(tris[1-[3,5-bis[4-(9H-carbazol-9-yl)butoxy]phenyl]-3-phenyl-
aLab. for Advanced Materials and Institute of Fine Chemicals,
East China University of Science & Technology, Shanghai 200237,
P. R. China
bInstitute of Advanced Materials, Fudan University, Shanghai 200433,
P. R. China
{ Electronic supplementary information (ESI) available: Synthesis
and full characterization of all dendritic ligands and complexes
Eu(MCPD)3(Phen), Eu(BCPD)3(Phen) and Eu(TCPD)3(Phen);
MALDI-TOF mass spectrum of dendritic europium complex
Eu(TCPD)3(Phen) (Fig. S1); schematic diagram of energy transfer in
Eu(III) complexes (Fig. S2); five peaks are corresponding to 5D0 A 7Fj
propane-1,3-dione](1,10-phenanthroline)europium(III))
and
(j
5 0–4) transitions (Fig. S3); CIE chromaticity coordinates
Eu(TCPD)3(Phen) (tris[1-[3,4,5-tris[4-(9H-carbazol-9-yl)butoxy]-
phenyl]-3-phenylpropane-1,3-dione](1,10-phenanthroline)euro-
pium(III)) (shown in Scheme 1) were synthesized. With
(0.333, 0.348) of EL device with the configuration of ITO/NPB/CBP :
Eu(TCPD)3(Phen)/BCP/Mg : Ag at the applied voltage of 16.2 V
(Fig. S4). See http://dx.doi.org/10.1039/b504738b
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J. Mater. Chem., 2005, 15, 3221–3228 | 3221

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carbazole units assembled around
a
europium ion to
as site-isolation and hole-transporting. The incorporation of
form a light-harvesting antenna, Eu(MCPD)3(Phen),
carbazole units into the corresponding hydroxyl-substituted
benzoates can occur smoothly in the presence of potassium
carbonate and 18-crown-6 in acetone under reflux. The result-
ing dendritic esters underwent condensation to obtain related
ligands. As in the case of Eu(TCPD)3(Phen) illustrated in
Scheme 2, the dendritic ligand was obtained by Claisen
condensation with acetophenone in the presence of sodium
hydride in anhydrous THF. The dendritic Eu(III) complex
was finally obtained in good yield by treatment of the
corresponding ligand with EuCl3?6H2O in ethanol–THF
mixed solvent. All dendritic ligands and complexes were fully
characterized by 1H NMR and FAB or MALDI-TOF mass
spectrometry as reported in electronic supplementary informa-
tion (ESI), Fig. S1.{
Eu(BCPD)3(Phen) and Eu(TCPD)3(Phen) are sensitive to UV
excitation light and emit a brighter red photoluminescence
than the reference dendritic compound Eu(BPPD)3(Phen) in
both solution and thin solid films. In our design, the
phenanthroline ligand was included to enhance the electron
transport, and the carbazole-grafted fragments were
appended onto the diketonate ligand with the intention of
improving hole-transporting ability. Generally, to obtain a
certain white light CIE it is necessary to use three phosphors
with three emission bands in the red, green and blue ranges.
Single lanthanide complexes are not able to produce white
light of special CIE coordinates but only one specific CIE
coordinate set. To the best of our knowledge, we have
observed for the first time unexpected white EL emission
using a single dopant, Eu(TCPD)3(Phen), in a device of
configuration ITO/NPB/CBP : Eu(TCPD)3(Phen)/BCP/Mg :
Ag. The dendron-grafted carbazole units are thought to be
the origin of blue and green emissions by forming an
exciplex and an electroplex with CBP (4,49-N,N9-dicarbazole
biphenyl) and b-diketonate, respectively. Our results show
that modifying ligands can not only tune the complex
carrier-transport properties, but also provide a useful clue to
the use of electroplexes or exciplexes8 to realize broad or
even white electroluminescence.
Light-harvesting antenna of grafted carbazole units in europium
complexes
The absorption and luminescence spectroscopy data for
europium complexes and reference carbazole in CH2Cl2
(2.0 6 1025 mol L21) and solid film are shown in Table 1.
In CH2Cl2, the absorption peaks at around 331 and 345 nm
are assigned to the p–p* transition of the carbazole moiety,
and the peak about 360 nm is that of b-diketonate. As shown
in Table 1, the absorbance of carbazole units is proportional
to the grafted carbazole number within experimental error,
which indicates that the desired number of carbazole units is
successfully incorporated into the b-diketonate ligand.
Results and discussion
Generally, because the central lanthanide(III) ion shows little
or no absorption in the visible light region, the luminescence
of lanthanide complexes is critically dependent on the energy
transfer from the ‘‘light-harvesting antennae’’ ligand (donor)
to the central lanthanide ion. Also the excitation mechanism of
the central metal ion is quite different from that of common
organic fluorescent compounds. For Eu(III) complexes with
p-conjugated ligands such as b-diketonate, Eu(III) ions are
excited via intramolecular energy transfer from triplet excited
states of ligands (shown in ESI Fig. S2{).3 To improve the
energy transfer to Eu(III) ions, the energy level of triplet states
Design and synthesis of dendritic europium complexes
To overcome the inherently weak absorption cross section
of Eu(III) ion and enhance the luminescence of europium
complexes, three dendritic europium complexes with func-
tional peripheries of carbazole chromophore were designed
and synthesized (Scheme 1). The incorporated carbazole
moiety here is expected to act as a light-harvesting antenna
to sensitize and populate the excited state of the central Eu(III)
ion. Meanwhile, their corresponding dendritic ligands can be
endowed with specific physical and chemical properties, such
Scheme 1 Chemical structures of the reference compound Eu(BPPD)3(Phen) and three target dendritic europium complexes containing grafted
carbazole units as their functional peripheries.
3222 | J. Mater. Chem., 2005, 15, 3221–3228
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Scheme 2 Synthetic route to dendritic complex Eu(TCPD)3(Phen).
Table 1 Data of absorption and luminescence spectra for europium complexes and reference N-ethylcarbazole in CH2Cl2 (2.0 6 1025 mol L21
and solid film
)
In CH2Cl2 (2.0 6 1025 mol L21
lmaxabs/nm
)
In solid film
gb
(lex 5 331 nm)
Wr2
(lex 5 331 nm)
a
a
Wr1
(lex 5 331 nm)
Complex
(e 6 1024/M21 cm21
)
lem/nm
labsmax/nm
lem/nm
Eu(BPPD)3(Phen)
Eu(MCPD)3(Phen)
360 (2.82)
348 (5.71)
360 (2.79)
331 (7.25)
346 (11.46)
360 (2.75)
334 (11.15)
347 (16.90)
331 (1.23)
345 (1.85)
615
615
1.0
1.1
360
346
615
615
1.0
3.3
0.998
0.990
Eu(BCPD)3(Phen)
615
615
1.8
1.6
346
360
615
615
7.9
4.5
Eu(TCPD)3(Phen)
0.992
346
360
N-Ethylcarbazole
350
366
a
The relative luminescence quantum efficiency (Wr) was calculated relative to the reference complex when these spectra were normalized at
331 nm (Wr1 in CH2Cl2; Wr2 in solid film). The emission quenching efficiency of peripheral donor carbazole.
b
of ligands should be closely matched to or slightly above
the emitting resonance levels of central Eu(III) ions. When
indirectly excited at the absorption peak of the grafted
carbazole units, the synthesized dendritic complexes in
CH2Cl2 or solid film emit a characteristic sharp luminescence
peak at 615 nm with four shoulders, which are ascribed to the
transitions between 4f states of Eu(III) ion (Figs. 1 and 2). All
five peaks at 586, 591, 615, 651 and 702 nm (show in ESI
Fig. S2 and S3{) correspond to Eu(III) characteristic transi-
complex (Figs. 1 and 2) is found in each spectrum ranging
from 400–550 nm. With respect to carbazole, the emission
quenching of peripheral donor carbazole units in the systems of
Eu(MCPD)3(Phen), Eu(BCPD)3(Phen) and Eu(TCPD)3(Phen)
reached as high as 99%, and almost completely quenched
under the excitation of 331 nm (Table 1). As shown in Fig. 3
(curves a and b), the luminescence of carbazole is overlapped
with the absorption of Eu(TCPD)3(Phen) to a certain extent,
thus an efficient Fo¨rster-type energy transfer in the complex
system can take place from the singlet excited state of grafted
carbazole units to the singlet excited state of b-diketonate
ligand, and followed by intersystem crossing to the excited
7
tions (5D0 A Fj, j 5 0–4).9
Notably, compared with the luminescence of free b-diketo-
nate ligand TCPD (Fig. 3, curves d), trace emission from each
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Fig. 1 PL spectra of Eu(MCPD)3(Phen) (curve a), Eu(BCPD)3(Phen)
(curve b) and Eu(TCPD)3(Phen) (curve c) in CH2Cl2 (2.0
1025 mol L21) excited at 331 nm.
6
Fig. 4 Absorption spectrum (solid line) and excitation spectrum
(dash-dot line) of Eu(TCPD)3(Phen) monitored at 615 nm. The
excitation spectrum is normalized at the absorption peak in the
b-diketonate region (360 nm). All measurements were in CH2Cl2
(1.2 6 1026 mol L21).
light-harvesting antenna. Moreover, the site isolation of
dendrons could also be favorable to enhance luminance.
As a result of the light harvesting and site isolation of
dendrons, if the luminescence intensity of reference complex
Eu(BPPD)3(Phen) in solid film excited at the carbazole
absorption maximum (331 nm) is arbitrarily defined as
one unit, the relative luminescence quantum yields (Table 1
and Fig. 2) of Eu(MCPD)3(Phen), Eu(BCPD)3(Phen) and
Eu(TCPD)3(Phen) are 3.3, 7.9 and 4.5, respectively. It
should be noted that the luminescence of Eu(TCPD)3(Phen)
containing three carbazole units is unexpectedly weaker than
that of Eu(BCPD)3(Phen) containing two carbazole units,
which might be attributed to the spatial hindrance of three
neighboring carbazole units incorporated into one end of
b-diketonate, thus leading to a less effective energy transfer
between carbazole and b-diketonate.
Fig. 2 PL spectra of Eu(BPPD)3(Phen) (curve a), Eu(MCPD)3(Phen)
(curve b), Eu(BCPD)3(Phen) (curve c), and Eu(TCPD)3(Phen) (curve d)
in solid film excited at 331 nm.
Luminescence dynamics and photo-physical properties
The luminescence dynamics of the europium complexes were
measured using the time-correlated single photon counting
technique (Fig. 5). Each of the Eu(III) complexes was excited
with a nanosecond flashlamp as excitation source at 331 and
360 nm, and the luminescence decays were monitored at the
emission maximum of each sample. The luminescent lifetimes
of complexes in CH2Cl2 are characteristic of phosphorescence,
and are given in Table 2. In the synthesized dendritic
complexes, the b-diketonate ligand is not symmetric. In
principle, the synthesized complexes should have two isomers,
i.e. trans and cis. The reference complex Eu(BPPD)3(Phen)
exhibits a bi-exponential decay with lifetimes of 67.9 and
242.7 ms. Similarly, Eu(MCPD)3(Phen) and Eu(BCPD)3(Phen)
upon 331 nm excitation are found to decay bi-exponentially
with major contributing decay components of 116.3 and 94.0 ms
as well as additional minor components of 281.8 and 209.0 ms,
respectively. Such bi-exponential luminescent decaying
profiles for Eu(MCPD)3(Phen) and Eu(BCPD)3(Phen) and
reference Eu(BPPD)3(Phen) are related to the presence of
Fig. 3 Absorption spectrum of Eu(TCPD)3(Phen) in CH2Cl2 (2.0 6
1025 mol L21, curve a), and PL spectra excited at 331 nm in CH2Cl2
(2.0 6 1025 mol L21) of N-ethylcarbazole (curve b), BPPD (curve c),
TCPD (curve d), and Eu(TCPD)3(Phen) (curve e).
triplet state of b-diketonate, where it is finally transferred to
the central Eu(III) ion. The efficient energy transfer is further
confirmed by the excitation spectrum of Eu(TCPD)3(Phen)
(Fig. 4). Thus it is possible to obtain a strong intense emission
from the central Eu(III) ion via sensitization from an excited
3224 | J. Mater. Chem., 2005, 15, 3221–3228
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the other hand, energy transfer efficiency can also be calculated
according to the following equation:13
tda
w
ET~1{
(1)
td
where tda and td refer to the donor (carbazole) lifetimes in the
presence and absence of the energy acceptor (Eu(III) ion),
respectively. Hence tda is easily calculated from eqn. (1). The
subsequent energy-transfer rate constant kET can be calculated
using the following relationship:14
ꢀ
ꢁ
1
1
:
kET
~
(2)
td ð1=wETÞ{1
Photoluminescent quantum yields were obtained by using
Rhodamine B in ethanol (2 6 1025 mol L21, Wref 5 0.5)15 as a
reference. The luminescence quantum yield (Wem) expresses
how well the radiative processes (characterized by rate
constant kr) compete with non-radiative processes (overall
rate constant knr). Both of the decay rate constants for the
excited states of Eu(III) ion were determined from the values of
average tem (excited-state lifetime of Eu(III) ion and equal to
(kr + knr)21) by using eqn. (3).16 All dynamic and photo-
physical data of complexes are listed in Table 2.
Fig. 5 Luminescence
decay
profiles
of
Eu(BPPD)3(Phen),
Eu(MCPD)3(Phen), Eu(BCPD)3(Phen) and Eu(TCPD)3(Phen) in
CH2Cl2 (2.0 6 1025 mol L21).
two conformations in each complex.10 In contrast, the
dendritic europium complex Eu(TCPD)3(Phen) shows
a
mono-exponential decay with lifetime 148.4 ms, which approxi-
mates to the average lifetime of Eu(BCPD)3(Phen) (171 ms).
Interestingly, here the mono-exponential luminescence
decay suggests the presence of a single conformation in
Eu(TCPD)3(Phen), which is quite different from the two
conformations identified for the other complexes. An attempt
to grow high quality single crystals for structure analysis
was not successful. We consider that the spatial hindrance of
three neighboring carbazole units incorporated into one end of
ligand b-diketonate in Eu(TCPD)3(Phen) might be favorable
for the formation of a single conformation.
wem 5 kr/(kr + knr) 5 tem?kr
(3)
From Table 2, the energy-transfer rate constant kET from
carbazole or ligand to the central Eu(III) ion is seven orders
of magnitude higher than radiative decay rate kr, and six
orders of magnitude higher than non-radiative decay rate knr,
respectively. That is, such fast energy transfer does not have a
distinct effect on the luminescence lifetime of the central
Eu(III) ion itself.
In order to obtain quantitative knowledge of energy transfer
between carbazole units and Eu(III) ion, the energy transfer
efficiencies (WET) of the carbazole-grafted europium complexes
are estimated by using both steady-state and time-resolved
fluorescence measurements. The first method used below relies
on a comparison of absorption spectra and corresponding
EL properties based on carbazole-grafted europium complex
EL devices with configuration ITO/NPB/complex : CBP
(2.5 wt%)/BCP/Mg : Ag were fabricated and studied. The
emitting layer is carbazole-grafted europium complex with a
doping concentration of 2.5 wt% in CBP (4,49-N,N9-dicarba-
zole biphenyl). As a matrix material, CBP has become an
excellent host for phosphorescent dopants due to its high
triplet energy of 2.56 eV and ambipolar charge-transporting
ability.17 To confine the exciton recombination inside the
emitting layer, a hole block layer BCP, which has HOMO and
PL excitation profile,11 which is shown in Fig.
4 for
Eu(TCPD)3(Phen). For complexes Eu(MCPD)3(Phen),
Eu(BCPD)3(Phen) and Eu(TCPD)3(Phen), the energy transfer
efficiencies (WET) in CH2Cl2 were calculated to be 97%, 96%
and 93%, respectively (Table 2). These values are very close to
the respective carbazole quenching efficiencies (Table 1).12 On
Table 2 Dynamic and photo-physical data of complexes determined in CH2Cl2 (2.0 6 1025 mol L21) at 25 uC
No.a
lex/nm
l
em/nm t1b/ms (Rel. %) t2b/ms (Rel %) temc/ms tdad/ns Wf
WET
kETg/1029 s21 krh/1022 s21 knri/1023 s21
e
f
1
2
360
331
360
331
360
331
360
331
615
615
615
615
615
615
615
350
67.9 (19.0)
116.3 (60.4)
123.4 (64.7)
94.0 (52.2)
110.0 (70.9)
148.4 (100)
139.0 (100)
8.4 ns (100)
242.7 (81.0)
281.8 (39.6)
331.5 (35.3)
209.0 (47.8)
402.3 (29.1)
232
218
194
171
285
148
139
0.054
0.059 0.97
0.058
0.097 0.96
0.089
2.32
2.71
2.98
5.67
3.12
5.80
5.68
4.06
4.32
4.84
5.23
4.94
6.16
6.62
0.25
0.34
0.59
3.8
2.9
1.6
3
4
0.086 0.93
0.079
5
a
Compound Nos. 1, 2, 3,
N-ethylcarbazole, respectively. Luminescence decay lifetime, ¡5%. Average lifetime. Donor (carbazole) lifetime in the presence of acceptor
4 and 5 correspond to Eu(BPPD)3(Phen), Eu(MCPD)3(Phen), Eu(BCPD)3(Phen), Eu(TCPD)3(Phen) and
c d
b
e
f
(Eu(III)), calculated by tda 5 (1 2 WET)td. Photoluminescent quantum yield relative to Rhodamine B. Energy transfer efficiency calculated
g
by a comparison of absorption spectra and PL excitation spectra. Energy-transfer rate constant, calculated by eqn. (2). Radiative decay rate
h
i
. Non-radiative decay rate calculated by knr 5 (1 2 Wem)/tem.
calculated by kr 5 Wem/tem
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Fig. 6 Schematic HOMO–LUMO energy level diagram of ITO/NPB/
CBP : Eu(III) complex/BCP/Mg : Ag.
LUMO levels at 6.4 and 2.9 eV, respectively, was inserted
between the cathode and the emitting layer. It was anticipated
that electrons can move freely from the cathode through BCP
to the emitting layer, whereas BCP effectively blocked the
passage of holes out of the emitting layer. Meanwhile, the
HOMO of Eu(TCPD)3(Phen) is reduced via the incorporation
of carbazole unit for a favorable hole injection. In addition,
the HOMO and LUMO of Eu(TCPD)3(Phen) are so close to
those of the host CBP that the electron and hole from CBP is
easily confined in the complex dopant. The energy levels of the
device are schematically shown in Fig. 6.
Fig. 8 EL efficiencies as a function of current density of device ITO/
˚
NPB(500 A)/CBP : Eu(TCPD)3(Phen)(2.5 wt%)(600 A)/BCP(500 A)/
˚
˚
˚
Mg : Ag(800 A).
229 cd m22 and current density of 194 mA cm22 was achieved
at the driving voltage of 20.5 V (Figs. 7 and 8).
Generally, rare earth metal complexes exhibit extremely
sharp emission bands in PL and EL spectra.3 Kido and his
colleagues described an OLED employing [Tb(acac)3] as a
green light phosphor.19 Hence it is difficult to get white light
of special CIE coordinates with a single lanthanide complex.
Instead, by the combination of Tb(III) and Eu(III) complexes
as green and red emitters a white EL device has been
achieved with a multilayer structure.20 Binuclear complex
EuxTb12x(acac)3phen was another alternative to realize white
light emission at an appropriate voltage whose emission
consists of three parts: red from Eu(III), green from Tb(III)
and blue from TPD (or NPB).21 Therefore, white light electro-
luminescence with using a single complex Eu(TCPD)3(Phen) is
our unexpected result (the inset of Fig. 7), which might come
from excimers22 and/or exciplexes.8
˚
A device with the configuration of ITO/NPB(500 A)/
˚
˚
CBP : Eu(TCPD)3(Phen)(2.5 wt%)(600 A)/BCP(500 A)/Mg :
˚
Ag(800 A) is extensively studied. Fig. 7 shows the current
density–voltage–luminance characteristics of the device.
Unexpectedly, as shown in the inset of Fig. 7, the EL spectrum
of the device gave a white emission with three peaks at 487, 563
and 610 nm, and the CIE coordinates (0.333, 0.348) at the
driving voltage of 16.2 V (Fig. S4 in ESI{). The luminance
drastically increased with increasing current density (Fig. 8).
Furthermore, the external quantum efficiency was relatively
high, and gradually decreased when the current density was
over 50 mA cm22, which is consistent with a previous report.18
With the current density reaching 100 mA cm22, the efficiency
still remained at more than 0.7%. Its external quantum effi-
ciency reached 1.1% with a luminous efficiency of 0.2 lm W21
at the driving voltage of 10.5 V. A maximum luminance of
For the sake of gaining insight into the possible exciplex
between CBP and carbazole, the luminance of a CBP film
doped with Eu(TCPD)3(Phen) (2.5 wt%) excited at different
wavelengths was compared. As seen in Fig. 3 (curve d), the
luminescence of ligand TCPD consists of blue and green
emission bands. The blue band might result from the emission
of carbazole units due to the similar spectrum profile in the
same region. The green emission band is ascribed to the
formed exciplex from TCPD intermolecular interactions
because of the concentration dependence. The green emission
from TCPD disappears when the concentration is less
than 1026 mol L21. In contrast to that of the neat film of
Eu(TCPD)3(Phen), a red-shift by around 30 nm in the blue
region is observed in the PL spectrum of the blend film
with Eu(TCPD)3(Phen) (50 wt%) in CBP (Fig. 9, curve e),
suggesting that the exciplex was also formed between the host
matrix CBP and carbazole units. Furthermore, a blue band
centered at 390 nm belonging to the emission of CBP is nearly
quenched due to non-radiative energy transfer from CBP to
Eu(TCPD)3(Phen).23 The conclusion is further supported by
the PL excitation spectrum (Fig. 9, curve a) with high intensity
at 298–310 nm corresponding to the absorption of CBP. Since
the distance between CBP and Eu(TCPD)3(Phen) is over the
Fo¨rster radius, the PL intensity of Eu(TCPD)3(Phen) in the
Fig. 7 Current density–voltage–luminance curves of device ITO/
˚
NPB(500 A)/CBP : Eu(TCPD)3(Phen)(2.5 wt%)(600 A)/BCP(500 A)/
˚
˚
˚
Mg : Ag(800 A). Note: the inset shows the EL spectrum.
3226 | J. Mater. Chem., 2005, 15, 3221–3228
This journal is ß The Royal Society of Chemistry 2005

View Article Online
curve d). It might be attributed to CBP irradiation due to the
incomplete energy transfer25 from CBP to the Eu(III) core.
Another possibility is an exciplex between the excited state of
carbazole unit and the ground state of CBP.
Conclusions
Three novel dendritic europium complexes Eu(MCPD)3(Phen),
Eu(BCPD)3(Phen) and Eu(TCPD)3(Phen) were synthesized
with peripheries of carbazole units surrounding the core. With
the light-harvesting and energy-transferring ability of the
periphery, the dendritic europium complexes emitted brighter
pure red light than the reference compound Eu(BPPD)3(Phen)
in both CH2Cl2 and film state. Furthermore, carbazole units
could tune the HOMO–LUMO levels of Eu(TCPD)3(Phen)
to match the hole transporter NPB with less interfacial
energy gap. Notably, white light electroluminescence (CIE:
0.333, 0.348) can be achieved by using a single complex
Eu(TCPD)3(Phen) as dopant emitter. Preliminary results
show that modifying ligands can not only tune the complex
carrier-transport properties, but also provide a useful clue to
the use of electroplexes or exciplexes to realize broad or even
white electroluminescence.
Fig. 9 PL excitation spectrum monitored at 615 nm of (a) CBP
film doped with Eu(TCPD)3(Phen) (2.5 wt%); PL spectra of (b) CBP
(2 6 1028 mol L21 in CH2Cl2, lex 5 290 nm), (c) CBP film doped with
Eu(TCPD)3(Phen) (2.5 wt%, lex 5 290 nm), (d) CBP film doped
with Eu(TCPD)3(Phen) (2.5 wt%, lex 5 331 nm), (e) CBP film doped
with Eu(TCPD)3(Phen) (50 wt%, lex 5 290 nm, 331 nm).
doped film does not increase much with respect to that of the
neat film.
It is certain that there exists energy transfer from CBP to the
central Eu(III) ion, and the energy for the excitation of Eu(III)
ion was nearly saturated until the voltage increased to 15.8 V.
When the driving voltage reached 16.2 V, EL in the blue band
increased, and the blue emission from CBP balanced the white
color. The possible EL mechanism of the white light-emitting
device might be elucidated as follows. (1) An original red
peak at 610 nm is from the central europium(III) ion. The
major energy generated by the carrier combination in the
host material CBP is transferred to the singlet excited state of
b-diketonate through dipole interaction, then undergoing an
intersystem crossing to its triplet state (T1). The central Eu(III)
ions obtain the energy from b-diketonate (T1) to the energy
Experimental
Sodium hydride was purchased from J&K and kept in a
vacuum drier at room temperature. EuCl3?6H2O (99.9%) and
carbazole were bought from Aldrich. Acetophenone from
Aldrich was dried with MgSO4 and redistilled prior to usage.
All the other starting materials and reagents were analytical
purity without treatment. Melting points were measured on an
X4 Micro-melting point apparatus. 1H NMR spectra were
recorded on a Bruker AM-500 spectrometer. MS were recorded
on FAB or MALDI-TOF mass spectrometers. Synthesis and
full characterization of all dendritic ligands and complexes are
available in electronic supplementary information (ESI).{
Absorption and luminescence spectra were recorded on
Varian Cary 500 and Varian Cary Eclipse spectrometers,
respectively. Luminescence lifetime study was performed by a
single photon counting spectrofluorimeter from Edinburgh
Instruments (FLS920). Each of the Eu(III) complexes was
excited with a nanosecond flashlamp as excitation source at
331 and 360 nm, and the luminescence decays were monitored
at the emission maximum of each sample. For N-ethylcarba-
zole, a Hamamatsu pulsed picosecond laser M8903 (maximum
operation frequency 100 MHz) was used as the excitation
source. The time-correlated single photon counting (TCSPC)
data were analyzed by iterative convolution of the lumines-
cence decay profile with the instrument response function
using software provided by Edinburgh Instruments.
level 5D0, and relax through 5D0
A
7Fx transitions to emit
sharp red light. (2) The broad EL emission band at 530–580 nm
in the spectrum is likely ascribed to the electroplex formed
within the ligand TCPD. Despite the similar green emission to
the above-mentioned PL spectrum, the mechanism is quite
different. In the case of a low ratio of Eu(TCPD)3(Phen)
dispersed in CBP, the exciplex did not form intermolecularly.
Moreover, the distance between carbazole and b-diketonate
within the ligand TCPD is larger than 0.4 nm, which does
not form an exciplex effectively.24 On the contrary, owning to
the incorporation of both the electron-donor (carbazole) and
electron-acceptor (b-diketonate) into TCPD, charge carriers
+
are subjected to separation (A2 D ) in TCPD, and state
(A2–D+) is developed subsequently, thus resulting in the green
emission band in the EL spectra. The green emission band is
red-shifted by around 30 nm with respect to the above-
mentioned exciplex emission in the PL spectrum of TCPD
(Fig. 3, curve d). In addition, the excimer possibility between
overcrowded carbazole units also cannot be ruled out in the
system. (3) As far as the blue band at 400–530 nm is concerned,
the origin is rather complicated. Notably, the blue band of
the EL spectrum is consistent with that of the PL spectrum
for a CBP film doped with 2.5 wt% Eu(TCPD)3(Phen) (Fig. 9,
…
All organic materials for device prior to use were purified by
train sublimation. In the device fabrication procedure, indium
tin oxide (ITO) substrate coated glasses with a sheet resistance
of 50 V square21 were cleaned ultrasonically with detergent,
de-ionized water and isopropanol (IPA). NPB (N,N9-bis(1-
naphthyl)-N,N9-diphenyl-1,19-biphenyl-4,49-diamine) served as
a hole transporter. Above the layer of hole transporter was an
emitting layer of CBP doped with 2.5 wt% Eu(TCPD)3(Phen).
This journal is ß The Royal Society of Chemistry 2005
J. Mater. Chem., 2005, 15, 3221–3228 | 3227

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7 S. F. Li, W. H. Zhu, J. F. Pan, Z. Y. Xu and H. Tian, Huaxue
Tongbao (in Chinese), 2004, 67, w86–89 (Chemistry Online
http://www.hxtb.org).
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M. Mazzeo, D. Pisignano, F. D. Sala, J. Thompson, R. I. R. Blyth,
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A layer of BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthro-
line) to serve as a hole blocker was deposited between the
emitting layer and cathode made of Mg : Ag (10 : 1) with a
thickness of 80 nm. The emissive area of the devices was
5 mm 6 6 mm. EL spectrum and brightness–current–voltage
characteristics of the device were measured with a PR650
Spectra Scan at room temperature and atmosphere. The
˚
device reported here had the configuration ITO/NPB(500 A)/
˚
CBP : Eu(TCPD)3(Phen)(2.5 wt %)(600 A)/BCP(500 A)/Mg :
˚
˚
Ag(800 A).
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Acknowledgements
This work was partially supported by NSFC/China (Project
No.: 20106006, 90401026), Education Committee of Shanghai
and Scientific Committee of Shanghai (04dz05102).
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