Emission color tuning in AIQ3 complexes with extended conjugated chromophores

Article abstract of DOI:10.1021/ol034693a

(Matrix presented) A new method for the synthesis of 5-arylethynyl-8-hydroxyquinoline ligands using Sonogashira-Hagihara coupling was developed. The electronic nature of arylethynyl substituents affects the emission color and quantum yield of the resulting AI(III) complex. Photophysical properties of the metallocomplexes correspond to the electron-withdrawing/-donating character of the arylethynyl substituents. Optical properties of such AI(III) complexes correlate with the Hammett constant values of the respective substituents. This strategy offers a powerful tool for the preparation of electroluminophores with predictable photophysical properties.

Full text of DOI:10.1021/ol034693a

ORGANIC
LETTERS
2003
Vol. 5, No. 16
2769-2772
Emission Color Tuning in AlQ
Complexes with Extended Conjugated
Chromophores
3
Radek Pohl and Pavel Anzenbacher, Jr.*
Center for Photochemical Sciences and Department of Chemistry,
Bowling Green State UniVersity, Bowling Green, Ohio 43403
paVel@bgnet.bgsu.edu
Received April 25, 2003
ABSTRACT
A new method for the synthesis of 5-arylethynyl-8-hydroxyquinoline ligands using Sonogashira−Hagihara coupling was developed. The electronic
nature of arylethynyl substituents affects the emission color and quantum yield of the resulting Al(III) complex. Photophysical properties of
the metallocomplexes correspond to the electron-withdrawing/-donating character of the arylethynyl substituents. Optical properties of such
Al(III) complexes correlate with the Hammett constant values of the respective substituents. This strategy offers a powerful tool for the preparation
of electroluminophores with predictable photophysical properties.
Emission properties of metallocomplexes are in general
controlled by a number of factors, including electronic
properties of the ligand and metal ion, coordination bond
lengths and angles that affect the energy mixing, and splitting
of electronic states involved in the emission. The design of
novel emissive materials with improved photonic properties
is predicated upon detailed understanding of the relationship
between the structure of a ligand and photonic properties of
the resulting metallocomplexes.
excited states.3 Light emission of the AlQ3 complexes
originates from the electronic π-π* transitions in the
quinolinolate ligands.4,5 The HOMO-orbitals are located on
the phenoxide side of the ligand, while the LUMO-orbitals
are located on the pyridyl side.2b This suggests that substit-
uents attached to the quinolinolate ligand may be used as a
handle for tuning of the complex emission. The literature,
however, provides spotty evidence supporting the correlation
of the substituent properties with the photophysical properties
of quinolinolate chromophores. In general, electron-donating
groups (EDGs) such as methyl attached to the pyridine ring
Among the luminescent metallocomplexes, tris(8-quino-
linolate)Al(III) (AlQ3) plays an important role as an elec-
troluminophore and the most stable electron-transporting
compound currently used in organic light-emitting diodes
(OLEDs).1,2 AlQ3-type complexes display ligand-centered
(2) (a) Wang, S. Coord. Chem. ReV. 2001, 215, 79-98. (b) Chen, C.
H.; Shi, J. Coord. Chem. ReV. 1998, 171, 161-174. (c) Organic Electrolu-
minescent Materials and DeVices; Miyata, S., Nalwa, H. S., Eds.; Gordon
and Breach Publishers: Amsterdam, 1997.
(3) 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, 2001.
(1) Kelly, S. M. Flat Panel Displays: AdVanced Organic Materials;
Connor, J. A., Series Ed.; The Royal Society of Chemistry: Cambridge,
2000.
10.1021/ol034693a CCC: $25.00 © 2003 American Chemical Society
Published on Web 07/16/2003

cause a blue shift in the complex emission1,6 (∼510 nm)
compared to the emission of the parent AlQ3 (526 nm), while
introduction of alkyls to the benzene ring causes a red shift
(g530 nnm).1,2b,7 The presence of electron-withdrawing
groups (EWGs) such as fluoro-, chloro-,8a,4 and cyano8b
groups in the 5- or 7-position of the benzene ring results in
almost negligible emission shifts (520-530 nm), while strong
EWGs such as sulfonamide (-SO2NR2) result in significantly
blue-shifted emission (∼480 nm).8c
Scheme 1. Strategy for Synthesis of Protected Quinolate
Ligands
In an effort to explore the magnitude of the substituent
effect and its possible use in the emission color tuning in
AlQ3 complexes, we decided to synthesize a series of 8-hy-
droxyquinoline ligands (Q) with various aryl substituents (Ar)
connected together via an ethynylene spacer (E). The inclu-
sion of the ethynylene spacer was also motivated by a
high degree of electronic communication between the ligand
and the substituted aryl moiety as well as avoiding a poten-
tial interaction of ortho-substituents on Ar with the Q-moiety
in the case of directly attached aryl moieties (Q-Ar).
Structures of the complexes prepared in this study are shown
in Figure 1.
The synthesis of complexes 1a-h departs from 5-bromo-
quinoline-8-hydroxyquinoline,9 which was converted to the
corresponding Boc-derivative 2 by reaction with di-tert-butyl
dicarbonate. The Boc group was selected over the published
methyl,10 benzyl,11 and TBDMS12 groups because of its easy
deprotection compared to Bn/Me protection and the crystal-
linity of the TBDMS derivatives.
According to pathway A, compound 2 underwent Sono-
gashira-Hagihara coupling13 with TMS-acetylene fol-
lowed by KF-mediated deprotection of the TMS group to
give 5-ethynyl-5-BocO-quinoline 3 in 69% overall yield
(Scheme 2).
Scheme 2. Synthesis of Quinolinolate Ligandsa
Figure 1. Structures of tris(5-arylethynyl-8-quinolinolate)Al(III)
complexes 1a-h.
The strategy for the synthesis of the ligands utilized in
complexes 1a-h is depicted in Scheme 1. Pathway A was
used for the attachment of electron-deficient arenes, while
pathway B was used for the synthesis of electron-rich ligands
4a,b,d. The only exception was the synthesis of the 2-(4-
pyridyl)ethynyl compound 4c, which was synthesized using
pathway B.
a Reaction conditions: (i) Pd(TPP)4 (5%), CuI (5%), DIPEA,
THF; (ii) KF (2 equiv), MeOH; (iii) piperidine (3 equiv), CH2Cl2;
(iv) TFA (5 equiv), CH2Cl2.
Electron-deficient arenes such as 2-chloro-4,6-dimethoxy-
1,3,5-triazine,14 pentafluoroiodobenzene, and p-bromoben-
(4) Jang, H.; Do, L.-M.; Kim, Y.; Kim, J. G.; Zyung, T.; Do, Y. Synth.
Met. 2001, 121, 1669-1670.
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1986, 25, 3858-3865.
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1965, 30, 1251.
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S. Jpn. J. Appl. Phys. 1999, 38, 6762-6763. (b) Kido, J.; Iizumi, Y. Chem.
Lett. 1997, 963-964.
(10) (a) Trecourt, F.; Mongin, F.; Mallet, M.; Queguiner, G. J. Heterocycl.
Chem. 1995, 32, 1261-1267. (b) Albrecht, M.; Blau, O.; Ro¨ttele, H. New
J. Chem. 2000, 24, 619-622.
(11) Nakano, Y.; Imai, D. Synthesis 1997, 1425-1428.
(12) Jotterand, N.; Pearce, D. A.; Imperiali, B. J. Org. Chem. 2001, 66,
3224-3228.
(13) (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975,
16, 4467-4470. (b) Sonogashira, K. in Metal-Catalyzed Cross-Coupling
Reactions; Diederich, F., Stang, P. J., Eds.; Wiley-VCH: New York, 1998.
(14) Cronin, J. S.; Ginah, F. O.; Murray, A. R.; Copp, J. D. Synth.
Commun. 1996, 26, 3491-3494.
(7) Hamada, Y.; Sano, T.; Fujita, M.; Fujii, T.; Nishio, Y.; Shibata, K.
Jpn. J. Appl. Phys. 1993, 32, 514-515.
(8) (a) Matsumura, M.; Akai, T. Jpn. J. Appl. Phys. 1996, 35, 5357-
5360. (b) 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. (c) 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.
2770
Org. Lett., Vol. 5, No. 16, 2003

zonitrile were then attached using standard conditions for
Sonogashira-Hagihara coupling to give 4a (83%), 4b (62%),
and 4d (97%), respectively. The same method was also used
for the reaction with p-bromonitrobenzene and p-bromopy-
ridine-N-oxide.15
Synthetic pathway B utilizes electron-rich ethynylarenes
such as ethynylbenzene, p-ethynylmethoxybenzene, 1-ethy-
nylpyrene, and p-ethynyldimethylaminobenzene to yield
ligands of general structures 4e-h in 69, 65, 76, and 59%
yields, respectively. The same approach was used to prepare
8-BocO-5-[(pyridin-4-yl)ethynyl]quinoline 4c from com-
mercially available p-ethynylpyridine hydrochloride, albeit
the yield was lower (49%). All Boc-protected ligands were
purified by flash chromatography and recrystallization
(acetone-hexane). This method also yielded monocrystals
of 4g. X-ray analysis of 4g (Figure 2) suggests effective
conjugation in the extended chromophore ligand.
Figure 3. Panel A: Emission of complexes 1a-h upon illumina-
tion with black light (365 nm). Panel B: Emission spectra of
complexes 1a-h.
Table 1. Optical Properties of Complexes 1a-ha
Figure 2. X-ray structure of the protected ligand 4g. Thermal
ellipsoids are scaled to the 30% probability level.
c
compound
Amax (ꢀ)b
λF [nm]
ΦF
τ [ns]
1a
b
c
d
e
f
g
h
407 (3.5 × 104)
410 (2.7 × 104)
414 (2.4 × 104)
414 (8.4 × 104)
421 (2.2 × 104)
421 (1.8 × 104)
385 (6.8 × 104)
425 (3.9 × 104)
388 (7.0 × 103)
520
538
541
545
561
569
573
600
526
0.317
0.228
0.235
0.203
0.088
0.047
0.036
0.009
0.171
11.85
8.67
8.95
6.61
3.56
1.86
1.28
0.89
15.38
Deprotection of the Boc group may be achieved in several
ways, including TFA- or piperidine-mediated deprotection.
For example, the dimethylamino derivative 4h was depro-
tected by both TFA and piperidine to give after crystalliza-
tion essentially the same yields of 71% (5h) and 73%
(6h), respectively. Piperidine deprotection yielded ligands
5a-c,h in 79, 79, 90, and 73% yields, respectively. Depro-
tection using TFA gave trifluoroacetate salts of ligands 6d-h
in 99, 67, 79, 94, and 71% yields. Ligands 5a-c,h and 6d-h
were converted to the corresponding complexes 1a-h by
reaction of ethanolic solutions of ligands with aqueous AlCl3
followed by neutralization with triethylamine and crystal-
lization from ethanol and dichloromethane-hexane. All
complexes 1a-h are soluble in a variety of solvents,
including dichloromethane, THF, and DMSO and less soluble
in alcohols.
AlQ3
a Used 30 µM solutions in dichloromethane. b Units ) mol-1 cm-1
.
c Determined using quinine sulfate as a standard.
with black light (365 nm). As expected, the emission shifts
from green-blue to yellow and red depending on the
electronic nature of the aryl (Figure 3).
Table 1 summarizes the optical properties of complexes
1a-h determined from UV-vis and fluorescence measure-
ments.
One can see that the emission maxima of complexes 1a-h
span ∼100 nm between 520 and 610 nm, and the emission
profiles at ca. 50% of the peaks intensity cover a significant
portion of the visible light spectrum (485-670 nm). Com-
plexes 1a-d also display reasonably high fluorescence
quantum yields, which makes them potentially useful as
electroluminophores. Because the emission of light by the
quinolinolate complexes originates from ligand-centered
excited states,3 we were intrigued by the possibility of
correlation of the photophysical properties with the Hammett
Successful emission color tuning in the complexes medi-
ated by the substituents may be observed by naked eye
examination of the solutions of the complexes and fluores-
cence spectroscopy. Complexes 1a-h show bright photo-
luminescence upon excitation of the dichloromethane solution
(15) Same method was also used for reaction with p-bromonitrobenzene
(76%) and p-bromopyridine-N-oxide (78%); however, these materials after
piperidine-mediated deprotection (97 and 73% yields, respectively) did not
yield complexes with appreciable luminescence properties and therefore
are not discussed in this communication.
Org. Lett., Vol. 5, No. 16, 2003
2771

Figure 4. Fluorescence quantum yield correlation with the
Hammett constant for complexes 1d-f,h with correlation coef-
ficients R2 ) 0.9923 (panel A) and R2 ) 0.9899 (panel B).
Figure 5. Energy gap correlation performed for complexes
1a-h. Correlation coefficient R2 ) 0.9320.
constants to predict the emission of the complexes before
their synthesis is performed. Although the pool of compounds
1a-h bearing substituents with a known Hammett constant
(σp)16 is limited, one can clearly see that the photophysical
properties, namely, the fluorescence quantum yield and
lifetime, show excellent correlation (Figure 4A,B).
Additionally, photochemical properties of the prepared
complexes were investigated in terms of the energy gap
law,17 which describes the exponential energy gap depen-
dence of the nonradiative decay processes in the chromo-
phore. As the energy gap between the lowest excited state
and ground state increased, the photoluminescence lifetime
and quantum yield increased. Because the energy-gap law
correlation is based on recorded fluorescence lifetime, quan-
tum yield, and emission maxima recorded for each com-
pound, the successful correlation for compounds 1a-h (Fig-
ure 5) attests to the high photonic quality of the prepared
materials.
We were concerned with the stability of compounds 1a-h
regarding their potential use in OLEDs given the potential
sensitivity of the acetylene units to the electron injection.
Simple OLEDs were fabricated using ITO-modified glass
(anode), Baytron-P (hole-transport layer), a complex spin-
coated from CH2Cl2, and In-Ga (cathode). Although the
performance of these simple OLEDs was somewhat incon-
sistent, our preliminary results indicate that compounds 1a-h
are potential electroluminophores. The OLEDs fabricated
using compounds prepared in this study showed turn-on
voltages at 6-7 V.
sion color and fluorescence quantum yield of the resulting
Al(III) complex. 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 electroluminophores
with predictable photophysical properties. Efforts toward
investigation of solid-state luminescence, fabrication, and
evaluation of OLEDs utilizing compounds 1a-h are cur-
rently under way.
Acknowledgment. Financial support from Bowling Green
State University, the donors of the Petroleum Research Fund,
administered by the American Chemical Society (Grant
38110-G4 to P.A.), and NSF (Grant DMR-0306117 to P.A.)
is gratefully acknowledged. We are grateful to Dr. Vince
M. Lynch of the University of Texas at Austin for solving
the X-ray structure of 4g and Drs. Kamil Lang and Pavel
Kubat of the Czech Academy of Sciences for their help with
recording the photophysical properties of 1a-h.
Supporting Information Available: Experimental pro-
cedures and spectroscopic data for complexes 1a-h, pro-
tected ligands 4a-h, deprotected ligands 5a-c,h and
6d-h, and crystallographic data for 4g in CIF format. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL034693A
In conclusion, a new class of electroluminescent com-
pounds based on tris(8-quinolinolate)Al(III) with aryleth-
ynyl substituents was synthesized using a Sonogashira-
Hagihara coupling procedure. We have shown that the elec-
tronic nature of the arylethynyl substituent affects the emis-
(16) Smith, M. B.; March, J. March’s AdVanced Organic Chemistry:
Reactions, Mechanisms, and Structure, 5th ed.; Wiley-Interscience: New
York, 2001.
(17) Caspar, J. V.; Meyer, T. J. Inorg. Chem. 1983, 22, 2444-2453.
2772
Org. Lett., Vol. 5, No. 16, 2003