First oxadiazole-functionalized terbium(III) β-diketonate for organic electroluminescence [1]

Full text of DOI:10.1021/ja004113u

J. Am. Chem. Soc. 2001, 123, 6179-6180
6179
Communications to the Editor
First Oxadiazole-Functionalized Terbium(III)
â-Diketonate for Organic Electroluminescence
Jiafu Wang,†,‡ Ruiyao Wang,† Jun Yang,† Zhiping Zheng,*,†
Michael D. Carducci,† Troy Cayou,†
Nasser Peyghambarian,*,‡ and Ghassan E. Jabbour*,‡
Department of Chemistry and Optical Sciences Center
UniVersity of Arizona, Tucson, Arizona 85721
ReceiVed NoVember 29, 2000
Considerable efforts are being devoted to the development of
organic light-emitting diodes (OLEDs) for energy efficient, full-
color, flat-panel displays.1 Electroluminescence (EL) across the
whole visible spectrum has been demonstrated, and prototype
devices meeting realistic specifications for applications have been
realized. However, obtaining pure and sharp emission from
currently used molecular or polymeric emissive materials is
difficult; their emission spectra are generally broad, which would
compromise the quality of colors in actual displays.
Figure 1. An ORTEP representation of 3 (50% probability ellipsoids).
H atoms are omitted for clarity.
Scheme 1
Luminescent lanthanide complexes are believed to be a
promising solution to this problem.2 Owing to the unique
f-electronic configurations,3 lanthanide-based materials can not
only generate extremely pure emission, but also offer an unlimited
theoretical ceiling for device efficiency. Furthermore, the physical
properties pertinent to the processability of these materials can
be conveniently altered without affecting the metal-based emission
characteristics. Despite these propitious features, only one of the
many potential advantages of using lanthanide-based emitting
materials, namely the monochromaticity, has been realized. One
key issue is that the high photoluminescence (PL) efficiency of
lanthanide complexes does not translate into EL efficiency of
comparable magnitude.2 It is agreed upon, although not proven,
that this underachievement is due to the poor ability of lanthanide
complexes to transport charge carriers (especially electrons);
unbalanced injection and transport of charge carriers would cause
their recombination at locations other than the emitting layer,
leading to low device efficiency and reduced lifetime.4
Aiming at actualizing OLEDs of high efficiency, sharp emis-
sion, and long lifetime, we sought lanthanide complexes with
chelating ligands that would promote balanced injection, transport,
and recombination of charge carriers. Our initial efforts have been
focused on the design and synthesis of oxadiazole-functionalized
â-diketone ligands and corresponding lanthanide complexes. The
EL applications of lanthanide â-diketonates have been well-
established,2,4 while oxadiazole derivatives are among the most
widely employed electron-transporting and hole-blocking materi-
als.5 Extensive work in both areas notwithstanding, 3-(5-phenyl-
1,3,4-oxadiazol-2-yl)-2,4-pentanedione6 (1) is the only known
oxadiazole-functionalized â-diketone, and lanthanide complexes
with such ligands have not yet been reported. Herein, we report
the high-yield synthesis7 of two oxadiazole-derivatized â-dike-
tones (1 and 2). The synthesis, characterization, and structural
determination of the first lanthanide complex (3) with such
ligands, formed by a Tb(III) ion with 1, are also described.
† Department of Chemistry.
‡ Optical Sciences Center.
(1) Selected recent reviews on OLEDs: (a) Tsutsui, T. MRS Bull. 1997,
June, 39-45. (b) Chen, C. H.; Shi, J.; Tang, C. W. Macromol. Symp. 1997,
125, 1-48. (c) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J.
H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bre´das,
J.-L.; Lgdlund, M.; Salaneck, W. R. Nature 1999, 397, 121-128. (d) Mitschke,
U.; Bauerle, P. J. Mater. Chem. 2000, 10, 1471-1507.
The syntheses of 1-3 are set out in Scheme 1. The compounds
1
were characterized7 by H and 13C NMR, FAB-MS, and mi-
croanalysis. The crystal structure of 3 (Figure 1) was established
by X-ray diffraction. The Tb(III) ion is surrounded by eight
oxygen atoms, six of which are from the bidentate â-diketonate
(2) Selected references of lanthanide-based OLEDs: (a) Kido, J.; Nagai,
K.; Ohashi, Y. Chem. Lett. 1990, 657-660. (b) Jabbour, G. E.; Wang, J. F.;
Kippelen, B.; Peyghambarian, N. Jpn. J. Appl. Phys., Lett. 1999, 38, L1553-
L1555. (c) Zhu, D.; Liu, Y.; Bai, F. Thin Solid Films 2000, 363, 51-54. (d)
Shipley, C. P.; Salata, O. V.; Capecchi, S.; Christou, V.; Dobson, P. J. AdV.
Mater. 1999, 11, 533-536. (e) Robinson, M. R.; O’Regan, M. B.; Bazan, G.
C. Chem. Commun. 2000, 1645-1646. (f) Yu, G.; Liu, Y.; Wu, X.; Zhu, D.;
Li, H.; Jin, L.; Wang, M. Chem. Mater. 2000, 12, 2537-2541.
(3) Sabbatini, N.; Guardogli, M.; Lehn, J.-M. Coord. Chem. ReV. 1993,
123, 201-228.
(5) (a) Adachi, C.; Tsutsui, T.; Saito, S. Appl. Phys. Lett. 1989, 55, 1489-
1491. (b) Wang, J. F.; Jabbour, G. E.; Mash, E. A.; Anderson, J.; Zhang, Y.
D.; Lee, P. A.; Armstrong, N. R.; Peghambarian, N.; Kippelen, B. AdV. Mater.
1999, 11, 1266-1269.
(6) Yamanaka, H.; Ohba, S.; Sakamoto, T. Heterocycles 1990, 31, 1115-
1127.
(7) Synthetic attempts by following the reported procedure (ref 6) failed
to produce any meaningful amount of 1. See Supporting Information for
synthetic details and characterization data.
(4) (a) Kido, J.; Hayase, H.; Hongawa, K.; Nagai, K.; Okuyama, K. Appl.
Phys. Lett. 1994, 65, 2124-2126. (b) Adachi, C.; Baldo, M. A.; Forrest S. R.
J. Appl. Phys. 2000, 87, 8049-8055.
10.1021/ja004113u CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/30/2001

6180 J. Am. Chem. Soc., Vol. 123, No. 25, 2001
Communications to the Editor
Figure 3. The EL spectra of the device ITO/PVK:PBD:3/Alpop/CsF/Al
at 8 (solid line) and 19 V (broken line) of driving voltage.
Figure 2. Luminance-voltage characteristics of the device with the
configuration ITO/PVK:PBD:3/Alpop/CsF/Al.
characterizing the performance of the present device are compared
with those of an otherwise identical device employing the
unsubstituted terbium tris(acetylacetonate) (Supporting Informa-
tion). Although green light was clearly visible with the control
device, the emission was too weak to be measured with the
experimental setup. Moreover, the current density of this device
is 0.6 and 1.3 mA/cm2 at driving voltages of 15 and 20 V,
respectively. The corresponding current density is 25 and 275
mA/cm2, respectively, when compound 3 was used. Such drastic
increases in current density strongly suggest that charge transport
across the thin-film structure can be significantly improved by
the introduction of the oxadiazolyl group and that the ligand
design approach elaborated in this work to improve OLED
performance is feasible.
In summary, we report for the first time the design, synthesis,
and EL application of a terbium(III) complex with oxadiazole-
functionalized â-diketonate ligands. Although neither the con-
figurational nor the compositional structure of the present device
is optimized, it is, to the best of our knowledge, the brightest
and the most efficient EL device fabricated by spin casting a
Tb(III) complex-doped polymer.10 Our results suggest that
incorporating oxadiazole moiety into the â-diketone platform is
a viable strategy for modifying the physical properties and
electronic structure of the corresponding lanthanide complex. This
in turn may lead to more stable devices with brighter and more
efficient emission due to more balanced injection, transport, and
recombination of charge carriers in an appropriately configured
device. Efforts are being directed toward increasing the volatility
of the complexes and further enhancing their electron transport
ability.
ligands and the other two from the coordinated water molecules.
The coordination polyhedron can be best described as square
antiprismatic. The Tb-O bond distances, ranging from 2.310(2)
to 2.466(3) Å (average 2.360(7) Å), are comparable with those
observed in other complexes of terbium with analogous ligands.8
Using 3 as the emitting material, a bright and highly efficient
green-emitting LED was fabricated. The device was constructed
on a glass slide precoated with indium-tin oxide (ITO). The first
layer was formed by spin casting a chloroform solution of hole-
transporting poly(N-vinylcarbazole) (PVK) and electron-transport-
ing 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD)
(1:1 weight) containing 5 wt % of 3. A thin layer of hole-blocking
and electron-transporting tris(2-(5-phenyl-1,3,4-oxadiazolyl)phe-
nonate) aluminum (Alpop)5b was then grown by means of vacuum
vapor deposition on top of the spin-coated layer. Finally, a thin
layer of electron injection-facilitating CsF (0.6 nm) was intro-
duced, followed by the deposition of 200 nm of aluminum as the
cathode. Doping of 3 is expected to further balance the injection
and transport of charges due to the electron transport-facilitating
oxadiazole substituent. Putative recombination of charge carriers
within the PVK:PBD:3 composite layer generates excitons that
effectively transfer their energy to the emission-responsible Tb(III)
center, thereby yielding bright luminance with high efficiency.
The forward light output-voltage (EL-V) characteristics for
the device are presented in Figure 2. Green light emitted was
visible at as low as 8 V. The emission comes solely from the
Tb(III) ion, as the EL spectrum is identical to that of the PL of
3. The brightness of the device increases with the current density.
At 15 V, the light output reaches 100 cd/m2 with an external EL
efficiency as high as 1.1% (0.4 cd/A), and 550 cd/m2 with an
efficiency of 0.6% (0.2 cd/A) at 20 V. It is worth noting that
starting from 19 V a broad peak appears at about 430 nm in the
EL spectrum (Figure 3). This appreciable blue emission is
attributable to exciplex formation between PVK and 3, as similar
observations are well documented9 for devices fabricated by spin
casting using Tb(III) complexes doped in PVK matrix.
Acknowledgment. This work was supported by the University of
Arizona (Z.Z.), DARPA (G.E.J.), and NSF (G.E.J. and N.P.). The CCD
based X-ray diffractometer was purchased through NSF Grant CHE-
96103474. We thank Professor Saavedra for use of the fluorescence
spectrophotometer.
Supporting Information Available: Synthetic details, characteriza-
tion data (1-3), crystallographic data (3), and control experiment results
using terbium tris(acetylacetonate) (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
The favorable role played by the oxadiazole ligand of facilitat-
ing charge transport is clearly established when the parameters
JA004113U
(8) (a) Drake, S. R.; Lyons, A.; Otway, D. J.; Williams, D. J. Inorg. Chem.
1994, 33, 1230-1233. (b) Zhou, D.; Li, Q.; Huang, C.; Yao, G.; Umetani,
S.; Matsui, M.; Ying, L.; Yu, A.; Zhao, X. Polyhedron 1997, 16, 1381-
1389.
(9) Liang, C.; Li, W.; Hong, Z.; Liu, X.; Peng, J.; Liu, L.; Lu, Z.; Xie, M.;
Liu, Z.; Yu, J.; Zhao, D. Synth. Met. 1997, 91, 151-154.
(10) The brightest green-emitting device based on a Tb(III) complex was
fabricated by the Vacuum Vapor deposition technique. See: Capecchi, S.;
Renault, O.; Moon, D.; Halim, M.; Etchells, M.; Dobson, P. J.; Salata, O. V.;
Christou, V. AdV. Mater. 2000, 12, 1591-1594.