Synthesis of 5-dialkyl(aryl)aminomethyl-8-hydroxyquinoline dansylates as selective fluorescent sensors for Fe3+

Article abstract of DOI:10.3390/12051191

A series of 5-dialkyl(aryl)aminomethyl□-8-hydroxyquinoline dansylates were synthesized and their fluoroionophoric properties toward representative alkali ions, alkaline earth ions and transition metal ions were investigated. Among the selected ions, Fe³⁺ caused considerable quenching of the fluorescence, while Cr³⁺ caused quenching to some extent. The absence of any significant fluorescence quenching effects of the other ions examined, especially Fe²⁺, renders these compounds highly useful Fe ³⁺-selective fluorescent sensors.

Full text of DOI:10.3390/12051191

Molecules 2007, 12, 1191-1201
molecules
ISSN 1420-3049
http://www.mdpi.org
Communication
Synthesis of 5-Dialkyl(aryl)aminomethyl-8-hydroxyquinoline
Dansylates as Selective Fluorescent Sensors for Fe3+
Ruogu Peng, Feng Wang and Yaowu Sha*
The Key Laboratory of Bioorganic Phosphorous Chemistry and Chemical Biology, Department of
Chemistry, Tsinghua University, Beijing 100084, P. R. China; E-mail: prg@mails.tsinghua.edu.cn (for
Ruogu Peng)
* Author to whom correspondence should be addressed. E-mail: shayw@tsinghua.edu.cn
Received: 4 April 2007; in revised form: 21 May 2007 / Accepted: 29 May 2007 / Published: 31 May
2007
Abstract: A series of 5-dialkyl(aryl)aminomethyl-8-hydroxyquinoline dansylates were
synthesized and their fluoroionophoric properties toward representative alkali ions,
alkaline earth ions and transition metal ions were investigated. Among the selected ions,
Fe3+ caused considerable quenching of the fluorescence, while Cr3+ caused quenching to
some extent. The absence of any significant fluorescence quenching effects of the other
ions examined, especially Fe2+, renders these compounds highly useful Fe3+-selective
fluorescent sensors.
Keywords: Fluorescent sensor, Fe3+, fluorescence quenching.
Introduction
A variety of metal ions are known to play vital roles in the structural, catalytic and regulatory
aspects of biological systems [1a], therefore, sensitive and selective detection of these metal ions is of
great importance. A large number of fluorescent sensors have been reported in recent years [1b-f].
Iron is one of the most important trace elements in the human body [2a]. Increased iron availability
in serum or tissues is associated with an increased risk of several tumors and may promote
carcinogenesis [2b]. Moreover, hereditary hemochromatosis is characterized by excess iron that causes
tissue damage and fibrosis with irreversible damage to various organs [2c]. Iron homeostasis is an
important factor involved in neuroinflammation and progression of Alzheimer’s disease [2d].

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Compared to the number of fluorescent sensors for other transition metal ions, such as Hg2+ [3] and
Zn2+ [4], Fe3+-specific fluorescent sensors are comparatively rare.
Regarding the Fe3+ sensors reported up to date (Figure 1), their detection of Fe3+ is always
accompanied by interference from Cu2+ and Cr3+, which affects their selectivity. For example,
4-amino-1,8-dicyanonaphthalene derivatives exhibit considerable fluorescence enhancement by both
Fe3+ and Cr3+, with a Fe3+:Cr3+ amplification ratio of 1:0.84 [5a]. Another naphthalene-derived
fluorescent sensor showed a Fe3+:Cr3+ fluorescence-quenching ratio of 1:0.83 [2a]. A
calix[4]arene-derived fluorescent sensor could selectively detect Fe3+ and Cu2+ [5b] with a Fe3+:Cu2+
fluorescence-quenching ratio of 1:1.17, depending on the pH. The best sensor reported so far is a
derivative of 1-oxa-4,10-dithia-7-aza-cyclododecane, which displayed significant Fe3+-amplified
fluorescence with slight interference from Cu2+ [5c]. However, the test for its interaction with Cr3+ was
not undertaken.
Figure 1. Reference Fe3+ sensors.
With an aim of developing more selective fluorescent sensors for Fe3+, we noticed that most
fluorescent sensors are composed of metal-binding sites and a proximal fluorophore, and function via a
photo-induced electron transfer (PET) mechanism [2a,3a,3j]. Herein, we report the investigation of the
8-hydroxyquinoline dansylate structure as a fluorescent sensor. The dansyl group is a commonly used
fluorophore that emits strong fluorescence [6], while 8-hydroxyquinoline is an excellent chelator of
metal ions and has many significant applications [7]. The sulfonic group in the dansylate group and the
nitrogen in the quinoline could cooperatively chelate metal ions. In the predicted chelation process,
concerted changes in the distances and angles between the two ligands are required. The structural
limitations of such conformational changes could lead to ion selection. Therefore, a series of

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8-hydroxyquinoline dansylate derivatives were synthesized and found to be highly sensitive and more
selective for Fe3+.
Results and Discussion
The synthesis of the title compounds is shown in Scheme 1. First, 8-hydroxyquinoline (1) was
chloromethylated to give crude 5-chloromethyl-8-hydroxyquinoline (2). High purity products were
obtained by washing with concentrated hydrochloric acid (37%) instead of acetone [8]. The residual
8-hydroxyquinoline was not removed completely by washing with acetone and a substitution reaction
occurred during the crystallization in ethanol. Next, 2 was reacted with different secondary amines in
dichloromethane at room temperature to give 3a-3d in high yields. Triethylamine was used as the base.
The use of inorganic bases, such as sodium carbonate, sodium hydrogen carbonate and sodium
hydroxide, led to either a very slow reaction rates or many unexpected byproducts. The final products,
4a-4d, were prepared by reaction of 3a-3d with dansyl chloride in tetrahydrofuran in good yield. 4e
1
was prepared by direct dansylation of 1. The structures were identified by H-NMR, 13C-NMR,
elemental analysis and ESI-MS.
Scheme 1.
The absorption spectra of representative compounds 4c and 4e are shown in Figure 2. When
excited at 360 nm, 4a-4e showed similar emission patterns. The maximum emission wavelengths were
466, 468, 465, 466 and 463 nm, respectively, for 4a-4e. Their fluoroionophoric properties toward
representative alkali ions (K+), alkaline earth ions (Mg2+, Ca2+, Ba2+) and transition metal ions (Fe3+,
Fe2+, Co2+, Ni2+, Cu2+, Ag+, Zn2+, Cd2+ Pb2+, Mn2+, Cr3+) were investigated in methanol solutions. The
results (Table 1) revealed that only Fe3+ and Cr3+ caused considerable fluorescence quenching, similar
to levels previously described in the literature [2a,5b]. However, there were differences in the
sensitivities of 4a-4e. The best result was obtained with 4e, which showed a quenching percentage
(calculated by [(I0-I)/I0] × 100%) of 91.2% for Fe3+. Compound 4d behaved in the same way as 4e,
while 4a-4c behaved similarly, but showed slightly poorer selectivity.

Molecules 2007, 12
Figure 2. Absorption spectra of 4c and 4e (100 µM) upon adding 1.0 equiv of Fe3+ in methanol.
1194
4c
2.5
2.0
1.5
1.0
0.5
0.0
3+
1 eq. Fe
3+
0 eq. Fe
250
300
350
400
450
500
550
W avelength(nm )
4e
2.5
2.0
1.5
1.0
0.5
0.0
3+
1 eq. Fe
3+
0 eq. Fe
250
300
350
400
450
500
550
Wavelength(nm)
Table 1. Fluorescence quenching of 4a-4e by 1.0 equiv of Fe3+.
Relevant
intensity
Quenching percent. of
Fe3+(%)
Quenching percent. of
Cr3+(%)
Quenching
Compd.
λ
max(nm)
ratio(Fe3+:Cr3+)
466
468
465
466
463
57.8
58.7
59.7
26.5
38.5
72.2
74.8
70.5
87.4
91.2
59.2
62.8
58.5
60.6
52.9
1:0.82
1:0.84
1:0.83
1:0.69
1:0.58
4a
4b
4c
4d
4e
The fluorescence quenching data for sensors 4c and 4e are shown in Figure 3. Fe3+ caused the
highest level of fluorescence quenching, while Cr3+ caused quenching to some extent. The other ions

Molecules 2007, 12
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showed little fluorescence quenching. However, the selectivity between Fe3+ and Cr3+ was improved
comparative with previously reported Fe3+ sensors. For example, the best quenching ratio was 1:0.58
(Fe3+:Cr3+) for 4e, which is better than the previously reported ratio of 1:0.83 for a naphthalene-derived
fluorescent sensor [5b]. Furthermore, the absence of any significant fluorescence quenching effects by
Cu2+, Mn2+, Co2+, Ni2+ and especially Fe2+ renders these derivatives highly useful Fe3+-selective
sensors.
Figure 3. Quenching percentages [(I0-I)/ I0]×100% of the fluorescence intensities of 4c
and 4e (100 µM) upon the addition of 1.0 equiv of metal ions in methanol.
In order to gain further insights into the analytical possibilities, fluorescence titration of 4e by Fe3+
was performed and the obtained curves are shown in Figure 4. The quenching was very effective and a
quenching efficiency of 91.2% was obtained at a ratio of 1:1. The break around 1.0 equiv of Fe3+
suggested a 1:1 stoichiometry for the 4e-Fe3+ complex system [3j]. By working on the Stern-Volmer
equation [9,3a], the association constant was calculated to be about 1.1×105 M-1.

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Figure 4. Fluorescence titration curves of 4e with Fe3+ in methanol. [4e]=100 µM. The
inset shows a plot of the fluorescence intensities against [Fe3+]/[4e].
Next, a competition experiment was carried out in the presence of 4.0 equiv of background metal
ions in order to confirm the practical applicability of 4e as a Fe3+ sensor (Figure 5). Most of the metal
ions, including Cr3+, did not show any obvious interference with the detection of Fe3+, while Cu2+,
Co2+, Zn2+ and Mn2+ showed only moderate interference. Therefore, 4e is a potential Fe3+-selective
fluorescent chemosensor.
Figure 5. Fluorescence quenching percentages [(I0-I)/I0]×100% of 4e (100 µM) upon the
addition of 1.0 equiv of Fe3+ and 4.0 equiv of background metal ions in methanol.
100
80
60
40
20
0
No ion K+ Mg2+Ca2+Ba2+Mn2+Fe2+Co2+Ni2+Cu2+Ag+ Zn2+Cd2+Pb2+Cr3+
In order to gain further insights into the interaction between Fe3+ and designed sensors, related
conformations were simulated using the MM2 method as implemented in Chem3D. The optimized
conformation of 4e is shown in Figure 4e and the 4e-Fe3+ complex in Figure 6. Among the changes,
the most important would be the naphthyl conformation. It actually changed from a plane form to a

Molecules 2007, 12
1197
twisted one. Therefore the conjugation system of dansyl moiety has been severely damaged and this
may be the actual cause of the observed fluorescence quenching.
Figure 6. Computer simulated conformation of 4e(a) and 4e-Fe3+ complex(b).
Conclusions
A series of novel Fe3+ fluorescent sensors based on 8-hydroxyquinoline and the dansyl group were
synthesized and their fluoroionophoric properties toward K+, Mg2+, Ca2+, Ba2+, Fe3+, Fe2+, Co2+, Ni2+,
Cu2+, Ag+, Zn2+, Cd2+, Pb2+, Mn2+ and Cr3+ were investigated in methanol solutions. Only Fe3+ caused
considerable quenching of the fluorescence emission, and the other metal ions showed little
interference with the detection of Fe3+. Among these potential metal ion fluorescent sensors, 4e
showed the best efficiency and selectivity toward Fe3+.
Experimental
General
All solvents and reagents were purchased from Aldrich and used without further purification.
1H-NMR (300 MHz) and 13C-NMR (75 MHz) spectra were recorded on a JOEL JNM-ECA300
spectrometer. Chemical shifts (δ) are reported in parts per million (ppm) relative to tetramethylsilane
(TMS) as an internal standard, and coupling constants (J) are given in hertz (Hz). Elemental analyses
were carried out on an Elementar Vario EL CHN element analysis instrument. All ESI-MS
experiments were undertaken on a Bruker ESQUIRE-LC. Fluorescence intensity was measured using a
HORABA Fluoromax 3.
General procedure for the preparation of 5-chloromethyl-8-hydroxyquinoline hydrogen chloride 2
A stream of hydrogen chloride gas was blown through a solution of 8-hydroxyquinoline 1 (0.1 mol)
and formaldehyde (20 mL, 37%) in 37% hydrochloric acid (50 mL) for 8 hours at 50°C. After
filtration, the product was washed with 37% hydrochloric acid and dried to afford 2 in 85% yield.

Molecules 2007, 12
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General procedure for the preparation of 5-dialkyl(aryl)aminomethyl-8-hydroxyquinoline 3a-3d
To a solution of 2 (10 mmol) and triethylamine (30 mmol) in dichloromethane (50 mL) was added
dropwise a solution of the appropriate dialkyl(aryl)amine (10 mmol) in dichloromethane (50 mL) at
room temperature over 30 minutes. After a further 15 minute of reaction, the solution was washed
thoroughly with brine, dried over magnesium sulfate and concentrated to afford 3a-3d without further
purification (yields are listed in Scheme 1).
General procedure for the preparation of 5-dialkylaminomethyl-8-hydroxyquinoline dansylate 4a-4e
To a solution of 3a-3d or 1 (1 mmol) in THF (10 mL) was added sodium hydroxide (5 mmol),
followed by stirring for 5 minutes. Next, a solution of dansyl chloride (1 mmol) in THF (10 mL) was
added dropwise at room temperature over 5 minutes. The reaction was kept at room temperature for a
further 10 minutes and then filtered. The filtrate was evaporated under vacuum and the crude product
was purified by silica gel column chromatography (EtOAc-Petroleum Ether).
1
5-Dimethylaminomethyl-8-hydroxyquinoline dansylate (4a). H-NMR (CDCl3) δ 8.68-8.71 (m, 2H), δ
8.53-8.60 (m, 2H), 8.17 (d, 1H, J=7.20), 7.58-7.63 (m, 1H), 7.44 (t, 1H, J=7.89), 7.31-7.35 (m, 2H),
13
7.17-7.25 (m, 2H), 3.70 (s, 2H), 2.87 (s, 6H), 2.21 (s, 6H); C-NMR (CDCl3) δ 151.7, 150.5, 145.6,
142.1, 134.6, 133.1, 132.5, 131.8, 130.7, 130.5, 129.9, 128.9, 128.8, 127.0, 123.0, 121.7, 121.0, 120.4,
115.6, 61.7, 45.6, 37.5; ESI-MS: calcd for (M+H)/z: 436.2. Found: (M+H)/z: 436.2. Anal. Calcd for
C24H25N3O3S: C 66.18, H 5.19, N 9.65; found C 65.92, H 5.36, N 9.55.
1
5-Diethylaminomethyl-8-hydroxyquinoline dansylate (4b). H-NMR (CDCl3) δ 8.70-8.73 (m, 2H), δ
8.57-8.66 (m, 2H), 8.18 (d, 1H, J=7.23), 7.58-7.63 (m, 1H), 7.44 (t, 1H, J=7.89), 7.31-7.36 (m, 2H),
7.17-7.21 (m, 2H), 3.87 (s, 2H), 2.87 (s, 6H), 2.51 (q, 4H, J=6.84), 1.00 (t, 6H, J=6.84); 13C-NMR
(CDCl3) δ 151.7, 150.4, 145.3, 142.1, 133.1, 132.5, 131.8, 130.7, 130.5, 129.9, 128.9, 128.8, 126.8,
125.6, 123.0, 121.4, 121.0, 120.4, 115.6, 55.5, 46.8, 45.6, 11.4; ESI-MS: calcd for (M+H)/z: 464.2.
Found: (M+H)/z: 464.1. Anal. Calcd for C26H29N3O3S: C 67.36, H 6.31, N 9.06; found C 67.13, H 6.44,
N 9.31.
5-Di-n-butylaminomethyl-8-hydroxyquinoline dansylate (4c). 1H-NMR (CDCl3) δ 8.75 (d, 1H, J=8.58),
8.70 (d, 1H, J=3.78), 8.64 (d, 1H, J=8.58), 8.59 (d, 1H, J=8.58), 8.17 (d, 1H, J=7.56), 7.60-7.66 (m,
1H), 7.43 (t, 1H, J=7.92), 7.30-7.34 (m, 2H), 7.19-7.23 (m, 2H), 3.84 (s, 2H), 2.88 (s, 6H), 2.38 (d, 4H,
J=7.20), 1.35-1.42 (m, 4H), 1.15-1.22 (m, 4H), 0.77 (t, 6H, J=7.23); 13C-NMR (CDCl3) δ 151.7, 150.4,
145.2, 142.1, 136.0, 133.3, 132.6, 131.8, 130.7, 130.6, 129.9, 128.9, 128.7, 126.6, 122.9, 121.2, 121.1,
120.5, 115.6, 57.1, 53.7, 45.6, 28.9, 20.7, 14.1; ESI-MS: calcd for (M+H)/z: 520.3. Found: (M+H)/z:
520.3. Anal. Calcd for C30H37N3O3S: C 69.33, H 7.18, N 8.09; found C 69.23, H 7.04, N 8.27.
1
5-Diphenylaminomethyl-8-hydroxyquinoline dansylate (4d). H-NMR (CDCl3) δ 8.62-8.83 (m, 3H), δ
8.20-8.31 (m, 2H), 7.61-7.66 (m, 1H), 7.39-7.50 (m, 3H), 7.22-7.26 (m, 6H), 6.95-7.17 (m, 6H), 5.34

Molecules 2007, 12
1199
(s, 2H), 2.91 (s, 6H); 13C-NMR (CDCl3) δ 151.7, 150.6, 147.8, 145.2, 142.3, 133.4, 132.6, 131.9, 130.9,
130.7, 130.5, 130.0, 129.5, 129.4, 128.9, 127.3, 123.1, 122.0, 121.8, 121.0, 120.8, 120.4, 115.7, 53.6,
45.6; ESI-MS: calcd for (M+H)/z: 560.2. Found: (M+H)/z: 560.3. Anal. Calcd for C34H29N3O3S: C
72.96, H 5.22, N 7.51; found C 72.70, H 5.05, N 7.68.
8-Hydroxyquinoline dansylate (4e). 1H-NMR (CDCl3) δ 8.71-8.75 (m, 2H), 8.60 (d, 1H, J=8.58), 8.18
(d, 1H, J=7.56), 8.06 (d, 1H, J=8.22), 7.60-7.67 (m, 2H), 7.45 (t, 1H, J=7.89), 7.31-7.40 (m, 2H),
13
7.20-7.25 (m, 2H), 2.88 (s, 6H); C-NMR (CDCl3) δ151.7, 150.9, 145.9, 141.9, 135.7, 132.4, 131.9,
130.7, 130.5, 129.9, 129.7, 128.8, 127.0, 125.9, 123.0, 122.0, 121.9, 120.4, 115.7, 45.6; ESI-MS: calcd
for (M+H)/z: 379.1. Found: (M+H)/z: 379.0. Anal. Calcd for C21H18N2O3S: C 66.65, H 4.79, N 7.40;
found C 66.88, H 5.26, N 7.38.
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Sample Availability: Compound 4e is available from the authors.
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