Hydroxynaphthyridine-derived group III metal chelates: Wide band gap and dee blue analogues of green Alq (tris(8-hydroxyquinolate)aluminum) and their versatile applications for organic light-emitting diodes

Article abstract of DOI:10.1021/ja807284e

A series of group III metal chelates have been synthesized and characterized for the versatile application of organic light-emitting diodes (OLEDs). These metal chelates are based on 4-hydroxy-1,5naphthyridine derivates as chelating ligands, and they are

Full text of DOI:10.1021/ja807284e

Published on Web 12/18/2008
Hydroxynaphthyridine-Derived Group III Metal Chelates: Wide
Band Gap and Deep Blue Analogues of Green Alq3
(Tris(8-hydroxyquinolate)aluminum) and Their Versatile
Applications for Organic Light-Emitting Diodes
Szu-Hung Liao,†,‡ Jin-Ruei Shiu,†,§ Shun-Wei Liu,†,| Shi-Jay Yeh,† Yu-Hung Chen,|
Chin-Ti Chen,*,†, Tahsin J. Chow,*,† and Chih-I Wu*,|
Institute of Chemistry, Academia Sinica, Taipei, Taiwan 11529, R.O.C., Department of
Chemistry, National Taiwan Normal UniVersity, Taipei, Taiwan 11677, R.O.C., Department of
Chemistry and Biochemistry, National Chung Cheng UniVersity, Chia-Yi, Taiwan 62102, R.O.C.,
Graduate Institute of Photonics and Optoelectronics and Department of Electrical Engineering,
National Taiwan UniVersity, Taipei, Taiwan 10617, R.O.C., and Department of Applied
Chemistry, National Chiao Tung UniVersity, Hsinchu, Taiwan 30050, R.O.C.
Received September 20, 2008; E-mail: cchen@chem.sinica.edu.tw; tjchow@chem.sinica.edu.tw
Abstract: A series of group III metal chelates have been synthesized and characterized for the versatile
application of organic light-emitting diodes (OLEDs). These metal chelates are based on 4-hydroxy-1,5-
naphthyridine derivates as chelating ligands, and they are the blue version analogues of well-known green
fluorophore Alq3 (tris(8-hydroxyquinolinato)aluminum). These chelating ligands and their metal chelates
were easily prepared with an improved synthetic method, and they were facially purified by a sublimation
process, which enables the materials to be readily available in bulk quantity and facilitates their usage in
OLEDs. Unlike most currently known blue analogues of Alq3 or other deep blue materials, metal chelates
of 4-hydroxy-1,5-naphthyridine exhibit very deep blue fluorescence, wide band gap energy, high charge
carrier mobility, and superior thermal stability. Using a vacuum-thermal-deposition process in the fabrication
of OLEDs, we have successfully demonstrated that the application of these unusal hydroxynaphthyridine
metal chelates can be very versatile and effective. First, we have solved or alleviated the problem of exciplex
formation that took place between the hole-transporting layer and hydroxynaphthyridine metal chelates, of
which OLED application has been prohibited to date. Second, these deep blue materials can play various
roles in OLED application. They can be a highly efficient nondopant deep blue emitter: maximum external
quantum efficiency ηext of 4.2%; Commision Internationale de L’Eclairage x, y coordinates, CIEx,y ) 0.15,
0.07. Compared with Alq3, Bebq2 (beryllium bis(benzoquinolin-10-olate)), or TPBI (2,2′,2′′-(1,3,5-phenyle-
ne)tris(1-phenyl-1H-benzimidazole), they are a good electron-transporting material: low HOMO energy level
of 6.4-6.5 eV and not so high LUMO energy level of 3.0-3.3 eV. They can be ambipolar and possess a
high electron mobility of 10-4 cm2/V s at an electric field of 6.4 × 105 V/cm. They are a qualified wide band
gap host material for efficient blue perylene (CIEx,y ) 0.14, 0.17 and maximum ηext 3.8%) or deep blue
9,10-diphenylanthracene (CIEx,y ) 0.15, 0.06 and maximum ηext 2.8%). For solid state lighting application,
they are desirable as a host material for yellow dopant (rubrene) in achieving high efficiency (ηext 4.3% and
ηP 8.7 lm/W at an electroluminance of 100 cd/m2 or ηext 3.9% and ηP 5.1 lm/W at an electroluminance of
1000 cd/m2) white electroluminescence (CIEx,y ) 0.30, 0.35).
and red fluorescent dopants.3 Moreover, due to the rigid ball-
like geometry, high glass transition temperature (Tg ∼175 °C),3c
and polymorphic nature,4 Alq3 is readily sublimed to form
amorphous thin films, which is beneficial to the fabrication and
operation lifetime of OLEDs. To tune its fluorescence color to
1. Introduction
Efficient electroluminescence (EL) was first reported by Tang
and Van Slyke using green light-emitting tris(8-hydroxyquino-
linato)aluminum (Alq3).1 Very few materials attract attention
as much as Alq3 does in organic light-emitting diodes (OLEDs).2
Alq3 has been used as a green emitter, a common electron-
transporting material, and a host material for saturated green
(3) (a) Tang, C. W.; Van Slyke, S. A.; Chen, C. H. J. Appl. Phys. 1989,
65, 3610. (b) Bulovic, V.; Baldo, M. A.; Forrest, S. R. In Organic
Electronic Materials: Conjugated Polymers and Low Molecular Weight
Organic Solids; Farchioni, R., Grosso, G., Eds.; Springer-Verlag: New
York, 2001; p 391. (c) Higginson, K. A.; Thomsen, D. L., III; Yang,
B.; Papadimitrakopoulos, F. In Organic Light-Emitting DeVices: A
SurVey; Shinar,J., Ed.; Springer-Verlag: New York, 2004; p 71. (d)
Chen, C.-T. Chem. Mater. 2004, 16, 4389.
† Academia Sinica.
‡ National Taiwan Normal University.
§ National Chung Cheng University.
| National Taiwan University.
National Chiao Tung University.
(1) Tang, C. W.; Van Slyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
(4) Brinkmann, M.; Gadret, G.; Muccini, M.; Taliani, C.; Masciocchi,
N.; Sironi, A. J. Am. Chem. Soc. 2000, 122, 5147.
(2) (a) Chen, C. H.; Shi, J. Coord. Chem. ReV. 1998, 171, 161.
9
10.1021/ja807284e CCC: $40.75
2009 American Chemical Society
J. AM. CHEM. SOC. 2009, 131, 763–777 763

A R T I C L E S
Liao et al.
Scheme 1. Chemical Structures of Alq3, Almq3, and AlND3
the red or blue region, some structurally modified Alq3
derivatives have been developed in past two decades to match
the requirement in the RGB full color display or white light
illumination application. However, most structurally modified
Alq3 derivatives become less thermally stable and less volatile
under reduced pressure. Very often, they fail in the fabrication
of OLEDs by a thermal-vacuum-deposition process.
Photophysical and theoretical studies have already provided
insight into the electron distribution of the HOMO/LUMO
orbitals of Alq3.2,5 Whereas the highest electron density of
LUMO is found on pyridine ring, the electrons of HOMO
orbitals are located mostly on the phenoxide side of the ligand.
An electron-withdrawing substituent on the para-position of the
phenoxide ring will deplete the HOMO electron density
lowering energy level of the filled states. Following the same
rationale, the substitution of a electron-donating group on the
para-position of the pyridine ring will promote the LUMO
electron density raising the energy level of the vacant states.
Either structural modification results in increasing energy of
electrons involved in frontier orbital transition (π-π*) and an
Scheme 2. Chemical Structures of Group III Metal Chelates of
8-Hydroxy-1,5-naphthyridine Derivatives
f
emission that is blue-shifted relative to that of parent Alq3 (λmax
∼ 514 nm in toluene and 524 in dichloromethane). Although
there are several blue or near-blue light-emitting aluminum
f
chelates, Alq3 derivatives showing deep blue fluorescence (λmax
< 450 nm) have not been realized yet.2,6 Surveying literature,
we have found that a methyl substituent on the pyridine moiety
or aza (nitrogen) replacement of “CH” of the phenoxide moiety
of Alq3 is most attractive in blue-shifting fluorescence color,
preserving the rigid and globular structure and good valtility of
parent Alq3. Examples are tris(4-methyl-8-quinolinato)aluminum
(AlmQ3) and tris(4-[1,5]naphthyridinolato)aluminum (AlND3)
showing blue-shifted fluorescence at 506 and 440 nm, respec-
tively (Scheme 1). Whereas the greenish AlmQ3 was reported
with EL performance,6c-f application of blue AlND3 for OLEDs
is still literature unknown to date.2 Herein, in addition to the
4-hydroxy-1,5-naphthyridine (ND) aluminum chelate, which was
efficiently prepared by our improved synthesis, we report the
facile synthesis and full characterization of 4-hydroxy-8-methyl-
1,5-naphthyridine (mND), 2,8-dimethyl-4-hydroxy-1,5-naph-
thyridine (mmND), and 4-hydroxy-2-phenyl-1,5-naphthyridine
(mpND) metal chelates (Scheme 2). For mND, mmND, and
mpND chelating ligands, a whole series of chelates with group
III metals (aluminum, gallium, and indium) were also synthe-
sized and characterized. We employ blue-shifting factors of both
a methyl substituent and aza (nitrogen element) as the replace-
ment of “CH” in one structure. To our surprise, these metal
chelates are in fact new substances and previously unknown.
In addition to the thermal stability, they were characterized for
deep blue fluorescence and EL in applications of OLEDs.
Furthermore, due to their wide band gap and electron-deficient
nature, we will demonstrate that these metal chelates are feasible
as electron-transporting layer (ETL) material, the host material
for highly efficient blue fluorescence dopants, or blue host
material for yellow dopants to generate white EL in solid state
lighting (SSL) applications.
2. Results and Discussion
2.1. Synthesis and Structural Characterization. Parent 4-hy-
droxy-1,5-naphthyridine (or 1,5-naphthyridin-4-ol) is best known
to be prepared by so-called “EMME synthesis” from 3-ami-
nopyridine and diethyl ethoxymethylenemalonate.7 After the
intramolecular cyclization, the hydrolysis of the resulting ester
and thermo-decarboxylation of the acid afford 4-hydroxy-1,5-
naphthyridine in unsatisfactory overall yields (18-31%).8
Alternatively, a shorter and more convenient procedure known
as the Cassis method in the synthesis of a wide range of 4-1H-
quinolones (tautomeric forms of hydroxyquinoline) is using
(5) (a) Van Slyke, S. A.; Brynn, P. S.; Levecchio, F. V. U.S. Patent No.
5150006, 1992. (b) Burrow, P. E.; Shen, Z.; Bulvoic, V.; McCarty,
D. M.; Forrest, S. R.; Cronin, J. A.; Thompson, M. E. J. Appl. Phys.
1996, 79, 7991. (c) Sugimoto, M.; Anzai, M.; Sakanoue, K.; Sakaki,
S. Appl. Phys. Lett. 2001, 79, 2348.
(6) (a) Hamada, Y.; Sano, T.; Fujita, M.; Fuji, T.; Nishio, Y.; Shibata, K.
Jpn. J. Appl. Phys. 1993, 32, L514. (b) Hopkins, T. A.; Meerholz, K.;
Shaheen, S.; Anderson, M. L.; Schmidt, A.; Kippelen, B.; Padias,
A. B.; Hall, H. K., Jr.; Peyghambarian, Armstrong, N. R. Chem. Mater.
1996, 8, 344. (c) Kido, J.; Iizumi, Y. Chem. Lett. 1997, 963. (d) Kido,
J.; Iizumi, Y. Appl. Phys. Lett. 1998, 73, 2721. (e) Mattoussi, H.;
Murata, H.; Merritt, C. D.; Iizumi, Y.; Kido, J. J. Appl. Phys. 1999,
86, 2642. (f) Sapochak, L. S.; Padmaperuma, A.; Washton, N.;
Endrino, F.; Schmett, G. T.; Marshall, J.; Forgarty, D.; Burrows, P. E.;
Forrest, S. R. J. Am. Chem. Soc. 2001, 123, 6300. (g) Yu, J.; Chen,
Z.; Sakuratani, Y.; Suzuki, H.; Tokita, M.; Miyata, S. Jpn. J. Appl.
Phys 1999, 38, 6762. (h) Pohl, R.; Anzenbacher, P., Jr. Org. Lett.
2003, 5, 2769. (i) Pohl, R.; Montes, V. A.; Shinar, J.; Anzenbacher,
P., Jr. J. Org. Chem. 2004, 69, 1723. (j) Montes, V. A.; Li, G.; Pohl,
R.; Shinar, J.; Anzenbacher, P., Jr. AdV. Mater. 2004, 16, 2001. (k)
Cheng, J.-A.; Chen, C. H. J. Mater. Chem. 2005, 15, 1179. (l) Montes,
V. A.; Pohl, R.; Shinar, J.; Anzenbacher, P., Jr. Chem.sEur. J. 2006,
12, 4523. (m) Pe´rez-Bolivar, C.; Montes, V. A.; Anzenbacher, P., Jr.
Inorg. Chem. 2006, 45, 9610.
(7) Paudler, W. W.; Kress, T. J. AdV. Heterocycl. Chem. 1970, 11, 123.
(8) Eck, T. D.; Wehry, E. L., Jr.; Hercules, D. M. J. Inorg. Nucl. Chem.
1966, 28, 2439.
(9) (a) Cassis, R.; Tapia, R.; Valderrama, J. A. Synth. Commun. 1985,
15, 125. (b) Chen, B.; Hung, X.; Wang, J. Synthesis 1987, 482. (c)
Singh, B.; Laskowski, S. C.; Lesher, G. Y. Synlett 1990, 549. (d)
Marcos, A.; Pedregel, C.; Avendaoˇo, C. Tetrahedron 1994, 50, 12941.
(e) Bontemps, N.; Delfourne, E.; Batide, J.; Francisco, C.; Bracher,
F. Tetrahedron 1997, 53, 1743. (f) Kitahara, Y.; Nakahara, S.;
Yonezawa, T.; Nagatsu, M.; Shibano, Y.; Kubo, A. Tetrahedron 1997,
53, 17029. (g) Jeon, M.-K.; Kim, K. Tetrahedron Lett. 2000, 41, 1943.
(h) Salon, J.; Milata, V.; Pronayova, N.; Lesko, J. Monatch Chem.
2000, 131, 293.
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Group III Metal Chelates for OLEDs
A R T I C L E S
Scheme 3. Synthetic Routes to a Series of 8-Hydroxy-1,5-naphthyridine Metal Chelates
2,2,6-trimethyl-4H-1,3-dioxin-4-one (Meldrum’s acid) as an
effective diketone equivalent precursor.9 Therefore, we adopted
the effective Cassis method in the synthesis of the new
naphthyrine derivatives, mND, mmND, and mpND (Scheme
3). For comparison purposes in the study herein, parent chelating
ligand ND was also synthesized by the more convenient Cassis
method.
displayed in Figure 1.10 Similar to that of Alq3, the meridional
(mer) configuration of AlND3 is clearly evident by the structure
diagram and it is consistent with its complicated H NMR
1
spectrum. Interestingly, unlike AlmND3 or GamND3, InmND3
1
exhibited a simple H NMR spectrum (four sharp and well-
separated proton resonances), an indication of the facial (fac)
configuration of InmND3. However, recent evidence has
1
demonstrated that the simple H NMR spectrum of Inq3 (and
In the first step of synthesis, four pyridylaminomethylene
Meldrum’s acid derivatives (pre-ND, pre-mND, pre-mmND,
pre-mpND) were all obtained in reasonably good yields, 87,
78, 71, and 58%, respectively. Among four species, relatively
low yields (58%) of pre-mpND can be attributed to the bulky
phenyl group that causes sterical hindrance in the formation of
the methylene-bridge-head between the amino substituent and
Meldrum’s acid. In the following step of a ring closure reaction,
heating Meldrum’s acid derivatives in diphenyl ether afforded
ND, mND, mmND, and mpND in 47-77% yields, respectively.
Among them, ND (1,5-naphthyridin-4-ol or 8-hydroxy-1,5-
naphthyridine) was obtained in lowest yields (47%), and it is
due to formation of an undesired structural isomer (1,7-
naphthyridin-4-ol) in the ring closure reaction. The same cause
is believed to occur in the previous “EMME synthesis” of ND,
which was prepared in even lower yields.8 Nevertheless, the
methyl or phenyl substituents of mND, mmND, or mpND are
simply from the starting materials of either 3-amino-4-picoline
or triethyl orthoacetate/orthobenzoate. A different alkyl group
or substituted aromatic ring can be readily incorporated onto
ND with appropriate starting materials. Through the case of ND,
it is conceivable that the Cassis method illustrated herein is more
convenient and versatile. From the Cassis method, we obtained
ND in ∼40% overall synthetic yields, which is in fact better
than 18-31% overall synthetic yields from the literature
reported “EMME synthesis”.8 Moreover, the advantage of the
Cassis method is the simpler and more reliable synthetic
operation, a two-step procedure instead of a four-step procedure.
In “EMME synthesis”, a tedious purification process has to be
performed four times after each step in the synthetic sequence.
The newly synthesized 4-hydroxy-1,5-naphthyridine derivatives
were successfully converted into group III metal chelates.
However, due to the different reactivity of the metal starting
material and the different solubility of the metal chelate product,
various reaction conditions and the isolation/purification method
were adopted in the final metal chelation reactions, which is
somewhat variant from the conventional method preparing Alq3
(see Experimental Section for details). Basically, the synthesis
and purification of hydroxynaphthyridine group III metal
chelates are not much difficult than those for the easily prepared
and purified Alq3. They can be readily obtained with volume
production in a conventional synthetic laboratory.
hence InmND3) is due to the rapid fluxional transitions between
mer and fac configurations on the NMR time scale.11
The higher level of electron deficiency of ND than 8-hy-
droxyquinoline is evident by the significantly smaller pKa 2.85
of ND than pKa 5.13 of 8-hydroxyquinoline.12 ND derivatives
have been known for keto-enol tautomerism in polar organic
solvents (see Scheme 3).7 We found that the extent of naph-
thyridone tautomeric forms (and hence the pKa value) can be
1
gauged by the H NMR signal (chemical shift) of the proton
next to the hydroxy substituent of 8-hydroxyquinoline, ND, and
mND, which locates at 7.09, 6.52, and 6.50 ppm, respectively
(see Figure 2). We expect that even smaller pKa values are
present for mmND and mpND. The smaller pKa values (due to
the electron deficiency) render ND derivatives weaker δ-donors
on the phenoxide side of the chelating ligand. As the result of
a weaker δ-donor, the average Al-O bond distance of AlND3
(1.867 ( 0.03 Å) is longer than that (1.856 ( 0.02 Å) of Alq3.4
On the pyridine side of the chelating ligand, the average Al-N
bond distance of AlND3 (2.025 ( 0.03 Å) is shorter than that
(2.051 ( 0.02 Å) of Alq3, and this can be attributed to the
“seesaw-like” binding mode of a bidentate chelating ligand.
Otherwise, it can be attributed to the stronger N δ-donor in
AlND3 than in Alq3 because of the electron-donating methyl
substituent para to the N δ-donor. We believe that the strength
of the δ-donor (or the electron deficiency) of chelating ligands
is one of the reasons why aluminum metal chelates of mND,
mmND, or mpND all have a shorter fluorescence peak
wavelength and are better in blue color purity than AlND3 or
Alq3 (see Table 1 for fluorescence data).
2.2. Photophysical Properties and Energy Levels. Whereas
fl
it is the desired deep blue color (λmax 415-417 nm) of
AlmND3, AlmmND3, and AlmpND3, a less satisfactory sky blue
fluorescence (λmaxfl 433 nm) of the previously known AlND3 is
clearly shown in their solution fluorescence images (see Figure
3). In the solid state, all four aluminum chelates display red-
shifted fluorescence wavelength λmaxfl 425 nm (AlmpND3), 431
(10) Crystal data for AlND3 ·CH2Cl2: C25H17AlCl2N6O3: Fw ) 547.33,
j
Triclinic, P1, Z ) 2, F(000) ) 560. Cell dimendions: a ) 7.9946(7)
Å, b ) 12.0879(11) Å, c ) 13.1153(12) Å, R ) 70.346(2)o, ꢀ )
82.950(2)°, γ ) 84.082(2)o, V ) 1181.98(18) Å3, 2θmax ) 50.0°, Fcacld
) 1.538 mg/m3. Of 8777 reflections, 4157 were independent, 334
parameters, R(Fo) ) 0.0588 (for reflections with I > 2σ(I)), Rw(Fo) )
0.1663 (for reflections with I > 2σ(I)). The GoF on F2 was 0.977.
(11) Sapnochak, L. S.; Ranasinghe, A.; Kohlmann, H.; Ferris, K. F.;
Burrows, P. E. Chem. Mater. 2004, 16, 401.
These metal chelates were fully characterized by 1H and 13
C
NMR, mass spectroscopy, and elemental analysis, and they were
consistent with proposed structures. Particularly, a single crystal
X-ray structure of AlND3 was obtained. Its ORTEP drawing is
(12) Mason, S. F. J. Chem. Soc. 1957, 5010.
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A R T I C L E S
Liao et al.
Figure 3. Solution (in dichloromethane) and solid state fluorescence image
of AlND3, AlmND3, AlmmND3, and AlmpND3 from left to right,
respectively.
Figure 1. Molecular structure of AlND3 determined by X-ray diffraction
analysis.
Figure 4. Energy alignment of AlmND3, GamND3, InmND3, Alq3, NPB,
and CBP in OLED devices based on ITO anode and LiF/Al cathode.
Among them, AlmpND3 has the most severe fluorescence
quenching in the solid state with a Φf of only 6%. Such solid-
state fluorescence quenching is universal for mpND-based
chelates (AlmpND3, GampND3, and InmpND3. This can be
rationalized by the molecular contact (probably π-π interaction)
through protruded phenyl substituents in the solid state. With
no exception of the three chelating ligands mND, mmND, and
mpND, fluorescence of these metal chelates was observed to
be reduced with increasing atomic number of the metal ion from
Al to Ga and then In (see Φf data in Table 1), known as the
heavy atom effect that increases in the rate of intersystem
crossing.2,13
Figure 2. 1H NMR spectra of 8-hydroxyquinoline, ND, and mND in
CD3OD from top to bottom, respectively.
Table 1. Optical and Thermal Properties of Metal Chelates of
4-Hydroxy-1,5-naphthyridine Derivatives
solutiona
solid
In the search for the origin of the wide band gap nature,
AlmND3, GamND3, and InmND3 were found to have a HOMO
energy level around 6.4-6.5 eV, which is significantly lower
than 5.9 eV of Alq3 (Figure 4). The LUMO energy level of
three mND metal chelates is around 3.0-3.3 eV, which is
similar to or just a bit lower than 3.1 eV of Alq3. This is
perfectly logical because the prominent electron deficient feature
(N aza substituent) locates on the HOMO of the molecule, which
is the pyridin-4-olate ring of the chelating ligand, instead of
the LUMO of three mND metal chelates, which is the para-
methylpyridine (or 4-picoline) ring of the chelating ligand. It
can be further identified that either HOMO or LUMO is
stabilized by the higher atomic number of the central metals,
Al, Ga, and In. Such a stabilization effect happens more
prominently in the LUMO than in the HOMO. As a result, the
energy band gap of three mND metal chelates decreses in the
order AlmND3 > GamND3 > InmND3. Relative to that of Alq3,
a significantly low HOMO energy level and moderately low
LUMO energy level enlarge the energy band gap of these
hydroxylnaphthyridine-based metal chelates.
ab
fl
fl
λmaxab, λon
(nm)
λmax Φf λmax Φf Tg
Tc
Tm LUMO/HOMO
(eV)b
set
metal chelates
(nm) (%) (nm) (%) (°C) (°C) (°C)
Alq3
AlND3
AlmND3
388, 443 524 20 516 40 174 367 412
341, 381 433 47 447 45 122 262 412
338, 370 415 45 431 43 196 277 420
3.1/5.9
-
3.0/6.4
387
c
AlmmND3
AlmpND3
GamND3
335, 372 416 45 419 39 233
326, 376 417 45 425
341, 374 431 45 439 41
436
-
6
204 278 370
-
c
c
c
414
430
3.2/6.5
-
GammND3 339, 376 432 42 434 52 212
GampND3
InmND3
InmmND3
InmpND3
327, 383 432 25 439 11 185 275 381
344, 382 436 37 445 12 183
332, 381 437 36 436 22 221
-
c
361
372
3.3/6.5
-
-
c
329, 386 437 23 446
8 186 224 370
a In dichloromethane. b HOMO energy was determined as the edge of
HOMO energy level or the ionization potential of the material; LUMO
energy was determined as the lowest photoexcitation state energy from
the on-set absorption energy in absorption spectra. c Not observed.
fl
nm (AlmND3), and 447 nm (AlND3), except λmax 419 nm of
AlmmND3 (Table 1). In dichloromethane solution, all four
aluminum chelates have reasonably good fluorescence quantum
yields (Φf) around 45-47% but more or less suffer from
fluorescence concentration quenching in the solid state
(Table 1).
(13) Burrow, P. E.; Sapochak, L. S.; McCarty, D. M.; Forrest, S. R.;
Thompson, M. E. Appl. Phys. Lett. 1994, 64, 2718.
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Group III Metal Chelates for OLEDs
A R T I C L E S
Figure 5. Hole and electron mobility vs the square root of the applied electric field of AlmND3 and AlmmND3 (a) and Alq3 and Bebq2 (b).
2.3. Charge-Transporting Properties and Charge Carrier
Mobilities. In terms of HOMO and LUMO energy levels,
hydroxynaphthyridine-based metal chelates are better hole-
blocking or electron-transporting materials than Alq3 or other
deep blue fluorophores. Most known deep blue fluorophores
are based on triarylamines, such as NPB, or nonheteroatom-
containing polycyclic aromatic hydrocarbons (PAHs), such as
anthracene, perylene, pyrene, or spirobifluorene compounds.
Compared with those of hydroxynaphthyridine metal chelates,
such nonmetal chelate deep blue fluorophores have significantly
higher HOMO and LUMO energy levels of 5.1-5.8 eV and
2.0-2.7 eV (assuming band gap energy is ca. 3.1 eV) below
vacuum level, respectively.14
The HOMO-LUMO energy level of the material shown
above is one of the determining factors for electron-transporting
or hole-transporting properties of OLED materials. The charge
carrier (electron or hole) mobility is the other decisive charac-
teristic that influences the charge-transporting nature and hence
the efficiency performance of OLEDs. The relative magnitude
of the electron and hole mobility of the material indicates the
extent of charge balancing, the efficiency of charge recombina-
tion, and hence the EL efficiency of OLEDs. We measured the
intrinsic charge carrier (hole or electron) mobility in a bulk film
(0.6-1 µm) of AlmND3 and AlmmND3 using the optical time-
of-flight (TOF) technique that has been described before. In
addition, we also took a measurement on Alq3 and Bebq2 with
the same measuring system for comparison and accuracy
checking. Figure 5 shows the field dependence of the hole and
electron drift mobility of AlmND3, AlmmND3 (Figure 5a), Alq3,
and Bebq2 (Figure 5b). First, the electron mobility of Alq3 was
determined to be ∼10-5 cm2/V s at an electric field of 6.4 ×
105 V/cm, which agrees with those obtained previously.15
Second, the electron mobility of Bebq2 was determined to be
∼10-4 cm2/V s at the same electric field, which is 1 order of
magnitude higher than that of Alq3 and consistent with the report
that Bebq2 is a better electron-transporting material than Alq3.16
Devices with an electron-transporting layer (ETL) of Bebq2 have
been demonstrated with lower driving voltage and longer
operation lifetime, when compared with ones with Alq3 as an
ETL.16 Third, we found that the charge carrier (either hole or
electron) mobility of AlmND3 is higher than that of AlmmND3
by nearly 1 order of magnitude (Figure 5a). In fact, checking
the extrapolated data in the range of electric field of (3.6-6.4)
× 105 V/cm, the charge carrier (either hole or electron) mobility
of AlmND3 is the highest among all (Figure 5a and 5b).
Therefore, AlmND3 is probably a better electron-transporting
material than AlmmND3 because of its high electron mobility.
In addition, AlmND3 seems to be the only material that is
ambipolar because of its very similar hole and electron mobility
determined by TOF technique. On the other hand, AlmmND3
may be a better nondopant deep blue emitter because of its low
hole mobility, which limits the amount of hole carrier on
AlmmND3 and enhances the charge balance in normally hole-
dominated devices, one crucial factor for high efficiency OLEDs.
As shown in the following sections (2.7 and 2.8), our OLED
results are consistent with the forgoing derivation from the TOF
data of AlmND3 and AlmmND3.
2.4. Thermal Properties. Figure 6 shows the differential
scanning calorimetry (DSC) thermograms of Alq3, AlND3,
AlmND3, GamND3, AlmmND3, and GammND3. In DSC
measurements, these metal chelates were first taken from the
sample that was purified by a sublimation process. With such
prethermal-annealed samples (sublimed-scarped samples), we
found that DSC thermograms often show nothing but weak
endothermic step transitions, indicative of the glass phase
transition temperature (Tg). To reveal DSC signals of other phase
transition temperatures, such as crystallization temperature (Tc),
melting temperature (Tm), or polymorphic phase transition
temperature (Tp), the sublimed-scarped samples of metal chelates
were redissolved in dichloromethane and then evaporated until
dryness under reduced pressure at room temperature. In Figure
6, each metal chelate is displayed with two types of DSC traces.
First the heating scan, positioned as the top scan, has a heating
temperature beyond the large endothermic signal, Tm. The other
scans marked with the sequence number of heating scans or
cooling scans in Figure 6 are DSC traces with a measuring
temperature less than Tm, except for AlmpND3, GampND3, and
InmpND3 (see Figure S1 for their DSC traces). Two types of
heating thermograms were displayed because these metal
chelates often exhibit thermal decomposition right after their
melting transition, which was evident from observations from
a polarized optical microscope (POM). With samples prepared
under such conditions, Alq3 shows Tg (on-set Tg) at 174 °C, a
broad and small endortherm peaking at ∼355 °C (assigned as
Tp) immediately followed by an exotherm peaked at 367 °C
(Tc), and finally a large endothermic signal at 412 °C (Tm).
Whereas a Tg of 174 °C and Tm of 412 °C are rather consistent
with literature data for Alq3,3c,4,6f,17 the coupled endortherm and
(14) Adachi, C.; Oyamada, T. Data Book on HOMO LeVels of Organic
Thin Films in Organic Semiconductor DeVices; CMC Publishing:
Tokyo, 2005.
(15) Tse, S. C.; Kwok, K. C.; So, S. K. Appl. Phys. Lett. 2006, 89, 262102.
(16) (a) Lee, J.-H.; Wu, C.-I.; Liu, S.-W.; Huang, C.-A.; Chang, Y. Appl.
Phys. Lett. 2005, 86, 103506. (b) Lee, J.-H.; Ho, Y.-H.; Lin, T.-C.;
Wu, C.-F. J. Electrochem. Soc. 2007, 154, J226.
(17) Higginson, K. A.; Zhang, X.-M.; Papadimitrakopoulos, F. Chem.
Mater. 1998, 10, 1017.
9
J. AM. CHEM. SOC. VOL. 131, NO. 2, 2009 767

A R T I C L E S
Liao et al.
Figure 6. DSC thermograms of Alq3, AlND3, AlmND3, GamND3, AlmmND3, and GammND3.
exotherm have been shown before, although they were reported
at higher temperatures of 393 and 396 °C, respectively.4,6f,18 It
is interesting to know that Alq3 will be vaporized (sublimed)
when heated around melting temperatures at atmospheric
pressure.6f We have a similar observation (under POM) for Alq3
and most metal chelates reported herein as well. Such an
observation is indirect evidence that these new metal chelates
preserve their superior volatility as well as Alq3. Unlike those
structurally modified Alq3 blue derivatives, volatile hydrox-
ynaphthyridine metal chelates reported herein have no problem
in the fabrication of OLED by thermal-vacuum-deposition
processes.
in great detail recently.4,6f,18 It has been suggested that the
exotherm observed for Alq3 in DSC traces is due to crystal-
lization, indicative of the instability of the glassy phase of Alq3.
Some new hydroxynaphthyridine metal chelates, such as Alm-
mND3, GamND3, GammND3, InmND3, and InmmND3, show
no discernible exotherm in DSC traces, a good sign of the
morphological stability of their glassy phase. Except for AlND3
(Tg ∼122 °C) and GamND3 (no detectable Tg in DSC traces),
all metal chetates reported herein show Tg values around
183-233 °C, which is higher than 174 °C for Alq3 (Table 1).
Higher Tg’s of metal chetates often imply a higher morphor-
logical stability or a glass phase stability, which is beneficial to
the operation lifetime of the multiple-thin-film OLEDs. Fur-
thermore, for all aluminum chelates but AlmpND3, higher
melting temperatures than Alq3 were observed by DSC (Tab-
le 1). This is a plus because the melting transition of these
aluminum chelates was rapidly followed by the thermal
decomposition. A higher melting temperature means a higher
thermal stability in the case of the aluminum chelates studied
herein.
Alq3 has been well-known for its polymorphic and racemic
nature and its multiple phase transitions that have been studied
(18) (a) Sano, K.; Kawata, Y.; Urano, T. I.; Mori, Y. J. Mater.Chem. 1992,
2, 767. (b) Braun, M.; Gmeiner, J.; Tzolov, M.; Co¨lle, M.; Meyer,
F. D.; Milius, W.; Hillebrecht, H.; Wendland, O.; von Schu¨tz, J. U.;
Bru¨tting, W. J. Chem. Phys. 2001, 114, 9625. (c) Co¨lle, M.; Gmeiner,
J.; Milius, W.; Hillebrecht, Br¨; utting, W. AdV. Funct. Mater 2003,
13, 108. (d) Muccini, M.; Loi, M. A.; Kenevey, K.; Zamboni, R.;
Masciocchi, N.; Sironi, A. AdV. Mater. 2004, 16, 861. (e) Levichkova,
M. M.; Assa, J. J.; Frob, H.; Leo, K. Appl. Phys. Lett. 2006, 88,
201912.
Metal chelates based on mpND are different from the rest.
Among all metal chelates, AlmpND3, GampND3, and InmpND3
9
768 J. AM. CHEM. SOC. VOL. 131, NO. 2, 2009

Group III Metal Chelates for OLEDs
A R T I C L E S
Table 2. Electroluminescence Characteristics of Nondoped
AlmND3 OLEDs Containing CBP with Different Layer Thicknessa
luminance,
efficiency,
voltage
[cd/m2, %, V]b
CBP
thickness
[nm]
max. luminance,
voltage
[cd/m2, V]
max.
efficiency
[%, cd/A, lm/W]
CIE 1931
chromaticity
[x, y]
el
λmax
[nm]
3
5
10
15
5555, 15
4971, 15
5070, 15
5606, 15
119, 0.39, 5.68 0.53, 0.79, 0.33 456 0.18, 0.20
162, 0.86, 5.63 1.08, 1.01, 0.45 450 0.15, 0.11
277, 1.63, 8.37 1.79, 1.51, 0.53 448 0.15, 0.10
315, 1.63, 9.55 1.74, 1.68, 0.53 452 0.16, 0.12
a Devices have the configuration of ITO/NPB(40 nm)/CBP(x nm)/
AlmND3(30 nm)/Alq3(20 nm)/LiF(0.5 nm)/Al(150 nm), x ) 3, 5, 10,
b
and 15, respectively. At current density of 20 mA/cm2.
a low HOMO energy level around 6.1 eV,20 arylamine mCP
(1,3-di(9H-carbazol-9-yl)benzene) is effective in preventing the
exciplex emission as evident in device ITO/NPB(30 nm)/
mCP(30 nm)/AlmND3(30 nm)/LiF(5 nm)/Al (150 nm), although
such a device is very poor in EL efficiency and brightness. Also
having a low HOMO energy level around 6.3 eV,21 another
hole-transporting material CBP (4,4′-di(9H-carbazol-9-yl)bi-
phenyl) was thus inserted between NPB and AlmND3. By the
variation of CBP layer thickness, we have successfully elimi-
nated the exciplex emission and optimize the performance of
the blue devices ITO/NPB(40 nm)/CBP(x nm)/AlmND3(30 nm)/
Alq3 (20 nm)/LiF (5 nm)/Al (150 nm) optimized with x ) 10
(Table 2). From such CBP-inserted nondoped AlmND3 OLEDs,
we have successfully achieved deep blue ELs (CIEx,y ) 0.15,
0.10) with a reasonably good external quantum efficiency (ηext)
reaching 1.79% (or 1.63% at 20 mA/cm2) and a maximum
brightness of 5070 cd/m2 (or 277 cd m-2 at 20 mA/cm2) (Figure
8). To the best of our knowledge, we believe that a similar
exciplex problem of AlmND3 has been found for AlND3 before
and the problem has not been solved until this study.22
2.6. As Electron-Transporting Material in Non-Doped
AlmND3 OLEDs. Similar to green emitter Alq3, deep blue
emitter AlmND3 potentially can be used as an electron-
transporting material in OLEDs. To gauge such viability, four
nondoped AlmND3 OLEDs containing four different ETL
materials were fabricated: ITO/NPB(40 nm)/CBP10 nm)/
AlmND3(30 nm)/ETL(20 nm)/LiF(5 nm)/Al(150 nm), where the
ETL material is TPBI (2,2′,2′′-(1,3,5-phenylene)tris(1-phenyl-
1H-benzimidazole), Bebq2 (beryllium bis(benzoquinolin-10-
olate), Alq3, or AlmND3. As data have shown in Table 3 and
Figure 9, in terms of ηext, nondoped AlmND3 OLEDs are most
efficient when AlmND3 is employed as the ETL material in
OLEDs, although it is not as bright as others. However, in terms
of power efficiency (ηP), nondoped AlmND3 OLEDs having
TPBI or Bebq2 are more efficient than ones using Alq3 or
AlmND3 as the ETL materials (Table 3). We notice that
AlmND3 provide OLEDs a relatively low current density and
a relatively high turn-on voltage (Figure 9), which are compa-
rable with those of Alq3 but inferor to those of TPBI or Bebq2.
Nevertheless, such results validate the usage of AlmND3 as an
ETL material in OLEDs.
Figure 7. EL spectra of four NPB/AlmND3-containing OLEDs: ITO/
multiple organic layers/LiF/Al.
were found with relatively low Tg’s of 185-204 °C, and they
are easily recognizable in repeated heating scans in DSC
measurements (Figure S1). Their Tg’s were always observed
prior to a cold crystallization peak without discernible poly-
morphic phase transition signals (as Tp marked for AlND3,
AlmND3, and AlmmND3 in Figure 6). AlmpND3, GampND3,
and InmpND3 are also a few exceptions that they do not
decompose following the melting transition. Such a difference
in thermal properties of mpND-based metal chelates can be all
attributed to the protruded phenyl substituent on a symmetric
and nearly globular structure of naphthyridinolate metal chelates.
2.5. Elimination of Adverse Exciplex Formation in Non-
Doped Blue OLEDs. Deep blue fluorescence of AlmND3 or
AlmmND3 is very attractive for highly demanding blue OLEDs.
A simple nondoped bilayer EL device ITO/NPB(30 nm)/
AlmND3(30 nm)/LiF(5 nm)/Al (150 nm) was first fabricated
by sequential thermal-vacuum-deposition of NPB, AlmND3,
LiF, and Al onto ITO (indium tin oxide)-coated glass substrate.
Unfortunately, in addition to the emission band peaking around
430-450 nm of AlmND3, a pronounced emission band around
480-540 nm appears in the EL spectrum of the device (Fig-
ure 7).
Such a long wavelength EL devastates the EL efficiency and
also impairs the blue color purity of OLEDs. A similar
observation was also found for AlmmND3 OLEDs. To clarify
the origin of the long wavelength EL, three more control devices
were then fabricated: ITO/NPB (30 nm)/mCP(30 nm)/
AlmND3(30 nm)/LiF(5 nm)/Al(150 nm), ITO/NPB(30 nm)/
AlmND3 (30 nm)/Alq3(30 nm)/LiF(5 nm)/Al(150 nm), and ITO/
NPB(30 nm)/AlmND3 (30 nm)/TPBI (30 nm)/LiF (5 nm)/Al
(150 nm). From their EL spectra (Figure 7), we can firmly
conclude that the EL around 480-540 nm is due to the exciplex
emission occurring at the interface of NPB and AlmND3. It is
rather common that electron-deficient materials (such as Alq3
and 1,3,4-oxadiazole compounds) have a propensity to form
exciplex with electron-rich hole-transporting materials (such as
NPB triarylamine species).19 By lowering the HOMO energy
level of the hole-transporting material (i.e., reducing the HOMO
energy level difference between the hole-transporting material
and AlmND3),19d the problem of the exciplex associated with
AlmND3 can be largely alleviated. As shown in Figure 7, having
2.7. AlND3, AlmND3, AlmmND3, and AlmpND3 for High
Efficiency Non-Doped Deep Blue OLEDs. Having solved the
problem of exciplex EL and demonstrated the electron-
(19) (a) Itano, K.; Ogawa, H.; Shirota, Y. Appl. Phys. Lett. 1998, 72, 636.
(b) Chan, L.-H.; Lee, R.-H.; Hsieh, C.-F.; Yeh, H.-C.; Chen, C.-T.
J. Am. Chem. Soc. 2002, 124, 6469. (c) Guan, M.; Bian, Z. Q.; Zhou,
Y. F.; Li, F. Y.; Li, Z. J.; Huang, C. H. Chem. Commun. 2003, 2708.
(d) Matsumoto, N.; Nishiyama, M.; Adachi, C. J. Phys. Chem. C 2008,
112, 7735.
(20) Wu, M.-F.; Yeh, S.-J.; Chen, C.-T.; Murayama, H.; Tsuboi, T.; Li,
W.-S.; Chao, I.; Liu, S.-W.; Wang, J.-K. AdV. Funct. Mater 2007, 17,
1887.
(21) Hill, I. G.; Rajagopai, A.; Kahn, A. J. Appl. Phys. 1998, 84, 3236.
(22) Chen, Chin Hsin, Department of Photonics and Display Institute,
National Chiao Tung University, private communication.
9
J. AM. CHEM. SOC. VOL. 131, NO. 2, 2009 769

A R T I C L E S
Liao et al.
Figure 8. EL characteristics of ITO/NPB(40 nm)/CBP(x nm)/AlmND3(30 nm)/Alq3(20 nm)/LiF(0.5 nm)/Al(150 nm), x ) 3, 5, 10, and 15, respectively.
Table 3. Electroluminescence Characteristics of Nondoped
that an insignificant methyl substituent or the number of methyl
substituents can exert that much on ηext and EL brightness.
AlmND3 OLEDs with Various ETL Materialsa
max.
luminance,
efficiency,
voltage
[cd/m2, %, V]b
Plausibly, among four aluminum chelates, we may recognize
AlmmND3 as an exceptional one based on the red-shifted
fluorescence from solution to the solid state. The red-shifting
fluorescence is 443f447 nm, 415f431 nm, 416f419 nm, and
417f425 nm for AlND3, AlmND3, AlmmND3, and AlmpND3,
respectively. Energywise, this corresponds to a red-shifting
energy of 723, 895, 172, and 451 cm-1, respectively for four
aluminum chelates. AlmmND3 has the smallest red-shifted
fluorescence among all. Accordingly, we surmise that, in the
solid state, AlmmND3 aggregates in a quite different fashion
from the other aluminum chelates. Compared with other
aluminum chelates, such aggregation enables high efficiency
of charge recombination (or charge balance) and thus the ηext
(or ηP) of AlmmND3 OLEDs. For AlmmND3, the low hole
mobility (lower than its electron mobility) helps in balancing
the charge of AlmmND3 OLEDs.
We have also examined the performance of blue OLEDs with
GamND3, GammND3, InmND3, or InmmND3 as the nondoped
light-emitting layer (Table S1). As it can be anticipated from
the fact that a heavy atom will quench the fluorescence, indium
chelates performed the worst in terms of EL efficiency or
brightness. However, this is not exactly the case for gallium
chelates. Comparing data in Table 4 and Table S1, the
GammND3 OLED is worse than the AlmmND3 OLED but the
GamND3 OLED is better than the AlmND3 OLED. Neverthe-
less, once again, we verified the same trend of OLED perfor-
mance; namely, mmND is better than mND for gallium or
indium metal chelates as nondoped deep blue light-emitting
materials for OLEDs.
max.
efficiency
[%, cd/A, lm/W]
CIE 1931
chromaticity
[x, y]
el
luminance
and voltage
[cd/m2, V]
λmax
[nm]
ETL
TPBI
Bebq2
Alq3
6000, 15 240, 1.15, 6.2 1.86, 1.79, 0.86 454 0.16, 0.13
7140, 15 340, 1.67, 5.6 1.81, 1.84, 0.98 458 0.15, 0.13
5070, 15 277, 1.63, 8.4 1.79, 1.51, 0.53 448 0.15, 0.10
AlmND3 5240, 15 310, 1.85, 6.8 1.96, 1.65, 0.84 452 0.15, 0.10
a ITO/NPB(40 nm)/CBP(10 nm)/AlmND3(30 nm)/ETL(20 nm)/LiF(5
b
nm)/Al(150 nm). At 20 mA/cm2.
transporting property of AlmND3, we are ready to examine and
find the best aluminum chelates of ND, mND, mmND, and
mpND for nondoped deep blue OLEDs. Having AlmND3 as
the electron-transporting layer, four nondoped OLEDs were
based on AlND3, AlmND3, AlmmND3, and AlmpND3: ITO/
NPB(40 nm)/CBP(10 nm)/Blue Al chelates(30 nm)/AlmND3(20
nm)/LiF(0.5 nm)/Al(150 nm) were fabricated and characterized
(Table 4 and Figure 10).
As it can be anticipated from their solution or solid-state
fluorescence wavelength, the AlmND3 OLED showed a much
deeper blue EL with 1931 Commision Internationale de
L’Eclairage x, y coordinates (CIEx,y ) 0.15, 0.09) than that of
AlND3 OLED (CIEx,y ) 0.15, 0.19). However, we do not
anticipate that AlmND3 OLED performs better than does AlND3
OLED considering its fluorescence quantum yield. Since
fluorescence quantum yields of AlND3 are higher than those of
AlmND3 either in solution or in the solid state, we attribute
such a result to the charge balancing in the OLED, which has
a great influence on the efficiency of the charge-recombination
in the OLED. More surprisingly, the AlmmND3 OLED shows
EL
2.8. Nondoped AlmmND3 OLEDs with Various Metal
Chelates as Electron-Transporting Layer. Knowing all group
III metal chelates are potential ETL material in OLEDs, we
fabricated a series of AlmmND3-based nondoped blue OLEDs
the shortest EL wavelength (λmax ∼436 nm), the highest ηext
of 4.18% (or 4.11% at 20 mA/cm2), and the brightest EL of
445 cd/m2 at 20 mA/cm2. Considering the structural difference
among AlND3, AlmND3, and AlmmND3, it is hard to conceive
9
770 J. AM. CHEM. SOC. VOL. 131, NO. 2, 2009

Group III Metal Chelates for OLEDs
A R T I C L E S
Figure 9. EL characteristics of ITO/NPB(40 nm)/CBP(10 nm)/AlmND3(30 nm)/ETL(20 nm)/LiF(0.5 nm)/Al(150 nm), where ETL is TPBI, Bebq2, Alq3,
and AlmND3, respectively.
fl
Table 4. Electroluminescence Characteristics of Nondoped OLEDs
fluorescent blue emitter with λmax 467 nm (in clyclohexane)
of AlND3, AlmND3, AlmmND3, and AlmpND3 blue Al chelatesa
and Φf 94%.23 However, in the solid state blue perylene
max. luminance
and voltage
[cd/m2, V]
luminance,
efficiency, voltage
CIE 1931
chromaticity
[x, y]
becomes a poor fluorophore, a yellow one, due to the severe
molecular aggregation that brings about concentration quench-
ing, and a diminished and red-shifting fluorescence takes place.
The fluorescence of DPA is even bluer and stronger showing
λmaxfl 438 nm (in cyclohexane) and Φf 100%.23 Similar to blue
perylene, deep blue DPA has the inherent problem of crystal-
lizing when deposited as a thin film that prohibits its nondopant
usage in a device.24 Whereas some reports are available for the
dopant usage of perylene,25 there is no known literature case
of the dopant usage of DPA in OLEDs to date. This can
be simply attributed to the lack of appropriate host material, a
wide band gap one, required for such a deep blue DPA dopant.
We took 0.5, 2, and 4 wt % dopant concentration of both
perylene and DPA in the fabrication of dopant-based AlmND3
OLEDs. The EL characteristics of both series of devices are
displayed in Figures 11 and 12, and their data are summarized
in Table 6.
el
Al
chelates
max. efficiency λmax
[%, cd/A, lm/W] [nm]
b
[cd/m2, %, V]
AlND3
AlmND3
AlmmND3
AlmpND3
2286, 15
3824, 15
3792, 15
4078, 15
216, 0.75, 8.31 0.76, 1.09, 0.45 466 0.15, 0.19
279, 1.86, 7.57 1.91, 1.43, 0.77 448 0.15, 0.09
445, 4.11, 9.36 4.18, 2.27, 0.86 436 0.15, 0.07
306, 1.58, 9.63 1.62, 1.58, 0.71 452 0.15, 0.12
a Devices have the configuration of ITO/NPB(40 nm)/CBP(10 nm)/
Blue Al chelates(30 nm)/AlmND3(20 nm)/LiF(0.5 nm)/Al(150 nm). b At
current density of 20 mA/cm2.
with a variation of ETL material, AlND3, AlmND3, AlmmND3,
AlmpND3, GamND3, or InmND3 (Table 5 and Figure S2).
Here, for a fair comparison, the AlmmND3 OLED having
AlmND3 as ETL material is a refabricated one, not the same
one in Table 4 and Figure 10. As the data show in Table 5, all
el
devices exhibit virtually the same λmax 432 nm and a very
similar deep blue color chromaticity CIEx,y ) 0.15-0.16,
0.07-0.08, consistent with all OLEDs having the same deep
blue nondoped emitter AlmmND3. Regardless of ETL materials,
all deep blue OLEDs have a relatively high EL efficiency with
ηext being more than 3.0%. In terms of EL efficiency, the
AlmmND3 OLED having AlmND3 as the ETL material is the
most outstanding. This OLED has ηext reaching 3.77% and ηP
over 0.80 lm/W at a current density of 20 mA/cm2. From OLED
results in section 2.7 and 2.8, it is very clear that AlmmND3 is
the most efficient nondoped deep blue emitter but AlmND3 is
the best ETL material for it.
2.9. AlmND3 as the Host Material for Blue Perylene and
Deep Blue 9,10-Diphenylanthracene Dopants. To demonstrate
the wide band gap nature of hydroxynaphthyridine metal
chelates, we have fabricated a series of AlmND3 OLEDs
containing perylene or 9,10-diphenylanthracene (DPA) as highly
efficient blue or deep blue dopant materials. Perylene is a highly
(23) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic
Molecules, 2nd ed.; Academic Press: New York, 1971; pp 264 and
399.
(24) Adachi, C.; Tsutsui, T.; Sato, S. Appl. Phys. Lett. 1990, 56, 799.
(25) (a) Kojima, H.; Ozawa, A.; Takahshi, T.; Nagaoka, M.; Homma, T.;
Nagatomo, T.; Omoto, O. J. Electrochem. Soc. 1997, 144, 3628. (b)
Mi, B. X.; Gao, Z. Q.; Lee, C. S.; Lee, C. T.; Kwong, H. L.; Wong,
N. B. App. Phys. Lett. 1999, 75, 4055. (c) Lu, P.; Hong, H.; Cai, G.;
Djurovich, P.; Weber, W. P.; Thompson, M. E. J. Am. Chem. Soc.
2000, 122, 7480. (d) Jiang, X.-Y.; Zhang, Z.-L.; Zheng, Z.-Y.; Wu,
Y.-Z.; Xu, S.-H. Thin Solid Films 2001, 401, 251. (e) Wu, C.-C.; Lin,
Y.-T.; Chiang, H.-H.; Cho, T.-Y.; Chen, C.-W.; Wong, K.-T.; Liao,
Y.-L.; Lee, G.-H.; Peng, S.-M. Appl. Phys. Lett. 2002, 81, 577. (f)
Ni, S. Y.; Wang, X. R.; Wu, Y. Z.; Chen, H. Y.; Zhu, W. Q.; Jiang,
X. Y.; Zhang, Z. L.; Sun, R. G. Appl. Phys. Lett. 2004, 85, 878. (g)
Jarikov, V. V. J. Appl. Phys. 2006, 100, 014901. (h) Tse, S.-C.; Tsung,
K.-K.; So, S.-K. Appl. Phys. Lett. 2007, 90, 213502. (i) Lee, R.-H.;
Hung, Y.-W.; Wang, Y.-Y.; Chang, H.-Y. Thin Solid Films 2008, 516,
5062.
9
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A R T I C L E S
Liao et al.
Figure 10. EL characteristics of ITO/NPB(40 nm)/CBP(10 nm)/Blue Al chelate(30 nm)/ETL(20 nm)/LiF(0.5 nm)/Al(150 nm), where Blue Al chelate is
AlND3, AlmND3, AlmmND3, and AlmpND3, respectively.
Table 5. Electroluminescence Characteristics of Nondoped OLEDs
deep blue color purity was elevated to a higher level, CIEx,y
) 0.15, 0.06. After the comparison with emission spectra of
of AlmmND3 with Various Metal Chelates as ETL Materialsa
max. luminance
and voltage
[cd/m2, V]
luminance,
efficiency, voltage
CIE 1931
chromaticity
[x, y]
solution DPA, solid state AlmND3, and solid state DPA
(fluorescence spectra shown in Figure 12), the EL spectra of
the DPA-doped AlmND3 OLED can be recognized as a
coemission from both DPA and AlmND3. Since the main
emission wavelength of DPA is shorter than that for AlmND3,
the coemission EL observed for the DPA-doped AlmND3
OLED gave rise to the deepest blue color (see inserted 1931
CIE chromaticity diagram in Figure 12). Considering the
inadequate overlapping of the absorption spectrum of DAP
and the emission spectrum of AlmND3 (not shown in Figure
12), we can conceive that Fo¨rster energy transfer between
el
max. efficiency λmax
[%, cd/A, lm/W] [nm]
ETL
b
[cd/m2, %, V]
AlND3
3313, 15
4444, 15
4879, 15
5487, 15
4694, 15
3848, 15
369, 3.40, 8.92 3.67, 2.00, 1.04 432 0.15, 0.07
401, 3.77, 7.75 3.79, 2.00, 0.94 432 0.15, 0.07
329, 3.01, 8.60 3.27, 1.78, 0.84 432 0.16, 0.07
376, 3.23, 8.24 3.41, 1.99, 1.13 432 0.16, 0.08
416, 3.67, 8.18 3.68, 2.09, 0.93 432 0.15, 0.07
376, 3.62, 8.62 3.67, 1.91, 0.91 432 0.15, 0.07
AlmND3
AlmmND3
AlmpND3
GamND3
InmND3
a Devices have the configuration of ITO/NPB(40 nm)/CBP(10 nm)/
AlmmND3(30 nm)/ETL(20 nm)/LiF(0.5 nm)/Al(150 nm). b At current
density of 20 mA/cm2.
Dopant concentration 0.5 wt% was found to be optimum
for either perylene or DPA. The vibronic emission observed
at the low energy side of the major EL band is the distinct
feature of a not so blue preylene EL, indicating a sufficient
Fo¨rster energy transfer between the AlmND3 host and
perylene dopant. Compared to the undoped device (second
entry of Table 4), the maximum EL efficiency of the
perylene-doped device was enhanced more than four times
to ∼3.06 lm/W, or more than three times to ∼4.67 cd/A. At
a current density of 20 mA/cm2, ηext was 3.13%, which was
enhanced 1.7 times compared to an undoped device. The
maximum electroluminance also increased to 12 420 cd/m2,
a greater than 3-fold enhancement than that of the undoped
device, although this is at a price of deep blue color purity.
(26) (a) Xie, Z. Y.; Huang, J. S.; Li, C. N.; Liu, S. Y.; Wang, Y.; Li, Y. Q.;
Shen, J. C. Appl. Phys. Lett. 1999, 74, 641. (b) Steuber, F.; Staudigel,
J.; Stro¨ssel, M.; Simmerer, J.; Winnacker, A.; Spreitzer, H.; Weisso¨rtel,
F.; Salbeck, J. AdV. Mater. 2000, 12, 130. (c) Xie, Z. Y.; Feng, J.;
Huang, J. S.; Liu, S. Y.; Wang, Y.; Shen, J. C. Synth. Met. 2000, 108,
81. (d) Zhang, Z.; Jiang, X.; Xu, S. Thin Solid Films 2000, 363, 61.
(e) Liu, S.; Hunag, J.; Xie, Z.; Wang, Y.; Chen, B. Thin Solid Films
2000, 363, 294. (f) Chuen, C. H.; Tao, Y. T. Appl. Phys. Lett. 2002,
81, 4499. (g) Li, G.; Shinar, J. Appl. Phys. Lett. 2003, 83, 5359. (h)
Cheng, G.; Zhao, Y.; Zhang, Y.; Liu, S.; He, F.; Zhang, H.; Ma, Y.
Appl. Phys. Lett. 2004, 84, 4457. (i) Liu, T.-H.; Wu, Y.-S.; Lee, M.-
T.; Chen, H.-H.; Liao, C.-H.; Chen, C. H. Appl. Phys. Lett. 2004, 85,
4304. (j) Cheng, G.; Xie, Z.; Zhao, Y.; Zhang, Y.; Xia, H.; Ma, Y.;
Liu, S. Thin Solid Films 2005, 484, 54. (k) Xie, W.; Meng, M.; Li,
C.; Zhao, Y.; Liu, S. Opt. Quant. Electron. 2005, 37, 943. (l) Li, M.;
Li, W.; Niu, J.; Chu, B.; Li, B.; Sun, X.; Zhang, Z.; Hu, Z. Solid-
State Electron. 2005, 49, 1956. (m) Tao, S.; Peng, Z.; Zhang, X.; Wu,
S. J. Lumin. 2006, 121, 568. (n) Zhang, G. H.; Hua, Y. L.; Petty,
M. C.; Wu, K. W.; Zhu, F. J.; Niu, X.; Hui, J. L.; Liu, S.; Wu, X. M.;
Yin, S. G.; Deng, J. C. Displays 2006, 27, 187. (o) Choukri, H.;
Fischer, A.; Forget, S.; Che´nais, S.; Castex, M.-C.; Ade´s, D.; Slove,
A.; Geffroy, B. Appl. Phys. Lett. 2006, 89, 183513. (p) Hsiao, C.-H.;
Lin, C.-F.; Lee, J.-H. J. Appl. Phys. 2007, 102, 094508. (q) Huang,
H.-H.; Chu, S.-Y.; Kao, P.-C.; Chen, Y.-C. Thin Solid Films 2008,
516, 5669. (r) Tang, S.; Liu, M.; Lu, P.; Cheng, G.; Zeng, M.; Xie,
Z.; Xu, H.; Wang, H.; Yang, B.; Ma, Y.; Yan, D. Org. Electron. 2008,
9, 241. (s) Duan, Y.; Mazzeo, M.; Maiorano, V.; Mariano, F.; Qin,
D.; Cingolani, R.; Gigli, G. Appl. Phys. Lett. 2008, 92, 113304.
The perylene-doped device had a chromaticity of CIEx,y
)
0.14, 0.17, less deep blue than CIEx,y ) 0.15, 0.09 of the
undoped device (second entry of Table 4). Nonetheless, the
performance of the 0.5 wt% perylene-doped AlmND3 OLED
reported herein is one of the best perylene dopant devices
known in literature.25 Similar enhancement of the OLED
performance was also observed for DPA-doped devices,
although the enhancement was to a smaller extent when
compared with that of perylene-doped OLEDs. However, the
9
772 J. AM. CHEM. SOC. VOL. 131, NO. 2, 2009

Group III Metal Chelates for OLEDs
A R T I C L E S
Figure 11. EL characteristics of ITO/NPB(40 nm)/CBP(10 nm)/AlmND3:perylene(x %, 30 nm)/AlmND3(20 nm)/LiF(0.5 nm)/Al(150 nm), where x is 0.5,
2, or 4, the weight percent of perylene dopant.
Figure 12. EL characteristics of ITO/NPB(40 nm)/CBP(10 nm)/AlmND3:DPA(x %, 30 nm)/AlmND3(20 nm)/LiF(0.5 nm)/Al(150 nm), where x is 0.5, 2,
or 4, the weight percent of DPA dopant
2.10. AlmND3 as the Host Material for High Efficiency
White OLEDs with Rubrene Yellow Dopant. Yellow fluoro-
phore rubrene is probably the most commonly used dopant
accompanying blue fluorescent host material in the fabrication
the AlmND3 host and DPA dopant is somewhat incomplete,
even though the HOMO energy level of the AlmND3 host is
low enough for the DPA dopant (but the LUMO energy level
of AlmND3 is not high enough for the DPA dopant). Despite
such a coemission, to the best of our knowledge, this is the
first observation of a deep blue EL from DPA.
of white OLEDs.26 Fluorescence spectrum of AlmND3 is
,27
partially overlapping with the absorption spectrum of rubrene
9
J. AM. CHEM. SOC. VOL. 131, NO. 2, 2009 773

A R T I C L E S
Liao et al.
Table 6. Electroluminescence Characteristics of AlmND3 OLEDs
with Various Dopant (Perylene or 9,10-Diphenylanthracene)
Concentrationsa
OLEDs, and it facilitates the charge balance and hence the EL
efficiency of OLEDs.
luminance,
efficiency,
voltage
3. Conclusion
dopant max. luminance
concn
[wt %]
max.
efficiency
[%, cd/A, lm/W]
CIE 1931
chromaticity
[x, y]
el
λmax
[nm]
and voltage
[cd/m2, V]
We have reported the first series of group III metal chelates
as the authentic deep blue analogues of green Alq3. A large
quantity of these deep blue metal chelates are readily available
now due to our improved synthesis and facial purification
method applicable for them. Unlike several currently known
blue analogues of Alq3, these deep blue group III metal chelates
are volatile and thermally stable enough for OLED fabrication
by a vacuum-thermal-deposition process. For OLEDs, we have
overcome the problem of exciplex formation, which has
hampered AlND3 from practical usage in OLEDs before. High
efficiency (maximum ηext > 4.0%) and deep blue (CIEx,y ) 0.15,
0.07) nondoped OLEDs were achieved for AlmmND3. The wide
band gap of these deep blue metal chelates, 6.4 and 3.0 eV for
AlmND3 HOMO and LUMO energy levels, respectively, enable
their usage as the host material for perylene blue dopant or deep
blue 9,10-diphenylanthracene. These group III metal chelates,
particularly AlmND3 and AlmmND3, have been characterized
for the charge carrier mobility by a time-of-flight technique.
Both AlmND3 and AlmmND3 exhibit high electron mobility,
comparable with or even higher than that of BeBq2 or Alq3.
Moreover, AlmND3 is ambipolar with a similar mobility of 10-4
cm2/Vs for both hole and electron. The success of highly
efficient white OLEDs (or yellow OLEDs) based on a rubrene
dopant is attributed to the high electron (and hole) mobility of
the host material, AlmND3. We have demonstrated the versatile
and effective application of hydroxynaphthyridine-based group
III metal chelates for OLEDs. More high performance OLEDs
can be anticipated now due to the availability of long-thought,
wide band gap, deep blue group III metal chelates.
[cd/m2, %, V]b
Perylene as Dopant
0.5
2
4
12420, 15
9610, 15
8060, 15
810, 3.13, 8.3 3.56, 4.67, 3.06 456
590, 1.98, 8.7 3.77, 3.51, 3.28 456
580, 1.38, 8.9 2.73, 5.45, 3.69 458
0.14, 0.17
0.15, 0.20
0.18, 0.31
9,10-Diphenylanthracene (DPA) as Dopant
0.5
2
4
5120, 15
4595, 15
3295, 15
278, 2.78, 5.8 2.80, 1.39, 0.89 448
259, 2.31, 5.4 2.47, 1.38, 0.87 446
203, 1.75, 5.6 1.90, 1.91, 0.69 446
0.15, 0.06
0.15, 0.07
0.18, 0.07
a ITO/NPB(40 nm)/CBP(10 nm)/AlmND3:Dopant (x %, 30 nm)/
AlmND3(20 nm)/LiF(5 nm)/Al(150 nm), where x ) 0.5, 2, and 4
(weight percent) for perylene or 9,10-diphenylanthracene (DPA) dopant.
b At 20 mA/cm2.
(Figure S3 left), and white fluorescence is feasible with an
appropriate ratio of mixed AlmND3 and rubrene (Figure S3
right).
We have demonstrated that the deep blue AlmND3 is a good
host material for rubrene in generating high EL efficiency white
OLEDs. A series of OLEDs ITO/NPB(40 nm)/CBP(10 nm)/
AlmND3:rubrene(x %, 30 nm)/AlmND3(20 nm)/LiF(0.5 nm)/
Al(150 nm) with a variation of rubrene dopant concentration
0.5-4 wt% and EL characteristics of such AlmND3:rubrene
OLEDs are shown in Figure 13 and their data are summarized
in Table 7.
Similar to most rubrene-based white OLEDs, an authentic
white color purity (CIEx, y ) 0.32, 0.38) was achieved at a very
low dopant concentration, 0.5 wt % of AlmND3 (first entry of
Table 7). Any higher dopant concentration simply impairs the
white color purity of OLEDs. For such white OLEDs at a
brightness of 100 cd/m2, ηext is 4.25%, which is equivalent to
an ηP of 8.67 lm/W, one of the highest among literature-known
rubrene-based white OLEDs.26,27 The color rendering index
(CRI) of such white (two-color-component) OLEDs was
determined to be in the range 45-50, and it is in general inferior
to CRI ∼80 of three-color-component white OLEDs.28 At
practical lighting conditions, i.e., 1000 cd/m2, such an AlmND3:
rubrene white OLED has an ηext of ∼3.9% or ηP of 5.1 lm/W
(Figure 13). With few exceptions,27 this EL efficiency at 1000
cd/m2 also outperforms most literature-known rubrene-based
two-element white OLEDs. Also, a doped device with a 4%
4. Experimental Section
General Information. Both solution and solid-state fluorescence
quantum yields (Φf’s) of the blue metal chelates were determined
by the integrating-sphere method.30 Photoluminescence (PL) spectra
were recorded on a Hitachi fluorescence spectrophotometer F-4500,
and the same spectrophotometer was used to record the EL spectra
of OLEDs. Melting points (Tms), glass transition temperatures (Tg’s),
and crystallization temperatures (Tc’s) of respective compounds
were measured via differential scanning calorimetry (DSC) using
a Perkin-Elmer DSC-6 differential scanning calorimeter. The
HOMO energy levels of the thin-film samples of metal chelates
were studied by ultraviolet photoemission spectroscopy (UPS). The
experimental detail of UPS measurement has been described
before.31 LUMO energy levels were estimated by subtracting the
energy gap (∆E) from HOMO energy levels. ∆E was determined
by the on-set absorption energy from the absorption spectra of the
materials. UV-visible electronic absorption spectra were recorded
on a Hewlett-Packard 8453 Diode Array spectrophotometer. The
method of time-of-flight (TOF) in measuring charge carrier mobility
has been reported before.32 Data collection of the X-ray crystal-
lography analysis was carried out on a Brucker X8APEX CCD
rubrene concentration is virtually a yellow OLED, CIEx,
)
y
0.45, 0.51 (Figure 13 and the fourth entry of Table 7). Its high
efficiency, 4.56%, 15.83 cd/A, or 11.94 lm/W, at 100 cd/m2
and high brightness (1930 cd/m2 at 20 mA/cm2) outperforms
most currently known yellow OLEDs.29 The high EL efficiency
of such rubrene-based white or yellow OLEDs can be attributed
to the high electron mobility of AlmND3. High electron mobility
is rarely observed for the host material in the white or yellow
(27) (a) Huang, J.; Li, G.; Wu, E.; Xu, Q.; Yang, Y. AdV. Mater. 2006, 18,
114. (b) Huang, J.; Hou, W.-J.; Li, J.-H.; Li, G.; Yang, Y. Appl. Phys.
Lett. 2006, 89, 133509. (c) Tsai, Y.-C.; Jou, J.-H. Appl. Phys. Lett.
2006, 89, 243521.
(30) (a) de Mello, J. C.; Wittmann, H. F.; Friend, R. H. AdV. Mater. 1997,
9, 230. (b) Chiang, C.-L.; Wu, M.-F.; Dai, D.-C.; Wen, Y.-S.; Wang,
J.-K.; Chen, C.-T. AdV. Funct. Mater 2005, 15, 231.
(28) (a) D’Andrade, B. W.; Forrest, S. R. AdV. Mater. 2004, 16, 1585.
(b) Misra, A.; Kumar, P.; Kamalasanan, Chandra, S. Semicomd. Sci.
Technol. 2006, 21, R35. (c) Yeh, S.-J.; Chen, H.-Y.; Wu, M.-F.; Chan,
L.-H.; Chiang, C.-L.; Yeh, H.-C.; Chen, Lee, J.-H. Org. Electron.
2006, 7, 137.
(31) Wu, C.-I.; Lee, G.-R.; Lin, C.-T.; Chen, Y.-H.; Hong, Y.-H.; Liu, W.-
G.; Wu, C.-C.; Wong, K.-T.; Chao, T.-C. Appl. Phys. Lett. 2005, 87,
242107.
(32) (a) Wu, M.-F.; Yeh, S.-J.; Chen, C.-T.; Murayama, H.; Tsuboi, T.;
Li, W.-S.; Chao, I.; Liu, S.-W.; Wang, J.-K. AdV. Funct. Mater. 2007,
17, 1887. (b) Liu, S.-W.; Lee, J.-H.; Lee, C.-C.; Chen, C.-T.; Wang,
J.-K. Appl. Phys. Lett. 2007, 91, 142106.
(29) See: Chiang, C.-L.; Tseng, S.-M.; Chen, C.-T.; Hsu, C.-P.; Shu, C.-F.
AdV. Funct. Mater 2008, 18, 248, and references therein.
9
774 J. AM. CHEM. SOC. VOL. 131, NO. 2, 2009

Group III Metal Chelates for OLEDs
A R T I C L E S
Figure 13. EL characteristics of ITO/NPB(40 nm)/CBP(10 nm)/AlmND3:rubrene (x %, 30 nm)/AlmND3 (20 nm)/LiF(0.5 nm)/Al(150 nm), where x is 0.5,
1, 2, or 4, the weight percent of rubrene dopant. EL efficiency (ηext or ηP) at 100 or 1000 cd/m2 electroluminance is arrow-marked for the device with 0.5
wt% rubrene dopant.
Table 7. Electroluminescence Characteristics of White OLEDs with AlmND3 Host and Various Dopant (Rubrene) Concentrationa
rubrene
concn
[wt %]
max. luminanceand
voltage
luminance,
efficiency, voltage
[cd/m2, %, V]b
efficiency at 100
and 1000 cd/m2
[%, cd/A, lm/W]
CIE 1931
chromaticity
[x, y]
el
λmax
[nm]
[cd/m2, V]
0.5
1
26 710, 15
24 810, 15
36 290, 15
30 970, 15
1830, 3.67, 6.58
1670, 3.73, 6.75
2260, 3.47, 7.02
1930, 2.79, 7.05
4.25, 11.55, 8.67
3.91, 9.65, 5.13
4.97, 11.17, 8.20
3.97, 8.93, 4.58
5.35, 17.46, 13.03
3.98, 12.97, 6.78
4.56, 15.83, 11.94
3.24, 11.25, 5.77
444, 550
0.30, 0.35
0.36, 0.42
0.41, 0.48
0.45, 0.51
444, 554
444, 558
444, 560
2
4
a Devices have the configuration of ITO/NPB(40 nm)/CBP(10 nm)/AlmND3:rubrene x %(30 nm)/AlmND(20 nm)/LiF(0.5 nm)/Al(150 nm), x ) 0.5,
1, 2, 4, respectively. b At current density of 20 mA/cm.
diffractometer at 100 K for AlND3 · CH2Cl2 single crystals. The
experimental detail of X-ray diffraction and their data process in
solving the crystal structure can be found elsewhere.30b
The fabrication of OLEDs and their EL characterization also have
been described before.19b,33 The device was placed close to the
photodiode such that all the forward light entered the photodiode.
The effective size of the emitting diodes was 3.14 mm2, which is
significantly smaller than the active area of the photodiode dectector,
a condition known as “under-fillling”, satisfying the measurement
protocol.34 This is one of the most conventional ways in measuring
the EL efficiency of OLEDs, although sometimes experimental
errors may arise due to the non-Lambertian emission of OLEDs.35
The color rendering index (CRI) of white OLEDs was measured
by a spectroradiometer (Specbos 1201, JETI Technishe Instrumente
GmbH).
1H and 13C NMR specra were recorded on a Bruker AMX-400
MHz or AVA-400 MHz Fourier-transform spectrometer at room
temperature. Elemental analyses (on a Perkin-Elmer 2400 CHN
Elemental Analyzer) and electron impact (EI), fast atom bombard-
ment (FAB), or matrix-assisted laser desorption/ionization time-
of-flight (MALDI-TOF) mass spectra (on a VA Analytical 11-250J
or 4800 MALDI TOF/TOF Analyzer) were recorded by the
Elemental Analyses and Mass Spectroscopic Laboratory in-house
service of the Institute of Chemistry, Academic Sinica.
Materials. For the materials used in device fabrication, Bebq2,
DPA (9,10-diphenylanthracene), perylene, and rubrene are com-
mercially available materials and they were used without further
purification. NPB (1,4-bis(1-naphthylphenylamino)biphenyl), CBP
(4,4′-bis(9-carbazolyl)-2,2′-biphenyl), mCP (1,3-bis(9-carbazolyl)-
benzene), TPBI (2,2′,2′′-(1,3,5-phenylene)tris[1-phenyl-1H-benz-
imidazole]), and Alq3 were prepared via published methods and
were subjected to gradient sublimation prior to use.
(33) Yeh, S.-J.; Wu, M.-F.; Chen, C.-T.; Song, Y.-H.; Chi, Y.; Ho, M.-H.;
Hsu, S.-F.; Chen, C. H. AdV. Mater. 2005, 17, 285.
(34) Forrest, S. R.; Bradley, D. D. C.; Thompson, M. E. AdV. Mater. 2003,
15, 1043.
Synthesis of Tris(4-hydroxy-1,5-naphthyridinato)aluminum
(AlND3). To a toluene solution (4.5 mL) was added ND (0.15 g,
1.0 mmol) and aluminum triisopropoxide (0.07 g, 0.34 mmol). The
(35) Tanaka, I.; Tokito, S. Jpn. J. Appl. Phys 2004, 43, 7733.
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J. AM. CHEM. SOC. VOL. 131, NO. 2, 2009 775

A R T I C L E S
Liao et al.
reaction mixture was heated at reflux under nitrogen for 1 h. During
the reaction, a formation of milky white solid emitting blue
fluorescence was observed. After cooling, the reaction solution was
filtered to isolate a white solid. After washing with n-hexanes, the
solid was subjected to zone-temperature sublimation. The pure
product was isolated as a white solid. Yield: 76% (0.12 g). 1H NMR
(400 MHz, CD2Cl2): δ 8.77 (d, 1H, J ) 4.75 Hz), 8.73-8.67 (m,
4H), 8.56 (d, 1H, J ) 8.54 Hz), 8.51-8.47 (m, 2H), 7.73 (dd, 1H,
J ) 8.56 Hz, J ) 4.72 Hz), 7.56 (dd, 1H, J ) 8.60 Hz, J ) 4.75
Hz), 7.49 (dd, 1H, J ) 8.50 Hz, J ) 4.74 Hz), 7.29 (d, 1H, J )
4.59 Hz), 6.93 (d, 1H, J ) 5.33 Hz), 6.90 (d, 1H, J ) 5.34 Hz),
6.81 (d, 1H, J ) 5.32 Hz). 13C NMR (100 MHz, CDCl3): δ 165.17,
164.98, 164.65, 156.28, 155.73, 155.70, 144.80, 144.43, 144.17,
144.07, 142.39, 141.87, 141.45, 141.37, 138.84, 138.18, 138.01,
125.13, 124.55, 109.80, 109.62, 109.08. FAB-MS: calcd 462.10,
m/z ) 463.1 (M+H+). Anal. Found (Calcd) for C24H15AlN6O3: C,
62.31 (62.34); H, 2.93 (3.27); N, 17.88 (18.17).
fluorescence was observed. After cooling, the reaction solution was
filtered to isolate the white solid. After washing with water, the
solid was vacuum-dried and finally subjected to zone-temperature
sublimation. The pure product was isolated as a white solid. Yield:
1
67% (0.11 g). H NMR (400 MHz, CDCl3): δ 8.70 (d, 3H, J )
5.36 Hz), 8.62 (d, 1H, J ) 4.83 Hz), 8.57 (d, 1H, J ) 4.90 Hz),
7.54 (d, 1H, J ) 4.80 Hz), 7.46 (d, 1H, J ) 4.71 Hz), 7.31 (d, 1H,
J ) 4.78 Hz), 7.27 (d, 1H, J ) 4.89), 6.96 (d, 1H, J ) 5.37 Hz),
6.93 (d, 1H, J ) 5.42 Hz), 6.92 (d, 1H, J ) 5.36 Hz), 2.84 (s, 3H),
2.80 (s, 3H), 2.78 (s, 3H). 13C NMR (100 MHz, CDCl3): δ 165.18,
164.98, 164.74, 154.43, 153.96, 153.51, 153.04, 153.00, 143.89,
143.81, 143.41, 143.15, 141.22, 136.08, 135.45, 135.34, 125.23,
125.14, 124.71, 109.61, 109.51, 108.97, 17.95, 17.85. FAB-MS:
calcd 546.09, m/z ) 547.0 (M+H+). Anal. Found (Calcd) for
C27H21GaN6O3: C, 59.03 (59.26); H, 4.00 (3.87); N, 15.09 (15.36).
Synthesis of Tris(4-hydroxy-2,8-dimethyl-1,5-naphthyridi-
nato)gallium (GammND3). This compound was synthesized in the
same manner as GamND3, except that mmND (0.60 g, 3.4 mmol)
was used instead of mND. The pure product was isolated as a white
Synthesis of Tris(4-hydroxy-8-methyl-1,5-naphthyridina-
to)aluminum (AlmND3). This compound was synthesized in the
same manner as AlND3, except that mND (1.00 g, 6.2 mmol) was
used instead of ND. The pure product was isolated as a white solid.
1
solid. Yield: 83% (0.56 g). H NMR (400 MHz, CDCl3): δ 8.51
(d, 1H, J ) 4.83 Hz), 8.48 (d, 1H, J ) 4.86 Hz), 7.45 (d, 1H, J )
4.78 Hz), 7.38 (d, 1H, J ) 4.90 Hz), 7.23 (d, 1H, J ) 4.89 Hz),
7.15 (d, 1H, J ) 4.90 Hz), 6.83-6.82 (m, 3H), 2.80 (s, 3H), 2.77
(s, 3H), 2.75 (s, 3H), 2.61 (s, 3H), 2.59 (s, 6H). 13C NMR (100
MHz, CDCl3): δ 164.63, 164.56, 164.36, 163.94, 163.40, 163.33,
152.32, 151.88, 143.31, 143.25, 142.20, 142.00, 140.07, 135.02,
134.38, 134.26, 125.10, 124.96, 124.62, 109.57, 109.32, 108.88,
26.31, 17.80, 17.71. FAB-MS: calcd 588.14, m/z ) 589.0 (M+H+).
Anal. Found (Calcd) for C30H27GaN6O3: C, 61.50 (61.14); H, 4.63
(4.62); N, 14.17 (14.26).
1
Yield: 86% (0.90 g). H NMR (400 MHz, CDCl3): δ 8.73-8.70
(m, 3H), 8.57 (d, 1H, J ) 4.90 Hz), 8.52 (d, 1H, J ) 4.91 Hz),
7.51 (d, 1H, J ) 4.81 Hz), 7.42 (d, 1H, J ) 4.90 Hz), 7.27 (d, 1H,
J ) 4.76 Hz), 7.11 (d, 1H, J ) 4.94), 6.94 (d, 1H, J ) 5.34 Hz),
6.91 (d, 2H, J ) 5.34 Hz), 2.84 (s, 3H), 2.79 (s, 3H), 2.77 (s, 3H).
13C NMR (100 MHz, CDCl3): δ 165.58, 165.44, 165.12, 154.74,
154.22, 154.20, 153.22, 152.70, 152.56, 143.95, 143.76, 143.70,
143.65, 143.62, 141.75, 137.99, 137.31, 137.18, 125.25, 125.23,
124.69, 109.68, 109.40, 108.91, 17.76, 17.67. FAB-MS: calcd
504.15, m/z
)
505.1 (M+H+). Anal. Found (Calcd) for
Synthesis of Tris(4-hydroxy-8-dimethyl-2-phenyl-1,5-naph-
thyridinato)gallium (GampND3). This compound was synthesized
in the same manner as GamND3, except that mpND (0.20 g, 8.5
mmol) was used instead of mND. The pure product was isolated
as a white solid. Yield: 69% (0.15 g). 1H NMR (400 MHz, CDCl3):
δ 8.62 (d, 1H, J ) 4.88 Hz), 8.59 (d, 1H, J ) 4.88 Hz), 8.15-8.12
(m, 6H), 7.53-7.40 (m, 14H), 7.33 (d, 1H, J ) 5.04 Hz), 7.30 (d,
1H, J ) 4.96 Hz), 2.93 (s, 3H), 2.92 (s, 3H), 2.86 (s, 3H). 13C
NMR (100 MHz, CDCl3): δ 165.42, 165.31, 165.13, 161.13, 160.78,
160.70, 153.50, 153.15, 153.11, 143.63, 143.59, 142.77, 142.62,
140.66, 140.39, 140.19, 139.91, 135.45, 134.82, 134.70, 129.64,
129.46, 129.32, 128.69, 128.67, 128.60, 127.69, 128.61, 125.38,
125.24, 124.87, 106.99, 106.61, 106.24, 17.73, 17.65. FAB-MS:
calcd 774.19, m/z ) 775.2 (M+H+). Anal. Found (Calcd) for
C45H33GaN6O3: C, 69.80 (69.69); H, 4.27 (4.29); N, 10.72 (10.84).
Synthesis of Tris(4-hydroxy-8-methyl-1,5-naphthyridinato)-
indium (InmND3). To a water solution (7.2 mL) were added mND
(0.15 g, 0.94 mmol) and indium chloride (0.07 g, 0.32 mmol). The
reaction mixture was stirred and heated at reflux temperature. An
excess amount of potassium acetate was added to the solution to
change the solution acidity from pH 3-4 to pH 7-8. During the
reaction (ca. 1 h), the formation of a milky white solid was observed.
However, the blue fluorescence from such a precipitate was barely
discernible. After the removal of water by vacuum distillation, the
resulting solid was subjected to Soxhlet extration by dichlo-
romethane for 24 h. The extracted dichloromethane solution was
evaporated until dryness, and the solid residue was further purified
by zone-temperature sublimation. The pure product was isolated
C27H21AlN6O3: C, 64.20 (64.28); H, 4.33 (4.20); N, 16.36 (16.66).
Synthesis of Tris(4-hydroxy-2,8-dimethyl-1,5-naphthyridina-
to)aluminum (AlmmND3). This compound was synthesized in the
same manner as AlND3, except that mmND (0.50 g, 2.9 mmol)
was used instead of ND. The pure product was isolated as a white
1
solid. Yield: 86% (0.45 g). H NMR (400 MHz, CDCl3): δ 8.46
(d, 1H, J ) 4.88 Hz), 8.43 (d, 1H, J ) 4.88 Hz), 7.42 (d, 1H, J )
4.88 Hz), 7.34 (d, 1H, J ) 4.88 Hz), 7.18 (d, 1H, J ) 4.92 Hz),
7.00 (d, 1H, J ) 4.96 Hz), 6.81 (s, 2H), 6.80 (s, 1H), 2.80 (s, 3H),
2.76 (s, 3H), 2.74 (s, 3H), 2.61 (s, 3H), 2.60 (s, 6H). 13C NMR
(100 MHz, CDCl3): δ 165.03, 164.76, 164.24, 163.59, 152.01,
151.57, 151.40, 143.09, 143.05, 142.99, 142.74, 142.48, 140.61,
136.89, 136.23, 136.09, 125.08, 124.60, 109.67, 109.26, 108.89,
26.36, 26.29, 17.60, 17.53. FAB-MS: calcd 546.20, m/z ) 547.2
(M+H+). Anal. Found (Calcd) for C30H27AlN6O3: C, 65.54 (65.93);
H, 4.94 (4.98); N, 15.37 (15.38).
Synthesis of Tris(4-hydroxy-8-dimethyl-2-phenyl-1,5-naph-
thyridinato)aluminum (AlmpND3). This compound was synthe-
sized in the same manner as AlND3, except that mpND (0.20 g,
0.9 mmol) was used instead of ND. The pure product was isolated
1
as a white solid. Yield: 77% (0.16 g). H NMR (400 MHz, d6-
DMSO): δ 8.59 (d, 1H, J ) 4.90 Hz), 8.49 (d, 1H, J ) 4.92 Hz),
8.23-8.20 (m, 6H), 7.78 (d, 1H, J ) 5.00 Hz), 7.69 (d, 1H, J )
4.91 Hz), 7.58-7.42 (m, 14H), 2.85 (s, 3H), 2.83 (s, 3H), 2.80 (s,
3H). 13C NMR (100 MHz, CDCl3): δ 165.79, 165.75, 165.51,
161.48, 161.04, 153.20, 152.82, 152.69, 143.43, 143.32, 143.12,
141.23, 140.46, 140.25, 139.94, 137.32, 136.68, 136.52, 129.65,
129.46, 129.29, 128.71, 128.67, 128.60, 127.72, 127.65, 127.62,
125.39, 124.88, 17.55, 17.47. FAB-MS: calcd 732.24, m/z ) 733.3
(M+H+). Anal. Found (Calcd) for C45H33AlN6O3: C, 73.84 (73.76);
H, 4.37 (4.54); N, 11.37 (11.47).
Synthesis of Tris(4-hydroxy-8-methyl-1,5-naphthyridinato)-
gallium (GamND3). To a water solution (7.2 mL) were added mND
(0.14 g, 0.87 mmol) and gallium chloride (0.06 g, 0.34 mmol).
The reaction mixture was heated and stirred at ∼40 °C. An excess
amount of potassium acetate was added to the solution to change
the solution acidity from pH 3-4 to pH 7-8. During the reaction
(ca. 1 h), the formation of a milky white solid emitting blue
1
as an off-white solid. Yield: 76% (0.14 g). H NMR (400 MHz,
CDCl3): δ 8.67 (d, 3H, J ) 5.40 Hz), 8.33 (d, 3H, J ) 4.80 Hz),
7.51 (dd, 3H, J ) 4.79 Hz, J ) 0.71 Hz), 6.95 (d, 3H, J ) 5.41
Hz), 2.83 (s, 9H). 13C NMR (100 MHz, CDCl3): δ 165.73, 153.71,
153.68, 144.30, 144.22, 136.46, 125.04, 110.75, 18.23. FAB-MS:
calcd 592.07, m/z ) 593.0 (M+H+). Anal. Found (Calcd) for
C27H21InN6O3: C, 54.79 (54.75); H, 3.46 (3.57); N, 14.07 (14.19).
Synthesis of Tris(4-hydroxy-2,8-dimethyl-1,5-naphthyridina-
to)indium (InmmND3). This compound was synthesized in the
same manner as InmND3, except that mmND (0.17 g, 0.98 mmol)
was used instead of mND. The pure product was isolated as an
9
776 J. AM. CHEM. SOC. VOL. 131, NO. 2, 2009

Group III Metal Chelates for OLEDs
A R T I C L E S
off-white solid. Yield: 81% (0.17 g). 1H NMR (400 MHz, CDCl3):
δ 8.22 (d, 3H, J ) 4.80 Hz), 7.43 (dd, 3H, J ) 4.82 Hz, J ) 0.68
Hz), 6.84 (s, 3H), 2.79 (s, 9H), 2.53 (s, 9H). 13C NMR (100 MHz,
CDCl3): δ 165.19, 163.02, 152.57, 143.74, 143.04, 135.36, 124.89,
110.57, 26.15, 18.04. FAB-MS: calcd 634.12, m/z ) 635.1
(M+H+). Anal. Found (Calcd) for C30H27InN6O3: C, 56.49 (56.80);
H, 4.24 (4.29); N, 12.96 (13.25).
821.15 (M+H+). Anal. Found (Calcd) for C45H33InN6O3: C, 65.78
(65.86); H, 3.95 (4.05); N, 10.26 (10.24).
Acknowledgment. This research was supported in part by the
National Science Council of Taiwan, Academia Sinica, National
Taiwan Normal University, National Chung Cheng University,
National Taiwan University, and National Chiao Tung University.
Supporting Information Available: The synthesis and struc-
tural characterization data of chelating ligands, ND, mND,
mmND, mpND, and their precursors are described. DSC
thermograms of AlmpND3, GampND3, and InmpND3 (Figure
S1); EL characteristics (Figure S2) and data (Table S1) of
nondoped GamND3, GammND3, InmND3, or InmmND3
OLEDs; absorption and photoluminescence spectra of AlmND3
and rubrene as well as their solution fluorescence images (Figure
S3) are given. The crystallographic information is listed in Table
S2 and file (.cif) also given for AlND3 ·CH2Cl2. This material
is available free of charges via the Internet at http://pubs.acs.org.
Synthesis of Tris(4-hydroxy-8-dimethyl-2-phenyl-1,5-naph-
thyridinato)indium (InmpND3). This compound was synthesized
in the same manner as InmND3, except that mpND (0.50 g, 2.1
mmol) was used instead of mND. However, the precipitate formed
during the reaction was observed for blue fluorescence, and it was
isolated by filtration. After washing with water, the solid was
vacuum-dried and finally subjected to zone-temperature sublimation.
The pure product was isolated as an off-white solid. Yield: 51%
1
(0.30 g). H NMR (400 MHz, CDCl3): δ 8.34 (d, 3H, J ) 4.80
Hz), 8.15-8.12 (m, 6H), 7.53-7.51 (m, 6H), 7.49-7.40 (m, 9H),
2.91 (s, 9H). 13C NMR (100 MHz, CDCl3): δ 165.99, 160.24,
153.78, 144.04, 143.62, 140.05, 135.77, 129.44, 128.63, 127.57,
125.15, 107.86, 17.99. MOLDI-TOF MS: calcd 820.17, m/z )
JA807284E
9
J. AM. CHEM. SOC. VOL. 131, NO. 2, 2009 777