Systematic study of the photoluminescent and electroluminescent properties of pentacoordinate carboxylate and chloro bis(8-hydroxyquinaldine) complexes of gallium(III)

Article abstract of DOI:10.1021/jp961770t

A detailed analysis of the structural, optical absorption, photoluminescent, and electroluminescent properties of a series of pentacoordinate bis(8-hydroxyquinaldine)gallium(III) complexes, i.e. q′₂GaX with X = acetate, dimethylpropionate, benzoate, and chloro, is presented. These materials are compared with tris(8-hydroxyquinaldine)Ga and the tris(8-hydroxyquinolate) complexes of aluminum (Alq₃) and gallium. Structural studies show that the pentacoordinate complexes have significantly less steric congestion than their hexacoordinate analogues, leading to comparatively smaller Franck-Condon shifts in their absorption and emission spectra. Relatively short π-π stacking distances are observed between the quinolate or quinaldine ligands of adjacent complexes (i.e., 3.3-3.5 A), which facilitates electron transport in the materials. All of the pentacoordinate complexes had high electroluminescent efficiencies at low drive currents. The benzoate derivative exhibits light output saturation at high drive currents when used in electroluminescent devices, while the other derivatives had characteristics comparable to that of Alq₃ at all drive currents.

Full text of DOI:10.1021/jp961770t

17766
J. Phys. Chem. 1996, 100, 17766-17771
Systematic Study of the Photoluminescent and Electroluminescent Properties of
Pentacoordinate Carboxylate and Chloro Bis(8-hydroxyquinaldine) Complexes of
Gallium(III)
Linda S. Sapochak,†,‡ Paul E. Burrows,§ Dimitri Garbuzov,§ Douglas M. Ho,†
Stephen R. Forrest,*,§ and Mark E. Thompson*,|
Department of Chemistry, Princeton UniVersity, Princeton, New Jersey 08544, AdVanced Technology Center
for Photonic and Optoelectronic Materials, Department of Electrical Engineering, Princeton UniVersity,
Princeton, New Jersey 08544, and Department of Chemistry, UniVersity of Southern California,
Los Angeles, California 90089-0744
ReceiVed: June 17, 1996X
A detailed analysis of the structural, optical absorption, photoluminescent, and electroluminescent properties
of a series of pentacoordinate bis(8-hydroxyquinaldine)gallium(III) complexes, i.e. q′2GaX with X ) acetate,
dimethylpropionate, benzoate, and chloro, is presented. These materials are compared with tris(8-
hydroxyquinaldine)Ga and the tris(8-hydroxyquinolate) complexes of aluminum (Alq3) and gallium. Structural
studies show that the pentacoordinate complexes have significantly less steric congestion than their
hexacoordinate analogues, leading to comparatively smaller Franck-Condon shifts in their absorption and
emission spectra. Relatively short π-π stacking distances are observed between the quinolate or quinaldine
ligands of adjacent complexes (i.e., 3.3-3.5 Å), which facilitates electron transport in the materials. All of
the pentacoordinate complexes had high electroluminescent efficiencies at low drive currents. The benzoate
derivative exhibits light output saturation at high drive currents when used in electroluminescent devices,
while the other derivatives had characteristics comparable to that of Alq3 at all drive currents.
Introduction
Alq3 is the most thoroughly studied molecular emitter material
for OLEDs. This hexacoordinate chelate complex exhibits a
very high PL quantum efficiency in the solid state (φ ) 0.32).6
The excited state for Alq3 is a localized Frenkel exciton,7
eliminating the intermolecular interactions that can lead to self-
quenching. Although a material must be fluorescent in the solid
state in order to exhibit electroluminescence, the efficiency of
the fluorescence is only one criterion for determining the
usefulness of a material as an emitter in an OLED. The criteria
for efficient EL include the material volatility, film forming
properties, charge transport properties, energy band offsets
relative to the carrier transporting electrodes and organic layers,
and environmental stability. These parameters are strongly
coupled to molecular structure and the bulk molecular packing
characteristics. An example of how these parameters can affect
electroluminescence is the comparison of Alq3 and Gaq3
OLEDs.8 Even though Alq3 has a 3-4 times larger PL
efficiency than Gaq3 in thin film form, their EL efficiencies
are comparable. Furthermore, the Gaq3 OLEDs exhibit a lower
turn-on voltage than the corresponding Alq3 devices, and thus,
a 50% higher power efficiency has been achieved.8 It is
important to gain a better understanding of how to optimize
these parameters in order to design superior emitter materials.
Considerable research is currently focused on the development
of new light-emitting device technologies for flat panel displays.
The primary motivation is to replace bulky and energy-
consuming cathode ray tubes (CRTs) with energy-efficient flat
panels. While liquid crystal displays (LCDs) are a reasonable
substitute for CRTs in some applications, it is advantageous
for ease of viewing in bright background environments to have
an emissive display, rather than the reflective or transmissive
panels of LCDs. One emissive technology that shows promise
involves organic light-emitting diodes (OLEDs). The interest
in organic molecular materials for use in (OLEDs) began with
the report of efficient green electroluminescence (EL) from
aluminum tris(8-hydroxyquinoline) (Alq3).1,2 Since those re-
ports, other metal chelate systems3,4 and polymers5 have been
found which efficiently produce EL throughout the visible
spectrum. Devices prepared with molecular and polymeric
materials can be fabricated on virtually any substrate by simple
vapor deposition or wet processes, making them economically
attractive as well. In this paper we present a systematic study
of the optical absorption, photoluminescent (PL), and electrolu-
minescent (EL) properties of a series of pentacoordinate bis-
(8-hydroxyquinaldine)gallium(III) complexes, which have po-
tential for use as efficient organic light emitters for flat panel
displays (FPDs). Results are compared with the hexacoordinate
tris(8-hydroxyquinoline) complexes of Al and Ga (Alq3 and
Gaq3) and tris(8-hydroxyquinaldine)gallium(III) (Gaq′3).
Experimental Section
Synthesis and Preparation of Materials. All reagents were
obtained from Aldrich Chemical Co. The Ga(NO3)3‚xH2O was
high purity grade (99.999%). 8-Hydroxyquinaldine was re-
crystallized from ethanol/water before use. The syntheses for
all of the compounds reported herein are shown schematically
in Figure 1. Alq3, Gaq3,9 and phenolatobis(8-hydroxyquinaldi-
ne)aluminum(III),10 as well as two of the pentacoordinate q′2-
GaX complexes (X ) OAc, Cl),11,12 were prepared using
published procedures. Spectroscopic and analytical data for
these materials matched those reported previously. All of the
† Department of Chemistry, Princeton University.
‡ Current address: Alpha Analytical, Inc., 2505 Chandler Ave., Suite 1,
Las Begas, NV 89120.
§ Department of Electrical Engineering, Princeton University.
| University of Southern California.
* To whom correspondence should be addressed. S.R.F.: e-mail,
forrest@ee.princeton.edu; voice, (609) 258-4532; FAX, (609) 258-1954.
M.E.T.: e-mail, thompson@chem1.usc.edu; voice, (609) 740-6402; FAX,
(609) 740-0930.
X Abstract published in AdVance ACS Abstracts, October 1, 1996.
S0022-3654(96)01770-4 CCC: $12.00 © 1996 American Chemical Society

Pentacoordinate Gallium(III) Complexes
J. Phys. Chem., Vol. 100, No. 45, 1996 17767
TABLE 1: Crystallographic Coordinates and Thermal
Parameters for All Nonhydrogen Atoms of Gaq′3
x
y
z
U(eq)a
Ga
N(1)
C(2)
C(3)
C(4)
C(4a)
C(5)
C(6)
C(7)
C(8)
O(8)
C(8a)
C(9)
N(1′)
C(2′)
C(3′)
C(4′)
C(4a′)
C(5′)
C(6′)
C(7′)
C(8′)
O(8′)
C(8a′)
C(9′)
N(1′′)
C(2′′)
C(3′′)
C(4′′)
C(4a′′)
C(5′′)
C(6′′)
C(7′′)
C(8′′)
O(8′′)
C(8a′′)
C(9′′)
3012(1)
2746(2)
2270(2)
2248(2)
2696(2)
3201(2)
3688(2)
4150(2)
4156(2)
3691(2)
3676(1)
3203(2)
1756(2)
3580(2)
3627(2)
4111(2)
4550(2)
4526(2)
4960(2)
4891(2)
4395(2)
3964(2)
3495(1)
4020(2)
3176(3)
2115(2)
1994(2)
1367(2)
866(2)
5481(1)
5960(2)
5774(3)
6196(3)
6807(3)
7024(3)
7655(3)
7828(3)
7386(3)
6765(2)
6353(2)
6576(3)
5115(3)
5013(2)
5239(2)
4863(3)
4284(3)
4041(3)
3449(3)
3264(3)
3651(3)
4238(2)
4636(2)
4427(2)
5916(3)
4561(2)
3755(3)
3387(3)
3842(3)
4696(3)
5209(3)
6020(3)
6365(3)
5880(3)
6176(2)
5019(3)
3219(3)
2183(1)
712(2)
-269(3)
-1066(3)
-878(3)
131(3)
34(1)
35(2)
43(2)
49(2)
49(3)
40(2)
50(3)
49(3)
41(2)
34(2)
40(1)
34(2)
60(3)
31(2)
36(2)
41(2)
44(2)
37(2)
44(2)
47(3)
41(2)
34(2)
36(1)
29(2)
53(3)
37(2)
45(2)
54(3)
56(3)
44(2)
60(3)
64(3)
53(3)
39(2)
40(2)
36(2)
70(4)
419(3)
1426(3)
2196(3)
1970(3)
2683(2)
914(3)
-510(3)
3717(2)
4576(3)
5520(3)
5599(3)
4713(3)
4705(3)
3805(3)
2883(3)
2853(3)
1998(2)
3786(3)
4521(3)
1688(2)
1446(3)
1137(3)
1067(3)
1344(3)
1347(4)
1614(4)
1892(3)
1905(3)
2164(2)
1641(3)
1471(5)
Figure 1. Synthetic scheme for the q′2GaX complexes investigated in
this study.
971(2)
497(2)
650(3)
1280(2)
1762(2)
2359(1)
1611(2)
2519(3)
added. The precipitate was filtered, washed with ethanol, air-
dried, and recrystallized from chlorobenzene to yield 1.982 g
1
(50%) of small green-yellow crystals, mp ) 340 °C (dec). H
NMR [250 MHz, CDCl3 data reported as assignment ) δ in
ppm (multiplicity) (coupling constant in Hz)]: H3 ) 7.39 (d)
(8.55); H4 ) 8.24 (d); H5 ) 7.16 (d) (8.24); H6 ) 7.46 (t)
(7.93); H7 ) 7.15 (d); CH3-q′ ) 3.04 (s); H8-ArCO2 ) 7.92
(d) (7.02); H9-ArCO2 ) 7.27 (t) (6.41); H10-ArCO2 ) 7.35-
7.50 (m). FT-IR (KBR): 1634, 1610,1509, 1467, 1453, 1433,
1389, 1340, 1305, 1274, 1112, 839, 755, 729, 693, 649, 528.
Anal. Calcd for C27H21GaN2O4: C, 63.94; H, 4.17; N, 5.52.
Found: C, 63.81; H, 4.11; N, 5.55.
X-ray Crystallographic Characterization of Gaq′3. The
X-ray diffraction analysis of Gaq′3 was performed on a Siemens
P4 four-circle diffractometer employing graphite-monochro-
mated Mo KR radiation (λ ) 0.710 73 Å). Unit cell constants
were determined by a least-squares fit of the 2θ values for 25
centered reflections having 25° < 2θ < 39°. Systematic
absences were consistent with the monoclinic space group C2/
c. A total of 4683 reflections were measured (ω scan mode,
4° < 2θ < 50°) and corrected for Lorentz-polarization and
absorption effects. Of these, 4394 were unique (Rint ) 1.32%)
and 3116 reflections having F > 3σ(F) were considered
observed.
The structure of Gaq′3 was solved by direct methods and
refined using the SHELXTL Plus package of programs. All of
the nonhydrogen atoms were refined with anisotropic displace-
ment coefficients; hydrogen atoms were included with a riding
model [C-H ) 0.96 Å, U(H) ) 1.2U(C)]. The methyl
hydrogen atoms were located in a difference-Fourier map and
idealized, and each CH3 group was refined as a rigid group.
The refinement converged to R ) 4.60%, wR ) 4.91%, and S
) 1.08, with 343 variables and 9.1 reflections per refined
parameter. The atom positions for all nonhydrogen atoms are
given in Table 1, and a thermal ellipsoid plot for Gaq′3 is shown
in Figure 2. The unit cell for Gaq′3 (C30H24GaN3O3) was refined
materials were purified by vacuum sublimation at 225 °C at
10-4 Torr, prior to OLED fabrication. Elemental analyses for
all of the materials were obtained from Robertson Microlit
Laboratories, Inc., Madison, NJ.
Bis(8-hydroxyquinaldine)gallium 2,2-dimethylpropionate (q′2-
GaDMP) was prepared by the following procedure. A solution
of 2.0 g (7.82 mmol) of Ga(NO3)3‚xH2O in 200 mL of water
was stirred rapidly with warming, while a solution of 2.452 g
(15.4 mmol) of 8-hydroxyquinaldine and 7.99 g (78.2 mmol)
of 2,2-dimethylpropionic acid in 600 mL of water was added
over 1 h. A precipitate formed immediately, and stirring was
continued for an additional 1 h after all of the reactants had
been added. The precipitate was filtered, washed with hot water,
air-dried, and recrystallized from methanol to yield 1.03 g (35%)
of large green-yellow crystals, mp ) 264-265 °C. 1H NMR
[250 MHz, CDCl3 data reported as assignment ) δ in ppm
(multiplicity) (coupling constant in Hz)]: H3 ) 7.42 (d) (8.55);
H4 ) 8.24 (d); H5 ) 7.14 (d) (8.24); H6 ) 7.43 (t) (7.93); H7
) 7.10 (d); CH3-q′ ) 3.03 (s); (CH3)3CCO2 ) 1.02 (s). FT-IR
(KBR): 1652, 1507, 1465, 1457, 1431, 1394, 1344, 1272, 1116,
833, 756, 528. Anal. Calcd for C25H25GaN2O4: C, 61.64; H,
5.17; N, 5.75. Found: C, 61.44; H, 5.13; N, 5.74.
Bis(8-hydroxyquinaldine)gallium benzoate (q′2GaBen) was
prepared by the following procedure. A solution of 1.0 g (3.9
mmol) of Ga(NO3)3‚xH2O in 50 mL of water was stirred rapidly
while a solution of 1.226 g (7.7 mmol) of 8-hydroxyquinaldine
and 6.185 g (38.6 mmol) of potassium benzoate in 280 mL of
ethanol/water (50:50) was added over 30 min. Precipitation of
the product occurred immediately, and the reaction mixture was
stirred an additional 30 min after all the reactants had been

17768 J. Phys. Chem., Vol. 100, No. 45, 1996
Sapochak et al.
recorded with an EG&G optical multichannel analyzer on a 0.25
focal length spectrograph.
Results and Discussion
Synthesis and Characterization. Substitution of a methyl
group into the 2-position of 8-hydroxyquinoline ligand (i.e.,
2-CH3-8-OH-C9H5N ) 8-hydroxyquinaldine, abbreviated q′)
sufficiently increases the steric bulk of the bidentate ligands
such that it is not possible to make a stable tris complex with
aluminum. However, trigonal bipyramidal aluminum complexes
can be prepared with q′ ligands, in which four sites are taken
up by two bidentate q′ ligands and the fifth site is filled by a
monodentate ligand, such as phenoxide.10 Due to its larger ionic
radius, it is possible to make a tris complex of q′ with gallium,
i.e. Gaq′3, as well as analogues of the trigonal bipyramidal Al
complexes, i.e. q′2GaX, where X ) carboxylate or halide. Gaq′3
is significantly less stable than either the Mq3 or q′2MX complex.
While Gaq′3 is initially stable in air, it degrades over a period
of weeks, giving an orange/brown solid. This is adequate for
the photophysical measurements described below but makes
Gaq′3 a poor candidate for OLEDs where a long device lifetime
is essential to their practical application.
Figure 2. Thermal ellipsoid figure of Gaq′3.
to a ) 23.712(3) Å, b ) 16.057(2) Å, c ) 15.722(2) Å, â )
124.049(8)°, volume ) 4959.9(10) Å3, Z ) 8, and density
(calcd) ) 1.458 g/cm3.
All q′2GaX complexes prepared in this work are air-stable,
pale green-yellow powders. The solubilities of the complexes
vary, for X ) OOCCH3 (OAc) and OOC(CH3)3 (DMP) are
soluble in methanol (similar to Alq3, Gaq3, and Gaq′3). Re-
crystallization of q′2GaOAc from methanol results in large pale
yellow prism-shaped crystals, which contain one methanol per
Optical Characterization. Absorption spectra were recorded
with a Hewlett-Packard spectrophotometer. Photoluminescent
and excitation spectra were obtained with a Perkin-Elmer LS-
100 Fluorimeter for both chloroform solution (filtered and
degassed) and thin film samples on quartz (thin films were
prepared as described previously).7,8 The emission spectra were
recorded using an excitation wavelength of 365 nm, and the
excitation spectra were recorded by detecting emission at 500
nm. Alq3 was used as the reference for quantum yield (φPL)
calculations based on the reported φPL(Alq3) in chloroform
(0.04).13 The thin film φPL for q′2GaCl was measured as
described previously.6
1
molecule of complex based on H NMR, elemental analysis,
and crystal structure data. Recrystallization of q′2GaDMP also
results in large pale yellow prisms. The complexes with X )
OOCC6H5 (Ben) and Cl are less soluble, and recrystallization
from chlorobenzene was the most advantageous, resulting in
small pale yellow microcrystals.
The crystal structures of q′2GaOAc,11 q′2GaCl,12 Alq3‚
(MeOH),9 and Gaq3‚(MeOH)9 have been reported. We have
determined the crystal structures of Gaq′3, shown in Figure 1.
Structural data for q′2GaOAc‚(MeOH),14 q′2GaOAc, and q′2-
GaCl show that the Ga-N bond lengths [2.093(3), 2.086(3),
2.110(8) Å, respectively] and Ga-O bond lengths [1.874(3),
1.877(2), 1.882(8) Å] are very similar, as expected for the three
closely related complexes. The average Ga-N bond length for
Gaq′3 [2.214(4) Å] is significantly longer that of Gaq3 [2.085
(4) Å] or for the q′2GaX complexes. The increase in Ga-N
bond length observed for Gaq′3 is due to steric hindrance of
the 2-Me substitution of the quinolate ring. The Ga-N bond
lengths of the q′2GaX complexes are close to the values seen
in Gaq3, suggesting that the absence of the third quinaldine
ligand in the complexes lessens the effects of steric congestion
due to the 2-Me group in the molecules. The average Ga-O
bond length is shortened in the Gaq′3 complex [1.904 (3) Å]
compared to the Ga-O bond length of Gaq3 [1.955(3) Å]. The
average Ga-O bond lengths observed in q′2GaX complexes
(1.875 Å) are shorter than the Ga-O bonds in either Gaq3 and
Gaq′3. The greater steric congestion of Gaq′3 may be respon-
sible for the lowered stability of the complex both in solution
and the solid-state relative to Gaq3 and the q′2GaX complexes.
An examination of the solid-state packing of Alq3, Gaq3, and
Gaq′3 shows that there are close π-π stacking interactions
between the quinolate ligands of adjacent molecules, with ligand
stacking distances ranging from 3.29(2) to 3.52(2) Å. Good
overlap between adjacent molecules facilitates carrier transport
in these materials. The packing observed in crystals of q′2-
GaX shows that the quinaldine ligands of adjacent molecules
also have short π-π distances, e.g. 3.38-3.45 Å for q′2GaOAc,
Devices were grown on glass slides precoated with ITO (sheet
resistance of 15 Ω/square). Substrates were ultrasonically
cleaned in detergent solution for about 1 min, followed by
thorough rinsing in deionized water. They were then boiled in
1,1,1-trichloroethane, rinsed in acetone followed by methanol,
and dried in pure nitrogen gas between each step. Devices were
formed by sequential, high vacuum (<2 × 10-6 Torr) vapor
deposition of of a 350 Å thick layer of the preferentially hole-
transporting organic material, N,N′-diphenyl-N,N′-bis(3-meth-
ylphenyl)-1,1-biphenyl-4,4′-diamine (TPD), and a 400 Å layer
of the emitter complex.7,8 Deposition was carried out by thermal
evaporation from a baffled Ta crucible at a nominal deposition
rate of 2-4 Å/s. An array of circular 250 µm diameter electron-
injecting electrodes of approximately 10:1 Mg:Ag atomic ratio
was subsequently deposited by coevaporation from separate Ta
boats at a vacuum of 10-5 Torr. Without breaking the vacuum,
a 500 Å layer of Ag was deposited to inhibit atmospheric
oxidation of the electrode. All possible steps, such as substrate
cleaning and electrode deposition, were performed in parallel
to minimize sample-to-sample variations.
Electrical pressure contact to the devices was by means of a
25 µm diameter Au wire. Current-voltage characteristics were
measured with a Hewlett-Packard HP4145 semiconductor
parameter analyzer, and EL intensity was measured with a
Newport 835 power meter with a broad spectral bandwidth
(400-1100 nm) photodetector placed directly below the glass
substrate. Although this measurement underestimates the total
power since much is lost by wave guiding to the edges of the
glass substrate, it nevertheless accurately measures the relatiVe
efficiency between devices. Electroluminescence spectra were

Pentacoordinate Gallium(III) Complexes
J. Phys. Chem., Vol. 100, No. 45, 1996 17769
TABLE 2: Photoluminescence and Absorbance Data for
q′2GaX Complexes
CHART 1
fluorescence quantum
λ
max (nm)
efficiency
emitter
Alq
Gaq
abs
PLa
∆ (cm-1
)
CHCl3
thin film
388
392
362
366
366
366
366
520
540
520
495
500
500
500
6542
7161
8393
7120
7322
7322
7322
0.04
0.01
-
0.08
0.05
0.07
0.06
0.36
-
bonding parameters observed in these complexes and similar
degrees of structural distortion in their excited states.17 A similar
situation is observed for the aluminum analogues of the q′2-
GaX complexes. The pentacoordinate q′2AlOPh complex gives
absorption and emission spectra that are blue-shifted relative
to Alq3 (by 1401 and 1602 cm-1, respectively).10 The energy
differences between the absorption and emission maxima for
each compound are very similar (6341 cm-1 for q′2AlOPh and
6542 cm-1 for Alq3), again consistent with similar levels of
steric congestion in the q′2MX and Mq3 complexes. All q′2-
GaX complexes, as well as Alq3 and Gaq3 exhibit excitation
spectra that are very similar to their optical absorption profiles.
The PL quantum yields (φPL) for Alq3, Gaq3, and q′2GaX in
chloroform and for Alq3 and q′2GaCl thin films are listed in
Table 2. The q′2GaX complexes exhibited a 1.3-2 times larger
solution φPL than Alq3, with q′2GaCl exhibiting the largest value
of all the compounds studied. Halogens and carbonyl moieties
are known to cause fluorescence quenching;18-20 however, the
chloro and carboxylate moieties in q′2GaX do not interact with
the quinaldine ligands where the PL is centered, and thus seem
to have little effect. The thin film quantum efficiency of q′2-
GaCl is 0.3. This value is very high relative to most organic
solids and similar to that found for Alq3, showing that self-
quenching is not important in these q′2GaX complexes.
Molecular orbital calculations are very valuable for probing
the origins of electronic transitions in small molecules. We have
used semiemperical INDO methods (INDO ) intermediate
neglect of differential overlap). The program has been param-
etrized by Zerner from electronic spectra of metal complexes
(referred to as ZINDO)21 and has been used to very effectively
probe the molecular orbital structure of Alq3 and related
complexes.7 The visible bands in the absorption and emission
spectra of these metal complexes are due to transitions involving
the π system of the quinolate ligands. Configuration interaction
leads to extensive mixing of the three quinolate π systems in
the low-energy transitions, giving a substantial red-shift for
absorption and emission bands of the trisquinolate complexes,
relative to those of a metal complex with a single quinolate
ligand.
ZINDO is not parametrized for Ga, preventing us from
examining q′2GaX complexes using ZINDO; however, calcula-
tions on analogous Al complexes, i.e. q′2AlX (X ) Cl, OAc),
have been carried out and suggest an explanation for the
observed blue-shift in q′2MX (M ) Al, Ga) complexes relative
to Mq′3. In these calculations, the first step is to adjust the
atom positions to give a minimum energy. An electronic
structure calculation is then carried out, including configuration
interactions (CI, 10 filled and 10 empty states used in CI). All
of the q′2MX compounds that have been crystallographically
characterized have a structure similar to that shown in Chart
1.11,12,14 This structure was used as the starting point for our
geometry minimizations of q′2AlCl and q′2AlOAc. The geometry-
minimized structures are very similar to those determined
crystallographically, all giving small deviations from the ideal
trigonal bipyramidal structure (N-Al-N angles ) 165°-170°
and O-Al-O ) 118°-125°). For both the chloro and acetate
derivatives, CI leads to complete mixing of the π systems of
both quinaldine ligands in the low-energy transitions, which is
very similar to the situation seen for metal trisquinolates.7 For
example, the lowest energy transition for q′2AlCl is calculated
Gaq′3
-
q′2GaCl
q′2GaAc
q′2GaDMP
q′2GaBen
0.3
-
-
-
a Emission wavelength of thin films with excitation wavelength of
365 nm.
Figure 3. Photoluminescence spectra of thin films of q′2GaX
complexes.
suggesting that carrier transport may be reasonable in the q′2-
GaX materials as well.
Electronic Properties. Sizable blue-shifts are observed in
both the absorption and emission spectra of Gaq′3 relative to
Gaq3 (see Table 2 and Figure 3). The absorption maxima for
Gaq′3 and Gaq3 are 362 and 392 nm, respectively,15,16 corre-
sponding to a difference of 2114 cm-1. A smaller difference
is observed in the PL spectra of these two complexes. Gaq′3
emits at a shorter wavelength (520 nm) than Gaq3 (545 nm),15
giving an energy difference of 883 cm-1. The energy difference
between the absorption and emission maxima for Gaq′3 (8393
cm-1) is significantly larger than for Gaq3 (7161 cm-1). This
is due to the structural differences between the ground- and
excited-state geometries for Gaq′3, which are larger than for
Gaq3. This is not surprising considering the degree of steric
congestion observed in Gaq′3.
The substitution of one of the q′ ligands in Gaq′3 with a
carboxylate or chloro ligand leads to a small red-shift in their
absorption spectra relative to Gaq′3 and a significant blue-shift
in the PL spectra, Table 2 and Figure 3. The pentacoordinate
q′2GaX complexes (X ) carboxylate, Cl) have absorption
maxima at 366 nm, and emission maxima at 500 nm for q′2-
Ga(carboxylate) complexes and 495 nm for q′2GaCl (see Figure
3). The energy differences between absorption and emission
maxima for these bisquinaldine complexes are 7322 cm-1 for
the carboxylate derivatives and 7120 cm-1 for the chloro
derivative. The fact that the energy differences between the
absorption and emission maxima observed for q′2GaX and that
reported for Gaq3 are very close is consistent with the similar

17770 J. Phys. Chem., Vol. 100, No. 45, 1996
Sapochak et al.
Figure 4. Current versus voltage plots for Alq3, Gaq3, and q′2GaX
OLEDs.
TABLE 3: Electroluminescene Data for q′2GaX OLEDs
Alq3 Gaq3 q′GaAc q′GaDMP q′GaBen q′GaCl
λ
φEL
VT (V)
EL (nm) 532
535
0.58
500
0.65
16.7
500
1.04
16.2
500
0.73
21
500
0.73
13.8
a
1.0
16.9 15
a EL quantum efficiencies are normalized to Alq ) 1 for clarity.
to be at 342 nm (oscillator strength ) 0.22), which is composed
of equal amounts of the π-π* transitions of each quinaldine.
The likely reason for the blue-shift observed in absorption and
luminescence of q′2MX relative to Mq3 is that, while CI leads
to extensive mixing of ligand states for both complexes, it
involves mixing of two quinaldine ligands for q′2MX and the
mixing of three such ligands for Mq3. The transitions for q′2-
MX originate from a smaller and thus higher energy system
than those of Mq3.
Figure 5. Light output versus voltage plots for Alq3, Gaq3, and q′2-
GaX OLEDs.
are responsible for the saturation behavior, It is interesting to
note that they do not lead to significant fluorescence quenching.
Further purification procedures are currently underway, and
reanalysis of these complexes is warranted to better understand
their device properties. We are also investigating the crystal
structures of the q′2GaX complexes to see if there are any
obvious relationships between the intermolecular interactions
and device properties.
Electroluminescence. The light outputs from the OLEDs
of the q′2GaX complexes are blue-green, while those of Alq3
and Gaq3 are green to green-yellow. The EL spectra of q′2-
GaX are identical with their PL spectra, consistent with hole-
electron recombination in the q′2GaX layer in the multilayer
device. The current versus voltage characteristics of the OLEDs
are shown in Figure 4. The device operational voltages (at fixed
current)sor VTsincrease in the order q′2GaCl < Gaq3 < q′2-
GaDMP < Alq3 ≈ q′2GaOAc < q′2GaBen (Table 3). Figure 5
shows the dependence of the optical output power on drive
current for each of the OLEDs. At low drive current (<100
µA, 2 mA/cm2), the EL output at a given current decreases in
the order q′2GaBen > q′2GaDMP > Alq3 > q′2GaCl > q′2-
GaOAc > Gaq3 (Figure 5a). At higher drive currents (>100
µA), the order is significantly different, where the EL output at
a given current level decreases in the order Alq3 > q′2GaDMP
> q′2GaCl > Gaq3 > q′2GaOAc > q′2GaBen (Figure 5b). The
q′2GaBen and q′2GaDMP complexes are significantly brighter
than Alq3 and Gaq3 at low drive currents; however, while the
q′2GaDMP complex consistently exhibits an increase in EL
output with increasing drive current, the q′2GaBen complex
exhibits saturation of the EL output above 200 µA. This
saturation is completely reversible, showing no decay or
hysteresis on cycling between 0 and 300 µA.
Conclusions
The PL and EL spectra for the q′2GaX complexes (X ) Cl,
OAc, DMP, Ben) are very similar and are blue-shifted substan-
tially from those of Alq3 and Gaq3. The solution PL quantum
efficiencies determined for the q′2GaX complexes are greater
than those reported for Alq3 and Gaq3. As observed for Mq3-
based devices; however, an increase in PL efficiency does not
lead directly to an increase in EL efficiency. While all of the
q′2GaX-based OLEDs performed well relative to Alq3 and Gaq3
at low drive currents, at higher drive currents, the Mq3 devices
showed higher EL quantum efficiencies. The q′2GaBen OLED
has the best EL quantum efficiency at low drive currents but
shows saturation behavior at higher currents.
Acknowledgment. The authors thank the Universal Display
Corp., ARPA (Contract No. F33615-94-1-1414), and the
National Science Foundation (Grant No. DMR94-00362) for
their financial support of this work.
The saturation behavior for the q′2GaBen complex and the
lower efficiencies for the other q′2GaX complexes may be an
intrinsic property of the material (low electron mobility or high
heterojunction energy barriers) or may be due to impurities.
While the materials were purified by sublimation prior to device
fabrication, we cannot rule out that the differences are extrinsic
in nature, i.e. they are due to impurities. However, if impurities
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