Synthesis of fluorescent dendritic 8-hydroxyquinoline ligands and investigation on their coordinated Zn(II) complexes

Article abstract of DOI:10.1016/j.tetlet.2004.03.106

A series of luminescent PAMAM dendrons emanating from 8-hydroxyquinoline have been synthesized and their coordination with Zn(II) was investigated for the first time. The obtained dendritic Zn(II) complexes were soluble in common organic solvents. It was found that the luminescence intensity of G2 dendron 6 was higher than that of G1 dendron 4. Furthermore, when they were coordinated with Zn(II), red-shift was observed and the intensities of the coordinated Zn(II) complexes were higher than that of the corresponding ligands.

Full text of DOI:10.1016/j.tetlet.2004.03.106

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 3961–3964
Synthesis of fluorescent dendritic 8-hydroxyquinoline ligands
and investigation on their coordinated Zn(II) complexes
a,
a
b
Liang Shen,a Fuyou Li,b,* Yaowu Sha, Xiaoyin Hong and Chunhui Huang
*
aThe Key Laboratory of Bioorganic Phosphorous Chemistry and Chemical Biology, Ministry of Education,
Department of Chemistry, Tsinghua University, Beijing 100084, PR China
bState Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University, Beijing 100871, PR China
Received 18 July 2003;revised 2 March 2004;accepted 5 March 2004
Abstract—A series of luminescent PAMAM dendrons emanating from 8-hydroxyquinoline have been synthesized and their coor-
dination with Zn(II) was investigated for the first time. The obtained dendritic Zn(II) complexes were soluble in common organic
solvents. It was found that the luminescence intensity of G2 dendron 6 was higher than that of G1 dendron 4. Furthermore, when
they were coordinated with Zn(II), red-shift was observed and the intensities of the coordinated Zn(II) complexes were higher than
that of the corresponding ligands.
ꢀ 2004 Elsevier Ltd. All rights reserved.
Dendrimers are highly ordered, globular monodisperse
macro-molecules composed of repeated branch units
emanating from a central core.1 Their physical proper-
ties depend mainly on their ending groups. Dendrimers
with an active site encapsulated inside, for example, the
core as a ligand of coordination, are very interesting.
Dendrimer, containing a photo- or/and an electro-active
unit as a core, was expected to have significant
enhancement in its photochemical and electrochemical
properties, in comparison with its core molecule.2;3
tection of hydroxyl group in 8-hydroxyquinoline was
efficiently avoided. Furthermore, the coordination of
ester-terminated dendrons 4 and 6 with Zn(II) in ethyl
acetate was investigated for the first time. Enhanced
green fluorescence emitting of the complexes was
observed.
As shown in Scheme 1, 5-amino-8-hydroxyquinoline 1,
was acylated using 1.0 equiv of acryloyl chloride in the
presence of 1.0 equiv of Et3N.9 The crude product was
washed with CH2Cl2 to yield 2 (50%) as a white powder.
The Michael reaction of 2 with ethylenediamine was
carried out with a large excess of ethylenediamine in
methanol, followed by chromatography of the crude
product with CH2Cl2–CH3OH (25:1 v/v), to yield 3
(80%) as a yellow powder. A further Michael reaction of
3 with methyl acrylate was carried out in methanol to
give 4 as yellow oil in 75% yield after column chroma-
tography (silica gel). The second generation of ester-
terminated dendritic ligand 6 (50%), as yellow oil, was
prepared from 4 by the previous method.10 Their
structures were assigned by ESI-MS, 1H NMR, 13C
NMR, 2D 13C–1H COSY and IR spectra.11 The experi-
mental result showed that the ester-terminated ligands 4
and 6 have good solubility in most common organic
solvent, such as CH3OH, CH2Cl2, CHCl3, THF,
CH3CN and EtOAc.
8-Hydroxyquinoline and its derivatives are of the most
extensively investigated ligands in the coordination
chemistry.4 They have been used in metallic ion detec-
tion,5 chromatography6 and especially in organic light-
emitting diodes (OLEDs).7;8 Most metallic complexes of
8-hydroxy-quinoline have poor solubility in organic
solvents, so that tedious vacuum-grafting technique had
to be applied to fabricate films for OLEDs. Therefore,
to synthesize readily soluble metallic complexes of
8-hydroxyquinoline is of great importance.
Here, we report a convenient method for the prepara-
tion of a group of solvent-soluble dendrons emanating
from 8-hydroxyquinoline. The protection and depro-
Keywords: Synthesis;Dendritic;8-Hydroxyquinoline;Luminescence.
* Corresponding authors. Tel./fax: +86-10-62795866 (Y.S.);e-mail:
shayw@tsinghua.edu.cn
Dendritic complexes Zn(4)2 and Zn(6)2 were formed by
treating dendron 4 and 6 (30 mL, 1.9 · 10ꢀ4 M, in ethyl
0040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.03.106

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L. Shen et al. / Tetrahedron Letters 45 (2004) 3961–3964
HO
NH
HO
NH
HO
NH
O
NH
HO
NH
O
N
OMe
O
2
b
a
c
O
N
O
N
N
N
NH
N
2
2
1
3
4
OMe
O
O
OMe
NH
MeO
NH
b
NH
2
N
O
HO
NH
O
N
O
HO
NH
O
N
OMe
c
N
O
N
O
N
MeO
N
N
O
O
O
HN
O
NH
HN
2
NH
O
6
5
NH
MeO
a: CH =CHCOCl/NEt /CH Cl
2 3 2 2
H N
2
b: Excess NH CH CH NH /MeOH
2
2
2
2
O
N
MeO
c: Excess CH =CHCO Me/MeOH
2
2
OMe
O
MeO
O
NH
N
OMe
O
O
O
MeO
O
N
O
N
Zn
O
N
O
O
N
OMe
O
MeO
N
HN
OMe
Zn(4)
2
O
O
N
MeO
MeO
NH
OMe
N
N
O
OMe
O
O
HN
N
O
NH
N
OMe
O
O
NH
O
N
O
MeO
N
N
O
O
Zn
O
O
OMe
O
N
HN
O
O
MeO
O
HN
N
HN
NH
MeO
O
O
N
O
N
MeO
OMe
OMe
Zn(6)2
O
Scheme 1. Synthesis of dendritic 8-hydroxyquinoline ligands and their complexes with Zn(II).
acetate), respectively, with stoichiometric amounts of
Zn(OAc)2 in CH3OH (0.50 mL). The resulting dendri-
mer complexes with Zn(II), Zn(4)2 and Zn(6)2, have
been confirmed by ESI-MS spectrum. At the first gen-
eration, the mass spectrum of 4 exhibits an intense signal
at m/z 533 corresponding to [4+H]þ and a weak signal at
m/z 555 corresponding to [4+Na]þ. The signal at m/z 533
almost disappears after addition of a stoichiometric
amount of Zn(OAc)2, whereas two intense signals at m/z
1128 and 1150 corresponding to [Zn(4)2+H]þ and
[Zn(4)2+Na]þ were emerged. For the second generation,
the mass spectrum of 6 exhibits an intense signal at m/z
1134 corresponding to [6+H]þ. The signal almost dis-
appears after addition of a stoichiometric amount of
Zn(OAc)2, whereas an intense signal at m/z 2351 corre-
sponding to [Zn(6)2+Na]þ appeared (Fig. 1).
The fluorescence spectra of the dendrons (4 and 6) and
the dendrimer complexes Zn(4)2 and Zn(6)2 were shown
in Figures 2 and 3. The intensity of 6 is higher than that
of 4 at 520 nm. The dendrimer complexes show stronger
fluorescent intensity and a red-shifted emission in

L. Shen et al. / Tetrahedron Letters 45 (2004) 3961–3964
3963
comparison with their dendrons. The dendrimer com-
plexes Zn(4)2 and Zn(6)2 in ethyl acetate emit at 536 and
533 nm, respectively. The intensity of Zn(4)2 is higher
than that of 4 and the intensity of Zn(6)2 is higher than
that of 6. From a spectroscopic and photophysical
viewpoint, they could consist of a core of bis(8-
hydroxyquinoline) zinc(II) (ZnQ2) chromophoric unit.
ZnQ2 in CH2Cl2 emits at 535 nm.12 The emission spectra
in visible region for ZnQ2, Zn(4)2 and Zn(6)2 are quite
similar, indicating that the shape and chemical proper-
ties of the dendrimer branches do not substantially affect
the ZnQ2-based chromophoric core. Figures 2 and 3
show that the higher generation dendron or dendrimer
complex exhibits stronger luminescence. All of the
luminescence measurements were carried out in air-
equilibrated solutions at room temperature with a Per-
kin–Elmer LS50B spectrofluorimeter.
In conclusion, an efficient divergent procedure for syn-
thesizing novel dendritic 8-hydroxyquinoline ligands
was described and their complexation with Zn(II) was
investigated. The ester-terminated dendrimer Zn(II)
complexes emit green fluorescence and have excellent
solubility in a wide-range of organic solvents. Work is
continuing in our laboratory to synthesize high genera-
tion of dendrons and their complexes with other metallic
ions.
Figure 1. (a) ESI-MS spectrum of 4 (1.9 · 10ꢀ4 M, in ethyl acetate)
before (left) and after (right) addition of a stoichiometric amount of
Zn(OAc)2;(b) ESI-MS spectrum of 6 (1.9 · 10ꢀ4 M, in ethyl acetate)
before (left) and after (right) addition of a stoichiometric amount of
Zn(OAc)2.
250
200
150
100
50
Acknowledgements
We thank the National Natural Science Foundation of
China (No 50233030, 20221101), NHTRDP of PR
China (2002AA 302403) and Basic Research Founda-
tion of Tsinghua University (No 2002030) for support of
this research.
0
450
500
550
600
650
References and notes
Wavelength (nm)
Figure 2. Luminescence of 4 (1.9 · 10ꢀ4 M, in ethyl acetate) before
(dashed line) and after (full line, kmax ¼ 536 nm) addition of a stoi-
chiometric amount of Zn(OAc)2 on excitation at 400 nm (room tem-
perature).
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450
500
550
600
650
Wavelength (nm)
Figure 3. Luminescence of 6 (1.9 · 10ꢀ4 M, in ethyl acetate) before
(dashed line) and after (full line, kmax ¼ 533 nm) addition of a stoi-
chiometric amount of Zn(OAc)2 on excitation at 400 nm (room tem-
perature).
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3964
L. Shen et al. / Tetrahedron Letters 45 (2004) 3961–3964
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138.2, 132.0, 124.2, 124.2, 124.1, 121.7, 109.4, 51.7, 51.5,
51.4, 50.8, 49.4, 49.3, 33.9, 32.0, 31.5;ESI-MS: calcd for
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(M+H)þ: 533. Found: m/z (M+H)þ: 533. Compound 6: IR
3413, 2955, 2835, 1734, 1649 cmꢀ1 1H NMR (400 MHz,
;
CDCl3) d 9.69 (s, 1H, ArNHC@O), 8.77 (dd, 1H, J ¼ 4:2,
1.4), 8.39 (d, 1H, J ¼ 8:3), 8.18 (br s, 1H, ArOH), 7.58 (d,
1H, J ¼ 8:2), 7.49 (dd, 1H, J ¼ 8:6, 4.2), 7.12 (d, 1H,
J ¼ 8:2), 7.02 (s, 2H, –NHC@O), 6.96 (s, 1H, –NHC@O),
3.65 (s, 12H, –OCH3), 3.63 (s, 6H, –OCH3), 3.26–3.22 (m,
4H, –CH2NHC@O), 2.87 (br, 4H, –NCH2–, 2H,
–CH2NHC@O), 2.77–2.70 (m, 4H, –NCH2–), 2.73 (t,
8H, –NCH2–), 2.64 (br, 4H, –NCH2CH2N–, 2H,
–CH2C@O), 2.56 (t, 4H, –NCH2–), 2.49 (t, 4H, –NCH2–),
2.41 (t, 12H, –CH2COOCH3), 2.30 (t, 6H, –CH2CONH–),
2.16 (t, 2H, –NCH2–); 13C NMR (100 MHz, CDCl3) d
173.0, 172.6, 172.3, 150.3, 147.8, 138.1, 132.6, 124.8, 124.3,
124.0, 121.6, 109.2, 52.9, 52.4, 51.8, 51.6, 51.6, 51.4, 51.1,
50.1, 49.8, 49.2, 49.0, 37.1, 36.9, 34.8, 33.8, 33.6, 32.6;ESI-
MS: calcd for (M+H)þ: 1133. Found: m/z (M+H)þ: 1133.
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11. Selected characterization data of the dendritic ligands.
Compound 4: IR 3263, 2952, 2833, 1735, 1682, 1657 cmꢀ1
;
1H NMR (400 MHz, CDCl3) d 9.68 (s, 1H, ArNHC@O),
8.79 (dd, 1H, J ¼ 4:2, 1.3), 8.22 (dd, 1H, J ¼ 8:5, 1.3), 8.21
(br s, 1H, ArOH), 7.56 (d, 1H, J ¼ 8:2), 7.49 (dd, 1H,
J ¼ 8:5, 4.2), 7.15 (d, 1H, J ¼ 8:2), 3.63 (s, 6H, –OCH3),
3.42 (s, 3H, –OCH3), 2.95 (t, 2H, –NCH2–), 2.89 (t, 2H,
–NCH2–), 2.73 (t, 4H, –NCH2–), 2.69 (t, 2H, –NCH2–),
2.66 (t, 2H, –NCH2–), 2.59 (t, 2H, –CH2C@O), 2.56 (t,
2H, –CH2C@O), 2.35 (t, 4H, –CH2C@O); 13C NMR
(100 MHz, CDCl3) d 172.9, 172.8, 171.9, 150.6, 147.8,