True blue: Blue-emitting aluminum(III) quinolinolate complexes

Article abstract of DOI:10.1021/ic061051l

Blue-emitting heteroleptic aluminum(III) bis(2-methyl-8-quinolinolate) phenolate complexes were synthesized. A tunable, blue-to-green emission is achieved by attaching electron-withdrawing modulators to the emisssive quinaldinate ligand. The electronic na

Full text of DOI:10.1021/ic061051l

Inorg. Chem. 2006, 45, 9610−9612
True Blue: Blue-Emitting Aluminum(III) Quinolinolate Complexes
Ce´sar Pe´rez-Bol´ıvar, Victor A. Montes, and Pavel Anzenbacher, Jr.*
Department of Chemistry and Center for Photochemical Sciences, Bowling Green State UniVersity,
Bowling Green, Ohio 43403
Received June 12, 2006
Blue-emitting heteroleptic aluminum(III) bis(2-methyl-8-quinolino-
late)phenolate complexes were synthesized. A tunable, blue-to-
green emission is achieved by attaching electron-withdrawing
modulators to the emisssive quinaldinate ligand. The electronic
nature of modulator substituents attached to the position of the
highest HOMO (highest occupied molecular orbital) density is used
to modulate ligand HOMO levels to achieve effective emission
tuning to obtain blue-emitting materials. Optical and electrochemical
properties of the resulting complexes were investigated and
compared to the results of density functional theory (DFT/B3LYP/
6-31G*) studies. The resulting materials may find application as
organic light-emitting device materials.
Figure 1. Structures of a mer isomer of Alq3 with numbered positions in
the ligand and 2Meq2AlOPh.
buffer layers on the cathode side, and the hole-transport layer
on the anode side. Few materials satisfy these conditions,
which makes them highly desirable.1
In the past, numerous research groups investigated alumin-
um(III) quinolinolate complexes in studies focused on unrav-
eling their photophysical and semiconductor properties.2 Both
the semiconductor and emissive properties are largely defined
by HOMO-LUMO levels of the quinolinolate ligand and
its lowest electronic π-π* transitions.3 A number of studies
aimed at HOMO-LUMO levels and emission color tuning in
the Alq3-type materials were attempted with a variable degree
of success. Theoretical studies using density functional theory
(DFT) calculations provided an insight into the distribution
of the HOMO-LUMO densities in the quinolinolate ligand
in the mer isomer of Alq3 and suggested that the HOMO
orbitals are located mostly on the phenoxide side of the ligand,
C8 and C5 in particular, while the highest LUMO density is
found at the pyridine ring, namely, on C2 and C4 (Figure 1).
The studies aimed at investigating the effect of quinoli-
nolate substituents on photophysical properties revealed that
the attachment of electron-donating substituents to the
phenolate ring results in red-shifted emission, while the
attachment of electron-donating substituents to the pyridine
ring results in blue-shifted emission from the complex. While
the parent Alq3 emits at 525 nm,2 a methyl group on C2,
C3, C4, and C5 results in emission shifting to 495,4 520,5
505,6 and 545 nm,5 respectively. Attachment of the π-electron-
Electroluminescent organometallic complexes are valued
emitters for the fabrication of small-molecule-based organic
light-emitting devices (SMOLEDs).1 The unique electron-
transport and emissive properties of tris(8-quinolinolate)alumin-
um(III) (Alq3) and its derivatives resulted in extensive use
of Alq3 in OLED fabrication (Figure 1). The advantage
of aluminum(III) quinolinolate complexes is in their semi-
conductor properties, namely, the ability to transport electrons
and act as an electron-transport layer material and a host for
various dopants. This, together with the ease of deposition
by thermal vacuum evaporation, made aluminum(III) quino-
linolate complexes widely used OLED materials. An obstacle
in the fabrication of SMOLED-based full-color displays so
far appears to be the limited availability of a pure blue color.1
Additionally, in the case of blue-emitting complexes, the blue
emission attests to the fact that the HOMO-LUMO gap
(energy difference between the highest occupied molecular
orbital and lowest unoccupied molecular orbital) is 3.2-3.3
eV regardless of the actual energy level of each molecular
orbital. However, for successful device operation, the HOMO
and LUMO energy levels of the electron-transporting mate-
rial must also be aligned with the levels of the other materials
in the OLED, namely, the dopant, both hole-blocking and
(2) Chen, C. H.; Shi, J. Coord. Chem. ReV. 1998, 171, 161-174.
(3) Vogler, A.; Kunkely, H. Luminescent Metal Complexes: Diversity
of Excited States. In Topics in Current Chemistry; Yersin, H., Ed.;
Springer-Verlag: Berlin, 2001; Vol. 213.
(4) Matsumura, M.; Akai, T. Jpn. J. Appl. Phys. 1996, 35, 5357-5360.
(5) Sapochak, L. S.; Padmaperuma, A.; Washton, N.; Endrino, F.; Schmett,
G. T.; Marshall, J.; Fogarty, D.; Burrows, P. E.; Forrest, S. R. J. Am.
Chem. Soc. 2001, 123, 6300-6307.
* To whom correspondence should be addressed. E-mail: pavel@
bgnet.bgsu.edu.
(1) (a) Organic electroluminescence; Kafafi, Z. H., Ed.; CRC Press: Boca
Raton, FL, 2005. (b) Shinar, J. Organic Light-Emitting DeVicessA
SurVey; Springer-Verlag: Berlin, 2003. (c) Kelly, S. M. In Flat Panel
Displays: AdVanced Organic Materials; Connor, J. A., Series Ed.;
Royal Society of Chemistry: Cambridge, U.K., 2000.
9610 Inorganic Chemistry, Vol. 45, No. 24, 2006
10.1021/ic061051l CCC: $33.50
© 2006 American Chemical Society
Published on Web 10/24/2006

COMMUNICATION
rich substituents in the C5 position resulted mostly in red-
shifted emission. For example, C5 phenyl, fluoro, chloro,
and cyano substituents resulted in complexes with emission
maxima at 550,7 535,8 540,2 and 520 nm,9 respectively. An
example of effective blue emission from an Alq3 derivative
is the C5 piperidine sulfonamide emitting at 480 nm.10
Recently, our group performed extensive studies aimed at
emission color tuning in Alq3-type materials.11 We used sim-
ple quinolinolate complexes with electron-donating or electron-
withdrawing aromatic moieties connected either directly12a
or via an acetylene spacer12b to the C5 of the ligand. We
have demonstrated that electron-deficient moieties cause a
blue shift in emission while the electron-donating substituents
result in red-shifted emission. We also showed that these
materials retain their semiconductor properties and may be
used for OLED fabrication.13
Figure 2. Structure of C5-substituted derivatives and 2Meq2AlOPh.
Unfortunately, none of the above efforts yielded a high-puri-
ty blue emitter, one with emission centered around 450-470
nm while possessing a high emission quantum yield. Such an
emitter, however, is required for the fabrication of full-color
displays to generate the red-green-blue signal.1 Also, re-
cently discovered high-efficacy (∼18%) white OLEDs for
interior lighting require a good-quality blue emitter.14 We have
realized that it is unlikely that a single substituent on the
quinolinolate ligand would exert such a strong effect to yield
a blue emitter of sufficient quality and that we may need to
use a combined effect of two substituents. The question was,
how much the substituent effects that modulate the quinolino-
late emission were additive, which is the subject of this study.
Here we explore the properties of fluorescent quinolinolate
complexes, in which the quinolinolate ligand bears two sub-
stituents to modulate the emission. First, we decided to use the
2-methyl substituent (2Meq), which is known to blue shift the
emission for ca. 0.2 eV compared to the parent Alq3 through
a combination of electronic and geometrical effects.2 The 2Meq
ligand, however, is too sterically demanding to form a tris-
complex. Therefore, the 2-methylquinolinolate complexes of
Al(III) are complexes comprising two 2Meq ligands and one
ancillary ligand. We used a phenolate as an ancillary ligand
and used the reaction of aluminum(III) phenoxide15 with two
2-methyl-8-hydroxyquinoline ligands and its C5-substituted
derivatives16 to prepare the complexes described in Figure 2.
The C5 substituents are divided into two groups: electron-
poor substituents were chosen to maximize the blue shift in
Figure 3. Top: Emission of 1a-f in CH2Cl2 solutions illuminated with
black light (365 nm). Bottom: Emission spectra of complexes 1a-f.
the emission, while the two electron-rich substituents were
included for the purpose of investigating the additivity of
the substituent effects. The 2-methyl induces a blue shift,
while the 5-phenyl and, namely, the 5-(4-dimethylaminophe-
nyl) induce a red shift.12,13
The substituent-mediated emission color tuning in com-
plexes 1a-f can be observed by a naked eye. Upon excitation
of CH2Cl2 solutions with black light (365 nm), complexes
1a-f show bright photoluminescence (PL) that shifts from
blue to green and red, depending on the electronic nature of
the aryl substituent (Figure 3).
Table 1 summarizes the properties of complexes 1a-f
determined from UV-visible and fluorescence measure-
ments. The data for the parent complex 2Meq2AlOPh
(substituent ) H) are also included for comparison.
The emission maxima of complexes 1a-f span over 120
nm between 455 and 575 nm, while showing strong emission
in the blue region of the visible light spectrum. In general,
we found that the C5 substituents provide a stronger
emission-tuning effect compared to 2-methyl. It appears that
the substituents on the phenoxide and pyridine rings induce
an additive effect on the emission wavelength. The combina-
(6) Kido, J.; Iizumi, Y. Chem. Lett. 1997, 963-964.
(7) Tokito, S.; Noda, K.; Tanaka, H.; Taga, Y.; Tsutsui, T. Synth. Met.
2000, 111-112, 393-396.
(8) Matsumura, M.; Akai, T. Jpn. J. Appl. Phys. 1996, 35, 5357-5360.
(9) Burrows, P. E.; Shen, Z.; Bulovic, V.; McCarty, D. M.; Forrest, S.
R.; Cronin, J. A.; Thompson, M. E. J. Appl. Phys. 1996, 79, 7991-
8006.
(10) Hopkins, T. A.; Meerholz, K.; Shaheen, S.; Anderson, M. L.; Schmidt,
A.; Kippelen, B.; Padias, A. B.; Hall, H. K., Jr.; Peyghambarian, N.;
Armstrong, N. R. Chem. Mater. 1996, 8, 344-351.
(11) Montes, V. A.; Pohl, R.; Shinar, J.; Anzenbacher, P., Jr. Chem.sEur.
J. 2006, 17, 4523-4535.
(12) (a) Pohl, R.; Montes, V. A.; Shinar, J.; Anzenbacher, P., Jr. J. Org.
Chem. 2004, 69, 1723-1725. (b) Pohl, R.; Anzenbacher, P., Jr. Org.
Lett. 2003, 5, 2769-2772.
(15) This procedure is an adaptation of the method described in: Bryan,
P. S.; Lovecchio, F. V.; Van Slyke, S. A. U.S. Patent 5,141,671, 1992.
This method uses aluminum(III) isopropoxide.
(16) The synthesis of the C5-substituted 2-methyl-8-hydroxyquinoline
ligands used to synthesize 1a-f is reported in the Supporting
Information.
(13) Montes, V. A.; Pohl, R.; Li, G.; Shinar, J.; Anzenbacher, P., Jr. AdV.
Mater. 2004, 16, 2001-2003.
(14) Sun, Y.; Giebink, N. C.; Kanno, H.; Ma, B.; Thompson, M. E.; Forrest,
S. R. Nature 2006, 440, 908-912.
Inorganic Chemistry, Vol. 45, No. 24, 2006 9611

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Table 1. Photophysical Properties of Complexes 1a-fa
optic band
λFSS (nm) gap (eV)
c
complex
Amax (ꢀ)b
λF (nm)
ΦF
H
357 (7.0 × 104)
377 (1.04 × 105)
364 (1.44 × 104)
363 (1.37 × 104)
368 (7.39 × 104)
370 (1.04 × 104)
353 (1.06 × 103)
495
455
468
484
504
515
574
0.40 492
0.60 467
0.40 487
0.60 485
0.30 504
0.10 513
>0.01 ND
3.47
3.29
3.40
3.41
3.37
3.35
3.51
1a
1b
1c
1d
1e
1f
a Absorption maximum (Amax), fluorescence emission maximum (λF),
and quantum yield (ΦF) in a CH2Cl2 solution at room temperature. Solid-
state fluorescence maximum (λFSS) measured in 20% (w/w) poly(methyl
methacrylate) in a CH2Cl2 suspension. Refer to the Supporting Information
for emission spectra. The optical band gap was estimated from the UV-
Figure 4. Left: HOMO-LUMO band gap for 1a-f and 2Meq2AlOPh,
estimated by CV. Right: Correlation between the HOMO level and
HOMO-LUMO obtained from redox potentials as a function of the
fluorescence emission energy. The asterisk in 1f indicates the potential
uncertainty due to the redox chemistry of the dimethylamino moiety.
visible spectra. b Units: mol-1 cm-1
.
c Determined using quinine sulfate
in 0.05 M H2SO4 as a standard.
3.08 eV calculated from electrochemistry measurements.
Figure 4 shows a systematic decrease in the HOMO-LUMO
gaps across the series 1a-f.
Table 2. Electrochemical Properties of Complexes 1a-fa
E ox
E red
HOMO
(eV)
LUMO HOMO-LUMO
There is a good correlation between the photophysical data
and the data derived from electrochemistry (Figure 4, right
panel). As mentioned before, this is partially due to the fact
that the photophysical properties of these materials are
defined by their HOMO-LUMO gap, with the
S0fS1 electronic transition having a large character of
HOMOfLUMO. The linear dependence of the HOMO
levels and the HOMO-LUMO gap on the S1-S0 emission
energy attests to the uniform photophysical and semiconduc-
tor behavior of the complexes within the series.
complex
(V)
(V)
(eV)
(eV)
H
1.70
1.90
1.86
1.85
1.57
1.57
-1.43
-1.36
-1.38
-1.37
-1.37
-1.37
-6.27
-6.47
-6.43
-6.42
-6.14
-6.14
-3.14
-3.21
-3.18
-3.19
-3.19
-3.20
-3.18
3.13
3.26
3.24
3.22
2.94
1a
1b
1c
1d
1e
1f
2.94
<1.69b -1.39 <-6.20b
<3.00b
a E ox and E red were determined using CV of 1.0 mM solutions in CH2Cl2
containing 0.1 M tetrabutylammonium perchlorate. See the Supporting
Information for more details. b The redox chemistry of the dimethylamino
moiety prevented the exact determination of the E ox, HOMO, and HOMO-
LUMO gap values.
Also, the above findings were supported by the DFT calcu-
lations of the HOMO levels and the HOMO-LUMO gap at
the B3LYP/6-31G* level of theory in vacuum. The absolute
values of the HOMO levels were uniformly overestimated
in the DFT calculations, presumably because of the fact that
the calculations were carried out for 1a-f in vacuum.
Otherwise, the DFT calculations showed the same decrease
in the HOMO levels as that observed in electrochemistry.
In conclusion, a new class of quinolinolate complexes
1a-f with tunable properties was prepared. Complexes 1a-d
show strong blue emission (ΦFL ) 30-60%) and emission
maxima at 455-504 nm, which makes them comparable with
most small-molecule blue emitters.1,2 We have shown that
the electronic nature of the aryl substituent affects the
emission color, predominantly via effective modification of
the HOMO levels located on C5 of the quinolinolate ligand.
The optical properties of the complexes 1a-f also correlate
well with the electrochemical properties. All of our observa-
tions suggest that the electronic effects that modulate the
emissive properties in quinolinolate complexes are additive
and the effects of individual substituents may be combined
to obtain emitters of desired properties. The charge-transport
and electroluminescence properties of the complexes 1a-f
are under investigation.
tion of both types of substituents may be used to achieve a
desired blue emission. For example, when compared to Alq3
(λF ∼ 525 nm), 1a (455 nm) shows a ∼0.4 eV blue shift.
This shift appears to be a combined effect of the 2-methyl
and 5-dimethoxytriazinyl substituents, both known to induce
a ∼0.2 eV shift when individually attached to Alq3 sepa-
rately.11 Also, in agreement with the energy gap law,17 we
found a decreasing fluorescence quantum yield with a
decreasing emission energy (Table 1).
While the photophysical data confirmed that the optical
HOMO-LUMO gap is sufficient for a blue emission, we
performed cyclic voltammetry (CV) experiments to deter-
mine the absolute HOMO and LUMO levels (Table 2).18
The data in Table 2 show that the reduction potentials and
the corresponding LUMO levels in 1a-f are not significantly
changed across the series. On the other hand, the oxidation
potentials and HOMO levels underwent systematic changes,
reflecting the tuning of the HOMO-LUMO gap (Figure 4,
left panel). We reason that this is due to the 2-methyl
substituent in the C2 position with the highest LUMO density
in all derivatives, while the varying substituents in the C5
position affect predominantly the HOMO levels because C5
is the site with high HOMO density. The magnitudes of the
HOMO-LUMO gap for 1a-f correlate with the observed
emission properties. Thus, the most blue complex 1a shows
the highest HOMO-LUMO gap of 3.26 eV, and the most
red-shifted emission of complex 1f corresponds to a gap of
Acknowledgment. This work was supported by the NSF
(Grant DMR-0306117) and BGSU (TIE and FRC grants).
Supporting Information Available: Experimental data for the
preparation of 1a-f, their UV-visible and PL spectra, CV curves,
and DFT calculations. This material is available free of charge via
the Internet at http://pubs.acs.org.
(17) Englman, R.; Jortner, J. Mol. Phys. 1970, 18, 145-164.
(18) Pommerehne, J.; Vestweber, H.; Guss, W.; Mahrt, R.; Ba¨ssler, H.;
Porsch, M.; Daub, J. AdV. Mater. 1995, 7, 551-554.
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