Novel fluorescent aluminum complexes based on N-hydroxy-3,6-diaryl-4- phenyl-2-pyridone ligands

Article abstract of DOI:10.1246/cl.2008.248

Novel fluorescent aluminium complexes of N-hydroxy-3,6-diaryl-4-phenyl-2- pyridone derivatives were synthesized and their fluorescent properties were investigated. The complexes having a trifluoromethyl group and a methoxy group at the para-position on th

Full text of DOI:10.1246/cl.2008.248

248
Chemistry Letters Vol.37, No.3 (2008)
Novel Fluorescent Aluminum Complexes
Based on N-Hydroxy-3,6-diaryl-4-phenyl-2-pyridone Ligands
Satoshi Minakata,Ã1 Hiroshi Inada,1 Mitsuo Komatsu,1 Hirotake Kajii,2 Yutaka Ohmori,Ã2
Manabu Tsumura,3 and Kiyoyuki Namura3
1Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871
2Center for Advanced Science and Innovation, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871
3Kaneka Corporation, 3-2-4 Nakanoshima, Kita-ku, Osaka 530-8288
(Received November 1, 2007; CL-071206; E-mail: minakata@chem.eng.osaka-u.ac.jp)
Novel fluorescent aluminium complexes of N-hydroxy-3,6-
diaryl-4-phenyl-2-pyridone derivatives were synthesized and
their fluorescent properties were investigated. The complexes
having a trifluoromethyl group and a methoxy group at the
para-position on the 3- and 6-phenyl groups exhibit purple
fluorescence in the solid state.
EWG
OEt
NH2OH HCl (50 equiv.)
O
NaH
EWG
EDG
Ph
O
(0.25 equiv.)
Et3N (50 equiv.)
+
O
EtOH, reflux, 5–17 h
THF, reflux, 3 h
O
Ph
EDG
Fluorescent organic compounds, especially organic electro-
luminescence (EL) devices, have attracted much interest be-
cause of applications such as large-area light-emitting displays
that operate at low-drive voltage.1 Thus, the development of
new types of fluorophores; small organic molecures2–4 as well
as organic metal complexes,5–11 has been actively pursued. For
metal organic complexes, emission properties are readily con-
trolled by a number of factors, including electronic properties
of metal ions, ligands, and coordination length. One of the most
representative emitting materials is Alq3 [tris(8-hydroxyquinoli-
nato)aluminum], which was originally reported by Tang’s group
in 1987.5 This complex plays an important role as an electrolu-
minophore and is the electron-transporting compound currently
used in organic light-emitting diodes (OLEDs). In addition,
metal–chelate materials having aromatic heterocycles and phe-
nolic oxygen moieties have been reported. These compounds
can be easily tuned by choosing a suitable ligand of the com-
plexes.7 Recently, elegant tuning of the emission of Alq3 deriv-
atives was achieved by the introduction of a variety of substitu-
ents at the 5-position of the ligand.11
On the other hand, the authors recently reported on a new
series of fluorophores that consist of 6-membered heterocycles,
pyrones12–14 and pyridones15 having triaryl subsituents at 3-,
4-, and 6-positions. These compounds exhibit intense fluores-
cence from blue to orange in the solid state, but this fluorescence
is weak in solution. Because the nitrogen atom on the pyridones
could have heteroatoms that would coordinate to metals, the
authors hypothesized that the introduction of hydroxy groups
to the nitrogen would newly form a metal-complex through a
5-membered chelating model including carbonyl oxygen.
This paper reports the formation of novel metal–chelate
complexes (especially aluminum complexes), based on N-
hydroxypyridone ligands, in which electron-withdrawing and
donating groups were introduced at the para-position on 3- and
6-substituted phenyl rings. The photoluminescence properties
of the complexes were studied, and blue and yellowish-green
emitting materials were obtained.
EWG
EWG
AlCl3 (1/3 equiv.)
EtOH, reflux, 3 h
Ph
O
Ph
O
O
Al
N
N
OH
3
EDG
EDG
Scheme 1. Synthesis of aluminum complexes.
in the presence of a catalytic amount of sodium hydride.15,16
The pyridones were readily synthesized by the reaction of the
pyrones with hydroxylamine hydrochloride in the presence of
triethylamine.
These pyridones were converted to the corresponding
aluminum complexes by reaction of ethanolic solutions of the
N-hydroxypyridones with aluminum chloride, followed by crys-
tallization from tetrahydrofuran and water. Table 1 listed the
yields of the last step reaction and the spectroscopic properties
in solution and in the solid state of the synthesized aluminum
complexes.
The absorption spectra of the complexes were measured in
dichloromethane (1:0 Â 10À4 M). Although the absorption max-
ima of complex 2a–2c, which had a methoxy group at the para-
Table 1. Yields and spectroscopic properties of Al complexes
In CH2Cl2 In the solid
state
ꢁ
abs/nma
solution
flu/nma
Complex EWG EDG Yield/%
ꢁ
ꢁ
flu/nm
1a
2a
H
H
H
83
60
80
72
52
43
37
—
335
339
334
333
355
358
354
380
413
413
417
414
465
472
485
514
415
417
421
416
493
519
479
494
OMe
OMe
2b
2c
Br
CF3 OMe
3a
3b
H
NMe2
NMe2
CF3 NMe2
Br
3c
Alq3
The synthesis of aluminum complexes is shown in Scheme 1.
3,6-Diaryl-4-phenyl-ꢀ-pyrones, precursors of pyridones, were
prepared from ethyl arylacetates and aroylphenylacetylenes
—
—
aIn a solution of CH2Cl2 (1:0 Â 10À4 M).
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.3 (2008)
249
80
3c
3a
2000
1000
3b
60
40
20
0
Alq3
2c
2a
1a
2b
0
300
400
500
600
700
800
400
450
500
550
600
wavelength/nm
wavelength/nm
Figure 3. The emission spectrum of Al complex 1a.
Figure 1. Fluorescence spectra of Al complexes in CH2Cl2
(1:0 Â 10À4 M).
in the solid state, and the complex having a dimethylamino
group on the para-position at the 6-phenyl group emitted yellow-
ish-green fluorescence in a dichloromethane solution. Our group
found a new type of metal complex fluorophore that emits blue
fluorescence both in the solid state and in solution.
120
2c
2a
80
Alq3
3a
1a
2b
References and Notes
3c
3b
1
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40
0
400
450
500
wavelength/nm
550
600
Figure 2. Fluorescence spectra of Al complexes in the solid
state.
2
position on the 6-substituted phenyl ring as an electron-donating
group, were almost the same as that of complex 1a, the introduc-
tion of a dimethylamino group (3a–3c) induced a significant
red-shift.
3
4
I. Aoki, T. Harada, T. Sakaki, Y. Kawahara, S. Shinkai, J.
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Complexes 1a and 2a–2c in dichloromethane solution had
fluorescence maxima at approximately 415 nm and a weakly
emitted blue fluorescence. However, complexes 3a–3c had fluo-
rescence maxima in the range of 465–485 nm and emitted green
or yellowish-green fluorescence (Figure 1). The explanation for
the red-shift might be that the strong electron-donating group de-
creased the torsion angle between the pyridone ring and the aryl
groups at the 6-position of 3a–3c.13 The fluorescence quantum
yields in solution of 3a–3c were ca. 0.7, which is about three
times higher than that of Alq3 (ꢀf 0.2 in CH2Cl2 solution).
In the solid state, complexes 1a and 2a–2c showed purple-blue
fluorescence (Figure 2). The fluorescence quantum yield of
complex 2c (ꢀf 0.25) was rather low compared with that of Alq3
(ꢀf 0.34 in the solid state).17
The emission spectrum of aluminum complex 1a is shown
in Figure 3. The EL device consists of an indium tin oxide
(ITO) a transparent electrode, a 100-nm-thick layer of aluminum
complex 1a, and a 1-nm-thick CsF carrier injection layer, a Mg
with silver electrode, and an Ag cap layer. The emission peaked
at 450 nm, and the full width at half maximum (FWHM) of the
emission peak was about 110 nm.17
5
6
C. W. Tang, S. A. Vanslyke, Appl. Phys. Lett. 1987, 51, 913.
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In summary, novel fluorescent aluminum complexes of N-
hydroxy-3,6-diaryl-4-phenyl-2-pyridones were synthesized and
their spectroscopic properties were described. The complexes
emit both in the solid state and in solution. The complex having
a trifluoromethyl group and a methoxy group on the para-posi-
tion at the 3- and 6-phenyl group exhibited purple fluorescence
16 a) F. G. Baddar, F. H. Al-Hajjar, N. R. El-Rayyes, J.
Heterocycl. Chem. 1976, 13, 195. b) Y. A. Al-Farkh, F. H.
Al-Hajjar, N. R. El-Rayyes, H. S. Hamoud, J. Heterocycl.
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17 Supporting Information is available electronically on the
CSJ-Journal web site, http//www.csj.jp/journals/chem-lett/.
Published on the web (Advance View) February 2, 2008; doi:10.1246/cl.2008.248