Red-Green-Blue Emission from Tris(5-aryl-8-quinolinolate)Al(III) Complexes

Article abstract of DOI:10.1021/jo035602q

A simple yet effective strategy for synthesis of 5-aryl-8-quinolinolate-based electroluminophores with tunable emission wavelengths is presented. Two different pathways for the attachment of electron-donating or electron-withdrawing aryl groups to the 5-p

Full text of DOI:10.1021/jo035602q

the oxygen),3b we decided to manipulate the HOMO-
LUMO energy gap responsible for the emission energy
via attaching electron-withdrawing (EWG) or electron-
donating groups (EDG) to the C5-aryl moieties of the
quinolinolate.
Red -Gr een -Blu e Em ission fr om
Tr is(5-a r yl-8-qu in olin ola te)Al(III)
Com p lexes
Radek Pohl,† Victor A. Montes,† J oseph Shinar,‡ and
Pavel Anzenbacher J r.*,†
At first glance, this approach may not seem as entirely
new. The literature reports that emission tuning via
EWG/EDGs attached to the C5 of 8-quinolinolate was
attempted; however, there did not seem to be a clear
connection between the electronic properties of the C-5
substituent and the emission of the resulting Al(III)
complex.3b For example, attaching the CtN group did
not result in an appreciable blue shift in the emission6a
compared to the parent Alq3 (525 nm). Similarly the
introduction of fluoro6b and chloro substituents6c resulted
only in a small red-shifted emission, to 535 and 540 nm,
respectively. The only example of a successful blue-
shifted emission was reported for piperidine-amide of
quinolinolate-5-sulfonic acid (emission maximum ≈ 480
nm).6d
Recently, we showed that the emission of Alq3 deriva-
tives with aryl-ethynylene moieties attached to the
quinolinolate ligand may allow for tuning of the emission
color in the resulting Al(III) complex.7 These materials
provided an insight into the emission tuning in the
Al(III) quinolinolates; however, the thermal stability
required for OLED fabrication was less satisfactory. Here
we present a series of 5-aryl-quinolinolate Al(III) com-
plexes, in which the electron-withdrawing/-donating
nature of the substituent is projected via the C-5 aryl
bridge to the quinolinolate chromophore.
Center for Photochemical Sciences, Bowling Green State
University, Bowling Green, Ohio 43403, and
Ames Laboratory-USDOE and Department of Physics,
Iowa State University, Ames, Iowa 50011
pavel@bgnet.bgsu.edu
Received October 30, 2003
Abstr a ct: A simple yet effective strategy for synthesis of
5-aryl-8-quinolinolate-based electroluminophores with tun-
able emission wavelengths is presented. Two different
pathways for the attachment of electron-donating or electron-
withdrawing aryl groups to the 5-position of the quinolino-
late ligand via Suzuki coupling were developed. A successful
tuning in the emission color was achieved: the emission
wavelength was found to correlate with the Hammett
constant of the respective substituents, providing a powerful
strategy for prediction of the optical properties of new
electroluminophores.
The availability of full-color displays based on the
small-molecule organic light-emitting diode (SMOLED)
technology is predicated upon a successful development
of red-, green-, and blue-emitting electroluminophores.1,2
Organometallic SMOLED materials are valued for their
stability and emission-color purity. The major obstacle
in the fabrication of SMOLED-based full color displays
is the so far limited availability of complexes that span
the whole visible spectrum, while possessing similar
emissive characteristics, physical properties, and pro-
cessability.
Complexes 1a -j were designed to provide a varying
degree of electronic density in the quinolinolate ligand,
which is modulated by the aryl moiety.
Here we present a method for tuning the tris(8-
quinolinolate)Al(III) (Alq3) emission from blue-green to
yellow and red via attaching electron rich/poor aryl
moieties to the 5-position of the quinolinolate ligand. We
selected Alq3 as the most important SMOLED material,
which is also the most stable electron-transporting
compound currently used.2,3 Photophysical properties of
Alq3-type complexes are dominated by ligand-centered
excited states4 originating from the electronic π-π*
transitions in the quinolinolate ligands.5 Because the
highest density HOMOs of the quinolinolates are located
on the phenoxide oxygen and the C5 (para position to
F IGURE 1. Novel 5-aryl Alq3-type complexes prepared.
The synthesis of complexes 1a -j (Scheme 1) departs
from that of 5-bromo-8-hydroxyquinoline,8 which was
* To whom correspondence should be addressed. Phone: (419) 372-
2080. Fax: (419) 372-9809.
† Bowling Green State University.
(5) Ballardini, R.; Varani, G.; Indelli, M. T.; Scandola, F. Inorg.
Chem. 1986, 25, 3858-3865.
‡ Iowa State University.
(1) Kelly, S. M. Flat Panel Displays: Advanced Organic Materials;
Connor, J . A., Series Ed.; The Royal Society of Chemistry: Cambridge,
UK, 2000.
(2) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913-
915.
(3) (a) Wang, S. Coord. Chem. Rev. 2001, 215, 79-98. (b) Chen, C.
H.; Shi, J . Coord. Chem. Rev. 1998, 171, 161-174.
(4) Vogler, A.; Kunkely, H. Luminescent Metal Complexes: Diversity
of Excited States. In Topics in Current Chemistry; Vol. 213, Transition
Metal and Rare Earth Compounds: Excited States, Transitions,
Interactions I; Yersin, H., Ed.; Springer-Verlag: Berlin, Germany,
2001.
(6) (a) 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. (b) Matsumura, M.; Akai, T. J pn. J . Appl. Phys. 1996, 35, 5357-
5360. (c) Organic Electroluminescent Materials and Devices; Miyata,
S., Nalwa, H. S., Eds.; Gordon and Breach Publishers: Amsterdam,
The Netherlands, 1997. (d) Hopkins, T. A.; Meerholz, K.; Shaheen, S.;
Anderson, M. L.; Schmidt, A.; Kippelen, B.; Padias, A. B.; Hall, H. K.,
J r.; Peyghambarian, N.; Armstrong, N. R. Chem. Mater. 1996, 8, 344-
351.
(7) Pohl, R.; Anzenbacher, P., J r. Org. Lett. 2003, 2769-2772.
(8) Prasad, R.; Coffer, H. L. D.; Fernando, Q.; Freiser, H. J . Org.
Chem. 1965, 30, 1251.
10.1021/jo035602q CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/30/2004
J . Org. Chem. 2004, 69, 1723-1725
1723

TABLE 1. P h otop h ysica l a n d Electr och em ica l
SCHEME 1. Syn th esis of
P r op er ties of Com p lexes 1a -ja
5-Ar yl-8-h yd r oxyqu in olin e Liga n d sa
Amax
HOMO-LUMO
complex (ꢀ [mol-1‚cm-1]) λF [nm] ΦF
τF [ns]
gap [eV]
b
Alq3
1a
1b
1c
1d
1e
1f
1g
1h
1i
388 (7.0 × 103)
390 (2.7 × 104)
388 (1.1 × 104)
397 (7.5 × 103)
398 (1.4 × 104)
394 (1.4 × 104)
402 (1.0 × 104)
402 (1.0 × 104)
385 (2.2 × 104)
410 (1.1 × 104)
422 (1.0 × 104)
526
490
516
530
534
537
541
545
551
564
612
0.171 15.38
0.533 29.50
0.453 20.31
0.301 16.57
0.298 14.53
0.234 12.76
0.201 11.13
2.570
3.255
3.266
3.246
2.908
2.752
2.797
2.718
2.801
2.534
2.473
0.100
0.098
0.057
0.008
9.72
6.53
4.73
1.49
1j
a
Absorption maximum (Amax), emission maximum (λF), fluo-
rescence quantum yield (ΦF), and lifetime (τF) in dichloromethane
at room temperature. HOMO-LUMO energy gap was estimated
by using cyclic voltammetry of 1.0 mM solutions in acetonitrile
a
Conditions: (i) Bn-Cl, K2CO3, AcCN, reflux; (ii) Pd(TPP)4 (5%),
b
containing 0.1 M tetrabutylammonium perchlorate; Determined
1 M aq K2CO3, toluene, TBACl, 90 °C; (iii) Pd(TPP)4 (3%), TEA/
THF, 90 °C; (iv) 1,4-cyclohexadiene, Pd-C (10%), isopropyl alcohol,
reflux.
with use of quinine sulfate (30 µM in 0.05 M H2SO4) as a standard.
Table 1 summarizes the properties of complexes 1a -j
determined from UV-vis, fluorescence, and electrochemi-
cal measurements.
converted to the corresponding benzyl-derivative 2.
Intermediate 2 was coupled by Suzuki reaction with a
suitable arylboronic acid bearing electron-donating group
(EDG) to yield 3 (Pathway A), followed by hydrogenolytic
deprotection.9 The introduction of aryl moieties bearing
EWGs was performed according to Pathway B: 2 was
converted to the pinacolato-boronate ester 4, then reacted
with an appropriate haloaromate to yield protected
ligands of a general structure 3. The final Al(III) com-
plexes were obtained by reacting the ligands 5a -j with
AlCl3‚6H2O in ethanol.
The emission maxima of complexes 1a -j span over 120
nm between 490 and 612 nm, and the emission profiles
cover almost the entire visible light spectrum. Examina-
tion of the data in Table 1 shows decreasing fluorescence
quantum yield and lifetime with decreasing emission
energy. This dependence was investigated in terms of the
optical energy gap law,10 which describes the exponential
dependence of the nonradiative decay rate constant (knr)
on the energy gap between singlet and ground states for
the chromophores (Figure 3).11a,b
Successful emission color tuning in complexes 1a -j
mediated by substituents is easily observed by a naked
eye examination of the complexes solution-based fluo-
rescence. Complexes 1a -j show bright photolumines-
cence upon excitation of dichloromethane solutions with
black light (365 nm). As expected, the emission shifts
from blue to yellow and red depending on the electronic
nature of the aryl group (Figure 2).
F IGURE 3. Plot of ln knr vs emission energy for the complexes
1a -j.
The graph in Figure 3 showed an excellent linear
relationship between ln knr and the relevant optical
energy gap (correlation coefficient ) 0.97), demonstrating
that the characteristics of the emissive excited state
remain comparable across the series.
(9) Nakano, Y.; Imai, D. Synthesis 1997, 1425-1428.
(10) Values for krad and knr were obtained by using data in Table 1
and the following equations: krad ) Φ/τF and knr ) 1/τF - krad. The
table of actual krad and knr values is included in the Supporting
Information.
(11) (a) Henry, B. R; Siebrand, W. Organic Molecular Photophysics;
Birks, J . B., Ed.; J ohn Wiley & Sons: London, UK, 1973; Vol. 1. (b)
Turro, N. J . Modern Molecular Photochemistry; Benjamin/Cummings
Publishing: Menlo Park, CA, 1978.
F IGURE 2. Panel A: Emission of complexes 1a -j upon
illumination with black light (365 nm). Panel B: Emission
spectra of complexes 1a -j.
1724 J . Org. Chem., Vol. 69, No. 5, 2004

Because we were interested in developing a method
for predicting emissive properties in Alq3-type complexes,
we decided to investigate also the relationship between
the photophysical properties of complexes 1 and the
Hammett constant values of the substituents (Figure
4A,B). Although the pool of complexes 1a -j bearing
substituents with a known Hammett constant (σp)12 is
limited, one can clearly see that the photophysical
properties, namely the fluorescence quantum yield and
lifetime, show excellent correlation (coefficients ) 0.97).
The results of these correlations provide unambiguous
proof that our initial notion that the C-5 aromatic
substituents would provide an effective tool for tuning
of the photophysical properties of the complexes 1a -j was
correct.
1i indicate that complexes 1a -j are electroluminescent
and, most importantly, that they can be processed via
vapor deposition. Figure 6 shows three OLEDs made with
Alq3, 1c, and 1i used as electroluminophores. The emis-
sion maxima of the OLEDs are very close to the maxima
recorded in solution suggesting that also the OLEDs of
the complexes 1a -j may span the visible spectrum from
blue to red.
F IGURE 6. Vapor-deposited OLEDs based on Alq3, 1c, and
1i.
In conclusion, a new class of electroluminescent com-
pounds with tunable emission based on tris(8-quinolino-
late)Al(III) with substituted aryl moieties in the 5-posi-
tion was prepared. We have shown that the electronic
nature of the aryl substituent affects the emission color
and fluorescence quantum yield of the resulting complex
presumably via effective modification of the levels of
HOMO located on C5 of the quinolinolate ligand. The
optical properties of the resulting Al(III) complexes
correlate with the values of the Hammett constant of the
respective substituents. This strategy offers a powerful
tool for the preparation of EL materials with predictable
photophysical properties. Complexes 1a -h also display
reasonably high fluorescence and electroluminescence
intensity, which makes them potentially useful as OLED
materials. Efforts focused on optimization and evaluation
of OLEDs utilizing compounds 1a -j are currently in
progress.
F IGURE 4. Correlation of fluorescence quantum yield (Panel
A) and lifetime (Panel B) with the Hammett constant for
complexes 1d ,e,f,g,i,j, respectively.
Additionally, the electrochemical HOMO-LUMO gap
for the complexes was estimated by cyclic voltametry.13
Figure 5 shows the correlation between the magnitude
of the electrochemical HOMO-LUMO gap in complexes
1d -g,i,j and the electronic nature of aryl substituents
as described by Hammett constants (σp).12 As expected,
the magnitude of the HOMO-LUMO gap directly cor-
relates with electronic properties of the aryl moieties
attached to the quinolinolate ligand expressed by Ham-
mett constants.
Ack n ow led gm en t. This work was supported by
NSF (DMR-0306117 and NER-0304320 grants to P.A.),
BGSU (TIE and FRC grants to P.A.), Kraft Foods, Inc
grant to P.A., and the Petroleum Research Fund (ACS-
PRF No. 38110-G to P.A.). We would like to thank Dr.
L. Zou, formerly of the USDOE Ames Laboratory and
the Physics and Astronomy Department of ISU, for
fabrication of the OLEDs.
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures for the preparation and characterization of compounds,
their UV-vis and fluorescence spectra, numeric values for kr
and knr used to construct energy gap law correlation, and the
crystallographic data for 4 in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
J O035602Q
(12) Smith, M. B.; March, J . March’s Advanced Organic Chemistry:
Reactions, Mechanisms, and Structure, 5th ed.; Wiley-Interscience:
New York, 2001.
(13) Pommerehne, J .; Vestweber, H.; Gusws, W.; Mahrt, R.; Ba¨ssler,
H.; Porsch, M.; Daub, J . Adv. Mater. 1995, 7, 551.
F IGURE 5. HOMO-LUMO gap correlation with the Hammett
constants for complexes 1d -g,i,j.
(14) The OLEDs were fabricated by using the standard vapor
deposition method. Device configuration: glass-ITO cathode, CuPh-
thalocyanine (100 Å), NPD (N,N′-di-1-naphthyl-N,N′-diphenyl-1,1′-
biphenyl-4,4′-diamine, 400 Å), electroluminophore (500 Å), CsF (10 Å),
Al (2000 Å). Forward bias for Alq3 ) 4.5 V, 1c ) 6 V, 1i ) 6 V.
Last but not least, the preliminary experiments with
fabrication of OLED devices14 utilizing complexes 1c and
J . Org. Chem, Vol. 69, No. 5, 2004 1725