Bright and monochromic red light-emitting electroluminescence devices
based on a new multifunctional europium ternary complex
Min Sun,a Hao Xin,b Ke-Zhi Wang,*a Yong-An Zhang,a Lin-Pei Jina and Chun-Hui Huang*b
a Department of Chemistry, Beijing Normal University, Beijing 100875, P. R. China.
E-mail: kzwang@bnu.edu.cn; Fax: + 86 10 6220 0567; Tel: +86 10 6220 9940
b State Key Laboratory of Rare Earth Materials Chemistry and Applications and the University of Hong
Kong Joint Laboratory on Rare Earth Materials and Bioinorganic Chemistry, Peking University, Beijing
100871, P. R. China
Received (in Cambridge, UK) 18th December 2002, Accepted 4th February 2003
First published as an Advance Article on the web 17th February 2003
A novel europium(III) complex, tris(dibenzoylmethanato)(2-
4A-triphenylamino)imidazo[4,5-f]1,10-phenanthroline)euro-
pium(III), Eu(DBM)3(TPIP), is synthesized. The light-emit-
ting center, hole-transporting triphenylamine and
electron-transporting phenanthroline fragments are inte-
grated into one molecule. A single-layer device of ITO/
Eu(DBM)3(TPIP)(60 nm)/Mg0.9Ag0.1/Ag exhibits EuIII
-
based pure red emission with a maximum brightness of 19 cd
m22 at 13.5 V and 280 mA cm22, and an onset driving
voltage of 8 V. A four-layer device of ITO/TPD (20 nm)/
Eu(DBM)3(TPIP) (40 nm)/BCP (20 nm)/AlQ(40 nm)/
Mg0.9Ag0.1/Ag gives a maximum EuIII-based pure red
emitting luminance of 1305 cd m22 at 16 V and 255 mA
cm22 with an onset driving voltage of 6 V; the maximum
external quantum yield and luminous yield are estimated to
be 0.85% and 1.44 lm W21, respectively, at 7.5 V and 0.25
Scheme 1 Synthetic route to TPIP and Eu(DBM)3(TPIP). Reagents and
conditions: i, 4-triphenylaminobenzaldehyde, HAc–NH4Ac, 4 h, 80%; ii,
dmf, NaH, C2H5Br, reflux 24 h, 53%; iii, CHCl3, Eu(DBM)32H2O, reflux
1 h.
7
nm assigned to 5D1 ? F1 transition,8 in addition to three peaks
mA cm22
.
5
7
for D0 ? Fj (j = 0–2) transitions at 579, 590 and 612 nm
5
respectively. For the four-layer device, three peaks due to D1
Transition metal and lanthanide complex-based electrolumines-
cence (EL) devices are currently being keenly pursued.
However, the EL device parameters reported for lanthanide
(e.g. EuIII and TbIII) complexes, have not been as encouraging
as expected, compared to those for certain transition metal
complexes,1,2 although EuIII or TbIII complexes show sharp
photoluminescence (PL) peaks with theoretical PL quantum
yields up to unity.3 For single-layer EL devices based on pure
EuIII complexes, few of them could attain brightness greater
7
7
? F1 transition at 538 nm, 5D0 ? F0 transition at 580 nm and
5D0
?
7F2 transition at 612 nm were strong enough to be
5
7
recognizable, while D0 ? F1 transition at ~ 580 nm almost
vanished compared to the single-layer device. It is worth
mentioning that the EL spectra for both of the EL devices were
of the feature with EuIII-centered electric dipolar transition (5D0
7
? F2) being the strongest, similarly to that observed for the
powder, indicating that the recombination of excitons and holes
occurred in the EuIII complex layer. As seen from Fig. 2, the
maximum luminance of 19 cd m22 which was achieved at 13.5
V and 280 mA cm22 for the single-layer device, compares
favourably to similar devices so far reported.6 The turn-on
voltage, which is defined as the voltage required to achieve the
luminance of 1 cd m22 for this device, was found to be ~ 8 V
which is comparable in magnitude to many organic EL devices,
and is obviously superior to EL devices made in our group with
simple bidentate N-heterocyclic ligand-containing EuIII ternary
than 10 cd m22 4-6
Hong and co-workers reported a highly
.
efficient EL device made of a mixed Eu(DBM)3(bath) and TPD
layer with an external quantum efficiency of 4.6%, but the
efficiency was attainable at current density of only 0.01 mA
cm22.4 The multilayer devices using TPD as hole-transporting
layer, AlQ as electron-transporting layer and BCP (2,9-dime-
thyl-4,7-diphenyl-1,10-phenanthroline)
layer,2 could obtain greatly improved brightness, but few of
them showed brightness greater than 1000 cd m22 5
By
as
hole-blocking
.
incorporating light-emitting center, hole-transporting tripheny-
lamine and electron-transporting phenanthroline fragments into
one molecule, we synthesized a new bidentate ligand and its
EuIII ternary complex. By using this newly synthesized EuIII
complex as dopant-free emitters, bright and high-efficient
electroluminescence were obtained.
The new ligand TPIP and its EuIII complex were synthesized
by following the synthetic route shown in Scheme 1.† Both a
single-layer device of ITO/Eu(DBM)3(TPIP)(60 nm)/
Mg0.9Ag0.1/Ag and a four-layer device of ITO/TPD(20 nm)/
Eu(DBM)3(TPIP)(40
nm)/BCP(20
nm)/AlQ(40
nm)/
Mg0.9Ag0.1/Ag were made by successive thermal evaporation of
organic materials and metal electrode materials in high vacuum
( < 8 3 1024 Pa) onto precleaned ITO substrate. The in-
strumentations for EL measurements were the same as before.7
EL spectra for the single- and four-layer devices, and PL spectra
for EuIII complex powder are compared in Fig. 1. The powder
exhibited four PL peaks at 579, 594, 612 and 653 nm,
Fig. 1 Normalized EL spectra for the four-layer device at bias voltage of 16
V (a) and the single-layer device at 12 V (b), and PL spectrum for the EuIII
complex powder (c).
5
7
corresponding to D0? Fj (j = 0–3) transitions, respectively.
The single-layer EL device showed a shoulder centered at 538
702
CHEM. COMMUN., 2003, 702–703
This journal is © The Royal Society of Chemistry 2003
Sun, Min
